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Rhythms of spontaneous firing rate and of responsiveness to pressure ejection of melatonin were recorded from neurons in the Syrian hamster suprachiasmatic nuclei (SCN) in a slice preparation. In animals taken from light-dark cycles (LD 14:10), SCN cells had high firing rates during the projected day and lower rates during the projected night. The proportion of melatonin-suppressed cells (35% overall) was also high during the day and fell during the night, while melatonin activated approximately 23% of cells at all phases. To assess the source of the melatonin-responsiveness rhythm, hamsters were exposed for approximately 48 h to constant illumination (LL) to suppress melatonin secretion. LL exposure before slice preparation altered both firing-rate and melatonin-responsiveness rhythms. Firing rates failed to show a morning peak and remained at low levels, with no indication of daily rhythmicity. Melatonin responsiveness also failed to show the usual rhythm and even tended to rise at night. Overall melatonin responsiveness rose after LL exposure so that 50% of cells were suppressed and 21% activated. LL exposure also increased the proportions of cells which showed regular baseline firing rates. Control studies indicated that pressure artifacts did not account for either suppression or activation by melatonin, while the composition of the saline vehicle appeared to be responsible for the activations recorded. The results indicate that brief LL exposure alters SCN sensitivity to melatonin and SCN rhythmicity in Syrian hamsters, perhaps as a result of the loss of the daily melatonin secretion rhythm. Physiological melatonin patterns may have important effects on the rodent circadian pacemaker.

As a test of the origin of the melatonin-sensitivity rhythm recorded from SCN cells in brain slices from intact Syrian hamsters, hamsters were either pinealectomized to remove the influence of endogenous melatonin, or sham operated. Cells from sham-operated hamsters showed a trough in responsiveness during the projected night. Pinealectomy eliminated the daily rhythm of melatonin responsiveness, reduced the proportions of cells responding to melatonin, and raised response thresholds in those cells that did not meet the criterion for responsiveness. Pinealectomy also altered the firing-rate rhythm so that the morning peak in firing rate was not sustained and the nocturnal trough was attenuated, leading to a firing-rate rhythm with reduced amplitude compared to those recorded from sham-operated or intact animals. These results indicate a role for endogenous melatonin in regulating both melatonin sensitivity and the integrity of the SCN firing-rate rhythm, and they suggest why pinealectomy can disrupt circadian organization in some situations.