DEVELOPMENTAL REGULATION OF MESSENGER-RNAS ENCODING RAT-BRAIN KAINATEAMPA RECEPTORS - A NORTHERN ANALYSIS STUDY

Functionally diverse lpha-amino-3-hydroxy-5-methyl-4-isoxazolepropioni c acid (AMPA) receptors are generated by assembly of glutamate recepto r (GluR)1, 2, and 3 subunits into homomeric and heteromeric channels. We examined GluR1, 2, and 3 gene expression in embryonic, neonatal, an d adult rat brain by northern analysis under conditions of high string ency. In the adult, hybridization to a GluR1 riboprobe revealed the pr esence of an abundant RNA species, 5.2 kb in size, and minor bands of 3.2 and 3.9 kb. GluR2 hybridized to two species, 3.9 and 5.9 kb, of co mparable abundance, presumably attributable to alternate splice produc ts. Hybridization to the GluR3 riboprobe showed a major species of 5.2 kb. This pattern of RNA species was invariant over all the brain regi ons examined. Examination of GluR expression in development revealed t hat in the postnatal period, GluR1, 2, and 3 mRNAs are regulated as a function of age. In adult rat brain, GluR1 and 2 mRNA expression was h ighest in hippocampus; GluR3 was expressed at highest density in hippo campus and frontal cortex. The three transcripts were first detected a t embryonic day 16 and then exhibited changes in expression levels in a region-specific manner. In hippocampus, all three transcripts exhibi ted elevated expression in the late neonatal period; in frontal cortex , elevated expression was observed for GluR2 and 3 only. In striatum, all three transcripts were expressed at relatively low levels througho ut development, with a modest peak at postnatal day 14. In cerebellum, the GluR1 mRNA level was high from postnatal day 28 to adult. Studies of recombinant receptors indicate that GluR1 and 3 subunits form chan nels that are Ca2+ permeable; heteromeric receptors containing the Glu R2 subunit are Ca2+ impermeable. These observations raise the possibil ity that a modification of GluR2 expression during development may reg ulate the level of Ca2+-permeable kainate channels. We therefore inves tigated the expression of the GluR2 subunit relative to those of the G luR1 and 3 subunits as a function of age. In hippocampus and frontal c ortex, the (GluR1 + GluR3)/GluR2 ratio declined with increasing age. T his result provides evidence for a developmental regulation of kainate /AMPA receptor gene expression in the brain during the first 2 weeks o f postnatal life and predicts a decrease in glutamate-operated Ca2+ pe rmeability mediated through kainate/AMPA receptors as a function of ag e.