Enteric Dysfunctions in Experimental Parkinson's Disease: Alterations of Excitatory Cholinergic Neurotransmission Regulating Colonic Motility in Rats.

Parkinson's disease is frequently associated with gastrointestinal symptoms, mostly represented by constipation and defecatory dysfunctions. This study examined the impact of central dopaminergic denervation, induced by injection of 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle, on distal colonic excitatory cholinergic neuromotor activity in rats. Animals were euthanized 4 and 8 weeks after 6-OHDA injection. In vivo colonic transit was evaluated by radiologic assay. Electrically induced and carbachol-induced cholinergic contractions were recorded in vitro from longitudinal and circular muscle colonic preparations, whereas acetylcholine levels were assayed in the incubation media. Choline acetyltransferase (ChAT), HuC/D (pan-neuronal marker), muscarinic M2 and M3 receptors were assessed by immunohistochemistry or western blot assay. As compared with control rats, at week 4, 6-OHDA-treated animals displayed the following changes: decreased in vivo colonic transit rate, impaired electrically evoked neurogenic cholinergic contractions, enhanced carbachol-induced contractions, decreased basal and electrically stimulated acetylcholine release from colonic tissues, decreased ChAT immunopositivity in the neuromuscular layer, unchanged density of HuC/D immunoreactive myenteric neurons, and increased expression of colonic muscarinic M2 and M3 receptors. The majority of such alterations were also detected at week 8 post 6-OHDA injection. These findings indicate that central nigrostriatal dopaminergic denervation is associated with an impaired excitatory neurotransmission characterized by a loss of myenteric neuronal ChAT positivity and decrease in acetylcholine release, resulting in a dysregulated smooth muscle motor activity, which likely contributes to the concomitant decrease in colonic transit rate.

Diet-induced regulation of bitter taste receptor subtypes in the mouse gastrointestinal tract.

Bitter taste receptors and signaling molecules, which detect bitter taste in the mouth, are expressed in the gut mucosa. In this study, we tested whether two distinct bitter taste receptors, the bitter taste receptor 138 (T2R138), selectively activated by isothiocyanates, and the broadly tuned bitter taste receptor 108 (T2R108) are regulated by luminal content. Quantitative RT-PCR analysis showed that T2R138 transcript is more abundant in the colon than the small intestine and lowest in the stomach, whereas T2R108 mRNA is more abundant in the stomach compared to the intestine. Both transcripts in the stomach were markedly reduced by fasting and restored to normal levels after 4 hours re-feeding. A cholesterol-lowering diet, mimicking a diet naturally low in cholesterol and rich in bitter substances, increased T2R138 transcript, but not T2R108, in duodenum and jejunum, and not in ileum and colon. Long-term ingestion of high-fat diet increased T2R138 RNA, but not T2R108, in the colon. Similarly, α-gustducin, a bitter taste receptor signaling molecule, was reduced by fasting in the stomach and increased by lowering cholesterol in the small intestine and by high-fat diet in the colon. These data show that both short and long term changes in the luminal contents alter expression of bitter taste receptors and associated signaling molecules in the mucosa, supporting the proposed role of bitter taste receptors in luminal chemosensing in the gastrointestinal tract. Bitter taste receptors might serve as regulatory and defensive mechanism to control gut function and food intake and protect the body from the luminal environment.

Radiological analysis of gastrointestinal dysmotility in a model of central nervous dopaminergic degeneration: comparative study with conventional in vivo techniques in the rat.

INTRODUCTION: Gastrointestinal (GI) motility disorders include many clinical manifestations associated with various pathologies. They are widespread and can be considered a primary symptom or can be associated to other diseases, such as Parkinson's disease. Understanding the type and site of GI dysmotility is crucial to identify the functional abnormality and to unravel the underlying mechanisms, in order to design adequate therapeutic interventions. METHODS: In the present study, we applied radiological analysis, a common tool in clinical practice, to follow up in vivo the progression of GI dysmotility over time and along the entire GI tract in an animal model of central nervous dopaminergic degeneration and compared these results to those obtained with standard techniques commonly used to assess GI motor functions in small rodents. RESULTS: Our radiological data, showing delayed gastric emptying and constipation, agree with and expand previous information obtained with other functional assays in the same model, suggesting that radiological analysis can be an appropriate method to explore GI dysmotility in animal models of human pathologies. DISCUSSION: In this study we have applied for the first time the GI radiological analysis to an animal model of central nervous dopaminergic degeneration providing a non-invasive/animal-preserving approach, ethically more acceptable and useful to follow up the development of GI dysmotility in pathologies evolving over time.

Effects of methylnaltrexone on guinea pig gastrointestinal motility.

The purpose of the present study was to compare the effects of methylnaltrexone (MNTX), a peripherally acting µ opioid receptor (µOR) antagonist, on gastrointestinal (GI) motility in naïve vs. opiate chronically treated guinea pigs in vitro and in vivo. We have used the electrically stimulated muscle twitch contractions of longitudinal muscle-myenteric plexus (LMMP) preparations and total GI transit as measure of GI motility. In LMMP preparations of naïve guinea pigs, MNTX (1-30 µM) induced a significant, dose-response reduction of morphine-induced inhibition of electrically stimulated muscle twitch contractions, with an IC50 of 9.4 10(-8)M. By contrast, MNTX abolished the inhibitory effect of acute morphine at any concentrations tested (1-30 µM) in the guinea pigs chronically treated with opiates. In vivo, MNTX (10-50 mg s.c.) did not affect GI transit in naïve guinea pigs when administered acutely or for five consecutive days, but reversed the GI transit delay induced by chronic morphine treatment. These findings show that MNTX is effective in reversing opiate-induced inhibition of GI motility acting at peripheral µ opioid receptors, but does not exert a pharmacologic effect on GI transit in the absence of opiate stimulation.