Effects of dithiothreitol, 5,5'-dithio bis(2-nitrobenzoic acid) and n-ethylmaleimide on synaptic transmission at sympathetic ganglion cells of frog

Abstract

The effects of disulfide bond and sulfhydryl group reagents on synaptic transmission were studied in the bullfrog sympathetic ganglion. Ten millimoles of dithiothreitol (DTT). a disulfide bond reducing agent, decreased the amplitude of the transmitted postganglionic compound action potential (CAP) to 41% of control. More than one hour wash-out of DTT with normal Ringer's solution did not reverse the block, but 1 mM 5.5'-dithio bis(2-nitrobenzoic acid) (DTNB), an oxidizing agent, rapidly restored the compound action potential amplitude. N-Ethylmalemide (NEM), an alkylating agent, added before DTNB, prevented the restoration of transmission by DTNB. Block of transmission induced by dithiothreitol was also reversed rapidly by 50 μm 3,4-diaminopyridine (3,4-DAP), to 87% of control compound action potential amplitude, and subsequent addition of DTNB restored transmission completely and this was accompanied by the stimulus-bound repetitive firing (SBR) which diaminopyridine produced in otherwise untreated ganglia. One to 3 mM dithiothreitol decreased the amplitude of excitatory post-synaptic potentials (EPSP), and greater concentrations slightly depolarized the cell membrane and decreased membrane resistance. Washout with Ringer's solution reversed the depolarization, but not the block of excitatory postsynaptic potentials. The amplitude of excitatory post-synaptic potentials was restored to threshold by 1 mM DTNB, which also slightly increased membrane resistance and the resting potential. It is concluded that the actions of dithiothreitol and DTNB in the bullfrog sympathetic ganglion are generally similar to those previously observed in other junctional tissues. Interactions between these drugs and 3,4-diaminopyridine in the present study also raise the question whether dithiothreitol and DTNB have any presynaptic effects.