Epithelial Na⁺ sodium channels in magnocellular cells of the rat supraoptic and paraventricular nuclei

Abstract

Teruyama R, Sakuraba M, Wilson LL, Wandrey NE, Armstrong WE. Epithelial Na ϩ sodium channels in magnocellular cells of the rat supraoptic and paraventricular nuclei. Am J Physiol Endocrinol Metab 302: E273-E285, 2012. First published November 1, 2011; doi:10.1152/ajpendo.00407.2011.-The epithelial Na ϩ channels (ENaCs) are present in kidney and contribute to Na ϩ and water homeostasis. All three ENaC subunits (␣, ␤, and ␥) were demonstrated in the cardiovascular regulatory centers of the rat brain, including the magnocellular neurons (MNCs) in the supraoptic nucleus (SON) and the paraventricular nucleus (PVN). However, the functional significance of ENaCs in vasopressin (VP) and oxytocin (OT) synthesizing MNCs is completely unknown. In this study, we show with immunocytochemical double-labeling that the ␣-ENaC is colocalized with either VP or OT in MNCs in the SON and PVN. In addition, parvocellular neurons in the dorsal, ventrolateral, and posterior subregions of the PVN (not immunoreactive to VP or OT) are also immunoreactive for ␣-ENaC. In contrast, immunoreactivity to ␤-and ␥-ENaC is colocalized with VP alone within the MNCs. Furthermore, immunoreactivity for a known target for ENaC expression, the mineralcorticoid receptor (MR), is colocalized with both VP and OT in MNCs. Using single-cell RT-PCR, we detected mRNA for all three ENaC subunits and MR in cDNA libraries derived from single MNCs. In whole cell voltage clamp recordings, application of the ENaC blocker benzamil reversibly reduced a steady-state inward current and decreased cell membrane conductance approximately twofold. Finally, benzamil caused membrane hyperpolarization in a majority of VP and about one-half of OT neurons in both spontaneously firing and quiet cells. These results strongly suggest the presence of functional ENaCs that may affect the firing patterns of MNCs, which ultimately control the secretion of VP and OT. aldodsterone; vasopressin; oxytocin THE NEUROHYPOPHYSIAL HORMONES vasopressin (VP) and oxytocin (OT) are synthesized in the magnocellular neurons (MNCs) located within the paraventricular nucleus (PVN) and the supraoptic nucleus (SON) of the hypothalamus and released from the neurohypophysis into the general circulation in response to physiological demands. The secretion of VP increases in response to hyperosmolality, hypovolemia, and hypotension and produces antidiuretic and pressor effects (59). In addition to the well-known effects of OT during parturition and lactation, plasma OT also increases with hyperosmolality and hypernatremia (32) and induces natriuresis (13, 31).The non-voltage-dependent, amiloride-sensitive epithelial Na ϩ channels (ENaCs) are present in the apical membrane of epithelial cells in a variety of tissues, such as urinary bladder, renal collecting duct, distal colon, sweat and salivary glands, lung, and taste buds, and are known to mediate the transport of Na ϩ across epithelia (7, 21). Thus, together with the Na ϩ /K ϩ -ATPase present in the basal membrane of epithelial cells, ENaCs regulate transe...

ENaC-expressing neurons in the sensory circumventricular organs become c-Fos activated following systemic sodium changes

“…In addition to voltage-dependent sodium channels, it is becoming increasingly clear that ENaCs also carry a significant portion of persistent sodium leak current. ENaCs have been shown to be a significant determinant of membrane resting potential in both nonneuronal cells (7) and neuronal cells (50) where they regulate excitability in magnocellular cells of the rat supraoptic and paraventricular hypothalamic nuclei. Overall, increased ENaC activity in CVO neurons would depolarize the membrane potential and change their electrical firing pattern.…”

“…To date, the bulk of the research done on ENaCs has focused on the kidney (25), but ENaCs are present in other tissues, including in the sodium taste receptors of the tongue where they function as sodium detectors (6,31). Besides central neurons (2,50), ENaCs are expressed in astrocytes, endothelial cells, and the choroid plexus (2) raising the possibility that these channels participate in a range of different functions including the maintenance of critical Na ϩ levels in the extracellular space of the brain and spinal cord, regulation of [Na ϩ ] in the cerebrospinal fluid, and potentially detecting changes in plasma sodium levels. We were especially interested in the latter possibility.…”

Integration of thermal and osmotic regulation of water homeostasis: the role of TRPV channels

“…For example, the sodium-sensitive, NaX channel is expressed in SFO and MnPO, regions that contain Na ϩ -sensitive neurons (33,68). Also, all subunits of the epithelial Na channels (ENaC) are expressed in VP neurons (79). Temperature sensitivity of the NaX channels remains to be evaluated, but at least in expression systems ENaC has been shown to be temperature sensitive (5).…”

Mineralocorticoid-Induced Sodium Appetite and Renal Salt Retention: Evidence for Common Signaling and Effector Mechanisms

“…In areas such as the SFO, immunoreactivity was prominent only for the α- and β -subunits of ENaC and MR whereas γ-ENaC was not detected; immunoreactivity for α- and β-ENaC subunits in these regions usually paralleled MR positivity, and colocalized well with neuron marker positive cells [67]. The three ENaC subunits and MR were also colocalized in vasopressin and oxytocin-positive cells in the SON and PVN in rats [90]. Single cell PCR confirmed mRNA co-expression of all three ENaC subunits and MR in MNCs from those areas, and, by using the ENaC blocker benzamil, indirect evidence was provided that ENaCs can affect the firing patterns of MNCs [90].…”

“…The three ENaC subunits and MR were also colocalized in vasopressin and oxytocin-positive cells in the SON and PVN in rats [90]. Single cell PCR confirmed mRNA co-expression of all three ENaC subunits and MR in MNCs from those areas, and, by using the ENaC blocker benzamil, indirect evidence was provided that ENaCs can affect the firing patterns of MNCs [90]. Studies in mice confirmed the expression of all three ENaC subunits in cortex, choroid plexus, SFO, SON, and PVN [91].…”