Besides their difference in the expression of DA receptors, the two major types of striatal MSNs express different neuropeptides in both rodents and primates21454, the D1 MSNs containing SP and DYN while the D2 MSNs are enriched with ENK20. As for the D1/D2 MSNs, a previous report has suggested that they express both ENK and DYN in the rat24, whereas the present findings clearly show that they display immunoreactivity only for DYN. Such a discrepancy might reflect a species difference, but most likely results from a variation in the methodological approach. We used highly specific transgenic fluorescent reporters to identify the D1, D2 and D1/D2 striatal MSNs, whereas Perreault, et al.24 employed antibodies against the D1 and D2 receptors, which are known to be also highly expressed in the striatal neuropil, a situation that might have hampered the proper identification of the various peptide expressing MSNs. Our data indicate that, in regard to neuropeptide content, the striatal D1/D2 neurons in mice have more in common with the D1 than with the D2 MSNs.
