Hypothalamic regulation of the cardiovascular and respiratory systems: role of specific opiate receptors.

Experiments were designed to evaluate the role of mu and delta opiate receptors in central cardiovascular control in the hypothalamic nucleus preopticus medialis of rats anaesthetized with pentobarbitone. The highly selective mu opiate receptor agonist D-Ala2-MePhe4-Gly5-ol-enkephalin was extremely potent in eliciting hypotension and bradypnoea; tachycardia was elicited by a low dose (0.064 nmol), but not by higher doses (0.64-6.4 nmol). Other selective mu receptor agonists (morphine sulphate, morphiceptin) caused tachycardia at lower doses (0.64, 6.4 nmol), hypotension and bradypnoea after the highest dose (64 nmol). The relatively selective delta receptor agonist D-Ala2-D-Leu5-enkephalin caused profound bradypnoea and hypotension at the high dose (64 nmol), tachycardia after the lowest dose (0.64 nmol), but bradycardia was found during the hypotension induced by the high dose (64 nmol). All of the opiate/opioid effects were reversed by naloxone (0.5 mg kg-1, i.v.). It is concluded that mu receptors may mediate the cardiovascular and respiratory effects of opiates and opioid peptides in the nucleus preopticus medialis of the rat.