The cellular representation of temperature across the somatosensory thalamus.

Although distinct thalamic nuclei encode sensory information for almost all sensory modalities, the existence of a thalamic representation of temperature with a role in thermal perception remains unclear. To address this, we performed high-density electrophysiological recordings across the entire forelimb somatosensory thalamus in awake mice, and identified an anterior and a posterior representation of temperature that spans three thalamic nuclei. We found that these parallel representations show fundamental differences in the cellular encoding of temperature which reflects their cortical output targets. While the anterior representation encodes cool only and the posterior both cool and warm; in both representations cool was more densely represented and showed shorter latency, more transient responses as compared to warm. Moreover, thalamic inactivation showed a major role in thermal perception. Our comprehensive dataset identifies the thalamus as a key structure in thermal processing and highlights a novel posterior pathway in the thalamic representation of warm and cool.

CaMKII Metaplasticity Drives Aß Oligomer-Mediated Synaptotoxicity.

Alzheimer's disease (AD) is emerging as a synaptopathology driven by metaplasticity. Indeed, reminiscent of metaplasticity, oligomeric forms of the amyloid-ß peptide (oAß) prevent induction of long-term potentiation (LTP) via the prior activation of GluN2B-containing NMDA receptors (NMDARs). However, the downstream Ca2+-dependent signaling molecules that mediate aberrant metaplasticity are unknown. In this study, we show that oAß promotes the activation of Ca2+/calmodulin-dependent kinase II (CaMKII) via GluN2B-containing NMDARs. Importantly, we find that CaMKII inhibition rescues both the LTP impairment and the dendritic spine loss mediated by oAß. Mechanistically resembling metaplasticity, oAß prevents subsequent rounds of plasticity from inducing CaMKII T286 autophosphorylation, as well as the associated anchoring and accumulation of synaptic AMPA receptors (AMPARs). Finally, prolonged oAß treatment-induced CaMKII misactivation leads to dendritic spine loss via the destabilization of surface AMPARs. Thus, our study demonstrates that oAß engages synaptic metaplasticity via aberrant CaMKII activation.

Activity-dependent control of NMDA receptor subunit composition at hippocampal mossy fibre synapses.

KEY POINTS: CA3 pyramidal cells display input-specific differences in the subunit composition of synaptic NMDA receptors (NMDARs). Although at low density, GluN2B contributes significantly to NMDAR-mediated EPSCs at mossy fibre synapses. Long-term potentiation (LTP) of NMDARs triggers a modification in the subunit composition of synaptic NMDARs by insertion of GluN2B. GluN2B subunits are essential for the expression of LTP of NMDARs at mossy fibre synapses. ABSTRACT: Single neurons express NMDA receptors (NMDARs) with distinct subunit composition and biophysical properties that can be segregated in an input-specific manner. The dynamic control of the heterogeneous distribution of synaptic NMDARs is crucial to control input-dependent synaptic integration and plasticity. In hippocampal CA3 pyramidal cells from mice of both sexes, we found that mossy fibre (MF) synapses display a markedly lower proportion of GluN2B-containing NMDARs than associative/commissural synapses. The mechanism involved in such heterogeneous distribution of GluN2B subunits is not known. Here we show that long-term potentiation (LTP) of NMDARs, which is selectively expressed at MF-CA3 pyramidal cell synapses, triggers a modification in the subunit composition of synaptic NMDARs by insertion of GluN2B. This activity-dependent recruitment of GluN2B at mature MF-CA3 pyramidal cell synapses contrasts with the removal of GluN2B subunits at other glutamatergic synapses during development and in response to activity. Furthermore, although expressed at low levels, GluN2B is necessary for the expression of LTP of NMDARs at MF-CA3 pyramidal cell synapses. Altogether, we reveal a previously unknown activity-dependent regulation and function of GluN2B subunits that may contribute to the heterogeneous plasticity induction rules in CA3 pyramidal cells.

Operation and plasticity of hippocampal CA3 circuits: implications for memory encoding.

The CA3 region of the hippocampus is important for rapid encoding of memory. Computational theories have proposed specific roles in hippocampal function and memory for the sparse inputs from the dentate gyrus to CA3 and for the extended local recurrent connectivity that gives rise to the CA3 autoassociative network. Recently, we have gained considerable new insight into the operation and plasticity of CA3 circuits, including the identification of novel forms of synaptic plasticity and their underlying mechanisms, and structural plasticity in the GABAergic control of CA3 circuits. In addition, experimental links between synaptic plasticity of CA3 circuits and memory are starting to emerge.

Control of Spike Transfer at Hippocampal Mossy Fiber Synapses In Vivo by GABAA and GABAB Receptor-Mediated Inhibition.

Despite extensive studies in hippocampal slices and incentive from computational theories, the synaptic mechanisms underlying information transfer at mossy fiber (mf) connections between the dentate gyrus (DG) and CA3 neurons in vivo are still elusive. Here we used an optogenetic approach in mice to selectively target and control the activity of DG granule cells (GCs) while performing whole-cell and juxtacellular recordings of CA3 neurons in vivo In CA3 pyramidal cells (PCs), mf-CA3 synaptic responses consisted predominantly of an IPSP at low stimulation frequency (0.05 Hz). Upon increasing the frequency of stimulation, a biphasic response was observed consisting of a brief mf EPSP followed by an inhibitory response lasting on the order of 100 ms. Spike transfer at DG-CA3 interneurons recorded in the juxtacellular mode was efficient at low presynaptic stimulation frequency and appeared insensitive to an increased frequency of GC activity. Overall, this resulted in a robust and slow feedforward inhibition of spike transfer at mf-CA3 pyramidal cell synapses. Short-term plasticity of EPSPs with increasing frequency of presynaptic activity allowed inhibition to be overcome to reach spike discharge in CA3 PCs. Whereas the activation of GABAA receptors was responsible for the direct inhibition of light-evoked spike responses, the slow inhibition of spiking activity required the activation of GABAB receptors in CA3 PCs. The slow inhibitory response defined an optimum frequency of presynaptic activity for spike transfer at ∼10 Hz. Altogether these properties define the temporal rules for efficient information transfer at DG-CA3 synaptic connections in the intact circuit. SIGNIFICANCE STATEMENT: Activity-dependent changes in synaptic strength constitute a basic mechanism for memory. Synapses from the dentate gyrus (DG) to the CA3 area of the hippocampus are distinctive for their prominent short-term plasticity, as studied in slices. Plasticity of DG-CA3 connections may assist in the encoding of precise memory in the CA3 network. Here we characterize DG-CA3 synaptic transmission in vivo using targeted optogenetic activation of DG granule cells while recording in whole-cell patch-clamp and juxtacellular configuration from CA3 pyramidal cells and interneurons. We show that, in vivo, short-term plasticity of excitatory inputs to CA3 pyramidal cells combines with robust feedforward inhibition mediated by both GABAA and GABAB receptors to control the efficacy and temporal rules for information transfer at DG-CA3 connections.

Molecular determinants for the strictly compartmentalized expression of kainate receptors in CA3 pyramidal cells.

Distinct subtypes of ionotropic glutamate receptors can segregate to specific synaptic inputs in a given neuron. Using functional mapping by focal glutamate uncaging in CA3 pyramidal cells (PCs), we observe that kainate receptors (KARs) are strictly confined to the postsynaptic elements of mossy fibre (mf) synapses and excluded from other glutamatergic inputs and from extrasynaptic compartments. By molecular replacement in organotypic slices from GluK2 knockout mice, we show that the faithful rescue of KAR segregation at mf-CA3 synapses critically depends on the amount of GluK2a cDNA transfected and on a sequence in the GluK2a C-terminal domain responsible for interaction with N-cadherin. Targeted deletion of N-cadherin in CA3 PCs greatly reduces KAR content in thorny excrescences and KAR-EPSCs at mf-CA3 synapses. Hence, multiple mechanisms combine to confine KARs at mf-CA3 synapses, including a stringent control of the amount of GluK2 subunit in CA3 PCs and the recruitment/stabilization of KARs by N-cadherins.

Rabphilin 3A retains NMDA receptors at synaptic sites through interaction with GluN2A/PSD-95 complex.

NMDA receptor (NMDAR) composition and synaptic retention represent pivotal features in the physiology and pathology of excitatory synapses. Here, we identify Rabphilin 3A (Rph3A) as a new GluN2A subunit-binding partner. Rph3A is known as a synaptic vesicle-associated protein involved in the regulation of exo- and endocytosis processes at presynaptic sites. We find that Rph3A is enriched at dendritic spines. Protein-protein interaction assays reveals that Rph3A N-terminal domain interacts with GluN2A(1349-1389) as well as with PSD-95(PDZ3) domains, creating a ternary complex. Rph3A silencing in neurons reduces the surface localization of synaptic GluN2A and NMDAR currents. Moreover, perturbing GluN2A/Rph3A interaction with interfering peptides in organotypic slices or in vivo induces a decrease of the amplitude of NMDAR-mediated currents and GluN2A density at dendritic spines. In conclusion, Rph3A interacts with GluN2A and PSD-95 forming a complex that regulates NMDARs stabilization at postsynaptic membranes.

Exclusive photorelease of signalling lipids at the plasma membrane.

Photoactivation of caged biomolecules has become a powerful approach to study cellular signalling events. Here we report a method for anchoring and uncaging biomolecules exclusively at the outer leaflet of the plasma membrane by employing a photocleavable, sulfonated coumarin derivative. The novel caging group allows quantifying the reaction progress and efficiency of uncaging reactions in a live-cell microscopy setup, thereby greatly improving the control of uncaging experiments. We synthesized arachidonic acid derivatives bearing the new negatively charged or a neutral, membrane-permeant coumarin caging group to locally induce signalling either at the plasma membrane or on internal membranes in ß-cells and brain slices derived from C57B1/6 mice. Uncaging at the plasma membrane triggers a strong enhancement of calcium oscillations in ß-cells and a pronounced potentiation of synaptic transmission while uncaging inside cells blocks calcium oscillations in ß-cells and causes a more transient effect on neuronal transmission, respectively. The precise subcellular site of arachidonic acid release is therefore crucial for signalling outcome in two independent systems.

Scribble1/AP2 complex coordinates NMDA receptor endocytic recycling.

The appropriate trafficking of glutamate receptors to synapses is crucial for basic synaptic function and synaptic plasticity. It is now accepted that NMDA receptors (NMDARs) internalize and are recycled at the plasma membrane but also exchange between synaptic and extrasynaptic pools; these NMDAR properties are also key to governing synaptic plasticity. Scribble1 is a large PDZ protein required for synaptogenesis and synaptic plasticity. Herein, we show that the level of Scribble1 is regulated in an activity-dependent manner and that Scribble1 controls the number of NMDARs at the plasma membrane. Notably, Scribble1 prevents GluN2A subunits from undergoing lysosomal trafficking and degradation by increasing their recycling to the plasma membrane following NMDAR activation. Finally, we show that a specific YxxR motif on Scribble1 controls these mechanisms through a direct interaction with AP2. Altogether, our findings define a molecular mechanism to control the levels of synaptic NMDARs via Scribble1 complex signaling.

Kainate receptors in the hippocampus.

Kainate receptors (KARs) consist of a family of ionotropic glutamate receptors composed of the combinations of five subunits, GluK1-GluK5. Although KARs display close structural homology with AMPA receptors, they serve quite distinct functions. A great deal of our knowledge of the molecular and functional properties of KARs comes from their study in the hippocampus. This review aims at summarising the functions of KARs in the regulation of the activity of hippocampal synaptic circuits at the adult stage and throughout development. We focus on the variety of roles played by KARs in physiological conditions of activation, at pre- and postsynaptic sites, in different cell types and through either metabotropic or ionotropic actions. Finally, we present some of the few attempts to link the role of KARs in the regulation of local hippocampal circuits to the behavioural functions of the hippocampus in health and diseases.

Membrane lipids tune synaptic transmission by direct modulation of presynaptic potassium channels.

Voltage-gated potassium (Kv) channels are involved in action potential (AP) repolarization in excitable cells. Exogenous application of membrane-derived lipids, such as arachidonic acid (AA), regulates the gating of Kv channels. Whether membrane-derived lipids released under physiological conditions have an impact on neuronal coding through this mechanism is unknown. We show that AA released in an activity-dependent manner from postsynaptic hippocampal CA3 pyramidal cells acts as retrograde messenger, inducing a robust facilitation of mossy fiber (Mf) synaptic transmission over several minutes. AA acts by broadening presynaptic APs through the direct modulation of Kv channels. This form of short-term plasticity can be triggered when postsynaptic cell fires with physiologically relevant patterns and sets the threshold for the induction of the presynaptic form of long-term potentiation (LTP) at hippocampal Mf synapses. Hence, direct modulation of presynaptic Kv channels by activity-dependent release of lipids serves as a physiological mechanism for tuning synaptic transmission.

Lack of the presynaptic RhoGAP protein oligophrenin1 leads to cognitive disabilities through dysregulation of the cAMP/PKA signalling pathway.

Loss-of-function mutations in the gene encoding for the RhoGAP protein of oligophrenin-1 (OPHN1) lead to cognitive disabilities (CDs) in humans, yet the underlying mechanisms are not known. Here, we show that in mice constitutive lack of Ophn1 is associated with dysregulation of the cyclic adenosine monophosphate/phosphate kinase A (cAMP/PKA) signalling pathway in a brain-area-specific manner. Consistent with a key role of cAMP/PKA signalling in regulating presynaptic function and plasticity, we found that PKA-dependent presynaptic plasticity was completely abolished in affected brain regions, including hippocampus and amygdala. At the behavioural level, lack of OPHN1 resulted in hippocampus- and amygdala-related learning disabilities which could be fully rescued by the ROCK/PKA kinase inhibitor fasudil. Together, our data identify OPHN1 as a key regulator of presynaptic function and suggest that, in addition to reported postsynaptic deficits, loss of presynaptic plasticity contributes to the pathophysiology of CDs.

CaMKII-dependent phosphorylation of GluK5 mediates plasticity of kainate receptors.

Calmodulin-dependent kinase II (CaMKII) is key for long-term potentiation of synaptic AMPA receptors. Whether CaMKII is involved in activity-dependent plasticity of other ionotropic glutamate receptors is unknown. We show that repeated pairing of pre- and postsynaptic stimulation at hippocampal mossy fibre synapses induces long-term depression of kainate receptor (KAR)-mediated responses, which depends on Ca(2+) influx, activation of CaMKII, and on the GluK5 subunit of KARs. CaMKII phosphorylation of three residues in the C-terminal domain of GluK5 subunit markedly increases lateral mobility of KARs, possibly by decreasing the binding of GluK5 to PSD-95. CaMKII activation also promotes surface expression of KARs at extrasynaptic sites, but concomitantly decreases its synaptic content. Using a molecular replacement strategy, we demonstrate that the direct phosphorylation of GluK5 by CaMKII is necessary for KAR-LTD. We propose that CaMKII-dependent phosphorylation of GluK5 is responsible for synaptic depression by untrapping of KARs from the PSD and increased diffusion away from synaptic sites.

NMDA receptor-dependent metaplasticity at hippocampal mossy fiber synapses.

Hippocampal mossy fiber synapses have been reported to lack NMDA receptor (NMDAR)-dependent long-term potentiation (LTP) of AMPA excitatory postsynaptic currents (EPSCs), unlike conventional glutamatergic synapses. An explanation for this difference may reside in the relatively low number of NMDARs at these synapses. Because mossy fiber synapses display LTP selective for NMDARs, we examined whether this would affect the plasticity rules at mossy fiber-CA3 synapses in mouse hippocampal slices. We found that LTP of NMDARs serves as a metaplastic switch making mossy fiber synapses competent for generating NMDAR-dependent LTP of AMPA EPSCs.

Synaptic localization and activity of ADAM10 regulate excitatory synapses through N-cadherin cleavage.

N-Cadherin has an important role during dendrite arborization, axon guidance, and synaptogenesis. In particular, at synaptic sites, N-cadherin is involved in the regulation of cell-cell adhesion and in morphology and plasticity control. Recent studies have shown that N-cadherin can be cleaved by the metalloproteinase ADAM10. Here we demonstrate that impairing ADAM10 localization and activity at synaptic sites decreases its processing of N-cadherin. This leads to an accumulation of the full-length form of N-cadherin, to an increase in spine head width, and to modifications of the number and function of glutamate receptors of AMPA type, both in vitro and in vivo. Our results indicate a key role for ADAM10 in the complex sequence of events through which N-cadherin affects spine maturation and controls structure and function of glutamatergic synapses.

Profilin II regulates the exocytosis of kainate glutamate receptors.

The trafficking of ionotropic glutamate receptors to and from synaptic sites is regulated by proteins that interact with their cytoplasmic C-terminal domain. Profilin IIa (PfnIIa), an actin-binding protein expressed in the brain and recruited to synapses in an activity-dependent manner, was shown previously to interact with the C-terminal domain of the GluK2b subunit splice variant of kainate receptors (KARs). Here, we characterize this interaction and examine the role of PfnIIa in the regulation of KAR trafficking. PfnIIa directly and specifically binds to the C-terminal domain of GluK2b through a diproline motif. Expression of PfnIIa in transfected COS-7 cells and in cultured hippocampal neurons from PfnII-deficient mice decreases the level of extracellular of homomeric GluK2b as well as heteromeric GluK2a/GluK2b KARs. Our data suggest a novel mechanism by which PfnIIa exerts a dual role on the trafficking of KARs, by a generic inhibition of clathrin-mediated endocytosis through its interaction with dynamin-1, and by controlling KARs exocytosis through a direct and specific interaction with GluK2b.

Synaptic activation of kainate receptors gates presynaptic CB(1) signaling at GABAergic synapses.

Glutamate can control inhibitory synaptic transmission through activation of presynaptic kainate receptors. We found that glutamate released by train stimulation of Schaffer collaterals could lead to either short-term depression or short-term facilitation of inhibitory synaptic transmission in mouse CA1 pyramidal neurons, depending on the presence of cannabinoid type 1 (CB(1)) receptors on GABAergic afferents. The train-induced depression of inhibition (t-Di) required the mobilization of 2-arachidonoylglycerol through postsynaptic activation of metabotropic glutamate receptors and [Ca(2+)] rise. GluK1 (GluR5)-dependent depolarization of GABAergic terminals enabled t-Di by facilitating presynaptic CB(1) signaling. Thus, concerted activation of presynaptic CB(1) receptors and kainate receptors mediates short-term depression of inhibitory synaptic transmission. In contrast, in inhibitory connections expressing GluK1, but not CB(1), receptors, train stimulation of Schaffer collaterals led to short-term facilitation. Thus, activation of kainate receptors by synaptically released glutamate gates presynaptic CB(1) signaling, which in turn controls the direction of short-term heterosynaptic plasticity.

Changes in expression and function of extrasynaptic GABAA receptors in the rat hippocampus during pregnancy and after delivery.

Pregnancy is associated with changes in mood and anxiety level as well as with marked hormonal fluctuations. Increases in the brain concentrations of neuroactive steroids during pregnancy in rats are accompanied by changes in expression of subunits of the GABA type A receptor (GABA(A)-R) in the brain. Granule cells of the dentate gyrus (DGGCs) exhibit two components of inhibitory GABAergic transmission: a phasic component mediated by synaptic GABA(A)-Rs, and a tonic component mediated by extrasynaptic GABA(A)-Rs. Recordings of GABAergic currents were obtained from hippocampal slices prepared from rats in estrus, at pregnancy day 15 (P15) or P19, or at 2 d after delivery. Exogenous GABA or 3alpha,5alpha-THP induced an increase in tonic current in DGGCs that was significantly greater at P19 than in estrus. Neither tonic nor phasic currents were affected by pregnancy in CA1 pyramidal cells. Immunohistochemical analysis revealed a marked increase in the abundance of the delta subunit of the GABA(A)-R and a concomitant decrease in that of the gamma(2) subunit in the hippocampus at P19. Expression of the alpha(4) subunit did not change during pregnancy but was increased 2 d after delivery. Treatment of rats from P12 to P18 with the 5alpha-reductase inhibitor finasteride prevented the changes in tonic current and in delta and gamma(2) subunit expression normally apparent at P19. These data suggest that the number of extrasynaptic GABA(A)-Rs is increased in DGGCs during late pregnancy as a consequence of the associated marked fluctuations in the brain levels of neuroactive steroids.

Alcohol impairs long-term depression at the cerebellar parallel fiber-Purkinje cell synapse.

Acute alcohol consumption causes deficits in motor coordination and gait, suggesting an involvement of cerebellar circuits, which play a role in the fine adjustment of movements and in motor learning. It has previously been shown that ethanol modulates inhibitory transmission in the cerebellum and affects synaptic transmission and plasticity at excitatory climbing fiber (CF) to Purkinje cell synapses. However, it has not been examined thus far how acute ethanol application affects long-term depression (LTD) and long-term potentiation (LTP) at excitatory parallel fiber (PF) to Purkinje cell synapses, which are assumed to mediate forms of cerebellar motor learning. To examine ethanol effects on PF synaptic transmission and plasticity, we performed whole cell patch-clamp recordings from Purkinje cells in rat cerebellar slices. We found that ethanol (50 mM) selectively blocked PF-LTD induction, whereas it did not change the amplitude of excitatory postsynaptic currents at PF synapses. In contrast, ethanol application reduced voltage-gated calcium currents and type 1 metabotropic glutamate receptor (mGluR1)-dependent responses in Purkinje cells, both of which are involved in PF-LTD induction. The selectivity of these effects is emphasized by the observation that ethanol did not impair PF-LTP and that PF-LTP could readily be induced in the presence of the group I mGluR antagonist AIDA or the mGluR1a antagonist LY367385. Taken together, these findings identify calcium currents and mGluR1-dependent signaling pathways as potential ethanol targets and suggest that an ethanol-induced blockade of PF-LTD could contribute to the motor coordination deficits resulting from alcohol consumption.