Partial agonistic effects of OPC-14597, a potential antipsychotic agent, on yawning behavior in rats

The present experiments were performed to examine the behavioral effects of OPC-14597, which acts on dopamine receptors in rats. OPC-14597 administered subcutaneously (SC) at doses of 0.1-5 mg/kg elicited yawning, as did OPC-4392 (0.5-2 mg/kg, SC) and (-)-3-PPP (2.5-10 mg/kg, SC). These yawning responses were blocked by intraperitoneal (IP) pretreatment with haloperidol (0.5 mg/kg) but were increased by pindolol (20 mg/kg, IP) or reserpine (5 mg/kg, IP), which per se did not elicit yawning. The yawning induced by talipexole, a selective dopamine D2 receptor agonist, was inhibited by OPC-14597 (0.5-5 mg/kg, SC) and (-)-3-PPP (10 mg/kg, SC). Apomorphine (0.5 mg/kg, SC), a dopamine D1/D2 receptor agonist, elicited stereotypy such as sniffing and licking but OPC-14597 (5-20 mg/kg, SC) did not induce this behavior. The stereotypy induced by apomorphine was inhibited not only by haloperidol (0.5 mg/kg, IP) and (-)-3-PPP (10 mg/kg, SC) but also by OPC-14597 (5-20 mg/kg, SC), without being affected by OPC-4392 (2 mg/kg, SC). In 6-hydroxydopamine (6-OHDA)-treated rats, apomorphine (0.5 mg/kg, SC) elicited rotation behavior whereas OPC-14597, OPC-4392 and (-)-3-PPP did not produce this behavior. These findings suggest that OPC-14597 provokes yawning without causing stereotypy and rotation but markedly antagonizes the talipexole-induced yawning and apomorphine-induced stereotypy, and that OPC-14597 thus exerts partial agonistic effects on yawning behavior but antagonistic effects on stereotypy in rats.