Dynamics of spontaneous beating of cardiomyocytes in vitro depend on the number of involved cells

The hypothalamic suprachiasmatic nucleus (SCN) is the predominant pacemaker of the mammalian brain that generates and controls circadian rhythms of various endocrine and behavioral processes. Different lines of evidence suggest that stress interferes with the maintenance of such rhythms. As a first approach to investigate whether the neuropeptide arginine vasopressin (AVP), which shows circadian rhythms of synthesis and release within the SCN, might contribute to this stress-induced alterations in circadian rhythms, we monitored acute effects of swim stress on the intra-SCN release of AVP in male rats by means of the microdialysis technique. A 10-min forced swimming session triggered a marked but relatively short-lasting increase in the intranuclear release of AVP (to approx. 440%). This effect was restricted to the area containing predominantly somata and dendrites of vasopressinergic neurons, since no changes in AVP release could be measured in one of their major projection areas, the nucleus of the dorsomedial hypothalamus. Our data provide evidence that the amount of AVP released within the SCN can vary widely not only in accordance with AVP's intrinsically regulated circadian rhythm but also in response to a physiologically relevant stressor. In this way, the neuropeptide may contribute to the regulation of endocrine and behavioral rhythms particularly in challenging situations associated with resettings of the endogenous clock.