In the present study, we investigated the influence of intra-Medial Septum (intra-MS) injections of dopamine D1 receptor agents on amnesia induced by intra-CA1 injections of a muscarinic acetylcholine receptor antagonist, scopolamine. This study used a step-through inhibitory (passive) avoidance task to assess memory in adult male Wistar rats. The results showed that in the animals that received post-training intra-MS injections of saline, intra-CA1 administrations of scopolamine (0.75, 1, and 2 µg/rat) decreased inhibitory avoidance (IA) memory consolidation as evidenced by a decrease in step-through latency on the test day, which was suggestive of drug-induced amnesia. Post-training intra-MS injections of a dopamine D1 receptor agonist, SKF38393 at doses of 0.1, 0.15, and 0.3 µg/rat had no effect, but at dose of 0.5 µg/rat impaired IA memory consolidation. Interestingly, intra-MS injections of SKF38393 (0.15, 0.3 and 0.5 µg/rat) significantly prevented amnesia induced by intra-CA1 injections of scopolamine (1 µg/rat). Intra-MS injections of a dopamine D1 receptor antagonist, SCH23390 (0.5 and 0.75 µg/rat) by itself impaired IA memory consolidation, and also at dose of 0.75 µg/rat increased amnesia induced by intra-CA1 administrations of an ineffective dose of scopolamine (0.5 µg/rat). Post-training intra-MS injections of ineffective doses of SCH23390 (0.1, 0.3 and 0.5 µg/rat) prevented an effective dose of SKF38393 response to the impaired effect of scopolamine. These results suggest that dopamine D1 receptors in the MS via projection neurons to the hippocampus affect impairment of memory consolidation induced by intra-CA injections of scopolamine.
