Ontogeny of the VIP system in the gastro-intestinal tract of the Axolotl, Ambystoma mexicanum: successive appearance of co-existing PACAP and NOS

Expression of neuropeptides and anoctamin 1 in the embryonic and adult zebrafish intestine, revealing neuronal subpopulations and ICC-like cells

This immunohistochemical study in zebrafish aims to extend the neurochemical characterization of enteric neuronal subpopulations and to validate a marker for identification of interstitial cells of Cajal (ICC). The expression of neuropeptides and anoctamin 1 (Ano1), a selective ICC marker in mammals, was analyzed in both embryonic and adult intestine. Neuropeptides were present from 3 days postfertilization (dpf). At 3 dpf, galanin-positive nerve fibers were found in the proximal intestine, while calcitonin gene-related peptide (CGRP)- and substance P-expressing fibers appeared in the distal intestine. At 5 dpf, immunoreactive fibers were present along the entire intestinal length, indicating a well-developed peptidergic innervation at the onset of feeding. In the adult intestine, vasoactive intestinal peptide (VIP), pituitary adenylate cyclase-activating peptide (PACAP), galanin, CGRP and substance P were detected in nerve fibers. Colchicine pretreatment enhanced only VIP and PACAP immunoreactivity. VIP and PACAP were coexpressed in enteric neurons. Colocalization stainings revealed three neuronal subpopulations expressing VIP and PACAP: a nitrergic noncholinergic subpopulation, a serotonergic subpopulation and a subpopulation expressing no other markers. Ano1-immunostaining revealed a 3-dimensional network in the adult intestine containing multipolar cells at the myenteric plexus and bipolar cells interspersed between circular smooth muscle cells. Ano1 immunoreactivity first appeared at 3 dpf, indicative of the onset of proliferation of ICC-like cells. It is shown that the Ano1 antiserum is a selective marker of ICC-like cells in the zebrafish intestine. Finally, it is hypothesized that ICC-like cells mediate the spontaneous regular activity of the embryonic intestine.

Changes in the control of gastric motor activity during metamorphosis in the amphibian Xenopus laevis, with special emphasis on purinergic mechanisms

The stomach of the amphibian Xenopus laevis is subject to extensive remodelling during metamorphosis. We investigated the changes in gastric activity control during this period using in vitro circular smooth muscle preparations mounted in organ baths. The nitric oxide synthase inhibitor L-NAME increased mean force in metamorphic and juvenile frogs but not in prometamorphic tadpoles. Serotonin (5-HT) relaxed stomach muscle prior to metamorphosis but elicited a biphasic response in juveniles consisting of contraction at low concentrations and relaxation at high concentrations. The effects of 5-HT were blocked by methysergide. In the prometamorphic tadpole, ATP elicited relaxation that was blocked by the ectonucleotidase inhibitor ARL67156 and the adenosine A(1) receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), suggesting adenosine as the mediator. Exogenous adenosine and the A(1) receptor agonist N(6)-cyclopentyladenosine (CPA) induced relaxation at all stages. After metamorphosis, the potency of ATP decreased and neither DPCPX nor ARL67156 could block ATP-induced relaxation. Uridine 5'-triphosphate (UTP) induced relaxation prior to metamorphosis, but caused contraction of muscle strips from metamorphosing tadpoles. Single doses of UTP blocked phasic contractions in juveniles in a tetrodotoxin (TTX)-sensitive manner while the simultaneous increase in muscle tension was TTX insensitive. The P2X(1)/P2X(3) receptor agonist alpha-beta-MeATP elicited pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS)-sensitive contractions at all stages investigated. These results indicate the development of an inhibitory nitrergic tonus during metamorphosis and a 5-HT receptor involved in muscle contraction. Also, the development of UTP receptors mediating increased tension and neural UTP receptors decreasing contraction frequency in juveniles is indicated. An adenosine A(1)-like receptor mediating relaxation and a P2X-like receptor mediating contraction is demonstrated at all stages.

Ontogeny of excitatory and inhibitory control of gastrointestinal motility in the African clawed frog,Xenopus laevis

The transparent body wall of Xenopus laevis larvae during the first developmental stages allows in vivo studies of gastrointestinal tract activity. The purpose of this study was to chart the ontogeny of gut motility in Xenopus larvae and to identify the most important control systems during the first developmental stages. Coordinated descending contraction waves first occurred in the gut at Nieuwkoop and Faber stage 43 [0.8 ± 0.1 contractions/min (cpm)] and increased to 4.9 ± 0.1 cpm at stage 47. The cholinergic receptor agonist carbachol (5–10 μM) increased contraction frequency already at stage 43, as did neurokinin A (NKA, 0.3–1 μM). The muscarinic antagonist atropine (100 μM) first affected contraction frequency at stage 45, which coincides with the onset of feeding. The tachykinin antagonist MEN-10,376 (6 μM) blocked NKA-induced contractions but not spontaneous motility. Both sodium nitroprusside [nitric oxide (NO) donor, 1–10 μM] and vasoactive intestinal peptide (VIP, 0.1–1 μM) inhibited contractions from the earliest stage onward. Blocking NO synthesis using NG-nitro-l-arginine methyl ester (100 μM) had no effect per se, but antagonized VIP evoked inhibition at stage 47. We conclude that gastrointestinal motility is well developed in the Xenopus laevis larvae before the onset of feeding. Functional muscarinic and tachykinin receptors are present already at the onset of motility, whereas a cholinergic tone develops around the onset of feeding. No endogenous tachykinin tone was found. Functional VIP receptors mediate inhibition at the onset of motility. NO seems to mediate the VIP effect at later stages.

The effects of endogenous and exogenous nitric oxide on gut motility in zebrafish Danio rerio embryos and larvae

Using motion analysis, the ontogeny of the nitrergic control system in the gut was studied in vivo in zebrafish Danio rerio embryos and larvae. For the first time we show the presence of a nitrergic tonus,modulating both anterograde and retrograde contraction waves in the intestine of developing zebrafish. At 4 d.p.f. (days post fertilisation), the nitric oxide synthase (NOS) inhibitor l-NAME (three boluses of 50–100 nl, 10–3 mol l–1) increased the anterograde contraction wave frequency by 0.50±0.10 cycles min–1. Subsequent application of the NO donor sodium nitroprusside (SNP; three boluses of 50–100 nl, 10–4mol l–1) reduced the frequency of propagating anterograde waves (–0.71±0.20 cycles min–1). This coincided with the first appearance of an excitatory cholinergic tonus, observed in an earlier study. One day later, at 5 d.p.f., in addition to the effect on anterograde contraction waves, application of l-NAME increased(0.39±0.15 cycles min–1) and following SNP application reduced (–1.61±0.36 cycles min–1) the retrograde contraction wave frequency. In contrast, at 3 d.p.f., when no spontaneous motility is observed, application of l-NAME did not induce contraction waves in either part of the gut, indicating the lack of a functional inhibitory tonus at this early stage. Gut neurons expressing NOS-like immunoreactivity were present in the distal and middle intestine as early as 2 d.p.f., and at 1 day later in the proximal intestine. In conclusion, the present study suggests that a nitrergic inhibitory tonus develops shortly before or at the time for onset of exogenous feeding.

Phylogenesis of constitutively formed nitric oxide in non-mammals

It is widely recognized that nitric oxide (NO) in mammalian tissues is produced from L-arginine via catalysis by NO synthase (NOS) isoforms such as neuronal NOS (nNOS) and endothelial NOS (eNOS) that are constitutively expressed mainly in the central and peripheral nervous system and vascular endothelial cells, respectively. This review concentrates only on these constitutive NOS (cNOS) isoforms while excluding information about iNOS, which is induced mainly in macrophages upon stimulation by cytokines and polysaccharides. The NO signaling pathway plays a crucial role in the functional regulation of mammalian tissues and organs. Evidence has also been accumulated for the role of NO in invertebrates and non-mammalian vertebrates. Expression of nNOS in the brain and peripheral nervous system is widely determined by staining with NADPH (reduced nicotinamide adenine dinucleotide phosphate) diaphorase or NOS immunoreactivity, and functional roles of NO formed by nNOS are evidenced in the early phylogenetic stages (invertebrates and fishes). On the other hand, the endothelium mainly produces vasodilating prostanoids rather than NO or does not liberate endothelium-derived relaxing factor (EDRF) (fishes), and the ability of endothelial cells to liberate NO is observed later in phylogenetic stages (amphibians). This review article summarizes various types of interesting information obtained from lower organisms (invertebrates, fishes, amphibians, reptiles, and birds) about the properties and distribution of nNOS and eNOS and also the roles of NO produced by the cNOS as an important intercellular signaling molecule.

Ontogeny of the gut motility control system in zebrafish Danio rerio embryos and larvae

Using digital motion analysis, the ontogeny of the cholinergic, tachykinin and pituitary adenylate cyclase-activating polypeptide (PACAP) control systems was studied in zebrafish Danio rerio larvae, in vivo. For the first time we show that the regular propagating anterograde waves that occur in the zebrafish larval gut before and around the onset [at 5–6 days post fertilization (d.p.f.)] of feeding are modulated by acetylcholine or atropine, PACAP and NKA (neurokinin A). At 3 d.p.f., when no spontaneous motility has developed, application of acetylcholine did not affect the gut. However, at 4 d.p.f., acetylcholine increased and atropine reduced the frequency of propagating anterograde waves. At 5 d.p.f., NKA increased and PACAP reduced the wave frequency. This suggests that both excitatory and inhibitory pathways develop at an early stage in the gut, independent of exogenous feeding. Immunohistochemistry established the presence of gut neurons expressing PACAP and NKA in the proximal part of the developing gut from the first stage investigated (2 d.p.f.) and before regular motility was observed. 1 d.p.f. (PACAP) or 2 d.p.f. (NKA) stages later the whole gut was innervated. This supports physiological results that gut motility is under neuronal control during the period when regular motility patterns develop.