The retrograde tracer fluoro-gold interferes with the expression of fos-related antigens

The retrograde tracer, fluoro-gold (FG) has been used in combination with immediate early gene (IEG) immunohistochemistry to identify neural circuits activated by pharmacological, physiological or behavioral manipulations. However, since FG has been shown to be toxic to cell bodies, axons and terminals at the injection site, the question arises as to whether FG alters the detection of IEG products. To examine this question, FG was microiontophoresed unilaterally into the nucleus accumbens (NAc) of rats and Fos-related antigens (FRAs) were examined in both hemispheres 12 days later. Approximately half as many FRA-positive nuclei were observed in the tracer-injected NAc as were found in the contralateral NAc. Similar results were observed in the ventral subiculum of the hippocampus and the basolateral and central amygdaloid nuclei, but not in the lateral septum or lateral habenula. These results suggest that FG microiontophoresed into the NAc interferes with the expression of FRAs at the injection site and also at other ipsilateral limbic sites.