Circadian organization of the estrous cycle of the golden hamster.

Abstract

In constant dim illumination the hamster estrous cycle free-runs with a period that is a quadruple multiple of the concurrently recorded rhythm of wheel-running activity; both activity and estrous cycles are generated by biological clocks. Maintenance of stable phase angle differences between heat onset and running onset before and after treatment with deuterium oxide suggests that a common circadian system generates periodicities in estrus and activity. An organization of the estrous cycle is proposed in which the stimulus for the ovulatory surge of luteinizing hormone is generated by a circadian system that includes the suprachiasmatic nuclei of the hypothalamus. Various possible interactions of estradiol and photoperiod with the neurogenic stimulus for the luteinizing hormone surge are described and implications of different types of circadian organization of the estrous cycle for theories of sexual differentiation are considered. Estrous and menstrual cycles are readily detectable in a wide variety of spontaneously ovulating polycyclic mammals (1, 2) and were among the first hormonally related rhythms described (3). Typically, the interval between successive recurrences of ovulation, behavioral receptivity (estrus), or release of pituitary ovulating hormone is relatively constant and species specific.Female reproductive cycles depend upon rhythmic release of hormones from the anterior pituitary. The underlying basis for this phenomenon "remains one of the more poorly understood phenomena in regulatory biology" (ref. 4, p. 607). One exciting possibility is that these cycles are manifestations of biological clocks, defined as endogenous self-sustained oscillators specialized for time measurement (5, 6). If one were to establish the clock-like nature of the mechanism underlying the mammalian estrous cycle, then studies of reproductive phenomena could benefit from substantial theoretical and empirical generalizations of the discipline of biochronometry (6).To establish that an observed rhythm is the manifestation of a biological clock it is adequate to demonstrate that the rhythm persists under constant environmental conditions with a period that differs significantly from that under entrained conditions. This has been accomplished for the estrous rhythm of one mammal. In a landmark study, Alleva et al. (7) observed that the period of the hamster estrous cycle was 96 hr when the animals were exposed to a 16 hr light:8 hr dark (LD 16:8) cycle; in constant illumination (LL), the estrous cycle persisted, with 18 of the 20 animals generating periods that differed significantly from 96 hr (range of 95. 35-97.54 hr). This approximately 4-day cycle (a circaquadridian rhythm) established the endogenous nature of the hamster estrous cycle. Alleva et al. (7) speculated that the clock for the estrous cycle functioned with a circadian rather than a circaquadridian frequency. This hypothesis is supported by several lines of evidence; e.g., spontaAbbreviations: LH, luteinizing hormone (lutropin); GnRH, gonadotrophin r...

Hamster activity and estrous cycles: control by a single versus multiple circadian oscillator(s).

“…A circadian system entrainable by the illumination cycle generates the estrous cycle and appears to be the mechanism responsible for the precise phase relationships among gonadotropin release [e.g., the luteinizing hormone (LH) surge], behavioral receptivity, and ovulation (3)(4)(5). Several findings suggest that the endogenous timing system of the hamster estrous cycle has a circadian (a 1-day) rather than a circaquadridian (i4-day) period.…”

“…In constant dim illumination, the estrous cycle persists and "free-runs" with a r value significantly longer than 96 hr (3,4).…”

“…Wheel-running activity was intermittently split into entrained and free-running components for one female: at T = 22.0 hr, estrous onset was coupled first with one and then with the other of the split activity components. These findings suggest that two or more separate circadian oscillators may control timing of the activity and estrous rhythmsŨ nmated female mammals typically undergo repeated estrous cycles; the interval between successive behavioral estrous onsets (EOs) is relatively constant and species specific (1, 2).Golden hamsters maintained in 16-hr light/8-hr dark (LD 16:8) or LD 14:10 photoperiods have estrous cycles with a period (X) of 96 hr (3,4). In constant dim illumination, the estrous cycle persists and "free-runs" with a r value significantly longer than 96 hr (3, 4).…”

“…Golden hamsters maintained in 16-hr light/8-hr dark (LD 16:8) or LD 14:10 photoperiods have estrous cycles with a period (X) of 96 hr (3,4). In constant dim illumination, the estrous cycle persists and "free-runs" with a r value significantly longer than 96 hr (3, 4).…”

“…Several findings suggest that the endogenous timing system of the hamster estrous cycle has a circadian (a 1-day) rather than a circaquadridian (i4-day) period. (i) The estrous cycles of hamsters housed in constant dim illumination free run with periods that are a quadruple multiple of the concurrently recorded circadian activity rhythms (4)…”