ALTERED FOS-LIKE IMMUNOREACTIVITY IN TERMINAL REGIONS OF THE MESOTELENCEPHALIC DOPAMINE SYSTEM IS ASSOCIATED WITH REAPPEARANCE OF TYROSINE-HYDROXYLASE IMMUNOREACTIVITY AT THE SITES OF FOCAL 6-HYDROXYDOPAMINE LESIONS IN THE NUCLEUS-ACCUMBENS

The present study was undertaken in order to determine whether unilate ral 6-hydroxydopamine (6-OHDA) lesions in the nucleus accumbens (Acb) affect basal Fos-like immunoreactivity (-LI) in terminal regions of th e mesotelencephalic dopamine system. It was hypothesized that dopamine depletion in the Acb would alter activation of mesotelencephalic dopa mine neurons perhaps via the striatomesencephalic GABAergic pathway, a nd that this may be detected as altered basal Fos-LI in mesotelencepha lic terminal regions. 6-OHDA treatment effectively depleted tyrosine h ydroxylase (TH)-LI in well-circumscribed areas of the Acb at 14 days p ost-lesion, but at 25 days post-lesion all animals showed a reappearan ce of TH-LI staining in the lesioned region. When data from a number o f mesotelencephalic terminals regions was pooled, Fos-LI cell density was higher in the sham and lesion 14-day groups and sham 25-day group than both the 25-day lesion group and untreated controls. The present study demonstrates that unilateral sham and 6-OHDA lesions in the Acb may have repercussions throughout the mesotelencephalic dopamine syste m. Further investigation is necessary to determine whether reappearanc e of TH-LI at the lesion site contributes to the return of Fos-LI to b asal levels.