Effects of acetylcholine agonists and antagonists on yawning and analgesia in the rat.
The ability of acetylcholine muscarinic agonists, injected subcutaneously (s.c.) to elicit yawning and analgesia (tail-flick response) in rats was examined. Yawning was elicited by physostigmine, RS86 and pilocarpine with an inverted 'U'-shaped dose-response relationship; maximal effects occurred at 0.1, 0.5 and 2.0 mg/kg respectively. Neostigmine (0.05-0.2 mg/kg); arecoline (0.5-2.0 mg/kg); bethanecol (0.1-10 mg/kg) and McN-A-343 (5-20 mg/kg) had marginal or no activity. In contrast, dose-related analgesia was obtained following oxotremorine (0.01-0.3 mg/kg) and arecoline (0.5-4.0 mg/kg) and physostigmine (0.1-0.4 mg/kg), RS86 (0.25-2.5 mg/kg) and pilocarpine (0.5-8.0 mg/kg). The effects of acetylcholine antagonists on physostigmine-induced yawning and physostigmine-induced analgesia were also investigated. Following their s.c. injection, trihexyphenidyl, atropine, dicyclomine, secoverine and methylatropine but not pirenzepine, inhibited both yawning and analgesia; there were clear differences in their potencies on the two responses. Pirenzepine, intracerebroventricularly (i.c.v.), inhibited yawning (ED50 value 5.7 micrograms/rat) but not analgesia (3-100 micrograms/rat). The results are discussed in terms of a possible functional differentiation of central muscarinic receptors.
D000109:Acetylcholine / Q000008:administration & dosage / Q000037:antagonists & inhibitors / Q000502:physiology*; D000698:Analgesia; D000818:Animals; D001285:Atropine / Q000494:pharmacology; D007276:Injections, Intraventricular; D008297:Male; D010890:Pirenzepine / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D011930:Reaction Time / Q000187:drug effects; D015000:Yawning / Q000187:drug effects*
D010890:Pirenzepine; D001285:Atropine; D000109:Acetylcholine