Molecular target size of an L-glutamate receptor from the cockroach central nervous system

Insect neurotransmission: neurotransmitters and their receptors

The roles of acetylcholine, dopamine, octopamine, tyramine, 5-hydroxytryptamine, histamine, glutamate, 4-aminobutanoic acid (gamma-aminobutyric acid) and a range of peptides as insect neurotransmitters are evaluated in terms of the criteria used to identify transmitters. Of the biogenic amines considered, there is good evidence that acetylcholine, dopamine, octopamine, 5-hydroxytryptamine, and histamine should be considered to be neurotransmitters, but the case for tyramine is less convincing at the moment. The evidence supporting neurotransmitter roles for glutamate and gamma-aminobutyric acid at specific insect synapses is overwhelming, but much work remains to be undertaken before the full significance of these molecules in the insect nervous system is appreciated. Attempts to characterise biogenic amine and amino acid receptors using pharmacological and molecular biological techniques have revealed considerable differences between mammalian and insect receptors. The number of insect neuropeptides isolated and identified has increased spectacularly in recent years, but genuine physiological or biochemical functions can be assigned to very few of these molecules. Of these, only proctolin fulfills the criteria expected of a neurotransmitter, and the recent discovery of proctolin receptor antagonists should enable the biology of this pentapeptide to be explored fully.

Receptors for L-glutamate and GABA in the nervous system of an insect (Periplaneta americana)

The nervous system of the cockroach Periplaneta americana is well suited to studies of invertebrate amino acid receptors. Using a combination of radioligand binding and electrophysiological techniques, several distinct receptors have now been identified. These include an L-glutamate-gated chloride channel which has no known counterpart in the vertebrate nervous system, and a putative kainate/quisqualate receptor with pharmacological properties different from those of the existing categories of vertebrate excitatory amino acid receptors. GABA receptors have also been characterized in the cockroach nervous system. Bicuculline, benzodiazepines and steroids have revealed important differences between certain insect GABA-gated chloride channels and vertebrate GABA receptors. Identifiable neurones may facilitate the allocation of specific functions to amino acid receptor subtypes. In view of the existence of subtypes of amino acid receptors in insects, it is of interest to examine how this is reflected at the molecular level in terms of receptor subunit composition and amino acid sequence. Preliminary molecular cloning studies on insect GABA receptors are described.

Isolation of glutamate-binding proteins from rat and bovine brain synaptic membranes and immunochemical and immunocytochemical characterization

We previously isolated two glutamate-binding proteins from rat brain synaptic membranes of 71,000 and 63,000 mol. wt [Chen et al. (1988) J. biol. Chem. 263, 417-426]. In the present study, the 71,000 and 63,000 mol. wt glutamate-binding proteins were purified from rat and bovine brain synaptic membranes by affinity chromatographic separation on an L-glutamate-derived Trisacryl matrix. The major protein component in the purified fractions was a 71,000 mol. wt protein as judged by sodium dodecyl sulfate gel electrophoresis. This fraction represented enrichment of bovine brain glutamate-binding proteins by a factor of 7000-8000 when compared with brain homogenates. The ligand binding characteristics of the proteins purified by this new procedure are very similar to those of the proteins purified by the procedures we described in previous studies. Polyclonal antibodies were raised in mice against the purified, native rat and bovine brain glutamate-binding proteins. These two sets of antibodies interacted specifically with the glutamate-binding proteins but not with any glutamate-metabolizing enzymes. Both sets of antibodies labeled a 71,000 mol. wt protein in Western blots of synaptic membranes obtained from either rat or bovine brain, an indication of homology between these proteins. Both sets of antibodies produced immunoprecipitation of approximately 70-75% of all glutamate-binding proteins from solubilized synaptic membrane proteins. These observations are an indication that the glutamate-binding proteins described above are the most common glutamate-binding entities in both bovine and rat brain synaptic membranes and that they can be easily purified in a one-step chromatographic procedure.

Solubilization and purification of a putative quisqualate-sensitive glutamate receptor from crustacean muscle

Two high-affinity glutamate-binding proteins have been solubilized and purified from crustacean muscle membranes. L-[3H]Glutamate binding to intact membranes gave IC50 values (concn. giving 50% inhibition) of 0.35 microM for glutamate. 9.0 microM for quisqualate and 36.2 microM for ibotenic acid. Kainate, domoate, N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate, D-glutamate and L-aspartate were poor inhibitors of glutamate binding. The two protein were purified approx. 600-fold from muscle membranes by use of concanavalin A and gel-filtration column chromatography. One protein showed an apparent Stokes radius of 5.4 nm, as determined by gel filtration, with a subunit of Mr 74000 as shown by SDS/polyacrylamide-gel electrophoresis. Analysis of glutamate binding to this protein revealed a KD of 2.4 microM and a Bmax, of 15.5 nmol/mg, and a k1 for quisqualate inhibition of glutamate binding of 1.0 microM. The second protein was eluted anomalously from the gel-filtration column and showed a subunit of Mr 65000 on SDS/polyacrylamide gels. Glutamate binding to this protein showed a KD of 1.3 microM and a Bmax, of 14.5 nmol/mg, and was only slightly sensitive to the presence of quisqualate (Ki = 0.8 mM). Glutamate binding to both proteins was insensitive to kainate. It is concluded that the Mr-74000 protein is a prime candidate for the quisqualate-sensitive receptor of the crustacean neuromuscular junction.