SLC18A2 and Parkinson disease: Thus, we used 12 months old vesicular monoamine transporter 2 (VMAT2) deficient mice (VMAT2 LO mice or LO mice), a well- established animal model of PD (Mooslehner et al., 2001; Caudle et al., 2007; Taylor et al., 2009), to test for an apathy-like phenotype by means of the burrowing test, nest building test and saccharin preference test, and for a possible depressive-like phenotype by means of the forced swimming test.