The excitation of frog motoneurones in vitro by the glutamate analogue, DI-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA), and the effect of amino acid antagonists

Effects of acromelic acid A on the binding of [3H]glutamic acid and [3H]kainic acid to synaptic membranes and on the depolarization at the frog spinal cord

Triton treatment of synaptic membranes from the frog spinal cord enhanced the specific binding of [3H]glutamic acid compared with non-treated fresh and frozen ones, but not that of [3H]kainic acid. Acromelic acid A specifically inhibited the binding of [3H]kainic acid, and was approximately 100 times more potent than kainic acid. Acromelic acid A and excitatory amino acids caused a depolarization in the ventral root of the frog spinal cord in a dose-dependent manner, and the effect of acromelic acid A was much superior to that of kainic acid or domoic acid. Acromelic acid A is one of the most potent kainic acid agonist at the frog spinal cord.

Characterization of quisqualate recognition sites in rat brain tissue using DL-[3H]alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and a filtration assay

The binding of [3H]AMPA (DL-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid), a ligand for the putative quisqualate excitatory amino acid receptor subtype, was evaluated using centrifugation and filtration receptor binding techniques in rat brain crude synaptosomal membrane preparations. Maximal specific binding of [3H]AMPA occurred in Triton X-100 treated membranes in the presence of the chaotropic agent potassium thiocyanate (KSCN). The effects of KSCN on binding were reversible and optimal at 100 mM. Supernatant obtained from detergent-treated membranes inhibited specific [3H]AMPA and [3H]kainic acid binding, suggesting the presence of an inhibitory agent which was tentatively identified as glutamate. Using centrifugation, saturation analysis revealed two distinct binding sites in both the absence and presence of KSCN. The chaotrope was most effective in increasing binding at the low affinity binding site, enhancing the affinity (Kd) without a concommitant change in the total number of binding sites. Using filtration, a single binding site was detected in Triton-treated membranes. Like the data obtained by centrifugation, KSCN enhanced the affinity of the receptor (Kd value = 10 nM) without altering the number of binding sites (Bmax = 1.2 pmol/mg protein). The rank order of potency of various glutamate analogs in the [3H]AMPA binding assay was quisqualate greater than AMPA greater than L-glutamate greater than kainate greater than D-glutamate, consistent with the labeling of a quisqualate-type excitatory amino acid receptor subtype. L-glutamic acid diethylester, and 2-amino-7-phosphonoheptanoic acid (AP7) were inactive. The present technique provides a rapid, reliable assay for the evaluation of quisqualate-type excitatory amino acid agonists and/or antagonists that may be used to discover more potent and selective agents.

AMPA is a powerful neurotoxin in the chicken retina

Intravitreal (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazoleacetic acid (AMPA) is a powerful excitotoxin on the chicken retina. Doses of 10 nmol/eye produce half-maximum destruction of cholinergic amacrine cells, making AMPA equipotent with kainic acid. The effects of AMPA can be distinguished from those of kainic acid morphologically, and from those of N-methyl-D-aspartic acid by the failure of 2-amino-5-phosphonopentanoic acid (2-AP5) to block those of AMPA. Both morphologically, and in response to 2-AP5, the effects of AMPA and quisqualic acid are indistinguishable, but AMPA is much more potent than quisqualic acid, presumably due to the uptake and inactivation of quisqualic acid.

Characterization of quisqualate-type L-glutamate receptors in the retina

In the vertebrate retina excitatory transmission seems to be mediated mainly by excitatory amino acids; glutamate and/or aspartate are the most viable candidates to subserve this function. Postsynaptic receptors for N-methyl-D-aspartate (NMDA), kainate (KA), quisqualate (QA) and 2-amino-4-phosphonobutyric acid have been electrophysiologically identified. In this work we have tried to identify and characterize QA receptors through the binding of the most specific analogue available for this receptor: [3H]alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid ([3H]AMPA). AMPA binding to retinal membranes was sodium- and temperature-independent, with optimum pH at 6-7. Ligand-receptor interaction was reversible and saturable. Pharmacologically, glutamate analogues were more active displacers than NMDA analogues: AMPA greater than (RS)-3-hydroxy-4,5,6,7-tetrahydro-isoxazolo-(5,4-C)-pyridine-7-car boxylic acid = L-Glu = QA; with IC50 in the low microM range. Glutamic acid diethylester was uneffective while KA and cis-2,3-piperidine dicarboxylate were potent inhibitors of binding. Binding was stereospecific, L-isomers being more effective displacers than D-forms. Subcellular distribution showed binding concentrated in the inner plexiform layer (IPL), but also present in the outer plexiform layer (OPL). Kinetics of [3H]AMPA binding showed a high affinity kB = 1-2 microM in membranes from complete retina, IPL and OPL, with binding sites concentrated in P2 (Bmax = 16.2 pmol/mg protein). Our results provide biochemical evidence for the presence and distribution of physiologically relevant QA receptors in the chick retina which is in agreement with previous physiological findings.

Pharmacological study of the cortical-induced excitation of subthalamic nucleus neurons in the rat: evidence for amino acids as putative neurotransmitters

Extracellular records were made from subthalamic nucleus neurons during microiontophoretic application of drugs and stimulation of the corticosubthalamic nucleus pathway. In 87% of the subthalamic nucleus cells, cortical stimulation induced a powerful excitation, consisting of a burst of 1-7 spikes. This projection must arise from a large area of the cortex since stimulation of nearly all the ipsilateral cortex and the rostral two-thirds of the contralateral cortex was found to influence the activity of subthalamic nucleus neurons. Experiments were undertaken in order to determine the identity of the neurotransmitter involved in the corticosubthalamic nucleus pathway. Glutamic acid diethyl ester reversibly suppressed subthalamic nucleus excitations induced by ipsi- or contralateral cortical stimulation or microiontophoretically applied glutamate. On the same cells, this compound had no effect on acetylcholine-evoked excitation and gamma-aminobutyric acid-evoked inhibition and subthalamic excitation induced by stimulation of the tegmenti pedunculopontine nucleus. Atropine at doses which antagonized the acetylcholine response, flupenthixol at dose which antagonized the dopamine response, and bicuculline at doses which antagonized the gamma-aminobutyric acid response failed to block excitations evoked by cortical stimulation and by glutamate. These experiments excluded a role for acetylcholine, dopamine and gamma-aminobutyric acid in the cortically evoked excitation of subthalamic nucleus cells. Since an amino acid seemed to play a role as neurotransmitter of the corticosubthalamic nucleus pathway, further experiments were designed to confirm these data and to determine the contribution of each amino acid receptor type in the cortical-induced excitation of subthalamic cells. All the subthalamic cells recorded were also excited by microiontophoretically applied N-methyl-D-aspartic, quisqualic and kainic acids. The cortical-evoked activation of subthalamic nucleus neurons was reversibly suppressed by kynurenic acid and cis-2,3-piperidine dicarboxylic acid, two broad-spectrum antagonists of excitatory amino acids, microiontophoretically applied at doses which also blocked excitations induced by N-methyl-D-aspartic, quisqualic and kainic acids. Application of 2-amino-5-phosphonovaleric acid inhibited excitation induced by N-methyl-D-aspartic acid but not those elicited by quisqualic or kainic acid, while glutamate excitation was only slightly affected. This compound had no effect on the cortically evoked excitation of subthalamic nucleus neurons.(ABSTRACT TRUNCATED AT 400 WORDS)