The ionic mechanism of gamma resonance in rat striatal fast-spiking neurons

Abstract

Striatal fast-spiking (FS) cells in slices fire in the gamma frequency range and in vivo are often phase-locked to gamma oscillations in the field potential. We studied the firing patterns of these cells in slices from rats ages 16-23 days to determine the mechanism of their gamma resonance. The resonance of striatal FS cells was manifested as a minimum frequency for repetitive firing. At rheobase, cells fired a doublet of action potentials or doublets separated by pauses, with an instantaneous firing rate averaging 44 spikes/s. The minimum rate for sustained firing was also responsible for the stuttering firing pattern. Firing rate adapted during each episode of firing, and bursts were terminated when firing was reduced to the minimum sustainable rate. Resonance and stuttering continued after blockade of Kv3 current using tetraethylammonium (0.1-1 mM). Both gamma resonance and stuttering were strongly dependent on Kv1 current. Blockade of Kv1 channels with dendrotoxin-I (100 nM) completely abolished the stuttering firing pattern, greatly lowered the minimum firing rate, abolished gamma-band subthreshold oscillations, and slowed spike frequency adaptation. The loss of resonance could be accounted for by a reduction in potassium current near spike threshold and the emergence of a fixed spike threshold. Inactivation of the Kv1 channel combined with the minimum firing rate could account for the stuttering firing pattern. The resonant properties conferred by this channel were shown to be adequate to account for their phase-locking to gamma-frequency inputs as seen in vivo.

Fig. 1.

Identification of striatal fast-spiking (FS) neurons. A: candidate FS cells were identified by their characteristic morphology after intracellular staining with Alexa Fluor 594. B: typical appearance of the FS cell action potential. FS cells' action potentials were brief with a large but short-duration afterhyperpolarization. C: at rheobase currents, FS cells typically fired a single doublet of action potentials preceded by subthreshold oscillations. D: frequency-intensity curve for the cell shown in C, E, and F. Delay interval is the interval between the early first action potential and the beginning of the burst for current levels that produced an early spike. First and last intervals refer to the first and last interspike interval in the burst. E and F: suprathreshold firing of the same cell in C and D, showing the early spike, delay interval, and burst. Below each trace is the instantaneous firing rate calculated for each interval. The minimum firing rate, as shown in the trace in C, is approximately equal to the firing rate at which firing fails at the end of the burst.

Fig. 2.

Structure of repeated bursts in response to long current pulses. A: firing of a striatal FS cell in response to a 5-s current pulse. Bursts occur at irregular intervals and are separated by periods of prolonged depolarization with subthreshold oscillations. The first action potential in each burst is of lower amplitude than the others. The termination of the current pulse is followed by a long-lasting afterhyperpolarization. B: instantaneous firing rate. Rates for intervals within a burst are connected by lines. Firing rate was elevated at the beginning of each burst and decayed to approximately the same failure point for each burst. C: action potential threshold plotted in the same way as in B. Note the elevated threshold of the first action potential in each burst. D: maximum rate of rise of the action potentials (dV/dt). Note the parallel evolution of threshold and rate of rise throughout the burst. E: frequency spectrum of subthreshold oscillations during the pauses. The peak of the spectrum corresponds to the minimum firing rate. The inset shows an example interburst membrane potential trajectory.

Fig. 3.

Effect of low doses of tetraethylammonium (TEA) on action potential duration and firing pattern. A: increase in action potential duration, reduction in slope of the falling phase of the action potential, and decrease in the early single spike afterhyperpolarization after 1 mM TEA. Note the lack of a change in the initial membrane potential preceding the first action potential, or in the duration of delay preceding the burst, or action potential threshold or firing pattern during the burst. B: dose-dependent effect of TEA on action potential duration, as measured by half-width. Iberiotoxin (ibTX; 50 nM) had no effect on action potential duration, suggesting the absence of a contribution from large-conductance calcium-dependent potassium (BK) channels. The number of cells tested is indicated above each data point. AP, action potential.

Fig. 4.

Repeated bursting after TEA application. A: an example showing stuttering firing of a striatal FS cell after application of TEA. Subthreshold oscillations between bursts and the long-lasting afterhyperpolarization at the end of a 5-s pulse continued to be evident. B: instantaneous firing rate during the current pulse shown in A. Inst., instantaneous. C: maximum rate of rise of action potentials. D: threshold voltage for action potential generation. None of these measures were altered by 1 mM TEA treatment, despite the change in action potential duration and single-spike afterhyperpolarization amplitude.

Fig. 5.

Effect of dendrotoxin-I (DTX; 100 nM) on firing pattern of striatal FS cells. A: control near-rheobase firing pattern in response to a 1-s current pulse. B: response of the same cell after application of DTX. Note not only the loss of the initial delay but also the loss of minimum firing rate and the stuttering firing pattern. C: the same neuron firing in response to a stronger 5-s current pulse before application of DTX. D: response to the same 5-s current pulse after application of DTX. The stuttering pattern was abolished. Note the long-lasting afterhyperpolarization at the end of the current pulse was not blocked by DTX. E: instantaneous firing rate for control (blue) and DTX traces (red) in C and D. Spike frequency adaptation was slowed but not abolished by DTX, and firing rate eventually was reduced below the control minimum firing rate.

Fig. 6.

Changes in threshold but not action potential rate of rise after DTX. A: initial firing (top trace) and onset of the last burst (bottom trace) from the same data shown in Fig. 5. Action potential threshold is indicated by red dots. Note the large negative shift in threshold in DTX trace (black). The elevated threshold for the first action potential in the control burst is in addition to this shift. B: action potential threshold (top) and maximum action potential rate of rise (bottom) for every action potential in both long traces shown in Fig. 5. C: threshold shifts and maximum action potential rate of rise for the first action potential in a sample of 8 cells treated with DTX. Lines represent individual cells, averaged over traces at all current levels. Red horizontal lines are medians. D: threshold and maximum rate of rise of the action potential for the second action potentials. Note the consistent difference in threshold but not in maximum rate of rise of action potentials. V_m, membrane potential.

Fig. 7.

Origin of the minimum rate. A single-compartment model is shown for the FS cell. Parameters are as listed in Table 1. A: steady state current-voltage (I-V) curve as measured using a 1-s voltage ramp from −80 to −40 mV. When Kv1 conductance (ḡ_Kv1) is intact (3 mS/cm²), the curve is monotonic. For interspike intervals comparable to the duration of the ramp, outward currents exceed inward ones and firing at this rate is impossible. Firing in this case (class 2 excitability) only occurs when the voltage changes more quickly so that kinetic differences between inward and outward currents can favor regenerative depolarization. Blockade of Kv1 removes an outward current in the −60- to −45-mV range of membrane potentials and creates a negative conductance region. Addition of a small constant current (red line, i_app = 0.85) causes loss of the subthreshold equilibrium potential, and firing can occur. Firing will slow asymptotically as the I-V curve nears the 0 current line, allowing firing at arbitrarily low rates. B: response of the model to a 2.5 μA/cm² current with Kv1 current at 3 mS/cm². C: response to a near-rheobase current (0.85 μA/cm²) in the absence of Kv1 current.

Fig. 8.

Computer simulation of the Kv1-dependent voltage attenuation in the axon and its affect on the apparent threshold seen at the soma. A: action potentials are triggered at the axon because of its greater excitability. Kv1 is localized to the initial segment of the axon, between the somatic recording and current injection site and the spike trigger zone. B: simulated response to a somatic current pulse, showing large voltage attenuation between the soma (red) and the spike trigger zone (black). The voltage difference is reduced as Kv1 current inactivates during the prolonged depolarization. Action potentials triggered in the axon propagate antidromically to the soma, which is depolarized because of current injection. The threshold measured at the soma has a large error that increases with increased Kv1 conductance. C: after removal of Kv1 current, the voltage attenuation along the axon is greatly reduced. Action potentials are still triggered in the axon, but the apparent threshold seen at the somatic end of the axon is nearly the same as at the trigger zone.

Fig. 9.

Phase-locking of the model FS cell to gamma-frequency components of a broadband signal. The model cell had a minimum firing rate of 35 spikes/s. A: sample of the noisy current waveform composed of randomly timed exponential excitatory postsynaptic currents used for the simulation. B: magnitude spectrum of the synaptic barrage. C: sample of the noisy current, bandpass filtered at 32.5–37.5 Hz to show a gamma-frequency component, and the resulting model cell membrane potential waveform. Action potentials occur irregularly but preferentially occur near zero-crossings in the noise waveform. D: histogram showing average phase of firing for 160 s of simulated firing at ∼10 spikes/s as in C. E: deviation of histograms like that in D from a uniform distribution, as measured by Kolmogorov-Smirnov (K-S) distance, for 5-Hz frequency bands from 5 to 110 Hz. Solid line represents an intact cell; dotted line represents a cell without Kv1 conductance. Note the prominent peak in phase-locking in the gamma range, which is lost with blockade of Kv1. F: spike-triggered average of the current waveform showing 35-Hz (29 ms) resonance.