Destruction of the nigrostriatal pathway increases Fos-like immunoreactivity predominantly in striatopallidal neurons

Unilateral destruction of the nigrostriatal pathway by injection of 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle (MFB) produces a long-lasting ( > 3 months) increase in the expression of Fos-like immunoreactivity (FLI) in medium-sized neurons (12-18 μm) of the ipsilateral striatum. In order to determine the nature of neurons which contain FLI in the 6-OHDA-denervated striatum, striatonigral and striatopallidal neurons were retrogradely labelled with the fluorescent tracer Fluoro-Gold. Nuclei displaying FLI were frequently found in striatopallidal neurons (72% overlap) but seldom in striatonigral neurons (11% overlap). These results are consistent with studies suggesting that dopamine tonically inhibits striatopallidal neurons which become more active in its absence. Moreover, the preferential localization of FLI in striatopallidal neurons supports the proposal that the AP-1 transcriptional regulating factor may contribute to neuropeptide and/or D2 dopamine receptor increases which occur in these neurons after 6-OHDA lesions.