Figure 3. Nicotinic receptor activity induces a positive feedback on ACh release.

(A) In order to obtain the nicotinic calcium signal in absence of DICR in Xenopus, synaptic events were transiently kept subthreshold either with curare (decreased nicotinic conductance, Gnico) or by dynamic-clamp injection of gK⁺ leaks (increased input conductance, Gin), or left suprathreshold while DICR was blocked by ryanodine. (B) Effect of presynaptic burst stimulation and curare on spontaneous (upper trace, sPSC) and evoked synaptic currents (ePSC). ePSCs recorded under voltage-clamp were evoked at low rate (0.03 Hz) and averaged by 30–40 events (lower traces). During conditioning presynaptic stimulations (3 bursts of 5 events at 30 Hz), the postsynaptic potential was released from clamp and curare transiently applied (middle trace). Upper trace: for clarity, ePSCs were removed from the continuous trace in order to display the sPSCs only. (C), In Xenopus, mean ePSC relative change 30 min after control chronic bursting activity ('chronic activity', n = 5, illustrated in Figure 1E), 45 min after subthreshold synaptic activity ('Curare', n = 9, illustrated in B; 'Dynamic-clamp', n = 5, illustrated in Figure 3—figure supplement 2A), transient curare application in absence of stimulation ('curare No Burst', n = 3), sub- ('curare-ryanodine', n = 3) and suprathreshold synaptic activity ('ryanodine', n = 6, illustrated in Figure 3— figure supplement 3) in muscle cells preloaded with ryanodine. (D), Relative change in amplitude and frequency of sPSCs after potentiation in Xenopus. (E), In FDB mice muscles, voltage reached by ePSPs in ryanodine treated (black dots, n = 60 fibers, 2 mice), and in ryanodine treated and burst stimulated preparations (red dots, n = 70 fibers, 2 mice). (F) Mean ePSP amplitude in control ('no stim') and high frequency nerve stimulation ('Pre stim') shown in Figure 1D, in non-stimulated ('Ryanodine') and in high frequency stimulated ('Pre stim Rya') ryanodine treated preparations shown in E. *, p<0.05; **, p<0.01; ***, p<0.001; t-test.

DOI: http://dx.doi.org/10.7554/eLife.12190.009

Figure 3—figure supplement 1. Decreasing muscle cell excitability by injection of artificial conductances.

(A) We used the K⁺ currents data of Figure 1—figure supplement 3 to establish and inject models of leak conductances into the muscle cell with the dynamic-clamp technique (see Materials and methods). Trace is a negative of the current output of the Kir conductance model when a ramp potential (125 mV/s) was used as input. (B) Time course of sPSPs in the presence of a simulated Kir conductance with dynamic-clamp (Kir DC). Artificial increase of the postsynaptic input conductance decreased the averaged sPSP amplitude.

DOI: http://dx.doi.org/10.7554/eLife.12190.010

Figure 3—figure supplement 2. LTP induction with dynamic-clamp or postsynaptic ryanodine.

(A) Potentiating effect of maintaining synaptic efficacy subthreshold by dynamic-clamp injection of gK⁺ leak (see Materials and methods). Middle trace: subthreshold nicotinic ePSPs in presence of the dynamic-clamp current (trace below). (B). Potentiating effect of intact synaptic activity generating muscle cell AP firing with DICR blocked (100 µM ryanodine). The muscle cell was pre-loaded with ryanodine using the classical whole-cell configuration of the patch-clamp technique. After removing the patch pipette, a perforated patch was performed for electrophysiological recordings.

DOI: http://dx.doi.org/10.7554/eLife.12190.011