Single unit response of neurons within the hamster suprachiasmatic nucleus to GABA and low chloride perfusate during the day and night.

Using the in vitro hamster hypothalamic slice preparation, the effects of GABA and 80% chloride (Cl-) reduced medium on the single unit activity of SCN neurons was investigated. GABA 10(-4) M produced inhibitory responses in 55% of the 69 SCN neurons examined. No statistically significant day-night difference was observed in either the percentage of SCN units responding to GABA, or in their threshold response. During the day 80% Cl- reduced medium had an excitatory effect on SCN neurons; however, following the return to normal Cl- concentrations a transient, but significant inhibition was observed. During the night, 80% Cl- reduced medium produced an excitatory response similar to that observed during the day, but no inhibition following return to the medium containing normal Cl- concentrations. Only during the night was Cl- reduced medium found to initiate activity in a dose-dependent manner in some silent cells. No significant day-night difference in response to 80% Cl- reduced medium occurred in neurons of the paraventricular nucleus of the hypothalamus. These results indicate that SCN neurons whose activity is mediated by Cl- may be involved in the control of circadian rhythms.