A variant of metabotropic glutamate receptor subtype 5: an evolutionally conserved insertion with no termination codon.

We herein report the presence of a variant of the rat metabotropic glutamate receptor subtype 5 (mGluR5) cDNA. In this variant, named mGluR5b, an extra 96 bp sequence is inserted into the originally reported mGluR5 cDNA. The position of this insertion corresponds to that of the 85 bp insertion reported for the alternative form of mGluR1, another subtype of the receptors (designated as mGluR1 beta). No sequence-similarity was found between the additional sequences of the mGluR1 beta and the mGluR5b. While the additional sequence of the mGluR1 beta cDNA contains a translation termination codon in the frame, that of the mGluR5b cDNA does not. This identical 96 bp insertion was also found in a human mGluR5 cDNA clone. Northern blot analyses with the mGluR5b-specific probe showed that the expression of mRNA for this species is regulated in a region-specific manner in the rat brain.