A local circuit interaction in the flight system of the locust

Most of the interneurons that have been identified in the locust flight system to date are spiking, intersegmental interneurons and the described interactions between them are spike-mediated postsynaptic potentials. We wished to discover whether the same interneurons also communicate via subthreshold interactions to form local circuits independent of their spike-mediated connections. Using a deafferented flight preparation of the locust and glass microelectrodes, we recorded simultaneously from the neuropil segments of different interneurons within a single thoracic ganglion. Interneuron 301 had an indirect connection with the contralateral 301 and an indirect connection feeding back to itself. The feedback circuit could be activated with subthreshold stimuli. Spikes in a 301 affected self and contralateral partner, whereas subthreshold stimuli affected only self. Thus, this paper demonstrates the existence of 2 pathways from a single interneuron that can be functionally separate depending on whether activity in the interneuron is subthreshold or suprathreshold for spiking. The results show that dendritic regions of spiking intersegmental interneurons in the locust can participate in local circuits whose operation could have a considerable role to play in neural integration.