Contributions of AMPA and GABA(A) receptors to the induction of NMDAR-dependent LTP in CA1

Dopamine D4 receptor activation restores CA1 LTP in hippocampal slices from aged mice

Normal aging is characterized with a decline in hippocampal memory functions that is associated with changes in long-term potentiation (LTP) of the CA3-to-CA1 synapse. Age-related deficit of the dopaminergic system may contribute to impairment of CA1 LTP. Here we assessed how the modulation of CA1 LTP by dopamine is affected by aging and how it is dependent on the Ca2+ source. In slices from adult mice, the initial slope of the field potential showed strong LTP, but in slices from aged mice LTP was impaired. Dopamine did not affect LTP in adult slices, but enhanced LTP in aged slices. The dopamine D1/D5 receptor (D1R/D5R) agonist SKF-81297 did not affect LTP in adult but caused a relative small increase in LTP in aged slices; however, although there was no difference in dopamine D4 receptor (D4R) expression, the D4R agonist PD168077 increased LTP in aged slices to a magnitude similar to that in adult slices. The N-Methyl-D-aspartate receptor antagonist D-AP5 reduced LTP in adult slices, but not in aged slices. However, in the presence of D-AP5, PD168077 completely blocked LTP in aged slices. The voltage-dependent calcium channel (VDCC) blocker nifedipine reduced LTP in adult slices, but surprisingly enhanced LTP in aged slices. Furthermore, in the presence of nifedipine, PD168077 caused a strong enhancement of LTP in aged slices to a magnitude exceeding LTP in adult slices. Our results indicate that the full rescue of impaired LTP in aging by the selective D4R activation and that a large potentiation role on LTP by co-application of D4R agonist and VDCC blocker may provide novel strategies for the intervention of cognitive decline of aging and age-related diseases.

Activation of Metabotropic Glutamate Receptor 7 Is Required for Induction of Long-Term Potentiation at SC-CA1 Synapses in the Hippocampus

Of the eight metabotropic glutamate (mGlu) receptor subtypes, only mGlu7 is expressed presynaptically at the Schaffer collateral (SC)-CA1 synapse in the hippocampus in adult animals. Coupled with the inhibitory effects of Group III mGlu receptor agonists on transmission at this synapse, mGlu7 is thought to be the predominant autoreceptor responsible for regulating glutamate release at SC terminals. However, the lack of mGlu7-selective pharmacological tools has hampered direct testing of this hypothesis. We used a novel, selective mGlu7-negative allosteric modulator (NAM), ADX71743, and a newly described Group III mGlu receptor agonist, LSP4-2022, to elucidate the role of mGlu7 in modulating transmission in hippocampal area CA1 in adult C57BL/6J male mice. Interestingly, although mGlu7 agonists inhibit SC-CA1 EPSPs, we found no evidence for activation of mGlu7 by stimulation of SC-CA1 afferents. However, LSP4-2022 also reduced evoked monosynaptic IPSCs in CA1 pyramidal cells and, in contrast to its effect on SC-CA1 EPSPs, ADX71743 reversed the ability of high-frequency stimulation of SC afferents to reduce IPSC amplitudes. Furthermore, blockade of mGlu7 prevented induction of LTP at the SC-CA1 synapse and activation of mGlu7 potentiated submaximal LTP. Together, these data suggest that mGlu7 serves as a heteroreceptor at inhibitory synapses in area CA1 and that the predominant effect of activation of mGlu7 by stimulation of glutamatergic afferents is disinhibition, rather than reduced excitatory transmission. Furthermore, this mGlu7-mediated disinhibition is required for induction of LTP at the SC-CA1 synapse, suggesting that mGlu7 could serve as a novel therapeutic target for treatment of cognitive disorders.

Disruption of Slc4a10 augments neuronal excitability and modulates synaptic short-term plasticity

Slc4a10 is a Na⁺-coupled Cl⁻-HCO₃⁻ exchanger, which is expressed in principal and inhibitory neurons as well as in choroid plexus epithelial cells of the brain. Slc4a10 knockout (KO) mice have collapsed brain ventricles and display an increased seizure threshold, while heterozygous deletions in man have been associated with idiopathic epilepsy and other neurological symptoms. To further characterize the role of Slc4a10 for network excitability, we compared input-output relations as well as short and long term changes of evoked field potentials in Slc4a10 KO and wildtype (WT) mice. While responses of CA1 pyramidal neurons to stimulation of Schaffer collaterals were increased in Slc4a10 KO mice, evoked field potentials did not differ between genotypes in the stratum radiatum or the neocortical areas analyzed. Paired pulse facilitation was diminished in the hippocampus upon disruption of Slc4a10. In the neocortex paired pulse depression was increased. Though short term plasticity is modulated via Slc4a10, long term potentiation appears independent of Slc4a10. Our data support that Slc4a10 dampens neuronal excitability and thus sheds light on the pathophysiology of SLC4A10 associated pathologies.

NMDA receptor expression and activity in osteoarthritic human articular chondrocytes

OBJECTIVE

Classical neuronal signalling molecules such as substance P and glutamate have been identified in cartilage and have roles in regulation of chondrocyte function. This study looks at expression and activity of the ionotropic glutamate NMDA (N-methyl-D-aspartic acid) receptor (NMDAR) in human osteoarthritic (OA) chondrocytes.

METHOD

Chondrocytes were obtained from human knee joint arthroplasty specimens. NMDAR subunits and PSD-95 (postsynaptic density protein 95) expression were analysed by reverse transcription-polymerase chain reaction and Western blotting. Activity of NMDAR was assayed by radioactive calcium(45) uptake and changes in membrane potential in the presence and absence of NMDA and NMDAR antagonists and blockade of cell membrane ion channels.

RESULTS

NMDAR 1, 2A, 2B and PSD-95 were detected in human OA chondrocytes whereas NR2B was absent from normal chondrocytes. NMDA induced calcium flux into OA chondrocytes and cell membrane depolarisation. These responses were blocked by NMDAR antagonists, removal of extracellular calcium, inhibition of nNOS (neuronal nitric oxide synthase) activity and uncoupling of NMDAR from PSD-95. Blockade of sodium channels by tetrodotoxin resulted in NMDA-induced membrane hyperpolarisation which was, in turn inhibited by apamin, a blocker of SK channels. NMDA-induced changes in cell membrane potential were not affected by l-type and stretch activated calcium channel inhibitors.

CONCLUSIONS

Human OA and normal articular chondrocytes differ in the expression of NMDAR subunits. In OA chondrocytes NMDAR signalling requires extracellular calcium, association with PSD-95, and nNOS activity. Downstream signalling results in activation of tetrodotoxin sensitive sodium channels and SK channels, a response that differs from that of normal chondrocytes suggesting altered activity of NMDAR in OA.

Altered synaptic dynamics and hippocampal excitability but normal long-term plasticity in mice lacking hyperpolarizing GABA A receptor-mediated inhibition in CA1 pyramidal neurons

GABA(A) receptor (GABA-AR)-mediated inhibition is critical for proper operation of neuronal networks. Synaptic inhibition either shifts the membrane potential farther away from the action potential firing threshold (hyperpolarizing inhibition) or via increase in the membrane conductance shunts the excitatory currents. However, the relative importance of these different forms of inhibition on the hippocampal function is unclear. To study the functional consequences of the absence of hyperpolarizing inhibition, we have used KCC2-deficient mice (KCC2hy/null) maintaining only 15-20% of the neuron-specific K-Cl-cotransporter. Gramicidin-perforated patch-clamp recordings in hippocampal CA1 pyramidal cells revealed that the reversal potential of the GABA-AR-mediated postsynaptic currents (E(GABA-A)) was approximately 20 mV more positive in KCC2hy/null mice than in wild-type (WT) animals. The basic glutamatergic transmission appeared unaltered in the KCC2hy/null mice, yet they displayed lowered threshold for stimulation-induced synchronous afterdischarges in the CA1 area. Also fatigue of field excitatory postsynaptic potentials/excitatory postsynaptic currents in response to repetitious stimulation was smaller in KCC2hy/null mice, indicating altered synaptic dynamics. Interestingly, this effect was present also under blockade of GABA-ARs and was dependent on the extracellular K+ concentration. Moreover, there were no differences in the levels of either long-term potentiation or long-term depression between the genotypes. The local hippocampal CA1 network can in several aspects maintain its functional viability even in the absence of hyperpolarizing inhibition in pyramidal cells. Our results underscore the central role of shunting type of inhibition in controlling the neuronal excitation/inhibition balance. Moreover, our data demonstrate a novel, unexpected role for the KCC2, namely the modulation of properties of glutamatergic transmission during repetitious afferent activity.

Developmental regulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor subunit expression in forebrain and relationship to regional susceptibility to hypoxic/ischemic injury. II. Human cerebral white matter and cortex

This report is the second of a two-part evaluation of developmental differences in alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor (AMPAR) subunit expression in cell populations within white matter and cortex. In part I, we reported that, in rat, developmental expression of Ca2+-permeable (GluR2-lacking) AMPARs correlated at the regional and cellular level with increased susceptibility to hypoxia/ischemia (H/I), suggesting an age-specific role of these receptors in the pathogenesis of brain injury. Part II examines the regional and cellular progression of AMPAR subunits in human white matter and cortex from midgestation through early childhood. Similarly to the case in the rodent, there is a direct correlation between selective vulnerability to H/I and expression of GluR2-lacking AMPARs in human brain. For midgestational cases aged 20-24 postconceptional weeks (PCW) and for premature infants (25-37 PCW), we found that radial glia, premyelinating oligodendrocytes, and subplate neurons transiently expressed GluR2-lacking AMPARs. Notably, prematurity represents a developmental window of selective vulnerability for white matter injury, such as periventricular leukomalacia (PVL). During term (38-42 PCW) and postterm neonatal (43-46 PCW) periods, age windows characterized by increased susceptibility to cortical injury and seizures, GluR2 expression was low in the neocortex, specifically on cortical pyramidal and nonpyramidal neurons. This study indicates that Ca2+-permeable AMPAR blockade may represent an age-specific therapeutic strategy for potential use in humans. Furthermore, these data help to validate specific rodent maturational stages as appropriate models for evaluation of H/I pathophysiology.

Divergent effects of Abeta1-42 on ionotropic glutamate receptor-mediated responses in CA1 neurons in vivo

Aggregated Abeta1-42 is hypothesized to be the central cause of Alzheimer's disease. However, early changes in synaptic activity may be detected in the disease long before a significant cell loss is manifested. Despite the fact that Abeta1-42 interference with long-term potentiation (LTP) and the field excitatory postsynaptic potential (fEPSP) is well documented, the exact mechanism of these events remains to be clarified. Here we studied the effects of iontophoretically applied Abeta1-42 on the neuronal firing evoked in vivo on the CA1 hippocampal neurons of Wistar rats by different agonists of the ionotropic glutamate receptors: N-methyl-d-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and kainic acid (KA). NMDA elicited firing enhanced in all of the measured cells; in contrast, the AMPA-mediated responses decreased significantly after Abeta1-42 ejection. The changes in KA-evoked responses to Abeta1-42 revealed two types of cells. In the first type, the KA-mediated firing remained at the control level, while in the second type, Abeta1-42 attenuated the KA-evoked responses. A protective pentapeptide, Leu-Pro-Tyr-Phe-Asp-amide, was used to verify the specificity of these beta-amyloid-elicited effects. The pentapeptide protected against the modulatory effects of Abeta1-42 on the NMDA and AMPA responses. In conclusion, we have shown that Abeta1-42 exerts divergent effects on the activity of the ionotropic glutamate receptors in vivo. These results suggest that the LTP disruption and fEPSP attenuation seen after Abeta1-42 application are in part due to the altered function of these receptors.

Coantagonism of glutamate receptors and nicotinic acetylcholinergic receptors disrupts fear conditioning and latent inhibition of fear conditioning

The present study investigated the hypothesis that both nicotinic acetylcholinergic receptors (nAChRs) and glutamate receptors (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors (AMPARs) and N-methyl-d-aspartate glutamate receptors (NMDARs)) are involved in fear conditioning, and may modulate similar processes. The effects of the nAChR antagonist mecamylamine administered alone, the AMPAR antagonist NBQX administered alone, and the NMDAR antagonist MK-801 administered alone on cued fear conditioning, contextual fear conditioning, and latent inhibition of cued fear conditioning were examined. In addition, the effects of coadministration of either mecamylamine and NBQX or mecamylamine and MK-801 on these behaviors were examined. Consistent with previous studies, neither mecamylamine nor NBQX administered alone disrupted any of the tasks. However, coadministration of mecamylamine and NBQX disrupted both contextual fear conditioning and latent inhibition of cued fear conditioning. In addition, coadministration of mecamylamine with a dose of MK-801 subthreshold for disrupting either task disrupted both contextual fear conditioning and latent inhibition of cued fear conditioning. Coadministration of mecamylamine and NBQX, and coadministration of mecamylamine with a dose of MK-801 subthreshold for disrupting fear conditioning had little effect on cued fear conditioning. These results suggest that nAChRs and glutamate receptors may support similar processes mediating acquisition of contextual fear conditioning and latent inhibition of fear conditioning.

AMPA receptor antagonists, GYKI 52466 and NBQX, do not block the induction of long-term potentiation at therapeutically relevant concentrations

The involvement of alpha-amino-3-hydroxy-5-methylizoxazole-4-propionic acid (AMPA) receptors in induction of long-term potentiation (LTP) was examined in rat hippocampal slice preparation. Using conventional extracellular recording, excitatory postsynaptic potentials (EPSPs) and population action potentials (PSs), evoked by low-frequency stimulation of the Schaffer collateral-commissural fibres, were recorded in the CA1 region. The effects of a competitive AMPA receptor antagonist, 6-nitro-7-sulfamoylbenzo(f)quinoxaline-2, 3-dione (NBQX), and that of a non-competitive blocker, 1-(4-aminophenyl)-4-methyl-7,8-methylendioxy-5H-2,3-benzodiazepine (GYKI 52466) have been examined. 0.25-0.5 microM of NBQX and 20-40 microM of GYKI 52466 did not suppress the induction of LTP. LTP was attenuated only at the highest concentrations tested (1 microM NBQX or 80 microM GYKI 52466). These in vitro concentrations, however, exceed the brain levels needed for in vivo anticonvulsant action. Furthermore, even at the highest concentrations both compounds suppressed only the expression but not the induction of LTP. Namely after their washout LTP reappeared. Thus, at pharmacologically relevant concentrations these AMPA receptor antagonists apparently do not suppress LTP, a cellular mechanism underlying memory formation. These experiments suggest that in clinical practice AMPA receptor blockade may have some advantage over N-methyl-D-aspartate receptor antagonism, which is accompanied by severe memory impairment.

Pharmacology of AMPA/kainate receptor ligands and their therapeutic potential in neurological and psychiatric disorders

It has been postulated, consistent with the ubiquitous presence of glutamatergic neurons in the brain, that defects in glutamatergic neurotransmission are associated with many human neurological and psychiatric disorders. This review evaluates the possible application of ligands acting on glutamate alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and kainate (KA) receptors to minimise the pathology and/or symptoms of various diseases. Glutamate activation of AMPA receptors is thought to mediate most fast synaptic neurotransmission in the brain, while transmission via KA receptors contributes only a minor component. Variants of the protein subunits forming these receptors greatly extend the pharmacological and electrophysiological properties of AMPA/KA receptors. Disease and drug use can differentially affect the expression of the subunits and their variants. Ligands bind to AMPA receptors by competing with glutamate at the glutamate binding site, or non-competitively at other sites on the proteins (allosteric modulators). Ligands showing selective competitive antagonist actions at the AMPA/ KA class of glutamate receptors were first reported in 1988, and the systemically active antagonist 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(F)quinoxaline (NBQX) was first shown to have useful therapeutic effects on animal models of neurological diseases in 1990. Since then, newer antagonists with increased potency, higher specificity, increased water solubility, and a longer duration of action in vivo have been developed. Negative allosteric modulators such as the prototype GYKI-52466 also block AMPA receptors but have little action at KA receptors. Positive allosteric modulators enhance glutamatergic neurotransmission at AMPA receptors. Polyamines and adamantane derivatives bind within the ion channel of calcium-permeable AMPA receptors. The latest developments include ligands selective for KA receptors containing Glu-R5 subunits. Evidence for advantages of AMPA receptor antagonists over N-methyl-D-aspartate (NMDA) receptor antagonists for symptomatic treatment of neurological and psychiatric conditions, and for minimising neuronal loss occurring after acute neurological diseases, such as physical trauma, ischaemia or status epilepticus, have been shown in animal models. However, as yet AMPA receptor antagonists have not been shown to be effective in clinical trials. On the other hand, a limited number of clinical trials have been reported for AMPA receptor ligands that enhance glutamatergic neurotransmission by extending the ion channel opening time (positive allosteric modulators). These acute studies demonstrate enhanced memory capability in both young and aged humans, without any apparent serious adverse effects. The use of these allosteric modulators as antipsychotic drugs is also possible. However, the long term use of both direct agonists and positive allosteric modulators must be approached with considerable caution because of potential adverse effects.