Modulation of pituitary somatostatin receptor subtype (sst1-5) messenger ribonucleic acid levels by changes in the growth hormone axis

The role of individual components of the hypothalamic-pituitary-GH axis in the modulation of pituitary somatostatin (SRIF) receptor subtype (sst1-5) s ynthesis was assessed using multiplex RT-PCR to measure receptor messenger RNA (mRNA) levels in normal rats and spontaneous dwarf rats (SDRs). In SDRs , a strain with no immunodetectable GH, pituitary sst1 and sst2 mRNA levels were elevated. sst5 mRNA levels were reduced, and sst3 and sst4 mRNA level s did not significantly differ from those in normal controls. Treatment of SDRs with GH (72 h), but not insulin-like growth factor I, significantly de creased sst2 mRNA levels and increased sst4 and sst5 mRNA levels above vehi cle-treated control levels. To test whether more rapid changes in circulati ng GH levels could alter SRIF receptor subtype expression, normal rats were infused (iv) with GH-releasing hormone (GHRH) for 4 h in the presence or a bsence of SRIF antiserum. GHRH infusion increased pituitary sst1 and sst2 a nd decreased sst5, but had no effect on sst3 and sst4 mRNA levels. Immunone utralization of SRIF, which produced a rise in circulating GH levels, did n ot alter basal or GHRH-mediated SRIF receptor subtype expression. These obs ervations indicate that acute suppression of SRIF tone does not regulate pi tuitary SRIF receptor subtype mRNA levels in vivo. The possibility that ele vated circulating GH concentrations induced by GHRH infusion were responsib le for the observed changes in SRIF receptor subtype mRNA levels was examin ed by infusing SDRs with GHRH for 4 h. GHRH did not increase sst1 mRNA leve ls in SDRs above their already elevated value. However, GHRH infusion produ ced an increase in sst2 and a decrease in sst5 mRNA levels similar to those observed in normal rats, indicating that the acute effects of GHRH on SRIF receptor subtype expression are independent of circulating GH levels. Prim ary rat pituitary cell cultures were incubated with GHRH (10 nM) or forskol in (10 mu M) for 4 h to determine whether GHRH could directly mediate SRIF receptor subtype mRNA. GHRH treatment increased sst1 and sst2 mRNA levels a nd decreased sst5 mRNA levels, but had no effect on sst3 and sst4, similar to the results in vivo. The effect of forskolin mimicked that of GHRH on ss t1, sst2, and sst5 mRNA, suggesting that GHRH acts through cAMP to directly mediate gene transcription or mRNA stability of these SRIF receptor subtyp es. In addition, forskolin reduced sst3 and sst4 expression. These results strongly suggest that rat pituitary sst1, sst2, and sst5 mRNA levels are re gulated both in vivo and in vitro by GHRH. The stimulatory action of GHRH o n sst1 and sst2 and the inhibitory action on sst5 indicate that these recep tor subtypes have independent and unique roles in the modulation of pituita ry GH release.