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Oestrual Phase at first exposure to Predator-induced Stress Determines Pattern of Alterations in Oestrous Cycle and Endocrine Response in the Rat Medubi, O.O.; Iranloye, B.O. & Adegoke, O.A. Abstract Summary: Stress has been acknowledged as one of the aetiologies of female reproductive dysfunction, yet the mechanisms involved are not totally elucidated. Based on the paucity of information on how predator-induced stress (PS) affects oestrous cycle in rats, this study was designed to investigate the effect of PS on the oestrous cycle in rats. Forty-eight (48) Sprague-Dawley rats were used for this study. They were randomly divided into Control and PS group. Each group was divided into four subgroups (n=6/group) according to the phases of oestrous cycle. Stress was induced by exposing rats to cat (predator) for 60 minutes/day for 14 consecutive days. PS caused significant disruption of the oestrous cycle. In animals subjected to PS at proestrus (PS-proestrus) and oestrus (PS-oestrus), percentage occurrence of proestrus, oestrus and metestrus phases were significantly reduced compared with control. In animals subjected to PS at metestrus (PS-metestrus) and diestrus (PSdiestrus), percentage occurrence of oestrus phase was not significantly affected. In all animals exposed to PS, percentage occurrence of diestrus was significantly increased regardless of the phase of first exposure compared with control. Corticosterone and prolactin levels were significantly elevated in PS groups compared with control. Progesterone was significantly increased in animals at diestrus phase compared with oestrus phase and respective phases in control. Oestradiol was significantly reduced in PS group compared with control at oestrus phase but not significantly different at diestrus phase. Luteinizing hormone (LH) and follicle stimulating hormone (FSH) levels were significantly lower in PS groups at oestrus phase compared with diestrus phase. This study shows that PS disrupts the oestrous cycle secondary to perturbation of hormonal control of female reproduction and is influenced by the phase at first exposure to stress. Keywords Predator; Vaginal Cytology; Oestrous Phases; Prolactin; Corticosterone; Stress

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