Depressor and bradycardic effect following intrathecal injection of [NMePhe3,D-Pro4]morphiceptin in rats.

The intrathecal injection of 1.25, 2.5 and 5 micrograms of [ NMePhe3 ,D-Pro4] morphiceptin (MC), a highly selective mu receptor agonist, in pentobarbital-anesthetized rats resulted in a dose-dependent decrease of mean arterial pressure, reaching a maximal effect 1 h after its administration. A marked bradycardic effect paralleled the depressor effect. These effects were totally reversed by prior intrathecal injection of 10 micrograms of naloxone. The effect of MC was fully present in bilaterally vagotomized rats and in rats under artificial respiration. These results provide evidence for the opioid control of blood pressure and heart rate via mu receptors in the spinal cord.