NMDA RECEPTOR MODULATION BY A CONDITIONED MEDIUM DERIVED FROM RAT CEREBELLAR GRANULE CELLS

Our previous studies have shown that the response to the excitotoxic a ction of glutamate by cultured cerebellar granule cells depends upon t he cell density or the volume of medium in which they have been grown: the higher the cell density or the lower the volume, the higher the r esponse to glutamate. We have hypothesized that this variable response is due to the formation in culture of a glutamate-sensitizing activit y GSA more abundantly in conditioned medium derived from high-density or low-volume cultures than that present in low-density or high volume cultures and capable of restoring sensitivity in previously resistant granule cells. In order to elucidate the mechanism of action of gluta mate-sensitizing activity, we measured the extent and function of NMDA receptors in low-and high-volume cultures and assessed the effect oi glutamate-sensitizing activity on the same receptors. We found that un der high-volume conditions the extent of MK-801 binding, the amount of NMDA receptor type 1, the currents evoked in whole cells after an NMD A pulse and the response of cultured cells to this ligand were markedl y reduced compared with low-volume cultures. Addition of glutamate-sen sitizing activity to high-volume cultures increased their glutamate se nsitivity the NMDA-evoked currents, the extent of MK-801 binding and t he amount of NMDA receptor type 1 protein present. The corresponding m RNA transcripts, on the contrary, were unchanged in high-volume, low-v olume and high-volume GSA-treated cultures.