Nocturnal synthesis of the pineal hormone melatonin (MEL) is regulated by the circadian clock in the suprachiasmatic nucleus (SCN) of the hypothalamus. We examined the hypothesis that MEL can feed back to regulate the SCN using a brain slice preparation from rat. We monitored the SCN ensemble firing rate and found that MEL advanced the time of peak firing rate by more than 3 h at restricted circadian times (CTs) near subjective dusk [CT 10-14 (10-14 h after lights on)] and dawn (CT 23-0) on days 2 and 3 after treatment. The effect of MEL at CT 10 was blocked by pertussis toxin. The protein kinase C (PKC) activator, 12-O-tetradecanoylphorbol 13-acetate, reset the SCN firing rate rhythm with a profile of temporal sensitivity congruent with that of MEL. Two specific PKC inhibitors, calphosin C and chelerythrine chloride, independently blocked MEL-induced phase advances at each sensitive period. Furthermore, MEL administration increased PKC phosphotransferase activity transiently o 200% at CT 10 and CT 23, but not at CT 6. These data demonstrate that 1) MEL can directly modulate the circadian timing of the SCN within two windows of sensitivity corresponding to dusk and and 2) MEL alters SCN cellular function via a pertussis toxin-sensitive G protein pathway that activates PKC.
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