Adenosine A2A receptors control generalization of contextual fear in rats.

Fear learning is essential to survival, but traumatic events may lead to abnormal fear consolidation and overgeneralization, triggering fear responses in safe environments, as occurs in post-traumatic stress disorder (PTSD). Adenosine A2A receptors (A2AR) control emotional memory and fear conditioning, but it is not known if they affect the consolidation and generalization of fear, which was now investigated. We now report that A2AR blockade through systemic administration of the A2AR antagonist SCH58261 immediately after contextual fear conditioning (within the consolidation window), accelerated fear generalization. Conversely, A2AR activation with CGS21680 decreased fear generalization. Ex vivo electrophysiological recordings of field excitatory post-synaptic potentials (fEPSPs) in CA3-CA1 synapses and of population spikes in the lateral amygdala (LA), showed that the effect of SCH58261 is associated with a reversion of fear conditioning-induced decrease of long-term potentiation (LTP) in the dorsal hippocampus (DH) and with increased amplitude of LA LTP in conditioned animals. These data suggest that A2AR are engaged during contextual fear consolidation, controlling long-term potentiation mechanisms in both DH and LA during fear consolidation, impacting on fear generalization; this supports targeting A2AR during fear consolidation to control aberrant fear processing in PTSD and other fear-related disorders.

The physiological effects of caffeine on synaptic transmission and plasticity in the mouse hippocampus selectively depend on adenosine A1 and A2A receptors.

Caffeine is the most consumed psychoactive drug worldwide and its intake in moderate amounts prevents neurodegenerative disorders. However, the molecular targets of caffeine to modulate activity in brain circuits are ill-defined. By electrophysiologically recording synaptic transmission and plasticity in Schaffer fibers-CA1 pyramid synapses of mouse hippocampal slices, we characterized the impact of caffeine using a concentration reached in the brain parenchyma upon moderate caffeine consumption. Caffeine (50 µM) facilitated synaptic transmission by 40%, while decreasing paired-pulse facilitation, and also decreased by 35% the amplitude of long-term potentiation (LTP). Clearance of extracellular adenosine with adenosine deaminase (2 U/mL) blunted all the effects of caffeine on synaptic transmission and plasticity. The A1R antagonist DPCPX (100 nM) only eliminated caffeine-induced facilitation of synaptic transmission while not affecting caffeine-induced depression of LTP; conversely, the genetic (using A2AR knockout mice) or the pharmacological blockade (with SCH58261, 50 nM) of A2AR eliminated the effect of caffeine on LTP while not affecting caffeine-induced facilitation of synaptic transmission. Finally, blockade of GABAA or of ryanodine receptors with bicuculline (10 µM) or dantrolene (10 µM), respectively, did not affect the ability of caffeine to alter synaptic transmission or plasticity. These results show that the effects of caffeine on synaptic transmission and plasticity in the hippocampus are selectively mediated by antagonizing adenosine receptors, where A1R are responsible for the impact of caffeine on synaptic transmission and A2AR regulate the impact of caffeine on LTP.

Blockade of adenosine A2A receptors recovers early deficits of memory and plasticity in the triple transgenic mouse model of Alzheimer's disease.

Alzheimer's disease (AD) begins with a deficit of synaptic function and adenosine A2A receptors (A2AR) are mostly located in synapses controlling synaptic plasticity. The over-activation of adenosine A2A receptors (A2AR) causes memory deficits and the blockade of A2AR prevents memory damage in AD models. We now enquired if this prophylactic role of A2AR might be extended to a therapeutic potential. We used the triple transgenic model of AD (3xTg-AD) and defined that the onset of memory dysfunction occurred at 4 months of age in the absence of locomotor or emotional alterations. At the onset of memory deficits, 3xTg mice displayed a decreased density of markers of excitatory synapses (10.6 ±â€¯3.8% decrease of vGluT1) without neuronal or glial overt damage and an increase of synaptic A2AR in the hippocampus (130 ±â€¯22%). After the onset of memory deficits in 3xTg-AD mice, a three weeks treatment with the selective A2AR antagonist normalized the up-regulation of hippocampal A2AR and restored hippocampal-dependent reference memory, as well as the decrease of hippocampal synaptic plasticity (60.0 ±â€¯3.7% decrease of long-term potentiation amplitude) and the decrease of global (syntaxin-I) and glutamatergic synaptic markers (vGluT1). These findings show a therapeutic-like ability of A2AR antagonists to recover synaptic and memory dysfunction in early AD.

Adenosine A2b receptors control A1 receptor-mediated inhibition of synaptic transmission in the mouse hippocampus.

Adenosine is a neuromodulator mostly acting through A1 (inhibitory) and A2A (excitatory) receptors in the brain. A2B receptors (A(2B)R) are G(s/q)--protein-coupled receptors with low expression in the brain. As A(2B)R function is largely unknown, we have now explored their role in the mouse hippocampus. We performed electrophysiological extracellular recordings in mouse hippocampal slices, and immunological analysis of nerve terminals and glutamate release in hippocampal slices and synaptosomes. Additionally, A(2B)R-knockout (A(2B)R-KO) and C57/BL6 mice were submitted to a behavioural test battery (open field, elevated plus-maze, Y-maze). The A(2B)R agonist BAY60-6583 (300 nM) decreased the paired-pulse stimulation ratio, an effect prevented by the A(2B)R antagonist MRS 1754 (200 nM) and abrogated in A(2B)R-KO mice. Accordingly, A(2B)R immunoreactivity was present in 73 ± 5% of glutamatergic nerve terminals, i.e. those immunopositive for vesicular glutamate transporters. Furthermore, BAY 60-6583 attenuated the A(1)R control of synaptic transmission, both the A(1)R inhibition caused by 2-chloroadenosine (0.1-1 µM) and the disinhibition caused by the A(1)R antagonist DPCPX (100 nM), both effects prevented by MRS 1754 and abrogated in A(2B)R-KO mice. BAY 60-6583 decreased glutamate release in slices and also attenuated the A(1)R inhibition (CPA 100 nM). A(2B)R-KO mice displayed a modified exploratory behaviour with an increased time in the central areas of the open field, elevated plus-maze and the Y-maze and no alteration of locomotion, anxiety or working memory. We conclude that A(2B)R are present in hippocampal glutamatergic terminals where they counteract the predominant A(1)R-mediated inhibition of synaptic transmission, impacting on exploratory behaviour.

Hyperactivation of D1 and A2A receptors contributes to cognitive dysfunction in Huntington's disease.

Stimulation of dopamine D1 receptor (D1R) and adenosine A2A receptor (A2AR) increases cAMP-dependent protein kinase (PKA) activity in the brain. In Huntington's disease, by essentially unknown mechanisms, PKA activity is increased in the hippocampus of mouse models and patients and contributes to hippocampal-dependent cognitive impairment in R6 mice. Here, we show for the first time that D1R and A2AR density and functional efficiency are increased in hippocampal nerve terminals from R6/1 mice, which accounts for increased cAMP levels and PKA signaling. In contrast, PKA signaling was not altered in the hippocampus of Hdh(Q7/Q111) mice, a full-length HD model. In line with these findings, chronic (but not acute) combined treatment with D1R plus A2AR antagonists (SCH23390 and SCH58261, respectively) normalizes PKA activity in the hippocampus, facilitates long-term potentiation in behaving R6/1 mice, and ameliorates cognitive dysfunction. By contrast, chronic treatment with either D1R or A2AR antagonist alone does not modify PKA activity or improve cognitive dysfunction in R6/1 mice. Hyperactivation of both D1R and A2AR occurs in HD striatum and chronic treatment with D1R plus A2AR antagonists normalizes striatal PKA activity but it does not affect motor dysfunction in R6/1 mice. In conclusion, we show that parallel alterations in dopaminergic and adenosinergic signaling in the hippocampus contribute to increase PKA activity, which in turn selectively participates in hippocampal-dependent learning and memory deficits in HD. In addition, our results point to the chronic inhibition of both D1R and A2AR as a novel therapeutic strategy to manage early cognitive impairment in this neurodegenerative disease.

The P2X7 receptor antagonist Brilliant Blue G attenuates contralateral rotations in a rat model of Parkinsonism through a combined control of synaptotoxicity, neurotoxicity and gliosis.

Parkinson's disease (PD) involves an initial loss of striatal dopaminergic terminals evolving into a degeneration of dopaminergic neurons in the substantia nigra (SN), which can be modeled by 6-hydroxydopamine (6-OHDA) administration. Since ATP is a danger signal acting through its P2X7 receptors (P2X7R), we now tested if a blood-brain barrier-permeable P2X7R antagonist, Brilliant Blue G (BBG), controlled the 6-OHDA-induced PD-like features in rats. BBG (45 mg/kg) attenuated the 6-OHDA-induced: 1) increase of contralateral rotations in the apomorphine test, an effect mimicked by another P2X7R antagonist A438079 applied intra-cerebroventricularly; 2) short-term memory impairment in the passive avoidance and cued version of the Morris Water maze; 3) reduction of dopamine content in the striatum and SN; 4) microgliosis and astrogliosis in the striatum. To grasp the mechanism of action of BBG, we used in vitro models exploring synaptotoxicity (striatal synaptosomes) and neurotoxicity (dopamine-differentiated neuroblastoma SH-SY5Y cells). P2X7R were present in striatal dopaminergic terminals, and BBG (100 nM) prevented the 6-OHDA-induced synaptosomal dysfunction. P2X7R were also co-localized with tyrosine hydroxylase in SH-SY5Y cells, where BBG (100 nM) attenuated the 6-OHDA-induced neurotoxicity. This suggests that P2X7R contribute to PD pathogenesis through a triple impact on synaptotoxicity, gliosis and neurotoxicity, highlighting the therapeutic potential of P2X7R antagonists in PD.