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Electronic Letters to:
- Cellular:
Hua Hu, Koen Vervaeke, and Johan F. Storm- M-Channels (Kv7/KCNQ Channels) That Regulate Synaptic Integration, Excitability, and Spike Pattern of CA1 Pyramidal Cells Are Located in the Perisomatic Region
J. Neurosci. 2007; 27: 1853-1867[Abstract] [Full text] [PDF]
eLetters: Submit a response to this article
Electronic letters published:
Control of somatic and dendritic excitability by Kv7 channels
- Yoel Yaari (4 March 2007)
- Johan F. Storm, Hua Hu, Koen Vervaeke (3 July 2007)
Control of somatic and dendritic excitability by Kv7 channels 4 March 2007 
Yoel Yaari,
Pofessor of Physiology and Neurobiology
Department of Physiology, Hebrew University School of Medicine, P.O. Box 12272, Jerusalem 91120Send letter to journal:
Re: Control of somatic and dendritic excitability by Kv7 channels
yaari{at}md.huji.ac.il Yoel Yaari
The paper by Hu et al reexamines the somato-dendritic distribution of functional Kv7 channels in CA1 pyramidal cells. By focally applying Kv7 channel modifiers to the perisomatic region or to the apical dendrites, the authors reproduce our finding, that proximal, but not apical dendritic, Kv7 channels control somatic excitability and spike output (Yue and Yaari, 2006). However, the effects we reported are strikingly more dramatic than those reported by Hu et al. In our hands linopirdine and XE991 markedly facilitated the spike afterdepolarization and converted single spikes to bursts of 3-8 spikes (Yue et al., 2004, 2006), whereas Hu et al. demonstrate only modest increases in spike discharge without a change in pattern (Fig. 3AB). One likely explanation for these differences is that Hu et al. conducted their experiments at resting potentials more negative than the ones we have used. Indeed, hyperpolarizing the neurons by 5-10 mV reduced the effects of linopirdine (Fig. 3 in Yue and Yaari, 2004). It is also very likely that differences in methodology (e.g., sharp microelectrode recording versus patch-clamping associated with cell dialysis), are responsible for Kv7 channels playing a more critical role in controlling spike output in our experiments than in those of Hu et al. This important issue requires further study.
We have also reported that apical dendritic Kv7 channels modulate local calcium spikes and associated bursting (Yue and Yaari, 2006). Hu et al. could not replicate these findings and dismiss them with the pretext that our application technique (puffing drugs on the slice surface) is so crude, that drugs applied on the distal apical dendrites diffuse to the perisomatic region. However, our control experiments showed that distal drug applications do not affect somatic excitability, whereas subsequent perisomatic applications produced dramatic effects (Figs. 1-3 in Yue and Yaari, 2006). There are several possible reasons for the apparent discrepancy between the two studies. Perhaps Kv7 channels in the apical dendrites are so dispersed, that blocking or opening them in a limited zone of the main dendritic shaft, as done by Hu et al., has no impact on overall dendritic excitability. Furthermore, the use of more hyperpolarized neurons by Hu et al. may also reduce the impact of Kv7 channels on dendritic excitability, as occurs at the soma. Finally, as mentioned above, differences in the recording techniques may contribute tothis discrepancy.
Yue C, Yaari Y (2004) KCNQ/M channels control spike afterdepolarization and burst generation in hippocampal neurons. J Neurosci 24: 4614-4624.
Yue C, Yaari Y (2004) Axo-somatic and apical dendritic Kv7/M channels differentially regulate the intrinsic excitability of adult rat CA1 pyramidal cells. J Neurophysiol 95: 3480-3495.
Re: Control of somatic and dendritic excitability by Kv7 channels 3 July 2007 
Johan F. Storm,
Professor
Department of Physiology, CMBN, University of Oslo, N-0317, Norway,
Hua Hu, Koen VervaekeSend letter to journal:
Re: Re: Control of somatic and dendritic excitability by Kv7 channels
j.f.storm{at}medisin.uio.no Johan F. Storm, et al.
We thank Yoel Yaari for his comments on our paper (Hu, Vervaeke, and Storm, J. Neurosci. 2007; 27: 1853-1867) in his e-letter 4 March 2007, which gives us an opportunity to explain our views more in detail.
Yaari’s first point concerns the effects of focally applying blockers or openers of Kv7/ KCNQ/M channels to different parts of CA1 pyramidal cells. Our results support the conclusion that this channel type is concentrated in the perisomatic region of these cells, as we saw no significant effects of applications in the apical dendrites. Thus, we agree with Yue and Yaari (2006) about the main point that persisomatic Kv7/ KCNQ/M channels control somatic excitability and spike output. But Yaari is concerned that the effects reported by them “are strikingly more dramatic than those reportedby Hu et al.”, since in their hands the Kv7 channel blockers linopirdine and XE991 “markedly facilitated the spike afterdepolarization and converted single spikes to bursts of 3-8 spikes (Yue et al. 2204, 2006), whereas Hu et al. (2007) demonstrate only modest increases in spike discharge without a change in pattern (Fig. 3AB).” It may well be that differences in “resting” potential and recording method (sharp microelectrode versus patch pipettes) may contribute to the differences. However, another major difference in methodology makes it hard to directly compare these results: whereas Yue and Yaari (2006) always evoked spikes by abrupt, square depolarizing steps, and often evoked single spikes by very brief pulses (see their Figures 2-10), Hu et al. (2007) evoked spike trains by very slow, depolarizing ramps whenever Kv7 channel blockers were applied (Figs 3-4). Because adaptive mechanisms make a slow, gradual ramp depolarization less likely to trigger an initial burst than an abrupt depolarizing step, the lack of bursting in our data may be partly due to this difference in stimulus shape. In agreement with this idea, adapted tonic firing after Kv7 channel blockade can be seen after the initial burst in the data of Yue and Yaari (2006) (e.g. Fig. 3AC) – resembling the adapted, tonic firing in response to our slow ramps.
Yaari’s second point concerns the contrasting effects of dendritic applications of Kv7 channel modifiers. Whereas the results of Yue and Yaari (2006) suggest that dendritic Kv7 channels modulate local calcium spikes and associated bursting, we could not replicate these findings. We agree with Yaari that there are several possible reasons for the apparent discrepancy between the two studies, and it remains possible that Kv7 channels in the apical dendrites may be so dispersed, that blocking or opening them in a limited zone of the dendrites has no detectable impact on dendritic excitability. However, when we tested whether our local dendritic application was too limited, by comparing bath-application of M- channel blockers (XE991 and linopirdine) or a M-channel opener (retigabine) (which affect all Kv7/KCNQ/M channels on the cell surface, not only “in a limited zone of the main dendritic shaft”) with local perisomastic applications, we found no significant difference in sodium spikes or on the more persistent dendritic depolarizations caused by long train of high-frequency excitatory synaptic input recorded as dendritic field EPSPs (see Figs 3, 5, 7 E, and 8 in Hu et al., 2002). This indicates that even a complete block of these channels in all parts of the dendrites did not appreciably change local effects of dendritic excitatory synaptic input, nor somatic spiking. Furthermore, because the latter test employed extracellar field potential recording, possible effects of dialysis by whole-cell recording (a concern discussed both in our paper (p.1864) and by Yaari) was eliminated in this case.
Obviously, every type of experiment has limitations, so we cannot (and did not) rule out that there might still be a low density of Kv7 channels in the distal apical dendrites if CA1 pyramidal cells – a density below the detection limit of our methods. Nor did we dismiss the results that Yue and Yaari (2006) obtained by puffing drugs on the slice surface. In fact, their results agree well with ours for nearly all cases, except for calcium spikes evoked under the rather artificial conditions when both A-channels and sodium channels had been fully blocked by high doses of 4- AP and TTX (Fig. 9 of Yue and Yaari). However, we mentioned the possibility that the different drug application methods may have contributed to the differences in our results at this point. Thus, although Yue and Yaari showed that perisomatic drug applications but not distal applications affected somatic excitability in their Figs 1-3, this type of local specificity was apparently not shown in the very same experiments in which calcium spikes were tested in the presence of 4-AP and TTX (their Fig. 9), and no time course of any of the effects were shown. We think this may be relevant, for several reasons. (1) The slices were 400 um thick (p.3481 of Yue and Yaari), the depth of the impaled cell somata was variable, and the depth of the dendrites 200-300 um away from the soma (p.3481 of Yue and Yaari) is likely to be even more variable, because the angle between the main dendritic axis and the slice surface was not controlled. (2) The effects of drop applications on top of the slice was quite slow (“attained maximum within 8–10 min”, p.3481 of Yue and Yaari), allowing considerable diffusion in various directions. (3) Thus, the drug, which was seen to cover “an area about 50 uM in diameter” on top of the slice (p.3481 of Yue and Yaari) is likely to reach a considerably larger area deeper down in the slice after 8-10 minutes and more, and may later reach (albeit at lower concentration) the soma, which is only 200-300 uM away. (4) We know from experience that the effects of 4 -AP, which is quite lipophilic and slow to equilibrate within the slice, can build up over long times, even during bath application. (5) Because these are all time-dependent processes that also may vary considerably from one experiment to the next, we think it is difficult - in the absence of time plots and control tests for each experiment - to rule out the possibility that the effects attributed by Yue and Yaari to dendritic M- channel blockade, might be partly due to some of these other factors.
For these reasons, at least until new data emerge, we see no strong reason to revise our main conclusion: that functional M-channels appear to be concentrated in the perisomatic region of CA1 pyramidal neurons, with little or no impact of this channel type in the distal apical dendrites.
Reference List
Hu, H., K. Vervaeke, and J. F. Storm. "M-channels (Kv7/KCNQ channels) that regulate synaptic integration, excitability, and spike pattern of CA1 pyramidal cells are located in the perisomatic region." J.Neurosci. 27.8 (2007): 1853-67.
Yue, C. and Y. Yaari. "Axo-somatic and apical dendritic Kv7/M channels differentially regulate the intrinsic excitability of adult rat CA1 pyramidal cells." J.Neurophysiol. 95.6 (2006): 3480-95.
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