Characterization of a novel osmotically-evoked phospholipase C pathway and Slack and Slick-mediated Na+-activated K+ currents in rat supraoptic neurons

Abstract

The magnocellular neurosecretory cells (MNCs) of the hypothalamus are important players in systemic osmoregulation that strives to stabilize water and salt levels inside the mammalian body. In accordance with the physiological osmotic needs, the MNCs adopt changes in their electrical activity to regulate the systemic release of vasopressin (VP) and oxytocin (OT) hormones, which act on the kidneys to control urine and sodium excretion, respectively. Although MNCs are known to exhibit osmotically-evoked changes in their membrane bound TRPV1 channels and intracellular cytoskeleton proteins, which confer them intrinsic osmosensitive properties, the identity and role of osmotically-evoked changes in their second messenger systems are less clear. In the first part of this Ph.D. thesis, I present evidence for the presence of a novel osmotically-evoked and Ca2+-dependent phospholipase C (PLC) signaling pathway in the rat MNCs using immunocytochemical methods. Using patch clamp methods, I have also shown that this osmotically-evoked PLC pathway acts in a feed forward manner to potentiate the intrinsic osmosensitivity of MNCs by contributing to the activation of TRPV1 channels. Although MNCs are known to adopt changes in their electrical activity in accordance with the physiological needs of VP and OT, the mechanisms regulating the transition in their electric behaviour are unclear. It is therefore important to identify all the different ion channels that are possibly present in MNCs and could potentially contribute to their electrical behave;/. In the second part of this Ph.D. thesis, using patch clamp methods I have demonstrated novel evidence for the presence of Na+-activated K+ (KNa) channels in rat MNCs. Since KNa channels contributes to activity-dependent after hyperpolarizations (AHPs) and shaping of firing behaviour in other neurons, it is possible that they could also play an important role in regulating the electrical behaviour of MNCs. In summary, this Ph.D. thesis contributes to a better understanding of mechanisms that regulates both the osmotic physiology and electric behaviour of the MNCs and thus the overall process of systemic osmoregulation in the body.