Twenty-Four-Hour Patterns in Serum Prolactin and Cortisol after Partial and Complete Isolation of the Hypothalamic-Pituitary Unit in Rhesus Monkeys*

Abstract

Serum concentrations of PRL and cortisol (F) exhibited dissimilar 24-h patterns in adult rhesus females subjected to 4-h sequential blood sampling for either 24 or 48 h. The changes in PRL concentrations appeared to show circadian rhythmicity, with daytime (0800-1600 h) levels of PRL (14.1 ± 2.6 ng⁄ml) being approximately half (27.8 ± 3.7 ng⁄ml) those found during the night (1900-0400 h). The variations in serum concentrations of F also showed a circadian rhythm; the zenith (36.5 ± 4.6 jug⁄100 ml) o ccurred near 0300-0400 h and the nadir (12.1 ± 3.2 ⁄xg⁄100 ml) occurred near 1900-2000 h. The influence of the brain on these hormonal patterns was examined by destroying various neural connections contributing to the hypothalamic- pituitary axis. After pituitary stalk transection (n = 4), both PRL and F 24-h rhythms were abolished. These losses in rhythmicity were accompanied by a 10-fold elevation in serum PRL levels and a decline in serum F to levels below those at the nadir in intact monkeys. Females (n = 5) ith an isolated medial basal hypothalamic-pituitary unit also showed disruption of the PRL and F rhythms. In three additional animals with posterior disconnections only, the normal 24-h patterns of serum PRL andF persisted. Bilateral lesions in the rostral hypothalamus (n = 6) that destroyed the suprachiasmatic nuclei in four monkeys and that partially ablated these nuclei in two animals had no influence on the 24-h patterns of either serum PRL or F. Subsequent posterior hypothalamic disconnections in five of these six monkeys disrupted the normal PRL and F rhythms. When patterns of serum F were synchronized by aligning peak steroid values, the F excursions suggested the presence of free-running rhythms. The extreme variability in serum PRL patterns within individual monkeys did not permit conclusions as to free-running rhythms. These data suggest that neither the isolated pituitary nor the isolated medial basal hypothalamic-pituitary unit is capable of supporting normal 24-h patterns in PRL and F. Removal of the neural input from the medial-anterior hypothalamic area or from posterior-lateral directions did not disrupt these two hormonal patterns. Thus, in the rhesus monkey, the rhythmic secretion of both PRL and F may require intact antero-lateral neural connections from regions outside the medial basal hypothalamus.