Basal expression of c-Fos and Zif268 in the rat basal ganglia: immunohistochemical characterization of striatal Zif268-positive neurons

Basal expression of the protein products of the inducible immediate early genes (IEGs), c‐Fos and Zif268, was investigated in five regions of the rat basal ganglia using immunohistochemistry. In particular, high basal levels of Zif268 but very low levels of c‐Fos were seen in the caudate‐putamen (CPu). Double immunostaining revealed that many of the constitutively expressed Zif268‐positive neurons were GABAergic but very few were cholinergic or neuronal nitric oxide synthase (nNOS)‐positive, and some of the Zif268‐positive neurons were also immunopositive for a glutamate NMDA receptor subunit NR1 or NR2A. No regional difference between the medial and lateral parts of the CPu was observed in the cellular phenotypes of Zif268‐positive neurons. Almost no basal levels of Zif268 were seen in the other four regions: the globus pallidus, entopeduncular nucleus, subthalamic nucleus and substantia nigra pars reticulata. As in the CPu, negligible levels of c‐Fos were seen in these four regions. Differential expression of these two IEGs may suggest gene‐specific and region‐specific functions of c‐Fos and Zif268 in the basal ganglia. Constitutive expression of Zif268 existing mainly in the GABAergic neurons in the CPu may at least in part be maintained by glutamatergic afferents.