M. Malcangio

Role of muscarinic receptor subtypes in central antinociception

1 The ability to modify the pain threshold by the two M1‐muscarinic agonists: McN‐A‐343 and AF‐102B and by the specific M2‐agonist arecaidine was examined in mice and rats by using three different noxious stimuli: chemical (writhing test), thermic (hot‐plate test) and mechanical (paw pressure test). 2 In the mouse hot‐plate test McN‐A‐343 (20–50 μg per mouse i.c.v.) and AF‐102B (1–10 mg kg−1 i.p.) produced significant antinociception which was prevented by atropine (1 μg per mouse i.c.v.) and by the two selective M1 antagonists: pirenzepine (0.01 μg per mouse i.c.v.) and dicyclomine (0.08 μg per mouse i.c.v. or 10 mg kg−1 i.p.) but not by the specific M2‐antagonist AFDX‐116 (0.1 μg per mouse i.c.v.), naloxone (1 mg kg−1 i.p.) or by the acetylcholine (ACh) depletor hemicholinium‐3 (HC‐3) (1 μg per mouse i.c.v.). McN‐A‐343 and AF‐102B were able to increase the pain threshold also in the mouse acetic acid writhing test and in rat paw pressure test. These antinociceptive effects were completely prevented by dicyclomine (0.08 μg per mouse i.c.v. or 10 mg kg−1 i.p.) but not by AFDX‐116 (0.1 μg per mouse or rat i.c.v.). 3 In contrast with the M1‐agonists, the M2‐agonist arecaidine (0.1–2 μg per mouse or rat i.c.v.) did not induce antinociception in all three analgesic tests. However, arecaidine, at the same i.c.v. doses, was able to reduce the pain threshold in the hot‐plate and paw pressure tests. 4 The site of muscarinic control of the pain threshold is localized in the CNS since drugs which do not cross the blood‐brain barrier such as McN‐A‐343, pirenzepine and arecaidine exerted their effects only if injected i.c.v. 5 On the basis of the above findings and existing literature we suggest that the postsynaptic muscarinic receptors involved in antinociception belong to the M1 subtype. Nevertheless, presynaptic autoreceptors (M2 subtype) may play a role in pain regulation since they are involved in modulation of endogenous ACh release.

CGP 35348, a new GABAB antagonist, prevents antinociception and muscle-relaxant effect induced by baclofen.

1 CGP 35348, a new GABAB antagonist, was examined on antinociception induced by (±)‐baclofen by use of the hot plate and writhing tests in mice and the paw pressure test in rats. CGP 35348 was also studied in mice on (±)‐baclofen‐induced impairment of rota‐rod performance. 2 CGP 35348, injected either i.p. (60–100 mg kg−1 in mouse) or intracerebroventricularly (i.c.v.) (0.5–2.5 μg per mouse; 25 μg per rat) prevented (±)‐baclofen‐induced antinociception. 3 CGP 35348 did not modify oxotremorine‐ and morphine‐induced antinociception in mice and rats. 4 CGP 35348 (2.5 μg i.c.v. per mouse) also prevented (±)‐baclofen‐induced impairment of the rota‐rod test. 5 Two other GABAB antagonists, phaclofen (50 μg i.c.v. per mouse) and 2‐OH‐saclofen (2.5 μg‐10 μg i.c.v. per mouse) did not modify (±)‐baclofen‐induced antinociception. 7 These results suggest that, at present, CGP 35348 is the only compound able to antagonize (±)‐baclofen‐induced antinociception.

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 μg per mouse, i.c.v.), but not by naloxone (3 mg kg−1, i.p.), α‐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 μg 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 μg), lignocaine (150 μg) and bupivacaine (25 μg), 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.

Effect of ICV taurine on the impairment of learning, convulsions and death caused by hypoxia.

The effect of the intracerebroventricular (ICV) administration of taurine on amnesia, convulsions and death caused by hypoxia was investigated in mice. Taurine in doses of 80–100 μg/mouse impaired acquisition of a single trial in passive avoidance performance, but protected mice from the learning impairment induced by hypoxia. Neither β-alanine nor saccarose were able to mimic the effects of taurine. Taurine had no effect on amnesia induced by scopolamine injected intraperitoneally. Taurine protected against the onset of convulsions induced by hypoxia, while convulsions induced by pentylenetetrazole (PTZ) and hyperbaric oxygen were unaffected. The survival time of mice exposed to hypoxia was significantly increased by taurine treatment. These data suggest that taurine may play a role as an antihypoxic agent.

Desensitization of GABAB receptors and antagonism by CGP 35348, prevent bicuculline- and picrotoxin-induced antinociception.

The effect of the GABAA antagonists, bicuculline and picrotoxin, in the hot plate and writhing tests in mice and the paw-pressure test in rats was assessed. Subconvulsant doses of bicuculline (1.3-4 mumol kg-1, s.c.) or picrotoxin (0.8-2.5 mumol kg-1, s.c.) induced a dose-related increase in latency of licking in the hot plate test in mice, whereas subconvulsant doses of strychnine and thiosemicarbazide (0.9 and 6 mg kg-1, s.c. respectively), did not modify the threshold to thermal stimuli in mice. The effects of bicuculline and picrotoxin were not modified by naloxone (3 mg kg-1, i.p., a dose which inhibited the antinociceptive effect of morphine) or by atropine (5 mg kg-1, i.p., a dose which prevented oxotremorine-induced antinociception) but were antagonized by the GABAB antagonist CGP 35348 (2.5 micrograms, i.c.v., a dose which prevented (+/-)baclofen-induced antinociception). Mice, rendered tolerant to baclofen-induced antinociception by twice daily injection of increasing doses of baclofen (5-18 mg kg-1, s.c.), were unresponsive to the antinociceptive effects of bicuculline and picrotoxin but still responded to morphine. Bicuculline and picrotoxin, in the same range of doses which affected the three models of antinociception used, inhibited pentobarbital-induced hypnosis. Large doses of bicuculline and picrotoxin (4 and 2.5 mumol kg-1, s.c. respectively), reduced locomotor activity and impaired rota-rod performance in mice. The changes in response to noxious stimuli, induced by bicuculline and picrotoxin, are interpreted as an antinociceptive effect. It is then suggested that this effect might depend on an indirect activation of GABAB receptors through release of GABA.(ABSTRACT TRUNCATED AT 250 WORDS)