5-HT and OA have no effect on GABA receptors of muscle fibers. A, 5-HT (A₁ ) and OA (A₂ ) do not changeg_GABA. g_GABAamplitudes were calculated from membrane potential changes in a muscle fiber activated by iontophoretically applied GABA.Insets, GABA-activated membrane potential changes before (1 and dashed line) and after (2 and solid line) modulator application. The traces represent the averages of 38 trials. B, IPSGs calculated from action potential-activated IPSPs in the same muscle fibers. Both 5-HT (B₁ ) and OA (B₂ ) increase the IPSG amplitude. ΔE_Cl and R_mmeasured before and after 5-HT application were 7.5 mV, 0.21 MΩ and 7.5 mV, 0.21 MΩ, respectively. The same parameters measured before and after OA application were 8.8 mV, 0.29 MΩ and 9 mV, 0.3 MΩ, respectively. All experiments were performed at 23°C.