Characteristics of yawning behavior induced by apomorphine, physostigmine and pilocarpine.

Apomorphine, a dopamine receptor agonist, physostigmine, an anticholinesterase agent and pilocarpine, a cholinergic receptor agonist, produced yawning in rats, with the most effective doses being 0.25 mg/kg, 0.2 mg/kg and 4 mg/kg, respectively. The yawning induced by these drugs is characterized by differences in direction of head moving, tongue protruding and duration of the yawn. The apomorphine-induced yawn was characterized by a slow and wide opening of the mouth with the head moving mainly downward and with a marked protrusion of the tongue. The apomorphine-induced yawn was 3.6 sec in duration. Physostigmine elicited a similar yawn to that seen with apomorphine, except for moving of the head in the upward direction. The pilocarpine-induced yawn was characterized by the head moving forward with a high frequency but without tongue protrusion and lasted 1.8 sec. The frequency of physiological yawning was low, but the behavioral posture was almost similar to that of physostigmine-induced yawning. The characteristics of yawns induced by these agents were unchanged at all doses of the drugs. Different doses altered the frequency of yawning. After pretreatment with mecamylamine, the apomorphine- and physostigmine-induced tongue protruding was inhibited and the duration of the yawning induced by the both drugs was shortened. The results suggest that the behavioral features of yawning induced by apomorphine, physostigmine and pilocarpine differ from one another, and that physiological yawning as well as physostigmine-induced yawning may be mediated by endogenous acetylcholine which stimulates both muscarinic and nicotinic receptors.