The NMDA receptor is thought to play a central role in some forms of neuronal plasticity, including the induction of long-term potentiation. NMDA receptor hypofunction can result in mnemonic impairment and has been implicated in the cognitive symptoms of schizophrenia. The activity of NMDA receptors is controlled by its endogenous coagonist glycine, and a local elevation of glycine levels is expected to enhance NMDA receptor function. Here, we achieved this by the generation of a novel mouse line (CamKII␣Cre;Glyt1tm1.2fl/ fl) with a neuron and forebrain selective disruption of glycine transporter 1 (GlyT1). The mutation led to a significant reduction of GlyT1 and a corresponding reduction of glycine reuptake in forebrain samples, without affecting NMDA receptor expression. NMDA (but not AMPA) receptor-evoked EPSCs recorded in hippocampal slices of mutant mice were 2.5 times of those recorded in littermate controls, suggesting that neuronal GlyT1 normally assumes a specific role in the regulation of NMDA receptor responses. Concomitantly, the mutants were less responsive to phencyclidine than controls. The mutation enhanced aversive Pavlovian conditioning without affecting spontaneous anxiety-like behavior in the elevated plus maze and augmented a form of attentional learning called latent inhibition in three different experimental paradigms: conditioned freezing, conditioned active avoidance, conditioned taste aversion. The CamKII␣Cre; Glyt1tm1.2fl/fl mouse model thus suggests that augmentation of forebrain neuronal glycine transmission is promnesic and may also offer an effective therapeutic intervention against the cognitive and attentional impairments characteristic of schizophrenia.

Pharmacotherapy of schizophrenia based on the dopamine hypothesis remains unsatisfactory for the negative and cognitive symptoms of the disease. Enhancing N-methyl-d-aspartate receptors (NMDAR) function is expected to alleviate such persistent symptoms, but successful development of novel clinically effective compounds remains challenging. Adenosine is a homeostatic bioenergetic network modulator that is able to affect complex networks synergistically at different levels (receptor dependent pathways, biochemistry, bioenergetics, and epigenetics). By affecting brain dopamine and glutamate activities it represents a promising candidate for restoring the functional imbalance in these neurotransmitter systems believed to underlie the genesis of schizophrenia symptoms, as well as restoring homeostasis of bioenergetics. Suggestion of an adenosine hypothesis of schizophrenia further posits that adenosinergic dysfunction might contribute to the emergence of multiple neurotransmitter dysfunctionscharacteristic of schizophrenia via diverse mechanisms. Given the importance of adenosine in early brain development and regulation of brain immune response, it also bears direct relevance to the aetiology of schizophrenia. Here, we provide an overview of the rationale and evidence in support of the therapeutic potential of multiple adenosinergic targets, including the high-affinity adenosine receptors (A1R and A2AR), and the regulatory enzyme adenosine kinase (ADK). Key preliminary clinical data and preclinical findings are reviewed.

The neuromodulator adenosine fulfills a unique role in the brain affecting glutamatergic neurotransmission and dopaminergic signaling via activation of adenosine A1 and A2A receptors, respectively. The adenosine system is thus ideally positioned to integrate glutamatergic and dopaminergic neurotransmission, which in turn could affect behavior and cognition. In the adult brain, adenosine levels are largely regulated by its key metabolic enzyme adenosine kinase (ADK), which may assume the role of an 'upstream regulator' of these two neurotransmitter pathways. To test this hypothesis, transgenic mice with an overexpression of ADK in brain (Adk-tg), and therefore reduced brain adenosine levels, were evaluated in a panel of behavioral and psychopharmacological assays to assess possible glutamatergic and dopaminergic dysfunction. In comparison to non-transgenic control mice, Adk-tg mice are characterized by severe learning deficits in the Morris water maze task and in Pavlovian conditioning. The Adk-tg mice also exhibited reduced locomotor reaction to systemic amphetamine, whereas their reaction to the non-competitive N-methyl-d-aspartate receptor antagonist MK-801 was enhanced. Our results confirmed that ADK overexpression could lead to functional concomitant alterations in dopaminergic and glutamatergic functions, which is in keeping with the hypothesized role of ADK in the balance and integration between glutamatergic and dopaminergic neurotransmission. The present findings are of relevance to current pathophysiological hypotheses of schizophrenia and its pharmacotherapy.

An emerging theory of schizophrenia postulates that hypofunction of adenosine signaling may contribute to its pathophysiology. This study was designed to test the "adenosine hypothesis" of schizophrenia and to evaluate focal adenosine-based strategies for therapy. We found that augmentation of adenosine by pharmacologic inhibition of adenosine kinase (ADK), the key enzyme of adenosine clearance, exerted antipsychotic-like activity in mice. Further, overexpression of ADK in transgenic mice was associated with attentional impairments linked to schizophrenia. We observed that the striatal adenosine A 2A receptor links adenosine tone and psychomotor response to amphetamine, an indicator of dopaminergic signaling. Finally, intrastriatal implants of engineered adenosine-releasing cells restored the locomotor response to amphetamine in mice overexpressing ADK, whereas the same grafts placed proximal to the hippocampus of transgenic mice reversed their working memory deficit. This functional double dissociation between striatal and hippocampal adenosine demonstrated in Adk transgenic mice highlights the independent contributions of these two interconnected brain regions in the pathophysiology of schizophrenia and thus provides the rationale for developing local adenosine augmentation therapies for the treatment of schizophrenia. IntroductionDespite extensive research over half a century, schizophrenia remains a major health concern, affecting more than one percent of the population. Two hypotheses, those of dopaminergic hyperfunction (1) and glutamatergic hypofunction (2), are widely accepted conceptual frameworks for understanding the pathophysiology of schizophrenia and for the development of drugs for the treatment of either dopamine-related or glutamate-related symptoms of the disease (2, 3). This study addresses a third hypothesis, the "adenosine hypothesis of schizophrenia," which has recently been proposed as a novel concept to integrate the dopaminergic hyperfunction and glutamatergic hypofunction hypotheses (4).The purine ribonucleoside adenosine modulates neurotransmission through activation of 4 types of G protein-coupled adenosine receptors (ARs), A 1 R, A 2A R, A 2B R, and A 3 R, which exert spatially distinct functions within the brain on presynaptic and postsynaptic sites (5, 6). Presynaptically, adenosine regulates the release of both dopamine and glutamate (7,8), whereas the output of dopaminergic and glutamatergic neurotransmission is regulated by heterodimerization of ARs with dopamine and glutamate receptors (9, 10). Through these mechanisms adenosine exerts upstream control over both dopaminergic and glutamatergic signaling. Consequently, any disruption in adenosine homeostasis is expected to affect those 2 transmitter systems, which play fundamental roles in the pathophysiology of schizophrenia. This regulatory function of adenosine might provide a missing link for the functional integration of the dopamine and glutamate hypotheses. Conventional antipsychotic

The adenosine A 2A receptor (A 2A R) is highly enriched in the striatum where it is uniquely positioned to integrate dopaminergic, glutamatergic, and other signals to modulate cognition. Although previous studies support the hypothesis that A 2A R inactivation can be pro-cognitive, analyses of A 2A R's effects on cognitive functions have been restricted to a small subset of cognitive domains. Furthermore, the relative contribution of A 2A Rs in distinct brain regions remains largely unknown. Here, we studied the regulation of multiple memory processes by brain region-specific populations of A 2A Rs. Specifically, we evaluated the cognitive impacts of conditional A 2A R deletion restricted to either the entire forebrain (i.e., cerebral cortex, hippocampus, and striatum, fb-A 2A R KO) or to striatum alone (st-A 2A R KO) in recognition memory, working memory, reference memory, and reversal learning. This comprehensive, comparative analysis showed for the first time that depletion of A 2A R-dependent signaling in either the entire forebrain or striatum alone is associated with two specific phenotypes indicative of cognitive flexibility-enhanced working memory and enhanced reversal learning. These selective pro-cognitive phenotypes seemed largely attributed to inactivation of striatal A 2A Rs as they were captured by A 2A R deletion restricted to striatal neurons. Neither spatial reference memory acquisition nor spatial recognition memory were grossly affected, and no evidence for compensatory changes in striatal or cortical D 1 , D 2 , or A 1 receptor expression was found. This study provides the first direct demonstration that targeting striatal A 2A Rs may be an effective, novel strategy to facilitate cognitive flexibility under normal and pathologic conditions.

Background Adenosine A2A receptors (A2ARs) are enriched in the striatum but are also present at lower levels in the extra-striatal forebrain (i.e., hippocampus, cortex), integrating dopamine, glutamate, and brain-derived neurotrophic factor (BDNF) signaling, and are thus essential for striatal neuroplasticity and fear and anxiety behavior. Methods We tested two brain region-specific A2AR knockout lines with A2ARs selectively inactivated either in the striatum only (st-A2AR KO) or the entire forebrain (striatum, hippocampus, and cortex, fb-A2AR KO) on fear and anxiety-related responses. We also examined the effect of hippocampus-specific A2AR deletion by local injection of AAV5-Cre into conditional (floxed)-A2AR knockout mice. Results Selective deletion of striatal A2ARs in st-A2AR KO mice increased Pavlovian fear conditioning (both context and tone), but when the A2AR deletion was extended to include extra-striatal regions in fb-A2AR KO mice, context fear conditioning was normalized and tone fear conditioning was attenuated. Moreover, focal deletion of hippocampal A2ARs by AAV5-Cre injection selectively attenuated context (but not tone) fear conditioning. Deletion of A2ARs in the entire forebrain in fb-A2AR KO mice also produced an anxiolytic phenotype in both the elevated plus maze and open field tests and increased the startle response. These extra-striatal forebrain A2AR behavioral effects were associated with reduced BDNF levels in the fb-A2AR KO hippocampus. Conclusions This study provides the first evidence that inactivation of striatal A2ARs facilitates Pavlovian fear conditioning while inactivation of extra-striatal A2ARs in the forebrain inhibits fear conditioning and also affects anxiety-related behavior.

We conclude that social isolation in mice represents a valuable tool for the examination of candidate genes within the context of the "two-hit" hypothesis of the aetiological processes in schizophrenia.

Selective deletion of glycine transporter 1 (GlyT1) in forebrain neurons enhances N-methyl-D-aspartate receptor (NMDAR)-dependent neurotransmission and facilitates associative learning. These effects are attributable to increases in extracellular glycine availability in forebrain neurons due to reduced glycine re-uptake. Using a forebrain- and neuron-specific GlyT1-knockout mouse line (CamKIIalphaCre; GlyT1tm1.2fl/fI), the authors investigated whether this molecular intervention can affect recognition memory. In a spontaneous object recognition memory test, enhanced preference for a novel object was demonstrated in mutant mice relative to littermate control subjects at a retention interval of 2 hr, but not at 2 min. Furthermore, mutants were responsive to a switch in the relative spatial positions of objects, whereas control subjects were not. These potential procognitive effects were demonstrated against a lack of difference in contextual novelty detection: Mutant and control subjects showed equivalent preference for a novel over a familiar context. Results therefore extend the possible range of potential promnesic effects of specific forebrain neuronal GlyT1 deletion from associative learning to recognition memory and further support the possibility that mnemonic functions can be enhanced by reducing GlyT1 function.