Inhibitory effect of activation of GABAA receptor in the central nucleus of amygdala on the sodium intake in the sodium-depleted rat

Knockdown of the type 1 cannabinoid receptor in the central amygdala increases both spontaneous and water deprivation-induced sodium intake in rats

Important inputs originating in the forebrain circumventricular organs and also in the central amygdala (CeA) trigger essential water deprivation (WD)-induced behaviors, such as thirst and sodium appetite. Together with the secretion of the neurohypophysial peptides arginine vasopressin (AVP) and oxytocin (OT), these behavioral responses seek to maintain the normalcy of extracellular fluid (ECF) osmolality and volume. Within this context, the main hypothesis tested by the present study was that CeA type 1 cannabinoid receptors (CB1Rs) were essential for the maintenance of body fluid homeostasis, particularly in response to WD challenge. We found that CeA CB1R knockdown increased spontaneous and WD-induced hypertonic saline intake, without significantly impacting water ingestion. In euhydrated rats, despite unaltered urinary volume, CB1R knockdown reduced urinary osmolality, and diminished urinary nitrate concentrations, suggesting reduced renal sodium excretion. No relevant changes were induced by CeA CB1R knockdown on urinary parameters following WD-induced rehydration, which is consistent with unaltered AVP and OT mRNA transcription and hormone release under the same experimental conditions. Taken together, the present data support the notion that CeA CB1Rs participate in both spontaneous and WD-induced NaCl intake, without significantly affecting neuroendocrine output. Given the well-described facilitatory CeA role on natriorexigenic responses, and the reported interplay between CB1Rs and γ-aminobutyric acid (GABA) within the CeA, the present findings suggest that CB1Rs may indirectly regulate sodium appetite through effects on CeA GABAergic neurotransmission.NEW & NOTEWORTHY CeA CB1R knockdown increased spontaneous and WD-induced hypertonic saline intake, without significantly impacting water ingestion. In euhydrated rats, despite unaltered urinary volume, CB1R knockdown reduced urinary osmolality, and diminished urinary nitrate concentrations, suggesting reduced renal sodium excretion. No relevant changes were induced by CeA CB1R knockdown on urinary parameters following WD-induced rehydration, which is consistent with unaltered AVP and OT mRNA transcription and hormone release under the same experimental conditions.

Central regulation of body fluid homeostasis

Extracellular fluids, including blood, lymphatic fluid, and cerebrospinal fluid, are collectively called body fluids. The Na⁺ concentration ([Na⁺]) in body fluids is maintained at 135-145 mM and is broadly conserved among terrestrial animals. Homeostatic osmoregulation by Na⁺ is vital for life because severe hyper- or hypotonicity elicits irreversible organ damage and lethal neurological trauma. To achieve "body fluid homeostasis" or "Na homeostasis", the brain continuously monitors [Na⁺] in body fluids and controls water/salt intake and water/salt excretion by the kidneys. These physiological functions are primarily regulated based on information on [Na⁺] and relevant circulating hormones, such as angiotensin II, aldosterone, and vasopressin. In this review, we discuss sensing mechanisms for [Na⁺] and hormones in the brain that control water/salt intake behaviors, together with the responsible sensors (receptors) and relevant neural pathways. We also describe mechanisms in the brain by which [Na⁺] increases in body fluids activate the sympathetic neural activity leading to hypertension.

The purinergic mechanism of the central nucleus of amygdala is involved in the modulation of salt intake in sodium-depleted rats

The central nucleus of the amygdala (CeA) is a critical region in regulating sodium intake, and interestingly, purinergic receptors reportedly related to fluid balance, are also expressed in CeA. In this study, we investigated whether the purinergic mechanisms of CeA were involved in regulating sodium intake. Male Sprague-Dawley rats had cannulas implanted bilaterally into the CeA and were sodium depleted with furosemide (FURO 20 mg/kg) plus 24 h-sodium deficient food fed. Bilateral injections of the P2X purinergic agonist, α,β-methyleneadenosine 5'-triphosphate (α,β-methylene ATP 1.0, 2.0, 4.0 nmol, respectively) into the CeA region induced dose-related reductions in sodium intake without affecting water intake. Injection of P2X purinergic antagonist, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS 4.0 nmol/0.5 μl) into the CeA region did not alter sodium and water intake, however, prior injection of PPADS into the CeA area abolished the inhibitory effects on sodium intake by α,β-methylene ATP. Interestingly, prior injection of γ-aminobutyric acid type A (GABA_A) receptor antagonist, bicuculline (4.0 nmol/0.5 μl) into the CeA region partially reversed the deficit of sodium intake induced by α,β-methylene ATP. These results suggest that purinergic receptors in the CeA are involved in the control of sodium intake in the sodium-depleted rats and this negative modulation may be, at least partly, mediated by the GABA_A receptor.

AT1 receptor blockade in the central nucleus of the amygdala attenuates the effects of muscimol on sodium and water intake

The blockade of the central nucleus of the amygdala (CeA) with the GABAA receptor agonist muscimol significantly reduces hypertonic NaCl and water intake by sodium-depleted rats. In the present study we investigated the effects of previous injection of losartan, an angiotensin II type-1 (AT1) receptor antagonist, into the CeA on 0.3M NaCl and water intake reduced by muscimol bilaterally injected into the same areas in rats submitted to water deprivation-partial rehydration (WD-PR) and in rats treated with the diuretic furosemide (FURO). Male Sprague-Dawley rats with stainless steel cannulas bilaterally implanted into the CeA were used. Bilateral injections of muscimol (0.2 nmol/0.5 μl, n=8 rats/group) into the CeA in WD-PR-treated rats reduced 0.3M NaCl intake and water intake, and pre-treatment of the CeA with losartan (50 μg/0.5 μl) reversed the inhibitory effect of muscimol. The negative effect of muscimol on sodium and water intake could also be blocked by pretreatment with losartan microinjected into the CeA in rats given FURO (n=8 rats/group). However, bilateral injections of losartan (50 μg/0.5 μl) alone into the CeA did not affect the NaCl or water intake. These results suggest that the deactivation of CeA facilitatory mechanisms by muscimol injection into the CeA is promoted by endogenous angiotensin II acting on AT1 receptors in the CeA, which prevents rats from ingesting large amounts of hypertonic NaCl and water.