Fig. 5.

Short-term forms of synaptic facilitation are greatly impaired at lethal Vol mutant NMJs. Short-term facilitation was evoked in 0.2 mmCa²⁺ with paired stimuli (20–100 msec intervals) and short trains (20 stimuli) at 2–20 Hz. A, PPF.Right, Traces show averages of 5–10 responses to paired stimuli delivered at 20 msec intervals. Examples are shown for two larvae each for control (yw, top) and lethal Vol mutant alleles (Vol³, middle;Vol⁴, bottom).Left, Mean PPF ratio (I₂/I₁) is plotted for each stimulus interval. For each stimulus interval, 5–10 responses (separated by 5 sec rest) were averaged.I₂/I₁ is defined as the ratio: mean amplitude of response 2/mean amplitude of response 1. I₂ peak amplitudes were determined relative to a baseline value immediately preceding the second EJC. Of the viable mutant alleles, Vol² PPF (shaded squares) is indistinguishable from control, whereasVol¹ (shaded triangles) exhibits reduced PPF at the briefest interval (20 msec). Both Vol³ andVol⁴ lethal alleles (open symbols) exhibit strongly impaired PPF at 20–30 msec intervals, at which facilitation is normally greatest. Number of larvae for each genotype is indicated in legends. B, STF.Right, Representative responses to 20 Hz stimulation trains. The first 10 consecutive EJCs are shown (response number indicated above traces), with the average of the last 10 of 20 responses in the train shown in the rightmost traces (average, 11–20). STF at each frequency was defined as the average amplitude of responses 11–20, normalized to mean EJC amplitude at the basal stimulation frequency (0.5 Hz). Stimulus trains were separated by 10–20 sec rest. EJC amplitude rapidly facilitates by several-fold at control yw or ry NMJs (yw, top).Vol³ andVol⁴ mutant NMJs (middle and bottom traces) have greater initial EJC amplitudes and weaker facilitation in response 20 Hz stimuli; in some cases facilitation is absent or replaced with depression (Vol⁴, bottom traces). Left, Mean STF at 2–20 Hz stimulation is plotted for each genotype. Vol²NMJs (shaded squares) have nearly normal STF, whereas Vol¹ (shaded triangles),Vol³ (open triangles), and Vol⁴ (open diamonds) genotypes, respectively, exhibit progressively weaker STF. In particular, Vol⁴ has on average no STF at 2–5 Hz stimulation, and STF reduced to ∼20% of control levels at 20 Hz.