GABA(A) receptor subunit expression in intrastriatal ventral mesencephalictransplants

To compare the expression of GABA, receptor subunits in the normal substant ia nigra and in fetal mesencephalic neurons ectopically transplanted into t he dopamine-depleted striatum, we have employed single and double immunocyt ochemical approaches using tyrosine hydroxylase (TH) and alpha1, alpha2, al pha3, and beta2/3 GABA(A) receptor subunit specific antibodies. In the subs tantia nigra, alpha1 and beta2/3 GABA(A) receptor subunits were labeled in processes in the pars compacta (SNc) and, more intensely, in both somata an d processes in the pars reticulata (SNr). There was no clear TH and al or b eta2/3 colocalization, with the exception of some TH-immunoreactive (-ir) n eurons that showed a weak immunoreactivity for beta2/3. Sections immunolabe led for alpha2 showed a faint diffuse labeling for this subunit both in the SNr and in the SNc. Scattered somata were immunopositive for alpha2, and s ome of them were also TH-ir. The labeling for alpha3 and TH showed that TH- positive neurons expressed intense alpha3 immunoreactivity, although some T H-negative somata in the SNr expressed weak alpha3 immunoreactivity. In the transplants, double immunostaining procedures showed that the labeling for alpha1 or beta2/3 appeared particularly concentrated in patches of intense ly immunoreactive neuronal processes that surrounded TH-ir cells, but these processes were not TH-ir. In the case of alpha2, diffuse immunostaining wa s observed all over the graft, with some scattered positive somata. Only a few of them were also TH positive. Sections immunoreacted for alpha3 and TH revealed that TH-ir neurons expressed intense alpha3 immunoreactivity, and that only a few TH-negative neurons were weakly positive for alpha3. These results show that mesencephalic tissue ectopically grafted into the striat um develops a pattern of GABA(A) receptor expression similar to that normal ly expressed in situ, and particularly that the grafted dopaminergic neuron s express similar GABA(A) receptors, including the alpha3 subunit. This mig ht be due to the similarity of GABAergic afferents to these neurons in the SNc and the graft, or that at the time of transplantation this expression h ad already been determined.