Investigations on day-night differences of vesicle densities in synapses of the rat suprachiasmatic nucleus.

The present study was conducted to test whether the well-known circadian alterations in physiological and metabolical parameters of the hypothalamic suprachiasmatic nucleus (SCN) are accompanied by day-night differences in the number of vesicles in intrinsic synapses of the nucleus. Two groups of 5 adult male rats each were killed at mid-light or mid-dark, respectively, by perfusion with Karnovsky's fluid. The SCN were removed and processed for routine electron microscopy. In medial parts of the nucleus, synapses were characterized as being of Gray type I (asymmetrical), Gray type II (symmetrical) or of intermediate form, and the vesicles per synaptic profile (VPSP) were counted over a defined area. It was found that the ratio of different types of synapses did not differ between day and night. Median VPSP numbers were slightly augmented in animals killed at mid-dark when compared to those obtained from mid-light rats, this difference, however, lacks statistical significance. The present results suggest that vesicle density as a morphological parameter does not clearly parallel the circadian changes in SCN metabolism.