Sexually dimorphic expression of sst1 and sst2 somatostatin receptor subtypes in the arcuate nucleus and anterior pituitary of adult rats

The pattern of growth hormone (GH) secretion and rate of somatic growth are markedly sexually dimorphic, but the underlying neuroendocrine mechanisms are far from clear. In the present study, we tested the hypothesis that the sexual dimorphism of GH secretion may be due to gender-related differences in the transduction of somatostatin's actions in brain and/or pituitary. T o accomplish this, we compared the distributional pattern and level of expr ession of two somatostatin receptor subtypes, sst1 and sst2, in the brain a nd pituitary of adult male and female rats by in-situ hybridization using S -35-labelled antisense riboprobes. In the brain, the hybridization pattern and labelling density of sst1 and sst2 mRNA-expressing cells, as revealed b y computer-assisted image analysis, in areas including the cerebral cortex, medial habenula (MHb) and ventromedial hypothalamic nucleus (VMN), were si milar in male and female rats. In contrast, there was a marked sex-related difference in sst1 expression in the arcuate nucleus of the hypothalamus; b oth the number and labelling density of sst1 mRNA-expressing cells were two - to threefold greater in males than in females and this significant increa se was homogenous throughout: the rostrocaudal extent of the nucleus. No ge nder-related differences in arcuate sst2 mRNA levels were found. At the lev el of the anterior pituitary, the labelling density of sst2 mRNA in males w as significantly higher than that of females. No sex-related difference in pituitary sst1 mRNA was observed. These results demonstrate a sexual dimorp hism in the expression of two somatostatin receptor subtypes, sst1 and sst2 , at the level of the arcuate nucleus and anterior pituitary, respectively. Such dimorphism suggests a differential involvement of sst1 and sst2 in GH regulation with respect to gender, and may imply roles for sst2 and sst1 i n transducing somatostatin's actions on pituitary somatotrophs and GH-relea sing hormone-containing arcuate neurones, respectively, to generate the low er basal and higher GH pulse levels characteristic of the male rat.