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Abstract
We evaluated the chemical coding of the myenteric plexus in the proximal and distal intestine of gilthead sea bream (Sparus aurata), which represents one of the most farmed fish in the Mediterranean area. The presence of nitric oxide (NO), acetylcholine (ACh), serotonin (5-HT), calcitonin-gene-related peptide (CGRP), substance P (SP) and vasoactive intestinal peptide (VIP) containing neurons, was investigated in intestinal whole mount preparations of the longitudinal muscle with attached the myenteric plexus (LMMP) by means of immunohistochemical fluorescence staining. The main excitatory and inhibitory neurochemicals identified in intestinal smooth muscle were ACh, SP, 5HT, and NO, VIP, CGRP. Some neurons displayed morphological features of ascending and descending interneurons and of putative sensory neurons. The expression of these pathways in the two intestinal regions is largely superimposable, although some differences emerged, which may be relevant to the morphological properties of each region. The most important variances are the higher neuronal density and soma size in the proximal intestine, which may depend on the volume of the target tissue. Since in the fish gut the submucosal plexus is less developed, myenteric neurons substantially innervate also the submucosal and epithelial layers, which display a major thickness and surface in the proximal intestine. In addition, myenteric neurons containing ACh and SP, which mainly represent excitatory motor neurons and interneurons innervating the smooth muscle were more numerous in the distal intestine, possibly to sustain motility in the thicker smooth muscle coat. Overall, this study expands our knowledge of the intrinsic innervation that regulates intestinal secretion, absorption and motility in gilthead sea bream and provides useful background information for rational design of functional feeds aimed at improving fish gut health.
Highlights
The control of the main digestive functions in fish, as in other vertebrates, largely depends upon the activation of intrinsic neuronal circuitries constituting the enteric nervous system (ENS)
In the proximal intestine, villi were generally branched and characterized by an elongated shape projecting into the intestinal lumen, whereas in the distal intestine villi were stubbier with a larger base (Fig 1, panels A-B)
We give a first description of myenteric plexus chemical coding in the juvenile gilthead sea bream (Sparus aurata) proximal and distal intestine
Summary
The control of the main digestive functions in fish, as in other vertebrates, largely depends upon the activation of intrinsic neuronal circuitries constituting the enteric nervous system (ENS). The myenteric plexus lays between the longitudinal and circular layers of the muscularis propria and is principally involved in the regulation of the gut motor function, whereas the submucosal plexus, which is less well developed in fish than in mammals, mainly regulates intestinal secretion [1,2]. The structural organization of fish myenteric plexus shows some peculiar differences with respect to the mammalian ENS. This latter consists of ganglia, composed of neurons and enteric glia, neuronal connections between ganglia, and nerve fibers supplying the effector tissues [3,4]. The fish ENS lacks such a well-organized network of ganglia and interconnecting fibers, instead, neurons are either scattered upon the longitudinal muscular layer or aggregated in small groups at the nodes of fiber connections. The pattern of neuron distribution over the muscular layer, is apparently not casual but follows nerve bundles along the length of the gut [1,2]
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