Human and animal studies have converged to suggest that caffeine consumption prevents memory deficits in aging and Alzheimer’s disease through the antagonism of adenosine A2A receptors (A2AR). To test if A2AR activation in hippocampus is actually sufficient to impair memory function and to begin elucidating the intracellular pathways operated by A2AR, we have developed a chimeric rhodopsin-A2AR protein (optoA2AR), which retains the extracellular and transmembrane domains of rhodopsin (conferring light responsiveness and eliminating adenosine binding pockets) fused to the intracellular loop of A2AR to confer specific A2AR signaling. The specificity of the optoA2AR signaling was confirmed by light-induced selective enhancement of cAMP and phospho-MAPK (but not cGMP) levels in HEK293 cells, which was abolished by a point mutation at the C-terminal of A2AR. Supporting its physiological relevance, optoA2AR activation and the A2AR agonist CGS21680 produced similar activation of cAMP and phospho-MAPK signaling in HEK293 cells, of pMAPK in nucleus accumbens, of c-Fos/pCREB in hippocampus and similarly enhanced long-term potentiation in hippocampus. Remarkably, optoA2AR activation triggered a preferential phospho-CREB signaling in hippocampus and impaired spatial memory performance while optoA2AR activation in the nucleus accumbens triggered MAPK signaling and modulated locomotor activity. This shows that the recruitment of intracellular A2AR signaling in hippocampus is sufficient to trigger memory dysfunction. Furthermore, the demonstration that the biased A2AR signaling and functions depend on intracellular A2AR loops, prompts the possibility of targeting the intracellular A2AR interacting partners to selectively control different neuropsychiatric behaviors.

Cognitive impairments in Huntington's disease (HD) are attributed to a dysfunction of the cortico-striatal pathway and significantly affect the quality of life of the patients, but this has not been a therapeutic focus in HD to date. We postulated that adenosine A(2A) receptors (A(2A)R), located at pre- and post-synaptic elements of the cortico-striatal pathways, modulate striatal neurotransmission and synaptic plasticity and cognitive behaviors. To critically evaluate the ability of A(2A)R inactivation to prevent cognitive deficits in early HD, we cross-bred A(2A)R knockout (KO) mice with two R6/2 transgenic lines of HD (CAG120 and CAG240) to generate two double transgenic R6/2-CAG120-A(2A)R KO and R6/2-CAG240-A(2A)R KO mice and their corresponding wild-type (WT) littermates. Genetic inactivation of A(2A)R prevented working memory deficits induced by R6/2-CAG120 at post-natal week 6 and by R6/2-CAG240 at post-natal month 2 and post-natal month 3, without modifying motor deficits. Similarly the A2(A)R antagonist KW6002 selectively reverted working memory deficits in R6/2-CAG240 mice at post-natal month 3. The search for possible mechanisms indicated that the genetic inactivation of A(2A)R did not affect ubiquitin-positive neuronal inclusions, astrogliosis or Thr-75 phosphorylation of DARPP-32 in the striatum. Importantly, A(2A)R blockade preferentially controlled long-term depression at cortico-striatal synapses in R6/2-CAG240 at post-natal week 6. The reported reversal of working memory deficits in R6/2 mice by the genetic and pharmacological inactivation of A(2A)R provides a proof-of-principle for A(2A)R as novel targets to reverse cognitive deficits in HD, likely by controlling LTD deregulation.

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