Effects of season and testosterone treatment on gonadotrophin secretion and pituitary responsiveness to gonadotrophin-releasing hormone in castrated Romney and Poll Dorset rams

Determinants of the annual pattern of reproduction in mature male Merino and Suffolk sheep: responses to a nutritional stimulus in the breeding and non-breeding seasons

This study was designed to test whether an acute improvement in diet would increase gonadotrophin secretion and testicular growth in strongly photoperiod-responsive Suffolk rams and weakly photoperiod-responsive Merino rams in both the breeding (February-March) and the non-breeding (July-August) seasons. Mature rams (n = 5 or 6) of these breeds were fed a maintenance diet (0.9 kg chaff + 100 g lupin grain) or the same diet supplemented with 1.5 kg lupin grain for 42 days in each season. Lupin grain is a rich source of both energy and protein. Testosterone, luteinizing hormone (LH) and follicle stimulating hormone (FSH) were measured in plasma from blood sampled every 20 min for 24 h on Days -1, 12 and 35 relative to the change in feeding. In rams supplemented with lupins, body mass increased in both breeds in both seasons (P < 0.001). Scrotal circumference and LH pulse frequency increased with lupin supplementation in both seasons (P < 0.003) in Merinos, but only during the breeding season (P < 0.003) in Suffolks. Plasma FSH concentrations were affected by diet only during the breeding season, being elevated on Day 12 in lupin-supplemented rams of both breeds (P < 0.05). It was concluded that Merino rams exhibit reproductive responses to improved nutrition irrespective of time of the year, whereas Suffolk rams respond to nutrition only when the hypothalamic reproductive centres are not inhibited by photoperiod. Thus, Suffolks do respond to nutrition, just as Merinos do, but only when photoperiod allows. This difference between breeds appears to be a result of differences in the neuroendocrine pathways that control pulsatile gonadotrophin-releasing hormone secretion.

Effect of testosterone and season on proenkephalin messenger RNA expression in the preoptic area of the hypothalamus in the ram

Enkephalin appears to exert an inhibitory action on LH secretion, but whether testosterone regulates enkephalin gene expression is unknown. This study tested the hypothesis that testosterone and/or season modulate preproenkephalin mRNA expression in specific areas of the hypothalamus. Romney Marsh rams were castrated (wethers) either during the breeding season or nonbreeding season and received intramuscular injections of either oil or testosterone propionate (five/group). Blood samples were taken for the assay of plasma LH and testosterone. Preproenkephalin mRNA expression was quantified in hypothalamic sections by in situ hybridization. Mean plasma LH concentrations were reduced and the interpulse interval for LH pulses was greater in testosterone propionate-treated wethers compared with oil-treated wethers, with no change in LH pulse amplitude. Testosterone propionate treatment reduced proenkephalin expression in the diagonal band of Broca, the caudal preoptic area, and the bed nucleus of the stria terminalis. Seasonal differences in proenkephalin expression were observed in the bed nucleus of the stria terminalis, lateral septum, periventricular nucleus, and paraventricular nucleus. No differences were observed between treatments in seven other regions examined. We conclude that testosterone and season regulate proenkephalin mRNA levels in the preoptic area/hypothalamus in the ram in a region-specific manner.

Negative feedback regulation of the secretion and actions of gonadotropin-releasing hormone in males

This minireview considers the state of knowledge regarding the interactions of testicular hormones to regulate the secretion and actions of GnRH in males, with special focus on research conducted in rams and male rhesus monkeys. In these two species, LH secretion is under the negative feedback regulation of testicular steroids that act predominantly within the central nervous system to suppress GnRH secretion. The extent to which these actions of testicular steroids result from the direct actions of testosterone or its primary metabolites, estradiol or dihydrotestosterone, is unclear. Because GnRH neurons do not contain steroid receptors, the testicular steroids must influence GnRH neurons via afferent neurons, which are largely undefined. The feedback regulation of FSH is controlled by inhibin acting directly at the pituitary gland. In male rhesus monkeys, the feedback regulation of FSH secretion is accounted for totally by the physiologically relevant form of inhibin, which appears to be inhibin B. In rams, the feedback regulation of FSH secretion involves the actions of inhibin and testosterone and interactions between these hormones, but the physiologically relevant form of inhibin has not been determined. The mechanisms of action for inhibin are not known.

Photoperiod and temperature interact to affect the GnRH neuronal system of male prairie voles (Microtus ochrogaster)

Individuals of numerous species limit energy expenditure during winter by inhibiting reproduction and other nonessential functions. To time these adaptations appropriately with the annual cycle, animals rely on environmental cues that predict, well in advance, the onset of winter. The most commonly studied environmental factor that animals use to time reproduction is photoperiod. Rodents housed in short photoperiods in the laboratory or in naturally declining day lengths exhibit pronounced alterations in reproductive function concomitant with alterations in the hypothalamic gonadotropin-releasing hormone neuronal system. Because animals in their natural environment use factors in addition to photoperiod to time reproduction, the present study sought to determine the independent effects of photoperiod and temperature, as well as the interaction between these factors, on reproductive parameters and the GnRH neuronal system. Male prairie voles were housed in either long (LD 16:8) or short (LD 8:16) day lengths for 10 weeks. Animals in each photoperiod were further subdivided into groups housed in either mild (i.e., 20 degrees C) or low (i.e., 8 degrees C) temperatures. As shown with immunohistochemistry, voles that underwent gonadal regression in response to short photoperiods and long-day voles housed in low temperatures (and maintained large gonads) exhibit higher GnRH-immunoreactive (GnRH-ir) neuron numbers in the preoptic area/anterior hypothalamus (POA/AH) relative to all other groups. In addition, voles that underwent gonadal regression in response to both short days and low temperatures did not exhibit an increase in GnRH-ir neuron numbers compared to long-day, mild-temperature controls. These data suggest that photoperiod and temperature interact to influence reproductive function potentially by alterations of the GnRH neuronal system.

Secretory activity of gonadotropin and the responsiveness of gonadotrophs to gonadotropin-releasing hormone during the annual reproductive cycle of male bats, Rhinolophus ferrumequinum: analysis by cell immunoblot assay

The purpose of this study was to examine secretory activity of gonadotropin (Gn) and the responsiveness of Gn secretion to Gn-releasing hormone (GnRH) in male horseshoe bats, Rhinolophus ferrumequinum, during the annual reproductive cycle. Anterior pituitary cells were monodispersed and subjected to cell immunoblot assay for Gn. Cell blots specific for follicle stimulating hormone (FSH) or luteinizing hormone (LH) were quantified using a microscopic image analyzer. The percentages of LH- or FSH-secreting cells detected as immunoreactive cell blots were markedly increased in the spermatogenic period (summer) and decreased in the hibernation period (winter). The mean Gn secretion from individual cells and total Gn secretion per unit area of the transfer membrane also showed similar changes. The responsiveness of Gn secretion to GnRH was greater in the spermatogenic period than in other seasons. On the other hand, although the secretory activity of Gn was markedly decreased during hibernation, a stimulatory effect of GnRH on Gn secretion was observed. These findings suggest that seasonal changes in the release of Gn required for gametogenesis and gonadal steroidogenesis varied depending on the reproductive activity and seasonal changes in Gn sensitivity to stimulatory effects of GnRH due to alterations in GnRH receptor numbers and/or in postreceptor events of gonadotrophs.

Effects of photoperiod and reproductive responsiveness on pituitary sensitivity to GnRH in male prairie voles (Microtus ochrogaster)

In order to promote survival and reproductive success, many nontropical rodents inhibit reproduction well in advance of winter in response to decreasing day lengths. Male prairie voles (Microtus ochrogaster), small temperate zone rodents, vary in their reproductive response to photoperiod. Some male voles undergo complete gonadal regression when housed in short days (responders) whereas others fail to inhibit reproduction when exposed to short (i. e., <12 h light/day) day lengths (nonresponders). Previous research has shown that phenotypic variation in reproductive response is reflected at the level of the hypothalamic gonadotropin-releasing hormone (GnRH) neuronal system. The present study sought to determine if photoperiod or reproductive condition alters pituitary responsiveness to a GnRH challenge. Animals were housed in either long (LD 16:8) or short (LD 8:16) photoperiods for 10 weeks. Subsequently, short-day voles were separated into responders and nonresponders based on testicular size. To reduce the influence of endogenous testosterone on luteinizing hormone concentrations, half of the animals in each group were castrated. All animals were injected (i.p.) with either 100 or 50 ng of GnRH and a blood sample was collected after 15 min. Although castration resulted in a significant increase in LH concentrations (P < 0.05), neither photoperiod nor reproductive condition affected LH concentrations in response to a GnRH challenge (P > 0.05). Taken together, these data support the hypothesis that short photoperiods lead to reproductive inhibition by acting at the level of the hypothalamus rather than the pituitary.

Effects of adrenocorticotrophin (ACTH) and progesterone on luteinising hormone (LH) secretion in recently castrated rams

The goal of the present study was to determine whether ACTH and progesterone have any effect on LH secretion and pulse frequency in recently castrated rams. Six 2-year-old Corriedale rams were castrated in the winter. The day before castration, blood samples were taken in order to establish the precastration LH levels. The rams were divided into an untreated group (group U: n = 2) and a treated group (group T: n = 4). The first treatment consisted of the i.v. administration of 0.5 mg of ACTH on day 20 post-castration, immediately after the first sample had been taken. During the second treatment, subcutaneous progesterone implants were given to group T for 5 days. Control samplings were performed one week before each treatment. Prior to castration, the testosterone levels were low, while after castration they were below the detection limit of the assay. Cortisol and progesterone concentrations were basal before castration in all of the animals and after castration in group U and also in the control samplings for group T. ACTH treatment caused a significant increase in both cortisol and progesterone levels for 3 h (P < 0.001). Progesterone implants raised progesterone levels in group T, but cortisol levels remained basal. Before castration, all animals had low LH levels and hardly any pulse activity was seen. After castration, both the number of LH pulses and the mean LH production increased significantly in all of the animals (P < 0.01). During the ACTH trial, LH pulse frequency was significantly reduced for the first 4 h following ACTH administration (P = 0.013), however, no such differences occurred in the prior control period. No effect was seen on mean LH concentration during the ACTH treatment. Progesterone treatment did not have any effect on either the number of LH pulses nor on LH concentrations (P > 0.05).

Annual variations of in vitro GNRH release by hypothalamic explants in intact and castrated male mink: relations with LH, FSH and testosterone circulating serum levels

In mink, a short-day breeder, testis growth begins in autumn (November), reaches a maximum in February, before matings occur, and decreases from March to very low volumes during spring and summer. To study the effects of season and testosterone feedback on gonadotrophin and GnRH secretion, the annual variations of LH, FSH, testosterone and GnRH were studied in intact and castrated mink. As portal blood sampling raised serious difficulties, an in vitro static incubation system was used for studying GnRH variations. In intact mink, serum LH concentrations did not vary significantly throughout the year, whereas FSH concentrations increased significantly between September and November then decreased to a minimum in January. Testosterone values rose significantly from November to a maximum from January to March, decreased very rapidly thereafter. Castration in November resulted in a significant increase in LH and FSH concentrations which remained higher than the values measured in intact males throughout the year. In long-term castrated mink, FSH concentrations did not fluctuate during the year, whereas LH concentrations showed an annual variation, with high values in April and August. For the study of in vitro GnRH liberation, medio-basal hypothalamic explants were incubated in Krebs-Ringer phosphate buffer for 3 periods of 15 min, and stimulated with copper complexed equimolarly with histidine (Cu/His, 200 microM) and prostaglandin E2 (PGE2, 10 microM). After Cu/His, the release of GnRH was 1 to 4 fold the basal release; after PGE2, the increase was 4-7 fold the basal release. The basal release of GnRH increased significantly between September and October to reach a maximum in November, decreased significantly in December to a minimum in February then increased progressively from May. The release of GnRH stimulated by Cu/His and PGE2 showed the same seasonal variation as the basal release. Castration 8 days before the sacrifice did not alter the release of GnRH, except in December: the release stimulated by PGE2 was then higher in intact than in castrated mink. Taken together, these results indicate that, with an in vitro static incubation system, it is possible to study the annual variations of GnRH release and to correlate these variations with those of serum gonadotrophin and testosterone concentrations. The synthesis and release of GnRH increased slightly from May, under long days, then more rapidly from September, resulting in an increased secretion of FSH in October, responsible for testis recrudescence. The annual pattern of basal and stimulated GnRH release was similar in intact and castrated mink, suggesting a direct effect of the season on the hypothalamus, rather than a negative feedback effect of the testis; however, testosterone seemed to feedback mainly at the pituitary level.

Effects of melatonin treatment on the gonadotropin-releasing hormone neuronal system and on gonadotropin secretion in male mink, Mustela vison, in the presence or absence of testosterone feedback

The effects of subcutaneous melatonin capsules on the gonadotropin-releasing hormone (GnRH) immunoreactive (ir) system and the secretion of follicle stimulating hormone (FSH) and luteinizing hormone (LH) have been tested in intact, castrated, and castrated adult male mink supplemented with testosterone. Animals were transferred in July, i.e., during the period of sexual rest, under a daily light:dark cycle of 16-hr light and 8-hr darkness and studied over 13 weeks. GnRH (ir) perikarya, visualized by immunocytochemistry, were counted on serial coronal sections from the diagonal band of Broca to the infundibulum. Serum FSH and LH concentrations were measured by radioimmunoassay. In intact mink, melatonin induced a significant increase in the number of (ir) perikarya and in FSH and LH concentrations 3 and 8 weeks, respectively, after melatonin capsule implantation. In castrated mink, the number of perikarya and the concentrations of FSH, which had increased within 2 weeks after castration, did not change during melatonin treatment. In contrast, the concentration of LH, which were not altered by castration, increased 3-6 weeks after the onset of melatonin administration, suggesting a stimulation of GnRH release. In castrated testosterone-treated mink, the number of perikarya was increased as in castrated males, but the elevation of FSH in response to castration was prevented. Within 2 weeks after melatonin capsule implantation, the concentrations of FSH decreased while those of LH remained low, indicating an inhibition of GnRH release. These results show that testosterone modulates the effect of melatonin on the activity of the GnRH-gonadotropin system.