Behavioral interactions produced by co-administration of 7-OH-DPAT with cocaine or apomorphine in the rat

Previous research from our laboratory suggests that low doses (<0.1 mg/kg) of the dopamine (DA) D₃-preferring agonist 7-hydroxy-N,N-di-n- propyl-2-aminotetralin (7-OH-DPAT) attenuate conditioned place preference (CPP) produced by the indirect DA agonist d-amphetamine, but enhance d- amphetamine-induced stereotypic behaviors. This study further examined the effects of 7-OH-DPAT on behaviors produced by the indirect DA agonist, cocaine, and the non-selective direct DA agonist, apomorphine. To examine whether 7-OH-DPAT would alter cocaine and apomorphine dose-response curves for motor behaviors and CPP, 0.1 mg/kg 7-OH-DPAT was co-administered with 0- 30 mg/kg cocaine and 0-3 mg/kg apomorphine. To establish place conditioning, drug injections were paired with one of two distinctly different compartments, whereas saline injections were paired with the other compartment. Locomotion, sniffing, oral stereotypy, and headbobbing were measured following acute and repeated drug administration during conditioning, and place conditioning was assessed 24 h following the last conditioning day. 7-OH-DPAT enhanced cocaine- and apomorphine-induced stereotypies following repeated administration. 7-OH-DPAT also attenuated cocaine-CPP, but potentiated apomorphine-CPP. Furthermore, 7-OHDPAT attenuated locomotion produced by high doses of apomorphine. The attenuation of cocaine-CPP by 7-OHDPAT likely involves stimulation of D₂/D₃ autoreceptors in the mesolimbic pathway, whereas the potentiation of apomorphine-CPP likely involves stimulation of D₂/D₃ postsynaptic receptors. Furthermore, it is suggested that attenuation of apomorphine- induced locomotion by 7-OH-DPAT likely involves stimulation of postsynaptic D₃ receptors in the mesolimbic pathway. Thus, if postsynaptic D₃ receptors are involved in mediating CPP and locomotion, then stimulation of D₃ receptors may facilitate CPP but inhibit locomotion.