| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Neuroendocrinology |
Department of Neurobiology and Physiology, Northwestern University, Evanston, Illinois 60208
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
behavior, luteinizing hormone, neuroendocrinology, pheromones, seasonal reproduction
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
In the present study, we investigated the source of the stimulus that triggers the LH release in photostimulated male Siberian hamsters. Female pheromones induce either LH and/or testosterone secretion in males of many species, although the source of the pheromone varies with species. In laboratory mice (Mus musculus), the increase in serum LH in males is stimulated by a chemical contained in female urine [5–9]. The response of male mice is specific to urine from female mice; urine from male mice or female hamsters failed to stimulate LH release [5]. In male golden hamsters (Mesocricetus auratus), postovulatory vaginal secretions are the source of pheromones that induce testosterone release and mating behaviors [10–12]. In Phodopus campbelli, a sister species to Phodopus sungorus, males exhibit preferences for odors derived from several distinct body areas of females [13]; however, the endocrine responses of males to these odors were not examined. In P. campbelli, the attractiveness of female odors varies with reproductive status of the female [13]. In the present study, we investigated whether the release of LH by male Siberian hamsters (Phodopus sungorus) is specific to the presence of a female and whether the ability of female odors to induce LH release in the male is dependent on the reproductive status of the female, as influenced by ovariectomy or exposure to different photoperiods. Serum concentrations of follicle stimulating hormone (FSH), testosterone (T), and cortisol also were measured. FSH was measured to verify our previous observation that the presence of a female stimulated secretion of LH, but not FSH [1]. Testosterone was measured to determine whether the amount of LH secreted was sufficient to induce testosterone release from the testis. Cortisol was measured to determine whether the experimental treatments or blood sampling procedures activated the hypothalamic-pituitary-adrenal axis, which might interfere with the objectives of the study. Finally, we investigated which regions of females produce the odors and whether other sensory stimuli are required to induce LH release in male P. sungorus.
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Siberian hamsters (P. sungorus) used in the present experiments were produced from breeding colonies maintained at either the University of Wisconsin-Parkside (UWP) or Northwestern University, as previously described [1]. Animals born at the UWP facility were transferred to Northwestern University at least 2 wk before entry into an experimental protocol.
All animals were weaned at 18–20 days of age and group-housed (three to five per cage for experiments 1 and 2 and two to three per cage for experiments 2, 3, and 4) in clear acrylic plastic cages (dimensions: [height] 19.5 x [width] 26 x [length] 46.5 cm) with same-sex littermates. The animals were kept in chambers where temperature (21°C), humidity (50%), and light intensity were monitored. A 40-W fluorescent bulb (Cool White, General Electric, Fairfield, CT) positioned on the ceiling of the chambers provided 50–300 lux illumination. Chrontrol software (Chrontrol Corporation, San Diego, CA) linked to digital timers controlled light onset and offset in each chamber. Food (Harlan Teklad 7012 Mouse/Rat diet; Madison, WI) and water were provided ad libitum. All experiments were conducted using protocols approved by the Northwestern University Animal Care and Use Committee, in accordance with the "Guide for the Care and Use of Laboratory Animals" [14].
Sexually naive male Siberian hamsters (4–5 mo old) that had been born and raised in a long photoperiod (16L:8D) were individually housed for 4 days before exposure to one of the test stimuli. The experiments were divided into two replicates because the large number of animals used for each experiment made it logistically difficult to collect blood samples from all males in an experiment on the same day. Blood samples were collected by cardiac puncture; the details of the sampling procedure are given in a following section. The experiments were conducted using a randomized complete block design. In each replicate, three to four males per group were exposed to one of the various stimuli in their home cages for 1.5 h. Each male was exposed to only one stimulus, and the reaction to the stimulus recorded. A previous study in our laboratory has shown that serum LH levels begin to increase in males within 15 min of exposure to a female, reach maximal levels within an hour, and remain elevated for at least 3 h [1]; therefore, blood samples were taken 1 and 1.5 h after introduction of a stimulus. Animals were killed by administration of an overdose of anesthesia after completion of the terminal blood sample. Body weights were measured following killing, and testes were removed and weighed to verify that the animals were reproductively mature. In addition to LH levels, FSH, T, and cortisol levels were measured to provide a profile of the responses of the hypothalamic-pituitary-gonadal and hypothalamic-pituitary-adrenal axes of the males to the various stimuli.
Experiment 1: Specificity of the Response
In this experiment, we tested the hypothesis that the release of LH by males that have been reared in a long photoperiod is triggered specifically by the presence of a female and verified that the release of LH is not a generalized response to an intruder (e.g., another male). We also examined whether the release of LH by males was dependent on the reproductive status of the female, whether vaginal secretions stimulated release of LH, and whether limited physical contact with a female was sufficient to induce LH release. This experiment was conducted as a randomized complete block design to screen a wide variety of potential stimuli while conserving the number of animals used as controls. Males were exposed to each stimulus for 1.5 h. Blood samples were taken 1 and 1.5 h after the introduction of the stimulus.
Stimuli Tested (Sample Size: 7–8 Males per Group)
1. Water on a cotton swab A cotton swab dipped in a vial containing tap water was used as a neutral control. In this and all other experiments in which a potential stimulus was presented to the male on a cotton swab, the swab was touched to the nostrils of the male and then left in the cage with the male for 1.5 h.
2. Ovary-intact female As a positive control and to verify our previous results, males were exposed to a female in their home cages for 1.5 h. Because group housing of female Siberian hamsters suppresses estrus [15], 3- to 6-month-old female hamsters that had been exposed to long photoperiods were individually housed for 4 days before presentation to a male to remove potential inhibitory effects of group housing on reproductive cyclicity and increase their effectiveness as a stimulus. Estrous cyclicity is difficult to characterize in Phodopus because vaginal cytology is not as reliable an indicator of the stage of the estrous cycle in Siberian hamsters as it is in other laboratory rodents and the precise relationship between vaginal cytology and ovarian follicular development is unclear [15]. However, in an attempt to standardize the stimulus provided by the female, the vaginal cytology of the females used in this experiment was monitored and only those females from which the lavages contained primarily nucleated epithelial cells with few to no leukocytes were used. In rats and mice, nucleated epithelial cells are associated with the increased estradiol levels on proestrus.
3. Ovariectomized female Females were ovariectomized 2 wk before use and were subsequently presented to males to determine whether the presence of ovarian hormones affected the ability of a female to trigger LH release in a male.
4. Female separated by a wire mesh This group was included to determine whether limited physical interactions between the experimental male and the stimulus female were sufficient to induce the release of LH in the experimental male. Males were presented with a female with intact ovaries as described for 2 above.
For treatments 4 and 7, the experimental male's cage was fitted with a single wire mesh at the time the experimental male was first housed in the cage. The mesh divided the cage in half along the long axis of the cage. The wire mesh permitted the two animals to touch each other through the mesh (e.g., nose to nose investigation, sniffing of body parts), but did not allow extended interactions such as chasing, fighting, or mounting.
5. Female hamster vaginal secretion (FHVS) To determine whether vaginal secretions from female Siberian hamsters (3–6 mo old) stimulate LH secretion in males, the copious white vaginal discharge that females produce intermittently was collected with a cotton swab. A similar discharge is secreted following ovulation by female Syrian hamsters and contains the protein Aphrodisin, which stimulates mating behavior [16–18]. Each male was presented with the discharge from a single female. The cotton swab was touched to the nostrils of the experimental males and then left in the cage with the male for 1.5 h.
6. Male To verify that the release of LH by a male was a specific response to the presence of a female and not a generalized response to the presence of an intruder in the cage, another male of similar age and photoperiod background (LD), was placed in the home cage of the experimental male for 1.5 h.
7. Male separated by a wire mesh This group was included as a control for any direct aggressive encounters between intruder and resident males that may have altered the endocrine response when intruders were placed directly in the cages of the resident (experimental) males (group 6).
Experiment 2: To Determine if Exposure of Females to Short Photoperiod Alters Their Ability to Induce LH Release in Males
The results of experiment 1 demonstrated that male Siberian hamsters released LH in response to ovariectomized females; therefore, we wished to determine whether females in which ovarian function was inhibited by exposure to short days would induce LH release in male Siberian hamsters that have been reared in LD.
Stimuli Tested (Sample Size: 9–10 Males per Group)
1. LD females Reproductively mature LD females (3–4 mo of age) were used as positive controls. Experiment 1 determined that the ability of LD females to trigger LH release in males did not depend on the presence of the ovaries or the reproductive status of the female; therefore, in this and subsequent experiments, females were group-housed before use in an experiment. The group-housed females were used at random stages of their estrous cycles and some females may have been anestrous due to the group housing conditions. The presence of a mixture of randomly cycling and anestrous females may have increased variability in the responses of the males. Males were exposed to females for 1.5 h.
2. SD females Females were placed in a short photoperiod (6L:18D, lights-on at 0800 CST; SD females) at weaning (18 days of age) and held in the short photoperiod for 10 wk before being used in the experiment. As expected, short photoperiod suppressed reproductive development; all females exhibited closed vaginas characteristic of anestrous females. These females were of similar age (3–4 mo old) as the LD females at the time of the experiments. Males were exposed to females for 1.5 h.
Experiment 3: To Determine if Auditory, Visual, Tactile, Behavioral, or Chemical Cues from Females Are Necessary and Sufficient to Trigger the Release of LH in Males
This experiment examined several potential sources of chemical stimuli from the female [13] and whether auditory, tactile, behavioral, and visual cues from the female were necessary and sufficient to stimulate LH release in males that have been raised in long days. Males were exposed to each stimulus for 1.5 h. When a potential stimulus was presented to the male on a cotton swab, the swab was touched to the nostrils of the male and then left in the cage with the male for 1.5 h. Blood samples were taken 1 and 1.5 h after the introduction of the stimulus into the home cage of the male.
Stimuli Tested (Sample Size: 6 Males per Group)
1. Water on a cotton swab See experiment 1.
2. A normally active female See experiment 1. The females used in this experiment were group housed before being presented to a male and were used at random stages of their estrous cycles or may have been anestrous. The presence of a mixture of randomly cycling and anestrous females may have increased variability in the responses of the males.
3. An anesthetized female To determine whether behavioral cues or vocalizations are required to trigger LH release, males were exposed to anesthetized females. Group-housed females were anesthetized with intraperitoneal injections of a cocktail of sodium pentobarbital (25 mg/kg) and ketamine hydrochloride (60 mg/kg) in saline (0.01 ml/g body weight). When fully anesthetized, an individual female was placed in the home cage with the male. The females remained anesthetized throughout the period of exposure.
4. A normally active female separated by a double wire mesh The use of a single wire mesh in experiment 1 permitted limited physical contact between males and females. The females and males could climb the wire-mesh and appose each other. They could also touch, lick, and sniff each other. To ensure complete physical separation, the male's cage was fitted with two wire meshes separated by 2.5 cm, which divided the cage in half along the long axis. The experimental and stimulus animals could not come into physical contact with each other; thus, any cues transmitted between them would be auditory, visual, or chemical. Chemical cues would need to be sufficiently volatile to be transmitted across the distance between the meshes.
5. Urine from a female Urine was collected from females on a cotton swab and presented to males by touching the swab to the male's nostrils. The urine was collected from females in random stages of the estrous cycle. Most females urinated spontaneously when briefly held ventral-side up. Occasionally, the abdominal region of the female was massaged to facilitate urination.
6. Oral secretions from a female A cotton swab was dipped in tap water, rubbed against the oral angle of the female and presented to the males by touching it to their nostrils. The swab could contain secretions from the sacculi located at the oral angle or saliva from the female's mouth.
7. Secretions from the midventral gland of a female Siberian hamsters possess a sebaceous gland located on the midline of their abdomen (see Lai et al. [13] for more information). A cotton swab was dipped in tap water, rubbed against the ventral gland of a female, and presented to the males by touching it to their nostrils.
8. Soiled bedding from a female's cage Bedding from a female's cage was scooped up using a piece of paper, placed in a plastic Petri dish, and the fecal pellets removed. The dish of soiled bedding was placed in the male's cage for 1.5 h.
9. Feces from a female Males were presented with freshly defecated feces from a group-housed female as well as fecal pellets collected from the female's cage. The feces were placed on a Petri dish and placed in the male's cage for 1.5 h.
Experiment 4: To Determine the Source of the Signaling Chemical in the Female's Feces That Triggers LH Release in the Male
The results of experiment 3 showed that feces from females housed in LD elicited a significant increase in serum LH concentrations in males. To identify the source of the fecal pheromones responsible for triggering release of LH, males raised in LD were exposed to feces removed from different regions of the large intestine and from swabs taken from the perineal and rectal regions of females. Males were exposed to each stimulus for 1.5 h. When a potential stimulus was presented to the male on a cotton swab, the swab was touched to the nostrils of the male and then left in the cage with the male for 1.5 h. Blood samples were taken 1 and 1.5 h after the introduction of the stimulus into the home cage of the male.
Stimuli Tested (Sample Size: 9–10 Males per Group)
1. Water on a cotton swab See experiment 1.
2. A normally active female See experiment 1. The females used in this experiment were group housed and used at random stages of their estrous cycles. Some females may have been anestrous because of the group-housing condition. The presence of a mixture of randomly cycling and anestrous females may have increased variability in the responses of the males.
3. Feces extruded normally by a female Freshly defecated feces were collected as described for experiment 3.
4. Fecal material from the large intestine of a female (i.e., unextruded feces) To collect fecal material without rectal or anal gland secretions, females were killed with an overdose of sodium pentobarbital and their abdominal cavity exposed. Fecal pellets were removed from the large intestine.
5. Perineal odors A cotton swab moistened with tap water was rubbed on and around the anal region of a female. This swab may contain discharges from skin glands located on and around the anus and the vagina, and urine or feces [13].
6. Rectal gland secretions A cotton swab moistened with tap water was rubbed against the rectal mucosa of females that had been killed for collection of fecal material (4 above).
Testing and Blood Sampling Procedures
For all experiments, males were individually housed for 4 days before testing. On the day before testing, a baseline blood sample (0.4 ml) was taken from each male. Blood samples were taken 1 h (0.4 ml) and 1.5 h (terminal sample, 0.4–1 ml) after introduction of the stimulus into the male's cage on the day of testing. Blood samples were collected by cardiac puncture after anesthetizing the males with Metofane (methoxyfluorane; Schering-Plough Animal Health Corp., Union, NJ; experiments 1 and 2) or Halothane (Halocarbon Laboratories, River Edge, NJ; experiments 3 and 4). A pilot study compared the effects of Halothane versus Metofane on LH release and no differences were observed (data not shown).
Hormone Measurements
Serum LH, FSH, cortisol, and testosterone levels were measured by radioimmunoassay (RIA). LH and FSH were measured using materials supplied by NIDDK (Rockville, MD). All assays were performed in the Ligand Assay Core of Northwestern University. LH and FSH were measured from 50-µl aliquots of serum. The LH standard used was rat LH RP-3 and the antibody used was rat LH-S-11. The intraassay coefficient of variation for LH was 7.48%, while the interassay coefficient of variance was 18.5%. The FSH standard used was Rat FSH RP-3 and the antibody used was rat FSH-S-11. The intraassay coefficient of variation for FSH was 4.95%, while the interassay coefficient of variance was 7.14%. The use of the heterologous (rat) reagents to measure LH and FSH in Siberian hamster serum has been validated previously by our laboratory and others [19, 20]; the regression lines for the standard curves using the current reagents and serial dilutions of pooled serum from castrated and intact males are parallel. The sensitivity of the assays, as determined from the minimum detectable levels on the standard curves, for LH and FSH were 0.2 ng/ml and 1 ng/ml, respectively.
Testosterone and cortisol were measured in 25-µl aliquots of serum using RIA kits purchased from ICN Biomedicals, Inc., Diagnostic Division (Costa Mesa, CA). The testosterone assay has been validated previously in our laboratory for use in Siberian hamsters [3, 21]. The intraassay coefficient of variation for testosterone was 7.14%, while the interassay coefficient of variance was 14.7%. The cortisol assay also has been validated previously for use in Siberian hamsters in our laboratory [1]. The intraassay coefficient of variation for cortisol was 10.4%, while the interassay coefficient of variance was 17.9%. Minimum detectable levels of testosterone and cortisol were 0.02 ng/ml and 1.5 ng/ml, respectively.
Statistical Analyses
All statistical tests comparing serum hormone levels were conducted using NCSS 97 (Number Cruncher Statistical Systems, Kaysville, UT). Two-way ANOVA with repeated measures was used for comparisons of hormone levels between the groups. Main effects of treatments or their interactions were judged to be statistically significant when P < 0.05. Post hoc comparisons were made with the Duncan multiple-comparisons test. The response within a group was judged significantly different when P < 0.05 for the post hoc comparisons between samples collected at baseline and following exposure to a stimulus.
|
RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
LH release in males was stimulated by both ovary-intact and ovariectomized females. Males released LH regardless of whether the female was placed directly in their cage or was separated by a single wire-mesh divider (Fig. 1A; ANOVA, P < 0.001). Baseline LH concentrations were low in all groups, and no increase in LH was seen in the males exposed to water. Neither exposure to hamster vaginal secretions nor exposure to an intruder male, whether placed in the same cage or when separated by a wire mesh, induced LH release.
|
Testosterone concentrations increased in a pattern similar to that of LH (Fig. 1B; ANOVA, P < 0.05). There was no increase in FSH (ANOVA, P = 0.19) or cortisol (ANOVA, P = 0.75) in response to any of the factors (data not shown).
Experiment 2: To Determine if Exposure of Females to Short Photoperiod Alters Their Ability to Induce LH Release in Males
Exposure of females to the short photoperiod inhibited growth, suppressed reproductive development (as monitored by lack of vaginal opening), and resulted in the development of the white coat color characteristic of Siberian hamsters in winter conditions. The behavior of the SD females toward males was very different from that of the females housed in LD. SD females were aggressive toward the males, while LD females allowed the males to sniff and groom them. Male hamsters released LH when presented with a female that had been housed in LD but not when presented with a female that had been housed in SD (Fig. 2A; ANOVA, P < 0.001). Serum T levels increased significantly in males exposed to females housed in LD, but not when exposed to females that had been housed in SD (Fig. 2B; ANOVA, P < 0.05). There was no increase in FSH or cortisol secretion in response to any treatment (data not shown).
|
Experiment 3: To Determine if Auditory, Visual, Tactile, Behavioral or Chemical Cues from Females Are Necessary and Sufficient to Trigger the Release of LH in Males
Male hamsters released LH when either a normally active female, an anesthetized female, or feces from a female were placed in the cage with the male (Fig. 3A; ANOVA, P < 0.05). Males exposed to other odor sources, including urine, oral and midventral gland secretions, and bedding did not show an increase in serum LH concentrations. LH concentrations also did not increase in males that were separated from the female by a double wire mesh, which prevented physical contact between the male and female. Serum T levels mirrored LH levels in all conditions (Fig. 3B; ANOVA, P < 0.05). There was no significant increase in FSH or cortisol associated with any of the treatment factors (data not shown).
|
Experiment 4: To Determine the Source of the Signaling Chemical in the Female's Feces That Triggers LH Release in the Male
Males exposed to the two control conditions exhibited the expected LH concentrations (low for males exposed to water on a cotton swab and high in males exposed to a female; Figure 4A; P < 0.05). Males presented with feces that were surgically removed from the large intestine of a female or that were presented with a swab containing perineal odors showed no increase in LH secretion. In contrast, males exposed to feces that were defecated by the female and to swabs taken from the rectal mucosa exhibited a slight increase in serum LH concentrations. This difference was not significant using the Duncan multiple-comparisons test, but was significant (P < 0.05) using the less stringent Fisher least significant difference multiple-comparison test.
|
Serum testosterone concentrations increased significantly in males exposed to females, to naturally defecated feces, to fecal material removed from the intestines, and to perineal odors. Testosterone concentrations were also elevated in males following exposure to swabs taken from the rectal mucosa but failed to reach significance because of high baseline levels in this group (Fig. 4B; ANOVA, P < 0.05). There was no significant increase in FSH or cortisol associated with any of the treatment factors (data not shown).
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
The production of odor cues by females varies in other species over the course of the estrous cycle and in response to seasonal changes in day length resulting in the production of odors that are more attractive when females are receptive to males. Changes in ovarian hormone production are thought to account for many [7, 13, 22–24], but not all, of these differences in the attractiveness of female odors [25]. The results of the present study indicate that ovariectomized LD female Siberian hamsters are able to trigger LH release in males, although ovary-intact SD females do not induce LH release. This suggests that ovarian hormones are not required for females to stimulate LH release from males, but that some factor associated with the exposure of females to long photoperiods is necessary (Figs. 1 and 2). Females that have been exposed to SD provide a different suite of cues as compared with females housed in LD, including differences in visual cues (white versus dark coat) and behavioral cues (aggressive rather than proceptive behavior). These differences might directly alter the ability of a female to trigger LH release in a male or might alter the ability of a male to touch the female, thus reducing the male's exposure to the chemical; however, the males had access to the feces deposited by the females during the 1.5-h exposure period. This suggests that exposure to short photoperiods may inhibit the production of a pheromone in females by a mechanism that is independent of ongoing secretion of ovarian hormones in the adult.
Previous studies by Johnston and colleagues have demonstrated that odors from several different glands of female Djungarian hamsters (Phodopus campbelli) elicit specific behaviors from males [13]; thus, the response of a male may be influenced by the source of the odor as well as the reproductive status of the female at the time odors are collected from a female. Lai et al. [13] have termed this anatomical specificity of odor signals an across-odor code. The present study examined the ability of male Siberian hamsters (P. sungorus) to release LH in response to many of these same odor sources (vaginal secretions, urine, oral and midventral gland secretions) plus bedding and feces from females. Of these various potential sources of pheromones, only fecal pellets stimulated a significant increase in LH in males (Fig. 3). Experiment 4 attempted to localize the source of the chemical in the female's feces that induced LH release in males (Fig. 4). Freshly defecated feces (Fig. 4, extruded feces) and secretions collected by swabbing the rectal mucosa of females produced a moderate increase in LH in males, which was significant using the Fisher least significant difference test but not the Duncan multiple comparisons test. Males held the females' naturally defecated fecal pellets between their forepaws and investigated them by sniffing and licking. In contrast, fecal pellets that had been removed surgically from the upper regions of the rectum did not generate such investigation; most males sniffed them briefly and then ignored them. It appears that female Siberian hamsters deposit a chemical capable of stimulating LH release in males as the fecal material passes through the final stages of the female's intestinal tract.
The observation that LH was released in response to feces but not in response to other odor sources was surprising in light of the considerable data that demonstrate a potent role for female hamster vaginal secretion (FHVS) in the induction of mating behaviors and androgen secretion in golden hamsters (Mesocricetus auratus) [10, 12, 26–29]. Many of these behaviors are dependent on GnRH acting as a neurotransmitter; thus, we anticipated that FHVS would trigger LH release. Similarly, urine from female hamsters did not trigger LH release in male Siberian hamsters. This is in contrast with the well-documented ability of urine from female mice to induce LH and testosterone secretion in male mice [5, 6, 9, 30, 31]. Finally, odors from many of the same regions tested in the current experiment were found to be attractive to males when assessed in behavioral tests using the closely related Djungarian hamster [13]. Behavioral observations made during the course of the present experiments indicated that male Siberian hamsters showed considerable interest in the cotton swabs when coated with urine, oral or midventral gland secretions, and anal or rectal gland secretions. Similarly, the responses of males to swabs treated with FHVS were quite dramatic; all males presented with FHVS on a cotton swab accepted the swabs with alacrity and ingested the FHVS. All swabs coated with potential sources of pheromones were shredded by the males. Bedding from the cages of females provoked extensive investigation and occasional digging. These results suggest that Siberian hamsters may employ a similar across-odor code as Djungarian hamsters [13], but that different neural pathways mediate effects of pheromones on behavior and neurosecretion.
Three other hormones, cortisol, follicle stimulating hormone, and testosterone, were measured from all samples in each of these experiments. There were no changes in serum concentrations of cortisol or FSH in any of the experiments. Cortisol was measured to determine whether any of the manipulations (i.e., exposure to intruders in cages, repeated cardiac punctures) imposed during the experiment induced a stress response. The absence of any significant increase in cortisol levels suggests that the treatments did not cause sufficient levels of stress to interfere with the results.
Serum FSH levels were measured in this study to verify and extend our previous observation that exposure of a male to a female triggers LH release, but not FSH release [1]. Treatment with exogenous gonadotropin releasing hormone (GnRH) stimulates the release of both LH and FSH from the pituitary of the Siberian hamster. In the Siberian hamster, as in other species, the ratio of LH to FSH is dependent on the frequency of GnRH pulses [3]. The female-induced release of LH is dependent on the secretion of GnRH; antagonists to GnRH block female-induced LH secretion [1]; thus, one would expect that exposure to a female would also induce FSH secretion. The current results confirm the results of our previous study indicating that, in the male Siberian hamster, the hypothesized release of GnRH in response to the presence of a female is insufficient to stimulate FSH secretion [1]. The current results extend this observation to a variety of stimuli from the female. The neuroendocrine mechanism responsible for this dramatic differential regulation of LH and FSH secretion is unknown.
Stimulation of LH secretion resulted in a concomitant increase in serum testosterone concentrations (Figs. 1–4). Testosterone levels were elevated in the absence of a detectable increase in LH in the baseline samples for some groups and in males exposed to some stimuli. It is possible that we detected testosterone secretion that may have occurred in response to spontaneous LH secretion that preceded the onset of blood sampling on a particular day. Spontaneous LH secretion in male rats occurs in a pulsatile fashion, but there is not a direct one-to-one correlation between LH release and testosterone secretion [32, 33]. Pierroz et al. [33] report that, in rats, a series of three to four LH pulses results in the onset of testosterone secretion 1–2 h after the initiation of the first LH pulse; elevated testosterone levels may persist for up to an hour after the completion of the final LH pulse. Alternatively, LH-independent testosterone secretion has been observed in rats (see [33–35]). Additional studies would be required to determine if these stimuli (urine [Fig. 3B] and feces extracted from the intestinal tract and anal secretions on a cotton swab [Fig. 4B]) consistently result in increased testosterone secretion in the absence of an increase in LH secretion. Alternatively, it is possible we detected testosterone secretion that may have occurred in response to spontaneous LH secretion that preceded the onset of the experiment on a particular day. Spontaneous LH secretion in male rats occurs in a pulsatile fashion, but there is not a direct one-to-one correlation between LH release and testosterone secretion [32]. The results of the present experiments and our previous study [1], however, clearly demonstrate that the presence of a female, or the pheromones she produces, induces sufficient LH secretion to stimulate testosterone secretion from the mature testes of male hamsters that have been housed in a long photoperiod.
To summarize, the results of these experiments indicate that social stimuli, specifically a chemical signal, can induce LH secretion in male Siberian hamsters. The present study indicates that photoperiod regulates the ability of the female to stimulate LH release in a male. We have demonstrated previously that the ability of males to release LH in the presence of a LD female is inhibited in SD males [1]. Thus, in this seasonal breeder, short photoperiods inhibit both the source of a signal (the female) and the response system to the signal (the male). These data suggest that female Siberian hamsters may advertise their entrance into a period of photoperiod-induced reproductive competence by means of chemical cues contained in the feces. Tactile, visual, auditory, and behavioral cues are dependent on the spatial and temporal proximity of individuals; chemosensory cues are not and can be broadcast widely. Several forms of chemical communication (i.e., flank marking and vaginal marking) by females have been well characterized in this and other species [13, 36–39]. In contrast, feces have not been noted to evoke significant changes in behavior in the golden hamster [39], nor has a role for feces in regulating territorial or mating behavior in Siberian hamsters been identified [13, 36–39]. However, previous studies did report that male Siberian hamsters expressed interest in feces from females [40, 41]. In the wild, feces could be deposited in middens or trails to advertise the presence of a sexually receptive female. This might be particularly important for a species like the Siberian hamster, which lives at low density; the female's trail of fecal pheromones may serve to inform the male of her proximity and entrance into a state of reproductive competence. The release of LH and testosterone in the male in response to the female's pheromones may facilitate the male's search for the female and facilitate the sexual response when they meet.
|
ACKNOWLEDGMENTS |
|---|
|
FOOTNOTES |
|---|
2 Correspondence: Teresa H. Horton, Department of Neurobiology and Physiology, Northwestern University, 2-160 Hogan, 2205 Tech Drive, Evanston, IL 60208. FAX: 847 491 5211; thorton{at}northwestern.edu
Received: 5 June 2003.
First decision: 20 June 2003.
Accepted: 25 November 2003.
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
