Procaine alters fast excitatory postsynaptic current decay in amphibian sympathetic ganglia

1--The effects of procaine (25-200 microM) on the fast excitatory postsynaptic currents (e.p.s.cs) of bullfrog sympathetic ganglion B cells were studied with a two-microelectrode voltage clamp system. 2--Procaine decreased peak e.p.s.c. size with no measurable decrease in quantal content. 3--The peak e.p.s.c.-voltage relationship was linear in control cells, but showed a marked nonlinearity in cells treated with procaine so that peak e.p.s.c. size decreased progressively at hyperpolarized values of membrane potential. The e.p.s.c. reversal potential was not altered in procaine. 4--Although the e.p.s.c. decay time course was well described by a single exponential in control cells, the decay phase often became complex in the presence of procaine. The decay phase consisted of two components in the presence of procaine which became more obvious with increasing concentration and membrane hyperpolarization. 5--In control cells, the e.p.s.c. time constant of decay increased with membrane hyperpolarization. In the presence of procaine, the first time constant of decay, tauf, increased with hyperpolarization up to -40mV, but then decreased with hyperpolarization between -40 and -100mV. 6--We conclude that procaine has two sites of action at postganglionic sympathetic neurones: (1) it reduces the number of activatable receptor-channel complexes and (2) procaine blocks open synaptic channels. Blockade of open channels became more important with hyperpolarization.