The posterior pituitary (neurohypophysis) is composed exclusively of neural tissue and is a direct extension of the hypothalamus. It lies directly adjacent to the adenohypophysis but is embryologically distinct, derived from an invagination of neuroectodermal tissue in the third ventricle. Axons in the posterior pituitary originate from neurons with cell bodies in two distinct regions of the hypothalamus, the supraoptic and paraventricular nuclei, named for their anatomic relationship to the optic chiasm and the third ventricle. Together these two nuclei compose the hypothalamic magnocellular system. These neurons can secrete their synthetic products directly from axonal boutons into the general circulation to act as hormones. This is the mechanism of secretion of the hormones of the posterior pituitary, oxytocin and arginine vasopressin (AVP). Although this is the primary mode of release for these hormones, numerous other secondary pathways were identified, including secretion into the portal circulation, intrahypothalamic secretion, and secretion into other regions of the CNS (46).
In addition to the established functions of oxytocin and vasopressin, several other diverse roles were suggested in animal models. These functions include modulation of sexual activity and appetite, learning and memory consolidation, temperature regulation, and regulation of maternal behaviors (47). In the human, these neuropeptides were linked to social attachment (48–50). Receptor variants for these two molecules were linked to the spectrum of autistic disorders, suggesting that proper function of these two neuropeptides with their receptors is required for positive group interactive behavior. This relationship is strengthened by a strong association between altruistic behavior and the length of the AVP-1a receptor promoter region (51). It is both surprising and humbling that complex human behaviors may be partially explained by such a relatively simple neuropeptide system. Continuing investigation should help elucidate this physiology and potential therapeutic interventions.
Oxytocin Oxytocin is a 9–amino acid peptide primarily produced by the paraventricular nucleus of the hypothalamus (Fig. 7.7). The primary function of this hormone in humans is the stimulation of two specific types of muscular contractions (Fig. 7.8). The first type, uterine muscular contraction, occurs during parturition. The second type of muscular contraction regulated by oxytocin is breast lactiferous duct myoepithelial contractions, which occur during the milk letdown reﬂex. Oxytocin release may be stimulated by suckling, triggered by a signal from nipple stimulation transmitted via thoracic nerves to the spinal cord and then to the hypothalamus, where oxytocin is released in an episodic fashion (45). Oxytocin release also may be triggered by olfactory, auditory, and visual clues, and it may play a role in the conditioned reﬂex in nursing animals. Stimulation of the cervix and vagina can cause significant release of oxytocin, which may trigger reﬂex ovulation (the Ferguson reﬂex) in some species.
Figure 7.7 Oxytocin and arginine-vasopressin (AVP) are 9–amino acid peptides produced by the hypothalamus. They differ in only two amino acids.
Figure 7.8 Oxytocin stimulates muscular contractions of the uterus during parturition and the breast lactiferous duct during the milk letdown reﬂex. Arginine-vasopressin (AVP) regulates circulating blood volume, pressure, and osmolality.