Comparison of Learning and Memory in Morphine Dependent Rats using Different Behavioral Models

There are several conflicting evidences showing the effect of morphine on learning and memory processes. In the present study the effect of chronic morphine administration on passive avoidance, active avoidance and spatial learning and memory of morphine dependent male rats using Passive Avoidance shuttle box and Morris Water Maze tasks were investigated, respectively. Male rats received morphine sulfate in their drinking water for 21 days. Morphine dependency was assessed by injection of naloxone HCl (2 mg/kg) showing the withdrawal signs. Our results showed that in the passive avoidance experiments although the learning of the morphine dependent group was lower than the sham and control groups, but was not statistically significant. Also no significant difference was observed between the memory retention of these groups. In the 2-way active avoidance task, learning was increased significantly in morphine dependent rats in the first day of training with respect to the sham and control groups. But, there was no significant difference in memory of these three groups. Our data in the Morris Water Maze showed that learning of the dependent group in the 3rd day of training decreased significantly with respect to the sham and control groups. But no significant difference was observed in their memory retention and also in their motor activity. Our results showed that in the male rats, chronic morphine administration decreased spatial learning, but had no effect on spatial memory and motor activity. On the contrary, it facilitated 2-way active avoidance learning but had no effect on active and passive avoidance memory. In conclusion, it seems that the effect of morphine dependency on learning and memory in rats is task dependent and depends on the types of experimental learning and memory paradigms.