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.     

Antinociceptive action of picrotoxin in the mouse

Picrotoxin, an antagonist of GABA-associated chloride ionophores with convulsant activity, possesses antinociceptive activity in the hot-plate and writhing tests in the mouse. Analgesia produced by a subconvulsant dose of picrotoxin (0.75 mg/kg, s.c.) was reversed by naloxone (1.0 mg/kg, s.c.), atropine (5 mg/kg, i.p.), and methysergide (10 mg/kg, i.p.) in the jumping reaction (hot-plate test). These data indicate that opiate pathways, as well as cholinergic and serotoninergic pathways could be involved in the mechanism that underlies picrotoxin-induced analgesia. Furthermore, such results should be considered when interpreting the behavioral effects of picrotoxin.     

H1- and H2-histamine receptor blockers and opiate analgesia in mice

In this paper, the interactions between opiates and antihistaminic compounds, both H1- and H2-blockers, were studied. CD1 mice were used, treated with saline, morphine CHl (5 mg/kg), and 1 or 10 mg/kg of the H1-antihistaminics tripelennamine, chlorpheniramine, diphenhydramine and cyclizine and the H2-antihistaminics ranitidine and cimetidine, all compounds by s.c. route. Using the hot-plate test, it was observed that the two doses of tripelennamine and the higher doses of chlorpheniramine and cimetidine had antinociceptive activity. This increase on analgesia was also observed after chronic treatment with all H1-antihistaminics (10 mg/kg, 3 times daily for 4 days). When antihistaminics were administered with morphine, it was observed that only ranitidine (10 mg/kg) blocked opiate analgesia. On the other hand, previous administration of the opiate antagonist naloxone (1 mg/kg) blocked the antinociceptive action of tripelennamine and chlorpheniramine (10 mg/kg) in control mice. In morphine-dependent mice (by s.c. implantation of a 75-mg morphine pellet), a single injection of diphenhydramine or ranitidine blocked the analgesic action of morphine. Chronic administration of all antihistaminics did not modify morphine analgesia. These data are discussed in relation to the possible binding to the opioid receptors by antihistaminics and their facility in crossing the blood-brain barrier.     

The antinociceptive effects of 3,4-methylenedioxymethamphetamine (MDMA) in the rat

The antinociceptive effects of MDMA and morphine were examined in rats using the tail-flick and hot-plate analgesiometric tests. MDMA, in the dose range of 1.5-6.0 mg/kg IP, produced a dose-dependent elevation in hot-plate latency, but did not elevate tail-flick latency. In contrast, morphine (2-8 mg/kg, IP) produced analgesia on both the tail-flick and hot-plate tests in a dose-dependent manner. Neither the opiate antagonist naltrexone nor the adrenoceptor antagonist phentolamine effectively attenuated MDMA-induced analgesia. Conversely, the serotonin antagonist methysergide significantly reversed the analgesic effects of MDMA on the hot-plate test. These findings suggest that the antinociceptive effects of MDMA are serotonergically mediated. Furthermore, the results verify earlier findings describing the test-specific effects of serotonin-induced pain modulation.     

Properties of the interaction between ketamine and opiate binding sites in vivo and in vitro

Analgesia induced by ketamine appears to be partially mediated by opiate mechanisms. Not only is its action attenuated by the narcotic antagonist naloxone, but the drug has a weak affinity for, and interacts stereoselectively at, opiate receptors. It also produces a classical narcotic action on the guinea-pig ileum. The present study showed that analgesic doses of the drug in rats yielded concentrations sufficient to interact effectively at opiate binding sites in vivo. A dose-dependent (80-120 mg/kg i.p.) inhibition of the binding of [3H]naloxone was observed in both brain and spinal cord. All regions of the brain (except the cerebellum) were affected, but the reduction was significant in the cortex, hippocampus, thalamus and striatum. Thus, a component of ketamine-induced analgesia could be related to a functional interaction with opiate receptors. Additionally, ketamine may be similar to morphine in its preference for the mu, rather than the delta sub-type of opiate receptors, and thus may promote mu-mediated pharmacological effects. For example, in vitro studies of radioligand binding showed that ketamine and morphine were four times more effective in inhibiting the binding of [3H]dihydromorphine than that of [3H] [D-Ala2, D-Leu5] enkephalin. On the other hand, ketamine also effectively interacted at a component of the sigma opiate/phencyclidine binding sites that appears to be relatively insensitive to morphine. This component may be involved in dysphoria induced by ketamine.     

Serotonin receptor antagonists induce hyperalgesia without preventing morphine antinociception

5-Hydroxytryptamine (5-HT) receptor blockade by administration of mianserin (1 mg/kg) or metergoline (0.25 mg/kg) shortened the response latencies of rats in the hot-plate (hind-paw lick response) and tail-flick tests, but did not consistently attenuate the antinociceptive effect of morphine (1.25--5.0 mg/kg). Injection of the opiate receptor antagonist naloxone (1 mg/kg) did not change tail-flick response latencies and did not interfere with the antinociceptive action of the 5-HT receptor agonist 5-methoxy-N,N-dimethyltryptamine (5-MeODMT). The antinociceptive effect of morphine was reduced in chronically spinal rats, although significant increases in tail-flick latencies were observed after 2.5 and 5.0 mg/kg. Concomitant administration of 5-MeODMT failed to restore the effect of morphine in spinal rats. In the hot-plate test, morphine did not reliably prolong latencies to forepaw lick, indicating that this response is not a useful measure of pain sensitivity. The results suggest that different mechanisms underlie the analgesia induced by systemic administration of morphine and 5-HT mediated tonic inhibition of nociception.     

Ketamine-induced potentiation of morphine analgesia in rat tail-flick test: role of opioid-, alpha2-adrenoceptors and ATP-sensitive potassium channels

Ketamine is known to improve opioid efficacy, reduce postoperative opioid requirement and oppose opioid associated pain hypersensitivity and tolerance. However, the mechanisms underlying these beneficial effects are not clear. This study investigated the effects of ketamine at a non-analgesic dose (30 mg/kg, i.p.) on analgesia induced by morphine (2.5, 5.0, 7.5 mg/kg, s.c.), using rat tail-flick test as an animal model of acute pain. Further, the role of opioid-, alpha2-adrenoceptors and ATP-sensitive potassium channels was examined on the potentiating effect of ketamine. Male rats received morphine alone at 5.0 and 7.5 but not at 2.5 mg/kg showed a dose-related increase in tail-flick latencies. The combination of morphine and ketamine resulted in dose-related increase in morphine analgesia, both on the intensity as well as on duration. The ketamine-induced potentiation of morphine (7.5 mg/kg) analgesia was unaffected by glibenclamide (3 mg/kg, s.c.) and only partially blocked by yohimbine (2 mg/kg, i.p.), but more completely abolished by naloxone (2 mg/kg, i.p.). Both morphine (5.0 mg/kg) and ketamine (30 mg/kg) alone did not evoke catalepsy in rats but on combination produced a synergistic effect, which was however, abolished by naloxone pretreatment. In the open-field test, while morphine (5.0 mg/kg) caused a depressant effect, ketamine (30 mg/kg) enhanced the locomotor activity. Nevertheless, in combination potentiated the morphine's depressant effect on locomotion, which was also antagonized by naloxone. These results indicate that ketamine at a non-analgesic dose can potentiate morphine analgesia, induce catalepsy and cause locomotor depression, possibly involving an opioid mechanism. This potentiation, although favorable in acute pain management, may have some adverse clinical implications.     

Interactions of ketamine, naloxone and morphine in the rat

The effect of naloxone on the ketamine-induced anesthesia and analgesia, and the development of tolerance to ketamine and the cross-tolerance to morphine (measured by an analgesic effect) were investigated in the rat. Ketamine produced a dose-dependent analgesia. Naloxone, 1 mg/kg, significantly inhibited analgesia induced by ketamine, 100 mg/kg, but even in a dose of 4 mg/kg it did not affect the duration of anesthesia. A chronic administration of ketamine (100 mg/kg twice a day (b.i.d.) for 7 days) resulted in the development of tolerance to analgesic effects of ketamine. The analgesic action of morphine was attenuated in rats receiving ketamine chronically, while the analgesic effects of ketamine were significantly potentiated in morphine-dependent rats. Ketamine, 25 mg/kg, significantly attenuated the withdrawal signs evoked by naloxone in morphine-dependent rats. The results corroborate the suggestion about the participation of the central opioid neurotransmission in the mechanism of ketamine action.     

Effects of intrathecal antagonists on the antinociception, hypotension, and bradycardia produced by intravenous administration of [D-Ala2]-methionine enkephalinamide (DALA) in the rat

DALA is a synthetic pentapeptide that produces inhibition of the tail-flick reflex evoked by radiant heat, as well as hypotension and bradycardia. Two experiments examined the effects of administration of various receptor antagonists into the subarachnoid space of the lumbar spinal cord on the antinociception produced by IV administration of DALA. Experiment 1 showed that intrathecal administration of 30 micrograms of phentolamine produced a significant reduction in the antinociceptive effect of DALA, while naloxone (30 micrograms), methysergide (30 micrograms), or vehicle control had no effect. Experiment 2 showed that intrathecal administration of combinations of either phentolamine and methysergide, or phentolamine and naloxone, were no more effective in reducing the antinociceptive effect of DALA than administration of phentolamine alone. These data demonstrate the involvement of descending noradrenergic systems in the production of antinociception by IV DALA. Further, they indicate that the antinociception produced by DALA is independent of a direct spinal action of the drug.     

Morphine analgesia and the bulbospinal noradrenergic system: Increase in the concentration of normetanephrine in the spinal cord of the rat caused by analgesics

1 Administration of an analgesic dose (10 mg/kg, s.c.) of morphine increased the concentration of a noradrenaline metabolite, normetanephrine (NM) in the spinal cord of normal rats. The time course of the change in the NM concentration corresponded approximately to that of the morphine analgesia. The concentration of noradrenaline was not affected.2 A similar effect on the NM concentration was also observed after the administration of pentazocine (30 mg/kg, s.c.) and nalorphine (20 mg/kg, s.c.).3 The NM increasing effect of morphine, pentazocine and nalorphine was found in the dorsal half of the spinal cord but not in the ventral half.4 The increase in the concentration of NM induced by morphine, pentazocine or nalorphine was completely suppressed by naloxone (1 mg/kg, s.c.) given 5 min before the administration of these drugs.5 When the spinal cord was transected at C1, the NM increasing effect of morphine disappeared, yet when the brain stem was transected at the inter-collicular level, the effect remained.6 In morphine-tolerant rats, the concentration of NM in the spinal cord was almost the same as that observed in normal rats, but the increase in the concentration of NM in the spinal cord after the acute administration of morphine did not take place.7 The NM concentration in the spinal cord of normal rats was not modified by aminopyrine (75 mg/kg, s.c.), chlorpromazine (10 mg/kg, s.c.), mephenesin (100 mg/kg, i.p.) or naloxone (25 mg/kg, s.c.).8 The possible relation between morphine analgesia and the descending noradrenergic neurones in the spinal cord of rats is discussed.     

The effects of midazolam and morphine on analgesic and sedative activity of ketamine in rats

The aim of this study was to investigate possible interactions between the analgesic activity of ketamine (an N-methyl-D-aspartate antagonist), midazolam (a benzodiazepine derivative) and morphine using the tail-flick test in rats. Animals were treated s.c. with ketamine (1.0-10.0 mg/kg), midazolam (0.3 mg/kg), or morphine (0.6 mg/kg) alone. or in combination The strongest analgesic effect of ketamine was observed after 3.0 mg/kg. In higher doses no enhancement of ketamine activity were found. After morphine and ketamine (3.0 mg/kg) or morphine, midazolam and ketamine co-administration. higher antinociceptive effects compared to ketamine activity were found. Rats administered midazolam and ketamine (3.0 mg/kg) showed a decrease of the effect of ketamine analgesia, and the antinociceptive effect of the three-component mixture was lower than after co-injection of morphine and ketamine. The interaction of these two compounds with ketamine (5.0 mg/kg) occurred in a different manner, because midazolam led to a strong enhancement of ketamine analgesia. After morphine and ketamine (5.0 mg/kg) administration, very weak increase of ketamine analgesia was observed. The results of this study allow better understanding of the alteration of the analgesic effects of low doses of ketamine under the influence of morphine and midazolam.     

Further studies on the role of cholinergic mechanisms in the development of increased naloxone potency in mice.

Using the abdominal constriction response in mice, it was shown that pretreatment with either pilocarpine (5.0 mg/kg, s.c.) or oxotremorine (0.05--0.10 mg/kg s.c) caused a small but significant potentiation of the antinociceptive effect of morphine and the antagonistic action of naloxone. The potentiation of naloxone was considerably augmented following pretreatment with morphine plus either pilocarpine (2.5--5.0 mg/kg) or oxotremorine (0.05--0.10 mg/kg) as compared with any of these drugs given alone. Pretreatment with atropine sulphate (2.0 mg/kg, s.c.) had no effect on naloxone potency nor on the antinociceptive activity of morphine, but somewhat reduced that of pilocarpine and oxotremorine. Atropine abolished the ability of morphine pretreatment to enhance naloxone potency, and greatly reduced the augmenting effect of morphine plus the muscarinic receptor agonist drugs. It was also shown that oxotremorine reduced the development of dependence on morphine as tested by naloxone-precipitated jumping. These results agreed well with those reported using anticholinesterase drugs and add further evidence that the cholinergic system does play a role in the development of increased naloxone potency caused by morphine pretreatment. The increased naloxone potency does not seem to be related to the development of "acute dependence" on morphine.     

Opioid mechanisms of some behavioral effects of ketamine

The effect of naloxone on the duration of sleep and on analgesia produced by ketamine, and on the development of tolerance and cross-tolerance with morphine to ketamine analgesic effects were investigated in mice. Ketamine produced a dose-dependent analgesia. Naloxone (4 mg/kg) significantly inhibited the analgesic effects of ketamine (40 mg/kg), but (given in a dose of 2 mg/kg) did not affect the duration of ketamine sleep. Chronic administration of ketamine (160 mg/kg twice daily for 7 days) resulted in a gradual shortening of ketamine sleep and in the development of tolerance to the analgesic action of ketamine. There also developed cross-tolerance between analgesic effects of morphine and ketamine. Ketamine (20 mg/kg) significantly inhibited symptoms of morphine abstinence produced in morphine-pelleted mice by naloxone administration or by pellet removal. The results suggest that at least some elements of the mechanism of action of ketamine and morphine may be common and related to the endogenous opioid system.     

THE EFFECT OF CHLORPROMAZINE PRETREATMENT ON THE NARCOTIC ANTAGONISTIC POTENCY OF NALOXONE IN MICE

Morphine pretreatment (8.0 mg/kg s.c.) induced no overt tolerance to its antinociceptive effect in mice 4 h later, but enhanced the antagonistic potency of naloxone. Pretreatment with chlorpromazine hydrochloride (0.5–2.0 mg/kg s.c.) potentiated the antinociceptive effect of morphine measured 4.5 h later. The antagonistic effect of naloxone was also enhanced. The observed effect of chlorpromazine on naloxone potency was augmented when naloxone hydrochloride 0.2 mg/kg was administered in the pretreatment regime. The enhanced naloxone potency induced by morphine pretreatment was inhibited by chlorpromazine administered 0.5 h before the morphine pretreatment. These results indicate that pretreatment with either morphine or chlorpromazine increased the antagonistic potency of naloxone. However, it appears that these two drugs act by different mechanisms.     

Effects of yohimbine on morphine analgesia and physical dependence in the rat.

The effects of yohimbine on morphine analgesia and on the development of opiate physical dependence were studied to find out more about the involvement of alpha 2-adrenergic mechanisms in opioid actions. Male Sprague-Dawley rats (250-300 g) were used. The acute effect of morphine (5 mg/kg i.p.) in the tail-flick test was reduced significantly by pretreatment with a single dose of yohimbine (2 mg/kg i.p.). Alone yohimbine, produced a slight hyperalgesia. Animals treated with a sustained-release preparation of morphine (300 mg/kg s.c.) showed the same sensitivity to opiate analgesia 72 h later whether they were treated concomitantly with yohimbine or not, but they exhibited fewer withdrawal symptoms upon naloxone injection after yohimbine (2 or 4 mg/kg i.p. 24, 28, 48 and 52 h after the start of systemic morphine treatment). The results obtained confirm previous data on the effects of yohimbine on morphine analgesia and reveal the importance of alpha 2-adrenergic activation in the development of opioid physical dependence.     

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.     

Comparison of the analgesic actions of THIP and morphine

The antinociceptive actions of intraperitoneally-administered 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) and morphine were compared using three strains of mice. With the hot-plate assay, ED50 values for the action of THIP were about 4 mg/kg in OF1, CD1 and NMRI strains, whereas ED50 values for morphine varied among strains, being 6.8 mg/kg for OF1, 16.9 mg/kg for CD1, and about 29 mg/kg for NMRI mice; thus, the genetic control of the analgesic action of THIP appears to differ from that of morphine. The analgesic action of THIP in the hot-plate test was not blocked by naloxone, bicuculline, phentolamine or methysergide, but was partially reversed by a high dose of atropine, indicating that classic opiate-receptors, bicuculline-sensitive GABA-receptors, alpha-adrenoceptors and serotonin-receptors do not appear to mediate the action of THIP but that cholinergic receptors might be indirectly involved. THIP was about equipotent or more potent than morphine in the phenylbenzoquinone writhing test, evasion test, and traction test. Since the ED50 values for THIP in OF1 mice were similar for hot-plate, evasion and traction tests, the analgesic action of THIP might not be readily dissociated from its sedative or myorelaxant action.     

Neurotransmitter-blocking agents influence antinociceptive effects of carbamazepine, baclofen, pentazocine and morphine on bradykinin-induced trigeminal pain

The influence of naloxone (a narcotic antagonist), bicuculline (a GABA antagonist), phentolamine (an alpha-blocking agent), propranolol (a beta-adrenergic blocking agent), haloperidol (a dopaminergic blocking agent), methysergide (a serotonergic blocking agent) and atropine (a muscarinic blocking agent), on the antinociceptive effects induced by carbamazepine, baclofen, pentazocine and morphine, were investigated with a new antinociception test, using the trigeminal pain induced by application of bradykinin onto the tooth pulp of the rat. The antinociceptive effect of carbamazepine was significantly inhibited by bicuculline, phentolamine, propranolol and haloperidol but not by naloxone, methysergide and atropine. The effect of baclofen was significantly reduced by naloxone, bicuculline, propranolol and atropine but not by phentolamine, haloperidol and methysergide. The antinociceptive actions of pentazocine and morphine on trigeminal pain were significantly reduced by naloxone and phentolamine, and by naloxone alone, respectively. These results suggest the involvement of different neurotransmitters in the antinociceptive effects of the four analgesic drugs on trigeminal pain induced by bradykinin.     

Influence of omega-conotoxin on morphine analgesia and withdrawal syndrome in rats.

The effect of omega-conotoxin on opiate analgesia and withdrawal syndrome was investigated in rats. omega-Conotoxin given i.c.v. and i.p. caused weak analgesia in the tail-flick test. When the toxin (20 ng/rat) was given i.c.v. immediately before morphine (1.5 micrograms/rat i.c.v.) the resultant analgesic effect was additive. In contrast, the analgesia elicited by morphine (3 micrograms/rat i.c.v.) was greatly reduced after 24-h pretreatment with the toxin (20 ng/rat i.c.v.). The systemic administration of the toxin (10 micrograms/kg i.p.) did not affect morphine analgesia whether omega-conotoxin was coadministered with morphine (2.5 mg/kg i.p.) or was given 24 h before the opiate (5 mg/kg i.p.). omega-Conotoxin i.c.v. injected in morphine-dependent rats 15 min before naloxone challenge significantly attenuated the abstinence syndrome. On the contrary systemic administration of omega-conotoxin failed to suppress the morphine withdrawal syndrome. The present results suggest that omega-conotoxin affects both acute and chronic effects of morphine.     

Enhancement of morphine analgesia by tricyclic antidepressants

Using a cat tail-flick analgetic testing procedure, enhancement of the antinociceptive action of morphine (0.25 mg/kg s.c.) was detected following pre-treatment (1 hour) with 10 mg/kg s.c. of either amitriptyline or nortriptyline. No analgesia was observed following s.c. administration of either antidepressant or saline alone. Both central (amine re-uptake blockade; anticholinergic) and peripheral (decreased hepatic biotransformation) actions of tricyclic antidepressants may have contributed to augmentation of morphine analgesia.