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

Abstract

The sensory circumventricular organs (CVOs) are specialized collections of neurons and glia that lie in the midline of the third and fourth ventricles of the brain, lack a blood-brain barrier, and function as chemosensors, sampling both the cerebrospinal fluid and plasma. These structures, which include the organum vasculosum of the lamina terminalis (OVLT), subfornical organ (SFO), and area postrema (AP), are sensitive to changes in sodium concentration but the cellular mechanisms involved remain unknown. Epithelial sodium channel (ENaC)-expressing neurons of the CVOs may be involved in this process. Here we demonstrate with immunohistochemical and in situ hybridization methods that ENaC-expressing neurons are densely concentrated in the sensory CVOs. These neurons become c-Fos activated, a marker for neuronal activity, after various manipulations of peripheral levels of sodium including systemic injections with hypertonic saline, dietary sodium deprivation, and sodium repletion after prolonged sodium deprivation. The increases seen c-Fos activity in the CVOs were correlated with parallel increases in plasma sodium levels. Since ENaCs play a central role in sodium reabsorption in kidney and other epithelia, we present a hypothesis here suggesting that these channels may also serve a related function in the CVOs. ENaCs could be a significant factor in modulating CVO neuronal activity by controlling the magnitude of sodium permeability in neurons. Hence, some of the same circulating hormones controlling ENaC expression in kidney, such as angiotensin II and atrial natriuretic peptide, may coordinate ENaC expression in sensory CVO neurons and could potentially orchestrate sodium appetite, osmoregulation, and vasomotor sympathetic drive.

5-HT neurons of the area postrema become c-Fos-activated after increases in plasma sodium levels and transmit interoceptive information to the nucleus accumbens

“…2A, sodium levels were maintained between 137 to149 meq/l, which approximates the normal range of 141-147 meq/l that has been reported for Sprague-Dawley male rats. Similar plasma sodium values were obtained in an earlier study in which plasma sodium levels were 146 Ϯ 0.58 meq/l, and plasma osmolality was 297.58 Ϯ 0 .72 mosmol/kgH 2 O 2 h after intraperitoneal injections of 2 M NaCl (29). In the present study, plasma osmolality ranged from 290 to 307 mosmol/ kgH 2 O, which closely matches the normal values (294 -313 mosmol/kgH 2 O), as reported by Hilltop Laboratories (http:// hilltoplabs.com/public/blood.html).…”

“…A chicken c-Fos antibody (1:1,250, ab14285; Abcam, Cambridge, MA) was also used, and it was made against a synthetic peptide corresponding to amino acids 168 -380 of human c-Fos. In an earlier study, the labeling efficiency of these two antibodies were compared; the chicken antibody colabeled ϳ90% of the c-Fos-labeled neurons detected with the rabbit antibody (29).…”

“…Nineteen rats were used for measurement of plasma sodium levels after various manipulations: sodium deprivation (8 days), 2 h of sodium repletion with 0.9% M NaCl after 8 days of sodium deprivation, and 2 h of sodium repletion with 3.0% M NaCl after 8 days of sodium deprivation. In addition, plasma sodium measurements from the hypertonic saline experiments were made in an earlier study (29), and all of these data are presented in Fig. 5.…”

Expression of mineralocorticoid and glucocorticoid receptors in preautonomic neurons of the rat paraventricular nucleus

“…Sympathetic drive is greater on a LS diet, compared with HS diet, including in humans (64,72,104). Neurons of the subfornical organ and organum vasculosum of the lamina terminalis, circumventricular organs that project to the PVN and modulate SNA activity, are also activated by a chronic LS diet (76), thus probably contributed significantly to the increased activity in the preautonomic neurons in response to a LS diet in our present studies. The moderate activation of preautonomic neurons of the PVN by chronic low salt intake is adaptive for terrestrial animals, most of which do not have ready access to large amounts of sodium and, thus, would experience situations in which the hemodynamic support by the SNS would be required to maintain blood pressure.…”

“…Activation of the SNS in response to a sodium intake is complex, multifactorial with multiple signals converging upon the preautonomic neurons, including efferents from neurons of the circumventricular organs outside of the blood-brain barrier that sense changes in plasma and cerebrospinal fluid osmolality and/or sodium concentrations, and mediators of the RAAS (76,96). c-Fos is a transcription factor commonly used as a functional marker of neuron activation.…”