Proximodistal segregation of nonspatial information in CA3: preferential recruitment of a proximal CA3-distal CA1 network in nonspatial recognition memory

Abstract

A prevailing view in memory research is that CA3 principally supports spatial processes. However, few studies have investigated the contribution of CA3 to nonspatial memory function. Interestingly, the proximal part of CA3 (close to the dentate gyrus) predominantly projects to distal CA1 (away from the dentate gyrus), which preferentially processes nonspatial information. Moreover, the cytoarchitecture and connectivity patterns in the proximal and distal parts of CA3 strongly differ, suggesting a functional segregation in this area. Here, we tested whether CA3 is recruited during nonspatial recognition memory, and whether nonspatial information is differentially represented along the proximodistal axis of CA3. Furthermore, we investigated whether the pattern of activation within CA3 would mirror that of CA1. We used a high-resolution imaging technique specifically designed to analyze brain activity in distant areas that is based on the detection of the expression of the immediate-early gene Arc, used as a marker of neuronal activation. We showed that proximal CA3 is strongly recruited during a nonspatial delayed nonmatching-to-sample recognition memory task in rats, while distal CA3 is not. In addition, distal CA1 was more activated than proximal CA1 in the same task. These findings suggest a functional segregation of CA3 that mirrors that of CA1, and potentially indicate the existence of a proximal CA3-distal CA1 hippocampal subnetwork that would preferentially process nonspatial information during recognition memory.

Figure 1.

Non spatial recognition memory task. A, Ten odors are presented to the animal during the study phase (one at a time). After a 20 min delay, the memory for the studied odors is tested by presenting the same odors intermixed with 10 new odors to the animals (also one at a time). B, C, Delayed nonmatching-to-sample rule. If the odor belonged to the study list (old odor), the rat was expected to refrain digging and turn around to get a food reward at the back of the cage (B). If the odor did not belong to the study list (new odor), the rat could dig in the stimulus cup to retrieve a buried reward (C). D, The percentage of correct choices (accuracy; means ± SEM) is shown for the last three sessions of testing. Comparison to chance level (50% correct): ^¶p < 0.005.

Figure 2.

Proximodistal nonspatial representations along the transverse axis of CA1 and CA3. A, Location of the counting frames along the transverse axis of the hippocampus: dorsal view of the CA1 hippocampal subfield [inspired by Henriksen et al. (2010); note that CA3, which lies under CA1, is not represented]. Estimated borders between proximal and distal CA1 along the transverse axis of the hippocampus are indicated by blue dotted lines (note that the complex three-dimensional conformation of the hippocampus makes any standard planes of sectioning inappropriate for a correct assessment of its transverse organization). B, To compare proximal and distal parts of CA1 (blue frames) or CA3 (red frames) at a corresponding dorsoventral level, and along the transverse axis of the hippocampus, counting frames were shifted from proximal CA1 (lateral) at the septal level to distal CA1 (medial) at the temporal level, and from distal CA3 (lateral) at the septal level to proximal CA3 (medial) at the temporal level (Andersen et al., 1971). C, The task-induced proportion of Arc-positive cells in the proximal and distal parts of CA3 and CA1 (left to right) in the DNMS group. The task-induced proportion of Arc-positive cells (means ± SEM) is significantly higher in proximal CA3 and distal CA1 than in distal CA3 and proximal CA1 (respectively), suggesting that nonspatial information is not uniformly represented along the proximodistal axis of CA3 and CA1. *p < 0.05. Comparisons to 0: °p < 0.05.

Figure 3.

Arc expression in proximal and distal parts of CA3 and CA1 along the transverse axis of the hippocampus. Representative images of Arc expression in proximal CA3 (A), distal CA3 (B), distal CA1 (C), and proximal CA1 (D) in the DNMS group. DAPI-stained nuclei are shown in blue and Arc intranuclear signal in red. Arrows show examples of Arc-positive cells, and arrowheads show examples of Arc-negative cells. Proximal CA3 and distal CA1 displayed significantly more Arc-positive cells than distal CA3 and proximal CA1, respectively. Scale bar, 20 μm.

Figure 4.

Proximodistal nonspatial representations along the mediolateral axis of CA3 and CA1 at the septal and temporal levels of the hippocampus. To investigate the proximodistal segregation of nonspatial information in CA3 and CA1 at the septal and temporal levels, four additional frames (dashed frames) were sampled: distal CA1 and proximal CA3 at the septal level (A), and proximal CA1 and distal CA3 at the temporal level (C), in addition to the four frames (plain frames) already described in Figure 2B. B, D, Results at the septal and temporal levels of the hippocampus mirror those along the transverse axis: the task-induced proportion of Arc-positive cells (means ± SEM) is significantly higher in proximal CA3 and distal CA1 when compared with that of distal CA3 and proximal CA1, respectively. This suggests that nonspatial information is not uniformly distributed along the proximodistal axis of CA3 and CA1 throughout the hippocampus. *p < 0.05, **p < 0.005. Comparison to 0: °p < 0.05.

Figure 5.

Task-induced Arc expression in proximal and distal parts of CA3 and CA1 in the random reward group. In the random reward group, the task-induced proportion of Arc-positive cells (means ± SEM) does not differ between the proximal and distal parts of CA3 and CA1 along the transverse axis (A) and mediotemporal axis at the septal (B) and temporal (C) levels, suggesting that testing conditions alone did not lead to the pattern of activation found in the DNMS group. Comparison to 0: °p < 0.05.

Figure 6.

Task-induced Arc expression in proximal and distal parts of CA3 and CA1 in the MECS group. The proportion of Arc-positive cells induced by MECS (means ± SEM) does not differ between proximal and distal parts of CA3 and CA1 along the transverse axis (A) and mediotemporal axis at the septal (B) and temporal (C) levels, showing that proximal and distal parts of CA3 and CA1 were capable of expressing Arc to a similar level at the time of death. Comparison to 0: °p < 0.0005.