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

AMPA receptors mediate most of the fast excitatory synaptic transmission in the mammalian CNS. Their ontogeny during embryonic (E) and postnatal (P) development is still poorly understood. We have studied the expression of the genes encoding for AMPA glutamate receptor subunits (GlurA, GlurB, GlurC and GlurD) in the rat ventral mesencephalon (MES) and striatum (STR) and in fetal midbrain primary cultures. Each receptor subunit shows unique area- and temporal-expression pattern. In MES, GluRA, GlurB and GlurC mRNA are detectable from the earliest embryonic stage studied (E13) and raise thereafter between E15 and E17, to plateau at E19 to adult values. Differently, GlurD mRNA increases throughout embryonic and postnatal development reaching its highest levels in the adult MES. The pattern of AMPA proteins corresponded to the mRNA levels for all subunits. In the STR, GlurA gene expression increases between E15 and E19, GlurB mRNA levels are sustained from the first embryonic stages analyzed (E15) until E19 and gradually decrease thereafter toward adult levels, GlurC gene expression increases gradually throughout ontogeny to reach its highest levels in the adult. STR GlurD transcripts remain at constant levels in all stages studied. In embryonic MES primary cultures, every subunit show a characteristic expression profile similar to that observed in vivo. They all decrease significantly during the second week in vitro. Thus, all the AMPA receptor subunit transcripts appear independently regulated during development, probably depending on the tissue-specific environment, which seems preserved in MES cultures.