Circadian properties of vasopressin and melatonin rhythms in cat cerebrospinal fluid

Using a method for continuous removal of cisternal cerebrospinal fluid (CSF) from freely moving cats, we delineated the circadian nature of the daily rhythm in CSF arginine vasopressin. The daily melatonin rhythm was also monitored in CSF as another marker of circadian function. Under diurnal lighting conditions, both hormones exhibited prominent daily rhythms; the CSF vasopressin rhythm was characterized by high daytime values, whereas the CSF melatonin rhythm was characterized by high nighttime levels. In contrast, drinking behavior exhibited a 24-h component in only one of four animals studied. Daily CSF rhythms of vasopressin and melatonin persisted for over 78 days of study in constant light. The vasopressin rhythm clearly free-ran in this environment, manifesting cycle lengths of slightly greater than 24 h. The daily melatonin pattern split into several components with increasing time in constant light. An acute 8-h phase delay in the daily light-dark cycle resulted in corresponding but gradual phase shifts in both rhythms. These results indicate that both the vasopressin and melatonin rhythms in cat CSF are endogenously generated and are entrained by the daily light-dark cycle.