Professor (Research), Psychiatry and Behavioral Sciences
Member, Wu Tsai Neurosciences Institute
Honors & Awards
Fellow, American Association for the Advancement of Science (2012)
Drug exposure induces cell and synaptic plasticity within the brain reward pathway that could be a catalyst for progression to addiction. Several cellular adaptations have been described in the ventral tegmental area (VTA), a central component of the reward pathway that is the major source of dopamine release. For example, administration of morphine induces long-term potentiation (LTP) of excitatory synapses on VTA dopamine cells and blocks LTP at inhibitory synapses. Drug-induced synaptic changes have a common endpoint of increasing dopamine cell firing and dopamine release. However, gaining a complete picture of synaptic plasticity in the VTA is hindered by its complex circuitry of efferents and afferents. Most studies of synaptic plasticity in the VTA activated a mixed population of afferents, potentially yielding an incomplete and perhaps misleading view of how drugs of abuse modify VTA synapses. Here, we use midbrain slices from mice and find that electrical stimulation in two different regions induces different forms of plasticity, including two new forms of LTP at inhibitory synapses. High frequency stimulation induces LTP independently of NMDA receptor activation, and surprisingly, some inhibitory inputs to the VTA also undergo NMDAR-independent LTP after a low frequency stimulation (LFS) pairing protocol.SIGNIFICANCE STATEMENT Synaptic plasticity of inhibitory inputs onto dopamine cells in the ventral tegmental area has a major influence on the circuits implicated in addictive behaviors. The location of electrical stimulation in an acute midbrain slice dictatedthe response of inhibitory inputs to plasticity induction protocols. We describe a new form of synaptic strengthening that occurs at an opioid-sensitive input to the ventral tegmental area.
View details for DOI 10.1523/ENEURO.0137-19.2019
View details for PubMedID 31619451
The trigeminal nucleus caudalis (TNc) receives extensive afferent innervation from peripheral sensory neurons of the trigeminal ganglion (TG), and is the first central relay in the circuitry underpinning orofacial pain. Despite the initial characterization of the neurons in the superficial laminae, many questions remain. Here we report on electrophysiological properties of 535 superficial lamina I/II TNc neurons. Based on their firing pattern, we assigned these cells to five main groups, including (1) tonic, (2) phasic, (3) delayed, (4) H-current, and (5) tonic-phasic neurons, groups that exhibit distinct intrinsic properties and share some similarity with groups identified in the spinal dorsal horn. Driving predominantly nociceptive TG primary afferents using optogenetic stimulation in TRPV1/ChR2 animals, we found that tonic and H-current cells are most likely to receive pure monosynaptic input, whereas delayed neurons are more likely to exhibit inputs that appear polysynaptic. Finally, for the first time in TNc neurons, we used unsupervised clustering analysis methods and found that the kinetics of the action potentials and other intrinsic properties of these groups differ significantly from one another. Unsupervised spectral clustering based solely on a single voltage response to rheobase current was sufficient to group cells with shared properties independent of action potential discharge pattern, indicating that this approach can be effectively applied to identify functional neuronal subclasses. Together, our data illustrate that cells in the TNc with distinct patterns of TRPV1/ChR2 afferent innervation are physiologically diverse, but can be understood as a few major groups of cells having shared functional properties.
View details for DOI 10.14814/phy2.14112
View details for PubMedID 31215180
In this issue of Neuron, Li etal. (2019) distinguish two separable GABAergic projections from the ventral tegmental area (VTA) to the dorsal raphe nucleus (DRN), with differential mu-opioid receptor regulation, each targeting different postsynaptic neurons and promoting opposing behavioral states.
View details for PubMedID 30790535
Of the fast ionotropic synapses, glycinergic synapses are the least well understood, but are vital for the maintenance of inhibitory signaling in the brain and spinal cord. Glycinergic signaling comprises half of the inhibitory signaling in the spinal cord, and glycinergic synapses are likely to regulate local nociceptive processing as well as the transmission to the brain of peripheral nociceptive information. Here we have investigated the rapid and prolonged potentiation of glycinergic synapses in the superficial dorsal horn of young male and female mice after brief activation of NMDA receptors (NMDARs). Glycinergic inhibitory postsynaptic currents (IPSCs) evoked with lamina II-III stimulation in identified GABAergic neurons in lamina II were potentiated by bath-applied Zn2+ and were depressed by the prostaglandin PGE2, consistent with the presence of both GlyRalpha1- and GlyRalpha3-containing receptors. NMDA application rapidly potentiated synaptic glycinergic currents. Whole-cell currents evoked by exogenous glycine were also readily potentiated by NMDA, indicating that the potentiation results from altered numbers or conductance of postsynaptic glycine receptors. Repetitive depolarization alone of the postsynaptic GABAergic neuron also potentiated glycinergic synapses, and intracellular EGTA prevented both NMDA-induced and depolarization-induced potentiation of glycinergic IPSCs. Optogenetic activation of trpv1 lineage afferents also triggered NMDAR-dependent potentiation of glycinergic synapses. Our results suggest that during peripheral injury or inflammation, nociceptor firing during injury is likely to potentiate glycinergic synapses on GABAergic neurons. This disinhibition mechanism may be engaged rapidly, altering dorsal horn circuitry to promote the transmission of nociceptive information to the brain.
View details for DOI 10.1371/journal.pone.0222066
View details for PubMedID 31498817
The peripheral trigeminovascular pathway mediates orofacial and craniofacial pain and projects centrally to the brainstem trigeminal nucleus caudalis (TNc). Sensitization of this pathway is involved in many pain conditions, but little is known about synaptic plasticity at its first central synapse. We have taken advantage of optogenetics to investigate plasticity selectively evoked at synapses of nociceptive primary afferents onto TNc neurons. Based on immunolabeling in the trigeminal ganglia, TRPV1-lineage neurons comprise primarily peptidergic and nonpeptidergic nociceptors. Optical stimulation of channelrhodopsin-expressing axons in the TRPV1/ChR2 mouse in TNc slices thus allowed us to activate a nociceptor-enriched subset of primary afferents. We recorded from lamina I/II neurons in acutely prepared transverse TNc slices, and alternately stimulated two independent afferent pathways, one with light-activated nociceptive afferents and the other with electrically-activated inputs. Low-frequency optical stimulation induced robust long-term depression (LTD) of optically-evoked EPSCs, but not of electrically-evoked EPSCs in the same neurons. Blocking NMDA receptors or nitric oxide synthase strongly attenuated LTD, whereas a cannabinoid receptor 1 antagonist had no effect. The neuropeptide PACAP-38 or the nitric oxide donors nitroglycerin or sodium nitroprusside are pharmacologic triggers of human headache. Bath application of any of these three compounds also persistently depressed optically-evoked EPSCs. Together, our data show that LTD of nociceptive afferent synapses on trigeminal nucleus neurons is elicited when the afferents are activated at frequencies consistent with the development of central sensitization of the trigeminovascular pathway.SIGNIFICANCE STATEMENT Animal models suggest that sensitization of trigeminovascular afferents plays a major role in craniofacial pain syndromes including primary headaches and trigeminal neuralgia, yet little is known about synaptic transmission and plasticity in the brainstem trigeminal nucleus caudalis (TNc). Here we used optogenetics to selectively drive a nociceptor-enriched population of trigeminal afferents while recording from superficial laminae neurons in the TNc. Low-frequency optical stimulation evoked robust long-term depression at TRPV1/ChR2 synapses. Moreover, application of three different headache trigger drugs also depressed TRPV1/ChR2 synapses. Synaptic depression at these primary afferent synapses may represent a newly identified mechanism contributing to central sensitization during headache.
View details for PubMedID 30054391
Short-term synaptic plasticity contributes to many computations in the brain and allows synapses to keep a finite record of recent activity. Here we have investigated the mechanisms underlying an intriguing form of short-term plasticity termed labile LTP, at hippocampal and PFC synapses in male rats and male and female mice. In the hippocampus, labile LTP is triggered by high-frequency activation of presynaptic axons and is rapidly discharged with further activation of those axons. However, if the synapses are quiescent, they remain potentiated until further presynaptic activation. To distinguish labile LTP from NMDAR-dependent forms of potentiation, we blocked NMDARs in all experiments. Labile LTP was synapse-specific and was accompanied by a decreased paired pulse ratio, consistent with an increased release probability. Presynaptic Ca2+ and protein kinase activation during the tetanus appeared to be required for its initiation. Labile LTP was not reversed by a PKC inhibitor and did not require either RIM1? or synaptotagmin-7, proteins implicated in other forms of presynaptic short-term plasticity. Similar NMDAR-independent potentiation could be elicited at synapses in mPFC. Labile LTP allows for rapid information storage that is erased under controlled circumstances and could have a role in a variety of hippocampal and prefrontal cortical computations related to short-term memory.SIGNIFICANCE STATEMENT Changes in synaptic strength are thought to represent information storage relevant to particular nervous system tasks. A single synapse can exhibit multiple overlapping forms of plasticity that shape information transfer from presynaptic to postsynaptic neurons. Here we investigate the mechanisms underlying labile LTP, an NMDAR-independent form of plasticity induced at hippocampal synapses. The potentiation is maintained for long periods as long as the synapses are infrequently active, but with regular activation, the synapses are depotentiated. Similar NMDAR-independent potentiation can also be induced at L2/3-to-L5 synapses in mPFC. Labile LTP requires a rise in presynaptic Ca2+ and protein kinase activation but is unaffected in RIM1? or synaptotagmin-7 mutant mice. Labile LTP may contribute to short-term or working memory in hippocampus and mPFC.
View details for PubMedID 29802202
Ventral tegmental area (VTA) dopaminergic neurons are key components of the reward pathway, and their activity is powerfully controlled by a diverse array of inhibitory GABAergic inputs. Two major sources of GABAergic nerve terminals within the VTA are local VTA interneurons and neurons in the rostromedial tegmental nucleus (RMTg). Here, using optogenetics, we compared synaptic properties of GABAergic synapses on VTA dopamine neurons using selective activation of afferents that originate from these two cell populations. We found little evidence of co-release of glutamate from either input, but RMTg-originating synaptic currents were reduced by strychnine, suggesting co-release of glycine and GABA. VTA-originating synapses displayed a lower initial release probability, and at higher frequency stimulation, short-term depression was more marked in VTA- but not RMTg-originating synapses. We previously reported that nitric oxide (NO)-induced potentiation of GABAergic synapses on VTA dopaminergic cells is lost after exposure to drugs of abuse or acute stress; in these experiments, multiple GABAergic afferents were simultaneously activated by electrical stimulation. Here we found that optogenetically-activated VTA-originating synapses on presumptive dopamine neurons also exhibited NO-induced potentiation, whereas RMTg-originating synapses did not. Despite providing a robust inhibitory input to the VTA, RMTg GABAergic synapses are most likely not those previously shown by our work to be persistently altered by addictive drugs and stress. Our work emphasises the idea that dopamine neuron excitability is controlled by diverse inhibitory inputs expected to exert varying degrees of inhibition and to participate differently in a range of behaviours.
View details for PubMedID 29480954
Stressful experiences potently activate kappa opioid receptors (?ORs). ?ORs in the ventral tegmental area regulate multiple aspects of dopaminergic and non-dopaminergic cell function. Here we show that at GABAergic synapses on rat VTA dopamine neurons, a single exposure to a brief cold-water swim stress induces prolonged activation of ?ORs. This is mediated by activation of the receptor during the stressor followed by a persistent, ligand-independent constitutive activation of the ?OR itself. This lasting change in function is not seen at ?ORs at neighboring excitatory synapses, suggesting distinct time courses and mechanisms of regulation of different subsets of ?ORs. We also provide evidence that constitutive activity of ?ORs governs the prolonged reinstatement to cocaine-seeking observed after cold water swim stress. Together, our studies indicate that stress-induced constitutive activation is a novel mechanism of ?OR regulation that plays a critical role in reinstatement of drug seeking.
View details for PubMedID 28402252
There is a high demand for in vitro models of the central nervous system (CNS) to study neurological disorders, injuries, toxicity, and drug efficacy. Three-dimensional (3D) in vitro models can bridge the gap between traditional two-dimensional culture and animal models because they present an in vivo-like microenvironment in a tailorable experimental platform. Within the expanding variety of sophisticated 3D cultures, scaffold-free, self-assembled spheroid culture avoids the introduction of foreign materials and preserves the native cell populations and extracellular matrix types. In this study, we generated 3D spheroids with primary postnatal rat cortical cells using an accessible, size-controlled, reproducible, and cost-effective method. Neurons and glia formed laminin-containing 3D networks within the spheroids. The neurons were electrically active and formed circuitry through both excitatory and inhibitory synapses. The mechanical properties of the spheroids were in the range of brain tissue. These in vivo-like features of 3D cortical spheroids provide the potential for relevant and translatable investigations of the CNS in vitro.
View details for PubMedID 26414693
View details for PubMedCentralID PMC4663656
Dopaminergic neurons in the ventral tegmental area of the brain are an important site of convergence of drugs and stress. We previously identified a form of long-term potentiation of gamma-aminobutyric acid (GABA)ergic synapses on these neurons (LTPGABA). Our studies have shown that exposure to acute stress blocks this LTP and that reversal of the block of LTPGABA is correlated with prevention of stress-induced reinstatement of cocaine-seeking behavior.Sprague-Dawley rats were subjected to cold-water swim stress. Midbrain slices were prepared following stress, and whole-cell patch clamp recordings of inhibitory postsynaptic currents were performed from ventral tegmental area dopamine neurons. Antagonists of glucocorticoid receptors and kappa opioid receptors (?ORs) were administered at varying time points after stress. Additionally, the ability of a kappa antagonist administered following stress to block forced swim stress-induced reinstatement of cocaine self-administration was tested.We found that an acute stressor blocks LTPGABA for 5 days after stress through a transient activation of glucocorticoid receptors and more lasting contribution of ?ORs. Even pharmacological block of ?ORs beginning 4 days after stress has occurred reversed the block of LTPGABA. Administration of a ?ORs antagonist following stress prevents reinstatement of cocaine-seeking behavior.A brief stressor produces changes in the reward circuitry lasting several days. Our findings reveal roles for glucocorticoid receptors and ?ORs as mediators of the lasting effects of stress on synaptic plasticity. ?ORs antagonists reverse the neuroadaptations underlying stress-induced drug-seeking behavior and may be useful in the treatment of cocaine addiction.
View details for PubMedID 24957331
View details for PubMedCentralID PMC4240751
In this issue of Neuron, Ma et al. (2014) show that long-term depression of two independent prefrontal cortical inputs to nucleus accumbens modifies behavioral responses controlling incubation of cocaine craving. Intriguingly, one input increases while the other attenuates behavioral responses, hinting that both "prorelapse" and "antirelapse" pathways are strengthened after cocaine self-administration.
View details for PubMedID 25233302
View details for Web of Science ID 000342748200021
Long-term potentiation (LTP) is a persistent increase in synaptic strength required for many behavioral adaptations, including learning and memory, visual and somatosensory system functional development, and drug addiction. Recent work has suggested a role for LTP-like phenomena in the processing of nociceptive information in the dorsal horn and in the generation of central sensitization during chronic pain states. Whereas LTP of glutamatergic and GABAergic synapses has been characterized throughout the central nervous system, to our knowledge there have been no reports of LTP at mammalian glycinergic synapses. Glycine receptors (GlyRs) are structurally related to GABAA receptors and have a similar inhibitory role. Here we report that in the superficial dorsal horn of the spinal cord, glycinergic synapses on inhibitory GABAergic neurons exhibit LTP, occurring rapidly after exposure to the inflammatory cytokine interleukin-1 beta. This form of LTP (GlyR LTP) results from an increase in the number and/or change in biophysical properties of postsynaptic glycine receptors. Notably, formalin-induced peripheral inflammation in vivo potentiates glycinergic synapses on dorsal horn neurons, suggesting that GlyR LTP is triggered during inflammatory peripheral injury. Our results define a previously unidentified mechanism that could disinhibit neurons transmitting nociceptive information and may represent a useful therapeutic target for the treatment of pain.
View details for PubMedID 24830427
While stressful experiences are a part of everyone's life, they can also exact a major toll on health. Stressful life experiences are associated with increased substance abuse, and there exists significant co-morbidity between mental illness and substance use disorders [N.D. Volkow & T.K. Li (2004) Nat. Rev. Neurosci., 5, 963-970; G. Koob & M.J. Kreek (2007) Am. J. Psych., 164, 1149-1159; R. Sinha (2008) Annals N.Y. Acad. Sci., 1141, 105-130]. The risk for development of mood or anxiety disorders after stress is positively associated with the risk for substance use disorders [R. Sinha (2008) Annals N.Y. Acad. Sci., 1141, 105-130], suggesting that there are common substrates for vulnerability to addictive and affective disorders. Understanding the molecular and physiological substrates of stress may lead to improved therapeutic interventions for the treatment of substance use disorders and mental illnesses.
View details for PubMedID 24712997
View details for PubMedCentralID PMC4019343
Neuronal arborization is regulated by cell-autonomous and nonautonomous mechanisms including endosomal signaling via BDNF/TrkB. The endosomal Na?/H? exchanger 6 (NHE6) is mutated in a new autism-related disorder. NHE6 functions to permit proton leak from endosomes, yet the mechanisms causing disease are unknown. We demonstrate that loss of NHE6 results in overacidification of the endosomal compartment and attenuated TrkB signaling. Mouse brains with disrupted NHE6 display reduced axonal and dendritic branching, synapse number, and circuit strength. Site-directed mutagenesis shows that the proton leak function of NHE6 is required for neuronal arborization. We find that TrkB receptor colocalizes to NHE6-associated endosomes. TrkB protein and phosphorylation are reduced in NHE6 mutant neurons in response to BDNF signaling. Finally, exogenous BDNF rescues defects in neuronal arborization. We propose that NHE6 mutation leads to circuit defects that are in part due to impoverished neuronal arborization that may be treatable by enhanced TrkB signaling.
View details for PubMedID 24035762
View details for PubMedCentralID PMC3830955
TRPV (transient receptor potential, vanilloid) channels are a family of nonselective cation channels that are activated by a wide variety of chemical and physical stimuli. TRPV1 channels are highly expressed in sensory neurons in the peripheral nervous system. However, a number of studies have also reported TRPV channels in the brain, though their functions are less well understood. In the hippocampus, the TRPV1 channel is a novel mediator of long-term depression (LTD) at excitatory synapses on interneurons. Here we tested the role of other TRPV channels in hippocampal synaptic plasticity, using hippocampal slices from Trpv1, Trpv3 and Trpv4 knockout (KO) mice. LTD at excitatory synapses on s. radiatum hippocampal interneurons was attenuated in slices from Trpv3 KO mice (as well as in Trpv1 KO mice as previously reported), but not in slices from Trpv4 KO mice. A previous study found that in hippocampal area CA1, slices from Trpv1 KO mice have reduced tetanus-induced long-term potentiation (LTP) following high-frequency stimulation; here we confirmed this and found a similar reduction in Trpv3 KO mice. We hypothesized that the loss of LTD at the excitatory synapses on local inhibitory interneurons caused the attenuated LTP in the mutants. Consistent with this idea, blocking GABAergic inhibition rescued LTP in slices from Trpv1 KO and Trpv3 KO mice. Our findings suggest a novel role for TRPV3 channels in synaptic plasticity and provide a possible mechanism by which TRPV1 and TRPV3 channels modulate hippocampal output.
View details for PubMedID 23536486
Stress facilitates reinstatement of addictive drug seeking in animals and promotes relapse in humans. Acute stress has marked and long-lasting effects on plasticity at both inhibitory and excitatory synapses on dopamine neurons in the ventral tegmental area (VTA), a key region necessary for drug reinforcement. Stress blocks long-term potentiation at GABAergic synapses on dopamine neurons in the VTA (LTPGABA), potentially removing a normal brake on activity. Here we show that blocking kappa opioid receptors (KORs) prior to forced-swim stress rescues LTPGABA. In contrast, blocking KORs does not prevent stress-induced potentiation of excitatory synapses nor morphine-induced block of LTPGABA. Using a kappa receptor antagonist as a selective tool to test the role of LTPGABA in vivo, we find that blocking KORs within the VTA prior to forced-swim stress prevents reinstatement of cocaine seeking. These results suggest that KORs may represent a useful therapeutic target for treatment of stress-triggered relapse in substance abuse.
View details for PubMedID 23473323
Endocannabinoids (eCBs) mediate various forms of synaptic plasticity at excitatory and inhibitory synapses in the brain. The eCB anandamide binds to several receptors including the transient receptor potential vanilloid 1 (TRPV1) and cannabinoid receptor 1 (CB1). We recently identified that TRPV1 is required for long-term depression at excitatory synapses on CA1 hippocampal stratum radiatum interneurons. Here we performed whole-cell patch clamp recordings from CA1 stratum radiatum interneurons in rat brain slices to investigate the effect of the eCB anandamide on excitatory synapses as well as the involvement of Group I metabotropic glutamate receptors (mGluRs), which have been reported to produce eCBs endogenously. Application of the nonhydrolysable anandamide analog R-methanandamide depressed excitatory transmission to CA1 stratum radiatum interneurons by ?50%. The Group I mGluR agonist DHPG also depressed excitatory glutamatergic transmission onto interneurons to a similar degree, and this depression was blocked by the mGluR5 antagonist MPEP (10 ?M) but not by the mGluR1 antagonist CPCCOEt (50 ?M). Interestingly, however, neither DHPG-mediated nor R-methanandamide-mediated depression was blocked by the TRPV1 antagonist capsazepine (10 ?M), the CB1 antagonist AM-251 (2 ?M) or a combination of both, suggesting the presence of a novel eCB receptor or anandamide target at excitatory hippocampal synapses. DHPG also occluded R-methanandamide depression, suggesting the possibility that the two drugs elicit synaptic depression via a shared signaling mechanism. Collectively, this study illustrates a novel CB1/TRPV1-independent eCB pathway present in the hippocampus that mediates depression at excitatory synapses on CA1 stratum radiatum interneurons.
View details for PubMedID 21069781
The reduction in synaptic transmission and plasticity in mice lacking the hippocampus-enriched AMPA receptor (AMPAR) auxiliary subunit TARP?-8 could be a result of a reduction in AMPAR expression or of the direct action of ?-8. We generated TARP?-8?4 knock-in mice lacking the C-terminal PDZ ligand. We found that synaptic transmission and AMPARs were reduced in the mutant mice, but extrasynaptic AMPAR expression and long-term potentiation (LTP) were unaltered. Our findings suggest that there are distinct TARP-dependent mechanisms for synaptic transmission and LTP.
View details for DOI 10.1038/nn.2952
View details for Web of Science ID 000296518600014
View details for PubMedID 22002768
View details for PubMedCentralID PMC3206644
Synaptic plasticity in the ventral tegmental area (VTA) is modulated by drugs of abuse and stress and is hypothesized to contribute to specific aspects of addiction. Both excitatory and inhibitory synapses on dopamine neurons in the VTA are capable of undergoing long-term changes in synaptic strength. While the strengthening or weakening of excitatory synapses in the VTA has been widely examined, the role of inhibitory synaptic plasticity in brain reward circuitry is less established. Here, we investigated the effects of drugs of abuse, as well as acute stress, on long-term potentiation of GABAergic synapses onto VTA dopamine neurons (LTP(GABA)). Morphine (10 mg/kg i.p.) reduced the ability of inhibitory synapses in midbrain slices to express LTP(GABA) both at 2 and 24 h after drug exposure but not after 5 days. Cocaine (15 mg/kg i.p.) impaired LTP(GABA) 24 h after exposure, but not at 2 h. Nicotine (0.5 mg/kg i.p.) impaired LTP(GABA) 2 h after exposure, but not after 24 h. Furthermore, LTP(GABA) was completely blocked 24 h following brief exposure to a stressful stimulus, a forced swim task. Our data suggest that drugs of abuse and stress trigger a common modification to inhibitory plasticity, synergizing with their collective effect at excitatory synapses. Together, the net effect of addictive substances or stress is expected to increase excitability of VTA dopamine neurons, potentially contributing to the early stages of addiction.
View details for PubMedID 20608969
GABAergic inhibitory interneurons are embedded in almost all central neuronal networks, where they act to influence cell excitability, spike timing, synchrony, and oscillatory activity, that is, almost every physiologically relevant process in the mammalian central nervous system. Consequently, presynaptic plasticity of the synaptic input onto, or the outputs from, a single inhibitory interneuron can have major ramifications for the activity of the often thousands of downstream target neurons. Here we discuss several recently described forms of presynaptic long-term potentiation (LTP) and long-term depression (LTD) occurring at synapses either made onto inhibitory interneurons, or at inhibitory synapses onto downstream targets in a number of central structures. As we will illustrate, the induction mechanisms underlying these disparate examples of presynaptic plasticity share few common features, however, their expression mechanisms converge on the presynaptic release machinery. We hypothesize that these varied forms of presynaptic plasticity can operate in a manner fundamentally distinct from most postsynaptic 'point to point' forms of plasticity, to achieve powerful modification of the integration and output of large-scale networks.
View details for DOI 10.1016/j.conb.2009.05.008
View details for Web of Science ID 000269765400004
View details for PubMedID 19581079
View details for PubMedCentralID PMC3121152
Drugs of abuse usurp the mechanisms underlying synaptic plasticity in areas of the brain, a process that may contribute to the development of addiction. We previously reported that GABAergic synapses onto dopaminergic neurons in the ventral tegmental area (VTA) exhibit long-term potentiation (LTP(GABA)) blocked by in vivo exposure to morphine. The presynaptically maintained LTP requires the retrogradely released nitric oxide (NO) to activate a presynaptic cGMP signaling cascade. Previous work reported that inhibitory GABA(A) receptor synapses in the VTA are also potentiated by cAMP. Here, we explored the interactions between cGMP-dependent (PKG) and cAMP-dependent (PKA) protein kinases in the regulation of these GABAergic synapses and LTP(GABA). Activation of PKG was required for NO-cGMP signaling and was also essential for the induction of synaptically elicited LTP(GABA), but not for its maintenance. Synapses containing GABA(A) receptors were potentiated by NO-cGMP signaling, whereas synapses containing GABA(B) receptors on the same cells were not potentiated. Moreover, although the cAMP-PKA system potentiated GABA(A) synapses, synaptically induced LTP(GABA) was independent of PKA activation. Surprisingly, however, raising cGMP levels saturated potentiation of these synapses, precluding further potentiation by cAMP and suggesting a convergent end point for both signaling pathways in the regulation of GABAergic release. We further found that persistent GABAergic synaptic modifications observed with in vivo morphine did not involve the presynaptic cAMP-PKA cascade. Taken together, our data suggest a synapse-specific role for NO-cGMP-PKG signaling pathway in opioid-induced plasticity of VTA GABA(A) synapses.
View details for PubMedID 19194373
View details for PubMedCentralID PMC2680921
TRPV1 (transient receptor potential, vanilloid) channels belong to a family of ligand-gated ion channels gated not only by the binding of certain lipophilic molecules but also by extracellular protons and physical stimuli such as heat or osmotic pressure changes. These nonselective cation channels are permeable to Na(+) and K(+) and are also very Ca(2+) permeable; in fact, TRPV1 is as Ca(2+) permeable as the NMDA receptor channel and can, thus, act as a trigger for Ca(2+)-mediated cell signaling. Although these channels are highly expressed in primary sensory afferents, accumulating evidence indicates that TRPV family channels are also present in the brain. Here, we review evidence that TRPV channels in the central nervous system might contribute to many basic neuronal functions including resting membrane potential, neurotransmitter release and synaptic plasticity.
View details for PubMedID 19285736
The development of drug addiction progresses along a continuum from acute drug use to compulsive use and drug seeking behavior. Many researchers have focused on identifying the physiological mechanisms involved in drug addiction in order to develop effective pharmacotherapies. Neuroplasticity, the putative mechanism underlying learning and memory, is modified by drugs of abuse and may contribute to the development of the eventual addicted state. Innovative treatments directly targeting these drug-induced changes in brain reward components and circuits may be efficacious in reducing drug use and relapse.
View details for PubMedID 19444729
Learning-related plasticity at excitatory synapses in the mammalian brain requires the trafficking of AMPA receptors and the growth of dendritic spines. However, the mechanisms that couple plasticity stimuli to the trafficking of postsynaptic cargo are poorly understood. Here we demonstrate that myosin Vb (MyoVb), a Ca2+-sensitive motor, conducts spine trafficking during long-term potentiation (LTP) of synaptic strength. Upon activation of NMDA receptors and corresponding Ca2+ influx, MyoVb associates with recycling endosomes (REs), triggering rapid spine recruitment of endosomes and local exocytosis in spines. Disruption of MyoVb or its interaction with the RE adaptor Rab11-FIP2 abolishes LTP-induced exocytosis from REs and prevents both AMPA receptor insertion and spine growth. Furthermore, induction of tight binding of MyoVb to actin using an acute chemical genetic strategy eradicates LTP in hippocampal slices. Thus, Ca2+-activated MyoVb captures and mobilizes REs for AMPA receptor insertion and spine growth, providing a mechanistic link between the induction and expression of postsynaptic plasticity.
View details for PubMedID 18984164
Excitatory synapses on dopamine neurons in the VTA can undergo both long-term potentiation and depression. Additionally, drug-induced plasticity has been found at VTA synapses, and is proposed to play a role in reward-related learning and addiction by modifying dopamine cell firing. LTP at these synapses is difficult to generate experimentally in that it requires an undisturbed intracellular milieu and is often small in magnitude. Here, we demonstrate the induction of LTP as a property of evoked field potentials within the VTA. Excitatory field potentials were recorded extracellularly from VTA neurons in acute horizontal midbrain slices. Using extracellular and intracellular recording techniques, we found that evoked field potentials originate within the VTA itself and are largely composed of AMPA receptor-mediated EPSPs and action potentials triggered by activation of glutamatergic synapses on both dopamine and GABA neurons. High-frequency afferent stimulation (HFS) induced LTP of the field potential. The induction of this LTP was blocked by application of the NMDAR antagonist, d-APV, prior to HFS. As reported previously, glutamatergic synapses on GABA neurons did not express LTP while those on dopamine neurons did. We conclude that the potentiation of glutamatergic synapses on dopamine neurons is a major contributor to NMDA receptor-dependent LTP of the field potential. Field potential recordings may provide a convenient approach to explore the basic electrophysiological properties of VTA neurons and the development of addiction-related processes in this brain region.
View details for PubMedID 17851541
One of the mechanisms by which the experience-dependent reorganization of neural circuitry can occur is through changes in synaptic strength. Almost every excitatory synapse in the mammalian brain exhibits LTP (long-term potentiation) or LTD (long-term depression), two cellular mechanisms of synaptic plasticity. However, LTP and LTD have been reported much more rarely at fast inhibitory GABA(A) receptor synapses. Our recent study suggests that in vivo morphine initiates a long-lasting alteration of GABAergic synapses in the ventral tegmental area (VTA) by blocking the mechanisms required for LTP of GABAergic synapses. Here we put this work into the context of other examples of synaptic plasticity at GABAergic synapses.
View details for PubMedID 18079157
TRPV1 receptors have classically been defined as heat-sensitive, ligand-gated, nonselective cation channels that integrate nociceptive stimuli in sensory neurons. TRPV1 receptors have also been identified in the brain, but their physiological role is poorly understood. Here we report that TRPV1 channel activation is necessary and sufficient to trigger long-term synaptic depression (LTD). Excitatory synapses onto hippocampal interneurons were depressed by either capsaicin, a potent TRPV1 channel activator, or the endogenously released eicosanoid, 12-(S)-HPETE, whereas neighboring excitatory synapses onto CA1 pyramidal cells were unaffected. TRPV1 receptor antagonists also prevented interneuron LTD. In brain slices from TRPV1-/- mice, LTD was absent, and neither capsaicin nor 12-(S)-HPETE elicited synaptic depression. Our results suggest that, in the hippocampus, TRPV1 receptor activation selectively modifies synapses onto interneurons. Like other forms of hippocampal synaptic plasticity, TRPV1-mediated LTD may have a role in long-term changes in physiological and pathological circuit behavior during learning and epileptic activity.
View details for PubMedID 18341994
View details for PubMedCentralID PMC2698707
Addiction is caused, in part, by powerful and long-lasting memories of the drug experience. Relapse caused by exposure to cues associated with the drug experience is a major clinical problem that contributes to the persistence of addiction. Here we present the accumulated evidence that drugs of abuse can hijack synaptic plasticity mechanisms in key brain circuits, most importantly in the mesolimbic dopamine system, which is central to reward processing in the brain. Reversing or preventing these drug-induced synaptic modifications may prove beneficial in the treatment of one of society's most intractable health problems.
View details for DOI 10.1038/nrn2234
View details for Web of Science ID 000251074700013
View details for PubMedID 17948030
Excitatory brain synapses are strengthened or weakened in response to specific patterns of synaptic activation, and these changes in synaptic strength are thought to underlie persistent pathologies such as drug addiction, as well as learning. In contrast, there are few examples of synaptic plasticity of inhibitory GABA (gamma-aminobutyric acid)-releasing synapses. Here we report long-term potentiation of GABA(A)-mediated synaptic transmission (LTP(GABA)) onto dopamine neurons of the rat brain ventral tegmental area, a region required for the development of drug addiction. This novel form of LTP is heterosynaptic, requiring postsynaptic NMDA (N-methyl-d-aspartate) receptor activation at glutamate synapses, but resulting from increased GABA release at neighbouring inhibitory nerve terminals. NMDA receptor activation produces nitric oxide, a retrograde signal released from the postsynaptic dopamine neuron. Nitric oxide initiates LTP(GABA) by activating guanylate cyclase in GABA-releasing nerve terminals. Exposure to morphine both in vitro and in vivo prevents LTP(GABA). Whereas brief treatment with morphine in vitro blocks LTP(GABA) by inhibiting presynaptic glutamate release, in vivo exposure to morphine persistently interrupts signalling from nitric oxide to guanylate cyclase. These neuroadaptations to opioid drugs might contribute to early stages of addiction, and may potentially be exploited therapeutically using drugs targeting GABA(A) receptors.
View details for PubMedID 17460674
Dopamine modulates the function of glutamatergic synapses in prefrontal cortex, modifying synaptic strength and influencing synaptic plasticity. Here we have explored the ability of endogenous dopamine, present in slices containing the prefrontal cortex, to influence excitatory synaptic transmission. We found that 10 microM amphetamine, which releases and blocks the reuptake of dopamine from dopaminergic nerve terminals, significantly depressed excitatory field potentials recorded in layer V during stimulation of layer II/III. The depression was reversible, dose dependent and correlated with increased paired pulse facilitation, suggesting that amphetamine inhibits the presynaptic release of glutamate. Pharmacological dissection of this response showed that dopamine D1 receptors are likely to mediate the effects of endogenous dopamine on excitatory synaptic transmission, with little effect of alpha2 adrenergic receptors, serotonin receptors, or D2 dopamine receptors. The time to peak amphetamine effect was longer than expected based on diffusion, suggesting that to raise dopamine levels in brain slices amphetamine may need to be transported into the presynaptic terminals. These results provide evidence that D1/D5 receptors depress glutamate release at this cortical synapse, and suggest that amphetamine will have profound and persistent effects on PFC functioning in vivo. Dysregulation of this mechanism could contribute to the impairment in cognitive performance associated with abnormal PFC dopamine receptor activation.
View details for DOI 10.1016/j.neuropharm.2006.07.004
View details for Web of Science ID 000243698200021
View details for PubMedID 16895728
View details for Web of Science ID 000233442100163
Drugs of abuse activate the reward circuitry of the mesocorticolimbic system, and it has been hypothesized that drug exposure triggers synaptic plasticity of glutamatergic synapses onto dopamine (DA) neurons of the ventral tegmental area. Here, we show that just a 2 h in vivo exposure to amphetamine is sufficient to potentiate these synapses, measured as an increase in the synaptic AMPAR/NMDAR ratio. We tested the prediction that an increase in GluR1-containing AMPA receptors would result in an increase in GluR1 homomeric receptors at synapses, but were unable to observe any evidence of the predicted rectification in DA neurons from animals treated with amphetamine. We also examined the possibility of increased AMPA receptor insertion in the membrane, but did not detect a significant increase in biotinylated surface GluR1. We conclude that amphetamine induces rapid changes in synaptic AMPAR/NMDAR ratios, suggesting that potentiation of glutamatergic synapses is a relatively early event in the series of neuroadaptations in response to drugs of abuse.
View details for PubMedID 15150533
Long-term potentiation (LTP) of synaptic strength, the most established cellular model of information storage in the brain, is expressed by an increase in the number of postsynaptic AMPA receptors. However, the source of AMPA receptors mobilized during LTP is unknown. We report that AMPA receptors are transported from recycling endosomes to the plasma membrane for LTP. Stimuli that triggered LTP promoted not only AMPA receptor insertion but also generalized recycling of cargo and membrane from endocytic compartments. Thus, recycling endosomes supply AMPA receptors for LTP and provide a mechanistic link between synaptic potentiation and membrane remodeling during synapse modification.
View details for Web of Science ID 000224136000052
View details for PubMedID 15448273
One of the central questions in neurobiology is how experience modifies neural function, and how changes in the nervous system permit an animal to adapt its behavior to a changing environment. Learning and adaptation to a host of different environmental stimuli exemplify processes we know must alter the nervous system because the behavioral output changes after experience. Alterations in behavior after exposure to addictive drugs are a striking example of chemical alterations of nervous system function producing long-lasting changes in behavior. The alterations produced in the central nervous system (CNS) by addictive drugs are of interest because of their relationship to human substance abuse but also because these CNS alterations produce dramatic, easily observed alterations in behavior in response to discrete stimuli. Considerable study has been given to behavioral and biochemical correlates of addiction over the past 50 or more years; however, our understanding of the cellular physiological responses of affected CNS neurons is in its infancy. This review focuses on alterations in cellular and synaptic physiology in the ventral tegmental area (VTA) in response to addictive drugs.
View details for PubMedID 14977410
The mesolimbic dopamine system is essential for reward-seeking behavior, and drugs of abuse perturb the normal functioning of this pathway. The nucleus accumbens (NAc) is a major terminal field of the mesolimbic dopamine neurons and modifications in neuronal structure and function in NAc accompany repeated exposure to psychomotor stimulants and other addictive drugs. Glutamatergic afferents to the NAc are thought to be crucial to the development of several aspects of addictive behavior, including behavioral sensitization and relapse to cocaine self-administration. Here we examine glutamatergic neurotransmission and synaptic plasticity in NAc neurons in vitro before and after repeated amphetamine treatment in vivo. We find that dopamine attenuates the response of NAc neurons to repetitive activation of glutamatergic afferents and thereby blocks long-term potentiation (LTP) induced by high-frequency afferent stimulation. Dopamine's effects are mimicked by dopamine receptor agonists and by amphetamine. In a second set of experiments, animals were treated with amphetamine daily for 6 days and brain slices were prepared after 8-10 days of withdrawal. In these slices, LTP in the NAc appears normal. However, acute exposure of such slices to amphetamine no longer modulates synaptic transmission or LTP induction. Thus, repeated exposure to amphetamine produces long-lasting changes in the modulation of glutamatergic synaptic transmission by amphetamine in the NAc. Our results support the notion that after psychostimulant exposure, excitatory synapses on NAc neurons alter their response to further psychostimulant for long periods of time.
View details for PubMedID 14579420
In this issue of Neuron, Saal et al. find that exposure to any of five addictive drugs or exposure to a brief stressor produces a shared cellular modification of excitatory synapses in the ventral tegmental area (VTA). This common response may represent a starting point for dissecting early changes that underlie addiction.
View details for PubMedID 12597851
Dopamine neurons of the ventral tegmental area (VTA) are critically involved in processing novel and rewarding information, and mediate the addictive properties of many drugs of abuse. Excitatory synapses on these neurons, like those in other brain regions, exhibit long-term depression (LTD). Amphetamine or dopamine block LTD at VTA synapses, indicating that both pathological and local physiological stimuli regulate LTD. Here we show that in common with other forms of LTD, VTA LTD results from a selective decrease in AMPA receptor function accompanied by a decrease in cell surface AMPA receptors. However, unlike the case for any previously described form of LTD, activation of cyclic AMP-dependent protein kinase (PKA) is necessary and sufficient to trigger LTD at synapses on VTA dopamine neurons.
View details for PubMedID 12467595
Genetic evidence indicates that cell adhesion molecules of the immunoglobulin superfamily (IgCAMs) are critical for activity-dependent synapse formation at the neuromuscular junction in Drosophila and have also been implicated in synaptic remodelling during learning in Aplysia (see  for review). In mammals, a widely adopted model for the process of learning at the cellular level is long-term potentiation (LTP) in the hippocampal formation. Studies in vitro have shown that antibodies to the IgCAMs L1 and NCAM reduce LTP in CA1 neurons of rat hippocampus, suggesting a role for these molecules in the modulation of synaptic efficacy, perhaps by regulating synaptic remodelling . A role for NCAM in LTP has been confirmed in mice lacking NCAM  (but see ), but similar studies have not been reported for L1. Here we examine LTP in the hippocampus of mice lacking L1 [5,6], using different experimental protocols in three different laboratories. In tests of LTP in vitro and in vivo we found no significant differences between mutant animals and controls. Thus, contrary to expectation, our data suggest that L1 function is not necessary for the establishment or maintenance of LTP in the hippocampus. Impaired performance in spatial learning exhibited by L1 mutants may therefore not be due to hippocampal dysfunction .
View details for DOI 10.1016/S0960-9822(00)00865-4
View details for Web of Science ID 000166807100024
View details for PubMedID 11137015
The mesolimbic dopamine system is essential for reward-seeking behavior, and drugs of abuse are thought to usurp the normal functioning of this pathway. A growing body of evidence suggests that glutamatergic synapses on dopamine neurons in the ventral tegmental area (VTA) are modified during exposure to addictive drugs, producing sensitization, a progressive augmentation in the rewarding properties of psychostimulant drugs with repeated exposure. We have tested the hypothesis that psychostimulant exposure interferes with the synaptic plasticity of glutamatergic inputs to the VTA. We find that excitatory synapses onto VTA dopamine neurons exhibit long-term depression (LTD) in response to low-frequency stimulation and modest depolarization. LTD in the VTA is NMDA receptor-independent but is dependent on intracellular Ca(2+) and can be induced by driving Ca(2+) into the dopamine neuron. Brief exposure to amphetamine entirely blocks LTD at glutamatergic synapses in the VTA, by releasing endogenous dopamine that acts at D2 dopamine receptors. The block of LTD is selective, because amphetamine has no effect on hippocampal LTD. The LTD we have discovered in the VTA is likely to be an important component of excitatory control of the reward pathway; amphetamine will inhibit LTD, removing this normal brake on the glutamatergic drive to dopamine neurons. This effect of amphetamine represents an important mechanism by which normal function of the brain reward system may be impaired during substance abuse.
View details for Web of Science ID 000088354000005
View details for PubMedID 10908593
The ventral tegmental area (VTA) is the origination zone for dopaminergic neurons involved in reward and addictive properties of a variety of abused substances. A major excitatory projection to VTA neurons originates in the medial prefrontal cortex, and several lines of evidence suggest that glutamatergic synapses on VTA neurons are activated and modified during exposure to psychostimulant drugs. Here, we report for the first time that amphetamine depresses excitatory glutamatergic synaptic transmission onto VTA neurons in the midbrain slice preparation. Unexpectedly, this depression is mediated not by activation of dopamine receptors, but instead by activation of serotonin receptors. Our findings suggest that an acute effect of amphetamine exposure is the release of serotonin in the VTA, which in turn modulates excitation of VTA neurons. This process may be an important early component of permanent changes occurring in the reward pathway that contribute to drug addiction.
View details for Web of Science ID 000083607700015
View details for PubMedID 10559387
Specific patterns of electrical stimulation trigger several forms of synaptic plasticity in hippocampal pyramidal cells, including a long-term potentiation (LTP) of excitatory synaptic transmission. I investigated the effect of commonly used stimulation protocols at different distances from the recording site. Sustained electrical stimulation (100 Hz, 1 s) delivered close to the recording site prevented LTP induction; the same stimulation from a second electrode placed farther away subsequently produced LTP at the same recording site. Strong stimulation near the recording site could also interfere with LTP triggered from a distal site. In contrast to sustained high-frequency stimulation, intermittent stimulation (theta burst pattern) delivered close to the recording site produced normal LTP. These data support the hypothesis that strong stimulation releases a factor that acts locally to prevent LTP.
View details for Web of Science ID 000078832800051
View details for PubMedID 10036292
Serotonin systems have been implicated in the regulation of hippocampal function. Serotonin 5-HT2C receptors are widely expressed throughout the hippocampal formation, and these receptors have been proposed to modulate synaptic plasticity in the visual cortex. To assess the contribution of 5-HT2C receptors to the serotonergic regulation of hippocampal function, mice with a targeted 5-HT2C-receptor gene mutation were examined. An examination of long-term potentiation at each of four principal regions of the hippocampal formation revealed a selective impairment restricted to medial perforant path-dentate gyrus synapses of mutant mice. This deficit was accompanied by abnormal performance in behavioral assays associated with dentate gyrus function. 5-HT2C receptor mutants exhibited abnormal performance in the Morris water maze assay of spatial learning and reduced aversion to a novel environment. These deficits were selective and were not associated with a generalized learning deficit or with an impairment in the discrimination of spatial context. These results indicate that a genetic perturbation of serotonin receptor function can modulate dentate gyrus plasticity and that plasticity in this structure may contribute to neural mechanisms underlying hippocampus-dependent behaviors.
View details for DOI 10.1073/pnas.95.25.15026
View details for Web of Science ID 000077436700079
View details for PubMedID 9844009
View details for PubMedCentralID PMC24569
During distinct behavioral states, the hippocampus exhibits characteristic rhythmic electrical activity. Evidence in vivo suggests that both principal pyramidal cells and GABAergic interneurons participate in generating oscillations. We found that during rhythmic oscillations in area CA3, functionally distinct classes of interneurons could be identified, although all recorded interneurons had similar dendritic and axonal arbors. One group of interneurons was powerfully excited by CA3 pyramidal cells, whereas two other interneuron groups were relatively unaffected by pyramidal cell firing. One of these groups of interneurons was potently inhibited by other local interneurons during the pyramidal cell bursts. Our findings emphasize that morphologically similar cells are wired together very differently within the local circuit. The classes of hippocampal interneurons we have tentatively defined may be used during distinct behavioral states to switch the local network from one oscillatory state to another.
View details for Web of Science ID 000074971800009
View details for PubMedID 9671655
Separating contributions of pre- and postsynaptic factors to the maintenance of long-term potentiation (LTP) and long-term depression (LTD) has been confounded by their experimental interdependence. To isolate the postsynaptic contribution, glutamate-receptor-mediated currents were elicited by localized photolysis of caged glutamate in small spots along the dendrites of CA1 hippocampal pyramidal cells. With synaptic transmission blocked, pairing depolarization of pyramidal cells with repeated photolysis of caged glutamate at one site markedly and persistently depressed subsequent responses to glutamate; responses at a second, unpaired site were unchanged. Like synaptically induced LTD at the CA3-CA1 synapse, this depression was site specific, NMDA-receptor dependent and blocked by protein-phosphatase inhibitors. Thus, robust, persistent alterations of postsynaptic glutamate receptor efficacy can occur without presynaptic neurotransmitter release.
View details for DOI 10.1038/368
View details for Web of Science ID 000076361100012
View details for PubMedID 10195126
Properties of carbachol-induced oscillatory activity in rat hippocampus. J. Neurophysiol. 78: 2631-2640, 1997. The recent resurgence of interest in carbachol oscillations as an in vitro model of theta rhythm in the hippocampus prompted us to evaluate the circuit mechanisms involved. In extracellular recordings, a regularly spaced bursting pattern of field potentials was observed in both CA3 and CA1 subfields in the presence of carbachol. Removal of the CA3 region abolished oscillatory activity observed in CA1, suggesting that the oscillatory generator is located in CA3. An alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX), blocked carbachol oscillations, indicating that AMPA receptor-mediated synaptic currents are necessary for the population oscillation. Moreover, the spread of oscillatory activity into CA1 required intact N-methyl--aspartate receptors. These data are more consistent with epileptiform bursting than with theta rhythm described in vivo. In the presence of carbachol, individual CA3 pyramidal cells exhibited a slow, rhythmic intrinsic oscillation that was not blocked by DNQX and that was enhanced by membrane hyperpolarization. We hypothesize that this slower oscillation is the fundamental oscillator that participates in triggering the population oscillation by exciting multiple synaptically connected CA3 neurons. gamma-aminobutyric acid-A (GABAA) receptors are not necessary for carbachol to elicit synchronous CA3 field events but are essential to the bursting pattern observed. Neither GABAB nor metabotropic glutamate receptors appear to be necessary for carbachol oscillations. However, both nicotinic and M1 and M3 muscarinic cholinergic receptors contribute to the generation of this activity. These results establish the local circuit elements and neurotransmitter receptors that contribute to carbachol-induced oscillations and indicate that carbachol-induced oscillations are fundamentally distinct from theta rhythm in vivo.
View details for Web of Science ID A1997YG93900036
View details for PubMedID 9356412
Hippocampal interneurons are excited via serotonin-gated ion channels. J. Neurophysiol. 78: 2493-2502, 1997. Serotonergic neurons of the median raphe nucleus heavily innervate hippocampal GABAergic interneurons located in stratum radiatum of area CA1, suggesting that this strong subcortical projection may modulate interneuron excitability. Using whole cell patch-clamp recording from interneurons in brain slices, we tested the effects of serotonin (5-HT) on the physiological properties of these interneurons. Serotonin produces a rapid inward current that persists when synaptic transmission is blocked by tetrodotoxin and cobalt, and is unaffected by ionotropic glutamate and gamma-aminobutyric acid (GABA) receptor antagonists. The 5-HT-induced current was independent of G-protein activation. Pharmacological evidence indicates that 5-HT directly excites these interneurons through activation of 5-HT3 receptors. At membrane potentials negative to -55 mV, the current-voltage (I-V) relationship of the 5-HT current displays a region of negative slope conductance. Therefore the response of interneurons to 5-HT strongly depends on membrane potential and increases greatly as cells are depolarized. Removal of extracellular calcium, but not magnesium, increases the amplitude of 5-HT-induced currents and removes the region of negative slope conductance, thereby linearizing the I-V relationship. The axons of 5-HT-responsive interneurons ramify widely within CA1; some of these interneurons also project to and arborize extensively in the dentate gyrus. The organization of these inhibitory connections strongly suggests that these cells regulate excitability of both CA1 pyramidal cells and dentate granule cells. As our results indicate that 5-HT may mediate fast excitatory synaptic transmission onto these interneurons, serotonergic inputs can simultaneously modulate the output of both hippocampus and dentate gyrus.
View details for Web of Science ID A1997YG93900024
View details for PubMedID 9356400
Individual GABAergic interneurons in hippocampus can powerfully inhibit more than a thousand excitatory pyramidal neurons. Therefore, control of interneuron excitability provides control over hippocampal networks. We have identified a novel mechanism in hippocampus that weakens excitatory synapses onto GABAergic interneurons. Following stimulation that elicits long-term potentiation at neighboring synapses onto excitatory cells, excitatory synapses onto inhibitory interneurons undergo a long-term synaptic depression (interneuron LTD; iLTD). Unlike most other forms of hippocampal synaptic plasticity, iLTD is not synapse specific: stimulation of an afferent pathway triggers depression not only of activated synapses but also of inactive excitatory synapses onto the same interneuron. These results suggest that high frequency afferent activity increases hippocampal excitability through a dual mechanism, simultaneously potentiating synapses onto excitatory neurons and depressing synapses onto inhibitory neurons.
View details for DOI 10.1016/S0896-6273(00)80269-X
View details for Web of Science ID A1997WK82200012
View details for PubMedID 9052799
The inferior colliculus receives excitatory and inhibitory input from parallel auditory pathways that differ in discharge patterns, latencies, and binaural properties. Processing in the inferior colliculus may depend on the temporal sequence in which excitatory and inhibitory synaptic inputs are activated and on the resulting balance between excitation and inhibition. To explore this issue at the cellular level, we used the novel approach of whole-cell patch-clamp recording in the midbrain of awake bats (Eptesicus fuscus) to record EPSCs or IPSCs. Sound-evoked EPSCs were recorded in most neurons. These EPSCs were frequently preceded by an IPSC, followed by an IPSC, or both. These findings help explain the large latency range and transient responses that characterize inferior colliculus neurons. The EPSC was sometimes followed by long-lasting oscillatory currents, suggesting that a single brief sound sets up a pattern of altered excitability that persists far beyond the duration of the initial sound. In three binaural neurons, ipsilateral sound evoked a large IPSC that partially or totally canceled the EPSC evoked by contralateral sound. In one binaural neuron with ipsilaterally evoked IPSCs, contralaterally evoked IPSCs occurred in response to frequencies above and below the neuron's best frequency. Thus, both monaural and binaural interactions can occur at single inferior colliculus neurons. These results show that whole-cell patch-clamp recording offers a powerful means of understanding how subthreshold processes determine the responses of auditory neurons.
View details for Web of Science ID A1996UF71100014
View details for PubMedID 8622130
Previous studies suggest that activation of metabotropic glutamate receptors (mGluRs) reduces GABA-mediated synaptic inhibition in area CA1 of the hippocampus. However, the mechanisms involved in this disinhibition are not known. We report that mGluR activation reduces both the GABAA and GABAB receptor-mediated components of evoked inhibitory postsynaptic potentials (IPSPs) by reducing transmission at excitatory synapses onto inhibitory interneurons and inhibitory synapses onto CA1 pyramidal cells.
View details for DOI 10.1016/0304-3940(94)90564-9
View details for Web of Science ID A1994PT18600019
View details for PubMedID 7898776
Based on responses to metabotropic glutamate receptor (mGluR) activation, we have characterized two distinct classes of interneuron in stratum (st.) oriens of the CA1 region of hippocampus. One type of interneuron was strongly excited by 1S,3R-aminocyclopentane dicarboxylic acid (ACPD), responding with a large inward current accompanied by increased baseline noise and prominent current oscillations. A second interneuron population responded with a modest inward current with no changes in baseline noise. These two classes of responses persisted in the presence of tetrodotoxin and antagonists of ionotropic glutamate and GABA receptors, suggesting that the inward currents result from mGluRs on the interneurons themselves. The two physiologically defined cell types correspond to two distinct morphological cell types in st. oriens/alveus, distinguished by very different patterns of local axonal connections. Large oscillatory inward current responses were recorded predominantly from an interneuron type whose axons heavily innervated st. lacunosum. The more modest inward current response was generally found in interneurons whose axons innervated the somata and proximal dendrites of CA1 pyramidal neurons. These differences in physiology and local circuitry imply that activation of mGluRs in st. oriens will cause very strong excitation of interneurons synapsing in st. lacunosum, and weaker excitation of interneurons innervating pyramidal cells at the soma and proximal dendrites. These data suggest that each interneuron population has a specific role in hippocampal function, and that mGluR activation will affect the local circuit differently for each interneuron type. Metabotropic GluR activation also markedly enhanced the amplitudes of the evoked and spontaneous EPSCs received by all interneurons in the region, independent of changes in the postsynaptic holding current and with no change in the kinetics of the EPSC. In contrast to the enhancement of evoked and spontaneous EPSCs, miniature EPSCs recorded in the presence of tetrodotoxin were not increased. These data suggest that ACPD acts at a presynaptic site to potentiate the EPSC. Taken together, these results highlight an important modulatory role for metabotropic receptors located at sites both pre- and postsynaptic to CA1 st. oriens interneurons.
View details for Web of Science ID A1994NZ62000037
View details for PubMedID 7517996