FUNCTIONAL N-METHYL-D-ASPARTATE RECEPTORS DEVELOP AND PERSIST IN HETEROTOPIC CORTICAL GRAFTS AND REGULATE EXPRESSION OF TRANSCRIPTION FACTOR KROX-24

N-Methyl-D-aspartate (NMDA) receptors are a major subfamily of glutama te receptors and thought to play a pivotal role in developmental plast icity and synaptogenesis, neuronal migration and differentiation. NMDA receptors have also been implicated in neuronal degeneration, as glut amate binding to the receptor initiates rapid excitotoxic signal trans duction. Molecular cloning of cDNAs has yielded different NMDA recepto r subtypes with an essential NR1 subunit associated with various modul atory NR2 subunits. The NR1 gene is expressed at high levels in virtua lly all brain structures, but to a distinctly higher extent in cortex than in striatum. Here we report on the development, maintenance and f unction of glutamate receptors in intrastriatally located cortical gra fts. Cortical primordia of rat fetuses (E14) were stereotactically gra fted into the rostral striatum of adult recipient rats. Expression of NR1 mRNA was examined by in situ hybridization after post transplantat ion periods of 2, 6 and 12 months. Analysis of NR1 mRNA expression in grafts after a differentiation period of 2 months revealed equal level s compared to the intact neocortex of the host rats and that of rats w ith the same ontogenetic age. No downregulation of NR1 mRNA was seen 6 and 12 months after transplantation. To ensure normal function of NMD A receptors in grafts, we studied the effects of a blockade of recepto r dependent gene expression, using Krox-24 as a reporter gene. In norm al brain tissue, constitutive expression of KROX-24 protein is thought to be maintained by NMDA receptor mediated physiological synaptic act ivity and can be abolished by the noncompetitive NMDA receptor antagon ist MK-801. Immunostaining of KROX-24 protein was almost identical in grafts compared to the corresponding neocortex. This constitutive expr ession of KROX-24 could be abolished by treatment with MK-801. Thus, o ur data indicate normal development and long term persistence of gluta mate receptors with intrinsic excitatory activity in transplants.