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Dehydration increases L-type calcium channel density in the somata of magnocellular neurosecretory cells in rats

Permission to Use In presenting this thesis in partial fulfilment of the requirements for a Postgraduate degree from the University of Saskatchewan, I agree that the Libraries of this University may make it freely available for inspection. I further agree that permission for copying of this thesis in any manner, in whole or in part, for scholarly purposes may be granted by the professor or professors who supervised my thesis work or, in their absence, by the Head of the Department or the Dean of the College in which my thesis work was done. It is understood that any copying or publication or use of this thesis or parts thereof for financial gain shall not be allowed without my written permission. It is also understood that due recognition shall be given to me and to the University of Saskatchewan in any scholarly use which may be made of any material in my thesis. iii ABSTRACT The magnocellular neurosecretory cells (MNCs) of the hypothalamus are responsible for the synthesis and secretion of vasopressin (VP), which is important for fluid homeostasis, and oxytocin (OT), which is responsible for uterine contraction during parturition and milk let-down during lactation. VP-ergic MNCs undergo a number of structural and functional changes during dehydration, including the adoption of a bursting pattern of firing, the retraction of glial processes from MNC somata and terminals, the translocation of κ-opioid receptors from internal stores to the plasma membrane, and the somatodendritic release of VP and OT. Since voltage-gated Ca 2+ channels have been found on intracellular granules, and since an increase in Ca 2+ current could regulate firing patterns and neuropeptide release, the surface expression of Ca 2+ channel subtypes in MNCs was tested to determine if it would be altered by 16-24 hours of water deprivation. Using radioligand binding of antagonists of N-type and L-type Ca 2+ channels, channel density was measured in the supraoptic nucleus (SON), which is largely composed of MNC somata, and in the neurohypophysis (NH), which is largely composed of MNC terminals. Dehydration caused an increase in the density of L-type channels in the SON, while causing no significant change in the N-type density. No change in density of either channel type was observed in the NH. Electrophysiological measurements in isolated MNC somata showed no change in total Ca 2+ current, but a significant increase in the nifedipine-sensitive current following dehydration. Reverse transcription-polymerase chain reaction …