Giuseppe Sciamanna and Charles J. Wilson J. Neurophysiol. 106:2936-2949.

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 aged 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 K_v3 current using TEA (0.1-1 mM).  Both gamma-resonance and stuttering were strongly dependent on K_v1 current.  Blockade of K_v1 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 K_v1 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. 

Figure 1.  Identification of striatal 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 AHP.  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 seen in trace C, is approximately equal to the firing rate at which firing fails at the end of the burst.

Figure 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 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 is elevated at the beginning of each burst and decays to approximately the same failure point for each burst.  C.  Action potential threshold plotted in the same way as B.  Note the elevated threshold of the first action potential in each burst.  D.  Maximum rate of rise of the action potentials.  Note 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 inter-burst membrane potential trajectory.

Figure 3.  Effect of low doses of 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 lack of a change in the initial membrane potential preceding the first action potential, or in 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 BK channels. Number of cells tested is indicated above each point.

Figure 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 continue to be evident. B.  Instantaneous firing rate during the current pulse shown in A.  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 AHP amplitude.

Figure 5.  Effect of dendrotoxin-I (100 nM) on firing pattern of striatal FS cells.

A.  Control near-rheobase firing pattern in response to a 1 s current pulse.  B.  The response of the same cell at the same current level after application of DTX.  Note not only the loss of the initial delay, but also 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 DTX.  D.  Response to the same 5 s current pulse after DTX.  The stuttering pattern is abolished.  Note the long lasting afterhyperpolarization at the end of the current pulse is not blocked by DTX.  E.  Instantaneous firing rate for control (blue) and DTX (red) traces in C and D.  Spike frequency adaptation is slowed, but not abolished by DTX, and firing rate eventually is reduced below the control minimum firing rate.

Figure 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 Figure 5.  Action potential threshold is indicated by red dots.  Note 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 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 consistent difference in threshold but not in maximum rate of rise of action potentials. 

Figure 7.  Origin of the minimum rate.  A single compartment model for the FS cell.

A.  Steady state I-V curve as measured using a 1 s voltage ramp from -80 to -40 mV.  When K_v1 conductance 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 K_v1, 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, iapp=0.85), causes the 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 K_v1 current at 3 mS/cm².  B.  Response to a near-rheobase current (0.85 µA/cm²) in the absence of K_v1 current.