STIMULATORY EFFECT OF N-METHYL-D-ASPARTATE ON SOMATOSTATIN GENE-EXPRESSION IN CULTURED HYPOTHALAMIC NEURONS

The aim of the present study was to determine whether N-methyl-D-aspartate (NMDA) stimulates somatostatin gene function in primary cultures of hypothalamic neurons. Neurons were either shortly (for 3, 8, 24 and 72 h) or chronically (for 11 days) exposed to NMDA (20 muM). Medium and cellular somatostatin contents were determined by radioimmunoassay, and steady-state preprosomatostatin mRNA levels by Northern blot analysis with an oligonucleotide probe. DNA content was measured as a cellular viability control. After 8 h incubation, NMDA induced a significant 2-fold increase in somatostatin mRNA accumulation, with a maximal 4-fold increase after 24 h incubation. A significant and dose-dependent (1.7-fold and 2.5-fold at 20 and 100 muM, respectively) stimulatory effect was also observed after chronic treatment. The kinetic patterns for medium and cellular somatostatin contents were similar to those obtained for somatostatin mRNA levels. Total DNA content was not modified under any experimental condition. The augmentations in cellular somatostatin and somatostatin mRNA determined after 24 h or chronic exposure to NMDA were blocked by (+)-5-methyl-10.11-dihydro-5H-dibenzo(ad')cyclohepten-5,10-imine hydrogen maleate (MK-801), an NMDA receptor antagonist. MK-801 alone significantly (P < 0.05) reduced somatostatin mRNA. The stimulatory effect of NMDA on somatostatin mRNA was specific since it was not accompanied by any change in glyceraldehyde-3-phosphate deshydrogenase (GAPDH) mRNA. After immunostaining with a specific antibody against somatostatin, no difference was observed in the number of immunostained neurons detected in control and NMDA exposed groups. However, further analysis of immunostained neurons revealed qualitative and quantitative staining differences between the two groups, with maximal immunostaining intensity detected in perikarya of neurons exposed to NMDA for 24 h. These results demonstrate that NMDA induces specific stimulation of somatostatin gene expression and somatostatin biosynthesis in vitro in hypothalamic neurons.