Hypothalamic–pituitary–prolactin axis

Tuberoinfundibular dopaminergic neurons The cell bodies of TIDA neurons are located in the arcuate nucleus (infundibular nucleus) of the mediobasal hypothalamus. Their axons project to the external zone of the median eminence, where dopamine is released into the fenestrated capillaries of the primary portal plexus. First identified by Kjell Fuxe in 1963–1964, TIDA neurons are distinguished from nigrostriatal and mesolimbic dopamine populations by their neuroendocrine function. TIDA neurons display intrinsic oscillatory activity and are regulated by gonadal steroids and by prolactin itself. In the normal adult pituitary, lactotrophs constitute approximately 15–25% of anterior pituitary cells; this proportion rises to as high as 50% during pregnancy and lactation owing to estrogen-driven hyperplasia. Lactotrophs are electrically excitable, firing spontaneous action potentials accompanied by calcium transients that sustain continuous exocytosis. Severing or compressing the pituitary stalk eliminates dopamine delivery and causes prolactin levels to rise sharply, an effect opposite to that seen with every other anterior pituitary hormone. == Regulation of prolactin secretion ==

Because lactotrophs are constitutively active, prolactin secretion operates through a "release from inhibition" model. Several inhibitory and stimulatory factors modulate the system. Inhibitory factors Dopamine Dopamine is the principal prolactin-inhibiting factor. It acts on D2 receptors (D2R) on the lactotroph membrane. Two D2R isoforms (D2L and D2S) couple to Gi/Go proteins and produce inhibition through multiple time-dependent mechanisms: Somatostatin Somatostatin acts as a secondary inhibitor of prolactin release, counteracting TRH- and VIP-stimulated secretion. GnRH-associated peptide GnRH-associated peptide (GAP), a 56-amino-acid peptide cleaved from the GnRH precursor, was shown in 1985 to inhibit prolactin secretion in rat pituitary cultures at a potency comparable to dopamine. Its physiological significance in vivo remains uncertain, as results have varied across species. Stimulatory factors No single dominant prolactin-releasing hormone has been identified, which reinforces the primacy of inhibitory control in this axis. In primary hypothyroidism, elevated TRH stimulates both TSH and prolactin, producing hyperprolactinaemia in approximately 20–40% of hypothyroid patients. However, TRH-knockout mice display normal prolactin levels, indicating that TRH is a modulator rather than an obligate releasing factor. During pregnancy, rising estrogen levels contribute to the physiological expansion of the lactotroph population. Over 12–16 hours, prolactin increases tyrosine hydroxylase expression and activity, elevating dopamine synthesis. == Physiological functions ==

Lactation Prolactin is the primary lactogenic hormone. It drives mammary gland development (mammogenesis), milk synthesis (lactogenesis), and maintenance of milk production. PRLR-knockout mice show absent mammary development. During pregnancy, high estrogen and progesterone promote ductal and lobuloalveolar growth but suppress milk secretion; withdrawal of these steroids at parturition permits prolactin-driven lactogenesis. Suckling activates a neuroendocrine reflex: afferent signals from mechanoreceptors inhibit TIDA dopamine release (raising prolactin) while simultaneously triggering oxytocin release for the milk ejection reflex. This mechanism underlies lactational amenorrhoea: elevated prolactin during breastfeeding physiologically inhibits ovulation. This is an ancient function of prolactin, conserved across vertebrate evolution. In mammals, the osmoregulatory role is less prominent but includes effects on amniotic fluid regulation and renal sodium handling. ==See also==