Tolerance, opioid-induced allodynia and withdrawal associated allodynia in infant and young rats.

Our laboratory has previously characterized age-dependent changes in nociception upon acute morphine withdrawal. This study characterizes changes in mechanical and thermal nociception following acute, intermittent, or continuous morphine administration in infant (postnatal days 5-8) and young (postnatal days 19-21) rats. Morphine was given as a single acute administration (AM), intermittently twice a day for 3 days (IM), or continuously for 72 h via pump (CM). AM did not produce long-term changes in mechanical or thermal nociception in either infant or young rats. CM produced changes in mechanical nociception that included the development of tolerance, opioid-induced mechanical allodynia and withdrawal-associated mechanical allodynia in young rats, but only tolerance and a prolonged withdrawal-associated mechanical allodynia in infant rats. IM produced withdrawal-associated mechanical allodynia in both infant and young rats. Measuring paw withdrawal responses to thermal stimuli, infant and young rats showed tolerance without opioid-induced thermal hyperalgesia or withdrawal-associated thermal hyperalgesia following CM. In contrast to CM, withdrawal-associated thermal hyperalgesia was seen in both ages following IM. In conclusion, CM versus IM differentially modified mechanical and thermal nociception, suggesting that opioid-dependent thermal hyperalgesia and mechanical allodynia can be dissociated from each other in infant and young rats. Furthermore, tolerance, opioid-induced hypersensitivity, and withdrawal-associated hypersensitivity are age-specific and may be mediated by distinct mechanisms.

Enflurane actions on spinal cords from mice that lack the beta3 subunit of the GABA(A) receptor.

BACKGROUND: 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. METHODS: 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. RESULTS: 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. CONCLUSIONS: 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.

Enflurane directly depresses glutamate AMPA and NMDA currents in mouse spinal cord motor neurons independent of actions on GABAA or glycine receptors.

BACKGROUND: 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. METHODS: 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. RESULTS: 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. CONCLUSIONS: 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.

Patch clamp studies of motor neurons in spinal cord slices: a tool for high-resolution analysis of drug actions.

AIM: To develop a tool for detailed analysis of spinally acting anesthetic and analgesic agents. METHODS: 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. RESULTS: 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). CONCLUSION: 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.

Ethanol directly depresses AMPA and NMDA glutamate currents in spinal cord motor neurons independent of actions on GABAA or glycine receptors.

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.

Minimum alveolar anesthetic concentration of fluorinated alkanols in rats: relevance to theories of narcosis.

UNLABELLED: The Meyer-Overton hypothesis predicts that the potency of conventional inhaled anesthetics correlates inversely with lipophilicity: minimum alveolar anesthetic concentration (MAC) x the olive oil/gas partition coefficient equals a constant of approximately 1.82 +/- 0.56 atm (mean +/- SD), whereas MAC x the octanol/gas partition coefficient equals a constant of approximately 2.55 +/- 0.65 atm. MAC is the minimum alveolar concentration of anesthetic required to eliminate movement in response to a noxious stimulus in 50% of subjects. Although MAC x the olive oil/gas partition coefficient also equals a constant for normal alkanols from methanol through octanol, the constant (0.156 +/- 0.072 atm) is one-tenth that found for conventional anesthetics, whereas the product for MAC x the octanol/gas partition coefficient (1.72 +/- 1.19) is similar to that for conventional anesthetics. These normal alkanols also have much greater affinities for water (saline/gas partition coefficients equaling 708 [octanol] to 3780 [methanol]) than do conventional anesthetics. In the present study, we examined whether fluorination lowers alkanol saline/gas partition coefficients (i.e., decreases polarity) while sustaining or increasing lipid/gas partition coefficients, and whether alkanols with lower saline/gas partition coefficients had products of MAC x olive oil or octanol/gas partition coefficients that approached or exceeded those of conventional anesthetics. Fluorination decreased saline/gas partition coefficients to as low as 0.60 +/- 0.08 (CF3[CF2]6CH2OH) and, as hypothesized, increased the product of MAC x the olive oil or octanol/gas partition coefficients to values equaling or exceeding those found for conventional anesthetics. We conclude that the greater potency of many alkanols (greater than would be predicted from conventional inhaled anesthetics and the Meyer-Overton hypothesis) is associated with their greater polarity. IMPLICATIONS: Inhaled anesthetic potency correlates with lipophilicity, but potency of common alkanols is greater than their lipophilicity indicates, in part because alkanols have a greater hydrophilicity--i.e., a greater polarity.

Differential sensitivities of TTX-resistant and TTX-sensitive sodium channels to anesthetic concentrations of ethanol in rat sensory neurons.

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.

Glutamate receptor-mediated hyperexcitability after ethanol exposure in isolated neonatal rat spinal cord.

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.

Propofol potentiates the depressant effect of alfentanil in isolated neonatal rat spinal cord and blocks naloxone-precipitated hyperresponsiveness.

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.

Ethanol as a general anesthetic: actions in spinal cord.

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.

Synergistic interactions between midazolam and alfentanil in isolated neonatal rat spinal cord.

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.

The NMDA receptor antagonist MK-801 differentially modulates mu and kappa opioid actions in spinal cord in vitro.

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.

Descending inhibition in neonatal rat spinal cord: actions of pentobarbital and morphine.

Descending inhibition plays an important role in modulating spinal nociceptive neurotransmission. Barbiturates have been suggested to be poor analgesics or anti-analgesic because they block descending inhibition from supraspinal centers to the spinal cord. Opiate analgesics, on the other hand, are postulated to increase descending inhibition. We tested this hypothesis in an isolated brain stem-spinal cord preparation from neonatal rats, using as the test response a nociceptive-related slow ventral root potential (sVRP) recorded in the lumbar region. Brain stem and spinal cord were separately perfused. Transecting the spinal cord, applying the local anesthetic lidocaine to the brain stem, or cooling the brain stem increased the area of the sVRP, thus demonstrating that tonic descending inhibition is present in this preparation. Pentobarbital (Pb) (1-10 microM) applied to the spinal cord depressed the sVRP in a dose-dependent fashion. Spinal cord transection did not significantly change Pb potency. Pb (5-10 microM) applied to the brain stem alone did not significantly increase sVRP amplitude. Morphine (15-35 nM) applied to the spinal cord also depressed the sVRP but had no effect when applied to the brain stem. The results show that there are functional synaptic connections mediating tonic descending inhibition in the neonatal rat. They do not support interaction with tonic descending inhibition as an explanation for morphine analgesia or as a reason for lack of analgesic properties in the barbiturates.

Anesthetic actions within the spinal cord: contributions to the state of general anesthesia.

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.

Long-term potentiation in an isolated peripheral nerve-spinal cord preparation.

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)

Selective effects of alfentanil on nociceptive-related neurotransmission in neonatal rat spinal cord.

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.

N-methyl-D-aspartate receptors are implicated in hyperresponsiveness following naloxone reversal of alfentanil in isolated rat spinal cord.

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.