Antipsychotic-like profile of CIQ isomers in animal models of schizophrenia

Mechanisms of Action Underlying Conductance-Modifying Positive Allosteric Modulators of the NMDA Receptor

N-methyl-D-aspartate receptors (NMDARs) are ionotropic glutamate receptors that mediate a slow, Ca2+-permeable component of excitatory neurotransmission. Modulation of NMDAR function has the potential for disease modification as NMDAR dysfunction has been implicated in neurodevelopment, neuropsychiatric, neurologic, and neurodegenerative disorders. We recently described the thieno[2,3-day]pyrimidin-4-one (EU1622) class of positive allosteric modulators, including several potent and efficacious analogs. Here we have used electrophysiological recordings from Xenopus oocytes, human embryonic kidney cells, and cultured cerebellar and cortical neurons to determine the mechanisms of action of a representative member of this class of modulator. EU1622-240 enhances current response to saturating agonist (doubling response amplitude at 0.2-0.5 μM), slows the deactivation time course following rapid removal of glutamate, increases open probability, enhances coagonist potency, and reduces single-channel conductance. We also show that EU1622-240 facilitates NMDAR activation when only glutamate or glycine is bound. EU1622-240-bound NMDARs channels activated by a single agonist (glutamate or glycine) open to a unique conductance level with different pore properties and Mg2+ sensitivity, in contrast to channels arising from activation of NMDARs with both coagonists bound. These data demonstrate that previously hypothesized distinct gating steps can be controlled by glutamate and glycine binding and shows that the 1622-series modulators enable glutamate- or glycine-bound NMDARs to generate open conformations with different pore properties. The properties of this class of allosteric modulators present intriguing therapeutic opportunities for the modulation of circuit function. SIGNIFICANCE STATEMENT: NMDA receptors are expressed throughout the central nervous system and are permeable to calcium. EU1622-240 increases open probability and agonist potency while reducing single-channel conductance and prolonging the deactivation time course. EU1622-240 allows NMDA receptor activation by the binding of one coagonist (glycine or glutamate), which produces channels with distinct properties. Evaluation of this modulator provides insight into gating mechanisms and may lead to the development of new therapeutic strategies.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

Cognitive Deficit in Schizophrenia: From Etiology to Novel Treatments

Schizophrenia is a major mental illness characterized by positive and negative symptoms, and by cognitive deficit. Although cognitive impairment is disabling for patients, it has been largely neglected in the treatment of schizophrenia. There are several reasons for this lack of treatments for cognitive deficit, but the complexity of its etiology-in which neuroanatomic, biochemical and genetic factors concur-has contributed to the lack of effective treatments. In the last few years, there have been several attempts to develop novel drugs for the treatment of cognitive impairment in schizophrenia. Despite these efforts, little progress has been made. The latest findings point to the importance of developing personalized treatments for schizophrenia which enhance neuroplasticity, and of combining pharmacological treatments with non-pharmacological measures.

Facilitation of GluN2C-containing NMDA receptors in the external globus pallidus increases firing of fast spiking neurons and improves motor function in a hemiparkinsonian mouse model

Globus pallidus externa (GPe) is a nucleus in the basal ganglia circuitry involved in the control of movement. Recent studies have demonstrated a critical role of GPe cell types in Parkinsonism. Specifically increasing the function of parvalbumin (PV) neurons in the GPe has been found to facilitate motor function in a mouse model of Parkinson’s disease (PD). The knowledge of contribution of NMDA receptors to GPe function is limited. Here, we demonstrate that fast spiking neurons in the GPe express NMDA receptor currents sensitive to GluN2C/GluN2D-selective inhibitors and glycine site agonist with higher efficacy at GluN2C-containing receptors. Furthermore, using a novel reporter model, we demonstrate the expression of GluN2C subunits in PV neurons in the GPe which project to subthalamic nuclei. GluN2D subunit was also found to localize to PV neurons in GPe. Ablation of GluN2C subunit does not affect spontaneous firing of fast spiking neurons. In contrast, facilitating the function of GluN2C-containing receptors using glycine-site NMDA receptor agonists, D-cycloserine (DCS) or AICP, increased the spontaneous firing frequency of PV neurons in a GluN2C-dependent manner. Finally, we demonstrate that local infusion of DCS or AICP into the GPe improved motor function in a mouse model of PD. Together, these results demonstrate that GluN2C-containing receptors and potentially GluN2D-containing receptors in the GPe may serve as a therapeutic target for alleviating motor dysfunction in PD and related disorders.

Discovery of Dihydropyrrolo[1,2-a]pyrazin-3(4H)-one-Based Second-Generation GluN2C- and GluN2D-Selective Positive Allosteric Modulators (PAMs) of the N-Methyl-d-Aspartate (NMDA) Receptor

The N-methyl-d-aspartate receptor (NMDAR) is an ion channel that mediates the slow, Ca²⁺-permeable component of glutamatergic synaptic transmission in the central nervous system (CNS). NMDARs are known to play a significant role in basic neurological functions, and their dysfunction has been implicated in several CNS disorders. Herein, we report the discovery of second-generation GluN2C/D-selective NMDAR-positive allosteric modulators (PAMs) with a dihydropyrrolo[1,2-a]pyrazin-3(4H)-one core. The prototype, R-(+)-EU-1180-453, exhibits log unit improvements in the concentration needed to double receptor response, lipophilic efficiency, and aqueous solubility, and lowers cLogP by one log unit compared to the first-generation prototype CIQ. Additionally, R-(+)-EU-1180-453 was found to increase glutamate potency 2-fold, increase the response to maximally effective concentration of agonist 4-fold, and the racemate is brain-penetrant. These compounds are useful second-generation in vitro tools and a promising step toward in vivo tools for the study of positive modulation of GluN2C- and GluN2D-containing NMDA receptors.