Autonomic nerve control of the swimbladder of the goldsinny wrasse, Ctenolabrus rupestris

Protein kinase A activity and NO are involved in the regulation of crucian carp (Carassius carassius) red blood cell osmotic fragility

Activation of the cAMP pathway by β-adrenergic stimulation and cGMP pathway by activation of guanylate cyclase substantially affects red blood cell (RBC) membrane properties in mammals. However, whether similar mechanisms are involved in RBC regulation of lower vertebrates, especially teleosts, is not elucidated yet. In this study, we evaluated the effects of adenylate cyclase activation by epinephrine and forskolin, guanylate cyclase activation by sodium nitroprusside, and the role of Na⁺/H⁺-exchanger in the changes of osmotic fragility and regulatory volume decrease (RVD) response in crucian carp RBCs. Western blot analysis of protein kinase A and protein kinase G substrate phosphorylation revealed that changes in osmotic fragility were regulated via the protein kinase A, but not protein kinase G signaling pathway. At the same time, the RVD response in crucian carp RBCs was not affected either by activation of adenylate or guanylate cyclase. Adenylate cyclase/protein kinase A activation significantly decreased RBC osmotic fragility, i.e., increased cell rigidity. Inhibition of Na⁺/H⁺-exchanger by amiloride had no effect on the epinephrine-mediated decrease of RBC osmotic fragility. NO donor SNP did not activate guanylate cyclase, however affected RBCs osmotic fragility by protein kinase G-independent mechanisms. Taken together, our data demonstrated that the cAMP/PKA signaling pathway and NO are involved in the regulation of crucian carp RBC osmotic fragility, but not in RVD response. The authors confirm that the study has no clinical trial.

Geriatric Freshwater and Marine Fish

As pain management finally becomes accepted for this last of the vertebrate taxa, fish medicine is finally reaching the sophistication of other vertebrates. The diseases of aging fish in captivity therefore need to be addressed. The degenerative organ/tissue changes and neoplasias of fish deserve the same diagnosis and treatments of their terrestrial counterparts including pain relief, anti-inflammatory medications, chemotherapy, surgery, joint supplements, regenerative cell therapy, and photobiomodulation. Besides the challenges of an aquatic environment, recognizing normal changes in older fish will be addressed in this article. Clinicians can appreciate the diversity of fishes and their unique anatomies, physiologies, and behaviors which translate to creative medicine.

Adrenergic control of swimbladder deflation in the zebrafish (Danio rerio)

Many teleosts actively regulate buoyancy by adjusting gas volume in the swimbladder. In physostomous fishes such as the zebrafish, a connection is maintained between the swimbladder and the oesophagus via the pneumatic duct for the inflation and deflation of this organ. Here we investigated the role of adrenergic stimulation of swimbladder wall musculature in deflation of the swimbladder. Noradrenaline (NA), the sympathetic neurotransmitter (dosage 10(-6) to 10(-5) mol l(-1)), doubled the force of smooth muscle contraction in isolated tissue rings from the anterior chamber, caused a doubling of pressure in this chamber in situ, and evoked gas expulsion through the pneumatic duct, deflating the swimbladder to approximately 85% of the pre-NA volume. These effects were mediated by beta-adrenergic receptors, representing a novel role for these receptors in vertebrates. No effects of adrenergic stimulation were detected in the posterior chamber. In a detailed examination of the musculature and innervation of the swimbladder to determine the anatomical substrate for these functional results, we found that the anterior chamber contained an extensive ventral band of smooth muscle with fibres organized into putative motor units, richly innervated by tyrosine hydroxylase-positive axons. Additionally, a novel arrangement of folds in the lumenal connective tissue in the wall of the anterior chamber was described that may permit small changes in muscle length to cause large changes in effective wall distensibility and hence chamber volume. Taken together, these data strongly suggest that deflation of the zebrafish swimbladder occurs primarily by beta-adrenergically mediated contraction of smooth muscle in the anterior chamber and is under the control of the sympathetic limb of the autonomic nervous system.

Autonomic control of the swimbladder

The swimbladder of teleost fishes is the primary organ for controlling whole-body density, and thus buoyancy. The volume of gas in the swimbladder is adjusted to bring the organism to near neutral buoyancy at a particular depth. Swimbladder morphology varies widely among teleosts, but all species are capable of inflating and deflating this organ under reflex control by the autonomic nervous system, to achieve neutral buoyancy. Here we review the control of effectors within the swimbladder, including acid-secreting cells, vasculature and musculature, that are involved in determining gas volume. This control system is complex. It incorporates the "classical" efferent elements of the autonomic nervous system, the spinal autonomic and cranial autonomic limbs and their neurotransmitters (typically noradrenaline (NA)/adrenaline (ADR), and acetylcholine, respectively), but also non-adrenergic, non-cholinergic neurotransmitters such as peptides, purines and nitric oxide. The detailed patterns of autonomic innervation of swimbladder effectors are not well understood, nor are the relationships of terminals releasing non-adrenergic, non-cholinergic neurotransmitters onto these effectors. Furthermore, in most cases the complement of postjunctional receptor subtypes activated by adrenergic, cholinergic and other neurotransmitters, and the biological effects of these neurochemicals, have not been completely established. In order to clarify some of these issues and to provide insight into basic principles underlying autonomic control of swimbladder function, we propose the zebrafish as a potentially useful model teleost.

Nervous control of fish swimbladders

The swimbladder of teleost fish receives a rich and complex innervation by nerve fibres of the autonomic nervous system. While an understanding of the form and function of a non-adrenergic, non-cholinergic innervation is slowly emerging, the pattern of control by the "classical" cholinergic and adrenergic innervation is becoming relatively well understood. This short review describes the autonomic innervation patterns, and attempts to summarise the role of cholinergic and adrenergic pathways in the control of gas secretion and resorption in the teleost swimbladder.

pH regulation and swimbladder function in fish

Gas gland cells of the European eel (Anguilla anguilla) are specialized for the production and secretion of acidic metabolites. Although typically exposed to high oxygen partial pressures, they convert glucose mainly into lactate, but also produce CO2 in the pentose phosphate shunt. Only a very small fraction of glucose is oxidized via aerobic metabolism. Although the buffer capacity of gas gland cells appears to be high, even at low extracellular pH values intracellular pH is always kept about 0.2-0.3 pH-units more acidic. Thus, under all physiological conditions proton concentration within gas gland cells is higher than in the extracellular fluid, facilitating proton extrusion. Diffusion of CO2, Na+/H+-exchange, sodium-dependent anion exchange and a V-ATPase represent the pathways available for proton secretion. While under resting conditions the sodium-dependent pathways and diffusion of CO2 appear to be the dominating mechanisms for acid secretion, at low intracellular pH the contribution of Na+/H+-exchange and of V-ATPase appear to increase, while sodium-dependent anion exchange becomes less important. The mechanisms regulating the activity of these acid-secreting pathways and of the metabolism responsible for the production of protons are largely unknown.

Vasodilation of swimbladder vessels in the european eel (Anguilla anguilla) induced by vasoactive intestinal polypeptide, nitric oxide, adenosine and protons

The effects of β-adrenergic stimulation, vasoactive intestinal polypeptide (VIP), adenosine, the nitric oxide (NO)-releasing agent sodium nitroprusside and of metabolic end-products of gas gland cell metabolism on swimbladder blood flow were investigated using saline- or blood-perfused swimbladder preparations of the freshwater European eel Anguilla anguilla. While β-adrenergic vasodilation was not detectable, a bolus injection of adenosine (100 microl, 10(−)7 mol l-1) and application of VIP (10(−)7 mol kg-1) caused a significant decrease in perfusion pressure in saline-perfused swimbladder preparations. Immunohistochemical analysis revealed the presence of VIP-immunoreactive nerve fibres in the swimbladder artery and in the swimbladder vein (seawater-adapted eels were used for immunohistochemical studies). Application of sodium nitroprusside also elicited a small, but significant, decrease in perfusion pressure in saline-perfused swimbladder preparations, while preincubation of swimbladder tissue with N(ω)nitro-l-arginine, a non-selective inhibitor of nitric oxide synthase, significantly enhanced the flow-induced increase in perfusion pressure. Lactate, the major metabolic end-product of gas gland cell metabolism, had no effect on perfusion pressure. In contrast, an increase in proton concentration in both saline- and blood-perfused preparations induced a vasodilation, as indicated by a significant decrease in perfusion pressure. The results demonstrate that VIP, NO, adenosine and protons may induce a vasodilation of swimbladder blood vessels. None of these effects, however, compares in time span with the previously described immediate, short-lasting vasodilation of swimbladder vessels elicited by pulse stimulation of the vagus nerve.

Effects of vasoactive intestinal polypeptide, substance P, 5-hydroxytryptamine, met-enkephalin and neurotensin on the swimbladder smooth muscle of two teleost species,Gadus morhua andAnguilla anguilla

The effects of vasoactive intestinal polypeptide (VIP), substance P (SP), 5-hydroxytryptamine (5-HT), neurotensin (NT) and met-enkephalin (mEnk) on the smooth muscles of the teleost swimbladder were studied in two teleost species, the Atlantic cod (Gadus morhua) and the eel (Anguilla anguilla). The study was made on isolated strip preparations of the muscularis mucosae, using putative transmitters corresponding to the immunoreactive materials that have previously been localized by immunohistochemical methods in nerves or endocrine/paracrine cells of the teleost swimbladder and/or the gastrointestinal canal. VIP was relaxatory on both cod and eel swimbladder smooth muscle, SP and 5-HT were constrictory in both species, and mEnk was excitatory on the eel swimbladder smooth muscle. Clear effects of these agents were usually seen at a concentration ofca. 10 nM in cumulative concentration/effect experiments. NT had no effect in either species. In the eel, the effects on the pneumatic duct were generally greater than on the swimbladder proper. The study indicates that the 5-HT and peptides previously observed by immunohistochemistry have physiological functions in the swimbladder.

Identification of beta-adrenergic receptors in teleost red blood cells

Beta-adrenoceptors have been identified in eel erythrocyte membranes using [3H]dihydroalprenolol binding. These receptors exhibit a mean dissociation constant (KD) of 1.36 nM and a mean maximum number of binding sites (Bmax) of 315 fmol/mg protein. These receptors do not appear to belong to the beta 1-subtype.

Ultrastructure and innervation of the swimbladder of Tetractenos glaber (Tetraodontidae)

The general structure, ultrastructure and innervation of the swimbladder of the smooth toadfish, Tetractenos glaber, were examined with light-microscopic, fluorescence-histochemical, and transmission electron-microscopic techniques. The structure of the swimbladder is similar to that of other euphysoclists. Fluorescence histochemistry showed adrenergic fibres in both the secretory and resorptive areas of the swimbladder. Transmission electron microscopy revealed two morphologically distinct axon profiles: type-I profiles containing many small, flattened vesicles; type-II profiles containing both large, granular vesicles and rounded, small clear vesicles in varying proportions. The gas-gland cells and surrounding muscularis mucosae are innervated by both type-I and type-II fibres. Type-I fibres also innervate pre-rete arteries. The rete- and gas-gland capillaries do not appear to be innervated. Arteries running to the resorptive area are innervated by type-I fibres. Both type-I and type-II profiles make contact with the muscularis mucosae in the resorptive area. Only type-I fibres innervate the radial dilator muscle in the oval sphincter region, whereas only type II fibres innervate the circular muscle of the oval sphincter. Type-I fibres took up alpha-methyl-noradrenaline, and could not be found after pre-treatment with 6-hydroxydopamine. They are, therefore, assumed to be adrenergic. Type-II fibres were tentatively identified, by exclusion, as cholinergic.

Neuropharmacology of adrenergic neurons in teleost fish

Although this brief review is based on relatively few types of experiments in few species of teleosts, it is possible to summarize some points of interest regarding the similarities and differences in the mechanisms of adrenergic neurotransmission in fish compared to the higher vertebrates. 1. There is a substantial mixing of cranial autonomic ("parasympathetic") and spinal autonomic ("sympathetic") pathways in the cranial nerves. This close relationship between the two systems and the differences in the nature of the neurons of cranial origin (cholinergic, and non-adrenergic, non-cholinergic) and spinal origin (adrenergic, cholinergic and mixed "polynergic") gives a basis in fish also for a complex pattern of innervation of the various organs. 2. Adrenaline is the major transmitter substance in the adrenergic neurons of most teleosts studied, but there are exceptions within the same species. For instance, in the swimbladder mucosa of the cod, noradrenaline dominates, while adrenaline is the major catecholamine in most other organs innervated by adrenergic neurons. The reasons for the regional differences are not known and further studies of the rate of catecholamine turn-over in the adrenergic neurons of fish are clearly indicated. 3. Adrenoceptors of both the alpha- and the beta-type show great similarities with those of mammals. Some differences in the potencies of certain compounds (e.g., clonidine and methoxamine) exist and receptor binding studies should add valuable information about the adrenoceptors of teleosts. The existence of a subtype of beta-adrenoceptor (beta 2) has been proposed and further work is needed to confirm or deny the applicability of the beta 1/beta 2 adrenoceptor terminology in fish. 4. There appears to be some differences in the mode of action of the so called "indirectly acting amines", such as tyramine, between teleosts and mammals. While the uptake of tyramine into the nerve terminals in mammals appears to take place via the cocaine-sensitive neuronal uptake system which is also responsible for catecholamine uptake (uptake 1), tyramine uptake in cod neurons appears to be via a separate pathway. 5. Presynaptic supersensitivity of the type seen in mammals has also been demonstrated in teleost adrenergic neurons. Both denervation (chemical or surgical) and blockade of the neuronal uptake mechanism by cocaine or desipramine produce this type of supersensitivity, while post-synaptic supersensitivity has so far not been described in teleosts. The effects of removal of the uptake system shows that the uptake process may be as important in teleosts as in mammals in the removal of adrenergic transmitter from the synaptic cleft. 6. In the total picture of adrenergic functions in fish, the circulating catecholamines take a special role...