The refinement of local circuitry within the developing CNS is dependent on a competitive synaptogenic mechanism through which depolarizations of target cells feed back to stabilize only those inputs that are effective in producing the depolarization. It is hypothesized that synaptically induced changes in post-synaptic Ca++ and excitatory amino acid receptors, particularly the N-methyl-D-aspartate (NMDA) receptor, because of its voltage-gated property, are critical to this developmental from of synaptic plasticity. Initial studies show that a developmental increase in the expression of mRNA for the NMDA receptor NMDAR1 in the superficial layers of the rat superior colliculus (SC) is associated with the postnatal period of refinement of the retinal and cortical inputs to that area. In addition, chronic treatment of SC with low levels of AP5 or MK801 in slow release plastic disrupts the structural refinement of the retinocollicular projection and simultaneously blocks the developmental increase in mRNA for NMDAR1. Proposed experiments will use the rat SC to pursue the molecular and functional implications of these findings. The goal is to define the relationship between activity level of glutamate receptors, structural synaptic changes and the expression of particular glutamate receptor subunits known to be associated with post-synaptic Ca++ changes in an effort to determine if message levels for the NMDA receptor subunits are selectively regulated to function at high levels during the period of synaptic change. Quantitative Northern analyses, and in situ hybridization will be used to define the time course of expression of mRNA for NMDA receptors relative to that of the metabotropic glutamate receptor mGluR1, GluR1 and GluR3, subunits of nonNMDA ionotropic receptors and GluR2, the subunit known to reduce Ca++ permeability of GluR1 and GluR2 when expressed with them in oocytes. These changes in the SC of normal rats will be compared with mRNA levels in rats chronically treated with AP5, LAP5, CNQX or NMDA. These treatments will be initiated at two ages, before or during the steep rise in NMDAR1 mRNA expression. Changes in mRNA expression with age or treatment will be correlated with voltage clamp physiological studies and Ca++ imaging with confocal microscopy in acute slices of SC to assess the functional effectiveness of NMDA and non-NMDA glutamate receptors in similar tissue. If discrepancies between functional assays and mRNA levels are observed, analyses at the protein level will be conducted using antibodies to specific receptor subunits, immunocytochemistry and immunoprecipitation. It is hoped that these investigation will provide basic insights into the normal regulation of the glutamate receptors that effect post-synaptic Ca++ and into the functional and synaptogenic consequences of that regulation. The findings could also impact on our understanding of childhood seizure disorders, neurodegenerative disease and recovery from CNS trauma. Glutamate receptors, particularly those that alter Ca++ levels, have been implicated in all of these instances of nervous system dysfunction.
