To study mGluR6 expression, the authors investigated two transgenic mouse lines that express enhanced green fluorescent protein (GFP) under control of mGluR6 promoter. In retina, GFP was expressed exclusively in all ON bipolar cell types, either uniformly across all cells of this class (line 5) or in a mosaic (patchy) fashion (line 1). In brain, GFP was found in certain cortical areas, superior colliculus, axons of the corpus callosum, accessory olfactory bulb, and cells of the subcommissural organ. Outside the nervous system, GFP was seen in the corneal endothelium, testis, the kidney's medulla, collecting ducts and parietal layer that surround the glomeruli, and B lymphocytes. Furthermore, RT-PCR showed that most tissues that expressed GFP in the transgenic mouse also transcribed two splice variants of mGluR6 in the wild-type mouse. The alternate variant was lacking exon 8, predicting a protein product of 545 amino acids that lacks the 7-transmembrane domains of the receptor. In cornea, immunostaining for mGluR6 gave strong staining in the endothelium, and this was stronger in wild-type than in mGluR6-null mice. Furthermore, calcium imaging with Fura-2 showed that application of L-AP4, an agonist for group III metabotropic glutamate receptors including mGluR6, elevated calcium in endothelial cells.