An inhibitor for calcineurin, FK506, blocks induction of long-term depression in rat visual cortex

Long-term depression (LTD) of synaptic transmission, often used as an essential component in synaptic models for learning, memory and forgetting, can be produced in layer II/III of the visual cortex by a prolonged, low-frequency stimulation (LFS) of layer IV. The activation of Ca2+/calmodulin-dependent protein phosphatase, calcineurin, has been postulated to play a role in the induction of LTD. The recent introduction of a specific inhibitor for calcineurin, FK506, prompted the investigation of the involvement of this phosphatase in the induction of LTD in visual cortex. Thus, we administered FK506 at 1 microM to visual cortical slices of young rats, and found that it did not significantly affect field responses of layer II/III evoked by test stimulation of layer IV at 0.1 Hz, but prevented LTD of the responses from being induced by LFS (1 Hz for 15 min) in all the 10 slices tested. Without FK506, significant LTD was induced by the same parameters of LFS in 8 of the 12 slices. These results suggest the critical involvement of calcineurin in producing LTD in visual cortex.