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Abstract
An extremely close association exists between the membranes of the neurosecretory
endings and the resident astrocytes (pituicytes) of the neurohypophysis. Indeed, synaptoid
contacts involving neurosecretory vesicle-containing axons contacting pituicytes have
been observed, suggesting pituicytes as targets of the products released from neurosecretory
axons. We have investigated the effects of various neural lobe peptides on pituicytes
in primary culture from adult neurohypophyses. Using Fura-2 loaded cells and dynamic
ratio imaging, we have determined that arginine vasopressin (AVP) or V1- but not V2-receptor
agonists, mobilise pituicyte intracellular Ca2+ ([Ca2+]i) in the absence of extracellular
Ca2+. AVP was consistently effective at concentrations of 10 nM or higher in elevating
[Ca2+]i by 200-1000 nM. These responses could be blocked by V1-antagonists and were
shown to be associated with accumulation of phosphoinositides. Oxytocin was also found
to mobilise [Ca2+]i but was effective only at higher concentrations than for AVP.
Oxytocin-evoked [Ca2+]i elevations were also blocked by V1-antagonists. Raising [K+]0
was ineffective in changing [Ca2+]i suggesting that these cells lack voltage-gated
Ca2+ channels. We conclude that pituicytes possess V1-receptors, activation of which
mobilises [Ca2+]i, possibly functioning to initiate a Ca(2+)-activated K+ conductance
which could contribute to further depolarisation of secretory terminals and facilitate
exocytosis.