Mechanism of an Intrinsic Oscillation in Rat Geniculate Interneurons

Abstract

Depolarizing current injections produced a rhythmic bursting of action potentials - a bursting oscillation - in a set of local interneurons in the lateral geniculate nucleus (LGN) of rats. The current dynamics underlying this firing pattern have not been determined, though this cell type constitutes an important cellular component of thalamocortical circuitry, and contributes to both pathologic and non-pathologic brain states. We thus investigated the source of the bursting oscillation using pharmacological manipulations in LGN slices in vitro and in silico. 1. Selective blockade of calcium channel subtypes revealed that high-threshold calcium currents $I_L$ and $I_P$ contributed strongly to the oscillation. 2. Increased extracellular K⁺ concentration (decreased K⁺currents) eliminated the oscillation. 3. Selective blockade of K⁺ channel subtypes demonstrated that the calcium-sensitive potassium current ( $I_{AHP}$ ) was of primary importance. A morphologically simplified, multicompartment model of the thalamic interneuron characterized the oscillation as follows: 1. The low-threshold calcium current $\left(I_T\right)$ provided the strong initial burst characteristic of the oscillation. 2. Alternating fluxes through high-threshold calcium channels and $I_{AHP}$ then provided the continuing oscillation's burst and interburst periods respectively. This interplay between $I_L$ and $I_{AHP}$ contrasts with the current dynamics underlying oscillations in thalamocortical and reticularis neurons, which primarily involve $I_T$ and $I_H$ , or $I_T$ and $I_{AHP}$ respectively. These findings thus point to a novel electrophysiological mechanism for generating intrinsic oscillations in a major thalamic cell type. Because local interneurons can sculpt the behavior of thalamocortical circuits, these results suggest new targets for the manipulation of ascending thalamocortical network activity.

Figure 1:

Generalized blockade of high-threshold calcium conductances abolishes bursting oscillations in vitro. A: Depolarizing current injection induced bursting oscillation. B: Bath application of a high-threshold calcium conductance blocker, ${\mathrm{C}\mathrm{d}}^{2+}\left(200\mu \mathrm{M}\right)$, blocked the oscillation. C: After washout of Cd²⁺, the oscillation was recovered. D: Subsequent to C, addition of Co²⁺ (1 mM), another high-threshold calcium conductance blocker, also blocked the oscillation.

Figure 2:

Combined blockade of specific high-threshold calcium channels blocks the oscillation. A-F: High-threshold calcium-channel manipulations in vitro. G-I (bottom row): in silico calcium channel manipulations. A: Control, in vitro. B: Selective L-channel blockade ($20\mu \mathrm{M}$ nifedipine) enhances the initial burst and subsequent oscillation rate in vitro. C: Nifedipine washout. D: Control, in vitro. E: Selective P-channel blockade ($50\mu \mathrm{M}$ MVIIC) alters the initial burst and increases oscillation rate in vitro. F: Combined L- and P-type calcium channel blockade ($20\mu \mathrm{M}$ nifedipine $+50\mu \mathrm{M}$ MVIIC) abolishes the oscillation. G: Control, in silico. H: Reduction of high-threshold calcium channel conductance to approximately ⅔ of the control value enhances the initial burst and increases the oscillation rate in silico. I: Reduction of high-threshold calcium channel conductance to approximately ⅓ of the control value abolishes the in silico oscillation.

Figure 3:

Low-threshold calcium-channel manipulation, in silico. A: Control. B: Voltage trace after 40% reduction in low-threshold calcium channel conductance.

Figure 4:

Elimination of the I_CAN conductance in silico increased the time to initial burst and reduced the overall oscillation frequency. Black: Control I_CAN density. Cyan: I_CAN density set to 0.

Figure 5:

Effect of reducing I_Naf density in silico. A: Control. B: 75% reduction reduces amplitude of spikes but maintains oscillation. C: Elimination of I_Naf eliminates oscillation.

Figure 6:

Increased extracellular potassium concentration eliminated the oscillation, resulting in a depolarized plateau potential of limited duration. Left (A, C): In vitro voltage traces. Right (B, D): In silico voltage traces. A: Control (2.5 mM K⁺), in vitro. B: Control, in silico. C: Increased extracellular [K⁺] (9 mM), in vitro. D: Simulated increase in extracellular [K⁺], in silico.

Figure 7:

I_C current blockade did not impact the oscillation. A. Depolarizing current injection induced bursting oscillation. B. Application of 60 nM CTX had little effect on the oscillation (n=4).

Figure 8:

Blockade of I_A current prevented fast intraburst repolarization without impacting oscillation frequency. A: Depolarizing current injection induced bursting oscillation in vitro. B: Application of $400\mu \mathrm{M}$ 4-AP created a square-wave firing pattern, eliminating intraburst action potentials but having little impact on the oscillation frequency (n=2). C: Washout did not recover original firing pattern.

Figure 9:

I_AHP contributes to the bursting oscillation. Left (A, C): Voltage traces, in vitro. Right (B, D): Voltage traces, in silico. A: Control, in vitro. B: Control, in silico. C: In vitro application of the I_AHP blocker apamin eliminated the bursting oscillation, leading to low-amplitude, high-frequency spiking. D: Elimination of the I_AHP conductance in silico similarly resulted in low-amplitude, high-frequency spiking.

Figure 10:

Reduced input resistance was simulated via two different in silico mechanisms, resulting in a shift in firing mode in both cases, from bursting oscillation to tonic firing. A: Control. B: Increased leak channel conductance. C: Control. D: Increased I_H, I_CAN, and I_AHP conductances.

Figure 11:

Origin of steady-state oscillatory bursting with 0.11 nA current clamp. Initial depolarization activates $I_{\mathrm{C}\mathrm{A}\mathrm{N}},I_{\mathrm{T}}$, and feeds $I_{\mathrm{T}}{\left[{\mathrm{C}\mathrm{a}}^{++}\right]}_i$ pool, leading to initial burst (green line), terminated through hyperpolarization due to $I_{\mathrm{A}\mathrm{H}\mathrm{P}}$. Then, the continuing depolarization inactivates $I_{\mathrm{T}}$, leaving $I_{\mathrm{L}}$ to provide the depolarizing $I_{\mathrm{C}\mathrm{a}}$ during the repeated bursts. (${\mathrm{V}}_{\mathrm{m}}$: soma voltage; Currents: $I_{\mathrm{T}},I_{\mathrm{L}}$: T-, L-type ${\mathrm{C}\mathrm{a}}^{2+}$ currents; $I_{\mathrm{C}\mathrm{A}\mathrm{N}}$: nonselective cation; $I_{\mathrm{N}\mathrm{a}\mathrm{f}}$ : fast ${\mathrm{N}\mathrm{a}}^{+};I_{\mathrm{A}\mathrm{H}\mathrm{P}}:{\mathrm{C}\mathrm{a}}^{2+}$-activated ${\mathrm{K}}^{+};{\mathrm{C}\mathrm{a}}^{2+}$ concentrations: separate pools served by $I_{\mathrm{T}}$ vs. $I_{\mathrm{L}}$)