Loss of enteric dopaminergic neurons and associated changes in colon motility in an MPTP mouse model of Parkinson's disease

Abstract

Gastrointestinal (GI) dysfunction is the most common non-motor symptom of Parkinson's disease (PD). Symptoms of GI dysmotility include early satiety and nausea from delayed gastric emptying, bloating from poor small bowel coordination, and constipation and defecatory dysfunction from impaired colonic transit. Understanding the pathophysiology and treatment of these symptoms in PD patients has been hampered by the lack of investigation into GI symptoms and pathology in PD animal models. We report that the prototypical parkinsonian neurotoxin, MPTP (1-methyl 4-phenyl 1,2,3,6-tetrahydropyridine), is a selective dopamine neuron toxin in the enteric nervous system (ENS). When examined 10 days after treatment, there was a 40% reduction of dopamine neurons in the ENS of C57Bl/6 mice administered MPTP (60 mg/kg). There were no differences in the density of cholinergic or nitric oxide neurons. Electrophysiological recording of neural-mediated muscle contraction in isolated colon from MPTP-treated animals confirmed a relaxation defect associated with dopaminergic degeneration. Behaviorally, MPTP induced a transient increase in colon motility, but no changes in gastric emptying or small intestine transit. These results provide the first comprehensive assessment of gastrointestinal pathophysiology in an animal model of PD. They provide insight into the impact of dopaminergic dysfunction on gastrointestinal motility and a benchmark for assessment of other PD model systems.

Figure 1. MPTP causes dopamine neuron loss in the ENS

A. Bar graph indicating the number of TH-positive neurons per ganglion in the myenteric plexus of the distal ileum in mice ten days after treatment with saline (N=8) or MPTP (60 mg/kg; N=7). *p < 0.05. B. Photograph of the myenteric plexus from mouse ileum immunostained for TH. The arrow denotes a TH-positive neuron shown at higher magnification in the inset. The dashed lines outline two ganglia.

Figure 2. MPTP does not affect numbers of nitric oxide or cholinergic neurons in the ENS

A. Bar graph indicating the number of NADPH-diaphorase-positive neurons per mm² in the myenteric plexus of the distal ileum in mice ten days after treatment with saline (N=7) or MPTP ( N=7). B. Bar graph indicating the number of ChAT-positive neurons per mm² in the myenteric plexus of the distal ileum in mice ten days after treatment with saline (N=4) or MPTP (N=7). C. Photograph of the myenteric plexus from mouse ileum immunostained for ChAT (brown) and histochemically stained for NADPH-diaphorase (blue). Note the relative abundance of both neuron types as compared to TH-positive cells. Inset shows a higher magnification view of one ChAT-positive and one NADPH-positive neuron.

Figure 3. Dopamine neuron loss causes impaired neural-mediated relaxation of proximal colon

A. Representative tracing of EFS-induced muscle contraction in circular muscle preparations from a saline- and an MPTP-treated mouse. B. Compiled data from 4 experiments like that shown in A quantifying enhanced contraction in MPTP-treated mice. C. Representative tracing of EFS-induced muscle relaxation in longitudinal muscle preparations from a saline- and an MPTP-treated mouse. D. Compiled data from 4 experiments like that shown in C quantifying impaired relaxation in MPTP-treated mice. *p <0.05.

Figure 4. Transiently increased colon motility after MPTP treatment

A. Stool frequency was dramatically higher in MPTP-treated animals 2-3 days after treatment, but was similar to saline-treated controls by 8-10 days after MPTP. (N=8 per group). *p < 0.05. B. Solid matter in stool correlates with stool frequency (R = −0.69). Since the colon functions to remove water, this confirms the utility of one-hour stool frequency as a measure of colon transit time. Data are from multiple one-hour collection periods from saline- (N=14) and MPTP-treated (N=12) animals across the entire time course of the experiment (from prior to injection through 10 days after).

Figure 5. Gastric emptying is unaffected by MPTP

A. Time course of gastric dye retention in saline- and MPTP-treated animals ten days after treatment. Amount of dye remaining in the stomach was normalized between the zero and 1 hour time points. (N=3 per group per time point). B. The amount of solid food remaining in the stomach after 2 hours was no different between groups (N=13 per group).

Figure 6. Small intestinal transit is unaffected by MPTP

Distance from the pylorus to the dye front is plotted against time. (N=3 per group per time point).