Long-term potentiation (LTP) at hippocampal CA3-CA1 synapses is often associated with increases in quantal size, traditionally attributed to enhanced availability or efficacy of postsynaptic glutamate receptors. However, augmented quantal size might also reflect increases in neurotransmitter concentration within the synaptic cleft since AMPA-type glutamate receptors are not generally saturated during basal transmission. Here we report evidence that peak cleft glutamate concentration ([glu]cleft) increases during LTP, as indicated by a lessening of the blocking effects of rapidly unbinding antagonists of AMPA. The efficacy of slowly equilibrating antagonists remained unchanged. The elevated [glu]cleft helps support the increased quantal amplitude of AMPA-type EPSCs (excitatory postsynaptic currents) during LTP.
alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid ; Glutamic Acid* ; Long-Term Potentiation ; Neurotransmitter Agents ; Receptors, Glutamate ; Synapses

Glutamate and gamma-aminobutyric acid (GABA) are major excitatory and inhibitory neurotransmitters in the vertebrate retina, respectively. Using the whole-cell patch clamp technique and a rapid solution changer, glutamate and GABA receptors have been extensively investigated in carp retina. Glutamate receptors on both horizontal and amacrine cells may be an AMPA preferring subtype, which predominantly consists of flop splice variants. GABAA and GABAC receptors coexist in bipolar cells and they both show significant desensitization. Kinetics analysis demonstrated that activation, deactivation and desensitization of the GABAC receptor-mediated response of these cells are overall slower than those of the GABAA response. Endogenous modulator Zn2+ in the retina was found to differentially modulate the kinetic characteristics of the GABAC and GABAA responses.
alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid ; Amacrine Cells ; Carps ; gamma-Aminobutyric Acid* ; Glutamic Acid* ; Kinetics ; Neurotransmitter Agents ; Receptors, GABA* ; Receptors, Glutamate ; Retina* ; Vertebrates

A central challenge in ischemia-induced neuronal death research is understanding the mechanisms by which apoptotic or necrotic cascades are initiated and affected. We tested potential roles for AMPA and NMDA receptor protein levels and activation of calpain, caspase-3 in the hippocampus at times after transient global ischemia when detectable necrotic or apoptotic cell damage was observed by neurofilament 200 (NF200) degradation, TUNEL, and H & E. We determined that the decrease in the AMPA receptor subunit, GluR2, in response to the transient global ischemia plays a major role in triggering the neuronal cell death in hippocampus. We also examined potential roles for calpain and caspase-3 in ischemic cell death and found that (1) calpain is activated at a time following caspase-3 activation and paralleled degradation of NR2A, NR2B, and GluR2 and irreversible necrotic neuronal changes, (2) caspase-3 is has their maximal expression at the time of highest apoptosis, (3) the NF200 degradation, one of the neuronal deathinducing factors was correlated well with the calpain activation and necrotic changes in the hippocampal CA1 neurons. These results suggest that the significant degradation of GluR2 subunits of AMPA receptor and calpain activation are possibly involved in NF 200 degradation-mediated necrotic hippocampal cell death after transient global ischemia.
alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid* ; Animals ; Apoptosis ; Calpain* ; Caspase 3* ; Cell Death* ; Hippocampus* ; In Situ Nick-End Labeling ; Ischemia* ; N-Methylaspartate ; Neurons* ; Rats* ; Receptors, AMPA

Acute hypotension induced excitation of electrical activities and expression of c-Fos protein and pERK in the vestibular nuclei. In this study, to investigate the excitatory signaling pathway in the vestibular nuclei following acute hypotension, expression of NR2A and NR2B subunits of glutamate NMDA receptor and GluR1 subunit of glutamate AMPA receptor was determined by RT-PCR and Western blotting in the medial vestibular nucleus 30 min after acute hypotension in rats. Acute hypotension increased expression of NR2A, NR2B, and pGluR1 in the medial vestibular nuclei. These results suggest that both of NMDA and AMPA glutamate receptors take part in transmission of excitatory afferent signals following acute hypotension.
alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid ; Animals ; Blotting, Western ; Glutamic Acid* ; Hypotension* ; N-Methylaspartate ; Rats ; Receptors, AMPA ; Receptors, Glutamate* ; Vestibular Nuclei*

We investigated the comparative effects of electroacupuncture (EA) with different frequency on the spinal c-fos and inotropic glutamate receptor expression in carrageenan-injected rats. Bilateral EA stimulation with 2, 15 and 120 Hz were delivered at those acupoints corresponding to Zusanli and Sanyinjiao in man via the needles. The inhibitory effects of the EA on the inflammatory process were investigated in the central nociceptive sites by immunohistochemical analysis. Three hours after carrageenan injection, the number of c-fos-like immunoreactive (LI) neurons was significantly increased in all layers of the ipsilateral spinal cord at L4-5 segment. But these immunoreactive neurons were markedly reduced in the spinal gray matter, especially in the superficial laminae, by all kinds of EA stimulation. The number of NMDA (NR-1 and NR-2A) and AMPA receptors (GluR-1, GluR-2/3) -LI neurons was also increased by carrageenan injection. But NR-2A-LI neuron was significantly reduced in superficial laminae of dorsal horn by 2 Hz EA stimulation. While GluR-1-LI neuron of 15 Hz and 120 Hz EA was increased in the nucleus proprius compared with carrageenan-injected group, GluR-2/3-LI neuron of these EA stimulated groups significantly reduced in superficial laminae of dorsal horn. In conclusion, EA treatment can attenuate spinal c-fos expression in carrageenan injected rats and regulates spinal inotropic glutamate receptor expression.
Acupuncture Points ; alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid ; Animals ; Carrageenan ; Electroacupuncture* ; Glutamic Acid* ; Horns ; N-Methylaspartate ; Needles ; Neurons ; Rats* ; Receptors, AMPA ; Receptors, Glutamate* ; Spinal Cord

We investigated the comparative effects of electroacupuncture (EA) with different frequency on the spinal c-fos and inotropic glutamate receptor expression in carrageenan-injected rats. Bilateral EA stimulation with 2, 15 and 120 Hz were delivered at those acupoints corresponding to Zusanli and Sanyinjiao in man via the needles. The inhibitory effects of the EA on the inflammatory process were investigated in the central nociceptive sites by immunohistochemical analysis. Three hours after carrageenan injection, the number of c-fos-like immunoreactive (LI) neurons was significantly increased in all layers of the ipsilateral spinal cord at L4-5 segment. But these immunoreactive neurons were markedly reduced in the spinal gray matter, especially in the superficial laminae, by all kinds of EA stimulation. The number of NMDA (NR-1 and NR-2A) and AMPA receptors (GluR-1, GluR-2/3) -LI neurons was also increased by carrageenan injection. But NR-2A-LI neuron was significantly reduced in superficial laminae of dorsal horn by 2 Hz EA stimulation. While GluR-1-LI neuron of 15 Hz and 120 Hz EA was increased in the nucleus proprius compared with carrageenan-injected group, GluR-2/3-LI neuron of these EA stimulated groups significantly reduced in superficial laminae of dorsal horn. In conclusion, EA treatment can attenuate spinal c-fos expression in carrageenan injected rats and regulates spinal inotropic glutamate receptor expression.
Acupuncture Points ; alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid ; Animals ; Carrageenan ; Electroacupuncture* ; Glutamic Acid* ; Horns ; N-Methylaspartate ; Needles ; Neurons ; Rats* ; Receptors, AMPA ; Receptors, Glutamate* ; Spinal Cord

This study examined the effects of N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and kainate on basal and electrically-evoked release of acetylcholine (ACh) from the rat hippocampal and striatal slices which were preincubated with [3H]choline. Unexpectedly, the basal and evoked ACh release were not affected at all by the treatment with NMDA (3~100microM), AMPA (1~100microM) or kainate (1~100microM) in hippocampal slices. However, in striatal slices, under the Mg2 -free medium, 30microM NMDA increased the basal ACh release with significant decrease of the electrically- evoked releases. The treatment with 1microM MK-801 not only reversed the 30microM NMDA-induced decrease of the evoked ACh release, but also attenuated the facilitatory effect of 30microM NMDA on the basal ACh release. The treatment with either 30microM AMPA or 100microM kainate increased the basal ACh release without any effects on the evoked release. The treatment with 10microM NBQX abolished the AMPA- or kainate-induced increase of the basal ACh release. Interestingly, NBQX significantly attenuated the evoked release when it was treated with AMPA, although it did not affect the evoked release alone without AMPA. These observations demonstrate that in hippocampal slices, ionotropic glutamate receptors do not modulate the ACh release in cholinergic terminals, whereas in striatal slices, activations of ionotropic glutamate receptors increase the basal ACh release though NMDA may decrease the electrically-evoked ACh release.
Acetylcholine* ; alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid* ; Animals ; Diethylpropion ; Dizocilpine Maleate ; Hippocampus ; Kainic Acid* ; N-Methylaspartate* ; Rats* ; Receptors, Ionotropic Glutamate

Conventional views of synaptic transmission generally overlook the possibility of "postfusional- control" the regulation of the speed or completeness of transmitter release upon vesicular fusion. However, such regulation often occurs in non-neuronal cells where the dynamics of fusion-pore opening is critical for the speed of transmitter release. In case of synapses, the slower the transmitter release, the smaller the size and rate-of-rise of postsynaptic responses would be expected if postsynaptic neurotransmitter receptors were not saturated. This prediction was tested at hippocampal synapses where postsynaptic AMPA-type glutamate receptors (AMPAR) were not generally saturated. Here, we found that the small miniature excitatory postsynaptic currents (mEPSCs) showed significantly slower rise times than the large mEPSCs when the sucrose-induced mEPSCs recorded in cyclothiazide (CTZ), a blocker for AMPAR desensitization, were sorted by size. The slow rise time of the small mEPSCs might result from slow release through a non-expanding fusion pore, consistent with postfusional control of neurotransmitter release at central synapses.
alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid* ; Excitatory Postsynaptic Potentials ; Neurotransmitter Agents ; Receptors, AMPA* ; Receptors, Glutamate ; Receptors, Neurotransmitter ; Synapses* ; Synaptic Transmission

The neuronal activity-dependent change in the manner in which light is absorbed or scattered in brain tissue is called the intrinsic optical signal (IOS), and provides label-free, minimally invasive, and high spatial (~100 µm) resolution imaging for visualizing neuronal activity patterns. IOS imaging in isolated brain slices measured at an infrared wavelength (>700 nm) has recently been attributed to the changes in light scattering and transmittance due to aquaporin-4 (AQP4)-dependent astrocytic swelling. The complexity of functional interactions between neurons and astrocytes, however, has prevented the elucidation of the series of molecular mechanisms leading to the generation of IOS. Here, we pharmacologically dissected the IOS in the acutely prepared brain slices of the stratum radiatum of the hippocampus, induced by 1 s/20 Hz electrical stimulation of Schaffer-collateral pathway with simultaneous measurement of the activity of the neuronal population by field potential recordings. We found that 55% of IOSs peak upon stimulation and originate from postsynaptic AMPA and NMDA receptors. The remaining originated from presynaptic action potentials and vesicle fusion. Mechanistically, the elevated extracellular glutamate and K⁺ during synaptic transmission were taken up by astrocytes via a glutamate transporter and quinine-sensitive K2P channel, followed by an influx of water via AQP-4. We also found that the decay of IOS is mediated by the DCPIB- and NPPB-sensitive anion channels in astrocytes. Altogether, our results demonstrate that the functional coupling between synaptic activity and astrocytic transient volume change during excitatory synaptic transmission is the major source of IOS.
Action Potentials ; alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid ; Amino Acid Transport System X-AG ; Astrocytes ; Brain ; Electric Stimulation ; Glutamic Acid ; Hippocampus ; Jupiter ; Neurons ; Receptors, N-Methyl-D-Aspartate ; Synaptic Transmission ; Water

The purpose of this study was, first, to devise a new model for topical application of excitatory amino acids(EAAs) to rat cerebral cortex that successfully and repeatdly initiate the cortical spreading depression(CSD). Then, by using this model, six major EAAs that are known to act on single or multiple subtypes of EAA receptor were examined; glutamate, kainate, aspartate, N-methyl-D-aspartate(NMDA), quisqualate, and alpha-amino-3-hydroxy-5-methyl-4-isoxazoie-proprite(AMPA). Through the model, with a cone-shaped well buried in 1.5mm depth of the cerebral cortex, these chemical agents were topically applied to the cortical gray matter. A total of 50 Sprague-Dawley rats were used and divided into seven groups including the sham group. Doses of each EAA between 10(-7) and 10(-4)M concentrations were escalated for triggering the CSD and its rate of consistency in triggering was also evaluated. In the overall results. CSDs were repeatedly initiated in all experimental groups with relatively consistent rates. Duration of CSDs were 1-4 minutes(mean 2.2+/-1.4) and amplitudes were 20-40mV. Effective dose(50)(ED(50)), that trigger over 50% of CSD was 10(-5)M(n=8) for glutamate, 10(-7)M(n=8) for aspartate, 10(-5)M(n=7) for AMPA, 10(-5)M(n=7) for quisqualate, and 10(-4)M(n=7) for NMDA and kainate group. Among those acting on the single receptor, AMPA was shown to be the most effective in triggering CSD, and NMDA, and kainate were in descending orders. Aspartate that was known to act on multiple EAA receptors, showed the highest rate of triggering CSD among all groups, but glutamate, known to act on all receptors of its subtypes, showed the most consistent rate of triggering CSD at dose escalation. These results revealed that those EAA acting on multiple receptors, namely aspartate and glutamate, showed the highest and most consistent rate of triggering CSD. Among those acting on single channel of receptors. AMPA was the most effective, although its consistency and rate of triggering of CSD was somewhat lower than.
alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid ; Animals ; Aspartic Acid ; Cerebral Cortex ; Cortical Spreading Depression* ; Excitatory Amino Acids* ; Glutamic Acid ; Kainic Acid ; N-Methylaspartate ; Quisqualic Acid ; Rats ; Rats, Sprague-Dawley ; Receptors, AMPA