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

Implants have been made of dissociated embryonic mesencephalic or medullary raphé cells into the adult rat striatum, previously depleted of its 5HT innervation. The transmitter complement and fibre outgrowth of the grafted neurones were analysed immunocytochemically. Serotonin-containing cells were found in both types of transplant, and the proportionate survival of the potential number of implanted 5HT cells was similar for each type of graft. However, these proportions were both greater than that described previously in transplants of mesencephalic raphé cells to the spinal cord. In addition, the proportionate survival of medullary substance P neurones grafted to the striatum was greater than that of medullary 5HT cells implanted in the same region. The transmitter complement of the medullary neurones was largely unaltered after transplantation. However, the mesencephalic grafts contained neurones storing 5HT- and/or substance P-, or TRH-like immunoreactivity. The 5HT/substance P and TRH neurones have so far not been encountered in the mesencephalon in situ using the present immunohistochemical methodology. Invasion of the host striatum by 5HT processes from the transplanted mesencephalic cells was extensive. Fibres from medullary raphé neurones, however, were restricted principally to within the graft itself. It is concluded that there may exist in the adult rat striatum a set of trophic factors for 5HT and substance P neurones different from those found in other regions of the central nervous system, such as spinal cord. Moreover, trophic agents in the host striatum appear to operate differentially on mesencephalic and medullary raphé 5HT neurones to regulate their axonal outgrowth. Lastly, the neurotransmitter phenotypic expression of the embryonic mesencephalic raphé cells may be susceptible to influences from the host environment.