A DENDRITIC GAIN-CONTROL MECHANISM IN AXONLESS NEURONS OF THE LOCUST, SCHISTOCERCA-AMERICANA

1. To understand the possible function of active dendritic currents in local neurons, the non-linear electrical properties of locust axonless non-spiking interneurons were considered in parallel with the properties of graded transmitter release from their dendrites. 2. The dendritic membrane of the non-spiking interneurons was least responsive to an applied or synaptic current at potentials between - 55 and - 45 mV. This is because, at these potentials, the input resistance of the dendrites is reduced by the activation of voltage-dependent K+ conductances. Conversely, the membrane of the non-spiking interneurons was most responsive to an applied or synaptic current at potentials more negative than - 55 mV (where the membrane behaves more or less passively), or more positive than - 45 mV (where the activation of a Ca2+ current can boost depolarizing potentials). 3. The threshold for detectable release at the non-spiking synapse was around - 65 mV. The dynamic gain of the synapse (slope of the synaptic transfer curve) was maximum around - 50 mV. Saturation was observed around - 40 mV. Synaptic transfer is therefore most efficient at presynaptic potentials where the non-spiking dendritic membrane is least responsive to incoming signals. 4. The possible consequences of this matching of membrane and synaptic nonlinearities was studied theoretically, with computer-assisted simulations, and experimentally, by recording simultaneously from the dendrites of synaptically connected non-spiking interneurons and motoneurons. This precise matching of nonlinearities was found to have two important consequences: (i) it allowed the effective gain of polysynaptic pathways via non-spiking dendrites to depend little on the state of the interposed interneuron (linearization) and (ii) it optimized coding by preventing undesired over-amplification and synaptic saturation.