Expression of GluR6/7 subunits of kainate receptors in rat adenohypophysis.

We have previously demonstrated the presence of unidentified [3H]glutamate (Glu) binding sites with stereo-selectivity, high affinity and saturability in rat peripheral excitable tissues such as the pituitary (Yoneda, Y., Ogita, K., 1986a. [3H]Glutamate binding sites in the rat pituitary. Neurosci. Res. 3, 430--435) and adrenal (Yoneda, Y., Ogita, K., 1986b. Localization of [3H]glutamate binding sites in rat adrenal medulla. Brain Res. 383, 387--391, 1986). In this study, peripheral binding sites were further evaluated for the ionotropic Glu receptor subtype insensitive to N-methyl-D-aspartate by using reverse transcription polymerase chain reaction (RT-PCR) and Western blotting, in addition to receptor binding using radiolabeled ligands other than [3H]Glu. Binding of [3H]kainate (KA) and [3H]DL-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate was detected in membrane preparations obtained from the rat pituitary and adrenal irrespective of prior treatment with Triton X-100. An RT-PCR analysis revealed constitutive expression of mRNA for GluR1, GluR3, GluR5, KA1 and KA2 subunits in the rat adrenal and pituitary, as well as the brain and retina. The pituitary also expressed mRNA for GluR2, GluR4, GluR6 and GluR7 subunits in contrast to the adrenal. Under our experimental conditions employed, however, Western blotting assays failed to confirm the expression of receptor proteins for GluR1, GluR2/3 and GluR4 subunits in the adrenal cortex, adrenal medulla, adenohypophysis and neurohypophysis. Immunoreactive GluR6/7 subunits were only detectable in the adenohypophysis, but not in the adrenal cortex, adrenal medulla and neurohypophysis. An intraperitoneal injection of KA doubled DNA binding activity of the nuclear transcription factor activator protein-1 in the rat pituitary, with concomitant more potent potentiation of that in the hippocampus. These results suggest that GluR6/7 subunits of KA receptors may be constitutively expressed with responsiveness to the systemic administration of an agonist at least in the rat adenohypophysis.