Retrograde and anterograde contextual fear amnesia induced by selective elimination of layer IV-Va neurons in the granular retrosplenial cortex (A29)

Differences in cytoarchitectural organization and connectivity distinguishes granular (or area 29, A29) and dysgranular (or area 30, A30) subdivisions of the retrosplenial cortex (RSC). Although increasing evidence supports the participation of RSC in contextual fear learning and memory, the contribution of each RSC subdivision remains unknown. Here we used orchiectomized rats and intraperitoneal (i.p.) injections of saline (control) or 5 mg/kg MK801, to trigger selective degeneration of pyramidal neurons in layers IV-Va of A29 (A29(MK801) neurons). These treatments were applied 3 days before or two days after contextual fear conditioning, and contextual fear memory was evaluated by scoring freezing in the conditioned context five days after training. Afterwards, brains were fixed and c-Fos and Egr-1 expression were assessed as surrogates of neuronal activity elicited by the recall in A29, A30 and in limbic areas. We found that eliminating A29(MK801) neurons after training reduces conditioned freezing to 43.1 +/- 9.9% respect to control rats. This was associated with a significant reduction of c-Fos and Egr-1 expression in A30, but not in other limbic areas. On the other hand, eliminating A29(MK801) neurons before training caused a mild but significant reduction of conditioned freezing to 79.7 +/- 6.8%, which was associated to enhanced expression of c-Fos in A29, A30 and CA1 field of hippocampus, while Egr-1 expression in caudomedial (CEnt) entorhinal cortex was not depressed as in control animals. These observations show that severeness of amnesia differs according to whether A29(MK801) neurons were eliminated before or after conditioning, likely because loss of A29(MK801) neurons after conditioning disrupt memory engram while their elimination before training allow recruitment of other neurons in A29 for partial compensation of contextual fear learning and memory. These observations add further support for the critical role of A29(MK801) neurons in contextual fear learning and memory by connecting limbic structures with A30.