The effect of neuropeptide Y (NPY) and vasopressin (VP) on the firing rates of neurons in the retinorecipient portion of the rat suprachiasmatic nucleus (SCN) was studied by superfusion onto hypothalamic slices in vitro. NPY produces either excitation or excitation followed by inhibition when applied during subjective day. With increasing doses of NPY, the number of neurons showing an excitatory response alone remains stable, and the neurons increase firing rates slightly. In contrast, the number of neurons showing an excitatory re sponse followed by inhibition is greater with increased NPY concentration, and the inhibitory effect is prolonged. VP produces only excitatory responses, and these are increased with increasing VP concentration. Prolonged administration of NPY produces an overall decrease in firing rate during subjective day, but has no effect during subjective night. In contrast, the excitatory effects of VP are essentially the same in both subjective day and subjective night. The responses to NPY and VP are preserved in calcium-free medium, indicating that the peptides act directly on SCN neurons. Optic nerve stimulation produces excitation in 36% of neurons in the retinorecipient component of the SCN, and 80% of these respond to NPY, whereas only 29% of neurons that do not respond to optic nerve stimulation are affected by NPY. In contrast, both groups respond equally well to VP. These observations indicate that neurons in the component of the rat SCN receiving retinal afferents respond differently to NPY and VP, and that responses to NPY are complex and dependent upon the circadian time at which NPY is administered.
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