Several genes that exhibited unique expression patterns within subareas of the VTA and SN were identified, including genes restricted within subpopulations of the VTA, e.g. NeuroD6, Grp and TrpV1. TrpV1-Cre transgenic mice were selected for functional optogenetics within the midbrain, while the human counterparts of the genes identified in the mouse were assessed in human brain material, including post-mortem tissue derived from PD patients. This evidence concerns the gene GRP and Parkinson disease.