Context-specific morphine withdrawal: Evidence that rearing reflects withdrawal, not exploration

Abstract

Several studies have demonstrated that contextual cues previously paired with morphine elicit conditioned withdrawal in the absence of the drug. Conditioned withdrawal signs have included circling, jumping, rearing, and wet dog shakes. One of these signs, rearing, may in fact reflect exploration rather than withdrawal. In those studies, rats had morphine paired or unpaired with distinctive contextual cues for several sessions and then were tested in that environment. During conditioning, the rats that received the drug in that context may have explored it less because they were sedated by the morphine. In the withdrawal test, this environment would have been more novel for these rats, as compared with the rats for which the contextual cues had been paired with saline. In the present study, rats were given eight injections of morphine, spaced 48 h apart, either paired or unpaired with distinctive contextual cues. During these sessions, the rats were physically restrained. Although both groups were equated for exploration of this context, the paired group, which received morphine in this context, displayed more rearing during the test session. These results suggest that rearing is an index of conditioned withdrawal.