METABOTROPIC GLUTAMATE-RECEPTOR HETEROGENEITY IN RAT-BRAIN

Metabotropic glutamate receptors (mGluRs) are G protein-linked recepto rs that operate through the formation of different second messengers. Utilizing quantitative autoradiographic techniques, we have characteri zed [H-3]glutamate binding to mGluRs in discrete regions of adult rat brain. [H-3]Glutamate binding, in the presence of high concentrations of alpha-amino-3-hydroxymethyl-4-isoxazolepropionic acid (10 mu M), N- methyl-D-aspartate (100 mu M), and 2.5 mM calcium chloride (CaCl2), wa s saturable. Scatchard plots were linear in all regions examined and r evealed similar affinity constants of about 500 nM. The largest number of sites was found in the outer cerebral cortical layers (10 pmol/mg of protein). [H-3]Glutamate binding was displaced by quisqualate, tran s-1-amino-1,3-cyclopentane dicarboxylic acid (t-ACPD) (racemic mixture ), and (1S,3R)-ACPD but not by (1R,3S)-ACPD. The guanine nucleotide an alogue guanosine-5'-O-(3-thio) triphosphate (100 mu M) reduced the bin ding by affecting the affinity but not the total number of sites, as p redicted for G protein-coupled receptor sites. Quisqualate displacemen t curves were always biphasic and resolved two binding sites, with K-i values in the low nanomolar (15 nM) and micromolar (63 mu M) ranges. (1S,3R)-ACPD displaced [H-3]glutamate binding both in the absence and in the presence of 2.5 mu M quisqualate, suggesting that both high and low affinity quisqualate sites are linked to mGluRs. (1S,3R)-ACPD com petition curves were broad (Hill coefficient = 0.73) but monophasic un der both conditions, with K-i values in the micromolar range (14-116 m u M), suggesting that (1S,3R)-ACPD acts on the two quisqualate sites w ith similar apparent affinities. The regional distributions of the two sites were different. The highest levels of the high affinity quisqua late binding site were found in the cerebellar molecular layer. The hi ghest levels of the low affinity quisqualate binding sites were found in the outer cerebral cortex. The pharmacological profile and regional distribution suggest that the high and low affinity quisqualate-sensi tive components of [H-3]glutamate binding sites might correspond to th e mGluR1/mGluR5 and mGluR2/mGluR3 subgroups of cloned mGluRs, respecti vely.