Shunting versus inactivation: Analysis of presynaptic inhibitory mechanisms in primary afferents of the crayfish

Primary afferent depolarizations (PADs) are associated with presynaptic inh ibition in both vertebrates and invertebrates. In the present study, we hav e used both anatomical and electrophysiological techniques to analyze the r elative importance of shunting mechanisms versus sodium channel inactivatio n in mediating the decrease of action potential amplitude, and thereby pres ynaptic inhibition. Experiments were performed in sensory afferents of a st retch receptor in an in vitro preparation of the crayfish. Lucifer yellow i ntracellular labeling of sensory axons combined with GABA immunohistochemis try revealed close appositions between GABA-immunoreactive (ir) fibers and sensory axons. Most contacts were located on the main axon at the entry ron e of the ganglion, close to the first branching point within the ganglion. By comparison, the output synapses of sensory afferents to target neurons w ere located on distal branches. The location of synaptic inputs mediating s pontaneous PADs was also determined electrophysiologically by making dual i ntracellular recordings from single sensory axons. Inputs generating PADs a ppear to occur around the first axonal branching point, in agreement with t he anatomical data. In this region, small PADs (3-15 mV) produced a marked reduction of action potential amplitude, whereas depolarization of the memb rane potential by current injection up to 15 mV had no effect. These result s suggest that the decrease of the amplitude of action potentials by single PADs results from a shunting mechanism but does not seem to involve inacti vation of sodium channels. Our results also suggest that GABAergic presynap tic inhibition may act as a global control mechanism to block transmission through certain reflex pathways.