BEHAVIORAL EVIDENCE THAT DIFFERENT NEUROCHEMICAL MECHANISMS UNDERLY STRETCHING-YAWNING AND PENILE ERECTION INDUCED IN MALE-RATS BY SND-919,A NEW SELECTIVE D-2 DOPAMINE-RECEPTOR AGONIST

The behavioural effects induced in male Wistar rats by SND 919, a new drug reputed to have selective agonistic activity at D-2 dopamine (DA) receptors, were studied. The following aspects of behaviour were cons idered: motor activity, stretching-yawning (SY), penile erection (PE) and stereotyped behaviour (SB). Intraperitoneal injection (IP) of the drug (0.01-20 mg/kg) induced an SY syndrome in the form of a bell-shap ed dose-response curve, the effect being maximal at the dose of 0.1 mg /kg and disappearing completely at 10 mg/kg. SND 919 also potently eli cited PE; this latter effect, however, was not coincident with SY indu ction, being maximal at 1 mg/kg and persisting at 10 and 20 mg/kg. SND 919-induced SY was potently antagonized by pretreatment not only with the D-2 antagonist, L-sulpiride (20 mg/kg), but also with the alpha(2 ) antagonist, yohimbine (1, 3 mg/kg), and the more selective alpha(2) antagonist, idazoxan (1, 2 and 5 mg/kg). While sulpiride also decrease d SND 919-induced PE, idazoxan at all doses and yohimbine at 1 mg/kg d id not affect this behaviour. Inhibition of motor activity was induced by the D-2 agonist at low doses (0.05, 0.1 mg/kg), while at high dose s (1, 10 and 20 mg/kg), it was actually replaced by a form of SB chara cterized by downward sniffing and licking. When, for comparison, the D -2 agonist, RU 24213 (0.1-20 mg/kg IF), was tested for PE, SY, motor a ctivity and SB, it displayed a behavioural pattern very similar to tha t obtained with SND 919. Idazoxan (2 mg/kg), administered before RU 24 213 (10 mg/kg), significantly antagonized the drug-induced SY, but not PE. The discussion centres on the specific neurochemical mechanisms p resumably underlying the various forms of SND 919-induced behaviour an d, in particular, PE and SY, which seem to differ, at least with respe ct to alpha(2) involvement.