Evidence for glutamatergic mechanisms in the vagal sensory pathway initiating cardiorespiratory reflexes in the shorthorn sculpin Myoxocephalus scorpius

Contrasting effects of acute and chronic stress on the transcriptome, epigenome, and immune response of Atlantic salmon

Stress experienced during early life may have lasting effects on the immune system, with impacts on health and disease dependent on the nature and duration of the stressor. The epigenome is especially sensitive to environmental stimuli during early life and represents a potential mechanism through which stress may cause long-lasting health effects. However, the extent to which the epigenome responds differently to chronic vs acute stressors is unclear, especially for non-mammalian species. We examined the effects of acute stress (cold-shock during embryogenesis) and chronic stress (absence of tank enrichment during larval-stage) on global gene expression (using RNA-seq) and DNA methylation (using RRBS) in the gills of Atlantic salmon (Salmo salar) four months after hatching. Chronic stress induced pronounced transcriptional differences, while acute stress caused few lasting transcriptional effects. However, both acute and chronic stress caused lasting and contrasting changes in the methylome. Crucially, we found that acute stress enhanced transcriptional immune response to a pathogenic challenge (bacterial lipopolysaccharide, LPS), while chronic stress suppressed it. We identified stress-induced changes in promoter and gene-body methylation that were associated with altered expression for a small proportion of immune-related genes, and evidence of wider epigenetic regulation within signalling pathways involved in immune response. Our results suggest that stress can affect immuno-competence through epigenetic mechanisms, and highlight the markedly different effects of chronic larval and acute embryonic stress. This knowledge could be used to harness the stimulatory effects of acute stress on immunity, paving the way for improved stress and disease management through epigenetic conditioning.

Air- breathing in fish: Air- breathing organs and control of respiration: Nerves and neurotransmitters in the air-breathing organs and the skin

In fishes, exploitation of aerial gas exchange has evolved independently many times, involving a variety of air-breathing organs. Indeed, air-breathing occurs in at least 49 known families of fish (Graham, 1997). Many amphibious vertebrates, at some stage of their development are actually trimodal breathers that use various combinations of respiratory surfaces to breath both water (skin and/or gill) and air (skin and/or lung). The present review examines the evolutionary implications of air-breathing organs in fishes and the morphology of the peripheral receptors and the neurotransmitter content of the cells involved in the control of air-breathing. Control of breathing, whether gill ventilation or air-breathing, is influenced by feedback from peripheral and/or central nervous system receptors that respond to changes in PO2, PCO2 and/or pH. Although the specific chemoreceptors mediating the respiratory reflexes have not been conclusively identified, studies in water-breathing teleosts have implicated the neuroepithelial cells (NECs) existing in gill tissues as the O2 sensitive chemoreceptors that initiate the cardiorespiratory reflexes in aquatic vertebrates. Some of the air-breathing fishes, such as Protopterus, Polypterus and Amia have been shown to have NECs in the gills and/or lungs, although the role of these receptors and their innervation in the control of breathing is not known. NECs have been also reported in the specialized respiratory epithelia of accessory respiratory organs (ARO's) of some catfish species and in the gill and skin of the mudskipper Periophthalmodon schlosseri. Unlike teleosts matching an O2-oriented ventilation to ambient O2 levels, lungfishes have central and peripheral H+/CO2 receptors that control the acid-base status of the blood.

Central and Peripheral Effects of Urotensin II and Urotensin II-Related Peptides on Cardiac Baroreflex Sensitivity in Trout

The baroreflex response is an essential component of the cardiovascular regulation that buffers abrupt changes in blood pressure to maintain homeostasis. Urotensin II (UII) and its receptor UT are present in the brain and in peripheral cardiovascular tissues of fish and mammals. Intracerebroventricular (ICV) injection of UII in these vertebrates provokes hypertension and tachycardia, suggesting that the cardio-inhibitory baroreflex response is impaired. Since nothing is known about the effect of UII on the cardiac baroreflex sensitivity (BRS), we decided to clarify the changes in spontaneous BRS using a cross spectral analysis technique of systolic blood pressure (SBP) and R-R interval variabilities after ICV and intra-arterial (IA) injections of trout UII in the unanesthetized trout. We contrasted the effects of UII with those observed for the UII-related peptides (URP), URP1 and URP2. Compared with vehicle-injected trout, ICV injection of UII (5-500 pmol) produced a gradual increase in SBP, a decrease in the R-R interval (reflecting a tachycardia) associated with a dose-dependent reduction of the BRS. The threshold dose for a significant effect on these parameters was 50 pmol (BRS; -55%; 1450 ± 165 ms/kPa vs. 3240 ± 300 ms/kPa; P < 0.05). Only the 500-pmol dose of URP2 caused a significant increase in SBP without changing significantly the R-R interval but reduced the BRS. IA injection of UII (5-500 pmol) caused a dose-dependent elevation of SBP. Contrasting with the ICV effects of UII, the R-R interval increased (reflecting a bradycardia) up to the 50-pmol dose while the BRS remained unchanged (50 pmol; 2530 ± 270 ms/kPa vs. 2600 ± 180 ms/kPa; P < 0.05). Nonetheless, the highest dose of UII reduced the BRS as did the highest dose of URP1. In conclusion, the contrasting effect of low picomolar doses of UII after central and peripheral injection on the BRS suggests that only the central urotensinergic system is involved in the attenuation of the BRS. The limited and quite divergent effects of URP1 and URP2 on the BRS, indicate that the action of UII is specific for this peptide. Further studies are required to elucidate the site(s) and mechanisms of action of UII on the baroreflex pathways. Whether such effects of central UII on the BRS exist in mammals including humans warrants further investigations.

Evolution and physiology of neural oxygen sensing

Major evolutionary trends in animal physiology have been heavily influenced by atmospheric O2 levels. Amongst other important factors, the increase in atmospheric O2 which occurred in the Pre-Cambrian and the development of aerobic respiration beckoned the evolution of animal organ systems that were dedicated to the absorption and transportation of O2, e.g., the respiratory and cardiovascular systems of vertebrates. Global variations of O2 levels in post-Cambrian periods have also been correlated with evolutionary changes in animal physiology, especially cardiorespiratory function. Oxygen transportation systems are, in our view, ultimately controlled by the brain related mechanisms, which senses changes in O2 availability and regulates autonomic and respiratory responses that ensure the survival of the organism in the face of hypoxic challenges. In vertebrates, the major sensorial system for oxygen sensing and responding to hypoxia is the peripheral chemoreflex neuronal pathways, which includes the oxygen chemosensitive glomus cells and several brainstem regions involved in the autonomic regulation of the cardiovascular system and respiratory control. In this review we discuss the concept that regulating O2 homeostasis was one of the primordial roles of the nervous system. We also review the physiology of the peripheral chemoreflex, focusing on the integrative repercussions of chemoreflex activation and the evolutionary importance of this system, which is essential for the survival of complex organisms such as vertebrates. The contribution of hypoxia and peripheral chemoreflex for the development of diseases associated to the cardiovascular and respiratory systems is also discussed in an evolutionary context.

Brainstem mechanisms controlling cardiovascular reflexes in channel catfish

Microinjections of kynurenic acid and kainic acid into the general visceral nucleus (nGV), homologous to the mammalian nucleus tractus solitarius of the medulla, in anesthestized, spontaneously breathing catfish were used to identify central areas and mechanisms controlling resting normoxic heart rate and blood pressure and the cardiovascular responses to hypoxia. Kynurenic acid, an antagonist of ionotropic glutamate receptors, significantly reduced resting normoxic heart rate but did not block the bradycardia associated with aquatic hypoxia. Kainic acid (an excitotoxic glutamatergic receptor agonist) also significantly reduced normoxic heart rate, but blocked the hypoxia-induced bradycardia. Neither kynurenic acid nor kainic acid microinjections affected blood pressure in normoxia or hypoxia. The results of this study indicate that glutamatergic receptors in the nGV are involved in the maintenance of resting heart rate and the destruction of these neurons with kainic acid abolishes the bradycardia associated with aquatic hypoxia.

Central ventilatory and cardiovascular actions of serotonin in trout

This study was undertaken to investigate the central actions of 5-HT on ventilatory and cardiovascular variables in the unanesthetized trout. Compared to vehicle, intracerebroventricular injection (ICV) of 5-HT elevated the total ventilation. This elevation was due to its stimulatory action on ventilatory amplitude. Moreover, 5-HT produced a dose-dependent increase in mean dorsal aortic blood pressure (PDA) without change in heart rate (fH). Methysergide, a 5-HT1/5-HT2 receptor antagonist, reduced the hyperventilatory and hypertensive actions of 5-HT. 8-OH-2-(di-n-propylamino) tetralin, a 5-HT1A receptor agonist, increased PDA while α-methyl-5-HT, a 5-HT2 receptor agonist, elevated all ventilatory variables and increased PDA without changing fH. Intra-arterial injection of 5-HT was without effect on ventilation, but 5-HT initially produced hypotension followed by hypertension. These changes were accompanied by tachycardia. It remains to be determined whether endogenous 5-HT within the brain of trout may act as a potent neuroregulator causing stimulatory effects on cardio-ventilatory functions. In the periphery, 5-HT may act as local modulator involved in vasoregulatory mechanisms.

Niche expansion of the shorthorn sculpin (Myoxocephalus scorpius) to Arctic waters is supported by a thermal independence of cardiac performance at low temperature

Cardiovascular adaptations that permit successful exploitation of polar marine waters by fish requires a capacity to negate or compensate for the depressive effects of low temperatures on physiological processes. Here, we examined the effects of acute and chronic temperature change on the maximum cardiac performance of shorthorn sculpin (Myoxocephalus scorpius (L., 1758)) captured above the Arctic Circle. Our aim was to establish if the sculpin’s success at low temperatures was achieved through thermal independence of cardiac function or via thermal compensation as a result of acclimation. Maximum cardiac performance was assessed at both 1 and 6 °C with a working perfused heart preparation that was obtained after fish had been acclimated to either 1 or 6 °C. Thus, tests were performed at the fish’s acclimation temperature and with an acute temperature change. Maximum cardiac output, which was relatively large (>50 mL·min−1·kg−1 body mass) for a benthic fish at a frigid temperature, was found to be independent of both acclimation temperature and test temperature. While maximum β-adrenergic stimulation produced positive chronotropy at both acclimation temperatures, inotropic effects were weak or absent. We conclude that thermal independence of cardiac performance at low temperature likely facilitated the exploitation of polar waters by the shorthorn sculpin.

Sensitivity of ventilation and brain metabolism to ammonia exposure in rainbow trout, Oncorhynchus mykiss

Ammonia has been documented as a respiratory gas that stimulates ventilation, and is sensed by peripheral neuroepithelial cells (NECs) in the gills in ammoniotelic rainbow trout. However, the hyperventilatory response is abolished in trout chronically exposed (1+ months) to high environmental ammonia [HEA; 250 μmol l(-1) (NH4)2SO4]. This study investigates whether the brain is involved in the acute sensitivity of ventilation to ammonia, and whether changes in brain metabolism are related to the loss of hyperventilatory responses in trout chronically exposed to HEA ('HEA trout'). Hyperventilation (via increased ventilatory amplitude rather than rate) and increased total ammonia concentration ([TAmm]) in brain tissue were induced in parallel by acute HEA exposure in control trout in a concentration-series experiment [500, 750 and 1000 μmol l(-1) (NH4)2SO4], but these inductions were abolished in HEA trout. Ventilation was correlated more closely to [TAmm] in brain rather than to [TAmm] in plasma or cerebrospinal fluid. The close correlation of hyperventilation and increased brain [TAmm] also occurred in control trout acutely exposed to HEA in a time-series analysis [500 μmol l(-1) (NH4)2SO4; 15, 30, 45 and 60 min], as well as in a methionine sulfoxamine (MSOX) pre-injection experiment [to inhibit glutamine synthetase (GSase)]. These correlations consistently suggest that brain [TAmm] is involved in the hyperventilatory responses to ammonia in trout. The MSOX treatments, together with measurements of GSase activity, TAmm, glutamine and glutamate concentrations in brain tissue, were conducted in both the control and HEA trout. These experiments revealed that GSase plays an important role in transferring ammonia to glutamate to make glutamine in trout brain, thereby attenuating the elevation of brain [TAmm] following HEA exposure, and that glutamate concentration is reduced in HEA trout. The mRNAs for the ammonia channel proteins Rhbg, Rhcg1 and Rhcg2 were expressed in trout brain, and the expression of Rhbg and Rhcg2 increased in HEA trout, potentially as a mechanism to facilitate the efflux of ammonia. In summary, the brain appears to be involved in the sensitivity of ventilation to ammonia, and brain ammonia levels are regulated metabolically in trout.

Brain neuropeptides in central ventilatory and cardiovascular regulation in trout

Many neuropeptides and their G-protein coupled receptors (GPCRs) are present within the brain area involved in ventilatory and cardiovascular regulation but only a few mammalian studies have focused on the integrative physiological actions of neuropeptides on these vital cardio-respiratory regulations. Because both the central neuroanatomical substrates that govern motor ventilatory and cardiovascular output and the primary sequence of regulatory peptides and their receptors have been mostly conserved through evolution, we have developed a trout model to study the central action of native neuropeptides on cardio-ventilatory regulation. In the present review, we summarize the most recent results obtained using this non-mammalian model with a focus on PACAP, VIP, tachykinins, CRF, urotensin-1, CGRP, angiotensin-related peptides, urotensin-II, NPY, and PYY. We propose hypotheses regarding the physiological relevance of the results obtained.

Time domains of the hypoxic ventilatory response in ectothermic vertebrates

Over a decade has passed since Powell et al. (Respir Physiol 112:123-134, 1998) described and defined the time domains of the hypoxic ventilatory response (HVR) in adult mammals. These time domains, however, have yet to receive much attention in other vertebrate groups. The initial, acute HVR of fish, amphibians and reptiles serves to minimize the imbalance between oxygen supply and demand. If the hypoxia is sustained, a suite of secondary adjustments occur giving rise to a more long-term balance (acclimatization) that allows the behaviors of normal life. These secondary responses can change over time as a function of the nature of the stimulus (the pattern and intensity of the hypoxic exposure). To add to the complexity of this process, hypoxia can also lead to metabolic suppression (the hypoxic metabolic response) and the magnitude of this is also time dependent. Unlike the original review of Powell et al. (Respir Physiol 112:123-134, 1998) that only considered the HVR in adult animals, we also consider relevant developmental time points where information is available. Finally, in amphibians and reptiles with incompletely divided hearts the magnitude of the ventilatory response will be modulated by hypoxia-induced changes in intra-cardiac shunting that also improve the match between O(2) supply and demand, and these too change in a time-dependent fashion. While the current literature on this topic is reviewed here, it is noted that this area has received little attention. We attempt to redefine time domains in a more 'holistic' fashion that better accommodates research on ectotherms. If we are to distinguish between the genetic, developmental and environmental influences underlying the various ventilatory responses to hypoxia, however, we must design future experiments with time domains in mind.

Control of respiration in fish, amphibians and reptiles

Fish and amphibians utilise a suction/force pump to ventilate gills or lungs, with the respiratory muscles innervated by cranial nerves, while reptiles have a thoracic, aspiratory pump innervated by spinal nerves. However, fish can recruit a hypobranchial pump for active jaw occlusion during hypoxia, using feeding muscles innervated by anterior spinal nerves. This same pump is used to ventilate the air-breathing organ in air-breathing fishes. Some reptiles retain a buccal force pump for use during hypoxia or exercise. All vertebrates have respiratory rhythm generators (RRG) located in the brainstem. In cyclostomes and possibly jawed fishes, this may comprise elements of the trigeminal nucleus, though in the latter group RRG neurons have been located in the reticular formation. In air-breathing fishes and amphibians, there may be separate RRG for gill and lung ventilation. There is some evidence for multiple RRG in reptiles. Both amphibians and reptiles show episodic breathing patterns that may be centrally generated, though they do respond to changes in oxygen supply. Fish and larval amphibians have chemoreceptors sensitive to oxygen partial pressure located on the gills. Hypoxia induces increased ventilation and a reflex bradycardia and may trigger aquatic surface respiration or air-breathing, though these latter activities also respond to behavioural cues. Adult amphibians and reptiles have peripheral chemoreceptors located on the carotid arteries and central chemoreceptors sensitive to blood carbon dioxide levels. Lung perfusion may be regulated by cardiac shunting and lung ventilation stimulates lung stretch receptors.

Sensory innervation of the Gills: O2-sensitive chemoreceptors and mechanoreceptors

Physical characteristics of water (O(2) solubility and capacitance) dictate that cardiovascular and ventilatory performance be controlled primarily by the need for oxygen uptake rather than carbon dioxide excretion, making O(2) receptors more important in fish than in terrestrial vertebrates. An understanding of the anatomy and physiology of mechanoreception and O(2) chemoreception in fishes is important, because water breathing is the primitive template upon which the forces of evolution have modified into the various cardioventilatory modalities we see in extant terrestrial species. Key to these changes are the O(2)-sensitive chemoreceptors and mechanoreceptors, their mechanisms and central pathways.

Osmotic stress sensing and signaling in fishes

In their aqueous habitats, fish are exposed to a wide range of osmotic conditions and differ in their abilities to respond adaptively to these variations in salinity. Fish species that inhabit environments characterized by significant salinity fluctuation (intertidal zone, estuaries, salt lakes, etc.) are euryhaline and able to adapt to osmotic stress. Adaptive and acclimatory responses of fish to salinity stress are based on efficient mechanisms of osmosensing and osmotic stress signaling. Multiple osmosensors, including calcium sensing receptor likely act in concert to convey information about osmolality changes to downstream signaling and effector mechanisms. The osmosensory signal transduction network in fishes is complex and includes calcium, mitogen-activated protein kinase, 14-3-3 and macromolecular damage activated signaling pathways. This network controls, among other targets, osmosensitive transcription factors such as tonicity response element binding protein and osmotic stress transcription factor 1, which, in turn, regulate the expression of genes involved in osmotic stress acclimation. In addition to intracellular signaling mechanisms, the systemic response to osmotic stress in euryhaline fish is coordinated via hormone- and paracrine factor-mediated extracellular signaling. Overall, current insight into osmosensing and osmotic stress-induced signal transduction in fishes is limited. However, euryhaline fish species represent excellent models for answering critical emerging questions in this field and for elucidating the underlying molecular mechanisms of osmosensory signal transduction.

Ventilatory and cardiovascular actions of centrally administered trout tachykinins in the unanesthetized trout

The brains of teleost fish contain members of the tachykinin family that are the products of orthologous genes expressed in mammalian nervous tissues,but little is known regarding the physiological effects of these peptides in their species of origin. The present study compares the central actions of trout neuropeptide gamma (NPγ), substance P (SP) and neurokinin A (NKA)(5–250 pmol) on ventilatory and cardiovascular parameters in the unanesthetized rainbow trout Oncorhynchus mykiss. Intracerebroventricular (ICV) injection of NPγ evoked a dose-dependent elevation of the ventilation rate (fV) but a reduction of the ventilation amplitude (VAMP) that was caused by a reduction of the magnitude of the adduction phase of the ventilatory signal. The net effect of NPγ was to produce an hypoventilatory response since the total ventilation (VTOT) was significantly reduced. The minimum effective dose for a significant effect of NPγ on fV and VAMP was 50 pmol. SP evoked a significant elevation of fV, a concomitant depression of VAMP, and a resultant decrease in VTOTbut only at the highest dose (250 pmol). NKA was without action on fV but significantly decreased VAMP at only the highest dose tested. In this case also, the net effect of NKA was to reduce VTOT. When injected centrally, none of the three peptides, at any dose tested, produced changes in heart rate or mean dorsal aortic blood pressure (PDA). Intra-arterial injection of the three tachykinins (250 pmol) produced a significant (P&lt;0.05)increase in PDA, but only SP and NKA induced concomitant bradycardia. None of the three peptides produced any change in fV or VAMP. In conclusion, our results demonstrate that centrally injected tachykinins, particularly NPγ,produce a strong hypoventilatory response in a teleost fish and so suggest that endogenous tachykinins may be differentially implicated in neuroregulatory control of ventilation.

Central actions of angiotensin II on spontaneous baroreflex sensitivity in the trout Onc orhynchus mykiss

The goal of the present study was to investigate the central action of native angiotensin II (ANG II) on the spontaneous baroreflex sensitivity (BRS) in unanesthetized trout. The animals were equipped with two subcutaneous electrocardiographic (ECG) electrodes, a dorsal aorta catheter and an intracerebroventricular (ICV) cannula which was inserted within the third ventricle of the brain. The ECG and the systolic blood pressure (SBP) signals were recorded during a pre-injection period of 5 min and during five post-injection periods of 5 min. All injections were made at the fifth minute of the test. The time-series were processed with a sequence technique in order to detect the sequences of three or more consecutive increases in the SBP pulse, or three or more decreases in the SBP pulse correlated respectively with one delay beat increase of the RR interval of the ECG signal or shortening of this interval. The slope of the average regression line between the SBP and the RR intervals for each type of sequence was taken as a measure of the spontaneous BRS. Compared with pre-injection values, the ICV injection of vehicle (0.5 microl) had no effect on heart rate (HR), SBP, the total number of positive or negative sequences or on the spontaneous BRS during the post-injection periods. By contrast, ANG II at doses of 5 and 50 pmol increased HR but only 50 pmol ANG II elevated SBP. For all doses, ANG II depressed the spontaneous BRS, but the peptide had no effect upon the number of each baroreflex sequences. Intra-arterial injections of atropine dramatically reduced the number of positive and negative baroreflex sequences and decreased the sensitivity of the few remaining sequences, suggesting that the autonomic control of the cardiac BRS was solely due to vagal parasympathetic control. In atropinized trout the ICV injection of 5 pmol ANG II had no effect upon HR, SBP and the baroreflex parameters. This study determines for the first time the spontaneous BRS in a non-mammalian species and demonstrates an inhibitory action of ICV injection of ANG II upon this variable through a probable control of the vagal parasympathetic activity.

Reflex Cardioventilatory Responses to Hypoxia in the Flathead Gray Mullet (Mugil cephalus) and Their Behavioral Modulation by Perceived Threat of Predation and Water Turbidity

In hypoxia, gray mullet surface to ventilate well-oxygenated water in contact with air, an adaptive response known as aquatic surface respiration (ASR). Reflex control of ASR and its behavioral modulation by perceived threat of aerial predation and turbid water were studied on mullet in a partly sheltered aquarium with free surface access. Injections of sodium cyanide (NaCN) into either the bloodstream (internal) or ventilatory water stream (external) revealed that ASR, hypoxic bradycardia, and branchial hyperventilation were stimulated by chemoreceptors sensitive to both systemic and water O2 levels. Sight of a model avian predator elicited bradycardia and hypoventilation, a fear response that inhibited reflex hyperventilation following external NaCN. The time lag to initiation of ASR following NaCN increased, but response intensity (number of events, time at the surface) was unchanged. Mullet, however, modified their behavior to surface under shelter or near the aquarium edges. Turbid water abolished the fear response and effects of the predator on gill ventilation and timing of ASR following external NaCN, presumably because of reduced visibility. However, in turbidity, mullet consistently performed ASR under shelter or near the aquarium edges. These adaptive modulations of ASR behavior would allow mullet to retain advantages of the chemoreflex when threatened by avian predators or when unable to perceive potential threats in turbidity.

Cardiorespiratory and tissue adenosine responses to hypoxia and reoxygenation in the short-horned sculpinMyoxocephalus scorpius

Adenosine is a product of adenylate phosphate breakdown that can exert protective effects on tissues during energy limitation. Accumulation of cardiac adenosine under hypoxia is well documented in mammals but has not been shown in fish. Adenosine content was measured in heart and brain tissue from short-horned sculpin Myoxocephalus scorpius L. exposed to acute hypoxia and to graded hypoxia and reoxygenation at 8°C. Cardiorespiratory parameters were recorded along with plasma lactate, K+,Ca2+ and Na+ levels and their relationship to adenosine levels investigated. Sculpin exhibited a large bradycardia during hypoxia,with a concomitant drop in cardiac output that recovers fully with reoxygenation. Ventilation rate also declined with hypoxia, suggesting a depression of activity. Plasma lactate concentration was significantly elevated after 4 h at 2.0 mg l-1 dissolved oxygen while K+ levels increased during acute hypoxia. Adenosine levels were maintained in heart under acute and graded hypoxia. Brain levels fluctuated under hypoxia and showed no change with reoxygenation. It is concluded that a depression of cardiac activity in conjunction with an adequate anaerobic metabolism allow sculpin to avoid excessive adenosine accumulation under conditions of moderate hypoxia. Cardiac adenosine levels decreased and plasma K+ levels and heart rate increased significantly at reoxygenation.

N-methyl-D-aspartate receptors mediate chemoreflexes in the shorthorn sculpin Myoxocephalus scorpius

Glutamate microinjected into the vagal sensory area in the medulla produces cardiorespiratory responses mimicking oxygen chemoreflexes in fish. Here we directly investigate whether these reflexes are dependent on the ionotropic N-methyl-D-aspartate (NMDA) glutamate receptor. Fish were equipped with opercular, branchial and snout cannulae for measurements of cardiorespiratory parameters and drug injections. Oxygen chemoreceptor reflexes were evoked by rapid hypoxia, NaCN added into the blood (internal, 0.3 ml, 50 microg ml(-1)) and the mouth (external, 0.5 ml, 1 mg ml(-1)), before and after systemic administration of the NMDA receptor antagonist MK801 (3 mg kg(-1)). Hypoxia produced an MK801-sensitive increase in blood pressure and ventilation frequency, whereas the marked bradycardia and the increased ventilation amplitude were NMDA receptor-independent. The fish appeared more responsive to externally applied cyanide, but the injections and MK801 treatment did not distinguish whether external or internal oxygen receptors were differently involved in the hypoxic responses. In addition, using single-labelling immunohistochemistry on sections from the medulla and ganglion nodosum, the presence of glutamate and NMDA receptors in the vagal oxygen chemoreceptor pathway was established. In conclusion, these results suggest that NMDA receptors are putative central control mechanisms that process oxygen chemoreceptor information in fish.