Synaptic responses evoked by tactile stimuli in Purkinje cells in mouse cerebellar cortex Crus II in vivo

Abstract

Background: Sensory stimuli evoke responses in cerebellar Purkinje cells (PCs) via the mossy fiber-granule cell pathway. However, the properties of synaptic responses evoked by tactile stimulation in cerebellar PCs are unknown. The present study investigated the synaptic responses of PCs in response to an air-puff stimulation on the ipsilateral whisker pad in urethane-anesthetized mice.

Methods and main results: Thirty-three PCs were recorded from 48 urethane-anesthetized adult (6-8-week-old) HA/ICR mice by somatic or dendritic patch-clamp recording and pharmacological methods. Tactile stimulation to the ipsilateral whisker pad was delivered by an air-puff through a 12-gauge stainless steel tube connected with a pressurized injection system. Under current-clamp conditions (I = 0), the air-puff stimulation evoked strong inhibitory postsynaptic potentials (IPSPs) in the somata of PCs. Application of SR95531, a specific GABA(A) receptor antagonist, blocked IPSPs and revealed stimulation-evoked simple spike firing. Under voltage-clamp conditions, tactile stimulation evoked a sequence of transient inward currents followed by strong outward currents in the somata and dendrites in PCs. Application of SR95531 blocked outward currents and revealed excitatory postsynaptic currents (EPSCs) in somata and a temporal summation of parallel fiber EPSCs in PC dendrites. We also demonstrated that PCs respond to both the onset and offset of the air-puff stimulation.

Conclusions: These findings indicated that tactile stimulation induced asynchronous parallel fiber excitatory inputs onto the dendrites of PCs, and failed to evoke strong EPSCs and spike firing in PCs, but induced the rapid activation of strong GABA(A) receptor-mediated inhibitory postsynaptic currents in the somata and dendrites of PCs in the cerebellar cortex Crus II in urethane-anesthetized mice.

Figure 1. PC somata and dendrites in response to tactile stimulation.

A, Whole-cell patch-clamp recording from a soma of a PC in response to hyperpolarizing (–100 µA) and depolarizing (+100 µA) current pulses. B, Under voltage-clamp (V_hold  =  –70 mV), superposition of five sweeps shows the effects of tactile stimuli (bar, 30 ms) on PC membrane currents. Notably, 3/5 IPSC traces expressed a second peak (arrow) at ∼30 ms after the first peak. C, Under current-clamp conditions (I = 0), superposition of five sweeps (upper) and the raster plot of simple spike events (lower) show how the PC responds to tactile stimuli (bar, 30 ms). D, The photomicrographs show the morphology of the cell, which is shown in A–C. The left column shows an overview of the location of the biocytin-labeled cell, which is indicated with a black circle. The right column shows the detail of the biocytin-labeled cell. E, Under current-clamp (I = 0), a representative dendritic patch-clamp recording shows the response of the PC dendrite to tactile stimuli. Upper, superposition of five sweeps presenting the dendrite in response to tactile stimuli (bar, 30 ms). Asterisks indicate complex spikes. Lower, superposition of three sweeps showing the different durations (30 ms, 60 ms, and 90 ms) of tactile stimulation-induced responses in the dendrites. Arrows indicate the offset responses. F, Ex vivo image of cerebellum with lobules indicated by letters, C₁ is Crus I, C₂ is Crus II; P, paramedian lobules; S, simple lobule. The overall distribution of the recorded PCs in this study is indicated by black circles; it is located in Crus II.

Figure 2. Response properties of the somata of PCs during tactile stimulation under current-clamp.

A, Representative examples (upper; arrows shown in middle), raster plot (middle) and instant frequency (lower) of simple spike events showing a PC response to tactile stimulation (black bar, 30 ms) before (black) and after application of 20 µM SR95531 (gray). The gray bar indicates the application of SR95531 (10 min). B, Pooled data (n = 6) indicate the time course of PC simple spike firing rate in response to tactile stimulation (black bar, 30 ms) in ACSF (black) and with SR95531 (gray). Note that the frequency of the simple spike firing was significant increased after stimulus, which was significantly higher than the basal simple spike firing rate (asterisk; p = 0.005; n = 6). Error bars indicate ± SEM.

Figure 3. Properties of the somata of PCs in response to tactile stimulation under voltage-clamp.

A. Under voltage-clamp (V_hold  =  −70 mV), the effects of SR95531 (20 µM) on the tactile stimulation (bar, 30 ms)-evoked somatic responses are shown. Arrows indicate the responses evoked by stimulation offset. Note that blockade inhibition revealed the stimulation-evoked action potentials, indicating a poor space clamping of this PC under in vivo conditions. B, Raster plot showing the peaks of the responses evoked by onset (On) and offset (Off) of the tactile stimuli (started at t = 0 ms). C, Representative current traces and mean values (± SEM) of the time to peak of the somatic responses evoked by tactile stimuli in ACSF (black; n = 12) and with SR95531 (gray; n = 6). The time point (0, left dashed line) indicates the onset of responses, and the right dashed line indicates the onset of inhibition. D, Under voltage-clamp (V_hold  =  −70 mV), the somatic response to tactile stimuli (bar, 30 ms) in ACSF, with NBQX (50 µM) and recovery (wash).

Figure 4. Pharmacological properties of PC dendrites in response to tactile stimulation.

A, Under current-clamp (I = 0), a PC dendritic patch-clamp recording demonstrates the spontaneous attenuated simple spike activity. B, Under voltage-clamp (V_hold  =  −70 mV), the effects of SR95531 (20 µM) on the tactile stimulation (bar, 30 ms)-evoked dendritic responses are shown. Asterisks denote the summation of PF-EPSCs. Arrows indicate the responses induced by stimulation offset. C, Representative current traces and mean values (± SEM) of the time to peak of the responses show the difference between the somata (black; n = 12) and dendrites (red; n = 19) in response to tactile stimulation. The time point (0) indicates the onset of the responses. The vertical scale denotes 200 pA (black) for the soma and 50 pA (red) for the dendrite, respectively. D, Under voltage-clamp (V_hold  =  −70 mV), the PC dendrites' response to tactile stimuli in ACSF, with SR95531, and with a mixture of SR95531 and NBQX is shown. (E) The photomicrographs depict the morphology of the recorded cell. The left column shows an overview of the location of the biocytin-labeled cell, which is indicated with a black circle. The right column shows the detail of the biocytin-labeled cell.