PLASMA CATECHOLAMINE CONCENTRATIONS DURING MORPHINE WITHDRAWAL IN CONSCIOUS GUINEA-PIGS

1. Plasma noradrenaline (NA) and adrenaline (AD) concentrations and locomotor activity were measured in conscious normal and adrenalectomized guinea-pigs before and after injection of a single dose of morphine (15 mg/kg) and following precipitation of withdrawal with naloxone 2 h later. Arterial blood samples were obtained via carotid catheters inserted 3 days before experiment. Normal guinea-pigs given saline instead of morphine were used as controls. 2. Increased locomotor activity occurred following naloxone injection in both intact and adrenalectomized morphine-treated guinea-pigs. 3. Plasma AD concentration was significantly elevated only during the naloxone-precipitated withdrawal period, whereas plasma NA concentration was also elevated during the morphine treatment period. No significant changes occurred in saline-treated guinea-pigs given naloxone. 4. In adrenalectomized guinea-pigs AD did not reach detectable levels in the plasma at any time during the experiments, thus showing that the increased plasma AD level during morphine withdrawal in normal animals reflected adrenal medullary release. 5. It was concluded that measurements of plasma AD concentration and locomotor activity in guinea-pigs should be useful for the study of the interactions between behavioural and autonomic mechanisms involved in opiate dependence and withdrawal, since the effects of morphine and morphine withdrawal on the autonomic nervous system are clearly different in this species.