Differences between D- and L-aspartate binding to the Na+-dependent binding sites on glutamate transporters in frozen sections of rat brain.

The Na+-dependent, "high-affinity" transport of L-glutamate (GluT) in brain tissue has become a significant focus of interest, particularly since it has been revealed that abnormalities of GluT may be associated with serious neurological disorders. Using quantitative autoradiography on 3H-sensitive films, we have studied, in thaw-mounted sections of rat brain, the distribution and pharmacology of radioligand binding to sites with characteristics of the substrate-recognition/binding locus on GluT. The technique makes it possible to determine not only the intensity of binding in brain regions but, with a high level of precision, pharmacological constants such as IC50 or nH. [3H]L-aspartate and [3H]D-aspartate are two classical radioligands used in studies of GluT. We have determined IC50 values for the inhibition of [3H]L- and [3H]D-aspartate binding by their non-radioactive counterparts in the cerebral neocortex. hippocampus, striatum, septal nuclei and the cerebellar cortex. The two radioligands did not appreciably differ from each other in their interactions with the binding sites in the forebrain, consistent with all Na+-dependent GluT binding sites in that region having no stereoselectivity for aspartate enantiomers. In the cerebellar cortex, however, the data indicated the presence of a GluT binding site that preferred L- over D-aspartate. These findings contrast with many previous observations and suggest that the pharmacological characteristics of the ligand binding sites on GluT in the mammalian cerebellar cortex may have to be re-assessed and/or a possibility of an existence of (a) hitherto unknown molecule(s) with properties of a glutamate transporter be considered.

Preparative electrophoresis: a general method for the purification of polyclonal antibodies.

Antibodies were purified from normal rabbit, sheep, goat, rat, human and bovine serum using preparative electrophoresis on a Gradiflow in a single-step process using an asymmetrical cartridge with three different pore size polyacrylamide membranes. Recoveries in each case were over 80% and were higher than those obtained using affinity chromatography on protein A, protein G or protein L. Degree of purity was at least comparable with these methods. These results suggest that preparative electrophoresis can be considered a general method for the purification of research quantities of antibodies from multiple serum sources and may be particularly useful where the reactivity with protein A, G or L is unknown.

Regional distribution and pharmacological characteristics of [3H]N-acetyl-aspartyl-glutamate (NAAG) binding sites in rat brain.

Autoradiographical studies revealed that 10 nM [3H]N-acetyl-aspartyl-glutamate (NAAG) labelled grey matter structures, particularly in the hippocamus, cerebral neocortex, striatum, septal nuclei and the cerebellar cortex. The binding was inhibited by (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)-glycine (DCG IV), an agonist at group II metabotropic glutamate receptors (mGluR II). (RS)-alpha-Methyl-4-tetrazolylphenylglycine (MTPG), (RS)-alpha-cyclopropyl-4-phosphonoglycine (CPPG) and (RS)-alpha-methylserine-O-phosphate monophenyl ester (MSOPPE), all antagonists at mGluR II and mGluR III, also inhibited [3H]NAAG binding. Other inhibitors were (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate (ACPD), a broad-spectrum mGluR agonist with preference for groups I and II and the mGluR I agonists/mGluR II antagonists (S)-3-carboxy-4-hydroxyphenylglycine (3,4-CHPG) and (S)-4-carboxy-3-hydroxyphenylglycine (4,3-CHPG). Neither the mGluR I specific agonist (S)-dihydroxyphenylglycine nor any of the ionotropic glutamate receptor ligands such as kainate, AMPA and MK-801 had strong effects (except for the competitive NMDA antagonist CGS 19755, which produced 20-40% inhibition at 100 microM) suggesting that, at low nM concentrations, [3H]NAAG binds predominantly to metabotropic glutamate receptors, particularly those of the mGluR II type. Several studies have indicated that NAAG can interact with mGluR II and the present study supports this notion by demonstrating that sites capable of binding NAAG at low concentrations and displaying pharmacological characteristics of mGluR II exist in the central nervous tissue. Furthermore, the results show that autoradiography of [3H]NAAG binding can be used to quantify the distribution of such sites in distinct brain regions and study their pharmacology at the same time.