Role of glucocorticoids in the regulation of pituitary somatostatin receptor subtype (sst1-sst5) mRNA levels: evidence for direct and somatostatin-mediated effects.

Glucocorticoids can differentially regulate somatostatin (SRIH) receptor subtype expression depending on the duration of treatment, dose used and tissue type examined. In order to determine if glucocorticoids are critical regulators of pituitary SRIH receptor synthesis in vivo, we examined the effect of adrenalectomy (ADX), with and without dexamethasone (DEX; 200 microg/day for 8 days) treatment, on the relative expression levels of the SRIH receptor subtypes, sst1-sst5, by multiplex RT-PCR. ADX increased pituitary sst2 mRNA levels, but did not significantly alter mRNA levels of the other SRIH receptor subtypes. These findings indicate that pituitary sst2 synthesis is normally under inhibitory control of endogenous glucocorticoids. High-dose DEX resulted in a decrease in sst1-sst4 mRNA and an increase in sst5 mRNA, independent of adrenal status. DEX also decreased sst2, sst3 and sst4 mRNA levels and increased sst5 mRNA levels by short-term in vitro application (10 nM, 4 h) in primary rat pituitary cell cultures, indicating DEX regulation of sst2-sst5 in vivo is at least in part due to a direct action at the level of the pituitary. However, the inhibitory actions of DEX on sst1 mRNA levels observed in vivo were not consistently replicated in vitro. In order to determine if the somatotrope population of the pituitary would display a similar response to DEX, fluorescent-activated cell sorting was used to obtain somatotrope-enriched cultures (>95% growth hormone immunopositive cells). DEX treatment (10 nM, 4 h) of somatotropes decreased sst2 and sst3, but did not alter sst5 mRNA levels. These results indicate that the effects of DEX on sst5 mRNA levels observed in unsorted pituitary cell cultures might be due to changes in sst5 expression in pituitary cell types other than somatotropes. Since excess glucocorticoids are thought to enhance SRIH tone, we also tested if ligand activation of SRIH receptor subtypes in vitro could mimic any of the actions of DEX on SRIH receptor mRNA levels observed in vivo. To this end, unsorted pituitary cell cultures and somatotrope-enriched cultures were treated with SRIH (1 and 100 nM) for 4 h. SRIH increased sst3 and sst5 mRNA levels, in both culture systems. These results suggest that the DEX-induced increase in endogenous SRIH tone may contribute to enhanced sst5 mRNA levels observed in vivo. However, the stimulatory actions of SRIH on sst3 mRNA levels observed in vitro might be overridden by direct inhibitory actions of DEX.