Stephen G. Reid

Modulation of catecholamine storage and release by the pituitary-interrenal axis in the rainbow trout, Oncorhynchus mykiss

This study examined the effects of pituitary-interrenal hormones on catecholamine storage and release in the rainbow trout Oncorhynchus mykiss. An extract of trout pituitary elicited the release of adrenaline, but not noradrenaline, using an in situ perfusion preparation. A variety of doses of adrenocorticotropic hormone (2–2000 mU) caused the release of both catecholamines in situ which was unaffected by pre-treatment with the ganglion blocker, hexamethonium, or the serotonergic receptor antagonist, methysergide, but was abolished in calcium-free media. Intra-arterial injections of adrenocorticotrophic hormone in vivo caused an elevation of plasma adrenaline but not noradrenaline levels. Injections of cortisol in situ did not elicit catecholamine release. Trout given an intraperitoneal implant of cortisol (50 mg·kg-1 body weight) had significantly higher plasma cortisol concentrations when compared to controls after 7 days of implantation. Increases in the levels of stored catecholamines were observed in various regions of the kidney and posterior cardinal vein following 3 and 7 days of cortisol treatment. The ability of the chromaffin cells to release catecholamines in response to cholinergic stimulation was assessed in situ after 7 days of treatment. Basal (non-stimulated) adrenaline outflowing perfusate levels were greater in the cortisol-treated fish. Cortisol treatment increased the responsiveness of the catecholamine release process to low doses of the cholinoceptor agonist carbachol. Three or 7 days of cortisol treatment did not alter the in vitro activity of the enzyme phenylethanolamine-N-methyl-transferase. The results of this study demonstrate that interactions within the pituitary-adrenal axis can influence both catecholamine storage and release in the rainbow trout.

The role of branchial and orobranchial O2 chemoreceptors in the control of aquatic surface respiration in the neotropical fish tambaqui (Colossoma macropomum): progressive responses to prolonged hypoxia.

SUMMARY The present study examined the role of branchial and orobranchial O2 chemoreceptors in the cardiorespiratory responses, aquatic surface respiration (ASR), and the development of inferior lip swelling in tambaqui during prolonged (6 h) exposure to hypoxia. Intact fish (control) and three groups of denervated fish (bilateral denervation of cranial nerves IX+X (to the gills), of cranial nerves V+VII (to the orobranchial cavity) or of cranial nerves V alone), were exposed to severe hypoxia (PwO2=10 mmHg) for 360 min. Respiratory frequency (fr) and heart rate (fh) were recorded simultaneously with ASR. Intact (control) fish increased fr, ventilation amplitude (VAMP) and developed hypoxic bradycardia in the first 60 min of hypoxia. The bradycardia, however, abated progressively and had returned to normoxic levels by the last hour of exposure to hypoxia. The changes in respiratory frequency and the hypoxic bradycardia were eliminated by denervation of cranial nerves IX and X but were not affected by denervation of cranial nerves V or V+VII. The VAMP was not abolished by the various denervation protocols. The fh in fish with denervation of cranial nerves V or V+VII, however, did not recover to control values as in intact fish. After 360 min of exposure to hypoxia only the intact and IX+X denervated fish performed ASR. Denervation of cranial nerve V abolished the ASR behavior. However, all (control and denervated (IX+X, V and V+VII) fish developed inferior lip swelling. These results indicate that ASR is triggered by O2 chemoreceptors innervated by cranial nerve V but that other mechanisms, such as a direct effect of hypoxia on the lip tissue, trigger lip swelling.

Glutamate receptors in the nucleus tractus solitarius contribute to ventilatory acclimatization to hypoxia in rat

Ventilation increases more with chronic than acute hypoxia and does not return to control levels when normoxia is restored, indicating plasticity in the reflexes that control breathing. Glutamate is the primary excitatory neurotransmitter between arterial chemoreceptors that sense hypoxia and neural circuits that control breathing in the brainstem. We microinjected specific glutamate receptor antagonists into the brainstem of awake unrestrained rats and found NMDA‐type glutamate receptors explain increased ventilatory sensitivity to hypoxia after chronic hypoxia. AMPA‐type glutamate receptors mediate increased ventilatory drive in normoxia after chronic hypoxia, as well as increased ventilation in acute hypoxia after chronic hypoxia and in control conditions. Phosphorylation of AMPA and NMDA receptors is increased by chronic hypoxia. The results indicate that plasticity in different glutamate receptors have unique effects on the reflexes that control breathing in chronic hypoxia and may share cellular mechanisms with other models of neural plasticity.

Cholinoceptor-mediated control of catecholamine release from chromaffin cells in the American eel, Anguilla rostrata.

The cholinergic agonist-induced secretion of catecholamines from chromaffin cells in the American eel, Anguilla rostrata, was assessed using a salineperfused posterior cardinal vein preparation. Direct membrane depolarization with 60 mmol·l-1 K+ caused a significant release of catecholamines (adrenaline + noradrenaline) into the perfusate which was unaffected by pre-treatment with the ganglion blocker, hexamethonium (final concentration = 10-3 mol · l-1). The nicotinic receptor agonist, 1,1-dimethyl-4-phenylpiperazinium iodide, evoked catecholamine release in response to several doses exceeding 10-7 mol; at 10-5 mol the response was abolished by pre-treatment with the ganglion blocker, hexamethonium (final concentration = 10-3 mol · l-1). The muscarinic receptor agonist, pilocarpine, did not elicit catecholamine release in response to any of the doses administered (10-8–10-4 mol). A single injection of the mixed nicotinic/muscarinic cholinoceptor agonist, carbachol (10-5 mol), caused the release of catecholamines which was abolished by pre-treatment with hexamethonium but which was unaffected by pre-treatment with the muscarinic receptor antagonist atropine (final concentration = 10-5 mol · l-1). The results of this study indicate that the process of cholinergic agonist-induced catecholamine secretion from the chromaffin cells in the American eel is mediated exclusively by activation of nicotinic receptors with no involvement of the muscarinic receptor.

The Cardiorespiratory System in Tropical Fishes: Structure, Function, and Control

Publisher Summary The high temperatures of the tropical aquatic environment, often accompanied by hypoxia and hypercarbia/acidosis, have also given rise to a tremendous adaptive radiation in cardiorespiratory strategies designed to enhance survival under these conditions. This chapter presents the structure, function, and control of the respiratory and circulatory systems in these fishes. The chapter focuses on recent advances in the understanding of the control of cardiorespiratory processes in these fish with a brief review of structure and function designed to place discussion of control mechanisms in perspective. The great majority of tropical fishes continue to breathe water like their temperate relatives. Many species of tropical fish have evolved no special mechanisms for dealing with harsh conditions such as hypoxia/anoxia but constantly sense and monitor environmental conditions and migrate to better areas. These migrations are usually short, moving between stagnant areas and areas with higher water flow. The mechanisms involved include regulation of different hemoglobin fractions, adjustment of intra-erythrocytic levels of organophosphates, changes in hematocrit/hemoglobin and metabolic suppression; almost all under catecholaminergic control. The primary adaptations seen in the respiratory organs of water-breathers living in oxygen-poor waters are associated with gill diffusing capacity. Here we see both interspecies and intraspecies adaptations.

Effects of afferent input on the breathing pattern continuum in the tambaqui (Colossoma macropomum)

This study used a decerebrate and artificially-ventilated preparation to examine the roles of various afferent inputs in breathing pattern formation in the tambaqui (Colossoma macropomum). Three general breathing patterns were observed: (1) regular breathing; (2) frequency cycling and (3) episodic breathing. Under normoxic, normocapnic conditions, 50% of control fish exhibited regular continuous breathing and 50% exhibited frequency cycling. Denervation of the gills and oro-branchial cavity promoted frequency cycling. Central denervation of the glossopharyngeal and vagus nerves produced episodic breathing. Regardless of the denervation state, hyperoxia produced either frequency cycling or episodic breathing while hypoxia and hypercarbia shifted the pattern to frequency cycling and continuous breathing. We suggest that these breathing patterns represent a continuum from continuous to episodic breathing with waxing and waning occupying an intermediate stage. The data further suggest that breathing pattern is influenced by both specific afferent input from chemoreceptors and generalised afferent input while chemoreceptors specific for producing changes in breathing pattern may exist in fish.

Modulation of breathing by phasic pulmonary stretch receptor feedback in an amphibian, Bufo marinus

This study examined the role of phasic pulmonary stretch receptor (PSR) feedback in ventilatory control, breath clustering and breath timing in decerebrate, paralysed and artificially-ventilated cane toads (Bufo marinus) under conditions designed to minimise tonic PSR feedback. Fictive breathing was recorded as trigeminal motor output to the buccal musculature. Artificial tidal ventilation, with hypercarbic gas mixtures, was either continuous or activated by the fictive breaths and was manipulated to provide differing amounts/patterns of phasic PSR feedback. The results demonstrate that increased amounts of phasic PSR feedback increase overall breathing frequency. Within multi-breath episodes there was an increase in the instantaneous breathing frequency during the later stages of the episode. The temporal relationship between a fictive breath and lung inflation influenced the duration of the pause between fictive breaths. The data indicate that phasic PSR feedback stimulates breathing by enhancing the occurrence of breathing episodes in this species but does not appear to modify the instantaneous breathing frequency during an episode.

Chronic hypercapnia modulates respiratory-related central pH/CO2 chemoreception in an amphibian, Bufo marinus.

SUMMARY Anuran amphibians have multiple populations of pH/CO2-sensitive respiratory-related chemoreceptors. This study examined in cane toads (Bufo marinus) whether chronic hypercapnia (CHC) altered the pH/CO2 sensitivity of central respiratory-related chemoreceptors in vitro and whether CHC altered the acute hypercapnic ventilatory response (HCVR; 5% CO2) in vivo. Toads were exposed to CHC (3.5% CO2) for 9 days. In vitro brainstem–spinal cord preparations were used to examine central respiratory-related pH/CO2 chemosensitivity. CHC augmented in vitro fictive breathing as the pH of the superfusate was lowered from 8.2 to 7.4. Midbrain transection in vitro (at a level known to reduce the clustering of breaths) did not alter this augmentation. In vivo, CHC did not alter the acute HCVR but midbrain transection changed the breathing pattern and increased the overall level of ventilation. CHC did not alter the effect of olfactory CO2 chemoreceptor denervation on the acute HCVR in vivo but did alter the response when returned to normal air. The results indicate that CHC increases the response of central pH/CO2 chemoreceptors to changes in cerebrospinal fluid pH in vitro yet this increase is not manifest as an increase in the HCVR in vivo.

Chemoreceptor and pulmonary stretch receptor interactions within amphibian respiratory control systems

The hypercapnic drive to breathe in amphibians is generally greater than hypoxic ventilatory drive and a variety of interdependent control systems function to regulate both the hypoxic and hypercapnic ventilatory responses. During exposure to hypercapnic conditions, breathing increases in response to input from central chemoreceptors (sensitive to CSF pH/CO(2) levels) and peripheral chemoreceptors (sensitive to arterial blood O(2) and CO(2)). On the other hand, olfactory CO(2) receptors in the nasal epithelium inhibit breathing during exposure to acute hypercapnia. Further complexity arises from the CO(2)-sensitive nature of the pulmonary stretch receptors (PSR) which provide both tonic (stimulates lung inflation at low lung volumes; deflation at higher volumes) and phasic (generally excitatory) feedback. This review focuses on interactions between the various populations of chemoreceptors and interactions between chemoreceptors and PSR. Differences between various levels of experimental reduction (i.e., in vitro; in situ; in vivo) are highlighted as are the effects of chronic respiratory challenges on acute hypoxic and hypercapnic chemoreflexes.

Reciprocal modulation of O2 and CO2 cardiorespiratory chemoreflexes in the tambaqui

This study examined the effect of acute hypoxic and hypercapnic cardiorespiratory stimuli, superimposed on existing cardiorespiratory disturbances in tambaqui. In their natural habitat, these fish often encounter periods of hypoxic hypercapnia that can be acutely exacerbated by water turnover. Tambaqui were exposed to periods of normoxia, hypoxia, hyperoxia and hypercapnia during which, externally oriented O2 and CO2 chemoreceptors were further stimulated, by administration into the inspired water of sodium cyanide and CO2-equilibrated water, respectively. Hyperoxic water increased the sensitivity of the NaCN-evoked increase in breathing frequency (f(R)) and decrease in heart rate. Hypoxia and hypercapnia attenuated the increase in f(R) but, aside from blood pressure, did not influence the magnitude of NaCN-evoked cardiovascular changes. Water PO2 influenced the magnitude of the CO2-evoked cardiorespiratory changes and the sensitivity of CO2-evoked changes in heart rate and blood flow. The results indicate that existing respiratory disturbances modulate cardiorespiratory responses to further respiratory challenges reflecting both changes in chemosensitivity and the capacity for further change.