Evidence of a role for catecholamines in the control of breathing in fish

Summary Our current knowledge of the control of ventilation in fish is incomplete at all levels. The respiratory rhythm originates in a medullary central pattern generator (CPG), which has yet to be clearly identified and characterized. Its activity is directly modulated by inputs from elsewhere in the CNS and from peripheral mechanoreceptors. The central location of respiratory motoneurones, innervating the various respiratory muscles, has been described in detail for some fish, particularly elasmobranchs. We are still unclear, however, about the link between the CPG and the sequential firing of the motoneurones, which result in coordinated contractions of the respiratory muscles, and about the mechanisms that result in recruitment of feeding muscles into forced ventilation. In teleosts, ventilation is matched to oxygen requirements by stimulation of gill chemoreceptors, which seem to respond to oxygen content or supply. There is little evidence of a role for these receptors in elasmobranchs.Chemoreceptor stimulation evokes a number of reflex changes in the respiratory and cardiovascular systems of fish that are rapid in onset and seem adaptive (e.g. increased ventilation and a bradycardia in response to hypoxia). Conditions that result in hypoxaemia and the consequent ventilatory changes also cause an elevation in circulating catecholamine levels. We have explored the possibility of a causal relationship between these levels and the ventilatory response. Strong evidence for this relationship arises from experiments on hypoxia and acid infusion, which trigger a ventilatory increase and a rise in circulating catecholamines. Both ventilatory responses are blocked by an injection of propranolol, indicating that adrenoreceptors are involved in the response.The ventilatory response to hypoxia, in teleosts at least, occurs very rapidly, perhaps before any marked increase in circulating catecholamines and almost certainly before any blood-borne catecholamines could reach the respiratory neurones. This argues for an immediate neuronal reflex based on chemoreceptors in the gill region responding to hypoxia. Clearly, circulating catecholamines also affect ventilation through some action in the medulla and could act in concert with a direct neuronal chemoreceptive drive during hypoxia. The studies on acid infusion during hyperoxia, where there is an acidosis but no increase in ventilation or blood catecholamines, would argue against any hydrogen ion receptor, either peripheral or central, being involved in the reflex ventilatory response to acidotic conditions in fish.The release of catecholamines into the circulation, therefore, seems to be an absolute requirement for the ventilatory response to acidosis in fish. Present evidence supports a role for -adrenergic receptors on respiratory neurones, stimulated by changes in the levels of circulating catecholamines, in the control of ventilatory responses to marked changes in oxygen availability in fish, such as those occurring in the post-exercise acidotic state.