Excitatory and inhibitory effects of prolactin release activated by nerve stimulation in rat anterior pituitary

The anterior pituitary of rat was sliced and stimulated with electrical field in a self-designed perfusion chamber. The perfusate was continuously collected in aliquots and measured by radioimmunoassay for prolactin levels. After statistic analysis, differences of prolactin concentrations within and between groups were outlined.The results showed that stimulation at frequency of 2 Hz caused a quick enhancement of prolactin release, when stimulated at 10 Hz, prolactin release was found to be inhibited which came slower and lasted longer. The effect of nerve stimulation on prolactin release is diphasic and frequency dependent.The present in vitro study offers the first physiological evidence that stimulation of nerve fibers can affect prolactin release in rat anterior pituitary. Low frequency stimulation enhances prolactin release and high frequency mainly inhibits it.It has been well acknowledged that the anterior pituitary is regulated via hypothalamic hormones released at the median eminence [1]. Although there are small amount of autonomic vascular nerve fibers in the gland, they do not play a role of direct regulation of hormone secretion [2,3]. Since the discovery of the presence of substantial amount of nerve fibers among the anterior pituitary gland cells of monkey in late 1980s [4,5], a series of studies have been conducted showing that most of these nerve fibers are closely related to the gland cells in several mammalian species [6-10], with their varicosities in close proximity to the latter. Electron microscopic studies have proven that the nerve terminals form synapses with the anterior pituitary gland cells, which serves as a golden morphological criterion that the nerve fibers can regulate the activity of gland cells [11,12]. In the rat, typical asymmetrical synapse can be found on every type of gland cells [13]. To evaluate the importance of synapses, their number per anterior pituitary of the rat has been quantitatively studied, demonstrating that the