Autoradiographic visualization of group III metabotropic glutamate receptors using [3H]-L-2-amino-4-phosphonobutyrate

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Abstract
1. In vitro receptor autoradiography using [3H]-L-2-amino-4-phosphonobutyrate ([3H]-L-AP4) binding to sections of rat brain has been characterized and shown to most likely represent labelling of group III metabotropic glutamate receptors. 2. Specific [3H]-L-AP4 binding to rat brain sections was observed at high densities in the molecular layer of the cerebellar cortex and the outer layer of the superior colliculus. Moderate levels were observed throughout the cerebral cortex, in the molecular layer of the hippocampal dentate gyrus, in thalamus, striatum, substantia nigra and in the medial geniculate nucleus. Low levels of [3H]-L-AP4 binding were found in other regions of the hippocampal formation, in the entorhinal cortex and the granule cell layer of cerebellum. 3. Inhibitors of sodium- or calcium/chloride-dependent glutamate uptake did not displace [3H]-L-AP4 binding to rat brain sections indicating that the observed binding does not represent [3H]-L-AP4 uptake via these carriers. Furthermore, in contrast to [3H]-L-AP4 uptake into cerebellar membranes, [3H]-L-AP4 binding to brain sections was sensitive to guanosine-5'-O-(3-thio)trisphosphate-gamma-S. 4. In the molecular layer of the cerebellar cortex, [3H]-L-AP4 binding showed a maximal binding density (Bmax) of 0.52+/-0.06 pmol mg(-1) tissue and an affinity (Kd) of 346 nM. The rank order of affinity for displacement of [3H]-L-AP4 binding to rat brain sections was: L-AP4 > L-serine-O-phosphate > glutamate > (L)-2-aminomethyl-4-phosphonobutanoate > (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate which is in agreement with a group III metabotropic glutamate receptor pharmacology.