DRD2 and Parkinson disease: Thus striatal D1 hypersensitivity could lead to an increased responsiveness of striatopallidal MSNs to D2R activation in the conditions of PD with and without LID, although only a small population of the striatonigral MSNs has been found to form collateral axon connections with striatopallidal MSNs in the mouse striatum (Taverna et al., 2008).