Daily rhythms in the hormone content of the neurohypophysial system and release of oxytocin and vasopressin in the male rat: effect of constant light

Abstract

Patterns of neurohypophysial hormone secretion and changes in the hormone content of the hypothalamus and posterior pituitary lobe were monitored in the male rat for cycles of 24 h in association with changes in food and water intake and fluid excretion. Plasma oxytocin and vasopressin concentrations were seen to rise significantly over the hours of daylight, decreasing during the night. Parallel changes were seen in the immunoreactive material in the hypothalamus, whilst the content of the neurohypophysis was inversely related to plasma concentrations. The ratio of plasma oxytocin:vasopressin reached a significant peak at about 02.00 h which might be related to the feeding activity of the rats, food and water intake being largely confined to the night, as was fluid excretion. On exposure to constant light, despite initial disruption hormonal rhythms were still seen but showed a phase shift. The relationships between plasma and tissue levels were maintained. Patterns of food and water intake and urinary excretion were little affected by exposure to constant light, remaining largely confined to the former night phase. The hormonal rhythms appeared to be more closely related to the activity of the rats, which also showed a phase shift during constant light.

Neurones in the supraoptic nucleus of the rat are regulated by a projection from the suprachiasmatic nucleus

“…The smaller proportion of responsive cells in vitro may reflect some damage to the projection during preparation of the slice. The data confirm earlier anatomical tracing studies by Stephan et al (1981) and identify a potential pathway by which the SCN might regulate the daily changes in plasma and pituitary content of oxytocin and vasopressin (Windle et al 1992). Although most SCN efferents are reported to be ipsilateral (Berk & Finkelstein, 1981;Watts et al 1987), the presence of contralateral projections from the SCN to the SON cannot be ruled out (Stephan et al 1981).…”

“…The difference suggests that neural projections from the SCN to vasopressin and oxytocin neurones in the SON are distinct. Such a difference was unexpected since the circadian profiles of plasma and pituitary levels of oxytocin and vasopressin are indistinguishable (Windle et al 1992). There are, however, other sensory inputs, including those involved with milk ejection (Lincoln & Wakerley, 1974), gastric distension (Renaud, Tang, McCann, Stricker & Verbalis, 1987) and baroreception (Harris, 1979), which are known to affect oxytocin and vasopressin cells differentially.…”

Diurnal rhythms in neurohypophysial function

“…Although there was some controversy in the literature, it is now established that secretion of oxytocin and vasopressin increases during the period of sleep in both the rat and the human . The changes in plasma hormone concentrations in the rat are reflected in changes in the posterior pituitary and hypothalamic hormone content, the pituitary content falling as plasma concentrations increase while the changes in hypothalamic content parallel the changes in circulating concentrations (Windle et al 1992). In the female the magnitude of the increase of both hormones depends on reproductive status (Peysner & Forsling, 1988;Windle & Forsling, 1993;Kostoglou-Athanassiou et al 1998a), although the changes seen in the rate of secretion of oxytocin and vasopressin with varying steroid background do not parallel each other.…”

“…A key finding in understanding possible mechanisms underlying the pattern of release was the disturbed pattern seen in rats exposed to constant light (Windle et al 1992). Because this would affect melatonin 0 Sham operated .------.…”

Inter-colony variation in fluid balance and its relationship to vasopressin secretion in male Sprague-Dawley rats

“…Since the secretion of AVP is a major factor in the control of body fluid homeostasis, this comparison also includes data for basal and stimulated plasma AVP concentrations. In addition, there is some evidence for a diurnal variation in AVP secretion (Windle et al, 1992) and therefore the patterns of water and electrolyte turnover have been investigated during the light and dark phases. An attempt was made to interpret the findings in relation to the available information on the pathogenic status and the conditions under which the animals were housed.…”