GABA(A)-MEDIATED LOCAL SYNAPTIC PATHWAYS CONNECT NEURONS IN THE RAT SUPRACHIASMATIC NUCLEUS

The suprachiasmatic nucleus (SCN) in mammals functions as the biologic al clock controlling circadian rhythms, but the synaptic circuitry of the SCN is largely unexplored. Most SCN neurons use the neurotransmitt er gamma-aminobutyric acid (GABA), and anatomic studies indicate many GABAergic synapses and local axon collaterals; however, physiological evidence for synaptic communication among SCN neurons is indirect. We have used three approaches to investigate local circuitry in the SCN i n acute hypothalamic slices from rat. First, tetrodotoxin was used to block action-potential-dependent synaptic release, which resulted in a decrease in the frequency of spontaneous synaptic currents in SCN neu rons, suggesting that spontaneously active neurons in the slice connec t synaptically to SCN neurons. Postsynaptic currents in SCN neurons we re also evoked by the selective stimulation of other SCN neurons with glutamate, which avoids direct activation of axons that might originat e outside the SCN. Two different methods of glutamate microapplication (i.e., pressure ejection and ultraviolet photolysis of caged glutamat e) indicated that SCN neurons receive GABA(A)-receptor-mediated synapt ic input from other SCN neurons. In contrast, glutamate-receptor-media ted synaptic connections between SCN neurons were not detected. The GA BAergic synapses that comprise the network described here could concei vably be a substrate for the synchronization and amplification of the circadian rhythm of SCN firing. Alternatively, this circuitry might me diate other aspects of clock function such as the integration of envir onmental and physiological information.