NaX Channel Is a Physiological [Na+] Detector in Oxytocin- and Vasopressin-Releasing Magnocellular Neurosecretory Cells of the Rat Supraoptic Nucleus

The Scn7A gene encodes NaX, an atypical noninactivating Na+ channel, whose expression in sensory circumventricular organs is essential to maintain homeostatic responses for body fluid balance. However, NaX has also been detected in homeostatic effector neurons, such as vasopressin (VP)-releasing magnocellular neurosecretory cells (MNCVP) that secrete VP (antidiuretic hormone) into the bloodstream in response to hypertonicity and hypernatremia. Yet, the physiological relevance of NaX expression in these effector cells remains unclear. Here, we show that rat MNCVP in males and females is depolarized and excited in proportion with isosmotic increases in [Na+]. These responses were caused by an inward current resulting from a cell-autonomous increase in Na+ conductance. The Na+-evoked current was unaffected by blockers of other Na+-permeable ion channels but was significantly reduced by shRNA-mediated knockdown of Scn7A expression. Furthermore, reducing the density of NaX channels selectively impaired the activation of MNCVP by systemic hypernatremia without affecting their responsiveness to hypertonicity in vivo These results identify NaX as a physiological Na+ sensor, whose expression in MNCVP contributes to the generation of homeostatic responses to hypernatremia.SIGNIFICANCE STATEMENT In this study, we provide the first direct evidence showing that the sodium-sensing channel encoded by the Scn7A gene (NaX) mediates cell-autonomous sodium detection by MNCs in the low millimolar range and that selectively reducing the expression of these channels in MNCs impairs their activation in response to a physiologically relevant sodium stimulus in vitro and in vivo These data reveal that NaX operates as a sodium sensor in these cells and that the endogenous sensory properties of osmoregulatory effector neurons contribute to their homeostatic activation in vivo.

High dietary salt amplifies osmoresponsiveness in vasopressin-releasing neurons

High dietary salt increases arterial pressure partly through activation of magnocellular neurosecretory cells (MNCVP) that secrete the antidiuretic and vasoconstrictor hormone vasopressin (VP) into the circulation. Here, we show that the intrinsic and synaptic excitation of MNCVP caused by hypertonicity are differentially potentiated in two models of salt-dependent hypertension in rats. One model combined salty chow with a chronic subpressor dose of angiotensin II (AngII-salt), the other involved replacing drinking water with 2% NaCl (salt loading, SL). In both models, we observed a significant increase in the quantal amplitude of EPSCs on MNCVP. However, model-specific changes were also observed. AngII-salt increased the probability of glutamate release by osmoreceptor afferents and increased overall excitatory network drive. In contrast, SL specifically increased membrane stiffness and the intrinsic osmosensitivity of MNCVP. These results reveal that dietary salt increases the excitability of MNCVP through effects on the cell-autonomous and synaptic osmoresponsiveness of MNCVP.

Trpv4 Mediates Hypotonic Inhibition of Central Osmosensory Neurons via Taurine Gliotransmission

The maintenance of hydromineral homeostasis requires bidirectional detection of changes in extracellular fluid osmolality by primary osmosensory neurons (ONs) in the organum vasculosum laminae terminalis (OVLT). Hypertonicity excites ONs in part through the mechanical activation of a variant transient receptor potential vanilloid-1 channel (dn-Trpv1). However, the mechanism by which local hypotonicity inhibits ONs in the OVLT remains unknown. Here, we show that hypotonicity can reduce the basal activity of dn-Trpv1 channels and hyperpolarize acutely isolated ONs. Surprisingly, we found that mice lacking dn-Trpv1 maintain normal inhibitory responses to hypotonicity when tested in situ. In the intact setting, hypotonicity inhibits ONs through a non-cell-autonomous mechanism that involves glial release of the glycine receptor agonist taurine through hypotonicity activated anion channels (HAAC) that are activated subsequent to Ca²⁺ influx through Trpv4 channels. Our study clarifies how Trpv4 channels contribute to the inhibition of OVLT ONs during hypotonicity in situ.

Peripubertal gonadal steroids are necessary for steroid-independent male sexual behavior in castrated B6D2F1 male mice

Gonadal steroids play an integral role in male sexual behavior, and in most rodent models, this relationship is tightly coupled. However, many other species, including humans, continue to demonstrate male sex behavior in the absence of gonadal steroids, and the mechanisms that regulate steroid-independent male sex behavior are not well understood. Approximately 30% of castrated male B6D2F1 hybrid mice display male sex behavior many months after castration, allowing for the investigation of individual variation in steroidal regulation of male sex behavior. During both the perinatal and peripubertal periods of development, the organizational effects of gonadal steroids on sexual differentiation of the neural circuits controlling male sex behavior are well-documented. Several factors can alter the normal range of gonadal steroids or their receptors which may lead to the disruption of the normal processes of masculinization and defeminization. It is unknown whether the organizational effects of gonadal hormones during puberty are necessary for steroid-independent male sex behavior. However, gonadal steroids during puberty were not necessary for either testosterone or estradiol to activate male sex behavior in adulthood. Furthermore, activation of male sex behavior was initiated sooner in hybrid male mice castrated prior to puberty that were administered estradiol in adulthood compared to those that were provided testosterone. The underlying mechanisms by which gonadal hormones, during both the perinatal and peripubertal developmental periods of sexual differentiation, organize the normal maturation of neural circuitry that regulates steroid-independent male sex behavior in adult castrated B6D2F1 male mice warrants further investigation.