Benzamide-type AMPA receptor modulators form two subfamilies with distinct modes of action

AMPA receptors play an important role in the biological consequences of spinal cord injury: Implications for AMPA receptor modulators for therapeutic benefit

Spinal cord injury (SCI) afflicts millions of individuals globally. There are few therapies available to patients. Ascending and descending excitatory glutamatergic neural circuits in the central nervous system are disrupted by SCI, making α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) a potential therapeutic drug target. Emerging research in preclinical models highlights the involvement of AMPARs in vital processes following SCI including breathing, pain, inflammation, bladder control, and motor function. However, there are no clinical trial data reported in this patient population to date. No work on the role of AMPA receptors in sexual dysfunction after SCI has been disclosed. Compounds with selective antagonist and potentiating effects on AMPA receptors have benefit in animal models of SCI, with antagonists generally showing protective effects early after injury and potentiators (ampakines) producing improved breathing and bladder function. The role of AMPARs in pathophysiology and recovery after SCI depends upon the time post injury, and the timing of AMPAR augmentation or antagonism. The roles of inflammation, synaptic plasticity, sensitization, neurotrophic factors, and neuroprotection are considered in this context. The data summarized and discussed in this paper document proof of principle and strongly encourage additional studies on AMPARs as novel gateways to therapeutic benefit for patients suffering from SCI. The availability of both AMPAR antagonists such as perampanel and AMPAR allosteric modulators (i.e., ampakines) such as CX1739, that have been safely administered to humans, provides an expedited means of clinical inquiry for possible therapeutic advances.

Acute ampakines increase voiding function and coordination in a rat model of SCI

Neurogenic bladder dysfunction causes urological complications and reduces the quality of life in persons with spinal cord injury (SCI). Glutamatergic signaling via AMPA receptors is fundamentally important to the neural circuits controlling bladder voiding. Ampakines are positive allosteric modulators of AMPA receptors that can enhance the function of glutamatergic neural circuits after SCI. We hypothesized that ampakines can acutely stimulate bladder voiding that has been impaired due to thoracic contusion SCI. Adult female Sprague-Dawley rats received a unilateral contusion of the T9 spinal cord (n = 10). Bladder function (cystometry) and coordination with the external urethral sphincter (EUS) were assessed 5 d post-SCI under urethane anesthesia. Data were compared to responses in spinal-intact rats (n = 8). The 'low-impact' ampakine CX1739 (5, 10, or 15 mg/kg) or vehicle (2-hydroxypropyl-beta-cyclodextrin [HPCD]) was administered intravenously. The HPCD vehicle had no discernible impact on voiding. In contrast, following CX1739, the pressure threshold for inducing bladder contraction, voided volume, and the interval between bladder contractions were significantly reduced. These responses occurred in a dose-dependent manner. We conclude that modulating AMPA receptor function using ampakines can rapidly improve bladder-voiding capability at subacute time points following contusion SCI. These results may provide a new and translatable method for therapeutic targeting of bladder dysfunction acutely after SCI.

Pattern sensitivity of ampakine-hypoxia interactions for evoking phrenic motor facilitation in anesthetized rat

Repeated hypoxic episodes can produce a sustained (>60 min) increase in neural drive to the diaphragm. The requirement of repeated hypoxic episodes (vs. a single episode) to produce phrenic motor facilitation (pMF) can be removed by allosteric modulation of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors using ampakines. We hypothesized that the ampakine-hypoxia interaction resulting in pMF requires that ampakine dosing precedes the onset of hypoxia. Phrenic nerve recordings were made from urethane-anesthetized, mechanically ventilated, and vagotomized adult male Sprague-Dawley rats during isocapnic conditions. Ampakine CX717 (15 mg/kg iv) was given immediately before (n = 8), during (n = 8), or immediately after (n = 8) a 5-min hypoxic episode (arterial oxygen partial pressure 40-45 mmHg). Ampakine before hypoxia (Aprior) resulted in a sustained increase in inspiratory phrenic burst amplitude (i.e., pMF) reaching +70 ± 21% above baseline (BL) after 60 min. This was considerably greater than corresponding values in the groups receiving ampakine during hypoxia (+28 ± 47% above BL, P = 0.005 vs. Aprior) or after hypoxia (+23 ± 40% above BL, P = 0.005 vs. Aprior). Phrenic inspiratory burst rate, heart rate, and systolic, diastolic, and mean arterial pressure (mmHg) were similar across the three treatment groups (all P > 0.3, treatment effect). We conclude that the presentation order of ampakine and hypoxia impacts the magnitude of pMF, with ampakine pretreatment evoking the strongest response. Ampakine pretreatment may have value in the context of hypoxia-based neurorehabilitation strategies.NEW & NOTEWORTHY Phrenic motor facilitation (pMF) is evoked after repeated episodes of brief hypoxia. pMF can also be induced when an allosteric modulator of AMPA receptors (ampakine) is intravenously delivered immediately before a single brief hypoxic episode. Here we show that ampakine delivery before hypoxia (vs. during or after hypoxia) evokes the largest pMF with minimal impact on arterial blood pressure and heart rate. Ampakine pretreatment may have value in the context of hypoxia-based neurorehabilitation strategies.

Acute ampakines increase voiding function and coordination in a rat model of SCI

Neurogenic bladder dysfunction causes urological complications and reduces the quality of life in persons with spinal cord injury (SCI). Glutamatergic signaling via AMPA receptors is fundamentally important to the neural circuits controlling bladder voiding. Ampakines are positive allosteric modulators of AMPA receptors that can enhance the function of glutamatergic neural circuits after SCI. We hypothesized that ampakines can acutely stimulate bladder voiding that has been impaired due to thoracic contusion SCI. Adult female Sprague Dawley rats received a unilateral contusion of the T9 spinal cord (n=10). Bladder function (cystometry) and coordination with the external urethral sphincter (EUS) were assessed five days post-SCI under urethane anesthesia. Data were compared to responses in spinal intact rats (n=8). The "low impact" ampakine CX1739 (5, 10, or 15 mg/kg) or vehicle (HPCD) was administered intravenously. The HPCD vehicle had no discernable impact on voiding. In contrast, following CX1739, the pressure threshold for inducing bladder contraction, voided volume, and the interval between bladder contractions were significantly reduced. These responses occurred in a dose-dependent manner. We conclude that modulating AMPA receptor function using ampakines can rapidly improve bladder voiding capability at sub-acute time points following contusion SCI. These results may provide a new and translatable method for therapeutic targeting of bladder dysfunction acutely after SCI.

Novel Nanomolar Allosteric Modulators of AMPA Receptor of Bis(pyrimidine) Series: Synthesis, Biotesting and SAR Analysis

Positive allosteric modulators (PAMs) of AMPA receptors represent attractive candidates for the development of drugs for the treatment of cognitive and neurodegenerative disorders. Dimeric molecules have been reported to have an especially potent modulating effect, due to the U-shaped form of the AMPA receptor's allosteric binding site. In the present work, novel bis(pyrimidines) were studied as AMPA receptor modulators. A convenient and flexible preparative approach to bis(pyrimidines) containing a hydroquinone linker was elaborated, and a series of derivatives with varied substituents was obtained. The compounds were examined in the patch clamp experiments for their influence on the kainate-induced currents, and 10 of them were found to have potentiating properties. The best potency was found for 2-methyl-4-(4-((2-methyl-5,6,7,8-tetrahydroquinazolin-4-yl)oxy)phenoxy)-6,7,8,9-tetrahydro-5H-cyclohepta[d]pyrimidine, which potentiated the kainate-induced currents by up to 77% in all tested concentrations (10^-12-10^-6 M). The results were rationalized via the modeling of modulator complexes with the dimeric ligand binding domain of the GluA2 AMPA receptor, using molecular docking and molecular dynamics simulation. The prediction of ADMET, physicochemical, and PAINS properties of the studied bis(pyrimidines) confirmed that PAMs of this type may act as the potential lead compounds for the development of neuroprotective drugs.

Spinally delivered ampakine CX717 increases phrenic motor output in adult rats

Ampakines are synthetic molecules that allosterically modulate AMPA-type glutamate receptors. We tested the hypothesis that delivery of ampakines to the intrathecal space could stimulate neural drive to the diaphragm. Ampakine CX717 (20 mM, dissolved in 10 % HPCD) or an HPCD vehicle solution were delivered via a catheter placed in the intrathecal space at the fourth cervical segment in urethane-anesthetized, mechanically ventilated adult male Sprague-Dawley rats. The electrical activity of the phrenic nerve was recorded for 60-minutes following drug application. Intrathecal application of CX717 produced a gradual and sustained increase in phrenic inspiratory burst amplitude (n = 10). In contrast, application of HPCD (n = 10) caused no sustained change in phrenic motor output. Phrenic burst rate, heart rate, and mean arterial pressure were similar between CX717 and HPCD treated rats. We conclude that intrathecally delivered ampakines can modulate phrenic motor output. This approach may have value for targeted induction of spinal neuroplasticity in the context of neurorehabiliation.

Ampakine pretreatment enables a single hypoxic episode to produce phrenic motor facilitation with no added benefit of additional episodes

Repeated short episodes of hypoxia produce a sustained increase in phrenic nerve output lasting well beyond acute intermittent hypoxia (AIH) exposure (i.e., phrenic long-term facilitation; pLTF). Pretreatment with ampakines, drugs which allosterically modulate AMPA receptors, enables a single brief episode of hypoxia to produce pLTF, lasting up to 90 min after hypoxia. Here, we tested the hypothesis that ampakine pretreatment would enhance the magnitude of pLTF evoked by repeated bouts of hypoxia. Phrenic nerve output was recorded in urethane-anesthetized, mechanically ventilated, and vagotomized adult male Sprague-Dawley rats. Initial experiments demonstrated that ampakine CX717 (15 mg/kg iv) caused an acute increase in phrenic nerve inspiratory burst amplitude reaching 70 ± 48% baseline (BL) after 2 min (P = 0.01). This increased bursting was not sustained (2 ± 32% BL at 60 min, P = 0.9). When CX717 was delivered 2 min before a single episode of isocapnic hypoxia (5 min, [Formula: see text] = 44 ± 9 mmHg), facilitation of phrenic nerve burst amplitude occurred (96 ± 62% BL at 60 min, P < 0.001). However, when CX717 was given 2 min before three, 5-min hypoxic episodes ([Formula: see text] = 45 ± 6 mmHg) pLTF was attenuated and did not reach statistical significance (24 ± 29% BL, P = 0.08). In the absence of CX717 pretreatment, pLTF was observed after three (74 ± 33% BL at 60 min, P < 0.001) but not one episode of hypoxia (1 ± 8% BL at 60 min, P = 0.9). We conclude that pLTF is not enhanced when ampakine pretreatment is followed by repeated bouts of hypoxia. Rather, the combination of ampakine and a single hypoxic episode appears to be ideal for producing sustained increase in phrenic motor output.NEW & NOTEWORTHY Pretreatment with ampakine CX717 created conditions that enabled an acute bout of moderate hypoxia to evoke phrenic motor facilitation, but this response was not observed when ampakine pretreatment was followed by intermittent hypoxia. Thus, in anesthetized and spinal intact rats, the combination of ampakine and one bout of hypoxia appears ideal for triggering respiratory neuroplasticity.

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.

Insufficiency of ventral hippocampus to medial prefrontal cortex transmission explains antidepressant non-response

BACKGROUND

There is extensive evidence that antidepressant drugs restore normal brain function by repairing damage to ventral hippocampus (vHPC) and medial prefrontal cortex (mPFC). While the damage is more extensive in hippocampus, the evidence of treatments, such as deep brain stimulation, suggests that functional changes in prefrontal cortex may be more critical. We hypothesized that antidepressant non-response may result from an insufficiency of transmission from vHPC to mPFC.

METHOD

Antidepressant non-responsive Wistar Kyoto (WKY) rats were subjected to chronic mild stress (CMS), then treated with chronic daily administration of the antidepressant drug venlafaxine (VEN) and/or repeated weekly optogenetic stimulation (OGS) of afferents to mPFC originating from vHPC or dorsal HPC (dHPC).

RESULTS

As in many previous studies, CMS decreased sucrose intake, open-arm entries on the elevated plus maze (EPM), and novel object recognition (NOR). Neither VEN nor vHPC-mPFC OGS alone was effective in reversing the effects of CMS, but the combination of chronic VEN and repeated OGS restored normal behaviour on all three measures. dHPC-mPFC OGS restored normal behaviour in the EPM and NOR test irrespective of concomitant VEN treatment, and had no effect on sucrose intake.

CONCLUSIONS

The synergism between VEN and vHPC-mPFC OGS supports the hypothesis that the antidepressant non-responsiveness of WKY rats results from a failure of antidepressant treatment fully to restore transmission in the vHPC-mPFC pathway.

Perspectives for therapy of treatment-resistant depression

A high proportion of depressed patients fail to respond to antidepressant drug treatment. Treatment-resistant depression (TRD) is a major challenge for the psychopharmacology of mood disorders. Only in the past decade have novel treatments, including deep brain stimulation (DBS) and ketamine, been discovered that provide rapid and sometimes prolonged relief to a high proportion of TRD sufferers. In this review, we consider the current status of TRD from four perspectives: the challenge of developing an appropriate regulatory framework for novel rapidly acting antidepressants; the efficacy of non-pharmacological somatic therapies; the development of an animal model of TRD and its use to understand the neural basis of antidepressant non-response; and the potential for rapid antidepressant action from targets (such as 5-HT_1A receptors) beyond the glutamate receptor.

Furazans in Medicinal Chemistry

Incorporation of heterocycles into drug molecules can enhance physical properties and biological activity. A variety of heterocyclic groups is available to medicinal chemists, many of which have been reviewed in detail elsewhere. Oxadiazoles are a class of heterocycle containing one oxygen and two nitrogen atoms, available in three isomeric forms. While the 1,2,4- and 1,3,4-oxadiazoles have seen widespread application in medicinal chemistry, 1,2,5-oxadiazoles (furazans) are less common. This Review provides a summary of the application of furazan-containing molecules in medicinal chemistry and drug development programs from analysis of both patent and academic literature. Emphasis is placed on programs that reached clinical or preclinical stages of development. The examples provided herein describe the pharmacology and biological activity of furazan derivatives with comparative data provided where possible for other heterocyclic groups and pharmacophores commonly used in medicinal chemistry.

AMPA receptors mediate the pro-cognitive effects of electrical and optogenetic stimulation of the medial prefrontal cortex in antidepressant non-responsive Wistar-Kyoto rats

BACKGROUND

The chronic mild stress (CMS) procedure is a widely used animal model of depression, and its application in Wistar-Kyoto (WKY) rats has been validated as a model of antidepressant-refractory depression. While not responding to chronic treatment with antidepressant drugs, WKY rats do respond to acute deep brain stimulation (DBS) of the medial prefrontal cortex (mPFC). In antidepressant-responsive strains there is evidence suggesting a role for AMPA subtype of glutamate receptor in the action mechanism of both antidepressants and DBS.

METHODS

Animals were subjected to CMS for 6 to 8 weeks; sucrose intake was monitored weekly and novel object recognition (NOR) test was conducted following recovery from CMS. Wistars were treated chronically with venlafaxine (VEN), while WKY were treated acutely with either DBS, optogenetic stimulation (OGS) of virally-transduced (AAV5-hSyn-ChR2-EYFP) mPFC or ventral hippocampus, or acute intra-mPFC injection of the AMPA receptor positive allosteric modulator CX-516. The AMPA receptor antagonist NBQX was administered, at identical sites in mPFC, immediately following the exposure trial in the NOR.

RESULTS

Sucrose intake and NOR were suppressed by CMS, and restored by VEN in Wistars and by DBS, OGS, or CX-516 in WKY. However, OGS of the ventral hippocampal afferents to mPFC was ineffective. A low dose of NBQX selectively blocked the procognitive effect of VEN, DBS and OGS.

CONCLUSIONS

These results suggest that activation of AMPA receptors in the mPFC represents a common pathway for the antidepressant effects of both conventional (VEN) and novel (DBS, OGS) antidepressant modalities, in both antidepressant responsive (Wistar) and antidepressant-resistant (WKY) rats.

The impact and mechanism of ampakine CX1739 on protection against respiratory depression in rats

Background: Abuse of analgesic and sedative drugs often leads to severe respiratory depression and sometimes death. Approximately 69,000 people worldwide die annually from opioid overdoses. Purpose: This work aimed to investigate whether CX1739 can be used for emergency treatment of acute respiratory depression due to drug abuse. Results: First, the results clarify that CX1739 is a low-impact ampakine that can safely activate α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors without causing excito-neurotoxicity. Second, CX1739 rapidly crossed the blood-brain barrier (T_max = 2 min), which meets the requirement of rapid onset of action in vivo. Our work provides preliminarily confirmation that high-dose intravenous administration of CX1739 can immediately reverse respiratory depression in animal models of respiratory depression caused by opioid agonist 030418, pentobarbital sodium and ethanol.

Ampakines stimulate phrenic motor output after cervical spinal cord injury

Activation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors increases phrenic motor output. Ampakines are a class of drugs that are positive allosteric modulators of AMPA receptors. We hypothesized that 1) ampakines can stimulate phrenic activity after incomplete cervical spinal cord injury (SCI), and 2) pairing ampakines with brief hypoxia could enable sustained facilitation of phrenic bursting. Phrenic activity was recorded ipsilateral (IL) and contralateral (CL) to C2 spinal cord hemisection (C2Hx) in anesthetized adult rats. Two weeks after C2Hx, ampakine CX717 (15 mg/kg, i.v.) increased IL (61 ± 46% baseline, BL) and CL burst amplitude (47 ± 26%BL) in 8 of 8 rats. After 90 min, IL and CL bursting remained above baseline (BL) in 7 of 8 rats. Pairing ampakine with a single bout of acute hypoxia (5-min, arterial partial pressure of O₂ ~ 50 mmHg) had a variable impact on phrenic bursting, with some rats showing a large facilitation that exceeded the response of the ampakine alone group. At 8 weeks post-C2Hx, 7 of 8 rats increased IL (115 ± 117%BL) and CL burst amplitude (45 ± 27%BL) after ampakine. The IL burst amplitude remained above BL for 90-min in 7 of 8 rats; CL bursting remained elevated in 6 of 8 rats. The sustained impact of ampakine at 8 weeks was not enhanced by hypoxia exposure. Intravenous vehicle (10% 2-Hydroxypropyl-β-cyclodextrin) did not increase phrenic bursting at either time point. We conclude that ampakines effectively stimulate neural drive to the diaphragm after cervical SCI. Pairing ampakines with a single hypoxic exposure did not consistently enhance phrenic motor facilitation.

Ampakine pretreatment enables a single brief hypoxic episode to evoke phrenic motor facilitation

Phrenic long-term facilitation (LTF) is a sustained increase in phrenic motor output occurring after exposure to multiple (but not single) hypoxic episodes. Ampakines are a class of drugs that enhance AMPA receptor function. Ampakines can enhance expression of neuroplasticity, and the phrenic motor system is fundamentally dependent on excitatory glutamatergic currents. Accordingly, we tested the hypothesis that combining ampakine pretreatment with a single brief hypoxic exposure would result in phrenic motor facilitation lasting well beyond the period of hypoxia. Phrenic nerve output was recorded in urethane-anesthetized, ventilated, and vagotomized adult Sprague-Dawley rats. Ampakine CX717 (15 mg/kg iv; n = 8) produced a small increase in phrenic inspiratory burst amplitude and frequency, but values quickly returned to predrug baseline. When CX717 was followed 2 min later by a 5-min exposure to hypoxia (n = 8; PaO2 ~45 mmHg), a persistent increase in phrenic inspiratory burst amplitude (i.e., phrenic motor facilitation) was observed up to 60 min posthypoxia (103 ± 53% increase from baseline). In contrast, when hypoxia was preceded by vehicle injection (10% 2-hydroxypropyl-β-cyclodextrin; n = 8), inspiratory phrenic bursting was similar to baseline values at 60 min. Additional experiments with another ampakine (CX1739, 15 mg/kg) produced comparable results. We conclude that pairing low-dose ampakine treatment with a single brief hypoxic exposure can evoke sustained phrenic motor facilitation. This targeted approach for enhancing respiratory neuroplasticity may have value in the context of hypoxia-based neurorehabilitation strategies.NEW & NOTEWORTHY A single brief episode of hypoxia (e.g., 3-5 min) does not evoke long-lasting increases in respiratory motor output after the hypoxia is concluded. Ampakines are a class of drugs that enhance AMPA receptor function. We show that pairing low-dose ampakine treatment with a single brief hypoxic exposure can evoke sustained phrenic motor facilitation after the acute hypoxic episode.

Optogenetic Stimulation of Prefrontal Glutamatergic Neurons Enhances Recognition Memory

Finding effective cognitive enhancers is a major health challenge; however, modulating glutamatergic neurotransmission has the potential to enhance performance in recognition memory tasks. Previous studies using glutamate receptor antagonists have revealed that the medial prefrontal cortex (mPFC) plays a central role in associative recognition memory. The present study investigates short-term recognition memory using optogenetics to target glutamatergic neurons within the rodent mPFC specifically. Selective stimulation of glutamatergic neurons during the online maintenance of information enhanced associative recognition memory in normal animals. This cognitive enhancing effect was replicated by local infusions of the AMPAkine CX516, but not CX546, which differ in their effects on EPSPs. This suggests that enhancing the amplitude, but not the duration, of excitatory synaptic currents improves memory performance. Increasing glutamate release through infusions of the mGluR7 presynaptic receptor antagonist MMPIP had no effect on performance.

SIGNIFICANCE STATEMENT These results provide new mechanistic information that could guide the targeting of future cognitive enhancers. Our work suggests that improved associative-recognition memory can be achieved by enhancing endogenous glutamatergic neuronal activity selectively using an optogenetic approach. We build on these observations to recapitulate this effect using drug treatments that enhance the amplitude of EPSPs; however, drugs that alter the duration of the EPSP or increase glutamate release lack efficacy. This suggests that both neural and temporal specificity are needed to achieve cognitive enhancement.

AMPAkines and morphine provide complementary analgesia

Glutamate signaling in the central nervous system is known to play a key role in pain regulation. AMPAkines can enhance glutamate signaling through α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. previous studies have shown that AMPAkines are effective analgesic agents, and their site of action is likely in the brain. It is not known, however, if AMPAkines can provide complementary analgesia in combination with opioids, the most commonly used analgesics. Here, we show that the co-administration of an AMPAkine with morphine can provide additional analgesia, both in naïve rats and in rats that experience postoperative pain. Furthermore, we show that this AMPAkine can be administered directly into the prefrontal cortex to provide analgesia, and that prefrontal AMPAkine infusion, similar to systemic administration, can provide added pain relief to complement morphine analgesia.

Chronic Ampakine Treatments Stimulate Dendritic Growth and Promote Learning in Middle-Aged Rats

Positive allosteric modulators of AMPA-type glutamate receptors (ampakines) have been shown to rescue synaptic plasticity and reduce neuropathology in rodent models of cognitive disorders. Here we tested whether chronic ampakine treatment offsets age-related dendritic retraction in middle-aged (MA) rats. Starting at 10 months of age, rats were housed in an enriched environment and given daily treatment with a short half-life ampakine or vehicle for 3 months. Dendritic branching and spine measures were collected from 3D reconstructions of Lucifer yellow-filled CA1 pyramidal cells. There was a substantial loss of secondary branches, relative to enriched 2.5-month-old rats, in apical and basal dendritic fields of vehicle-treated, but not ampakine-treated, 13-month-old rats. Baseline synaptic responses in CA1 were only subtly different between the two MA groups, but long-term potentiation was greater in ampakine-treated rats. Unsupervised learning of a complex environment was used to assess treatment effects on behavior. Vehicle- and drug-treated rats behaved similarly during a first 30 min session in the novel environment but differed markedly on subsequent measures of long-term memory. Markov sequence analysis uncovered a clear increase in the predictability of serial movements between behavioral sessions 2 and 3 in the ampakine, but not vehicle, group. These results show that a surprising degree of dendritic retraction occurs by middle age and that this can be mostly offset by pharmacological treatments without evidence for unwanted side effects. The functional consequences of rescue were prominent with regard to memory but also extended to self-organization of behavior.

SIGNIFICANCE STATEMENT

Brain aging is characterized by a progressive loss of dendritic arbors and the emergence of impairments to learning-related synaptic plasticity. The present studies show that dendritic losses are evident by middle age despite housing in an enriched environment and can be mostly reversed by long-term, oral administration of a positive allosteric modulator of AMPA-type glutamate receptors. Dendritic recovery was accompanied by improvements to both synaptic plasticity and the encoding of long-term memory of a novel, complex environment. Because the short half-life compound had no evident negative effects, the results suggest a plausible strategy for treating age-related neuronal deterioration.

Rational Design of a Novel AMPA Receptor Modulator through a Hybridization Approach

The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are a family of glutamate ion channels of considerable interest in excitatory neurotransmission and associated disease processes. Here, we demonstrate how exploitation of the available X-ray crystal structure of the receptor ligand binding domain enabled the development of a new class of AMPA receptor positive allosteric modulators (7) through hybridization of known ligands (5 and 6), leading to a novel chemotype with promising pharmacological properties.

Enhanced AMPA receptor function promotes cerebellar long-term depression rather than potentiation

Ampakines are allosteric modulators of AMPA receptors that facilitate hippocampal long-term potentiation (LTP) and learning, and have been considered for the treatment of cognition and memory deficits. Here, we show that the ampakine CX546 raises the amplitude and slows the decay time of excitatory postsynaptic currents (EPSCs) at cerebellar parallel fiber (PF) to Purkinje cell synapses, thus resembling CX546 effects described at hippocampal synapses. Using the fluorescent calcium indicator dye Oregon Green BAPTA-2 and an ultra-high-speed CCD camera, we also monitored calcium transients in Purkinje cell dendrites. In the presence of CX546 in the bath, PF-evoked calcium transients were enhanced and prolonged, suggesting that CX546 not only enhances synaptic transmission, but also boosts dendritic calcium signaling at cerebellar synapses. In contrast to previous observations in the hippocampus, however, CX546 applied during cerebellar recordings facilitates long-term depression (LTD) rather than LTP at PF synapses. These findings show that ampakines selectively modify the LTP–LTD balance depending on the brain area and type of synapse, and may provide tools for the targeted regulation of synaptic memories.

Pharmacological enhancement of memory or cognition in normal subjects

The possibility of expanding memory or cognitive capabilities above the levels in high functioning individuals is a topic of intense discussion among scientists and in society at large. The majority of animal studies use behavioral endpoint measures; this has produced valuable information but limited predictability for human outcomes. Accordingly, several groups are pursuing a complementary strategy with treatments targeting synaptic events associated with memory encoding or forebrain network operations. Transcription and translation figure prominently in substrate work directed at enhancement. Notably, the question of why new proteins would be needed for a now-forming memory given that learning-driven synthesis presumably occurred throughout the immediate past has been largely ignored. Despite this conceptual problem, and some controversy, recent studies have reinvigorated the idea that selective gene manipulation is a plausible route to enhancement. Efforts to improve memory by facilitating synaptic encoding of information have also progressed, in part due of breakthroughs on mechanisms that stabilize learning-related, long-term potentiation (LTP). These advances point to a reductionistic hypothesis for a diversity of experimental results on enhancement, and identify under-explored possibilities. Cognitive enhancement remains an elusive goal, in part due to the difficulty of defining the target. The popular view of cognition as a collection of definable computations seems to miss the fluid, integrative process experienced by high functioning individuals. The neurobiological approach obviates these psychological issues to directly test the consequences of improving throughput in networks underlying higher order behaviors. The few relevant studies testing drugs that selectively promote excitatory transmission indicate that it is possible to expand cortical networks engaged by complex tasks and that this is accompanied by capabilities not found in normal animals.

Development of antipsychotic medications with novel mechanisms of action based on computational modeling of hippocampal neuropathology

A large number of cellular level abnormalities have been identified in the hippocampus of schizophrenic subjects. Nonetheless, it remains uncertain how these pathologies interact at a system level to create clinical symptoms, and this has hindered the development of more effective antipsychotic medications. Using a 72-processor supercomputer, we created a tissue level hippocampal simulation, featuring multicompartmental neuron models with multiple ion channel subtypes and synaptic channels with realistic temporal dynamics. As an index of the schizophrenic phenotype, we used the specific inability of the model to attune to 40 Hz (gamma band) stimulation, a well-characterized abnormality in schizophrenia. We examined several possible combinations of putatively schizophrenogenic cellular lesions by systematically varying model parameters representing NMDA channel function, dendritic spine density, and GABA system integrity, conducting 910 trials in total. Two discrete “clusters” of neuropathological changes were identified. The most robust was characterized by co-occurring modest reductions in NMDA system function (-30%) and dendritic spine density (-30%). Another set of lesions had greater NMDA hypofunction along with low level GABA system dysregulation. To the schizophrenic model, we applied the effects of 1,500 virtual medications, which were implemented by varying five model parameters, independently, in a graded manner; the effects of known drugs were also applied. The simulation accurately distinguished agents that are known to lack clinical efficacy, and identified novel mechanisms (e.g., decrease in AMPA conductance decay time constant, increase in projection strength of calretinin-positive interneurons) and combinations of mechanisms that could re-equilibrate model behavior. These findings shed light on the mechanistic links between schizophrenic neuropathology and the gamma band oscillatory abnormalities observed in the illness. As such, they generate specific falsifiable hypotheses, which can guide postmortem and other laboratory research. Significantly, this work also suggests specific non-obvious targets for potential pharmacologic agents.

Glutamatergic Approaches for the Treatment of Schizophrenia

Glutamate is the major excitatory neurotransmitter in the mammalian central nervous system and plays a key role in most aspects of normal brain function including cognition, learning and memory. Dysfunction of glutamatergic neurotransmission has been implicated in a number of neurological and psychiatric disorders with a growing body of evidence suggesting that hypofunction of glutamatergic neurotransmission via the N-methyl-d-aspartate (NMDA) receptor plays an important role in the pathophysiology of schizophrenia. It thus follows that potentiation of NMDA receptor function via pharmacological manipulation may provide therapeutic utility for the treatment of schizophrenia and a number of different approaches are currently being pursued by the pharmaceutical industry with this aim in mind. These include strategies that target the glycine/d-serine site of the NMDA receptor (glycine transporter GlyT1, d-serine transporter ASC-1 and d-amino acid oxidase (DAAO) inhibitors) together with those aimed at enhancing glutamatergic neurotransmission via modulation of AMPA receptor and metabotropic glutamate receptor function. Such efforts are now beginning to bear fruit with compounds such as the GlyT1 inhibitor RG1678 and mGlu2 agonist LY2140023 proving to have clinical meaningful effects in phase II clinical trials. While more studies are required to confirm long-term efficacy, functional outcome and safety in schizophrenic agents, these agents hold real promise for addressing unmet medical needs, in particular refractory negative and cognitive symptoms, not currently addressed by existing antipsychotic agents.

Benzobistriazinones and related heterocyclic ring systems as potent, orally bioavailable positive allosteric AMPA receptor modulators

AMPA receptors (AMPARs) are an important therapeutic target in the CNS. A series of substituted benzobistriazinone, benzobispyrimidinone and related derivatives have been prepared with high potency and selectivity for the allosteric binding site of AMPARs. Further improvements have been made to previously reported series of positive AMPAR modulators and these compounds exhibit excellent in vivo activity and improved in vivo metabolic stability with up to 100% oral bioavailability in rat.

Benzotriazinone and benzopyrimidinone derivatives as potent positive allosteric AMPA receptor modulators

AMPA receptors (AMPARs) have been demonstrated to be an important therapeutic CNS target. A series of substituted benzotriazinone and benzopyrimidinone derivatives were prepared with the aim to improve in vivo activity over the previously reported bis-benzoxazinone based AMPAKINE series from our laboratory. These compounds were shown to be potent, positive allosteric AMPAR modulators that have better in vivo activity and improved metabolic stability over the analogous benzoxazinone derivatives.

The likelihood of cognitive enhancement

Whether drugs that enhance cognition in healthy individuals will appear in the near future has become a topic of considerable interest. We address this possibility using a three variable system (psychological effect, neurobiological mechanism, and efficiency vs. capabilities) for classifying candidates. Ritalin and modafinil, two currently available compounds, operate on primary psychological states that in turn affect cognitive operations (attention and memory), but there is little evidence that these effects translate into improvements in complex cognitive processing. A second category of potential enhancers includes agents that improve memory encoding, generally without large changes in primary psychological states. Unfortunately, there is little information on how these compounds affect cognitive performance in standard psychological tests. Recent experiments have identified a number of sites at which memory drugs could, in principle, manipulate the cell biological systems underlying the learning-related long-term potentiation (LTP) effect; this may explain the remarkable diversity of memory promoting compounds. Indeed, many of these agents are known to have positive effects on LTP. A possible third category of enhancement drugs directed specifically at integrated cognitive operations is nearly empty. From a neurobiological perspective, two plausible candidate classes have emerged that both target the fast excitatory transmission responsible for communication within cortical networks. One acts on nicotinic receptors (alpha7 and alpha4) that regulate release of the neurotransmitter glutamate while the other ('ampakines') allosterically modulates the glutamate receptors mediating the post-synaptic response (EPSCs). Brain imaging in primates has shown that ampakines expand cortical networks engaged by a complex task; coupled with behavioral data, these findings provide evidence for the possibility of generating new cognitive capabilities. Finally, we suggest that continuing advances in behavioral sciences provide new opportunities for translational work, and that discussions of the social impact of cognitive enhancers have failed to consider the distinction between effects on efficiency vs. new capabilities.

Substituted benzoxazinones as potent positive allosteric AMPA receptor modulators: part II

AMPA receptors (AMPARs) are an important therapeutic target in the CNS. A series of substituted benzoxazinone derivatives with good to very good in vitro activity as positive allosteric AMPAR modulators was synthesized and evaluated. The appropriate substituent choice on the benzoxazinone fragment improved the affinity towards the AMPA receptor significantly in comparison to our lead molecule CX614.

Benzoxazinones as potent positive allosteric AMPA receptor modulators: part I

AMPA receptors (AMPARs) are an increasingly important therapeutic target in the CNS. Aniracetam, the first identified potentiator of AMPARs, led to the rigid and more potent CX614. This lead molecule was optimized in order to increase affinity towards the AMPA receptor. The substitution of the dioxine with a benzoxazinone ring system increased the activity and allowed further investigation of the sidechain SAR.

Structural and functional analysis of two new positive allosteric modulators of GluA2 desensitization and deactivation

At the dimer interface of the extracellular ligand-binding domain of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors a hydrophilic pocket is formed that is known to interact with two classes of positive allosteric modulators, represented by cyclothiazide and the ampakine 2H,3H,6aH-pyrrolidino(2,1-3',2')1,3-oxazino(6',5'-5,4)benzo(e)1,4-dioxan-10-one (CX614). Here, we present structural and functional data on two new positive allosteric modulators of AMPA receptors, phenyl-1,4-bis-alkylsulfonamide (CMPDA) and phenyl-1,4-bis-carboxythiophene (CMPDB). Crystallographic data show that these compounds bind within the modulator-binding pocket and that substituents of each compound overlap with distinct moieties of cyclothiazide and CX614. The goals of the present study were to determine 1) the degree of modulation by CMPDA and CMPDB of AMPA receptor deactivation and desensitization; 2) whether these compounds are splice isoform-selective; and 3) whether predictions of mechanism of action could be inferred by comparing molecular interactions between the ligand-binding domain and each compound with those of cyclothiazide and CX614. CMPDB was found to be more isoform-selective than would be predicted from initial binding assays. It is noteworthy that these new compounds are both more potent and more effective and may be more clinically relevant than the AMPA receptor modulators described previously.

Glutamate receptor ion channels: structure, regulation, and function

The mammalian ionotropic glutamate receptor family encodes 18 gene products that coassemble to form ligand-gated ion channels containing an agonist recognition site, a transmembrane ion permeation pathway, and gating elements that couple agonist-induced conformational changes to the opening or closing of the permeation pore. Glutamate receptors mediate fast excitatory synaptic transmission in the central nervous system and are localized on neuronal and non-neuronal cells. These receptors regulate a broad spectrum of processes in the brain, spinal cord, retina, and peripheral nervous system. Glutamate receptors are postulated to play important roles in numerous neurological diseases and have attracted intense scrutiny. The description of glutamate receptor structure, including its transmembrane elements, reveals a complex assembly of multiple semiautonomous extracellular domains linked to a pore-forming element with striking resemblance to an inverted potassium channel. In this review we discuss International Union of Basic and Clinical Pharmacology glutamate receptor nomenclature, structure, assembly, accessory subunits, interacting proteins, gene expression and translation, post-translational modifications, agonist and antagonist pharmacology, allosteric modulation, mechanisms of gating and permeation, roles in normal physiological function, as well as the potential therapeutic use of pharmacological agents acting at glutamate receptors.

Positive modulation of AMPA receptors prevents downregulation of GluR2 expression and activates the Lyn-ERK1/2-CREB signaling in rat brain ischemia

alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) are responsible for excitotoxicity induced by ischemic injury in hippocampal CA1 neurons, whereas the molecular mechanisms responsible for their neurotrophic activities are much less studied. Here, we examined the neuroprotective effect of positive modeulation of AMPARs by coapplication of AMPA with PEPA, an allosteric potentiator of AMPARs. We showed that coapplication of AMPA with PEPA protected hippocampal CA1 neurons from brain ischemia-induced death. Coapplication of AMPA with PEPA could prevent downregulated expression of GluR2 subunit caused by ischemia and increase BDNF expression via Lyn-ERK1/2-CREB signaling. Furthermore, TrkB receptor-mediated PI3K/Akt signal pathway was activated after coapplication of AMPA with PEPA, which was related to MAPK pathway and protected CA1 neurons against ischemic insults through depression of JNK3 activity, release of cytochrome c to cytosol and depression of capase-3 activity. Our results revealed that positive modulation of AMPARs could exert neuroprotective effects and the possible signaling pathways underlied.

Opiate-induced suppression of rat hypoglossal motoneuron activity and its reversal by ampakine therapy

Background

Hypoglossal (XII) motoneurons innervate tongue muscles and are vital for maintaining upper-airway patency during inspiration. Depression of XII nerve activity by opioid analgesics is a significant clinical problem, but underlying mechanisms are poorly understood. Currently there are no suitable pharmacological approaches to counter opiate-induced suppression of XII nerve activity while maintaining analgesia. Ampakines accentuate α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) receptor responses. The AMPA family of glutamate receptors mediate excitatory transmission to XII motoneurons. Therefore the objectives were to determine whether the depressant actions of μ-opioid receptor activation on inspiratory activity includes a direct inhibitory action at the inspiratory premotoneuron to XII motoneuron synapse, and to identify underlying mechanism(s). We then examined whether ampakines counteract opioid-induced depression of XII motoneuron activity.

Methodology/Principal Findings

A medullary slice preparation from neonatal rat that produces inspiratory-related output in vitro was used. Measurements of inspiratory burst amplitude and frequency were made from XII nerve roots. Whole-cell patch recordings from XII motoneurons were used to measure membrane currents and synaptic events. Application of the μ-opioid receptor agonist, DAMGO, to the XII nucleus depressed the output of inspiratory XII motoneurons via presynaptic inhibition of excitatory glutamatergic transmission. Ampakines (CX614 and CX717) alleviated DAMGO-induced depression of XII MN activity through postsynaptic actions on XII motoneurons.

Conclusions/Significance

The inspiratory-depressant actions of opioid analgesics include presynaptic inhibition of XII motoneuron output. Ampakines counteract μ-opioid receptor-mediated depression of XII motoneuron inspiratory activity. These results suggest that ampakines may be beneficial in countering opiate-induced suppression of XII motoneuron activity and resultant impairment of airway patency.

Modulation of agonist binding to AMPA receptors by 1-(1,4-benzodioxan-6-ylcarbonyl)piperidine (CX546): differential effects across brain regions and GluA1-4/transmembrane AMPA receptor regulatory protein combinations

Ampakines are cognitive enhancers that potentiate alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor currents and synaptic responses by slowing receptor deactivation. Their efficacy varies greatly between classes of neurons and brain regions, but the factor responsible for this effect remains unclear. Ampakines also increase agonist affinity in binding tests in ways that are related to their physiological action. We therefore examined 1) whether ampakine effects on agonist binding vary across brain regions and 2) whether they differ across receptor subunits expressed alone and together with transmembrane AMPA receptor regulatory proteins (TARPs), which associate with AMPA receptors in the brain. We found that the maximal increase in agonist binding (E(max)) caused by the prototypical ampakine 1-(1,4-benzodioxan-6-ylcarbonyl)piperidine (CX546) differs significantly between brain regions, with effects in hippocampus and cerebellum being nearly three times larger than that in thalamus, brainstem, and striatum, and cortex being intermediate. These differences can be explained at least in part by regional variations in receptor subunit and TARP expression because combinations prevalent in hippocampus (GluA2 with TARPs gamma3 and gamma8) exhibited E(max) values nearly twice those of combinations abundant in thalamus (GluA4 with gamma2 or gamma4). TARPs seem to be critical because GluA2 and GluA4 alone had comparable E(max) and also because hippocampal and thalamic receptors had similar E(max) after solubilization with Triton X-100, which probably removes associated proteins. Taken together, our data suggest that variations in physiological drug efficacy, such as the 3-fold difference previously seen in recordings from hippocampus versus thalamus, may be explained by region-specific expression of GluA1-4 as well as TARPs.

Neto1 is a novel CUB-domain NMDA receptor-interacting protein required for synaptic plasticity and learning

The N-methyl-D-aspartate receptor (NMDAR), a major excitatory ligand-gated ion channel in the central nervous system (CNS), is a principal mediator of synaptic plasticity. Here we report that neuropilin tolloid-like 1 (Neto1), a complement C1r/C1s, Uegf, Bmp1 (CUB) domain-containing transmembrane protein, is a novel component of the NMDAR complex critical for maintaining the abundance of NR2A-containing NMDARs in the postsynaptic density. Neto1-null mice have depressed long-term potentiation (LTP) at Schaffer collateral-CA1 synapses, with the subunit dependency of LTP induction switching from the normal predominance of NR2A- to NR2B-NMDARs. NMDAR-dependent spatial learning and memory is depressed in Neto1-null mice, indicating that Neto1 regulates NMDA receptor-dependent synaptic plasticity and cognition. Remarkably, we also found that the deficits in LTP, learning, and memory in Neto1-null mice were rescued by the ampakine CX546 at doses without effect in wild-type. Together, our results establish the principle that auxiliary proteins are required for the normal abundance of NMDAR subunits at synapses, and demonstrate that an inherited learning defect can be rescued pharmacologically, a finding with therapeutic implications for humans.

The fundamental unit for information processing in the brain is the synapse, a highly specialized site of communication between the brain's multitude of individual neurons. The strength of the communication at each synapse changes in response to neuronal activity—a process called synaptic plasticity—allowing networks of neurons to adapt and learn. How synaptic plasticity occurs is a major question in neurobiology. A central player in synaptic plasticity is an assembly of synaptic proteins called the NMDA receptor complex. Here, we discovered that the protein Neto1 is a component of the NMDA receptor complex. Neto1-deficient mice had a dramatic decrease in the number of NMDA receptors at synapses and consequently, synaptic plasticity and learning were impaired. By indirectly enhancing the function of the residual NMDA receptors in Neto1-deficient mice with a small molecule, we restored synaptic plasticity and learning to normal levels. Our findings establish the principle that inherited abnormalities of synaptic plasticity and learning due to NMDA receptor dysfunction can be pharmacologically corrected. Our discoveries also suggest that synaptic proteins that share a molecular signature, called the CUB domain, with Neto1 may be important components of synaptic receptors across species, because several CUB-domain proteins in worms have also been found to regulate synaptic receptors.

Spatial learning and memory depend on the N-methyl-D-aspartic acid receptor, a synaptic ion channel regulated by Neto1. Impaired cognition due to the absence of Neto1 can be rescued pharmacologically, a finding with implications for the therapy of inherited learning defects in humans.

Antagonism of glutamate receptors in the CA1 to perirhinal cortex projection prevents long-term potentiation and attenuates levels of brain-derived neurotrophic factor

The CA1 to perirhinal cortex projection is one of multiple hippocampal-neocortical projections considered to be involved in memory consolidation. This projection has been shown to sustain long-term potentiation (LTP) following stimulation of CA1. Here we examined the pharmacological properties underpinning the plasticity observed in this projection. A stimulating electrode was inserted into the area CA1 and a recording electrode was inserted into the perirhinal cortex of urethane-anaesthetised Wistar rats. Rats (n=6 in each drug group) were administered with either saline (0.09%), MK-801 (NMDA antagonist; 0.1 mg/kg) or CNQX (AMPA/kainate antagonist; 1.5 mg/kg). Baseline recordings were made for 10 min by stimulating area CA1 (0.05 Hz stimulation protocol). High-frequency stimulation (HFS; 250 Hz) was performed and post-HFS fEPSP recordings were made for 1 h (0.05 Hz, as above). Baseline and post-HFS paired-pulse facilitation (PPF) recordings were performed across six different interpulse intervals. CA1 and perirhinal cortex tissue samples were taken from the stimulated and unstimulated hemispheres of each rat brain and analysed using a brain-derived neurotrophic factor (BDNF) ELISA. Results indicate that LTP was induced in the saline and MK-801 groups but not in the CNQX group; fEPSPs in the latter group rapidly returned to baseline levels following a short period of post-tetanic potentiation. Drug treatment and HFS had no effect on PPF levels. Drug treatment significantly reduced concentrations of both CA1 and perirhinal BDNF and prevented stimulation-induced increases in BDNF in CA1. This molecular and electrophysiological data suggests that LTP in the CA1-perirhinal cortex projection may require activation of postsynaptic AMPA/kainate receptors in order to sustain LTP.

The role of kisspeptin and GPR54 in the hippocampus

The granule cells of the dentate gyrus form the input stage of the hippocampal trisynaptic circuit and their function is strongly influenced by peptidergic systems. GPR54 is highly and discretely expressed in these cells. We have found that activation of GPR54 with kisspeptin-10 causes a rapid and large increase in the amplitude of excitatory synaptic responses in granule cells, without changing membrane properties. The effect was suppressed by the G-protein inhibitor GDP-beta-S and the calcium chelator BAPTA, and analysis of miniature EPSCs revealed an increase in mean amplitude but not event frequency, indicating that GPR54 and the mechanisms for enhancing EPSCs are postsynaptic, possibly involving changes in AMPA receptor number or conductance. The kisspeptin-induced synaptic potentiation was abolished by inhibitors of ERK1/2, tyrosine kinase, and CaMKII. RT-PCR experiments showed that KiSS-1 is expressed in the dentate gyrus. KiSS-1 mRNA was significantly increased by seizure activity in rats and when neuronal activity in organotypic hippocampal slice cultures was enhanced by kainate or picrotoxin, while mRNA for GPR54 remained essentially unchanged. These results suggest that kisspeptin may be locally synthesized and act as an autocrine factor. In separate experiments, hippocampal KiSS-1 mRNA in male rats was increased after gonadectomy. In summary, kisspeptin is a novel endogenous factor which is dynamically regulated by neuronal activity and which, in marked distinction from other neuropeptides, increases synaptic transmission in dentate granule cells through signaling cascades possibly linked to the MAP kinase system. This novel peptide system may play a role in cognition and in the pathogenesis of epilepsy.

Mechanisms underlying cognitive enhancement and reversal of cognitive deficits in nonhuman primates by the ampakine CX717

RATIONALE

Performance of cognitive tasks in nonhuman primates (NHPs) requires specific brain regions to make decisions under different degrees of difficulty or "cognitive load."

OBJECTIVE

Local cerebral metabolic activity ([18F]FDG PET imaging) in dorsolateral prefrontal cortex (DLPFC), medial temporal lobe (MTL), and dorsal striatum (DStr) is examined in NHPs performing a delayed-match-to-sample (DMS) task with variable degrees of cognitive load.

MATERIALS AND METHODS

Correlations between cognitive load and degree of brain metabolic activity were obtained with respect to the influence of the ampakine CX717 (Cortex Pharmaceuticals), using brain imaging and recordings of neuronal activity in NHPs and measures of intracellular calcium release in rat hippocampal slices.

RESULTS

Activation of DLPFC, MTL, and DStr reflected changes in performance related to cognitive load within the DMS task and were engaged primarily on high load trials. Similar increased activation patterns and improved performance were also observed following administration of CX717. Sleep deprivation in NHPs produced impaired performance and reductions in brain activation which was reversed by CX717. One potential basis for this facilitation of cognition by CX717 was increased firing of task-specific hippocampal cells. Synaptic mechanisms affected by CX717 were examined in rat hippocampal slices which showed that N-methyl-D-aspartic acid-mediated release of intracellular calcium was reduced in slices from sleep-deprived rats and reversed by application of CX717 to the bathing medium.

CONCLUSIONS

The findings provide insight into how cognition is enhanced by CX717 in terms of brain, and underlying neural, processes that are activated on high vs. low cognitive load trials.

Glutamate receptor-mediated restoration of experience-dependent place field expansion plasticity in aged rats

Place fields of hippocampal pyramidal cells expand asymmetrically when adult rats repeatedly follow the same route. This behaviorally induced expression of neuronal plasticity uses an NMDAR-dependent, LTP-like mechanism and could be used by hippocampal networks to store information. Aged spatial memory-impaired rats exhibit defective experience-dependent place field expansion plasticity. One possible explanation for this aged-associated deficit is alterations in glutamatergic function. In fact, both NMDAR- and AMPAR-mediated field excitatory postsynaptic potentials in CA1 decrease with aging. The current study investigated whether modulation of either AMPA or NDMA receptor activity could restore this experience-dependent plasticity by prolonging AMPAR activity with the ampakine CX516 and modulating the NMDAR with the noncompetitive antagonist memantine. The spatial firing characteristics of multiple CA1 pyramidal cells were monitored under both treatment conditions as aged rats repeatedly traversed a circular track. Compared to the saline baseline condition, acute administration of memantine, but not CX516, reinstated experience-dependent place field expansion. Taken together, these data suggest that pharmacological manipulation of the NMDAR can improve the function of hippocampal networks critical to optimal cognition in aging.

Physiological significance of high- and low-affinity agonist binding to neuronal and recombinant AMPA receptors

Radioligand binding studies have shown that AMPA receptors exist in two variants that differ about twenty-fold in their binding affinities, with brain receptors being mainly of the low-affinity type and recombinantly expressed receptors having almost exclusively high affinity. However, the physiological correlate of high- and low-affinity binding is not yet known. In this study we examined if physiological experiments similarly reveal evidence for two distinct receptor variants. We therefore measured equilibrium desensitization by glutamate and determined IC(50) values for neuronal receptors and for the homomeric receptors GluR1-4 expressed in HEK293 cells. Contrary to the prediction that these IC(50) values exhibit large differences commensurate with those of high- and low-affinity binding, values for homomeric receptors (1-18 microM) were on an average not different from those of neuronal receptors (3-10 microM). Moreover, simulations with kinetic receptor models suggest that the IC(50) values for neuronal and recombinant receptors correspond to the binding affinity of the low-affinity receptor variant. These findings indicate that the high-affinity binding measured in heterologous expression systems represents an immature receptor variant that does not contribute to the currents recorded from these cells, and that the functional low-affinity receptors are present in such small number that they are effectively masked in binding assays by the high-affinity receptors. Thus, in order to compare experimentally determined saturation binding profiles with those predicted by kinetic receptor models and with dose-response curves from physiological studies, it will be imperative to develop methods for isolating first the low-affinity receptors.

A placebo-controlled add-on trial of the Ampakine, CX516, for cognitive deficits in schizophrenia

AMPA-receptor-positive modulators (Ampakines) facilitate learning and memory in animal models and in preliminary trials in human subjects. CX516 is the first Ampakine to be studied for cognitive enhancement in schizophrenia. Stable schizophrenia patients treated with clozapine (n=52), olanzapine (n=40), or risperidone (n=13) were randomly assigned to add-on treatment with CX516 900 mg three times daily or placebo for 4 weeks. Subjects were assessed with a cognitive battery at baseline, week 4, and at 4-week follow-up. Clinical scales and safety monitoring were also performed. The primary endpoint was the change from baseline in a composite cognitive score at week 4 for the intent-to-treat sample. Additional analyses examined change in symptom rating scores and examined drug effects on patients treated with clozapine separately from patients treated with either olanzapine or risperidone. A total of 105 patients were randomized and 95 (90%) completed the 4-week trial. Patients treated with CX516 did not differ from placebo in change from baseline on the composite cognitive score, or on any cognitive test at weeks 4 or 8. The between groups effect size at week 4 for the cognitive composite score was -0.19 for clozapine-treated patients and 0.24 for patients treated with olanzapine or risperidone. The placebo group improved more on the PANSS total score than the CX516 group; no other clinical rating differed between treatment groups. CX516 was associated with fatigue, insomnia and epigastric discomfort compared to placebo, but was generally well tolerated. CX516 was not effective for cognition or for symptoms of schizophrenia when added to clozapine, olanzapine, or risperidone.

Excitotoxic protection by polyanionic polysaccharide: evidence of a cell survival pathway involving AMPA receptor-MAPK Interactions

Growing numbers of studies indicate that polysaccharides influence signaling events important for brain function. It has been speculated that such polysaccharide modulation of neuronal signals can promote synaptogenesis and cell maintenance. Here, we tested whether dextran sulfate, a polyanion that mimics natural mucopolysaccharides, protects hippocampal neurons against excitotoxic insults. An excitotoxin was applied to primary hippocampal cultures in the absence or presence of a large 500-kDa dextran sulfate (DS-L), a smaller 5-8-kDa species (DS-S), or sulfate-free dextran of 500 kDa. Only DS-L prevented neuronal damage as determined by a membrane permeability assay and phase contrast morphology. The sulfate and size dependence is also characteristic of DS-L's modulatory action on the channel activity of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type glutamate receptors. The extent of neuroprotection correlates with the level of modulation of AMPA responses, and DS-L exhibits comparable EC(50) values for the two effects (3-7 nM). DS-L also modulates the link between AMPA receptors and mitogen-activated protein kinase (MAPK) involving extracellular signal-regulated protein kinase (ERK), well known for its involvement in cell survival and repair. Correspondingly, protection against N-methyl-D-aspartate (NMDA) excitotoxicity was evident in hippocampal slice cultures when DS-L was applied 30 min postinsult. These findings suggest that polysaccharides elicit neuroprotection in the brain, including enhanced repair responses through the AMPA receptor-MAPK axis.

S18986: a positive modulator of AMPA-receptors enhances (S)-AMPA-mediated BDNF mRNA and protein expression in rat primary cortical neuronal cultures

The present study describes the effect of (S)-2,3-dihydro-[3,4]cyclopentano-1,2,4-benzothiadiazine-1,1-dioxide (S18986), a positive allosteric modulator of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors, on (S)-AMPA-mediated increases in brain-derived neurotrophic factor (BDNF) mRNA and protein expression in rat primary cortical neuronal cultures. (S)-AMPA (0.01-300 microM) induced a concentration-dependent increase in BDNF mRNA and protein expression (EC(50)=7 microM) with maximal increases (50-fold) compared to untreated cultures observed between 5 and 12 h, whereas for cellular protein levels, maximal expression was detected at 24 h. S18986 alone (< or =300 microM) failed to increase basal BDNF expression. However, S18986 (300 microM) in the presence of increasing concentrations of (S)-AMPA maximally enhanced AMPA-induced expression of BDNF mRNA and protein levels (3-5-fold). S18986 (100-300 microM) potentiated BDNF mRNA induced by 3 microM (S)-AMPA (2-3-fold). Under similar conditions, the AMPA allosteric modulator cyclothiazide induced a potent stimulation of (S)-AMPA-mediated BDNF expression (40-fold; EC(50)=18 microM), whereas IDRA-21 was inactive. Kinetic studies indicated that S18986 (300 microM) in the presence of 3 microM (S)-AMPA was capable of enhancing BDNF mRNA levels for up to 25 h, compared to 3 microM (S)-AMPA alone. On the other hand, S18986 only partially enhanced kainate-mediated expression of BDNF mRNA, but failed to significantly enhance N-methyl-D-aspartate-stimulated BDNF expression levels. In support of these observations, the competitive AMPA receptor antagonist NBQX (1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide) but not the selective NMDA-receptor antagonist, (+)-MK-801 [(5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine], abrogated S18986-induced effects on BDNF expression. S18986-mediated enhancement of (S)-AMPA-evoked BDNF protein expression was markedly attenuated in Ca(2+)-free culture conditions. Furthermore, from a series of kinase inhibitors only the Calmodulin-Kinase II/IV inhibitor (KN-62, 25 microM) significantly inhibited (-85%, P<0.001) AMPA+S18986 stimulated expression of BDNF mRNA. The present study supports the observations that AMPA receptor allosteric modulators can enhance the expression of BDNF mRNA and protein expression via the AMPA receptor in cultured primary neurones. Consequently, the long-term elevation of endogenous BDNF expression by pharmacological intervention with this class of compounds represents a potentially promising therapeutic approach for behavioural disorders implicating cognitive deficits.

Targeting AMPA receptor gating processes with allosteric modulators and mutations

Allosteric modulators and mutations that slow AMPAR desensitization have additional effects on deactivation and agonist potency. We investigated whether these are independent actions or the natural consequence of slowing desensitization. Effects of cyclothiazide (CTZ), trichlormethiazide (TCM), and CX614 were compared at wild-type GluR1 and "nondesensitizing" GluR1-L497Y mutant receptors by patch-clamp recording with ultrafast perfusion. CTZ, TCM, or L/Y mutation all essentially blocked GluR1 desensitization; however, the effects of L/Y mutation on deactivation and glutamate EC50 were three to five times greater than for modulators. CTZ and TCM further slowed desensitization of L/Y mutant receptors but paradoxically accelerated deactivation and increased agonist EC50. Results indicate that CTZ and TCM target deactivation and agonist potency independently of desensitization, most likely by modifying agonist dissociation (koff). Conversely, CX614 slowed desensitization and deactivation without affecting EC50 in both wild-type and L/Y receptors. The S750Q or combined L497Y-S750Q mutations abolished all CTZ and TCM actions without disrupting CX614 activity. Notably, the S/Q mutation also restored L/Y deactivation and EC50 to wild-type levels without restoring desensitization, further demonstrating that desensitization can be modulated independently of deactivation and EC50 by mutagenesis and possibly by allosteric modulators.

Ampakines and the threefold path to cognitive enhancement

Ampakines are the first peripherally administered drugs that increase excitatory monosynaptic responses in the brain. Because of this effect, the compounds improve communication in complex networks, potently facilitate long-term potentiation (LTP) and induce the expression of neurotrophic factors. Ampakines are effective in animal models of neuropsychiatric disorders and have produced positive results in a small number of human studies. Neurobiological considerations and modeling studies suggest that the drugs, in addition to their effects on disturbed behavior, will alter the encoding and organization of information in normal brains. Results from physiological and behavioral studies accord with this prediction. Building on these findings, this article considers how the threefold effects of ampakines will modify, and enhance, cognition.