The exact function of the D1/D2 MSNs of the striatum remains elusive. Colocalization of the D1 and D2 DA receptors has been reported in axons located in various basal ganglia components24. Whether this observation indicates a distinct striatofugal pathway remains unclear66. However, the existence of such unique striatofugal projections would imply a complementary functional role of the D1/D2 MSNs, working in concert with the D1 and the D2 MSNs for harmonious basal ganglia functioning. Single-axon tracing studies in rodents667 and monkeys768 have emphasized the highly collateralized nature of the striatofugal projections, challenging the concept of a simple dual (direct/indirect) striatofugal system. Whether the D1/D2 MSNs contribute to these highly collateralized striatofugal projections is currently unknown and single-axon tracing of D1/D2 MSNs is obviously needed to better appreciate their functional role in the basal ganglia functioning.
