Presynaptic inhibition and antidromic spikes in primary afferents of the crayfish: A computational and experimental analysis

Primary afferent depolarizations (PADs) are associated with presynaptic inh ibition and antidromic discharges in both vertebrates and invertebrates. In the present study, we have elaborated a realistic compartment model of a p rimary afferent from the coxobasipodite chordotonal organ of the crayfish b ased on anatomical and electrophysiological data. The model was used to tes t the validity of shunting and sodium channel inactivation hypotheses to ac count for presynaptic inhibition. Previous studies had demonstrated that GA BA activates chloride channels located on the main branch close to the firs t branching point. We therefore focused the analysis on the effect of GABA synapses on the propagation of action potentials in the first axonal branch . Given the large diameters of the sensory axons in the region in which PAD s were likely to be produced and recorded, the model indicates that a relat ively large increase in chloride conductance (up to 300 nS) is needed to si gnificantly reduce the amplitude of sensory spikes. The role of the spatial organization of GABA synapses in the sensory arborization was analyzed, de monstrating that the most effective location for GABA synapses is in the ar ea of transition from active to passive conduction. This transition is like ly to occur on the main branch a few hundred micrometers distal to the firs t branching point. As a result of this spatial organization, antidromic spi kes generated by large-amplitude PADs are prevented from propagating distal ly.