No abstract

Background: The noradrenergic innervation of the spleen is implicated in the autonomic control of inflammation and has been the target of neurostimulation therapies for inflammatory diseases. However, there is no real-time marker of its successful activation, which hinders the optimization of anti-inflammatory neurostimulation therapies and mechanistic studies in anti-inflammatory neural circuits. Methods: In mice, we performed fast-scan cyclic voltammetry (FSCV) in the spleen during intravascular injections of norepinephrine (NE), or during stimulation of the vagus, splanchnic, or splenic nerves. We defined the stimulus-elicited charge generated at the oxidation potential for NE (~0.8 V) as the “NE voltammetry signal” and quantified the dependence of the signal on NE or nerve stimulation dose. We correlated the NE voltammetry signal in response to splenic nerve stimulation (SpNS) with the latter’s anti-inflammatory effect in a model of lipopolysaccharide- (LPS) induced endotoxemia, quantified as suppression of TNF release. Results: We found that the NE voltammetry signal is proportional to injected amount and estimated peak NE concentration, with 0.3 μM detection threshold. In response to SpNS, the signal increases within seconds, returns to baseline minutes later and is blocked by interventions that deplete NE or inhibit NE release. The signal is elicited by efferent, but not afferent, electrical or optogenetic vagus nerve stimulation, and by splanchnic nerve stimulation. The magnitude of the signal during SpNS is inversely correlated with subsequent TNF suppression in endotoxemia and explains 40% of the variance in TNF measurements. Conclusion: FSCV in the spleen provides a marker for real-time monitoring of anti-inflammatory activation of the splenic innervation during autonomic stimulation.

Bioelectronic medicine uses neurostimulation devices to modulate the activity of autonomic nerves and restore function in visceral organs. The autonomic, noradrenergic, innervation of the spleen is implicated in the neural control of inflammation; for that reason, the spleen is the target of autonomic stimulation therapies tested in rheumatoid arthritis and other inflammatory diseases. However, there are no real time markers for successful engagement of the spleen by autonomic stimulation. Here, we use fast-scan cyclic voltammetry (FSCV) in the spleen to assess, in real time, the release of norepinephrine (NE) in response to autonomic stimulation in mice. We show that spleen voltammograms demonstrate oxidative current peaks at a voltage of ~0.8 V that is proportional up to a NE blood concentration of 30 to 300nM. The oxidative current increases within seconds in response to electrical splenic nerve stimulation and is blocked by interventions that deplete NE or inhibit NE release. Oxidative current increases in response to electrical vagus nerve stimulation (VNS) and is elicited by efferent, but not afferent, fiber-selective optogenetic VNS. Spleen cyclic voltammetry can potentially provide a real-time physiological marker for dose calibration and monitoring of the anti-inflammatory effectiveness of autonomic stimulation therapies targeting the spleen.