ARREST OF AFFERENT AXON EXTENSION BY TARGET NEURONS IN-VITRO IS REGULATED BY THE NMDA RECEPTOR

Cerebellar granule neurons in vitro specifically arrest the extension of their appropriate presynaptic axons, messy fibers. This ''stop-grow ing signal'' may be an essential step in the formation and specificity of synapses. Here, we have tested whether ionotropic glutamate recept ors are involved in the stop-growing signal. When explants of basilar pontine nuclei, a messy fiber source, were cultured on granule neurons , most pontine neurites terminated <200 mu m from their explant of ori gin, a criterion for the stop-growing signal. In contrast, treatment w ith the NMDA antagonist D(-)-2-amino-5-phosphonopentanoic acid (D-APS) greatly increased the number of pontine neurites extending beyond 300 mu m, whereas treatment with NMDA reduced the number of pontine neuri tes extending beyond 200 mu m. A non-NMDA agonist (AMPA) and antagonis t (6-cyane-7-nitroquinoxaline-2,3-dione) did not alter pontine neurite lengths. None of these agents affected neurite outgrowth from pontine explants in the absence of granule neurons, nor did any agent affect the survival of granule neurons. These results indicate that NMDA and D-APS specifically perturb an interaction between axons and target cel ls necessary for the stop-growing signal, and that NMDA receptors are critical for the development of a major cerebellar afferent system. Th ese findings also suggest that NMDA-sensitive refinement of axon arbor s during later development may involve the direct regulation of axon e xtension by target neurons.