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.

Introduction

Gastrointestinal dysfunction is a prominent non-motor feature of Parkinson's disease (PD). PD patients experience symptoms that span the entire alimentary tract including abnormal salivation, dysphagia, delayed gastric emptying, constipation, and defecatory dysfunction (Pfeiffer, 2003, Pfeiffer and Quigley, 1999). Dysfunctional motility is the pathophysiological mechanism underlying many of these symptoms. This dysmotility contributes directly to the morbidity of PD and complicates the disease's clinical management. For example, in the stomach, delayed emptying leads to nausea, contributes to weight loss because of decreased food intake, and adds to fluctuations in motor impairment from variable absorption of medication (Djaldetti et al., 1996, Goetze et al., 2006, Goetze et al., 2005, Kurlan et al., 1988). In the colon, longer transit time due to poor motility causes harder stools and constipation by increasing the absorption of water (Ashraf et al., 1995, Edwards et al., 1992, Ueki and Otsuka, 2004). The exact mechanism of motility dysfunction in PD is poorly understood. Lack of understanding of the changes in the gastrointestinal tract in PD has led to limited success in the treatment of gastrointestinal dysfunction in PD.

Control of gastrointestinal motility arises from both local and central locations. Local control is directed by the intrinsic enteric nervous system (ENS), a semiautonomous network of nerves that consists of a deep myenteric and more superficial submucosal plexus (Johnson, 2001). The myenteric plexus is the more important of the two in terms of controlling motility. Circuitry in this plexus controls the contraction and relaxation of the circular and longitudinal smooth muscle that line the length of the gastrointestinal tract. Though neurons producing virtually every neurotransmitter seen in the central nervous system have been identified within the ENS, acetylcholine serves as the primary excitatory neurotransmitter, while nitric oxide and vasoactive intestinal peptide are the prominent inhibitory transmitters (Johnson, 2001). Effective peristalsis depends on precise temporal coordination of inhibition and excitation of the two layers of smooth muscle. Central control arises from the autonomic inputs imposed on the local enteric nervous system. Parasympathetic, cholinergic innervation originates primarily in the dorsal motor nucleus of the vagus (DMV) in the medulla and generally promotes increased motility. Sympathetic input originates in paravertebral and sacral ganglia and generally serves to inhibit gastrointestinal motility.

Although PD has traditionally been considered a disease of dopaminergic neurons in the substantia nigra, pathological analyses of brain and gastrointestinal samples from PD patients have suggested neuronal loss in other areas. Lewy bodies have been described in both the myenteric and submucosal plexuses of the ENS (Braak et al., 2006, Kupsky et al., 1987, Singaram et al., 1995, Wakabayashi et al., 1988, Wakabayashi et al., 1989, Wakabayashi et al., 1990), and other research has found lower levels of enteric dopaminergic neurons in ENS of PD patients (Singaram et al., 1995). Central areas associated with gastrointestinal motility have also been implicated. For example, Braak et al. (2003) suggest that the DMV is perhaps the earliest central site affected in PD because α-synuclein pathology in the DMV was found in brains without damage to the substantia nigra (Braak et al., 2003).

One factor that has made research into the etiology of gastrointestinal dysfunction and evaluation of effective treatments difficult has been the absence of an animal model for this aspect of PD. The aim of the present research was to evaluate gastrointestinal dysfunction in a well-described animal model of PD using the selective dopamine neurotoxin MPTP (1-methyl 4-phenyl 1,2,3,6-tetrahydropyridine) (Jackson-Lewis et al., 1995). Animals treated with MPTP were behaviorally evaluated for signs of dysmotility in the stomach, small intestine, and colon. In addition, samples were examined for evidence of neuronal dysfunction and loss in the ENS. The results suggest that MPTP is a selective dopaminergic neurotoxin in the ENS and causes changes in colonic motility.