Activity-dependent enhancement of synaptic transmission in hippocampal slices treated with the phosphatase inhibitor calyculin A.

The role of protein phosphatases in regulating synaptic transmission in the CA1 region of the hippocampus was examined using slices pretreated with calyculin A, a specific inhibitor of protein phosphatases 1 and 2A. Stimulation of afferents at 1 Hz (but not 0.1 Hz) for periods of 5-10 min caused a long-lasting enhancement of synaptic transmission. The increase in synaptic responses was not due to a change in fiber excitability, as there was a shift to the left in the input-output curve following the synaptic enhancement. The enhancement was observed only in the input that received the 1 Hz stimulation and not in an independent control pathway, indicating that the increase in synaptic strength is input specific and limited to repetitively activated synapses. Applying 1 Hz stimulation when synaptic transmission was blocked by replacing extracellular Ca2+ with Mg2+ prevented or significantly reduced any change in synaptic efficacy after reperfusion with normal Ca(2+)-containing medium. In contrast, 1 Hz stimulation given when synaptic transmission was blocked by non-NMDA and NMDA glutamate receptor antagonists still caused a synaptic enhancement following washout of the antagonists. The enhancement of synaptic transmission also was not blocked by loading CA1 cells with the calcium chelator BAPTA. Thus, influx of Ca2+ into presynaptic elements is required for the synaptic enhancement elicited by 1 Hz stimulation in calyculin A-treated hippocampal slices. Consistent with the activation of processes that cause an increase in transmitter release, the magnitude of paired-pulse facilitation decreased following the synaptic enhancement, and the NMDA receptor-mediated component of the synaptic response was increased by 1 Hz stimulation. These results suggest that when protein phosphatases are inhibited by calyculin A, prolonged periods of 1 Hz stimulation lead to activation of presynaptic Ca(2+)-dependent protein kinases, resulting in a persistent increase in evoked transmitter release. They also indicate that the activity of presynaptic protein phosphatases is critically important for limiting increases in synaptic strength following repetitive afferent activity.