Gut-Brain Gateways to Parkinson's
Once it was thought that Parkinson’s disease (PD) was primarily a central nervous system condition that impinges upon motor function, but now we know that it entails critical inputs from the enteric nervous system (ENS), intestinal and brain barriers, exposome, enteric immune response, and microbiome. Deeper digging into the founding mechanisms of Parkinson’s suggests that it could actually originate in the intestines, employing immune dysregulation and the resulting inflammation as a chronic distress signal for recruiting the broader whole-body response; even the concept of the vagus nerve serving as a direct physical vector for conveying substances between the brain and gut is no longer revolutionary,
The relative and absolute makeup of gut microbe metabolites (fatty acids, neurotransmitters, peptides, hormones, etc.) affects immune cell metabolism and cytokine production, and the microbiome is, in turn, profoundly influenced by dietary inputs and environmental exposures. Consistent changes in relative abundance of several bacterial phyla has been noted in the PD microbiome, and may relate to inflammation and altered production of ghrelin (a hormonal factor in brain dopamine function), B vitamins, and short-chain fatty acids, especially butyrate (a crucial energy substrate for enteric health); some scientists feel that smoking and coffee drinking may impact PD through the microbiome. One well-known microbial candidate, Helicobacter pylori, may contribute to PD through multiple mechanisms, including altering gut microbiota composition, exacerbating inflammation, altering dopamine metabolism, worsening motor symptoms, and producing neurotoxic substances. Intestinal bacterial overgrowth is more common in those with PD, and PD patients with dysbiosis have shown greater symptomatology that improves with antimicrobial treatment. Animal models additionally present evidence for negative effects of microbiome transplants from PD patients on motor function.
Gastrointestinal difficulties like constipation and reduced motility characterize early Parkinson’s disease to some degree, and overexpression and aggregation of the distinctive Parkinson’s protein alpha-synuclein occurs in the intestines as well as the brain. The enteric nervous system (and particularly the inflammation-sensitive dopaminergic neurons in the intestinal musculature), which interacts in a profusely intimate fashion with the microbiome, appears to be a focal point in Parkinson’s pathology. Heightened intestinal permeability can additionally compromise the blood-brain and blood-cerebrospinal fluid barriers, introducing or aggravating neuroinflammatory processes, and the presence of pesticides in the individual exposome clearly contributes to PD risk.