Prior morphine facilitates the occurrence of immobility and anhedonia following stress.

The role of the activation of the opiate system either induced by a 120-min restraint session or by a single morphine administration (10 mg/kg, i.p.) on the behaviors performed in a subsequent forced-swim test has been evaluated. In addition, animals were pretreated with naloxone (2 mg/kg, i.p.) prior to restraint or to morphine. Furthermore, in order to evaluate if this opioid mechanism could participate in the effect of stress on the response to a rewarding stimulus, rats were administered with morphine (10 mg/kg, i.p.)--whether associated or not with prior naloxone (2 mg/kg, i.p.) administration--and subsequently exposed to a 90-min restraint period. Following stress, all rats were submitted to a sucrose (1%) preference test. Both morphine and restraint enhanced the time spent in immobility in the forced-swim test. Both behavioral effects were attenuated by naloxone pretreatment thus suggesting that the increased immobility is probably modulated by the previous activation of an opiate mechanism. Furthermore, only animals with the associated treatment with morphine and restraint showed a clear reduction in sucrose preference. The fact that this effect was blocked by naloxone suggests the involvement of an opiate process in this decreased response to reward. These behavioral data suggest that the activation of an endogenous opiate mechanism facilitates the occurrence of enhanced immobility and anhedonia in response to a subsequent stress experience.