Magnocellular neurosecretory cells are largeneuroendocrine cells within thesupraoptic nucleus andparaventricular nucleus of thehypothalamus. They are also found in smaller numbers in accessory cell groups between these two nuclei, the largest one being thecircular nucleus.[1] There are two types of magnocellularneurosecretory cells,oxytocin-producing cells andvasopressin-producing cells, but a small number can produce bothhormones. These cells areneuroendocrineneurons, areelectrically excitable, and generateaction potentials in response to afferent stimulation.[2] Vasopressin is produced from the vasopressin-producing cells via theAVP gene, a molecular output of circadian pathways.[citation needed]
Magnocellular neurosecretory cells in rats (where these neurons have been most extensively studied) in general have a single long varicose axon, which projects to theposterior pituitary. Each axon gives rise to about 10,000 neurosecretory terminals and many axon swellings that store very large numbers of hormone-containing vesicles.[3] These vesicles are released from the axon swellings and nerve terminals byexocytosis in response to calcium entry throughvoltage-gated ion channels, which occurs whenaction potentials are propagated down the axons.[4]
The cells typically have two or three long dendrites, which also contain large dilations and a very high density of hormone-containing vesicles. Oxytocin and vasopressin can, thus, be released within the brain from these dendrites, as well as into the blood from the terminals in the posterior pituitary gland.[5] However, the release of oxytocin and vasopressin from dendrites is not consistently accompanied by peripheral secretion, as dendritic release is regulated differently. Dendritic release can be triggered by depolarisation, but can also be triggered by the mobilisation of intracellular calcium stores. The dendrites receive most of the synaptic inputs from afferent neurons that regulate the magnocellular neurons; typically a magnocellular neuron receives about 10,000 synapses from afferent neurons.
The activity of magnocellular neurosecretory cells is regulated by local glial cells as well as through themselves (intrinsically). Their activity is also dependent on reproductive, osmotic, and cardiovascular inputs.[6]