The effects of putative non-adrenergic, non-cholinergic autonomic transmitters on isolated strips from the stomach of the rainbow trout. Salmo gairdneri

Dietary Lipid Modulation of Intestinal Serotonin in Ballan Wrasse (Labrus bergylta)-In Vitro Analyses

Serotonin (5-HT) is pivotal in the complex regulation of gut motility and consequent digestion of nutrients via multiple receptors. We investigated the serotonergic system in an agastric fish species, the ballan wrasse (Labrus bergylta) as it represents a unique model for intestinal function. Here we present evidence of the presence of enterochromaffin cells (EC cells) in the gut of ballan wrasse comprising transcriptomic data on EC markers like adra2a, trpa1, adgrg4, lmxa1, spack1, serpina10, as well as the localization of 5-HT and mRNA of the rate limiting enzyme; tryptophan hydroxylase (tph1) in the gut epithelium. Second, we examined the effects of dietary marine lipids on the enteric serotonergic system in this stomach-less teleost by administrating a hydrolyzed lipid bolus in ex vivo guts in an organ bath system. Modulation of the mRNA expression from the tryptophan hydroxylase tph1 (EC cells isoform), tph2 (neural isoform), and other genes involved in the serotonergic machinery were tracked. Our results showed no evidence to confirm that the dietary lipid meal did boost the production of 5-HT within the EC cells as mRNA tph1 was weakly regulated postprandially. However, dietary lipid seemed to upregulate the post-prandial expression of tph2 found in the serotonergic neurons. 5-HT in the intestinal tissue increased 3 hours after "exposure" of lipids, as was observed in the mRNA expression of tph2. This suggest that serotonergic neurons and not EC cells are responsible for the substantial increment of 5-HT after a lipid-reach "meal" in ballan wrasse. Cells expressing tph1 were identified in the gut epithelium, characteristic for EC cells. However, Tph1 positive cells were also present in the lamina propria. Characterization of these cells together with their implications in the serotonergic system will contribute to broad the scarce knowledge of the serotonergic system across teleosts.

Autonomic control of gut motility: a comparative view

Gut motility is regulated to optimize food transport and processing. The autonomic innervation of the gut generally includes extrinsic cranial and spinal autonomic nerves. It also comprises the nerves contained entirely within the gut wall, i.e. the enteric nervous system. The extrinsic and enteric nervous control follows a similar pattern throughout the vertebrate groups. However, differences are common and may occur between groups and families as well as between closely related species. In this review, we give an overview of the distribution and effects of common neurotransmitters in the vertebrate gut. While the focus is on birds, reptiles, amphibians and fish, mammalian data are included to form the background for comparisons. While some transmitters, like acetylcholine and nitric oxide, show similar distribution patterns and effects in most species investigated, the role of others is more varying. The significance for these differences is not yet fully understood, emphasizing the need for continued comparative studies of autonomic control.

Evidence for P2-purinoceptor-mediated uptake of Ca2+ across a fish (Oreochromis mossambicus) intestinal brush border membrane

We have studied the effect of ATP on Ca2+ uptake in intestinal brush border membrane vesicles (BBMVs) of the teleost tilapia (Oreochromis mossambicus). ATP stimulated Ca2+ uptake 12-fold over the control, with a linear time course. Ionomycin and detergent treatment did not reduce BBMVs' Ca2+ content, indicating the binding of Ca2+ to a membrane component. A rank order of ATP > ADP > AMP was established for the stimulation of Ca2+ uptake. Adenosine, vanadate, adenosine 5'-[alpha, beta-methylene]triphosphate (a P2x purinoceptor agonist) and adenosine 5'-[gamma-thio]triphosphate (a P-type ATPase inhibitor) were without effect. 2-Methylthioadenosine 5'-triphosphate, a P2y purinoceptor agonist, mimicked the stimulation by ATP. As judged from a kinetic comparison, ATP hydrolysis and the stimulation by ATP of Ca2+ uptake were not compatible. The P2 purinoceptor antagonist suramin and the P2y purinoceptor antagonist Reactive Blue-2 inhibited the Ca2+ uptake stimulated by 1 mM ATP (IC50 0.17 mM and 58 microM respectively). We conclude that ATP-stimulated Ca2+ uptake in tilapia intestine is dissociated from ATPase activity and is mediated through a P2 purinoceptor.

Regulatory peptides in gastric endocrine cells of the rainbow trout Oncorhynchus mykiss: general distribution and colocalizations

The endocrine cells of the rainbow trout (Oncorhynchus mykiss) stomach have been investigated using the immunocytochemical techniques of peroxidase-anti-peroxidase and avidin-biotin-peroxidase complexes on paraffin sections. 33 antisera were tested and eight immunoreactivities were detected: somatostatin-, glucagon- bombesin-, substance P-, serotonin-, met-enkephalin-, CCK/gastrin-, and chromogranin-like containing cells. All of them were present throughout the gastric mucosa except CCK/gastrin-like containing cells that were restricted to the pyloric epithelium. Somatostatin 25 and chromogranin immunoreactive cells are described for the first time in fish stomach. Serotonin immunoreactive cells were also positive for the Grimelius technique and some of them were immunoreactive to anti substance P or anti CCK/gastrin. Immunoreactivities for gastrin 17, gastrin 34 and CCK appeared in the same cells and the absorption controls showed that a molecule containing the carboxi-terminal pentapeptide of this family was present in trout stomach.

Tachykinins and intestinal motility in different fish groups

The presence and function of tachykinins were studied in the intestine of hagfish (Myxine glutinosa), lamprey (Lampetra fluviatilis), starry ray (Raja radiata), lungfish (Lepidosiren paradoxa), bichir (Polypterus senegalensis), and rainbow trout (Oncorhynchus mykiss), which represent different systematic groups of fish. Immunohistochemistry revealed cells containing substance P (SP)-like material in the intestine of lamprey and lungfish, and in the stomach of the ray. The intestinal motility was studied using isolated muscle strip preparations. SP had no effect on hagfish or lamprey intestine. In the other four species SP produced intestinal contractions. In ray, bichir, and lungfish the tachykinins may be released from endocrine cells and act, at least in the bichir and lungfish, directly onto the smooth muscle cells. In the rainbow trout intestine, where SP-like material may be released from both nerve fibres and endocrine cells, it is indicated that the contractile effect is in part direct upon the smooth muscle and in part via stimulation of cholinergic and serotonergic neurons.

5-Hydroxytryptamine is a possible neurotransmitter of the non-cholinergic excitatory nerves in the longitudinal muscle of rainbow trout stomach (Salmo gairdneri)

1. The neurotransmitter of the non-cholinergic excitatory nerves in the rainbow trout stomach was identified on the basis of the pharmacological properties of the contractile responses to transmural stimulation (TMS) and nicotine. 2. TMS caused tetrodotoxin-sensitive contractions of rainbow trout stomach strips in a frequency-dependent manner (0.5-50 Hz). Atropine (1 microM) significantly decreased the contractile response to low-frequency stimulation (0.5-2 Hz), but did not affect that to high-frequency stimulation (3-20 Hz). 3. The atropine-resistant contractile response to TMS (20 Hz) was unaffected by hexamethonium (100 microM), phentolamine (5.4 microM), pyrilamine (1 microM), naloxone (1 microM) or substance P-induced desensitization. 4. 5-Hydroxytryptamine (5-HT, 3 nM-3 microM) caused atropine-resistant contractions in a concentration-dependent manner. In the presence of atropine, methysergide (1 microM) decreased the contractile responses to TMS and 5-HT. 5. Nicotine (3 microM-500 microM) induced atropine-resistant contractions that were completely abolished by tetrodotoxin or hexamethonium. Also methysergide inhibited the contractile responses to nicotine. 6. An acid extract of rainbow trout stomach exhibited atropine-resistant contractions that were decreased by methysergide, in both rainbow trout stomach and guinea-pig ileum longitudinal smooth muscle preparations. 7. The present results indicate that, in longitudinal muscle strips of the rainbow trout stomach, 5-HT is one of the mediators (neurotransmitters) of the non-cholinergic excitatory contractions induced by TMS and nicotine.

Control of gallbladder motility in the rainbow trout,Salmo gairdneri

Immunohistochemistry revealed nerves containing VIP-like and 5-HT-like material in both gallbladder wall and bile duct of the rainbow trout, while endocrine cells containing gastrin/CCK-like and substance P-like material were present in the mucosa of the bile duct and the duodenum. Fluorescence histochemistry showed adrenergic nerves close to the muscle layer of the gallbladder.Sulphated CCK8, caerulein and non-sulphated CCK8 (in this order of potency), 5-HT and acetylcholine were excitatory on isolated strip preparations, while VIP and adrenergic agonists were inhibitory. The adrenergic drugs were probably actingvia a beta-adrenergic receptor, while the effects of 5-HT and cholinergic drugs were antagonized by methysergide and atropine respectively.Electrical stimulation of the gallbladder nervesin situ failed to show any effect or under certain conditions induced a rebound effect.It is concluded that the motility control of the rainbow trout gallbladder may involve an inhibitory innervation by adrenergic and possibly VIP-releasing nerves, while 5-HT, acetylcholine and a CCK-like substance may be involved in the excitatory control.

Neuropeptide functions in the fish gut

Several neuropeptides, more or less related to their mammalian counterparts, are present in the enteric nervous system in fish, and may function as neurotransmitters, cotransmitters or neuromodulators in this system. Excitatory effects on fish gut smooth muscle are found with bombesin, substance P and neurotensin. VIP and bombesin are inhibitory in some preparations and metenkephalin and somatostatin give varying responses. Of the more closely studied neuropeptides, a bombesin-like peptide may be involved in a cofunction with acetylcholine in the excitation of gut motility. One or more tachykinins may be involved in different mechanisms controlling the gut motility, directly or indirectly via, e.g., serotonergic neurons. Bombesin, a tachykinin and VIP may control gastric secretion, and VIP may have vasodilatory functions as well as inhibitory functions on gut motility at least in some species.