Changes in Nicotinic Neurotransmission during Enteric Nervous System Development

Acetylcholine-activating pentameric nicotinic receptors (nAChRs) are an essential mode of neurotransmission in the enteric nervous system (ENS). In this study, we examined the functional development of specific nAChR subtypes in myenteric neurons using Wnt1-Cre; R26R-GCaMP3 mice, where all enteric neurons and glia express the genetically encoded calcium indicator, GCaMP3. Transcripts encoding alpha 3, alpha 4, alpha 7, beta 2, and beta 4 nAChR subunits were already expressed at low levels in the E11.5 gut and by E14.5 and, thereafter, alpha 3 and beta 4 transcripts were the most abundant. The effect of specific nAChR subtype antagonists on evoked calcium activity in enteric neurons was investigated at different ages. Blockade of the alpha 3 beta 4 receptors reduced electrically and chemically evoked calcium responses at E12.5, E14.5, and P0. In addition to the alpha 3 beta 4 antagonist, antagonists to alpha 3 beta 2 and alpha 4 beta 2 also significantly reduced responses by P10-11 and in adult preparations. Therefore, there is an increase in the diversity of functional nAChRs during postnatal development. However, an alpha 7 nAChR antagonist had no effect at any age. Furthermore, at E12.5 we found evidence for unconventional receptors that were responsive to the nAChR agonists 1-dimethyl-4-phenylpiperazinium and nicotine, but were insensitive to the general nicotinic blocker, hexamethonium. Migration, differentiation, and neuritogenesis assays did not reveal a role for nAChRs in these processes during embryonic development. In conclusion, there are significant changes in the contribution of different nAChR subunits to synaptic transmission during ENS development, even after birth. This is the first study to investigate the development of cholinergic transmission in the ENS.