The other cells only displayed a mild increase in the calcium transient amplitude, probably due to insufficient penetration of the toxin in the slice. As anticipated,

the voltage-dependence of the spike number and of the peak CFCT amplitude in the three cells responding to the toxin was greatly reduced compared to DHPG (Figures 8D and 3F) (22% decrease of CFCT amplitude and 36% decrease in number of spike between −59 ± 0.5 mV and −81 ± 0.5 mV in Phrixotoxin; 80% decrease of CFCT amplitude and 94% decrease in number of spike between −62 ± 0.1 mV and −80 ± 0.3 mV in DHPG). Hence, activation of low-threshold Kv4 channels limits the initiation of high-threshold spike in proximal dendrites Androgen Receptor pathway Antagonists during CF-evoked dendritic

EPSPs. Increased inactivation of Kv4 channels by activity-dependent signals (depolarization and mGluR1 activation) can fully account for the observed dendritic spike unlocking. Because PF stimulations can both activate mGluR1 receptors PI3K inhibitor and depolarize the distal dendrites, we tested whether PF stimulations similar to the ones used for LTD induction protocols could produce spike unlocking. As previously shown (Hildebrand et al., 2009), a burst of ten PF beam stimulations at 100–200 Hz produced a calcium transient mediated by T-type voltage-gated calcium channels in a circumscribed region of the dendrites. Pairing of the PF stimulation with the CF stimulation (5–20 ms after PF offset) at depolarized potentials (spontaneous firing) increased the CFCT measured within the PF responsive region from 0.071 ± 0.006 ΔG/R to 0.094 ± Idoxuridine 0.005 ΔG/R (p = 0.016, paired t test, n = 5) (Figures 8B and 8C). This potentiation resulted in a calcium flux shift from subthreshold regime (0.087 ± 0.013 ΔG/R·ms−1)

to suprathreshold regime (0.211 ± 0.021 ΔG/R·ms−1) in all the cells (n = 5) (Figures 8C and 8D, circles). Multiple spikes were never observed. This milder effect can be explained by the persistence of Kv4 channels in the dendrites outside of the stimulated PF beam. The spatial restriction of the effect was further tested by PF stimulation of extremely distal spiny branchlets (soma distance above 150 μm). At these locations, the sensitivity to somatic depolarization appeared reduced (hyperpolarized 0.021 ± 0.008 ΔG/R, depolarized 0.031 ± 0.005 ΔG/R) and the CFCT was only mildly potentiated by PF pairing (0.039 ± 0.004 ΔG/R n = 4), remaining well below spike threshold (control calcium flux 0.017 ± 0.003 ΔG/R·ms−1; paired calcium flux 0.028 ± 0.009 ΔG/R·ms−1) (Figures 8D and 8E, triangles). Hence, focal PF beam stimulations can unlock local nonpropagated CF induced P/Q spikes but only if the PF input is not too remote from the proximal initiation sites in the smooth dendrites. Widespread PF input over the whole dendritic tree would probably be necessary to achieve global unlocking.