Novel anxiolytics discriminate between postsynaptic serotonin receptors mediating different physiological responses on single neurons of the rat hippocampus

The effects of buspirone on hippocampal pyramidal cells of the CA1 region were examined by means of intracellular recordings in in vitro hippocampal brain slices. Bath administration of buspirone elicited a long lasting hyperpolarization which was mediated by an increase in potassium conductance and resembled the hyperpolarizing component of the response to 5-HT (5-hydroxytryptamine). Buspirone, however, failed to mimic the depolarizing action of 5-HT or to reduce the calcium-activated after hyperpolarization. Quantitative comparisons of the hyperpolarizing responses of 5-HT and buspirone revealed that the maximal hyperpolarization induced by buspirone was significantly smaller than that induced by 5-HT. Since the buspirone induced hyperpolarization was also accompanied by a surmountable antagonism of 5-HT responses, these results indicate that buspirone behaves as a partial agonist at a subpopulation of 5-HT receptors in the CA1 region of the hippocampus. Administration of the buspirone congeners gepirone and isapirone also elicited a hyperpolarization and reduced 5-HT responses, although they lack antidopaminergic activity, indicating that the effects observed with buspirone are unlikely to be mediated through dopamine receptors. These results indicated that novel anxiolytics can discriminate between functional 5-HT receptors. In conjunction with previous biochemical and electrophysiological studies, the present results suggest that their administration might alter the balance of serotonergic actions on postsynaptic neurons.