O. Sacchi et al., Nicotinic EPSCs in intact rat ganglia feature depression except if evoked during intermittent postsynaptic depolarization, J NEUROPHYS, 83(6), 2000, pp. 3254-3263
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.