Involvement of the Postrhinal and Perirhinal Cortices in Microscale and Macroscale Visuospatial Information Encoding

Abstract

Whereas the postrhinal cortex (POR) is a critical center for the integration of egocentric and allocentric spatial information, the perirhinal cortex (PRC) plays an important role in the encoding of objects that supports spatial learning. The POR and PRC send afferents to the hippocampus, a structure that builds complex associative memories from the spatial experience. Hippocampal encoding of item-place experience is accompanied by the nuclear expression of immediate early gene (IEGs). Subfields of the Cornus ammonius and subregions of the hippocampus exhibit differentiated and distinct encoding responses, depending on whether the spatial location and relationships of large highly visible items (macroscale encoding) or small partially concealed items (microscale encoding), is learned. But to what extent the PRC and POR support hippocampal processing of different kinds of item-place representations is unclear. Using fluorescence in situ hybridization (FISH), we examined the effect of macroscale (overt, landmark) and microscale (subtle, discrete) item-place learning on the nuclear expression of the IEG, Arc. We observed an increase in Arc mRNA in the caudal part of PRC area 35 and the caudal part of the POR after macroscale, but not microscale item-place learning. The caudal part of PRC area 36, the rostral and middle parts of PRC areas 35 and 36, as well as the middle part of the POR responded to neither type of item. These results suggest that macroscale items may contain a strong identity component that is processed by specific compartments of the PRC and POR. In contrast small, microscale items are not encoded by the POR or PRC, indicating that item dimensions may play a role in the involvement of these structures in item processing.

Keywords: fluorescence in situ hybridization; immediate early gene; perirhinal cortex; postrhinal cortex; visuospatial information processing.

Figure 1

Schema of item-place configurations and object exploration behavior. (A,B) Animals either explored (A) three small novel objects (microscale cues) that were placed within three of four holes of a holeboard, or (B) three large novel objects (macroscale cues) that were placed on the floor of the chamber, for 5 min. (C) During microscale item-place exploration, the animals explored holes 2, 3, and 4 that contained microscale cues and hole 1 that did not contain an object, equally. ANOVA F_(3,60) = 0.2514, p = 0.860057. (D) During macroscale item-place exploration, no preference was evident concerning the time spent exploring objects 1, 2, and 3. ANOVA F_(2,36) = 1.9840, p = 0.152276. (E) No difference was evident between the total exploration times inside the chamber during microscale or macroscale item-place exposure. ANOVA F_(1,27) = 0.000, p = 0.98764. (F) Animals that explored microscale item-place configurations engaged in a significantly higher number of rears compared to animals that were exposed to macroscale item-place configurations. ANOVA F_(1,27) = 24.8538, p = 0.000032. (C–F) Mean ± SEM. (F) ANOVA: ***p < 0.001.

Figure 2

Exploration of item-place configurations does not trigger immediate early gene (IEG) expression in the rostral part of the perirhinal cortex (PRC). (A) Illustration of the rostral part of the PRC (outlined by a red square) in a DAPI-stained coronal section (ca. −3.24 mm posterior to Bregma) of the rat brain (left) and enlargement of the outlined area to show the rostral PRC (right) in which outlines of area 36 (top) and 35 (bottom) are included (indicated by red rectangles). (B–G) Representative images of Arc mRNA expression (red dots, indicated by white arrows) in rostral PRC area 36 (B–D) and area 35 (E–G) after novel exploration of microscale (C,F), or macroscale (D,G) item-place configurations, compared to controls (B,E). Images were taken using a 63× objective. Nuclei were stained with DAPI (blue). (H,I) Bar charts show the relative percentage of Arc-positive nuclei in the rostral PRC area 36 (H) and area 35 (I) following item-place exploration, compared to controls (mean ± SEM). Nuclear Arc mRNA expression in rostral PRC area 36 and area 35 remained unchanged after novel item-place exploration compared to control animals (ANOVA; area 36: F_(2,20) = 0.0756, p = 0.927454; area 35: F_(2,19) = 0.13803, p = 0.871941).

Figure 3

IEG expression in the middle part of perirhinal cortex area 35 and area 36 is unaffected by exposure to novel item-place configurations. (A) The left panel shows a DAPI-stained coronal section (ca. −4.56 mm posterior to Bregma) of the rat brain and the outline of the middle PRC (red square). The right panel shows an enlargement of the outlined area to show the middle PRC in which outlines of area 36 and 35 (red squares) are indicated, where z-stacks were taken for analysis. (B–G) Representative images of nuclear Arc mRNA positive nuclei (red dots, indicated by white arrows) in the middle PRC, area 36 (B–D) and PRC area 35 (E–G) following exploration of microscale cues (C,F), or macroscale cues (D,G) compared to controls (B,E). Blue: nuclear counterstaining with DAPI. Images were obtained using a 63× objective. (H,I) Bar charts describe the relative percentage of nuclei expressing Arc mRNA in the middle PRC after novel item-place exploration, compared to responses detected in control animals (mean ± SEM). No significant changes in nuclear Arc mRNA expression occured in PRC area 36 (H) or area 35 (I) in both experimental conditions, compared to controls (ANOVA; middle PRC area 36 F_(2,19) = 0.0630, p = 0.939180; middle PRC area 35 F_(2,19)= 0.2444, p = 0.785604).

Figure 4

Exposure to a novel macroscale item-place configuration induces an increase in IEG expression in the caudal part of perirhinal cortex area 35, but not in area 36. (A) The left panel shows a DAPI-stained coronal section (ca. −5.52 mm posterior to Bregma) of the rat brain with the caudal PRC highlighted by a red rectangle. The right panel shows a magnification of the caudal PRC area. Z-stacks were created in the caudal PRC area 36 and 35, indicated by red squares. (B–G) Photomicrographs, taken using a 63× objective, show nuclear Arc mRNA expression (red points, indicated by white arrows) in the caudal PRC (B–D) area 36 and (E–G) area 35 following (C,F) microscale and (D,G) macroscale item-place exploration, or IEG expression in corresponding brain sections from control animals (control; B,E). Cell nuclei (blue) are stained with DAPI. (H–I) Bar charts represent the mean percentage (mean ± SEM) of Arc mRNA positive nuclei in caudal PRC area 36 (H) or PRC area 35 (I) under control or test conditions (ANOVA; caudal PRC area 35: F_(2,21) = 7.9498, p = 0.002689; area 36: F_(2,20) = 1.18071, p = 0.327571). Exploration of microscale item-place cues does not lead to significant changes in Arc mRNA expression in caudal PRC areas 35 and 36, relative to controls (microscale vs. control, post hoc Fisher’s LSD test, p > 0.05, each). Caudal PRC area 35, but not area 36, responds to macroscale item-place information (post hoc Fisher’s LSD test; area 36 p > 0.05; area 35: ***p < 0.001 for macroscale vs. control and *p < 0.05 for macroscale vs. microscale).

Figure 5

Differentiated IEG expression in the middle and caudal parts of the postrhinal cortex (POR) following the exploration of novel item-place configurations. (A,B) DAPI-stained coronal sections of the rat brain show the regions of interest examined in the (A) middle and (B) caudal parts of the POR, as indicated by red squares and the respective magnified images (middle). The red squares in enlarged images indicate areas where z-stacks were obtained for the middle POR (ca. −6.96 mm posterior to Bregma) and for the caudal POR (ca. −7.8 mm posterior to Bregma). (C–H) Photomicrographs represent nuclear Arc mRNA expression (red dots, indicated by white arrows) in the middle POR (C–E) and caudal POR (F–H) of control animals (C,F) or animals that participated in microscale (D,G) or macroscale item-place exploration (E,H). Nuclei (blue) were stained with DAPI. Images were taken using a 63× objective. (I,J) Bar charts showing the relative percentage of positive Arc mRNA nuclei in the middle POR (I) and caudal POR (J) of controls and for both experimental conditions (mean ± SEM). No significant changes can be observed in the middle POR (I) following microscale and macroscale item-place exploration compared to the control group (ANOVA F_(2,19) = 1.19376, p = 0.324814). Interestingly, in caudal POR (J), a significant difference in Arc mRNA expression can be detected in animals that participated in macroscale item-place exploration compared to the control group (post hoc Fisher’s LSD test, *p < 0.05), whereas exposure to microscale cues does not change Arc mRNA expression (p > 0.05).