Interaction of cholinergic and dopaminergic influences on yawning behavior.

The possible interaction between cholinergic and dopaminergic influences in the induction of yawning behavior in the rat is explored resorting to several experimental approaches: comparison of the ontogeny of yawning behavior induced by physostigmine (0.15 mg/kg) and apomorphine (0.05 mg/kg); simultaneous injection of both drugs; "crossed blocking" experiments, in which the action of the cholinomimetic agent is examined after injection of spiroperidol (0.05 mg/kg) and that of apomorphine after scopolamine (0.25 mg/kg). While physostigmine-elicited yawning is highest in early postnatal days and tends to decline from the 7th day onwards, reaching its lowest level around 3 wk, yawning induced by apomorphine begins around the 9th day and increases thereafter to a plateau that is reached in the third week. No synergism on yawning behavior is observed by simultaneous injection of optimal or suboptimal doses of physostigmine and apomorphine. Scopolamine blocks apomorphine-induced yawning; spiroperidol blocks apomorphine- but potentiates physostigmine-induced yawning, both in 15-day-old and young adult rats. Two 5-HT uptake blockers, citalopram (10-20 mg/kg) and fluoxetin (10-20 mg/kg) potentiate physostigmine - but not apomorphine-elicited yawning. On the basis of these results a tentative model of "in series" organization of dopaminergic and cholinergic influences on yawning behavior is proposed.