The external layer of the median eminence: A neurovascular synapse

The external layer of the median eminence is comprised of nerve fibres that terminate on the primary plexus of the hypophysial portal vessels. This system is a unique neurovascular synapse which can be used as a window through which may be studied the characteristics of central neurotransmission. The heterogeneity of the neuropil of the external layer of the median eminence, in terms of neurotransmitter types, carries the advantage that interactions between different classes of neurons can be investigated. These points are illustrated by physiological and pharmacological studies on the release into hypophysial portal blood of several neuropeptides including luteinizing hormone releasing hormone, thyrotrophin releasing hormone, vasoactive intestinal peptide, somatostatin 14, 28 and (1-12) 28, vasopressin and oxytocin. The portal vessels convey these neurohormones to the anterior pituitary gland where they stimulate or inhibit the release of pituitary hormones. The way in which the pituitary gland may be used to investigate the "post synaptic" effects of neuropeptides is illustrated with special reference to luteinizing hormone releasing hormone. In addition to the "established" neurohormones, attention is also focused on a recently discovered hypothalamic pituitary system that controls the release of plasminogen activator and on thyrocalcitonin and the calcitonin gene related peptide which demonstrate the way in which studies of the hypothalamus have led to the discovery of new neural control mechanisms.