OxytocinSynthesis, Secretion, and Reproductive Functions

This chapter focuses on the peptide hormone oxytocin and its reproductive functions. Oxytocin plays a highly integrated role in reproductive physiology and allows the hormone to control reproductive functions at multiple levels. The integrative reproductive functions are based on multiple modes of action. First, oxytocin acts like the uterus and mammary gland. The oxytocin ergicmagno cellular neurons of the hypothalamus express the oxytocin gene at a high rate and store the mature peptide hormone in the posterior pituitary gland. One level of control is the synthesis and secretion of oxytocin. Whereas the transcription of the oxytocin gene is affected by external factors and physiological conditions, the regulatory range of gene expression and biosynthesis is limited to a few-fold, due to the high basal oxytocin mRNA content. Only after chronic repression of oxytocin gene transcription, as during long-lasting hyponatremia, can gene expression be up-regulated dramatically. In contrast, the release of oxytocin from the posterior pituitary gland is highly dynamic with large amplitude. The synchronized bursting of oxytocin neurons results in pulses of massively released oxytocin. These are required for milk ejection on demand and the promotion of parturition. The activities of oxytocin as a releasing hormone for prolactin and luteinizing hormone (LH) in the anterior pituitary gland can also be linked to the same hypothalamic neurons and augment milk production indirectly through stimulated synthesis and secretion of prolactin.