Presynaptic short-term plasticity is thought to play a major role in the process of spike transfer within local circuits. Mossy fiber (Mf) synapses between the axons of dentate gyrus (DG) granule cells (GCs) and cornu ammonis area 3 pyramidal cells (CA3-PCs) (Mf-CA3 synapses) display a remarkable extent of presynaptic plasticity. Here, we have investigated the role of short-term presynaptic facilitation at Mf synapses in the operation of CA3 circuits in vivo and in memory encoding by analyzing mice with selective abrogation of synaptotagmin 7 (Syt7) in DG granule cells (DG-Syt7 knockout [KO] mice), hence in all presynaptic Mf targets, including mossy cells and interneurons (INs). We extend previous studies to show that short-term presynaptic facilitation is suppressed at Mf-CA3 PC synapses in the absence of Syt7, without any impact on basal synaptic properties and on long-term potentiation (LTP). Short-term plasticity was found to be crucial for spike transfer between the DG and CA3 in conditions of naturalistic patterns of presynaptic firing. At the network level, in awake head-fixed mice, the abrogation of short-term facilitation was associated with reduced co-activity of CA3-PCs. Finally, DG-Syt7 KO mice show deficits in spatial memory tasks that rely on the process of pattern completion, but not on pattern separation, and display altered emotional processes. These results give important insights into how short-term presynaptic facilitation of DG-CA3 synapses may contribute to hippocampal function at the circuit and behavioral levels.

We developed a high content screening to investigate how Alzheimer disease (AD) genetic risk factors may affect synaptic mechanisms in rat primary neuronal cultures. Out of the target genes identified, we found that Plcg2 downregulation in mouse dentate gyrus neurons consistently disrupted dendritic morphology and synaptic function. In human neuronal cultures (hNCs), PLCG2 downregulation also impaired synaptic function and increased Aβ levels and Tau phosphorylation. Very rare PLCG2 loss-of-function (LoF) variants were associated with a 10-fold increased AD risk. PLCG2 LoF carriers exhibit low mRNA/protein PLCG2/PLCγ2 levels and the R953* LoF mutation compromised synaptic function and increased AD hallmarks in hNCs. Single nuclei RNAseq analyses confirmed that the downregulation of PLCG2 impacted pathways related to synaptic and neuronal functions, potentially through neurexin in neurons. In conclusion, PLCγ2 downregulation could increase AD risk by impairing synaptic functions and increasing the Aβ levels and Tau phosphorylation in neurons.

Abstract The ability to flexibly adapt behavior to changing environmental contingencies is a core component of brain function and relies on experience-dependent remodeling of neural circuits. While cognitive flexibility has been primarily attributed to prefrontal–striatal networks, the contribution of hippocampus and their underlying molecular substrates remains less understood. Here, we show that the dorsal hippocampus has a key role in cognitive flexibility. In particular, Ring Finger Protein 10 (RNF10)-mediated signaling, linking activation of synaptic NMDARs to specific transcriptional programs in the dorsal CA1, is necessary for cognitive flexibility. In fact, in vivo downregulation, through gene deletion and silencing of RNF10, resulting in impaired long-term synaptic plasticity, suppressed cognitive flexibility. This was reflected in the impaired ability to disengage from previously acquired contextual, visual, and spatial information and to adapt behavior to changed context. Overall, our results identified RNF10 as a key in vivo player necessary for the balance between cognitive stability and flexibility.

We developed a high-content screening to investigate how Alzheimer's disease (AD) genetic risk factors may affect synaptic mechanisms in rat primary neuronal cultures. Of the target genes identified, we found that Plcg2 downregulation in mouse dentate gyrus neurons consistently disrupted dendritic morphology and synaptic function. In human neuronal cultures (hNCs), PLCG2 downregulation also impaired synaptic function and increased amyloid-β (Aβ) levels and Tau phosphorylation. Very rare PLCG2 loss-of-function (LoF) variants were associated with a tenfold increased AD risk. PLCG2 LoF carriers show low mRNA/protein PLCG2/PLCγ2 levels and the R953* LoF mutation compromised synaptic function and increased AD hallmarks in hNCs. Single-nucleus RNA sequencing analyses confirmed that the downregulation of PLCG2 impacted pathways related to synaptic and neuronal functions, potentially through neurexins in neurons. In conclusion, PLCγ2 downregulation could increase AD risk by impairing synaptic functions and by increasing Aβ levels and Tau phosphorylation in neurons.

This is a maximal intensity projection of CA1 pyramidal cell transfected with plasmid with the reporter GFP using single cell electroporation technique. In this particular case the organotypic slices were prepared from p5-7 pups in a tissue chopper (McIlwain). And maintained in MEM bases media with added glutamax with a change in 2 alternative dyas at 37°C and 5% CO 2 for 4 days in-vitro (DIV) before electroporating with a glass pipette of 7-10mΩ resistance by applying 4 square pulses of -ve voltage of −2.5 V, for 25 ms duration at 1 Hz frequency. Then maintained in the same medium for one four more days before fixing them in PFA(4‰ v/v) and amplified with anti-GFP antibody (ab290, 1:1000 dil) and imaged in SP2 confocal microscope (Leica) for z-stack.

The ionotropic glutamate receptors comprise members of the NMDA (N-methyl-D-aspartate), AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid) and kainate receptor classes, named originally according to their preferred, synthetic, agonist [60, 99, 163]. Receptor heterogeneity within each class arises from the homo-oligomeric, or hetero-oligomeric, assembly of distinct subunits into cation-selective tetramers. Each subunit of the tetrameric complex comprises an extracellular amino terminal domain (ATD), an extracellular ligand binding domain (LBD), 3 TM domains (M1, M3 and M4), a channel lining re-entrant 'p-loop' (M2) located between M1 and M3 and an intracellular carboxy- terminal domain (CTD) [106, 74, 115, 163, 88]. The X-ray structure of a homomeric ionotropic glutamate receptor (GluA2- see below) has recently been solved at 3.6Å resolution [151] and although providing the most complete structural information current available may not be representative of the subunit arrangement of, for example, the heteromeric NMDA receptors [77]. It is beyond the scope of this supplement to discuss the pharmacology of individual ionotropic glutamate receptor isoforms in detail; such information can be gleaned from [60, 72, 34, 83, 44, 122, 27, 71, 163, 120, 121, 171]. Agents that discriminate between subunit isoforms are, where appropriate, noted in the tables and additional compounds that distinguish between receptor isoforms are indicated in the text below.The classification of glutamate receptor subunits has been re-addressed by NC-IUPHAR [31]. The scheme developed recommends a nomenclature for ionotropic glutamate receptor subunits that is adopted here.NMDA receptorsNMDA receptors assemble as obligate heteromers that may be drawn from GluN1, GluN2A, GluN2B, GluN2C, GluN2D, GluN3A and GluN3B subunits. Alternative splicing can generate eight isoforms of GluN1 with differing pharmacological properties. Various splice variants of GluN2B, 2C, 2D and GluN3A have also been reported. Activation of NMDA receptors containing GluN1 and GluN2 subunits requires the binding of two agonists, glutamate to the S1 and S2 regions of the GluN2 subunit and glycine to S1 and S2 regions of the GluN1 subunit [43, 28]. The minimal requirement for efficient functional expression of NMDA receptors in vitro is a di-heteromeric assembly of GluN1 and at least one GluN2 subunit variant, as a dimer of heterodimers arrangement in the extracellular domain [50, 106, 77]. However, more complex tri-heteromeric assemblies, incorporating multiple subtypes of GluN2 subunit, or GluN3 subunits, can be generated in vitro and occur in vivo. The NMDA receptor channel commonly has a high relative permeability to Ca2+ and is blocked, in a voltage-dependent manner, by Mg2+ such that at resting potentials the response is substantially inhibited.AMPA and Kainate receptorsAMPA receptors assemble as homomers, or heteromers, that may be drawn from GluA1, GluA2, GluA3 and GluA4 subunits. Transmembrane AMPA receptor regulatory proteins (TARPs) of class I (i.e. γ2, γ3, γ4 and γ8) act, with variable stoichiometry, as auxiliary subunits to AMPA receptors and influence their trafficking, single channel conductance gating and pharmacology (reviewed in [45, 110, 161, 70]). Functional kainate receptors can be expressed as homomers of GluK1, GluK2 or GluK3 subunits. GluK1-3 subunits are also capable of assembling into heterotetramers (e.g. GluK1/K2; [94, 127, 126]). Two additional kainate receptor subunits, GluK4 and GluK5, when expressed individually, form high affinity binding sites for kainate, but lack function, but can form heteromers when expressed with GluK1-3 subunits (e.g. GluK2/K5; reviewed in [127, 71, 126]). Kainate receptors may also exhibit 'metabotropic' functions [94, 139]. As found for AMPA receptors, kainate receptors are modulated by auxiliary subunits (Neto proteins, [126, 95]). An important function difference between AMPA and kainate receptors is that the latter require extracellular Na+ and Cl- for their activation [13, 128]. RNA encoding the GluA2 subunit undergoes extensive RNA editing in which the codon encoding a p-loop glutamine residue (Q) is converted to one encoding arginine (R). This Q/R site strongly influences the biophysical properties of the receptor. Recombinant AMPA receptors lacking RNA edited GluA2 subunits are: (1) permeable to Ca2+; (2) blocked by intracellular polyamines at depolarized potentials causing inward rectification (the latter being reduced by TARPs); (3) blocked by extracellular argiotoxin and joro spider toxins and (4) demonstrate higher channel conductances than receptors containing the edited form of GluA2 [147, 69]. GluK1 and GluK2, but not other kainate receptor subunits, are similarly edited and broadly similar functional characteristics apply to kainate receptors lacking either an RNA edited GluK1, or GluK2, subunit [94, 126]. Native AMPA and kainate receptors displaying differential channel conductances, Ca2+ permeabilities and sensitivity to block by intracellular polyamines have been identified [33, 69, 98]. GluA1-4 can exist as two variants generated by alternative splicing (termed ‘flip’ and ‘flop’) that differ in their desensitization kinetics and their desensitization in the presence of cyclothiazide which stabilises the nondesensitized state. TARPs also stabilise the non-desensitized conformation of AMPA receptors and facilitate the action of cyclothiazide [110]. Splice variants of GluK1-3 also exist which affects their trafficking [94, 126].

Ligand-gated ion channels (LGICs) are integral membrane proteins that contain a pore which allows the regulated flow of selected ions across the plasma membrane. Ion flux is passive and driven by the electrochemical gradient for the permeant ions. These channels are open, or gated, by the binding of a neurotransmitter to an orthosteric site(s) that triggers a conformational change that results in the conducting state. Modulation of gating can occur by the binding of endogenous, or exogenous, modulators to allosteric sites. LGICs mediate fast synaptic transmission, on a millisecond time scale, in the nervous system and at the somatic neuromuscular junction. Such transmission involves the release of a neurotransmitter from a pre-synaptic neurone and the subsequent activation of post-synaptically located receptors that mediate a rapid, phasic, electrical signal (the excitatory, or inhibitory, post-synaptic potential). However, in addition to their traditional role in phasic neurotransmission, it is now established that some LGICs mediate a tonic form of neuronal regulation that results from the activation of extra-synaptic receptors by ambient levels of neurotransmitter. The expression of some LGICs by non-excitable cells is suggestive of additional functions.By convention, the LGICs comprise the excitatory, cation-selective, nicotinic acetylcholine [959, 210], 5-HT3 [68, 1441], ionotropic glutamate [856, 1375] and P2X receptors [659, 1330] and the inhibitory, anion-selective, GABAA [1066, 83] and glycine receptors [878, 1539]. The nicotinic acetylcholine, 5-HT3, GABAA and glycine receptors (and an additional zinc-activated channel) are pentameric structures and are frequently referred to as the Cys-loop receptors due to the presence of a defining loop of residues formed by a disulphide bond in the extracellular domain of their constituent subunits [966, 1357]. However, the prokaryotic ancestors of these receptors contain no such loop and the term pentameric ligand-gated ion channel (pLGIC) is gaining acceptance in the literature [573]. The ionotropic glutamate and P2X receptors are tetrameric and trimeric structures, respectively. Multiple genes encode the subunits of LGICs and the majority of these receptors are heteromultimers. Such combinational diversity results, within each class of LGIC, in a wide range of receptors with differing pharmacological and biophysical properties and varying patterns of expression within the nervous system and other tissues. The LGICs thus present attractive targets for new therapeutic agents with improved discrimination between receptor isoforms and a reduced propensity for off-target effects. The development of novel, faster screening techniques for compounds acting on LGICs [359] will greatly aid in the development of such agents.

PhospholipaseC γ2 (PLCG2) is known to have direct link with genetic risk factors for Alzheimer’s like dementia (AD). PLCG2 has been previously demonstrated to have association with Aß uptake through microglia. And mostly expressed in dentate gyrus (DG) network of hippocampus. WT mouse were transfected with sh-RNA of LV-PLCG2 (U6 promotor, GFP as reporter; shRNA) or scrambled (used as control; shNT). Whole-cell patch-clamp recordings were performed on acute slices with glass pipette (<3-4MΩ) at -70mv, which was accompanied with morphometry of perforant pathway (PP)-DG circuitry. Two-tailed Mann-Whitney test was used for statistics unless otherwise mentioned, p<0.05 was considered to be significant. We found significant increase in AP half width (p = 0.0274), rheobase (p = 0.0046) in shRNA on the other hand, input resistance (p = 0.0159), number of spike vs current injection (p = 0.0268) was significantly decreased. However, there were no significant change was observed in AP onset (p = 0.6105), AP amplitude (p = 0.7345), RMP (p = 0.7591) and firing threshold (p = 0.5247) in shRNA group when compared to shNT. mEPSC amplitude was found to be reduced (p = 0.0288) in shRNA condition, even the cumulative probability of mEPSC amplitude was also found to be significantly altered after silencing of PLCG2 gene in DG cells (p = 0.0014; Kolmogorof-Smirnov test). mEPSC frequency of the miniature activity of transfected DG cells were did not show significant change (p = 0.7707). Sholl analysis of the DG cells expressing shRNA showed a decrease in the number of intersections vs distance from soma (P<0.05), accompanied by significant decrease in dendritic length (p = 0.0501), dendritic volume (p<0.0001) and dendritic surface area (p = 0.0273). When spine density was measured we have found a significant decrease in cumulative distribution of spine density (p<0.0001; Kolmogorof-Smirnov test), which is tightly associated with decrease in spine length (p = 0.0209) and spine head diameter (P<0.0001). Furthermore, these dendritic and spine morphometric alteration is also accompanied with the decrease in the volume of mossy fiber gigantic bouton (mfB) with alteration in the number of filopodia per mfBs (p<0.05). Results from the study, reflect critical insight of PLCG2 regulating neuronal and/or synaptic activity, which may underlie the cognitive dysfunction (viz. recognition memory) and synaptic alteration to the PP-DG circuitry seen in AD.

The ionotropic glutamate receptors comprise members of the NMDA (N-methyl-D-aspartate), AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid) and kainate receptor classes, named originally according to their preferred, synthetic, agonist [60, 99, 163]. Receptor heterogeneity within each class arises from the homo-oligomeric, or hetero-oligomeric, assembly of distinct subunits into cation-selective tetramers. Each subunit of the tetrameric complex comprises an extracellular amino terminal domain (ATD), an extracellular ligand binding domain (LBD), 3 TM domains (M1, M3 and M4), a channel lining re-entrant 'p-loop' (M2) located between M1 and M3 and an intracellular carboxy- terminal domain (CTD) [106, 74, 115, 163, 88]. The X-ray structure of a homomeric ionotropic glutamate receptor (GluA2- see below) has recently been solved at 3.6Å resolution [151] and although providing the most complete structural information current available may not representative of the subunit arrangement of, for example, the heteromeric NMDA receptors [77]. It is beyond the scope of this supplement to discuss the pharmacology of individual ionotropic glutamate receptor isoforms in detail; such information can be gleaned from [60, 72, 34, 83, 44, 122, 27, 71, 163, 120, 121, 171]. Agents that discriminate between subunit isoforms are, where appropriate, noted in the tables and additional compounds that distinguish between receptor isoforms are indicated in the text below.The classification of glutamate receptor subunits has been re-addressed by NC-IUPHAR [31]. The scheme developed recommends a nomenclature for ionotropic glutamate receptor subunits that is adopted here.NMDA receptorsNMDA receptors assemble as obligate heteromers that may be drawn from GluN1, GluN2A, GluN2B, GluN2C, GluN2D, GluN3A and GluN3B subunits. Alternative splicing can generate eight isoforms of GluN1 with differing pharmacological properties. Various splice variants of GluN2B, 2C, 2D and GluN3A have also been reported. Activation of NMDA receptors containing GluN1 and GluN2 subunits requires the binding of two agonists, glutamate to the S1 and S2 regions of the GluN2 subunit and glycine to S1 and S2 regions of the GluN1 subunit [43, 28]. The minimal requirement for efficient functional expression of NMDA receptors in vitro is a di-heteromeric assembly of GluN1 and at least one GluN2 subunit variant, as a dimer of heterodimers arrangement in the extracellular domain [50, 106, 77]. However, more complex tri-heteromeric assemblies, incorporating multiple subtypes of GluN2 subunit, or GluN3 subunits, can be generated in vitro and occur in vivo. The NMDA receptor channel commonly has a high relative permeability to Ca2+ and is blocked, in a voltage-dependent manner, by Mg2+ such that at resting potentials the response is substantially inhibited.AMPA and Kainate receptorsAMPA receptors assemble as homomers, or heteromers, that may be drawn from GluA1, GluA2, GluA3 and GluA4 subunits. Transmembrane AMPA receptor regulatory proteins (TARPs) of class I (i.e. γ2, γ3, γ4 and γ8) act, with variable stoichiometry, as auxiliary subunits to AMPA receptors and influence their trafficking, single channel conductance gating and pharmacology (reviewed in [45, 110, 161, 70]). Functional kainate receptors can be expressed as homomers of GluK1, GluK2 or GluK3 subunits. GluK1-3 subunits are also capable of assembling into heterotetramers (e.g. GluK1/K2; [94, 127, 126]). Two additional kainate receptor subunits, GluK4 and GluK5, when expressed individually, form high affinity binding sites for kainate, but lack function, but can form heteromers when expressed with GluK1-3 subunits (e.g. GluK2/K5; reviewed in [127, 71, 126]). Kainate receptors may also exhibit 'metabotropic' functions [94, 139]. As found for AMPA receptors, kainate receptors are modulated by auxiliary subunits (Neto proteins, [126, 95]). An important function difference between AMPA and kainate receptors is that the latter require extracellular Na+ and Cl- for their activation [13, 128]. RNA encoding the GluA2 subunit undergoes extensive RNA editing in which the codon encoding a p-loop glutamine residue (Q) is converted to one encoding arginine (R). This Q/R site strongly influences the biophysical properties of the receptor. Recombinant AMPA receptors lacking RNA edited GluA2 subunits are: (1) permeable to Ca2+; (2) blocked by intracellular polyamines at depolarized potentials causing inward rectification (the latter being reduced by TARPs); (3) blocked by extracellular argiotoxin and joro spider toxins and (4) demonstrate higher channel conductances than receptors containing the edited form of GluA2 [147, 69]. GluK1 and GluK2, but not other kainate receptor subunits, are similarly edited and broadly similar functional characteristics apply to kainate receptors lacking either an RNA edited GluK1, or GluK2, subunit [94, 126]. Native AMPA and kainate receptors displaying differential channel conductances, Ca2+ permeabilites and sensitivity to block by intracellular polyamines have been identified [33, 69, 98]. GluA1-4 can exist as two variants generated by alternative splicing (termed ‘flip’ and ‘flop’) that differ in their desensitization kinetics and their desensitization in the presence of cyclothiazide which stabilises the nondesensitized state. TARPs also stabilise the non-desensitized conformation of AMPA receptors and facilitate the action of cyclothiazide [110]. Splice variants of GluK1-3 also exist which affects their trafficking [94, 126].

VAMP7 is a vesicular SNARE of the longin family that localizes to axons and dendrites during development, where it is important in neurite growth. In the adult brain, VAMP7 is enriched in a subset of nerve terminals, particularly in hippocampal mossy fibres (Mfs) originating from the dentate gyrus. We analysed the VAMP7 function in neurotransmitter release by detailed functional characterization of Mf synapses onto CA3 pyramidal cells in knockout mutant mice for VAMP7. We have evaluated the role of VAMP7 in different forms of short-term synaptic plasticity and the potential contribution of the co-release of glutamate and zinc. This analysis has not revealed any significant impact of the loss of VAMP7 for basal properties of synaptic transmission, for short-term plasticity, for asynchronous release and for the ability of Mf vesicles to release ionic zinc. Based on these findings, the potential role of VAMP7 in the regulation of presynaptic mechanisms is discussed.

In the hippocampus, dentate gyrus granule cells connect to CA3 pyramidal cells via their axons, the mossy fibers (Mf). The synaptic terminals of Mfs (Mf boutons, MfBs) form large and complex synapses with thorny excrescences (TE) on the proximal dendrites of CA3 pyramidal cells (PCs). MfB/TE synapses have distinctive “detonator” properties due to low initial release probability and large presynaptic facilitation. The molecular mechanisms shaping the morpho-functional properties of MfB/TE synapses are still poorly understood, though alterations in their morphology are associated with Down syndrome, intellectual disabilities, and Alzheimer’s disease. Here, we identify the core PCP gene Vangl2 as essential to the morphogenesis and function of MfB/TE synapses. Vangl2 colocalises with the presynaptic heparan sulfate proteoglycan glypican 4 (GPC4) to stabilise the postsynaptic orphan receptor GPR158. Embryonic loss of Vangl2 disrupts the morphology of MfBs and TEs, impairs ultrastructural and molecular organisation, resulting in defective synaptic transmission and plasticity. In adult, the early loss of Vangl2 results in a number of hippocampus-dependent memory deficits including characteristic flexibility of declarative memory, organisation and retention of working / everyday-like memory. These deficits also lead to abnormal generalisation of memories to salient cues and diminished ability to form detailed contextual memories. Together, these results establish Vangl2 as a key regulator of DG-CA3 connectivity and functions.

Forming new memories after a one-time experience requires initial encoding then consolidation over time. During learning, multimodal information converges onto the hippocampus, activating sparse neuronal assemblies which are thought to form a memory representation through concerted activity and synaptic interconnectivity. In this work, we use a novel tool for fast fluorescent labeling of engram neurons (FLEN). FLEN is based on c-Fos activity-dependent transient expression of a destabilized fluorescent marker ZsGreen1 rapidly after one-trial learning. With FLEN, we explore the electrophysiological properties of c-Fos activated CA3 pyramidal neurons (PNs) a few hours following one-trial learning of an episodic-like memory. In parallel, we employ the Robust Activity Marker (RAM) system, which provides activity-dependent labeling 24 hr following a novel experience. Comparing FLEN+ and RAM+ neurons allows us to characterize how the properties of neuronal assemblies evolve during an initial phase of consolidation. Whereas no difference was observed in the excitability of FLEN+ vs. FLEN− neurons, RAM+ neurons were more excitable than RAM− neurons. This suggests that CA3 PNs recruited in an engram progressively acquire increased excitability as compared to neurons which were not activated by the one-trial contextual memory task. On the other hand, like RAM+ neurons, FLEN+ CA3 neurons show an increased number of excitatory inputs. Overall, with the FLEN strategy, we can show that both the intrinsic excitability and the synaptic properties of CA3 PNs undergo progressive plastic changes over the first day following a one-trial memory task.

We developed a high content screening to investigate how Alzheimer disease (AD) genetic risk factors may impair synaptic mechanisms in rat primary neuronal cultures. Out of the gene targets identified, we found that shRNA-mediated downregulation of Plcg2 in mouse dentate gyrus neurons consistently impaired dendritic morphology and synaptic function. In human neuronal cultures (hNCs), PLCG2 downregulation also impaired synaptic function and was associated with increased levels of Aβ and Tau phosphorylation, potentially via the AKT/GSK3β axis. Very rare PLCG2 loss-of-functon(LoF) variants were associated with a 10-fold increased AD risk. PLCG2 LoF carriers exhibit low mRNA/protein PLCG2/PLCγ2 levels, consistent with nonsense-mediated mRNA decay mechanisms. Restoring PLCγ2 levels in shPLCG2-hNCs fully reversed the disease-related phenotypes. Our findings indicate that the downregulation of PLCγ2 increases the risk of AD by impairing synaptic function and increasing the levels of Aβ and Tau phosphorylation in neurons.

Short-term synaptic plasticity refers to the regulation of synapses by their past activity on time scales of milliseconds to minutes. Hippocampal mossy fiber synapses onto CA3 pyramidal cells (Mf-CA3 synapses) are endowed with remarkable forms of short-term synaptic plasticity expressed as facilitation of synaptic release by a factor of up to ten-fold. Three main forms of short-term plasticity are distinguished: 1) Frequency facilitation, which includes low frequency facilitation and train facilitation, operating in the range of tens of milliseconds to several seconds; 2) Post-tetanic potentiation triggered by trains of high frequency stimulation, which lasts several minutes; 3) Finally, depolarization-induced potentiation of excitation, based on retrograde signaling, with an onset and offset of several minutes. Here we describe the proposed mechanisms for short-term plasticity, mainly based on the kinetics of presynaptic Ca2+ transients and the Ca2+ sensor synaptotagmin 7, on cAMP-dependent mechanisms and readily releasable vesicle pool, and on the regulation of presynaptic K+ channels. We then review evidence for a physiological function of short-term plasticity of Mf-CA3 synapses in information transfer between the dentate gyrus and CA3 in conditions of natural spiking, and discuss how short-term plasticity counteracts robust feedforward inhibition in a frequency-dependent manner. Although DG-CA3 connections have long been proposed to play a role in memory, direct evidence for an implication of short-term plasticity at Mf-CA3 synapses is mostly lacking. The mechanistic knowledge gained on short-term plasticity at Mf-CA3 synapses should help in designing future experiments to directly test how this evolutionary conserved feature controls hippocampal circuit function in behavioural conditions.

Twenty-one simplified analogues of the natural product domoic acid were designed, synthesized, and then characterized at homomeric kainic acid (KA) receptors (GluK1-3,5). LBG20304 displays a high affinity for homomeric GluK5 receptors (IC50 = 432 nM) with a >40-fold selectivity over homomeric GluK1-3 subtypes and ≫100-fold selectivity over native AMPA and N-methyl d-aspartate receptors. Functional studies of LBG20304 on heteromeric GluK2/5 receptors show no agonist or antagonist functional response at 10 μM, while a concentration of 100 μM at neuronal slices (rat) shows low agonist activity. A molecular dynamics simulation of LBG20304, in a homology model of GluK5, suggests specific interactions with the GluK5 receptor and an occluded ligand binding domain, which is translated to agonist or partial agonist activity. LBG20304 is a new compound for the study of the role and function of the KA receptors with the aim of understanding the involvement of these receptors in health and disease.

Mesial temporal lobe epilepsy (mTLE) is the most prevalent type of epilepsy in adults. First and subsequent generations of anti-epileptic therapy regimens fail to decrease seizures in a large number of patients suffering from mTLE, leaving surgical ablation of part of the hippocampus as the only therapeutic option to potentially reach seizure freedom. GluK2 has recently been identified as a promising target for the treatment of mTLE using gene therapy. Here, we engineered an adeno-associated virus serotype 9 vector expressing a cluster of two synthetic microRNAs (miRNAs), expressed from the human synapsin promoter, that target GRIK2 mRNA. Intra-hippocampal delivery of this vector in a mouse model of mTLE significantly reduced GRIK2 expression and daily seizure frequency. This treatment also improved the animals' health, reduced their anxiety, and restored working memory. Focal administration of the vector to the hippocampus of cynomolgus monkeys in GLP toxicology studies led to the selective transduction of hippocampal neurons with little exposure elsewhere in the brain and no transduction outside the central nervous system. Expression of miRNAs in hippocampal neurons resulted in substantially decreased GRIK2 mRNA expression. These data suggest that the intra-hippocampal delivery of a GMP-grade AAV9 encoding a synthetic miRNAs targeting GRIK2 is a promising treatment strategy for mTLE.

Q/R editing of the kainate receptor (KAR) subunit GluK2 radically alters recombinant KAR properties, but the effects on endogenous KARs in vivo remain largely unexplored. Here, we compared GluK2 editing-deficient mice that express similar to 95% unedited GluK2(Q) to wild-type counterparts that express similar to 85% edited GluK2(R). At mossy fiber-CA3 (MF-CA3) synapses GluK2(Q) mice displayed increased postsynaptic KAR function and KAR-mediated presynaptic facilitation, demonstrating enhanced ionotropic function. Conversely, GluK2(Q) mice exhibited reduced metabotropic KAR function, assessed by KAR-mediated inhibition of slow after-hyperpolarization currents (I-SAHP). GluK2(Q) mice also had fewer GluA1-and GluA3-containing AMPA receptors (AMPARs) and reduced postsynaptic AMPAR currents at both MF-CA3 and CA1-Schaffer collateral synapses. Moreover, long-term potentiation of AMPAR-mediated transmission at CA1-Schaffer collateral synapses was reduced in GluK2(Q) mice. These findings suggest that GluK2 Q/R editing influences ionotropic/metabotropic balance of KAR signaling to regulate synaptic expression of AMPARs and plasticity.