Importance of 6 p.m. in Hamster Timekeeping Evidenced by Computer Analysis of Wheel-Running Activity

We addressed the question whether the clock signal for hamsters to become active occurs at sundown throughout summer or at some constant time after noon (p.m. time). Ten female golden hamsters housed in wheel cages in a windowless room were exposed to 24-h light/dark (LD) cycles simulating the equinoxes (LD 12: 12), when the sun sets at 6 p.m. and rises at 6 a.m., and summer (LD 14: 10, 16: 8, and 18: 6), when the sun sets after 6 p.m. and rises before 6 a.m. The onset of behavioral estrus, a mask-free phase marker of the same clock that controls wheel-running, was observed every 4 days, and wheel revolutions were recorded every 5 min for 52 days. Computer analysis of the 5-min values averaged for all 10 hamsters revealed a clear onset of running for each LD exposure. Time in the windowless room is referenced to mid-L (room “noon”) of the LD cycles. Although L-off ranged from 6 p.m. in LD 12: 12 (6 h after mid-L) to 9 p.m. in LD 18: 6, estrus began close to 4 p.m. and running close to 6 p.m. in every LD cycle. In a second study, 13 females not tested for estrus began running closer to 7 p.m. in LD 16: 8 (L-off, 8 p.m.), but when L-off was advanced to 4 p.m. they also began running on that day at 6 p.m. Testing for estrus may have made the first group of hamsters less fearful of light and therefore more responsive to a 6 p.m. clock signal to become active. It is conceivable that these nocturnal rodents voluntarily suppress, to varying degrees, overt activity from 6 p.m. standard time to sundown to avoid predators. It is noteworthy that 6 p.m. room time also marks the onset of the clock's 12-h light-sensitive period underlying hamster timekeeping.     

The hamster clock phase-response curve from summerlike light:dark cycles and its role in daily and seasonal timekeeping

We address the subject of entrainment of the hamster clock by the day:night cycle in summer when the sun sets after 6 PM and rises before 6 AM (nights < 12 h). Summer day:night cycles were simulated by 6 light:dark (LD) cycles with D < 12 h (summerlike, SLD) ranging from SLD 12.5 h:11.5 h (D, 6:15 PM-5:45 AM) to 18 h:6 h (D, 9 PM-3 AM). These are the near limiting SLDs for constant PM timing (entrainment) of behavioral estrus and wheel running in hamsters. The onset of estrus was observed every 4 d in the same hamsters as a phase marker of their 24 h clock. On the day before an experimental estrus, preceded and followed by control onsets, a dark period was imposed to cover a putative 6 PM-6 AM light-sensitive period (LSP). This was scanned with a light pulse (and periodic 5 sec bell alarms) lasting 5-240 min. Shifts in onset of estrus on the next day were plotted vs. the end of the light pulse for PM times ("dusk") and its onset for AM times ("dawn"). The resulting phase shifts from the six SLDs were similar, permitting their combination into a single phase-response curve (PRC) of 1605 shifts. This SLD composite PRC rose at 10:15 PM, peaked at 2 AM (81 min advanced shift), fell linearly to 5:55 AM, and then abruptly to normal at 6 AM (no shift). Peak shift was unaffected by light pulse duration or intensity, or hamster age. The SLD composite PRC lacked the 6 PM-9 PM curve of delayed shifts present in reported PRCs from LD 12 h:12 h and DD. However, a two-pulse experiment showed that all light from 6 PM to L-off was needed to block (balance) the advancing action of a 5 min morning light pulse, thereby maintaining entrainment. A working hypothesis to explain daily entrainment and seasonal fertility in the golden hamster is illustrated. A nomenclature for labeling the phases of the hamster clock (circadian time) is proposed.     

THE HAMSTER CLOCK PHASE-RESPONSE CURVE FROM SUMMERLIKE LIGHT:DARK CYCLES AND ITS ROLE IN DAILY AND SEASONAL TIMEKEEPING

We address the subject of entrainment of the hamster clock by the day:night cycle in summer when the sun sets after 6 PM and rises before 6 AM (nights<12 h). Summer day:night cycles were simulated by 6 light:dark (LD) cycles with D<12 h (summerlike, SLD) ranging from SLD 12.5h:11.5h (D, 6:15 PM–5:45 AM) to 18h:6h (D, 9 PM–3 AM). These are the near limiting SLDs for constant PM timing (entrainment) of behavioral estrus and wheel running in hamsters. The onset of estrus was observed every 4 d in the same hamsters as a phase marker of their 24h clock. On the day before an experimental estrus, preceded and followed by control onsets, a dark period was imposed to cover a putative 6 PM–6 AM light-sensitive period (LSP). This was scanned with a light pulse (and periodic 5sec bell alarms) lasting 5–240 min. Shifts in onset of estrus on the next day were plotted vs. the end of the light pulse for PM times (“dusk”) and its onset for AM times (“dawn”). The resulting phase shifts from the six SLDs were similar, permitting their combination into a single phase-response curve (PRC) of 1605 shifts. This SLD composite PRC rose at 10:15 PM, peaked at 2 AM (81min advanced shift), fell linearly to 5:55 AM, and then abruptly to normal at 6 AM (no shift). Peak shift was unaffected by light pulse duration or intensity, or hamster age. The SLD composite PRC lacked the 6 PM–9 PM curve of delayed shifts present in reported PRCs from LD 12h:12h and DD. However, a two-pulse experiment showed that all light from 6 PM to L-off was needed to block (balance) the advancing action of a 5min morning light pulse, thereby maintaining entrainment. A working hypothesis to explain daily entrainment and seasonal fertility in the golden hamster is illustrated. A nomenclature for labeling the phases of the hamster clock (circadian time) is proposed.     

Field and laboratory studies provide insights into the meaning of day-time activity in a subterranean rodent (Ctenomys aff. knighti), the tuco-tuco

South American subterranean rodents (Ctenomys aff. knighti), commonly known as tuco-tucos, display nocturnal, wheel-running behavior under light-dark (LD) conditions, and free-running periods >24 h in constant darkness (DD). However, several reports in the field suggested that a substantial amount of activity occurs during daylight hours, leading us to question whether circadian entrainment in the laboratory accurately reflects behavior in natural conditions. We compared circadian patterns of locomotor activity in DD of animals previously entrained to full laboratory LD cycles (LD12:12) with those of animals that were trapped directly from the field. In both cases, activity onsets in DD immediately reflected the previous dark onset or sundown. Furthermore, freerunning periods upon release into DD were close to 24 h indicating aftereffects of prior entrainment, similarly in both conditions. No difference was detected in the phase of activity measured with and without access to a running wheel. However, when individuals were observed continuously during daylight hours in a semi-natural enclosure, they emerged above-ground on a daily basis. These day-time activities consisted of foraging and burrow maintenance, suggesting that the designation of this species as nocturnal might be inaccurate in the field. Our study of a solitary subterranean species suggests that the circadian clock is entrained similarly under field and laboratory conditions and that day-time activity expressed only in the field is required for foraging and may not be time-dictated by the circadian pacemaker.     

Synergistic Inhibition by Benzodiazepine and Barbiturate Drugs of the Clock Control of Ovulation in Hamsters

A surge of pituitary luteinizing hormone (LH) into the bloodstream occurs in hamsters every 4 days between 1:30 p.m. and 3 p.m. in response to a signal from a biological clock. This surge initiates behavioral estrus ∼2 h later and ovulation ∼12 h later. Phenobarbital at a dose ≥100 mg/kg consistently blocks LH release. Barbiturate and benzodiazepine drugs have separate binding sites in the GABA_A receptor/chloride channel complex. Binding of either drug increases GABA-mediated chloride conductance, which suppresses the postsynaptic neuron. Barbiturate binding also increases benzodiazepine binding. This suggested that these drugs might synergize to inhibit LH release. A combination of triazolam and phenobarbital at doses of 10 mg/kg injected s.c. at 1:30 p.m. inhibited ovulation and extended the 4-day vaginal cycle in all treated hamsters. Either drug dose injected alone at 1:30 p.m., or the combination at 3 p.m., was completely ineffective. Bicuculline prevented inhibition by the combination at 1:30 p.m. The clock signal for LH release may act by antagonizing GABA transmission, which may be chronically inhibiting LH release. The combination delimited a 75-min period (1:30-2:45 p.m.) within which the clock signal for LH release occurred in all individuals (ET₅₀ = 2:08 p.m.). This period appears to arise from individuals with different but constant clock settings rather than from a 75-min variation in the clock setting of the individual.     

Effect of different light-dark schedules on survival from heart failure

Our earlier work showed that life in constant light prolonged life for hamsters with an inherited cardiomyopathy when compared to littermates spending their lives in 24 hour days (lights on 12 hr each day). This study was designed to begin evaluating the mechanism for this effect. 4-5.5 month old cardiomyopathic hamsters (CMHs) were placed on one of 5 different light-dark (LD) schedules for the remainder of their lives: (1) LD 12:12 (moderate total light exposure, short photoperiodic effect, 24 hr daylength); (2) LD 12:13 (same total light as LD 12:12, long photoperiodic effect, non-24 hr daylength); (3) LD 6:30 (less total light than LD 12:12, long photoperiodic effect, non-24 hr daylength); (4) LD 18:6 (more total light than LD 12:12, long photoperiodic effect, 24 hr daylength); (5) constant light (high total light exposure, long photoperiodic effect, non-24 hr daylength). CMHs living on the first two non-24 hr schedules lived longer than LD 12:12 controls. This study therefore suggests that manipulating the biological clock can have positive therapeutic consequences. However, in contrast to our earlier studies, hamsters living in constant light were not protected--perhaps because the hamsters began the treatment later in their lives or because their inherited disease was less severe than had been the case in hamsters used in previous studies. Defining the conditions that diminish or enhance the photobiological effect is an important challenge for future research.     

Effects of wheel running on photoperiodic responses of Djungarian hamsters (Phodopus sungorus)

Djungarian hamsters (Phodopus sungorus) were exposed to artificial short days either with access to a running wheel (RW) or without. Within 6 weeks RW hamsters considerably increased their body mass, whereas controls showed the typical body mass reduction. Estimation of paired testis weights indicated a decelerated testis regression in RW hamsters. Subsequent locking of RWs for 9 weeks led to a decline in body mass of RW animals in parallel to controls. Daily torpor was almost completely missing in hamsters with initially unlocked wheels. During the final phase when RWs were again unlocked (3 weeks), body mass of exercising hamsters increased again, while controls reached the nadir in body mass. In comparison to equiponderate long-day (LD) controls the relative liver weight of RW hamsters was significantly increased unlike the relative heart weight. However, the latter tended to be higher than in sedentary LD hamsters. A growth-stimulating effect of wheel running was proven by elongated femora in exercising short-day (SD) hamsters compared to SD controls and suggested by exercise-induced elevation of body mass in a further experiment under continuous LD conditions, indicating a growth-promoting effect of wheel running independent from the photoperiod.     

Delay of behavioral estrus in hamsters and phenobarbital

The onset of behavioral estrus was used as a phase marker of the hamster timing system in SLD 16:8 (dark 20:00-04:00). TZ was injected between 11:00 of cycle day 3 and noon of cycle day 4 when onset of estrus was determined. At no time did injection of TZ cause a phase advance in SLD 16:8. Small delays of estrus resulted from 11:00-16:00 injections but marked delays began with the 17:00 injection. Phenobarbital was injected between noon and 19:30 on cycle day 3. Injections between noon and 16:00 had no effect but all later injections beginning at 17:00 delayed estrus, the 17:30 injection causing the greatest delay. Diazepam also markedly delayed estrus when tested at 17:30. These results with three drugs support results with light pulses that 18:00 in SLD 16:8 marks the same phase of the 24-h hamster timing system as the onset of wheel running does in DD, LL, and WLD. These findings with three GABA potentiators extend to SLD previous evidence based on the onset of wheel running in DD, LL and WLD that GABA may be involved in hamster timekeeping and its responses to light and drugs.     

Nonphotic enhancement of adjustment to new light-dark cycles: masking interpretation discounted

The adjustment of hamsters to advanced light-dark (LD) cycles can be greatly accelerated by scheduling a single 3-hr bout of extra activity in a novel running wheel, starting about 7 hr before the time when the animals become active in the preceding LD cycle. The present experiments were designed to provide stronger evidence that this effect depends on a shift in the pacemaker rather than on masking. It was shown that when hamsters were put into continuous darkness (DD) 1 day after the exercise-accelerated phase shift, their free-running rhythms took off from a time nearer to the onset of darkness in the new LD cycle than in the preceding LD cycle. An incidental finding was that in DD the free-running period of the hamsters with the accelerated phase shifts was longer than that of the control animals. Further evidence that the 3-hr exercise pulse had produced a greater phase advance than that occurring in undisturbed control animals was obtained by giving a light pulse at the same clock time to all animals after they had been in DD for 8 days. The animals that had previously exercised for the additional 3-hr phase-advanced in response to the light pulse, while the undisturbed control animals phase-delayed.     

Effect of cage enrichment on the daily use of running wheels by Syrian hamsters

Institutional animal care committees may one day require for the welfare of captive hamsters more floor space and the introduction of tunnels and toys. As hamsters are popular animal subjects in chronobiological research, and as clock phase is usually measured through running wheel activity, it is important to determine what effect cage enrichment might have on daily wheel use. Here the daily number of wheel revolutions, the daily duration of the running activity phase, the phase relationship between lights-off and onset of running activity, and the free-running period of circadian activity rhythms were measured in Syrian hamsters, Mesocricetus auratus, housed in single cages or in multiple cages linked by tunnels and supplied with commercial wooden toys. Free-running periodicity was not affected by cage enrichment. In multiple-cage systems, there were fewer daily revolutions, shorter wheel-running activity phases, and delayed running activity onsets. These effects, however, were small as compared to interindividual and week-to-week variation. They were statistically significant only under a light:dark cycle, not in constant darkness, and only when interindividual variation was eliminated through a paired design or when the number of cages was increased to five (the maximum tested). Daily wheel use is thus affected by cage enrichment, but only slightly.     

Daily novel wheel running reorganizes and splits hamster circadian activity rhythms.

The phenomenon of splitting of locomotor activity rhythms in constant light has implied that the mammalian circadian pacemaker is composed of multiple interacting circadian oscillators. Exposure of male Syrian hamsters to novel running wheels also induces splitting in some reports, although novel wheel running (NWR) is better known for its effects on altering circadian phase and the length of the free-running period. In three experiments, the authors confirm and extend earlier reports of split rhythms induced by NWR. Male Syrian hamsters, entrained to LD 14:10, were transferred for 6 to 11 consecutive days to darkened novel Wahmann wheels at ZT 4 and were returned to their home cages at ZT 9. All hamsters ran robustly in the novel wheels. NWR caused a marked reorganization of home cage wheel-running behavior: Activity onsets delayed progressively with each additional day of NWR. After 11 days, activity onset in the nighttime scotophase was delayed by 7 h and disappeared completely in 2 hamsters (Experiment 1). After 6 to 7 days of NWR (Experiment 2), activity onset delayed by 5 h. Transfer of hamsters to constant darkness (DD) after 7 days of NWR revealed clearly split activity rhythms: The delayed nighttime activity bout was clearly identifiable and characterized by a short duration. A second bout associated with the former time of NWR was equally distinct and exhibited a similarly short duration. These components rejoined after 3 to 5 days in DD accomplished via delays and advances of the nighttime and afternoon components, respectively. The final experiment established that rejoining of activity components could be prevented by perpetuating the light-dark:light-dark cycle used to induce split rhythms. The data suggest that NWR causes selective phase shifting of some circadian oscillators and that component oscillators interact strongly in constant darkness.     

Inhibition of hibernation by exercise is not affected by intergeniculate leaflets lesion in hamsters

The circadian clock of mammals, located in the suprachiasmatic nuclei (SCN) of the hypothalamus, has been demonstrated to integrate day length change from long (LP) to short photoperiod (SP). This photoperiodic change induces in Syrian hamsters a testicular regression through melatonin action, a phenomenon that is inhibited when hamsters have free access to a wheel. The intergeniculate leaflets (IGL), which modulate the integration of photoperiod by the SCN, are a key structure in the circadian system, conveying nonphotic information such as those induced by novelty-induced wheel running activity. We tested in hamsters transferred from LP to a cold SP the effects of wheel running activity on a photoperiod-dependent behavior, hibernation. Lesions of the IGL were done to test the role of this structure in the inhibition induced by exercise of photoperiod integration by the clock. We show that wheel running activity actually inhibits hibernation not only in sham-operated animals, but also in hamsters with a bilateral IGL lesion (IGLX). In contrast, IGL-X hamsters without a wheel integrate slower to the SP but hibernate earlier compared with sham-operated animals. Moreover, some hibernation characteristics are affected by IGL lesion. Throughout the experiment at 7 degrees C, IGL-X hamsters were in hypothermia during 18% of the experiment vs. 32% for sham-operated hamsters. Taken together, these data show that the IGL play a modulatory role in the integration of photoperiodic cues and modulate hibernation, but they are not implicated in the inhibition of hibernation induced by wheel running activity.     

Recording and Analysis of Circadian Rhythms in Running-wheel Activity in Rodents

When rodents have free access to a running wheel in their home cage, voluntary use of this wheel will depend on the time of day^1-5. Nocturnal rodents, including rats, hamsters, and mice, are active during the night and relatively inactive during the day. Many other behavioral and physiological measures also exhibit daily rhythms, but in rodents, running-wheel activity serves as a particularly reliable and convenient measure of the output of the master circadian clock, the suprachiasmatic nucleus (SCN) of the hypothalamus. In general, through a process called entrainment, the daily pattern of running-wheel activity will naturally align with the environmental light-dark cycle (LD cycle; e.g. 12 hr-light:12 hr-dark). However circadian rhythms are endogenously generated patterns in behavior that exhibit a ~24 hr period, and persist in constant darkness. Thus, in the absence of an LD cycle, the recording and analysis of running-wheel activity can be used to determine the subjective time-of-day. Because these rhythms are directed by the circadian clock the subjective time-of-day is referred to as the circadian time (CT). In contrast, when an LD cycle is present, the time-of-day that is determined by the environmental LD cycle is called the zeitgeber time (ZT).Although circadian rhythms in running-wheel activity are typically linked to the SCN clock^6-8, circadian oscillators in many other regions of the brain and body^9-14 could also be involved in the regulation of daily activity rhythms. For instance, daily rhythms in food-anticipatory activity do not require the SCN^15,16 and instead, are correlated with changes in the activity of extra-SCN oscillators^17-20. Thus, running-wheel activity recordings can provide important behavioral information not only about the output of the master SCN clock, but also on the activity of extra-SCN oscillators. Below we describe the equipment and methods used to record, analyze and display circadian locomotor activity rhythms in laboratory rodents.     

Vitamin B12 accelerates re-entrainment of activity rhythms in rats.

The effects of methyl vitamin B12 (5-6 mg/kg, p.o.) on the entrainment of circadian running wheel activity rhythm to a new lighting schedule were measured in rats. After the light-dark (LD) cycle was abruptly reversed, rats given vitamin B12 took less time to entrain their circadian locomotor activity rhythm to the new cycle than did controls. This result indicates that vitamin B12 accelerates the reentrainment of the mammalian circadian activity rhythm following an abrupt change in the environmental LD cycle.     

Entrainment of the circadian activity rhythm to the light-dark cycle can be altered by a short-acting benzodiazepine, triazolam

The circadian rhythm of locomotor activity in hamsters maintained in either constant darkness or constant light can be phase-shifted by a single injection of the short-acting benzodiazepine, triazolam. These results suggest that treatment with triazolam may also alter the entrainment pattern of circadian rhythms in animals that are synchronized to a light-dark (LD) cycle. To test this hypothesis, hamsters maintained on an LD 6:18 light cycle received daily injections of triazolam (or vehicle) for 10-12 days, and any subsequent effects on the phase relationship between the onset of activity and the LD cycle were determined. Daily injections of triazolam (but not vehicle) induced pronounced advances or delays in the phase relationship between the entrained activity rhythm and the LD cycle; the direction of the shift was dependent on the time of the injection. Taken together with data from previous studies, these results suggest that triazolam, and perhaps other short-acting benzodiazepines, can be used to manipulate the mammalian circadian clock under a variety of experimental conditions.     

Changes in phase-angle under light–dark cycles influenced by nonphotic stimulation

ABSTRACT

Most work looking at nonphotic effects on circadian rhythms is conducted when animals are held under freerunning conditions, usually constant darkness. However, for nonphotic effects to be functionally significant, they should be demonstrable under conditions in which most animals live, i.e., a 24-hr light–dark cycle (LD). Syrian hamsters held in LD 6:18 were administered nonphotic stimulation in the form of a 3-hr confinement to a novel wheel starting about 6 hr before the start of their normal nightly activity bout. This resulted in a 2.5-hr advance of their activity rhythm on the next day that gradually receded to about 1.5 hr over the next 10 days. When hamsters held in LD 6:18 were given five novel wheel confinements over 13 days their nightly activity onset advanced 3 hr and remained at that phase for at least 2 weeks. Home cage wheel deprivation experiments indicated that high levels of home cage activity are necessary to maintain the advanced phase. These results show that nonphotic stimulation can have large, long-lasting effects on daily rhythms in LD and suggest a possible mechanism whereby nocturnal rodents might achieve phase flexibility in response to seasonal changes.     

Effect of time of ovulation and sperm concentration on fertilization rate in gilts

In normal production practices, sows and gilts are inseminated at least twice during estrus because the timing of ovulation is variable relative to the onset of estrus. The objective of this study was to determine if a normal fertilization rate could be achieved with a single insemination of low sperm number given at a precise interval relative to ovulation. Gilts (n=59) were randomly assigned to one of three treatment groups: low dose (LD; one insemination, 0.5 x 10(9) spermatozoa), high dose (HD; one insemination, 3 x 10(9) spermatozoa) or multiple dose (MD; two inseminations, 3 x 10(9) spermatozoa per insemination). Twice daily estrus detection (06:00 and 18:00 h) was performed using fenceline boar contact and backpressure testing. Transrectal ultrasonography was performed every 6 h beginning at the detection of the onset of standing estrus and continuing until ovulation. Gilts in the LD and HD groups were inseminated 22 h after detection of estrus; MD gilts received inseminations at 10 and 22 h after detection of estrus. Inseminations were administered by using an insemination catheter and semen was deposited into the cervix. The uterus was flushed on Day 5 after the onset of estrus and the number of corpora lutea, oocytes, and embryos were counted. Time of insemination relative to ovulation was designated as 40 to >24 h, 24 to >12 h, and 12 to 0 h before ovulation and >0 h after ovulation. The LD gilts had fewer embryos (P<0.04), more unfertilized oocytes (P<0.05) and a lower fertilization rate (P<0.07) compared to MD gilts. The effects of time of insemination relative to ovulation and the treatment by time interaction were not significant. We conclude that a cervical insemination with low spermatozoa concentration may not result in acceptable fertility even when precisely timed relative to ovulation.     

Time of the preovulatory LH surge in the gilt and sow relative to the onset of behavioral estrus

Two experiments were conducted to study the time of occurrence of the preovulatory LH surge in pigs. Sampling every ten minutes in six cycling gilts before and after onset of standing estrus revealed the preovulatory surge began from 8 hr before to 12 hr after the lordosis reflex was elicited. Three of six gilts initiated the preovulatory LH release coincident with the onset of estrus. Data from 28 postpartum sows, with samples drawn every six hours commencing with the onset of estrus, indicated maximum LH levels were present at the first observance of estrus. Six of the 28 sows had an LH peak 18-24 hr after the onset of estrus.     

Changes in the phase response curve of the circadian clock to a phase-shifting stimulus.

Experiments were conducted in hamsters to determine whether the phase response curve (PRC) to injections of the short-acting benzodiazepine triazolam is a fixed or a labile property of the circadian clock. The results indicated that (1) both the shape and the amplitude of the PRC to triazolam generated on the first day of transfer from a light-dark cycle (LD 14:10) to constant darkness (DD) (i.e., PRCLD) were different from those of the PRC generated after many days in DD (PRCDD); and (2) the phase-shifting effects of triazolam on the activity rhythms of hamsters transferred from LD 14:10 or 12:12 to DD changed dramatically within the first 8-9 days spent in DD. In an attempt to accelerate the resynchronization of the circadian clock of hamsters subjected to an 8-hr advance in the LD cycle, triazolam was given to the animals at a time selected on the basis of the characteristics of PRCLD. The activity rhythms of five of eight triazolam-treated animals were resynchronized to the new LD cycle within 2-4 days after the shift, whereas those of most of the control animals were resynchronized 21-29 days after the shift. These findings suggest that attempts to use pharmacological or nonpharmacological tools to phase-shift circadian clocks under entrained conditions should take into account information derived from PRCs generated at the time of transition from entrained to free-running conditions.     

Time of ovulations in dairy goats induced to superovulate with porcine follicle stimulating hormone during and out of the breeding season

Alpine dairy goats were induced to superovulate at the end of a progestagen treatment with porcine follicle stimulating hormone (pFSH) during the breeding season (n = 10 goats) and out of the breeding season (n = 10 goats). Occurrence of estrus and of the luteinizing hormone (LH) peak were checked every 4 h. Ovulations were determined every 6 h by ovarian laparoscopic examination. Among the parameters studied, the mean interval from sponge removal to the onset of estrus did not differ whatever the season of treatment, but the variability was higher for females treated out of the breeding season. Ovulations began during the laparoscopic control period for nine of ten goats during the breeding season vs seven of ten goats out of the breeding season. For these 16 females, on which the LH peak and beginning of ovulation were known, the season did not affect the intervals between the onset of estrus and the LH peak and between the LH peak and the beginning of ovulation. When ovulations are observed by laparoscopy every 6 h, for any given goat 54.9% of total ovulations (counted 7 d after estrus) occurs in less than 6 h, and 87.1% in less than 12 h. Although the interval between the LH peak and the ovulation is quite constant, the additive variabilities of the intervals between the sponge removal and the onset of estrus and between the onset of estrus and the LH peak precluded the determination of an optimal time for artificial insemination (AI) by timing sponge removal or onset of estrus.