Electrophysiologically recorded C-fiber reflexes in intact and acute decerebrate-spinal cats: absence of naloxone facilitation in intact cats

A C-fiber reflex was obtained from stimulating and recording electrodes attached respectively to the superficial peroneal and posterior biceps semitendinosus nerves in intact cats. Naloxone in a dose dependent manner increased vocalizations produced by nerve stimulation, but it did not facilitate the C-fiber reflex in the intact cat. However, naloxone facilitated C-fiber reflexes in decerebrate-spinal cats under identical stimulating and recording conditions as used in the intact cat. It was concluded that naloxone causes increased nociception to cutaneous nerve stimulation in intact cats but for naloxone to facilitate the C-fiber reflex, removal of supraspinal control is necessary. Many C-fiber afferents transmit nociception to the CNS (Bessou and Perl, 1969). Electrical stimulation of the superficial peroneal nerve that activates C fibers produces a segmental reflex in the unanesthetized decerebrate-spinal cat (Koll, Hasse, Schutz and Muhlberg, 1963). This C-fiber reflex is manifested by a long latency discharge recorded from an ipsilateral L7 or S1 ventral root. The C-fiber reflex is considered nociceptive because only intense stimulation evokes it and morphine depresses it in doses lower than those that depress other spinal reflexes (Koll et al., 1963). Low doses of the opiate antagonists naloxone and naltrexone consistently facilitate the C-fiber reflex (Bell and Martin, 1977). These results support the hypothesis that released endogenous opioids inhibit the C-fiber reflex. However, the facilitative effects of the opiate antagonists could be confined to the decerebrate-spinal preparation where invasive experimental procedures (decerebration, dissection, etc.) may release endogenous opioids.(ABSTRACT TRUNCATED AT 250 WORDS)     

Interaction of morphine and naloxone in acute spinal cats

Naloxone (0.2 mg/kg i.v.) increased both monosynaptic and polysynaptic reflex amplitudes in acute spinal cats. The increase in monosynaptic reflex amplitude was greater than that for polysynaptic reflexes. Morphine administered intravenously. 36 min following naloxone, depressed monosynaptic reflexes at 3 and 12 mg/kg, and polysynaptic reflexes at 12 mg/kg. A subsequent 0.2 mg/kg dose of naloxone was administered 54 min after beginning the morphine injection. The action of naloxone on polysynaptic reflexes was enhanced by intervening morphine; that on monosynaptic reflexes was not. The actions of morphine and naloxone in these studies do not appear to involve cholinergic synapses in the spinal cord.     

Effects of enkephalin analogue and naloxone on cat spinal cord dorsal root potentials

A systemically active enkephalin analogue, FK33824, given intravenously depressed dorsal root potentials in cat spinal cord. The negative DR V and positive DR VI, measured by computer, were both decreased; this effect was reversed by small doses of intravenous naloxone. Naloxone, given alone, with no previous analogue produced no changes in dorsal root potentials suggesting the absence of a basal enkephalin tone. A second injection of FK33824 was much less effective than the first dose. The results were discussed in relation to presynaptic mechanisms for analgesia: we proposed that FK33824 causes presynaptic inhibition by modulation rather than by depolarization of primary afferent fibers.     

Effect of opiate peptides on interneuronal transmission in the spinal cord

It has been shown that leucin- and methionine-enkephalins (200 microgram into the lateral brain ventricle), as well as synthetic opiate peptides FK 33-824 (2 mg/kg) and tetrapeptide (25 mg/kg) disturb the excitation transmission in the cat ventrolateral columns. The effects of opiate receptors was more pronounced in animals with the intact central nervous system as compared with spinal animals. Naloxone (1 mg/kg) eliminated the effect of opiate peptides.     

Pharmacological and electrophysiological studies of morphine and enkephalin on rat supraspinal neurones and cat spinal neurones

1 The actions of morphine, methionine and leucine enkephalin, administered electrophoretically, were studied on supraspinal neurones in the cortex and brainstem of the rat anaesthetized with urethane and on spinal Renshaw cells and dorsal horn interneurones in the cat anaesthetized with pentobarbitone.

2 The majority of Renshaw cells and cortical and brainstem neurones were excited by all three compounds although some supraspinal neurones were depressed.

3 Naloxone reversibly antagonized both excitatory and depressant actions of morphine and enkephalin. Acetylcholine-induced excitation but not amino acid-induced excitation was also antagonized by naloxone.

4 Neither morphine nor the enkephalins had any naloxone-reversible action on dorsal horn neurones when ejected from conventional multibarrelled electrodes. However, morphine but not enkephalin, administered into the substantia gelatinosa region of the spinal cord selectively reduced responses to noxious stimuli of neurones in deeper laminae. Naloxone administered into the same region antagonized this action of morphine.

5 Intravenous morphine also antagonized responses of dorsal horn neurones to noxious stimuli and subsequent intravenous naloxone reversed this effect.

6 It was concluded that the excitatory and inhibitory effects of morphine and enkephalin on central neurones may be mediated by actions on different opiate receptors and that depression of noxious responses of dorsal horn neurones may be relevant to the analgesic action of morphine.

     

Naloxone antagonizes behavioural effects of d-amphetamine in mice and rats

The influence of naloxone on two characteristic behavioural effects of d-amphetamine has been studied in rodents. Naloxone (0.3, 1 and 3 mg/kg, s.c.) antagonized the increase in spontaneous locomotor activity that is associated with administration of d-amphetamine (2 mg/kg, s.c.) to mice. Naloxone, at 0.3, 1 and 3 mg/kg, subcutaneously (but not at 10 and 30 mg/kg), antagonized d-amphetamine-induced ipsilateral turning in rats lesioned unilaterally with 6-hydroxydopamine in the substantia nigra. It may be postulated that the effect of d-amphetamine in these behavioural models is brought about partially by its activity in releasing an endogenous opiate which acts via an opiate receptor influencing dopaminergic activity (naloxone sensitive) and partially by its activity on dopaminergic neurones (naloxone insensitive).     

Buprenorphrine: demonstration of physical dependence liability

This study was designed to assess the dependence-producing capacity of the opiate partial agonist, buprenorphine. Rats chronically treated with buprenorphine for 4 days showed only very weak signs of withdrawal upon cessation of buprenorphine treatment or upon challenge with naloxone, although complete tolerance had developed to the drug at this time. However, more intense withdrawal could be induced when buprenorphine treatment was followed by substitution treatment with morphine. Even one injection of morphine given 12 h after the last buprenorphine treatment enabled the precipitation of withdrawal with naloxone. Naloxone could not precipitate signs of withdrawal in naive rats treated with this dose of morphine. Thus, contrary to some claims in the literature, buprenorphine, like other opiate agonists and partial agonists, induces dependence. The fact that only few signs of withdrawal are seen in direct dependence tests, probably reflects the slow dissociation of the drug from the receptor - which probably limits the intensity of withdrawal by preventing the rapid uncovery of the receptor upon discontinuance of treatment with the drug or upon injection of an antagonist. In addition, the maximum degree of dependence induced by buprenorphine - in comparison to pure agonists is limited, like that of other partial agonists.     

Suppression of transmission of nociceptive impulses by morphine: Selective effects of morphine administered in the region of the substantia gelatinosa

1 In spinal cats anaesthetized with α-chloralose, a study was made of the effects of morphine and naloxone, administered electrophoretically from micropipettes, on the responses of dorsal horn neurones to noxious (raising of skin temperature above 45°C) and innocuous (deflection of hairs) peripheral stimuli.

2 Administered near cell bodies, morphine reduced the nociceptive responses of only 2 of 37 cells. Excitation occurred more commonly than depression and abnormalities in action potentials were commonly observed following ejection of morphine. None of these effects of morphine was antagonized by electrophoretically applied naloxone.

3 Administered in the substantia gelatinosa from one micropipette while recording responses of deeper neurones with a second micropipette, morphine reduced the nociceptive responses of 15 of 19 neurones. Firing in response to deflection of hairs was not reduced by morphine. Depression of nociceptive responses by morphine was long lasting (>20 minutes). Naloxone ejected into the substantia gelatinosa or given intravenously in doses as low as 0.1 mg/kg antagonized the effects of morphine. The effectiveness of this dose of intravenous naloxone suggests that the concentrations of morphine in the substantia gelatinosa which reduced nociceptive responses were not unlike those present after analgesic doses of systemic morphine. Naloxone alone, and excitant and depressant amino acids ejected into the substantia gelatinosa had little effect on cell firing.

4 Both the selective action of morphine on nociceptive responses and the reversal of this action by intravenous naloxone suggest that the opiate receptor present in the substantia gelatinosa is relevant to analgesia produced by opiates given systemically.

     

Naloxone in experimental spinal cord ischemia: dose-response studies

Temporary aortic occlusion produces a consistent degree of spinal cord injury in the unanesthetized rabbit. This 'spinal stroke' model was utilized to examine the potential therapeutic effects of the opiate antagonist naloxone in central nervous system ischemia. Naloxone treatment resulted in dose-related enhancement of motor recovery; greatest functional recovery was observed in rabbits treated with a dose of 2 mg/kg per h. This dose compares well with the high doses of naloxone shown to have a beneficial effect in other experimental models of stroke and spinal injury. In contrast, clinical stroke studies, which have been largely unsuccessful, have utilized naloxone doses which are several orders of magnitude lower than those successfully employed in experimental models.     

Antagonism by naloxone of tolerance and dependence in mice given a single dose of morphine

The effect of naloxone given at various times after morphine administration on the development of tolerance to and dependence on a single dose of morphine was studied. Naloxone antagonized the analgesic effect of morphine and the development of tolerance to and dependence on morphine, dose dependently. The time course of the development of tolerance to a single dose of morphine almost paralleled that of dependence on morphine but the time course of the disappearance of tolerance did not coincide with that of dependence. When start of the duration of action of morphine was blocked by naloxone for various time intervals, the degree of tolerance to and dependence on morphine was antagonized, time dependently. When the end of the duration of action of morphine was antagonized by naloxone for various time intervals, tolerance and dependence which developed up to that time was completely antagonized by naloxone.     

A local serotonergic component involved in the spinal antinociceptive action of morphine

Participation of opiate, serotonergic and noradrenergic components in the antinociceptive action of intrathecally administered morphine was evaluated by measuring the ability of subcutaneously administered doses of naloxone, methysergide and phentolamine to alter analgesia. Morphine produced a dose-dependent elevation of the tail-flick latency, due exclusively to local spinal actions. For example, 10 nmol of the drug, when administered intrathecally in rats with bilateral lesions of the dorsolateral funiculus, produced an increase in the tail-flick latency, that was similar to that observed in intact animals. Furthermore, morphine was ineffective when administered intracerebroventricularly into the fourth ventricle of intact rats. The spinal antinociceptive action of the opiate was antagonized by naloxone (ID50 = 0.035 mg/kg, s.c.) but was also significantly attenuated by methysergide (ID50 = 4.28 mg/kg, s.c.). Phentolamine was ineffective. Doses of methysergide that were most effective in reversing the spinal action of morphine also produced hyperalgesia when administered alone. On the other hand, when the dorsolateral funiculus was lesioned, the hyperalgesia was no longer observed, yet the antagonist remained effective against morphine. These data suggested that the doses of methysergide needed to antagonize the action of morphine were in the same range as those needed to block the synaptic actions of serotonin (5-HT) released from the tonically-acting, descending pain inhibitory nerves. The results demonstrate that local opiate, as well as serotonergic, mechanisms mediate the antinociceptive action of morphine in the spinal cord. The recruitment of a serotonergic component may be related to an action of opiates within the spinal cord, to cause the release of serotonin from the terminal fields of the spinipetal serotonergic nerves.     

Naloxone and [Met5]enkephalin effects on retention: attenuation by adrenal denervation

This study investigated the effect of posttraining administration (i.p.) of naloxone and [Met5]enkephalin on retention in intact and adrenal-denervated mice. Naloxone (0.3 mg/kg) enhanced and [Met5]enkephalin (0.5 or 2.0 micrograms/kg) impaired retention in both inhibitory avoidance and Y-maze discrimination tasks. Further, [Met5]enkephalin antagonized the facilitative effect of naloxone on retention while naloxone attenuated the retention deficit produced by [Met5]enkephalin. However, neither naloxone nor [Met5]enkephalin affected retention in adrenal denervated mice. These findings suggest that endogenous hormones of the adrenal medulla, including [Met5]enkephalin, may be involved in the modulating influence of opiate peptides on memory storage.     

The action of para-methoxyphenylethylamine (PMPEA) on monosynaptic reflex transmission in the cat

1. The intravenous injection of para-methoxyphenylethylamine (PMPEA) into cats produces an increase in the size of the spinal cord monosynaptic reflex. The reflex elevation occurs within 30 s of drug administration, reaches a peak within 2 min, and lasts about 20 min.2. The action of PMPEA is similar for extensor (gastrocnemius-soleus) and flexor (posterior biceps-semitendinosus) monosynaptic reflexes.3. Repeated doses of PMPEA give comparable effects. The degree of monosynaptic reflex elevation is dose related.4. The action of PMPEA is antagonized by phenoxybenzamine and by methysergide or cyproheptadine. The combination of phenoxybenzamine with either of the latter is particularly effective in preventing the reflex facilitation by PMPEA.5. It is concluded that PMPEA has a central action on the spinal cord. It seems likely that monoaminergic synapses are involved.     

Cold acclimation increases physiological responsiveness to intraventricular injection of β-endorphin in pentobarbital anaesthetized rats. 1. Cardiovascular functions

Intraventricular injection of β-endorphin (3, 7, 10 and 30 nmol/kg) into the third ventricular of pentobarbital-anaesthetized male Sprague-Dawley rats resulted in a dose-dependent increase in mean arterial pressure (MAP) while injection of the same volume of 0.9% NaC1 solution did not cause significant changes in MAP. Naloxone, which did not produce any significant change in MAP, antagonized the vasopressor effect of β-endorphin, indicating that the response is mediated via the naloxone sensitive opiate receptors. Rats acclimated to cold (5°) for 3 weeks showed a potentiated and prolonged increase in MAP following β-endorphin injection, indicating an increased responsiveness to the peptide. This increased responsiveness in the cardiovascular system is probably of adaptive value in cold acclimation. Naloxone itself did not alter MAP either, but abolished the cardiovascular response to β-endorphin completely in cold acclimated rats, indicating an increased effectiveness in its antagonistic effect following cold acclimation as well.     

Enkephalin receptors in the emetic chemoreceptor trigger zone of the dog

1 The emetic action of Met-enkephalin, morphine and naloxone was studied following their administration into the cerebral ventricles of dogs through chronically implanted cannulae and the effect on the responses of ablating the chemorceptor trigger zone (CTZ) was investigated. The opiate antagonist, naloxone, was used to determine the role of enkephalin receptors in emetic responses.2 Administration of Met-enkephalin (1.0 mug/kg) into the IVth ventricle regularly evoked emesis with an average latency of 35 s. A dose of morphine (2.5 mug/kg) which was five times larger was required for a consistent emetic response when introduced into the lateral cerebral ventricle (i.c.v.) as compared to the dose required by the IVth ventricular route. The latency of emetic responses by the latter route of injection of morphine was shorter. This is in accord with an action of morphine on the emetic CTZ.3 After bilateral ablation of the CTZ, intraventricular injections of Met-enkephalin and morphine failed to produce emesis even when given in doses that were 5 to 10 times the dose which regularly elicited emesis in animals with intact CTZ. The emesis produced in dogs by intraventricular Met-enkephalin and morphine is thus fully accounted for by an action on the CTZ.4 Naloxone (i.c.v.) in doses up to 10.0 mug/kg did not cause emesis. However, higher doses of naloxone elicited dose-dependent emesis in dogs. The 100% emetic dose of naloxone was found to be 160 mug/kg and the latency of emesis was 180 s. Unlike Met-enkephalin and morphine, naloxone continued to elicit emesis in CTZ-ablated animals.5 Pretreatment with intraventricular naloxone (1 to 8 mug/kg) blocked the emetic responses induced by intraventricular Met-enkephalin and morphine but not that to apomorphine. The selective protective action of the opiate antagonist against Met-enkephalin and morphine supports the presence of enkephalin receptors in the emetic CTZ.     

Dual action of methadone on 5-HT synthesis and metabolism

Summary The main purpose of these experiments was to compare the effects of methadone and morphine on cerebral 5-hydroxytryptamine (5-HT) synthesis and 5-hydroxyindoleacetic acid (5-HIAA) formation. In addition the rate of catecholamine synthesis and the concentrations of tyrosine and tryptophan in the brain were measured, as well as the effects of naloxone were investigated.Morphine (34 mg/kg, 2h) increased the synthesis of 5-HT and catecholamines, determined by measuring the accumulation of 5-hydroxytryptophan (5-HTP) and dopa in the whole brain of rats treated with an inhibitor of the aromatic l-amino acid decarboxylase (3-hydroxybenzylhydrazine hydrochloride, NSD 1015). Morphine also increased the cerebral 5-HIAA concentration both in rats treated with NSD 1015 or probenecid. Naloxone antagonized all these effects of morphine. A lower dose of naloxone was needed to antagonize the effect of morphine on 5-HT than on catecholamine synthesis, Similarly to morphine methadone (9 mg/kg, 2 h) increased the cerebral 5-HIAA concentration, but methadone alone did not alter the rate of formation of 5-HTP. However, in combination with naloxone methadone decreased the concentration of 5-HIAA and the accumulation of 5-HTP depending both on the dose of methadone and that of naloxone. Similarly to morphine, methadone stimulated and never reduced the catecholamine synthesis; naloxone antagonized this effect. Both morphine and methadone increased the cerebral concentrations of tryptophan and tyrosine and naloxone antagonized these effects. In addition naloxone alone (2+2 mg/kg, 1+2h) decreased the cerebral tyrosine concentration significantly suggesting that the opiate receptors are involved in the control of cerebral tyrosine concentration.Our results suggest that methadone similarly to morphine stimulates the cerebral 5-HT and catecholamine synthesis, and that these effects are most probably mediated via opiate receptors. However, when opiate receptors are blocked, methadone is able to decrease the cerebral 5-HT synthesis and cerebral 5-HIAA concentration probably via a feedback mechanism produced by blockade of 5-HT reuptake.