Pauses in Cholinergic Interneuron Activity Are Driven by Excitatory Input and Delayed Rectification, with Dopamine Modulation

Abstract

Cholinergic interneurons (ChIs) of the striatum pause their firing in response to salient stimuli and conditioned stimuli after learning. Several different mechanisms for pause generation have been proposed, but a unifying basis has not previously emerged. Here, using in vivo and ex vivo recordings in rat and mouse brain and a computational model, we show that ChI pauses are driven by withdrawal of excitatory inputs to striatum and result from a delayed rectifier potassium current (I_Kr) in concert with local neuromodulation. The I_Kr is sensitive to K_v7.2/7.3 blocker XE-991 and enables ChIs to report changes in input, to pause on excitatory input recession, and to scale pauses with input strength, in keeping with pause acquisition during learning. We also show that although dopamine can hyperpolarize ChIs directly, its augmentation of pauses is best explained by strengthening excitatory inputs. These findings provide a basis to understand pause generation in striatal ChIs. VIDEO ABSTRACT.

Keywords: basal ganglia; cholinergic interneuron; corticostriatal; delayed rectification; dopamine; excitatory input; nigrostriatal; pause response; striatum; thalamostriatal.

Figure 1

In Vivo Firing Rate of pChIs Reflects Changes in Excitatory Input, with Pauses Accompanying Withdrawal of Excitation (A) Firing of pChIs with (left, purple) or without (right, blue) short-latency-evoked action potentials before a pause. Top: example; bottom: average, pause (blue arrow), rebound (green arrow). Firing rate correlation, r² = 0.78, p < 0.001 (n = 4–5). (B) Mean ± SEM for amplitude (top) and duration (bottom) of pause and rebound. (C and D) Top: example striatal LFP; middle top: mean ± SEM of inverted LFP (iLFP); middle: mean firing rate (black) ± SEM (gray) in pChIs (n = 5, n = 9); bottom: mean firing rate (black) ± SEM (gray) in SPNs (n = 5) aligned to maximum of spontaneous iLFP (dashed red line) (C) or contralateral cortical stimulation (0.2 Hz; solid red lines) (D). Purple dashed line, firing rate maxima; shaded blue, pChI firing rate below baseline, a “pause.” (E) Phase plot of firing rates for ChIs and SPNs. Data were extracted during slow LFP oscillation in (D).

Figure 2

ChI Firing Ex Vivo Rate Reflects Changes to Excitatory Input and Pauses Are Driven by Withdrawal of Excitation (A) Characteristic ChI physiology. Immunocytochemical co-labeling: neurobiotin fill; ChAT-immunoreactivity (scale bar, 20 μm). (B) Example sweep, example firing rate histogram (20 sweeps), and mean firing rate histogram ± SEM (n = 6) of ChI response to a sine-wave current. Highest firing rate (purple dashed line), input current maximum (red dashed line), reduced firing rate versus baseline, ^∗p < 0.05, t test. (C and D) Responses to trapezoid current injections for depolarizing (C) and hyperpolarizing (D) input. (C) Top to bottom: example sweep, example firing rate histogram (20 sweeps), and representative and mean membrane potential ± SEM in presence of TTX 1 μM (red). Correlation, firing rate and membrane potential, r² = 0.91, 100 bins. ^∗∗∗p < 0.001, paired t tests for maximum versus plateau and minimum versus baseline (n = 10). (D) Example sweep plus membrane potential in presence of TTX (red).

Figure 3

I_Kr Underlies Hyperpolarization Induced by Excitatory Input Withdrawal in ChIs (A) Representative membrane potential in presence of TTX (1 μM) and I_h blocker Zd7288 (50 μM) without (dark gray) and with (light gray) resting membrane potential (RMP) restored to −55 mV (n = 6) during trapezoid current injections. (B) Representative membrane potential in presence of TTX (black) and either (gray) I_h blocker Zd7288 or Kv7.2/7.3 blocker XE-991 (100 μM, n = 6). RMP was held at pre-drug condition. (C and D) Mean ± SEM of amplitude of hyperpolarization below RMP (C) or ratio of trough:peak (black versus gray vertical dashed lines in B) (D), normalized to control. Riluzole, 100 μM; cadmium, 200 μM; CsCl, 2 mM; Zd7288, 50 μM; 4-AP, 1 mM; TEA, 20 mM. n = 4–6. Typical traces shown in Figure S3. ^∗∗∗p < 0.001, one-sample t test versus control.

Figure 4

I_Kr Underlies Hyperpolarization and Interacts with Dopamine in a Model ChI (A) Current-time responses of conductances, 20 mV steps from −100 mV; inset: single-compartment neuron model. (B) Membrane potential response to synaptic input (current), with I_leak and I_Kr and/or I_A. (C) I_Kr current density (green) and membrane potential (dark blue) showing overshoot (pink area) and undershoot (blue area). (D) Undershoot lost (red) at membrane potential −80 mV is restored with depolarization to normal RMP (blue). (E) Response to short sine-wave input. Maximum current injection (red dash); undershoot (green dash). Inset: trough amplitude (blue), but not latency (orange), scales with amplitude of current injected. (F) Effect of D₂ current and I_Kr after separate and combined activation after a stimulation (gray area) starting at time zero. (G and H) Membrane potential response (top) to input to ChIs (middle) flanking a DA neuron burst (bottom) with 100% (light blue) or 25% (dark blue) of D₂ currents identified in Straub et al. (2014) before (G) and after (H) enhanced excitatory input following learning (Suzuki et al., 2001). In (G), response without rebound (green). I_leak present throughout.