A comparison of the antinociceptive responses to the GABA-receptor agonists THIP and baclofen

The antinociceptive action of the gamma-aminobutyric acid (GABA) agonists THIP and baclofen was evaluated in mice using hot-plate (48 and 55 degrees C) and tail-immersion (50 degrees C) procedures. It was found that atropine reversed antinociception induced by THIP but not that induced by baclofen in the 48 degrees C test, whereas the anticholinergic drug blocked the response to both GABA agonists when the stimulus was provided by a 55 degrees C hot-plate. Atropine methylnitrate, mecamylamine, picrotoxin and bicuculline had no effect on antinociception induced by THIP or baclofen. Prior treatment with haloperidol enhanced only the response to baclofen on the 55 degrees C hot-plate. A reciprocal cross-tolerance was found between THIP and baclofen in the tail-immersion assay, although only THIP exhibited cross-tolerance to morphine. These results suggest that while the analgesic response to THIP and baclofen is partially mediated by a common system, the two agents act by independent mechanisms as well.     

Antinociception induced by systemic administration of local anaesthetics depends on a central cholinergic mechanism.

1 The antinociceptive effects of systemically-administered procaine, lignocaine and bupivacaine were examined in mice and rats by using the hot-plate, writhing and tail flick tests. 2 In both species all three local anaesthetics produced significant antinociception which was prevented by atropine (5 mg kg-1, i.p.) and by hemicholinium-3 (1 microgram per mouse, i.c.v.), but not by naloxone (3 mg kg-1, i.p.), alpha-methyl-p-tyrosine (100 mg kg-1, s.c.), reserpine (2 mg kg-1, i.p.) or atropine methylbromide (5.5 mg kg-1, i.p.). 3 Atropine (5 mg kg-1, i.p.) which totally antagonized oxotremorine (40 micrograms kg-1, s.c.) antinociception did not modify morphine (5 mg kg-1, s.c.) or baclofen (4 mg kg-1, s.c.) antinociception. On the other hand, hemicholinium, which antagonized local anaesthetic antinociception, did not prevent oxotremorine, morphine or baclofen antinociception. 4 Intracerebroventricular injection in mice of procaine (200 micrograms), lignocaine (150 microgram) and bupivacaine (25 micrograms), doses which were largely ineffective by parenteral routes, induced an antinociception whose intensity equalled that obtainable subcutaneously. Moreover, the i.c.v. injection of antinociceptive doses did not impair performance on the rota-rod test. 5 Concentrations below 10(-10) M of procaine, lignocaine and bupivacaine did not evoke any response on the isolated longitudinal muscle strip of guinea-pig ileum, or modify acetylcholine (ACh)-induced contractions. On the other hand, they always increased electrically-evoked twitches. 6 The same concentrations of local anaesthetics which induced antinociception did not inhibit acetylcholinesterase (AChE) in vitro. 7 On the basis of the above findings and the existing literature, a facilitation of cholinergic transmission by the local anaesthetics is postulated; this could be due to blockade of presynaptic muscarinic receptors.     

The involvement of central cholinergic system in (+)-matrine-induced antinociception in mice

The antinociceptive effect of (+)-matrine was examined in mice by writhing, tail-pressure and hot-plate tests. (+)-Matrine (5, 10 and 20 mg/kg s.c.) produced antinociception in a dose-dependent manner. In hot-plate test, the antinociception produced by (+)-matrine (10 mg/kg s.c.) was attenuated by muscarinic receptor antagonists atropine (5 mg/kg i.p.) and pirenzepine (0.1 mug/mouse i.c.v.) and acetylcholine depletor hemicholinium-3 (HC-3) (1 mug/mouse i.c.v.), but not by opioid receptor antagonist naloxone (2 mg/kg i.p.), dopamine D(2) receptor agonist (-)-quinpirole (0.1 mg/kg i.p.) or catecholamine depletor reserpine (2.5 mg/kg i.p.). Radioligand binding assay demonstrated that (+)-matrine had no affinity for mu-, kappa- or delta-opioid receptors in a wide concentration range (1 x 10(-11)-1 x 10(-3) M). The results suggest that (+)-matrine exerts its antinociceptive effect through multiple mechanism(s) such as increasing cholinergic activation in the CNS rather than acting on opioid receptors directly.     

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.     

L-NAME, a nitric oxide synthase inhibitor, modulates cholinergic antinociception.

Systemic administration of sumatriptan and buspirone (20 mg/kg: 5-HT1A agonists) produced antinociception against acetic acid-induced writhing. The antinociceptive effect was potentiated by cholinomimetic physostigmine (0.05 mg/kg i.p.) and blocked by the muscarinic antagonist atropine (5 mg/kg i.p.). Naloxone, an opiate antagonist, failed to reverse the sumatriptan- or buspirone-induced antinociception, but pindolol (10 mg/kg), a nonselective 5-HT1A antagonist, blocked this response. Sumatriptan- or buspirone-induced antinociception was significantly potentiated by L-NAME (a nitric oxide [NO] synthase inhibitor) although L-NAME (20 mg/kg) given alone had no effect on the nociceptive threshold. Recent studies have suggested that the L-arginine/NO/cGMP pathway is involved in the modulation of pain perception. The present results suggest that NO may play a role in cholinergic antinociception-mediated 5-HT1A receptor stimulation and that NO exerts an inhibitory action on cholinergic analgesia.     

The involvement of endogenous opioid mechanisms in the antinociceptive effects induced by antidepressant drugs, desipramine and trimipramine

The present study was designed to investigate the involvement of endogenous opioid systems in the antinociception induced by the antidepressant drugs, desipramine and trimipramine. For this purpose, the antinociceptive effects of desipramine (7.5 and 15.0 mg/kg i.p.) and trimipramine (5.0 and 10.0 mg/kg i.p.) were compared to that induced by morphine (0.2 and 2.0 mg/kg i.p.) in the tail-clip model in mice. Naloxone (0.3 and 3.0 mg/kg i.p.), a non-specific opioid receptor antagonist, inhibited morphine-induced antinociception in mice, whereas the antinociceptive effects of antidepressant drugs were found to be resistant to naloxone blockade to some extent, since only the higher concentration of naloxone (3.0 mg/kg i.p.) caused significant inhibition of the effects of antidepressant drugs. In contrast, naltrindole (1.0 mg/kg i.p.), a specific delta-receptor antagonist, inhibited antinociception induced by desipramine and trimipramine in this test, while it inhibited the antinociceptive effect of morphine only partly. None of the opioid antagonists produced a significant effect in the tail-clip experiment when they were injected alone. Based on these findings, we concluded that endogenous opioids are involved in the antinociceptive effects of the antidepressant drugs using different mechanisms.     

Combined use of tertiary amine parasympathomimetics with a quaternary amine parasympatholitic--a new perspective to use parasympathomimetic drugs for systemic analgesia

The interactions on antinociception between a muscarinic agonist arecoline (arec), an anticholinesterase physostigmine (physo) which both cross CNS, and a peripherally acting antimuscarinic hyoscine-N-butyl bromide (hyo), were assessed by tail flick test in mice. All drugs were administered intraperitoneally (i.p.). While hyoscine-N-butyl bromide (0.15 and 4.00 mg/kg, i.p.) did not produce antinociception, physostigmine salicylate (0.3 mg/kg, i.p.) and arecoline hydrobromide (8.00 mg/kg, i.p.) exerted significant antinociceptive effect. In combined applications, physo + hyo (0.075 + 0.15; 0.15 + 0.30; 0.30 + 0.60 mg/kg) and arec + hyo (1.00 + 0.50; 2.00 + 1.00; 4.00 + 2.00; 8.00 + 4.00 mg/kg), respectively, produced significant antinociception and the tail flick latencies produced by physo 0.30 + hyo 0.60 mg/kg and arec 8.00 + hyo 4.00 mg/kg were not significantly different from those of physo 0.30 mg/kg and arec 8.00 mg/kg, respectively, showing that hyo did not antagonise the antinociceptive effects of physo and arec. We believe that combining an centrally acting cholinergic drug applied systemically with a peripherally acting (quaternary amine) antimuscarinic compound might be used as an effective analgesic in clinical practice.     

TRK-820, a selective kappa-opioid agonist, produces potent antinociception in cynomolgus monkeys

TRK-820 ((-)-17-cyclopropylmethyl-3,14b-dihydroxy-4,5a-epoxy-6b-[N-methyl-trans-3-(3-furyl)acrylamide]morphinan hydrochloride) has been shown to be a potent opioid kappa-receptor agonist with pharmacological properties different from those produced by kappa1-opioid receptor agonists in rodents. To ascertain whether or not these properties of TRK-820 would be extended to primates, the antinociceptive effect of TRK-820 was evaluated in cynomolgus monkeys by the hot-water tail-withdrawal procedure. TRK-820 given intramuscularly (i.m.) produced a potent antinociceptive effect that was 295- and 495-fold more potent than morphine with the 50 degrees C and 55 degrees C hot-water tests, respectively, and 40-fold more potent than U-50,488H and 1,000-fold more potent than pentazocine in the 50 degrees C hot-water test. The duration of antinociceptive effects of TRK-820 treatment (0.01 and 0.03 mg/kg, i.m.) lasted more than 6 h, which was much longer than those of U-50,488H. The antinociception produced by the higher dose (0.03 mg/kg, i.m.) of TRK-820 was not inhibited by nor-binaltorphimine (3.2 and 10 mg/kg, s.c.) or by naloxone (0.1 mg/kg, s.c.), although the antinociception induced by a lower dose of TRK-820 (0.01 mg/kg, i.m.) was inhibited by nor-binaltorphimine (10 mg/kg, s.c.). The same doses of nor-binaltorphimine and naloxone effectively inhibited the antinociception induced by the higher doses of U-50,488H (1.0 mg/kg, i.m.) and morphine (10 mg/kg, i.m.), respectively. These results indicate that the antinociception induced by TRK-820 is less sensitive to nor-binaltorphimine and suggest that it is mediated by the stimulation of a subtype of kappa-opioid receptor different from the kappa-opioid receptor in cynomolgus monkeys.     

The morphine sparing effect of physostigmine

The antinociceptive effect of morphine was studied in tail-flick- and acetic acid-induced writhing in mice. Morphine effect was dose-related (1, 2 and 5 mg/kg s.c.). Physostigmine (0.05 and 0.1 mg/kg i.p.) potentiated the antinociceptive effect of morphine, and the anticholinergic, scopolamine (1 mg/kg i.p.), reversed the potentiating effect of physostigmine, indicating the involvement of the cholinergic system in pain. Coadministration of physostigmine would increase the therapeutic index of morphine thereby sparing the dose of morphine and also possibly the side effects including the development of tolerance and addiction.     

Antagonism of antinociception in mice by glucose and fructose: comparison of subcutaneous and intrathecal morphine

Determination of the ED50s of glucose and fructose, administered i.p., for antagonizing the antinociceptive action of morphine (4 mg/kg s.c. or 0.5 micrograms i.t.) and determination of the ED50s for i.t. morphine after i.p. pretreatment with saline, glucose (5 g/kg) or fructose (5 g/kg) in the mouse tail-flick test indicated that fructose was more potent than glucose in antagonizing antinociception after either route or morphine administration. It is concluded that the antagonism of morphine-induced antinociception by glucose and fructose is due to a direct effect of these sugars or their metabolic products within the central nervous system.     

Antinociceptive effect of the combination of pentazocine with morphine in the tail-immersion and scald-pain tests in rats

We investigated the antinociceptive effect of pentazocine hydrochloride (pentazocine) in combination with morphine hydrochloride (morphine) using two antinociceptive tests; i.e., the tail-immersion and scald-pain tests, in rats. In the tail-immersion test, the rat's tail was immersed in warm water at 47 degrees C, and the latency to a nociceptive response was measured. In the scald-pain test, the right hind foot was scalded by immersion into hot water at 57 degrees C. Two hours later, additional thermal stimulus was applied to the same foot, and the latency to a nociceptive response was measured. Subcutaneous treatment with either pentazocine (6, 12, 24 mg/kg) or morphine (1.5, 3, 6 mg/kg) alone dose-dependently showed antinociceptive effects in both tests. The ED50 values (95% confidence limit) of pentazocine and morphine were 13.0 (5.4-31.5) and 2.4 (1.6-3.7) mg/kg in the tail-immersion test and 11.0 (4.5-26.6) and 3.8 (1.8-7.2) mg/kg in the scald-pain test, respectively. Simultaneous treatment with pentazocine at the similar dose augmented the morphine (1.5 mg/kg)-induced antinociception, but did not diminish the morphine (6 mg/kg)-induced antinociception in both tests. These results suggest that the simultaneous administration of pentazocine at the antinociceptive dose and morphine exerts additional antinociceptive activity against thermal and scald-induced inflammatory pain.     

GABAA-antagonists and baclofen analgesia

1. Intraperitoneal (i.p.) injection of different doses of baclofen (5, 7.5 and 10 mg/kg) induced analgesia in tail-flick test. The effect was dose-dependent. 2. The antinociception induced by baclofen (10 mg/kg, i.p.) was decreased in animals pretreated with bicuculline (1.5 mg/kg, i.p., 30 min), but not with naloxone (1.5 mg/kg, i.p., 30 min). 3. In picrotoxin (1 mg/kg, i.p., 15 min) pretreated mice, baclofen (5 mg/kg, i.p.) showed a significant analgesic effect. 4. Morphine (6 mg/kg, s.c.) induced analgesia which was antagonized by naloxone pretreatment (1.5 mg/kg, i.p.), while bicuculline or picrotoxin did not alter the morphine response. 5. These data suggest that a part of analgesic effect of baclofen may be mediated through GABAA receptor sites, and differs from that of morphine.     

Antinociception induced by amitriptyline and imipramine is mediated by alpha2A-adrenoceptors

The involvement of alpha2-adrenoceptors in the antinociception induced by the tricyclic antidepressants amitriptyline and imipramine was investigated in mice by using the hot-plate and abdominal constriction tests. The antinociception produced by amitriptyline (15 mg/kg, i.p.) and imipramine (15 mg/kg, i.p.) was prevented by reserpine (2 mg/kg, i.p.) and yohimbine (3-10 mg/kg, i.p.) but not by naloxone (1 mg/kg, i.p.), atropine (5 mg/kg, i.p.), CGP 35348 (100 mg/kg, i.p.) and prazosin (1 mg/kg, i.p.). On the basis of the above data, it can be postulated that amitriptyline and imipramine exerted their antinociceptive effect by activation of alpha2-adrenoceptors. Administration of the alpha2A-adrenoceptor antagonist BRL 44408 (1 mg/kg, i.p.) prevented amitriptyline and imipramine antinociception, whereas the alpha2B/C-adrenoceptor antagonist ARC 239 (10 mg/kg, i.p.) was ineffective. These data indicate that the enhancement of the pain threshold produced by amitriptyline and imipramine is mediated by activation of alpha2A-adrenoceptors. Neither tricyclic antidepressants nor the antagonists used impaired mouse performance evaluated by the rota-rod and hole-board tests.     

GABAergic and cholinergic mediation in the antinociceptive action of homotaurine.

1. The role of GABAergic and cholinergic mechanisms in the antinociceptive effect of homotaurine (22.25-111.24 mg/kg i.p.) in chemical (acetic acid) and thermal (tail flick, tail immersion) tests has been studied by means of the interaction with baclofen and anticholinergic drugs. 2. Baclofen (2 mg/kg p.o.) and scopolamine sulfate (2.5 mg/kg i.p.) potentiate the antinociceptive effect of the amino acid in the chemical test. 3. Bicuculline (1 mg/kg i.p.) pretreatment does not modify the antinociceptive effect of homotaurine in the tail immersion and tail flick tests. 4. Scopolamine sulfate and methylnitrate (1 mg/kg i.p.) antagonise the effect of homotaurine (111.24 mg/kg i.p.) in the tail flick test. 5. The above results imply that peripheral GABAB and central cholinergic mechanisms play a role in the antinociceptive effect of homotaurine.     

Antinociceptive effect of R-(+)-hyoscyamine on the conjunctival reflex test in rabbits

R-(+)-Hyoscyamine (1-10 microg/kg, s.c.) dose-dependently increased the local anesthetic effect of procaine (50 microg/ml) and lidocaine (50 microg/ml) in the conjunctival reflex test in the rabbit. This potentiating effect is completely prevented by the M1 antagonist dicyclomine (10 mg/kg, s.c.). The intensity of R-(+)-hyoscyamine antinociception was comparable to that induced by morphine (2 mg/kg, s.c.) and minaprine (15 mg/kg, s.c.), used as analgesic reference drugs. In the same experimental conditions, the S-(-)-enantiomer of atropine (0.1-10 microg/kg, s.c.), was completely ineffective. The present results confirm the ability of R-(+)-hyoscyamine to produce a paradoxical antinociceptive effect mediated by a cholinergic mechanism not only in rodents but also in the rabbit.     

Involvement of Cholinergic and Opioid Receptor Mechanisms in Nicotine-Induced Antinociception

Abstract: In this work we have studied the influences of nicotinic agents on the antinociception of morphine in formalin test. Nicotine (0.001-0.1 mg/kg) induced antinociception in mice in a dose-dependent manner in the early phase of formalin test, and also potentiated the morphine effect. The nicotinic receptor antagonist, mecamylamine (0.5 mg/kg), but not hexamethonium decreased the antinociception induced by nicotine (0.1 mg/kg) in both phases. The muscarinic receptor antagonist atropine (5 and 10 mg/kg) also decreased the response of nicotine. Mecamylamine, hexamethonium or atropine did not alter morphine antinociceptive response, while naloxone decreased responses induced by nicotine or morphine. The antagonists by themselves did not elicit any response in formalin test, however, high doses of mecamylamine tend to increase pain response. It is concluded that central cholinergic and opioid receptor mechanisms may be involved in nicotine-induced antinociception.     

The antinociceptive effects of prostaglandin antagonists in the rat

This study examined the antinociceptive effects of two prostaglandin antagonists, SC-25469 and SC-19220 in the rat. SC-25469 and SC-19220 inhibited acetic acid-induced writhing with ED50 s of 6.9 and 6.8 mg/kg p.o., respectively. When compared to other analgesics, the rank order of potency in the writhing test was morphine greater than pentazocine = U-50,488 greater than SC-25469 = SC-19220 greater than ibuprofen greater than aspirin greater than acetaminophen. SC-25469 (150 and 300 mg/kg p.o.) and SC-19220 (50-300 mg/kg p.o.) also suppressed the behavioral response to s.c. injection of formalin, as did aspirin (50-150 mg/kg p.o.), ibuprofen (25-100 mg/kg p.o.) and acetaminophen (300 mg/kg p.o.). However, the suppression was not of the magnitude observed after administration of morphine (ED50: 0.9 mg/kg s.c.), pentazocine (ED50: 2.4 mg/kg s.c.) or U-50,488 (ED50: 0.8 mg/kg s.c.). This study demonstrates the antinociceptive properties of prostaglandin antagonists in two distinct tests of nociception.     

Morphine inhibits dopaminergic and cholinergic induced ejaculation in rats

1. Subcutaneous injection (s.c.) of apomorphine (0.1–0.5 mg/kg) and intraperitoneal administration (i.p.) of quinpirole (0.01–0.25 mg/kg), physostigmine (0.05–0.2 mg/kg) and philocarpine (0.75–3 mg/kg, i.p.) but not neostigmine (0.1–1 mg/kg) induced ejaculation in rats.2. The responses of drugs were reduced by morphine (1–6 mg/kg, s.c.) pretreatment.3. The inhibitory effect of morphine was reversed by naloxone (1.5 mg/kg, s.c.).4. Naloxone (0.75–3 mg/kg, s.c.) alone induced slight but significant ejaculation.5. Ejaculatory responses induced by apomorphine and quinpirole but not those by physostigmine and pilocarpine were reduced by sulpiride (100 mg/kg, i.p.) pretreatment.6. Domperidone (1–30 mg/kg, i.p.) did not change the response induced by apomorphine.7. Pretreatment of animals with the cholinergic antagonist atropine (10 mg/kg, i.p.) decreased the frequency of ejaculation induced by apomorphine, quinpirole, physostigmine or pilocarpine.8. It may be concluded that D-2 activation induces ejaculation through influence on cholinergic mechanisms and morphine inhibits the ejaculation induced by activation of both cholinergic and dopaminergic systems via opiate receptor sites.     

Clonidine and guanfacine-induced antinociception in visceral pain: possible role of alpha 2/I2 binding sites

Visceral pain is one of the most common forms of pain which is poorly understood. We now studied the influence of imidazoline/guanidinium compounds such as clonidine and guanfacine on visceral pain in the presence or absence of yohimbine and benazoline. To produce visceral pain-related behaviours, formalin (10%) was administered by inserting a fine cannula into the colon via the anus. Each experiment took 1 h. Clonidine (0.001, 0.01 and 0.1 mg/kg, i.p.) and guanfacine (2.5, 5 and 10 mg/kg, i.p.) produced analgesia dose dependently. The clonidine response was inhibited by yohimbine (0.2 mg/kg, i.p.). On the other hand, benazoline (5 mg/kg, i.p.) blocked the antinociceptive effect of guanfacine (5 mg/kg). Benazoline (2.5 and 5 mg/kg) itself also induced analgesia in inflammatory colonic pain. In this study, we used morphine to ensure that the behavioural responses were pain-related. Our results showed that morphine (2.5, 5 and 10 mg/kg, s.c.) produced a dose-dependent antinociception. The morphine (7 mg/kg, s.c.) response was reduced by naloxone (2 mg/kg, i.p.). However, we concluded that both imidazoline (I(2)) and alpha(2)-adrenoceptors may play a role in producing analgesia in visceral pain.     

Mechanism of antinociceptive effect of nimodipine in experimental diabetic neuropathic pain

This study used streptozotocin-(STZ; 50 mg/kg, i.v.) diabetic rats and monitored the weekly thermal nociceptive thresholds for 8-week diabetes. Nimodipine (10 mg/kg i.p.) treatment initiated after 8 weeks of diabetes antagonized the hyperalgesic response in diabetic rats. However, insulin treatment showed a partial response in these animals. Thermal hyperalgesia showed reduced sensitivity to the antinociceptive effect of morphine (5 mg/kg, i.p.). Furthermore, a reduced sensitivity to the antinociceptive effect of baclofen (GABAB agonist; 4 mg/kg i.p.) was observed. Five days of treatment with MK-801 (N-methyl-D-aspartate [NMDA] receptor antagonist 0.5 mg/kg i.p.) completely reversed 8-week diabetes-induced thermal hyperalgesia. These data suggest that diabetes-induced hyperalgesia may be the consequence of increased excitatory tone within the spinal cord. An increased release of glutamate and activation of the NMDA receptor would maintain the hyperalgesic state. Reduced activity of both opioidergic and GABAB ergic inhibitory systems might accelerate the increased excitation, thus contributing to the ongoing pain in diabetic rats.