Effects of anterior and posterior hypothalamic deafferentation on gonadotrophin and prolactin secretion in ovariectomized ewes under hormonally induced oestrous cycles

Oestradiol benzoate (OB), when given to ovariectomized ewes under hormo- nally induced oestrous cycles, caused both negative and positive feedback effects, i. e. decreased FSH levels were followed by an FSH surge 15 to 18 hrs later concomitant with a preovulatory-like LH release.This injection also caused a prolactin surge with a spiking release pattern between 9 to 18 to 30 to 42 hrs later.Basal FSH levels, as well as those obtained after OB injection, progressively increased from December to April.Between December and February, no significant variations in prolactin secretion were detected, while later on (February to April) both basal prolactin levels, and those obtained after OB injection rose significantly.These long-term variations of both FSH and prolactin secretion indicate seasonal effects.After anterior deafferentation of the mediobasal hypothalamus (MBH), FSH levels decreased drastically, but remained detectable.The OB-induced surges were totally suppressed in some animals, while in others they showed a very small residual response at the time of the residual LH release.Thus, FSH secretion could be regulated by the same hypothalamic hormone as LH secretion, i. e. LH-RH.The tonic and/or cyclic regulation was located in the part of the brain anterior to the cut, probably in the septopreoptic-anterior hypothalamic areas.Residual secretion could be dueto the activity of the mediobasal areas, still connected to the pituitary gland and participating in both tonic and cyclic mechanisms.Posterior deafferentation of the MBH, without effect on LH and FSH secretion, was performed in January and resulted in an early increase in prolactin levels in February.Anterior deafferentation of the MBH caused drastic damping of the OB-induced prolac- tin response, while the basal level remained unchanged.It is suggested that the posterior areas of the ewe MBH may play a role in the regulation of basal prolactin levels and related seasonal changes.Septopreoptic-anterior hypothalamic areas may have a major part in the control of the OB-induced prolactin response.The quantitatively different effects and localizations suggest the existence of at least two separate neural hormones.In the ewe, oestrogens are reported to induce an FSH discharge during the anoestrus season (Pant et al., 1973 ; Jonas et al., 1973).Annales de Biologie animale.