PRESENCE OF MESSENGER-RNA FOR GLUTAMIC-ACID DECARBOXYLASE IN BOTH EXCITATORY AND INHIBITORY NEURONS

Neurons in very low density hippocampal cultures that are physiologica lly identified as either GABAergic inhibitory or glutamatergic excitat ory all contain mRNA for the gamma-aminobutyric acid (GABA) synthetic enzyme, glutamic acid decarboxylase (GAD), as detected by single cell mRNA amplification and PCR. However, consistent with the physiology, i mmunocytochemistry revealed that only a subset of the neurons stain fo r either GAD protein or GABA. A similar fraction hybridize with RNA pr obes for GAD65 and GAD67. Hippocampal CA1 pyramidal neurons in slice p reparations, which are traditionally thought to be excitatory, also co ntain mRNA for GAD65 and GAD67. Hippocampal neurons in culture did not contain mRNA for two other neurotransmitter synthesizing enzymes, tyr osine hydroxylase, and choline acetyl transferase. These data suggest that in some neurons, presumably the excitatory neurons, GAD mRNA is s electively regulated at the level of translation. We propose that neur otransmitter phenotype may be posttranscriptionally regulated and neur ons may exhibit transient phenotypic plasticity in response to environ mental influences.