Caspase-3 and calpain: Differently directed involvement in presynaptic long-term plasticity

Abstract

We studied the effects of the proteolytic enzymes caspase-3 and calpain on the changes in LTP-dependent modifications of paired plasticity in the CA1 field of hippocampal slices. Paired pulse facilitation (PPF) was measured for 1 h after high-frequency stimulation of Schaffer collaterals (100 Hz, 1 s). After LTP induction, we observed a significant decrease in PPF, whereas when LTP was not induced after tetanization a decrease in PPF was also absent. At 1 hour after tetanization, the residual decline in PPF significantly correlated with the efficacy of LTP maintenance. After the end of the experiments we measured the activity of caspase-3 and calpain in the same slices. Potentiation rarely occurred with the low activity of both enzymes and most often came to the end with depression. The probability of LTP induction did not depend on specific activation of any proteases; however, the probability of long-term maintenance of the modifications was significantly higher when the activity of caspase-3, but not of calpain, was elevated. High activity of both proteases simultaneously did not influence the probability of LTP induction; however, under these conditions, LTP maintenance was worse compared to the slices with higher activity of caspase-3. The effect of an LTP-dependent long-term decrease in PPF was predominant in slices with high activity of caspase-3. In contrast, a high activity level of calpain was associated with a less expressed and short-term decrease in PPF. A low activity level of calpain was related to a stable decrease in PPF after LTP induction. Both these effects disappeared under the conditions of high activity of both proteases. In these slices, potentiation was followed by a gradual PPF increase without its initial decrease. Analysis using “caspase-3,” “calpain,” and “LTP maintenance” as factors revealed a significant interaction between the factors. Our data suggest that caspase-3 promotes whereas calpain prevents the maintenance of presynaptic mechanisms of plasticity. The specific effects of proteases may be related to different loci of plasticity or concurrent interaction in the presynaptic zone.