In order to elucidate the role of cyclic nucleotides in the suprachiasmatic nucleus, we examined the effect of dibutyryl cyclic AMP (c-AMP) and dibutyryl cyclic GMP (c-GMP) on single neuronal activity of the suprachiasmatic nucleus of the rat. A hypothalamic slice about 300 μm thick, including the suprachiasmatic nucleus, was cut coronally using a vibratome. By means of bath- as well as iontophoretic application, c-AMP and c-GMP caused mainly an inhibition and facilitation in suprachiasmatic neuronal activity, respectively. The present results suggest that c-AMP and c-GMP play an important role in regulation of suprachiasmatic neuronal activity as a second messenger and that these two nucleotides may function as a reciprocal messenger.
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