The Journal of Neurophysiology Vol. 83 No. 6 June 2000, pp. 3254-3263
Copyright ©2000 by the American Physiological Society
Nicotinic EPSCs in Intact Rat Ganglia Feature Depression Except
If Evoked During Intermittent Postsynaptic Depolarization
Oscar
Sacchi,
Maria Lisa
Rossi, and
Rita
Canella
Department of Biology, Sezione di Fisiologia Generale, University
of Ferrara, 44100 Ferrara, Italy
Sacchi, Oscar,
Maria Lisa Rossi, and
Rita Canella.
Nicotinic EPSCs in Intact Rat Ganglia Feature Depression Except
If Evoked During Intermittent Postsynaptic Depolarization. J. Neurophysiol. 83: 3254-3263, 2000. The involvement of the postsynaptic membrane potential level in
controlling synaptic strength at the ganglionic synapse was studied by
recording nicotinic 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 preganglionic sympathetic trunk over long periods;
their peak amplitude (or synaptic charge transfer) over time appeared
to depend on the potential level of the neuronal membrane where the
nicotinic receptors are embedded. EPSC amplitude remained constant
(n = 6) only if ACh was released within repeated
depolarizing steps of the postganglionic neuron, which constantly
varied between 
50 and 20 mV in consecutive 10-mV steps, whereas it
decreased progressively by 45% (n = 9) within 14 min when the sympathetic neuron was held at constant membrane
potential. Synaptic channel activation, channel ionic permeation and
depolarization of the membrane in which the nicotinic receptor is
localized must occur simultaneously to maintain constant synaptic
strength at 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 potential during low-rate stimulation,
the depressed EPSC showed posttetanic potentiation, recovering ~23%
of the mean pretetanic values (n = 10). The maximum effect was immediate in 40% of the neurons tested and developed over a
3- to 6-min period in the others; thereafter potentiation vanished
within 40 min of 0.05-Hz stimulation. In contrast, no statistically
significant synaptic potentiation was observed when EPSC amplitudes
were kept constant by repeated 50/20-mV command cycles
(n = 12). It is suggested that, under these
conditions, posttetanic potentiation could represent an attempt at
recovering the synaptic strength lost during inappropriate functioning
of the ganglionic synapse.
