Differential actions of intracerebroventricular (ICV) opioid receptor agonists on the activity of dorsal horn neurons (DHN) in the cat spinal cord

ICV infusion of morphine (MOR) produces strong analgesia in man and animals. The analgesic effect is thought to be mediated by the centrifugal inhibtory control, But neural mechanisms of the analgesic effect of ICV morphine are not well understood. For example, in the previous studies, ICV morphine does not inhibit nociceptive transmission in the spinal cord. On the contrary, ICV MOR often excites activity of dorsal horn neuron in the spinal cord. In the present study, we found that ICV MOR had dust actions on activity of dorsal horn neuron that it produced both inhibition and excitation of dorsal horn neurons. Since MOR exerts i Is action via three different types of opioid receptors, we further sought to investigate if there are differential effects of opioid receptor agonists on dorsal horn neurons when administered ICV.