Neuron
Previews The Map and the Territory: Mapping the Territory Regulated by Serotonergic Signaling at Striatal Projection Neurons Andrew F. Scheyer1,2,3,* and Olivier J. Manzoni1,2,3 1INMED,
INSERM U901, Marseille, France University, Marseille, France 3Cannalab Cannabinoids Neuroscience Research International Associated Laboratory, INSERM-Indiana University Bloomington, Bloomington, IN, USA *Correspondence:
[email protected] https://doi.org/10.1016/j.neuron.2018.04.037 2Aix-Marseille
In this issue of Neuron, Cavaccini et al. (2018) identify and thoroughly describe a previously unknown role for hyper-localized serotonergic signaling in the modulation of striatal projection neuron plasticity using electrophysiological, chemogenetic, and optogenetic approaches in addition to advanced imaging technology.
Navigating the behavioral landscape relies on a fine balance of reward and punishment, pleasure and aversion, excitation and inhibition. In turn, these experiences of positive and negative association regulate goal-directed actions that define our interactions with the external environment. The minutia of this balance necessitates such fine-tuning that aberrations in the equilibrium leave one at odds with his or her ability to regulate behavior, manifesting in such disorders as addiction (Grueter et al., 2012) and obsessivecompulsive disorder (Figee et al., 2011). Furthermore, in some cases, the regions governing the regulation of reward- and aversion-based learning similarly regulate the execution of behaviors guided by these learning mechanisms. This overlapping responsibility explains why dysfunction may similarly lead to such movement disorders as Parkinson’s disease (Surmeier et al., 2014). Monoaminergic signaling has long been known to serve a crucial role in the striatal control of learning and directed actions, in particular in the dorsal striatum (DS), where competing inputs onto striatal projection neurons (SPNs) act as a quasirheostat in fine-tuning motor output and reward-based learning behaviors. Indeed, the DS receives significant inputs from both cortical and thalamic projections that tightly regulate the direct and indirect pathways, which in turn stimulate and inhibit, respectively, both movement and reward (Kreitzer and Malenka, 2008). The focus of this regulatory control has
traditionally been on the role of dopamine (DA) while understanding that serotonin’s (5-HT) role has received comparatively limited attention. In this issue of Neuron, Cavaccini et al. (2018) embarked on an ambitious exploratory mission at cortico- and thalamostriatal connections. The result of this formidable effort is a new map of the subcellular territory regulated by serotonergic signaling at direct pathway SPNs (dSPNs). Using a combination of chemogenetic, optogenetic, and electrophysiological techniques coupled with transgenic mouse lines, Cavaccini et al. (2018) elucidate the mechanistic underpinnings of spike-timing-dependent plasticity (STDP) in the form of long-term depression (t-LTD) at thalamostriatal synapses. They demonstrate a negative modulation by 5-HT4A receptors on dendritic calcium signaling and, through the use of advanced imaging techniques, show that this mechanistic control of plasticity is tightly localized at dendritic shafts of the dSPNs. The primary question asked by Cavaccini et al. (2018) is simple: what mechanisms comprise serotonin’s role in governing SPN activity in the DS? Using transgenic hM4DiR DREADD receptor-expressing mice combined with an mCherry reporter to positively identify affected cells, they effectively silence serotonergic neurons of the dorsal raphe nucleus and show that t-LTD is inducible only under these conditions, whereas without the inhibition of serotonergic
signaling, the STDP fails to engage this form of plasticity. Noting the density and known contributions to striatal circuitry of the 5-HTR subtypes 5-HT4AR and 5-HT6R in this region (De Deurwaerde`re et al., 1997), they further dissect this modulatory control using bath perfusion of antagonists for either of these two receptors, concluding that only antagonism of the 5-HT4R recapitulates their chemogenetic findings. They continue by confirming that this t-LTD is not perturbed by the addition of a cannabinoid receptor antagonist, differentiating this form of plasticity from other LTD seen at striatal synapses (Manzoni et al., 1997). Finally, through additional recordings to observe paired-pulse ratio measurements as well as intracellular peptide infusion to probe the necessity of AMPA receptor (AMPAR) endocytosis, they confirm that this form of plasticity is indeed postsynaptically localized and requires AMPAR internalization. This thorough set of experiments provides a working model for dSPN t-LTD gated by 5-HT4Rs lacking cannabinoid receptor involvement and requiring non-constitutive postsynaptic AMPAR internalization. In an additional series of electrophysiological experiments, Cavaccini et al. (2018) use two-photon Ca2+ imaging coupled with whole-cell current clamp under the previously defined chemogenetic or pharmacological conditions’ permitting of t-LTD in order to elucidate the downstream mechanisms that mediate this plasticity. In contrast to previous reports that activation of Gs-coupled
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Previews receptors increases action-potentialgenerated, voltage-gated Ca2+ currents, Cavaccini et al. (2018) find that the application of the previously employed 5-HT4R antagonist enhances dendritic Ca2+ influx induced by their STDP protocol. To determine whether this is the result of 5-HT4R’s coupling to PKA or CaMKII, the latter of which positively regulates the Ca2+-activated K+ channel BK, they use a BK channel blocker alone or in conjunction with an inhibitor of CAMKII while also monitoring its phosphorylation levels. Indeed, they find that activation of 5-HT4R during STDP negatively regulates BK channel activation via CAMKII signaling, thereby altering Ca2+ signaling during back-propagating action potentials. Knowing that dSPNs may be modulated by both cortical and thalamic inputs, Cavaccini et al. (2018) sought to further define the input specificity of this 5-HT4R-modulated plasticity by using optogenetic stimulation of either corticostriatal or thalamostriatal afferents. First finding that optogenetic stimulation of cortical afferents fails to induce t-LTD even in the presence of a 5-HT4R antagonist, they move on to demonstrate that, conversely, the STDP protocol successfully does so at thalamostriatal synapses. To confirm their model of 5-HT4R negatively modulating BK channels and thereby permitting the induction of t-LTD, they recapitulate the optogenetically induced t-LTD at thalamostriatal synapses with neither the previously used 5-HT4R antagonists nor chemogenetic 5-HT inhibition, though in the presence of a BK channel blocker. Then, in a final confirmation of this model, they use a separate optogenetic protocol to induce 5-HT release during the STDP paradigm, effectively blocking the induction of t-LTD at these thalamostriatal synapses. In a final feat of thoroughly defining the underpinnings of this synaptic plasticity, Cavaccini et al. (2018) use a combination of immunohistochemistry, 3D segmentation, and morphometric analysis in two mouse lines expressing fluorescent markers at VGLUT1 or VGLUT2 in combination with a thalamic-injected retrovirus to identify glutamatergic thala-
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mostriatal synapses and observe colocalization with 5-HT4R. Their measurements of morphological data confirm that 5-HT4Rs are preferentially localized at these synapses, though to further refine their localization, they move on to use dendritic and spine head 3D reconstruction to measure 5-HT4R localization within the postsynaptic domain. Their data confirm that 5-HT4Rs are densely localized at both axo-spinous and axo-dendritic sites of thalamostriatal synapses. Together, the findings of the current paper make significant headway in defining the conditions, environment, and geography of modulatory control over t-LTD in the dorsolateral striatum (DLS). While previous research has focused largely on the role of dopamine and, in many cases, failed to differentiate between the multiple afferents onto SPNs, Cavaccini et al. (2018) thoroughly define a model wherein tightly localized 5-HT4Rs at thalamostriatal synapses negatively regulate t-LTD through BK channel activation via CAMKII, leading eventually to postsynaptic internalization of AMPARs. These findings differentiate this form of plasticity from those previously identified as being CB1R or NMDAR dependent. This added nuance in the already complex landscape of plasticity occurring in the DLS serves to challenge accepted dogmas as well as broaden our collective understanding of hyper-localized, synapse-specific forms of regulatory control. While the exact behavioral correlates of this plasticity remain to be defined, the findings herein potentially illuminate a means by which thalamostriatal control over ongoing motor behavior can be fine-tuned in order to better align with incoming signals governing attention, reward, and aversion. These findings hold additional implications for the actions of 5-HT-targeting drugs, as 5-HT4Rs have been identified as potential targets for the treatment of major depressive disorder (Vidal et al., 2014). Indeed, chronic elevation of 5-HT as a result of selective serotonin reuptake inhibitors treatment of obsessive-compulsive disorder (Goddard et al., 2008) or other disorders likely alters SPN plasticity via the mecha-
nisms herein unearthed. Further studies will be required to identify the consequences of altered 5-HT tone in such cases, as prolonged elevation of 5-HT results in complicated outcomes due to its complex role in plasticity modulation. Finally, having identified functional properties and localization for 5-HT4Rs begs the question of what role these receptors may play in other brain regions previously identified as having 5-HT-regulated forms of plasticity, such as the ventral striatum (Burattini et al., 2014). REFERENCES Burattini, C., Battistini, G., Tamagnini, F., and Aicardi, G. (2014). Low-frequency stimulation evokes serotonin release in the nucleus accumbens and induces long-term depression via production of endocannabinoid. J. Neurophysiol. 111, 1046– 1055. Cavaccini, A., Gritti, M., Giorgi, A., Locarno, A., Heck, N., Migliarini, S., Bertero, A., Mereu, M., Margiani, G., Trusel, M., et al. (2018). Serotonergic signaling controls input-specific synaptic plasticity at striatal circuits. Neuron 98, this issue, 801–816. De Deurwaerde`re, P., L’hirondel, M., Bonhomme, N., Lucas, G., Cheramy, A., and Spampinato, U. (1997). Serotonin stimulation of 5-HT4 receptors indirectly enhances in vivo dopamine release in the rat striatum. J. Neurochem. 68, 195–203. Figee, M., Vink, M., de Geus, F., Vulink, N., Veltman, D.J., Westenberg, H., and Denys, D. (2011). Dysfunctional reward circuitry in obsessivecompulsive disorder. Biol. Psychiatry 69, 867–874. Goddard, A.W., Shekhar, A., Whiteman, A.F., and McDougle, C.J. (2008). Serotoninergic mechanisms in the treatment of obsessive-compulsive disorder. Drug Discov. Today 13, 325–332. Grueter, B.A., Rothwell, P.E., and Malenka, R.C. (2012). Integrating synaptic plasticity and striatal circuit function in addiction. Curr. Opin. Neurobiol. 22, 545–551. Kreitzer, A.C., and Malenka, R.C. (2008). Striatal plasticity and basal ganglia circuit function. Neuron 60, 543–554. Manzoni, O., Michel, J.M., and Bockaert, J. (1997). Metabotropic glutamate receptors in the rat nucleus accumbens. Eur. J. Neurosci. 9, 1514–1523. Surmeier, D.J., Graves, S.M., and Shen, W. (2014). Dopaminergic modulation of striatal networks in health and Parkinson’s disease. Curr. Opin. Neurobiol. 29, 109–117. Vidal, R., Castro, E., Pilar-Cue´llar, F., PascualBrazo, J., Dı´az, A., Rojo, M.L., Linge, R., Martı´n, A., Valdiza´n, E.M., and Pazos, A. (2014). Serotonin 5-HT4 receptors: a new strategy for developing fast acting antidepressants? Curr. Pharm. Des. 20, 3751–3762.
