Pharmacological characteristics of dopamine receptors involved in the dual effect of dopamine agonists on yawning behaviour in rats

Increasing doses of apomorphine (APO) induced the dose-dependent appearance of yawns in rats at doses up to 0.1 mg · kg − and their disappearance from 0.1 to 0.6 mg · kg −. A similar biphasic effect on yawning was observed with increasing doses of n-propyl norapomorphine, piribedil, S 584, bromocriptine, lergotrile, lisuride, CQ 32084 and L-DOPA. APO, n-propyl norapomorphine, piribedil and CQ 32084 had similar ED 50 on the induction of sniffing and on the disappearance of yawns. All the neuroleptics tested antagonized the yawns induced by 0.1 mg · kg − APO. Increasing doses of haloperidol, chlorpromazine, mezilamine, metoclopramide and thioridazine made the yawns reappear in rats injected with 0.6 mg · kg −1 APO. The ID 50 were similar to those for the antagonism of sniffing. On the other hand, increasing doses of clozapine, (±)- or (−)-sulpiride, veralipride and DAN 2163 did not make the yawns reappear in rats injected with 0.6 mg · kg −1 APO although sniffing was antagonized. These results are discussed in terms of the ability of sulpiride, veralipiride and DAN 2163 to distinguish between the dopamine (DA) receptors involved in the appearance of yawns at low doses of DA agonists and in their disappearance at higher doses. The decreased APO-induced yawning observed concomitantly with increased sniffing in rats with 6-hydroxydopamine-lesioned olfactory tubercles suggests that yawning and sniffing could be mutually exclusive.