Thiocyanate ions inhibit AMPA-activated currents in recombinant non-NMDA receptors expressed in Xenopus laevis oocytes: the role of the GluR2 subunit

Dual roles in feeding for AMPA/kainate receptors: receptor activation or inactivation within distinct hypothalamic regions elicits feeding behavior

We have previously shown that hypothalamic injections of glutamate, or agonists of its ionotropic receptors (iGluRs), elicit intense feeding responses in satiated rats [Brain Res. 613 (1993) 88, Brain Res. 630 (1993) 41]. While attempting to clarify the role of the AMPA and kainate (KA) receptor subtypes in glutamatergic feeding systems, we discovered that lateral hypothalamic (LH) injection of high doses of the competitive AMPA/KA receptor antagonist, NBQX (10 and 30 nmol), elicited a pronounced feeding response. We questioned whether this effect was due to inactivation of AMPA or possibly KA receptors. To determine whether other AMPA/KA antagonists can also elicit feeding, we tested whether injection of CNQX, another AMPA/KA receptor antagonist, also stimulates eating and whether these feeding stimulatory effects were due to antagonists' actions in the LH or in other hypothalamic sites. Here we report that NBQX and CNQX elicit feeding in a dose dependent manner and are most effective when injected into the perifornical hypothalamus (PFH), or into the paraventricular nucleus (PVN) and, to a lesser extent, into the LH of satiated rats. In contrast, AMPA was most effective in stimulating feeding when injected into the LH, confirming previous reports. These data suggest that either activation or inactivation of AMPA/KA receptors in distinct but overlapping hypothalamic sites may be sufficient to induce feeding behavior, indicating a broadened role for glutamate in hypothalamic feeding mechanisms.

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.

Characterization of AMPA receptor populations in rat brain cells by the use of subunit-specific open channel blocking drug, IEM-1460

Dicationic adamantane derivative, IEM-1460, which selectively blocks GluR2-lacking, Ca2+-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors, was used to characterize the distribution of AMPA receptors among populations of rat brain cells. IEM-1460 inhibited kainate-induced inward currents (at -80 mV) in a dose-dependent manner. IEM-1460 concentrations producing 50% inhibition of kainate-induced current amplitude (IC50) varied greatly depending on the cell type studied. Striatal giant cholinergic interneurons and putative Bergmann glial cells isolated from the cerebellum were found to be highly sensitive to IEM-1460 block (IC50=2.6 microM), indicating the expression of GluR2-lacking AMPA receptor subtype. Among hippocampal and cortical non-pyramidal neurons, there were cell-to-cell differences in the pattern of AMPA receptor subtype expression. Some cells which are known to express AMPA receptors lacking GluR2 subunit exhibited high sensitivity of IEM-1460 block (IC50 about 1 microM) but in the others, the part of AMPA receptor population seemed to be represented by GluR2-having receptor subtype. The latter subtype was mainly expressed by pyramidal neurons isolated from hippocampus (IC50=1102 microM) and sensorimotor cortex (IC50=357 microM) which showed low affinity for IEM-1460 block. In conclusion, IEM-1460 can be utilized as an indicator of the distribution of AMPA receptor subtypes among populations of rat brain cells, and pharmacological detection of the absence of GluR2 subunit in AMPA receptor assembly can provide useful information for the interpretation of physiological events.

Allosteric regulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate receptors by thiocyanate and cyclothiazide at a common modulatory site distinct from that of 2,3-benzodiazepines

Allosteric regulators of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) receptors include 2,3-benzodiazepines such as GYKI 52466 and GYKI 53655 and the chaotropic anion thiocyanate that inhibit, and benzothiadiazines such as cyclothiazide that potentiate AMPA receptor currents. Here we sought to determine whether the allosteric regulators modulate AMPA receptors at a common or distinct allosteric sites by comparing their actions on AMPA- and kainate-evoked currents in cultured rat hippocampal neurons and Xenopus oocytes expressing recombinant AMPA receptor subunits. GYKI 52466 and thiocyanate blocked AMPA-evoked currents in a concentration-dependent manner (IC50 values, 8.2 microM and 1.1 mM, respectively); in contrast, kainate-evoked currents were blocked by GYKI 52466, but were potentiated by high concentrations of thiocyanate (> or = 3 mM). Thiocyanate enhanced the rate of desensitization and slowed recovery from desensitization of AMPA-evoked currents, whereas GYKI 52466 failed to affect desensitization. Among neurons in the hippocampal cultures, there was cell-to-cell variability in the sensitivity to block of AMPA-evoked currents by thiocyanate that was correlated with the degree of potentiation by cyclothiazide. Moreover, cyclothiazide caused a parallel rightward shift in the concentration-response curve for thiocyanate block, and slowed the onset of thiocyanate block to a rate that was similar to that of cyclothiazide dissociation. Together, these observations suggest that thiocyanate and cyclothiazide act at non-distinct allosteric sites. GYKI 52466 blocked AMPA receptor responses to a similar extent, irrespective of the degree of cyclothiazide potentiation. Moreover, the kinetics of GYKI 53655 block in the presence of cyclothiazide were not consistent with a competitive interaction. As is the case for cyclothiazide, SCN- exhibited greater affinity for flip than for flop AMPA receptor splice variants. In particular, GluR1flip/GluR2flip was especially sensitive to thiocyanate block. We conclude that thiocyanate, a flip-preferring allosteric modulator like cyclothiazide, appears to act by enhancing desensitization at a site that may overlap the site where cyclothiazide reduces desensitization, whereas 2,3-benzodiazepines act at a distinct site and the block does not involve a modification of desensitization.