Shunting versus Inactivation: Analysis of Presynaptic Inhibitory Mechanisms in Primary Afferents of the Crayfish

Abstract

Primary afferent depolarizations (PADs) are associated with presynaptic inhibition in both vertebrates and invertebrates. In the present study, we have used both anatomical and electrophysiological techniques to analyze the relative importance of shunting mechanisms versus sodium channel inactivation in mediating the decrease of action potential amplitude, and thereby presynaptic inhibition. Experiments were performed in sensory afferents of a stretch receptor in an in vitro preparation of the crayfish. Lucifer yellow intracellular labeling of sensory axons combined with GABA immunohistochemistry revealed close appositions between GABA-immunoreactive (ir) fibers and sensory axons. Most contacts were located on the main axon at the entry zone of the ganglion, close to the first branching point within the ganglion. By comparison, the output synapses of sensory afferents to target neurons were located on distal branches. The location of synaptic inputs mediating spontaneous PADs was also determined electrophysiologically by making dual intracellular recordings from single sensory axons. Inputs generating PADs appear to occur around the first axonal branching point, in agreement with the anatomical data. In this region, small PADs (3-15 mV) produced a marked reduction of action potential amplitude, whereas depolarization of the membrane potential by current injection up to 15 mV had no effect. These results suggest that the decrease of the amplitude of action potentials by single PADs results from a shunting mechanism but does not seem to involve inactivation of sodium channels. Our results also suggest that GABAergic presynaptic inhibition may act as a global control mechanism to block transmission through certain reflex pathways.

Contributions of Voltage- and Ca2+-Activated Conductances to GABA-Induced Depolarization in Spider Mechanosensory Neurons

“…It is not well understood why there is such a different effect in different cells. Inactivation of Na channels and increased membrane conductance (shunting) contribute to the inhibitory effect in at least some of the mechanosensory neurons (Cattaert and El Manira 1999;French et al 2006), but other ionic mechanisms have also been suggested (Graham and Redman 1994;Kriz et al 1974;Walmsley et al 1995). In the present study, we demonstrated that a slow voltage-activated Na ϩ current contributes to the depolarization resulting from activation of GABA A receptors in spider VS-3 mechanosensory neurons.…”

“…Although the increase in Cl Ϫ conductance in response to GABA A -receptor activation is generally accepted as the primary mechanism behind the primary afferent depolarization, there is still controversy about the ionic mechanism that leads to inhibition. Previous results in the VS-3 neurons indicated that inactivation of voltage-gated Na channels during GABA-mediated depolarization and membrane "shunting" can both cause inhibition Panek et al 2001), agreeing with results in many other neurons (e.g., AlvarezLeefmans et al 1998;Cattaert and El Manira 1999). Kritz et al (1974) suggested that in the spinal cord accumulation of K ϩ in the extracellular space could also produce depolarization leading to inhibition.…”

“…Cellular mechanisms of shunting and depolarization in centrally located afferent terminals have been difficult to unravel. However, GABA has similar effects on the peripherally located parts of arachnid and crustacean mechanosensory neurons making these models more experimentally accessible (Cattaert and El Manira 1999;Panek et al 2002).…”

Random Stimulation of Spider Mechanosensory Neurons Reveals Long-Lasting Excitation by GABA and Muscimol

“…Membrane depolarization and increased conductance (shunting) have been previously shown to contribute to the inhibitory effects of GABA, including presynaptic inhibition, on primary afferent mechanosensory neurons in many vertebrate and invertebrate preparations (Cattaert and El Manira 1999;Rudomin and Schmidt 1999). Since similar depolarization and shunting occur in the peripheral parts of spider VS-3 neurons and have previously been shown to contribute to the inhibitory effect (French et al 2006;Panek et al 2002), we asked whether they could also explain the increase in sensitivity when the neurons are stimulated with random signals.…”

Independent Control of Presynaptic Inhibition by Reticulospinal and Sensory Inputs at Rest and during Rhythmic Activities in the Cat

“…Presynaptic inhibition is produced by spinal interneurons with GABAergic axo-axonic synapses on primary afferent terminals (Eccles et al, 1962;Rudomin and Schmidt, 1999;Hughes et al, 2005). With GABAergic activation, Cl Ϫ anions escape the negative intra-axonal environment, thus producing a primary afferent depolarization (PAD) (Eccles et al, 1963;Nicoll and Alger, 1979;Cattaert and El Manira, 1999).…”