The expression of GAP-43 mRNA in developing embryonic striatal tissue grafts.

We have studied by quantitative in situ hybridization histochemistry the mRNA expression of the growth associated phosphoprotein (GAP-43) in grafts of foetal striatal neurones implanted into the ibotenic acid lesioned adult rat neostriatum. Animals were studied at 7, 15, 30 and 90 days after implantation. Using a 35S-labelled specific oligonucleotide probe we observed the highest levels of GAP-43 mRNA in 15-day-old grafts; levels were significantly reduced at 30 days and in 3-month-old grafts GAP-43 mRNA expression was not significantly different from the very low levels in the normal rat striatum. The data indicate that GAP-43 mRNA is highly expressed in developing but not mature neurones and suggest that embryonic striatal grafts may develop a relatively adult phenotype within the host brain by about 4 weeks; by which time most of the synaptic connections are established.