Regulation of chloride co-transporter expression and function in magnocellular vasopressin and oxytocin neurons by osmotic stress
Magnocellular vasopressin and oxytocin neurons are critically involved in fluid homeostasis. Studies have shown that a chronic osmotic challenge can induce a depolarizing shift in the GABA equilibrium potential (EGABA) in vasopressin neurons, which modulates the strength and sometimes even the valence of GABAergic synaptic signaling. I sought to elucidate the molecular mechanisms that regulate EGABA in vasopressin and oxytocin neurons. Specifically, I studied the molecular regulation of the potassium-chloride and sodium-potassium chloride co-transporters KCC2, NKCC1, and NKCC2, whose activity is responsible for setting the EGABA in neurons and whose expression and activity are highly regulated. Immunohistochemical analysis revealed low expression of the chloride exporter KCC2 in vasopressin neurons of normally hydrated rats, suggesting that the EGABA is shifted positive in vasopressin neurons under baseline conditions; the expression levels of the chloride importers NKCC1 and NKCC2 were relatively low and comparable between oxytocin and vasopressin neurons. Following osmotic challenge with chronic salt loading, a Western blot analysis showed no change in KCC2 expression, but an increase in NKCC1 expression. Despite the lack of change in total KCC2 expression after salt loading, there was an enhanced membrane-localization, suggesting trafficking of KCC2 to the membrane. Surprisingly, this was specific to oxytocin neurons and correlated with an increase in phosphorylation of KCC2 at serine 940 and at threonine 1007. Consistent with regulation of oxytocin neuron activity, salt loading caused a significant decrease in circulating oxytocin, and no change in circulating vasopressin. These data reveal a profound molecular regulation of oxytocin neurons by a chronic osmotic stress, and suggest that oxytocin may be a valid target for the treatment of hypertension accompanied by hypernatremia.