Discrete responses of myenteric neurons to structural and functional damage by neurotoxins in vitro

Damage to the enteric nervous system is implicated in human disease and animal models of inflammatory bowel disease, diabetes, and Parkinson's disease, but the mechanism of death and the response of surviving neurons are poorly understood. We explored this in a coculture model of myenteric neurons, glia, and smooth muscle during exposure to the established or potential neurotoxins botulinum A, hydrogen peroxide, and acrylamide. Neuronal survival, axonal degeneration and regeneration, and neurotransmitter release were assessed during acute exposure (0-24 h) to neurotoxin and subsequent recovery (96-144 h). Unique and selective responses to each neurotoxin were found with acrylamide (0.5-2.0 mM) causing a 30% decrease in axon number without neuronal loss, whereas hydrogen peroxide (1-200 microM) caused a parallel loss in both axon and neuron number. Immunoblotting identified the loss of synaptic vesicle proteins that paralleled axon damage and was associated with marked suppression of depolarization-induced release of acetylcholine (ACh). The caspase inhibitor zVAD, but not DEVD, significantly prevented neuronal death, implying a largely caspase-3/7-independent mechanism of apoptotic death that was supported by staining for annexin V and cleaved caspase-3. In contrast, botulinum A (2 microg/ml) caused a 40% decrease in ACh release without effect on neuronal survival or axon structure. By 96 h after exposure to acrylamide or hydrogen peroxide, axon number was restored to or even surpassed the level of time-matched controls, regardless of partial neuronal loss, but ACh release remained markedly suppressed. Neural responses to toxic factors are initially unique but then converge upon robust axonal regeneration, whereas neurotransmitter release is both vulnerable to damage and slow to recover.