Motor dysfunction and altered synaptic transmission at the parallel fiber-Purkinje cell synapse in mice lacking potassium channels Kv3.1 and Kv3.3

Abstract

Micelacking both Kv3.1 and both Kv3.3 K+ channel alleles display severe motor deficits such as tremor, myoclonus, and ataxic gait. Micelacking one to three alleles at the Kv3.1 and Kv3.3 loci exhibit in an allele dose-dependent manner a modest degree of ataxia. Cerebellar granule cells coexpress Kv3.1 and Kv3.3 K+ channels and are therefore candidate neurons that might be involved in these behavioral deficits. Hence, we investigated the synaptic mechanisms of transmission in the parallel fiber-Purkinje cell system. Action potentials of parallel fibers were broader in mice lacking both Kv3.1 and both Kv3.3 alleles and in mice lacking both Kv3.1 and a single Kv3.3 allele compared with those of wild-type mice. The transmission of high-frequency trains of action potentials was only impaired at 200 Hz but not at 100 Hz in mice lacking both Kv3.1 and Kv3.3 genes. However, paired-pulse facilitation (PPF) at parallel fiber-Purkinje cell synapses was dramatically reduced in a gene dose-dependent manner in mice lacking Kv3.1 or Kv3.3 alleles. Normal PPF could be restored by reducing the extracellular Ca2+ concentration indicating that increased activity-dependent presynaptic Ca2+ influx, at least in part caused the altered PPF in mutant mice. Induction of metabotropic glutamate receptor-mediated EPSCs was facilitated, whereas longterm depression was not impaired but rather facilitated in Kv3.1/Kv3.3 double-knockout mice. These results demonstrate the importance of Kv3 potassium channels in regulating the dynamics of synaptic transmission at the parallel fiber-Purkinje cell synapse and suggest a correlation between short-term plasticity at the parallel fiber-Purkinje cell synapse and motor performance.

Figure 2.

Parallel-fiber action potential shape is affected by lack of Kv3.1 and Kv3.3 alleles. A, Fluorescence image of the lobule V/VI cerebellar surface stained with the voltage-sensitive dye. The stimulation electrode (asterisk) was placed in lobule VI. B, Color-coded fluorescence change representing depolarization 1.11 (1), 1.48 (2), and 1.85 (3) msec after stimulation in area outlined by dotted line in A. Fluorescence change (ΔF) is expressed as percentage of baseline fluorescence (F). C, Fluorescence time course at the point marked by a black square in B. Width of compound action potential is measured at half-maximum fluorescence change relative to a baseline. The stimulation time point is marked by a black triangle. Calibration: 2 msec, 0.2%. Data shown in A-C were obtained from a WT mouse. D, Color-coded maps of action potential width obtained from a WT mouse, a TM mouse, and a DKO mouse. E, Action potential width plotted against distance from the stimulation electrode in a WT preparation and a DKO preparation. Note small increase in the width of the measured compound action potential with increasing distance from the stimulation site, indicating some inhomogeneity in AP conduction velocities of individual action potentials in both genotypes. F, Mean ± SEM action potential shape (resampled at 5 kHz) (see Materials and Methods). G, Mean ± SEM action potential width for WT(917 ± 36 μsec; n = 7), TM (1088 ± 56 μsec; n = 6), and DKO (1280 ± 89 μsec; n = 7). FWHM indicates fluorescence width at half-maximum. Asterisks indicate significant differences from WT (^*p < 0.05; ^**p < 0.01).

Figure 3.

High-frequency firing is impaired in Kv3.1/Kv3.3 DKO mice. A, Examples of the fluorescence time course on the parallel-fiber trajectory after stimulating parallel fibers with 10 pulses at 100 Hz in WT and DKO mice (stimulation times are marked by triangles). Calibration: 20 msec, 0.2%. B, Action potential amplitude during 100 Hz stimulation train. Data points represent mean ± SEM values of the amplitudes of the nth action potential in the train normalized to the amplitude of the first action potential in each train. Both wild-type and DKO mice can follow stimulation at 100 Hz. C, Examples of the fluorescence time course on the beam after stimulating parallel fibers with 20 pulses at 200 Hz in WT and DKO mice. Calibration: 20 msec, 0.2%. D, Normalized action potential amplitude with stimulation at 200 Hz. Asterisks indicatesignificantdifferences from WT (^*p < 0.05).

Figure 1.

Walking trajectories of mice lacking Kv3.1 or Kv3.1 alleles. A, Walking trajectories of a WT control mouse (Kv3.1^+/+Kv3.3^+/+), a Kv3.1 single mutant (SKO) (Kv3.1^-/-Kv3.3^+/+), a mouse lacking both Kv3.1 alleles and one Kv3.3 allele (TM) (Kv3.1^-/-Kv3.3^+/-), and a mouse lacking all of the Kv3.1 and Kv3.3 alleles (DKO) (Kv3.1^-/-Kv3.3^-/-). Dots indicate center of force of the mouse measured at 50 Hz and connected by lines. Note increased fluctuations perpendicular to the main direction of movement with increasing lack of Kv3.1 and Kv3.3 alleles. B, Ataxia indices obtained from mice lacking varying numbers of functional Kv3.1 and Kv3.3 alleles. Error bars indicate mean ± SEM values obtained from 21 WT, 30 Kv3.1^+/-Kv3.3^+/+ (SHET), 14 Kv3.1^+/-Kv3.3^+/- (DHET), 7 SKO, 12 TM, and 5 DKO mice. Asterisks indicate statistical significant differences from WT (^*p < 0.05; ^**p < 0.01).

Figure 4.

Short-term plasticity is altered in mice lacking Kv3.1 and Kv3.3 alleles. A, Purkinje cell EPSC PPF at intervals of 50, 100, 150, 200, and 250 msec in Kv3.1^+/+Kv3.3^+/+ (WT), Kv3.1^+/-Kv3.3^+/+ (SHET), Kv3.1^+/-Kv3.3^+/- (DHET), Kv3.1^-/-Kv3.3^+/+ (SKO), Kv3.1^-/-Kv3.3^+/- (TM), and Kv3.1^-/-Kv3.3^-/- (DKO) mice. Stimulus artifacts were suppressed for clarity. B, PPF ratio values (mean ± SEM; n = 6 cells for each category) plotted against paired-pulse interval. Asterisks indicate significant differences (p < 0.01) from WT. Note the gradual reduction of PPF with reduction of functional Kv3.1 and Kv3.3 alleles.

Figure 5.

Reduced PPF of DKO mice can be reversed by lowering extracellular Ca²⁺ concentration. PPF was measured at an interval of 50 msec in the presence of different concentrations of extracellular Ca²⁺ in wild-type and DKO mice. Each point is the mean ± SEM of separate experiments in four WT mice and four DKO mice. Asterisks indicate significant differences (p < 0.01) from WT. PPF ratios at different interpulse intervals are shown in Table 1.

Figure 6.

Reduced PPF with application of low concentration of TEA. PPF was measured in the presence of different concentrations of TEA in WT (open circles) and DKO (filled circles) mice. Each point is the mean ± SEM of separate experiments in two WT mice and three DKO mice (2-10 cells per animal) at paired-pulse intervals of 50 (A), 150 (B), and 250 (C) msec.

Figure 7.

Facilitated induction of metabotropic glutamate receptor-mediated slow EPSCs. A, B, mGluR-mediated EPSCs evoked in Purkinje cells by trains of 2, 4, 6, 8, and 10 pulses (at 100 Hz) in a WT and a DKO mouse. C, Amplitudes of mGluR-mediated EPSCs versus number of pulses. Each data point represents the mean ± SEM (7 cells from 4 WT mice and 18 cells from 6 DKO mice) normalized to the response evoked by a train of 10 stimuli. Asterisks indicate significant differences (^*p < 0.005; ^**p < 0.001) from WT.

Figure 8.

Long-term depression in WT and DKO mice. A, B, Pairing of parallel-fiber and climbing-fiber stimulation depressed the EPSCs elicited by parallel-fiber stimulation in WT and DKO mice. A, Records from single experiments. Left column, Baseline EPSCs. Right column, EPSCs 20 min after application of pairing protocol (dotted line indicates baseline EPSC). Calibration: 15 msec, 500 pA. B, Time course of EPSC amplitudes (mean ± SEM) of 3 of 6 and 5 of 10 experiments in which LTD (long-lasting decrease of EPSC to <90% of control) was induced in Purkinje cells of WT and DKO mice, respectively. C, D, Pairing of high-frequency (5 pulses at 100 Hz) parallel-fiber stimulation with Purkinje cell depolarization (to 0 mV for 50 msec) increased EPSCs of Purkinje cells of WT mice and depressed EPSCs of Purkinje cells of DKO mice. C, Records from single experiments. Calibration: 15 msec, 500 pA. D, Time course of EPSC amplitudes (mean ± SEM) for all of the experiments (4 of 4) in Purkinje cells of WT mice and all of the experiments (4 of 4) in Purkinje cells of DKO mice. Asterisks indicate significant differences (p < 0.05) from WT. Error bars indicate ± SEM, and dotted horizontal lines indicate baseline values (100%).