Introduction

It should not have been surprising that gonadal steroids exert powerful influences on cell–cell interactions in the magnocellular hypothalamoneurohypophyseal system (HNS) of the rat, but somehow it was. Since this system is responsible for the manufacture and release of oxytocin during parturition and lactation, times when the levels of circulating gonadal steroids show dramatic variations, some steroid involvement in the functioning of HNS could have been anticipated. Perhaps such anticipation was dulled by the observation that the dynamic interactions taking place among the cells of the HNS also occurred in response to manipulations of the animal's hydrational state, which have not been associated traditionally with variations in gonadal steroid output. However, gonadal steroids appear to exert some control over the cellular mechanisms that release both oxytocin and vasopressin in response to dehydration. Estrogens and androgens, under comparable conditions, often have opposite effects on the HNS. This chapter reviews the main structure–function relationships of the HNS, the dynamics of these relationships under physiological conditions of altered peptide hormone demand, and some of the roles possibly played in these functions by gonadal steroids in both males and females.

The magnocellular HNS

The magnocellular HNS is constituted chiefly by the supraoptic (SON) and paraventricular (PVN) nuclei, accessory nuclei in the anterior hypothalamus, and the neurohypophysis or neural lobe (NL) of the posterior pituitary to which the neurons of those hypothalamic nuclei send axonal projections (Fig. 18.1).