Trace amine-associated receptor 1 agonist reduces aggression in brain serotonin-deficient tryptophan hydroxylase 2 knockout rats

Abstract

Introduction: Aggression and self-harm disproportionately occur in youths preoccupied with social status tracking. These pathological conditions are linked to a serotonin (5-HT) deficit in the brain. Ablation of 5-HT biosynthesis by tryptophan hydroxylase 2 knockout (TPH2-KO) increases aggression in rodents. Remarkably, deletion of the trace amine-associated receptor 1 (TAAR1) results in the same consequences. Unlike the nuanced dynamics of social status cues in young people, the social ranks of rats mainly advance when they dominate larger opponents in combat.

Methods: This study explored whether the potent TAAR1 agonist RO5263397 reduces aggression caused by 5-HT depletion, and whether social rank advancement motivates this aggression. The resident-intruder paradigm was applied with larger and smaller intruders to evaluate whether social rank advancement motivates aggressive behaviors in TPH2-KO rats.

Results: When a smaller intruder was introduced, 5-HT-deficient rats did not differ from wild type littermates. However, when the intruders were larger, the mutants extended their aggressive efforts, refusing to submit. Importantly, RO5263397 selectively abolished this abnormal form of aggression in TPH2-KO rats.

Discussion: Results supported social rank advancement as the main incentive. These data also suggest that TAAR1 is a promising target for the development of new treatments for aggression; independent data also support this conclusion.

Keywords: 5HT; RO5263397; TAAR1; TAAR1 agonist; TPH2; aggression; serotonin; social dominance.

Figure 1

Experimental design of comparative analysis between WT and TPH2-KO rats (n = 8 per group).

Figure 2

Evaluation of baseline behavioral phenotype of TPH2-KO rats. (A, B) TPH2-KO rats showed decreased hole exploration in circular open-field test (p = 0.0260). Other parameters do not reach significant differences. (C) In the novel object recognition test (ORT) TPH2-KO rats showed significant changes compared to WT. In front of similar objects (Day 1), TPH2-KO rats demonstrated significantly increased grooming (p = 0.0006) and decreased exploration of the right object (p = 0.0017). (D) Day 2 of ORT revealed increased learning of the new object (p = 0.0293), but decreased time with right one in TPH2 compared to WT rats (p = 0.0093). (E) TPH2-KO rats spent less time in rearing exploration in comparison to WT (p = 0.0008). (F) The marble burying test revealed a slightly obsessive-compulsive behavioral phenotype in the TPH2-KO group represented in a buried endpoint (p = 0.0065). Data are presented as mean ± SEM (n = 8). *p<0.05, **p<0.01, ***p<0.001 vs. control, Mann-Whitney U-test. WT, wild type rats.

Figure 3

TPH2-KO rats have slightly decreased percent of correct transitions in grooming microstructure. (A) Comparative self-grooming microstructure analyses of TPH2 knockout (KO) rats vs. wild type (WT) control group in the grooming test (n = 8 per group). The diameter of circles and line thickness reflect mean frequency of grooming bouts or transitions, respectively. All grooming bouts, but only ‘correct’ grooming transitions adhering to the cephalo-caudal progression (paws > face > head > body > tail/genitals) were statistically assessed. (B) TPH2-KO rat had decreased percent of correct grooming transitions (p = 0.048) supporting its higher sensitivity to changes in grooming behavior. *p < 0.05 vs. control, Mann–Whitney U-test. WT, wild type control rat.

Figure 4

TPH2-KO rats cannot accept social defeat in the resident-intruder test. (A) Represents scheme of resident-intruder test with different types of intruders. TPH2-KO and WT rats opposed in front of SI and BI groups. (B) Radar-plot with total activity demonstrates a significant difference in TPH2-KO rats’ behavior versus the BI group. Minimal alterations were revealed in the SI group experiment. TPH2 gene knockout causes significantly increased aggressive behavior (p = 0.0002) in front of the larger opponent, decreased non-social exploration (p = 0.0031), and locomotor activity (p = 0.0216). (C–F) In the BI experiment, TPH2-KO vs. WT also demonstrated significantly decreased attack latency (p < 0.0001) and increased aggressive activity (p = 0.0029). Data are presented as mean ± SEM (n=8). *p<0.05, **p<0.01, ***p<0.01, ****p ≤ 0.0001 vs. control; Two-way repeated measures ANOVA (Factor 1 – genotype: WT/TPH2-KO; Factor 2 – intruder type: big/small) followed by Dunnett’s post-hoc test. WT, wild type control rats. .

Figure 5

TAAR1 agonist decreases social aggression in TPH2 knockout rats. (A) RO5263397 (5 mg/kg) or vehicle were administered intraperitoneally 10 min before the start of the resident intruder interaction with TPH2-KO and WT rats opposed in front of the BI group. (B) RO5263397 normalizes the behavioral profile in the resident-intruder test, by decreasing aggressive activity (p = 0.0087) and slightly increasing mounting activity (p = 0.0338). (C-F) In the experiment with TPH2-KO vs. WT RO5263397 normalize alterations in attack latency and aggressive activity endpoints. Data are presented as mean ± SEM (n = 8). *p<0.05, **p<0.01, ***p<0.01 vs. control; Two-way repeated measures ANOVA (Factor 1 – genotype: WT/TPH2-KO; Factor 2 - drug effect: RO5263397/TWEEN80) followed by Dunnett’s post-hoc test. WT, wild type control rats.