Social Behavioral Deficits Coincide with the Onset of Seizure Susceptibility in Mice Lacking Serotonin Receptor 2c

Abstract

The development of social behavior is strongly influenced by the serotonin system. Serotonin 2c receptor (5-HT2cR) is particularly interesting in this context considering that pharmacological modulation of 5-HT2cR activity alters social interaction in adult rodents. However, the role of 5-HT2cR in the development of social behavior is unexplored. Here we address this using Htr2c knockout mice, which lack 5-HT2cR. We found that these animals exhibit social behavior deficits as adults but not as juveniles. Moreover, we found that the age of onset of these deficits displays similar timing as the onset of susceptibility to spontaneous death and audiogenic-seizures, consistent with the hypothesis that imbalanced excitation and inhibition (E/I) may contribute to social behavioral deficits. Given that autism spectrum disorder (ASD) features social behavioral deficits and is often co-morbid with epilepsy, and given that 5-HT2cR physically interacts with Pten, we tested whether a second site mutation in the ASD risk gene Pten can modify these phenotypes. The age of spontaneous death is accelerated in mice double mutant for Pten and Htr2c relative to single mutants. We hypothesized that pharmacological antagonism of 5-HT2cR activity in adult animals, which does not cause seizures, might modify social behavioral deficits in Pten haploinsufficient mice. SB 242084, a 5-HT2cR selective antagonist, can reverse the social behavior deficits observed in Pten haploinsufficient mice. Together, these results elucidate a role of 5-HT2cR in the modulation of social behavior and seizure susceptibility in the context of normal development and Pten haploinsufficiency.

Fig 1. Three-chamber social approach and social novelty test in Htr2c ^-/Y juvenile male mice.

(A) Time spent in each chamber. (B) Approach-avoidance scores. (C) Preference index: [(number of mice where the time in the mouse chamber was ≥ 10% than the time spent in the tube chamber)–(number of mice where the time in the mouse chamber was < 10% than the time spent in the tube chamber)]/(total number of mice). (D) Velocity and distance traveled. n = 9 per genotype. *: p<0.05, NS: non-significant difference with paired samples t-test (A) and independent-samples t-test (B and D).

Fig 2. Three-chamber social approach and social novelty test in Htr2c ^-/Y adult male mice.

(A) Time spent in each chamber. (B) Approach-avoidance scores. (C) Individual approach-avoidance scores of adult males. (D) Preference index: [(number of mice where the time in the mouse chamber was ≥ 10% than the time spent in the tube chamber)–(number of mice where the time in the mouse chamber was < 10% than the time spent in the tube chamber)]/(total number of mice). (E) Velocity and distance traveled. n = 16 per genotype. *: p<0.05, **: p<0.01, ***: p<0.001 with paired samples t-test (A) and independent-samples t-test (B and E).

Fig 3. Audiogenic seizures in Htr2c ^-/Y adult male mice.

(A) Htr2c ^-/Y adult mice (P90) show a severe response to audiogenic stimulus (RA: n = 16) while Htr2c ^-/Y juvenile mice (P25) or wild-type adult and juvenile did not exhibit any response (NR: n = 4, 16, 4 respectively). NR, no reponse; WR, wild running; CS, clonic seizure; TS, tonic seizure; RA, respiratory arrest/death. (B) The seizure severity score indicated a fully penetrant phenotype in adult Htr2c ^-/Y mice.

Fig 4. Spontaneous death in Htr2c ^-/Y mice is accelerated by a second-site mutation in Pten.

Time course of spontaneous death in Htr2c ^-/Y (n = 15), Pten ^+/- (n = 21) and Htr2c ^-/Y; Pten ^+/- (n = 12) mice.

Fig 5. Rescue of social behavior deficits in Pten ^+/- mice by SB 242084 administration.

Results of three-chamber social approach after administration of SB 242084 or vehicle to wild-type and Pten ^+/- adult female mice. (A) Time spent in each chamber. For vehicle treated groups, n = 16 wild-type and 17 Pten ^+/-; for SB 242084-treated groups, n = 15 wild-type and 16 Pten ^+/-. *** p < 0.001 using paired-samples t-test (B) Approach-avoidance score. ** p < 0.01 using planned comparisons. (C) Preference index.