time profile of morphine effect on different phases of inhibitory avoidance memory in rat

The amnesic effect of morphine is well known in the laboratory animals. But, it is unclear that morphine at what times can exactly affect different phases of memory, including acquisition, consolidation, and retrieval. Therefore, we investigated the time profile of morphine's amnesic effect on passive [inhibitory] avoidance learning and memory in male Wistar rats. In order to evaluate the outcomes of pre- and post-training administrations of morphine, the animals were trained in a step-through type of passive avoidance task at various time points, and were tested 24 h after training to measure memory retrieval. The results showed that acquisition of memory was impaired in the animals that received a dose of 7.5 mg/kg of morphine [Intraperitoneally] at 0, 30 min, and 1 h before training, as evidenced by a decrease in step-through latency on the test day. Post-training administrations of morphine at 30 min and 1h, 4h except for the time immediately after training, did not impair memory consolidation. The results also showed that pre-test administrations of morphine at 0 and 30 min before the test, impaired retrieval of inhibitory avoidance memory. Taken together, the results suggest that morphine, when injected at different time points before training, after training, or before testing affects different phases of inhibitory avoidance memory. With regard to the time of injections related to each phase, other experiments can be designed to investigate molecular mechanisms involved in the impairing effect of morphine in each phase

Repeated histamine pretreatment decreased amnesia induced by post-training administration of the drug in a step-down inhibitory avoidance test in mice

Repeated administration of certain drugs could result in an enhancement of the behavioral effects of those drugs. In the present study, the effect of repeated administration of histamine on amnesia induced by post-training administration of the drug was examined. A single trial step-down inhibitory [passive] avoidance task was used for memory assessment in male NMRI mice. The results showed that post-training administration of different doses of histamine [5, 10, and 20 micro g/mouse, i.c.v.] decreased the step-down latency on the test day. Repeated pretreatment of histamine [10 and 20 micro g/mouse] for three days followed by five days of no drug treatment prevented amnesia due to post-training histamine [20 micro g/mouse]. In contrast, repeated administration of histamine H1 receptor antagonist, pyrilamine [5, 10, and 20 mg/kg] or histamine H2 receptor antagonist, ranitidine [12.5 and 25 mg/kg] 10 minutes prior to histamine injections, decreased the effect of repeated histamine administration. Moreover, a similar pattern was seen in animals which received dopamine D1 receptor antagonist, SCH 23390 [0.025, 0.5, and 1 mg/kg] or dopamine D2 receptor antagonist, sulpiride [0.2, 1, and 5 mg/kg] 10 minutes prior to histamine injections during the repeated pretreatment. The results indicated that both the histamine and dopamine receptor mechanisms may be involved in the effects of repeated pretreatment of histamine on drug induced amnesia