Altering extracellular [Ca^2<b>+</b>]o shows that BotoxA reduction in LTD is not due to reduced transmitter release probability.

<p><b>A</b>: Time course of the increase in normalized fEPSP slope (each point mean ± SEM of 8 slices) elicited by raising extracellular [Ca²⁺]o from 2.6 mM (light gray bar) to 4 mM (dark gray bar). After responses had plateaued in high [Ca²⁺]o, a low-frequency stimulus train (2 Hz/10 min, black bar) elicited marked LTD. <b>B</b>: Time course of the effect on fEPSP slope of raising extracellular [Ca²⁺]o to 4 mM (dark gray bar), and of LTD elicited by LFS (2 Hz/10 min, black bar), in slices (<i>n</i> = 4) pretreated for 90 min with BotoxA (200 ng/ml). Significantly less LTD was evoked (<i>P</i><0.05, Student’s t-test compared to untreated control slices in A). <b>C</b>: Time course of the effect of lowering [Ca²⁺]o from 2.6 mM (dark gray bar) to 1.3 mM (light gray bar), and of LTD elicited by LFS (2 Hz/10 min, black bar, <i>n</i> = 6). <b>D</b>: Mean ± SEM % LTD of Schaffer collateral-evoked fEPSP in <i>stratum radiatum</i> of field CA1 in slices treated with 4, 2.6 or 1.3 mM [Ca²⁺]o (hatched bars), compared to slices pre-treated with BoNT/A in 4 or 2.6 mM [Ca²⁺]o (open bars). BoNT/A-treated slices exhibited significantly less LTD than slices in either 4 mM [Ca²⁺]o (*, <i>P</i><0.01; Student’s t-test) or 2.6 mM [Ca²⁺]o (*, <i>P</i><0.05, Student’s t-test).