Endocannabinoid dynamics gate spike-timing dependent depression and potentiation

Thank you for submitting your work entitled "Spike-timing-dependent dynamics of 2-arachidonoylglycerol gates endocannabinoid-mediated LTP and LTD" for consideration by eLife. Your article has been reviewed by three peer reviewers, one of whom is a member of our Board of Reviewing Editors. The evaluation has been overseen by the Reviewing Editor and Eve Marder as the Senior Editor.

The reviewers have discussed the reviews with one another and the Reviewing Editor has drafted this decision to help you prepare a revised submission.

Summary:

This paper proposes a model involving pre and postsynaptic signaling components involved in corticostriatal synaptic plasticity. Key components of the model include presynatpic endocannabinoid signaling with three thresholds for different forms of plasticity, and a signaling network depending on calcium for the postsynaptic network. The paper is distinctive in presenting several high-level tests of the signaling model, including some non-obvious predictions. The authors present MAGL activity as a key controller of synaptic plasticity, and suggest that endocannabinoid receptors may be important in bidirectional regulation of plasticity.

Essential revisions:

1) The reviewers appreciated the experimental and theory combination of approaches to study plasticity.

2) The model assumes a number of model free parameters such as sharp thresholds for plasticity. The authors should test model outcomes of shallow thresholds to see if the results still hold, and also conduct parameter sensitivity analysis to examine how sensitive these parameters are.

3) The predictions should extend to a somewhat broader range of stimulus cases, such as modification of pairing frequencies and number.

4) All reviewers had questions on the details of the model pathways and role of sub-parts of the overall model. The authors should clarify this, with special attention to stating which pathways are really needed for which outcome of the model.

Reviewer #1:

1) A key assumption in the model is the presence of three thresholds for LTD start, LTD stop, and LTP start, for the endocannabinoid system. The key step here (thresholding) is not implemented biophysically or biochemically, but by a mathematical threshold. I think this weakens their case for a mechanistic account of STDP. The authors discuss this and have a schematic in Figure 9—figure supplement 1, but I don't feel that the proposed sharp thresholds are physiological. I would have liked to see a chemical implementation of the thresholds. Specifically, I am concerned that the presence of a shallow (more chemical-like) turn on rather than a sharp all-or-none threshold may invalidate the results. The authors should address this.

2) A related point from Figure 3 C1: The threshold positioning and the extra bump by CB1R at the start is quite finicky. I am dubious about the dependence on such fine-tuning. Even a small shift of either the threshold or the response would invalidate the prediction that such a mechanism could account for the properties of eCB-tLTP. Here, in fact, a shallower activation function to replace the threshold might prove to be more robust, but less precise.

3) Figure 4 is impressive in its match to experiments. I wonder if it is possible for the authors to achieve a similar match for Figure 6,Figure 7,Figure 8, which also test a series of model predictions. All look good but the prediction is just a single time-point. In all cases it would be nice if the simulations went the extra step to match the experimental time-course, rather than just predict amplitude of LTP or LTD.

4) The authors should comment on what stochasticity would do to their analysis. I am concerned that this would further weaken any analysis depending on sharp thresholds.

5) The CaMKII activity seems to be a bit of an orphan in the study. It turns on for sufficient pairings for pre-post pairings, but how does this impact the plasticity? I was not able to clearly see where this happened.

6) The authors get CaMKII to turn on (Figure 2) but I don't see that the LTD stimulus (or any other) gets it to turn off. This seems incomplete.

7) ModelDB does not seem to have this model. It isn't really possible to assess the implementation without it. The authors should present the model and any simulation files needed to generate the figures, as supplementary material.

Reviewer #2:

Cui et al. use a phenomenological, but quantitative model to increase the understanding of activity patterns leading to LTP/LTD. The model predictions agree with experiments. The main contribution is to illustrate how 2AG can control both LTP and LTD.

There are things which can strengthen the study, and make the model more transparent. The latter issue is important as others then can further improve it.

1) The experimental paradigm builds on repetition with 1 Hz pre- and postsynaptic pairings in different orders/delays. As the model mechanisms for 'many-pairing' LTP builds on successive activation of CamKII while the 'many-pairing' LTD builds on 2AG production with presynaptic effects. For the latter, postsynaptic depletion of ER_Ca and presynaptic receptor desensitization contribute, thus it would be interesting to see what is predicted if 1 Hz pairing frequency is modified to e.g. 2 or 0.1 Hz. From Figure 3C the ER depletion seems significant after only 20 pairings, is that realistic? Also is this depletion needed for the model to work?

2) It seems that the model free parameters are fitted to reproduce experimental outcome (see subheading “Parameters”), thus the model is not predicting the LTP and LTD results (since the LTP and LTD outcome is used to tune the model), but rather the model can work as a quantitative hypothesis on important subcellular mechanisms. Please specify which model parameters are considered free and their sensitivity to variations.

3) Details on the model that need to be made more transparent are, for example:

A) The activation of DAGL. A Ca dependent phosphorylation reaction assumed but is it not a more direct Ca activation of DAGL and presence of DAG sufficient to produce 2AG for example? Likely no consequences for the outcome but please clarify;

B) Which and how different Ca sources (NMDA_Ca, Ca via TRPV1R, ER_Ca, L_Ca, etc.) contribute to the total pool of Ca used to activate CamKII, DAGL, etc. (add e.g. a supplementary figure following Figure 2 or Figure 3 showing how the total Ca elevation is the sum of several sources). How is the 'unintuitive' result achieved that Ca is larger if post-pre stimulation is used compared to pre-post? If only NMDA_Ca is considered pre-post should give rise to more Ca influx into the cell as compared to post-pre;

C) Please plot separately the W_pre and W_pro to see how it compares to W_total plotted in several figures;

D) In paragraph three, subheading “Postsynaptic element” the postsynaptic model from Graupner and Brunel is adjusted for MSNs and it is assumed that PKA is indirectly activated by Ca via PP2A, motivate this or maybe remove the Ca dependency of PKA as it is hard to see how the AC5-PKA reactions is effectively stimulated by Ca in MSNs, even in the presence of DARPP75 disinhibition via PP2A). That CaMKII is activated following a sufficient number of pairings in MSNs is reasonable, but it probably happens in a slightly different way as in hippocampus.

Essential revisions:

1) The reviewers appreciated the experimental and theory combination of approaches to study plasticity.2) The model assumes a number of model free parameters such as sharp thresholds for plasticity. The authors should test model outcomes of shallow thresholds to see if the results still hold, and also conduct parameter sensitivity analysis to examine how sensitive these parameters are.

The revised manuscript includes two new sets of results to account for this point.

1) We implemented an alternative version of the model, where the sharp plasticity thresholds are replaced by a smooth function based on combinations of Hill functions. For clarity of the main text, the exact equation of this function is given in the new Supplementary file 2. In this function, the sharpness of the plasticity transition is controlled by a parameter k_S. The new Figure 4—figure supplement 1 (panel B) shows that model output is essentially conserved with this smooth threshold function, even though we did not change any of the model parameters outside the threshold function. We illustrate this for k_S =2 on Figure 4 but other values of k_S essentially lead to the same conclusion. Therefore our choice of a sharp thresholding for eCB-dependent plasticity is not crucial for the model output.

2) We now provide in new Figure 4—figure supplement 1 (panel C) a proper analysis for the sensitivity of the parameters, using standardized linear-regression coefficients (see Methods). As expected, the most sensitive parameters are those related to reactions that are known from pharmacological experiments to be indeed crucial to STDP and the dynamics of CB1R desensitization, in agreement with the importance of CB1R desensitization in the decay of eCB-LTP above 15-20 post-pre stimulations. More surprising is the sensitivity of the model to the dynamics of glutamate in the synaptic cleft. We therefore conclude that alterations of the dynamics of glutamate release and uptake could play an important role in the control of STDP at the corticostriatal synapse.

Changes made: Figure 4—figure supplement 1 shows the results with smooth threshold (panel B) and sensitivity analysis (panel C). Both results (threshold smoothness and sensitivity) are presented is a new subsection of the revised version (paragraphs 2-4, subheading “The mathematical model accounts for bidirectional eCB- and NMDAR- mediated STDP”). The definition of the smooth threshold function is given in the new Supplementary file 2. A new subsection of the Methods section explains the methodology used for sensitivity analysis ("Sensitivity analysis"). We also added new text on the choice of sharp thresholds in Methods (subheading “Synaptic plasticity and synaptic weights”).

3) The predictions should extend to a somewhat broader range of stimulus cases, such as modification of pairing frequencies and number.

Here too, we have taken this point into account with two new sets of results:

1) The revised version features a new set of modeling and experimental results obtained by varying both the spike timing and the pairing frequency (from 0.1 Hz to 4 Hz) with 10 pairings. Those results (experimental and modeling) are shown in the new Figure 5. The main prediction of the model are that, when frequency increases above 1Hz, the tLTP triggered by post-pre stimulations (ΔtSTDP < 0) persists and is even observed for an increasingly large ΔtSTDP range. Unexpectedly, the model also predicts the expression of another tLTP, triggered by 10 pre-post stimulations (ΔtSTDP > 0) for frequency larger than 2Hz. We present an entire new set of experiments at 0.1, 1.0, 2.5 and 4.0 Hz that validates these predictions (Figure 5B-E). In terms of pharmacology, we also show in the supplementary figure of Figure 5 that again, model and experiments agree about the fact that the tLTP at high frequencies (4 Hz) becomes of mixed origin, both eCB- and NMDAR-dependent.

2) Regarding the modification of pairing numbers, we now show in the panel A of the new Figure supplement of Figure 4, what is the output of the model when more than 100 pairings are used. The behavior observed with 100 pairings is conserved for larger pairings numbers.

Changes made: Figure 4—figure supplement 1 shows the model output with > 100 pairings (panel A, presented in the Results section, subheading “The mathematical model accounts for bidirectional eCB- and NMDAR- mediated STDP”). The frequency-dependence results (model and experiments) are shown in a new figure (Figure 5) and commented in a new subsection of the Results section ("Frequency dependence of eCB-tLTP") and discussed in the new Discussion section (paragraphs five and six).

4) All reviewers had questions on the details of the model pathways and role of sub-parts of the overall model. The authors should clarify this, with special attention to stating which pathways are really needed for which outcome of the model.

We give a detailed reply to those points in our response to the reviewers below. But to summarize:

1) We simplified the model at the level of DAG-Lipase activation. In the original version, DAG-Lipase activation by calcium occurred via DAG-Lipase phosphorylation catalyzed by a kinase (and the reverse phosphatase), that was itself calcium-activated. In the revised version, we replaced this unnecessary complex scheme by direct activation of DAG-Lipase by calcium (equation 12,). Note that all the model figures in the article have been revised to correspond to this new, simplified model.

2) We kept TRPV1 in the model because we have shown in a previous paper (Cui et al., 2015) that blocking TRPV1 inhibits eCB-dependent LTP. However, from a modeling perspective, the TRPV1 current can be ignored. The resulting model would essentially yield the same output as obtained with the standard model, as long as absence of TRPV1 is compensated for by a slight increase of NMDAR or VSCC conductances. We have added new text regarding this point in the Methods section (subheading “Postsynaptic element”).

3) The importance of each separate pathway can now be better judged through the sensitivity analysis of the model parameters featured in the revised version (see our reply to point 1. above). In particular, our interpretation of the sensitivity analysis in the revised version clearly points to the most important elements of pathways (paragraph four, subheading “The mathematical model accounts for bidirectional eCB- and NMDAR- mediated STDP”).

Reviewer #1:

1) A key assumption in the model is the presence of three thresholds for LTD start, LTD stop, and LTP start, for the endocannabinoid system. The key step here (thresholding) is not implemented biophysically or biochemically, but by a mathematical threshold. I think this weakens their case for a mechanistic account of STDP. The authors discuss this and have a schematic in Figure 9—figure supplement 1, but I don't feel that the proposed sharp thresholds are physiological. I would have liked to see a chemical implementation of the thresholds. Specifically, I am concerned that the presence of a shallow (more chemical-like) turn on rather than a sharp all-or-none threshold may invalidate the results. The authors should address this.

2) A related point from Figure 3 C1: The threshold positioning and the extra bump by CB1R at the start is quite finicky. I am dubious about the dependence on such fine-tuning. Even a small shift of either the threshold or the response would invalidate the prediction that such a mechanism could account for the properties of eCB-tLTP. Here, in fact, a shallower activation function to replace the threshold might prove to be more robust, but less precise.

In the revised version, we implemented an alternative version of the model, where the sharp plasticity thresholds are replaced by a smooth function based on combinations of Hill functions. For clarity of the main text, the exact equation of this function is given in the new Supplementary file 2. In this function, the sharpness of the plasticity transition is controlled by a parameter k_S. New Figure 4—figure supplement 1 (panel B) shows that model output is essentially conserved with this smooth threshold function, even though we did not change any of the model parameters outside the threshold function. We illustrate this for k_S =2 on Figure 4 but other values of k_S essentially lead to the same conclusion. Therefore our choice of a sharp thresholding for eCB-dependent plasticity is not crucial for the model output.

Changes made: Figure 4—figure supplement 1 shows the results with the smooth threshold (panel B). These results are presented is a new subsection of the revised version (paragraph 3, subheading “The mathematical model accounts for bidirectional eCB- and NMDAR- mediated STDP”). The definition of the smooth threshold function is given in the new Supplementary file 2. We also added new text on the choice of sharp thresholds in Methods (subheading “Synaptic plasticity and synaptic weights”).

3) Figure 4 is impressive in its match to experiments. I wonder if it is possible for the authors to achieve a similar match for Figure 6,Figure 7,Figure 8, which also test a series of model predictions. All look good but the prediction is just a single time-point. In all cases it would be nice if the simulations went the extra step to match the experimental time-course, rather than just predict amplitude of LTP or LTD.

We thank the reviewer for her/his nice comment on the match of Figure 4 with experiments. As pointed out by the reviewer, in its current state, the model does a very good job in predicting the final (long-term) amplitude of STDP. But it cannot reliably be used to predict the time course of the synaptic weight after the pairings because our knowledge of many of the molecular processes involved in plasticity maintenance and expression downstream of CaMKII and CB1R are still not known with enough detail. For the NMDAR-CaMKII part, the signaling pathway from glutamate to CaMKII activation is rather well characterized but the molecular mechanisms leading from CaMKII activation to changes of the synaptic weights are still unclear, especially in MSNs (direct AMPAR activation, alterations of AMPAR or NMDAR trafficking, other modifications of the anchoring/scaffolding properties of the PSD?). Likewise, for the mGluR-eCB-CB1R pathway it is not clear what are the molecular targets that are set in motion following CB1R-triggered activation of PKA or/and inhibition of VGCCs and that lead to changes of the presynaptic weight. In these conditions, it would seem rather pointless to propose a model to predict the temporal evolution of the synaptic weights. By contrast, our results strongly suggest the validity of the assumption that the steady-state (long-term) values of the pre- and post-synaptic weight is proportional to the amount of activated CB1R and CaMKII, respectively, so that predicting the (long-term) amplitude of STDP is still possible.

4) The authors should comment on what stochasticity would do to their analysis. I am concerned that this would further weaken any analysis depending on sharp thresholds.

We thank the reviewer for this very interesting comment. Actually, we have studied the issue of stochasticity on STDP protocols. As a first approach, we have studied and applied STDP protocols where spike timing, i.e. the time between pre and post pairings is not deterministic/Dirac distributed but a random variable. With increasing variance of these random spike timings and at 1 Hz, the model shows a striking differential behavior: whereas NMDAR-LTP is very sensitive and disappears quickly when variance increases, eCB-dependent plasticity (eCB-LTD and eCB-LTP) remains expressed for a much larger amount of noise. This results is however also frequency-dependent since for larger frequencies, NMDAR-LTP becomes much more resilient. Importantly, here again, the predictions of our model have been validated by experiments, which confirm the major trends of the model. Thus, we agree that stochasticity counter STDP expression in the model, but more importantly: i) noise differentially affects eCB-dependent and NMDAR-dependent plasticity and ii) this effect is also observed experimentally. We did not include these results in the present version of this manuscript because we feel the revised version already contains a large amount of information so adding the study of stochasticity on top of it would only go against the clarity of the paper. In addition, to be fully presented, these results (model + experiments) would require a full paper.

5) The CaMKII activity seems to be a bit of an orphan in the study. It turns on for sufficient pairings for pre-post pairings, but how does this impact the plasticity? I was not able to clearly see where this happened.

6) The authors get CaMKII to turn on (Figure 2) but I don't see that the LTD stimulus (or any other) gets it to turn off. This seems incomplete.

As explained above, the molecular mechanisms, leading from CaMKII activation to changes of the synaptic weights are still unclear. In these conditions, we adopted the hypothesis, already used by Graupner and Brunel, 2007 and others before, that the long-term (steady state) increase of the post-synaptic weight is proportional to the fraction CaMKII that is activated (phosphorylated). Considering the good match with our experiments, this seems a posteriori a valid hypothesis. Regarding bidirectionality, since our model for the NDMAR pathway is based on the model by Graupner and Brunel, 2007, it has inherited its potentiality to implement both LTP and LTD, depending on the stimulation. However, we have been undertaking experimental studies of STDP of the corticostriatal synapses in slices for some time now, but we never observed notable NMDAR-dependent LTD (at least in "control" conditions, i.e. in the absence of GABA or transporter blockers). This is the reason why we do not mention NMDAR-dependent LTD in the manuscript.

Changes made: the Methods section now contains a clearer and longer statement about how CaMKII was used to model post-synaptic plasticity (paragraph three, subheading “Postsynaptic element”).

7) ModelDB does not seem to have this model. It isn't really possible to assess the implementation without it. The authors should present the model and any simulation files needed to generate the figures, as supplementary material.

The model has now been uploaded in the ModelDB database (accession # 187605).

Reviewer #2:

1) The experimental paradigm builds on repetition with 1Hz pre- and postsynaptic pairings in different orders/delays. As the model mechanisms for 'many-pairing' LTP builds on successive activation of CamKII while the 'many-pairing' LTD builds on 2AG production with presynaptic effects. For the latter, postsynaptic depletion of ER_Ca and presynaptic receptor desensitization contribute, thus it would be interesting to see what is predicted if 1 Hz pairing frequency is modified to e.g. 2 or 0.1 Hz. From Figure 3C the ER depletion seems significant after only 20 pairings, is that realistic? Also is this depletion needed for the model to work?

In order to take this comment into account, the revised version now features a new set of modeling and experimental results obtained by varying both the spike timing and the pairing frequency (from 0.1 Hz to 4 Hz) with 10 pairings. Those results (experimental and modeling) are shown in the new Figure 5. The main prediction of the model are that, when frequency increases above 1Hz, the tLTP triggered by post-pre stimulations (ΔtSTDP < 0) persists and is even observed for increasingly large ΔtSTDP ranges. Unexpectedly, the model also predicts the expression of another tLTP, triggered by 10 pre-post stimulations (ΔtSTDP > 0) for frequency larger than 2Hz. We carried out an entire new set of experiments at 0.1, 1.0, 2.5 and 4.0 Hz that validates (Figure 5B-E) these predictions. In terms of pharmacology, we also show in the supplementary figure of Figure 5 that again, model and experiments agree about the fact that the tLTP at high frequencies (4 Hz) becomes of mixed origin, both eCB- and NMDAR-dependent.

Changes made: This frequency-dependence results (model and experiments) are shown in a new figure (Figure 5), commented in a new subsection of the Results section ("Frequency dependence of eCB-tLTP") and discussed in the new Discussion section (paragraph six).

2) It seems that the model free parameters are fitted to reproduce experimental outcome (see subheading “Parameters”), thus the model is not predicting the LTP and LTD results (since the LTP and LTD outcome is used to tune the model), but rather the model can work as a quantitative hypothesis on important subcellular mechanisms. Please specify which model parameters are considered free and their sensitivity to variations.

The revised manuscript provides in the new Figure 4—figure supplement 1 (panel C), a proper analysis for the sensitivity of the parameters, using standardized linear-regression coefficients (see Methods). As expected, the most sensitive parameters are those related to reactions that are known from pharmacological experiments to indeed be crucial to STDP and the dynamics of CB1R desensitization, in agreement with the importance of CB1R desensitization in the decay of eCB-LTP above 15-20 post-pre stimulations. More surprising is the sensitivity of the model to the dynamics of glutamate in the synaptic cleft. We therefore conclude that alterations of the dynamics of glutamate release and uptake could play an important role in the control of STDP at the corticostriatal synapse.

Changes made: Figure 4—figure supplement 1 shows the results of our sensitivity analysis (panel C) and is presented in a new subsection of the revised version (paragraph four, subheading “The mathematical model accounts for bidirectional eCB- and NMDAR- mediated STDP”). A new subsection of the Methods section explains the methodology used for sensitivity analysis ("Sensitivity analysis").

3) Details on the model that need to be made more transparent are, for example:

A) The activation of DAGL. A Ca dependent phosphorylation reaction assumed but is it not a more direct Ca activation of DAGL and presence of DAG sufficient to produce 2AG for example? Likely no consequences for the outcome but please clarify;

We thank the reviewer for her/his insightful comment. In the revised version, we have simplified how DAG-Lipase is activated in the model. In the original version, DAG-Lipase activation by calcium occurred via DAG-Lipase phosphorylation catalyzed by a kinase (and the reverse phosphatase), that was itself calcium-activated. In the revised version, we replaced this unnecessary complex scheme by direct activation of DAG-Lipase by calcium (equation 12). Note that we could have pushed further in this simplification effort by removing DAG-Lipase entirely and e.g. assuming a simple unimolecular reaction for 2-AG production, such as DAG → 2-AG. However, since our previous results (Cui et al., 2015) show that specific inhibition of DAG-lipase suppresses eCB-LTP, we decided to keep the explicit account of DAG-Lipase activation in the revised model.

Changes made: All the model figures in the article have been replotted using this new, simplified model.

B) Which and how different Ca sources (NMDA_Ca, Ca via TRPV1R, ER_Ca, L_Ca, etc.) contribute to the total pool of Ca used to activate CamKII, DAGL, etc. (add e.g. a supplementary figure following Figure 2 or Figure 3 showing how the total Ca elevation is the sum of several sources). How is the 'unintuitive' result achieved that Ca is larger if post-pre stimulation is used compared to pre-post? If only NMDA_Ca is considered pre-post should give rise to more Ca influx into the cell as compared to post-pre;

Author response image 1 illustrates the contribution of each calcium source to the total intracellular calcium pool during a single post-pre (left) or pre-post (right) pairing.

In both cases, the contribution of CICR is almost zero. Indeed, the effects of CICR develop progressively as IP3 accumulates during the successive pairings and is not sensible for the first stimulation. Contribution through TRPV1 is as well negligible since TRPV1 needs AEA to open and AEA starts to be large enough for this only in the next pairings. Calcium flux through VDCCs is important for post-pre pairings but also for pre-post ones, since its absence annihilates the global calcium influx during the bAP. Finally NMDAR is responsible for the majority of the calcium influx during the EPSP. We propose not to include those results in the revised manuscript, because we feel the revised version already contains a large amount of information and adding this figure would too much interrupt the reading flow of the manuscript. But we would accept to integrate it in the manuscript, in case the reviewer and editor find it necessary.

C) Please plot separately the W_pre and W_pro to see how it compares to W_total plotted in several figures;

We have modified Figure 4 (panels B and C) to take this issue into account. Actually, in the control case, the information about W_preand W_post is contained in our in silico "Knock out models" of Figure 4. W_post relies entirely on CaMKII activation, so our "NMDAR signaling knockout" model (Figure 4B1) corresponds to a situation where the contribution of W_post is absent and only W_precontributes to W_total. Likewise, because W_predepends on CB1R activation only, the "CB1R knockout model" actually reflects the case where only W_post contributes to W_total.

Changes made: In Figure 4, the title of the panels for the "NMDAR signaling KO" (Figure 4B1) and the "CB1R signaling KO" models (Figure 4C1) now read "NMDAR signaling KO (W_preonly)" and "CB1R signaling KO (W_post only)", respectively. We have also added new text to explain this (paragraph two, subheading “The mathematical model accounts for bidirectional eCB- and NMDAR- mediated STDP”).

D) In paragraph three, subheading “Postsynaptic element” the postsynaptic model from Graupner and Brunel is adjusted for MSNs and it is assumed that PKA is indirectly activated by Ca via PP2A, motivate this or maybe remove the Ca dependency of PKA as it is hard to see how the AC5-PKA reactions is effectively stimulated by Ca in MSNs, even in the presence of DARPP75 disinhibition via PP2A). That CaMKII is activated following a sufficient number of pairings in MSNs is reasonable, but it probably happens in a slightly different way as in hippocampus.

For the NMDAR-CaMKII signaling pathway, the major difference between MSNs and hippocampal neurons is indeed the enrichment of DARPP32 in MSNs in place of I-1 in hippocampus. But another specificity of MSNs is that they express large amounts of the B72 regulatory subunit of PP2A in lieu of the usual B56 one. This striatum-specific regulatory subunit provides B72-PP2A with calcium-activation properties (see e.g. Ahn et al., 2007). Therefore, calcium elevations in MSNs are expected to activate PP2A. Active PP2A can then dephosphorylate DARPP-32 on its pThr75 site, which disinhibits PKA (see e.g. the modeling studies by Lindskog et al., PLoS Comput Biol 2006 or Gutierrez-Arenas et al., PLoS Comput Biol 2014).

Changes made: We have added new text to explain this phenomenon (subheading “Synaptic plasticity and synaptic weight”).