Influence of ovarian hormones on endocrine activity of gonadotroph cells in the adenohypophysis of lambs during the postnatal transition to prepuberty

There is juvenile hiatus during maturation of larger mammals with relatively long life spans. Using histomorphological and functional criteria we describe the feedback mechanisms which could play a role in the regulation of the gonadotrophic axis during the postnatal transition to the quiescent prepubertal period in sheep. The aim of this study was to determine the influence of ovarian factors on the endocrine activity of gonadotroph cells, the site of synthesis, storage and release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH), in adenohypophyses of weanling and weaned prepubertal lambs. The examination was made in (i) 9-week-old infantiles, suckling lambs undergoing weaning, ovary-intact (OVI) and ovariectomised (OVX) at the 6th week of age, and (ii) 16-week-old juveniles OVI and OVX at the 12th week of age (n = 5 per group). Changes in gonadotrophs were assayed with hybridohistochemistry, immunohistochemistry and radioimmunoassay. The percentage of the adenohypophyseal area (PA) occupied by gonadotrophs containing LHβ-mRNA and immunoreactive for LHβ was lower (P < 0.05), whereas the PA occupied by cells containing FSHβ-mRNA and immunoreactive for FSHβ was higher (P < 0.05) in the 16-week-old OVI lambs in comparison with the 9-week-old ones. The mean concentration and basal level of LH in the peripheral blood plasma were greater (P < 0.05) in the 16-week-old OVI lambs in comparison with the 9-week-old group, whereas the circulating FSH was not different. In the OVX 9-week-old lambs, the PA occupied by gonadotrophs containing LHβ-mRNA and the plasma LH concentration, basal level, pulse frequency and amplitude were greater (P < 0.05), whereas the PA occupied by cells immunoreactive for LHβ was lower (P < 0.05) in comparison with the OVI group. In the OVX 16-week-old lambs, the PA occupied by gonadotrophs containing LHβ-mRNA and immunoreactive for LHβ, the LH plasma concentration, basal level and pulse frequency were (P < 0.05) greater in comparison with the OVI group. The PA occupied by gonadotrophs containing FSHβ-mRNA and the plasma FSH concentration were greater (P < 0.05) in the OVX 9- and 16-week-old lambs in comparison with the OVI ones. The ovariectomy had no effect on the PA occupied by cells immunoreactive for FSHβ in both age stages. In conclusion, ovarian factors may play inhibitory role in regulating both LH and FSH synthesis rate and release and stimulatory role in regulating LH storage in adenohypophyseal gonadotrophs in infantile lambs. In lambs at the beginning of the juvenile period, ovarian factors may play only inhibitory role in regulating both LH and FSH synthesis and release and LH storage. The effects of ovarian hormones on the gonadotrophin storage, i.e. releasable pools in adenohypophyseal cells, are specific for LH, no such effects are apparent on FSH in lambs during the postnatal transition to prepuberty. Thus, the initiation of the juvenile period in female sheep is a function of change of the stimulatory role of ovarian hormones in regulating LH storage to the inhibitory one.