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Differential Stimulation of Somatostatin but Not Neuropeptide Y Gene Expression by Quinolinic Acid in Cultured Cortical Neurons

Abstract: Somatostatin (SS) and neuropeptide Y (NPY) are coproduced in a subpopulation of neurons that are selectively resistant to NMDA neurotoxicity. We have previously reported that quinolinic acid (QUIN), an NMDA receptor agonist, augments SS mRNA in cultured fetal rat cortical neurons. This study examines coregulation of SS and NPY by QUIN and NMDA in cultured cortical neurons and compares the effects of these agents with those of forskolin and phorbol 12‐myristate 13‐acetate (PMA), known to activate SS and NPY gene transcription by protein kinase A‐ and protein kinase C‐dependent mechanisms. In addition, transcriptional regulation of the SS gene was investigated by acute transfection of cortical cultures with an SS promoter‐chloramphenicol acetyltransferase (CAT) construct. QUIN and NMDA displayed dose‐dependent fourfold augmentation of levels of mRNA for SS but not for NPY. In contrast, forskolin and PMA increased both SS and NPY mRNA levels. QUIN‐ and NMDA‐mediated induction of SS mRNA was blocked by the NMDA receptor antagonist (−)‐2‐amino‐5‐phosphonovaleric acid and displayed regional brain specificity because it was not observed in fetal hypothalamic cell cultures. In time course studies, the effects of QUIN/NMDA on SS mRNA occurred after a latency of 8 h, indicating a delayed effect. Cortical cells transfected with pSS‐750 CAT showed three‐ to fourfold stimulation of CAT activity with forskolin but not by QUIN or NMDA. These data reveal a dose‐dependent, tissue‐specific, NMDA receptor‐mediated stimulation of SS but not NPY mRNA. Such stimulation of SS gene expression does not require activation of the protein kinase A or protein kinase C signaling pathways, is not transcriptionally mediated, and is likely posttranscriptional, as also suggested by the delay in SS mRNA induction.