5-HT₄Receptor-Mediated Neuroprotection and Neurogenesis in the Enteric Nervous System of Adult Mice

Although the mature enteric nervous system (ENS) has been shown to retain stem cells, enteric neurogenesis has not previously been demonstrated in adults. The relative number of enteric neurons in wild-type (WT) mice and those lacking 5-HT(4) receptors [knock-out (KO)] was found to be similar at birth; however, the abundance of ENS neurons increased during the first 4 months after birth in WT but not KO littermates. Enteric neurons subsequently decreased in both WT and KO but at 12 months were significantly more numerous in WT. We tested the hypothesis that stimulation of the 5-HT(4) receptor promotes enteric neuron survival and/or neurogenesis. In vitro, 5-HT(4) agonists increased enteric neuronal development/survival, decreased apoptosis, and activated CREB (cAMP response element-binding protein). In vivo, in WT but not KO mice, 5-HT(4) agonists induced bromodeoxyuridine incorporation into cells that expressed markers of neurons (HuC/D, doublecortin), neural precursors (Sox10, nestin, Phox2b), or stem cells (Musashi-1). This is the first demonstration of adult enteric neurogenesis; our results suggest that 5-HT(4) receptors are required postnatally for ENS growth and maintenance.