The functions of the D 4 receptor, a newly cloned D2-1ike receptor, as well as the identity of cells expressing it, are still poorly defined. Using quantitative polymerase chain reaction we detected the messenger RNA of the D4, but not other D2-1ike receptor, in cultured granule cells from neonatal rat cerebellum. In these neurons, dopamine reduced high-voltage-activated calcium current, with a pharmacology corresponding to that of the D 4 receptor. The response declined from one to three days, when calcium currents were mostly sensitive to nifedipine, to 15 days, when nifedipine-insensitive calcium currents were also present and D 4 receptor messenger RNA had declined. The dopamine response was abolished after pretreatment of the cells by pertussis toxin, was potentiated and made irreversible by infusion of guanosine 5'-O-(3-thiotriphosphate) but persisted in the presence of cyclic AMP and isobutylmethylxanthine.

The molecular mechanisms regulating GABA, receptor activity in cultured frog rnelanotrophs were studied using the patch-clamp technique. In the whole-cell configuration, application of GABA evoked a dose-related increase of inward chloride currents. The ED, value, estimated from the sigmoidal dose-response curve was 2 x M and the Hill coefficient was 1.55. The amplitude of the GABA-induced current decayed with time, Kinetics analysis of the desensitization revealed that the time-course of the current decrement was fitted by one exponential. Graded doses of GABA or association of GABA with the benzodiazepine receptor agonist flunitrazepam accelerated the desensitization process. In contrast, the time-course of the current did not significantly vary at different holding potentials. In the outside-out configuration, GABA was found to activate channels which displayed three unitary conductance levels (8, 15 and 30 pS). The channel openings of the more frequent conductance level (30 pS) exhibited short and long lasting open states (1.2 and 28.3 ms at -60mV). Altogether these data reveal that frog melanotrophs possess a single population of GABA, receptors which interconvert into a higher affinity state in the presence of benzodiazepine receptor agonists. Two GABA molecules must bind to the receptor to trigger long lasting channel openings. In addition, the activity of the GABA,receptor appears to be independent of the accumulation of intracellular chloride ions.