Circling evoked by intranigral SKF 38393: a GABA-mediated D-1 response?

Intranigral injection of microgram doses of the dopamine D-1 receptor agonist SKF 38393, in rats acutely anaesthetised with halothane, did not overtly alter the animals' behaviour. The adenylate cyclase activator forskolin, the GABA uptake inhibitor nipecotic acid and the GABA potentiator pentobarbital, were similarly ineffective when administered singly to one substantia nigra pars reticulata. However, all three treatments interacted synergistically with coinjected SKF 38393 to promote active circling. It is suggested this SKF 38393-induced behavioural response is mediated by GABA released from D-1 receptor-bearing striatonigral neurones.