Neural mechanisms involved in pituitary control

The final common pathway of hypothalamo-hypophyseal regulation is composed of neurosecretory neurons elaborating over 20 different neurotransmitters or neuropeptides. Cell bodies of these neurons are located in four major hypothalamic structures (supraoptic and paraventricular nuclei, the preoptic hypothalamic area, and the arcuate-ventromedial region). Their axons build up the tuberoinfundibular bundle, which innervates the median eminence and the posterior pituitary. In those structures, neurosecretory nerve terminals release their secretion product into a microcirculation across neurovascular junctions. Each of the five adenohypophyseal cell types, as well as secretory cells of the intermediate lobe, express specific receptors for several neurotransmitters, many of which are colocalised in the same neuron. In addition, most pituitary neuropeptide receptors are located on more than one cell type. Consequently, pituitary secretion is controlled by multiple neural signals. Integration of these signals by the cell is achieved by reciprocal interactions between receptor coupling mechanisms. Those involve protein complexes which activate or inhibit adenylate cyclase, as well as mediation by phospholipases. Depending upon its particular mode of coupling, each transmitter-receptor complex can determine activation of phospholipase A or C, phosphoinositide-induced opening of Ca(2+) channels, or formation of arachidonic acid, a precursor of prostaglandins and leukotrienes. The present chapter reviews the cellular distribution and the coupling pathways of major neural signals driving pituitary functions, and discusses the functional consequences of reciprocal interactions between adenylate cyclase and phospholipase modulation.