Day-night differences in the sensitivity of adrenoceptors in the Syrian hamster pineal gland: an in vivo iontophoretic study.

Investigations on the regulation of pineal melatonin synthesis in the Syrian hamster revealed distinct differences compared to this well-understood mechanism in rat. E.g., a circadian profile of pineal norepinephrine (NE) is absent, there is no beta-adrenoceptor sensitivity during daytime and adrenergic receptor supersensitivity is not easily achieved. To elucidate the action of NE on pineal receptor sites, the effects of iontophoretic application of adrenergic compounds on spontaneous electrical discharge rates of pinealocytes were investigated during day- and nighttime. Following application of either NE, isoproterenol or clonidine, cells were activated, inhibited or not affected. Whereas about one-third of the units responded to iontophoretic application of sympathomimetics at daytime, the number of affected cells was doubled during the night. These results demonstrate the involvement of adrenoceptors in the regulation of circadian rhythms in electrophysiological properties of Syrian hamster pinealocytes. Since relatively few cells responded during daytime and inhibitory adrenergic mechanisms dominated at night, the classification of receptors by means of iontophoresis may provide a basis to explain the difficulty to influence pineal melatonin synthesis by sympathomimetics in the Syrian hamster.