A number of [(3-chlorophenyl)piperazinylpropyl]pyridazinones and the corresponding isoxazolopyridazinones, showing the arylpiperazinyl substructure present in very potent antinociceptive agents reported in the literature, were synthesized and tested for their analgesic activity. The investigated compounds showed antinociceptive properties in the mouse hot-plate test (thermal nociceptive stimulus) after systemic administration with an efficacy similar to that exerted by morphine. The increase of the pain threshold induced by the compounds labeled 5a, 7, 8, and 11 was prevented by reserpine, suggesting the involvement of the noradrenergic and/or serotoninergic system in their mechanism of action. Among them, 7 and 11 showed the highest analgesic potency and efficacy together with a good ratio (133 and 200, respectively) of the minimal nontoxic dose (MNTD) to the minimal analgesic dose (MAD). Furthermore, they were also active after icv administration and in the presence of a chemical, painful stimulus (abdominal constriction test).

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. 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.). 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. 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. 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. The same concentrations of local anaesthetics which induced antinociception did not inhibit acetylcholinesterase (AChE) in vitro. 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.

AbstractöThe e¡ect of the i.c.v. administration of antisense oligodeoxynucleotides directed against the K subunit of di¡erent Gi-proteins (anti-GiK 1 , anti-GiK 2 , anti-GiK 3 ) on the antinociception induced by the H 1 -antihistamines was evaluated in the mouse hot-plate test. The administration of diphenhydramine (20 mg kg 31 s.c.), pyrilamine (15 mg kg 31 s.c.) and promethazine (6 mg kg 31 s.c.) produced an increase of the pain threshold which peaked 15 min after injection. Pretreatment with anti-GiK 1 (12.5 Wg per mouse i.c.v.), anti-GiK 2 (25 Wg per mouse i.c.v.) and anti-GiK 3 (25 Wg per mouse i.c.v.), administered 24 and 18 h before test, prevented the antihistamine-induced antinociception. At the highest e¡ective doses, none of the compounds used impaired motor coordination, as revealed by the rota rod test, nor modi¢ed spontaneous motility and inspection activity, as revealed by the hole board test.These results suggest an important role played by the Gi-protein pathway in the transduction mechanism involved in the enhancement of the pain threshold produced by H 1 -antihistamines. ß

These results suggest the important role played by the Gi 1 -and Gi 2 -protein pathway in the transduction mechanism involved in the impairment of memory processes produced by the H 1 -antihistamine diphenhydramine.

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