Dynamic changes in the immunoreactivity of neuropeptide systems of the suprachiasmatic nuclei in golden hamsters during the sleep-wake cycle.

The sleep-wake cycle is virtually the most prominent circadian rhythm in mammals. In the timing system of sleep and wakefulness, the intrinsic neuropeptide systems of the suprachiasmatic nuclei (SCN) may play an important role. To elucidate this possible influence in the golden hamster, the immunoreactivity patterns of the suprachiasmatic gastrin-releasing peptide (GRP), vasoactive intestinal polypeptide (VIP), and arginine-vasopressin (AVP) systems were investigated in relation to the day-night and sleep-wake cycle by use of immunocytochemistry combined with semiquantitative planimetric analysis. For the GRP system, the highest level of immunoreactivity (expressed as area density) was observed in sleeping hamsters. Intermediate levels were found in awake, motorically active evening animals, whereas the lowest levels of immunoreactivity were detected in awake, motorically inactive hamsters studied in the morning. The immunoreactivity of the VIP system showed a completely opposite pattern, indicating highest area density in awake morning, intermediate area density in awake evening and lowest area density in sleeping golden hamsters. The immunoreactivity pattern of the AVP system, displaying highest levels in sleeping individuals, was virtually identical to that of the GRP system. Together with the related signs of neuronal activity, the present results favor an important role of these neuropeptide systems for the integration of central nervous information related to the sleep-wake cycle with photic information of the retinal input.