Facilitation of glutamate receptors enhances memory

A novel class of positive allosteric modulators of AMPA receptors: Design, synthesis, and structure–activity relationships of 3-biphenyl-4-yl-4-cyano-5-ethyl-1-methyl-1H-pyrrole-2-carboxylic acid, LY2059346

The synthesis and structure-activity relationship (SAR) of novel and highly potent positive allosteric modulators of AMPA receptors, 3-biphenyl-4-yl-4-cyano-5-ethyl-1-methyl-1H-pyrrole-2-carboxylic acid, are described. These studies indicated that higher potency was achieved with ortho substitution of the distal (D) phenyl of the 3-biphenyl ring and resulted in the discovery of a potent pyrrole LY2059346 (23q), that was selected for further evaluation in in vitro native tissue assays and in vivo experiments.

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.

Synaptic plasticity in early aging

Studies of how aging affects brain plasticity have largely focused on old animals. However, deterioration of memory begins well in advance of old age in animals, including humans; the present review is concerned with the possibility that changes in synaptic plasticity, as found in the long-term potentiation (LTP) effect, are responsible for this. Recent results indicate that impairments to LTP are in fact present by early middle age in rats but only in certain dendritic domains. The search for the origins of these early aging effects necessarily involves ongoing analyses of how LTP is induced, expressed, and stabilized. Such work points to the conclusion that cellular mechanisms responsible for LTP are redundant and modulated both positively and negatively by factors released during induction of potentiation. Tests for causes of the localized failure of LTP during early aging suggest that the problem lies in excessive activity of a negative modulator. The view of LTP as having redundant and modulated substrates also suggests a number of approaches for reversing age-related losses. Particular attention will be given to the idea that induction of brain-derived neurotrophic factor, an extremely potent positive modulator, can be used to provide long periods of normal plasticity with very brief pharmacological interventions. The review concludes with a consideration of how the selective, regional deficits in LTP found in early middle age might be related to the global phenomenon of brain aging.

Pharmacological characterization of DM232 (unifiram) and DM235 (sunifiram), new potent cognition enhancers

DM232 (unifiram) and DM235 (sunifiram) are potent cognition-enhancers, which are four order of magnitude more potent than piracetam. These compounds, although not showing affinity in binding studies for the most important central receptors or channels, are able to prevent amnesia induced by modulation of several neurotransmission systems. These compounds are able to increase the release of acetylcholine from rat cerebral cortex, and, as far as unifiram is concerned, to increase the amplitude of fEPSP in rat hippocampal slices. In vitro experiments, performed on hippocampal slices, also supported the hypothesis of a role of the AMPA receptors for the cognition-enhancing properties of unifiram and sunifiram.

Therapeutic potential of positive AMPA receptor modulators in the treatment of neuropsychiatric disorders

Drugs that potentiate the activity of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor cause a complex cascade of consequences in experimental models, ranging from enhancement of long-term potentiation to induction of neurotrophic factors. Animal studies characterising the pharmacological and behavioural effects of these substances have provided the rationale for several initial attempts to use these drugs in neuropsychiatric clinical settings. Applications in schizophrenia, Alzheimer's disease and mild cognitive impairment have been initiated. Other trials with these compounds include the treatment of Fragile X syndrome, and possible future applications may be in the field of Parkinson's disease. The literature published to date is limited mostly to small phase I or II trials, so there is no conclusive evidence for or against the use of these drugs. Substantial questions remain concerning which compounds to use, in what dose, for what condition and for how long.

Stargazin controls the pharmacology of AMPA receptor potentiators

Glutamate is the major excitatory neurotransmitter in brain, and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors (AMPARs) mediate the majority of postsynaptic depolarization. AMPAR ion channels display rapid gating, and their deactivation and desensitization determine the timing of synaptic transmission. AMPAR potentiators slow channel deactivation and desensitization, and these compounds represent exciting therapies for mental and neurodegenerative diseases. Previous studies showed that the AMPAR potentiators cyclothiazide and 4-[2-(phenylsulfonylamino)ethylthio]-2,6-difluorophenoxyacetamide display a preference for flip and flop alternatively spliced versions of glutamate receptor subunits, respectively. Here, we find that the AMPAR auxiliary subunit stargazin changes this pharmacology and makes both spliced forms of glutamate receptor subunit 1 sensitive to both classes of potentiator. Stargazin also enhances the effect of AMPAR potentiators on channel deactivation. This work demonstrates that stargazin controls AMPAR potentiator pharmacology, which has important implications for development of AMPAR potentiators as therapeutic agents.

Neurocognitive Enhancement: Ethical Considerations for an Emerging Subspecialty

Clinical neuropsychology has traditionally focused on the diagnosis and treatment of pathology. However, recent technological advances have begun to make possible the enhancement of healthy neurocognitive functioning. The potential to improve normal neurocognitive abilities, and the options for providing such improvement, raise complex ethical issues. To best serve consumers, the profession, and society, neuropsychologists must begin to consider the ethical issues related to neurocognitive enhancement. This article reviews neurocognitive enhancement methods, introduces and examines ethical issues that emerge when the enhancement of healthy neurocognitive functioning is considered, and offers recommendations for neuropsychologists entering this emerging subspecialty.

Restoration of long-term potentiation in middle-aged hippocampus after induction of brain-derived neurotrophic factor

Restoration of neuronal viability and synaptic plasticity through increased trophic support is widely regarded as a potential therapy for the cognitive declines that characterize aging. Previous studies have shown that in the hippocampal CA1 basal dendritic field deficits in the stabilization of long-term potentiation (LTP) are evident by middle age. The present study tested whether increasing endogenous brain-derived neurotrophic factor (BDNF) could reverse this age-related change. We report here that in middle-aged (8- to 10-mo-old) rats, in vivo treatments with a positive AMPA-type glutamate receptor modulator both increase BDNF protein levels in the cortical telencephalon and restore stabilization of basal dendritic LTP as assessed in acute hippocampal slices 18 h after the last drug treatment. These effects were not attributed to enhanced synaptic transmission or to facilitation of burst responses used to induce LTP. Increasing extracellular levels of BDNF by exogenous application to slices of middle-aged rats was also sufficient to rescue the stabilization of basal dendritic LTP. Finally, otherwise stable LTP in ampakine-treated middle-aged rats can be eliminated by infusion of the extracellular BDNF scavenger TrkB-Fc. Together these results indicate that increases in endogenous BDNF signaling can offset deficits in the postinduction processes that stabilize LTP.

Procholinergic and memory enhancing properties of the selective norepinephrine uptake inhibitor atomoxetine

Atomoxetine has been approved by the FDA as the first new drug in 30 years for the treatment of attention deficit/hyperactivity disorder (ADHD). As a selective norepinephrine uptake inhibitor and a nonstimulant, atomoxetine has a different mechanism of action from the stimulant drugs used up to now for the treatment of ADHD. Since brain acetylcholine (ACh) has been associated with memory, attention and motivation, processes dysregulated in ADHD, we investigated the effects of atomoxetine on cholinergic neurotransmission. We showed here that, in rats, atomoxetine (0.3-3 mg/kg, i.p.),--increases in vivo extracellular levels of ACh in cortical but not subcortical brain regions. The marked increase of cortical ACh induced by atomoxetine was dependent upon norepinephrine alpha-1 and/or dopamine D1 receptor activation. We observed similar increases in cortical and hippocampal ACh release with methylphenidate (1 and 3 mg/kg, i.p.)--currently the most commonly prescribed medication for the treatment of ADHD--and with the norepinephrine uptake inhibitor reboxetine (3-30 mg/kg, i.p.). Since drugs that increase cholinergic neurotransmission are used in the treatment of cognitive dysfunction and dementias, we also investigated the effects of atomoxetine on memory tasks. We showed that, consistent with its cortical procholinergic and catecholamine-enhancing profile, atomoxetine (1-3 mg/kg, p.o.) significantly ameliorated performance in the object recognition test and the radial arm-maze test.

AMPA receptor modulators have different impact on hippocampal pyramidal cells and interneurons

Positive modulators of AMPA receptors enhance synaptic plasticity and memory encoding. Facilitation of AMPA receptor currents not only results in enhanced activation of excitatory neurons but also increases the activity of inhibitory interneurons by up-modulating their excitatory input. However, little is known about the effects of these modulators on cells other than pyramidal neurons and about their impact on local microcircuits. This study examined the effects of members from three subfamilies of modulators (mainly CX516, CX546 and cyclothiazide) on excitatory synaptic responses in four classes of hippocampal CA1 neurons and on excitatory and disynaptically induced inhibitory field potentials in hippocampal slices. Effects on excitatory postsynaptic currents (EPSCs) were examined in pyramidal cells, in two types of inhibitory interneurons located in stratum radiatum and oriens, and in stratum radiatum giant cells, a novel type of excitatory neuron. With CX516, increases in EPSC amplitude in pyramidal cells were two to three times larger than in interneurons and six times larger than in radiatum giant cells. The effects of CX546 on response duration similarly were largest in pyramidal cells. However, this drug also strongly differentiated between stratum oriens and radiatum interneurons with increases being four times larger in the latter. In contrast, cyclothiazide had similar effects on response duration in all cell types. In field recordings, CX516 was several times more potent in enhancing excitatory postsynaptic potentials (EPSPs) than feedback or feedforward circuits, as expected from its larger influence on pyramidal cells. In contrast, BDP-20, a CX546 analog, was more potent in enhancing feedforward inhibition than either EPSPs or feedback inhibition. This preference for feedforward over feedback circuits is probably related to its higher potency in stratum radiatum versus oriens interneurons. Taken together, AMPA receptor modulators differ substantially in their potency and/or efficacy across major classes of neurons which is likely to have consequences with regard to their impact on circuits and behavior.

Three-dimensional structure of an AMPA receptor without associated stargazin/TARP proteins

Most excitatory synaptic transmissions in the central nervous system are mediated by the neurotransmitter glutamate. Binding of glutamate released from the presynaptic membrane causes glutamate receptors in the postsynaptic membrane to open, which results in a transient depolarization of the postsynaptic membrane. The AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) subtype of glutamate receptors is responsible for the majority of excitatory postsynaptic currents and is thought to play a central role in synaptic plasticity. Because modulation of glutamate receptors is believed to be involved in the basic mechanism underlying information storage in the brain, the molecular architecture of native AMPA receptors (AMPA-Rs) is of great interest. Previously, we have shown that AMPA-Rs purified from the brain are tightly associated with members of the stargazin/TARP (transmembrane AMPA receptor regulatory protein) family of membrane proteins [Nakagawa et al., Nature 433 (2005), pp. 545-549]. Here, we present a three-dimensional (3D) density map of the hetero-tetrameric AMPA-R without associated stargazin/TARP proteins as determined by cryo-negative stain single-particle electron microscopy. In the absence of stargazin/TARP proteins, the density representing the transmembrane region of the AMPA-R particles is substantially smaller, corroborating our previous analysis that was based solely on projection images.

Impaired maze performance in aged rats is accompanied by increased density of NMDA, 5-HT1A, and α-adrenoceptor binding in hippocampus

Using quantitative receptor autoradiography, we assessed binding site densities and distribution patterns of glutamate, GABA(A), acetylcholine (ACh), and monoamine receptors in the hippocampus of 32-month-old Fischer 344/Brown Norway rats. Prior to autoradiography, the rats were divided into two groups according to their retention performance in a water maze reference memory task, which was assessed 1 week after 8 days of daily maze training. The animals of the inferior group showed less long-term retention of the hidden-platform task but did not differ from superior rats in their navigation performance during place training and cued trials. The decreased retention performance in the group of inferior learners was primarily accompanied by increased alpha(1)-adrenoceptors in all hippocampal subregions under inspection (CA1-CA4 and dentate gyrus), while elevated alpha(2)-adrenoceptor binding was observed in the CA1 region and DG. Furthermore, inferior learners had higher NMDA binding in the CA2 and CA4 and increased 5-HT(1A) binding sites in the CA2, CA3, and CA4 region. No significant differences between inferior and superior learners were evident with regard to AMPA, kainate, GABA(A), muscarinergic M(1), dopamine D(1), and 5-HT(2) binding densities in any hippocampal region analyzed. These results show that increased NMDA, 5-HT(1A), and alpha-adrenoceptor binding in the hippocampus is associated with a decline in spatial memory. The increased receptor binding observed in the group of old rats with inferior maze performance might be the result of neural adaptation triggered by age-related changes in synaptic connectivity and/or synaptic activity.

Molecular and synaptic changes in the hippocampus underlying superior spatial abilities in pre-symptomatic G93A+/+ mice overexpressing the human Cu/Zn superoxide dismutase (Gly93 --> ALA) mutation

Although amyotrophic lateral sclerosis (ALS) is mainly considered as a motor disease, extramotor neural and cognitive alterations have also been reported in ALS patients. There is evidence that mutations in the Cu/Zn superoxide dismutase (SOD1) gene are implicated in about 20% of familiar ALS and transgenic mice overexpressing the human Cu/Zn superoxide dismutase (GLY(93) --> ALA) mutation show an ALS-like phenotype. However, while motor behavior has been extensively analyzed in these mutants, little is known on their cognitive abilities. To characterize the pre-symptomatic cognitive profile of G93A+/+ mice, we estimated their capability to detect spatial novelty and examined several indexes of their hippocampal function. We found an enhancement of spatial abilities in mutant mice associated with (1) a higher expression of hippocampal AMPA subunit GluR1 mRNA and of GluR1 protein levels, and (2) an increased induction and maintenance of long-term potentiation (LTP) at Schaffer collateral-CA1 synapses. Thus, before leading to extensive neuronal excitotoxicity, the high endogenous levels of glutamate present in the brain of pre-symptomatic G93A+/+ mice could mediate site-specific molecular and synaptic changes providing favorable conditions to spatial information processing. These findings suggest that identification of pre-symptomatic behavioral changes in murine models of ALS may point to early neural abnormalities selectively associated with mutations in the Cu/Zn superoxide dismutase (SOD1) gene.

Use of [3H]fluorowillardiine to study properties of AMPA receptor allosteric modulators

Compounds which modulate AMPA receptor function through allosteric mechanisms were examined for their effect on the binding of the agonist [3H]fluorowillardiine (FW). Benzamide-type positive modulators (ampakinestrade mark) under all experimental circumstances increased [3H]FW binding to native receptors in rat brain membranes. Benzothiadiazide drugs had more variable effects ranging from large reductions with cyclothiazide and JM-13 to increases produced by more recent compounds like PEPA, D1 and LY392098. These effects on binding were moderately influenced by the assay conditions, including temperature and the presence or absence of thiocyanate. Significant changes in agonist binding were also produced by other modulatory agents such as noncompetitive blockers (GYKI 53655, SYM 2206), polycationic compounds (spermine, Naspm, philanthotoxin) and polyanionic compounds (Evans Blue, suramin, PPNDS). EC50 values usually were similar to those from physiological studies, which validates using binding tests to assess drug potencies. Moreover, direction and magnitude of the binding change (Emax) provide information about which kinetic aspects are affected by a drug. For example, the magnitude of the binding increase produced by positive modulators was strongly correlated with their ability to slow response deactivation in excised patch recordings. Binding also provides a reliable method to examine whether interactions between agents are competitive. Thus, thiocyanate did not significantly influence the EC50 of cyclothiazide, suggesting distinct sites of action. Taken together, [3H]FW binding can yield important information about drug-receptor and drug-drug interactions for a wide range of modulatory agents. One potential limitation of [3H]FW is a large preference for subunits GluR1 and GluR2 (KD 4-10 nM) over GluR3 and GluR4 (160-600 nM) which implies that tests with brain membranes preferentially reveal drug effects produced at the former two subunits. Lastly, data are shown which highlight the importance of optimizing experimental conditions in filtration assays, for instance by always including thiocyanate in wash buffers.

Modulation of memory and visuospatial processes by biperiden and rivastigmine in elderly healthy subjects

RATIONALE

The central cholinergic system is implicated in cognitive functioning. The dysfunction of this system is expressed in many diseases like Alzheimer's disease, dementia of Lewy body, Parkinson's disease and vascular dementia. In recent animal studies, it was found that selective cholinergic modulation affects visuospatial processes even more than memory function.

OBJECTIVE

In the current study, we tried to replicate those findings. In order to investigate the acute effects of cholinergic drugs on memory and visuospatial functions, a selective anticholinergic drug, biperiden, was compared to a selective acetylcholinesterase-inhibiting drug, rivastigmine, in healthy elderly subjects.

METHODS

A double-blind, placebo-controlled, randomised, cross-over study was performed in 16 healthy, elderly volunteers (eight men, eight women; mean age 66.1, SD 4.46 years). All subjects received biperiden (2 mg), rivastigmine (3 mg) and placebo with an interval of 7 days between them. Testing took place 1 h after drug intake (which was around Tmax for both drugs). Subjects were presented with tests for episodic memory (wordlist and picture memory), working memory tasks (N-back, symbol recall) and motor learning (maze task, pursuit rotor). Visuospatial abilities were assessed by tests with high visual scanning components (tangled lines and Symbol Digit Substitution Test).

RESULTS

Episodic memory was impaired by biperiden. Rivastigmine impaired recognition parts of the episodic memory performance. Working memory was non-significantly impaired by biperiden and not affected by rivastigmine. Motor learning as well as visuospatial processes were impaired by biperiden and improved by rivastigmine.

CONCLUSIONS

These results implicate acetylcholine as a modulator not only of memory but also of visuospatial abilities.

Functional characterization of des-IGF-1 action at excitatory synapses in the CA1 region of rat hippocampus

Insulin-like growth factor-1 (IGF-1) and growth hormone play a major role in the growth and development of tissues throughout the mammalian body. Plasma IGF-1 concentrations peak during puberty and decline with age. We have determined that chronic treatments to restore plasma IGF-1 concentrations to adult levels attenuate spatial learning deficits in aged rats, but little is known of the acute actions of IGF-1 in the brain. To this end, we utilized hippocampal slices from young Sprague-Dawley rats to characterize the acute effects of des-IGF-1 on excitatory synaptic transmission in the CA1 region. We observed a 40% increase in field excitatory postsynaptic potential (fEPSP) slope with application of des-IGF-1 (40 ng/ml) and used whole cell patch-clamp recordings to determine that this enhancement was due to a postsynaptic mechanism involving alpha-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate (AMPA) but not N-methyl-D-aspartate receptors. Furthermore, the enhancement was completely blocked by the broad-spectrum tyrosine kinase inhibitor, genistein (220 microM), and significantly reduced by the PI3K blockers wortmannin (1 microM) and 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (10 microM), suggesting that the effect was predominantly dependent on PI3K activation. This characterization of the acute actions of des-IGF-1 at hippocampal excitatory synapses may provide insight into the mechanism by which long-term increases in plasma IGF-1 impart cognitive benefits in aged rats. Increases in AMPA receptor-mediated synaptic transmission may contribute directly to cognitive improvement or initiate long-term changes in synthesis of proteins such as brain-derived neurotrophic factor that are important to learning and memory.

Chronic treatment with sulbutiamine improves memory in an object recognition task and reduces some amnesic effects of dizocilpine in a spatial delayed-non-match-to-sample task

The effect of a sulbutiamine chronic treatment on memory was studied in rats with a spatial delayed-non-match-to-sample (DNMTS) task in a radial maze and a two trial object recognition task. After completion of training in the DNMTS task, animals were subjected for 9 weeks to daily injections of either saline or sulbutiamine (12.5 or 25 mg/kg). Sulbutiamine did not modify memory in the DNMTS task but improved it in the object recognition task. Dizocilpine, impaired both acquisition and retention of the DNMTS task in the saline-treated group, but not in the two sulbutiamine-treated groups, suggesting that sulbutiamine may counteract the amnesia induced by a blockade of the N-methyl-D-aspartate glutamate receptors. Taken together, these results are in favor of a beneficial effect of sulbutiamine on working and episodic memory.

Stargazin modulates AMPA receptor gating and trafficking by distinct domains

AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) receptors mediate fast excitatory synaptic transmission in the brain. These ion channels rapidly deactivate and desensitize, which determine the time course of synaptic transmission. Here, we find that the AMPA receptor interacting protein, stargazin, not only mediates AMPA receptor trafficking but also shapes synaptic responses by slowing channel deactivation and desensitization. The cytoplasmic tail of stargazin determines receptor trafficking, whereas the ectodomain controls channel properties. Stargazin alters AMPA receptor kinetics by increasing the rate of channel opening. Disrupting the interaction of stargazin ectodomain with hippocampal AMPA receptors alters the amplitude and shape of synaptic responses, establishing a crucial function for stargazin in controlling the efficacy of synaptic transmission in the brain.

Therapeutic potential of positive AMPA modulators and their relationship to AMPA receptor subunits. A review of preclinical data

BACKGROUND

Positive alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) modulators enhance glutamate transmission via the AMPA receptor by altering the rate of desensitization; alone they have no intrinsic activity. They are the only class of compounds known that may pharmacologically separate AMPA subtypes.

OBJECTIVE

This manuscript will review preclinical work on positive AMPA modulators, with clinical examples where relevant.

RESULTS

The activity of these compounds appears to be determined by the AMPA receptor subunit composition. Studies have shown that splice variant and/or subunit combinations change the desensitization rate of this receptor. Also, these subunits are heterogeneously expressed across the central nervous system. Therefore, the functional outcome of different positive AMPA modulators could indeed be different. The origins of this pharmacological class come from hippocampal long-term potentiation studies, so quite naturally they were first studied in models of short- and long-term memory (e.g., delayed match to sample, maze performance). In general, these agents were procognitive. However, more recent work with different chemical classes has suggested additional therapeutic effects in models of schizophrenia (e.g., amphetamine locomotor activity), depression (e.g., forced swim test), neuroprotection (e.g., NMDA agonist lesions) and Parkinson's disease (e.g., 6-hydroxydopamine lesion).

CONCLUSIONS

In conclusion, positive modulation of AMPA may offer numerous therapeutic avenues for central nervous system drug discovery.

LY503430: pharmacology, pharmacokinetics, and effects in rodent models of Parkinson's disease

Glutamate is the major excitatory transmitter in the brain. Recent developments in the molecular biology and pharmacology of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-subtype of glutamate receptors have led to the discovery of selective, potent and systemically active AMPA receptor potentiators. These molecules enhance synaptic transmission and play important roles in plasticity and cognitive processes. In the present studies we characterized a novel AMPA receptor potentiator, LY503430, on recombinant human GLU(A1-4) and native preparations in vitro, and then evaluated the potential neuroprotective effects of the molecule in rodent models of Parkinson's disease. Results indicated that at submicromolar concentrations LY503430 selectively enhanced glutamate-induced calcium influx into HEK293 cells transfected with human GLU(A1), GLU(A2), GLU(A3), or GLU(A4) AMPA receptors. The molecule also potentiated AMPA-mediated responses in native cortical, hippocampal and substantia nigra neurones. LY503430 had good oral bioavailability in both rats and dogs. We also report here that LY503430 provided dose-dependent functional and histological protection in animal models of Parkinson's disease. The neurotoxicity following unilateral infusion of 6-hyrdoxydopamine (6-OHDA) into either the substantia nigra or the striatum of rats and that following systemic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in mice were reduced. Interestingly, LY503430 also had neurotrophic actions on functional and histological outcomes when treatment was delayed until well after (6 or 14 days) the lesion was established. LY503430 also produced some increase in brain derived neurotrophic factor (BDNF) in the substantia nigra and a dose-dependent increase in growth associated protein-43 (GAP-43) expression in the striatum. Therefore, we propose that AMPA receptor potentiators such as LY503430 offer the potential of a new disease modifying therapy for Parkinson's disease.

SR141716A, a cannabinoid CB1 receptor antagonist, improves memory in a delayed radial maze task

An endogenous cannabinoid system may play an important role in controlling memory processes. SR141716A (N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamidehydrochloride), a selective cannabinoid CB(1) receptor antagonist, was studied in an eight-arm radial maze task in which either deficits or improvements in memory could be detected. This task required well-trained rats to recall after either a relatively short (3 h) or long (7 h) delay period where they had received rewards during an information phase in order to obtain the remaining rewards during a retention phase. SR141716A was administered intraperitoneally immediately after the information phase in order to determine the drug's effects on memory consolidation. Although SR141716A had no effect on the number of errors committed after a short interval, SR141716A significantly reduced the number of errors that occurred after 7 h. These results suggest that a cannabinoid CB(1) receptor antagonist can improve consolidation processes and thus may be useful in treating memory disorders.

The AMPA receptor potentiator LY404187 increases cerebral glucose utilization and c-fos expression in the rat

AMPA receptor potentiators enhance AMPA receptor-mediated glutamatergic neurotransmission and may have therapeutic potential as cognitive enhancers or antidepressants. The anatomical basis for the action of AMPA receptor potentiators is unknown. The aim of this study was to determine the effects of the biarylpropylsulfonamide AMPA receptor potentiator, LY404187 (0.05 to 5 mg/kg subcutaneously), upon cerebral glucose utilization and c-fos expression using 14C-2-deoxglucose autoradiography and c-fos immunocytochemistry. LY404187 (0.5 mg/kg) produced significant elevations in glucose utilization in 28 of the 52 anatomical regions analyzed, which included rostral neocortical areas and the hippocampus, as well the dorsal raphe nucleus, lateral habenula, and locus coeruleus. No significant decreases in glucose utilization were observed in any region after LY404187 administration. The increases in glucose utilization with LY404187 (0.5 mg/kg) were blocked by pretreatment with the AMPA receptor antagonist LY293558 (25 mg/kg), indicating that LY404187 acts through AMPA receptor-mediated mechanisms. LY404187 (0.5 mg/kg) also produced increases in c-fos immunoreactivity in the cortex, locus coeruleus, and the dorsal raphe nucleus. These studies demonstrate neuronal activation in key brain areas that are associated with memory processes and thus provide an anatomical basis for the cognitive enhancing effects of AMPA receptor potentiators.

Nootropic Agents Enhance the Recruitment of Fast GABAA Inhibition in Rat Neocortex

It is widely believed that nootropic (cognition-enhancing) agents produce their therapeutic effects by augmenting excitatory synaptic transmission in cortical circuits, primarily through positive modulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate receptors (AMPARs). However, GABA-mediated inhibition is also critical for cognition, and enhanced GABA function may be likewise therapeutic for cognitive disorders. Could nootropics act through such a mechanism as well? To address this question, we examined the effects of nootropic agents on excitatory and inhibitory postsynaptic currents (EPSCs and IPSCs) recorded from layer V pyramidal cells in acute slices of somatosensory cortex. Aniracetam, a positive modulator of AMPA/kainate receptors, increased the peak amplitude of evoked EPSCs and the amplitude and duration of polysynaptic fast IPSCs, manifested as a greater total charge carried by IPSCs. As a result, the EPSC/IPSC ratio of total charge was decreased, representing a shift in the excitation-inhibition balance that favors inhibition. Aniracetam did not affect the magnitude of either monosynaptic IPSCs (mono-IPSCs) recorded in the presence of excitatory amino acid receptor antagonists, or miniature IPSCs (mIPSCs) recorded in the presence of tetrodotoxin. However, the duration of both mono-IPSCs and mIPSCs was prolonged, suggesting that aniracetam also directly modulates GABAergic transmission. Cyclothiazide, a preferential modulator of AMPAR function, enhanced the magnitude and duration of polysynaptic IPSCs, similar to aniracetam, but did not affect mono-IPSCs. Concanavalin A, a kainate receptor modulator, had little effect on EPSCs or IPSCs, suggesting there was no contribution from kainate receptor activity. These findings indicate that AMPAR modulators strengthen inhibition in neocortical pyramidal cells, most likely by altering the kinetics of AMPARs on synaptically connected interneurons and possibly by modulating GABA(A) receptor responses in pyramidal cells. This suggests that the therapeutic actions of nootropic agents may be partly mediated through enhanced cortical GABAergic inhibition, and not solely through the direct modification of excitation, as previously thought.

Cosmetic neurology

Advances in cognitive neuroscience and neuropharmacology are yielding exciting treatments for neurologic diseases. Many of these treatments are also likely to have uses for people without disease. Here, I review the ways in which medicine might make bodies and brains function better by modulating motor, cognitive, and affective systems. These potential "quality of life" interventions raise ethical concerns, some related to the individual and others related to society. Despite these concerns, I argue that major restraints on the development of cosmetic neurology are not likely. Neurologists and other clinicians are likely to encounter patient-consumers who view physicians as gatekeepers in their own pursuit of happiness.

Psychopharmacology in fragile X syndrome--present and future

In addition to cognitive disability, fragile X syndrome (FXS) is associated with behavioral problems that are often functionally limiting. There are few controlled trials to guide treatment; however, available information does suggest that medications can be quite helpful for a number of categories of behavioral disturbance in FXS. Specifically, stimulants appear to be quite useful for management of distractibility, hyperactivity, and impulsive behavior; antidepressants help with anxiety, obsessive-compulsive behaviors and mood dysregulation; and antipsychotics can reduce aggression. These medications are supportive and help minimize dysfunctional behaviors and maximize functioning. As more is learned about the neural functions of FMRP, medications in the future will be expected to target specific synaptic mechanisms dysregulated in FXS brain and thus ameliorate the cognitive deficit with resultant behavioral improvements. This article summarizes knowledge about effectiveness and approaches to management of currently available psychopharmacology for behavior in FXS and discusses early leads to future treatments for cognition.

S 18986, a positive modulator of AMPA receptors with cognition-enhancing properties, increases ACh release in the hippocampus of young and aged rat

The effect of S 18986, positive AMPA receptor modulator, on acetylcholine (ACh), gamma-aminobutyric acid (GABA) and glutamate (Glu) release from the hippocampus of freely moving young and aged rats was investigated by microdialysis coupled to HPLC. The cognition-enhancing properties were evaluated by a passive avoidance test. In 3 month-old rats, S 18986 (10 mg/kg i.p.) increased by 70% ACh release, which returned to basal level within 2 h, while 3 mg/kg had no effect. In 22 month-old rats, both 3 and 10 mg/kg i.p. induced a long lasting increase in ACh release, as large as that induced by 10 mg/kg in young rats. S 18986 did not modify GABA and glutamate release. No effect on general behavior was observed, but S 18986 at both doses prevented the disrupting effect of scopolamine (1 mg/kg i.p.) on passive avoidance acquisition.

Effects of dehydroevodiamine exposure on glutamate release and uptake in the cultured cerebellar cells

Dehydroevodiamine has been reported to have neuroprotective and antiamnesic effects. This study examined the effects of dehydroevodiamine on glutamate release and uptake in cultured cerebellar cells. Chronic dehydroevodiamine exposure decreased the viability of granule cells. The basal and N-methyl-D-aspartate (NMDA)-induced release of glutamate from granule cells were decreased (26 and 14%) by dehydroevodiamine. The NMDA-induced release of glutamate was concentration-dependently inhibited in the granule cells. The basal and NMDA-induced releases of glutamate in chronically dehydroevodiamine-preexposed granule cells were unaffected by dehydroevodiamine. Glutamate uptake in the glial cells incubated without and with cAMP was inhibited (31% and 8%, respectively) by dehydroevodiamine. In the chronically dehydroevodiamine-preexposed glial cells, glutamate uptake was increased (8%) in the cAMP-coexposed glial cells by dehydroevodiamine but was unaffected in the naive cells. In addition, dehydroevodiamine potentiated (from 20% to 34%) the inhibition of L-pyrollidine-2,4-dicarboxylic acid (PDC) on glutamate uptake in naive glial cells, but this inhibition was reduced (from 41% to 26%) in cAMP-coexposed glial cells. These results suggest that dehydroevodiamine inhibits glutamate uptake and release. Furthermore, the results suggest that the characteristics of glutamate release and uptake in granule and glial cells may be altered by chronic exposure to dehydroevodiamine.

Sulfate- and size-dependent polysaccharide modulation of AMPA receptor properties

Previous work found evidence that alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-type glutamate receptors interact with and are functionally regulated by the glycosaminoglycan heparin. The present study tested whether dextran species affect ligand binding, channel kinetics, and calcium permeability of AMPA receptors. Dextran sulfate of 500 kDa markedly reduced high affinity [3H]AMPA binding in solubilized hippocampal membranes. In isolated receptors reconstituted in a lipid bilayer, the same dextran sulfate prolonged the lifetime of open states exhibited by AMPA-induced channel fluctuations. The large polysaccharide further changed the single channel kinetics by increasing the open channel probability five- to sixfold. Such modulation of channel activity corresponded with enhanced levels of calcium influx as shown in hippocampal neurons loaded with Fluo3AM dye. With an exposure time of <1 min, AMPA produced a dose-dependent increase in intracellular calcium that was blocked by 6-cyano-7-nitroquinoxaline-2,3-dione disodium (CNQX). Dextran sulfate, at the same concentration range that modified ligand binding (EC50 of 5-10 nM), enhanced the AMPA-induced calcium influx by as much as 60%. The enhanced influx was blocked by CNQX, although unchanged by the N-methyl-D-aspartate (NMDA) receptor antagonist AP5. Confocal microscopy showed that the increase in calcium occurred in neuronal cell bodies and their processes. Interestingly, smaller 5-8-kDa dextran sulfate and a non-sulfated dextran of 500 kDa had little or no effect on the binding, channel, and calcium permeability properties. Together, these findings suggest that synaptic polysaccharide species modulate hippocampal AMPA receptors in a sulfate- and size-dependent manner.

Long-term potentiation and memory

One of the most significant challenges in neuroscience is to identify the cellular and molecular processes that underlie learning and memory formation. The past decade has seen remarkable progress in understanding changes that accompany certain forms of acquisition and recall, particularly those forms which require activation of afferent pathways in the hippocampus. This progress can be attributed to a number of factors including well-characterized animal models, well-defined probes for analysis of cell signaling events and changes in gene transcription, and technology which has allowed gene knockout and overexpression in cells and animals. Of the several animal models used in identifying the changes which accompany plasticity in synaptic connections, long-term potentiation (LTP) has received most attention, and although it is not yet clear whether the changes that underlie maintenance of LTP also underlie memory consolidation, significant advances have been made in understanding cell signaling events that contribute to this form of synaptic plasticity. In this review, emphasis is focused on analysis of changes that occur after learning, especially spatial learning, and LTP and the value of assessing these changes in parallel is discussed. The effect of different stressors on spatial learning/memory and LTP is emphasized, and the review concludes with a brief analysis of the contribution of studies, in which transgenic animals were used, to the literature on memory/learning and LTP.

Neurotrophic actions of the novel AMPA receptor potentiator, LY404187, in rodent models of Parkinson's disease

Recent developments in the molecular biology and pharmacology of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors has led to the discovery of selective, potent and systemically active AMPA receptor potentiators. These molecules enhance synaptic transmission and evidence suggests that they play important roles in plasticity and cognitive processes. Activation of AMPA receptors also increases neuronal activation and activity-dependent signalling, which may increase brain-derived neurotrophic factor (BDNF) expression and enhance cell proliferation in the brain. We therefore hypothesised that an AMPA receptor potentiator may provide neurotrophic effects in rodent models of Parkinson's disease. In the present studies we report that the potent and selective AMPA receptor potentiator, R,S-N-2-(4-(4-Cyanophenyl)phenyl)propyl 2-propanesulfonamide (LY404187), provides both functional, neurochemical and histological protection against unilateral infusion of 6-hydroxydopamine into the substantia nigra or striatum of rats. The compound also reduced 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced toxicity in mice. Interestingly, we were also able to observe large functional and histological effects when we delayed treatment until after cell death had occurred (3 or 6 days after 6-hydroxydopamine infusion), supporting a neurotrophic mechanism of action. In addition, LY404187 provided a dose-dependent increase in growth-associated protein-43 expression in the striatum. Therefore, we propose that AMPA receptor potentiators offer the potential of a new therapy to halt the progression and perhaps repair the degeneration in Parkinson's disease.

Multiple molecular determinants for allosteric modulation of alternatively spliced AMPA receptors

Positive allosteric regulation of glutamate AMPA receptors involves conformational changes that can attenuate receptor desensitization and enhance ion flux through the channel pore. Many allosteric modulators (e.g., cyclothiazide and aniracetam) preferentially affect the flip (i) or flop (o) alternatively spliced isoform of AMPA receptors, implicating residues in the flip-flop domain as critical determinants of splice variant sensitivity. Indeed, previous mutational analyses have demonstrated that the differential sensitivity to cyclothiazide and aniracetam depends on a single amino acid, Ser (flip) and Asn (flop), suggesting that this residue may be solely responsible for differences in modulation of AMPA receptor isoforms. The present studies tested this hypothesis by investigating the molecular determinants of modulation of AMPA receptor splice variants by a structurally distinct compound, LY404187, which displays strikingly different and opposing kinetics of allosteric regulation characterized by a time-dependent enhancement in potentiation of homomeric GluR1-GluR4i and a time-dependent reduction in potentiation of GluR1-GluR4o. Site-directed mutagenesis of residues in the flip-flop domain of GluR2 revealed that, although exchange of Asn775 for Ser in GluR2o was sufficient to confer the GluR2i phenotype of potentiation, the corresponding mutation, Ser775Asn, in GluR2i did not impart the GluR2o response. In fact, the GluR2o kinetics of modulation depended on a novel set of substitutions in GluR2i, including Thr765Asn, Pro766Ala, and Val779Leu in combination with Ser775Asn. Collectively, these results show that, unlike cyclothiazide and aniracetam, the residues that confer splice variant differences in modulation by LY404187 are not identical and indicate that allosteric regulation of AMPA receptors can arise from multiple molecular determinants.

Multiple molecular determinants for allosteric modulation of alternatively spliced AMPA receptors

Positive allosteric regulation of glutamate AMPA receptors involves conformational changes that can attenuate receptor desensitization and enhance ion flux through the channel pore. Many allosteric modulators (e.g., cyclothiazide and aniracetam) preferentially affect the flip (i) or flop (o) alternatively spliced isoform of AMPA receptors, implicating residues in the flip-flop domain as critical determinants of splice variant sensitivity. Indeed, previous mutational analyses have demonstrated that the differential sensitivity to cyclothiazide and aniracetam depends on a single amino acid, Ser (flip) and Asn (flop), suggesting that this residue may be solely responsible for differences in modulation of AMPA receptor isoforms. The present studies tested this hypothesis by investigating the molecular determinants of modulation of AMPA receptor splice variants by a structurally distinct compound, LY404187, which displays strikingly different and opposing kinetics of allosteric regulation characterized by a time-dependent enhancement in potentiation of homomeric GluR1-GluR4i and a time-dependent reduction in potentiation of GluR1-GluR4o. Site-directed mutagenesis of residues in the flip-flop domain of GluR2 revealed that, although exchange of Asn775 for Ser in GluR2o was sufficient to confer the GluR2i phenotype of potentiation, the corresponding mutation, Ser775Asn, in GluR2i did not impart the GluR2o response. In fact, the GluR2o kinetics of modulation depended on a novel set of substitutions in GluR2i, including Thr765Asn, Pro766Ala, and Val779Leu in combination with Ser775Asn. Collectively, these results show that, unlike cyclothiazide and aniracetam, the residues that confer splice variant differences in modulation by LY404187 are not identical and indicate that allosteric regulation of AMPA receptors can arise from multiple molecular determinants.

Cyclothiazide potently inhibits gamma-aminobutyric acid type A receptors in addition to enhancing glutamate responses

Ionotropic glutamate and gamma-aminobutyric acid type A (GABAA) receptors mediate critical excitatory and inhibitory actions in the brain. Cyclothiazide (CTZ) is well known for its effect of enhancing glutamatergic transmission and is widely used as a blocker for alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptor desensitization. Here, we report that in addition to its action on AMPA receptors, CTZ also exerts a powerful but opposite effect on GABAA receptors. We found that CTZ reversibly inhibited both evoked and spontaneous inhibitory postsynaptic currents, as well as GABA application-induced membrane currents, in a dose-dependent manner. Single-channel analyses revealed further that CTZ greatly reduced the open probability of GABAA receptor channels. These results demonstrate that CTZ interacts with both glutamate and GABAA receptors and shifts the excitation-inhibition balance in the brain by two independent mechanisms. Understanding the molecular mechanism of this double-faceted drug-receptor interaction may help in designing new therapies for neurological diseases.

Ampakines reduce methamphetamine-driven rotation and activate neocortex in a regionally selective fashion

It has been proposed that glutamatergic and dopaminergic systems are functionally opposed in their regulation of striatal output. The present study tested the effects of drugs that enhance AMPA-receptor-mediated glutamatergic transmission (ampakines) for their effects on dopamine-related alterations in cortical activity and locomotor behavior. Rats with unilateral 6-hydroxydopamine lesions of the ascending nigro-striatal dopamine system were sensitized to methamphetamine and then tested for methamphetamine-induced circling behavior in the presence and absence of ampakines CX546 and CX614. Both ampakines produced rapid, dose-dependent reductions in circling that were evident within 15 min and sustained through 1 h of behavioral testing. In situ hybridization maps of c-fos mRNA expression showed that in the intact hemisphere, ampakine cotreatment markedly increased c-fos expression in parietal, sensori-motor neocortex above that found in rats treated with methamphetamine alone. Ampakine cotreatment did not augment c-fos expression in frontal, sensori-motor cortex or striatum. Still larger ampakine-elicited effects were obtained in parietal cortex of the dopamine-depleted hemisphere where labeling densities were increased by approximately 60% above values found in methamphetamine-alone rats. With these effects, the hemispheric asymmetry of cortical activation was less pronounced in the ampakine-cotreatment group as compared with the methamphetamine-alone group. These results indicate that positive modulation of AMPA-type glutamate receptors 1) can offset behavioral disturbances arising from sensitized dopamine receptors and 2) increases aggregate neuronal activity in a regionally selective manner that is probably dependent upon behavioral demands.

Chronic elevation of brain-derived neurotrophic factor by ampakines

The ampakine CX614 positively modulates alpha-amino-3-hydroxy-5methyl-4-isoxazolepropionic acid (AMPA) receptor-gated currents and increases brain-derived neurotrophic factor (BDNF) expression. In rat hippocampal slice cultures, CX614 rapidly increases BDNF gene expression but with time, mRNA levels fall despite the continued presence of active drug. The present study examined this apparent refractory period and the possibility that spaced ampakine treatments could sustain elevated BDNF protein levels. In cultured hippocampal slices, CX614, a second ampakine CX546, and the cholinergic agonist carbachol each increased BDNF mRNA levels with acute (3-h) treatment. After 4-day pretreatment with CX614, fresh ampakine (CX614 or CX546) did not induce BDNF mRNA, whereas carbachol did. Western blots confirmed that after an extended period of ampakine treatment, AMPA receptor protein levels are indeed reduced, suggesting that with longer treatments receptor down-regulation mediates ampakine insensitivity. Finally, using a "24-h on/24-h off" CX614 treatment protocol, the ampakine refractory state was circumvented, BDNF mRNA was induced with each ampakine application, and elevated BDNF protein levels were maintained through 5 days in vitro. These results suggest that spaced ampakine treatments can be used to sustain elevated neurotrophin levels and to test the utility of this manipulation for neuroprotection by endogenous neurotrophins.

Individual differences in the expression of a "general" learning ability in mice

Human performance on diverse tests of intellect are impacted by a "general" regulatory factor that accounts for up to 50% of the variance between individuals on intelligence tests. Neurobiological determinants of general cognitive abilities are essentially unknown, owing in part to the paucity of animal research wherein neurobiological analyses are possible. We report a methodology with which we have assessed individual differences in the general learning abilities of laboratory mice. Abilities of mice on tests of associative fear conditioning, operant avoidance, path integration, discrimination, and spatial navigation were assessed. Tasks were designed so that each made unique sensory, motor, motivational, and information processing demands on the animals. A sample of 56 genetically diverse outbred mice (CD-1) was used to assess individuals' acquisition on each task. Indicative of a common source of variance, positive correlations were found between individuals' performance on all tasks. When tested on multiple test batteries, the overall performance ranks of individuals were found to be highly reliable and were "normally" distributed. Factor analysis of learning performance variables determined that a single factor accounted for 38% of the total variance across animals. Animals' levels of native activity and body weights accounted for little of the variability in learning, although animals' propensity for exploration loaded strongly (and was positively correlated) with learning abilities. These results indicate that diverse learning abilities of laboratory mice are influenced by a common source of variance and, moreover, that the general learning abilities of individual mice can be specified relative to a sample of peers.

LY503430, a novel alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor potentiator with functional, neuroprotective and neurotrophic effects in rodent models of Parkinson's disease

Glutamate is the major excitatory transmitter in the brain. Recent developments in the molecular biology and pharmacology of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) subtype of glutamate receptors have led to the discovery of selective, potent, and systemically active AMPA receptor potentiators. These molecules enhance synaptic transmission and play important roles in plasticity and cognitive processes. In the present study, we first characterized a novel AMPA receptor potentiator, (R)-4'-[1-fluoro-1-methyl-2-(propane-2-sulfonylamino)-ethyl]-biphenyl-4-carboxylic acid methylamide (LY503430), on recombinant human GLUA1-4 and native preparations in vitro and then evaluated the potential neuroprotective effects of the molecule in rodent models of Parkinson's disease. Results indicated that submicromolar concentrations of LY503430 selectively enhanced glutamate-induced calcium influx into human embryonic kidney 293 cells transfected with human GLUA1, GLUA2, GLUA3, or GLUA4 AMPA receptors. The molecule also potentiated AMPA-mediated responses in native cortical, hippocampal, and substantia nigra neurons. We also report here that LY503430 provided dose-dependent functional and histological protection in animal models of Parkinson's disease. The neurotoxicity after unilateral infusion of 6-hydroxydopamine into either the substantia nigra or the striatum of rats and that after systemic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in mice were reduced. Interestingly, LY503430 also had neurotrophic actions on functional and histological outcomes when treatment was delayed until well after (6 or 14 days) the lesion was established. LY503430 also produced some increase in brain-derived neurotrophic factor in the substantia nigra and a dose-dependent increases in growth associated protein-43 (GAP-43) expression in the striatum. Therefore, we propose that AMPA receptor potentiators offer the potential of a new disease modifying therapy for Parkinson's disease.

Comparison of the effects of antipsychotics on a delayed radial maze task in the rat

RATIONALE

The cognitive impairments evident in many schizophrenics are related to the severity of their negative symptoms and ability to function in society. Drugs that alleviate cognitive impairments, in addition to other psychotic symptoms, may have an important influence on treatment outcome and the course of the illness.

OBJECTIVES

A delayed non-match to sample task conducted in an eight-arm radial maze was used to determine the influence of four atypical antipsychotics (olanzapine, ziprasidone, risperidone, and clozapine), as well as a typical neuroleptic (haloperidol) on consolidation processes in healthy rats.

METHOD

Well-trained rats were required to recall after a 7-h delay where they had received food pellets during an information phase (first four arm choices) in order to obtain the remaining food pellets during a retention phase (second four arm choices).

RESULTS

The total number of errors that occurred during the retention session increased with increasing delay periods from 0 to 7 h. When administered orally immediately after the information phase, olanzapine (3 and 5 mg/kg) and risperidone (0.1 mg/kg) significantly reduced the number of errors made during the retention phase. Under the same conditions, clozapine, ziprasidone and haloperidol failed to affect the total number of retention phase errors.

CONCLUSION

Some atypical antipsychotics, such as olanzapine and risperidone, improve consolidation processes and may alleviate the cognitive impairments associated with schizophrenia.

Individual differences in the expression of a "general" learning ability in mice

Human performance on diverse tests of intellect are impacted by a "general" regulatory factor that accounts for up to 50% of the variance between individuals on intelligence tests. Neurobiological determinants of general cognitive abilities are essentially unknown, owing in part to the paucity of animal research wherein neurobiological analyses are possible. We report a methodology with which we have assessed individual differences in the general learning abilities of laboratory mice. Abilities of mice on tests of associative fear conditioning, operant avoidance, path integration, discrimination, and spatial navigation were assessed. Tasks were designed so that each made unique sensory, motor, motivational, and information processing demands on the animals. A sample of 56 genetically diverse outbred mice (CD-1) was used to assess individuals' acquisition on each task. Indicative of a common source of variance, positive correlations were found between individuals' performance on all tasks. When tested on multiple test batteries, the overall performance ranks of individuals were found to be highly reliable and were "normally" distributed. Factor analysis of learning performance variables determined that a single factor accounted for 38% of the total variance across animals. Animals' levels of native activity and body weights accounted for little of the variability in learning, although animals' propensity for exploration loaded strongly (and was positively correlated) with learning abilities. These results indicate that diverse learning abilities of laboratory mice are influenced by a common source of variance and, moreover, that the general learning abilities of individual mice can be specified relative to a sample of peers.

Differential involvement of NMDA and AMPA receptors within the nucleus accumbens in consolidation of information necessary for place navigation and guidance strategy of mice

Recent evidence now points to a role of glutamate transmission within the nucleus accumbens (Nacc) in spatial learning and memory. Unfortunately, the role of the distinct classes of glutamate receptors within this structure in mediating the different steps of the memorization process is not clear. The aim of this study therefore was to further investigate this issue, trying to assess the involvement of the two classes of glutamate receptors within the Nacc in consolidation of spatial information using an associative spatial task, the water maze. For this purpose, focal injections of the NMDA antagonist, AP-5, and of the AMPA antagonist, DNQX, have been performed immediately after the training phase, and mice have been tested for retention 24 h later. Two different versions of the water-maze task have been used: In the place version, animals could learn the position of the platform using visual distal cues, and in the cue version, the location of the platform was indicated by a single proximal cue. The results demonstrated that posttraining NMDA receptor blockade affects mice response in the place but not in the cue water-maze task. On the contrary, AMPA receptor blockade induced no effect in either version of the task. These data confirm a functional dissociation between glutamate receptors located in the Nacc in modulating spatial memory consolidation and indicate that they are specifically involved in consolidation of information necessary to acquire a place but not to a guidance strategy.

Neuroadaptive processes in GABAergic and glutamatergic systems in benzodiazepine dependence

Knowledge of the neural mechanisms underlying the development of benzodiazepine (BZ) dependence remains incomplete. The gamma-aminobutyric acid (GABA(A)) receptor, being the main locus of BZ action, has been the main focus to date in studies performed to elucidate the neuroadaptive processes underlying BZ tolerance and withdrawal in preclinical studies. Despite this intensive effort, however, no clear consensus has been reached on the exact contribution of neuroadaptive processes at the level of the GABA(A) receptor to the development of BZ tolerance and withdrawal. It is likely that changes at the level of this receptor are inadequate in themselves as an explanation of these neuroadaptive processes and that neuroadaptations in other receptor systems are important in the development of BZ dependence. In particular, it has been hypothesised that as part of compensatory mechanisms to diazepam-induced chronic enhancement of GABAergic inhibition, excitatory mechanisms (including the glutamatergic system) become more sensitive [Behav. Pharmacol. 6 (1995) 425], conceivably contributing to BZ tolerance development and/or expression of withdrawal symptoms on cessation of treatment, including increased anxiety and seizure activity. Glutamate is a key candidate for changes in excitatory transmission mechanisms and BZ dependence, (1) since there are defined neuroanatomical relationships between glutamatergic and GABAergic neurons in the CNS and (2) because of the pivotal role of glutamatergic neurotransmission in mediating many forms of synaptic plasticity in the CNS, such as long-term potentiation and kindling events. Thus, it is highly possible that glutamatergic processes are also involved in the neuroadaptive processes in drug dependence, which can conceivably be considered as a form of synaptic plasticity. This review provides an overview of studies investigating changes in the GABAergic and glutamatergic systems in the brain associated with BZ dependence, with particular attention to the possible differential involvement of N-methyl-D-aspartate and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors in these processes.

Positive allosteric modulators of AMPA receptors are neuroprotective against lesions induced by an NMDA agonist in neonatal mouse brain

Four positive modulators of AMPA-type glutamate receptors (cyclothiazide, CX614, LY404187 and S18986-1) given in acute or chronic manner exerted a neuroprotective effect in lesions induced in postnatal day 5 (P5) mice by intracerebral injection of ibotenate, an NMDA agonist. The neuroprotective effects were mediated via the MAPK pathway since coinjection of the MEK inhibitor, PD98059, blocked the neuroprotective effects. Administration of CX614 to neonatal mice was followed by upregulation of hippocampal and cortical BDNF expression.

Protein kinase A mediates voltage-dependent facilitation of Ca2+ current in presynaptic hair cells in Hermissenda crassicornis

The simplest cellular model for classical conditioning in the nudibranch mollusk, Hermissenda crassicornis, involves the presynaptic hair cells and postsynaptic photoreceptors. Whereas the cellular mechanisms for postsynaptic photoreceptors have been studied extensively, the presynaptic mechanisms remain uncertain. Here, we determined the phenotype of the voltage-dependent Ca(2+) current in the presynaptic hair cells that may be directly involved in changes in synaptic efficacy during classical conditioning. The Ca(2+) current can be classified as a P-type current because its activation voltage under seawater recording conditions is approximately -30 mV, it showed slow inactivation, and it is reversibly blocked by omega-agatoxin-IVA. The steady-state activation and inactivation curves revealed a window current, and the single-channel conductance is approximately 20 pS. The P-type current was enhanced by cAMP analogs (approximately 1.3-fold), and by forskolin, an activator of adenylyl cyclase (approximately 1.25-fold). In addition, the P-type current showed voltage-dependent facilitation, which is mediated by protein kinase A (PKA). Specifically, the PKA inhibitor peptide [PKI(6-22)amide] blocked the enhancement of the Ca(2+) current produced by conditioning depolarization prepulses. Because neurotransmitter release is mediated by Ca(2+) influx via voltage-gated Ca(2+) channels, and because of the nonlinear relationship between the Ca(2+) influx and neurotransmitter release, we propose that voltage-dependent facilitation of the P-type current in hair cells would produce a robust change in synaptic efficacy.

Elementary properties of axonal calcium currents in type B photoreceptors in Hermissenda crassicornis

Axons of the type B photoreceptors form synapses with hair cells and interneurons that are involved in classical conditioning in Hermissenda. We examined the differences in the Ca2+ channels expressed in the soma and axons of the B photoreceptors by direct functional recordings of single-channel currents. Although the soma of the B cells express two Ca2+ current subtypes, a transient BayK 8644-insensitive (approximately 17 pS) current and a sustained BayK 8644-sensitive (approximately 10 pS) current, the axons expressed only the latter. The axonal Ca2+ current activated at potentials positive to -20 mV. Moreover, the Ca2+ channels are distributed heterogeneously along the length of the axon, with the higher channel density (approximately 10-15 channel microm(-2)) occurring at the distal one-third of the isolated axons, with respect to the soma. The regions of Ca2+ channel clusters may represent the presynaptic site of the photoreceptor-interneuron synapses. Furthermore, the high-density clusters of Ca2+ channels may augment postsynaptic responses. The results of the present study represent the first direct recordings of Ca2+ currents at presumed synaptic sites. Expression of different Ca2+ channel subtypes at distinct compartments of the type B photoreceptors may generate diverse Ca2+ domains that may be required for neuronal plasticity in Hermissenda.

Conventional and Atypical Antipsychotics in the Elderly

Psychoses are major mental disorders marked by derangement of personality and loss of contact with reality, and are common in the elderly. Various hypotheses suggest the pivotal role of abnormal neurotransmitter and neuropeptide systems in psychotic patients, the most studied of which are the dopaminergic, serotonergic and glutamatergic systems. In particular, long-term treatment with antagonists at dopamine (D) and serotonin (5-hydroxytryptamine; 5-HT) receptors and agonists at glutamate receptors may improve symptoms. Treatment with antipsychotics is very common in the elderly and often indispensable. However, for successful treatment it is essential to have an adequate multidimensional assessment of the geriatric patient and of his or her polypathology and polypharmacy, together with knowledge of age-dependent pharmacokinetics and pharmacodynamic changes and drug-drug interactions.Conventional antipsychotics such as haloperidol, chlorpromazine, promazine, tiapride and zuclopenthixol are D(2)-receptor antagonists and inhibit dopaminergic neurotransmission in a dose-related manner. They decrease the intensity of all psychotic symptoms, although not necessarily to the same extent and with the same time course. Negative symptoms may persist to a much more striking extent than delusions, hallucinations and thought disorders, and there is a dose-related incidence of extrapyramidal side effects (EPS). Newer antipsychotics, such as clozapine, olanzapine, risperidone, quetiapine and ziprasidone, have a different receptor-binding profile, interacting with both D and 5-HT receptors; they less frequently cause EPS and are better tolerated in the elderly. Their use is advantageous because they are effective both on positive and negative symptoms of schizophrenia and may also be used in the treatment of behavioural disturbances in elderly and/or demented individuals. The use of clozapine is limited by the onset of agranulocytosis, whereas olanzapine, risperidone, quetiapine and, more recently, ziprasidone are widely used, with good results in the above-mentioned diseases.

Elementary properties of axonal calcium currents in type B photoreceptors in Hermissenda crassicornis

Axons of the type B photoreceptors form synapses with hair cells and interneurons that are involved in classical conditioning in Hermissenda. We examined the differences in the Ca2+ channels expressed in the soma and axons of the B photoreceptors by direct functional recordings of single-channel currents. Although the soma of the B cells express two Ca2+ current subtypes, a transient BayK 8644-insensitive (approximately 17 pS) current and a sustained BayK 8644-sensitive (approximately 10 pS) current, the axons expressed only the latter. The axonal Ca2+ current activated at potentials positive to -20 mV. Moreover, the Ca2+ channels are distributed heterogeneously along the length of the axon, with the higher channel density (approximately 10-15 channel microm(-2)) occurring at the distal one-third of the isolated axons, with respect to the soma. The regions of Ca2+ channel clusters may represent the presynaptic site of the photoreceptor-interneuron synapses. Furthermore, the high-density clusters of Ca2+ channels may augment postsynaptic responses. The results of the present study represent the first direct recordings of Ca2+ currents at presumed synaptic sites. Expression of different Ca2+ channel subtypes at distinct compartments of the type B photoreceptors may generate diverse Ca2+ domains that may be required for neuronal plasticity in Hermissenda.