Ionic basis of presynaptic inhibitory potentials at crayfish claw opener

Abstract

Intracellular recordings from the claw opener excitor axon of the crayfish, P. clarkii, were obtained near the terminal arborizations of the axon on the surface of the opener muscle. Rest potential in the excitor axon averaged -80 mV over 20 cells. Action-potential amplitude and duration averaged 100 mV and 2 ms, respectively. A single action potential in the opener inhibitor axon produces a hyperpolarizing synaptic potential (average amplitude 0.3 mV) in the excitor axon. The apparent reversal potential of this inhibitory synaptic potential is approximately 5 mV more negative than rest in control saline. No excitor axons had depolarizing synaptic potentials at rest. A decrease in external chloride concentration from 240-24 mM causes the apparent reversal potential to depolarize an average of 12 mV, with no change in rest potential. In low-chloride saline, the synaptic potential evoked by stimulation of the inhibitor axon becomes depolarizing. An increase in external K concentration from 5-10 mM causes the apparent reversal potential to depolarize by 16 mV; rest potential depolarizes by 10 mV. Low external K has the opposite effects, causing both rest potential and the apparent reversal potential to hyperpolarize. Presynaptic inhibition at the Procambarus claw opener neuromuscular junction appears to be mediated by a hyperpolarizing synaptic potential. Chloride serves as the charge carrier for the presynaptic potential. The evidence for direct involvement of K as a charge carrier is equivocal due to Donnan equilibrium effects involving Cl.