The substantia nigra modulates proximal colon tone and motility in a vagally-dependent manner in the rat

Previous studies have shown that a monosynaptic pathway connects the substantia nigra pars compacta (SNpc) to neurons of the dorsal motor nucleus of the vagus (DMV). This monosynaptic pathway modulates the vagal control of gastric motility. The present study was designed to test the hypothesis that this nigro-vagal pathway also modulates the tone and motility of the proximal colon.In rats, concurrent microinjection of a retrograde tracer in the proximal colon and an anterograde tracer in the SNpc (n=4) showed that bilaterally labelled colonic-projecting preganglionic vagal motoneurons in the DMV received inputs from SNpc neurons. Activation of SNpc neurons via microinjection of the ionotropic glutamate selective agonist, NMDA, increased proximal colonic motility (187±52.6% of baseline, n=13) and tone (323±97.2mg, n=14) as measured via a strain gauge aligned with the colonic circular smooth muscle (paired t test, p<0.0001). Following transfection of SNpc neurons with pAAV-hSyn-hM3D(Gq)-mCherry, chemogenetic activation of nigro-vagal nerve terminals by brainstem application of CNO increased proximal colon motility (157±24.5% of baseline; n=3; unpaired t test; p=0.0399). The increase in colon motility following SNpc neuronal activation was abolished by brainstem pretreatment with the dopaminergic DA1-like antagonist SCH23390 (n=4; one-way ANOVA, p=0.9762 vs baseline; p=0.0159 vs NMDA alone) as well as by acute subdiaphragmatic vagotomy (94±47.9% of baseline; n=3; One-way ANOVA, p=0.9816 vs baseline and p=0.0105 vs NMDA alone). In contrast to the excitatory effects observed upon activation of brainstem D1R following SNpc stimulation, DMV microinjection of DA decreased both the tone (-81±27.3mg, n=7) and motility (49±21.6% of baseline; n=7; paired t test: p=0.0004) of the proximal colon. These inhibitory DA effects were attenuated by application of the DA2-like antagonist L741646 (tone -100±58.9mg vs 99±100.1mg; n=4; paired t test: t3=2.567, p=0.0414 and motility 63±15% vs 144±38% of baseline; n=5; one-way ANOVA, p=0462 in the absence and presence of L741646).These data suggest that a monosynaptic nigro-vagal pathway activates brainstem D1 receptors to increase proximal colon tone and motility in a vagally-dependent manner. This study provides the anatomical and mechanistic basis to explain how loss of SNpc neurons may lead to gastrointestinal dysfunction, including constipation, observed in patients with Parkinson’s Disease. no disclose This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.