Differential expression of neurotrophin and neurotrophin receptor mRNAs in and adjacent to fetal midbrain grafts implanted into the dopamine-denervated striatum

This study examined the expression of neurotrophins and neurotrophin receptors in the lesion/transplanted striatum at four different time points after transplantation. The ventral mesencephalic region was dissected from a single rat fetus at embryonic day 14 (E14) and implanted into the denervated striatum of rats with unilateral 6-hydroxydopamine lesions. Transplanted rats were killed at 1, 2, 3, or 4 weeks after transplantation surgery and the brains subsequently prepared for semiquantitative in situ hybridization analysis of neurotrophin and neurotrophin trk receptors. Hybridization of cRNA probes for trkB or trkC showed a time-dependent reduction within the transplant during the first 4 weeks after transplantation; hybridization of brain-derived neurotrophic factor or tyrosine hydroxylase mRNA probes within the transplant did not change significantly during the same posttransplantation period. Hybridization of the trkB mRNA probe in host striatum adjacent to the transplant was significantly higher than probe hybridization in the corresponding region of the intact striatum during the first 2 weeks after transplantation, but by the 3rd and 4th week, probe hybridization in the denervated/transplanted and intact striatum were the same. Lesioned animals without transplants maintained higher trkB mRNA probe hybridization in the denervated striatum than in the intact striatum at the same postlesion time points suggesting that lesioned/transplanted animals show a normalization of trkB mRNA probe hybridization. Hybridization of the trkC mRNA probe in the lesioned/transplanted striatum was significantly lower than that observed in the intact striatum 4 weeks after transplantation; however, at this same time point we observed a similar reduction of trkC probed hybridization in lesioned animals without transplants. The results of the study show dynamic neurotrophic activity occurring within the transplant and host tissue during the first month of transplant development.