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Biology of Reproduction 67, 591-598 (2002)
© 2002 Society for the Study of Reproduction, Inc.

Regular Article

Changes in Plasma Concentrations of Luteinizing Hormone, Progesterone, and Estradiol-17ß in Peripubertal Turkey Hens under Constant or Diurnal Lighting1

Wayne L. Bacon2,,a, Jorge A. Vizcarrab, James L.M. Morganb, Jingying Yangb, Han-Ken Liua, David W. Longa, and John D. Kirbyb

a Department of Animal Sciences, Ohio Agricultural Research and Development Center, The Ohio State University, Wooster, Ohio 444691 b Department of Poultry Science, University of Arkansas, Fayetteville, Arkansas 72701


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Possible circadian fluctuations and long-term changes in concentrations of reproductive hormones in peripubertal female birds is poorly documented in comparison with mammalian species. Our objective was to document changes in concentrations of several reproductive hormones the several days before and after initial pubertal preovulatory surges of LH in turkey hens photostimulated with either constant (24L:0D) or diurnal (14L:10D) lighting. The hens were cannulated for hourly blood sampling, starting 10 days after photostimulation and continuing until all hens had laid at least two eggs. First eggs were oviposited between 16 and 24 days after photostimulation, and egg production ranged from two to nine eggs/hen during the experimental period. With both lighting treatments, concentrations of LH declined slightly, concentrations of progesterone (P4) increased, and concentrations of estradiol-17ß (E2) were constant the 3–4 days prior to initial LH surges with no circadian fluctuations in hormone concentrations. Most (10 of 13) initial preovulatory surges of LH were coupled with ovulations, and all LH surges were coupled with P4 surges. Those LH and P4 surges not coupled with ovulations (blind surges) occurred with both lighting treatments, but the incidence of blind surges was higher with diurnal lighting. The interval between LH and P4 surges was longer between the first and second surges than between subsequent surges, when the interval was approximately 26 h. The duration of LH surges (7.4 ± 3.0 h) was shorter than that of P4 surges (10.0 ± 2.0 h). We conclude that, in the peripubertal female turkey, 1) prior to puberty (first LH-P4 surges), there are no circadian fluctuations in concentrations of LH, P4, and E2, 2) 3 days prior to initial LH surges, E2 concentrations are stable, LH concentrations decline slightly, and P4 concentrations increase, and 3) surges of LH are coupled to surges of P4 but LH-P4 surges are not always coupled to ovipositions (blind surges), possibly because of internal ovulations.

estradiol, luteinizing hormone, ovulation, progesterone, puberty


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Nocturnal increases in LH have been reported to occur prior to puberty in the female rat [1], sheep [2], and various primates [3], all spontaneously ovulating species. Also, an increase in estradiol-17ß (E2) occurs 48–24 h prior to ovulations in rats [1], sheep [2], primates [3], pigs [4], and a marsupial, the tammar wallaby [5]. This preovulatory increase in E2 has been shown to be the primary inducer of preovulatory surges of LH in these spontaneously ovulating mammalian species. In some induced-ovulator mammalian species, such as the rabbit [6], plasma E2 levels are high at first estrus and remain high through mating, which may not occur until after several successive waves of follicular development and regression. High-resolution data on changes in the concentration of E2 in female birds around the time of first ovulations are not available. Also, to the best of our knowledge, the possible occurrence of circadian fluctuations in LH, E2, and progesterone (P4) concentrations during the peripubertal period of turkey hens has not been reported.

The reproductive system of the turkey hen changes from an immature nonfunctional state to a mature functional state during the 2- to 4-wk period following photostimulation of photosensitive hens [7–9]. Photostimulation induces an increase in plasma LH [10, 11] and FSH [10] during the first long-day scotophase following the first long-day photophase. The increase in circulating LH is associated with an increase in the baseline level of the hormone and is followed by an increase in circulating E2, from about 0.050 ng/ml to about 0.300 ng/ml, within 3–7 days following photostimulation [9, 12, 13]. The increase in circulating E2 is associated with initiation of the vitellogenic stage of ovarian follicular [12, 14] and oviduct development [7, 9], which last 12–14 days in turkey hens [15, 16]. An increase in plasma P4, from about 0.10 to 1.00 ng/ml, occurs over the 4 days prior to oviposition of the first egg [17]. Thus, prior to initiation of the first preovulatory surge, LH, FSH, P4, and E2 all increase to levels observed between subsequent surges, with the order of the increases after photostimulation being LH = FSH > E2 >> P4. Most of these observations on peripubertal changes in LH, FSH, E2, and P4 are based on daily blood samples taken during the photophase of the daily lighting cycle.

Serial blood sampling studies, where blood was collected every 5–15 min, have shown that most of the observed increase in LH 1–2 days after photostimulation is associated with an increase in the baseline concentration rather than an increase in frequency, amplitude, or duration of pulses [11, 14, 18]. From the third day after photostimulation until the first oviposition, only a few low-amplitude pulses of LH were detected [14]. After initiation of egg production, the dominant feature of the secretory pattern of LH became the preovulatory surges of LH, superimposed on a nearly constant but relatively high baseline level of the hormone [14, 19, 20]. In laying turkeys, preovulatory surges of LH are characterized by steadily increasing plasma concentrations over 2–3 h to peak levels and then steadily declining concentrations over a 4- to 6-h period to baseline levels [14, 19, 20]. The onset of preovulatory P4 surges is coincident to the onset of LH surges [17, 20], but the duration of P4 surges is about 2 h longer [17]. Concentrations of both LH and P4 return to baseline levels by ovulation, where they remain until initiation of the next preovulatory LH and P4 surges. Additional intrasequence (intervals between surges of <30–34 h) and intersequence (intervals between surges of >30–34 h) preovulatory LH surges continue to occur as the reproductive period progresses [19, 20, 21].

Preovulatory LH surges normally occur during the dark periods (scotophases) during open periods in laying turkey [14, 19] and chicken [22] hens exposed to diurnal lighting. Constant lighting is also photostimulatory in turkey hens [23], but with constant lighting, the time of ovipositions spreads throughout the solar 24-h day [19, 22], as in chicken hens [24]. In both turkeys [14, 25] and chickens [26], egg production is spontaneous and not dependent on the presence of males or mating.

For the present study, we hypothesized that, in turkey hens, plasma concentrations of LH and E2 would be relatively constant for the several days just prior to initiation of egg production and that P4 concentrations would increase with the establishment of mature follicles in the follicular hierarchy prior to the initial ovulation. We also hypothesized that the initiation of preovulatory LH and P4 surges in turkey hens would be tightly coupled and would occur when a mature follicle capable of ovulation is present in the ovary during the open period of the LH secretion cycle. To determine whether circadian changes in these hormones occur prior to first ovulations, turkey hens were photostimulated with either constant lighting or photostimulatory diurnal lighting and hourly blood samples collected. Serial blood sampling was continued until all hens had oviposited at least two eggs, so peripubertal intervals between ovulatory surges of LH and P4 and patterns of LH and P4 surges could also be determined.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Turkey Hens and Management

Day-old female turkey poults of a line selected for increased egg production for 38 generations (Egg line, [27]) were utilized. The turkeys were grown in a single light-controlled room using wood shavings as litter. During brooding to 3 wk of age, the poults were given continuous light with heat lamps. From 3 to 16 wk of age, they were given 12L:12D lighting. At 16 wk of age, they were given short-day lighting of 6L:18D. At 19 wk of age, they were housed in individual enclosures (60 x 60 cm) bedded with wood shavings [8] in two identical rooms, 12 hens/room. At 26 wk of age, eight hens from each room were cannulated during photoperiods as previously described [14, 19]. After cannulation, the hens were returned to their individual enclosures, connected to a tether and swivel system with an extension cannula passing to an alcove outside the animal room, and placed on a continuous infusion of 0.7% NaCl, 0.5% Na citrate, 0.5 mg/ml Gentamicin at a rate of 10 ml/day. These procedures allowed the hens free movement and access to feed and water within the individual enclosures while infusion and blood collection operations were performed outside the animal room. At 27 wk of age, the hens were photostimulated with either diurnal lighting (14L:10D) or constant lighting (24L:0D). After 10 days of photostimulation, serial blood sampling of the hens began, with a blood sample (1.0 ml/sample) collected each hour for 320 h as previously described [19]. Serial blood sampling continued until all birds had laid at least two eggs (range, 2–9 eggs). The cannulas were then removed and the hens' egg production monitored on 1 additional week. Plasma was separated from blood cells by centrifugation at 4°C and harvested. To guard against hemodilution, the blood cells were reconstituted to original volume with sterile saline (0.90% NaCl) and returned to the hen of origin approximately every 3 h [19, 20, 22]. The hens were digitally palpated for presence of an egg in the shell gland and checked for the presence of oviposited eggs approximately every 8 h but not during scotophases. This method of palpation and egg collection allowed timing of ovulations and ovipositions into windows of 8 ± 2 h. An animal-use protocol for these procedures was approved by the Institutional Laboratory Animal Care and Use Committee (protocol 97 AG005).

Hormone Assays

The LH radioimmunoassay was conducted as previously described [11, 19]. The intra- and interassay coefficients of variation (CV) of a pool of laying turkey hen plasma (mean = 2.59 ng/ml) were 8.9% and 12.8%, respectively. The P4 radioimmunoassay was conducted as previously described [17], and the intra- and interassay CV of a pool of laying turkey hen plasma (mean = 1.41 ng ml-1) were 15.5 and 15.8%, respectively. The E2 radioimmunoassay was conducted as previously described [13], and the intra- and interassay CVs of a pool of laying turkey hen plasma (mean = 0.254 ng/ml) were 19.8% and 21.5%, respectively.

Statistical Analyses

The individual hen LH and P4 data were subjected to PULSAR analysis [28]. The G values used in the PULSAR analyses for LH and P4 were G(1) = 50.00, G(2) = 2.60, G(3) = 1.90, G(4) = 1.50, and G(5) = 1.20. The high G(1) value precludes detection of single-point pulses by the algorithm. The assay SD values used in the PULSAR analyses of LH were 7.53X + 8.90/100, where X is the concentration of LH measured in an individual sample. The assay SD values used in the PULSAR analyses of P4 were 15.0X + 0.0/100, where X is the concentration of P4 measured in an individual sample. Determination of frequency (period) of LH surges was done by calculating the intervals between peaks (highest values within pulses identified by PULSAR analysis) of consecutive LH surges. The number of surges of LH and P4 observed varied between three and nine within individual birds. Significance of differences between lighting treatments, interval or surge number, and their interaction were evaluated by ANOVA using the GLM procedure (SAS Institute, Cary, NC). Analyses were conducted for overall mean hormone concentration, calculated baseline hormone concentration, surge amplitude hormone concentration, and interval between hormone surges. For these analyses, surge number or interval was treated as a repeated measure.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Requirements set for inclusion of a hen in the study were that it must have laid at least two eggs during serial blood sampling and must have remained in egg production for at least 1 wk after completion of the 320 h of blood sampling. Seven of eight hens given constant lighting and six of eight hens given diurnal lighting met these inclusion criteria. The one hen given constant lighting that was excluded was successfully serially bled but stopped laying during serial blood sampling after producing six eggs, a highly unusual pattern of egg production for the egg line of turkeys. The two hens given diurnal lighting that were excluded lost cannula patency during serial blood sampling prior to laying two eggs.

There was no difference (P > 0.05) between lighting treatments for elapsed time from photostimulation to first eggs (range 16–24 and 17–22 days [mean ± SD, 19.3 ± 2.3 and 20.0 ± 2.0 days] for constant and diurnal lighting groups, respectively). Also, there was no difference in egg production during serial blood sampling (ranges 2–9 and 3–6 eggs [means ± SD, 6.0 ± 2.8 and 4.0 ± 1.3 eggs] for constant and diurnal lighting groups, respectively).

Representative examples of individual hen plasma LH and P4 concentration patterns are presented for three hens from each lighting treatment (Fig. 1). The data chosen for presentation were from hens within each lighting treatment with the shortest elapsed time from photostimulation to first eggs (Fig. 1, a and d), from hens with a duration of this period close to the mean for each group (Fig. 1, b and e), and from hens with the longest duration of this period for each group (Fig. 1, c and f). The hormone profiles for individual birds were similar between lighting treatments. All of the LH surges were associated with P4 surges for the profiles presented in Figure 1 and for the hormone profiles from the other seven birds of the experiment (profile data not presented). Each oviposited egg was coupled with a surge of LH and P4 (LH-P4), but not all LH-P4 surges were coupled with oviposition of an egg 1–2 days later (blind surges). In Figure 1, blind surges are marked with a question mark (?). No circadian fluctuations in concentrations of either LH or P4 prior to first eggs were apparent for individual birds given either lighting treatment. After first surges of LH-P4, repeating surges of both hormones occurred at intervals slightly longer than 24 h. The baseline concentrations of LH appeared to be relatively stable prior to and after first eggs, but the baseline concentrations of P4 appeared to increase the several days prior to first eggs in most of the hens.



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  		FIG. 1. Changes in concentrations of luteinizing hormone (LH, upper) and progesterone (lower) over 13+ d of hourly serial blood sampling of peripubertal turkey hens. a–c) Patterns of hens photostimulated with constant lighting (24L:0D). d–f) Patterns of hens photostimulated with diurnal lighting (14L:10D). a, d) Patterns from hens that started laying early after initiation of serial blood sampling, b, e) patterns from hens that started laying about midway through serial blood sampling, and c, f) patterns from hens that started laying late after initiation of serial blood sampling. The closed circles (•) represent approximate times of ovipositions. The question marks (?) in some panels represent occurrence of blind surges of LH

The interval between surges of LH was not affected by lighting treatment (Fig. 2), but the interval between the first and second LH surges was longer than intervals between subsequent surges. The mean interval between the second and later surges was 26.1 ± 2.9 h (mean ± SD). There was no interaction between lighting treatment and interval number. The amplitude of LH surges was not different between lighting treatments (Fig. 2), but the first surge had a higher amplitude than the third and subsequent surges. The mean amplitude of the third and later LH surges was 3.88 ± 1.87 ng/ml. There was no interaction between lighting treatment and surge number for amplitude of LH surges. The duration of LH surges was not different between lighting treatments (Fig. 2). The duration of surges did decrease from the first through the fifth surge. The mean duration of fifth and later surges was 7.4 ± 3.0 h. There was no interaction between lighting treatment and surge number for duration of surges (Fig. 2). The baseline concentration of LH was slightly higher for the hens given constant lighting than for the hens given diurnal lighting, but there was no change in baseline concentration of LH with increasing surge number. The mean baseline LH concentrations were 2.52 ± 0.68 and 2.22 ± 0.44 ng/ml for constant and diurnal lighting groups, respectively. There was no interaction between lighting treatment and surge number for baseline concentration of LH.



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  		FIG. 2. Effects of lighting treatment (C, constant lighting [24L:0D]; D, diurnal lighting [14L:10D]) and surge interval or number on a) interval between LH surges, b) duration of LH surges, c) amplitude of LH surges, and d) baseline of LH under surges. Columns with no same letters (x, y for lighting treatment and a, b, c, d, and e for interval or surge number) are different (P < 0.05). Data presented are means ± SD

The interval between surges of P4 was not affected by lighting treatment, but the interval between the first and second P4 surges was longer than intervals between subsequent surges (Fig. 3). The mean interval between the second and later surges was 25.8 ± 3.8 h. There was no interaction between lighting treatment and interval number for interval between surges. The amplitude of P4 surges was not different between lighting treatment or surge number (Fig. 3). The mean amplitude of P4 surges was 6.4 ± 2.0 ng/ml. There was no interaction between lighting treatment and surge number for amplitude of surges. The duration of P4 surges was longer with constant lighting than with diurnal lighting (Fig. 3). The duration of surges decreased from the first through the third surge. The mean duration of the third and later surges was 10.0 ± 2.0 h. There was no interaction between lighting treatment and surge number for duration of surges (Fig. 3). The baseline concentration of P4 was not different between lighting treatments. However, there was an increase in baseline concentration of P4 from the first through the third surge. The mean baseline P4 concentration of the third and later surges was 2.05 ± 0.45 ng/ml. There was no interaction between lighting treatment and surge number for baseline concentration of P4.



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  		FIG. 3. Effects of lighting treatment (C, constant lighting [24L:0D]; D, diurnal lighting [14L:10D]) and surge interval or number on a) interval between progesterone surges, b) duration of progesterone surges, c) amplitude of progesterone surges, and d) baseline of progesterone under surges. Columns with no same letters (x, y for lighting treatment and a, b, c, d, and e for interval or surge number) are different (P < 0.05). Data presented are means ± SD

The intervals between surges of LH and P4 after the first interval were not different (26.1 ± 2.9 h and 25.8 ± 3.8 h, respectively; P = 0.27). The duration of P4 surges after the fourth surge was longer than the duration of LH surges after the fourth surge (10.0 ± 2.0 h and 7.4 ± 3.0 h, respectively; P, < 0.01).

All LH surges were coupled with P4 surges, and the number of observed surges was not different between lighting treatment groups (6.2 ± 2.5 surges per hen for constant lighting and 5.2 ± 2.1 surges per hen for diurnal lighting). The incidence of surges of LH-P4 not coupled with palpation of an egg in the oviduct and oviposition of an egg 1 or 2 days later (blind surges; see Fig. 1 for examples) was 2 of 44 surges (4.5%) for the seven hens given constant lighting and 10 of 31 surges (32%) for the six hens given diurnal lighting (P < 0.01). Blind surges occurred in two of the seven hens given constant lighting and in all six hens given diurnal lighting. First surges of LH-P4 were associated with palpation of an egg in the oviduct within 18 h and oviposition of an egg within 36 h in six of the seven (86%) hens given constant lighting and in four of the six hens (67%) hens given diurnal lighting. Thus, with both lighting treatments, in most hens, first surges of LH-P4 were coupled with oviposition of eggs.

Data from four birds of each lighting treatment group were used to examine whether the concentration of LH, P4, and E2 changed during the 3–4 days prior to first LH-P4 surges. Physiological Time 0 for each hen and hormone was the peak sample of the first LH surge identified by PULSAR analysis. Figure 4 presents plots of the mean calculated baseline and mean observed values for LH, P4, and E2 for both lighting treatments. The calculated baseline values prior to initiation of first surges were examined by linear regression to detect if there was a change with time over the 3–4 days prior to initiation of first LH and P4 and E2 surges. The calculated baseline concentrations of LH decreased slightly with both lighting treatments as the hens approached first LH preovulatory surges (Fig. 4 and Table 1). While there was a significant decline with both lighting treatments, the relative change during the approximately 4 days prior to initiation of first LH surges was small (14% and 7%, respectively, for diurnal and constant lighting groups). The calculated baseline concentrations of P4 increased with both lighting treatments as the hens approached first LH preovulatory surges (Fig. 4 and Table 1). The relative increases of the calculated P4 baseline during the approximately 3 days prior to first preovulatory surges of LH were approximately 148% and 198%, respectively, for diurnal and constant lighting groups. The calculated baseline concentrations of E2 did not change in either lighting treatment group during the approximately 2–3 days prior to first preovulatory surges of LH.



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  		FIG. 4. Changes in concentrations of LH, progesterone, and estradiol-17ß the 3–4 days prior to, during, and 10–15 h after initial preovulatory surges of LH. The upper panels are from hens photostimulated with diurnal lighting (14L:10D) and the lower panels are from hens photostimulated with constant light (24L:0D). Time 0 for each hormone is the peak of the first LH surge identified by PULSAR analysis. Data presented are means ± SD


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  		TABLE 1. Linear regression intercepts and slopes and correlation coefficients between time (independent variable) and hormone concentration (dependent variable) for LH, P\in\4\r\, and E\in\2\r\ prior to puberty in turkey hens


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Initiation of ovulations in most of the turkey hens was associated with the first apparent surges of LH and P4 following photostimulation. We hypothesized that circadian variations or ghost surges of LH and P4 could precede the first preovulatory LH surge and that these variations could function to drive the maturation of the initial F1 follicle. The requirement for increased circulating levels of the gonadal steroids E2 and P4 in the induction of vitellogenesis by the liver and maturation of the oviduct has been well established. While increased circulating levels of E2 and P4 are associated with puberty in female birds, temporal changes in P4 have been suggested to act as a priming mechanism for the initiation of preovulatory LH surges [29]. However, unlike what has been observed in some spontaneously ovulating mammalian species, the first LH surge predominately resulted in an ovulation coupled to an oviposition. Thus, the initial F1 follicle at puberty matures although it is not exposed to prior surges or circadian fluctuations of LH, P4, or E2. Also, no such temporal variations in LH, E2, and P4 were identified prior to subsequent preovulatory surges of LH and P4 in turkey hens. These observations were the same with either constant or diurnal lighting. As this is the first study in birds to chronicle the hourly changes in LH, E2, and P4 at the onset of egg production, whether this is a common theme among avian species or unique to the turkey hen is unknown.

The photosensitive turkey hens utilized in this study were photostimulated with either constant or diurnal lighting. Only slight differences in plasma concentrations of LH, E2, and P4 were associated with lighting treatment. The baseline concentration of LH was slightly increased with constant lighting; however, this was not associated with a change in the baseline concentration of P4 or E2 but with a slight increase in duration of surges of P4. The intervals between surges of both LH and P4 were not affected by lighting treatment. The absence of light-dark cycles thus had only minor effects on hormone concentrations before initial surges at puberty and during the first few oviposition cycles immediately after puberty.

The amplitude and duration of LH surges declined after the initial surges at puberty. In a previous study with the Egg line of turkeys [21], LH surge amplitude declined further between 3 and 40 wk of egg production, but duration of surges did not change with progression of the reproductive period. Surges of LH are induced by surge secretion of GnRH in birds as in mammals [29]. We suggest that changes in duration and amplitude of LH surges may be directly related to changes in GnRH surge amplitude and duration during the reproductive period. However, GnRH surge secretion parameters have not been measured in birds. An alternative explanation of these changes in LH surge amplitude and duration could be related to changes in sensitivity of the anterior pituitary to surges of GnRH. This relationship also remains to be examined in detail in laying turkey hens.

The intervals between LH and P4 surges showed little variation and no change after the first interval and was not affected by lighting treatment. Also, no long-duration intersequence intervals (intervals > 30–34 h) ascribed to pauses between sequences [21] were observed in the present experiment. In a longitudinal study using the Egg line of turkeys [19], a slightly longer interval between LH surges (27.9 and 25.7 h, respectively) was found with constant lighting than with 14L:10D diurnal lighting. In this previous study [19], the hens were utilized at an older age (50 wk of age) and only intrasequence intervals between surges were included in the comparison between lighting treatments. Several intersequence intervals were detected in this previous study [19]. In a more recent study with Egg line turkey hens under constant lighting [21], the relative incidence of intrasequence intervals between LH surges decreased from 94% with a mean interval of 25.3 h early (3–5 wk) in a reproductive period to 60% with a mean interval of 29.0 h late (38–40 wk) in a reproductive period. We conclude from the present and previous studies [19–21] 1) that the relative incidence of intersequence intervals between LH-P4 surges is relatively low peripubertally but increases late in the egg production period [21], 2) that the interval between short-duration intrasequence LH surges is longer in a line of turkeys with poor egg production [20], and 3) that blind LH-P4 surges may occur both early (this study, [21]) and late [15] in the egg production period and at different rates among lines of turkeys [20].

In turkey hens, most ovarian E2 secretion has been shown to originate from the theca externa cells of prehierarchical and hierarchical follicles [30, 31] and to be sensitive to LH and FSH [32, 33] stimulation. In contrast, most ovarian P4 secretion has been shown to originate from granulosa cells of primarily the largest and most mature F1 hierarchical follicle [30, 31], which is also the follicle most sensitive to LH-induced ovulation. Thus, in the laying hen, most E2 is produced by theca cells under the dual control of LH and FSH, while most P4 is produced by granulosa cells of the F1 hierarchical follicle under the control of LH. This suggests that the observed increase in basal concentrations of P4 during the several days prior to first preovulatory surges of LH and P4 is associated with this increased maturation of hierarchical follicles and associated increased sensitivity to the relatively constant concentrations of LH during this time. These relationships also suggest that, in the laying turkey hen, the relatively high baseline level of P4 is maintained by the most mature follicle(s) in the ovarian hierarchy, while the preovulatroy surge of P4 is secreted by the F1 follicle in association with a preovulatory surge of LH [34].

Recently, the relationships among baseline plasma P4, ovulation rate, and number of mature ovarian follicle(s) have been reported [35]. High baseline levels of P4 (>2.0 ng/ml) were associated with a large number of mature ovarian follicles, an arrest in laying, retention of a hard-shelled egg in the shell gland by some hens, and a cessation of LH surges in hens photostimulated with constant lighting in comparison with diurnal lighting. In this previous report, it was suggested that high baseline P4 is associated with an arrest in laying. No difference in baseline P4 level was detected between lighting treatments in the current study, which was terminated after only about 1 wk of egg production, but the baseline concentration of P4 increased peripubertally, suggesting an increase in the number of large hierarchical follicles peripubertally.

Most (10 of 13) of the initial preovulatory LH-P4 surges were associated with oviposition of normal eggs 1 or 2 days later, but several (3 of 13, blind LH-P4 surges) were not. These initial blind LH-P4 surges also were not associated with the presence of an egg being held in the shell gland for an extended period of time. Other blind LH-P4 surges were noted after the initial LH-P4 surges with both lighting treatments, suggesting that some ovulations resulted in internal ovulations, where the ovulated yolk did not enter the infundibulum of the oviduct for completion of egg formation. The incidence of internal ovulations has been reported to be common at initiation of egg production in growth-selected lines of turkeys [7] and to be greater in commercial lines selected almost exclusively for growth characteristics than in commercial lines selected for a combination of growth and reproduction characteristics [7]. Blind LH-P4 surges have been observed previously in turkey hens at older ages [21] than the age of the hens used in the current study.

Prior to and during initial LH-P4 surges, no long-term change in concentration of E2 was detected. An increase in E2 concentration has been reported to occur 2–3 wk prior to first eggs in turkey hens [9, 12, 17] in association with oviductal development [7, 9], synthesis and secretion of yolk lipoprotein precursors by the liver, and the accretion of these E2-induced lipoproteins by hierarchical follicles [36]. The current study showed no nocturnal or circadian fluctuations in E2 immediately prior to initial ovulations at puberty or prior to subsequent ovulations of the laying hens.

In turkey hens, highest frequency of matings occurs approximately 1 wk prior to first eggs [37], when E2 levels in plasma are relatively high [13]. In the rabbit, an induced ovulator, plasma E2 levels are constantly high during behavioral estrus, which may be prolonged through numerous waves of follicular development, and mating induces a surge in LH and associated ovulation [25]. The turkey hen is thus similar to the rabbit in that high plasma levels of E2 are present and maintained for various periods of time prior to ovulation, but unlike the rabbit, the turkey is a spontaneously ovulating species that does not require the presence of a male or mating for ovulation. The turkey is also unlike spontaneously ovulating mammalian species where plasma E2 increases 24–48 h prior to first pubertal ovulations in that plasma E2 increases 2–3 wk prior to initial ovulations during the peripubertal period, and the plasma concentration is then relatively stable and without circadian fluctuations.

In conclusion, in the peripubertal female turkey, photostimulation with constant lighting had only minor effects on concentrations of LH, P4, and E2 or on induction of egg laying in comparison with photostimulation with diurnal lighting. All surges of LH were coupled to surges of P4, but LH-P4 surges were not always coupled to subsequent ovipositions. In most hens, first surges of LH-P4 were associated with oviposition of eggs. There were no circadian fluctuations in concentrations of LH, P4, and E2 prior to first eggs. Finally, the 3 days prior to first ovulations, the concentrations of E2 were steady, while the concentrations of LH declined and the concentrations of P4 increased 1.5- to 2-fold.


   ACKNOWLEDGMENTS
 
We thank Mr. John W. Anderson and Ms. Ruthi A. Patterson for help in collection of blood samples. We also thank Dr. John Proudman (USDA) for supplying the LH RIA reagents.


   FOOTNOTES
 
First decision: 9 January 2002.

1 Salaries and research support provided by state and federal funds appropriated to the Ohio Agricultural Research and Development Center, The Ohio State University, and the Arkansas Agricultural Experiment Station. Supported in part by the George and Edna Jaap Endowment for Poultry Research, The Ohio State University. Back

2 Correspondence: Wayne L. Bacon, Department of Animal Sciences, The Ohio State University, 1680 Madison Avenue, Wooster, OH 44691-4096. FAX: 330 263 3949; bacon.2{at}osu.edu Back

Accepted: March 12, 2002.

Received: December 14, 2001.


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TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
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