Dorsal horn opiate administration attenuates the perceived intensity of noxious heat stimulation in behaving monkey

In monkeys trained to detect and discriminate noxious heat stimuli, morphine microinjected into the medullary dorsal horn attenuated the perceived intensity of noxious heat in a dose- and stimulus-dependent fashion. These data demonstrate a pharmacologically specific effect of opiates on the sensory intensity component of pain at the earliest central relay pathway transmitting noxious information.