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Dendritic Inhibitory Synapses Punch above Their Weight

2015; Cell Press; Volume: 87; Issue: 3 Linguagem: Inglês

10.1016/j.neuron.2015.07.018

1097-4199

Lea Goetz, Arnd Roth, Michael Häusser,

Nanofabrication and Lithography Techniques

Müllner et al., 2015Müllner F.E. Wierenga C.J. Bonhoeffer T. Neuron. 2015; 87 (this issue): 576-589Abstract Full Text Full Text PDF PubMed Scopus (51) Google Scholar show that single inhibitory synapses placed in the right location on the dendritic tree can exert a powerful impact on backpropagating action potentials in hippocampal pyramidal neurons by controlling local Ca2+ influx with μm and ms precision. Müllner et al., 2015Müllner F.E. Wierenga C.J. Bonhoeffer T. Neuron. 2015; 87 (this issue): 576-589Abstract Full Text Full Text PDF PubMed Scopus (51) Google Scholar show that single inhibitory synapses placed in the right location on the dendritic tree can exert a powerful impact on backpropagating action potentials in hippocampal pyramidal neurons by controlling local Ca2+ influx with μm and ms precision. In the book The Hobbit—recently developed into an epic cinematic version—the gloriously evil dragon Smaug appears to be invincible, coated by impenetrable scales that cover his entire body. And yet he has a weakness: a tiny bare patch on his chest, not covered by a scale. Bard the Bowman, tipped off by Bilbo Baggins, fires a single arrow directly into the spot at exactly the right moment and thus manages to slay the dragon. This idea of a “weak point” that renders an apparently invincible foe vulnerable to even a modest weapon runs throughout mythology and literature, most famously the proverbial “Achilles’ heel.” There are also examples to be found in biology, and in this issue of Neuron, Müllner and colleagues show that even a single inhibitory synapse can “slay” a backpropagating action potential when placed at the right location in the dendritic tree. Understanding the power of single inhibitory synapses is essential if we are to understand the intricate interplay of excitation and inhibition that is the basis of synaptic integration in vivo. Inhibitory interneurons play a crucial role in orchestrating ongoing activity patterns in neural circuits (Klausberger and Somogyi, 2008Klausberger T. Somogyi P. Science. 2008; 321: 53-57Crossref PubMed Scopus (1432) Google Scholar) and in shaping the responses of individual neurons to sensory input, and defects in inhibitory control of circuit activity can cause disease, such as epilepsy. Importantly, inhibition also regulates synaptic plasticity rules, allowing the brain to learn while ensuring that learning-related changes in neural circuits do not lead to unstable activity patterns. To understand how inhibition achieves its diverse roles, it is therefore essential to understand the impact of single inhibitory synapses on synaptic integration and neuronal output. Early theoretical work provided key insights into the importance of both location and timing for the efficacy of inhibitory synapses. Inhibition was shown to be most effective when it is placed “on-path” between an excitatory synapse and the soma (Jack et al., 1975Jack J.J.B. Noble D. Tsien R.W. Electric current flow in excitable cells. Oxford UP, Oxford1975Google Scholar). This enables logical AND-NOT operations if the inhibitory conductance is large, placed between the excitatory synapse and the soma, and if excitation and inhibition overlap in time (Koch et al., 1983Koch C. Poggio T. Torre V. Proc. Natl. Acad. Sci. USA. 1983; 80: 2799-2802Crossref PubMed Scopus (356) Google Scholar). Supporting this mechanism, it was shown experimentally that for excitatory and inhibitory synaptic inputs in pyramidal cell dendrites to interact, they must be located in close vicinity on the same dendritic branch and must also temporally overlap (Liu, 2004Liu G. Nat. Neurosci. 2004; 7: 373-379Crossref PubMed Scopus (237) Google Scholar). Thus, to harness these capabilities, excitatory and inhibitory synapses need to spatially target individual dendrites in a specific way. Indeed, across many neural circuits, different types of interneurons have been shown to innervate specific subregions of the dendritic tree of their postsynaptic counterparts (Klausberger and Somogyi, 2008Klausberger T. Somogyi P. Science. 2008; 321: 53-57Crossref PubMed Scopus (1432) Google Scholar). A notable example are the somatostatin-expressing interneurons in the neocortex, which target some of their axonal boutons to individual spine heads, where highly specific interactions between the inhibitory input and an excitatory synapse made on the same spine can occur (Chiu et al., 2013Chiu C.Q. Lur G. Morse T.M. Carnevale N.T. Ellis-Davies G.C. Higley M.J. Science. 2013; 340: 759-762Crossref PubMed Scopus (186) Google Scholar). These findings have defined the rules for interactions between excitatory and inhibitory inputs in the dendritic tree. However, they have not addressed the issue of whether single interneurons—and in particular, single inhibitory synapses—can be powerful enough to inhibit the action potential, the dominant electrical signal of the neuron that also forms its output. Paired recordings have demonstrated that single presynaptic interneurons can delay spontaneously generated action potentials in Purkinje cells (Häusser and Clark, 1997Häusser M. Clark B.A. Neuron. 1997; 19: 665-678Abstract Full Text Full Text PDF PubMed Scopus (508) Google Scholar) and can also delay spikes in pyramidal cells that are close to threshold (Miles et al., 1996Miles R. Tóth K. Gulyás A.I. Hájos N. Freund T.F. Neuron. 1996; 16: 815-823Abstract Full Text Full Text PDF PubMed Scopus (748) Google Scholar). This can act as a mechanism to synchronize multiple pyramidal cells (Cobb et al., 1995Cobb S.R. Buhl E.H. Halasy K. Paulsen O. Somogyi P. Nature. 1995; 378: 75-78Crossref PubMed Scopus (1186) Google Scholar). However, these inhibitory effects were generated by interneurons making multiple contacts with the postsynaptic cells. It appears unlikely that single inhibitory contacts will be sufficiently strong to prevent initiation of an action potential. Nevertheless, one possible “Achilles’ heel” for the action potential is to prevent its spread into the dendritic tree after it has been initiated. Voltage-gated channels in the dendritic tree allow the action potential to backpropagate actively into the dendritic tree of many types of neurons (Stuart et al., 1997Stuart G. Spruston N. Sakmann B. Häusser M. Trends Neurosci. 1997; 20: 125-131Abstract Full Text Full Text PDF PubMed Scopus (595) Google Scholar). However, since dendrites are only weakly excitable, the propagation of the action potential is decremental in most cell types studied so far. Therefore, unlike somatic action potentials, backpropagating action potentials (bAPs) are not all-or-none events but are bidirectionally modifiable by synaptic inhibition (Tsubokawa and Ross, 1996Tsubokawa H. Ross W.N. J. Neurophysiol. 1996; 76: 2896-2906PubMed Google Scholar) and excitation (Stuart and Häusser, 2001Stuart G.J. Häusser M. Nat. Neurosci. 2001; 4: 63-71Crossref PubMed Scopus (278) Google Scholar). This therefore raises the question of whether single inhibitory synapses may be sufficiently powerful to act as a brake on the bAP. This is the question addressed by Müllner et al., 2015Müllner F.E. Wierenga C.J. Bonhoeffer T. Neuron. 2015; 87 (this issue): 576-589Abstract Full Text Full Text PDF PubMed Scopus (51) Google Scholar. Instead of measuring dendritic action potential amplitude directly, the authors used dendritic Ca2+ signals as a proxy (building on the pioneering work of Tsubokawa and Ross, 1996Tsubokawa H. Ross W.N. J. Neurophysiol. 1996; 76: 2896-2906PubMed Google Scholar), since the backpropagating action potential activates voltage-gated Ca2+ channels (Stuart et al., 1997Stuart G. Spruston N. Sakmann B. Häusser M. Trends Neurosci. 1997; 20: 125-131Abstract Full Text Full Text PDF PubMed Scopus (595) Google Scholar). This imaging approach allowed them to measure the impact of a single inhibitory synapse on a backpropagating action potential and also to measure its footprint in space and time. The approach used by the authors to study the effect of single identified inhibitory synapses is technically virtuosic: they made simultaneous whole-cell recordings from a pyramidal neuron and a dendrite-targeting GFP-positive interneuron in organotypic hippocampal slice cultures prepared from GAD65-GFP mice and confirmed monosynaptic connections by eliciting unitary IPSCs in the voltage-clamped pyramidal cell, followed by anatomical identification of whether the inhibitory connection consisted of just a single synapse or multiple synapses. Next, they activated action potentials in both neurons and imaged the [Ca2+] transient evoked by the bAP in the postsynaptic neuron at and nearby the inhibitory contact. Müllner and colleagues found that activation of a single inhibitory synapse can reduce the bAP-evoked [Ca2+] transient by up to 70%. The level of inhibition depends both on properties of the inhibitory synapses as well as the [Ca2+] transient itself. The level of inhibition by a single synapse depends on the contact area, and the correlation between impact and contact area also holds for multiple nearby synapses. Interestingly, the amount of Ca2+-transient inhibition by a given inhibitory synapse crucially depends on properties of the inhibited bAP itself—in particular, its amplitude at the location of the inhibitory synapse and their relative timing. The authors measured the spatiotemporal profile of Ca2+-transient inhibition at different distances from the activated inhibitory contact and, under the assumption of an exponential decay, derive space constants for Ca2+-transient inhibition—the distance over which inhibition drops to 1/e of its peak value—of 23–25 μm in the proximal and 23–28 μm in the distal direction from the contact site, respectively. These experimental results are supported by simulations using a biophysical model of a CA1 pyramidal cell, which show that the impact of the inhibitory synapse indeed falls off exponentially along the dendrite. Going further, the authors show that the inhibitory effect is not just local but also branch specific: Ca2+-transient inhibition drops more between branches than predicted by the average electrotonic length of those branches. Finally, the authors analyzed the difference between the inhibition seen in the spine head and the dendritic shaft. While experimentally they find no difference between inhibition of the bAP-evoked [Ca2+] signal on the shaft or spines, their simulations suggest that in the presence of an excitatory input on the spine, an inhibitory input placed on the dendritic shaft can result in a larger Ca2+-transient inhibition in the spine compared to the shaft. Given its high spatial precision, how temporally precise is inhibition by a single inhibitory synapse? To study the timing dependence of Ca2+-transient inhibition, the authors varied the timing between the presynaptic and postsynaptic AP. They find optimal inhibition with zero delay between bAP and inhibitory activation, and their biophysical model suggests that the spike-timing dependence of Ca2+-transient inhibition is a consequence of the fast synaptic kinetics. Interestingly, the temporal profile of Ca2+-transient inhibition displays a familiar shape: it follows the time course of the synaptic current, as has been observed previously for the boosting of bAPs by excitatory synaptic inputs (Stuart and Häusser, 2001Stuart G.J. Häusser M. Nat. Neurosci. 2001; 4: 63-71Crossref PubMed Scopus (278) Google Scholar). Notably, both studies find that the effect on the bAP—boosting or inhibiting—depends on the local amplitude of the bAP. This suggests a mechanism, explored by Müllner et al., 2015Müllner F.E. Wierenga C.J. Bonhoeffer T. Neuron. 2015; 87 (this issue): 576-589Abstract Full Text Full Text PDF PubMed Scopus (51) Google Scholar in a detailed model of CA1 pyramidal cells and illustrated using a simple ball-and-stick model in Figure 1: the single inhibitory synapse hyperpolarizes the local membrane potential to prevent recruitment of voltage-gated channels that are responsible for the regenerative propagation of the bAP along the dendrite. Consequently, [Ca2+] transients triggered by bAPs propagating just above the regenerative threshold can be reduced non-linearly if insufficient channels can be recruited due to the inhibition. This would “kill” the regenerative bAP, rendering its propagation passive and leading to a steep drop in amplitude (Figure 1A). On the other hand, bAPs sufficiently above the threshold for regenerative propagation, which tend to be associated with larger dendritic [Ca2+] transients, will experience only a temporary and local “dent” in amplitude as they pass the inhibitory synapse (Figure 1B). Finally, combining the spatial and temporal properties of Ca2+-transient inhibition, the authors find configurations where a single inhibitory synapse can paradoxically boost the [Ca2+] transient, presumably as a result of reduced inactivation of voltage-gated calcium and/or sodium channels. The results of Müllner et al., 2015Müllner F.E. Wierenga C.J. Bonhoeffer T. Neuron. 2015; 87 (this issue): 576-589Abstract Full Text Full Text PDF PubMed Scopus (51) Google Scholar therefore elegantly show how single inhibitory synapses can “punch above their weight” and have a significant impact even on the apparently invincible action potential: a well-timed inhibitory input, placed in the dendritic tree just where the backpropagating action potential is most vulnerable—namely near where active backpropagation is about to fail—can actually stop the action potential in its tracks. Inhibitory synapses placed elsewhere in the dendritic tree can have more subtle and spatially precise influences on dendritic [Ca2+] signals associated with backpropagating action potentials. These findings dramatically confirm earlier experimental and theoretical work about the importance of the location of inhibitory synapses for their efficacy and complement recent work showing that the interaction of inhibition with active properties of dendrites can have some surprising and counter-intuitive effects, such as “action at a distance” (Gidon and Segev, 2012Gidon A. Segev I. Neuron. 2012; 75: 330-341Abstract Full Text Full Text PDF PubMed Scopus (132) Google Scholar, Jadi et al., 2012Jadi M. Polsky A. Schiller J. Mel B.W. PLoS Comput. Biol. 2012; 8: e1002550Crossref PubMed Scopus (75) Google Scholar). These results lead to a number of predictions and suggestions for future experiments. First, since the threshold for regenerative propagation of bAPs also depends on dendritic morphology (Vetter et al., 2001Vetter P. Roth A. Häusser M. J. Neurophysiol. 2001; 85: 926-937PubMed Google Scholar), the impact of single inputs will be stronger at dendritic locations where propagation is particularly vulnerable, such as branch points with large impedance mismatches. Thus, inhibitory synapses could be strategically targeted to such locations to maximize their impact on the bAP. Second, bAPs will encounter different conditions in vivo compared to the dendrites of cultured neurons. In future experiments, modulation of AP backpropagation in vivo could be investigated by activation of individual interneurons or specific interneuron populations. For example, a precise activation of individual synaptic contacts could be achieved by targeted optogenetic activation of a single interneuron axon, or via GABA uncaging. Finally, the precise inhibition of backpropagating action potentials in dendrites observed by Müllner et al., 2015Müllner F.E. Wierenga C.J. Bonhoeffer T. Neuron. 2015; 87 (this issue): 576-589Abstract Full Text Full Text PDF PubMed Scopus (51) Google Scholar highlights the importance of controlling bAPs as a key mechanism in certain forms of synaptic plasticity. In particular, during STDP the backpropagating AP tells dendritic synapses whether they contributed successfully to generating the postsynaptic AP, implementing Hebb’s rule. The findings by Müllner et al., 2015Müllner F.E. Wierenga C.J. Bonhoeffer T. Neuron. 2015; 87 (this issue): 576-589Abstract Full Text Full Text PDF PubMed Scopus (51) Google Scholar therefore make an important prediction: synaptic plasticity can be regulated by a single inhibitory synapse. With precise spatial and temporal modulation of Ca2+-transient inhibition, plasticity could be vetoed very locally in individual dendritic branches, facilitating storage of new patterns of synaptic weights in some branches while preventing the weights of synapses on the inhibited dendritic branch from being overwritten. To test this prediction, future experiments could explore whether plasticity induction can be blocked specifically in dendritic branches contacted by individual inhibitory contacts, but not in nearby branches. Together, these experiments will help to show how single inhibitory synapses, like Bard the Bowman’s arrow, can have an unexpectedly powerful influence on their targets, on both brief and longer timescales. Precision of Inhibition: Dendritic Inhibition by Individual GABAergic Synapses on Hippocampal Pyramidal Cells Is Confined in Space and TimeMüllner et al.NeuronAugust 05, 2015In BriefBy imaging action potential-evoked dendritic calcium signals and simultaneously activating identified inhibitory synapses, Müllner et al. measured the spatio-temporal profile of inhibition exerted by individual GABAergic synapses, which fills a gap in the biophysical understanding of dendritic inhibition. Full-Text PDF Open Archive