Chronically lowering sympathetic activity protects sympathetic nerves in spleens from aging F344 rats

Possible organ-protective effects of renal denervation: insights from basic studies

Inappropriate sympathetic nervous activation is the body's response to biological stress and is thought to be involved in the development of various lifestyle-related diseases through an elevation in blood pressure. Experimental studies have shown that surgical renal denervation decreases blood pressure in hypertensive animals. Recently, minimally invasive catheter-based renal denervation has been clinically developed, which results in a reduction in blood pressure in patients with resistant hypertension. Accumulating evidence in basic studies has shown that renal denervation exerts beneficial effects on cardiovascular disease and chronic kidney disease. Interestingly, recent studies have also indicated that renal denervation improves glucose tolerance and inflammatory changes. In this review article, we summarize the evidence from animal studies to provide comprehensive insight into the organ-protective effects of renal denervation beyond changes in blood pressure.

Ageing and the Autonomic Nervous System

The vertebrate nervous system is divided into central (CNS) and peripheral (PNS) components. In turn, the PNS is divided into the autonomic (ANS) and enteric (ENS) nervous systems. Ageing implicates time-related changes to anatomy and physiology in reducing organismal fitness. In the case of the CNS, there exists substantial experimental evidence of the effects of age on individual neuronal and glial function. Although many such changes have yet to be experimentally observed in the PNS, there is considerable evidence of the role of ageing in the decline of ANS function over time. As such, this chapter will argue that the ANS constitutes a paradigm for the physiological consequences of ageing, as well as for their clinical implications.

Renal denervation mitigates atherosclerosis in ApoE-/- mice via the suppression of inflammation

Atherosclerosis is a chronic pathological process characterized by the accumulation of inflammation. Overactivation of the sympathetic nervous system accelerates the progression of atherosclerosis. Renal denervation (RDN) reduces the activity of the sympathetic nerve system (SNS) by disrupting sympathetic nerves surrounding renal arteries. We sought to determine whether RDN could mitigate atherosclerosis through the suppression of inflammation. First, we investigated the correlation between plasma norepinephrine concentrations and circulatory inflammation in the progression of atherosclerosis. Then, forty ApoE-/- mice underwent renal denervation or a sham operation after 6 weeks or 12 weeks of feeding with a high-fat diet. The effects of RDN on atherosclerosis in mice were explored. In the development of atherosclerosis, positive correlations were found between SNS activation and the accumulation of circulatory myeloid cells and inflammatory cytokines. In the second part of the study, inhibition of the increase in plaque size was found in both RDN groups compared with that in the sham operation (SO) groups (P<0.05), and RDN also ameliorated inflammation in plaques. Furthermore, RDN attenuated the accumulation of circulating neutrophils and monocytes (P<0.05), which is associated with a significant reduction in levels of several circulating inflammatory cytokines related to hemopoiesis (P<0.05). Flow cytometry analysis revealed comparable levels of neutrophils and monocytes in the bone marrow between all four groups. However, RDN decreased the production and proportions of neutrophils and monocytes in the spleen and reduced splenic sympathetic activity (P<0.05). In summary, our study reveals a novel link between SNS activity and inflammation in atherosclerosis and identifies RDN as a potential anti-inflammatory therapeutic strategy for the treatment of atherosclerosis by restricting the production of splenic immune cells.

Sympathetic neurotransmission in spleens from aging Brown-Norway rats subjected to reduced sympathetic tone

Senescence of innate and adaptive responses and low-grade inflammation (inflammaging) hallmarks normal aging, which increases vulnerability to infectious diseases, autoimmunity and cancer. In normal aging, sympathetic dysregulation contributes to the dysregulation of innate and adaptive immunity and inflammaging. Sympathetic innervation of immune cells in secondary immune organs regulates immune responses. Previously in Fischer 344 (F344) rats, we reported an age-related increase in sympathetic tone and sympathetic dysfunction in beta-adrenergic receptor (AR) signaling of splenic lymphocytes that contributes to immune senescence, although the responsible mechanisms remains unexplored. In this study, we extend our previous findings using the much longer-lived Brown-Norway (BN) rats, whose behavior and immune response profile differ strikingly from F344 rats. Here, we investigated whether increased sympathetic nerve activity (SNA) in the aging spleen contributes to age-related sympathetic neuropathy and altered neurotransmission in splenic lymphocytes in BN rats. Fifteen-month male BN rats received 0, 0.5 or 1.5 μg/kg/day rilmenidine intraperitoneally for 90 days to lower sympathetic tone. Untreated young and age-matched rats controlled for effects of age. We found that elevated SNA in the aging BN rat spleen does not contribute significantly to sympathetic neuropathy or the aging-induced impairment of canonical β-AR signal transduction. Despite the rilmenidine-induced increase in β-AR expression, splenocyte c-AMP production was comparable with age-matched controls, thus dampening nerve activity had no effect on receptor coupling to adenylate cyclase. Understanding how aging affects neuroimmune regulation in healthy aging rodent models may eventually lead to strategies that improve health in aging populations vulnerable to immunosenescence and low-grade systemic inflammation.

Sympathetic Nerve Hyperactivity in the Spleen: Causal for Nonpathogenic-Driven Chronic Immune-Mediated Inflammatory Diseases (IMIDs)?

Immune-Mediated Inflammatory Diseases (IMIDs) is a descriptive term coined for an eclectic group of diseases or conditions that share common inflammatory pathways, and for which there is no definitive etiology. IMIDs affect the elderly most severely, with many older individuals having two or more IMIDs. These diseases include, but are not limited to, type-1 diabetes, obesity, hypertension, chronic pulmonary disease, coronary heart disease, inflammatory bowel disease, and autoimmunity, such as rheumatoid arthritis (RA), Sjőgren's syndrome, systemic lupus erythematosus, psoriasis, psoriatic arthritis, and multiple sclerosis. These diseases are ostensibly unrelated mechanistically, but increase in frequency with age and share chronic systemic inflammation, implicating major roles for the spleen. Chronic systemic and regional inflammation underlies the disease manifestations of IMIDs. Regional inflammation and immune dysfunction promotes targeted end organ tissue damage, whereas systemic inflammation increases morbidity and mortality by affecting multiple organ systems. Chronic inflammation and skewed dysregulated cell-mediated immune responses drive many of these age-related medical disorders. IMIDs are commonly autoimmune-mediated or suspected to be autoimmune diseases. Another shared feature is dysregulation of the autonomic nervous system and hypothalamic pituitary adrenal (HPA) axis. Here, we focus on dysautonomia. In many IMIDs, dysautonomia manifests as an imbalance in activity/reactivity of the sympathetic and parasympathetic divisions of the autonomic nervous system (ANS). These major autonomic pathways are essential for allostasis of the immune system, and regulating inflammatory processes and innate and adaptive immunity. Pathology in ANS is a hallmark and causal feature of all IMIDs. Chronic systemic inflammation comorbid with stress pathway dysregulation implicate neural-immune cross-talk in the etiology and pathophysiology of IMIDs. Using a rodent model of inflammatory arthritis as an IMID model, we report disease-specific maladaptive changes in β₂-adrenergic receptor (AR) signaling from protein kinase A (PKA) to mitogen activated protein kinase (MAPK) pathways in the spleen. Beta₂-AR signal "shutdown" in the spleen and switching from PKA to G-coupled protein receptor kinase (GRK) pathways in lymph node cells drives inflammation and disease advancement. Based on these findings and the existing literature in other IMIDs, we present and discuss relevant literature that support the hypothesis that unresolvable immune stimulation from chronic inflammation leads to a maladaptive disease-inducing and perpetuating sympathetic response in an attempt to maintain allostasis. Since the role of sympathetic dysfunction in IMIDs is best studied in RA and rodent models of RA, this IMID is the primary one used to evaluate data relevant to our hypothesis. Here, we review the relevant literature and discuss sympathetic dysfunction as a significant contributor to the pathophysiology of IMIDs, and then discuss a novel target for treatment. Based on our findings in inflammatory arthritis and our understanding of common inflammatory process that are used by the immune system across all IMIDs, novel strategies to restore SNS homeostasis are expected to provide safe, cost-effective approaches to treat IMIDs, lower comorbidities, and increase longevity.

Autonomic nervous system and immune system interactions

The present review assesses the current state of literature defining integrative autonomic-immune physiological processing, focusing on studies that have employed electrophysiological, pharmacological, molecular biological, and central nervous system experimental approaches. Central autonomic neural networks are informed of peripheral immune status via numerous communicating pathways, including neural and non-neural. Cytokines and other immune factors affect the level of activity and responsivity of discharges in sympathetic and parasympathetic nerves innervating diverse targets. Multiple levels of the neuraxis contribute to cytokine-induced changes in efferent parasympathetic and sympathetic nerve outflows, leading to modulation of peripheral immune responses. The functionality of local sympathoimmune interactions depends on the microenvironment created by diverse signaling mechanisms involving integration between sympathetic nervous system neurotransmitters and neuromodulators; specific adrenergic receptors; and the presence or absence of immune cells, cytokines, and bacteria. Functional mechanisms contributing to the cholinergic anti-inflammatory pathway likely involve novel cholinergic-adrenergic interactions at peripheral sites, including autonomic ganglion and lymphoid targets. Immune cells express adrenergic and nicotinic receptors. Neurotransmitters released by sympathetic and parasympathetic nerve endings bind to their respective receptors located on the surface of immune cells and initiate immune-modulatory responses. Both sympathetic and parasympathetic arms of the autonomic nervous system are instrumental in orchestrating neuroimmune processes, although additional studies are required to understand dynamic and complex adrenergic-cholinergic interactions. Further understanding of regulatory mechanisms linking the sympathetic nervous, parasympathetic nervous, and immune systems is critical for understanding relationships between chronic disease development and immune-associated changes in autonomic nervous system function.