TH and obesity due to melanocortin 4 receptor deficiency: In obesity, decreased TH in VTA dopamine neurons is responsible for a reduction in the release of dopamine from nerve terminals in the NAcb (Geiger et al. 2009; Li et al. 2009), whereas decreased dopamine biosynthesis in somatodendritic regions of VTA dopamine neurons attenuates D2 receptor-mediated autoinhibition, and recovers excitability of these neurons.