Catecholaminergic activity and 3',5'-cyclic adenosine monophosphate concentrations in the right ventricle after acute and chronic morphine administration in the rat.

We have examined possible regulation of norepinephrine and dopamine concentrations and turnover in the right ventricle of the rat after acute administration of saline i.p. or morphine 30 mg kg-1 i.p. to placebo (naïve) or morphine (tolerant) pretreated rats. We also assessed concentrations of 3',5'-cyclic adenosine monophosphate (cAMP) in the right ventricle after the same treatments. Concentrations of catecholamines and their metabolites in the heart were measured by high-pressure liquid chromatography with electrochemical detection (HPLC/DE). Concentrations of cAMP in the heart were measured by radioimmunoassay (RIA). Administration of morphine to naïve rats did not modify concentrations of norepinephrine (NE), normetanephrine (NMN) or NMN/NE ratio in the right ventricle. However, dopamine concentrations increased whereas dopamine turnover decreased. In addition, cAMP concentrations decreased after acute administration morphine to naïve rats. In rats pretreated with morphine chronically, there was an increase in norepinephrine concentrations with no change in normetanephrine concentrations or norepinephrine turnover after acute injection of morphine. In contrast, dopamine turnover increased in the tolerant groups after acute injection of saline or morphine compared with the nave group given morphine, indicating that tolerance develops to the acute effects of the opioid. Concentrations of cAMP increased after chronic morphine administration. Our results demonstrate that chronic morphine pretreatment leads to up-regulation of the cAMP system in the heart and suggest that this up-regulation may be involved in the cellular mechanisms implicated in the adaptive changes of dopaminergic neurones in the heart observed during chronic treatment with morphine.