Chloride-dependent binding sites for L-[3H]glutamate on dendrodendritic synaptosomal membranes of rat olfactory bulb

✍ Scribed by Dr. M.R. Quinn; P.A. Spraguer

• Publisher: John Wiley and Sons
• Year: 1986
• Tongue: English
• Weight: 601 KB
• Volume: 16
• Category: Article
• ISSN: 0360-4012
• DOI: 10.1002/jnr.490160208

✦ Synopsis

Dendrodendritic synapses occur between granule cell dendrites and secondary dendrites of mitral cells within the olfactory bulb and are attainable in a subcellular fraction (DDS). Since the mitral cells are thought to utilize an excitatory amino acid as a neurotransmitter, we determined the pharmacologic specificity of Nafindependent L-[3H]glutamate binding to fresh membranes of DDS in 50 mM Tris-HC1, pH 7.1. Binding of L-glutamate to membranes of DDS was specific, C1-dependent, and saturable. Scatchard plots were analyzed by nonlinear regression analyses using the computer program LIGAND, and the data was best-fitted to a one-site model with KD of 0.56 + 0.04 pM and an apparent B,,, of 48 * 5 pmol/mg protein. Hill plots also indicated the presence of one site and no cooperativity (nH = 0.99 * 0.03). However, the relative effectiveness of several compounds in inhibiting L-glutamate binding to membranes of DDS clearly demonstrated the presence of more than one site. Electrophysiological studies suggest that 2-amino-4-phosphonobutyrate (APB) is a potent antagonist of evoked responses elicited by stimulation of mitral cell axons and that quisqualate is a potent agonist; both of these compounds were highly effective inhibitors of Lglutamate binding to DDS membranes. APB displaced about 70% of the sites labeled with 200 nh4 L-glutamate with a KI of 1.6 pM, whereas quisqualate inhibition of L-glutamate binding yielded a line that was curvilinear in the Scatchard plot and was resolved into two sites of relatively high affinity (KI values of 0.02 and 0.65 pM). Several other compounds, including N-methyl-D-aspartate and kainate, were relatively ineffective inhibitors. Although a small amount of C1--independent L-glutamate binding was demonstrable in the preparation, C1-dependent sites predominated and were stimulated by calcium. These results suggest an important role of L-glutamate, or a closely related compound, at dendrodendritic synapses of the olfactory bulb.