Arterial baroreflexes are not essential in mediating sympathoadrenal activation in conscious endotoxic rats

Neural regulation of gastrointestinal inflammation: role of the sympathetic nervous system

The sympathetic innervation of the gastrointestinal (GI) tract regulates motility, secretion and blood flow by inhibiting the activity of the enteric nervous system (ENS) and direct vasoconstrictor innervation of the gut microvasculature. In addition to these well-established roles, there is evidence that the sympathetic nervous system (SNS) can modulate GI inflammation. Postganglionic sympathetic neurons innervate lymphoid tissues and immune cells within the GI tract. Furthermore, innate and adaptive immune cells express receptors for sympathetic neurotransmitters. Activation of these receptors can affect a variety of important immune cell functions, including cytokine release and differentiation of helper T lymphocyte subsets. This review will consider the neuroanatomical evidence of GI immune cell innervation by sympathetic axons, the effects of blocking or enhancing SNS activity on GI inflammation, and the converse modulation of sympathetic neuroanatomy and function by GI inflammation.

Septic shock induces distinct changes in sympathetic nerve activity to the heart and kidney in conscious sheep

Sepsis and septic shock are the chief cause of death in intensive care units, with mortality rates between 30 and 70%. In a large animal model of septic shock, we have demonstrated hypotension, increased cardiac output, and tachycardia, together with renal vasodilatation and renal failure. The changes in cardiac sympathetic nerve activity (CSNA) that may contribute to the tachycardia have not been investigated, and the changes in renal SNA (RSNA) that may mediate the changes in renal blood flow and function are unclear. We therefore recorded CSNA and RSNA during septic shock in conscious sheep. Septic shock was induced by administration of Escherichia coli, which caused a delayed hypotension and an immediate, biphasic increase in heart rate (HR) associated with similar changes in CSNA. After E. coli, RSNA decreased for over 3 h, followed by a sustained increase (180%), whereas renal blood flow progressively increased and remained elevated. There was an initial diuresis, followed by oliguria and decreased creatinine clearance. There were differential changes in the range of the arterial baroreflex curves; it was depressed for HR, increased for CSNA, and unchanged for RSNA. Our findings, recording CSNA for the first time in septic shock, suggest that the increase in SNA to the heart is not driven solely by unloading of baroreceptors and that the increase has an important role to increase HR and cardiac output. There was little correlation between the changes in RSNA and renal blood flow, suggesting that the renal vasodilatation was mediated mainly by other mechanisms.

Endotoxemia causes central downregulation of sympathetic vasomotor tone in healthy humans

Experimental endotoxemia as a model of the initial septic response affects the autonomic nervous system with profound cardiovascular sequelae. Whether the postsynaptic sympathoneural activity to the muscle vascular bed is altered in the early septic phase remains to be determined. The present study aimed to elucidate the early effects of LPS on muscle sympathetic nerve activity (MSNA) and cardiovascular regulation in healthy humans. Young, healthy volunteers randomly received either an LPS bolus (4 ng/kg body wt, n = 11) or placebo (saline; n = 7). Experimental baroreflex assessment (baseline measurements followed by infusion of vasoactive drugs nitroprusside/phenylephrine) was done prior to and 90 min following LPS or placebo challenge. MSNA, heart rate, blood pressure, and blood levels of catecholamines, TNF-α and IL-6 were measured sequentially. Endotoxin but not placebo-induced flu-like symptoms and elevated cytokine levels. In contrast to placebo, LPS significantly suppressed MSNA burst frequency 90 min after injection [mean ± SE: 12.1 ± 2.9 vs. 27.5 ± 3.3 burst/min (post- vs. pre-LPS); P < 0.005] but increased heart rate [78.4 ± 3.1 vs. 60.6 ± 2.0 beats/min (post- vs. pre-LPS); P < 0.001]. Baseline blood pressure was not altered, but baroreflex testing demonstrated a blunted MSNA response and uncoupling of heart rate modulation to blood pressure changes in the endotoxin group. We conclude that endotoxin challenge in healthy humans has rapid suppressive effects on postsynaptic sympathetic nerve activity to the muscle vascular bed and alters baroreflex function which may contribute to the untoward cardiovascular effects of sepsis.

Involvement of CB1-receptors and beta-adrenoceptors in the regional hemodynamic responses to lipopolysaccharide infusion in conscious rats

A possible involvement of endocannabinoids in a chronic model of endotoxemia was assessed by measuring the regional (renal, mesenteric, hindquarters) hemodynamic responses to continuous 24-h LPS infusion (150 microg.kg(-1).h(-1)) in conscious, male Sprague-Dawley rats, in the absence or presence of the cannabinoid (CB1) receptor antagonist AM-251 (3 mg/kg). AM-251 inhibited the tachycardic and hindquarters vasodilator effects of LPS, but did not influence the other hemodynamic changes. In subsequent experiments, it was shown that the tachycardic and hindquarters vasodilator effects of LPS were also inhibited by the nonselective beta-adrenoceptor antagonist propranolol. In addition, the late (at 24 h) hindquarters vasodilator effects of LPS were inhibited by the beta2-adrenoceptor antagonist ICI-118551. Against the background of our previous work showing beta-adrenoceptor involvement in the cardiovascular effects of exogenous cannabinoids, we conclude that AM-251 may have been inhibiting endocannabinoid-modulated, sympathoadrenal-mediated activation of vasodilator beta-adrenoceptors in LPS-infused rats rather than suppressing a direct vasodilator action of endocannabinoids.

Dynamic norepinephrine alterations in bone marrow: evidence of functional innervation

Efferent sympathetic nerve activity has been hypothesized to regulate the proliferation and maturation of leukocytes in the bone marrow. Although there is histological evidence for bone marrow innervation and documentation of measurable neurotransmitter, functional activation of these nerves to external stimulation has never been demonstrated. The present study was designed to assess the dynamics of norepinephrine (NE) release in bone marrow in response to well-established protocols known to elevate sympathetic activity. Toward this end, norepinephrine turnover was measured using isotopic and non-isotopic methods in mice in response to cold exposure and bacterial challenge. Cold exposure increased NE turnover rate in bone marrow by 36% from 0.33 to 0.45 ng g(-1) h(-1), while peritoneal Pseudomonas aeruginosa infection increased bone marrow NE turnover rate by 131% from 0.13 to 0.30 ng g(-1) h(-1). These results demonstrate that the adrenergic innervation of the bone marrow is functionally dynamic and is responsive to generalized stress. Furthermore, these results lend credence to the premise that neural mechanisms participate in regulation of lympho- and myelopoietic cellular events.

Regulation of cytokine and chemokine production by transmitters and co-transmitters of the autonomic nervous system

The sympathetic nervous system innervates immune organs and, when activated, releases its signaling molecules in the vicinity of immune cells. The released molecules include the "classical" transmitters norepinephrine and epinephrine and the co-transmitters ATP and adenosine. Immune cells express various adrenergic and purinergic receptors that are sensitive to these molecules, and the production of immune/inflammatory mediators (cytokines, chemokines, and free radicals) is modulated by activation of these receptors. Notably, the production of tumor necrosis factor-alpha, interleukin-6, -10, and -12, and the chemokine macrophage inflammatory protein 1alpha and the production of the free radical nitric oxide, produced by the inducible nitric oxide synthase, have been shown to be altered by activation of these receptors. Alterations in the production of the immune mediators may contribute to the development of various diseases. On the other hand, novel experimental therapies based on the modulation of adrenergic or purinergic receptors on immune cells are emerging. Such approaches may have beneficial effects in limiting tissue injury and suppressing symptoms in certain pathophysiological states.

Renal regulation of phosphate excretion in endotoxaemic rats

1. Maintenance of phosphate homeostasis is essential for energy producing and oxygen delivery systems, particularly, when the energy requirements are increased in certain conditions, such as septicaemia. We investigated the phosphaturic response to parathyroid hormone (PTH) in endotoxin (ETx)-treated rats in order to clarify the renal regulation of phosphate excretion during endotoxaemia. 2. Wistar rats that had undergone thyroparathyroidectomy were challenged with either Escherichia coli ETx (n = 8) or saline vehicle (n = 9). Thirty-minute renal clearance tests were done before and after PTH infusion. Rats infused with saline instead of PTH served as time controls for the ETx- (n = 7) and saline-treated (n = 8) rats. 3. In time control rats, ETx administration enhanced phosphate excretion progressively and this was associated with an obvious increase in the level of kidney adenosine 3', 5'-cyclic mono-phosphate (P < 0.005) compared with levels following saline vehicle administration. However, this phosphaturia in late-phase endotoxaemia was not observed in rats infused with PTH; ETx, but not saline vehicle, blunted the PTH-mediated increase in phosphate excretion (P < 0.005). Increased urinary noradrenaline and constant dopamine excretion were observed in endotoxaemic rats. Endotoxin administration produced marked metabolic acidosis and hypocapnia in comparison with the administration of the saline vehicle. 4. To test whether renal tubular sensitivity to parathyroid hormone related-protein (PTHrP) was enhanced during endotoxaemia, phosphaturic response to PTHrP in ETx- (n = 7) and saline-treated rats (n = 7) was examined. Parathyroid hormone related-protein infusion produced phosphaturia in both groups. However, the severity of the phosphaturia after PTHrP infusion was less in ETx-than in saline-treated rats. 5. In summary, although ETx administration causes a progressive increase in phosphate excretion in the absence of PTH, this is overcome by the antiphosphaturic effect of ETx, attenuating PTH-mediated phosphaturia after PTH infusion.

Autonomic control of the heart and peripheral vessels in human septic shock

OBJECTIVE

Circulating endotoxin impairs the sympathetic regulation of the cardiovascular system in animals. We studied the changes in the autonomic control of the heart and circulation during septic shock in humans.

DESIGN

12 patients (age 43.0 +/- 6, 17-83 years) were investigated during septic shock (mean duration: 3.5 +/- 0.5 days) and during recovery, fluctuations in R-R interval, invasive arterial pressure (AP) and peripheral arteriolar circulation (PC, photoplethysmography) were evaluated by spectral analysis as a validated noninvasive measure of sympathovagal tone. Apache II score was adopted as the disease severity index. Low frequency components (0.03-0.15 Hz) of the frequency spectra were expressed as relative to the overall variability (LFnu) for each cardiovascular variable.

RESULTS

LFnu were low or absent during shock but, in the 10 patients who recovered, increased by the time of discharge (post-shock). R-R LFnu increased from 17 +/- 6 to 47 +/- 9 (p < 0.03), AP LFnu from 6 +/- 3 to 35 +/- 4 (p < 0.02) and PC LFnu from 18 +/- 3 to 66 +/- 4 (p < 0.001). Apache II fell from 23.1 +/- 1, at admission, to 14.8 +/- 1.8 at discharge (p < 0.005). Two patients died showing no LFnu increase.

CONCLUSION

Reduced LF components of the variability of cardiovascular signals are characteristic of septic shock, confirming the presence of abnormal autonomic control. Restored sympathetic (LF) modulation seems to be associated with a favourable prognosis.

Systemic endotoxin induces Fos-like immunoreactivity in rat spinal sympathetic regions

Immunocytochemical detection of Fos protein was used to evaluate the activation of neurons in sympathetic preganglionic regions of rat spinal cord after systemic treatment with endotoxin. Administration of relatively low doses of bacterial lipopolysaccharide (LPS) to conscious rats caused transient hypotension and stress hormone elevation. Three hours after LPS injection, Fos protein was detected in large numbers of neurons throughout the thoracic spinal cord. Fos-immunoreactive neurons were found in spinal cord segments T3-T13 in the four sympathetic preganglionic nuclei: the intermediolateral cell column (77.7%), the intercalated nucleus (10.6%), the central autonomic nucleus (10.1%) and the lateral funiculus (1.5%). These regions in control animals showed no Fos staining. We conclude that sublethal endotoxemia is a potent stimulus causing Fos expression in sympathetic preganglionic regions.

The effects of ricin on the sympathetic vascular neuroeffector system of the rabbit

Ricin is a toxic lectin that inhibits protein synthesis. Because ricin decreases arterial pressure and causes cardiovascular collapse, its effects on the vascular neuroeffector system were investigated. Rabbits were given either of two doses of ricin, and then norepinephrine (NE) release from aorta to transmural stimulation, NE uptake into aorta, NE content of aorta, monoamine oxidase activity, and catechol-O-methyl transferase activity in aorta were determined 18 hours, 4 days or 7 days later. Norepinephrine uptake and enzyme activities in the aorta were not altered by ricin administration. Norepinephrine release and content of aorta were increased at most time periods following ricin administration, significantly so for NE content at 4 days and for release at 18 hours following the lower dose of ricin. We conclude that the mechanisms involved in the release of NE from sympathetic nerves in the vasculature are not impaired by ricin administration, but rather show changes that indicate increased compensatory activity.