Rapid reduction in somatostatin mRNA expression by hypothalamic neurons induced by dexamethasone

We have previously reported that peripherally administered dexamethasone induces a rapid increase in hypothalamic somatostatin release. Here we investigated whether somatostatin synthesis could also be affected by this treatment and the potential involvement of glutamate in this effect. Male rats received a saline or a dexamethasone injection (300 microg/100 g body weight) and were killed 30 min later. Thirty minutes prior to dexamethasone treatment, another group received an i.p. injection of MK-801, a NMDA receptor antagonist. Cells expressing somatostatin mRNA in the periventricular nucleus were analyzed by in situ hybridization using digoxigenin-labeled somatostatin oligonucleotide probe. Dexamethasone decreased the number of digoxigenin-labeled cells expressing somatostatin mRNA in the periventricular nucleus as compared to the same histological sections from control rats. The dexamethasone effect was reversed by pretreatment with MK-801, which alone also decreased the number of cells expressing somatostatin mRNA. In summary, dexamethasone administration induces a significant rapid decrease in periventricular cells expressing somatostatin mRNA and this effect is partly abolished by MK-801.