We continued to monitor vaginal patency of females daily while they were living continuously with a male. and 6 days after estrus without mating. Progesterone concentrations were highest in the second half of the cycle when corpora lutea were present and estradiol concentrations peaked on the day of estrus. Thus, female degus appear to exhibit a spontaneous reproductive cycle consistent with other Hystricognathi rodents.Octodon degusis a novel model with which to examine the mechanisms underlying different reproductive cycles. Keywords:circadian rhythm, corpus luteum, female mating behavior, ovary, ovulatory cycle The hystricognath rodentOctodon degushas a spontaneous ovarian cycle that lasts 1721 days and has a true luteal phase, thus, it is a unique animal model for studies of mechanisms of female reproductive cyclicity. == INTRODUCTION == The estrous cycle of female mammals is a cyclical change in hormone secretion and ovulation that can be divided into two phases based on ovarian structures and the hormones they produce. During the follicular phase, maturing follicles in the ovary release increasing concentrations VL285 of VL285 estrogen. When estradiol concentrations reach a threshold, a surge of gonadotropin-releasing hormone (GnRH) is released from cells in the hypothalamus. This surge in GnRH in turn triggers a surge of luteinizing hormone (LH) from the anterior pituitary, which then induces ovulation. The luteal phase occurs when the ruptured follicles develop into the corpora lutea and these structures primarily secrete progesterone. Maximal behavioral receptivity, or estrus, is typically associated with high blood concentrations of estradiol, although some species are able to display receptive behavior KT3 tag antibody across the reproductive cycle [1]. Female reproductive cycles can be categorized with respect to whether ovulation, production of functional corpora lutea, and behavioral receptivity can occur spontaneously or be induced by external stimuli in a species-specific manner. Despite these known cycle variations, our knowledge of reproductive patterns is limited, in part because of the lack of knowledge about reproductive biology from a broad range of species. The analysis of hormonal secretion and sexual behavior patterns in additional species will aid in determining commonalities underlying female reproductive cycles as well as important species differences [2]. Generally speaking, however, species within a genus or order have reproductive cycles that share similar characteristics [3]. The most-studied animal models of female estrous cyclicity are myomorph rodents (i.e., rat, hamster, and mouse) [4]. These rodents exhibit spontaneous estrous cycles, follicular development, and ovulation. Corpora lutea form for 13 days, but they do not become functional in the absence of mating or vaginal cervical stimulation [5]. These species have a relatively short cycle length (4 or 5 VL285 5 days). The LH surge occurs 615 h prior to ovulation, and both events occur at precise circadian intervals [68]. For example, in hamsters housed in a light:dark cycle, the estrous cycle occurs every 96 h (four periods of 24 h). When animals are housed in constant light, this rhythm continues (range, 95.3597.54 h), indicating an endogenous clock (rather than the environmental light:dark cycle) is regulating the timing of the LH surge during the estrous cycle [6]. Laboratory rodents have been extensively studied and have provided valuable information as to the mechanisms underlying estrous cyclicity; however, not all mammals share these same features [4,9]. The study of additional species can provide novel additional information and breadth to our understanding of female reproductive cycles and behavior [2]. For example, many species with spontaneous ovulation exhibit functional, progesterone-secreting corpora lutea independent of mating stimulation (i.e., guinea pigs, coypu, sheep, and primates). In these species, the estrous cycle is much longer relative to laboratory rodents (1435 days), and the luteal phase typically lasts for 1213 days [5,10,11]. Most females in this group, including most Hystricognathi rodents, exhibit behavioral receptivity VL285 (estrus) only during the periovulatory period [12]. Although species with a long luteal phase exhibit spontaneous cycles of ovulation, it is unknown if the circadian system is involved in the timing or regulation of estrous-related events. The Hystricognathi infraorder contains South American rodent species, including tuco-tucos, guinea pigs, coypus, cuis, grasscutters, and chinchillas. The Hystricognathi rodentOctodon deguis one of the most numerous and well-studied mammals of central Chile [13]. This precocious rodent species is small (200 g), social, and breeds readily in the laboratory [13]. Pups are born fully furred and mobile, and by age 2 wk they are eating solid food, although they continue to nurse. In both the field and laboratory, animals are weaned at age 45 wk [14,15]. Females exhibit.