G protein-coupled receptor (GPR) 55 is sensitive to certain cannabinoids, it is expressed in the brain and, in cell cultures, it triggers mobilization of intracellular Ca 2+ . However, the adaptive neurobiological significance of GPR55 remains unknown. Here, we use acute hippocampal slices and combine two-photon excitation Ca 2+ imaging in presynaptic axonal boutons with optical quantal analysis in postsynaptic dendritic spines to find that GPR55 activation transiently increases release probability at individual CA3-CA1 synapses. The underlying mechanism involves Ca 2+ release from presynaptic Ca 2+ stores, whereas postsynaptic stores (activated by spot-uncaging of inositol 1,4,5-trisphosphate) remain unaffected by GPR55 agonists. These effects are abolished by genetic deletion of GPR55 or by the GPR55 antagonist cannabidiol, a constituent of Cannabis sativa. GPR55 shows colocalization with synaptic vesicle protein vesicular glutamate transporter 1 in stratum radiatum. Short-term potentiation of CA3-CA1 transmission after a short train of stimuli reveals a presynaptic, Ca 2+ store-dependent component sensitive to cannabidiol. The underlying cascade involves synthesis of phospholipids, likely in the presynaptic cell, but not the endocannabinoids 2-arachidonoylglycerol or anandamide. Our results thus unveil a signaling role for GPR55 in synaptic circuits of the brain. E ndocannabinoids play a major regulatory role in the functioning of neural circuitry. Classically, they are discharged by postsynaptic cells and target presynaptic cannabinoid type 1 (CB 1 ) receptors inhibiting neurotransmitter release (1, 2). Another documented cannabinoid target, the CB 2 receptor, is expressed almost entirely outside the brain, and little is known about any other cannabinoid signaling pathways. The cannabinoid-sensitive receptor G protein-coupled receptor 55 (GPR55) was identified and cloned a decade ago (3): Its presence in the brain, including the hippocampus, has been shown using quantitative PCR (4, 5). Although GPR55 activity can be modulated by certain phytoand endocannabinoids (4, 6), recent studies have suggested that L-α-lysophosphatidylinositol (LPI), which activates GPR55 but not CB 1 or CB 2 receptors, could be its endogenous ligand (7,8). Conversely, cannabidiol (CBD), a major constituent of Cannabis sativa, is a GPR55 antagonist, with low affinity for CB 1 receptors (4, 9). In cell cultures, activation of GPR55 evokes intracellular Ca 2+ oscillations through an inositol 1,4,5-trisphosphate (IP 3 )-sensitive mechanism mobilizing Ca 2+ stores (8, 10, 11) whereas CBD can modulate neuronal Ca 2+ depending on cell excitability (12). However, the GPR55 pharmacology is enigmatic, and its adaptive role in the brain remains unknown.In the hippocampus, presynaptic Ca 2+ stores contribute to repetitive release of glutamate (13,14). Because GPR55 action has been attributed to Ca 2+ stores, we asked whether these receptors influence transmission at CA3-CA1 synapses. To probe this mechanism at the single-synapse level, we combin...

Protein aggregation and abnormal lipid homeostasis are both implicated in neurodegeneration through unknown mechanisms. Here we demonstrate that aggregate-membrane interaction is critical to induce a form of cell death called ferroptosis. Importantly, the aggregate-membrane interaction that drives ferroptosis depends both on the conformational structure of the aggregate, as well as the oxidation state of the lipid membrane. We generated human stem cell-derived models of synucleinopathy, characterized by the intracellular formation of α-synuclein aggregates that bind to membranes. In human iPSC-derived neurons with SNCA triplication, physiological concentrations of glutamate and dopamine induce abnormal calcium signaling owing to the incorporation of excess α-synuclein oligomers into membranes, leading to altered membrane conductance and abnormal calcium influx. α-synuclein oligomers further induce lipid peroxidation. Targeted inhibition of lipid peroxidation prevents the aggregate-membrane interaction, abolishes aberrant calcium fluxes, and restores physiological calcium signaling. Inhibition of lipid peroxidation, and reduction of iron-dependent accumulation of free radicals, further prevents oligomer-induced toxicity in human neurons. In summary, we report that peroxidation of polyunsaturated fatty acids underlies the incorporation of β-sheet-rich aggregates into the membranes, and that additionally induces neuronal death. This suggests a role for ferroptosis in Parkinson’s disease, and highlights a new mechanism by which lipid peroxidation causes cell death.

A Rs, which can detect extracellular GABA. Such tonic GABA A R-mediated currents are particularly evident in dentate granule cells in which they play a major role in regulating cell excitability. Here we show that in rat dentate granule cells in ex vivo hippocampal slices, tonic currents are predominantly generated by GABA-independent GABA A receptor openings. This tonic GABA A R conductance is resistant to the competitive GABA A R antagonist SR95531 (gabazine), which at high concentrations acts as a partial agonist, but can be blocked by an open channel blocker, picrotoxin. When slices are perfused with 200 nM GABA, a concentration that is comparable to CSF concentrations but is twice that measured by us in the hippocampus in vivo using zero-net-flux microdialysis, negligible GABA is detected by dentate granule cells. Spontaneously opening GABA A Rs, therefore, maintain dentate granule cell tonic currents in the face of low extracellular GABA concentrations.

The synaptic response waveform, which determines signal integration properties in the brain, depends on the spatiotemporal profile of neurotransmitter in the synaptic cleft. Here, we show that electrophoretic interactions between AMPA-receptor-mediated excitatory currents and negatively charged glutamate molecules accelerate the clearance of glutamate from the synaptic cleft, speeding-up synaptic responses. This phenomenon is reversed upon depolarization and diminished when intra-cleft electric fields are weakened through a decrease in the AMPA receptor density. In contrast, the kinetics of receptor-mediated currents evoked by direct application of glutamate are voltage-independent, as are synaptic currents mediated by the electrically neutral neurotransmitter GABA. Voltage-dependent temporal tuning of excitatory synaptic responses may thus contribute to signal integration in neural circuits.Although ion currents through postsynaptic receptors are small (~10 −11 A), they can exert a lateral voltage gradient (electric field) of ~10 4 V/m inside the synaptic cleft (1, 2) raising the possibility that they can affect the dwell time of electrically charged neurotransmitters (3). Does electrodiffusion therefore play any role in synaptic transmission?The excitatory neurotransmitter glutamate is negatively charged at physiological pH (pK = 4.4), implying that postsynaptic depolarization should in principle retard its escape from the synaptic cleft (Fig. 1, A). AMPA-receptor-mediated excitatory postsynaptic currents (AMPAR EPSCs) decay more slowly at positive than at negative holding voltages in hippocampal basket cells (4) and in cerebellar granule cells (5). However, this has not been reported for AMPAR EPSCs generated at perisomatic synapses on CA1 or CA3 pyramidal cells (6-8). We evoked dendritic AMPAR EPSCs in CA1 pyramidal cells by stimulating Schaffer collaterals: the EPSC decay time ⊺ (defined here as the area/peak ratio) increased monotonically with depolarization ( Fig. 1, B). The ratio between ⊺ recorded at +40 mV and at −70 mV (⊺ +40 / ⊺ −70 ) was consistently above one (average ± SEM: 2.17 ± 0.09, n = 49, p < 0.001; fig. S1, A). This asymmetry was independent of the EPSC amplitude, glutamate transport or recording temperature, and could not be accounted for by unknown voltagedependent properties of receptor antagonists ( fig. S1, A A trivial possible explanation for this phenomenon is that AMPARs themselves have voltage-dependent kinetics. This has indeed been reported for AMPARs activated by brief pulses of glutamate applied to outside-out patches excised from brainstem neurons (9, 10), but not from hippocampal or dentate granule neurons (11). We confirmed that the decay of AMPAR currents evoked by 1 ms / 1 mM glutamate pulses in outside-out patches excised from somata (n = 9) or dendrites (n = 6) of CA1 pyramidal cells was indistinguishable at positive and negative voltages. Symmetrical decay kinetics were also observed when the AMPAR density was decreased in the patch with 0.1 μM NBQX (Fig. 1, C)...