Changes in the kinetics of the NMDA type glutamate receptor (NMDAR) have been implicated in synapse maturation in several brain regions including the superior colliculus (SC). While the evidence is compelling that the subunit composition of the NMDAR does regulate the kinetics of the NMDAR current decay during development, we have recently reported that 35-45% of the change in the NMDAR current occurs over a 24 hour period between postnatal day 10 (P10) and P11 in the rat SC. In that study (SFN'99) we concluded that activity of the protein phosphatase 2B (calcineurin) could account for this rapid alteration of the current decay. Here we demonstrate that this abrupt change in the NMDA receptor function is activity dependent. Stimulation of the optic tract for one hour on P10 induces a precocious abbreviation of the NMDAR current decay, and is reversed by inhibition of calcineurin (CaN). In contrast silencing activity in SC slices on P11 for 3 hours with TTX lengthens the NMDAR decay time. Stimulation on P10...