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Krauter, Eric Michael

Anatomy and Neurobiology

2007

PhD

The functions of the gastrointestinal (GI) tract are coordinated by the enteric nervous system (ENS), which resides within the gut wall. The myenteric plexus of the ENS is involved with regulating intestinal motor activities of the gut that serve to mix luminal contents and transport them along the intestines. Inflammation associated with inflammatory bowel disease (IBD) causes intestinal dysmotility. Additionally, this altered intestinal motility can persist following recovery from inflammation. To understand the underlying mechanisms of this gut dysfunction, we used the trinitrobenzene sulfonic acid (TNBS) model of colitis. Previously we have demonstrated that TNBS-induced inflammation causes facilitation of fast excitatory postsynaptic potential (EPSP) amplitudes in interneurons and motor neurons and hyperexcitability of sensory neurons. Changes in the neuronal properties can contribute to the uncoordinated peristaltic reflex resulting in dysmotility. The focus of this dissertation project was to elucidate the mechanism(s) that underlie the facilitation of the EPSP and to test whether changes in the myenteric neurons persist after the resolution of inflammation. The first study tested the hypothesis that the inflammation-induced fast synaptic facilitation is due to presynaptic mechanisms. Intracellular recordings from S neurons of control and inflamed tissues were evaluated. No differences in the amplitudes of responses to exogenously applied neurotransmitters or in the pharmacological profile of fEPSPs were detected. 5-HT₄ receptor antagonist, GR125487, reduced the fEPSP amplitudes in the neurons from control and inflamed tissue to the same extent. The PKA inhibitor, H89, reduced the fEPSP in inflamed tissue to a normal level, but did not affect the fEPSP in control tissue. The BK channel blocker, iberiotoxin, increased the EPSPs in control tissue but not in inflamed tissue. Less rundown of fEPSPs and a higher paired pulse ratio was detected in inflamed tissue as compared to control. No change in the synaptic contact density was detected between control and inflamed tissue. These data suggest that inflammation-induced facilitation of fEPSP amplitudes involves increased activity of PKA, independent of the 5-HT₄ receptor, possibly leading to inhibition of the BK channel, and inflammation also leads to an increase in the readily releasable pool.
The second study tested the hypothesis that the inflammation-induced changes in the electrophysiological properties of myenteric neurons and in intestinal motility persist after the resolution of inflammation. Myeloperoxidase activity and gross damage scores return to normal levels four weeks after TNBS administration. Altered motility patterns and changes in the electrophysiological properties of myenteric neurons persisted 4 weeks following recovery from inflammation. These data suggest that the inflarnrnationinduced changes in the myenteric neurons occur during active TNBS colitis and potentially contribute to the intestinal dysmotility observed following recovery from inflammation. These investigations indicate that inflammation can lead to synaptic plasticity involving multiple mechanisms, and that inflammation does not need to persist in order for the inflammation-induce changes in the myenteric neurons to be sustained. These studies may assist in the development of new therapeutics to alleviate the symptoms of IBD or patients in remission from IBD.