PdN6 disconnection impairs synaptic actions of C2 onto VSI PdN6 disconnection impairs synaptic actions of C2 onto VSI. A–C, Schematic drawings of a dorsal view of the Tritonia brain with the three CPG neurons (left) and simultaneously recorded activity of the three CPG neurons, while stimulating DSI and/or C2 (righ... PdN6 disconnection impairs synaptic actions of C2 onto VSI. A–C, Schematic drawings of a dorsal view of the Tritonia brain with the three CPG neurons (left) and simultaneously recorded activity of the three CPG neurons, while stimulating DSI and/or C2 (right) are shown. For stimulation, repetitive current pulses (8 nA, 20 ms) were injected into C2 or DSI, or both, at 10 Hz for 2 s while recording the membrane potential responses from VSI. A, In an intact brain, C2 makes synaptic connections onto VSI in both pedal ganglia, as indicated by dotted circles. Stimulation of C2 (Ai), or C2 and DSI together (Aiii), evoked a burst of action potentials in VSI. It was previously shown that VSI spikes are generated in the distal pedal ganglion (PdG), and they propagate antidromically to the cell body, where the intracellular recording was made (Sakurai and Katz, 2009; Sakurai et al., 2014). Stimulation of DSI depolarized C2 and hyperpolarized VSI (Aii). Although these synaptic interactions among the CPG neurons were determined to be monosynaptic, unitary synaptic potentials corresponding one to one to the presynaptic spikes have never been recorded (Getting, 1981, 1983). B, C, After PdN6 disconnection, the distal synaptic action was eliminated, and C2 stimulation mainly produced a hyperpolarizing response in VSI in the proximal pedal ganglion (Bi, Ci). The effect of DSI showed little change after PdN6 disconnection (Bii, Cii). Although C2 and DSI stimulation both had inhibitory actions onto VSI, the combination of both (C2+DSI stim.) produced a slightly larger depolarizing component than C2 alone (Biii), which grew larger in the next day (Ciii). A dotted line on each trace indicates −50 mV membrane potential. All recordings were obtained in normal saline from the same preparation. Akira Sakurai et al. eneuro 2016;3:ENEURO.0056-16.2016 ©2016 by Society for Neuroscience