The serotonergic system modulates brain functions that are considered to underlie affective states, emotion and cognition. Several lines of evidence point towards a strong lateralization of these mental processes, indicating similar asymmetries in associated neurotransmitter systems.

To investigate a potential brain asymmetry of the serotonin transporter (SERT) distribution using Positron Emission Tomography (PET).

As brain asymmetries differ between sexes, we aimed to compare serotonin transporter asymmetry between females, males and male-to-female transsexuals whose brains are considered to be partly feminized.

36 subjects aged 19-54 years (9 female controls, 13 male controls and 14 male-to-female transsexuals) were measured with PET and [11C]DASB. Whole-brain voxel-wise SERT binding potential (BPND) maps were computed using a tracer-specific symmetric template. Statistics comprised repeated measures ANOVA with group as the between subjects factor, voxel-wise SERT asymmetry as repeated factor and group*asymmetry as interaction term.

SERT binding in all groups showed both strong left and rightward asymmetries in several cortical and subcortical structures including temporal and frontal cortices, anterior cingulate, hippocampus, caudate and thalamus (p< 0.05 FDRcorrected). Further, male controls showed a rightward asymmetry in the midcingulate cortex (p>0.05 FDR-corrected) which was absent in females and male-to-female transsexuals.

Our data support the notion of a lateralized serotonergic system, which is in line with previous findings of asymmetric serotonin-1A receptor distributions, extracellular serotonin concentrations, serotonin turnover and uptake. The absence of serotonin transporter asymmetry in the midcingulate in male-to-female transsexuals may be attributed to an absence of brain masculinization in this region.