Differentiation of L-aspartate and L-glutamate high-affinity binding sites in a protein fraction isolated from rat cerebral cortex

Localization of putative transmitters in the hippocampal formation: with a note on the connections to septum and hypothalamus

Biochemical assays on microdissected samples, denervation studies, subcellular fractionation, and light and electron microscopic autoradiography of high affinity uptake have been performed to study the cellular localization of transmitter candidates in the rat hippocampal formation. High affinity uptake of glutamate and aspartate is localized in the terminals of several excitatory systems, such as the entorhino-dentate fibres (perforant path), mossy fibres (from granular cells) and pyramidal cell axons. Thus, in stratum radiatum and oriens of CA1, 85% of glutamate and asparate uptake and 40% of glutamate and aspartate content are lost after lesions of ipsilateral plus commissural fibres from CA3/CA4. Hippocampal efferents also take up aspartate and glutamate, since these activities are heavily reduced in the lateral septum and mamillary bodies after transection of fimbria and the dorsal fornix. The synthesis (by glutamic acid decarboxylase), content and high affinity uptake of gamma-aminobutyrate (GABA) are not reduced after lesions of these or other projection fibre systems. A localization in intrinsic neurons is confirmed by a selective loss of glutamic acid decarboxylase after local injections of kainic acid. Peak concentrations of the enzyme occur near the pyramidal and granular cell bodies, corresponding to the site of the inhibitory basket cell terminals, and in the outer parts of the molecular layers. Some 85% of glutamic acid decarboxylase is situated in 'nerve ending particles'. Acetylcholine synthesis (by choline acetyltransferase) disappears after lesions of septo-hippocampal fibres. Since 80% of the hippocampal choline acetyltransferase is in 'nerve ending particles', the characteristic topographical distribution of this enzyme should reflect the distribution of cholinergic septo-hippocampal afferents. Serotonin, noradrenaline, dopamine and histamine are located/synthesized in afferent fibre systems. Some monoamine-containing afferents to the hippocampal formation pass via the septal area, others via the amygdala. The hippocampal formation also contains nerve elements reacting with antibodies against neuroactive peptides, such as enkephalin, substance P, somatostatin and gastrin/cholecystokinin.

Inhibition by N-acetyl-aspartate of aspartate binding to a proteolipid fraction from rat cerebral cortex

Many roles have been suggested for N-acetyl-aspartate in brain function because of it being located almost exclusively in that organ. However, its true role remains to be demonstrated. We show here that N-acetyl-aspartate: 1) binds to a hydrophobic protein fraction from the cerebral cortex of the rat, which specifically binds L-aspartate, L-glutamate, and gamma-amino-butyric acid; and 2) has a marked inhibitory effect on the aspartate binding sites of this proteolipid fraction. Structural analogs of N-acetyl-aspartate, i.e. N-carbamyl-aspartate and N-methyl-aspartate also inhibit the L-aspartate binding by the brain protein fraction used.

Receptors for the excitatory amino acids in the mammalian central nervous system

On the basis largely of neuropharmacological analysis, three different receptors mediating neuronal excitation can be identified. The first is activated by quisqualate and other "flexible" molecules including L-glutamate and appears to bind its ligands in a folded configuration. The second is excited by NMDA and has a more extended conformation, the spacing between the amino and the omega-carboxylate groups being the determinant of specificity. The third type accepts kainate and appears to possess a reactive site for the unsaturated side chain which is essential to the operation of this receptor. All three classes appear to be implicated in synaptic events [although some kainate receptors at least are certainly extra-synaptic (Watkins et al., 1981)] and each appears to activate different ionophores in neuronal membranes. Of the endogenous amino acids which may function as synaptic transmitters, L-glutamate and L-cysteate seem to react preferentially with quisqualate receptors (McLennan and Lodge, 1979), while L-aspartate is more of a mixed agonist capable of reaction both with quisqualate and with the NMDA types. Whether folate has a physiological role involving kainate receptors is unknown; and the same is true of any action possessed by quinolinate. The fact that there are amino acid excitants which are pharmacologically distinct from those reacting with any of the three best known receptors suggests that at least one more class of receptor may also exist, but no further information is available at the present time. Other sites with which the pharmacologically active acidic amino acids react are identifiable neurochemically in membrane preparations derived from tissues of the central nervous system. Kinetic studies and analysis of inhibition of sodium-independent binding indicate that there are sites which accept glutamate, others binding aspartate and a third which binds kainate. However, the first does not correspond completely to the quisqualate excitatory receptor, and NMDA does not react with any of the binding sites. It is difficult to conclude then that any of these binding sites can be fully identified with the excitatory receptors. Finally, there are a number of systems which in their patterns of activity again appear completely distinct, but which presumably mediate uptake of amino acids.(ABSTRACT TRUNCATED AT 400 WORDS)

A comparison of sodium-dependent glutamate binding with high-affinity glutamate uptake in rat striatum

The specific sodium-dependent binding of [3H]glutamate to membranes of the rat striatum was examined and a comparison made with high affinity glutamate uptake. In the presence of sodium, [3H]glutamate binding was saturable and of high affinity. No binding could be detected in the absence of sodium. Removal of the cortical afferents to the striatum resulted in a parallel decrease in Na+-dependent glutamate binding and in high-affinity glutamate uptake. After the injection of the neurotoxin kainic acid into the striatum, the density of Na+-dependent glutamate binding sites was reduced by 40%, while high-affinity uptake showed no significant decrease. Drugs which inhibit high-affinity uptake were also effective at inhibiting Na+-dependent binding. The results suggest that about half the Na+-dependent glutamate binding sites in the striatum represent high-affinity uptake sites on the corticostriatal terminals. The remainder of the binding sites are located on striatal neurons and may also be uptake sites.

The convulsant drugs 3-mercaptopropionate and methionine sulfoximine inhibit L-glutamate and L-aspartate binding to a hydrophobic protein fraction from rat cerebral cortex

The action of the convulsant drugs, methionine sulfoximine (MSO), 3-mercaptopropionate (3-MP), megimide (MG), and allylglycine on the binding of L-[14C]aspartate, L-[14C]glutamate and [14C]GABA to a hydrophobic protein fraction isolated from rat cerebral cortex was studied. Using the convulsant at 10(-4) M concentration and the radioactive ligands as 10(-6) M the binding of L-[14C]glutamate was inhibited 60% by 3-MP and 40% by MSO, while MG and allylglycine had no effect. The binding of L-[14C]aspartate was inhibited 55%, and 10--20% by 3-MP and MSO, respectively, while MG and allylglycine had not effect. None of the drugs mentioned, except for a minimal inhibition by MG, altered the binding of [14C]GABA. Neither MSO nor 3-MP affected the high-affinity sites for L-[14C]glutamate of L-[14C]aspartate, but they had a strong inhibitory action on the medium affinity site. These results are discussed in relation to the possible mechanism of action of these drugs on L-glutamate and L-aspartate receptors.