The functional role of heteromers of G-protein-coupled receptors is a matter of debate. In the present study, we demonstrate that

The physiological conditions under which adenosine A2A receptors modulate synaptic transmission are presently unclear. We show that A2A receptors are localized postsynaptically at synapses between mossy fibers and CA3 pyramidal cells and are essential for a form of long-term potentiation (LTP) of NMDA-EPSCs induced by short bursts of mossy fiber stimulation. This LTP spares AMPA-EPSCs and is likely induced and expressed postsynaptically. It depends on a postsynaptic Ca2+ rise, on G protein activation, and on Src kinase. In addition to A2A receptors, LTP of NMDA-EPSCs requires the activation of NMDA and mGluR5 receptors as potential sources of Ca2+ increase. LTP of NMDA-EPSCs displays a lower threshold for induction as compared with the conventional presynaptic mossy fiber LTP; however, the two forms of LTP can combine with stronger induction protocols. Thus, postsynaptic A2A receptors may potentially affect information processing in CA3 neuronal networks and memory performance.

Single-wavelength fluorescent reporters allow visualization of specific neurotransmitters with high spatial and temporal resolution. We report variants of the glutamate sensor iGluSnFR that are functionally brighter, detect sub-micromolar to millimolar glutamate, and have blue, cyan, green, or yellow emission profiles. These variants allow in vivo imaging where original iGluSnFR was too dim, can resolve glutamate transients in dendritic spines and axonal boutons, and permit kilohertz imaging.

Decades after the discovery that ionic zinc is present at high levels in glutamatergic synaptic vesicles, where, when, and how much zinc is released during synaptic activity remains highly controversial. Here we provide a quantitative assessment of zinc dynamics in the synaptic cleft and clarify its role in the regulation of excitatory neurotransmission by combining synaptic recordings from mice deficient for zinc signaling with Monte Carlo simulations. Ambient extracellular zinc levels are too low for tonic occupation of the GluN2A-specific nanomolar zinc sites on NMDA receptors (NMDARs). However, following short trains of physiologically relevant synaptic stimuli, zinc transiently rises in the cleft and selectively inhibits postsynaptic GluN2A-NMDARs, causing changes in synaptic integration and plasticity. Our work establishes the rules of zinc action and reveals that zinc modulation extends beyond hippocampal mossy fibers to excitatory SC-CA1 synapses. By specifically moderating GluN2A-NMDAR signaling, zinc acts as a widespread activity-dependent regulator of neuronal circuits.

No abstract

The function of striatal adenosine A2Areceptors (A2ARs) is well recognized because of their high expression levels and the documented antagonistic interaction between A2ARs and dopamine D2receptors in the striatum. However, the role of extrastriatal A2ARs in modulating psychomotor activity is largely unexplored because of the low level of expression and lack of tools to distinguish A2ARs in intrinsic striatal versus nonstriatal neurons. Here, we provided direct evidence for the critical role of A2ARs in extrastriatal neurons in modulating psychomotor behavior using newly developed striatum-specific A2AR knock-out (st-A2AR KO) mice in comparison with forebrain-specific A2AR KO (fb-A2AR KO) mice. In contrast to fb-A2AR KO (deleting A2ARs in the neurons of striatum as well as cerebral cortex and hippocampus), st-A2AR KO mice exhibited Cre-mediated selective deletion of the A2AR gene, mRNA, and proteins in the neurons (but not astrocytes and microglial cells) of the striatum only. Strikingly, cocaine- and phencyclidine-induced psychomotor activities were enhanced in st-A2AR KO but attenuated in fb-A2AR KO mice. Furthermore, selective inactivation of the A2ARs in extrastriatal cells by administering the A2AR antagonist KW6002 into st-A2AR KO mice attenuated cocaine effects, whereas KW6002 administration into wild-type mice enhanced cocaine effects. These results identify a critical role of A2ARs in extrastriatal neurons in providing a prominent excitatory effect on psychomotor activity. These results indicate that A2ARs in striatal and extrastriatal neurons exert an opposing modulation of psychostimulant effects and provide the first direct demonstration of a predominant facilitatory role of extrastriatal A2ARs.

The anti-Parkinsonian effect of glutamate metabotropic group 5 (mGluR5) and adenosine A 2A receptor antagonists is believed to result from their ability to postsynaptically control the responsiveness of the indirect pathway that is hyperfunctioning in Parkinson's disease. mGluR5 and A 2A antagonists are also neuroprotective in brain injury models involving glutamate excitotoxicity. Thus, we hypothesized that the antiParkinsonian and neuroprotective effects of A 2A and mGluR5 receptors might be related to their control of striatal glutamate release that actually triggers the indirect pathway. The A 2A agonist, CGS21680 (1-30 nM) facilitated glutamate release from striatal nerve terminals up to 57%, an effect prevented by the A 2A antagonist, SCH58261 (50 nM). The mGluR5 agonist, CHPG (300-600 lM) also facilitated glutamate release up to 29%, an effect prevented by the mGluR5 antagonist, MPEP (10 lM). Both mGluR5 and A 2A receptors were located in the active zone and 57 ± 6% of striatal glutamatergic nerve terminals possessed both A 2A and mGluR5 receptors, suggesting a presynaptic functional interaction. Indeed, submaximal concentrations of CGS21680 (1 nM) and CHPG (100 lM) synergistically facilitated glutamate release and the facilitation of glutamate release by 10 nM CGS21680 was prevented by 10 lM MPEP, whereas facilitation by 300 lM CHPG was prevented by 10 nM SCH58261. These results provide the first direct evidence that A 2A and mGluR5 receptors are co-located in more than half of the striatal glutamatergic terminals where they facilitate glutamate release in a synergistic manner. This emphasizes the role of the modulation of glutamate release as a likely mechanism of action of these receptors both in striatal neuroprotection and in Parkinson's disease.

Interpretation: A 2A R activity in forebrain neurons is critical to the control of motor activity, whereas brain cells other than forebrain neurons (likely glial cells) are important components for protection against acute MPTP toxicity.