Neurochemical Circuits Subserving Fluid Balance and Baroreflex A Role for Serotonin, Oxytocin, and Gonadal Steroids

Changes in body water/sodium balance are tightly controlled by the central nervous system (CNS) to avoid abnormal cardiovascular function and the development of pathological states. Every time there is a disturbance in extracellular sodium concentration or body sodium content, there is also a change in extracellular fluid volume and, depending on its magnitude, this can be associated with an adjustment in arterial blood pressure (BP). The process of sensory integration takes place in different nuclei, with diverse phenotypes and at different levels of the CNS. To control those several changes, the CNS receives continuous input about the status of extracellular fluid osmolarity, sodium concentration, sense of taste, fluid volume, and BP (Figure 9.1). Signals detected by taste receptors, peripheral osmo-sodium, volume receptors, and arterial/cardiopulmonary baroreceptors reach the nucleus of the solitary tract (NTS) by the VIIth, IXth, and Xth cranial nerves. The other main brain entry of the information related to fluid and cardiovascular balance are the lamina terminalis (LT) and one of the sensory circumventricular organs (CVOs), the area postrema (AP). The LT, consisting of the median preoptic nucleus (MnPO) and the other two sensory CVOs—i.e., subfornical organ (SFO) and organum vasculosum of the lamina terminalis (OVLT) —is recognized as a site in the brain that is crucial for the physiological regulation of hydroelectrolyte balance. The SFO and OVLT lack a blood– brain barrier and contain cells that are sensitive to humoral signals, such as changes in plasma and cerebrospinal fluid sodium concentration (Vivas et al. 1990), osmolality (Sladek and Johnson 1983), and angiotensin II (ANG II) levels (Ferguson and Bains 1997; Simpson et al. 1978). Such unique features make the SFO and OVLT key brain regions for sensing the status of the body fluids and electrolytes. Humoral and neural signals that arrive to the two main brain entries—that is, the CVOs of the LT and within the hindbrain the AP-NTS—activate a central circuit that includes integrative areas such as the MnPO, the paraventricular (PVN), the supraoptic (SON), lateral parabrachial nucleus (LPBN), dorsal raphe nucleus (DRN), and neurochemical systems such as the angiotensinergic, vasopressinergic, oxytocinergic (OT), and serotonergic (5-HT) systems (Figures 9.1 and 9.2). Once these signals act on the above-mentioned neurochemical networks, they trigger appropriate sympathetic, endocrine, and behavioral responses. Therefore, after a body fluid deficit, water and sodium intake and excretion need to be controlled to minimize disturbances of hydromineral homeostasis. In this context, hypovolemia and hyponatremia …