Robert B. Felder

The central projections of carotid baroreceptors and chemoreceptors in the cat: a neurophysiological study.

The medullary projections of afferent neurons with cell bodies in the petrosal ganglion have been investigated using an antidromic mapping technique. Of the ninety‐three units studied, fifty‐eight were shown to have patterns of discharge indicating that they were baroreceptors and thirty‐five showed responses to stimuli indicating that they were arterial chemoreceptors. Twelve baroreceptor and thirteen chemoreceptor afferents had sufficiently stable unitary discharges to permit a detailed estimation of some of their central projections using stimulation through monopolar tungsten micro‐electrodes to evoke antidromic spikes. In order to estimate their pattern of projection, depth‐threshold contours for each penetration through the dorsomedial medulla and the values of antidromic latency were considered. Baroreceptor afferent fibres with myelinated (six units) and non‐myelinated (six units) axons showed similar patterns of central projection. All could be activated from the ipsilateral nucleus of the tractus solitarius (n.t.s.), most often from its lateral divisions rostral to the obex. The dorsolateral and dorsomedial portions of the n.t.s. were most often innervated, with the commissural subnucleus receiving an innervation in seven of the twelve neurones studied. Stimulation of the ventrolateral subnucleus was effective in activating two afferent fibres whilst stimulation of the ventral subnucleus was effective in only one case. All chemoreceptor afferent fibres had calculated conduction velocities less than 4 m/s and all were activated from the dorsomedial and medial subnuclei of the ipsilateral n.t.s. In twelve of the thirteen neurones investigated in detail there was evidence of an innervation of the commissural nucleus both at the level of the obex and behind it. In three cases this extended into the contralateral portion of the commissural nucleus. In four cases a sparse innervation of the lateral subnucleus, comprising its dorsolateral aspects, was seen. The potential significance of these distinctive patterns of projection of arterial baroreceptors and chemoreceptors is discussed in relation to cardiovascular and respiratory control.

Neuroendocrine and cytokine profile of chronic mild stress-induced anhedonia

A bidirectional relationship exists between depression and cardiovascular disease. Patients with major depression are more likely to develop cardiac events, and patients with myocardial infarction and heart failure are more likely to develop depression. A feature common to both clinical syndromes is activation of proinflammatory cytokines and stress hormones, including the hypothalamic-pituitary-adrenal axis and the renin-angiotensin-aldosterone system. In the present study we examined the hypothesis that exposure to chronic mild stress (CMS), an experimental model of depression that induces anhedonia in rats, is sufficient to activate the production of proinflammatory cytokines and stress hormones that are detrimental to the heart and vascular system. Four weeks of exposure of male, Sprague-Dawley rats to mild unpredictable environmental stressors resulted in anhedonia which was operationally defined as a reduction in sucrose intake without a concomitant effect on water intake. Humoral assays indicated increased plasma levels of tumor necrosis factor-alpha (TNF-alpha), interleukin-1beta (IL-1beta), plasma renin activity, aldosterone, and corticosterone in the CMS exposed rats. Tissue TNF-alpha and IL-1beta were increased in the hypothalamus, and TNF-alpha was increased in the pituitary gland. These humoral responses to CMS, associated with anhedonia as an index of depression in the rat, are likely to be associated with neurohumoral mechanisms that may contribute to adverse cardiac events. The findings provide a basis for examining more directly the interactions among the central, endocrine, and immune systems in depression associated with heart disease.

Novel Effect of Mineralocorticoid Receptor Antagonism to Reduce Proinflammatory Cytokines and Hypothalamic Activation in Rats With Ischemia-Induced Heart Failure

Blocking brain mineralocorticoid receptors (MRs) reduces the high circulating levels of tumor necrosis factor (TNF)-&agr; in heart failure (HF) rats. TNF-&agr; and other proinflammatory cytokines activate neurons in the paraventricular nucleus (PVN) of hypothalamus, including corticotropin-releasing hormone (CRH) neurons, by inducing cyclooxygenase (COX)-2 activity and synthesis of prostaglandin E2 by perivascular cells of the cerebral vasculature. We tested the hypothesis that systemic treatment with a MR antagonist would reduce hypothalamic COX-2 expression and PVN neuronal activation in HF rats. Rats underwent coronary ligation to induce HF, confirmed by echocardiography, or sham surgery, followed by 6 weeks treatment with eplerenone (30 mg/kg per day, orally) or vehicle (drinking water). Eplerenone-treated HF rats had lower plasma TNF-&agr;, interleukin (IL)-1&bgr; and IL-6, less COX-2 staining of small blood vessels penetrating PVN, fewer PVN neurons expressing Fra-like activity (indicating chronic neuronal activation), and fewer PVN neurons staining for TNF-&agr;, IL-1&bgr;, and CRH than vehicle-treated HF rats. COX-2 and CRH protein expression in hypothalamus were 1.7- and 1.9-fold higher, respectively, in HF+vehicle versus sham+vehicle rats; these increases were attenuated (26% and 25%, respectively) in HF+eplerenone rats. Eplerenone-treated HF rats had less prostaglandin E2 in cerebrospinal fluid, lower plasma norepinephrine levels, lower left ventricular end-diastolic pressure, and lower right ventricle/body weight and lung/body weight ratios, but no improvement in left ventricular function. Treatment of HF rats with anticytokine agents, etanercept or pentoxifylline, produced very similar results. This study reveals a previously unrecognized effect of MR antagonism to minimize cytokine-induced central neural excitation in rats with HF.

Does Aldosterone Upregulate the Brain Renin-Angiotensin System in Rats With Heart Failure?

The brain renin-angiotensin system (RAS) contributes to increased sympathetic drive in heart failure (HF). The factors upregulating the brain RAS in HF remain unknown. We hypothesized that aldosterone (ALDO), a downstream product of the systemic RAS that crosses the blood-brain barrier, signals the brain to increase RAS activity in HF. We examined the relationship between circulating and brain ALDO in normal intact rats, in adrenalectomized rats receiving subcutaneous infusions of ALDO, and in rats with ischemia-induced HF and sham-operated controls. Brain ALDO levels were proportional to plasma ALDO levels across the spectrum of rats studied. Compared with sham-operated controls rats, HF rats had higher plasma and hypothalamic tissue levels of ALDO. HF rats also had higher expression of mRNA and protein for angiotensin-converting enzyme and angiotensin type 1 receptors in the hypothalamus, increased reduced nicotinamide-adenine dinucleotide phosphate oxidase activity and superoxide generation in the paraventricular nucleus of the hypothalamus, increased excitation of paraventricular nucleus neurons, and increased plasma norepinephrine. HF rats treated for 4 weeks with intracerebroventricular RU28318 (1 &mgr;g/h), a selective mineralocorticoid receptor antagonist, had less hypothalamic angiotensin-converting enzyme and angiotensin type 1 receptor mRNA and protein, less reduced nicotinamide-adenine dinucleotide phosphate–induced superoxide in the paraventricular nucleus, fewer excited paraventricular nucleus neurons, and lower plasma norepinephrine. RU28318 had no effect on plasma ALDO or on angiotensin-converting enzyme or angiotensin type 1 receptor expression in brain cortex. The data demonstrate that ALDO of adrenal origin enters the hypothalamus in direct proportion to plasma levels and suggest that ALDO contributes to the upregulation of hypothalamic RAS activity and sympathetic drive in heart failure.

Interaction between Cardiac Receptors and Sinoaortic Baroreceptors in the Control of Efferent Cardiac Sympathetic Nerve Activity during Myocardial Ischemia in Dogs

The purpose of this study was to determine the relative influence of arterial baroreceptors and of cardiac receptors with vagal afferents on efferent cardiac sympathetic nerve activity during coronary artery occlusion. Changes in heart rate (beats/min), arterial pressure (mm Hg), and integrated cardiac sympathetic nerve activity (CSNA, percent change from control; recorded from the cut central end of the left ventral ansa subclavia) were determined during transient (90-second) circumflex (Cx) and anterior descending (LAD) coronary artery occlusions. In dogs with carotid and aortic baroreceptors intact, increases (mean ± SE) in CSNA during Cx (7 ± 2%) and LAD (9 ± 5%) occlusions were similar despite a significantly greater fall in arterial pressure during Cx (-14 ± 3 mm Hg) than during LAD (−5 ± 2 mm Hg) coronary artery occlusion. Heart rate did not change during these occlusions. In three dogs, hypotension induced by inferior vena caval occlusion resulted in greater increases in CSNA than did comparable decreases in arterial pressure resulting from occlusion of LAD or Cx. In dogs with sinoaortic denervation, Cx coronary occlusion resulted in decreases in CSNA (-14 ± 4%), arterial pressure (−38 ± 6 mm Hg), and heart rate (-13 ± 5 beats/min), whereas LAD occlusion resulted in a small decrease in arterial pressure (-12 ± 5 mm Hg) and no change in CSNA or heart rate. Vagotomy abolished the decreases in CSNA and heart rate and attenuated the arterial pressure responses to Cx occlusion. We conclude that cardiac receptors with vagal afferents exert an inhibitory influence on cardiac sympathetic nerve activity during myocardial ischemia, particularly during inferoposterior ischemia, and that this influence limits the arterial baroreceptor-mediated increases in CSNA resulting from ischemia-induced hypotension. Circ Res 45: 728-736, 1979

POST-SYNAPTIC ACTIVITY EVOKED IN THE NUCLEUS TRACTUS SOLITARIUS BY CAROTID-SINUS AND AORTIC NERVE AFFERENTS IN THE CAT

Post‐synaptic responses evoked in neurones of the nucleus tractus solitarius by electrical stimulation of the carotid sinus, aortic and vagal nerves, alone or in combination, have been studied in anaesthetized cats using both extracellular and intracellular recording techniques. A total of 292 neurones received an input from at least one of the three nerves tested. The activity of the large majority of these cells (249) could only be shown to be altered by stimulation of one of these nerves and in 222 of these cases this was an excitatory response. These responses showed the expected post‐synaptic characteristics including temporal summation and, in intracellular records, a summation of evoked excitatory post‐synaptic potentials (e.p.s.p.s). The minimum latency to onset of these responses was variable, both for individual cells and for the population as a whole and varied within the range 2‐124 ms. In a small number of cells (twenty‐seven), the input was purely inhibitory in nature. In neurones showing a tonic discharge this produced a decrease in the rate of firing. This influence was most marked in intracellular records where membrane hyperpolarizations were noted. Again, the latency to onset was variable, in the range 4‐27 ms. Convergent inputs from two or more of the nerves were identified in forty‐three neurones. The effects of these were always excitatory. They could be observed both as a facilitation of spike activity recorded extracellularly and as summation of subliminally evoked e.p.s.p.s recorded intracellularly. On the basis of threshold voltages and latency to onset, the afferents to these neurones are indistinguishable from those providing an exclusive input. It can be concluded that at least some of the neurones in the nucleus tractus solitarius and its vicinity receive inputs from more than one source. The implications of these observations on the role of this brain‐stem area in cardiorespiratory reflexes is discussed.

Angiotensin II upregulates hypothalamic AT1 receptor expression in rats via the mitogen-activated protein kinase pathway.

ANG II type 1 receptors (AT(1)R) mediate most of the central effects of ANG II on cardiovascular function, fluid homeostasis, and sympathetic drive. The mechanisms regulating AT(1)R expression in the brain are unknown. In some tissues, the AT(1)R can be upregulated by prolonged exposure to ANG II. We examined the hypothesis that ANG II upregulates the AT(1)R in the brain by stimulating the intracellular mitogen-activated protein kinase (MAPK) signaling pathway. Using molecular and immunochemical approaches, we examined expression of the AT(1)R and phosphorylated MAPK in the paraventricular nucleus of the hypothalamus (PVN) and the subfornical organ (SFO) of rats receiving a chronic (4-wk) subcutaneous infusion of ANG II (0.6 microg/h) or saline (vehicle control), with or without concomitant (4-wk) intracerebroventricular (ICV) infusions of MAPK inhibitors or the AT(1)R blocker losartan. Subcutaneous infusion of ANG II markedly increased phosphorylation of MAPK and expression of AT(1)R mRNA and protein and AT(1)R-like immunoreactivity in the PVN and SFO. ANG II-induced AT(1)R expression was blocked by ICV infusion of the p44/42 MAPK inhibitor PD-98059 (0.025 microg/h) and the JNK inhibitor SP-600125 (0.125 microg/h), but not by the p38 MAPK inhibitor SB-203580 (0.125 microg/h). Upregulation of the AT(1)R in the PVN and SFO by peripheral ANG II was abolished by ICV losartan (10 microg/h). The data indicate that blood-borne ANG II upregulates brain AT(1)R by activating intracellular p44/42 MAPK and JNK signaling pathways.

Inhibition of brain proinflammatory cytokine synthesis reduces hypothalamic excitation in rats with ischemia-induced heart failure

The expression of proinflammatory cytokines increases in the hypothalamus of rats with heart failure (HF). The pathophysiological significance of this observation is unknown. We hypothesized that hypothalamic proinflammatory cytokines upregulate the activity of central neural systems that contribute to increased sympathetic nerve activity in HF, specifically, the brain renin-angiotensin system (RAS) and the hypothalamic-pituitary-adrenal (HPA) axis. Rats with HF induced by coronary ligation and sham-operated controls (SHAM) were treated for 4 wk with a continuous intracerebroventricular infusion of the cytokine synthesis inhibitor pentoxifylline (PTX, 10 microg/h) or artificial cerebrospinal fluid (VEH). In VEH-treated HF rats, compared with VEH-treated SHAM rats, the hypothalamic expression of proinflammatory cytokines was increased, along with key components of the brain RAS (renin, angiotensin-converting enzyme, angiotensin type 1 receptor) and corticotropin-releasing hormone, the central indicator of HPA axis activation, in the paraventricular nucleus (PVN) of the hypothalamus. The expression of other inflammatory/excitatory mediators (superoxide, prostaglandin E(2)) was also increased, along with evidence of chronic neuronal excitation in PVN. VEH-treated HF rats had higher plasma levels of norepinephrine, ANG II, interleukin (IL)-1beta, and adrenocorticotropic hormone, increased left ventricular end-diastolic pressure, and increased wet lung-to-body weight ratio. With the exception of plasma IL-1beta, an indicator of peripheral proinflammatory cytokine activity, all measures of neurohumoral excitation were significantly lower in HF rats treated with intracerebroventricular PTX. These findings suggest that the increase in brain proinflammatory cytokines observed in rats with ischemia-induced HF is functionally significant, contributing to neurohumoral excitation by activating brain RAS and the HPA axis.

Modulation of carotid sinus afferent input to nucleus tractus solitarius by parabrachial nucleus stimulation.

There is increasing evidence that the parabrachial nucleus (PBN) may be integrally involved in cardiovascular reflex regulation. In cats in which anesthesia was induced with pentobarbital and maintained with alpha-chloralose, we studied the effects of PBN stimulation on cardiovascular afferent inputs to nucleus tractus solitarius (NTS), the site of first central termination for cardiovascular afferent fibers. Electrical stimulation of PBN resulted in an initial excitation followed by prolonged inhibition of the spontaneous activity of NTS neurons activated by ipsilateral carotid sinus nerve (CSN) stimulation. In 53 units recorded extracellularly in and around NTS, the number of action potential responses to ipsilateral CSN stimulation was reduced 73 +/- 3% by a prior conditioning stimulus to PBN at an interval of 30-60 msec. CSN input to 10 units excited by selective baroreceptor stimulation was inhibited by the PBN conditioning stimulus, as were convergent inputs from contralateral CSN, vagus, and renal nerves. The inhibitory influence of the PBN stimulus lasted as long as 450 msec. We examined the mechanism for these phenomena in additional intracellular recording experiments. In 57 units, PBN stimulation evoked a long lasting (65-359 msec) membrane potential hyperpolarization. In 42 cells, the PBN evoked inhibitory postsynaptic potential (IPSP) was preceded by an excitatory postsynaptic potential (EPSP). CSN and convergent inputs were inhibited when timed to occur during the PBN induced IPSP. Conversely, CSN and convergent afferent nerves inhibited PBN input to NTS neurons with no associated change in membrane potential (n = 9 of 14). These data demonstrate for the first time a potent modulatory influence of PBN on NTS neurons processing cardiovascular afferent input.

11β-Hydroxysteroid Dehydrogenase Type 2 Activity in Hypothalamic Paraventricular Nucleus Modulates Sympathetic Excitation

Aldosterone stimulates the sympathetic nervous system by binding to a select population of brain mineralocorticoid receptors (MR). These MR have an equal affinity for corticosterone that is present in substantially higher concentrations, but are held in reserve for aldosterone by activity of the enzyme 11&bgr;-hydroxysteroid dehydrogenase type 2 (11&bgr;-HSD-2), which converts corticosterone to an inactive metabolite. Thus, colocalization of MR and 11&bgr;-HSD-2 activity may help identify brain regions that mediate the effects of aldosterone. The present studies tested the hypothesis that 11&bgr;-HSD-2 activity regulates MR-mediated responses in the paraventricular nucleus (PVN) of the hypothalamus, a forebrain region implicated in sympathetic regulation. Real-time–polymerase chain reaction revealed the presence of 11&bgr;-HSD-2 mRNA in PVN. In anesthetized adult male Sprague-Dawley rats, microinjection of the 11&bgr;-HSD-2 inhibitor carbenoxolone (CBX) into PVN increased mean arterial pressure, heart rate, and renal sympathetic nerve activity. Intracerebroventricular injections of CBX excited PVN neurons and increased mean arterial pressure, heart rate, and renal sympathetic nerve activity. The ability of CBX to increase sympathetic activity by inhibiting 11&bgr;-HSD-2, thereby permitting corticosterone to activate MR, was confirmed by the following: Intracerebroventricular glycyrrhizic acid, another 11&bgr;-HSD-2 inhibitor, mimicked the sympathoexcitatory effects of CBX; the sympathoexcitatory effects of CBX were blocked by spironolactone, a MR antagonist. Neither CBX nor glycyrrhizic acid elicited a response in adrenalectomized rats. These findings suggest that MR in PVN contribute to sympathetic regulation and may be activated by aldosterone or corticosterone (or cortisol in humans) depending on the state of 11&bgr;-HSD-2 activity.