Excitotoxicity in the Enteric Nervous System

Abstract

Glutamate, the major excitatory neurotransmitter in the CNS, is also an excitatory neurotransmitter in the enteric nervous system (ENS). We tested the hypothesis that excessive exposure to glutamate, or related agonists, produces neurotoxicity in enteric neurons. Prolonged stimulation of enteric ganglia by glutamate caused necrosis and apoptosis in enteric neurons. Acute and delayed cell deaths were observed. Glutamate neurotoxicity was mimicked by NMDA and blocked by the NMDA antagonist D-2-amino-5-phosphonopentanoate. Excitotoxicity was more pronounced in cultured enteric ganglia than in intact preparations of bowel, presumably because of a reduction in glutamate uptake. Glutamate-immunoreactive neurons were found in cultured myenteric ganglia, and a subset of enteric neurons expressed NMDA (NR1, NR2A/B), AMPA (GluR1, GluR2/3), and kainate (GluR5/6/7) receptor subunits. Glutamate receptors were clustered on enteric neurites. Stimulation of cultured enteric neurons by kainic acid led to the swelling of somas and the growth of varicosities ("blebs") on neurites. Blebs formed close to neurite intersections and were enriched in mitochondria, as revealed by rhodamine 123 staining. Kainic acid also produced a loss of mitochondrial membrane potential in cultured enteric neurons at sites where blebs tended to form. These observations demonstrate, for the first time, excitotoxicity in the ENS and suggest that overactivation of enteric glutamate receptors may contribute to the intestinal damage produced by anoxia, ischemia, and excitotoxins present in food. Key words: necrosis; apoptosis; NMDA; kainic acid; glutamate transporters; bleb formation; rhodamine 123Excessive exposure to glutamate causes cell death ("excitotoxicity") in C NS neurons (Choi, 1988(Choi, , 1995. E xcitotoxicity consists of necrosis and apoptosis and is thought to occur via a breakdown in ionic homeostasis mediated by NMDA and non-NMDA glutamate receptor subtypes. Neurons can be protected from excitotoxicity by C a 2ϩ buffers (T ymianski et al., 1993); therefore, increases in [C a 2ϩ ] i are involved in causing excitotoxic cell death (Choi, 1988(Choi, , 1992. Consistent with this idea, both NMDA and kainic acid increase [C a 2ϩ ] i in central neurons (MacDermott et al., 1986;Brorson et al., 1994). Moreover, excessive exposure to either glutamate receptor agonist produces neuronal cell loss . Increases in [C a 2ϩ ] i produced by these excitotoxins occur at sites that are rich in mitochondria and produce a loss of mitochondrial membrane potential (Ankarcrona et al., 1995;Bindokas and Miller, 1995). Mitochondrial f unction seems to determine the mode of neuronal death in excitotoxicity. Early necrosis develops in neurons that lose mitochondrial membrane potential. Delayed apoptosis develops in neurons that recover mitochondrial potential and energy levels.Substantial evidence suggests that glutamate is an excitatory neurotransmitter in the enteric nervous system (ENS). The bowel contains glutamate-immunoreactive neurons, enteric neurons express ...

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