Emeritus Faculty, Acad Council, Anesthesiology, Perioperative and Pain Medicine
My laboratory tries to find out how pharmacologic agents used in the practice of anesthesia (general anesthetic and analgesic agents) lead to therapeutically desireable endpoints including unconsciousness, immobility and absence of pain. The old idea that general anesthetics are uniformly non-specific "membrane stabilizers" has given way to the realization that these agents exert specific actions on particular ion channels and intracellular signalling systems. Currently we are identifying anesthetic effects on ligand-gated and second messenger-operated ion channels in mammalian neurons, using both receptor-specific evoked potentials from isolated superfused spinal cord and whole cell patch clamp of neurons in situ in spinal cord slices. The goal of the research program is to construct a manageable set of actions which alone or in combination are both necessary and sufficient to bring about an anesthetic state.Most recently we have focused attention on mechanisms of spinal sensitization that contribute to chronic pain following injury and also to the development of tolerance to anesthetic and analgesic agents. We have described and partially characterized long term potentiation (LTP) in isolated spinal cord. We have also discovered long-lasting increases in spinal cord excitability following exposure to opioids such as morphine and to ethanol; these changes may be related to mechanisms of tolerance, dependence, and withdrawal.
We performed experiments in spinal cords isolated from neonatal rats to probe the mechanisms responsible for hyperresponsiveness of the population excitatory evoked potential (pEPSP) observed on washout of the volatile anesthetics halothane and isoflurane (1 minimal alveolar anesthetic concentration equivalent, MAC) compared with that observed after an anesthetic concentration of ethanol. After 30 min exposure to each anesthetic and washout, pEPSP area increased to levels significantly more than control (P < 0.01-0.001). Exposure to a very small (0.025 MAC) concentration of isoflurane over the same period itself produced a similarly exaggerated pEPSP (P < 0.05) in the continued presence of the drug, suggesting that the phenomenon is a direct excitatory effect of the small concentrations of anesthetic on washout, unlike the true withdrawal observed with ethanol. Isoflurane, but not halothane, significantly increased the amount of potassium-stimulated release of the excitatory neurotransmitters glutamate, aspartate, and substance P, suggesting the hyperresponsiveness for that drug is the result of a presynaptically mediated increase in transmitter release. A broad spectrum specific protein kinase C inhibitor, GF109203X, blocked ethanol withdrawal hyperresponsiveness but not hyperresponsiveness after halothane. If the behavioral symptoms of emergence from anesthesia are based on excitatory actions similar to those observed in the spinal cord, the results show that they represent direct excitatory actions rather than withdrawal and are attributable to direct actions on ion channels or receptors, rather than indirect effects mediated by protein kinase C.
View details for DOI 10.1213/01.ANE.0000142128.29660.AE
View details for Web of Science ID 000226567000021
View details for PubMedID 15673868
The present studies were designed to test the hypothesis that neuronal-specific protein kinase Cgamma (PKCgamma) plays a critical role in acute ethanol withdrawal hyper-responsiveness in spinal cord. Patch-clamp studies were carried out in motor neurons in neonatal rat spinal cord slices. Postsynaptic currents were evoked by brief pulses of 2 mM N-methyl-D-aspartic acid (NMDA) in the presence of bicuculline methiodide 10 microM; strychnine 5 microM and tetrodotoxin 0.5 microM. Both ethanol depression and withdrawal hyper-responsiveness of NMDA-evoked currents are dependent on increases in intracellular Ca(2+). Blocking intracellular increase in Ca(2+) by 30 mM 1,2-bis(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA) not only decreased the ethanol-induced depression of NMDA-evoked currents (33+/-5% in control vs 20+/-3% in BAPTA, P<0.05) but also eliminated acute ethanol withdrawal hyper-responsiveness. Immunohistochemistry studies revealed that neonatal spinal cord motor neurons contain an abundance of nuclear PKCgamma. Exposure to ethanol (100 mM) induced PKCgamma translocation from the nucleus to cytoplasm in motor neurons. Pretreatment with the gamma-isozyme-specific peptide PKC inhibitor, gammaV5-3, blocked ethanol-induced translocation and also blocked withdrawal hyper-responsiveness. The results show that PKCgamma mediates ethanol withdrawal hyper-responsiveness in spinal motor neurons; the results may be relevant to some symptoms of ethanol withdrawal in vivo.
View details for DOI 10.1038/sj.bjp.0706033
View details for Web of Science ID 000226848700001
View details for PubMedID 15655532
We have previously reported that withdrawal from acute ethanol (EtOH) exposure lowers mechanical thresholds in post-natal day 7 (P7) and post-natal day 21 (P21) rats. The present study tested the hypothesis that daily administration of 4 g/kg 15% EtOH for 5 days in rats during the human developmental equivalent of the third trimester, but not at a later time in development, would alter mechanical thresholds and formalin-induced pain behaviors. A transient decrease in mechanical thresholds (allodynia) was observed in P7 rats upon withdrawal from repeated EtOH between P3 and P7. When challenged with intraplantar formalin on P11, rats exposed to acute or chronic EtOH had enhanced phase II pain behaviors. In contrast to chronic EtOH administration to rats between P3 and P7, prolonged mechanical allodynia was observed in P21 rats upon withdrawal from chronic EtOH between P17 and P21. Formalin responses were unchanged in P25 rats exposed to acute or chronic EtOH. The affects of EtOH on somatosensory processing are dependent upon the age at which exposure occurs.
View details for DOI 10.1016/j.neulet.2004.05.079
View details for Web of Science ID 000223531800007
View details for PubMedID 15308291
We have previously found that in post-natal day 7 rats withdrawal from acute and chronic ethanol (EtOH) exposure lowers mechanical thresholds during withdrawal and exacerbates spontaneous pain responses to an inflammatory injury 4 days post-withdrawal. These findings suggested alterations in somatosensory pathways following EtOH exposure during the third trimester developmental equivalent. In this study we wanted to determine whether EtOH exposure during the third trimester equivalent exacerbates mechanical allodynia and thermal hyperalgesia produced by an incisional model of post-operative pain at post-natal day 21. The extent and duration of mechanical allodynia and thermal hyperalgesia following incision was measured and found to be unaffected by prior EtOH exposure.
View details for DOI 10.1016/j.neulet.2004.05.062
View details for Web of Science ID 000223438400022
View details for PubMedID 15288445
On withdrawal from opioids many patients experience a heightened sensitivity to stimuli and an exaggerated pain response. The phenomenon has been little studied in infants. We present evidence that in postnatal day 7 rats an exaggerated nociceptive ventral root response of spinal cords in vitro and withdrawal-associated thermal hyperalgesia in vivo are dependent on protein kinase C (PKC), and we document the roles of PKC and gamma isozymes. In vitro, the slow ventral root potential (sVRP) is a nociceptive-related response in spinal cord that is depressed by morphine and recovers to levels significantly above control on administration of naloxone. A broad-spectrum PKC antagonist, GF109213X, blocked withdrawal hyperresponsiveness of the sVRP whereas an antagonist specific to Ca(++)-dependent isozymes, Go69076, did not. Consistent with this finding, a specific peptide inhibitor of calcium-independent PKC, but not an inhibitor of calcium-dependent PKC gamma, blocked withdrawal hyperresponsiveness of the sVRP. Similarly, in vivo in 7-day-old rat pups, inhibition of PKC, but not PKC gamma, prevented thermal hyperalgesia precipitated by naloxone at 30 min post-morphine. In contrast, thermal hyperalgesia during spontaneous withdrawal was inhibited by both PKC and gamma inhibitors. The consistency between the in vivo and in vitro findings with respect to naloxone-precipitated withdrawal provides further evidence that the sVRP reflects nociceptive neurotransmission. In addition the difference between naloxone-precipitated and spontaneous withdrawal in vivo suggests that in postnatal day 7 rats, morphine exposure produces an early phase of primary afferent sensitization dependent upon PKC translocation, followed by a later phase involving spinal sensitization mediated by PKC gamma.
View details for DOI 10.1016/j.pain.2004.04.004
View details for Web of Science ID 000223232800034
View details for PubMedID 15275778
Anesthetic effects on receptor or ion channel phosphorylation by enzymes such as protein kinase C (PKC) have been postulated to underlie some aspects of anesthesia. In vitro studies show that anesthetic effects on several receptors are mediated by PKC. To test the importance of PKC for the immobility produced by inhaled anesthetics, we measured the effect of intrathecal injections of PKC-epsilon and -gamma inhibitors on halothane minimum alveolar anesthetic concentration (MAC) in 7-day-old and 21-day-old Sprague-Dawley rats. The inhibitors were made as solutions of 100 pmol/5 microL and were given in a volume of 5 microL (7-day-old [P7] rats) or 10 microL (21-day-old [P21] rats). Controls were saline injections or injections of the peptide carrier at the same concentration and volumes; there were six animals in each group. In P7 rats, MAC values (in percentage of an atmosphere) were 1.63 +/- 0.0727 (mean +/- SEM) in saline controls, 1.55 +/- 0.141 in carrier controls, 1.54 +/- 0.0800 in rats given PKC-epsilon, and 1.69 +/- 0.0554 in rats given PKC-gamma. In P21 animals, the values were 1.20 +/- 0.0490, 1.31 +/- 0.0124, 1.27 +/- 0.0367, and 1.15 +/- 0.0483, respectively. Injection of the inhibitors did not change MAC in either age group. These results do not support an anesthetic effect on phosphorylation as a mechanism underlying the capacity of inhaled anesthetics to prevent movement in response to noxious stimulation, and they indirectly support a direct action on receptors or ion channels.
View details for DOI 10.1213/01.ANE.0000118293.91808.38
View details for Web of Science ID 000222256400017
View details for PubMedID 15281508
Upon withdrawal from opioids many patients experience a heightened sensitivity to stimuli and an exaggerated pain response. We present evidence that neonatal rats exhibit allodynia and hyperalgesia on acute opiate withdrawal. Postnatal 7 and 21 day rats were used to approximately model a full term human infant and a human child, respectively. The opiate antagonist naloxone was used to precipitate withdrawal at 30 or 120 min after a single acute administration of morphine. Alternatively, rats were allowed to undergo spontaneous withdrawal. Behavioral manifestations of withdrawal syndrome were not observed when naloxone was administered at 30 min post-morphine, but were present when withdrawal was precipitated at 120 min. Spontaneous and precipitated withdrawal from a single acute administration of morphine produced mechanical allodynia and thermal hyperalgesia in postnatal day 7 rats and mechanical allodynia in postnatal day 21 rats. A higher dose of morphine was required to produce mechanical allodynia in postnatal day 21 versus 7 rats but this increase was independent of the analgesic efficacy of morphine at these two ages. The present work illustrates the need to examine the phenomenon of hypersensitivity upon opioid withdrawal in the human pediatric population.
View details for DOI 10.1016/j.pain.2004.04.003
View details for Web of Science ID 000223232800033
View details for PubMedID 15275777
The central nervous system undergoes dynamic changes as it matures. However, until recently, very little was known about the impact of these changes on pain and analgesia. This study tested the hypothesis that the epsilon and gamma isozymes of protein kinase C (PKC) contribute to formalin-induced nociception in an age-dependent manner. Expression of epsilon and gamma PKC and the contributions of these isozymes in formalin-induced nociception was examined in postnatal day 7, 15, and 21 rats. epsilonPKC expression in dorsal root ganglion neurons and gammaPKC expression in lamina II of the spinal cord increased from the first to the third postnatal week. Coupling immunohistochemical and Western analysis, translocation of epsilonPKC followed intraplantar formalin in all ages. In contrast, formalin-induced gammaPKC translocation was observed only in postnatal day 21 rats. Behaviorally, intrathecal administration of the epsilonPKC-specific inhibitor (epsilonV1-2) attenuated phase 1 and phase 2 formalin behaviors at all ages. In contrast, intrathecal administration of the gammaPKC-specific inhibitor (gammaV5-3) attenuated only phase 2 responses in postnatal day 15 and 21 rats. Functionally, inhibition of epsilonPKC decreased capsaicin-stimulated release of glutamate and calcitonin gene-related peptide in spinal cords isolated from postnatal day 7 rats. These results suggest that epsilonPKC age independently mediates inflammatory pain produced by intraplantar formalin. In contrast, gammaPKC contributes to formalin-induced nociception in an age-dependent manner. Identifying the molecular mechanisms responsible for age-specific patterns of nociception is necessary for the rational development of novel therapeutic strategies for treating pediatric pain.
View details for DOI 10.1124/jpet.103.060350
View details for Web of Science ID 000220972900024
View details for PubMedID 14762097
1. Following ethanol (EtOH) exposure, population excitatory postsynaptic potentials (pEPSPs) in isolated spinal cord increase to a level above control (withdrawal hyper-responsiveness). The present studies were designed to characterize this phenomenon and in particular to test the hypothesis that protein kinases mediate withdrawal. 2. Patch-clamp studies were carried out in motor neurons in rat spinal cord slices. Currents were evoked by brief pulses of glutamate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) or N-methyl-D-aspartic acid (NMDA). 3. Of 15 EtOH-sensitive neurons in which currents were evoked by glutamate, four (27%) displayed withdrawal hyper-responsiveness in the washout period. Mean current area after washout was 129.6+/-5% of control. 4. When currents were evoked by AMPA, two of 10 neurons (20%) displayed withdrawal hyper-responsiveness, with a mean current area 122+/-8% of control on washout. 5. Of a group of 11 neurons in which currents were evoked by NMDA, nine (82%) displayed withdrawal hyper-responsiveness. Mean increase in current area at the end of the washout period was to 133+/-6% of control (n=9, P<0.001). When NMDA applications were stopped during the period of EtOH exposure, mean area of NMDA-evoked responses on washout was only 98.0+/-5% of control (n=6, P>0.05). 6. The tyrosine kinase inhibitor genistein (10-20 microM) blocked withdrawal hyper-responsiveness. Of six EtOH-sensitive neurons, the mean NMDA-evoked current area after washout was 89+/-6% of control, P>0.05. 7 The protein kinase A (PKA) inhibitor Rp-cAMP (20-500 microM) did not block withdrawal hyper-responsiveness. On washout, the mean NMDA-evoked current area was 124+/-6% of control (n=5, P<0.05). 8 Two broad-spectrum specific protein kinase C (PKC) inhibitors, GF-109203X (0.3 microM) and chelerythrine chloride (0.5-2 nM), blocked withdrawal hyper-responsiveness. Responses on washout were 108+/-7%, n=5 and 88+/-4%, n=4 of control, respectively, P>0.05. 9 NMDA activation during EtOH exposure is necessary for withdrawal hyper-responsiveness. Both tyrosine kinase and PKC, but not PKA, appear to be essential for EtOH withdrawal hyper-responsiveness mediated by postsynaptic NMDA receptors in spinal cord motor neurons.
View details for Web of Science ID 000183125500009
View details for PubMedID 12746225
We have previously reported volatile anesthetic actions on glycinergic inhibitory transmission to spinal motor neurons. The present study is a comparable set of experiments on glutamatergic excitatory transmission. We tested the hypothesis that the balance between excitation and inhibition is shifted toward inhibition by larger depressant actions on excitation. Patch-clamp techniques were used to study spontaneous and evoked glutamate alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid currents in rat spinal cord slices. Enflurane (0.6 mM, 1 minimum alveolar anesthetic concentration) significantly decreased spontaneous miniature current frequencies either when sodium channels were blocked (miniature excitatory postsynaptic currents, mEPSCs), or when sodium channels were not blocked (spontaneous excitatory postsynaptic currents, sEPSCs). Enflurane did not affect mEPSC or sEPSC amplitude or kinetics. The effects on mEPSCs and sEPSCs did not differ. Enflurane significantly decreased both amplitude and area of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-evoked currents with no change in kinetics (P < 0.05 and 0.01, respectively). In contrast, enflurane increased miniature glycinergic current frequency when sodium channels were blocked, and prolonged glycinergic current duration. Enflurane actions on glutamatergic excitatory transmission are purely depressant both pre- and postsynaptically, whereas glycinergic inhibition is enhanced presynaptically under some conditions, and always prolonged postsynaptically. Thus, enflurane shifts the balance between synaptic excitation and inhibition in the direction of inhibition.Explanations proposed for anesthetic-induced central nervous system depression include enhancement of synaptic inhibition and depression of excitation. The results reported herein suggest that, in the case of enflurane, the mechanism is a shift in the balance toward inhibition. Excitation is uniformly depressed by multiple mechanisms, whereas some anesthetic actions tend to enhance inhibition.
View details for DOI 10.1213/01.ANE.0000055649.06649.D2
View details for Web of Science ID 000182456900021
View details for PubMedID 12707133
1. Ethanol (EtOH) tachyphylaxis (acute tolerance), a time-dependent decrease in apparent potency, is known in vivo and in some neuronal preparations. The present studies characterize EtOH tachyphylaxis in spinal motorneurons and test the hypothesis that metabotropic glutamate receptors (mGluRs) play a role. 2. Patch clamp studies were carried out in motorneurons in rat spinal cord slices. Currents were evoked by pulses of glutamate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) or N-methyl-D-aspartic acid (NMDA). 3. In nine of 15 cells, ethanol depression of glutamate-evoked currents was time-dependent. EtOH depressed current area 36.9+/-3% at 8-10 min, but only 16.8+/-3% at 20 min. Mean reduction in depression was 20.1+/-1%, N=9. Tachyphylaxis was less prominent in currents evoked by AMPA or NMDA, appearing in two of 10 AMPA and three of 11 NMDA currents. 4. The mGluR agonist trans-(1S,3R)-1-amino-1,3-cyclopentanedicarboxylic acid (ACPD) increased, the antagonist (+/-)-alpha-methyl-4-carboxyphenylglycine (MCPG) decreased the area of glutamate-evoked currents. ACPD also increased the area of NMDA- and AMPA-evoked currents. 5. ACPD increased the incidence of tachyphylaxis in glutamate-evoked currents to 100% (N=9); MCPG markedly reduced tachyphylaxis. ACPD also increased the incidence of tachyphylaxis in currents evoked by NMDA and AMPA to five of eight and four of seven neurons, respectively. 6. Block of G-protein pathways by intracellular GDP-beta-s abolished tachyphylaxis in glutamate-evoked currents (N=8); however, currents recovered only partially following EtOH washout. 7. Activation of mGluRs contributes to neuronal tachyphylaxis to EtOH in spinal cord motorneurons, probably via G-protein pathways.
View details for DOI 10.1038/sj.bjp.0705175
View details for Web of Science ID 000182969900005
View details for PubMedID 12721096
1. A common anaesthetic endpoint, prevention of withdrawal from a noxious stimulus, is determined primarily in spinal cord, where glycine is an important inhibitory transmitter. To define pre- and postsynaptic anaesthetic actions at glycinergic synapses, the effects of volatile anaesthetic agents on spontaneous and evoked glycinergic currents in spinal cord motor neurons from 6 - 14-day old rats was investigated. 2. The volatile anaesthetic agents enflurane, isoflurane and halothane significantly increased the frequency of glycinergic mIPSCs, enflurane to 190.4% of control+/-22.0 (mean+/-s.e.m., n=7, P<0.01), isoflurane to 199.0%+/-28.8 (n=7, P<0.05) and halothane to 198.2%+/-19.5 (n=7, P<0.01). However without TTX, isoflurane and halothane had no significant effect and enflurane decreased sIPSC frequency to 42.5% of control+/-12.4 (n=6, P<0.01). All the anaesthetics prolonged the decay time constant (tau) of both spontaneous and glycine-evoked currents without increasing amplitude. With TTX total charge transfer was increased; without TTX charge transfer was unchanged (isoflurane and halothane) or decreased (enflurane). 3. Enflurane-induced mIPSC frequency increases were not significantly affected by Cd(2+) (50 microM), thapsigargin (1 - 5 microM), or KB-R7943 (5 microM). KB-R7943 and thapsigargin together abolished the enflurane-induced increase in mIPSC frequency. 4. There are opposing facilitatory and inhibitory actions of volatile anaesthetics on glycine release dependent on calcium homeostatic mechanisms and sodium channels respectively. Under normal conditions (no TTX) the absolute amount of glycinergic inhibition does not increase. The contribution of glycinergic inhibition to anaesthesia may depend on its duration rather than its absolute magnitude.
View details for Web of Science ID 000176685200005
View details for PubMedID 12086976
Extensive studies on anesthetic mechanisms have focused on the nicotinic acetylcholine receptor, and to a lesser extent on the muscarinic receptor. We designed the present study to test the hypothesis that cholinergic receptors mediate some of the depressant actions of a volatile anesthetic in rat spinal cord. The cord was removed from 2- to 7-day-old rats and superfused in vitro; ventral root potentials were evoked by stimulating a lumbar dorsal root and recording from the corresponding ipsilateral ventral root. Both nicotine and muscarine depressed the nociceptive-related slow ventral root potential (sVRP). The nicotinic antagonists mecamylamine, methyllycaconitine, dihydro-beta-erythroidine, and the muscarinic antagonist atropine blocked the depressant effects of the respective agonists. Isoflurane 0.3 mini- mum alveolar anesthetic concentration depressed the sVRP area to approximately 40% of control. None of the antagonists changed the extent of isoflurane depression of the sVRP. The depressant actions of cholinergic agonists suggest that cholinergic receptors are important in spinal neurotransmission, but the lack of interaction between antagonists and isoflurane suggests that cholinergic receptors have little part in mediating the actions of this anesthetic in spinal cord. Because minimum alveolar anesthetic concentration is determined primarily in spinal cord, cholinergic receptors may be eliminated as molecular targets for this anesthetic end-point.Neither nicotinic nor muscarinic acetylcholine receptor antagonists altered spinal cord actions of isoflurane, suggesting that these receptors have little role in isoflurane actions in spinal cord. Cholinergic receptors thus may be eliminated as molecular targets in determining the anesthetic end-point of immobility in response to a noxious stimulus (minimum alveolar anesthetic concentration).
View details for Web of Science ID 000175890900023
View details for PubMedID 12032014
Gamma-aminobutyric acid type A (GABA(A)) receptors are considered important in mediating anesthetic actions. Mice lacking the beta3 subunit of this receptor (beta3-/-) have a higher enflurane minimum alveolar concentration (MAC) than wild types (+/+). MAC is predominantly determined in spinal cord.The authors measured three population-evoked responses in whole spinal cords, namely, the excitatory postsynaptic potential (pEPSP), the slow ventral root potential (sVRP), and the dorsal root potential. Synaptic and glutamate-evoked currents from motor neurons in spinal cord slices were also measured.Sensitivity of evoked responses to enflurane did not differ between +/+ and -/- cords. The GABA(A) receptor antagonist bicuculline significantly (P < 0.05) attenuated the depressant effects of enflurane on pEPSP, sVRP and glutamate-evoked currents in +/+ but not -/- cords. The glycine antagonist strychnine elevated the pEPSP to a significantly greater extent in -/- than in +/+ cords, but the interactions between strychnine and enflurane did not differ between -/- and +/+ cords.Similar enflurane sensitivity in spinal cords from -/- and +/+ mice was coupled with a decreased role for GABA(A) receptors in mediating the actions of enflurane in the former. This finding implies that other anesthetic targets substitute for GABA(A) receptors. Increase in glycine receptor-mediated inhibition was found in -/- cords, but the glycine receptor does not appear to be a substitute anesthetic target. This mutation thus led to a quantitative change in the molecular basis for anesthetic depression of spinal neurotransmission in a fashion not predicted by the mutation itself. The results argue against an immutable dominant role for GABA(A) receptors in mediating spinal contributions to MAC.
View details for Web of Science ID 000169657700022
View details for PubMedID 11465553
The spinal cord is an important anatomic site at which volatile agents act to prevent movement in response to a noxious stimulus. This study was designed to test the hypothesis that enflurane acts directly on motor neurons to inhibit excitatory synaptic transmission at glutamate receptors.Whole-cell recordings were made in visually identified motor neurons in spinal cord slices from 1- to 4-day-old mice. Excitatory postsynaptic currents (EPSCs) or potentials (EPSPs) were evoked by electrical stimulation of the dorsal root entry area or dorsal horn. The EPSCs were isolated pharmacologically into glutamate N-methyl-d-aspartate (NMDA) receptor- and non-NMDA receptor-mediated components by using selective antagonists. Currents also were evoked by brief pulse pressure ejection of glutamate under various conditions of pharmacologic blockade. Enflurane was made up as a saturated stock solution and diluted in the superfusate; concentrations were measured using gas chromatography.Excitatory postsynaptic currents and EPSPs recorded from motor neurons by stimulation in the dorsal horn were mediated by glutamate receptors of both non-NMDA and NMDA subtypes. Enflurane at a general anesthetic concentration (one minimum alveolar anesthetic concentration) reversibly depressed EPSCs and EPSPs. Enflurane also depressed glutamate-evoked currents in the presence of tetrodotoxin (300 nm), showing that its actions are postsynaptic. Block of inhibitory gamma-aminobutyric acid A and glycine receptors by bicuculline (20 micrometer) or strychnine (2 micrometer) or both did not significantly reduce the effects of enflurane on glutamate-evoked currents. Enflurane also depressed glutamate-evoked currents if the inhibitory receptors were blocked and if either D,L-2-amino-5-phosphonopentanoic acid (50 micrometer) or 6-cyano-7-nitroquinoxaline-2,3-dione disodium (10 micrometer) was applied to block NMDA or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-kainate receptors respectively.Enflurane exerts direct depressant effects on both alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and NMDA glutamate currents in motor neurons. Enhancement of gamma-aminobutyric acid A and glycine inhibition is not needed for this effect. Direct depression of glutamatergic excitatory transmission by a postsynaptic action on motor neurons thus may contribute to general anesthesia as defined by immobility in response to a noxious stimulus.
View details for Web of Science ID 000089671900027
View details for PubMedID 11020764
To develop a tool for detailed analysis of spinally acting anesthetic and analgesic agents.Studies were done on visually identified motor neurons in 400 microns thick spinal cord slices from 14-23 d old rats using patch clamp techniques. Ethanol was used as a prototype general anesthetic agent.Cell bodies in the ventrolateral horn identified as motor neurons by retrograde fluorescent labeling had a mean dimension of 32 +/- 5 microns (x +/- s, n = 25). Mean resting potential was -62.8 +/- 2.4 mV; input resistance was 44 +/- 24 M omega (n = 19). Threshold was -44 +/- 7 mV, and action potential amplitude 101 +/- 9 mV from baseline. Ethanol concentrations at and below 50-200 mmol/L decreased motor neuron excitability to the injected current; there was no effect on resting potential, but a variable reversible increase in input resistance. Ethanol reversibly depressed the excitatory postsynaptic potential, with a dose-response relationship similar to that previously observed for the population excitatory postsynaptic potential in intact spinal cord in vitro. Ethanol also reversibly depressed currents evoked by glutamate, reducing total charge transfer to 40% +/- 26% of control (x +/- s; n = 4).Reduction of connectivity in this relatively thick slice preparation does not significantly modify drug actions. The actions of ethanol on excitatory synaptic transmission observed in intact spinal cord are in part due to postsynaptic effects on motor neurons.
View details for Web of Science ID 000087561300004
View details for PubMedID 11360684
Ethanol is a general anesthetic agent as defined by abolition of movement in response to noxious stimulation. This anesthetic endpoint is due to spinal anesthetic actions. This study was designed to test the hypothesis that ethanol acts directly on motor neurons to inhibit excitatory synaptic transmission at glutamate receptors. Whole cell recordings were made in visually identified motor neurons in spinal cord slices from 14- to 23-day-old rats. Currents were evoked by stimulating a dorsal root fragment or by brief pulses of glutamate. Ethanol at general anesthetic concentrations (50-200 mM) depressed both responses. Ethanol also depressed glutamate-evoked responses in the presence of tetrodotoxin (300 nM), showing that its actions are postsynaptic. Block of inhibitory gamma-aminobutyric acidA and glycine receptors by bicuculline (50 microM) and strychnine (5 microM), respectively, did not significantly reduce the effects of ethanol on glutamate currents. Ethanol also depressed glutamate-evoked currents when the inhibitory receptors were blocked and either D, L-2-amino-5-phosphonopentanoic acid (40 microM) or 6-cyano-7-nitroquinoxaline-2,3-dione disodium (10 microM) were applied to block N-methyl-D-aspartate or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate receptors, respectively. The results show that ethanol exerts direct depressant effects on both alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and N-methyl-D-aspartate glutamate currents in motor neurons. Enhancement of gamma-aminobutyric acidA and glycine inhibition is not required for this effect. Direct depression of glutamatergic excitatory transmission by a postsynaptic action on motor neurons thus may contribute to general anesthesia as defined by immobility in response to a noxious stimulus.
View details for Web of Science ID 000081039900049
View details for PubMedID 10381800
Ethanol at concentration of 200 mM induces anesthesia in experimental animals and depresses neurotransmission in isolated spinal cords. To determine whether actions on primary afferent nerve terminals contribute to ethanol's depressant effects on spinal cord, a study was undertaken to test whether ethanol blocks sodium currents (I(Na)) in dorsal root ganglion neurons (DRGn). Whole-cell patch clamp was used to examine I(Na) in DRGn isolated from 1- to 15-day-old rats. At a holding potential of -80 mV ethanol (200 mM) decreased peak tetrodotoxin-resistant (TTX-R) and tetrodotoxin-sensitive (TTX-S) I(Na) by 19.0% +/- 2.7 (mean +/- SEM) and 8.5% +/- 2.2, respectively. Maximal available I(Na) was reduced to 82 +/- 4% (TTX-R) and 93 +/- 1% (TTX-S) of control. Steady-state inactivation curves were shifted in the hyperpolarizing direction by 2.1 +/- 0.2 mV (TTX-R) and 1.1 +/- 0.1 mV (TTX-S). At prepulse potentials of -30 mV (TTX-R) and -70 mV (TTX-S), these shifts contributed an additional 17 +/- 1% (TTX-R) and 7 +/- 1% (TTX-S) reduction in available I(Na). Ethanol thus selectively induced both voltage-independent and voltage-dependent block of TTX-R I(Na) in DRGn. Because DRGn TTX-R sodium channels are associated with small-diameter primary afferent fibers, these results are consistent with a role for ethanol actions on sodium channels in depression of nociceptive-related neurotransmission in spinal cord.
View details for Web of Science ID 000076758300001
View details for PubMedID 9822154
This study examined the mechanism for hyperexcitability after ethanol withdrawal from isolated neonatal rat spinal cord. Ethanol (65-130 mM, 30 min) significantly depressed the glutamate receptor-mediated population excitatory postsynaptic potential (pEPSP) underlying the monosynaptic reflex. On washing with drug-free solution the response recovered to levels significantly above control. Minimum ethanol exposure time required for induction of withdrawal hyperexcitability was approximately 15 min. A second application of ethanol after washout depressed the pEPSP to an extent similar to the first, and a second wash did not elevate response significantly more than the initial wash. Ethanol-induced hyperexcitability thus develops with a time course of minutes and plays a role in determining apparent initial ethanol potency. Both alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate and N-methyl-D-aspartate receptor-mediated components of the pEPSP were necessary for the expression of hyperexcitability on withdrawal but not for its induction. Butanol withdrawal also was associated with hyperexcitability, methanol was not. The case with octanol is uncertain because of slow recovery from this more lipophilic agent. Hyperexcitability on ethanol withdrawal was specific to the glutamate receptor-mediated pEPSP and not generalized to other evoked potentials. These results may be relevant to rapid and/or very rapid acute functional tolerance and to ethanol withdrawal.
View details for Web of Science ID 000072972200025
View details for PubMedID 9536011
Ethanol, usually studied in relation to intoxication, is also capable of producing general anesthesia. The most common standard of anesthetic potency is the concentration which produces immobility in response to a noxious stimulus. This concentration will be referred to as the anesthetic concentration. Immobilization is a spinal effect. Ethanol effects were studied in spinal cord from 2-7-day-old rats at concentrations which included the anesthetic concentration in both adult rats (97 mM) and 6-7-day-old rats (235 mM). At neonatal but not adult anesthetic concentrations, ethanol depressed monosynaptic reflex amplitude (mediated by glutamate AMPA receptors + compound action potential). At both neonatal and adult anesthetic concentrations ethanol reversibly depressed the population excitatory postsynaptic potential (pEPSP) (glutamate AMPA and NMDA receptors), the slow ventral root potential (NMDA + metabotropic receptors), and the dorsal root potential (GABA(A) receptors, via glutamate-excited interneurons). Effects were greater on NMDA receptor-mediated components than on AMPA-receptor-mediated components of the pEPSP and greater on NMDA than on metabotropic receptor-mediated components of the slow ventral root potential. The profile of ethanol effects on spinal cord resembles that of inhalation general anesthetics. The results show that both AMPA and NMDA receptor-mediated transmission are sensitive to ethanol and that enhancement of GABAergic neurotransmission is overridden by depression of excitation to the interneurons. They provide no obvious explanation for ethanol's lower general anesthetic potency in the neonate.
View details for Web of Science ID A1997XH48500003
View details for PubMedID 9226403
Our previous studies have shown that a benzodiazepine potentiates opioid actions on spinal cord by blocking a hyperresponsiveness that may be related to the development of opioid tolerance and withdrawal. The present study was designed to test whether propofol, which like benzodiazepines acts on GABA(A) receptors, displays similar interactions with opioids. Spinal cords isolated from 1-7 day old rats were arranged to record the slow ventral root potential (sVRP) elicited by stimulating a lumbar dorsal root. A concentration of propofol which by itself did not depress sVRP significantly enhanced the apparent potency of alfentanil and blocked the increase in sVRP observed when alfentanil is followed by naloxone. The results suggest that enhancement of GABA inhibition may increase opioid potency by inhibiting the development of acute tolerance.
View details for Web of Science ID A1997XK37000003
View details for PubMedID 9224789
Benzodiazepines, which may themselves have analgesic properties, display complex interactions with opioids. This study was designed to investigate the effects of midazolam on nociceptive neurotransmission in isolated neonatal rat spinal cord, and the interactions between midazolam and alfentanil. Slow ventral root potentials (sVRP) were recorded from a lumbar root of spinal cords isolated from 1-7-day-old rats and superfused at 27-28 degrees C. Midazolam (35 nmol litre-1 to 15 mumol litre-1) significantly (P < 0.05) depressed sVRP area in a concentration-dependent manner. Midazolam depression was antagonized by flumazenil, bicuculline and naloxone. Midazolam and alfentanil interacted synergistically, as determined by a combination index of less than 1. Midazolam blocked the rebound hyperexcitability observed when alfentanil was reversed by naloxone. The results of the study are relevant to benzodiazepine-opioid analgesia and to the effectiveness of benzodiazepines in mitigating the development of opioid tolerance and dependence.
View details for Web of Science ID A1996VG79000016
View details for PubMedID 8949814
We have examined the interactions between NMDA receptors and opioid effects in isolated neonatal rat spinal cord. Electrical stimulation of a lumbar dorsal root evoked a nociceptive-related slow ventral root potential (sVRP) recorded at the corresponding ipsilateral ventral root. The kappa opiate receptor agonist U69,593 (2.5 nM-1 microM) depressed sVRP area by a maximum of 80%, EC50 was approximately 33 nM. Both the non-specific antagonist naloxone and the kappa-specific antagonist nor-binaltorphimine (nor-BNI) antagonized the effects of U69,593. Morphine, a mu agonist, (1 nM-1 microM) depressed sVRP area with an approximate EC50 of 90 nM. The effects of both mu and kappa opioid agonists were selective for the very slow metabotropically mediated components of the sVRP, compared to the relatively fast NMDA receptor-mediated components. The non-competitive N-methyl-D-aspartate (NMDA) antagonist MK-801 (20 nM) had no effect on sVRP area when applied alone but co-applied with morphine significantly potentiated the depressant effects of morphine. In contrast, MK-801 either had no effect on or slightly antagonized the depressant effects of U69,593. Naloxone following morphine produced a significant increase in sVRP area above pre-morphine control values; the increase lasted 30 min or more. Neither naloxone nor nor-BNI was associated with an increase in sVRP area when given alone or following U69,593. MK-801 co-applied with morphine blocked the rebound increase in sVRP area following naloxone. These results suggest that (1) both mu and kappa receptor agonists exert similar selective depressant effects on spinal nociceptive neurotransmission; (2) mu but not kappa agonists exert prolonged excitatory effects that oppose the depression; and (3) NMDA receptors play a role in determining opioid analgesic potency and naloxone-precipitated hyperresponsiveness. The results may be related to initial steps in the development of acute tolerance to mu opioids, and suggest that tolerance to kappa opioids may have a different mechanism.
View details for Web of Science ID A1996VG56100029
View details for PubMedID 8880858
The behavioral state known as general anesthesia is the result of actions of general anesthetic agents at multiple sites within the neuraxis. The most common end point used to measure the presence of anesthesia is absence of movement following the presentation of a noxious stimulus. The actions of general anesthetics within the spinal cord have been shown to contribute significantly to the suppression of pain-evoked movements, an important component of clinical anesthesia. Studies in the spinal cord are likely to increase our understanding of the pharmacology by which general anesthetics alter the transmission of somatomotor information. It now appears that the pharmacology responsible for the production of anesthesia is agent- and site-selective, and not the result of a unitary mechanism of action.
View details for Web of Science ID A1995TG69800010
View details for PubMedID 8638296
1. Long-lasting increases in synaptic efficacy following repetitive stimulation have been demonstrated at several sites in the CNS, where they are collectively termed long-term potentiation (LTP). LTP is of interest with respect to its presumptive relationship to learning and memory in hippocampus. In the spinal cord in vivo, an LTP-like phenomenon is thought to underlie the allodynia and hyperalgesia that follows some peripheral injuries. 2. We investigated the capacity of the isolated neonatal rat spinal cord to sustain a long-lasting increase in a nociceptive-related slow ventral root potential (sVRP) recorded from a lumbar root after a tetanic train of stimuli to the peripheral cutaneous saphenous nerve. Stimuli were delivered at a low constant (0.02 s-1) frequency during a 30-min control period. A tetanic stimulus train (10 s-1 for 60 s) was then given followed by a resumption of low (0.02 s-1) frequency stimulation. Potentiation was defined as an increase in sVRP area > 2 SD above control mean. 3. Twenty of 20 preparations showed immediate posttetanic potentiation. In 13 of the 20, potentiation was maintained for > or = 1 h after the tetanic stimulus train. 4. Potentiation was dependent on activation of C fibers during the inducing train; stimuli below C-fiber threshold, activating only A fibers, were ineffective. Potentiation was selectively expressed by a long-latency component of the sVRP elicited by stimuli at a strength that evoked both A- and C-fiber responses in the nerve.(ABSTRACT TRUNCATED AT 250 WORDS)
View details for Web of Science ID A1995RU95200008
View details for PubMedID 7500126
We have examined the effects of alfentanil on nociceptive-related neurotransmission in isolated neonatal rat spinal cord, with particular attention to acute tolerance. Electrical stimulation of a lumbar dorsal root was used to evoke the monosynaptic reflex (MSR), a slow ventral root potential (sVRP), and the dorsal root potential (DRP). Alfentanil (0.5 nmol litre-1 to 1 mumol litre-1) depressed sVRP area by a maximum of 85%; EC50 was approximately 2 nmol litre-1. The effects of alfentanil were selective for very slow, metabotropically mediated sVRP components compared with faster NMDA receptor-mediated components. The MSR was unaffected. Alfentanil depressed DRP area by a maximum of 50% at 1 mumol litre-1. Naloxone antagonized all alfentanil effects. Morphine depressed sVRP area with an approximate EC50 of 90 nmol litre-1, giving an alfentanil:morphine potency ratio of 45:1. The effects of alfentanil on sVRP showed no biphasic time dependence up to 60 min. Naloxone administered after alfentanil produced a significant rebound in sVRP area to a level of 143 (SD 21.3)% above control. Thus, in this study there was no evidence for acute tolerance, as measured by a decrease in effectiveness over time, but there was evidence as measured by rebound following naloxone.
View details for Web of Science ID A1995RB86900013
View details for PubMedID 7640126
In isolated neonatal rat spinal cord, naloxone administered after an opioid increases a nociceptive-related slow ventral root potential (sVRP) to levels above pre-drug controls. We studied the role of N-methyl-D-aspartate (NMDA) receptors in this phenomenon, which may be related to acute tolerance and to hyperalgesia on antagonist-precipitated withdrawal. Naloxone (200 nM) alone produced no significant effect on sVRP area, while naloxone (560 nM) increased area to 121 +/- 17.7% of control (mean +/- SD). Following 200 nM alfentanil, naloxone (200 nM) was associated with a significant rebound in sVRP area to 138 +/- 18.0% of pre-drug control. Hyperresponsiveness developed within 7 min of initial alfentanil exposure. The non-competitive NMDA antagonist MK-801 (20 nM) had no effect on sVRP area when applied alone; higher concentrations produced irreversible depression. MK-801 (20 nM) co-applied with 200 nM alfentanil blocked the rebound increase in sVRP area following naloxone 200 nM and also the increase following naloxone alone (560 nM). The results suggest that alfentanil induces a rapid NMDA receptor-dependent change in spinal cord neuronal excitability.
View details for Web of Science ID A1995QV06300017
View details for PubMedID 7609918
Many ion channels have been proposed as target sites for anaesthetic action; for some agents multiple receptors-ion channels may be implicated. In addition to acting as a non-competitive antagonist at glutamate NMDA receptors, ketamine also affects other ion channels. The present study was undertaken to determine if the effects of ketamine in an integrated portion of the central nervous system involve multiple actions at glutamate non-NMDA, glutamate NMDA, and GABAA receptor. The effects of ketamine 1-50 mumol litre-1 were examined on three pharmacologically distinct responses in isolated superfused neonatal rat spinal cord: the monosynaptic reflex (glutamate non-NMDA); a slow ventral root potential (VRP) with a large NMDA-mediated component; and the dorsal root potential (DRP) (GABAA). Ketamine, at concentrations relevant to anaesthesia (1-50 mumol litre-1), reversibly depressed the area under the curve of the slow VRP in a concentration-dependent fashion. The effects of ketamine were selective for the early (0-1 s) component of the slow VRP. The monosynaptic reflex was unaffected at these concentrations. The actions of ketamine resembled those of the NMDA antagonist APV. Dorsal root potentials evoked by dorsal root stimulation or by muscimol were either unaffected or reversibly depressed by ketamine 1-20 mumol litre-1. The concentrations tested include the anaesthetic range for both rats and humans. The effects of ketamine on neurotransmission in this preparation can be accounted for entirely by its action at NMDA receptors. Glutamate non-NMDA receptors were unaffected and GABAA transmission was not enhanced.(ABSTRACT TRUNCATED AT 250 WORDS)
View details for Web of Science ID A1995QB73000017
View details for PubMedID 7880712
Two halogenated cyclobutanes, one anesthetic and one not, were compared on receptor-specific pathways in isolated neonatal rat spinal cord. The anesthetic 1-chloro-1,2,2-trifluorocyclobutane depressed the monosynaptic reflex (glutamate non-NMDA receptors) and abolished a slow ventral root potential (glutamate NMDA, non-NMDA and tachykinin receptors). This compound slightly enhanced the muscimol-evoked dorsal root potential (GABAA) but reversibly depressed the dorsal root potential elicited by dorsal root stimulation. The non-anesthetic 1,2-dichlorohexafluorocyclobutane increased monosynaptic reflex, depressed slow ventral root potential approximately 50%, had little effect on muscimol-evoked dorsal root potential, and irreversibly depressed dorsal root-evoked dorsal root potential. Hypoxia accounts for slow ventral root potential depression, but not monosynaptic reflex enhancement. In this preparation and for this pair of compounds, anesthetic properties are related to blockade of transmission at glutamate synapses, with a small component of GABAA enhancement. Monosynaptic reflex increase may be related to the non-anesthetic cyclobutane's convulsant and anti-anesthetic properties.
View details for Web of Science ID A1994PQ79500026
View details for PubMedID 7698184
Barbiturates are often described as non-analgesic or even hyperalgesic agents; the newer intravenous anesthetic agent propofol is said to be non-analgesic. Both propofol and barbiturates occupy sites on the GABAA receptor. The present study was designed to compare the effects of propofol and barbiturates on nociceptive-related neurotransmission in neonatal rat spinal cord; to search for actions that might be hyperalgesic; and to determine the extent to which propofol depression of nociceptive neurotransmission is mediated by GABAA receptors. The monosynaptic reflex, a slow ventral root potential (slow VRP) and the dorsal root potential (DRP) were recorded from isolated neonatal (1-5 days old) superfused rat spinal cords in response to electrical stimulation of a lumbar dorsal root. The slow VRP and the DRP are related to nociception. Propofol (0.5-10 microM), pentobarbital (1-10 microM), and thiopental (1-10 microM) reversibly depressed the slow VRP. Dose-response curves were monophasic and linear over this range. The monosynaptic reflex was unaffected. The GABAA agonist muscimol (0.2-1 microM) also depressed the slow VRP. Propofol and barbiturate slow VRP depression was antagonized by the GABAA antagonist bicuculline (1 microM). Propofol depressed the response evoked by direct application of substance P. The DRP is a GABAA-mediated depolarization of primary afferent nerve terminals that diminishes the effectiveness of nociceptive input. Propofol and thiopental increased electrically evoked DRP amplitude and increased the DRP evoked by application of muscimol. Both propofol and barbiturates thus depressed the nociceptive-related slow VRP and enhanced the antinociceptive DRP; their effective concentrations are at or close to the general anesthetic range for these agents. No anti-analgesic or hyperalgesic effect was observed. (ABSTRACT TRUNCATED AT 250 WORDS)
View details for Web of Science ID A1992KC86500016
View details for PubMedID 1334637
Slow ventral root potentials (slow VRP's) recorded from 1- to 5-day-old rat spinal cords are implicated in nociception, but there is controversy over their origin and persistence in the adult. The present study investigated changes in the role of substance P and NMDA receptors in slow VRP generation during the postnatal period (1-21 days). Through 9 days, dorsal root stimulation elicits slow VRP's with typical peak amplitudes at 3-4 s, decay time constants of 18-20 s, and durations > 20 s. After 11 days, peak amplitude shortens to < 1 s, decay time constant 4-5 s, and duration < 10 s. At 1-6 days, slow VRP's are sensitive to the NMDA receptor antagonist APV and the substance P antagonists spantide and CP 96,345. After 11 days, APV sensitivity is retained, but spantide and ability of substance P to evoke a response are diminished. Abbreviated slow VRP's in post-11-day spinal cords appear to correspond to the early APV-sensitive component of long-duration slow VRP's in younger animals. Attempts to restore long-duration slow VRP's in 12- to 14-day-old rat cords by blocking various inhibitory mechanisms were not successful. The results suggest that a substance P response, some of which is mediated by NK1 receptors, is lost with maturation of the cord. Either a developmental role played by substance P changes with maturity, or the motor neurons of the isolated post-11-day cord lose the capacity to sustain large long-duration plateau potentials.
View details for Web of Science ID A1992JZ04700009
View details for PubMedID 1281736
Substance P and glutamate actions have separately been implicated in the generation of nociceptive-related slow ventral root potentials (slow VRPs). We report that slow VRPs are dependent on both substance P and NMDA receptor-mediated neurotransmission. Slow VRPs of 10-40 s duration were evoked by electrically stimulating a lumbar dorsal root and recorded at the corresponding ipsilateral ventral root in spinal cords isolated from 1- to 5-day-old rats; the monosynaptic reflex was also recorded. The NMDA receptor antagonist APV (5-20 microM) and the substance P antagonist spantide (10-20 microM) both reversibly depressed the slow VRP without affecting the monosynaptic reflex; spantide and APV applied together nearly abolished the slow VRP. The quisqualate-kainate receptor antagonist CNQX (1-5 microM) reduced the monosynaptic reflex and an early component of the slow VRP. A slow VRP could be elicited by brief (0.1-1.0 s) focal applications of either substance P (2-20 microM) or NMDA (10 microM), and also by CGRP (2-20 microM). Substance P-evoked and NMDA-evoked responses were blocked by their respective antagonists spantide and APV. Each was also cross-sensitive to the other antagonist. Both excitatory amino acids, acting on an NMDA receptor, and substance P, acting on a tachykinin receptor, thus appear to be involved in generating this slow potential. Both NMDA and tachykinin receptors are necessary to generate a full response.
View details for Web of Science ID A1991GH08300003
View details for PubMedID 1723644
Alpha 2-adrenoceptor agonists such as clonidine are sedatives and enhance the effectiveness of several different kinds of anesthetics. This study was performed to quantitate the effect of dexmedetomidine, a novel alpha 2-adrenoceptor agonist, on the action of the volatile anesthetic agent isoflurane in rats in vivo. A separate set of experiments in rat hippocampal slices was designed to determine whether isoflurane and dexmedetomidine exerted similar effects on synaptic transmission in vitro and to examine the interaction between the two agents. In vivo, dexmedetomidine (100 micrograms/kg i.p.) reduced isoflurane minimum alveolar anesthetic requirement (MAC), determined by loss of response to tail pinch, by approximately 90%. In hippocampal CA1 neurons, on the other hand, there was a relatively small potentiation of the effects of isoflurane at the maximally effective dexmedetomidine concentration (1 nM). The hippocampal CA1 area, at least in the slice preparation, may thus not be representative of the CNS site(s) at which alpha 2 adrenoceptor agonists lessen anesthetic requirement in vivo.
View details for Web of Science ID A1991FP17000004
View details for PubMedID 1678296
Alpha 2-Adrenoceptors mediate analgesia in vivo. The present study explored the actions of the alpha 2-adrenoceptor agonists dexmedetomidine and clonidine on a nociceptive response in isolated neonatal rat spinal cord. Stimulation of a dorsal root generates a slow ventral root potential (slow VRP) at the corresponding ipsilateral ventral root. The slow VRP meets several criteria for a nociceptive response. Dexmedetomidine (10 nM) and clonidine (200 nM) depressed the slow VRP by approximately 80%. Dexmedetomidine's action was approximately linear over the concentration range 0.5-500 nM, whereas clonidine (20 nM-5 microM) exerted biphasic effects. The profile of agonist and antagonist effectiveness characterized the receptor(s) as alpha 2-adrenoceptors; the subtype could not be identified as either alpha 2A or alpha 2B. Naloxone pretreatment partially blocked dexmedetomidine's effect, suggesting a possible endogenous opiate involvement. Dexmedetomidine (0.5-2.0 nM) also depressed the VRP evoked by application of substance P to the cord, implicating postsynaptic as well as possible presynaptic actions. At high concentrations, dexmedetomidine (50-500 nM) depressed the monosynaptic reflex, probably through non-alpha 2-receptor(s). Results from the neonatal spinal cord correlate well with those from in vivo analgesia studies. They suggest an important direct spinal contribution to alpha 2-adrenoceptor-mediated analgesia.
View details for Web of Science ID A1991EW15200011
View details for PubMedID 1674474
Membrane hyperpolarization (increase in resting potential) together with a conductance increase has been suggested as a common mechanism of anesthetic action. The current study compared the effects of halothane, enflurane, and isoflurane on resting membrane potential and conductance of hippocampal CA1 neurons in vitro. At 1 MAC, halothane produced significant (P less than 0.01) hyperpolarization (-2.8 +/- 1.3 mV, mean +/- SD) accompanied by a conductance increase (6.2 +/- 2.7%). Enflurane also produced a significant (P less than 0.001) hyperpolarization (-3.15 +/- 1.2 mV); however, this was accompanied by a conductance decrease (-4.5 +/- 1.5%). Isoflurane produced variable effects. Anesthetic-induced hyperpolarization was maximal in neurons with more negative initial resting potentials and was reduced by depolarization. Across agents, these relatively small changes in resting potential were not correlated with decreases in excitability as measured by synaptically evoked population spike depression. The results are not consistent with a common action of the three agents on a single ionic channel.
View details for Web of Science ID A1991ET50400014
View details for PubMedID 1986663
The basis for the hyperexcitability and seizure activity associated with enflurane anaesthesia was investigated using extracellular and intracellular recording in rat hippocampal brain slices. Enflurane produced seizure-like burst discharges in CA1 pyramidal neurones, accompanied by depressed field potential amplitudes and a reduced threshold for synaptically evoked population spikes. However, threshold for action potentials evoked by intracellular current injection did not change, nor did action potential amplitude, duration or spike frequency accommodation in single neurones. Enflurane 2.0 vol% hyperpolarized CA1 neurones (3.1 (SD 1.3)mV), decreased membrane conductance (12 (6)% below control), and depressed EPSP amplitudes (34% of control) (P less than 0.01). Enflurane appeared to enhance both intrinsic and synaptically mediated inhibitory potentials. The N-methyl-D-aspartate (NMDA) receptor antagonist amino-phosphonovalerate (APV) 5-20 mumol litre-1 completely blocked seizure-like burst discharge of CA1 neurones in the presence of enflurane, and the enflurane-induced reduction of population spike threshold; it did not alter anaesthetic depression of EPSP amplitude. Thus enflurane-induced burst discharge of CA1 neurones appeared to involve an enhancement of excitatory synaptic transmission rather than depression of intrinsic or synaptic inhibition.
View details for Web of Science ID A1989AP06500009
View details for PubMedID 2572247
The synaptic integrative properties of facilitation and potentiation are important determinants of cortical neurone excitability. The present study measured the effects of halothane and methoxyflurane on synaptic facilitation, paired-pulse potentiation, and long-term potentiation (LTP) in CA1 pyramidal neurones of rat hippocampal slices. Methoxyflurane 0.16vol% and halothane 1.2 vol% depressed population spike amplitudes by approximately 100%, but halothane did so with relatively little (less than 10%) depressant effect on the field excitatory postsynaptic potential (EPSP). Both agents enhanced EPSP facilitation, halothane more than methoxyflurane. Halothane consistently enhanced paired-pulse population spike potentiation; methoxyflurane sometimes diminished it. Halothane reduced the probability of LTP induction to an extent correlated with block of population spike responses in the inducing stimulus train (r = 0.92); methoxyflurane did not block LTP. The results suggest that these agents affect cortical excitability by multiple actions, the distribution of actions being specific to each agent.
View details for Web of Science ID A1989T649000013
View details for PubMedID 2539171
1. Because hyperbaric pressure profoundly depresses excitatory synaptic transmission, it has proved difficult to account for its excitatory effects in the CNS. We tested the hypothesis that hyperbaric pressure might increase excitation by enhancing facilitation and potentiation during repetitive synaptic activation, and/or by selectively depressing inhibitory synaptic transmission. Intracellular microelectrode recordings were obtained from crustacean muscle fibers innervated by single identifiable excitor and inhibitor motor neurons; the preparations were exposed to pressures of 0.1-10.1 MPa. 2. Hyperbaric pressure reduced the amplitude of the singly evoked excitatory junctional potential (EJP), enhanced paired-pulse facilitation, and increased the potentiation elicited by trains of stimuli. The potentiated EJP at 10.1 MPa approached the comparable response evoked at normobaric pressure. 3. Hyperbaric pressure also depressed inhibitory synaptic transmission, measured as depression of the EJP by the inhibitor motor neuron. However, pressure depressed excitatory and inhibitory synaptic transmission to the same extent. Thus there appears to be no selective effect of pressure on the GABA-activated chloride channel. The amplitude of the inhibited EJP at 10.1 MPa remained below that at normobaric pressure, even during repetitive stimulation. 4. The results do not support the hypothesis that pressure increases central excitation by selectively depressing inhibitory transmission per se; enhancement of potentiation, however, probably plays an important role. In this preparation, in which inhibitory transmission also displays facilitation, pressure did not increase overall excitation or alter the balance between excitation and inhibition. 5. These results predict that a pressure-excitable network should encompass excitatory synaptic connections which exhibit pronounced facilitation and inhibitory synapses with little or no facilitation.
View details for Web of Science ID A1988Q605400019
View details for PubMedID 2848102
Hyperbaric pressure induces seizures and increases anaesthetic requirements ("pressure reversal of anaesthesia"), but both pressure and anaesthetic agents depress excitatory synaptic transmission. The present study has attempted to resolve this paradox. The interaction between helium pressure to 10.1 MPa and anaesthetic agents (pentobarbitone, halothane, methoxyflurane) was investigated at a crustacean glutaminergic excitatory neuromuscular junction which can be modulated by GABA inhibition. Both pressure and the anaesthetics depressed the singly evoked excitatory junctional potential (EJP). During repetitive stimulation, both pressure and pentobarbitone antagonized their own depressant effects by enhancing tetanic potentiation. The additive enhancement at 10.1 MPa was sufficient to increase the pentobarbitone-depressed response above the corresponding normobaric level. No significant antagonism between pressure and any of the anaesthetics was observed on the properties of EJP amplitude and time course, facilitation, potentiation or inhibition. Additivity rather than antagonism is the basis for pressure reversal of anaesthetic depression at this model synapse. The functional antagonism is therefore indirect, and probably involves multiple sites of action for both pressure and anaesthetics.
View details for Web of Science ID A1988N798100011
View details for PubMedID 2840107
Previous studies have demonstrated the existence of receptors for gamma-aminobutyric acid (GABA), beta-adrenergic catecholamines and acetylcholine in vertebrate peripheral nerve, and provided functional correlates for activation of both GABA and beta-adrenergic receptors. The present studies show that a cholinergic receptor present on the nerve can also modify impulse pattern. In frog sciatic nerve, both carbamylcholine and dibutyryl cyclic GMP increased the amplitude of the response to the second stimulus of a train at very short interstimulus intervals. The effect of carbamylcholine was blocked by 4-aminopyridine. The results are consistent with the hypothesis that cholinergic agonists mediate an increase in endogenous cyclic guanosine monophosphate (cGMP), which increases the ability of the nerve to follow closely spaced stimuli by inhibiting potassium channels.
View details for Web of Science ID A1987L058500004
View details for PubMedID 3427454