Research ReportFinding the place without the whole: Timeline involvement of brain regions

- We assessed reference memory in MWM under different spatial cues conditions.
- There are not behavioral differences between experimental conditions.
- Prefrontal, striatum, hippocampus, amygdala and mammillary bodies activation differed in PC.
- Striatum and thalamus activation was found in OPC group.
- Higher c-Fos counts in prefrontal, striatum, thalamus and CA3 in the PC.

Mastering the Morris water maze (MWM) requires the animal to consolidate, retain and retrieve spatial localizations of relevant visual cues. However, it is necessary to investigate whether a reorganization of the neural networks takes place when part of the spatial information is removed. We conducted four experiments using the MWM. A classical reference memory procedure was performed over five training days, RM5 (n=7), and eight days, RM8 (n=7), with the whole room and all the spatial cues presented. Another group of animals were trained in the same protocol, but they received an additional day of training with only partial cues, PC (n=8). Finally, a third group of animals performed the classical task, followed by an overtraining with partial cues for four more days, OPC (n=8). After completing these tasks, cytochrome c-oxidase activity (CO) in several brain limbic system structures was compared between groups. In addition, c-Fos positive cells were measured in the RM5, RM8, PC and OPC groups. No significant differences were found among the four groups in escape latencies or time spent in the target quadrant. CO revealed involvement of the prefrontal and parietal cortices, dorsal and ventral striatum, CA1 and CA3 subfields of the dorsal hippocampus, basolateral and lateral amygdala, and mammillary nuclei in the PC group, compared to the RM group. In the OPC group, involvement of the ventral striatum and anteroventral thalamus and the absence of amygdala involvement were revealed, compared to the PC group. C-Fos results highlighted the role of the prefrontal cortex, dorsal striatum, anterodorsal thalamus and CA3 in the PC group, compared to the OPC, RM5 and RM8 groups. The animals were able to find the escape platform even when only a portion of the space where the cues were placed was available. Although the groups did not differ behaviorally, energetic brain metabolism and immediate early gene expression revealed the engagement of different neural structures in the groups that received more training without the entire surrounding space.