Ontogeny and sexual differentiation of somatostatin biosynthesis and secretion in the hypothalamic periventricular-median eminence pathway

The biosynthesis of somatostatin (SRIH) in the hypothalamic periventricular nucleus (PeN) is sexually differentiated in neonatal and adult rats by vir tue of the organizational and activational actions, respectively, of sex st eroid hormones, Little information exists, however, on the normal pattern o f maturation of these neurones or on how the sexually differentiated biosyn thesis may relate to ontogenetic changes in somatostatin secretion during t he neonatal and pubertal periods of development. Hence in the present study we determined the postnatal developmental profile of SRIH mRNA and peptide levels in the PeN-median eminence (ME) pathway as well as SRIH secretion, using an acute explant preparation, from the day of birth, through puberty and into adulthood in male and female rats. The results demonstrate that: ( 1) developmental sex differences in SRIH biosynthesis in PeN neurones occur red in an orderly cascade with differences observed for mRNA expression at postnatal day 5, for peptide content in the perikarya at postnatal day 10 a nd for peptide content in the nerve terminal (ME) by postnatal day 25; (2) sex differences in SRIH release were not evident prior to postnatal day 40; and (3) the developmental profile of SRIH biosynthesis in PeN neurones is unique compared with other hypothalamic (ventromedial nucleus) and extrahyp othalamic (parietal cortex) populations. Specific developmental changes in the biosynthetic and secretory activity of the hypothalamic SRIH PeN-ME pat hway may have a functional importance in the maturation of hypothalamic SRI H pathways involved in the regulation of GH secretion.