ABSTRACT
Magnocellular neurosecretory neurons in the supraoptic nucleus (SON) and paraventricular nucleus (PVN) send their axons into the posterior lobe. These neurons secrete the nonapeptides arginine vasopressin (AVP), also known as antidiuretic hormone, and oxytocin. AVP is secreted from the posterior lobe into the systemic circulation in response to an increase in extracellular fluid osmolality or reduced blood volume. The vascular organ of the lamina terminalis (OVLT) is one of the circumventricular organs of the brain which lie on the blood side of the blood–brain barrier situated at the anterior end of the hypothalamus. Osmolality-sensitive neurons in the OVLT synapse with the PVN and SON cells, increasing their background discharge when osmolality rises. Spinothalamic input causes oxytocin secretion via an undefined neural pathway from midbrain to the PVN and SON. Neurons that secrete oxytocin are distinct from those that secrete AVP. Suckling causes burst firing of oxytocinergic cells, each burst causing the release of a pulse of oxytocin.
