Pituicyte modulation of neurohormone output

Abstract

Pituicytes have long been suspected to play a role in the regulation of neurohypophysial hormone output. This role has been mainly ascribed to morphological changes in these cells and subsequent modifications of their tight structural relationships with surrounding nerve terminals and capillaries. These entirely reversible changes are brought about by physiological states such as parturition, lactation, or dehydration, and it was inferred that they should facilitate neurohormone output, based on concerted analyses of in vitro, in situ, and ex vivo experiments. Pituicyte stellation, the in vitro counterpart of these morphological changes, can be induced by beta-adrenergic or A1-adenosine receptor activation, and appears to result from inhibition of the small GTPase RhoA. Actin depolymerization is the key event allowing stellation. Vasopressin and oxytocin reverse stellation and return pituicytes to their basal shape by activating Cdc42, another small GTPase that reorganizes the actin cytoskeleton in a cortical position. Adenosine and neurohormones also have opposite actions on the efflux of taurine, a local messenger that is released by pituicytes in hypotonic conditions and accordingly inhibits vasopressin output from axon terminals. As adenosine is likely generated from endogenous ATP co-released with neurohormones and broken down by local ectoATPases, these data suggest a subtle balance between a positive and a negative feedback on vasopressin output operated, respectively, by adenosine and vasopressin to maintain hydromineral homeostasis. A theoretical scenario is presented to account for the putative sequence of pituicyte-related events following disturbance of the hydromineral system.

Adaptation of <i>Camelus dromedarius</i> pars nervosa of the hypophysis to winter and summer living conditions

“…Virard et al [44] suggested that the rat NL contains glial progenitors. The pituicyte is also considered an osmotic sensor involved in the neurohypophysis hormone output [45]. The heterogeneity of dromedary NL glial cells, with four different types: light and dark in winter and phagocytic and granular in summer, suggests a relationship between the light type and the protoplasmic pituicyte, and the phagocytic type and a mi- crogliocyte or a subpopulation of pituicytes.…”

Immunohistochemical Localization of the Water Channels AQP4 and AQP5 in the Rat Pituitary Gland

“…In contrast, AQP4 in the pituicytes does not seem to accumulate at the perivascular area, suggesting a different function from that in brain astrocytes. Pituicytes show remarkable morphological changes in response to physiological stimuli such as dehydration [8, 23]. Under hypotonic conditions on the other hand, it has been suggested that pituicytes sense hypotonicity, causing taurine release which inhibits vasopressin release from the axon terminal [23].…”

“…Pituicytes show remarkable morphological changes in response to physiological stimuli such as dehydration [8, 23]. Under hypotonic conditions on the other hand, it has been suggested that pituicytes sense hypotonicity, causing taurine release which inhibits vasopressin release from the axon terminal [23]. Hence, pituicytes appear to play an important role as osmotic sensors [23] and AQP4 might be necessary for pituicytes to sense hypotonic or hypertonic conditions.…”

Cellular and Subcellular Aquaporin-4 Distribution in the Mouse Neurohypophysis and the Effects of Osmotic Stimulation

“…This implicates AQP4 water channels in neurohypophyseal neuroglial interactions affecting water homeostasis not only during pathologies such as brain edema but also under physiological conditions. Indeed, pituicytes appear to be key elements in the osmoregulation process (Hussy 2002;Hussy et al 2000): These cells are sensitive to osmolar changes and have been recently described as osmotic sensors (Rosso and Mienville 2009). Possibly, AQP4 in pituicyte plasma membranes is involved in this sensor effect during osmoregulation.…”