Dendritic spikes mediate negative synaptic gain control in cerebellar Purkinje cells

Abstract

Dendritic spikes appear to be a ubiquitous feature of dendritic excitability. In cortical pyramidal neurons, dendritic spikes increase the efficacy of distal synapses, providing additional inward current to enhance axonal action potential (AP) output, thus increasing synaptic gain. In cerebellar Purkinje cells, dendritic spikes can trigger synaptic plasticity, but their influence on axonal output is not well understood. We have used simultaneous somatic and dendritic patch-clamp recordings to directly assess the impact of dendritic calcium spikes on axonal AP output of Purkinje cells. Dendritic spikes evoked by parallel fiber input triggered brief bursts of somatic APs, followed by pauses in spiking, which cancelled out the extra spikes in the burst. As a result, average output firing rates during trains of input remained independent of the input strength, thus flattening synaptic gain. We demonstrate that this "clamping" of AP output by the pause following dendritic spikes is due to activation of high conductance calcium-dependent potassium channels by dendritic spikes. Dendritic spikes in Purkinje cells, in contrast to pyramidal cells, thus have differential effects on temporally coded and rate coded information: increasing the impact of transient parallel fiber input, while depressing synaptic gain for sustained parallel fiber inputs.

Fig. 1.

Single dendritic spikes enhance AP output on short, but not long, timescales. (A) Simultaneous whole-cell recordings were made from the soma and dendrite of the same Purkinje cell while stimulating PFs close to the dendritic recording site. (B) Stimulating PFs at the threshold for dendritic spike generation resulted in subthreshold EPSPs (black) or dendritic spikes (red, same traces as in C and D). (C) Somatic (dotted line) and dendritic (thick line) voltage recording during a single parallel fiber stimulus (arrow) not triggering a dendritic spike. The raster plot and the PSTH contain 10 trials; bin size is 2 ms. (D) Same as in C, except the synaptic stimulus triggered a dendritic spike. The raster plot shows 10 trials, the PSTH contains 45 trials; bin size is 2 ms. Note the somatic AP burst associated with the dendritic spike and the following pause in somatic firing. (E) Pooled averages of maximum somatic instantaneous firing rates and maximum somatic ISIs (n = 9). Black bars show the effect of EPSPs without dendritic spikes, red bars show the effect of EPSPs with dendritic spikes, and the blue bar represents the average maximal somatic ISI during spontaneous activity showing no significant pauses are present without dendritic spikes. (F) The PSTHs in C and D were integrated then normalized to trial number and spontaneous firing rate, thus only showing the stimulus-evoked spikes. (G) Bar graphs showing the pooled averages of maximum number of stimulus added spikes and the number of sustained added spikes (in a 100-ms window starting 100 ms after the stimulus, n = 9). *P < 0.02.

Fig. 2.

Multiple synaptically triggered dendritic spikes suppress AP output. (A) Somatic (gray) and dendritic (red) voltage traces during 10 PF stimuli delivered at 100 Hz. The instantaneous somatic firing rate (blue) is increasing as long as the synaptic input is active. (B) Same as in A except with stronger synaptic stimulus. The onset of dendritic spikes (*) in the dendritic voltage trace (orange line) coincides with a sharp drop in the somatic instantaneous firing rate. (C) Overlay of instantaneous somatic firing rates with and without dendritic spikes. (D) Comparison of the average somatic firing rate during the first and last 50 ms of the stimulus when no dendritic spikes were triggered, showing a clear increase during the stimulus (five cells). (E) Same as in D, except for trials with dendritic spikes (five cells).

Fig. 3.

Current injection-evoked dendritic spikes suppress AP output. (A) Somatic (gray) and dendritic (red) voltage trace during dendritic injection of synaptic-like current (green). (B) Superimposed somatic (gray) and average dendritic (red) traces triggered by the current injection. AP probability (blue) follows the input strength. (C) Histogram of somatic interspike intervals triggered by current injection below dendritic spike threshold. (D) As in A but with stronger current injection triggering dendritic spikes (asterisks). (E) Superimposed somatic (gray) and average dendritic (red) traces triggered at dendritic spikes. Somatic AP probability (blue) is strongly reduced following dendritic spikes. (F) Histogram of somatic interspike intervals triggered by current injection above dendritic spike threshold. (G) Average (black), maximal (red), and minimal (blue) somatic firing rates and the number of dendritic spikes (green) during the current injection is plotted versus the average injected current. Note the linear f/I relationship before dendritic spikes occur and the input independence of somatic firing during dendritic spikes. (H) Pooled data from five cells demonstrating the linearity of the f/I curve below (weak input) and above (strong input) dendritic spike threshold and the firing rate where dendritic spikes appear ensuing the clamping effect. Average values marked with red, filled circles. Dotted lines connect measurements from the same cell (black, open circles). (A–G) Data from the same cell.

Fig. 4.

The clamping of the input-output relationship by dendritic spikes requires BK channels and maintains axonal output below the propagation limit. (A) Somatic (gray) and dendritic (red) recording during dendritic current injection (green). BK channels were blocked by 100 nM penitrem A. Note the lack of dendritic afterhyperpolarization and somatic pause following dendritic spikes. The slow AHP of somatic spikes is also reduced. (B) Average somatic firing rate (black) and the number of dendritic spikes (red) during the current injection is plotted versus the peak of the injected current. Note that with the appearance of dendritic spikes, the somatic output continues to correlate positively with the input, although with a reduced gain. (C) Pooled values of the linearity of the f/I curve below (black) and above (red) dendritic spike threshold (n = 3). (D) The interspike interval histogram is shifted to the left and remains unimodal with the appearance of dendritic spikes when BK channels are blocked. (A, B, and D) Data from the same cell. (E) Somatic (gray) and dendritic (red) voltage trace during synaptically triggered dendritic spikes. BK channels were blocked by 100 nM penitrem A. The instantaneous somatic firing rate (blue) keeps increasing when the synaptic input is active despite the presence of dendritic spikes. (F) Pooled average of maximal sustained somatic firing rates during synaptic-like current injections. (G) Pooled average of the maximal somatic instantaneous firing rates. Here again, the BK channel-dependent dampening mechanism invoked by dendritic spikes keeps the instantaneous somatic firing rate below the axonal propagation limit.