Regulation of melatonin-sensitivity and firing-rate rhythms of hamster suprachiasmatic nucleus neurons: pinealectomy effects.

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.