Facilitation of glutamate receptors enhances memory

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

CX516 (BDP-12) and CX546, two first-generation benzamide-type AMPA receptor modulators, were compared with regard to their influence on AMPA receptor-mediated currents, autaptic responses in cultured hippocampal neurons, hippocampal excitatory postsynaptic currents, synaptic field potentials, and agonist binding. The two drugs exhibited comparable potencies in most tests but differed in their efficacy and in their relative impact on various response parameters. CX546 greatly prolonged the duration of synaptic responses, and it slowed 10-fold the deactivation of excised-patch currents following 1-ms pulses of glutamate. The effects of CX516 on those measures were, by comparison, small; however, the drug was equally or more efficacious than CX546 in increasing the amplitude of synaptic responses. This double dissociation suggests that amplitude and duration of synaptic responses are governed by different aspects of receptor kinetics, which are differentially modified by the two drugs. These effects can be reproduced in receptor simulations if one assumes that CX516 preferentially accelerates channel opening while CX546 slows channel closing. In binding tests, CX546 caused an approximately 2-fold increase in the affinity for radiolabeled agonists, whereas CX516 was ineffective. More importantly, even millimolar concentrations of CX516 did not influence the dose-response relation for CX546, suggesting the possibility that they bind to different sites. Taken together, the evidence suggests that benzamide modulators from the Ampakine family form two subgroups with different modes and sites of action. Of these, CX516-type drugs may have the greater therapeutic utility because of their limited efficacy in prolonging synaptic responses and in attenuating receptor desensitization.

Memory enhancement: the search for mechanism-based drugs

Rapid progress has been made in understanding the synaptic changes required for memory encoding. Several companies are now attempting to use information about the induction and consolidation phases of this process to build memory-enhancing drugs. These efforts have produced novel compounds that improve retention scores across a broad range of tests and species. Initial clinical results are encouraging. Issues now arise about appropriate applications of candidate drugs and optimal cellular targets for future development.

Further postmortem autoradiographic studies of AMPA receptor binding in schizophrenia

Previous research indicated an increased binding of AMPA receptor ligand [(3)H]CNQX at 50 nM in the caudate nucleus of schizophrenics and suicides relative to normal and neuroleptic-treated controls. The current work aimed to replicate this finding in a larger, independent sample of schizophrenics and controls. In addition to neostriatal structures, the hippocampal region and amygdala were also studied. Postmortem frozen sections from 15 schizophrenics (four suicides), 15 normal controls, 15 bipolars (eight suicides), and 15 unipolar nonpsychotic depressed (seven suicides) subjects were studied with quantitative autoradiographic procedures at 5, 20, and 50 nM [(3)H]CNQX in the striatum and at 20 nM in medial temporal structures. [(3)H]KA (kainic acid) binding was also examined. Instead of an expected increase, schizophrenics in this sample have a lower degree of [(3)H]CNQX binding in caudate and nucleus accumbens at 20 nM and in the nucleus accumbens at 50 nM, with suicided schizophrenics having higher binding than nonsuicided schizophrenics at the 20 nM concentration of [(3)H]CNQX in the caudate. [(3)H]CNQX binding was uniform across diagnostic categories at 5 nM in the striatum and at 20 nM in amygdala and hippocampal structures. KA receptor binding also did not differ among groups in any structures examined. These assays in a larger sample at three different concentrations do not support the previously reported increase in binding to AMPA receptors in schizophrenia but rather indicate an abnormal decrease in binding to this receptor in this sample. Possible explanations for the disparity in results between the two studies are considered. The data continue to indicate pathology of AMPA glutamate receptors in striatal structures in schizophrenia; however, it may be variable and its precise nature remains to be clarified.

Effects of 5'-alkyl-benzothiadiazides on (R,S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor biophysics and synaptic responses

Alkyl-substituted benzothiadiazides (BTDs) were tested for their effects on (R,S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors. In excised patches, the 5'-ethyl derivative "D1" blocked the desensitization of AMPA receptor currents during prolonged application of glutamate (EC(50), 36 microM), and it slowed deactivation of responses elicited by 1-ms glutamate pulses greater than 10-fold. [(3)H]Fluorowillardiine binding to rat synaptic membranes was increased by D1 by a factor of 3.6 (EC(50), 17 microM) with a Hill coefficient near 2. In hippocampal slices, the compound reversibly increased excitatory postsynaptic currents and field excitatory postsynaptic potentials (EPSPs) with thresholds around 10 microM. The size of the alkyl substituent influenced both the potency and nature of the drug effect on synaptic currents: 5'-methyl compounds had a 2-fold greater effect on response amplitude than on response duration, whereas 5'-ethyl compounds like D1 caused greater increases in duration than amplitude. In tests with recombinantly expressed AMPA receptor subunits, D1 preferred the glutamate receptor (GluR) subunit GluR4 flip (0.64 microM) over GluR4 flop (5.3 microM); similar affinities but with smaller flip-flop differences were obtained for GluR1 through 3. These results show that D1 and congeners are significantly more potent than the parent compound IDRA-21 and that they differ in two fundamental aspects from cyclothiazide, the most widely studied BTD: 1) D1 markedly increases the agonist affinity of AMPA receptors and 2) it has immediate and large effects on field EPSPs. The large gain in potency conferred by alkyl substitution suggests that the 5' substituent is in intimate contact with the receptor, with the size of the substituent determining the way in which receptor kinetics is changed.

Combinatorial chemistry and high-throughput screening in drug discovery: different strategies and formats

Different strategies for the discovery of novel leads interacting with therapeutically relevant targets are thoroughly presented and discussed, using also three recent examples. Emphasis is given to approaches which do not require extensive resources and budgets, but rather prove how cleverness and creativity can provide active compounds in drug discovery.

Randomized, double-blind, placebo-controlled international clinical trial of the Ampakine CX516 in elderly participants with mild cognitive impairment: a progress report

This progress report briefly describes the rationale and study design for the first cross-national clinical study of a positive AMPA-type glutamate receptor modulator in subjects with mild cognitive impairment (MCI). The study medication for the double-blind, placebo-controlled trial, the AMPAKINE CX516, represents a novel pharmacological approach to the treatment of memory disorders. Previous preclinical and pilot clinical studies have shown that CX516 has the ability to enhance memory and cognition. Design of the trial, including outcome measures and inclusion criteria, was aided by an international panel of experts in the newly emerging field of MCI.

A desensitization-selective potentiator of AMPA-type glutamate receptors

1: We examined the effects of PEPA, an allosteric potentiator of AMPA receptors, on AMPA receptor kinetics. 2: PEPA did not affect the deactivation of glutamate responses but potently attenuated the extent of receptor desensitization without slowing the onset of desensitization in most of the recombinant AMPA receptors (GluR1-flip, GluR1-flop, GluR3-flip, GluR3-flip+GluR2-flip, and GluR3-flop+GluR2-flop) expressed in Xenopus oocytes. For the GluR3-flop subunit, PEPA attenuated the extent of desensitization and only weakly prolonged deactivation (1.3 fold). 3: PEPA did not significantly affect recovery from desensitization in oocytes expressing GluR3-flip, GluR1-flop, and GluR1-flop, but weakly accelerated (2.6 fold) recovery from desensitization in oocytes expressing GluR3-flop. 4: PEPA's effect on desensitization of GluR3-flop-containing receptors is unique in that onset is very slow. 5: Simulation studies using simplified kinetic models for AMPA receptors are utilized to explore the differential effects of PEPA on GluR3-flip and -flop. It is possible to simulate the action on GluR3-flip by modulating two rate constants in a 12-state kinetic model. For simulation of the action on GluR3-flop, the 12-state kinetic model is not enough, and it is necessary to invoke a 13th state, a PEPA-bound receptor to which glutamate cannot bind. 6: These results suggest that attenuation of extent of desensitization represents the principal mechanism underlying the potentiation of AMPA receptors by PEPA, and that PEPA exhibits different mechanisms with respect to GluR3-flip and GluR3-flop.

Antidepressant activity of memory-enhancing drugs in the reduction of submissive behavior model

The present study tests the activity of nootropic drugs in a behavioral test linked to depression. This test measures the reduction of submissive behavior in a competition test as the relative success of two food-restricted rats to gain access to a feeder. Nootropic drugs tested include piracetam (2-oxo-1-pyrrolidineacetamide), aniracetam (1-(4-methoxybenzoyl)-2-pyrrolidinone), the Ampakine, Ampalex, 1-(quinoxalin-6-ylcarbonyl)piperidine, and analogs were compared to the antidepressants, fluoxetine ((+/-)-N-methyl-gamma-(4-[trifluoromethyl]phenoxy)-benzenepropanamine) and desimpramine (5H-dibenz[b,f]azepine-5-propanamine, 10,11-dihydro-N-methyl-, monohydrochloride), while the anxiolytic diazepam (7-chloro-1-methyl-5-phenyl-3H-1,4-benzodiazepin-2(1H)-one) served as a control. Drugs were given intraperitoneally for 3 weeks. The antidepressant and nootropic drugs reduced submissive behavior over time. The effect was dose dependent as measured for fluoxetine and Ampakines. The reduction of submissive behavior by Ampakines gradually faded after cessation of treatment and had a more rapid onset of activity (during the 1st week of treatment) than fluoxetine (after 2 weeks). The results suggest that Ampakines may have antidepressant activity. The potential of depression treatment with memory-enhancing drugs is hypothesized and the link between cognition and depression is discussed.

Survival signaling and selective neuroprotection through glutamatergic transmission

In the brain, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors mediate glutamatergic neurotransmission and, when intensely activated, can induce excitotoxic cell death. In addition to their ionotropic properties, however, AMPA receptors have been functionally coupled to a variety of signal transduction events involving Src-family kinases, G-proteins, and the mitogen-activated protein kinase (MAPK). In the present study, we tested whether AMPA receptors are linked to appropriate signaling events in order to prevent neuronal injury and/or enhance recovery. AMPA stimulation in hippocampal slice cultures caused the selective activation of MAPK through the upstream activator MAPK kinase (MEK). Inhibition of either component of the AMPA receptor--MAPK pathway potentiated cellular damage due to serum deprivation, suggesting that this pathway facilitates compensatory signals in response to injury. Correspondingly, positive modulation of AMPA receptors with the Ampakine 1-(quinoxalin-6-ylcarbonyl)piperidine (CX516) enhanced MAPK activation and reduced the extent of synaptic and neuronal degeneration resulting from excitotoxic episodes. CX516 was neuroprotective when infused into slices either before or after the insult. The Ampakine derivative also elicited neuroprotection in an in vivo model of excitotoxicity as evidenced by reduction in lesion size and preservation of two different types of neurons. Interestingly, the AMPA receptor--MAPK pathway selectively protects against excitotoxicity since enhancing the pathway did not protect against the nonexcitotoxic, slow pathology initiated by lysosomal dysfunction. The results indicate that glutamatergic communication is important for cellular maintenance and that AMPA receptors activate survival signals to counterpoise their own excitotoxic potential.

LY404187: a novel positive allosteric modulator of AMPA receptors

LY404187 is a selective, potent and centrally active positive allosteric modulator of AMPA receptors. LY404187 preferentially acts at recombinant human homomeric GluR2 and GluR4 versus GluR1 and GluR3 AMPA receptors. In addition, LY404187 potentiates the flip splice variant of these AMPA receptors to a greater degree than the flop splice variant. In both recombinant and native AMPA receptors, potentiation by LY404187 displays a unique time-dependent growth that appears to involve a suppression of the desensitization process of these ion channels. LY404187 has been shown to enhance glutamatergic synaptic transmission both in vitro and in vivo. This augmentation of synaptic activity is due to the direct potentiation of AMPA receptor function, as well as an indirect recruitment of voltage-dependent NMDA receptor activity. Enhanced calcium influx through NMDA receptors is known to be a critical step in initiating long-term modifications in synaptic function (e.g., long-term potentiation, LTP). These modifications in synaptic function may be substrates for certain forms of memory encoding. Consistent with a recruitment of NMDA receptor activity, LY404187 has been shown to enhance performance in animal models of cognitive function requiring different mnemonic processes. These data suggest that AMPA receptor potentiators may be therapeutically beneficial for treating cognitive deficits in a variety of disorders, particularly those that are associated with reduced glutamatergic signaling such as schizophrenia. In addition, LY404187 has been demonstrated to be efficacious in animal models of behavioral despair that possess considerable predictive validity for antidepressant activity. Although the therapeutic efficacy of AMPA receptor potentiators in these and other diseases will ultimately be determined in the clinic, evidence suggests that the benefit of these compounds will be mediated by multiple mechanisms of action. These mechanisms include direct enhancement of AMPA receptor function, secondary mobilization of intracellular signaling cascades, and prolonged modulation of gene expression.

Mechanism and impact of allosteric AMPA receptor modulation by the ampakine CX546

Glutamate release at central synapses is transduced into a characteristic fast postsynaptic response by AMPA receptor gating and agonist affinity. The effect of two classes of modulators of AMPA receptor desensitization, the benzothiadiazides (cyclothiazide and IDRA 21) and the benzoylpiperidines (CX516 and CX546), were studied on gating kinetics of recombinant, native AMPA receptors and on synaptic currents. CX546 reduced the degree of desensitization more potently than CX516 or IDRA 21, but not as efficiently as cyclothiazide. In presence of CX516/CX546, desensitization of GluR2(flip) receptors was inhibited more than of GluR1(flip), whereas they had no effect upon response shape or conductance. CX546 increased agonist affinity threefold on nondesensitizing AMPA receptors by slowing agonist unbinding. Analysis of modulatory action suggests that, in contrast to cyclothiazide or IDRA 21, the Ampakine CX546 binds specifically to the agonist bound nondesensitized receptor, most likely acting by destabilizing the desensitized receptor conformation. All modulators tested showed higher efficiency on native receptors as compared to homomeric receptors. At the glutamatergic synapse, evoked synaptic amplitudes were weakly potentiated, while EPSC decay was slowed by nearly a factor of three in the presence of CX546 or cyclothiazide. In the presence of CX546, the current induced by short pulses of glutamate from recombinant GluR2 receptors decayed with a time course that was approximately twentyfold faster than EPSCs. The unique properties of CX546 may be beneficial for therapeutical use.

A placebo-controlled pilot study of the ampakine CX516 added to clozapine in schizophrenia

CX516, a positive modulator of the glutamatergic alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor, improves performance in tasks requiring learning and memory in animals. CX516 was added to clozapine in 4-week, placebo-controlled, dose-finding (N = 6) and fixed-dose (N = 13) trials. CX516 was tolerated well and was associated with moderate to large, between-group effect sizes compared with placebo, representing improvement in measures of attention and memory. These preliminary results suggest that CX516 and other "ampakines" hold promise for the treatment of schizophrenia.

Changes in the glutamate release and uptake of cerebellar cells in perinatally nicotine-exposed rat pups

Cerebellar granule and glial cells were cultured from 7 day-old rat pups after pre- and post-natal nicotine treatment. Ten days later, the basal release of glutamate in the granule cells prepared from the pre- and post-natally nicotine-exposed pups was higher and lower than the controls, respectively. The N-methyl-D-aspartate-induced release of glutamate was higher in the granule cells of post-natal nicotine exposed rats. However, the nicotine-induced glutamate release was either unchanged or was lower in the granule cells of all nicotine-treated pups. The basal glutamate uptake was higher in the glial cells from those exposed pre-natally and lower in the continuously nicotine-exposed pups. The sensitivities of L-trans-pyrrolidine-2,4-dicarboxylic acid on glutamate uptake were higher in all nicotine treated groups. There was a higher number of specific [3H]dizocilpine binding sites in the pre- or continuously nicotine-exposed group. These results suggest that the cerebellar cell properties are altered after perinatal nicotine exposure and that the development of an excitatory amino acid system might be affected differently depending on the nicotine exposure time.

The AMPA receptor allosteric potentiator PEPA ameliorates post-ischemic memory impairment

PEPA (4-[2-(Phenylsulphonylamino)ethylthio]-2,6-difluorophenoxyacetamide) is a recently developed allosteric potentiator of AMPA receptors that preferentially affects flop splice variants. We tested the effects of PEPA on ischemia-induced memory deficit in rats. Permanent unilateral occlusion of the middle cerebral artery induced severe impairment of performance of rats in the Morris water maze test. Repeated intravenous administration of PEPA (1, 3, 10 mg/kg/day for 10 days) improved test performance. In contrast, a corresponding dose of aniracetam, a representative potentiator of AMPA receptor, did not significantly improve test performance. Thus, PEPA is more effective than aniracetam in reversing impaired memory function as assessed by the Morris water maze test; and PEPA may be an effective compound for the treatment of impaired memory.

Potentiation of NMDA receptor function through somatostatin release: a possible mechanism for the cognition-enhancing activity of GABA(B) receptor antagonists

CGP 36742 is a weak GABA(B) receptor antagonist. However, it improves cognitive performances at low doses; it blocks GABA(B) receptors potently and selectively on somatostatinergic terminals; it prevents kynurenate from antagonising NMDA-induced release of noradrenaline from rat brain slices potently. We here investigated whether and how somatostatin plays a role in the CGP 36742 activity. CGP 36742 increased the somatostatin-like immunoreactivity (SRIF-LI) release from hippocampal slices exposed to NMDA. In the kynurenate test with rat hippocampal slices SRIF-14 mimicked the effect of CGP 36742. CGP 36742 lost its activity in rats whose somatostatin content had been depleted with cysteamine. Exogenous SRIF-14 reverted kynurenate antagonism in somatostatin-depleted slices. L362855, an sst(5) receptor agonist, but not the selective sst(1)-sst(4) agonists, L797591, L779976, L796778 and L803087, displayed activity in the kynurenate test. The effects of CGP 36742, SRIF-14 and L362855 were antagonised by the sst(5)-preferring antagonist BIM-23056. The protein kinase C inhibitor GF 109203X prevented the reversal of the kynurenate antagonism by CGP 36742 or SRIF-14. In conclusion, by selectively blocking GABA(B) receptors on somatostatinergic terminals, CGP 36742 may disinhibit somatostatin release; the consequent activation of sst(5) receptors would potentiate the function of NMDA receptors coexisting with sst(5) receptors on noradrenergic neurons.

Applications of the Morris water maze in the study of learning and memory

The Morris water maze (MWM) was described 20 years ago as a device to investigate spatial learning and memory in laboratory rats. In the meanwhile, it has become one of the most frequently used laboratory tools in behavioral neuroscience. Many methodological variations of the MWM task have been and are being used by research groups in many different applications. However, researchers have become increasingly aware that MWM performance is influenced by factors such as apparatus or training procedure as well as by the characteristics of the experimental animals (sex, species/strain, age, nutritional state, exposure to stress or infection). Lesions in distinct brain regions like hippocampus, striatum, basal forebrain, cerebellum and cerebral cortex were shown to impair MWM performance, but disconnecting rather than destroying brain regions relevant for spatial learning may impair MWM performance as well. Spatial learning in general and MWM performance in particular appear to depend upon the coordinated action of different brain regions and neurotransmitter systems constituting a functionally integrated neural network. Finally, the MWM task has often been used in the validation of rodent models for neurocognitive disorders and the evaluation of possible neurocognitive treatments. Through its many applications, MWM testing gained a position at the very core of contemporary neuroscience research.

LY392098, a novel AMPA receptor potentiator: electrophysiological studies in prefrontal cortical neurons

The present experiments investigated the ability of LY392098, a novel positive allosteric modulator of AMPA receptors, to potentiate AMPA receptor-mediated currents of neurons in the prefrontal cortex (PFC). Co-application of LY392098 (0.03-10 microM) with AMPA (5 microM) enhanced current through AMPA receptor/channels in acutely isolated PFC neurons in a concentration-dependent manner. Estimates of the potency (EC(50)) and efficacy for LY392098 yielded an EC(50) value of 1.7+/-0.5 microM and a maximal potentiation of a 31.0+/-4.1-fold increase relative to current evoked by AMPA alone. The potentiation was activity-dependent, becoming evident only in the presence of agonist, and time-dependent, continuously developing over prolonged application times. An extracellular site of action was inferred by the absence of potentiation when the compound was applied intracellularly. LY392098 also increased the potency of agonist for the receptor by approximately sevenfold. Selectivity assays showed that the effects of LY392098 were exclusive for AMPA receptors, having no activity at N-methyl-D-aspartate (NMDA) receptors in PFC neurons. Extracellular recordings from single PFC neurons in vivo showed that administration of LY392098 (0.001-10 microg/kg, i.v.) enhanced the probability of evoked action potential discharge in response to stimulation of glutamatergic afferents from the ventral subiculum of the hippocampal formation. Spontaneous activity of PFC neurons was also increased. Collectively, these results demonstrate that LY392098 is a highly potent, selective and centrally active positive modulator of native AMPA receptors.

Member of the Ampakine class of memory enhancers prolongs the single channel open time of reconstituted AMPA receptors

Ampakines are small benzamide compounds that allosterically produce the positive modulation of AMPA receptors and improve performance on a variety of behavioral tasks. To test if the native synaptic membrane is necessary for the effects of such positive modulators, the mechanism of action of the Ampakine 1-(1,3-benzodioxol-5-ylcarbonyl)-1,2,3,6-tetrahydropyridine (CX509) was investigated in isolated rat brain AMPA receptors reconstituted in lipid bilayers. The drug increased the open time of AMPA-induced single channel current fluctuations with an EC(50) of 4 microM. The action of CX509 was highly selective since it had no effect on the amplitude or close time of channel events. The open time effect had a maximum enhancement of 70-fold and the modulated currents were blocked by CNQX. It is concluded that the synaptic membrane environment is not necessary for Ampakine effects. In fact, CX509 was about 100 times more potent on the reconstituted AMPA receptors than on receptors in their native membrane. These findings indicate that centrally active Ampakines modulate specific kinetic properties of AMPA currents. They also raise the possibility that AMPA receptors are regulated by factors present in situ, thus explaining the more efficient modulatory effects of CX509 when acting on receptors removed from their synaptic location.

Endocrine and environmental control of reproduction in Polychaeta

In Polychaeta, as in many invertebrates, reproduction is controlled by both environmental and endocrine factors. Although the effects of environmental factors on reproductive behaviour are briefly discussed, this review focuses on the endocrinology of reproduction. As Nereidae are the most intensively studied polychaetes, their epigamic monotelic strategy is discussed first in this review. Although a large number of physiological observations have been made, biochemical data have been greatly lacking until recent years, except, however, for the recent isolation of several pheromones. These substances, such as uric acid and L-cysteine gluthathione disulfide, occur widely and must be present at high concentrations in order to exert their physiological effects. Results obtained from iteroparous species are also considered. The stolonization strategy of Syllidae, the control of vitellogenesis in Nephtyidae, Phyllodocidae, Polynoidae, and Cirratulidae, and the regulation of gamete maturation in Arenicolidae and Pectinariidae are discussed. As with Nereidae, our knowledge of endocrine control is mainly based on experimental data, since only sperm-maturation factor in the genus Arenicola has been identified. Therefore, despite numerous interesting experimental studies in which functional roles for polychaete reproductive hormones have been described, their nature, their primary targets, and their mechanism of action are unfortunately still largely unknown.

Structural determinants for AMPA agonist activity of aryl or heteroaryl substituted AMPA analogues. Synthesis and pharmacology

We have previously reported the synthesis and pharmacological characterization of analogues of 2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA, 1a), in which the methyl group was replaced by a phenyl group (APPA, 1b) or heteroaryl groups. While 2b and its 3-pyridyl analogue 2-amino-3-[3-hydroxy-5-(3-pyridyl)-4-isoxazolyl]propionic acid (3-Py-AMPA, 3) show very low affinity for AMPA receptors, introduction of heteroaryl substituents containing heteroatom in the 2-position provides potent AMPA receptor agonists. We here report the synthesis and pharmacology of 2-amino-3-(3-hydroxy-5-pyrazinyl-4-isoxazolyl)propionic acid (7) (IC50 = 1.2 microM), which is weaker as an AMPA agonist than AMPA (IC50 = 0.040 microM; EC50 = 3.5 microM) but comparable in potency with 2-Py-AMPA (4) (IC50 = 0.57 microM; EC50 = 7.4 microM), as determined in radioligand binding and electrophysiological experiments, respectively. The AMPA analogues 8a-c, containing 2-, 3-, or 4-methoxyphenyl substituents, respectively, and the corresponding hydroxyphenyl analogues, 9a-c, were also synthesized and evaluated pharmacologically. With the exception of 2-amino-3-[3-hydroxy-5-(2-hydroxyphenyl)-4-isoxazolyl]propionic acid (9a), which is a very weak AMPA agonist (IC50 = 45 microM; EC50 = 324 microM), none of these compounds showed detectable effect at AMPA receptors.

Characterization of the binding site for a novel class of noncompetitive alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor antagonists

The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor is an ionotropic glutamate receptor that mediates fast excitatory synaptic transmission throughout the central nervous system. In addition to the glutamate binding site, allosteric modulatory sites on the receptor are inferred from the ability of synthetic compounds to affect channel function without interaction with the glutamate binding site. We have identified a novel class of potent, noncompetitive AMPA receptor antagonists typified by CP-465, 022 and CP-526,427. The latter compound was radiolabeled and used to elucidate the pharmacology of one allosteric modulatory site. [(3)H]CP-526,427 labels a single binding site in rat forebrain membranes with a K(d) value of 3.3 nM and a B(max) of 7.0 pmol/mg of protein. The [(3)H]CP-526,427 binding site does not seem to interact directly with the glutamate binding site but overlaps with that for another class of AMPA receptor antagonists, the 2,3-benzodiazepines. This binding site is distinct from that for the antagonist Evans blue and for several classes of compounds that modulate AMPA receptor desensitization. These results indicate the existence of at least two physically distinct allosteric sites on the AMPA receptor through which channel activity or desensitization is modulated.

Regional changes in the hippocampal density of AMPA and NMDA receptors across the lifespan of the rat

The current study dissected the fascia dentata (FD) and hilar region from the CA and subicular cell fields of the rat and conducted in vitro determinations of the number of binding sites for N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole (AMPA) glutamate receptors across the lifespan. We determined the density of binding of [3H]-glutamate or [3H]-AMPA to NMDA or AMPA receptor sites, respectively. The changes reported might be due to either a change in receptor number or an alteration in the binding characteristics of the receptor site with aging. We found an age-related decline in the number of NMDA receptors in the CA1, CA3 and subicular cell regions of the hippocampus, but not in the FD/hilar region, and an age-related decline in the number of AMPA receptors in the FD/hilar region, but not in the CA fields. The decline in the number of NMDA or AMPA receptors that occurs with aging was not a continuous or homogeneous process. These changes in receptor number might underlie selected age-associated changes in sensitivity to drugs that influence hippocampal function as well as to changes in NMDA-dependent long-term potentiation. A thorough understanding of the mechanisms underlying changes in glutamate receptor function in discrete brain regions, using combined neurochemical and electrophysiological methods, may ultimately provide insight into the fundamental substrates of age-associated memory disorders related to hippocampal dysfunction.

Effects of the potent ampakine CX614 on hippocampal and recombinant AMPA receptors: interactions with cyclothiazide and GYKI 52466

R,S-alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor up-modulators of the benzamide type ("ampakines") have previously been shown to enhance excitatory synaptic transmission in vivo and in vitro and AMPA receptor currents in excised patches. The present study analyzed the effects of an ampakine (CX614; 2H,3H, 6aH-pyrrolidino[2",1"-3',2']1,3-oxazino[6',5'-5,4]benz o[e]1, 4-dioxan-10-one) that belongs to a benzoxazine subgroup characterized by greater structural rigidity and higher potency. CX614 enhanced the size (amplitude and duration) of field excitatory postsynaptic potentials in hippocampal slices and autaptically evoked excitatory postsynaptic currents in neuronal cultures with EC(50) values of 20 to 40 microM. The compound blocked desensitization (EC(50) = 44 microM) and slowed deactivation of responses to glutamate by a factor of 8.4 in excised patches. Currents through homomeric, recombinant AMPA receptors were enhanced with EC(50) values that did not differ greatly across GluR1-3 flop subunits (19-37 microM) but revealed slightly lower potency at corresponding flip variants. Competition experiments using modulation of [(3)H]fluorowillardiine binding suggested that CX614 and cyclothiazide share a common binding site but cyclothiazide seems to bind to an additional site not recognized by the ampakine. CX614 did not reverse the effect of GYKI 52466 on responses to brief glutamate pulses, which indicates that they act through separate sites, a conclusion that was confirmed in binding experiments. In sum, these results extend prior evidence that ampakines are effective in enhancing synaptic responses, most likely by slowing deactivation, and that their effects are exerted through sites that are only in part shared with other modulators.

(S)-2,3-dihydro-[3,4]cyclopentano-1,2,4-benzothiadiazine-1,1-dioxide: (S18986-1) a positive modulator of AMPA receptors enhances (S)-AMPA-mediated [3H]noradrenaline release from rat hippocampal and frontal cortex slices

The present study describes the effect of (S)-2,3-dihydro-[3, 4]cyclopentano-1,2,4-benzothiadiazine-1,1-dioxide (S18986-1), a positive allosteric modulator of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors with cognitive-enhancing effects, on (S)-AMPA-induced [3H]noradrenaline release in rat hippocampal and frontal cortex slices. (S)-AMPA significantly increased [3H]noradrenaline release in rat hippocampus and frontal cortex slices, whereas S18986-1 (3-1000 microM) alone, was inactive. However, S18986-1 between 30 and 1000 microM potently enhanced (+200%) (S)-AMPA-mediated [3H]noradrenaline release in both hippocampal and frontal cortex slices. The capacity of S18986-1 to potentiate [3H]noradrenaline release was specific for AMPA receptors as S18986-1 failed to potentiate either kainate and N-methyl-D-aspartate (NMDA)-mediated release of [3H]noradrenaline in rat hippocampal slices. Moreover, 1, 2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide (NBQX) and 1-(4-aminophenyl)-3-methylcarbamoyl-4-methyl-3, 4-dihydro-7,8-methylenedioxy-5H-2,3-benzodiazepine (GYKI-53655) but not (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5, 10-imine ((+)-MK-801), inhibited (S)-AMPA and S18986-induced stimulation of (S)-AMPA-mediated [3H]noradrenaline release. In addition, S18986-1-induced stimulation of (S)-AMPA-evoked [3H]noradrenaline release was markedly attenuated in the presence of tetrodotoxin (1 microM) and in Ca(2+)-free buffer. S18986-1 enhanced (S)-AMPA-mediated [3H]noradrenaline release to a greater extent than its corresponding (R)-enantiomer S19024-1 and racemic mixture S17951-1. However, positive allosteric modulators of AMPA receptors such as aniracetam failed to potentiate AMPA-mediated noradrenaline release in hippocampal slices, whereas cyclothiazide potently enhanced (S)-AMPA-mediated [3H]noradrenaline release. These results suggest that the capacity of S18986-1 to enhance AMPA receptor-mediated release of noradrenaline in rat hippocampus and frontal cortex, could contribute to the cognition enhancing mechanisms of S18986-1.

Effects of S 18986-1, a novel cognitive enhancer, on memory performances in an object recognition task in rats

(S)-2,3-dihydro-[3,4]cyclopentano-1,2,4-benzothiadiazine-1,1-dioxi de (S 18986-1) is a new compound that facilitates post-synaptic responses by modulating alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor-mediated synaptic responses and thus promotes long-term potentiation and potentiates (S)-AMPA-induced release of noradrenaline in rat brain slices. In the present study, the effects of S 18986-1 were evaluated on cognitive functions by using a one-trial object-recognition test in the Wistar rat, a test which measures a form of episodic memory in rodents. Recognition was measured by the ability of treated rats to discriminate between a familiar and a new object after a 24-h retention delay. Oral administrations with S 18986-1 (0.3 to 100 mg/kg) 1 h before each session of the test improved object recognition at concentrations as low as 0.3 mg/kg. Under the same conditions, the nootropic drug aniracetam was active at a dose of 10 mg/kg by i.p. route. S 18986-1 was still effective on the object-recognition test when it was administered 4 h before each of the three sessions. Furthermore, subchronic oral pretreatment (7 days) with S 18986-1 (0.3 to 30 mg/kg) also increased the recognition of the familiar object indicating that the animals failed to develop tolerance to repeated administrations with S 18986-1. Finally, the recognition of the familiar object was improved when S 18986-1 was administered before the recognition trial whereas the rats failed to recognise the familiar object when S 18986-1 was administered before the sample presentation trial only. Taken together, the results indicated that S 18986-1 facilitated a form of episodic memory in the rat, by improving the recognition of a familiar information (retention). Furthermore, S 18986-1 was long-acting and demonstrated a good oral bioavailability. These data confer on S 18986-1, a potential role in improving episodic memory impaired in neurodegenerative diseases and during aging.

Theta oscillations and the ERP old/new effect: independent phenomena?

OBJECTIVES

The hypothesis is examined whether a memory-related change in induced band power (oscillatory old/new effect) is functionally related to a memory-related increase in ERP positivity (ERP old/new effect).

METHODS

In order to avoid a confounding on the measurement level, induced band power (IBP) was used as a measure that is devoid of the influence of evoked components. The EEG was recorded during a recognition memory task.

RESULTS

The results show that compared to correctly rejected words, targets (remembered words) elicit a significantly larger P300. An oscillatory old/new effect was found for the delta and theta but not for the alpha band. It is manifested by an increase in delta and theta IBP which is significantly larger for targets than for correctly rejected words. It can be observed during the same time interval and shows the same topographic distribution as the ERP old/new effect. Most importantly, however, the ERP old/new effect (as well as the P300 itself) is generated by very slow frequencies which lie below the delta band.

CONCLUSIONS

These findings demonstrate that the two types of old/new effects are functionally related. Possible physiological mechanisms underlying this relationship are discussed in terms of a threshold change in the cortex (generating the P300) that occurs during an increase in hippocampal theta activity (generating an increase in induced theta power).

Synaptic efficacy is commonly regulated within a nervous system and predicts individual differences in learning

The hypothesis that an individual's capacity for learning might be predicted or influenced by basal levels of synaptic efficacy has eluded empirical tests, owing in part to the inability to compare between animals single identified synaptic responses in the mammalian brain. To overcome this limitation, we have focused our analysis on the invertebrate Hermissenda, whose nervous system is composed of identifiable cells and synaptic interactions. Hermissenda were exposed to paired presentations of light and rotation such that the light came to elicit a learned defensive motor response. An animal's rate of learning was strongly correlated with the amplitude of the synaptic potential evoked in that animal's visual (light sensitive) receptors in response to stimulation of presynaptic vestibular (rotation sensitive) hair cells. In naive animals, strong correlations between the amplitude of both inhibitory and excitatory synaptic potentials were observed between synapses distributed throughout an animal's nervous system, and this conservation of synaptic efficacy was largely attributable to a common influence on transmitter release. These observations suggest that basal synaptic efficacy may be uniformly regulated throughout a nervous system, and provide direct evidence that the basal efficacy of synaptic transmission predicts, and possibly contributes to, individual differences between animals in their capacity to learn.

Therapeutic potential of positive AMPA receptor modulators in the treatment of neurological disease

Excitatory neurotransmission in the CNS depends heavily upon alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)-type glutamate receptors. Derangements in AMPA receptor mediated synaptic transmission may be a contributing factor in neurological and neurodegenerative diseases and could be a target for therapeutic intervention. Recently, drugs that positively modulate AMPA receptors have been identified, having differential effects upon certain AMPA receptor subunits and different effects upon physiological properties of AMPA receptors. These drugs facilitate AMPA receptor mediated processes and may have beneficial therapeutic effects. For example, certain AMPA modulators facilitate long-term potentiation, which is considered a cellular mechanism that may be important for memory storage and they also facilitate memory encoding in behavioural experiments. Thus, AMPA modulators might ameliorate memory deficits that occur in dementia, such as Alzheimer's disease (AD). However, AMPA receptor mediated excitotoxicity may occur with excessive AMPA receptor activation which occurs in seizures or ischaemia and positive AMPA modulators could promote neuronal injury in those conditions. Ultimately, the clinical utility of positive AMPA modulators will be dependent upon understanding the role of AMPA receptors in certain neurological disorders, identifying receptor subtypes involved in specific neurological disorders and developing drugs with selective actions upon specific AMPA receptor properties that also possess receptor subtype specificity. Currently available drugs have provided significant insight into the physiology and structural determinants of important AMPA receptor properties and some insight into potential clinical uses as well as potential dangers of such drugs.

Positive modulation of AMPA receptors increases neurotrophin expression by hippocampal and cortical neurons

This study investigated whether positive modulators of AMPA-type glutamate receptors influence neurotrophin expression by forebrain neurons. Treatments with the ampakine CX614 markedly and reversibly increased brain-derived neurotrophic factor (BDNF) mRNA and protein levels in cultured rat entorhinal/hippocampal slices. Acute effects of CX614 were dose dependent over the range in which the drug increased synchronous neuronal discharges; threshold concentrations for acute responses had large effects on mRNA content when applied for 3 d. Comparable results were obtained with a second, structurally distinct ampakine CX546. Ampakine-induced upregulation was broadly suppressed by AMPA, but not NMDA, receptor antagonists and by reducing transmitter release. Antagonism of L-type voltage-sensitive calcium channels blocked induction in entorhinal cortex but not hippocampus. Prolonged infusions of suprathreshold ampakine concentrations produced peak BDNF mRNA levels at 12 hr and a return to baseline levels by 48 hr. In contrast, BDNF protein remained elevated throughout a 48 hr incubation with the drug. Nerve growth factor mRNA levels also were increased by ampakines but with a much more rapid return to control levels during chronic administration. Finally, intraperitoneal injections of CX546 increased hippocampal BDNF mRNA levels in aged rats and middle-aged mice. The present results provide evidence of regional differences in mechanisms via which activity regulates neurotrophin expression. Moreover, these data establish that changes in synaptic potency produce sufficient network level physiological effects for inducing neurotrophin genes, indicate that the response becomes refractory during prolonged ampakine exposure, and raise the possibility of using positive AMPA modulators to regulate neurotrophin levels in aged brain.

Positive modulation of AMPA receptors increases neurotrophin expression by hippocampal and cortical neurons

This study investigated whether positive modulators of AMPA-type glutamate receptors influence neurotrophin expression by forebrain neurons. Treatments with the ampakine CX614 markedly and reversibly increased brain-derived neurotrophic factor (BDNF) mRNA and protein levels in cultured rat entorhinal/hippocampal slices. Acute effects of CX614 were dose dependent over the range in which the drug increased synchronous neuronal discharges; threshold concentrations for acute responses had large effects on mRNA content when applied for 3 d. Comparable results were obtained with a second, structurally distinct ampakine CX546. Ampakine-induced upregulation was broadly suppressed by AMPA, but not NMDA, receptor antagonists and by reducing transmitter release. Antagonism of L-type voltage-sensitive calcium channels blocked induction in entorhinal cortex but not hippocampus. Prolonged infusions of suprathreshold ampakine concentrations produced peak BDNF mRNA levels at 12 hr and a return to baseline levels by 48 hr. In contrast, BDNF protein remained elevated throughout a 48 hr incubation with the drug. Nerve growth factor mRNA levels also were increased by ampakines but with a much more rapid return to control levels during chronic administration. Finally, intraperitoneal injections of CX546 increased hippocampal BDNF mRNA levels in aged rats and middle-aged mice. The present results provide evidence of regional differences in mechanisms via which activity regulates neurotrophin expression. Moreover, these data establish that changes in synaptic potency produce sufficient network level physiological effects for inducing neurotrophin genes, indicate that the response becomes refractory during prolonged ampakine exposure, and raise the possibility of using positive AMPA modulators to regulate neurotrophin levels in aged brain.

Potentiating effects of 4-[2-(phenylsulfonylamino)ethylthio]-2,6-difluoro-phenoxyaceta mide (PEPA) on excitatory synaptic transmission in dentate granule cells

A novel sulfonylamino compound, 4-[2-(phenylsulfonylamino)-ethylthio]-2,6-difluoro-phenoxyaceta mide (PEPA) has been shown to selectively potentiate glutamate-induced currents in Xenopus oocytes expressing recombinant AMPA receptor subunits, GluR1-GluR4, by attenuation of desensitization. Here, we examined the effects of PEPA on responses to excitatory amino acids as well as on excitatory synaptic transmission in dentate granule cells of rat hippocampal slices using the whole-cell patch clamp technique. PEPA at 100 microM produced a 3-4-fold increases in the peak amplitude of current responses to AMPA and glutamate applied iontophoretically in the dentate granule cells, whereas it showed no effect on NMDA-induced currents. Excitatory postsynaptic currents (EPSCs) evoked in these neurons by stimulation of the perforant path had fast and slow components mediated by AMPA and NMDA receptors, respectively. PEPA at concentrations between 10 and 100 microM potentiated only the AMPA component of the EPSC (AMPA EPSC) in a dose-dependent manner without affecting the NMDA component. Although the potentiating effect of PEPA on the amplitude of the AMPA EPSC was weaker than that on the AMPA-induced current, it clearly prolonged the duration of the EPSC. PEPA at 100 microM increased the peak amplitude of the AMPA EPSC by 17%, and increased the area enclosed by the AMPA EPSC by 72%.

Two phases of behavioral plasticity in rats following unilateral excitotoxic lesion of the hippocampus

We investigated the effects of dizocilpine, a non-competitive N-methyl-D-aspartate receptor antagonist, on spatial reference and working memory in a radial arm maze task in rats with a unilateral hippocampal lesion. At a dose of 0.2 mg/kg to intact rats, dizocilpine significantly impaired both reference and working memory, and produced ataxia and impairment of food intake; at 0.1 mg/kg, dizocilpine had no effect on performance. Unilateral hippocampal lesion induced by quinolinic acid produced a marked working memory deficit concomitant with a slight but significant impairment of reference memory when mnemonic ability was examined one week after the lesion. The spatial memory deficits in the rats with a unilateral hippocampal lesion were ameliorated by repeated daily trainings over a 21-day period. Following recovery of the spatial memory deficits produced by the brain lesion (four weeks after the brain lesion), dizocilpine (0.1 mg/kg) significantly impaired both reference and working memory, without affecting general behavior or food intake in the brain-lesioned rats. An impairment of working memory, but not reference memory, by dizocilpine was observed six weeks after the brain lesion. However, the disrupting effect of dizocilpine at 0.1 mg/kg on spatial working memory had disappeared at eight weeks after the lesion. Ten weeks after the brain lesion, dizocilpine at 0.2 mg/kg was necessary to induce spatial memory impairment, which was accompanied by motor and food intake deficits, as in intact rats. In sham-operated rats, the dose-response effects of dizocilpine did not differ from those in intact rats at any time after the operation. These results suggest that two phases of behavioral plasticity take place, depending on demand, to compensate for brain dysfunction after the unilateral lesion of the hippocampus in rats.

Activation of muscarinic M3-like receptors and beta-adrenoceptors, but not M2-like muscarinic receptors or alpha-adrenoceptors, directly modulates corticostriatal neurotransmission in vitro

The aim of this study was to characterize the modulation of synaptic transmission in the glutamatergic corticostriatal pathway by cholinergic and adrenergic receptors. In coronal slices of mouse brain, negative-going field potentials were recorded in the dorsal striatum in response to stimulation of the overlying white matter, and their susceptibility to various pharmacological manipulations was studied. The responses were mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors, since they were augmented by aniracetam (0.5-1.5 mM), a positive modulator of AMPA-type glutamate receptors, and blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (> or = 10 microM), a selective antagonist of AMPA receptors. Carbachol (10 microM), a muscarinic agonist, reduced the size of responses and abolished paired-pulse depression; these effects being consistent with previous studies indicating that muscarinic activation inhibits release of glutamate in the corticostriatal pathway. Muscarinic antagonists could block the effect of carbachol. Their rank order was: 10 microM scopolamine (a non-selective muscarinic antagonist) > or = 1 microM 4-diphenylacetoxy-N-methyl-piperidine (M3/M1 antagonist)>1 microM pirenzepine (M1 antagonist)>10 microM methoctramine (M2 antagonist). McN-A-343 (1-10 microM), an M1 muscarinic agonist, was ineffective in this preparation. In contrast, isoproterenol (10-30 microM), a beta-adrenergic agonist, slightly increased the synaptic responses, but it did not affect paired-pulse depression. None of alpha-adrenergic agents (30 nM-1.0 microM dexmedetomidine, an alpha2-adrenergic agonist, 0.3 microM atipamezole, an alpha2-adrenergic antagonist or 30 microM phenylephrine, an alpha1-adrenergic agonist) influenced the size of the responses; neither did these drugs alter paired-pulse depression. These results indicate that the activation of striatal M3-like muscarinic receptors and beta-adrenoceptors, but not M2-like muscarinic receptors and alpha-adrenoceptors, modulates directly corticostriatal glutamatergic neurotransmission.

EEG alpha and theta oscillations reflect cognitive and memory performance: a review and analysis

Evidence is presented that EEG oscillations in the alpha and theta band reflect cognitive and memory performance in particular. Good performance is related to two types of EEG phenomena (i) a tonic increase in alpha but a decrease in theta power, and (ii) a large phasic (event-related) decrease in alpha but increase in theta, depending on the type of memory demands. Because alpha frequency shows large interindividual differences which are related to age and memory performance, this double dissociation between alpha vs. theta and tonic vs. phasic changes can be observed only if fixed frequency bands are abandoned. It is suggested to adjust the frequency windows of alpha and theta for each subject by using individual alpha frequency as an anchor point. Based on this procedure, a consistent interpretation of a variety of findings is made possible. As an example, in a similar way as brain volume does, upper alpha power increases (but theta power decreases) from early childhood to adulthood, whereas the opposite holds true for the late part of the lifespan. Alpha power is lowered and theta power enhanced in subjects with a variety of different neurological disorders. Furthermore, after sustained wakefulness and during the transition from waking to sleeping when the ability to respond to external stimuli ceases, upper alpha power decreases, whereas theta increases. Event-related changes indicate that the extent of upper alpha desynchronization is positively correlated with (semantic) long-term memory performance, whereas theta synchronization is positively correlated with the ability to encode new information. The reviewed findings are interpreted on the basis of brain oscillations. It is suggested that the encoding of new information is reflected by theta oscillations in hippocampo-cortical feedback loops, whereas search and retrieval processes in (semantic) long-term memory are reflected by upper alpha oscillations in thalamo-cortical feedback loops.

Effects of enadoline on the development of pentylenetetrazol kindling, learning performance, and hippocampal morphology

Opioids are involved in the development of epileptic seizures. Recently, interest has been focused on the role of the kappa-opioid receptor agonists as novel approaches to the treatment of epilepsy. In the present study we investigated the effects of the kappa-opioid receptor agonist enadoline (Ena) on pentylenetetrazol (PTZ) induced seizures, PTZ kindling, shuttle-box performance and hippocampal neuromorphology. Ena injected i.c.v. in doses of 1 and 10 nmol did not affect acute PTZ seizures. In the course of PTZ kindling development, co-treatment (1 nmol) with the kappa-opioid receptor agonist suppressed seizure strength. Eight days after kindling completion the animals received a challenge dose of PTZ. In reaction to challenge, kindled animals which were pretreated with Ena reached significantly lower seizure scores. Kindling resulted in diminished shuttle-box performance. Learning performance in kindled animals pretreated with Ena was not normalised. Kindling resulted in increased glutamate binding. Interestingly, in comparison with the saline/saline group, neither in the Ena/saline nor in the Ena/PTZ treated groups changes in glutamate binding were found. That means that Ena prevented the increase in glutamate binding in the kindled group. In kindled animals significant cell loss in CA1 of the dorsal hippocampus was found and this was efficaciously counteracted by Ena. However, Ena alone did induce similar cell loss compared to kindled animals. It is hypothesised that the effects of enadoline are mainly due to interferences with glutamatergic systems.

Effects of vasopressin and related peptides on neurons of the rat lateral septum and ventral hippocampus

The effects of vasopressin (VP), VP fragments and propressophysin glycopeptide on neuronal activities in the septum-hippocampus complex of rats were studied in vitro and in vivo. The frequency of the hippocampus theta rhythm in Brattleboro rats homozygous for diabetes insipidus was significantly slower than that of heterozygous litter mates and normal rats. Intracerebroventricular micro-injection of des-glycine-amide vasopressin corrected for several hours the frequency deficit of the theta rhythm in the homozygous Brattleboro rats and the centrally administered VP slowed down theta rhythm in normal rats. Microinotophoretically administered VP excited single neurons in the lateral septum of ventral hippocampus, and/or facilitated the responses of these neurons to glutamate and to stimulation of the glutamatergic afferent fibers in the fimbria bundle. The excitatory effects of VP vanished within seconds after termination of the peptide administration, however, the peptide-induced enhancement of glutamate and syntatically induced excitations were sustained for up to 60 min after the peptide administration. In vitro, pM concentrations of VP, VP 4-8 and C-terminus glycopeptide of propresophysin facilitated for 30-60 min the glutamate-mediated EPSPs in neurons of the lateral septum or the ventral hippocampus. The EPSPs increase in the lateral septum neurons was not prevented by pretreatment with antagonist of the V1a type of the vasopressin receptor. The resting membrane potential and input resistance were not affected by the peptides. A low-frequency electrical stimulation in the diagonal Band of Broca or in the Bed nucleus of the stria terminals, sources of the vasopressinergic innervation of the septum, facilitated the negative wave of the filed potentials responses evoked in the lateral septum by stimulating the fimbria bundle fibers in control Long-Evans and Brattleboro rats heterozygous for diabetes insipidus. The field potential increase was sustained for several hours after the stimulation, and it was not occluded by long-term potentiation elicited by high frequency stimulation of the fimbria bundle afferent fibers. Brattleboro rats homozygous for diabetes insipidus failed to show the filed potential increase after the diagonal band stimulation. It is suggested that the long-lasting facilitation of glutamate-mediated excitations might be a physiological action of the propressophysin-derived peptides in the septum-hippocampus complex which, in concert with other forms of synaptic plasticity like the long-term potentiation, facilitates the hippocampus-mediated forms of learning and memory. This action is presumably related to the memory enhancing effect of the propressophysin-derived peptides.

Vasopressin metabolites: a link between vasopressin and memory?

The effects of endogenous metabolites of the neuropeptide vasopressin (VP) in behavioural tests led to the hypothesis that VP metabolites have a more selective function than VP. In contrast to VP, no peripheral effects have been found thus far with VP metabolites and their function seems to be associated with memory-related behaviour. VP metabolites can improve both consolidation and retrieval of memory. Effects on autonomic and electrophysiological parameters and interactions with other neurotransmitter systems have provided some information about the processes that could underlie the effects of VP metabolites on memory-related behaviour. There is evidence that the effects of VP metabolites could be mediated by a VP metabolite receptor, which is different from the known VP receptors. The VP metabolite receptor could be a link between the neuropeptide VP and memory-related behaviour.

Heparin modulates the single channel kinetics of reconstituted AMPA receptors from rat brain

Glutamate receptors specifically activated by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) have been reported to interact with the highly sulfated glycosaminoglycan, heparin, and to subsequently express lower binding affinity for [3H]AMPA. The present study examined whether heparin also modifies the kinetic properties of single channel activity expressed by isolated AMPA receptors from rat forebrain. Upon application of 280 nM AMPA, the partially purified receptors reconstituted in lipid bilayers expressed bursting channel activity that was inhibited by dinitroquinoxaline-2-3,-dione (DNQX). Treating the receptors with heparin (10 microg/ml) produced no change in conductance but the mean burst length for 280 nM AMPA was nearly doubled. Heparin also prolonged the lifetime of open states of the individual ion channels 3-5-fold, perhaps by causing a decrease in the closing rate constant for channel gating. Heparin had no effect on the lifetime of the closed state or on the amplitude of currents. The single channel open time was voltage-dependent and an increase of applied voltage caused a decrease in the heparin effect on channel open times. While the lifetime of the open channel was increased 3-4 times by heparin at 20 mV, there was no significant change induced at 43 mV. The equivalent electric charge of the channel gate was increased by 40%. The heparin effects were specific as another polysaccharide, dextran, and a monomeric constituent of heparin, glucosamine 2,3-disulfate, failed to have any effect on the receptors. These findings suggest that heparin-containing extracellular matrix components can interact with AMPA receptors and influence their functional properties.

Nucleus basalis injections of N-methyl-D-aspartate enhance memory of rats in the double Y-maze

N-methyl-D-aspartate (NMDA) receptors have been implicated in learning and memory. Many findings show that NMDA receptor antagonists impair memory. Few studies, however, have investigated the role of NMDA receptor agonists in mnemonic function. The present study examined the effects of nucleus basalis magnocellularis (nbm) injections of NMDA on memory. Rats were trained in a two-component double Y-maze task consisting of a spatial discrimination and a delayed alternation. Rats (n = 7) were surgically implanted with bilateral cannulae in the nbm prior to maze training. Once trained, animals received bilateral nbm injections (0.5 microl) of saline (0.9%), NMDA (50, 75, and 100 ng/side), and the benzodiazepine receptor partial inverse agonist N-methyl-beta-carboline-3-carboxamide (FG 7142; 200 ng/side), in a counterbalanced order. During testing, delays (0, 30, 60 s) were introduced. Nbm FG 7142 or NMDA (50 ng/side) produced an improvement in the delayed alternation task. Results support the hypothesis that nbm NMDA receptors are involved in cognitive processes mediating memory.

AF-DX 116, a presynaptic muscarinic receptor antagonist, potentiates the effects of glucose and reverses the effects of insulin on memory

Male Swiss mice were tested 24 h after training in a one-trial step-through inhibitory avoidance task. Low subeffective doses of d-(+)-glucose (10 mg/kg, ip), but not its stereoisomer l-(-)-glucose (30 mg/kg,ip), administered immediately after training, and AF-DX 116 (0.3 mg/kg,ip), a presynaptic muscarinic receptor antagonist, given 10 min after training, interact to improve retention. Insulin (8 IU/kg, ip) impaired retention when injected immediately after training, and the effects were reversed, in a dose-related manner, by AF-DX 116 (0.3, 1.0, or 3.0 mg/kg, ip) administered 10 min following insulin. Since AF-DX 116 possibly blocks autoreceptors mediating the inhibition of acetylcholine release from cholinergic nerve terminals, the present data support the view that changes in the central nervous system glucose availability, subsequent to modification of circulating glucose levels, influence the activity of central cholinergic mechanisms involved in memory storage of an inhibitory avoidance response in mice.

The relationship between adhesion molecules and neuronal plasticity

1. It is presently widely assumed that structural reorganization of synaptic architectures subserves the functional gains that define certain neuronal plasticities. 2. While target molecules thought to participate in such morphological dynamics are not well defined, growing evidence suggests a pivotal role for cell adhesion molecules. 3. Herein, brief discussions are presented on (i) the history of how adhesion molecules became implicated in plasticity and memory processes, (ii) the general biology of some of the major classes of such molecules, and (iii) the future of the adhesion molecule/plasticity relationship.

The diazoxide derivative 7-chloro-3-methyl-3,4-dihydro-2H-1,2,4-benzothiadiazine-S,S-dioxide augments AMPA- and GABA-mediated synaptic responses in cultured hippocampal neurons

The diazoxide derivative 7-chloro-3-methyl-3,4-dihydro-2H-1,2,4-benzothiadiazine-S,S-dioxide (IDRA21) enhances memory and learning in rodents, most likely by potentiating AMPAergic synaptic activity. We examined IDRA21's effect upon AMPAergic synaptic currents and whole-cell glutamate currents in cultured rat hippocampal neurons to determine whether IDRA21 was a partial modulator of AMPA receptor desensitization and deactivation. Comparable to cyclothiazide, IDRA21 prolonged AMPAergic autaptic currents (5.6 times control, EC50 150 microM) and slowed the rate of AMPA deactivation (3 times control) following 1-ms applications of 1 mM glutamate to excised, outside-out membrane patches. IDRA21 also augmented autaptic GABA currents by 27 +/- 8.1%, although it had two opposing effects, reducing the peak amplitude versus prolonging autaptic GABA currents. IDRA21 (200 microM) inhibited whole-cell GABA currents elicited by exogenously applied 1 mM GABA by 41 +/- 11%. At sufficient concentrations, IDRA21 reduced AMPA receptor desensitization and slowed the rate of deactivation, most consistent with full agonist activity with lower potency compared to cyclothiazide. IDRA21 slightly augments GABAergic synaptic currents.

Learning-specific, time-dependent increases in hippocampal Ca2+/calmodulin-dependent protein kinase II activity and AMPA GluR1 subunit immunoreactivity

Ca2+/calmodulin-dependent protein kinase II (CAMK II) and one of its target, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), glutamate receptors have been shown to participate in both long-term potentiation (LTP) in the hippocampus, and in spatial, as well as in a variety, of learning paradigms. Recently, we were able to demonstrate that the intrahippocampal infusion of a specific inhibitor of CAMK II (KN62) provoked full retrograde amnesia of an inhibitory avoidance learning in rats when given immediately, but not 120 or 240 min, after training. Furthermore, this task is accompanied by a rapid, selective and reversible increase in hippocampal [3H] AMPA receptor binding. Here we report the effect of this aversively motivated learning task on CAMK II activity, and AMPA GluR1 subunit phosphorylation and immunoreactivity in the hippocampus. One trial inhibitory avoidance training is associated with a learning-specific, time-dependent increase (25-78%) in both total and Ca2+-independent activities of CAMK II in the hippocampus of rats killed immediately (0 min), but not 120 min, after training. In addition, immunoblotting experiments showed an increment in the amount of the alpha-subunit of CAMK II at 0, 30 and 120 min after training. An increase in the in vitro phosphorylation of alpha- and beta-subunits of CAMK II was also observed in hippocampal synaptosomal membranes (SPM) of trained rats killed immediately and 30 min post-training. In addition, inhibitory avoidance is accompanied by a 20% increase in GluR1 phosphorylation and a 33% increase in GluR1 immunoreactivity 120 min after training. No significant changes were observed in shocked animals. Phosphorylation of hippocampal SPM from naive control animals in conditions suitable for CAMK II activation resulted in a large increase in the density of [3H] AMPA binding (+ 100%). Taken together, these findings confirm and extend previous data suggesting that CAMK II and AMPA glutamate receptors in the hippocampus participate in the early phase of memory formation of an inhibitory avoidance learning.