The local monoaminergic dependency of spinal ketamine.

The effects of s.c. doses of naloxone, methysergide and phentolamine on ketamine-induced spinal analgesia were assessed to determine the involvement of opiate, serotonergic and noradrenergic components mediating ketamine's antinociceptive action. Ketamine administered intrathecally (i.t.) produced a significant elevation in tail-flick latency in rats. The spinal antinociceptive effects of ketamine were dose dependently reversed by methysergide (ID50 = 0.008 mg/kg s.c.), phentolamine (ID50 = 0.88 mg/kg s.c.) and naloxone (ID50 = 3.0 mg/kg s.c.). Unlike morphine, which remains analgesic and dependent on opiate interactions following bilateral lesions of the dorsolateral funiculus (DLF), ketamine analgesia was absent following DLF lesions. Thus, ketamine appears to produce an antinociceptive response which is dependent upon the neuronal activity of the descending pain-inhibitory pathways. The monoaminergic components comprising the descending pathways appear to be more prominent in the action of ketamine than they are in the spinal action of morphine. Furthermore, the spinal opioid receptors involved in ketamine's effect may be different from the mu subtype preferred by morphine.     

Increased antinociception by alpha-adrenoceptor drugs after spinal cord noradrenaline depletion

Animals depleted of the bulbospinal NA fiber tracts have been reported to be supersensitive to antinociceptive effects of intrathecally administered noradrenaline (NA) in vivo. In the present investigation, the antinociceptive effects were determined after systemic or intrathecal injections of noradrenergic agents. NA and the selective alpha 2-adrenoceptor agonists guanfacine and clonidine were used. NA depletion was performed by treatment neonatally with 6-hydroxydopamine (6-OHDA), or in adult animals by intrathecal 6-OHDA administration or systemic N-2-chloroethyl-N-ethyl-2-bromobenzylamine hydrochloride (DSP4). The neurotoxins were found to cause a severe depletion of spinal NA without affecting dopamine (DA) or 5-hydroxytryptamine (5-HT) levels. The antinociceptive effects of intrathecal injection of NA, clonidine and guanfacine were more strongly enhanced in the depleted than in the control rats. It was also found that clonidine and guanfacine given systemically had a stronger effect in depleted than in control animals. In conclusion, depletion of descending NA pathways induces functional supersensitivity both to intrathecally administered NA and to the selective alpha 2-adrenoceptor agonists clonidine and guanfacine. It was also found that systemically administered clonidine and guanfacine had a stronger effect in NA-depleted than in control animals.     

Role of adenosine in the spinal antinociceptive and morphine modulatory actions of neuropeptide FF analogs

The neuropeptide FF (Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH(2)) and its synthetic analogs bind to specific receptors in the spinal cord to produce antinociceptive effects that are partially attenuated by opioid antagonists, and at sub-effective doses neuropeptide FF receptor agonists augment spinal opioid antinociception. Since adenosine plays an intermediary role in the production of spinal opioid antinociception, this study investigated whether this purine has a similar role in the expression of spinal effects produced by neuropeptide FF receptor agonists. In rats bearing indwelling spinal catheters, injection of adenosine receptor agonists, N6-cyclohexyladenosine (CHA, 1.72 nmol) and N-ethylcarboxiamidoadenosine (NECA, 1.95 nmol), as well as morphine (13.2 nmol) elicited antinociception in the tail-flick and paw-pressure tests. Pretreatment with intrathecal 8-phenyltheophylline (5.9 and 11.7 nmol), an adenosine receptor antagonist, blocked the effect of all three agents without influencing baseline responses. Administration of two synthetic neuropeptide FF (NPFF) analogs, [D-Tyr(1),(NMe)Phe(3)]NPFF (1DMe, 0. 86 nmol) and [D-Tyr(1),D-leu(2),D-Phe(3)]NPFF (3D, 8.6 nmol) produced sustained thermal and mechanical antinociception. Pretreatment with doses of intrathecal 8-phenyltheophylline (5.9, 11. 7 and 23.5 nmol), producing adenosine receptor blockade, significantly inhibited the antinociceptive effects of 1DMe or 3D. Injection of a sub-antinociceptive dose of 1DMe (0.009 nmol) significantly augmented the antinociceptive effect of intrathecal morphine (13.2 nmol) in the tail-flick and paw-pressure tests. Intrathecal 8-phenyltheophylline (11.7 nmol) reduced the effect of this combination. Administration of low dose of 1DMe (0.009 nmol) or 3D (0.009 nmol) very markedly potentiated the antinociceptive actions of the adenosine receptor agonist, N6-cyclohexyladenosine (0. 43, 0.86 and 1.72 nmol) in the tail-flick and paw-pressure tests 50 min after injection. The results suggest that the antinociceptive and morphine modulatory effects resulting from activation of spinal NPFF receptors could be due to an increase in the actions or availability of adenosine.     

Antinociceptive effect of intrathecal morphine in tolerant and nontolerant spinal rats

The antinociceptive effect of intrathecal morphine on the tail-flick (TF) reflex of rats was significantly enhanced within one day after spinal transection (ED50 = 0.125 microgram) relative to the effect obtained in intact rats (ED50 = 5.9 micrograms). By 20-30 days after spinalization the potency of intrathecally administered morphine had substantially declined. Intact rats, made tolerant to the antinociceptive effect of systemic morphine (3.0 mg/kg, SC on each of seven consecutive days), were not tolerant to intrathecal morphine (ED50 = 6.5 micrograms). In contrast, rats that were pretreated with either morphine alone, repeated TF tests alone, or both of these treatments, were tolerant to intrathecal morphine when tested one day after spinal transection. The results suggest first, that the antinociceptive effect of intrathecal morphine in intact rats is tonically inhibited by descending supraspinal input and that removal of this input is responsible for the enhanced antinociceptive effect of intrathecal morphine in spinal rats. Second, the data suggest that tolerance to the antinociceptive effect of intrathecal morphine in intact rats may also be tonically inhibited by supraspinal input, because spinal opiate tolerance is expressed after spinal transection.     

Serotonin contributes to the spinal antinociceptive effects of morphine.

This study was designed to determine if morphine administered intrathecally (IT) interacts with serotonergic or noradrenergic nerve terminals in the spinal cord to produce analgesia on the spinally mediated tail-flick test. Male Sprague-Dawley rats were fitted with IT catheters. One week later, animals were spinally pretreated with receptor antagonists selective for opioid, serotonin or alpha-adrenoceptors, and the ability of these agents to alter spinal morphine-induced antinociception was assessed. Morphine dose-dependently elevated tail-flick latency in a naltrexone-reversible manner. The serotonin receptor antagonists spiroxatrine (5-HT1A), pindolol (5-HT1B), ritanserin (5-HT2) and ICS 205-930 (5-HT3) attenuated the spinal analgesic effects of morphine. In contrast, the alpha 1 and alpha 2-adrenoceptor antagonists prazosin and yohimbine, respectively, did not alter morphine-induced elevations in tail-flick latency. These data substantiate earlier reports that spinal morphine-induced antinociception relies on an opioid receptor-mediated component in addition to a local serotonergic component. The finding that the alpha-adrenoceptor antagonists did not alter the antinociceptive effects of IT morphine suggests that spinal norepinephrine does not contribute to the analgesic effects of the opiate.     

Antagonism by phenoxybenzamine and pentazocine of the antinociceptive effects of morphine in the spinal cord

In contrast to morphine, intrathecally administered phenoxybenzamine (PBX: 0.1–400 μg) or pentazocine (1–400 μg) had no effect on the rat hot plate or tail flick response. The failure to observe any effect of either drug on the hot plate or tail flick response, or on the locomotor behavior of the animals rules out a local anesthetic effect of these agents given intrathecally under the present conditions. Pretreatment with intrathecal PBX or pentazocine, however, antagonized the antinociceptive effects of a large (15 μg), but not a small (3 μg) dose of morphine. The antagonism was observed to occur only with small intrathecal doses of PBX (1–10 μg) and pentazocine (10–30 μg) with the PBX being approx. 10 times more potent than pentazocine in its ability to antagonize. At higher dose levels of either agent, neither antagonism nor enhancement of the morphine effect was observed. Phentolamine (1–200 μg) had no effect on the antinociceptive effects of intrathecal morphine. The failure of intrathecal pentazocine and PBX to produce a change in the hot plate response latency suggests that they do not act as full agonists upon the category of receptor acted upon by morphine and that the antinociceptive effects produced by systemically administered pentazocine and PBX must be mediated by an agonist action upon a different receptor population located within supraspinal structures. The biphasic activity described for the interaction of partial opiate agonists with full agonists is similar to that observed in the present experiments and leads to the conclusion that both pentazocine and the α-adrenergic antagonist, PBX, can serve as partial agonists of the spinal opiate receptor.     

Blockade and reversal of spinal morphine tolerance by peptide and non-peptide calcitonin gene-related peptide receptor antagonists

This study examined the effects of the peptide CGRP receptor antagonist CGRP(8-37) and the newly-developed non-peptide CGRP receptor antagonist BIBN4096BS for their potential to both inhibit the development and reverse tolerance to the antinociceptive action of morphine. Repeated administration of intrathecal morphine (15 microg), once daily, produced a progressive decline of antinociceptive effect and an increase in the ED(50) value in the tailflick and paw pressure tests. Co-administration of CGRP(8-37) (4 microg) or BIBN4096BS (0.05, 0.1 microg) with morphine (15 microg) prevented the decline of antinociceptive effect and increase in ED(50) value in the tailflick test. Intrathecal administration of the CGRP receptor antagonists did not alter the baseline responses in either tests. Acute CGRP(8-37) also did not potentiate the acute actions of spinal morphine. In animals rendered tolerant to intrathecal morphine, subsequent administration of CGRP(8-37) (4 microg) with morphine (15 microg) partially restored the antinociceptive effect and ED(50) value of acute morphine, reflecting the reversal of tolerance. Animals tolerant to intrathecal morphine expressed increased CGRP and substance P-like immunostaining in the dorsal horn of the spinal cord. The increase in CGRP, but not substance P-like immunostaining, was blocked by a co-treatment with CGRP(8-37) (4 microg). In animals already tolerant to morphine, the increase in CGRP but not substance P-like immunostaining was partially reversed by CGRP(8-37) (4 microg). These data suggest that activation of spinal CGRP receptors contributes to both the development and expression of spinal opioid tolerance. CGRP receptor antagonists may represent a useful therapeutic approach for preventing as well as reversing opioid tolerance.     

Lack of effect of naltrexone on the spinal synergism between morphine and non steroidal anti-inflammatory drugs

To enhance analgesia, the combinatorial use of analgesic drugs with proven efficacies is a widely-used strategy to reduce adverse side effects. The present study charcaterizes the antinociceptive interaction of intrathecal morphine co-administered with different NSAIDs using isobolographic analysis.Antinoceptive activity was evaluated using a model for acute visceral pain, the writhing test of mice. The possible involvement of opioid receptors in the mechanism of action of the intrathecal co-administration of morphine and NSAIDs was investigated using the non-selective receptor antagonist naltrexone. The study demonstrated a synergistic antinociception of intrathecal administered combinations of morphine with the following NSAIDs: diclofenac, ketoprofen, meloxicam, metamizol, naproxen, nimesulide, parecoxib and piroxicam. The supra additive effect was obtained with very low doses of each drug and it appeared to be independent of the COX-1 or COX-2 inhibition selectivity of each NSAID and was not significantly modified by intrathecal naltrexone. The findings of the present work suggest that the combination of opioids and NSAIDs has a direct action on spinal nociceptive processing, which may be achieved via mechanisms that are independent of the activation of opioid receptors. The ineffectiveness of naltrexone to reverse the analgesic activity of opioids + NSAIDs combinations indicates that other complex pain regulatory systems are involved in this effect.     

Adenosine release may mediate spinal analgesia by morphine

Spinal analgesia produced by morphine is blocked by methylxanthine adenosine receptor antagonists. In biochemical studies, morphine releases adenosine from spinal cord synaptosomes prepared from the dorsal spinal cord, as well as from the intact spinal cord in vivo. Adenosine release is reduced by intrathecal and neonatal pretreatment with capsaicin but not by intrathecal pretreatment with 6-hydroxydopamine or 5,7-dihydroxytryptamine, indicating that adenosine originates from small-diameter primary afferent neurons but not descending monoaminergic pathways. In this Viewpoint Jana Sawynok and colleagues review the evidence supporting the hypothesis that the spinal analgesic action of morphine is due to the release of adenosine from primary afferent nerve terminals and subsequent activation of A1 and A2 adenosine receptors.     

Potentiation of morphine antiallodynic efficacy by ACPT-III, a Group III metabotropic glutamate receptor agonist, in rat spinal nerve ligation-induced neuropathic pain

Despite the importance of spinal metabotropic glutamate receptors (mGluRs) and opioid receptors in nociceptive processing, the roles of these receptors in the modulation of neuropathic pain at the spinal level have not been thoroughly investigated. The purpose of this study was to investigate the effects of spinal mGluR agents and opioids (morphine) on neuropathic pain. Male Sprague-Dawley rats underwent L5 and L6 spinal nerve ligation to induce neuropathic pain and intrathecal catheterization for drug administration. A paw-withdrawal threshold to mechanical stimulus was measured using the “up and down” method. When administered intrathecally, neither Group I mGluR antagonists nor Group II or III agonists modified the withdrawal threshold after spinal nerve ligation. Intrathecal administration of morphine dose-dependently increased the withdrawal threshold. Whereas ACPT-III, a Group III mGluR agonist, enhanced the antiallodynic action of morphine, other mGluR agents did not. Collectively, mGluRs may not directly modulate the processing of spinal nerve ligation-induced neuropathic pain at the spinal level. However, Group III mGluR agonists in the spinal cord may indirectly contribute to the potentiation of morphine antiallodynia, indicating that these agonists might be used as adjuvants for spinal morphine.     

Interaction between gamma-aminobutyric acid GABAB and cannabinoid CB1 receptors in spinal pain pathways in rat

Antinociceptive effects of cannabinoids are mediated, in part, at the spinal level. Cannabinoid CB1 receptors are co-localized with dorsal horn interneurons containing gamma-aminobutyric acid (GABA). In this study, we investigated the interaction between intrathecally administered cannabinoid and GABA(B) receptor agonists and antagonists in the modulation of formalin-induced pain at the spinal level. Intrathecal pretreatment of rats with a cannabinoid receptor antagonist [N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1-H-pyrazole-3-carboxamide] (SR141716A, 30 microg) decreased the analgesic effect of the intrathecal administration of the GABA(B) receptor agonist, baclofen (0.125 microg and 0.25 microg). Intrathecal administration of the GABA(B) receptor antagonist, saclofen (30 microg), 10 min before administration of the cannabinoid receptor agonist (-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)-phenyl]-trans-4-(3-hydroxy-propyl)-cyclohexano (CP55940), did not affect the analgesia produced by the cannabinoid receptor agonist. Our results confirm that intrathecal administration of cannabinoid and GABA(B) receptor agonists have analgesic effects and that spinal antinociceptive effects of GABA(B) receptor agonists are likely through endocannabinoid modulation.     

Spinal cord effects of drugs used in anaesthesia.

1. Measurement of nociceptive thresholds at two skin sites in rats with chronic intrathecal catheters has allowed study of drug actions demonstrated to be at spinal level. 2. Using this preparation we have demonstrated segmental antinociceptive effects following intrathecal administration of agonists selective for benzodiazepine receptors, mu, kappa and delta opioid receptors, 5-hydroxytryptamine (5-HT) receptors and alpha adrenoceptors. 3. Such effects were dose related and were suppressed by appropriate selective antagonists. 4. Antagonist dose-response curves have been constructed for suppression of antinociceptive effects of various agonists. This has revealed complex interactions between spinal systems; for example midazolam produces its actions by activating an opioid system involving delta receptors.     

Acetaminophen: a central analgesic drug that involves a spinal tropisetron-sensitive, non-5-HT(3) receptor-mediated effect

The reversal of the antinociceptive effect of systemically administered acetaminophen (paracetamol) by intrathecal administration of the potent 5-HT(3) receptor antagonist tropisetron has been reported in rats subjected to the paw pressure test, suggesting that acetaminophen action is mediated through spinal 5-HT(3) receptors. However, more recent data, showing differences between the pharmacological profiles of various 5-HT(3) receptor antagonists, led us to reconsider the involvement of spinal 5-HT(3) receptors. To address this question, two different approaches were used: 1) electrophysiological recordings to assess whether acetaminophen directly modulates 5-HT(3) receptor activity and 2) pharmacological investigations with various 5-HT(3) receptor antagonists and spinal 5-HT(3) receptors antisense oligodeoxynucleotides (AODNs) to determine how those treatments might affect the antinociceptive action of acetaminophen. Electrophysiological studies demonstrated that acetaminophen had no direct agonist or antagonist effects on 5-HT(3A) receptors. Unlike tropisetron, other 5-HT(3) receptor antagonists, such as ondansetron and granisetron, injected intrathecally were unable to reverse the antinociceptive effect of acetaminophen. Moreover, pretreatment with AODNs did not reverse the acetaminophen-induced antinociceptive effect, although it suppressed the antinociceptive effect of m-chlorophenylbiguanide, a specific agonist of 5-HT(3) receptors, and significantly reduced (30%) the expression of these receptors in the dorsal horn of the spinal cord. These results suggest that acetaminophen-induced antinociceptive action involves a spinal tropisetron-sensitive receptor that is not the 5-HT(3) receptor and that remains to be identified.     

Low doses of alpha 2-adrenoceptor antagonists augment spinal morphine analgesia and inhibit development of acute and chronic tolerance

Background and purpose:

Ultra-low doses of opioid receptor antagonists augment spinal morphine antinociception and block the induction of tolerance. Considering the evidence demonstrating functional and physical interactions between the opioid and α₂-adrenoceptors, this study investigated whether ultra-low doses of α₂-adrenoceptor antagonists also influence spinal morphine analgesia and tolerance.

Experimental approach:

Effects of low doses of the competitive α₂-adrenoceptor antagonists—atipamezole (0.08, 0.8 ng), yohimbine (0.02, 2 ng), mirtazapine (0.02 ng) and idazoxan (0.08 ng) were investigated on intrathecal morphine analgesia, as well as acute and chronic morphine antinociceptive tolerance using the rat tail flick and paw pressure tests.

Key results:

At doses markedly lower than those producing α₂-adrenoceptor blockade, atipamezole, yohimbine, mirtazapine and idazoxan, prolonged the antinociceptive effects of morphine. When co-administered with repeated acute spinal injections of morphine, all four agents blocked the induction of acute tolerance. Co-injection of atipamezole with morphine for 5 days inhibited the development of tolerance in a chronic treatment paradigm. Spinal administration of atipamezole also reversed established antinociceptive tolerance to morphine as indicated by the restoration of morphine antinociceptive potency. The effects of atipamezole on spinal morphine tolerance were not influenced by treatment with 6-hydroxydopamine.

Conclusions and implications:

Low doses of competitive α₂-adrenoceptor antagonists can augment acute morphine analgesia and block or reverse tolerance to spinal administration of morphine. These actions are interpreted in terms of their interaction with an opioid-α₂-adrenoceptor complex, whose activity may have a function in the genesis of analgesic tolerance.     

Peripheral nerve injury alters spinal nicotinic acetylcholine receptor pharmacology

Nicotinic acetylcholine receptors are widely expressed in the rat spinal cord and modulate innocuous and nociceptive transmission. The present studies were designed to investigate the plasticity of spinal nicotinic acetylcholine receptors modulating mechanosensitive information following spinal nerve ligation. A tonic inhibitory cholinergic tone mediated by dihydro-beta-erythroidine- (DHbetaE) and methyllycaconitine- (MLA) sensitive nicotinic acetylcholine receptors was identified in the normal rat spinal cord and cholinergic tone at both populations of nicotinic acetylcholine receptors was lost ipsilateral to spinal nerve ligation. The administration of intrathecal nicotinic acetylcholine receptor agonists reduced mechanical paw pressure thresholds with a potency of epibatidine=A-85380>nicotine>choline in the normal rat. Following spinal nerve ligation, intrathecal epibatidine and nicotine produced an ipsilateral antinociception, but intrathecal A-85380 and choline did not. The antinociceptive response to intrathecal nicotine was blocked with the alpha7 and alpha9alpha10-selective nicotinic acetylcholine receptor antagonist, MLA, and the alphabeta heteromeric nicotinic acetylcholine receptor antagonist, DHbetaE. The antinociceptive effects of both intrathecal nicotine and epibatidine were mediated by GABA(A) receptors. Spinal [(3)H]epibatidine saturation binding was unchanged in spinal nerve-ligated rats, but spinal nerve ligation did increase the ability of nicotine to displace [(3)H]epibatidine from spinal cord membranes. Spinal nerve ligation altered the expression of nicotinic acetylcholine receptor subunits ipsilaterally, with a large increase in the modulatory alpha5 subunit. Taken together these results suggest that pro- and antinociceptive populations of spinal nicotinic acetylcholine receptors modulate the transmission of mechanosensitive information and that spinal nerve ligation-induced changes in spinal nicotinic acetylcholine receptors likely result from a change in subunit composition rather than overt loss of nicotinic acetylcholine receptor subtypes.     

Augmentation of spinal morphine analgesia and inhibition of tolerance by low doses of mu- and delta-opioid receptor antagonists

BACKGROUND AND PURPOSE: Ultralow doses of naltrexone, a non-selective opioid antagonist, have previously been found to augment acute morphine analgesia and block the development of tolerance to this effect. Since morphine tolerance is dependent on the activity of micro and delta receptors, the present study investigated the effects of ultralow doses of antagonists selective for these receptor types on morphine analgesia and tolerance in tests of thermal and mechanical nociception. EXPERIMENTAL APPROACH: Effects of intrathecal administration of mu-receptor antagonists, CTOP (0.01 ng) or CTAP (0.001 ng), or a delta-receptor antagonist, naltrindole (0.01 ng), on spinal morphine analgesia and tolerance were evaluated using the tail-flick and paw-pressure tests in rats. KEY RESULTS: Both micro and delta antagonists augmented analgesia produced by a sub-maximal (5 microg) or maximal (15 microg) dose of morphine. Administration of the antagonists with morphine (15 microg) for 5 days inhibited the progressive decline of analgesia and prevented the loss of morphine potency. In animals exhibiting tolerance to morphine, administration of the antagonists with morphine produced a recovery of the analgesic response and restored morphine potency. CONCLUSIONS AND IMPLICATIONS: Combining ultralow doses of micro- or delta-receptor antagonists with spinal morphine augmented the acute analgesic effects, inhibited the induction of chronic tolerance and reversed established tolerance. The remarkably similar effects of micro- and delta-opioid receptor antagonists on morphine analgesia and tolerance are interpreted in terms of blockade of the latent excitatory effects of the agonist that limit expression of its full activity.     

The antinociceptive effects of alpha7 nicotinic agonists in an acute pain model

Nicotinic receptors have been found to play a role in modulating pain transmission in the CNS. Activation of cholinergic pathways by nicotine and nicotinic agonists has been shown to elicit antinociceptive effects in a variety of species and pain tests. The involvement of alpha(7) nicotinic receptors in nicotinic analgesia was assessed after spinal (i.t.) and intraventricular (i.c.v.) administration in mice. Dose-dependent antinociceptive effects were seen with the alpha(7) agonist choline after spinal and supraspinal injection using the tail-flick test. Furthermore, alpha(7) antagonists MLA and alpha-BGTX significantly blocked the effects of choline. Dihydro-beta-erythroidine and mecamylamine failed to block choline-induced antinociception. These results strongly support the involvement of alpha(7) subunits in choline's antinociceptive effects. DMXB and 4-OH-DMXB, partial alpha(7) agonists, failed to elicit a significant antinociceptive effect. However, they blocked choline-induced antinociception in a dose-dependent manner following i.t. injection. This antagonism is probably related to their partial agonistic properties of the alpha(7) receptors. These studies suggest that activation of alpha(7) receptors in the CNS elicits antinociceptive effects in an acute thermal pain model.     

Morphine analgesia after intrathecal administration of a narcotic agonist, chloroxymorphamine and antagonist, chlornaltrexamine

The spinal effects of two opiate compounds with nonequilibrium properties, chloroxymorphamine (COA) and chlornaltrexamine (CNA), were studied in the rat. Permanent indwelling cannulas to the spinal subarachnoid space allowed repeated injections of drugs to be made into a circumscribed receptor population. At various times after injection of the alkylating agents, the analgesic efficacy of morphine was measured. A single intrathecal injection of the antagonist, CNA, was found to dramatically inhibit the analgesic effect of intrathecally administered morphine challenges from 2 to 13 days. Pretreatment with CNA also reduced the analgesic effect of subcutaneously administered morphine, but only mildly compared to the potent inhibition of intrathecally administered morphine. In contrast to CNA, pretreatment with naltrexone intrathecally did not significantly inhibit the analgesic effect of intrathecal morphine challenges. Unlike the pure antagonistic effect of CNA, a single intrathecal injection of COA produced an immediate dose-related analgesia. Subsequently, the analgesic effect was absent, and replaced by a potent and long-term antagonism of morphine analgesia, which was still present as long as 21 days after COA. As was found with CNA, the inhibition by COA of the analgesic effect of morphine administered intrathecally was much more potent than that of subcutaneously administered morphine. The intrathecal injection of chlorambucil, a non-specific alkylating agent, produced no analgesia and did not affect the analgesic efficacy of subsequent treatment with morphine. The non-opioid analgesic effect of serotonin injected intrathecally was not affected by pretreatment with COA, and only slightly inhibited by the previous spinal injection of CNA. Using this test system, our results indicate that COA and CNA possess a much greater potency and longer duration of inhibition of narcotic receptors than previously demonstrated.     

GABAergic agents-induced antinociceptive effect in mice

The effect of GABA agonists, namely gamma aminobutyric acid, muscimol, sodium valproate and baclofen was studied on radiant heat-induced nociception in mice. All of the drugs, with the exception of sodium valproate, enhanced the reaction time to radiant heat as effect per se. Concomitant administration of any of these agents with morphine showed a potentiation of morphine-induced analgesia. The GABA antagonists bicuculline and picrotoxin failed to reverse the antinociceptive effect; paradoxically both these agents showed antinociceptive effect per se and also enhanced the response due to morphine.     

Lack of reciprocity between opioid and 5-HT3 receptors for antinociception in rat spinal cord

We examined the properties of the drug interaction between morphine and 5-HT(3) receptor antagonist at the spinal level. The nociceptive state was induced by subcutaneously injecting formalin solution (5%, 50 microl) into the hindpaw of the rats. Intrathecal morphine and m-CPBG (5-HT(3) receptor agonist) dose-dependently decreased the flinching response during phase 1 and phase 2 in the formalin test. Intrathecal 5-HT(3) receptor antagonists (LY-278,584 and ondansetron) did not reverse the antinociceptive effect of intrathecal morphine. Intrathecal naloxone had little effect on attenuation of the antinociception of intrathecal m-CPBG. Taken together, no reciprocal interaction was noted between 5-HT(3) receptor and opioid receptors at the spinal level. Thus, the 5-HT(3) receptor antagonist may be useful to manage opioid-induced emesis at the spinal level.