The effects of glutamate on membrane potential and discharge rate of suprachiasmatic neurons

The suprachiasmatic nucleus (SCN) is a major pacemaker for circadian rhythms in mammals. Photic entrainment of the circadian pacemaker is mediated by the retinohypothalamic tract (RHT). Most likely, excitatory amino acids function as neurotransmitters in this pathway. We have now investigated the effect of glutamate on the membrane potential of cultured SCN cells of the rat with the aid of the patch clamp technique. It was found that 1 mM glutamate depolarizes the cells by about +44 mV. In spontaneously active neurons, the glutamate induced depolarization caused either an increase in discharge or a depolarization block. We then investigated the effect of 1 mM glutamate on SCN discharge in the acutely prepared hypothalamic slice of the hamster. In most cells glutamate induced an increase in discharge whilst in a few cells discharge was suppressed. Both series of experiments indicate that glutamate in the used dosage was effective and its effect reversible. The data are discussed with respect to the failure of 1 mM glutamate injections to mimic the effect of light on the circadian activity rhythm of the hamster.