This chapter describes peptidergic transmitters of the suprachiasmatic nuclei and the control of circadian rhythmicity and the plasma corticosterone rhythm, because corticotropin-releasing factor (CRF) neurons, regulating the release of adrenocorticotropin hormone (ACTH) and corticosterone, are located in the paraventricular nucleus (PVN). The close relationship between the changes in plasma ACTH and corticosterone levels indicates that the inhibitory effect of vasopressin (VP) involves the hypothalamo-pituitary pathway. Together with the data on the circadian rhythmicity of VP levels in the cerebrospinal fluid (CSF), which are inversely related to the rhythm of plasma corticosterone, the results show how the suprachiasmatic nucleus (SCN) might be able to induce the circadian fluctuations in the plasma levels of this important steroid hormone. The chapter proposes a hypothesis that VP released from the SCN projections may serve to inhibit arousal during the inactive part of an animal's sleep/waking cycle. However, studies involving other transmitters of the SCN may provide further insight in how this small nucleus in the basal hypothalamus is able to modulate such a variety of physiologically important functions. © 1992, Academic Press Inc.
