GABA(A) receptor subunit expression in intrastriatal striatal grafts comparison between normal developing striatum and developing striatal grafts

Expression of the alpha1, alpha2 and beta2/3 GABA(A) receptor subunits in maturing cell-suspension striatal grafts and in normal developing striatum was studied by immunocytochemistry. During normal postnatal development, the alpha1 subunit was present in the striatum only at very low density, while the alpha2 and beta2/3 subunits were present with a patchy distribution, in some patches at high density. Double-staining techniques indicated that DARPP-32 (a marker of striatal projection neurons) was not colocalized with alpha1, but was present in some beta2/3-positive areas and all alpha2-positive areas. In striatal grafts, alpha1 immunoreactivity was first detected 2 weeks post-grafting (p.g.), and by 3-10 weeks p.g. the pattern was similar to that observed in mature grafts (1 year p.g.), in which alpha1-immunopositive patches surrounding DARPP-32-positive (i.e. striatum-like) areas are observed. Alpha2 and beta2/3 immunoreactivity was observed within the first week p.g., and by 3-10 weeks p.g. was similar to that observed in mature grafts (i.e. immunoreactivity throughout the graft but with patches of different intensity). During graft maturation there was a marked decline in alpha2 immunoreactivity in DARPP-32-negative areas, as is observed during normal development of the globus pallidus and ventral pallidum. Interestingly, alpha1- and beta2/3-positive fibers (perhaps mostly dendrites) entered DARPP-32-positive patches from DARPP-32-negative areas. This study indicates that the time course of expression of GABA(A) receptor subunits in grafted striatal neurons, closely matches that of morphological maturation of the transplant, that of the development of functional synaptic activity and that of GABA(A) receptor subunit immunoreactivity in normal developing striatum. Our results also suggest that there are significant interactions between DARPP-32-positive and DARPP-32-negative areas with respect to the expression of GABA(A) receptors, and support the suggestion that miniature 'striatopallidal systems' may develop within grafts; such interactions may be important for the functional integration of striatal grafts with the host brain.