Pituicyte

Pituicytes are the main cell type within the posterior pituitary gland, a neuroendocrine gland at the base of the brain from which the hormones oxytocin and vasopressin are secreted. The pituicytes do not themselves make these hormones; oxytocin and vasopressin are synthesised by large neuroendocrine neurons of the hypothalamus whose axons project into the posterior pituitary gland. The pituicytes are closely associated with these axons, they express receptors by which they respond directly to oxytocin and vasopressin, and their morphology can change dramatically in different physiological states. Exactly what their role is remains incompletely understood. They are thought to have an influence upon secretion of vasopressin and oxytocin, in particular by releasing the inhibitory amino acid taurine in response to hypo-osmotic challenge . They are thought to be important for the growth of the neurosecretory axons, including regrowth of axons after damage . They are also thought to be involved in the autophagic destruction of damaged axons.

Pituicytes are the principal cell type intrinsic to the posterior pituitary gland, making up 25-30% of the volume of the gland. Most express glial fibrillary acidic protein, which identifies them as a subpopulation of astrocytic glial cells. Like other astrocytes, they are extensively interconnected by prominent gap junctions, via which they are electrotonically coupled. The cytoplasm contains abundant Golgi bodies, free ribosomes and lipid bodies. The pituicytes have long processes that extend among the neurosecretory axons that innervate the lobe and onto the basement lamina of the perivascular space. The morphology of the pituicytes changes dramatically in different physiological states.

Because of this morphological plasticity, pituicytes have long been suspected to have a role in the regulation of neurohypophysial hormone secretion. These morphological changes are apparent in physiological states such as parturition, lactation, and dehydration. In pituicytes cultured in vitro, similar morphological changes ("stellation") involving actin depolymerization can be induced by beta-adrenergic or A1-adenosine receptor activation, and appear to result from inhibition of the small GTPase RhoA. Pituicytes express specific receptors for the neurohypophysial hormones vasopressin and oxytocin, both of which can reverse stellation and return pituicytes to their basal shape by activating Cdc42, another small GTPase that reorganizes the actin cytoskeleton. Pituicytes also respond to dynorphin, an opioid peptide that is co-secreted with both vasopressin and oxytocin Adenosine and neurohormones also have opposite actions on the efflux of taurine, a local messenger that is released by pituicytes in hypotonic conditions and which can inhibits vasopressin secretion from axon terminals.