Effects of Photoperiod and Age on Secretory Patterns of Luteinizing Hormone and Testosterone and Semen Production in Male Domestic Turkeys¹

^a Department of Animal Sciences, Ohio Agricultural Research and Development Center, The Ohio State University, Wooster, Ohio 44691 ^b Department of Veterinary Biosciences, The Ohio State University, Columbus, Ohio 43210

	ABSTRACT

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The objective of this study was to determine effects of photoperiod and age on the circulating concentrations of LH, testosterone (T), thyroxine (T4), and semen production in male turkeys. Male turkeys from 10 or 12 wk of age were maintained under either a long-day (LD) photoperiod of 16L:8D up to 35 wk (LL), or a short-day (SD) photoperiod of 6L:18D to 29 wk and then an LD photoperiod of 16L:8D up to 35 wk of age (SL). Plasma concentrations of both LH and T increased by 17 or 18 wk of age under both photoperiods, but higher levels were attained in the LL group prior to 29 wk of age. Both LH and T levels increased significantly within days in the SL group after the group was switched to LD at 29 wk of age. Higher levels of T4 were present in the LL group prior to sexual maturation. No differences were observed in T4 concentration between lighting treatments after sexual maturation. The LL group first produced semen at 20–22 wk of age, which was about 1 wk earlier than first semen production in the SL group. A significantly larger volume of semen was produced in the LL group at most ages. No further increase in semen production was observed in the first 6 wk after the SL group was switched to the LD photoperiod at 29 wk of age. Pulsatile patterns of LH and T were characterized by serial blood sampling at 13, 23, and 35 wk of age under both the LL and SL photoperiods. The baseline levels of both LH and T in male turkeys were influenced by age and photoperiod. However, pulse characteristics (numbers, duration, and amplitude) of LH did not change with age or lighting treatment, while pulse characteristics of T did change with age. We conclude that exposing male turkeys to an LD photoperiod from 10 or 12 wk of age advanced the age of sexual maturation and induced earlier increased concentrations of LH and T.

	INTRODUCTION

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Changes in gonadotropins and sexual steroids have been determined in the male of many species in order to advance understanding of regulation of reproduction. Determination of changes in hormone concentrations in animals serially bled for several hours shows that both LH and testosterone (T) are secreted in pulsatile patterns in male mammals, including humans [1–3], and in avian species [4–8]. In avian species, the occurrence and duration of reproduction are primarily influenced by photoperiods. Reproduction of some mammals is also influenced by photoperiods, but responses to photoperiods in mammals and birds may be different [9]. Effects of photoperiod on changes in LH and T secretion have been studied in chickens [10, 11], turkeys [7, 8], and Japanese quail [12]. However, detailed studies utilizing serial bleeding to determine pulse characteristics in response to photoperiod changes have been not conducted in male avian species during development and early reproduction. The mechanism through which photoperiods induce changes in circulating hormone levels remains largely unknown in male birds. Interactions between LH and T have been proposed in a number of species, including chickens [13] and humans [14]; however, the way in which these interactions affect reproductive performance is not clear. In order to understand effects of photoperiod and age on secretory patterns of LH and T, as well as the regulatory mechanism of these hormones on semen production, three trials were conducted to study secretory patterns of LH and T and semen production in male turkeys exposed to different photoperiods, starting at 10 or 12 wk of age.

Thyroxine (T4) affects the reproductive cycles in mammals and birds. In birds, photoresponses may involve T4 [15, 16], since T4 may be necessary to induce and maintain the photorefractory state [9]. However, little information is available on the relationships of T4 to reproduction in male turkeys. To investigate potential relationships of T4 to male turkey reproduction, plasma T4 was also measured during growth and early reproduction.

	MATERIALS AND METHODS

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Trial 1

Male turkeys 10 wk of age (RBC-3 line, Ohio State University; [17]) were housed in light-tight rooms and given one of two lighting treatments: either LL (long-long), in which the turkeys (n = 13) were maintained under a long-day (LD) photoperiod of 16L:8D throughout the trial; or SL (short-long), in which the turkeys (n = 13) were maintained under a photoperiod of 6L:18D (SD) until 29 wk of age, when they were switched to LD. Food and water were available ad libitum. Blood samples (1.5 ml) were collected weekly from 10 to 29 wk of age from the jugular vein, then daily for 2 days after the switch to LD, when the trial was terminated. Semen collections were attempted weekly on all birds beginning at 20 wk of age. Semen production was measured gravimetrically. Body weights were recorded weekly.

Trial 2

To confirm the results from trial 1, male turkeys (RBC-3 line) 12 wk of age were used and treated as described for trial 1. The LL group had 8 birds; the SL group had 9 birds. After switching the SL group to LD at 29 wk of age, daily blood sampling was conducted for 5 days, and sampling then was performed weekly until 35 wk of age, when the trial was terminated. Semen collections were attempted from all birds weekly beginning at 18 wk of age. Semen production was determined gravimetrically. Body weights were recorded weekly. When the trial was terminated, the birds were killed and testes weights were determined.

Trial 3

Male turkeys (RBC-3 line) 10 wk of age were used and treated as described for trial 1. Two weeks prior to serial bleeding, males were chosen at random and caged individually in floor pens (60 x 82 cm) with wood shavings as litter. Serial bleeding was conducted on birds at 13, 23, and 35 wk of age under either LD or SD lighting. At each age, 8 birds, 4 given each lighting treatment, were serially bled. Blood samples were withdrawn from the jugular vein through a cannula system described by Chapman et al. [18]. Briefly, the cannulas (medical-grade silicon tubing) were inserted percutaneously about 10 cm into the right jugular vein and affixed in the skin of the neck. The cannula was fastened to a swivel system and then passed through a hole in the room wall to the outside of the animal room, where blood sampling was performed. This system allowed the individual birds free range and access to feed and water within the individual cages, as well as exposure to the experimental lighting program without interruption during blood sampling. Blood withdrawal operations were performed outside the room housing the animals so as not to disturb the birds or alter their behavior. Blood samples (1.0 ml each time) were collected every 10 min for 6 h, then every 5 min for an additional 3 h. Serial bleeding started at the beginning of the photophase for both the SD and LD photoperiods. Saline (0.7% NaCl plus 0.5% sodium citrate), equivalent in volume to the blood sample volume, was infused through the cannula after each sample. After serial bleeding, the birds were killed. The testes were then removed and weighed, and a portion of the right testis was fixed, embedded, sectioned, and stained for histological examination as previously described [19].

Hormone Assays

Concentrations of LH, T, and T4 were determined by validated RIA. The LH RIA has been described by Bacon and Long [20]. The LH used for iodination and standard was of chicken origin (USDA-cLH-K-3), and the primary antiserum was raised in rabbits using this LH as antigen. The sample volume was 100 µl for this assay. The intra- and interassay coefficients of variation (CV) were 6.0% and 8.4%, respectively. Extraction and RIA for T were as described by Bacon et al. [7]. Aliquots of 20 µl (mature toms) or 50 µl (immature toms) of plasma were used. The intra- and interassay CV were 8.1% and 12.0%, respectively. The T4 RIA was conducted using an ICN T4 RIA kit (ICN Pharmaceuticals, Inc., Costa Mesa, CA). The intra- and interassay CV were 3.6% and 6.4%, respectively.

Data Analysis

For trials 1 and 2, data were analyzed with repeated measures ANOVA with age and treatment within age as factors. Single degree of freedom comparisons of treatments at each age were used to determine age-specific treatment effects. For trial 3, the PULSAR algorithm [21] was used to calculate the overall mean level, baseline level, pulse number/9 h, and pulse amplitude. The G values for PULSAR analysis for LH and T were G(1) = 3.80, G(2) = 2.60, G(3) = 1.90, G(4) = 1.50, G(5) = 1.20. The assay SD calculated and used for LH and T PULSAR analysis were 5.67x + 1.40/100 and 7.15x + 2.10/100, respectively, where x is the concentration of LH and T in the individual sample. Testes weight, and overall mean concentration, baseline concentration, pulse amplitude concentration, and pulse number/9 h of the hormones were analyzed by ANOVA with age and treatment within age as factors. Statistical significance among means was evaluated with Duncan's multiple range test. Coincidence of LH and T pulses was determined manually based on results from PULSAR analysis [22].

	RESULTS

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In trial 1, the plasma concentrations of LH were not different between lighting treatments from 10 to 18 wk of age but were then generally higher in the LL group until 29 wk of age (Fig. 1A). In both the LL and the SL treatments, concentrations of LH increased significantly after 18 wk of age, but a higher sustained increase was noted in the LL group. This increased level of LH was maintained in the LL group until 29 wk of age, when the SL group was switched to LD. After the switch to LD, LH levels increased significantly in the SL group at the last samples taken 2 days later and LH was higher in the SL group. A pattern similar to that of LH was observed for T (Fig. 1B). Increases in T levels occurred at 19 wk of age in both groups and were then sustained. Levels of T were not different between lighting treatments but increased sharply in the SL group by 2 days after the switch to LD, at which time T became higher in the SL group. Production of semen was first detected in an individual bird in the LL group at 22 wk of age, and 13 of 13 birds were producing semen by 25 wk of age (Fig. 2A). Semen was first detected in an individual bird in the SL group at 23 wk of age, but only 4 of 14 birds were producing semen at 29 wk of age. Overall mean semen volume was significantly greater (0.30 ± 0.03 g/ejaculate, n = 8) from the LL birds than from the SL birds (0.14 ± 0.01 g/ejaculate, n = 7). Mean weekly semen volumes are presented in Figure 2B.

View larger version (32K): [in this window] [in a new window]   FIG. 1. Mean concentrations (mean ± SD) of LH (A and C) and T (B and D) in male turkeys maintained under either LL (open symbols) or SL (solid symbols). A and B from trial 1 (n = 13 for LL or SL); C and D from trial 2 (n = 8 for LL, n = 9 for SL). Different letters with thin lines indicate significant differences of hormone levels between ages (p < 0.05). * Differences between treatment groups at each age (p < 0.05).

View larger version (27K): [in this window] [in a new window]   FIG. 2. Percentage of male turkeys producing semen at each age (A and C) and volume of semen production (B and D, mean ± SD). Turkeys were maintained under either LL (open symbols) or SL (solid symbols). A and B from trial 1 (n = 13 for LL or SL); C and D from trial 2 (n = 8 for LL, n = 9 for SL).

In trial 2, changes in hormone concentrations and semen production confirmed and extended the results from trial 1. The concentrations of LH were not different between lighting treatments from 12 to 15 wk of age (Fig. 1C). The level of LH was then higher in the LL group until 29 wk of age, when the level of LH increased sharply in the SL birds by 2 days after the switch to LD, at which time LH became higher in the SL group. The level of LH then remained higher in the SL group until the trial was terminated at 35 wk of age. Levels of T increased at 17 wk of age in both groups and were then sustained (Fig. 1D). Levels of T were higher in the LL group until 29 wk of age, but increased sharply in the SL group by 2 days after the switch to LD, in agreement with results from trial 1. The level of T then remained higher in the SL group until the trial was terminated at 35 wk of age. Semen production was first detected in an individual bird in the LL group at 20 wk of age; 7 of 8 birds produced semen at 26 wk of age, and 8 of 8 birds did so at 35 wk of age (Fig. 2C). Semen was first detected in an individual bird in the SL group at 22 wk of age; 5 of 9 birds were producing semen at 29 wk of age, and 9 of 9 birds at 35 wk of age, or 6 wk after the switch from SD to LD lighting. Overall mean semen volume was significantly greater (0.29 ± 0.02 g/ejaculate, n = 16) from the LL birds than from the SL birds (0.21 ± 0.02 g/ejaculate, n = 14). Mean weekly semen volumes are presented in Figure 2D. Testes weights were significantly greater in the SL group than in the LL group at 35 wk of age, or 6 wk after the switch from SD to LD lighting (Fig. 3A). Overall mean body weight from both groups increased from 7.84 ± 0.9 kg at 12 wk of age to 23.36 ± 1.94 kg at 35 wk of age, with no difference observed between lighting treatments.

View larger version (13K): [in this window] [in a new window]   FIG. 3. Testes weights (mean ± SD). A) Trial 2 (n = 8 for LL, n = 9 for SL); all birds were 35 wk of age. B) From turkeys 13, 23, or 35 wk of age maintained under either LL (n = 4 for each age) or SL (n = 4 for each age), trial 3. Different letters within a group (age or treatment within age) indicate significant differences (p < 0.05).

In trial 3, changes in the pulsatile patterns of LH and T concentrations between lighting treatments were determined by serially bleeding birds over 9 h at 13, 23, and 35 wk of age. At 13 wk of age, both lighting treatment groups were predicted to be sexually immature; at 23 wk of age, most of the LL group was predicted to be sexually mature and most of the SL group sexually immature; while at 35 wk, both the LL and the SL groups were predicted to be sexually mature. At 13 wk of age, testes were small in size (Fig. 3B) and histologically immature in all 4 birds from each treatment. At 23 wk of age, testes were small in size and histologically immature in all 4 birds from the SL treatment, and none of the birds were producing semen. However, the testes were larger and histologically mature in all 4 birds from the LL treatment, and 3 of the 4 birds were producing semen. At 35 wk of age, testes were large and histologically mature in all 8 birds from both lighting treatments, and all birds were producing semen. Spermatozoa were present in the lumen of seminiferous tubules of all LL birds at 23 and 35 wk and in the SL birds at 35 wk (Fig. 4A). No spermatozoa were present in the lumen of seminiferous tubules of the LL and the SL groups of birds at 13 wk, or of the SL group at 23 wk (Fig. 4B).

View larger version (66K): [in this window] [in a new window]   FIG. 4. Histology of testis. A) Mature testis, representative of turkeys from the LL group at 23 wk of age and both the LL and SL groups at 35 wk of age; B) immature testis, representative of turkeys from both the LL and SL groups at 13 wk of age and the SL group at 23 wk of age (x200).

Pulses of both LH and T were detected in birds serially bled at all three ages in both lighting treatments (Fig. 5). Concordances between LH and T pulses were 34% at 13 wk, 78% at 23 wk, and 71% at 35 wk, based on overlapping of pulses of LH and T with pulses determined using the PULSAR algorithm. Most pulses of T were preceded by LH pulses at all ages. However, some pulses of T were detected without detectable prior or post-pulses of LH, and vice versa, especially at 13 wk of age (Fig. 5).

View larger version (33K): [in this window] [in a new window]   FIG. 5. Profiles of LH and T concentrations from individual turkeys at 13 (B#4, 8), 23 (B#12, 15), and 35 (B#18, 21) wk of age. B#4, 12, and 18 are from the LL group; B#8, 15, and 21 are from the SL group. The line with solid symbols indicates LH; the line with open symbols indicates T. Presence of symbols indicates points within peaks detected by PULSAR analysis. Thin lines in each panel indicate the presence of LH peaks, and thick lines indicate the presence of T peaks. The heavy bar at the top indicates the scotophase in the SL group.

Overall mean concentrations of LH were lower at 13 wk of age than at 23 and 35 wk of age (p < 0.05) but not different between 23 and 35 wk of age (Fig. 6A). Higher overall mean concentrations were present in the LL group at 23 wk of age, but no differences were detected between lighting treatments at 13 and 35 wk of age. Baseline levels of LH were significantly different between the three ages (p < 0.05). No differences were found between lighting treatments at 13 wk of age (p > 0.05), but the baseline level was significantly higher at 23 wk of age and significantly lower at 35 wk of age (p < 0.05) in the LL group than in the SL group (Fig. 6B). LH pulse number and pulse amplitudes were highly variable between birds within groups and were not different between ages and between lighting treatments within ages (p > 0.05; Fig. 6, C and D).

View larger version (35K): [in this window] [in a new window]   FIG. 6. Overall mean (A), baseline (B), pulse numbers (C), and amplitude concentrations (D) of LH (mean ± SD), trial 3. Different letters within a group (age or treatment within age) indicate significant differences (p < 0.05); n = 8 for each age, n = 4 for each age/treatment.

Overall mean and baseline levels of T were lower at 13 wk of age than at 23 and 35 wk of age (p < 0.05) but not different between 23 and 35 wk of age (Fig. 7, A and B). Overall mean and baseline concentrations of T were not different between the LL and SL groups at 13 wk of age but were higher in the LL group than the SL group at 23 wk of age. At 35 wk of age, the LL group demonstrated a higher overall mean level of T with no difference in T baseline level (Fig. 7, A and B). The number of pulses of T increased from 13 to 23 wk of age but decreased to an intermediate level at 35 wk of age (Fig. 7C). Pulse amplitude of T was higher in the LL group at 23 wk of age (Fig. 7D) but was not influenced by lighting treatments at other ages.

View larger version (29K): [in this window] [in a new window]   FIG. 7. Overall mean (A), baseline (B), pulse numbers (C), and amplitude concentrations (D) of T (mean ± SD), trial 3. Different letters within a group (age or treatment within age) indicate significant differences (p < 0.05); n = 8 for each age, n = 4 for each age/treatment.

Concentrations of plasma T4 were determined in all samples of trial 2 (Fig. 8A) and for the first samples of serial bleedings of trial 3 (Fig. 8B). In trial 2, concentrations of T4 were higher at most ages in the LL group than in the SL group before 23 wk of age. After the SL group was switched to LD at 29 wk, no difference in concentration of T4 was observed between lighting treatments at any age. No differences in T4 concentration between lighting treatments were detected in trial 3. However, T4 concentration was higher at 13 wk than at 23 and 35 wk.

View larger version (17K): [in this window] [in a new window]   FIG. 8. T4 concentrations (mean ± SD). A) From 12 to 35 wk of age, trial 2 (open symbols indicate LL, n = 8; solid symbols indicate SL, n = 9). * Difference between treatment groups at each age in A. B) From 13, 23, and 35 wk of age, trial 3 (n = 8 for each age, n = 4 for each age/treatment). Different letters within a group (age or treatment within age) indicate significant differences (p < 0.05).

	DISCUSSION

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The present studies examined the effects of photoperiod changes during growth and early reproduction on secretory patterns of LH and T in male turkeys. The results demonstrated age-related and photoperiod-related changes in plasma LH and T levels. This is the first study to report in detail the pulsatile changes in circulating LH and T in male birds in relation to the initiation of reproduction. Circulating levels of LH and T were characterized into three different patterns: 1) before sexual maturation, low baseline levels of both LH and T with detectable pulses of both hormones; 2) after sexual maturation, high baseline levels of both LH and T with detectable pulses of both hormones; and 3) after photostimulation of photosensitive male turkeys, high baseline levels of both LH and T with detectable pulses of LH but few detectable pulses of T.

Before secretory patterns of LH and T were determined, two trials were conducted to ascertain developmental patterns of the hormones in male turkeys exposed to the various photoperiods. In trial 1, increases in levels of LH and T occurred at about 18 wk of age under either LD or SD lighting, and the results of trial 1 were confirmed and extended in trial 2. This observation suggests that some turkeys have entered puberty by 18 wk of age under either the SD or LD lighting program. Similar developmental changes in LH and T have been observed in male chickens [13, 23, 24]. The increased hormone concentrations at about 18 wk of age are possibly associated with testis growth and semen production since semen production was first observed in some individual birds 1 or 2 wk subsequent to increased plasma LH and T.

The increased level of LH and T at 18 wk of age may be due to decreased sensitivity of GnRH neurons to negative feedback of steroids [5], but no direct evidence for this mechanism is available in birds. In mammals, similar increases in LH are observed, and a hypothesis has been proposed that at the onset of puberty, decreased negative feedback of steroids (estradiol) on GnRH-secreting neurons results in increases of LH release frequency that might induce sexual maturation [25]. The turkeys exposed to LD from the early age of 10–12 wk produced semen earlier, and with a higher percentage of birds producing semen at an earlier age, than those exposed to the SD photoperiod. This earlier onset of puberty was associated with higher levels of LH after 18 wk of age. Semen production at an early age was also observed in male turkeys in the study of Etches et al. [26], but no correlated hormone data were presented by these authors. There are reports that FSH in male chickens increases during development [23, 24]; this might induce sexual maturation and semen production. No information on FSH in turkeys is available because of the unavailability of a validated FSH assay in this species.

After 18 wk of age, the LL and SL lighting treatments were associated with different hormone levels. Higher levels of LH and T were detected in the LL group and were associated with earlier semen production and production of a larger volume of semen. Some of the turkeys maintained under SD photoperiods produced semen, which indicates that LD photoperiods are not absolutely necessary for sexual maturation in male turkeys; but it appears that SD photoperiods delay the onset of sexual maturation and decrease the quantity of semen produced (or that LD photoperiods advance sexual maturation and enhance semen production). SD are also associated with lower circulating concentrations of LH and probably T up to 29 wk of age, the most advanced age of SD lighting examined in the present experiments.

The levels of LH and T increased sharply after turkeys were switched from SD to LD at 29 wk of age. These photostimulated increases in LH and T were maintained for about 4 wk, then gradually declined until 35 wk of age, the latest age examined in the present trials. In male turkeys switched from SD to LD, the response in LH has previously been reported to be very rapid [27]. In turkey hens, an increase of LH level after photostimulation is also rapid, occurring during the first scotophase after the first LD photophase [28]. Photostimulation also stimulated an increase in testis growth resulting in the higher testis weight in the SL group in comparison to the LL group at 35 wk of age, which was 6 wk after photostimulation. However, no associated increase in semen production was observed during the 6 wk after photostimulation, even though the spermatogenesis from spermatogonia to spermatozoa is short in male birds; in Japanese quail this requires only 11 days [29]. The response to photostimulation in turkey hens is different. Good egg production is induced within about 3 wk when turkey hens are switched to an LD photoperiod from an SD photoperiod [30]. Compared to the relatively low-amplitude increases in plasma LH and T levels occurring at about 18 wk of age, increases in plasma LH and T levels induced by LD photostimulation at 29 wk were both rapid and of relatively high amplitude. The mechanism of quick response to photostimulation is possibly due to rapid increase in GnRH secretion. Rapid increase in GnRH is observed within 23 h after a single LD photostimulation in quail [31].

Results from trials 1 and 2 show obvious age-related changes in LH and T that are independent of, but modified by, photoperiods. To determine whether pulsatile patterns of LH and T at different ages during growth and development are present or are modified by photoperiods, secretory patterns of LH and T were determined at 13, 23, and 35 wk of age in male turkeys exposed to lighting treatments LL and SL. The turkeys at 13 wk of age were not sexually mature, but equivalent robust pulses of LH and pulses of T were detected in the two photoperiod groups. This observation suggests that photoperiods have little influence on controlling the secretion of LH and T in the male turkeys at prepubertal or immature ages. In contrast, photoperiod does influence levels of plasma LH in starlings during pubertal development. Increases in plasma LH level were observed in starlings exposed to an SD photoperiod during development but not under an LD photoperiod from hatch [32, 33], and the authors concluded that this species is photorefractory at hatch. In contrast, the present data indicate that male turkeys, similar to male chickens, are not photorefractory at hatch. The photoperiod did influence the onset of sexual development (trials 1 and 2) and patterns of secretion of LH and T (trial 3) in male turkeys by 23 wk of age. The birds maintained under LD photoperiods (the LL group) were sexually mature at 23 wk of age based on testis weight, testis histology, and production of semen by most of the turkeys. At 23 wk of age the LL group was characterized by high overall mean and baseline levels of LH and T without an increased number of pulses or an increase in amplitude above baseline of pulses. The birds maintained under SD photoperiods (the SL group, trial 3) were all sexually immature at 23 wk of age and had relatively low overall mean and baseline levels of LH and T. The most striking difference in secretory pattern of LH and T between the lighting treatments at 23 wk of age was the increased baseline concentrations of LH and T and the presence of LH and T pulses in the sexually mature birds. This suggests the importance of baseline concentration of LH and T and the presence of LH and T pulses in regulating the onset of sexual maturation in male turkeys. High baseline levels of LH and T with fewer pulses were detected in the SL group at 35 wk of age after the birds were stimulated at 29 wk of age. The increased baseline level of LH and fewer pulses of T seen in the SL group at 35 wk may be related to poorer reproductive performance at even later ages [34]. What relationships exist between pulsatile secretions of the hormones and semen production at various ages throughout a normal 30-wk reproductive period during which semen production usually declines, should be an interesting question for further work.

Interactions between LH and T have been reported in mammals [3, 14] and birds [13]. LH stimulates T secretion with a time delay of about 10–15 min, and T has a negative feedback on LH secretion. In the present study, concordance between LH and T pulses was 34%, 78%, and 71% at 13, 23, and 35 wk of age, respectively. Most pulses of T were preceded by LH pulses. Some pulses of T were detected without prior or post-pulses of LH, and vice versa. Pulse occurrences between LH and T in this study might reflect this negative feedback relationship: that is, LH stimulates secretion of T without T feedback effects, or the two might change independently, since some LH pulses were not followed by pulses of T or pulses of T were not associated with pulses of LH. Concordance rate was higher at 23 wk of age and later, indicating that age rather than reproductive state is associated with concordance of pulses at 35 wk of age. It cannot be concluded that interactions between LH and T exist based on only secretory pattern analysis, because the results from statistical analysis might not really reflect physiological relationships between LH and T secretion. For example, few pulses of T were detected in the SL birds at 35 wk of age in comparison to the number of pulses of LH.

It has been reported that T4 secretion by the thyroid gland may be involved in photoresponses in birds [9, 35, 36] and in mammals [37]. T4 could be necessary for the onset and continuation of photorefractoriness, since increased levels of T4 were associated with occurrence of photorefractoriness in starlings [35, 36] and Japanese quail [15]. Sustained high levels of T4 in immature turkeys prior to 23 wk of age were observed in the LL group in trial 2 in the present study. After LD photostimulation of SL birds, T4 concentrations decreased as the birds matured. These data suggest that T4 might somehow be involved in regulation of reproduction in male turkeys, with high T4 levels prior to 23 wk of age associated with maintenance of the juvenile state [9].

In conclusion, age-related increases in both LH and T occurred independently of photoperiods at about 18 wk of age in male turkeys. Sexual maturation was delayed in birds maintained under SD, but these birds did respond to LD photostimulation at 29 wk by rapid increases in LH and T. Pulsatile secretory patterns of LH and T were detectable in sexually immature turkeys as well as in mature turkeys. The baseline levels of LH and T were much higher in sexually mature turkeys. The physiological importance of differing pulsatile secretory patterns of LH and T on regulation of reproduction needs to be further studied. Photostimulation of turkeys at a very early age was associated with enhanced secretion of LH and T after 18 wk of age and with precocious semen production. The plasma T4 concentration may be related to the onset of sexual maturation.

	ACKNOWLEDGMENTS

 
The authors thank Dr. J. Proudman, USDA, ARS, Beltsville, MD, for providing antiserum and LH used in the RIA. The authors also thank Dennis Hartzler for helping with semen collection and Ruthi Patterson for helping with the statistical analysis.

	FOOTNOTES

 
¹ Salaries and research support provided by State and Federal Funds appropriated to the Ohio Agricultural Research and Development Center, The Ohio State University. Manuscript number 20–98.

² Correspondence: Wayne L. Bacon, Department of Animal Sciences, 1680 Madison Ave, Wooster, OH 44691. FAX: 330 2633949; bacon.2{at}osu.edu

Accepted: July 6, 1998.

Received: March 17, 1998.

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