Figure 4. High background inhibition has little affect on tonic activity of SNr neurons.

(A) Whole-cell recording of spontaneous IPSCs (sIPSCs) onto SNr neurons in control conditions (Cntrl; black trace) and following addition of tetrodotoxin (TTX) to isolate miniature events (+TTX; red trace, V_h 0 mV). (B) Cumulative histogram of IPSC amplitude in control conditions and following addition of TTX (n = 4 cells). (C) Box and whisker plot of IPSC amplitude for control and following addition of TTX. (D) Spiking output of SNr neurons following addition of high background excitation (upper) or inhibition (lower) via the dynamic clamp. (E) Summary data of change in firing rate of SNr neurons (n = 11 cells) following an increasing the relative frequency of inhibitory (red) or excitatory conductances (blue). (F) The slope of the change in firing rate as a function of change in conductance was significantly greater following increases in excitatory conductance compared to inhibitory conductance.

DOI: http://dx.doi.org/10.7554/eLife.02397.008

Figure 4—figure supplement 1. Intrinsic, net inward currents and a positive slope conductance allows feedback gain control of SNr neurons.

(A) Whole-cell recording of spontaneously spiking SNr neurons in vitro in the presence (upper black trace; Cntrl: AP5 & NBQX) and absence (lower red trace; +Gbz) of inhibition. Left shows tonic spiking and right shows ∼20 action potential waveforms (gray) and average action potential waveforms (black, Cntrl; red, Gbz). (B) No significant change in spike frequency was observed following addition of Gbz (n = 11 cells; n.s. p>0.05, dotted red line represents unitary line). (C) Phase plot of example average spike waveform from an individual SNr neuron in control conditions (black trace) and following removal of inhibition (red trace), lower plot focuses on the perithreshold membrane potential dynamics. (D and E) Summary data of perithreshold and threshold slopes measured in control conditions and following pharmacological blockade of inhibition (+Gbz, n = 11 cells).

DOI: http://dx.doi.org/10.7554/eLife.02397.009

Figure 4—figure supplement 2. Low release probability and sustained depression at feedback inhibitory synapse.

(A) Representative IPSC during first second of photostimulation delivered at a range of frequencies (10, 20, 50, 100 Hz, indicated by labels at right of traces and cyan line) during whole-cell voltage clamp recordings from SNr GABA neuron. (B) Average peak IPSC amplitude, normalized to IPSC₁ (left axis; dark blue trace) and average tonic IPSC amplitude (right axis; graygray trace) plotted as a function of stimulus number for 10, 20, 50, and 100 Hz photostimulation (n = 4 cells; shading represents SEM). (C) Peak Steady State (S.S.) IPSC ratio (IPSC₁₀₀/IPSC₁) and (D) S.S. tonic IPSC amplitudes (IPSC₁₀₀) plotted as a function of stimulation frequency (n = 4 cells). Error bars represent SEM.

DOI: http://dx.doi.org/10.7554/eLife.02397.010