α7-cholinergic receptor mediates vagal induction of splenic norepinephrine

Abstract

Classically, sympathetic and parasympathetic systems act in opposition to maintain the physiological homeostasis. In this article, we report that both systems work together to restrain systemic inflammation in life-threatening conditions such as sepsis. This study indicates that vagus nerve and cholinergic agonists activate the sympathetic noradrenergic splenic nerve to control systemic inflammation. Unlike adrenalectomy, splenectomy and splenic neurectomy prevent the anti-inflammatory potential of both the vagus nerve and cholinergic agonists, and abrogate their potential to induce splenic and plasma norepinephrine. Splenic nerve stimulation mimics vagal and cholinergic induction of norepinephrine and re-establishes neuromodulation in α7 nicotinic acetylcholine receptor (α7nAChR)-deficient animals. Thus, vagus nerve and cholinergic agonists inhibit systemic inflammation by activating the noradrenergic splenic nerve via the α7nAChR nicotinic receptors. α7nAChR represents a unique molecular link between the parasympathetic and sympathetic system to control inflammation.

FIGURE 1

Efferent vagus nerve induces norepinephrine. A, C57BL/6J WT or (B) α7nAChR knockout (α7KO) mice underwent sham surgery (control) or VNS. Plasma epinephrine (E) and norepinephrine (NE) were analyzed at 20 min after stimulation. *p < 0.01 versus control (n = 3; Mann–Whitney U test). C, α7nAChR knockout (α7KO) mice underwent sham surgery (control) or cervical vagotomy (VGX). Twenty-four hours later, nonsectioned intact nerve (control) or the distal trunk of sectioned vagus nerve (VGX) was stimulated for 10 min before LPS. ^#p < 0.01 versus LPS (n = 4; one-way ANOVA with Bonferroni’s corrections). D, Adult C57BL/6J WT mice were treated with reserpine (RES; 5 mg/kg) to inhibit the release of catecholamines. Serum TNF levels were analyzed 90 min later. *p < 0.01 versus LPS (n = 4; one-way ANOVA with Bonferroni’s corrections).

FIGURE 2

α7nAChR mediates vagal induction of norepinephrine. A and C, Adult male C57BL/6J WT littermate or (B, D) α7nAChR knockout (α7KO) mice underwent vagotomy 24 h before stimulation. Animals received (A, B) VNS or (C, D) selective α7nAChR agonist, choline (CHO; 15 mg/kg i.p.). Plasma levels of epinephrine (E) and norepinephrine (NE) were analyzed at 20 min after stimulation. *p < 0.01 versus control (n = 3; Mann–Whitney U test)

FIGURE 3

Splenectomy abrogates vagal induction of plasma norepinephrine. Adult male Sprague Dawley rats underwent sham surgery (sham), (A) adrenalectomy (ADX), or (B–D) splenectomy (SPX) 5 d before endotoxemia (LPS). Cervical vagotomy was performed 24 h before endotoxemia (LPS) and VNS. A and B, Serum TNF levels were analyzed at 90 min after LPS. Plasma levels of (C) epinephrine (E) and (D) norepinephrine (NE) were analyzed at 15 min after stimulation. ^#p < 0.01 versus LPS (n = 3; one-way ANOVA with Bonferroni’s corrections), *p < 0.01 versus control (n = 3; Mann–Whitney U test).

FIGURE 4

Vagus nerve and α7nAChR agonists control systemic inflammation through the splenic nerve. A, C57BL/6J WT mice underwent vagotomy (VGX) and sham (control) or splenic neurectomy (SNX) 24 h before endotoxemia (LPS) and cervical vagal stimulation (VNS). Serum TNF levels were analyzed at 90 min after endotoxemia. B and C, C57BL/6J WT mice underwent SNS, and (B) serum TNF (C) or plasma epinephrine (E) and norepinephrine (NE) were analyzed by ELISA. D, C57BL/6J WT mice underwent sham or splenic neurectomy (SNX) 24 h before treatments. Selective α7nAChR agonist choline (CHO; 15 mg/kg i.p.) was administered 30 min before LPS (6 mg/kg i.p.). E, SNS re-establishes neuromodulation in α7nAChR knockout mice. Knockout mice underwent vagotomy 24 h before endotoxemia (LPS). Animals received VNS or SNS. ^#p < 0.01 versus LPS (n = 3; one-way ANOVA with Bonferroni’s corrections), *p < 0.01 versus control (n = 4; Mann–Whitney U test

FIGURE 5

Vagus nerve modulates systemic norepinephrine via the α7nAChR, the splenic nerve, and the spleen. Vagotomy (VGX) prevents afferent vagus nerve toward the CNS. VNS and α7nAChR agonists activate the splenic nerve to produce norepinephrine (NE) via the α7nAChR. Our results suggest that acetylcholine released by the vagus nerve in the celiac mesenteric ganglia (CMG) activates postsynaptic α7nAChR of the splenic nerve, leading to the release of norepinephrine in the spleen. The anti-inflammatory potential of the vagus nerve and α7nAChR agonists is inhibited by splenectomy or splenic neurectomy (SNX). SNS mimics the anti-inflammatory mechanism of the vagus nerve and α7nAChR agonists to induce norepinephrine and prevent LPS-induced serum TNF levels. SNS re-establishes neuromodulation in α7nAChR knockout mice.