Fig. 7. Modulation of ovipositor muscle contractions by proctolin or high frequency pre-stimulation. (A) Application of 10^–9 mol l^–1 proctolin increases contraction force in all experimental groups. (B) To test for endogenous release of proctolin, contractions evoked by control stimulations are compared with those evoked by test stimulations. Shortly before the test stimulation, a high frequency pre-stimulation (50 Hz) was applied, as shown in the inset. Pre-stimulation was intended to release endogenous proctolin from neuronal elements innervating the muscle. Significant increase of contraction forces is observed in mature females only.