Acute opioid dependence in the cardiovascular system of the spinal rat

This article analyzes and characterizes the early development of opioid dependence in the cardiovascular system of the spinal rat. The main part of the study deals with changes in mean arterial pressure and heart rate induced by naloxone (NLX) in spinal rats pretreated with single doses of morphine. The results show that the cardiovascular system of the spinal rat is highly sensitive to the abstinence-precipitating actions of NLX after a single dose of morphine. Dose-response curves for precipitated abstinence evaluated as changes in mean arterial pressure and heart rate show a gradual increase in maximum followed by a progressive shift to the left as dependence progresses. Cardiovascular abstinence is mostly mediated by catecholaminergic systems. NLX also precipitates noncardiovascular signs of abstinence. Interestingly, morphine-free spinal rats gave some abstinence-like responses to NLX, probably because of endogenous release of opioids. The spinal rat seems to be a valuable system for a rapid quantitative pharmacological characterization of the abstinence-precipitating actions of opioid antagonists and for the study of the changes that occur during the acute development of opioid dependence.