Dopamine D2 receptor blockade has been an obligate mechanism of action present in all medications that effectively treat positive symptoms of schizophrenia (e.g., delusions and hallucinations) and have been approved by regulatory agencies since the 1950s. Blockade of 5-hydroxytrypatmine 2A receptors plays a contributory role in the actions of the second generation of antipsychotic drugs, the so-called atypical antipsychotics. Nevertheless, substantial unmet medical needs remain for the treatment of negative symptoms and cognitive dysfunction. Recognition that dissociative anesthetics block the N-methyl-D-aspartate (NMDA) receptor channel has inspired a search for glutamatergic therapeutic mechanisms because ketamine and phencyclidine are known to induce psychotic-like symptoms in healthy volunteers and exacerbate the symptoms of patients with schizophrenia. Current pathophysiological theories of schizophrenia emphasize that hypofunction of NMDA receptors at critical sites in local circuits modulate the function of a given brain region or control projections from one region to another (e.g., hippocampal-cortical or thalamocortical projections). The demonstration that a metabotropic glutamate 2/3 (mGlu2/3) receptor agonist prodrug decreased both positive and negative symptoms of schizophrenia raised hopes that glutamatergic mechanisms may provide therapeutic advantages. In addition to discussing the activation of mGlu2 receptors with mGlu2/3 receptor agonists or mGlu2 receptor positive allosteric modulators (PAMs), we discuss other methods that may potentially modulate circuits with hypofunctional NMDA receptors such as glycine transporter inhibitors and mGlu5 receptor PAMs. The hope is that by modulating glutamatergic neurotransmission, the dysfunctional circuitry of the schizophrenic brain (both local circuits and long-loop pathways) will be improved. Dopamine and serotonin, as opposed to emerging interest in glutamate, played a dominant role during the formative years of modern psychopharmacology when thinking about the pathophysiology and treatment of schizophrenia. Models derived from studying the behavioral effects of amphetamine and serotonergic hallucinogens were emphasized (Table 1). The seminal observations by Arvid Carlsson and his colleagues in the late early 1960s that first postulated functional dopamine receptor blockade as a necessary therapeutic action by the phenothiazine and butyrophenone structural classes set the stage for our current mechanistic understanding of all antipsychotic drugs currently used today (Carlsson and Lindqvist, 1963). The emergence of in vitro receptor binding and positron emission topography receptor occupancy studies in healthy volunteers and patients identified dopamine D2 receptor Article, publication date, and citation information can be found at

In the forebrain, synaptic glycine concentrations are regulated through the glycine transporter GlyT1. Because glycine is a coagonist of the N-methyl-D-aspartate (NMDA) receptor (NMDAR), which has been implicated in schizophrenia, inhibition of GlyT1 is thought to provide an option for the treatment of schizophrenia.

Antagonists at the ionotropic non-NMDA [AMPA (amino-methyl proprionic acid)/kainate] type of glutamate receptors have been suggested to possess several advantages compared to NMDA (N-methyl-D-aspartate) receptor antagonists, particularly in terms of risk/benefit ratio, but the non-NMDA receptor antagonists available so far have not fulfilled this promise. From a large series of pyrrolyl-quinoxalinedione derivatives, we selected six new competitive non-NMDA receptor antagonists. The basis of selection was high potency and selectivity for AMPA and/or kainate receptors, high in vivo potency after systemic administration, and an acceptable ratio between neuroprotective or anticonvulsant effects and adverse effects. Pharmacological characteristics of these novel compounds are described in this study with special emphasis on their effects in the kindling model of temporal lobe epilepsy, the most common type of epilepsy in humans. In most experiments, NBQX and the major antiepileptic drug valproate were used for comparison with the novel compounds. The novel non-NMDA receptor antagonists markedly differed in their AMPA and kainate receptor affinities from NBQX. Thus, while NBQX essentially did not bind to kainate receptors at relevant concentrations, several of the novel compounds exhibited affinity to rat brain kainate receptors or recombinant kainate receptor subtypes in addition to AMPA receptors. One compound, LU 97175, bound to native high affinity kainate receptors and rat GluR5-GluR7 subunits, i.e. low affinity kainate binding sites, with much higher affinities than to AMPA receptors. All compounds potently blocked AMPA-induced cell death in vitro and, except LU 97175, AMPA-induced convulsions in vivo. In the kindling model, compounds with a high affinity for GluR7 (LU 97175) or compounds (LU 115455, LU 136541) which potently bind to AMPA receptors and low affinity kainate receptor subunits were potent anticonvulsants in the kindling model, whereas the AMPA receptor-selective LU 112313 was the least selective compound in this model, indicating that non-NMDA antagonists acting at both AMPA and kainate receptors are more effective in this model than AMPA receptor-selective drugs. Three of the novel compounds, i.e. LU 97175, LU 115455 and LU 136541, exerted potent anticonvulsant effects without inducing motor impairment in the rotarod test. This combination of actions is thought to be a prerequisite for selective anticonvulsant drug action.

The research domain criteria (RDoC) matrix has been developed to reorient psychiatric research towards measurable behavioral dimensions and underlying mechanisms. Here, we used a new genetic rat model with a loss-of-function point mutation in the dopamine transporter (DAT) gene (Slc6a3_N157K) to systematically study the RDoC matrix. First, we examined the impact of the Slc6a3_N157K mutation on monoaminergic signaling. We then performed behavioral tests representing each of the five RDoC domains: negative and positive valence systems, cognitive, social and arousal/regulatory systems. The use of RDoC may be particularly helpful for drug development. We studied the effects of a novel pharmacological approach metabotropic glutamate receptor mGluR2/3 antagonism, in DAT mutants in a comparative way with standard medications. Loss of DAT functionality in mutant rats not only elevated subcortical extracellular dopamine concentration but also altered the balance of monoaminergic transmission. DAT mutant rats showed deficits in all five RDoC domains. Thus, mutant rats failed to show conditioned fear responses, were anhedonic, were unable to learn stimulus-reward associations, showed impaired cognition and social behavior, and were hyperactive. Hyperactivity in mutant rats was reduced by amphetamine and atomoxetine, which are well-established medications to reduce hyperactivity in humans. The mGluR2/3 antagonist LY341495 also normalized hyperactivity in DAT mutant rats without affecting extracellular dopamine levels. We systematically characterized an altered dopamine system within the context of the RDoC matrix and studied mGluR2/3 antagonism as a new pharmacological strategy to treat mental disorders with underlying subcortical dopaminergic hyperactivity.

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