Article Figures & Data
Figures
- Figure 1.
Contribution of voltage-gated calcium channels on the action potential waveform in Purkinje neurons. a, Whole-cell current-clamp recordings from a spontaneously firing Purkinje neuron under control conditions in the presence of blockers for T-, R-, N-, and L-type calcium channels or for P/Q-type calcium channels. The action potential has been truncated to show the AHP more clearly; complete action potentials are shown in the inset. Each trace is the average of 50-100 action potentials. Blockers were mibefradil (1 μm) for T- and R-type channels,ω conotoxin GVIA (3 μm) for N-type channels, nimodipine (2 μm) for L-type channels, and ω conotoxin MVIIC (2 μm), ω agatoxin IVA (1-2 μm), or Cd2+ (100 μm) for P/Q-type channels. b, Diagram showing the parameters of the action potential (AP) waveform that were measured. Vm threshold, Membrane potential 500 msec before the peak of the action potential; Vm AHP, membrane potential at the peak of the AHP; AHP amplitude, Vm AHP-Vm threshold; AP width at 50%, width of the action potential at one-half the difference between the peak potential and Vm threshold. c, Summary of the effects of calcium channel blockers on the action potential waveform. Values were significantly different for *p < 0.05 and **p < 0.005 by one-way ANOVA. Differences between all other values were not significant (ns). Error bars are ± SEM; n = 7.
- Figure 2.
Effect of blockers of voltage-gated calcium channels on spontaneous firing in Purkinje neurons. Spontaneous firing of individual Purkinje neurons was monitored with extracellular recording. a, Block of T- and R-type calcium channels does not alter the spontaneous firing rate. Left, Average firing rate versus time for a Purkinje neuron. Inset, Raw data recording. Right, Histograms comparing average firing rate and coefficient of variation of interspike intervals under control conditions and after channel block for five Purkinje neurons. Error bars are ± SEM; n = 5. Predominant firing rate is the firing rate corresponding to the peak of the distribution of interspike intervals. b, Block of L- and R-type calcium channels does not alter the spontaneous firing rate. Left, Average firing rate versus time for a single neuron. Right, Histograms comparing average firing rate and coefficient of variation of interspike intervals under control conditions and after channel block for five neurons. Error bars are ± SEM; n = 5. c, Partial P/Q-type channel block increases the average firing rate. EC50 for the lot of toxin was determined by the supplier to be 200 nm. Left, Average firing rate versus time for a single neuron. Block of P/Q channels increased the firing rate and caused the cell to burst and occasionally go silent (apparent in the abrupt increases in the firing rate and the quiescent periods). Right, Histograms comparing average firing rate and coefficient of variation of interspike intervals under control conditions and after channel block. Error bars are ± SEM; n = 5. Values were significantly different for *p < 0.02 and **p < 0.005 by one-way ANOVA. Other differences were not significant (ns).
- Figure 3.
Effect of increased calcium entry on selective coupling between calcium channels and calcium-activated potassium channels. a-c, Increasing the extracellular calcium concentration decreases the firing rate by activating BK channels. a, Spontaneous activity recorded from a Purkinje neuron. After SK channels were blocked with apamin (100 nm), increasing the extracellular calcium concentration from 2 to 5 mm decreased the firing rate. The firing rate decrease was reversed by blocking BK channels with iberiotoxin (100 nm). b, Distribution of interspike intervals for the cell in a. c, Average of the predominant firing rate from five Purkinje neurons. Error bars are ± SEM; n = 5. d, e, Activation of SK channels by increased extracellular calcium concentration was not reversed by block of T- and R-type (d) or combined block of T-, R-, N-, and L-type calcium channels (e). All recordings were made in the presence of iberiotoxin to block BK channels. Error bars are ± SEM; n = 7 (d) and n = 6 (e). f, g, Activation of BK channels by increasing extracellular calcium concentration was not reversed by block of T- and R-type (f) or combined block of T-, R-, N-, and L-type calcium channels (g). All recordings were made in the presence of apamin to block SK channels. Error bars are ± SEM; n = 6 (f) and n = 6 (g). Values were significantly different for *p < 0.05 by one-way ANOVA. Other differences were not significant (ns).
