Regional Heterogeneity of 5-HT1A Receptors in Human Cerebellum As Assessed by Positron Emission Tomography

Two measures used in brain imaging are binding potential (BP) and the specific to nonspecific equilibrium partition coefficient (V(3)''). V(3)'' determined using the 5-HT(1A) ligand [(11)C]WAY-100635 is sensitive to changes in the free and nonspecific binding of the ligand in the reference region (V(2)). Healthy female volunteers have higher 5-HT(1A) BP but not V(3)'' compared with men, because V(2) is higher in women. While there could be several explanations for this observation, we hypothesized that women have more 5-HT(1A) receptors in the cerebellum. We explore the cerebellum to define a subregion that more accurately represents the free and nonspecific binding, potentially allowing the use of V(3)''. A quantitative autoradiogram in human brain using [(3)H]WAY-100635 identified a cerebellar subregion devoid of 5-HT(1A) receptors. In vivo 5-HT(1A) receptors were evaluated using [(11)C]WAY-100635 in 12 healthy women and 13 healthy men. Each subject had a metabolite-corrected arterial input function. The autoradiogram demonstrates the lowest concentration of 5-HT(1A) receptors in the cerebellar white matter (CW) and highest concentration in the cerebellar vermis (CV). The CW volume of distribution (V(T)) is lower than CV. Cerebellar white matter is adequately modeled by a one-tissue compartmental model, while a two-tissue model is necessary to model CV or the total cerebellum (CT). Women have a higher CW V(T) compared with men, suggesting a difference in V(2). Use of CW improves identifiability and time stability of BP in cortical regions. Cerebellar white matter might be a better reference region for use in future 5-HT(1A) studies using [(11)C]WAY-100635. With CW as a reference region, V(3)'' cannot be used to detect differences in 5-HT(1A) receptors between men and women, suggesting the need for arterial input functions to determine BP.