The memory function of the hippocampal formation (Hip) and the marginal division (MrD) of neostriatum was compared. Rats with bilateral lesions of the MrD either immediate or 24 h after training in Y-maze were found to have decrease in correct runs in both groups. However, animals with transected afferent and efferent nerve bundles to isolate the Hip immediately or 24 h after training in Y-maze were found to show a decrease in correct runs only in the group injured immediately after Y-maze training but not in the 24 h group suggesting that MrD is likely involved in the entire process of long-term memory consolidation whereas the Hip only contributes to memory in the early stage. In addition, animals treated with a NMDA receptor (NMDAR) blocker, e.g. MK-801, showed decreased correct runs in Y-maze test and in expression level of phosphorylated CREB (pCREB) in neurons of the MrD but not in the Hip. Furthermore, animals treated with okadaic acid (OA), a potent protein phosphatase 1 inhibitor, showed increased correct runs in the Y-maze test. The expression level of pCREB and c-Fos and c-Jun was found increased in neurons of the MrD and the Hip in response to OA treatment. In conclusion, NMDAR and pCREB are involved in memory functions of both the Hip and the MrD. NMDAR might regulate pCREB level in neurons of the MrD but not in the Hip. Hence, the processes and mechanism of learning and memory involved in the MrD and the Hip may be different.
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