Transmitter expression and morphological development of embryonic medullary and mesencephalic raphé neurones after transplantation to the adult rat central nervous system. I. Grafts to the spinal cord.

Suspensions of cells derived from the mesencephalic raphé or medullary raphé regions of the 13-14 day old embryonic rat brain were injected into the spinal cord of adult rats which had been previously denervated with 5,7-dihydroxytryptamine. At periods of up to 12 months after grafting, the spinal cords were taken for immunohistochemical analysis of 5-hydroxytryptamine (5HT), substance P (SP) and thyrotropin releasing hormone (TRH). In nearly all cases, surviving transplants were found. The grafts derived from mesencephalic raphé cells contained neurones which were immunoreactive to 5HT, or SP, but not both together. On average 4% of the total possible number of the available embryonic mesencephalic serotoninergic cells were found. A very dense outgrowth of 5HT positive fibres from the transplant was observed, extending up to 1.5 cm in both the caudal and rostral directions from the graft locus. Some SP immunoreactive fibres were also apparent near the implant. The grafts derived from the medullary transplant also contained 5HT-immunoreactive cells, comprising on average 25% of the total 5HT neurones available from the embryonic medullary primordium. In addition, neurones co-localizing 5HT together with SP and TRH were visible, closely reflecting the situation found in the medullary raphé in situ. Dense plexi of fibres containing 5HT-LI extended both caudally and rostrally up to 12-15 mm from the transplant. Outgrowth of SP and TRH varicose fibres was also demonstrable, although to a lesser degree than for 5HT. It was also possible to find many motoneurones surrounded by varicose fibres containing both 5HT and SP, in contrast to the situation with the mesencephalic grafts, where no such patterns of innervation were seen. The experiments indicate that the milieu of the spinal cord may compromise the survival of mesencephalic raphé 5HT neurones far more than of medullary serotonin cells. However, despite this effect on cell survival, the outgrowth of fibres from the remaining mesencephalic 5HT neurones was apparently unaffected by their ectopic position. Similarly, the transmitter content of both classes of raphé cells was largely unaltered, either by the transplantation process or by the environment into which they were placed. It is concluded that although the adult denervated spinal cord can selectively affect neuronal survival, it is incapable either of inducing in other serotoninergic cells placed within it the transmitter phenotype typical of medullary raphé neurones, or of causing those ectopically located 5HT cells to form connections appropriate to the descending serotonin fibres.