Interaction between synaptic enhancement mediated by F2 facilitation and modulators. A, IPSG₅₀measured from both control (○) and facilitated (•) IPSGs is decreased by 100 nm 5-HT (A₁ ) and 2.5 μm OA (A₂ ). The dotted lines are used to highlight the fact that 5-HT did not decrease IPSG₅₀ beyond the level achieved by F2 facilitation, whereas OA was able to reduce IPSG₅₀ beyond that level.B, The effect of F2 facilitation is quantified by calculating the difference between control and facilitated IPSG₅₀ (ΔIPSG₅₀). Both 5-HT (B₁ ) and OA (B₂ ) decrease ΔIPSG₅₀. The averaged percentage change in ΔIPSG₅₀ is shown also, with SD (see Results). Membrane characteristics of the muscle fiber used in the 5-HT experiment were 10.4 mV, 0.89 MΩ, 13.5 msec and 10.4 mV, 0.88 MΩ, 13.3 msec. The same parameters measured from the OA experiment were 6.6 mV, 0.38 MΩ, 9.0 msec and 6.8 mV, 0.39 MΩ, 9.3 msec.