The electrophysiological effects of melatonin and a putative melatonin antagonist (N-acetyltryptamine) on rat suprachiasmatic neurones in vitro.

Several studies have implicated the suprachiasmatic nuclei (SCN) as a target for the action of melatonin in its regulation of seasonal and circadian behaviour. Single-unit activity from the SCN and adjacent paraventricular area and anterior hypothalamus was recorded using the in vitro rat hypothalamic slice preparation. Neurones were tested for responses to iontophoresed or pressure ejected melatonin (n = 62) and serotonin (5-HT, n = 80). The majority (n = 24-26) of melatonin-sensitive SCN neurones were inhibited by melatonin in a dose-dependent manner during the latter portion of the projected light phase of the circadian light-dark cycle. A putative melatonin antagonist, N-acetyltryptamine, exhibited concentration dependent mixed agonist-antagonist effects on melatonin-evoked responses.