Megan E. Bardgett

Glutamatergic Receptor Activation in the Rostral Ventrolateral Medulla Mediates the Sympathoexcitatory Response to Hyperinsulinemia

Hyperinsulinemia increases sympathetic nerve activity (SNA) and has been linked to cardiovascular morbidity in obesity. The rostral ventrolateral medulla (RVLM) plays a key role in the regulation of SNA and arterial blood pressure (ABP). Many sympathoexcitatory responses are mediated by glutamatergic receptor activation within the RVLM, and both the central renin–angiotensin and melanocortin systems are implicated in the sympathoexcitatory response to hyperinsulinemia. Therefore, we hypothesized that one or more of these neurotransmitters in the RVLM mediate the sympathoexcitatory response to insulin. Hyperinsulinemic–euglycemic clamps were performed in &agr;-chloralose anesthetized, male Sprague–Dawley rats by infusion of insulin (3.75 mU/kg per minute, IV) and 50% dextrose solution for 120 minutes. Physiological increases in plasma insulin elevated lumbar SNA, with no change in renal SNA, ABP, or blood glucose. Microinjection of the ionotropic glutamate receptor antagonist kynurenic acid into the RVLM significantly reduced lumbar SNA and ABP. Selective blockade of NMDA but not non-NMDA glutamate receptors resulted in similar reductions of lumbar SNA. In marked contrast, microinjection of the angiotensin II type 1 receptor antagonist losartan or the melanocortin 3/4 antagonist SHU9119 had no effect on lumbar SNA or ABP. Western blot analysis showed that insulin receptor expression is significantly lower in the RVLM than the hypothalamus, and direct microinjection of insulin into the RVLM did not significantly increase lumbar SNA. These findings suggest that hyperinsulinemia increases lumbar SNA by activation of a glutamatergic NMDA-dependent projection to the RVLM.

Ventral Lamina Terminalis Mediates Enhanced Cardiovascular Responses of Rostral Ventrolateral Medulla Neurons During Increased Dietary Salt

Increased dietary salt enhances sympathoexcitatory and sympathoinhibitory responses evoked from the rostral ventrolateral medulla (RVLM). The purpose of the present study was to determine whether neurons of the forebrain lamina terminalis (LT) mediated these changes in the RVLM. Male Sprague-Dawley rats with and without LT lesions were fed normal chow and given access to water or 0.9% NaCl for 14 to 15 days. Unilateral injection of l-glutamate into the RVLM produced significantly larger increases in renal sympathetic nerve activity and arterial blood pressure of sham rats ingesting 0.9% NaCl versus water. However, these differences were not observed between ventral LT-lesioned rats drinking 0.9% NaCl versus water. Similar findings were observed when angiotensin II or γ-aminobutyric acid was injected into the RVLM. Interestingly, a subset of animals drinking 0.9% but with damage restricted to the organum vasculosum of the lamina terminalis did not show enhanced responses to l-glutamate or γ-aminobutyric acid. In marked contrast, RVLM injection of l-glutamate or γ-aminobutyric acid produced exaggerated sympathetic nerve activity and arterial blood pressure responses in animals drinking 0.9% NaCl versus water after an acute ventral LT lesion or chronic lesion of the subfornical organ. Additional experiments demonstrated that plasma sodium concentration and osmolality were increased at night in rats ingesting 0.9% NaCl. These findings suggest that neurons of the ventral LT mediate the ability of increased dietary salt to enhance the responsiveness of RVLM sympathetic neurons.

Ang II–Salt Hypertension Depends on Neuronal Activity in the Hypothalamic Paraventricular Nucleus but Not on Local Actions of Tumor Necrosis Factor-α

Development of angiotensin II (Ang II)–dependent hypertension involves microglial activation and proinflammatory cytokine actions in the hypothalamic paraventricular nucleus (PVN). Cytokines activate receptor signaling pathways that can both acutely grade neuronal discharge and trigger long-term adaptive changes that modulate neuronal excitability through gene transcription. Here, we investigated contributions of PVN cytokines to maintenance of hypertension induced by subcutaneous infusion of Ang II (150 ng/kg per min) for 14 days in rats consuming a 2% NaCl diet. Results indicate that bilateral PVN inhibition with the GABA-A receptor agonist muscimol (100 pmol/50 nL) caused significantly greater reductions of renal and splanchnic sympathetic nerve activity (SNA) and mean arterial pressure in hypertensive than in normotensive rats (P<0.01). Thus, ongoing PVN neuronal activity seems required for support of hypertension. Next, the role of the prototypical cytokine tumor necrosis factor-&agr; was investigated. Whereas PVN injection of tumor necrosis factor-&agr; (0.3 pmol/50 nL) acutely increased lumbar and splanchnic SNA and mean arterial pressure, interfering with endogenous tumor necrosis factor-&agr; by injection of etanercept (10 &mgr;g/50 nL) was without effect in hypertensive and normotensive rats. Next, we determined that although microglial activation in PVN was increased in hypertensive rats, bilateral injections of minocycline (0.5 &mgr;g/50 nL), an inhibitor of microglial activation, failed to reduce lumbar or splanchnic SNA or mean arterial pressure in hypertensive or in normotensive rats. Collectively, these findings indicate that established Ang II–salt hypertension is supported by PVN neuronal activity, but short term maintenance of SNA and arterial blood pressure does not depend on ongoing local actions of tumor necrosis factor-&agr;.

Glucocorticoids attenuate the central sympathoexcitatory actions of insulin

Insulin acts within the central nervous system to regulate food intake and sympathetic nerve activity (SNA). Strong evidence indicates that glucocorticoids impair insulin-mediated glucose uptake and food intake. However, few data are available regarding whether glucocorticoids also modulate the sympathoexcitatory response to insulin. Therefore, the present study first confirmed that chronic administration of glucocorticoids attenuated insulin-induced increases in SNA and then investigated whether these effects were attributed to deficits in central insulin-mediated responses. Male Sprague-Dawley rats were given access to water or a drinking solution of the glucocorticoid agonist dexamethasone (0.3 μg/ml) for 7 days. A hyperinsulinemic-euglycemic clamp significantly increased lumbar SNA in control rats. This response was significantly attenuated in rats given access to dexamethasone for 7, but not 1, days. Similarly, injection of insulin into the lateral ventricle or locally within the arcuate nucleus (ARC) significantly increased lumbar SNA in control rats but this response was absent in rats given access to dexamethasone. The lack of a sympathetic response to insulin cannot be attributed to a generalized depression of sympathetic function or inactivation of ARC neurons as electrical activation of sciatic afferents or ARC injection of gabazine, respectively, produced similar increases in SNA between control and dexamethasone-treated rats. Western blot analysis indicates insulin produced similar activation of Akt Ser(473) and rpS6 Ser(240/244) in the ventral hypothalamus of control and dexamethasone-treated rats. Collectively, these findings suggest that dexamethasone attenuates the sympathoexcitatory actions of insulin through a disruption of ARC neuronal function downstream of Akt or mammalian target of rapamycin (mTOR) signaling.

Coping with dehydration: sympathetic activation and regulation of glutamatergic transmission in the hypothalamic PVN

Autonomic and endocrine profiles of chronic hypertension and heart failure resemble those of acute dehydration. Importantly, all of these conditions are associated with exaggerated sympathetic nerve activity (SNA) driven by glutamatergic activation of the hypothalamic paraventricular nucleus (PVN). Here, studies sought to gain insight into mechanisms of disease by determining the role of PVN ionotropic glutamate receptors in supporting SNA and mean arterial pressure (MAP) during dehydration and by elucidating mechanisms regulating receptor activity. Blockade of PVN N-methyl-D-aspartate (NMDA) receptors reduced (P < 0.01) renal SNA and MAP in urethane-chloralose-anesthetized dehydrated (DH) (48 h water deprivation) rats, but had no effect in euhydrated (EH) controls. Blockade of PVN α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors had no effect in either group. NMDA in PVN caused dose-dependent increases of renal SNA and MAP in both groups, but the maximum agonist evoked response (Emax) of the renal SNA response was greater (P < 0.05) in DH rats. The latter was not explained by increased PVN expression of NMDA receptor NR1 subunit protein, increased PVN neuronal excitability, or decreased brain water content. Interestingly, PVN injection of the pan-specific excitatory amino acid transporter (EAAT) inhibitor DL-threo-β-benzyloxyaspartic acid produced smaller sympathoexcitatory and pressor responses in DH rats, which was associated with reduced glial expression of EAAT2 in PVN. Like chronic hypertension and heart failure, dehydration increases excitatory NMDA receptor tone in PVN. Reduced glial-mediated glutamate uptake was identified as a key contributing factor. Defective glutamate uptake in PVN could therefore be an important, but as yet unexplored, mechanism driving sympathetic hyperactivity in chronic cardiovascular diseases.

Activation of corticotropin-releasing factor receptors in the rostral ventrolateral medulla is required for glucose-induced sympathoexcitation

Energy expenditure is determined by metabolic rate and diet-induced thermogenesis. Normally, energy expenditure increases due to neural mechanisms that sense plasma levels of ingested nutrients/hormones and reflexively increase sympathetic nerve activity (SNA). Here, we investigated neural mechanisms of glucose-driven sympathetic activation by determining contributions of neuronal activity in the hypothalamic paraventricular nucleus (PVN) and activation of corticotropin-releasing factor (CRF) receptors in the rostral ventrolateral medulla (RVLM). Glucose was infused intravenously (150 mg/kg, 10 min) in male rats to raise plasma glucose concentration to a physiological postprandial level. In conscious rats, glucose infusion activated CRF-containing PVN neurons and TH-containing RVLM neurons, as indexed by c-Fos immunofluorescence. In α-chloralose/urethane-anesthetized rats, glucose infusion increased lumbar and splanchnic SNA, which was nearly prevented by prior RVLM injection of the CRF receptor antagonist astressin (10 pmol/50 nl). This cannot be attributed to a nonspecific effect, as sciatic afferent stimulation increased SNA and ABP equivalently in astressin- and aCSF-injected rats. Glucose-stimulated sympathoexcitation was largely reversed during inhibition of PVN neuronal activity with the GABA-A receptor agonist muscimol (100 pmol/50 nl). The effects of astressin to prevent glucose-stimulated sympathetic activation appear to be specific to interruption of PVN drive to RVLM because RVLM injection of astressin prior to glucose infusion effectively prevented SNA from rising and prevented any fall of SNA in response to acute PVN inhibition with muscimol. These findings suggest that activation of SNA, and thus energy expenditure, by glucose is initiated by activation of CRF receptors in RVLM by descending inputs from PVN.

Blood pressure is maintained during dehydration by hypothalamic paraventricular nucleus-driven tonic sympathetic nerve activity

At normal resting mean arterial pressure (MAP), sympathetic nerve activity (SNA) mostly consists of respiratory and cardiac rhythmic bursts of action potentials. In animal models of sympathetic hyperactivity, elevated SNA and MAP become reliant on activity of neurones in the hypothalamic paraventricular nucleus (PVN). Dehydrated (DH) rats (48 h water deprived) were used as a model of sympathetic hyperactivity. As expected, acute PVN inhibition reduced MAP and integrated splanchnic SNA (sSNA) in DH rats, but had no effect in euhydrated controls. Unexpectedly, the fall of sSNA in DH rats was due to a reduction of irregular, tonic activity as neither respiratory nor cardiac rhythmic bursting was significantly affected. We conclude that MAP is largely maintained during dehydration by PVN‐driven tonic SNA and speculate that a normally quiescent tonic component of SNA might also be recruited in chronic diseases (hypertension, heart failure, obesity) where PVN activation drives sympathetic hyperactivity.