c-Fos expression in brain in response to hypotension and hypertension in conscious rats

Mediation by N-methyl- d-aspartate and non- N-methyl- d-aspartate receptors in the expression of fos protein at the nucleus tractus solitarii in response to baroreceptor activation in the rat

We investigated the role of glutamatergic synapses in the expression of Fos protein at the nucleus tractus solitarii following baroreceptor activation in rats anaesthetized with pentobarbital sodium. Microinjection of L-glutamate (1 nmol) bilaterally into the nucleus tractus solitarii elicited significant hypotension and bradycardia. There was a concurrent increase, as determined immunohistochemically, in the expression of Fos protein at the commissural, medial and dorsomedial subnuclei of the caudal nucleus tractus solitarii. These effects were blunted when L-glutamate was co-administered with either the selective N-methyl-D-aspartate or non-N-methyl-D-aspartate glutamate receptor antagonist, dizocilpine maleate (200 pmol) or 6-cyano-7-nitroquinoxaline-2,3-dione (8 pmol), into the caudal nucleus tractus solitarii. Repeated and scheduled transient hypertension evoked by phenylephrine (2.5, 5.0 or 10.0 microg/kg, i.v.) also appreciably increased the number of Fos-immunoreactive neurons at the commissural, medial and dorsomedial subnuclei of the caudal nucleus tractus solitarii. The expression of Fos protein in this fashion was reduced, simultaneous with a discernible depression in baroreceptor reflex response, when baroreceptor activation was coupled with microinjection bilaterally of dizocilpine maleate (200 pmol) or 6-cyano-7-nitroquinoxaline-2,3-dione (8 pmol) into the nucleus tractus solitarii. Regression analysis showed that the depressive action on the baroreceptor reflex response by both glutamate receptor antagonists correlated positively to the reduction in Fos-immunoreactivity in the nucleus tractus solitarii after baroreceptor activation. Double immunohistochemical staining revealed that nucleus tractus solitarii neurons that showed Fos immunoreactivity were generally also immunoreactive to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor subunit 1. On the other hand, Fos immunoreactivity was usually absent from neurons in the nucleus tractus solitarii that were immunoreactive to N-methyl-D-aspartate receptor subunit 1. These results suggest that glutamatergic neurotransmission plays an active role, via comparable contributions from both N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors, in the expression of Fos protein at the caudal nucleus tractus solitarii in response to baroreceptor activation.

Circumventricular organs and fever

We have examined the roles of three circumventricular organs, the area postrema, the subfornical organ, and the organum vasculosum of the lamina terminalis (OVLT), as possible access points for circulating pyrogens to cause fever. In conscious, unrestrained rats prepared with telemetry devices, intracerebroventricular cannulas, and intravenous catheters, body temperature was monitored after intravenously administered lipopolysaccharide and, on a different occasion, after intracerebroventricular prostaglandin E1. Lipopolysaccharide-induced fevers in sham control lesioned rats were indistinguishable from those observed in animals with lesions of the area postrema, the OVLT, or the tissue immediately adjacent to this structure (peri-OVLT). In contrast, rats with lesions of the subfornical organ displayed reduced fevers. In none of the groups of lesioned animals were prostaglandin E1 fevers reduced. Thus lesions did not interfere with central thermogenic pathways responsive to prostaglandin. Our results indicate that subfornical organ neurons respond to circulating pyrogens and through their efferent projections activate central pathways involved in fever.

Decreases in arterial pressure activate oxytocin neurons in conscious rats

Hemorrhage and nonhypotensive hypovolemia are known to increase plasma levels of oxytocin (OT) and vasopressin (VP) in rats. The present experiments demonstrated that secretion of OT and VP also are stimulated by acute drug-induced hypotension. Injection of hydralazine abruptly decreased arterial blood pressure in conscious rats and induced Fos expression, a marker of neuronal activation, within OT and VP neurons in the hypothalamus. Hydralazine also elicited substantial increases in plasma levels of both OT and VP. Injection of chlorisondamine similarly elicited acute hypotension and increased plasma levels of OT and VP. Furthermore, when the hypotensive effect of chlorisondamine was blunted by coinfusion of phenylephrine, the induced increases in OT and VP were markedly attenuated. Across all treatments, arterial blood pressure was inversely related to plasma levels of OT and VP. Plasma osmolality was not increased by hydralazine, nor was there evidence of gastric malaise, two known stimuli for OT secretion in rats. These results suggest that arterial hypotension increases neurohypophysial release of OT and VP in conscious rats.

Activation by hypotension of neurons in the hypothalamic paraventricular nucleus that project to the brainstem

To investigate the involvement of neuronal nitric oxide (NO) in the response of the brain to changes in blood pressure, we studied the activation of putative NO-producing neurons in the paraventricular nucleus of the hypothalamus (PVN) in rats whose mean arterial pressures (MAPs) were decreased by 40-50% with hemorrhage (HEM) or infusion of sodium nitroprusside (NP). Activation was assessed on the basis of expression of the immediate early gene, c-fos; putative NO-producing neurons were identified with the histochemical stain for nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d); and the proportions of neurons projecting to the nucleus of the tractus solitarius (NTS) and/or caudal ventrolateral medulla (CVLM) were determined with retrograde tracing techniques. No differences were found for results obtained from HEM and NP animals. Three to four percent of activated PVN neurons projected to the NTS or CVLM. Conversely, approximately 33% and 16% of neurons projecting to the NTS and CVLM, respectively, were activated. About 43% of NADPH-d neurons in the PVN were activated. Of PVN neurons projecting to the NTS or CVLM, 38% and 32%, respectively, were NADPH-d positive. About 11% of NADPH-d PVN neurons projected to the NTS or CVLM. An average of 3 NADPH-d neurons per section were activated and projected to either target. Finally, 7 PVN cells per section sent collateral branches to the NTS and CVLM; 2 or 3 of these cells per section were also activated by decreases in arterial pressure. No NADPH-d cells were found that sent collateral branches to the NTS and CVLM. This study shows that decreases in MAP activate PVN neurons that project, singly and through collaterals, to the NTS and CVLM. A relatively high proportion of the singly projecting neurons is NADPH-d positive. These results support the contention that descending projections from the PVN to the brainstem play an important role in the physiological response to decreases in arterial pressure and suggest that NO may participate in this response.

Fos immunohistochemical determination of brainstem neuronal activation in the muskrat after nasal stimulation

Stimulation of the nasal passages of muskrats with either ammonia vapours or retrogradely-flowing water produced cardiorespiratory responses (an immediate 62% decrease in heart rate, 29% increase in mean arterial blood pressure, and sustained expiratory apnoea). We used the immunohistological detection of Fos, the protein product of the c-fos gene, as a marker of neuronal activation to help elucidate the brainstem circuitry of this cardiorespiratory response. After repeated ammonia stimulation of the nasal passages, increased Fos expression was detected within the spinal trigeminal nucleus (ventral laminae I and II of the medullary dorsal horn, ventral paratrigeminal nucleus, and spinal trigeminal nucleus interpolaris), an area just ventromedial to the medullary dorsal horn, the caudal dorsal reticular formation and the area of the A5 catecholamine group compared to control animals. Repeated water stimulation of the nasal passages produced increased Fos expression only in the A5 catecholamine group. There was an increase in the number of Fos-positive cells in the ammonia group in the ventral laminae I and II of the medullary dorsal horn and the ventral paratrigeminal nuclei compared with the water group. We conclude that ammonia stimulation of the nasal passages produces a different pattern of neuronal activation within the brainstem compared with water stimulation. We also conclude that Fos immunohistochemistry is a good technique to determine functional afferent somatotopy, but that immunohistochemical detection of Fos is not a good technique to identify the medullary neurons responsible for the efferent aspects of an intermittently produced cardiorespiratory reflex.

Central amygdala Fos expression during hypotensive or febrile, nonhypotensive endotoxemia in conscious rats

The distribution and time course of Fos expression in neurons in the central nucleus of the amygdala (CeA) were studied in endotoxemic rats in two separate experiments. In each case, the severity of the endotoxin (lipopolysaccharide; LPS) challenge was characterized by using physiological outcome variables, including blood pressure and heart rate. Throughout the rostrocaudal extent of the CeA, extensive Fos staining was found 3 hours after injection with a hypotensive dose of Salmonella enteritidis LPS. Hypotension alone has been reported to induce Fos in the CeA; therefore, the remaining experiments were performed by using a lower dose of Escherichia coli LPS that did not cause hypotension. The nonhypotensive dose of E. coli LPS also induced Fos in large numbers of neurons of the CeA, with peak staining at 2 hours and Fos staining persisting for 6 hours after LPS injection. Tachycardia and fever after LPS also persisted for at least 6 hours. CeA Fos staining during nonhypotensive endotoxemia was predominantly located in the lateral subnucleus, although Fos-stained medial sub-nucleus neurons were also present. Additional forebrain regions that showed persistent Fos staining 6 hours after LPS included the parvocellular paraventricular nucleus of the hypothalamus, the bed nucleus of the stria terminalis, and the medial preoptic area. Forebrain regions that contained Fos-stained nuclei at earlier time points, but not at 6 hours, included the supraoptic nucleus, magnocellular regions of the paraventricular nucleus of the hypothalamus, the subfornical organ, and the organum vasculosum of the lamina terminalis. Few CeA neurons showed Fos staining in rats that were restrained in a ventilated plastic tube. Neurons in the lateral septal nucleus and the medial amygdaloid nucleus, which have numerous Fos-stained nuclei after stressors such as footshock or restraint, did not show Fos staining above control levels after LPS administration. Activation of CeA neurons after intravenous LPS may indicate persistent drive from vagal afferents and may implicate the CeA in the autonomic, neuroendocrine, and/or behavioral responses to this treatment.

Regional expression of c-fos in rat brain following stimulation of the ventral tegmental area

This study has investigated the effect of stimulating the region of origin of the mesolimbic dopaminergic system, the ventral tegmental area (VTA), with the substance P analogue DiMe-C7 on the regional expression of c-fos in the rat forebrain. We have previously shown this treatment produced a prolonged increase in blood pressure and heart rate which was mediated by both dopaminergic mechanisms and vasopressin release. Stimulation of the VTA resulted in increased levels of c-Fos immunostaining in several target regions of the mesolimbic dopaminergic system (such as the frontal cortex, olfactory tubercle, islands of Calleja and amygdala), with the notable exception of the nucleus accumbens. A marked increase in c-fos expression was also found in the supraoptic nucleus but not the paraventricular nucleus in the hypothalamus. These results support a role for a number of target areas of the mesolimbic dopaminergic system and vasopressin release in the increase in blood pressure and heart rate produced by stimulation of the VTA.

Losartan inhibition of angiotensin-related drinking and Fos immunoreactivity in hypertensive and hypotensive contexts

We assessed the ability of acute peripheral administration of the AT-1 receptor antagonist, losartan, to reverse both the water intake and Fos-immunoreactivity (Fos-ir) induced in rats by either peripheral or cerebroventricular (i.c.v.) administration of angiotensin (Ang) II. We compared this with endogenous generation of Ang II during either hypovolemia or hypotension. Relatively low doses of losartan blocked the dipsogenic effect of peripherally administered exogenous Ang II, but a higher dose (20 mg/kg) was needed to block the dipsogenic effect of i.c.v.-administered Ang II. Fos-ir induced by i.c.v. Ang II was attenuated in SFO and SON by 10-20 mg losartan/kg given peripherally, but Fos-ir in the MnPO and PVN was unaffected. These findings suggest that losartan has limited permeability into the brain. We used peripheral losartan to assess the contribution of Ang II to water intake and Fos-ir responses to peripheral injection of either polyethylene glycol (PEG; a colloid that produces non-hypotensive hypovolemia) or isoproterenol (hypotensive agent). Water intakes were unaffected by the higher dose of losartan given s.c. Intraperitoneal injection of EXP 3174, the active metabolite of losartan that may more readily penetrate the blood-brain barrier, inhibited isoproterenol-, but not PEG-induced water intakes. Fos-ir was induced by PEG and isoproterenol in several regions of the brain also activated by Ang II. Fos-ir was greatly attenuated in the SFO by losartan following administration of PEG, but not isoproterenol, and was either unaffected or increased in SON and PVN after either agent. These data suggest that the increased circulating Ang II following PEG or isoproterenol acts at the SFO and is more readily reversible by losartan in normotensive (PEG) than in hypotensive (isoproterenol) states. Non-Ang neural input to the SON and PVN, presumably from baroreceptors, appears to be sufficient to produce strong Fos-ir in these regions, as well as to engage drinking.