Pre & postsynaptic tuning of action potential timing by spontaneous GABAergic activity

Abstract

Frequency and timing of action potential discharge are key elements for coding and transfer of information between neurons. The nature and location of the synaptic contacts, the biophysical parameters of the receptor-operated channels and their kinetics of activation are major determinants of the firing behaviour of each individual neuron. Ultimately the intrinsic excitability of each neuron determines the input-output function. Here we evaluate the influence of spontaneous GABAergic synaptic activity on the timing of action potentials in Layer 2/3 pyramidal neurones in acute brain slices from the somatosensory cortex of young rats. Somatic dynamic current injection to mimic synaptic input events was employed, together with a simple computational model that reproduce subthreshold membrane properties. Besides the well-documented control of neuronal excitability, spontaneous background GABAergic activity has a major detrimental effect on spike timing. In fact, GABA(A) receptors tune the relationship between the excitability and fidelity of pyramidal neurons via a postsynaptic (the reversal potential for GABA(A) activity) and a presynaptic (the frequency of spontaneous activity) mechanism. GABAergic activity can decrease or increase the excitability of pyramidal neurones, depending on the difference between the reversal potential for GABA(A) receptors and the threshold for action potential. In contrast, spike time jitter can only be increased proportionally to the difference between these two membrane potentials. Changes in excitability by background GABAergic activity can therefore only be associated with deterioration of the reliability of spike timing.

Figure 1. Spontaneous GABAergic activity affects excitability and spike timing of pyramidal cells.

a, Superimposed (5) membrane potential (Vm) fluctuations of a gramicidin-perforated current-clamped L2/3 pyramidal cell in response to a DC step (1 s, 170 pA) before (control, blue) and after extracellular application of picrotoxin (100 µM, PTX, green). The cell was held at around −80 mV in both conditions. Same cell and colour code for b, c and d. b, Waterfall view of Vm when the 5^th spike was set as the time reference. c, Mean firing rate vs DC step. d, Coefficient of Variation of the Inter Spike Interval (CV_ISI) vs firing rate. e, Normalised changes in excitability observed when adding PTX (n = 7). f, Changes in spike jitter for an interpolated firing rate of 10 spikes/s.

Figure 2. Frequency-dependent tuning of pyramidal cell excitability and spike timing.

a, From upper to lower, an example of a GABA_A conductance (G_GABA-A) pattern of 33 events/s, GABA_A current (I_GABA-A), Vm and the DC protocol in a dynamic clamp recording of a L2/3 pyramidal cell. b, Superimposed (5) Vm fluctuations in response to a DC step (1 s, 170 pA) in control (green), in the presence of 33 (blue) or 100 (red) randomly occurring GABA_A events/s dynamically injected with E_GABA set at −70 mV. Same cell and colour code for c,d and e. c, Waterfall view of Vm when the 5^th spike was set as the time reference. d, Mean Firing rate vs DC step. e, CV_ISI vs mean spike firing rate. f, Normalised changes in excitability observed when neurons receive different levels of randomly occurring GABA_A events/s. g, Normalised changes in spike jitter for an interpolated firing rate of 10 spikes/s.

Figure 3. Modelling frequency-dependent tuning of neuronal excitability and spike timing.

a, From upper to lower, an example of G_GABA-A pattern of 33 events/s, I_GABA-A, R_in, Vm and a DC step (1 s, 120 pA) when running the LIF model. b, Superimposed (5) Vm fluctuations in response to a DC step (1 s, 160 pA) in control (green), in the presence of 33 (blue) or 100 (red) randomly occurring GABA_A events/s with E_GABA set at −70 mV. Same cell and colour code for c,d and e. c, Waterfall view of Vm when the 5^th spike was set as time reference. d, Mean Firing rate vs DC step. e, CV_ISI vs firing rate. f, Normalised changes in excitability observed when LIF received 1 to 100 randomly occurring GABA_A events/s. g, Normalised changes in spike jitter for a firing rate of 10 spikes/s.

Figure 4. Randomly occurring GABA_A conductance transients underlie the frequency-dependent tuning of pyramidal cell discharge fidelity.

a, Waterfall view of conductance fluctuations injected into the LIF model in control conditions (no conductance, green), when the LIF model received 33 randomly occurring GABA_A events/s (blue), an invariable pattern of 33 GABA_A events/s (red), or when the neuron model received a constant G_GABA-A equivalent to the average conductance for a rate of 33 events/s (orange). Same cell and colour code for b,c,d and e. b, Superimposed (5) Vm fluctuations of the LIF model in response to a DC step (1 s, 160 pA) in the various conditions depicted in a. c, Waterfall view of Vm when the 5^th spike was set as the time reference. d, Mean firing rate vs DC step. e CV_ISI vs firing rate. f, Mean firing rate displayed on a pseudocolor scale vs DC step and rate of GABA_A activity when LIF model received random (left), invariable (middle) or constant conductance (right). g, CV_ISI displayed on a pseudocolor scale vs firing and GABA_A activity rate. Same conditions as in f.

Figure 5. Randomly occurring GABA_A conductance transients underlie the frequency-dependent tuning of pyramidal cell discharge fidelity.

a, Superimposed (10) Vm fluctuations of the LIF model in response to a DC step (2 s) that allows a firing rate of 5 spikes/s, in control conditions (green, 95 pA), when the LIF model received 33 randomly occurring GABA_A events/s (blue, 111 pA), an invariable pattern of 33 GABA_A events/s (red, 110 pA), or when the neuron model received a constant G_GABA-A equivalent to the average conductance for a rate of 33 events/s (orange, 110 pA). b up, Raster plot of spike times collected from 50 consecutive trials in the various conditions depicted in a; down, Spike probability over time for these 50 consecutive trials.

Figure 6. Reversal potential-dependent tuning of pyramidal cell excitability and discharge fidelity.

a, Superimposed (5) Vm fluctuations of a whole-cell L2/3 pyramidal neurone in response to a DC step (1 s, 170 pA) in control conditions (green), when 33 randomly occurring GABA_A events/s were dynamically injected at E_GABA = −70 mV (blue) or −30 mV (red). Same cell and colour code for b, c and d. b, Waterfall view of Vm when the 5^th spike was set as the time reference. c, Mean firing rate vs DC step. d, CV_ISI vs firing rate. e, Normalised changes in excitability observed when neurons received 33 randomly occurring GABA_A dynamic currents/s at various differences between E_GABA and the calculated threshold for AP in each individual cell (n = 9). f, Normalised changes in spike jitter observed for a firing rate of 10 spikes/s. Same conditions as in e.

Figure 7. Frequency and reversal potential-dependent tuning of pyramidal cell excitability and discharge fidelity.

a, Superimposed (5) Vm fluctuations of the LIF model in response to a DC step (1 s, 160 pA) in control conditions (green), when the LIF model received 33 randomly occurring GABA_A events/s at E_GABA = −70 mV (blue) or −35 mV (red). Same colour code for b, c and d. b, Waterfall view of Vm when the 5^th spike was set as the time reference. c, Mean firing rate vs DC step. d, CV_ISI vs firing rate. e, Normalised changes in excitability observed when LIF model received 33 randomly occurring GABA_A events/s at various E_GABA in control conditions (black), when GABA activity induced only transient changes in I_GABA-A (gray) or when GABA activity induced only transient changes in R_in (light gray). f, Normalised changes in spike jitter observed for a rate of 10 spikes/s. Same conditions as in e. g, From left to right firing rate displayed on a pseudocolor scale vs DC step and E_GABA when the LIF model received 33 randomly occurring GABA_A events/s in control conditions (left), when GABA_A activity induced only transient changes in I_GABA-A (middle) or when GABA activity induced only transient changes in R_in (right). h, CV_ISI displayed on a pseudocolor scale vs firing rate and E_GABA. Same conditions as in g.