COREGULATION OF AMPA AND NMDA RECEPTORS DURING ACTIVITY BLOCKADE IN THE DEVELOPING FERRET LGN S. Oray*; C.A. Leamey; M. Sur Brain & Cognitive Sciences, MIT, Cambridge, MA, USA In the lateral geniculate nucleus (LGN) of the developing ferret, retinal axons segregate into eye specific laminae during the first two postnatal weeks and subsequently into ON/OFF sublaminae during the third and fourth postnatal weeks. During this period of sublamination, AMPA and NMDA receptors maintain stable electrophysiological properties as well as a constant AMPA to NMDA ratio (Hohnke, Oray, and Sur, J Neurosci 20:8051, 2000). However, on a short time scale, the expression of AMPA and NMDA receptors can be dramatically altered by activity blockade. To further investigate this short-term change in receptor expression, pharmacological manipulations of the NMDA receptor (NMDA and AP5) were performed. Receptor expression for GluR1 (AMPA) and NR1 (NMDA) subunits was quantified with immunocytochemistry and confocal laser microscopy. While there is significant variation in the effects of each drug, the degree to which AMPA and NMDA receptors were colocalized remained remarkably consistent irrespective of drug treatment. Additionally, both receptor subunits increased or decreased together in response to drug application. These effects were seen throughout the period of sublamination (postnatal days 14, 21 and 28). To specifically examine receptors at retinogeniculate synapses, the anterograde tracer cholera toxin subunit B (CTB) was injected into the eyes and CTB, NR1, and GluR1 was visualized in triple-labeling experiments. In these experiments, NR1 and GluR1 subunits were again coregulated regardless of drug treatment. These data argue that the expression of AMPA and NMDA receptors throughout the LGN, and at retinogeniculate terminals in particular, is tightly coupled. Supported by: NIH Grant EY 11512