Opioid peptide involvement in the bulbar inhibition of electrodermal activity in the cat

By analogy with supraspinal and spinal inhibitory controls of pain, it was hypothesized that an opioid mechanism could be involved in the bulbar inhibitory control of the electrodermal activity. This activity was evoked as skin potential responses on the footpads of 13 cats by the central tegmental field stimulation (control responses) and inhibited by the simultaneous stimulation of bulbar reticular formation (experimental responses). Then, naloxone, an opioid peptide antagonist, was injected intravenously or intrathecally and its effects were analyzed on both control and experimental responses. Intravenous injections of naloxone increased significantly the amplitude of experimental responses from 6 to 12 min after the injection and had no effect on the amplitude of control responses. Intrathecal injections of naloxone induced significant increases of amplitude of experimental responses from 6 to 42 min after the injection. These results showed that a spinal opioid peptide link could be involved in bulbar inhibition mechanisms of electrodermal activity.