Introduction
This topic covers two parts of chemical signalling in the brain. The first is the neuroendocrine pathway, the route by which hypothalamic neurons release hormones into the blood, either from the posterior pituitary or through the hypophyseal portal system to the anterior pituitary. The second is the central transmitter systems: small groups of neurons whose axons spread widely through the brain and use one transmitter each.
The hypothalamus as a neuroendocrine organ
The magnocellular neurons of the supraoptic and paraventricular nuclei produce vasopressin (antidiuretic hormone, ADH) and oxytocin. Their axons form the hypothalamo-hypophyseal tract, which runs through the infundibulum to the posterior pituitary (neurohypophysis). The hormones are stored in axon terminals there and released into the blood when the neurons fire. The posterior pituitary is therefore a storage and release site, not a gland that makes its own hormones.
The hypophyseal portal system
Parvocellular neurons in the hypothalamic nuclei, especially the arcuate and periventricular nuclei, release releasing and inhibiting hormones into the primary capillary plexus of the median eminence. The hypophyseal portal veins carry these hormones down the infundibular stalk to the secondary capillary plexus of the anterior pituitary (adenohypophysis), where they control secretion of its hormones. The anterior pituitary is therefore regulated through the blood, not by direct nerve endings.
The portal hormones act as follows. TRH releases TSH, GnRH releases LH and FSH, CRH releases ACTH, and GHRH stimulates growth hormone release, while somatostatin inhibits it. Dopamine acts as the prolactin-inhibiting factor, so prolactin secretion is normally held down.