J Infertil Reprod Biol, 2020, Volume 8, Issue 4, Pages: 68-72. https://doi.org/10.47277/JIRB/8(4)/68 68 Determination of the Ovulation Time in the Laboratory Rats (Rattus norvegicus) Joy Iyojo Itodo 1 *, Agnes Ifeoma Nwannenna 2 , John Shiradiyi Bugau 2 , Kenneth Owoicho Abah 3 , Danjuma Friday Audu 4 , Grace Imaben Opaluwa-Kuzayed 4 , Simon Azubuike Ubah 3 , Mohammed Babashani 5 , Kuje, Althea Agbi 2 1 Department of Animal Science Faculty of Agriculture Federal University of Lafia, Nasarawa, Nigeria 2 Department of Theriogenology and Production, Faculty of Veterinary Medicine, Ahmadu Bello University Zaria, Kaduna Nigeria 3 Department of Theriogenology, Faculty of Veterinary Medicine, University of Abuja, F.C.T. Nigeria 4 Department of Theriogenology and Production, Faculty of Veterinary Medicine, University of Jos, Plateau, Nigeria 5 Veterinary Teaching Hospital, Ahmadu Bello University Zaria, Kaduna Nigeria Received: 10/08/2020 Accepted: 29/10/2020 Published: 20/09/2020 Abstract The rat has been elected as the main animal model in different studies involving reproduction. However, there are scarce and conflicting data related to its estrous cycle. The aim of the experiment was to determine the time of ovulation in the primiparous laboratory rats (Rattus Norvegicus) by counting the number of graafian follicles present in the ovary at the time of oestrus, determining the percentage of these follicles that eventually ovulate and determining the 'spread' of ovulation during the oestrus period. Fifty (50) albino laboratory rats were observed in oestrus with the help of males to determine the time the female first stood to be mounted. This time was considered the onset of oestrus and used as a landmark for timing of oestrus period. From this onset of oestrus, the period was divided into 10 x 1-hour intervals into which the female rats were grouped. Each hour interval had five rat members. Ovaries of rats harvested at the end of their group-hour intervals were studied histologically, for functional structures. Observed structures were counted for the calculation of ovulation rates. All oestrus rats had far shorter periods, the ovulation in this study were found to be widespread from the onset to the tenth hour peaking maximally at the sixth and seventh hour of oestrous. Follicular activity was found to be more in the left ovaries while ovulatory activity was more in the right ovaries. It was concluded that rats presented far shorter oestrus periods and all ovulations in the albino rats used in our laboratory occurred at the onset of oestrus and that the mechanism responsible for the first "stance for mounting" is responsible for the ovulation and that ovulation is induced in these rats. Keywords: Corpora lutea, Histology, Rats, Estrous, Ovulation Introduction The rat has been elected as the main animal model in several studies involving reproduction. However, there are scarce and conflicting data related to its estrous cycle. It comprises phases characterized by different cell types in vaginal smears (proestrus, estrus, metestrus and diestrus) (1). Ovulation is the release of matured eggs from the ovaries. It is the culminating effect of the event of the oestrous cycle which is the rupture of the follicle and the shedding of the ovum (2). Since the laboratory rat (Rattus norvigecus) is the most used animal in various scientific researches worldwide especially in reproductive analysis, the improvement of its reproductive results and fertility rates seems to be a worthy objective. The basic principles for detection of ovulation in warm climates are similar to those for cool or temperate climates (3). Rats which have been adapted to tropical conditions are generally considered to have a regular oestrous cycle. Basic information concerning the spread and timing of ovulation and the optimum time of mating is needed to obtain maximum fertility both in natural and in reproductive research purposes. Information relative to such needs are lacking in Nigerian laboratories. Accurate detection of ovulation and its correct timing are, two prerequisites for the successful use of laboratory rats as reproductive research models. Behavioural observations measuring ovulatory manifestations and adequate description of the behavioural characteristics known as oestrous ('heat') should be provided in the studies of ovulation. Studies have shown that in rats as well as other animals, conception will only occur in those animals that are ovulating. Detection of the onset of ovulation, the seasonal patterns of rats' reproduction and breeding behavior are remarkably varied, therefore, it is very important to achieving high conception rates. However, detection of the onset of ovulation is not very easy, particularly in the tropics (4). Therefore, the goal of this present investigation is to study the ovulation time in primiparous laboratory rats through the counting the number of graafian follicles present in the ovary at the time of oestrus, determining the percentage of these follicles that eventually ovulate, determining the 'spread' of ovulation during the oestrus period. Materials and Methods Animals For this study, 50 albino laboratory female rats weighing between 110g - 185g and some males, bred in Theriogenology laboratory of the Faculty of Veterinary Medicine, Ahmadu Bello University, Zaria, were used. Selected female rats must have whelped. Approval for this study was sought and gotten *Corresponding author: Joy Iyojo Itodo, Department of Animal Science, Faculty of Agriculture Federal University of Lafia, Nasarawa, Nigeria. Email: iyojojoy@gmail.com