Ontogeny of kainate receptor gene expression in the developing rat midbrain and striatum

Kainate (KA) receptors are a family of ionotropic glutamate receptors, which mediate the excitatory synaptic transmission in various areas of the mammalian CNS. We have studied the expression pattern of the genes encoding for KA receptor subunits (Glur5-1, Glur5-2, Glur6, Glur7, KA1 and KA2) in rat prenatal (E), postnatal and adult ventral mesencephalon (MES) and striatum (STR) and in fetal midbrain primary cultures. Each receptor subunit shows a unique area- and temporal-expression pattern. In MES the onset of both Glur5 subunits is delayed when compared to the other subunits. In addition, most of the transcripts for KA subunits gradually increase during embryonic development and show a slight decrease during the first postnatal week. Differently, Glur6 and KA2 mRNAs show a sharp increase at E14.5 and decrease thereafter, reaching the lowest levels during late embryonic and postnatal development. In the STR, the gene expression of all KA subunit mRNAs is higher during embryonic development than after birth, except KA1 transcripts, that show a peak at P5. In embryonic MES primary cultures, Glur5-2, Glur6 and KA2 mRNAs are higher at the beginning of the culture when compared to older cultures, while the other subunit mRNAs do not show significant variation throughout the days in vitro. Thus, all the KA receptor subunit transcripts appear independently regulated during MES and STR development, probably contributing to the establishment of the fine tuning of the excitatory circuits reciprocally established between these CNS areas.