Increased Plasma Testosterone Concentrations Research Articles

Melatonin improves testicular haemodynamics, echotexture and testosterone production in Ossimi rams during the breeding season.

The objective was to determine effects of a single parenteral dose of melatonin on testicular blood flow indices, testicular echogenicity and plasma testosterone concentrations in rams during the physiological breeding season. We hypothesized that melatonin enhances testicular blood flow, echogenicity and plasma testosterone concentrations during the breeding season in rams. During the breeding season, 12 sexually mature Ossimi rams were randomly allocated to either a melatonin group (n=8) that received 18mg of melatonin in 1ml of corn oil (injected SC) or a control group (n=4) that received 1ml corn oil only. Blood collection and ultrasonographic assessment of the testes and supratesticular arteries were conducted immediately before treatment (W0) and once weekly for 6weeks after melatonin injection (W1-W6). Mean plasma testosterone concentrations were greater (p<.05; at least 1ng/ml) in the melatonin-treated group compared to the control group from W4 to W6 after treatment. A decrease (p<.05) in both resistive index (RI) and pulsatility index (PI) began 1week after melatonin injection (W1) and persisted until the end of the experiment, with mean RI and PI values in the melatonin group lower (p<.05) than those in the control group on W3 and W4. Furthermore, plasma testosterone concentrations in melatonin-treated rams were inversely correlated to both RI and PI (r=-.7 and -.6, respectively, p<.01). Testicular echogenicity decreased (p<.05) 1week after melatonin injection (W1) and remained lower (p<.05) in the melatonin-treated group compared to the control group until the end of the study (W6). In conclusion, melatonin administration significantly altered testicular blood flow and echogenicity and increased plasma testosterone concentrations in Ossimi rams during the breeding season.

Gonadal steroid hormone receptors in the medial amygdala contribute to experience-dependent changes in stress vulnerability

Social experience can generate neural plasticity that changes how individuals respond to stress. Winning aggressive encounters alters how animals respond to future challenges and leads to increased plasma testosterone concentrations and androgen receptor (AR) expression in the social behavior neural network. In this project, our aim was to identify neuroendocrine mechanisms that account for changes in stress-related behavior following the establishment of dominance relationships over a two-week period. We used a Syrian hamster model in which acute social defeat stress increases anxiety-like responses in a conditioned defeat test in males and in a social avoidance test in females. First, we administered flutamide, an AR antagonist, via intraperitoneal injections daily during the establishment of dominance relationships in male hamsters. We found that pharmacological blockade of AR prevented a reduction in conditioned defeat in dominant males and blocked an upregulation of AR in the posterior dorsal medial amygdala (MePD) and posterior ventral medial amygdala (MePV), but not in the ventral lateral septum. Next, we administered flutamide into the posterior aspects of the medial amygdala (MeP) prior to acute social defeat stress or prior to conditioned defeat testing in males. We found that pharmacological blockade of AR in the MeP prior to social defeat, but not prior to testing, increased the conditioned defeat response in dominant males and did not alter behavior in subordinates. Finally, we developed a procedure to establish dominance relationships in female hamsters and investigated status-dependent changes in plasma steroid hormone concentrations, estrogen receptor alpha (ERα) immunoreactivity, and defeat-induced social avoidance. We found that dominant female hamsters showed reduced social avoidance regardless of social defeat exposure as well as increased ERα expression in the MePD, but no status-dependent changes in the concentration of plasma steroid hormones. Overall, these findings suggest that achieving and maintaining stable social dominance leads to sex-specific neural plasticity in the MeP that underlies status-dependent changes in stress vulnerability.

Extended day length in late winter/early spring, with a return to natural day length of shorter duration, increased plasma testosterone and sexual performance in rams with or without melatonin implants.

Sixteen rams were used to quantify the effects of long days, imposed during late winter/early spring, with or without exogenous melatonin, on plasma testosterone concentrations and ram serving capacity. Rams were assigned to two groups: photoperiod-treated rams (Artificial Photoperiod, AP; n=8), exposed to 2months of long days (16hr of light/day) between 22 December and 22 February, and control rams (Natural Photoperiod, NP; n=8). At the end of the long-day period, AP rams were returned to the natural photoperiod, and each ram in the two groups either did (+M) or did not (-M) receive three subcutaneous melatonin implants. Four groups were created as follows: AP+M (n=4), AP-M (n=4), NP+M (n=4) and NP-M (n=4). Thirty days after of the onset of photoperiodic treatment, AP rams (13.5±2.8ng/ml) had significantly (p<.05) lower testosterone levels than NP rams (36.7±1.0), and similar differences were not apparent at the end of the photoperiod treatment. A month later, AP rams (24.3±7.9) had higher (p<.10) testosterone levels than NP rams (13.1±5.0), with no effect of melatonin treatment. Fifty days after melatonin implantations, rams were exposed for 20min to three oestrous ewes. AP rams (2.50±0.42) exhibited significantly (p<.05) more serves than did NP rams (1.11±0.39), and melatonin treatment had no significant effect; however, the interaction between treatments was significant. Time to first serve was significantly (p<.05) shorter in AP (2.30±1.20min) than it was in NP rams (5.58±0.68min). In conclusion, exposure to 2months of long days in late winter/early spring, with a return to natural day length of shorter duration, increased plasma testosterone concentrations and sexual performance in rams with or without exogenous melatonin. This particular management is an option if a non-hormonal reproductive strategy is scheduled; yet, if the use of exogenous hormones is feasible, melatonin implants increase the mating efficiency of rams.

Reproductive axis gene regulation during photostimulation and photorefractoriness in Yangzhou goose ganders

BackgroundThe Yangzhou goose is a long-day breeding bird that has been increasingly produced in China. Artificial lighting programs are used for controlling its reproductive activities. This study investigated the regulations of photostimulation and photorefractoriness that govern the onset and cessation of the breeding period.ResultsIncreasing the daily photoperiod from 8 to 12 h rapidly stimulated testis development and increased plasma testosterone concentrations, with peak levels being reached 2 months after the photoperiod increase. Subsequently, testicular activities, testicular weight, spermatogenesis, and plasma testosterone concentrations declined steadily and reached to the nadir at 5 months after the 12-hour photoperiod. Throughout the experiment, plasma concentrations of triiodothyronine and thyroxine changed in reciprocal fashions to that of testosterone. The stimulation of reproductive activities caused by the increasing photoperiod was associated with increases in gonadotropin-releasing hormone (GnRH), but decreases in gonadotropin-inhibitory hormone (GnIH) and vasoactive intestinal peptide (VIP) gene messenger RNA (mRNA) levels in the hypothalamus. In the pituitary gland, the levels of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) mRNA abruptly increased during the longer 12-hour photoperiod. The occurrence of photorefractoriness was associated with increased GnIH gene transcription by over 250-fold, together with increased VIP mRNA levels in the hypothalamus, and then prolactin and thyroid-stimulating hormone in the pituitary gland. FSH receptor, LH receptor, and StAR mRNA levels in the testis changed in ways paralleling those of testicular weight and testosterone concentrations.ConclusionsThe seasonal reproductive activities in Yangzhou geese were directly stimulated by a long photoperiod via upregulation of GnRH gene transcription, downregulation of GnIH, VIP gene transcription, and stimulation of gonadotrophin. Development of photorefractoriness was characterized by hyper-regulation of GnIH gene transcription in the hypothalamus, in addition of upregulation of VIP and TRH gene transcription, and that of their receptors, in the pituitary gland.

Effects of dietary seaweed (Ulva lactuca) supplementation on the reproductive performance of buck and doe rabbits

The aim of this study was to investigate the effect of dietary supplementation of seaweed on the reproductive performance of rabbits. Two trials were performed during this study. In the first trial, semen quality was evaluated in 15 buck rabbits with mean body weight of 4.80±0.41 kg and six month of age. In the second trial, prolificacy was determined in 30 artificially inseminated does with a mean body weight of 4.84±0.50 kg and five to six months of age. Rabbits in each trial were randomly allocated to one of three dietary seaweed levels; commercial pelleted diet (C), pelleted diet supplemented with 1% seaweed (T1), and pelleted diet supplemented with 2% seaweed (T2). Dietary supplementation of seaweed significantly increased plasma testosterone concentration and improved various sperm motility parameters. Analysis of acrosomal membrane integrity using electron microscopy revealed no significant influences of dietary seaweed supplementation on quality grade (I–IV) of all tested samples. These results reflected in positive prolificacy response of does artificially inseminated with semen samples pooled from bucks fed on T2, and offered diet supplemented with 2% seaweed one week prior to their insemination and throughout the gestation period. Feeding diets supplemented with 2% seaweed to doe rabbits improved their kindling rate, litter size, and their offspring ratio. Seaweed supplementation to the diets of rabbits raised under summer conditions had improved their reproductive performance by improving the semen fertility characteristics of bucks and the prolificacy characteristics of does. Hematological and biochemical parameters investigated in this study did not reveal any pathological signs in both rabbit's genders due to dietary seaweed supplementation.

Thermal treatments prior to and during the beginning of incubation affect phenotypic characteristics of broiler chickens posthatching

The significance and importance of the preincubation and incubation temperatures for broiler chickens has been elucidated by altering normal incubation conditions to study the effects on embryo development. Furthermore, only recently has convincing evidence that temperature could influence the sex ratio of avian offspring become available. The objective of this study was to elucidate the effects of temperature before or during (or both) the sex determination period of incubation on hatchability, apparent sex ratio, growth and development posthatching, and secondary sexual phenotypic characteristics. Two experiments were conducted in winter and summer using Cobb 500 fertile eggs that had been stored for 4 and 9 d, respectively. Four treatments of 180 eggs each were applied: control, preheating (Pre) 30.2°C for 12 h before incubation, heating (38.1°C) the embryos between embryonic d 0 (E0) and E5 (M) of incubation, and a combination of both (Pre+M). All 3 thermal treatments increased early embryonic deaths, but improved hatchability in both experiments. The point of 50% hatchability was achieved more rapidly in the treated eggs. The BW of males and females at 35 d of age in both experiments was numerically or significantly greater in the broilers that had been exposed to thermal treatments, which was coincident with a similar trend for increased relative breast muscle weight. Secondary sexual characteristics (comb, wattles, testes in males) were also affected by thermal treatments, being heavier in most cases, which may be attributed to the finding that the 3 thermal treatments resulted in numerically or significantly increased plasma testosterone concentration in both sexes and experiments. Differences in the level of significance between the experiments probably related to the length of storage period and the season in which each experiment took place. It was concluded that thermal treatments preincubation or during the sex determination period of incubation had, in general, a positive effect on hatchability, growth performance, and secondary sexual characteristics of broiler males and females, probably caused by the increase of plasma testosterone concentration in both sexes.

Role of testosterone and photoperiod on seasonal changes in horn growth and sperm variables in the Iberian ibex: a model for polygynous wild bovids

This work examines the effect of testosterone secretion and photoperiod on seasonal changes in horn growth and sperm variables in the Iberian ibex (Capra pyrenaica), here used as a model for polygynous wild bovids. The hypothesis that high levels of testosterone provide an endocrine signal that inhibits horn growth in autumn was tested by assessing the effect of cyproterone acetate (CA), an anti-androgen, administered in October - coinciding with the period of natural increases in plasma testosterone concentrations - under different photoperiodic conditions (natural photoperiod and artificial long days). The persistence of horn growth during autumn in all ibexes held under the long-day photoperiodic conditions clearly shows that horn growth regulation in the mating season is primarily modulated by day length and not by a fall in testosterone concentration. A retrospectively designed second experiment involving testosterone propionate (TP) administration in April (when horns are growing) was then undertaken to confirm that high levels of testosterone do not inhibit horn growth. Overall, the results strongly suggest that the rise in testosterone secretion during the autumn mating season does not act as an endocrine signal for the arrest of horn growth, although the rate of horn growth before the mating season may be related to springtime testosterone levels. A direct relationship was seen between the rate of horn growth and the incidence of sperm abnormalities. Neither CA treatment in October nor TP administration in April affected the studied sperm variables. By contrast, CA treatment plus artificial long days in autumn had a negative effect on sperm motility and sperm morphology.

Effects of gonadotropin-releasing hormone administration on the pituitary-gonadal axis in male and female dogs before and after gonadectomy

GnRH-stimulation tests were performed in 14 female and 14 male client-owned dogs of several breeds, before and 4 to 5 mo after gonadectomy. The aim of the study was to obtain more insight into the pituitary-gonadal axis in intact and neutered dogs and to establish reference values. Basal plasma luteinizing hormone (LH) and follicle-stimulating hormone (FSH) concentrations were increased significantly after gonadectomy in both bitches and male dogs. In both males and females ranges of the basal plasma FSH concentrations, before and after gonadectomy, did not overlap as opposed to the overlap in ranges of the basal plasma LH concentrations. Before gonadectomy basal plasma LH concentrations were lower and basal plasma FSH concentrations were higher in bitches than in male dogs. After gonadectomy these basal values did not differ significantly. GnRH administration before gonadectomy resulted in an increase in plasma LH and FSH concentrations in both genders. GnRH administration after gonadectomy produced an increase only in plasma LH concentrations in both genders, and a just significant increase in plasma FSH in castrated male dogs. GnRH administration before gonadectomy resulted in a significant increase in plasma testosterone concentration in both genders. In males ranges of basal and GnRH-stimulated plasma testosterone concentrations before and after gonadectomy did not overlap. Basal plasma estradiol concentrations were significantly higher in intact males than in castrated males and their ranges did not overlap. The basal estradiol concentrations in bitches before and after ovariectomy were not significantly different. At 120 min after GnRH administration, ranges of plasma estradiol concentration of intact and ovariectomized bitches no longer overlapped. In conclusion, basal plasma FSH concentration appears to be more reliable than basal plasma LH concentration for verification of neuter status in both male and female dogs. The basal plasma testosterone concentration appears to be reliable for verification of neuter status in male dogs. The plasma estradiol concentration at 120 min after GnRH administration can be used to discriminate between bitches with and without functional ovarian tissue.

Diet exposure to technical hexabromocyclododecane (HBCD) affects testes and circulating testosterone and thyroxine levels in American kestrels (Falco sparverius)

Hexabromocyclododecane (HBCD) is a high-production-volume, brominated flame-retardant that is used in items such as polystyrene foams. HBCD has been detected in the environment, wildlife tissues and in humans globally with some of the highest recorded levels in predatory birds. This study examined the effects of exposure to environmentally relevant levels of HBCD on the reproductive physiology of captive male American kestrels (Falco sparverius), a predatory bird. Two sets of males were used: one group not housed with females (unpaired: nc=12, nHBCD=10) and the second group housed with females (breeding: nc=10, nHBCD=20). All treatment birds were exposed to 0.51μg HBCD/g kestrel/day technical HBCD, and controls to safflower oil only, injected into their food during seasonal testicular development. Unpaired males were exposed for 3 weeks and euthanized for testicular analysis. Breeding males were exposed for 3 weeks prior to pairing and throughout the courtship period. The HBCD-exposed unpaired males had heavier testes (p≤0.017) and a trend towards more seminiferous tubules containing elongated spermatids (p=0.052). There was also a moderate increase in plasma testosterone concentrations (p=0.056) compared to controls. In breeding males, testosterone levels increased during courtship to culminate in higher levels than controls by the time the first egg was laid (p=0.010) and circulating free and total T4 was reduced throughout. The number of sperm cells reaching the perivitelline layer of the first egg for breeding males did not differ between the two groups. This study is the first report that HBCD exposure at environmentally relevant levels alters reproductive physiology in male birds and suggests that birds may be more sensitive to HBCD than mammals.

Effect of artificial long days and/or melatonin treatment on the sexual activity of Mediterranean bucks

The aim of this work was to determine whether treatments using photoperiod (long days) and/or exogenous melatonin are effective in enhancing reproductive activity during the natural anestrous season in Mediterranean bucks. Two experiments were carried out. In the first experiment, 19 adult bucks were allotted to 3 treatment groups. Fourteen bucks were exposed to 16 h of light/day (long day) for 98 days (late autumn–winter). At the end of the long day treatment, 7 bucks received 3 s.c. melatonin implants (LD-M group), and the other bucks remained a long day group (LD group, n = 7). The control group (C1, n = 5), remained under natural photoperiod. In the second experiment, 7 bucks received 3 s.c. melatonin implants (M group, n = 7), while the other 5 bucks acted as the control group (C2, n = 5) during the spring equinox. During the natural sexual anestrous season (spring), the treated groups recorded higher plasma testosterone concentrations than the control groups. Reactivation of the natural reproductive season was delayed only in the LD-M group. All treated bucks recorded a higher mean semen volume, than the control groups (Exp 1: 1.16 ± 0.04 mL, 1.16 ± 0.04 mL, and 1.08 ± 0.04 mL, for the LD-M, LD, and C1 groups, respectively P < 0.01; Exp 2: 1.04 ± 0.07 mL vs 0.77 ± 0.05 mL, for M and C2 groups, respectively; P < 0.01). The percentage of bucks that ejaculated was modified by treatment in the first experiment (89 ± 1.7%, 71 ± 2.4%, and 88 ± 1.9% for LD-M, LD, and C1 groups, respectively, P < 0.001). Results demonstrate that treating bucks with artificial long days and/or melatonin at Mediterranean latitudes can induce an increase in plasma testosterone concentration during the natural anestrous season. The long day treatment alone resulted in a less effective method of inducing adequate out-of-season buck sexual activity.

Reduced Sertoli cell number and altered pituitary responsiveness in male lambs undernourished in utero

The nutritional status of females during pregnancy can have profound effects on the fetus reproductive system development that could affect the reproductive potential as later as in adulthood (fetal programming). The aim of the present study was to examine the effects of maternal nutrient restriction imposed during different periods of gestation on pituitary responsiveness at different ages postnatal and on Sertoli cell number in male offspring. Pregnant ewes were fed to 100% of Metabolizable Energy and Crude Protein requirements throughout pregnancy (Control) or to 50% from 0 to 30 (R1) or from 31 to 100 days of gestation (R2). Male lambs were selected and fed to appetite throughout the study. At 2, 5.5 and 10 months of age a GnRH challenge was conducted. At slaughter (10 months) testes were removed and examined histologically. Maternal undernutrition did not affect the time of the onset of puberty, defined as the first increase in plasma testosterone concentrations ≥1 ng/ml. The LH and FSH response to GnRH challenge did not differ between groups at 2 and 5.5 months but at 10 months of age a higher ( P < 0.05) FSH response was found in R2 group. Testes weight did not differ between groups at slaughter. Mean Sertoli cell number was significantly lower in animals of R2 group compared with Control ( P < 0.01). A smaller seminiferous tubules diameter was detected in R2 group ( P < 0.05), while the diameter of the lumen was similar in all groups. Collectively, these results provide clear evidence for a direct effect of nutrient restriction during pregnancy on Sertoli cell number in adulthood. The lower number of Sertoli cells is the most candidate factor for the higher pituitary activity through a reduction in the suppressive effect of inhibin.

Horn growth related to testosterone secretion in two wild Mediterranean ruminant species: The Spanish ibex ( Capra pyrenaica hispanica) and European mouflon ( Ovis orientalis musimon)

Seasonal variations in the horn development and testicular activity of the Spanish ibex ( Capra pyrenaica hispanica) ( n = 6) and European mouflon ( Ovis orientalis musimon) ( n = 5) were monitored to determine the role of increasing testosterone concentration on the arrest of horn growth during the rutting season. Marked seasonal variations in the rate of horn growth ( P < 0.01) and testicular activity ( P < 0.001) were seen in both species, although the magnitude and timing of these changes were different ( P < 0.01). Horn growth rate was inversely correlated to seasonal levels in testosterone plasma concentration in both species (ibex: R = −0.45, P < 0.01; mouflon R = −0.51, P < 0.01). In the mouflon, the increase in plasma testosterone concentration recorded in September ( P < 0.05 compared with the lowest concentration) coincided with a significant reduction in horn growth ( P < 0.05). In the ibex, the increase in plasma testosterone concentration in October ( P < 0.05 compared with the lowest concentration) was associated with a significant arrest of horn growth in November ( P < 0.05). These results appear to support the hypothesis that high peripheral plasma levels of testosterone are linked with the seasonal arrest of horn growth during the rutting period.

The Effects of Subacute Inhalation of Di (2‐ethylhexyl) Phthalate (DEHP) on the Testes of Prepubertal Wistar Rats

In animal studies using oral dosing for short periods, di (2-ethylhexyl) phthalate (DEHP) is well known for its reproductive toxicity, especially for its testicular toxicity. However, extending the period of DEHP exposure in prepubertal rats resulted in significant increases in testosterone. This suggests that the reproductive effect of DEHP might be associated with the timing and the term of exposure. Moreover, the route of exposure may induce differences in its effect because tissue levels of metabolites of DEHP after inhalation are thought to be different from those after oral administration. We researched the effects of inhalation of DEHP on testes of prepubertal rats. Our results showed that inhalation of DEHP by 4-wk-old male Wistar rats at doses of 5 or 25 mg/m(3), 6 h per day, for 4 and 8 wk significantly increased the concentration of plasma testosterone and weight of seminal vesicles. However, the concentration of luteinizing hormone (LH), follicular stimulating hormone (FSH) and the expression of mRNAs of androgen biosynthesis enzyme, cytochrome P450 cholesterol side-chain-cleavage enzyme (P450scc), 3beta-hydroxysteroid dehydrogenase (3beta-HSD), cytochrome P450 17alpha-hydroxylase/17, 20 lyase (CYP17) and aromatase (CYP19) did not change. Rats with precocious testes did not increase in any of the DEHP groups. We also found that the estimated effective dose in this study was less than those reported in previous studies which used oral dosing. Our study showed that inhaled DEHP increased plasma testosterone concentrations in prepubertal rats and suggested that their effects were more sensitive to inhalation of DEHP than oral dosing.

Territoriality and testosterone in an equatorial population of rufous-collared sparrows, Zonotrichia capensis

Territorial aggression, displayed by male vertebrates in a reproductive context, is generally thought to be mediated by testosterone. The challenge hypothesis predicts that in socially monogamous species, territorial challenges should induce an increase in plasma testosterone concentrations, which will enhance aggressive behaviour and territory defence. This hypothesis is based on northern latitude birds and needs to be tested in tropical birds before it can be universally accepted. We tested the challenge hypothesis in an equatorial population of rufous-collared sparrows in Papallacta, Ecuador. This population shows an extended breeding period during which males aggressively guard territories. During the early breeding season, males were challenged with conspecific or heterospecific simulated territorial intrusions (STIs) lasting 10 min. Conspecific-challenged males responded more aggressively than heterospecific-challenged males. However, there was no increase in plasma testosterone in response to the conspecific STI. During the breeding season, males were challenged with conspecific STIs lasting 0, 10 or 30 min. Males behaved aggressively regardless of STI duration, and did not differ in plasma testosterone or luteinizing hormone concentrations. During the breeding season, males were implanted with testosterone-filled or empty silastic tubes and subsequently challenged with a conspecific STI. Testosterone implants significantly raised plasma testosterone concentrations, but testosterone-implanted males were not more aggressive than blank-implanted controls. Combined, these findings suggest that testosterone concentrations above breeding baseline are not related to territorial aggression in this population and therefore do not support the challenge hypothesis.

LH and testosterone responses to five doses of a GnRH analogue (buserelin acetate) in 12-month-old Thoroughbred colts

To determine the responsiveness of the pituitary–gonadal axis of peri-pubertal colts to GnRH, buserelin (0.5, 1, 5, 10 and 40 μg) was given to 13 male Thoroughbred yearlings ( n=3–8 colts per dose). Jugular venous blood samples were taken at −10, 0, 10, 20, 30, 40, 60, 120 and 180 min relative to buserelin administration. Increases ( P<0.05) in LH concentrations occurred in colts that received 5, 10, or 40 μg buserelin, but not in those that received 0.5 or 1 μg. Peak LH concentrations and mean area under the curve were higher ( P<0.05) in colts receiving 40 μg buserelin than in those that received 0.5 or 1 μg. Increases ( P<0.05) in testosterone concentrations occurred in some, but not all, colts that received 1, 5, 10, or 40 μg buserelin. Neither peak concentration nor area under the curve of testosterone differed significantly among doses of buserelin. The percentage of horses that responded to the buserelin increased with increasing dose, with only the highest dose eliciting LH and testosterone responses in all colts. In conclusion, peri-pubertal colts exhibited a dose–response release of LH following buserelin treatment, but individual colts responded in an “all or nothing” manner, such that each either had an LH response or did not. Some colts that exhibited a significant LH response had no subsequent increase in plasma testosterone concentrations; perhaps the pituitary LH response may not have been great enough to stimulate the Leydig cells in these individuals.

Reproductive seasonality of the male Florida gar, Lepisosteus platyrhincus

The objective of this study was to characterize the reproductive seasonality of a wild population of male Florida gar, Lepisosteus platyrhincus. We measured the gonadosomatic index, reproductive stage of the testes, seminiferous tubule area, and plasma concentrations of testosterone and 11-ketotestosterone for each fish. A seasonal pattern was observed in the reproduction of male gar. Following a quiescent period in the summer, there was an increase in plasma testosterone concentration during the fall, which was associated with the onset of active spermatogenesis as indicated by an increase in seminiferous tubule area and gonadosomatic index. Plasma concentrations of 11-ketotestosterone remained basal during the summer and winter and then increased during the early spring. These data, taken together with data from female gar collected in a parallel study by our lab, suggest that spawning occurred during the early spring (February–March), followed by a general decrease in reproductive parameters to summer levels. Recognizing that peaks of hormone concentrations and spawning period could vary with annual seasonal variation, this study is important because it is the first characterization of the seasonal morphological and endocrinological reproductive pattern in a holostean species. Furthermore, it increases our understanding of the reproductive biology of semitropical fishes.

Behavioural and physiological consequences of home advantage resource holding in male guinea pigs

This study tested the physiological and behavioural effects of a resource asymmetry between guinea pig males. Asymmetry was defined via cohabitation with a female in a home area during a 5-day period. An experimental setup was designed in which the same male alternated between being in this home area (and monopolizing a female therein) and playing the role of an intruder. The confrontation phase between the resource holder and the intruder lasted 5 days. Behavioural data were collected for 0.5 h at the start of the confrontation and again for 0.5 h at 3, 24, 72, and 120 h after the onset of confrontation. Blood samples were taken before the test started and after 3, 72, and 120 h. Body mass was registered before and after the confrontation phase. The results indicate that resource holders were more highly motivated than intruders in displaying aggressiveness. They exhibited more chasing behaviour and won more confrontations. Both male groups performed almost identical frequencies of threat displays and courtship behaviour. Physiologically, the resource holders lost less body mass and showed increased plasma testosterone concentrations after 120 h. Resource holders also exhibited steady plasma corticosteroid concentrations, whereas the intruders showed elevated corticosteroid levels at 72 and 120 h. The results show that when acting as a resource holder, males react according to the resident-always-wins hypothesis, and when acting as an intruder, lose most confrontations. We conclude that the behavioural and physiological reaction of males during confrontations is determined to a greater extent by environmental asymmetries than by experiences or physical conditions before the conflict.

Influence of placentally mediated fetal growth restriction on the onset of puberty in male and female lambs.

The onset of puberty in prenatally growth-restricted versus normally grown lambs of both sexes, born in April and housed under natural photoperiod, was examined. Singleton pregnancies were established and adolescent ewes were offered a high or moderate nutrient intake throughout gestation. Placental mass was reduced (P < 0.001) in high compared with moderate intake dams and resulted in the birth of growth-restricted and normal birth weight offspring, respectively. At birth, female lambs weighed 3.43 kg versus 5.03 kg (P < 0.001; n = 14 per group) and male lambs weighed 2.75 kg versus 5.18 kg (P < 0.001; n = 7 per group) in growth-restricted and normal birth weight groups, respectively. Lambs suckled for 12 weeks and thereafter were fed ad libitum until week 43 of age. Growth-restricted lambs had lower preweaning live weight gains and this difference was more pronounced in male (P < 0.05) than in female lambs (P = 0.07). Thereafter, live weight remained lower (P < 0.05) in growth-restricted than in normally grown lambs of both sexes until week 25 of age. In females, the time of onset of puberty was similar in the two groups. All females ovulated and there were no differences in the number of ovarian cycles recorded or in the incidence of aberrant ovarian function. In males, testosterone concentrations and testicular volume were lower in growth-restricted compared with normally developed lambs from birth until weeks 28 and 35 of age, respectively (P < 0.05). The seasonal increase in plasma testosterone concentrations occurred later in growth-restricted than in normally developed lambs (P < 0.01) but the timing of maximum peak concentrations was similar. Peak testosterone concentrations were lower (P < 0.05) in growth-restricted than in normal male lambs.

Effects of recent sexual experience and melatonin treatment of rams on plasma testosterone concentration, sexual behaviour and ability to induce ovulation in seasonally anoestrous ewes

The aim of this study was to determine whether advancing the seasonal changes associated with rams by treatment with exogenous melatonin and allowing the rams previous sexual experience would increase the proportion of anoestrous ewes ovulating in early July. North Country Mule ewes (n = 225) were grouped by live body weight and body condition score and allocated randomly to the following treatments: (i) isolated from rams (control; n = 25); (ii) introduced to rams (treatment 2); (iii) introduced to rams that had mated with ewes during the previous 2 days (treatment 3); (iv) introduced to rams implanted with melatonin (treatment 4); and (v) introduced to rams that were implanted with melatonin and had mated with ewes during the previous 2 days (treatment 5). Treatments 2-5 were replicated (2 x 25 ewes) and two rams were introduced to each replicate group. Introductions began on 4 July and were completed by 11 July. The rams were withdrawn from the ewes after 8 days. Melatonin was administered as a subcutaneous implant (Regulin((R))) on 22 May and again on 20 June. Blood samples were taken from all rams to determine plasma melatonin and testosterone concentrations (19 samples in 6 h). The behaviour of the sheep was videotaped continuously during the first 3 h after the ram was introduced. Ovulation was detected by an increase in plasma progesterone concentrations from < 0.5 ng ml(-1) to > 0.5 ng ml(-1). Mean +/- SE plasma melatonin concentrations were 649.7 +/- 281.4 and 18.3 +/- 2.4 pg ml(-1) in rams with and without melatonin implants, respectively (P < 0.001). Melatonin implants also increased plasma testosterone concentrations from 4.30 +/- 1.88 to 10.10 +/- 1.10 ng ml(-1) (P < 0.01), the libido of the rams and the proportion of ewes that ovulated in response to the rams (43 and 56% (treatments 4 and 5) versus 24% (treatments 2 and 3)). In conclusion, implanting rams with melatonin before introducing them to seasonally anoestrous ewes increases the proportion of ewes that ovulate in response to introduction of a ram, but previous sexual experience of rams appears to have little or no effect.

Effect of naloxone on the plasma levels of LH, FSH, prolactin and testosterone in Beetal bucks

Ten adult male Beetal goats were used for the study to elucidate the modulation of gonadotrophin, prolactin and testosterone secretion by endogenous opioid peptides. An indwelling catheter was placed in the jugular vein of each buck 20 h before the onset of the experiment. Bucks were divided randomly into two groups: Group I ( n = 5) received naloxone at a dose rate of 1 mg/kg body weight (BW) and Group II ( n = 5) received naloxone at a dose rate of 2 mg/kg BW intravenous. Blood samplings were done from 2 h before treatment until 2 h after treatment at 15 min intervals. Blood samples were quantified for plasma LH, FSH and prolactin concentration using a heterologous double antibody radioimmunoassay (RIA) and testosterone concentration was quantified by coat-a-count RIA kit. The mean plasma LH levels during pretreatment phase were 0.41 ± 0.03 ng/ml in Group I and 0.44 ± 0.02 ng/ml in Group II which significantly ( p < 0.05) increased to 0.91 ± 0.05 ng/ml in Group I and 1.53 ± 0.07 ng/ml in Group II. The mean plasma FSH levels did not show a difference in pre- and post-treatment animals in both groups. A significant ( p < 0.05) increase in plasma testosterone concentration was observed in both groups after naloxone treatment, whereas, a decrease ( p < 0.05) was observed in plasma prolactin levels after naloxone treatment. Thus, it can be concluded that endogenous opioids do play an important role in modulating plasma LH, prolactin and testosterone concentrations in male goats.