Activation of dorsal motor nucleus cholinergic neurons ameliorates the severity of acute pancreatitis

Abstract

Clusters of neuron cell bodies, termed nuclei, residing in the brainstem are the origin of cranial nerves which transmit action potentials controlling organ function. Neurons residing in the brainstem dorsal motor nucleus (DMN) project in the vagus nerve to communicate with the pancreas, liver, gastrointestinal tract and other organs. Cholinergic signaling in the vagus nerve‐mediated inflammatory reflex also controls immune system responses by inhibiting the production of cytokines in the spleen, although the function of DMN neurons in regulating pancreatic inflammation is not known. Here, we demonstrate that selective activation of cholinergic neurons in the DMN attenuates pancreatitis severity. Pancreatitis was induced with two intraperitoneal injections of the cholecystokinin analogue caerulein (50 mcg/kg), given one hour apart. A fiber‐optic cannula was inserted under stereotactic guidance into the DMN in transgenic mice expressing light‐responsive ion channel, channelrhodopsin, in cholinergic neurons under the choline acetyltransferase promoter. Animals were subjected to either optogenetic stimulation (473nm, 20 Hz, 25% duty cycle, 8‐12 mW total power) or sham stimulation (no light) for five minutes. Optogenetic stimulation but not sham stimulation of the cholinergic neurons in the DMN significantly attenuates serum amylase (4043mU/mL ± 264.4 vs. 2940mU/mL ± 170.9, p=0.0074; n=9‐11 mice/group) and pancreatic IL‐6 levels (1907pg/mg ± 879.3 vs. 1046pg/mg± 288.1, p=0.0330; n=9‐11 mice/group). The decreased severity of pancreatitis in optogenetically stimulated mice is further demonstrated by a significant improvement in pancreatic histologic severity score (6.86 ± 1.67 vs. 5.35 ± 1.44, p = 0.0462; n = 9‐11). Pharmacological blockade and surgical ablation of the vagus nerve signaling inhibit protective effects of DMN cholinergic neurons activation. Pre‐treatment with mecamylamine, a nicotinic receptor antagonist, attenuates the reduction in serum amylase following light stimulation (Sham stimulation vs. DMN vs DMN + Mecamylamine: 5047 ± 315.4 vs. 3769 ± 241.5 vs. 5620 ± 385.9, p = 0037; n = 10 mice/group). Additionally, mice with subdiaphragmatic vagotomy has no difference in serum amylase levels following optogenetic stimulation (2761mU/mL ± 787.3 vs. 2517mU/mL, p= 0.300; n=3), whereas sham surgery controls show a significant reduction in serum amylase levels (3224mU/mL ± 593.1 vs. 2087 mU/mL ± 951.2, p=0.0303; n=6 mice/group). These studies identify DMN as an important locus that regulates the severity of acute pancreatitis in vagus‐nerve‐dependent fashion, and provide new insights into the identity and central origin of the efferent vagus nerve fibers regulating acute pancreatitis.

Dorsal Motor Vagal Neurons Can Elicit Bradycardia and Reduce Anxiety-Like Behavior

“…Despite historical controversy about the capacity of CVN DMV to impact HR, our results demonstrate that optogenetic and chemogenetic activation of Chat+ DMV neurons each caused robust bradycardia (~56% and ~65.5% of resting HR, respectively) in awake, behaving mice. Cardioinhibitory responses using optogenetic techniques in urethane-anesthetized rats and ketamine/xylazine-anesthetized mice demonstrated variable strength of the response (Machhada et al, 2020;Thompson et al), implicating differences in anesthesia for differences in effect size. Alternatively, previous species used (namely cats and rats) may exhibit a lesser degree of DMV-activation driven bradycardia than mice because of differences in vagal tonus.…”

“…Despite these anatomical studies, functional studies remain conflicting. Some argue that direct electrical stimulation of DMV does not change chronotropy (Geis and Wurster, 1980;Laughton and Powley, 1987), while others demonstrate local activation (chemical, electrical, optogenetic, or chemogenetic) elicits bradycardias even if only modestly (Falvey et al, 2023;Machhada et al, 2020;Machhada et al, 2016;Sporton et al, 1991;Thompson et al, 2023). However, no activation technique in awake animals used to date rules out incidental stimulation of either neighboring nucleus tractus solitarius (NTS) neurons or inhibitory interneurons within DMV.…”