Relative Lack of Volumetric Differences in the Brain of Mouse Models Involving the Serotonin Transporter Gene

Background:   Serotonin (5-HT) is a neurotransmitter that plays a key role in brain development (Sodhi and Sanders-Bush, 2004). Further, the 5-HT transporter gene (SLC6A4, SERT) has been associated with both 5-HT levels and autism susceptibility.

Objectives: The purpose of this work was to assess the volumetric differences in the brains of three different SERT mouse models and corresponding wildtype littermates using MRI.

Methods: In total, 120 fixed mouse brains were examined from three different SERT mouse models. The first group was a SERT KO model, and the other two groups were SERT Ala56 KI models on two different backgrounds (129S6/S4 and C57BL/6).  Each group consisted of 40 mice, 20 wildtype littermates (WT) and 20 SERT mice (Age = P60, 10 Male and 10 Female).

MRI Acquisition – Scan parameters: T2- weighted, 3D fast spin-echo sequence (TR - 2000 ms, TEs - 14 ms, 6 echoes, 2 averages, FOV - 14 x 14 x 25 mm³, Matrix size = 250 x 250 x 450).  This sequence yielded an image with 0.056 mm isotropic voxels (3D pixel). Total imaging time ~12 h (Lerch et al., 2011).

Data Analysis– To visualize and compare any differences, the images from each of the three groups are registered together. The goal of the registration is to model how the deformation fields relate to genotype (Lerch et al., 2008). Volume differences can then be calculated for 62 regions in each groups (Dorr et al., 2008). Multiple comparisons were controlled for using the False Discovery Rate (FDR) (Genovese et al., 2002).

Results:   No significant volume differences were found in the brains of either SERT Ala56 KI models, measured in both absolute (mm³) and relative (% total brain) volume.  Minimal differences were found in the SERT KO model, and these differences were driven solely by the female mice.  Several volume differences in the female SERT KO mice were found with absolute volume, but only two regions, the cerebral peduncle and lateral septum, were found to be significantly different with relative volume in the female SERT KO mice. 

Conclusions:   It has been shown previously that behavioural phenotypes are associated with neuroanatomical phenotypes in 87% of models (Nieman et al., 2007).  Furthermore, in the 30+ autism models we have currently examined >75% of the models have neuroanatomical phenotypes. Therefore, these results are quite interesting, as both the SERT KO and Ala56 KI models have known behavioural and physiological phenotypes; although the phenotype of the SERT KI on the B6 is different from the 129S6/S4 (Moy et al., 2009; Kalueff et al., 2010; Veenstra-VanderWeele et al., 2012; Kerr et al., 2013). These null findings may indicate that developmental effects of altered extracellular 5-HT levels do not result in structural brain changes at a mesoscopic scale; although they do not rule out finer grained changes in connectivity or neuronal migration. Effects of sex and sex hormones are important to consider in future studies of serotonin’s role in brain development and function.