Growth hormone secretagogues (GHSs) are a family of synthetic peptidyl and non-peptidyl compounds with biological effects in several endocrine and non-endocrine target tissues, including activation of the hypothalamic-pituitary-adrenal axis. The physiological relevance of the GHS-signalling system was recently substantiated by the identification of the specific receptor for GHSs, namely GHS-R, and its endogenous cognate ligand, ghrelin. We have recently reported evidence for the novel expression and functional role of GHS signalling system in a major steroidogenic tissue, the testis (Tena-Sempere et al. 2002). In the present study, we aimed at evaluating the expression and homologous regulation of the GHS-receptor gene in rat adrenal. For comparative purposes, homologous regulation of GHS-receptor gene expression in rat pituitary and testis was also assessed. The animals were humanely killed and then we prepared tissue incubations.

Our RT-PCR analysis demonstrated expression of the GHS-R mRNA in adult rat adrenal gland. To note, expression of this message in adrenal tissue appeared to be under the regulation of homologous signals in vitro, as short-term incubation of adrenal samples in serum-free medium induced a significant increase (P < 0.01, ANOVA followed by Tukey’s test) in GHS-R mRNA levels, that was inhibited by exposure to different doses of GHRP-6 (10^-9 to 10^-5 M) or ghrelin (10^-7 M). In contrast, a strikingly different pattern of homologous regulation was observed in the pituitary and testis, where serum-free medium incubation decreased GHS-R mRNA levels, which were restored to basal in vivo values by stimulation with GHRP-6 and/or ghrelin.

In conclusion, our study provides novel evidence for the expression and regulation of the GHS-R gene in rat adrenal, and evidences clear-cut differences in the regulation of GHS-R mRNA expression by homologous signals between pituitary, testis and adrenal gland in the rat.

This work was supported by grant BFI2000.0419-C03.