Objective: To assess the cardiorespiratory effects of morphine sulfate and evaluate whether morphine blocks cardiac responses to a noxious stimulus in winter flounder.
Animals: 42 winter flounder (Pseudopleuronectes americanus) that were acclimated at 10 degrees C.
Procedures: Each fish was fitted with a Doppler flow probe around the ventral aorta; cannulae were placed for injection of drug or saline (0.9% NaCl) solution and assessments of respiration. Selected cardiorespiratory variables were measured in morphine-injected (40 mg/kg, IP [n = 18] or 17 mg/kg, IV [2]) or saline solution-injected (1.6 mL [22]) fish at various intervals. Heart rate and cardiac output (CO) were also measured in flounder that were injected with saline solution (n = 19) or morphine (10) and received a noxious or innocuous stimulus (injection of 5% acetic acid or saline solution SC into a cheek) 50 minutes later.
Results: Morphine administration promptly induced marked bradycardia (and a concomitant reduction in CO), followed by prolonged (> 48 hours) increases in CO and heart rate. Morphine injection only transiently affected respiratory rate. Application of a noxious stimulus to control flounder resulted in a significant (10%) but transient (< 5 minutes' duration) increase in CO, which was completely blocked by prior administration of morphine.
Conclusions and clinical relevance: Although morphine blocked the response to a noxious stimulus in fish, its cardiovascular effects might preclude its use in many research situations. Investigation of the dose dependency of these cardiovascular effects and their interspecific variation is required to determine the applicability of morphine for use in fish.