An opiate mechanism involved in conditioned analgesia influences forced swim-induced immobility.

The modulatory effect of conditioned opiate analgesia on immobility during a forced swim situation was studied. Animals submitted to inescapable shock (IS) were exposed 6 days later to a similar or different shock application context and, immediately after, tested in either hot plate test or forced swim test. A conditioned analgesia was observed only on animals submitted to the shock context. This conditioned analgesia was blocked by naloxone administration injected either before IS or before context exposure. In the same way, animals exposed to shock context and immediately forced to swim showed an increase in the immobility time, which was sensitive to naloxone injection before IS as well as before context exposure. These results and additional data referring to naloxone effect on inactivity during IS are discussed in terms of the possible role of endogenous opiate in analogous behavior expressed during different aversive experiences.