Developmental onset distinguishes three types of spontaneous recognition memory in mice

Abstract

Spontaneous recognition memory tasks build on an animal's natural preference for novelty to assess the what, where and when components of episodic memory. Their simplicity, ethological relevance and cross-species adaptability make them extremely useful to study the physiology and pathology of memory. Recognition memory deficits are common in rodent models of neurodevelopmental disorders, and yet very little is known about the expression of spontaneous recognition memory in young rodents. This is exacerbated by the paucity of data on the developmental onset of recognition memory in mice, a major animal model of disease. To address this, we characterized the ontogeny of three types of spontaneous recognition memory in mice: object location, novel object recognition and temporal order recognition. We found that object location is the first to emerge, at postnatal day (P)21. This was followed by novel object recognition (24 h delay), at P25. Temporal order recognition was the last to emerge, at P28. Elucidating the developmental expression of recognition memory in mice is critical to improving our understanding of the ontogeny of episodic memory, and establishes a necessary blueprint to apply these tasks to probe cognitive deficits at clinically relevant time points in animal models of developmental disorders.

Figure 1

Experimental design for object recognition tasks. (A) Schematic diagram of the object location task (OL). Mice of different ages underwent a 10 min sample phase and, following a 1 h delay period, underwent a 5 min test phase in which one object was moved to a novel location. (B) Schematic diagram of the novel object recognition task (NOR). Mice of different ages underwent a sample phase that terminated once a criterion of 20 s total object exploration time was met. Following a 24 h delay, mice underwent a 5 min test phase in which one object was replaced with a novel object. (C) Schematic diagram of the temporal order recognition task (TOR). Mice of different ages underwent two sample phases with an inter-phase delay of approximately 1 hour. Following another hour delay, mice underwent a 5 min test phase where they interacted with one object presented in sample phase 1 and one object presented in sample phase 2.

Figure 2

Ontogeny of object location recognition memory. C57/129J mice were tested in the OL task at P16, P21 or P25. (A) Schematic of the OL task. (B) Object exploration during the test phase of the OL task. Only P21 and P25 mice spent significantly more time exploring the novel location compared to the familiar location. (C) Relative preference for the novel location was calculated by a discrimination index (DI) dividing the time spent exploring the new location by the total object exploration time throughout the first 20 s of object interaction. A preference for the novel location was observed only at P21 and P25. Female (cyan) and male (magenta) data points are identified, indicating the lack of observed sex differences. *p < 0.05. P16, n = 33 (19 females, 14 males); P21, n = 29 (14 females, 15 males); P25, n = 25 (12 females, 13 males).

Figure 3

Ontogeny of novel object recognition memory. C57/129J mice were tested in the NOR task at P16, P21, P25 or P28. (A) Schematic of the NOR task. (B) Object exploration during the test phase of the NOR task. Only P25 and P28 mice spent significantly more time exploring the novel object compared to the familiar object. (C) Relative preference for the novel object was calculated as a discrimination index (DI) dividing the time spent exploring the new object by the total object exploration time throughout the first 20 s of object interaction. A preference for the novel object was observed only at P25 and P28. (D) Schematic of the NOR task with immediate delay. P21 mice underwent the same NOR protocol except with an immediate delay (under 2 min). (E) Object exploration during the test phase of the NOR immediate delay task during the first 20 s of object exploration. (F) Relative preference for the novel object during the test phase of the NOR immediate delay task expressed as a DI. P21 animals that underwent an immediate delay displayed preference for the novel object. Female (cyan) and male (magenta) data points are identified, indicating the lack of observed sex differences. *p < 0.05. P16, n = 23 (12 females, 11 males); P21, n = 17 (9 females, 8 males); P25, n = 16 (8 females, 8 males); P28, n = 19 (9 females, 10 males). P21 immediate delay, n = 9 (2 females, 7 males).

Figure 4

Ontogeny of temporal order recognition memory. C57/129J mice were tested in the TOR task at P16, P21, P25, P28 or P35. (A) Schematic of the TOR task. (B) Object exploration during the test phase of the TOR task. Only P28 and P35 mice spent significantly more time exploring the old object compared to the recent object. (C) Relative preference for the old object was calculated by a discrimination index (DI) dividing the time spent exploring the old object by the total object exploration time throughout the first 20 s of object interaction. A preference for the old object was observed only at P28 and P35. Female (cyan) and male (magenta) data points are identified, indicating the lack of observed sex differences. *p < 0.05. P16, n = 21 (12 females, 9 males); P21, n = 23 (14 females, 9 males); P25, n = 27 (13 females, 14 males); P28, n = 30 (13 females, 17 males); P35, n = 26 (10 females, 16 males).

Figure 5

Total object exploration across ages and tasks. Total object exploration during the test phase for (A) Object location (OL), (B) Novel object recognition (NOR) and (C) Temporal order recognition (TOR). We found no differences in object exploration time among any of the age groups for OL and TOR. P16 mice showed increased object exploration compared to P21 and P25 mice in NOR. (D–G) Correlation between total object exploration during the 5 min test phase and the discrimination index (DI) for all ages of the NOR task. No correlation was found for any of the age groups suggesting that behavioral performance in NOR is not influenced by differences in object exploration. *p < 0.05. OL: P16, n = 33; P21, n = 29; P25, n = 25; NOR: P16, n = 23; P21, n = 17; P25, n = 16; P28, n = 19; TOR: P16, n = 21; P21, n = 23; P25, n = 27; P28, n = 30; P35, n = 26.