Pre- and post-synaptic dopaminergic receptors involved in apomorphine-induced yawning.

The temporal course of yawning behavior elicited by increasing doses of apomorphine (APO), from 0.01 to 10 mg/kg, was studied experimentally in adult albino rats. In the higher dose range a great prolongation of drug induced yawning latency is observed. This result is explainable by postulating differences in sensitivity of two sets of dopaminergic (DA) receptors: low threshold presynaptic DA receptors which, when activated, disinhibit yawning, and high threshold postsynaptic DA receptors inhibiting yawning. Apomorphine in high doses can entirely suppress physostigmine-elicited yawning.