There are no effective therapeutics for cognitive impairments associated with schizophrenia (CIAS), which includes deficits in executive functions (working memory and cognitive flexibility) and episodic memory. Compounds that have entered clinical trials are inadequate in terms of efficacy and/or tolerability, highlighting a clear translational bottleneck and a need for a cohesive preclinical drug development strategy. In this review we propose hippocampal−prefrontal-cortical (HPC−PFC) circuitry underlying CIAS-relevant cognitive processes across mammalian species as a target source to guide the translation-focused discovery and development of novel, procognitive agents. We highlight several G protein-coupled receptors (GPCRs) enriched within HPC−PFC circuitry as therapeutic targets of interest, including noncanonical approaches (biased agonism and allosteric modulation) to conventional clinical targets, such as dopamine and muscarinic acetylcholine receptors, along with prospective novel targets, including the orphan receptors GPR52 and GPR139. We also describe the translational limitations of popular preclinical cognition tests and suggest touchscreen-based assays that probe cognitive functions reliant on HPC−PFC circuitry and reflect tests used in the clinic, as tests of greater translational relevance. Combining pharmacological and behavioral testing strategies based in HPC−PFC circuit function creates a cohesive, translationfocused approach to preclinical drug development that may improve the translational bottleneck currently hindering the development of treatments for CIAS.

The orphan Gαs-coupled receptor GPR52 is expressed exclusively in the brain, predominantly in circuitry relating to symptoms of neuropsychiatric and cognitive disorders such as schizophrenia. While GPR52 agonists have displayed antipsychotic and procognitive efficacy in murine models, there remains limited evidence delineating the molecular mechanisms of these effects. Indeed, previous studies have solely reported canonical cAMP signaling and CREB phosphorylation downstream of GPR52 activation. In the present study, we demonstrated that the synthetic GPR52 agonist, 3-BTBZ, equipotently induces cAMP accumulation, ERK1/2 phosphorylation, and β-arrestin-1 and -2 recruitment in transfected HEK293T cells. In cultured frontal cortical neurons, however, 3-BTBZ-induced ERK1/2 phosphorylation was significantly more potent than cAMP signaling, with a more prolonged signaling profile than that in HEK293T cells. Furthermore, knock down of β-arrestin-2 in frontal cortical neurons abolished 3-BTBZ-induced ERK1/2 phosphorylation, but not cAMP accumulation. These results suggest a β-arrestin-2-dependent mechanism for GPR52-mediated ERK1/2 signaling, which may link to cognitive function in vivo. Finally, these findings highlight the context-dependence of GPCR signaling in recombinant cells and neurons, offering new insights into translationally relevant GPR52 signaling mechanisms.

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GPR52 is an orphan G protein-coupled receptor (GPCR) expressed in brain regions subject to dopaminergic and glutamatergic dysregulation in schizophrenia. GPR52 is highly expressed in the striatum, exclusively on dopamine D 2 receptor-expressing medium spiny neurons (MSNs), and also on dopamine D 1 receptor-expressing cortical pyramidal neurons. To examine the neurophysiological role of GPR52, we measured cAMP, ERK1/2 phosphorylation and βarrestin recruitment in cells transfected with human GPR52 and/or mouse primary cultured embryonic striatal or cortical neurons treated with the selective synthetic agonist 3-[2-(3-chloro-5-uorobenzyl)-1-benzothiophen-7-yl]-N-(2methoxyethyl)benzamide (3-BTBZ). We also investigated 3-BTBZ-regulated phosphorylation of the striatal signal integrator, DARPP-32, at Thr34 and Thr75 in D 1-and D 2 receptor-expressing MSNs, by quantitative immunohistochemistry in acute mouse brain slices (Figure 1). In vivo activity of 3-BTBZ (3-30 mg/kg, i.p.) was assessed against amphetamine or phencyclidine (both 3 mg/kg, i.p.) induced hyperactivity in mice and also for effects on spontaneous motor activity. 3-BTBZ stimulated cAMP accumulation in CHO-hGPR52 cells (pEC 50 = 7.5 ± 0.2) and in striatal neurons (pEC 50 = 8.3 ± 0.2). Surprisingly, 3-BTBZ significantly increased Thr75 phosphorylation in D 1-, but not D 2-expressing MSNs, with no effect on Thr34 phosphorylation (Figure 1). The modulatory effect on Thr75 phosphorylation was lost in slices lacking cortical projection neurons, indicative of extra-striatal GPR52 signalling. Accordingly, we showed that 3-BTBZ stimulated both cAMP (pEC 50 = 7.2 ± 0.3) and ERK1/2 phosphorylation (pEC 50 = 8.2 ± 0.2) in cortical neurons, the latter (but not the former) response fully sensitive to knockdown of β-arrestin-2. Systemic administration of 3-BTBZ (i.p.) significantly reduced amphetamine-induced hyperactivity at only 30 mg/kg, which also significantly reduced spontaneous motor activity. However 3-BTBZ significantly inhibited phencyclidineinduced hyperactivity (MED = 3 mg/kg). These data show that GPR52 activation, likely via cAMP and ERK1/2 signalling, modulates DARPP-32 phosphorylation and PCP-stimulated behaviours in a mouse model of psychosis.