Biomedical subjects

G L Jackson

Publications and source records attributed to G L Jackson.

At least 55 records · Page 3Linked to original sources

PubMed1993

Seasonal changes in the relationships between secretion of gonadotropin-releasing hormone, luteinizing hormone, and testosterone in the ram.

In this study we examined the relationship between GnRH, LH, and testosterone (T) in intact rams during the nonbreeding season, the breeding season, and the transition between breeding and nonbreeding season. Blood was collected from the hypophyseal portal and jugular veins at 10-min intervals for 12 h from 3 groups of rams in May, September, and March. Individual LH pulses in May and September occurred at relatively regular intervals whereas in March, groups of pulses were separated by relatively long intervals without pulses. No significant effect of season on GnRH pulse parameters was found, but there was a significant effect of season on LH (p < 0.04) and T (p < 0.001) pulse amplitude and on mean T concentrations (p < 0.001). LH pulse amplitude was highest in March, and T pulse amplitude was highest in September. Simple indices of pituitary and testicular responsiveness were obtained by calculating the ratios of LH to GnRH and T to LH. The ratio of LH to GnRH pulse amplitude was significantly higher in March than in September (p < 0.01) or May (p < 0.05). The ratio of T to LH pulse amplitude was highest (p < 0.01) in September. Release of LH in response to exogenous GnRH also varied significantly (p < 0.01) with month, being higher in March than in September (p < 0.01) or May (p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

Circannual rhythms in the ewe: patterns of ovarian cycles and prolactin secretion under two different constant photoperiods.

The purpose of this experiment was to determine whether circannual rhythms of reproductive activity and prolactin secretion are expressed differently in ewes housed under two different constant photoperiods and restricted temperatures. Eleven ovary-intact ewes housed previously under constant 12L:12D were used. One group (n = 6) was switched to 8L:16D while the other group (n = 5) remained on 12L:12D. Ovarian cycles and prolactin concentrations were monitored for more than 3 yr. The switch from 12L:12D to 8L:16D caused a change from cycles of varied length to regular 17-day cycles within 25 +/- 3 days in all 6 ewes. Subsequently, one complete circannual cycle (300.8 +/- 10.2 days) of breeding and nonbreeding activity was expressed in all ewes, but it was repeated in only 2 ewes. Ewes maintained on 12L:12D cycled at irregular intervals, and only 2 exhibited a distinct circannual rhythm of anestrus and breeding cycles. Episodic variations in ovarian cycle density were observed in most ewes in both groups. The period of this rhythm was similar between groups (290 +/- 19 and 278 +/- 16 days) and differed from 365 days (p < 0.05). Plasma prolactin exhibited a circannual rhythm under both 8L:16D and 12L:12D with a periodicity different from 365 days. We conclude that photoperiod length does not influence the expression of circannual reproductive and prolactin rhythms in the ewe. The absence of a circannual rhythm of breeding and nonbreeding activity, in the presence of circannual changes in ovarian cycle density, suggests that expression of an underlying endogenous oscillator controlling LH secretion was masked or damped.

Anestrus↗

PubMed1993

Neuroendocrine mechanisms that control seasonal changes of luteinizing hormone secretion in sheep are sexually differentiated.

The apparent earlier onset of seasonal breeding activity of adult male versus adult female sheep suggests that the neuroendocrine mechanisms that control onset of the breeding season may be sexually differentiated in this species. However, it is difficult to compare gradual onset of testicular growth to abrupt changes in ovarian activity. To determine the relative timing of seasonal transitions in neuroendocrine reproductive activity, we compared temporal changes of LH concentrations in adult, gonadectomized, estradiol (E)-treated (experiment 1), or testosterone (T)-treated (experiment 2) Suffolk male and female sheep held outdoors for 18 mo. In addition, we compared the rates at which E-treated males and females become photorefractory to a winter solstice hold (WSH) photoperiod. In all three experiments, blood samples were collected weekly. In addition, serial blood samples were collected every 12 min for 4 h at approximately 2-mo intervals for experiment 1 and at monthly intervals for experiment 2. Neither plasma E nor T concentrations differed (p > 0.05) between sexes. In E-treated animals, mean LH changed (p < 0.001) over time in both sexes, and there was a sex x time interaction (p < 0.001). LH pulse amplitude changed (p < 0.001) over time in both sexes, but there was no detectable (p > 0.05) sex x time interaction. LH pulse frequency varied significantly over time in E-treated females (p < 0.001) but not in males (p < 0.05). The amplitude of the annual oscillation of mean LH was greater (p < 0.01) in E-treated females than in males.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

Olfactory bulb removal does not prevent gonadotropin or prolactin responses to changing photoperiod in the ewe.

The purpose of this study was to determine if bilateral olfactory bulb removal (Bulbx) alters photoperiod-induced changes of gonadotropin and prolactin secretion in the ewe. Ovariectomized (group 1; n = 12) or ovariectomized estradiol-treated (group 2; n = 12) Suffolk ewes underwent Bulbx (7 per group) or sham operations (5 per group). All ewes subsequently were placed into photochambers and exposed to a photoregimen of alternating 16 h light/8 h dark and 10 h light/14 h dark photoperiods. Plasma concentrations of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and prolactin were determined in blood samples taken twice weekly from group 2 ewes. At the end of each 90-day photoregimen blood samples from all ewes in both groups were taken at frequent intervals for 4 h to determine LH pulse parameters. During the initial 16 h light/8 h dark photoperiod, plasma melatonin concentrations were determined for group 2 ewes during a 24-hour period. The completeness of Bulbx was determined at time of necropsy for all Bulbx ewes. In addition, the functional completeness of Bulbx was determined in group 2 ewes by exposing them to ram's wool and measuring changes in LH secretion. Bulbx did not affect either basal or photoperiod-induced changes in LH, FSH and prolactin in group 2 ewes. LH pulse parameters varied with photoperiod but were not significantly (p > 0.05) affected by Bulbx in either group 1 or group 2 ewes. The normal nocturnal elevation in plasma melatonin concentrations was unaffected by Bulbx.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

Comparison of the effects of pulsatile and constant testosterone on the secretion of gonadotropins in the ram.

The objective of the present study was to compare the effectiveness of a pulsatile vs. a constant pattern of testosterone (T) infusion to suppress LH and FSH secretion in wethers. Two separate experiments were conducted. In Exp 1, animals were subjected to each of four different iv infusion regimens for 3 days: 1) constant diluent, 2) constant T (768 micrograms/kg.24 h), 3) pulsatile (one pulse every 4 h) diluent, and 4) pulsatile T (768 micrograms/kg.24 h). Blood samples were collected at 10-min intervals for 4 h both before infusion and during the last 4 h of the infusion. Compared to diluent, T decreased (P < 0.001) mean LH and increased (P < 0.001) LH interpulse interval. The LH interpulse interval was increased more (P < 0.005) by constant T than by pulsatile T. Mean LH was slightly more suppressed (P = 0.052) by constant T than by pulsatile T. LH pulse amplitude did not differ between constant T and pulsatile T. In Exp 2, animals were subjected to each of three different iv infusion regimens for 4 days: 1) constant diluent, 2) constant T (384 micrograms/kg.24 h), and 3) pulsatile (one pulse every 6 h) T (384 micrograms/kg.24 h). Both LH the interpulse interval (P = 0.001) and LH pulse amplitude (P = 0.04) were increased more by constant T than by pulsatile T. Mean LH was suppressed more (P = 0.002) by constant T than by pulsatile T. In both Exp 1 and 2, none of the treatments significantly affected mean FSH. These results indicate that constant T is more effective than pulsatile T in suppressing LH secretion in the ram.

Animals↗

PubMed1992

Quality of care for colorectal cancer in a fee-for-service and health maintenance organization practice.

This study assessed the effectiveness of two types of health plans, offered by the same health care provider, in the diagnosis and treatment of colorectal cancer. Data on 330 cases diagnosed from 1984 through 1989 were abstracted from medical records. Of these, 205 (62%) used fee-for-service (FFS) and 125 (38%) used health maintenance organization (HMO) plans. Overall, there were no differences between FFS and HMO cases for duration of symptoms before diagnosis, training of physician who diagnosed the tumor, anatomic location of the tumor, type of primary treatment, Dukes' stage at final diagnosis, or survival. There were differences between the groups for age, presence of symptoms at diagnosis, time from detection to treatment, and method of detection. Cox regression analysis showed no difference in survival by type of health plan before or after adjusting for age and stage at diagnosis. The findings from this study are consistent with those from studies reporting little or no difference in the process or outcome of care for patients with different types of medical insurance coverage.

Colorectal Neoplasms↗

PubMed1992

Breast cancer screening behaviors and attitudes in three racial/ethnic groups.

Data from a multiethnic sample of women participating in the American Cancer Society 1987 Texas Breast Screening Project was used to compare attitudes and behaviors related to breast cancer screening for whites, blacks, and Hispanics. In general, similar patterns of association were observed across racial/ethnic groups between a number of demographic and risk factors and prior mammography and recent clinical breast examination (CBE), although the magnitude of the associations varied somewhat across groups. Reasons for not having had prior mammography also were similar across groups, with lack of physician referral and cost cited as the two most important reasons. However, Hispanics were less likely than blacks or whites to report prior breast cancer screening, including mammography, CBE, and breast self-examination (BSE). This study demonstrated that women of different racial/ethnic backgrounds can be successfully recruited to participate in a patient-initiated, community-based program. However, this programmatic approach requires augmentation with other intervention strategies designed to reach low-income women because women with more years of education and higher family income were overrepresented in all three groups.

Adult↗

PubMed1992

Different neuroendocrine systems modulate pulsatile luteinizing hormone secretion in photosuppressed and photorefractory ewes.

The objective of this study was to determine whether two photoperiod regimens that induce anestrus in the ewe-short-day photorefractoriness (SDPR) and long-day photosuppression (LDPS)--act by different neuronal mechanisms. In separate experiments, ovary-intact (INTACT), ovariectomized (OVX), and ovariectomized estradiol-treated (OVX + E) ewes were subjected to three different photoperiodic regimens that resulted in reproductive quiescence: (1) exposure to long days (16L:8D), which caused photosuppression (INTACT, n = 9; OVX, n = 6; OVX + E, n = 5; (2) prolonged exposure to short days (10L:14D)), which caused photorefractoriness (INTACT, n = 10; OVX, n = 6; OVX + E, n = 5); (3) exposure to natural photoperiod, which induced seasonal anestrus (INTACT, n = 11; OVX, n = 6; OVX + E, n = 5). Effect of photoregimen was monitored by measuring progesterone or LH. Drug challenges were made after two sequential estrous cycles were missed in INTACT ewes, after mean LH concentrations dropped below 1 ng/ml in OVX + E ewes, and after LH interpulse intervals increased in OVX ewes. Effects of drug on LH pulse pattern were determined by taking blood samples at 12-min intervals for 8 h after i.v. diluent injection; then for 8 h after i.v. injection of cyproheptadine, a serotonin antagonist (3 mg/kg); and again 7 days later after i.v. injection of diluent or pimozide, a dopamine antagonist (0.25 mg/kg). Cyproheptadine had little effect except to decrease (p = 0.05) mean LH in INTACT anestrous ewes and decrease (p less than 0.01) pulse amplitude in OVX + E SDPR ewes. Pimozide did not affect LH pulse frequency in LDPS ewes. However, pimozide increased LH pulse frequency (p less than 0.005) and mean concentrations (p less than 0.005) in SDPR OVX + E ewes, whereas it suppressed LH pulse frequency (p less than 0.05) and amplitude (p less than 0.03) in SDPR INTACT and SDPR OVX ewes. The results suggest that (1) the role of the dopaminergic system differs in SDPR and LDPS ewes, and that different neuronal systems may effect SDPR and LDPS, (2) the effect of pimozide in SDPR ewes is altered by ovarian steroids, and (3) the serotonergic system has relatively little role in regulating pulsatile LH secretion in any of the three different states of anestrus.

Animals↗

PubMed1992

Divergent roles of protein kinase C in luteinizing hormone biosynthesis versus release in rat anterior pituitary cells.

We previously demonstrated that protein kinase C (PKC) activators, i.e. L-alpha-1,2-dioctanoyl glycerol (C8) and phorbol 12-myristate 13-acetate (PMA), mimic the stimulatory effects of GnRH on both LH glycosylation and release. To further evaluate the roles of PKC, we determined: 1) the interaction between PKC activator and GnRH; and 2) the effects of depleting cellular PKC with a high dose of PMA on LH glycosylation vs. release. Anterior pituitaries excised from ovariectomized rats were enzymatically dispersed and cultured. In series 1 experiments, day 3 monolayer cells were incubated in the presence of radiolabeled precursors and GnRH (0, 1, or 100 nM), with or without C8 (200 microM). In series 2 experiments, day 2 cells were pretreated with either PMA (1 microM) or vehicle (0.08% dimethyl sulfoxide) for 24 h and then incubated with diluent, GnRH (1 nM), or PMA (20 nM), and radiolabeled precursors for 4 h. LH translation and glycosylation were monitored by measuring incorporation of [14C]alanine ([14C]A) and [3H]glucosamine ([3H]GA), respectively, into LH. Immunoreactive LH (IRLH) was measured by RIA. In series 1 experiments, C8 increased basal release of IRLH, potentiated IRLH release stimulated by 1 nM GnRH, but not by 100 nM GnRH. C8 elevated total [3H]GA-LH but had no additive effects with GnRH. In series 2 experiments, PMA pretreatment inhibited subsequent PMA-stimulated IRLH release. However, PMA pretreatment did not affect GnRH-induced IRLH release even though PMA pretreatment decreased cellular IRLH content. In comparison, PMA pretreatment reduced both GnRH- and PMA-stimulated total [3H]GA-LH. PMA pretreatment had no effects on total [14C]A-LH in the presence of GnRH or PMA, but reduced the basal level. In summary, PKC activators had no additive effects on either IRLH release or LH glycosylation stimulated by a maximal dose of GnRH. However, PMA pretreatment decreased GnRH-induced LH glycosylation without depressing LH release. These results suggest differential roles of PKC in the actions of GnRH on LH glycosylation vs. LH release.

Alanine↗

PubMed1991

N-methyl-D, L-aspartate induces a transient increase in LH secretion in the seasonally anestrous ewe.

The primary objective of this study was to determine the LH response to an excitatory amino acid agonist, N-methyl-D, L-aspartate (NMA) in the seasonally anestrous ewe. In experiment 1, 3 i.v. injections of NMA were given; doses of 0.5, 1.5 and 4.5 mg/kg BW were tested. LH response to NMA depended on the dose. There was little response to the lowest dose. All animals responded to the first injection of the intermediate and the highest doses (mean pulse amplitude: 9.2 +/- 0.4 and 6.8 +/- 1.2 ng ml, respectively). The responses to the second or third injections of both doses were variable and were either absent or reduced compared to that of the first. In experiments 2 and 3, ewes were given 3 injections of normal saline (NS) followed by 3 injections of NMA (1.25 and 4.5 mg/kg BW, respectively) at 2 hr intervals. The last injection of NMA was followed 2 hr later by an injection of GnRH (3.0 ng/kg BW). In experiment 2, the first NMA injection induced an immediate LH pulse (mean pulse amplitude: 8.0 +/- 1.6 ng/ml) in all ewes, however, the second and third injections induced LH pulses in only 25% and 75% (mean pulse amplitude: 2.2 and 2.4 +/- 0.6 ng/ml) of the ewes, respectively. In experiment 3, NMA increased mean LH release (P less than 0.05) after all injections, but responsiveness to the third injection was reduced in some ewes. GnRH injections induced LH release in all ewes in experiments 2 and 3 (mean pulse amplitude: 6.9 +/- 1.8 and 6.4 +/- 2.2 ng/ml, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Anestrus↗

PubMed1991

Persistence of a circannual rhythm of plasma prolactin concentrations in ewes exposed to a constant equatorial photoperiod.

Circulating concentrations of prolactin were monitored for 3 yr in intact ewes kept either outdoors or indoors in a fixed equatorial photoperiod (12L:12D) and restricted range of environmental temperatures. Prolactin data were analyzed by spectral analysis. In all ewes kept outdoors, concentrations of prolactin showed robust circannual rhythms with a single predominant period of 359 days. In ewes kept indoors, the range of significant periods varied from 35 to 532 days. Although all ewes kept indoors showed a significant rhythm with a period of 354 days, this clearly was not the predominant period in all. The amplitude of the rhythm in ewes kept indoors was significantly lower (p less than 0.01) than that of ewes kept outdoors. Although the annual rhythm of circulating prolactin typical of ewes kept outdoors was significantly compromised in animals kept under a constant 12L:12D photoperiod and restricted environmental temperature range, there was evidence of an endogenous circannual rhythm.

Animals↗

PubMed1991

Testosterone inhibits gonadotropin-releasing hormone pulse frequency in the male sheep.

The objective was to determine the effect of chronic testosterone (T) treatment on GnRH and LH secretion in wethers. Rams were either castrated only or castrated and immediately treated with Silastic implants containing T. Several weeks later, a device for collecting hypophyseal-portal blood was surgically implanted. Six to seven days later, blood samples were collected simultaneously and continuously from the portal vessels and jugular vein of pairs of conscious animals. Samples were divided at 10-min intervals for 6-12 h. One hour before the end of collection, all animals received i.v. injections of 250 ng of GnRH. In samples collected simultaneously from 6 pairs of animals, T reduced the frequency of both GnRH pulses (1.8 +/- 0.2 vs. 0.9 +/- 0.3/h, p less than 0.03) and LH pulses (1.6 +/- 0.1 vs. 0.8 +/- 0.3/h, p less than 0.03). T did not alter amplitude of either GnRH or LH pulses. Testosterone reduced mean GnRH (9.7 +/- 0.6 vs. 7.9 +/- 0.5 pg/ml, p less than 0.05), whereas mean LH was not significantly reduced (9.6 +/- 1.4 vs. 6.1 +/- 1.8 ng/ml, p = 0.16). These results support the hypothesis that T reduces GnRH pulse frequency.

Animals↗

PubMed1991

Identification of colon cancer-predisposed individuals: a cytogenetic analysis.

Peripheral blood lymphocyte cultures from 23 individuals with polyps and 10 age-matched controls were cytogenetically studied. Of 23 individuals with polyps, 13 were males and 10 females. The age distribution of these patients was between 22 and 84 yr, with a medium age of 49.6 yr. Chromosome 5 was involved in structural rearrangements in 10 of the 18 patients with adenomatous polyps (55.5%). All those who showed involvement of chromosome 5 alterations had either adenomatous polyps or Gardner's syndrome, and two had colon adenocarcinoma. Of the remainder who did not show involvement of chromosome 5, eight had adenomatous polyps and five had hyerplastic polyps (non-neoplastic). Of the 10 control samples, only one showed alteration of chromosome 5. Of the 10 adenomatous polyp subjects that showed chromosome 5 defects, two had translocations of chromosome 5 and the short arm of 17. Chromosome 5 was broken 13 times (5 times in the q arm and 8 times in the p arm). Based on our present cytogenetic data, we tentatively conclude that lymphocyte metaphases may be used to identify colon cancer-predisposed individuals.

Adenoma↗

PubMed1990

Participation in breast screening programs: a review.

Despite recommendations by the American Cancer Society and other organizations for use of screening mammography, data on reported utilization of this procedure by American women show that these guidelines are not being met. We reviewed published studies that reported participation rates or that examined factors associated with participation in selected breast screening programs. In general, women at high risk due to age and family or personal history of breast disease were not more likely to participate in breast screening programs than women without those risk factors. The one group of variables that was fairly consistently associated with participation was the practice of other preventive health behaviors. Women who expressed more concern about their health and who were more knowledgeable about breast cancer screening and its benefits also were more likely to complete mammography. Approaches to increasing participation are discussed in the context of the literature on this subject.

Attitude to Health↗

PubMed1990

Continuous exposure of Suffolk ewes to an equatorial photoperiod disrupts expression of the annual breeding season.

The objective was to determine if "clamping" ewes onto a 12L:12D photoperiod resulted in expression of circannual rhythms of reproductive activity. On 24 February, 1986, two groups of 6 yearling ewes each were placed in isolated adjacent photochambers under a 12L:12D photoperiod and controlled temperature. Six control ewes were kept outdoors. Blood samples taken thrice weekly were analyzed for progesterone. Data from Days 0-1056 are reported. The mean number of cycles by control and 12L:12D ewes did not differ (32.8 +/- 1.7 vs. 29.7 +/- 4.0). The ranges were 27-39 vs. 4-51, respectively. Ten 12L:12D ewes started cycling coincidentally or later than the controls, and then cycled either regularly or irregularly throughout the study. Two of the 12L:12D ewes cycled continuously. The mean number of cycles during the period 15 April-15 August (anestrus) in Years 1, 2, and 3 were 0.7, 0.7, 0.2 for controls versus 0.3, 5.1, and 4.5 for 12L:12D ewes. The mean number of cycles during the period 15 September-15 January (breeding season) in Years 1, 2, and 3 were 7.3, 7.7, and 7.3 for controls versus 2.8, 4.8, and 4.0 for 12L:12D ewes. All controls showed distinct, alternating annual periods of anestrus and ovarian cycles whereas only two 12L:12D ewes showed a similar pattern. Estrous cycles were distributed nonrandomly in all controls and in 2 ewes exposed to 12L:12D. In the 12L:12D ewes, melatonin concentrations rose immediately after the lights-off and fell immediately after on. Lengths of the luteal phases of the cycles did not differ between groups. In summary, estrous cycles of most ewes clamped on a 12L:12D photoperiod occurred throughout the year at variable intervals rather than in distinct breeding seasons.

Anestrus↗

PubMed1990

17-beta-estradiol potentiates luteinizing hormone glycosylation and release induced by veratridine, diacylglycerol, and phospholipase C in rat anterior pituitary cells.

We determined the effect of 17 beta-estradiol (E2) on synthesis and release of luteinizing hormone (LH) induced by drugs which activate intracellular signal transduction mechanisms in rat anterior pituitary cells. Cells were pretreated with E2 (6 x 10(-10) M) or diluent for 24 h, then washed and incubated for 4 h with E2 or diluent, respectively, in the presence or absence of drugs. LH translation and glycosylation were monitored by measuring incorporation of [14C]alanine and [3H]glucosamine, respectively, into total (medium plus cells) immunoprecipitable LH. Immunoreactive LH (IRLH) was measured by radioimmunoassay. Gonadotropin-releasing hormone (GnRH, 1 nM), veratridine (5 microM), L-alpha-1,2-dioctanoyl glycerol (C8, 200 microM), and phospholipase C (PLC, 0.24 U/ml) all increased (p less than 0.01) medium IRLH, [3H]glucosamine-LH, and [14C]alanine-LH, and total [3H]glucosamine-LH in both E2- and diluent-treated cells. Total IRLH or [14C]alanine-LH were not increased by any treatment. E2 alone slightly increased (p less than 0.05) basal medium IRLH and [3H]glucosamine-LH. The stimulatory effects of E2 on basal medium [14C]alanine-LH and total [3H]glucosamine-LH were inconsistent. E2 potentiated (p less than 0.01) the effects of veratridine, C8, PLC, and GnRH on medium IRLH, and medium and total [3H]glucosamine-LH. E2 also potentiated (p less than 0.01) the effects of veratridine, PLC, and GnRH, but not of C8, on medium [14C]alanine-LH. In contrast, E2 did not increase either precursor uptake or incorporation of precursor into total protein in the presence of any secretagogue.(ABSTRACT TRUNCATED AT 250 WORDS)

Alanine↗

PubMed1990

Multiple polyposis coli associated with Gardner's syndrome and chromosomal mosaicism: a family analysis.

A family in which the proband (father) has multiple polyposis coli associated with Gardner's syndrome and his only son, also with multiple polyps, has been studied cytogenetically. The karyotypes of the proband and the son have shown deletion in the short arm of one chromosome 12 in a small percentage of cells. Normal cells and cells with a deletion of a 12p were noted, not only in the lymphocytes, but also in the skin fibroblast culture of the son. The karyotypes of the asymptomatic mother and their unaffected daughter were considered to be normal.

Adenomatous Polyposis Coli↗

PubMed1989

Reproductive refractoriness in the Welsh Mountain ewe induced by a short photoperiod can be overridden by exposure to a shorter photoperiod.

The objectives were to determine if relative lengths of photoperiods that induce reproductive cycles in ewes affect the length of the subsequent breeding season, if duration of the refractoriness that terminates breeding is affected by photoperiod length, and if the resulting refractoriness to an inductive photoperiod is absolute. Groups of Welsh Mountain ewes were exposed to either 12L:12D (n = 12) or 8L:16D (n = 6) photoperiods beginning at the summer solstice when daylengths reach a maximum of 17.5 h at Bristol, England. A control group (n = 10) was exposed to natural daylengths. Ovarian cycles in the controls, as judged by monitored plasma progesterone levels, commenced in early October, about 1 mo later (p less than 0.001 in both cases) than in sheep exposed to 12L:12D or 8L:16D. The advancement in cycle onset was similar under 12L:12D and 8L:16D (69 +/- 2 and 77 +/- 4 days after the summer solstice compared with 102 +/- 2 days in the controls). Duration of the breeding season (100 +/- 4 days) in ewes exposed to 12L:12D was significantly shorter (p less than 0.001 in both cases) than in ewes exposed to natural daylengths or 8L:16D (153 +/- 3 and 133 +/- 5 days, respectively). Approximately 70 days after the ending of ovulatory cycles in the 12L:12D group, half of the animals (n = 6) were transferred to 8L:16D. This treatment greatly (p less than 0.001) reduced the duration of anestrus and cycles began again 62 +/- 4 days after transfer to 8L:16D, or about 90 days earlier than in ewes (n = 6) remaining in 12L:12D.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗