Nicotinic EPSCs in intact rat ganglia feature depression except if evoked during intermittent postsynaptic depolarization

The involvement of the postsynaptic membrane potential level in controlling synaptic strength at the ganglionic synapse was studied by recording nicot inic fast synaptic currents (EPSCs) from neurons in the intact, mature rat superior cervical ganglion, using the two-electrode voltage-clamp technique . EPSCs were evoked by 0.05-Hz supramaximal stimulation of the preganglioni c sympathetic trunk over long periods; their peak amplitude (or synaptic ch arge transfer) over time appeared to depend on the potential level of the n euronal membrane where the nicotinic receptors are embedded. EPSC amplitude remained constant (n = 6) only if ACh was released within repeated depolar izing steps of the postganglionic neuron, which constantly varied between - 50 and -20 mV in consecutive 10-mV steps, whereas it decreased progressivel y by 45% (n = 9) within 14 min when the sympathetic neuron was held at cons tant membrane potential. Synaptic channel activation, channel ionic permeat ion and depolarization of the membrane in which the nicotinic receptor is l ocalized must occur simultaneously to maintain constant synaptic strength a t the ganglionic synapse during low-rate stimulation (0.03-1 Hz). Different posttetanic (20 Hz for 10 s) behaviors were observed depending on the mode of previous stimulation. In the neuron maintained at constant holding pote ntial during low-rate stimulation, the depressed EPSC showed posttetanic po tentiation, recovering similar to 23% of the mean pretetanic values (n = 10 ). The maximum effect was immediate in 40% of the neurons tested and develo ped over a 3- to 6-min period in the others; thereafter potentiation vanish ed within 40 min of 0.05-Hz stimulation. In contrast, no statistically sign ificant synaptic potentiation was observed when EPSC amplitudes were kept c onstant by repeated -50/-20-mV command cycles (n = 12). It is suggested tha t, under these conditions, posttetanic potentiation could represent an atte mpt at recovering the synaptic strength lost during inappropriate functioni ng of the ganglionic synapse.