Cardiovascular responses to naloxone challenge in opiate-dependent individuals.

Vagally mediated tachycardia appears to be a common response to abused drugs and, therefore, has implications for abuse liability. To test the specificity of this common factor, we determined whether the tachycardia to naloxone in opiate-dependent individuals has a significant vagal component. Naloxone challenge (0.4 mg, IM) in 19 opiate-dependent men and women was associated with highly reliable tachycardia, but no significant change in vagal tone index, a noninvasive measure of parasympathetic inhibitory control of the heart. We conclude that tachycardia during naloxone-precipitated withdrawal is not vagally mediated. Thus, there is some degree of specificity to the common factor of vagally mediated tachycardia to abused drugs because it was ruled out in at least one drug (naloxone) with aversive subjective effects.