Gender differences in the cardiovascular responses to morphine and naloxone in spinal rats.

Putative gender differences in opiate cardiovascular effects were evaluated in spinal rats. After a 4-h exposure to a single dose of morphine (30 mg/kg, i.v.), abstinence was precipitated by naloxone (0.03-3 mg/kg, i.v.). Morphine produced a long-lasting bradycardia and a transient increase in arterial pressure that was similar in both genders. Thereafter, blood pressure decreased both in males and females. Naloxone precipitated a similar dose-dependent heart rate increase in both sexes and a gender-dependent increase in blood pressure. This sex difference appeared in the shape of the response. Prazosin (0.2 mg/kg), prior to naloxone, reduced the pressor response in all animals, suggesting a similar participation of the noradrenergic system in both genders. The present results extend to acute dependence the notion of a sex-dependent differential effect of morphine. The need to consider gender as a factor when studying the effects of opioids is highlighted.