Autonomic neural control of motor activity in the intestine of freshwater barramundi (Lates calcarifer).

Evolutionary pressure on the gastrointestinal (GI) tract of teleost fish differs substantially from those of terrestrial animals. The intestine is a pivotal organ in teleost, for maintaining osmotic balance against the surrounding aquatic environment; primarily through its role in water absorption. Intestinal sensory and motor functions of vertebrates are largely mediated by the enteric nervous system (ENS) embedded within the gut wall. Although the ENS has been described in several teleost species, the group comprised of more than 25,000 species displays remarkable ecological, anatomical, and physiological diversity. As such, species that display distinct adaptations, i.e., euryhalinity, may provide valuable comparative insights. Here we show the structure of the ENS within the intestine of barramundi (Lates calcarifer), a catadromous perch species with a unique life history and growing commercial relevance. Immunohistochemical labelling identified enteric neurons synthesising nitric oxide synthase (NOS) and calcitonin-gene-related peptide (CGRP), both of which were typically uniaxonal with smooth cell bodies. Qualitatively, these neuronal populations formed a weakly arranged enteric plexus, analogous to the myenteric plexus found in terrestrial animals. Quantitatively, the proportion of NOS immunoreactive neurons decreased along the rostro-caudal axis of the intestine without accompanying changes to the overall neuronal density. Video imaging of intestinal wall movements ex vivo identified multiple recurrent motility patterns which were hexamethonium-sensitive, suggesting regulation by nicotinic synaptic transmission within enteric pathways. Together, these findings show that enteric neurons are present in the barramundi intestine but form a comparatively less defined plexus than in higher terrestrial vertebrates. These enteric neurons are involved in the regulation of intestinal motility via nicotinic transmission.