| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Letters to the Editor
Department of Neuroscience University of New Mexico School of Medicine Albuquerque, NM 87131 University of Alabama Department of Neurobiology Birmingham, AL 35294-0021
To the Editor: Over the past few years the investigation of intracellular Ca2+ release driven by synaptic input has become a surprisingly competitive area of research. We would call to your reader's notice, the following, from Zhou and Ross (2002
, p 1803), which was recently brought to our attention:
"Until our recent experiments (Nakamura et al. 1999), synaptically activated Ca2+ waves were not observed even though postsynaptic [Ca2+]i changes in CA1 pyramidal neurons have been measured for more than 10 yr (Regehr et al. 1989) and waves have been described often in other preparations for almost as long (e.g., Lechleiter et al. 1991). Jaffe and Brown (1994) observed waves in pyramidal cells following focal application of t-ACPD, but their observations were never followed up. Pozzo-Miller et al. (1996) detected Ca2+ release from intracellular stores in CA3 pyramidal neurons but found that the stores were ‘either insufficiently filled to provide appreciable Ca2+ increases, or are inaccessible to triggering events at the synapse’."
The underlined excerpt, from the discussion section of Pozzo-Miller et al. (1996), is a quote out of context that misstates our work. In fact, the title of our paper was "Ca2+ Release from Intracellular Stores Induced by Afferent Stimulation of CA3 Pyramidal Neurons in Hippocampal Slices," indicating that Ca2+ release was evoked by synaptic stimulation. Our paper showed, among other things, large, delayed, propagating Ca2+ elevations in response to synaptic stimulation in the presence of ionotropic glutamate receptor blockers that were generated by release from intracellular stores, all characteristics of later work (e.g., Larkum et al. 2003; Nakamura et al. 1999). We had purposely not used the term "Ca-wave" to avoid confusion with its prior use to describe inter-cellular Ca2+ spread in astrocytes. A Ca2+-store preloading protocol greatly increased the magnitude of the Ca2+ responses in neurons of acute slices, but it was not necessary in all cases, as shown in Fig. 5 of Pozzo-Miller et al. (1996). It is worth noting that a similar inter-trial Ca2+ loading protocol has also been employed in order "to make it easier to evoke Ca2+ release in the subsequent trial" (Larkum et al. 2003). In this case, such a procedure does not seem to have obviated the results.
With the spirit of avoiding confusion in an already controversial area, we respectfully point out to you that the present case is not an error of omission, e.g., a non-citation of preceding work, but rather a misinterpretation of our conclusions.
REFERENCES
Larkum ME, Watanabe S, Nakamura T, Lasser-Ross N, and Ross WN. Synaptically activated Ca2+ waves in layer 2/3 and layer 5 rat neocortical pyramidal neurons. J Physiol 549: 471-488, 2003.
Nakamura T, Barbara JG, Nakamura K, and Ross WN. Synergistic release of Ca2+ from IP3-sensitive stores evoked by synaptic activation of mGluRs paired with backpropagating action potentials. Neuron 24: 727-737, 1999.[ISI][Medline]
Pozzo-Miller LD, Petrozzino JJ, Golari G, and Connor JA. Ca2+ release from intracellular stores induced by afferent stimulation of CA3 pyramidal neurons in hippocampal slices. J Neurophysiol 76: 554-562, 1996.
Zhou S and Ross WN. Threshold conditions for synaptically evoking Ca2+ waves in hippocampal pyramidal neurons. J Neurophysiol 87: 1799-1804, 2002.
Department of Physiology New York Medical College Valhalla, NY 10595
To the Editor: John Connor and Lucas Pozzo-Miller make several points. First, they claim that we misinterpreted and took out of context an excerpt from their paper (Pozzo-Miller et al. 1996). Second, they feel that the statement in Zhou and Ross (2002) that "until our recent experiments (Nakamura et al. 1999) synaptically activated Ca2+ waves in neurons were not observed" is unfair to their paper, which they claim presented evidence for Ca2+ waves even though they were not called by that name. Third, by claiming that we used a "similar inter-trial Ca2+ loading protocol" in experiments described in our recent paper (Larkum et al. 2003), they are suggesting that we needed to achieve similar artificial conditions (as they did in their experiments) in order to see repetitive Ca2+ waves in our experiments. I have a different interpretation.
First, I repeat the acknowledgment that we made in our first paper on this subject (Nakamura et al. 1999) that their paper showed that "synaptically activated release contributes to the rise in postsynaptic [Ca2+]i in pyramidal neurons." They were the ones who noted that in their experiments release occurred under non-standard conditions, a conclusion they reemphasized elsewhere (Connor et al. 1999). In response, a recent paper (Kapur et al. 2001) made a point of showing "that release can occur in CA3 neurons under more physiological conditions..." A more complete excerpt from the Pozzo-Miller et al. (1996) paper follows. The reader can judge if we were unfair.
"The results demonstrate that synaptically addressed Ca2+ stores exist in CA3 neurons but, in general, are either insufficiently filled to provide appreciable Ca2+ increases, or are inaccessible to triggering events at the synapse. After intense Ca2+ loading, a condition which may or may not occur in vivo, a significant amount of Ca2+ can be released from these intracellular stores by high intensity presynaptic input."
The second issue is whether they described waves. Their letter claims that they showed that in conditions when ionotropic glutamate receptors are blocked "propagating Ca2+ elevations... were generated..." I do not see the evidence for this statement. At best, Figs. 1C and 5B5 of their paper show [Ca2+]i increases under ionotropic blockade that appear later at some locations than others. No mention of this delay appears in the text or figure legends. There is no suggestion of propagation in any form in the paper; it does not appear in the Abstract, Introduction, or Discussion.
Third, in our experiments analyzing synaptically activated Ca2+ release in hippocampal pyramidal neurons (Nakamura et al. 1999, 2002; Zhou and Ross 2002) we never used a loading protocol, yet we were able to stimulate Ca2+ release repeatedly in many cells. Furthermore, we did not use intense stimulation. There are many records in those papers where the summating EPSPs did not reach threshold for action potential generation. There was some variability in the responses. The variability was greater in cortical pyramidal cells (Larkum et al. 2003) so we used a loading protocol in some experiments to try to eliminate this problem. This procedure (action potential generation at 5 Hz) was chosen to be more physiological than the sustained voltage clamp steps to +40 mV used by Pozzo-Miller et al., although we made no mention of this point in the paper.
Finally, the quoted paragraph from our paper appeared in a part of the Discussion entitled "Why synaptically activated waves have been difficult to observe." This question is of interest since the different forms of Ca2+ signaling (entry through ligand-gated channels, entry through voltage-gated channels, release from internal stores) have different spatial, temporal and amplitude profiles in the dendrites. Different ways of stimulation or observation could bias the perception of the relative importance of these pathways. We had some ideas on this subject that we thought were worth discussing.
REFERENCES
Connor JA, Petrozzino J, Pozzo-Miller LD, and Otani S. Calcium signals in long-term potentiation and long-term depression. Can J Physiol Pharmacol 77: 722-734, 1999.[Medline]
Kapur A, Yeckel M, and Johnston D. Hippocampal mossy fiber activity evokes Ca2+ release in CA3 pyramidal neurons via a metabotropic glutamate receptor pathway. Neuroscience 107: 59-69, 2001.[ISI][Medline]
Larkum ME, Watanabe S, Nakamura T, Lasser-Ross N, and Ross WN. Synaptically activated Ca2+ waves in layer 2/3 and layer 5 rat neocortical pyramidal neurons. J Physiol 549: 471-488, 2003.
Pozzo-Miller LD, Petrozzino JJ, Golari G, and Connor JA. Ca2+ release from intracellular stores induced by afferent stimulation of CA3 pyramidal neurons in hippocampal slices. J Neurophysiol 76: 554-562, 1996.
Nakamura T, Barbara JG, Nakamura K, and Ross WN. Synergistic release of Ca2+ from IP3-sensitive stores evoked by synaptic activation of mGluRs paired with backpropagating action potentials. Neuron 24: 727-737, 1999.[ISI][Medline]
Nakamura T, Lasser-Ross N, Nakamura K, and Ross WN. Spatial segregation and interaction of calcium signalling mechanisms in rat hippocampal CA1 pyramidal neurons. J Physiol 543: 465-480, 2002.
Zhou S and Ross WN. Threshold conditions for synaptically evoking Ca2+ waves in hippocampal pyramidal neurons. J Neurophysiol 87: 1799-1804, 2002.
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Visit Other APS Journals Online |
