Involvement of different opioid receptor subtypes in electric shock-induced analgesia and motor suppression in the rat

We have investigated the correlation of electric shock-induced behavioral changes and functional alterations of endogenous opioid receptor subtypes. The degree of electric shock-induced behavioral changes, such as analgesia and motor suppression, was dependent on the duration of and time after electric shock application. The electric shock-induced behavioral changes were completely antagonized by naloxone. The apparent development of tolerance to both behavioral effects as a result of successive daily electric shock was different: Tolerance to electric shock-induced analgesia developed after 2 days' successive electric shock application, while tolerance to motor suppression was not observed even after 7 days' successive electric shock application. There was a decrease of [³H][D-Ala², Met⁵]enkephalinamide ([³H]DAMEA, δ agonist) binding and an increase of [³H]naloxone (μ antagonist) binding when potent electric shock-induced analgesia developed. On the other hand, the binding of [³H]DAMEA and [³H]ethylketocyclazocine (κ agonist) was significantly changed when locomotion was suppressed. These results suggest strongly that different opioid systems may participate in electric shock-induced analgesia and motor suppression: electric shock-induced analgesia and motor suppression may be mediated by μ/δ and κ/δ receptors, respectively.