These changes may possibly be explained by 5-HT being now expressed or taken up more strongly in these PPL1 neurons or, as observed in vertebrate models for PD, where denervation of DA neurons was found to potentiate 5-HT IR at neuronal terminals (Zhou et al., 1991; Rylander et al., 2010; Zeng et al., 2010; Niederkofler et al., 2015), derive from altered 5-HT plasticity. Here, HTR5A is linked to Parkinson disease.