Figure 7. Long-term depression evoked by 8-Hz electrical stimulation of mouse whiskers. (A) Continuous recordings arranged in cascades show single traces of evoked local field potential components induced by whisker electrical stimulation. (Top group) 15 min in control conditions; (middle group) 10 min of 8-Hz stimulation; (bottom group) 30 min after the stimulation protocol. Vertical dotted lines indicate the peak latencies of N1 presynaptic and N2 and N3 postsynaptic components under control conditions. During the 8-Hz stimulation, the postsynaptic components almost disappeared, and the presynaptic input remained stable. Long-term depression is evident after the 8-Hz stimulation protocol (bottom group), when the latency of the N2 components (curved black lines next to dotted lines) increased and the amplitude of the N3 component strongly decreased. These effects were maximal just after the 8-Hz stimulation and persisted for at least 30 min. (B) Single traces selected from (A) to compare latencies and amplitudes of evoked field potential components; (top) before 8-Hz stimulation protocol, (middle) during 8-Hz stimulation, and Cerebellar LTD in Alert Mice (bottom) just after 8-Hz stimulation protocol. Arrowheads indicate shifted latencies of postsynaptic components after 8-Hz stimulation. Dashed lines indicate the locations of components in the control condition to facilitate comparison. Calibrations are indicated for each figure. doi:10.1371/journal.pone.0036184.g007 Worth noting, 8 Hz is the dominant frequency of whisker movements during exploration in rats [44]. In an important study about long-term plasticity in the mouse sensorimotor cortex, Me´gevand et al. [45] demonstrated that passive stimulation of whiskers at 8 Hz induces plasticity of whisker-evoked cortical responses. In that relevant work, the authors produced rhythmic movement of whiskers in the anaesthetized animals for 10 min. In addition, when mice were exposed to environmental enrichment, the authors observed a lasting increase in the amplitude of whisker-evoked responses in the somatosensory cortex that occluded further potentiation. Relationship between PC firing and LFP Recent work showed that both SS and CS were evoked in the same PCs in response to whisker dermatome stimulation in both alert and anesthetized mice [22,25,46]. The dominant SS response consisted of a short excitation followed by inhibition, as originally described in the rat [26] and confirmed in other stimulation protocols [22,25,46–48]. This SS pattern was explained by the feedforward excitation–inhibition sequences described during direct PF stimulation in vitro [49,50] or in vivo [28]. The same peripheral stimulation produced a CS at about 20 ms in alert mice [25,46], similar to our present observations. This evidence demonstrated that climbing and mossy fibers are simultaneously recruited by a single peripheral stimulation. The fact that the first SS response occurred very early (,3.0 ms) and coincided with N2–N3 peak latency strongly supports the proposal that these components had a postsynaptic nature and, in the case of the N3, originated at the PF–PC synapses as the inversion and collision data suggest. The timing of the SS synchronized on the N3 field reinforced this fact. This N3 plasticity is also supported by the finding that PF–PC synapses display short- and long-term synaptic plasticity, as shown in different preparations [2,8,9,19]. Is the present LTD dependent on CF activity? Previous work has shown that in slices, LTD is induced when PF activation precedes CF activation by 250 ms; moreover, simultaneous activation or reversal of the temporal sequence impairs LTD [51]. However, both the blockade of GABAA receptors and the increase of the pairings toward 600 stimuli could induce LTD. The latter situation could have arisen in the present study because we used a total of 4,800 electrical stimuli (8 Hz for 10 min), which would give rise to about 3,180 conjunctive PF–CF activations (CF stimulation at ,5.3 Hz for 10 min) for inducing LTD in alert animals. We also confirmed conservation of the temporal order between the climbing and parallel fiber activations during the 8-Hz stimulation. Furthermore, the SS mean frequency doubled, and the evoked CS frequency reached 5.3 Hz compared to 0.69 Hz in the control condition. This finding corresponds to the basic requirement of the classical LTD view [52]. However, we note that a strong activation of the PF alone is sufficient to produce, in vitro, a form of LTD that is sustained by a Ca2+ transient [43]; nevertheless, the existence of these forms of plasticity in alert animals remains to be shown. It is difficult to compare an LFP recorded in alert animals to an excitatory postsynaptic potential recorded in a slice; the LFP amplitude is greatly dependent on the number of synchronized neuronal PLoS ONE | www.plosone.org 10 April 2012 | Volume 7 | Issue 4 | e36184