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a Division of Neurotoxicology, National Center for Toxicological Research, Jefferson, Arkansas 72079
b Department of Neurobiology and Physiology, Northwestern University, Evanston, Illinois 60208
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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In the present studies we have tested the hypothesis that GnRH pulse frequency may play an important role in the differential release of the gonadotropins in this species. Our working hypothesis holds that photostimulatory signals, likely transmitted via alterations in the duration of melatonin secretion [6], ultimately impact the level of activity in the reproductive axis through alterations in the frequency of pulsatile GnRH release; it is further hypothesized that this change in pulsatility is one in which the frequency of GnRH pulses increases from very slow in the photoinhibited state, to moderately fast in the early stages of photostimulation, and ultimately to a relatively rapid rate in the fully stimulated state. Thus, this hypothetical transition from slow, to moderate, to fast endogenous GnRH pulse frequencies may direct the temporally dissociated patterns of FSH and LH secretion in the male hamster after photostimulation. There is ample precedence for the idea that alterations in GnRH pulse frequency can differentially evoke FSH and LH secretions. Indeed, studies in other species have clearly demonstrated that manipulation of exogenous GnRH pulse frequency can differentially stimulate LH and FSH [7, 8], with lower GnRH pulse frequencies generally favoring FSH secretion and higher GnRH pulse frequencies facilitating LH secretion.
To test this idea, we have administered artificial GnRH pulse regimens consisting of either slow, medium, or fast GnRH pulse rates to Siberian hamsters maintained on the inhibitory, short-day photoperiod. We reasoned that the administration of exogenous GnRH pulses to photoinhibited animals, in regimens that may approximate the endogenous pulse frequency at various points following photostimulation, should produce divergent patterns of FSH and LH secretions. To this end, we have taken photoinhibited Siberian hamsters and given them various pulse frequencies of GnRH to determine in this photoperiodic species whether slower GnRH pulse frequencies can differentially induce release of FSH and whether faster frequencies favor release of LH.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Male and female Siberian hamsters were born in our breeding colony under a 16L:18D photoperiod (lights-on from 0500 to 2100 h). Siberian hamsters were chosen for this series of experiments as their reproductive axis responds more rapidly to photic stimulus than that of Syrian hamsters. At 18 ± 2 days of age all animals were weaned, group housed 2–4 per cage, and transferred to a light-tight box with an 8L:16D photoperiod (lights-on from 0900 to 1500 h). All animals were exposed to 25–35 days of short photoperiod. In male Siberian hamsters, testicular regression was assessed by gentle palpation of the scrotum of unanesthetized animals. Male animals with palpable testes were removed from the study. The estimated paired testes weight was less than 50 mg for the remaining male animals with nonpalpable testes. In female Siberian hamsters, uterine weight was determined after the experiment, and animals with uterine weights greater than 35 mg were removed from the study. Animals undergoing GnRH or vehicle injections were housed singly to avoid damage to catheters and swivel mechanisms.
Pump Infusion System
The s.c. catheter used in the GnRH infusion experiment was adapted from a design developed by Goldman et al. [6]. The portion of the catheter that was proximal to the animal consisted of a 5-mm-diameter aluminum washer attached to 10 cm of 16-gauge stainless steel tubing. A 17-inch piece of polyethylene tubing (PE-20) was threaded through the steel tubing and washer unit. The PE-20 tubing was then bent at a right angle so approximately 1 cm was exposed at the washer end. The washer and PE-tubing were then cemented to the stainless steel tubing. After surgery the distal end of the PE-tubing was connected to a 5-ml polystyrene syringe filled with the GnRH or vehicle. The syringe was mounted onto an infusion pump (#975; Harvard Apparatus, Millis, MA) that was placed on a small platform inside the light-tight boxes. The pump was connected to a variable cycling timer (Industrial Timer Co., Waterbury, CT) to allow for repeated timed s.c. 50-µl injections.
GnRH Preparation
Synthetic GnRH was purchased from Sigma Chemical Co. (St. Louis, MO). GnRH was dissolved in 0.9% sterile saline to a concentration of 1 µg/ml. This stock solution was then divided into 1-ml aliquots and stored at -70°C until used. At time of use, GnRH stock was diluted to 240 ng/ml with 0.9% sterile saline. GnRH for infusion was changed every morning throughout the duration of the experiment.
Surgical Procedures
Surgical procedures are only for those experiments in which the animals underwent repeated injections. Animals were anesthetized using methoxyflurane, and the cranium was exposed. A 21-gauge needle was passed through the scruff of the neck to emerge from the head incision. The distal end of the catheter assembly was attached to the needle. The needle was retracted, drawing the catheter assembly under the skin to exit through the scruff until it was retained by the washer, which came to rest subdermally. A small amount of lidocaine was placed around the incision, and the wound was closed using silk suture. After surgery, animals were housed individually and were allowed free access to food and water.
Physiological LH pulse amplitudes and frequencies have not yet been determined in Siberian hamsters because of technical difficulties associated with their small size. Instead, we based our GnRH infusion frequencies on physiological GnRH and LH frequencies measured in rats [9–11], LH frequencies measured in Syrian hamsters [12], and the LH responses to GnRH injections in this experiment. The upper end of our GnRH pulse frequency (1/45 min) was limited by the time required to clear the exogenous GnRH from the serum (20 min) and by the length of the LH response (
40 min). Based on these data it appeared that LH levels had enough time to return to baseline between GnRH injections.
Terminal Procedures
Blood sampling After experimental procedures, animals were anesthetized with methoxyflurane and were killed by exsanguination via cardiac puncture. The blood was collected into individual microcentrifuge tubes containing 50 µl of 50 IU heparin. Samples were centrifuged and the plasma was stored at -70°C for subsequent hormone RIAs.
Testes removal After exsanguination, testes were removed and paired testes weights obtained. Testes were then placed in individual vials containing Bouin's fixative. After 24 h in Bouin's fixative, testes were cut in half and placed into Bouin's fixative for another 72 h. On Day 4, testes were placed into repeated solutions of 70% alcohol plus lithium carbonate to remove excess Bouin's fixative. Testes were stored in 70% alcohol until histological preparation.
Hormone Assays
Serum LH and FSH levels were determined by RIA using materials supplied by NIDDK (Rockville, MD). The LH standard used was rat LH RP-3, and the antiserum was LH-S-11. The level of assay sensitivity was 20 pg/tube with an intraassay coefficient of variation of 16.59% at 5.02 ng/ml. The FSH standard used was rat FSH RP-2, and the antiserum was FSH-S-11. The level of assay sensitivity was 150 pg/tube with an intraassay coefficient of variation of 12.8% at 20.3 ng/ml. Serial dilutions of male Siberian hamster pools were run concurrently with each LH and FSH assay and revealed concentration curves parallel to standard LH and FSH curves in the RIAs. Testosterone RIAs were conducted using an RIA kit obtained from ICN Biomedical (Costa Mesa, CA). The level of assay sensitivity was 0.1 ng/ml and the intraassay coefficient of variation was 3.5% at 4.7 ng/ml.
Histology
After fixation, testes tissue was paraffin embedded and cut into 10-µm sections. Sections were stained using a modified hematoxylin and eosin stain. Testes morphology and spermatogenesis were then quantified using light microscopy.
Statistics
Data are expressed as mean ± SEM. Two-way ANOVA was used to assess differences between treatments of 1) serum LH, FSH, and testosterone and 2) paired testes weights, in photoinhibited animals that received pulsatile GnRH and vehicle infusions. In photostimulated animals, one-way ANOVA was used for all measurements. Pair-wise multiple comparisons were carried out using Student-Newman-Keuls method. Results were considered significant if p < 0.05.
Protocols
Experiment 1: Time course of LH response to a single s.c. GnRH injection Sixty female Siberian hamsters (n = 4) were used for this portion of the experiment due to limited availability of male Siberian hamsters within our breeding colony. Short-day (SD) female Siberian hamsters were given 1 s.c. injection of either 50, 100, or 200 ng/kg GnRH. Animals were killed immediately after injection or 10, 20, 30, or 40 min postinjection. After exsanguination, uteri were removed and weighed to determine the state of the reproductive axis. Only animals that were reproductively quiescent were used for LH analysis.
Experiment 2: Dose response for s.c. injection of GnRH Twenty SD male Siberian hamsters (n = 4) were given 1 s.c. injection of either 100, 200, 400, or 800 ng/kg GnRH in saline or saline alone. Animals were killed at 10 min postinjection. Blood was collected for LH RIA, and testes were removed and weighed.
Experiment 3: Siberian hamster responses to photostimulation Forty-two SD male Siberian hamsters (n = 6) were placed on a stimulatory photoperiod consisting of 18L:6D. Animals were killed immediately prior to entering the long-day (LD) photoperiod or after 3, 5, 10, 20, 25, or 30 days of continuous exposure to the LD photoperiod.
Experiment 4: Effects of GnRH pulse frequency on the Siberian hamster reproductive axis Ninety-six SD male hamsters (n = 6) were kept on an SD photoperiod and were given 1-min injections of 400 ng/kg GnRH via the s.c. catheter. The interinjection intervals were either 45 (fast pulse frequency), 90 (medium pulse frequency), or 180 (slow pulse frequency) min. Animals were killed immediately before the regimen of GnRH injections (Day 0) or after 3, 5, 10, 20, or 30 days of continuous exposure to one of the GnRH injection frequencies at 10 min postinjection. The control animal group (SD controls) consisted of 36 photoinhibited animals (n = 6) injected with vehicle once every 90 min via the s.c. catheter; these animals were animals killed at Days 0, 3, 5, 10, 20, and 30.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Subcutaneous GnRH injections (50, 100, and 200 ng/kg) produced peak LH levels 10 min after GnRH administration. LH levels then declined until reaching baseline 40 min after GnRH injection (Fig. 1A). On the basis of these results, blood samples for the GnRH pulse frequency experiments were collected 10 min postinjection.
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Experiment 2: Dose Response for Subcutaneous Injection of GnRH
LH responses to all the GnRH doses were significantly different from LH responses to the vehicle (Fig. 1B). On the basis of these data, we initially used pulsed infusions of GnRH at the 100 ng/kg dose. However, this dose paradigm did not produce any effects on the reproductive axis (data not shown). Therefore, the 400 mg/kg dose of GnRH was used for all experiments. In addition, the 400 mg/kg GnRH dose produced consistent LH responses similar in amplitude to endogenous LH pulses found in Syrian hamsters [12] and rats [9].
Experiment 3: Siberian Hamster Responses to Photostimulation
Ten days after photostimulation, paired testes weights were significantly greater than those of Day 0 animals. Paired testes weights increased linearly, reaching a peak weight of 589.30 ± 53.21 mg after 30 days of photostimulation (Fig. 2A). However, histological examination of the testes revealed that the Leydig cells had not redifferentiated and that interstitial space had not increased. Serum testosterone (Fig. 2B) and LH levels (Fig. 2C) did not change over the course of 30 days of photostimulation. On the other hand, 5 days of photostimulation significantly increased FSH levels compared to those in Day 0 animals (12.51 ± 2.85 vs. 1.25 ± 0.00). FSH reached peak values by Day 10 (14.07 ± 1.56) and then fell gradually to 6.72 ± 0.48 by Day 30 (Fig. 2D).
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Experiment 4: Effects of GnRH Pulse Frequency on the Siberian Hamster Reproductive Axis
Testes weight There was a clear effect of GnRH pulse frequency, with fast and medium pulse frequencies being more effective than slow pulse frequency in producing an increase in paired testes weight (Fig. 3A). Pulse frequency vs. time interactions revealed that animals maintained on any of the GnRH pulse frequencies did not exhibit increased paired testes weights through Day 5. However, by Day 10, paired testes weights were significantly increased compared to those of SD control animals in the fast and medium pulse frequency groups and by Day 20 in the slow pulse frequency group. Paired testes weights continued to rise in all GnRH pulse groups, reaching peak values by Day 30. At Days 20 and 30 the fast and medium pulse frequency paired testes weights were significantly greater than the slow pulse frequency paired testes weights. Paired testes weights in the SD control animals did not change over the course of the 30-day time period.
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Serum testosterone There was a clear effect of GnRH pulse frequency on serum testosterone levels, with fast frequency being more effective than medium or slow pulse frequencies in producing an increase in serum testosterone (Fig. 3B). The rate of rise in response to the fast GnRH pulse frequency increased significantly from Day 20 to 30, yielding supraphysiological levels of 32.94 ± 6.59 ng/ml. Serum testosterone levels did not change over the experimental time course in the SD control animals.
Serum LH There was similarly a direct relationship between GnRH pulse frequency and serum LH levels with the following rank-order potency: fast pulse frequency > medium pulse frequency > slow pulse frequency > SD controls (Fig. 3C). Furthermore, Day 10 LH levels in the fast pulse frequency group increased dramatically to a peak of 3.26 ± 0.30, which was significantly greater than the other 10-day LH levels. SD control serum LH levels did not change over the course of the experiment.
Serum FSH The levels of serum FSH also varied among groups receiving different GnRH pulse infusions, although there was not a direct frequency-response relationship. Rather, the rank-order efficacy of each regimen was found to be medium GnRH pulse frequency > fast GnRH pulse frequency > SD controls (Fig. 3D). The slow GnRH pulse frequency produced FSH levels that fell between the medium and fast pulse frequencies and that were not different from them. All of the GnRH pulse frequencies produced FSH levels that were significantly greater than those of their SD controls. Serum FSH did not change over the course of the experiment in the SD control animals.
Testes histology All of the animals exposed to GnRH infusions showed complete organization of the tubules in the process of spermatogenesis, including spermatogonia, late pachytene spermatocytes, acrosome-capped spermatids, and mature sperm within the lumen of the tubules (Fig. 4, B–D). However, animals that were exposed to slow GnRH pulse frequencies did not have any observable redifferentiation of the Leydig cells or an increase in the interstitial space (Fig. 4B). This is in agreement with the findings that these animals also had undetectable testosterone and LH levels. On the other hand, animals maintained on the medium and fast GnRH pulses had observable increases in both Leydig cell redifferentiation and interstitial space (Fig. 4, C and D). Examination of control animals (Fig. 4A) revealed few gonocytes, spermatogonia, and leptotene spermatocytes as well as a poorly organized germinal epithelium, undifferentiated Leydig cells, and small interstitial space that were consistent with observations from previous studies.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The differential release of LH and FSH in a variety of physiological situations has been attributed alternatively to differential gonadal feedback mechanisms [13, 14], the actions of a specific FSH-releasing factor (FSH-RF) [15, 16], and to the differential sensitivity of FSH and LH secretory processes to variations in the pattern of GnRH pulsatility [7, 8, 17–23]. We considered the latter hypothesis most compelling in the case of the singular FSH secretion after photostimulation, since dependence of this phenomenon on some characteristic of GnRH release was clearly shown in experiments using a specific GnRH receptor antagonist [5]. Among all of the possible features of pulsatile GnRH release that may be critically important in this regard, the frequency of GnRH pulsatility has received most attention.
GnRH pulse frequency appears to be physiologically regulated in a variety of circumstances, such as during the ovulatory cycle [24, 25], after gonadectomy [9–11], throughout sexual maturation [26], and in response to photoperiodic stimuli [12, 27]. Moreover, modulation of the frequency of exogenous GnRH stimuli has been clearly shown to evoke differential patterns of gonadotropin secretions and synthesis [8, 17–23]. For example, in sheep pretreated with a GnRH antagonist, the pulsatile administration of a GnRH agonist at a given frequency can induce an increase in the release of LH but not FSH [7]. Furthermore, in monkeys with hypothalamic lesions, high-frequency GnRH administration induces preferential release of LH, while slow GnRH pulse frequencies are detrimental to LH release but stimulate the release of FSH [8]. Recent studies on the regulation of rat gonadotropin subunits are consistent with these studies, demonstrating that faster GnRH pulse frequencies favor an increase in LH-ß transcription rates and mRNA levels while slower GnRH pulse frequencies favor FSH-ß transcription and mRNA levels [20, 22, 23].
In the present study, LH and FSH levels in regressed Siberian hamsters exposed to GnRH infusions differed according to the GnRH frequency. It is clear that the fast GnRH pulse frequency produced significantly higher serum LH levels than either the medium or the slow GnRH pulse frequency (Fig. 3C). The fast-frequency GnRH pulses also produced a clear increase in serum LH on Day 10. FSH levels, on the other hand, were significantly elevated by exposure to the medium GnRH pulse frequency. In contrast to the situation with the serum LH levels, the fast GnRH pulse frequency consistently resulted in the lowest serum FSH levels (Fig. 3D). These data are consistent with studies showing that pulsatile GnRH (as opposed to constant infusion) is necessary for the regulation of LH secretion in monkeys [28] and sheep [7]. In addition, these data are in direct agreement with differential regulation of LH and FSH release in sheep [7] and ß subunit regulation in rats [22, 23] by pulsatile GnRH.
Fully regressed Siberian hamsters exposed to pulsatile infusions of GnRH showed a steady increase in paired testes weight from Day 10 onward (Fig. 3A). However, from Day 20 on, a split in the growth curve of the testes occurred between the various groups. The slow-frequency GnRH regimen produced testes that were almost identical in size to those of the photostimulated animals. On the other hand, by Day 30, the two faster-frequency GnRH regimens produced supraphysiological testicular sizes.
Testosterone levels in the medium- and fast-frequency GnRH groups remained stable until 30 days, at which time serum testosterone levels in the fast-frequency group rose to levels that were 10–15 times greater than normal (Fig. 3B). Interestingly, the testosterone levels in the medium- and slow-frequency group were similar to those in the photostimulated group throughout the 30 days. LH is the main stimulator of testosterone, and we observed a clear correlation between serum LH levels and testosterone levels.
While the testes size and serum testosterone levels appeared to differ according to the type of GnRH pulse frequency, there were no observable differences in the organization of the tubules in the process of spermatogenesis between these groups (Fig. 4, B–D). From these data, it appears that the FSH levels established by each of the GnRH pulse paradigms were sufficient to maintain normal spermatogenic functions. On the other hand, the differences observed in Leydig cells and interstitial space suggest that the slow GnRH pulse frequencies did not produce an adequate LH stimulus for the maintenance of testosterone secretion. This was similar to observations in the photostimulated group of animals and suggests that in these two groups perhaps GnRH pulses had not attained a frequency that was adequate for Leydig cell redifferentiation and subsequent testosterone secretion.
Most of the neuroendocrine responses to photostimulation paralleled those seen in adult Siberian hamsters [1, 2]; the lack of a rise in LH and testosterone secretion was the exception in this regard. Since the testes size and the FSH levels noted in the present study are consistent with data from the previous studies, it is unlikely that the lack of LH and testosterone stimulation was due to inadequate photic stimulus. One possibility is that the age of the animals may render them less capable of achieving maximal activation of the GnRH pulse generator after photostimulation. Thus, FSH may be stimulated as the photic stimulus prompts a moderate acceleration of GnRH pulsatility but LH levels remain unstimulated due to the limited capacity of the juvenile animal to attain a high-frequency GnRH secretion pattern. This is supported by our recent observation that pubertal animals exhibited a rise in FSH at the expected time after photostimulation but failed to exhibit any significant rise in LH and testosterone [5].
The design of the present study necessitated that exogenous GnRH pulse amplitude be held constant while pulse frequency was manipulated. This resulted in exposure of animals receiving fast-frequency GnRH pulses to four times as much total GnRH as compared to animals receiving the slow-frequency GnRH pulse regimen. It is possible that the differences in serum LH and FSH levels were due to the total amount of GnRH that the animals received. To test for this possibility, we adjusted the total daily amount of GnRH by decreasing the dose of the high-frequency treatment, such that the total dose of GnRH given was equivalent to the total amount given to the medium-frequency group. Under these conditions, the high-frequency (but equivalent total dose) GnRH treatment was not as effective in stimulating either gonadotropin in comparison to the medium GnRH frequency regimen (data not shown). From these data it appears unlikely that differences among groups in the total daily amount of GnRH were important in the differing responses seen for the three frequency regimens.
The possibility remains that GnRH pulse amplitude may play a role in the differential regulation of the gonadotropins. There have been a few studies in which GnRH pulse amplitude appeared to play a role with regard to the differential control of the gonadotropins. In hypothalamic-lesioned monkeys it has been shown that an increase in the amplitude of exogenous GnRH administration can cause a concomitant decrease in FSH levels while LH levels remain unchanged [8]. In vitro studies using pituitary cell superfusion have shown that GnRH pulses of varying amplitude and durations given to lamb [19] and rat [17, 18] pituitary cells can be selective for either LH [17–19] or FSH [29] secretion. However, it is possible that changes in GnRH pulse amplitude are secondary to changes in pulse frequency, such that as GnRH pulse frequency increases, pulse amplitude decreases, and vice versa. We are focusing our attention in future studies on whether changes in GnRH pulse amplitude also play a significant role in neuroendocrine responses to changing day length.
In summary, we have adapted an infusion model in the photoinhibited Siberian hamster that allows for continuous s.c. pulsatile administration of GnRH over several weeks. Using this technique, we demonstrated that modulation of the frequency of GnRH pulses administered to photoinhibited Siberian hamsters can differentially evoke FSH and LH secretion, with FSH being stimulated to a greater extent by moderate or slow frequency, and LH levels being raised to a much greater extent at high-frequency stimulation. We have also demonstrated that modulation of GnRH pulse frequency is one route through which steroidogenesis and testicular growth can be stimulated independently. Since photostimulated FSH secretion is clearly dependent upon GnRH stimulation [5], our data are consistent with the idea that the differential release of the gonadotropins after transfer to stimulatory photoperiod is stimulated by a change in the frequency of endogenous GnRH release. This idea remains to be verified, pending development of methods for chronic measurement of GnRH pulse generator activity or GnRH secretion in this species.
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FOOTNOTES |
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2 Correspondence. FAX: (847) 491–5211; jlevine{at}nwu.edu
Accepted: May 20, 1998.
Received: November 13, 1997.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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