Naloxone does not modify the hemodynamic and neuroendocrine effects of clonidine in normal humans

The relationship between central opioidergic and noradrenergic central control mechanisms of blood pressure was investigated in normal men by evaluating the interference exerted by naloxone, a specific opiate antagonist, on the cardiovascular (blood pressure and heart rate) and neuroendocrine [human growth hormone (HGH) stimulation] effects of clonidine, a centrally acting alpha-adrenergic agonist, according to two different protocols. In series 1, the effects of placebo (normal saline), clonidine (0.15 mg i.v.), and naloxone (0.4 mg i.v.) were compared with that of clonidine plus naloxone (0.15 and 0.4 mg i.v., respectively), in seven normal male subjects. Clonidine decreased blood pressure and heart rate, and increased HGH levels. Naloxone administered alone (0.4 mg i.v.) did not modify blood pressure, heart rate, and HGH levels, while naloxone (0.4 mg) pretreatment left unaltered the hemodynamic and neuroendocrine effects of clonidine. In series 2, in five additional normal males, the effect of increasing doses of naloxone (0.4, 2.0, and 8.0 mg i.v.) on the pharmacodynamic activity of clonidine (0.15 mg i.v.) was further evaluated. Clonidine alone decreased blood pressure and heart rate and increased HGH levels, while naloxone pretreatment, in the whole range of doses studied, did not significantly modify the action of clonidine. These data suggest that a central opioidergic tone does not modulate the effect of central alpha-noradrenergic stimulation in normal humans.