Rearing Behavior as Indicator of Spatial Novelty and Memory in Developing Rats

Abstract

Among the various forms of exploration, rearing-where rodents stand on their hind legs-reflects the animal's processing of spatial information and response to environmental novelty. Here, we investigated the developmental trajectory of rearing in response to spatial novelty in a standard object-place recognition (OPR) task, with the OPR retrieval phase allowing for a direct comparison of measures of rearing, object exploration, and locomotion as indicators of spatial novelty and memory. Groups of male rats were tested on postnatal day (PD) 25, PD31, PD38, PD48, and at adulthood (PD84). The OPR task comprised a 5-min encoding phase with the rat exposed to an arena with two identical objects and, 3 h later, a 5-min retrieval phase in the same arena with one object being displaced to another arena zone. Rearing increased in response to spatial novelty (i.e., the displaced object) at retrieval relative to encoding, with this increase occurring first on PD31, and thus later than preferential object exploration-based responses emerging already on PD25. Importantly, zone-specific analyses during retrieval revealed an increase in rearing events in the (now empty) zone where the displaced object is used to be at encoding. This increase was only observed in adult rats (PD84) and likely indicates the presence of specific object-place associations in memory. These findings evidence rearing as behavior covering aspects of spatial novelty complementary to those of object exploration, thereby enabling a more comprehensive characterization of the emergence of spatial episodic memory during early life.

Keywords: developmental trajectory; exploratory behavior; object–place recognition; spatial episodic memory; spatial learning.

FIGURE 1

Novelty response to change in object configuration across development. (A) Experimental design and timeline. After 10‐min habituation sessions on the first 3 days, the rats were tested in a standard object–place recognition (OPR) task, comprising a 3‐h retention interval between the 5‐min encoding and 5‐min retrieval phases. During the retrieval phase one of the two (identical) objects placed in the arena during encoding was displaced to a novel location (arrow). (B) Illustration of the open field with distal cues at the surrounding walls, along with an illustration of the rat's exploratory rearing and object exploration behaviors. (C) Rats at different ages (juvenile: PD25, peri‐adolescent: PD31; adolescent: PD38, PD48, and young adult: PD84), were tested in the OPR task with the size of the open field being smaller for the younger (PD25, PD31) than older groups (PD38, PD48, PD84). Based on the encoding of a persistent memory during the encoding phase, the change in the spatial configuration of the two objects is expected to induce a novelty response, which expresses itself in an increase in exploratory rearing and an increased exploration of the displaced object. (D) mean rearing duration (%) and (E) total rearing number (% change from encoding phase), (F) Object discrimination index (as a measure of object exploration), and (G) distance travelled (% change from encoding phase) for each of the five age groups during the first 1‐min (white bars) and 3‐min intervals (gray bars) of the retrieval phase. Mean ± SEM values with overlaid dot plots are shown. #p < 0.05, ##p < 0.01, ###p < 0.001 for one sample t‐test against 0. *p < 0.05, **p < 0.01, ***p < 0.001, for Holm–Sidak post hoc tests. Data in F are adapted from Contreras et al. (2019).

FIGURE 2

Rearing activity in the Never vs Former zones of the arena. (A) For the analyses, the arena as used during the retrieval phase was divided into 4 quadrants (zones): The Same zone was defined by containing the nondisplaced object, the Novel zone by containing the displaced object, the Never zone by containing no object during both the encoding and retrieval phases, and the Former zone by having contained the displaced object during the encoding phase and being empty during the retrieval phase. (B) Mean rearing duration (s) and total rearing number in the Former and Never zones in the different age groups (PD25, PD31, PD38, PD48, PD84) for the first 1 min and (C) 3 min of the retrieval phase. Mean ± SEM values with overlaid dot plots are shown. *p < 0.05, **p < 0.01, for paired samples t‐test.

FIGURE 3

Rearing duration increases in response to change in the object configuration at OPR retrieval testing. (A) Experimental procedure: Following a within‐subject comparison, adult rats were tested in the standard OPR task condition (the same as in Experiment 1, white bars) and in a “stationary” task control condition (gray bars) where the two objects remained at the same location during the Encoding and Retrieval phases. (B) Mean rearing duration (%) and (C) total rearing number (%) during the Retrieval phase, as percent change from values during encoding (set to 100%). (D) Mean rearing duration (s, left) and total rearing number (right) as absolute values during the retrieval phase. Mean ± SEM values with overlaid dot plots are shown for the first 1 min and 3 min of the Retrieval phase. # p < p < 0.05, ## p < 0.01 for one sample t‐test against 0. *p < 0.05 for paired samples t‐test.

FIGURE 4

Rearing activity in the four different zones of the arena during the retrieval phase of the OPR task condition. (A) Discrimination of arena zones, see legend to Figure 2A. (B) Mean rearing duration (s, left) and total rearing number (right) in the Former and Never zones for the first 1 min and 3 min of the retrieval phase. (C) The same as in B, for the Same and Novel zones. Mean ± SEM values with overlaid dot plots are shown. *p < 0.05, **p < 0.01, ***p < 0.001 for paired samples t‐test.

FIGURE 5

Evolution of spatial exploration behavior during early development. The schema illustrates the animal's dominant response to spatial novelty at different developmental stages, that is, with a nonspecific increase in general locomotion around PD16 (blue line, Shan et al. 2022) with an increase in the object discrimination index (ODI) towards familiar object–locations around PD25 (orange line), with a general and persisting increase in rearing at PD31 (purple), with an increase in the ODI towards novel object locations around PD38 (black), and with a zone specific increase in rearing at previously occupied locations (as an indicator of object–location memory) only after PD48 (green).