Effects of a thioreactive agent, diamide, on neuromuscular transmission in lobster

Diamide[diazine-dicarboxylic acid-bis(dimethylamide)], a thiol-oxidizing agent, has both pre- and postsynaptic actions on the glutaminergic neuromuscular junction of the lobster walking leg. Postsynaptically, diamide produced an increase in the response to exogenously applied glutamate, whereas the effect of diamide on presynaptic transmitter release involved two major changes: 1) a decrease in excitatory junction potential amplitude and 2) an increase in miniature junction potential frequency. Short-term facilitation also decreased. Equilibration with 1,4-dithiothreitol (a sulfhydryl-reducing agent) reversed the decline in excitatory junction potential frequency, and the fall in short-term facilitation. The miniature junction potential frequency increase induced by diamide was independent of external Ca2+, as diamide in a Ca2+-free solution produced a similar response to that in a Ca2+-containing solution. We propose that the action of diamide on transmitter release is similar to the action of polyvalent cations, i.e., diamide has two sites of action, a blockade of inward Ca2+ flux and an increased release of Ca2+ inside the terminal.