Adverse Social Experiences in Adolescent Rats Result in Enduring Effects on Social Competence, Pain Sensitivity and Endocannabinoid Signaling

Abstract

Social affiliation is essential for many species and gains significant importance during adolescence. Disturbances in social affiliation, in particular social rejection experiences during adolescence, affect an individual's well-being and are involved in the emergence of psychiatric disorders. The underlying mechanisms are still unknown, partly because of a lack of valid animal models. By using a novel animal model for social peer-rejection, which compromises adolescent rats in their ability to appropriately engage in playful activities, here we report on persistent impairments in social behavior and dysregulations in the endocannabinoid (eCB) system. From postnatal day (pd) 21 to pd 50 adolescent female Wistar rats were either reared with same-strain partners (control) or within a group of Fischer 344 rats (inadequate social rearing, ISR), previously shown to serve as inadequate play partners for the Wistar strain. Adult ISR animals showed pronounced deficits in social interaction, social memory, processing of socially transmitted information, and decreased pain sensitivity. Molecular analysis revealed increased CB1 receptor (CB1R) protein levels and CP55, 940 stimulated [³⁵S]GTPγS binding activity specifically in the amygdala and thalamus in previously peer-rejected rats. Along with these changes, increased levels of the eCB anandamide (AEA) and a corresponding decrease of its degrading enzyme fatty acid amide hydrolase (FAAH) were seen in the amygdala. Our data indicate lasting consequences in social behavior and pain sensitivity following peer-rejection in adolescent female rats. These behavioral impairments are accompanied by persistent alterations in CB1R signaling. Finally, we provide a novel translational approach to characterize neurobiological processes underlying social peer-rejection in adolescence.

Keywords: CB1 receptor; adolescence; adverse experience; endocannabinoid system; female rats; peer-rejection; social behavior; social play.

Figure 1

Experimental design. Female Wistar rats (light gray) were either reared with same-strain partners during adolescence, from postnatal day (pd) 21–50, considered as adequate social rearing (control), or with three age-matched rats from the Fischer strain (blue), considered as inadequate social rearing (ISR). Beginning on pd 50, all Wistar rats were again group housed with same strain partners and only the Wistar rats of both social rearing conditions (control and ISR) were further examined. Behavioral testing (pd 70–150) and molecular analysis (pd 150) were performed in adult animals.

Figure 2

Social play behavior during adolescence on pd 30 and pd 35 in Wistar females subjected to adequate (control) or ISR conditions. The probability of a defensive response received by the play partner after an attack by the focus animal was significantly reduced in females of the ISR condition on both time points. In addition, ISR Wistar rats also responded significantly less when being attacked by the play partner than controls which was further reduced on pd 35. Locomotor play was significantly increased in ISR animals compared to control rats only on pd 30. Data are indicated as means ± SEM (control: n = 9; ISR: n = 8; **P ≤ 0.001; *P ≤ 0.05).

Figure 3

Comparison of blood corticosterone levels in adolescent (A) and adult (B) females of the adequate (control) and the ISR condition. Baseline corticosterone levels in rats of both rearing conditions did not differ over the course of adolescence between the two rearing conditions. Data are indicated as means ± SEM (control/ISR: n = 8 each).

Figure 4

Behavioral performance in adult rats after adequate (control) or ISR experiences in adolescence. During social interaction testing (A) anogenital sniffing and following/approach behavior were significantly reduced in ISR rats. Similarly, rats of the ISR condition showed a deficient performance in the social food transfer task and showed a decreased preference for the socially transmitted food source. (B) The ability to recognize social partners was significantly affected in ISR rats, as indicated by a decrease in % discrimination and the discrimination index compared to controls. (C) Object recognition abilities (% discrimination or discrimination index) did not differ between females of either rearing condition. (D) Pain sensitivity to a thermal stimulus measured in adult rats of the control and the ISR condition (E) showed higher response latency on the hot plate in ISR rats, indicating significantly decreased pain sensitivity. Data are indicated as means ± SEM (control: n = 18; ISR: n = 15; object recognition: control/ISR n = 10; **P ≤ 0.001; *P ≤ 0.05).

Figure 5

CB1 receptor (CB1R) protein levels in adult rats from the adequate (control) and the ISR condition. CB1R protein levels (A) were increased in the amygdala of ISR rats. A trend (p = 0.06) for increased CB1R levels was detected in the thalamus of ISR rats, with no further changes in the medial prefrontal cortex (mPFC) and striatum. CP 55, 940 stimulated (% over basal) [³⁵S]GTPγS binding (B) was significantly in the amygdala and thalamus (control n = 10; ISR n = 9) of the ISR females, while remaining unchanged in mPFC (control n = 8; ISR n = 9) and striatum. Data are indicated as means ± SEM (control/ISR: n = 10 each; *P ≤ 0.05; ^#P ≤ 0.1).

Figure 6

Fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL) protein levels in adult rats from the adequate (control) and the ISR condition. FAAH protein levels (A) were decreased specifically in the amygdala of ISR rats but were unchanged in the mPFC, striatum and thalamus. MAGL protein levels (B) were significantly reduced in the striatum of ISR rats compared to control rats but were unchanged in the mPFC, amygdala and thalamus. Data are indicated as means ± SEM (control/ISR: n = 10 each; *P ≤ 0.05).

Figure 7

Endocannabinoid (eCB) levels in adult rats from the adequate (control) and the ISR condition. Levels of the eCB analysis of anandamide (AEA; A) were significantly increased in the amygdala and the striatum, while levels of 2-AG (B) were increased in the striatum of ISR animals as compared to controls. No further significant changes were observed for other brain regions tested (mPFC and thalamus). Data are indicated as means ± SEM (control/ISR: n = 10 each; *P ≤ 0.05, ^#P ≤ 0.1).