The effects of histamine (HA) and serotonin (5-HT) on single neurons of the suprachiasmatic nucleus (SCN) in hypothalamic slices were investigated by iontophoretic application. Sixty-two of 181 cells were inhibited by HA, whereas 30 were excited. Forty-nine of 163 cells were inhibited by 5-HT, whereas 9 were excited. Pyrilamine, an H1-antagonist, slightly (2/7), and cimetidine, an H2-antagonist, strongly (10/13) antagonized the inhibitory effect of HA. The excitatory effect of HA was not antagonized by cimetidine (0/4), but was perfectly antagonized by pyrilamine (5/5). The present results suggest that HA and 5-HT play mainly an inhibitory role in the SCN neuronal activity, and that inhibition and excitation induced by HA may be mediated by H2- and H1-receptors, respectively.
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