Integration of Hypernatremia and Angiotensin II by the Organum Vasculosum of the Lamina Terminalis Regulates Thirst.

The organum vasculosum of the lamina terminalis (OVLT) contains NaCl-sensitive neurons to regulate thirst, neuroendocrine function, and autonomic outflow. The OVLT also expresses the angiotensin II (AngII) type1 receptor, and AngII increases Fos expression in OVLT neurons. The present study tested whether individual OVLT neurons sensed both NaCl and AngII to regulate thirst and body fluid homeostasis. A multifaceted approach, including in vitro whole-cell patch recordings, in vivo single-unit recordings, and optogenetic manipulation of OVLT neurons, was used in adult, male Sprague Dawley rats. First, acute intravenous infusion of hypertonic NaCl or AngII produced anatomically distinct patterns of Fos-positive nuclei in the OVLT largely restricted to the dorsal cap versus vascular core, respectively. However, in vitro patch-clamp recordings indicate 66% (23 of 35) of OVLT neurons were excited by bath application of both hypertonic NaCl and AngII. Similarly, in vivo single-unit recordings revealed that 52% (23 of 44) of OVLT neurons displayed an increased discharge to intracarotid injection of both hypertonic NaCl and AngII. In marked contrast to Fos immunoreactivity, neuroanatomical mapping of Neurobiotin-filled cells from both in vitro and in vivo recordings revealed that NaCl- and AngII-responsive neurons were distributed throughout the OVLT. Next, optogenetic excitation of OVLT neurons stimulated thirst but not salt appetite. Conversely, optogenetic inhibition of OVLT neurons attenuated thirst stimulated by hypernatremia or elevated AngII but not hypovolemia. Collectively, these findings provide the first identification of individual OVLT neurons that respond to both elevated NaCl and AngII concentrations to regulate thirst and body fluid homeostasis.SIGNIFICANCE STATEMENT Body fluid homeostasis requires the integration of neurohumoral signals to coordinate behavior, neuroendocrine function, and autonomic function. Extracellular NaCl concentrations and the peptide hormone angiotensin II (AngII) are two major neurohumoral signals that regulate body fluid homeostasis. Herein, we present the first compelling evidence that individual neurons located in the organum vasculosum of the lamina terminalis detect both NaCl and AngII. Furthermore, optogenetic interrogations demonstrate that these neurons play a pivotal role in the regulation of thirst stimulated by NaCl and AngII. These novel observations lay the foundation for future investigations for how such inputs as well as others converge onto unique organum vasculosum of the lamina terminalis neurons to coordinate body fluid homeostasis and contribute to disorders of fluid balance.