Changes In Prolactin Secretion Research Articles

Influence of Prolactin Secretion Changes on Sperm Head Size and Freezability in Ibex and Mouflon.

Aim: This work examined the influence of induced changes in prolactin (PRL) secretion on sperm cryoresistance of ibex and the mouflon. Materials and Methods: PRL secretion was modified in a first experiment by the use of bromocriptine (BCR, dopamine agonist) during the non-breeding season, and in a second experiment by the use of sulpiride (SLP, dopamine D2-receptor antagonist) during the rutting season. Slow and ultra-rapid freezing protocols were used to cryopreserve sperm samples. Results: BCR decreased blood plasma PRL concentrations, whereas SLP increased them. Cryoresistance ratios (CRs) for curvilinear velocity (VCL), straight-line velocity (VSL), and average path velocity (VAP) in BCR-treated mouflons were lower than in controls using slow-freezing (p < 0.05), while CRs of motility and morphologically normal sperm of BCR-treated mouflons were greater than controls with ultra-rapid freezing (p < 0.05). BCR increased the head sperm dimensions in ibexes (p < 0.001); conversely, BCR decreased the head dimensions in mouflons (p < 0.001). CR-motility, CR-amplitude of lateral head displacement (ALH), CR-viability, and CR-acrosome integrity in SLP-treated mouflons were lower than in controls with slow-freezing (p < 0.01); CR-viability and CR-acrosome were lower than controls with ultra-rapid freezing (p < 0.05). In ibexes, CR-ALH was lower for SLP-treated (p < 0.05). SLP treatment increased head dimensions in ibexes (p < 0.001) but did not affect the sperm head of mouflons. Conclusion: Our findings show that high levels of blood plasma PRL negatively affect the cryoresistance of ibex and mouflon sperm.

Total barley maiya alkaloids inhibit prolactin secretion by acting on dopamine D2 receptor and protein kinase A targets

Ethnopharmacological relevance: Barley maiya from gramineous plants (Hordeum vulgare L.) is obtained from ripe fruits through germination and drying. It is often used to treat diseases associated with high prolactin levels. Objective: To investigate the anti-hyperprolactinemia (anti-HPRL) mechanisms of total barley maiya alkaloids (TBMA) and hordenine. Methods: This experiment included 9 groups: Normal group, TBMA group, hordenine group, TBMA + haloperidol group, TBMA + forskolin group, TBMA + 8-bromo-cAMP group, hordenine + haloperidol group, hordenine + forskolin group, and hordenine + 8-bromo-cAMP group. The prolactin (PRL) concentration in the supernatant and the total cAMP concentration in the cells were detected by ELISA. The expression levels of PRL, dopamine D2 receptor (DRD2) and cAMP/PKA/CREB protein were measured by Western Blot. Results: In the TBMA group and the hordenine group, the PRL level in MMQ cells was significantly decreased, but in GH3 cells there was no change. DRD2 expression level was markedly increased, cAMP concentration was decreased, and the activity of PKA and CREB declined in MMQ cells. Compared with the TBMA group, there was a significant decrease of DRD2 expression level, a remarkable increase of PRL secretion and an increase of cAMP/PKA/CREB expression in MMQ cells within the TBMA + haloperidol group. Compared with the forskolin group, there was no significant change in PRL secretion and cAMP/PKA/CREB expression level in MMQ cells within the TBMA + forskolin group. There was a decrease in PRL secretion and cAMP/PKA/CREB expression level in MMQ cells within the TBMA + 8-bromo-cAMP group compared with the 8-bromo-cAMP group. Compared with the hordenine group, DRD2 expression level was significantly decreased, PRL secretion was markedly increased, and cAMP/PKA/CREB expression level was increased in MMQ cells within the hordenine + haloperidol group. There was no significant change in PRL secretion and cAMP/PKA/CREB expression level in MMQ cells within the hordenine + forskolin group compared with the forskolin group and within the hordenine + 8-bromo-cAMP group compared with the 8-bromo-cAMP group. Conclusion: TBMA and hordenine can both play an anti-HPRL role via DRD2, and TBMA can also act on PKA targets to exert its anti-HPRL effect. TBMA and hordenine may be potential treatment strategies for HPRL.

Menstrual Disorders and Main Indicators of Hormonal and Metabolic Homeostasis in Teenage Girls Against the Background of Endemic Goiter

Along with the widespread prevalence of goiter endemics, the medical and social significance of this problem is determined by the multifaceted spectrum of pathological abnormalities associated with asymptomatic course or hypothyroxinemia, causing numerous disorders at almost all stages of menstrual and reproductive function development.&#x0D; The objective of the research was to substantiate and conduct a comprehensive analysis of the prevalence, structure and leading factors of dishormonal disorders of menstrual function in adolescent girls with diffuse endemic goiter, and to develop a diagnostic algorithm based on the risk of the formation of this pathology.&#x0D; Materials and Methods. Two study groups were formed and a clinical laboratory examination was performed in 210 patients: 120 of them were diagnosed with menstrual dysfunction against the background of thyroid dysfunction (the main group); 60 girls were diagnosed with menstrual dysfunction and healthy thyroid gland (the comparison group); 30 apparently healthy girls made up the control group. Experimental and psychological methods, hormonal investigation of the level of thyroid, gonadotropic and steroid hormones, lipid and carbohydrate metabolism and vitamin D content in blood plasma were used.&#x0D; Results and Discussion. Hypomenstrual syndrome, secondary amenorrhea, dysmenorrhea, manifestations of premenstrual syndrome and anovulatory conditions are the main menstrual disorders in adolescents suffering from endemic goiter. One third of the girls in the main group had an increase in the proportion of delayed sexual development, while in the control group, physiological rates of sexual development occurred in 96.7% of observations. The presented hormonal studies made it possible to identify differentiated changes taking into account different clinical forms of menstrual disorders in this category of patients. The transition to the “mature” type of reproductive system functioning in girls with endemic goiter is associated with changes in prolactin secretion from low levels to the indicators that exceed the reference values, which is characterized by increased functional hyperprolactinemia. Generalization of the frequencies of the main psycho-emotional characteristics and indicators of the personal profile in the girls of the studied cohort allowed revealing the constant dominance of psycho-emotional manifestations of moderate degree, as well as high levels of psycho-emotional stress and depressive disorders against the background of dishormonal disorders and thyroid gland dysfunction. Obtained deviations of 25-hydroxyvitamin D level were noticed at the initial stages of thyroid disorder, and low 25-hydroxyvitamin D levels contributed to the manifestation of thyroid pathology.&#x0D; Conclusions. Dissociation of the thyroid system and the hypothalamic-pituitary-ovarian axis initiates menstrual disorders, development of functional cysts and multifollicular structure of the ovarian tissue, and, in case of hypothyroidism, is considered as a factor of metabolic disorders and polycystic ovary syndrome.

Serotonin and Antidepressant SSRIs Inhibit Rat Neuroendocrine Dopamine Neurons: Parallel Actions in the Lactotrophic Axis.

Selective serotonin reuptake inhibitors (SSRIs) are commonly prescribed for depression, but sexual side effects often compromise compliance. These reproductive dysfunctions are likely mediated by elevations of the hormone prolactin. Yet, how serotonin (5-HT) and SSRIs cause changes in prolactin secretion is not known. Here, using in vitro whole-cell patch-clamp recordings, we show that 5-HT hyperpolarizes and abolishes phasic discharge in rat neuroendocrine tuberoinfundibular dopamine (TIDA) neurons, the main inhibitor of prolactin secretion. This process is underpinned by 5-HT1A receptor-mediated activation of G-protein-coupled inwardly rectifying K(+)-like currents. We further demonstrate that the SSRIs, fluoxetine and sertraline, directly suppress TIDA neuron activity through parallel effects, independent of 5-HT transmission. This inhibition involves decreased intrinsic excitability and a slowing of TIDA network rhythms. These findings indicate that SSRIs may inhibit neuroendocrine dopamine release through both 5-HT-dependent and -independent actions, providing a mechanistic explanation for, and potential molecular targets for the amelioration of, the hyperprolactinemia and sexual dysfunction associated with these drugs. Depression affects approximately one-tenth of the population and is commonly treated with selective serotonin reuptake inhibitors (SSRIs; e.g., Prozac). Yet, many patients withdraw from SSRI therapy due to sexual side effects (e.g., infertility, menstrual disturbances, and impotence). Although it is generally accepted that sexual side effects are due to the ability of these drugs to elevate blood levels of the hormone prolactin, the mechanism for this hormonal imbalance is not known. Here, we show that SSRIs can inhibit hypothalamic dopamine neurons that normally suppress the secretion of prolactin. Intriguingly this inhibition can be explained both by increased serotonin activity and also by parallel serotonin-independent actions.

Regulation of male prolactin levels in an opportunistically breeding species, the African striped mouse

AbstractThe timing of reproduction of many species depends on seasonal changes in prolactin secretion. Photoperiod coincides with annual seasonal changes and typically regulates prolactin secretion. However, when environmental conditions are unpredictable, other ecological factors may contribute to prolactin regulation. In African striped mice (Rhabdomys pumilio), males show seasonal changes in reproduction and in prolactin levels, but unexpected increases of food availability out of the regular breeding season can also induce reproduction. We measured prolactin levels in free‐ranging male African striped mice during three periods: (1) the natural breeding in spring with increasing photoperiod; (2) the natural non‐breeding season in summer (dry season) with decreasing photoperiod; and (3) during two summers with unexpected rainfall inducing breeding in the population. Here, we report that breeding males showed increased prolactin levels when they were breeding independently of increases and decreases in day length. Also, we found a positive correlation (P = 0.05) between the availability of food plants and prolactin levels. Changes in prolactin levels in opportunistically breeding species like the African striped mouse are not strictly regulated by photoperiod, but seem to respond to cues from food availability.

Environmental regulation of pregnancy-specific protein B concentrations during late pregnancy in dairy cattle1

Environmental factors, such as photoperiod and heat stress, can be manipulated during the dry period to influence health, productivity, and reproductive performance of dairy cows in their subsequent lactation. The impacts of photoperiod and heat stress on subsequent lactation are related to alterations in prolactin (PRL) signaling and may affect the expression of pregnancy-specific protein B (PSPB). Additionally, exposure of cows to heat stress during the dry period decreases gestation length; however, the mechanism involved in this process is unknown. The objective of these experiments was to evaluate the influence of environmental factors (i.e., heat stress and photoperiod) during late gestation (i.e., dry period) on PSPB concentrations in plasma of dairy cows. In Exp. 1, cows were dried off in the summer months approximately 46 d before expected calving and assigned randomly to heat stress (HT; n=30) or cooling (CL; n=30) treatment. Cooling cows were housed with sprinklers, fans, and shade, whereas HT cows were provided only shade. In Exp. 2, cows were dried off at approximately 60 d before expected calving in summer/fall months and randomly assigned to 3 treatments: long day photoperiod (LDPP: 16L:8D; n=15), short day photoperiod (SDPP: 8L:16D; n=14) and SDPP+PRL implant (12 mg/d of PRL at 28 d or 16 mg/d of PRL at 39 d; n=11). In both experiments, plasma samples were collected at dry off and at -32, -18, -7, -3 and 0 d relative to calving. In Exp. 1, greater concentrations of PSPB were detected in plasma of CL versus HT cows (388.3±24.7 vs. 287.4±23.8 ng/mL; P<0.01). Concentrations of PSPB did not differ between -46 to -18 d before calving (66.0 ng/mL). However, PSPB concentrations were greater (P<0.01) for CL cows at d -7 (534.7>357.2 ng/mL), -3 (807.2>572.2 ng/mL) and 0 (800.8>563.5 ng/mL) relative to calving. Additionally, HT cows in Exp. 1 had increased PRL plasma concentrations compared with CL cows (21.01±1.6 vs. 13.78±1.6 ng/mL). In Exp. 2, no differences were detected in plasma concentrations of PSPB (ng/mL) among LDPP, SDPP, or SDPP+PRL groups on d -60 (41.5), -32 (51.7), -18 (58.5), -7 (532.9), -3 (838.2), and 0 (729.4) relative to parturition. Photoperiodic PRL concentrations were 10.81, 7.84, and 4.22 ng/mL for LDPP, SDPP+ PRL, and SDPP, respectively. Results indicate that HT alters PSPB concentrations in late pregnancy, suggesting that placental activity is altered in cows exposed to excessive elevated temperatures around the time of calving. However, the mechanism involved likely is not associated with changes in PRL secretion.

Studies on the Endocrine and Neuroendocrine Control of Broodiness in the Yuehuang Hen

The hypothesis that serotonin (5-HT) and dopamine stimulate prolactin (PRL) release, either directly from the pituitary gland or by acting through Vasoactive Intestinal Peptide (VIP), was investigated by assessing the role of hypothalamic dopamine and 5-HT in the control of broodiness and PRL release in Cantonese native chicken breed called the Yuehuang hen. A second objective was to assess the involvement of hypothalamic VIP in the control of broodiness and PRL release by using dopamine and serotonin receptor antagonists respectively. In Experiment 1, two hundred laying hens from battery cages were transferred to floor pens with nest boxes to induce broodiness. Starting from the second day after the onset of broodiness, sixty hens were allotted into groups 1, 2 and 3 respectively, corresponding to chlorpromazine-treated (n = 20), cyproheptadine-treated (n = 20) and control (n = 20) groups. Blood samples were collected from the wing veins of hens in each group on days 2, 5, 9, 14 and 19 respectively after the onset of broodiness for radioimmunoassay of PRL and Luteinizing Hormone (LH). Results indicate that in the drug treated hens which terminated broodiness, the concentration of plasma PRL decreased significantly (p 0.05) changes between the chlorpromazine and cyproheptadine treated hens. Within the chlorpromazine and cyproheptadine treated hens, there were significant (p 0.05) changes throughout the blood sampling periods. Sixteen (80%) chlorpromazine treated hens terminated broodiness on an average of 4.6±0.8 days but four hens (20%) did not. However, thirteen (65%) cyproheptadine treated hens terminated broodiness on a n average of 2.3±0.2 days while seven (35%) did not. In Experiment 2, sixty animals were used but their management and drug treatment for group 1 (n = 10) and group 2 (n = 10) were the same as described in Experiment 1. Group 3 (n = 10) served as the control. Eight hens each from groups 1, 2 and 3 were randomly selected for immunohistochemical studies. On day 7 after the onset of broodiness, hypothalamus from anaesthetized chlorpromazine and cyproheptadine treated hens as well as control hens were processed for immunohistochemical localization of VIP neurons in the hypothalamus of Yuehuang hens. Morphological observation showed a higher number of VIP neurons in the hypothalamus of the control hens. A few VIP neurons which were very faint were also found in the hypothalamus of the chlorpromazine and cyproheptadine treated hens. Results of these studies indicate a relationship between the functions of dopamine and 5-HT neurons in the hypothalamus and reproductive activities in domestic hens. They are consistent with the view that hypothalamic dopamine and 5-HT are regulators of PRL release and that using drugs which inhibit the functional activities of these neurotransmitters can inhibit PRL release to disrup t broodiness in hens to maintain egg production. The results also indicate a causal relationship between hypothalamic VIP and changes in PRL secretion associated with reproductive activities in domestic hens. This is consistent with the view that VIP might be an important hypothalamic PRL releasing neuropeptide and also indicate that VIP might be a physiological PRL regulatory hormone in domestic hens.

Changes in prolactin secretion in the short- and long-term after adrenalectomy

To evaluate the modulation of the hypothalamus-pituitary-adrenal axis (HPA) on prolactin secretion in rats after adrenalectomy (ADX). Plasma corticosterone, ACTH, and prolactin concentrations were measured by radioimmunoassay in rats after bilateral ADX in the short- (3 hours and 1 day) and long-term (3, 7, and 14 days). Animals that underwent ADX showed undetectable corticosterone levels and a triphasic ACTH response with a transient increase (3h), a decrease (1d), and further increase in the long-term after ADX. Sham animals showed a marked increase in corticosterone and ACTH levels three hours after surgery, with a decrease to basal levels thereafter. Plasma prolactin levels were not changed after ADX. There are different points of equilibrium in the HPA axis after the glucocorticoid negative feedback is removed. Prolactin plasma secretion is not altered in the short or long- term after ADX, suggesting that the peptidergic neurons essential for prolactin release are not activated after ADX.

Dose-Response of Prolactin to Increasing Doses of the Dopamine Receptor Antagonist, Sulpiride, in Horses: Effect of Season in Mares and Stallions of Estradiol Pretreatment in Geldings

Two experiments were performed to determine whether dopaminergic input to the adenohypophysis (1) differs across seasons in mares and stallions proportionally with changes in prolactin secretion and (2) is altered by estradiol administration in geldings. In experiment 1, prolactin responses to increasing doses of l-sulpiride in eight mares and eight stallions in March, June, September, and December were used to estimate the theoretical dose equivalent to 50% of maximal response. Prolactin areas increased (P < .001) with increasing doses of sulpiride and were greatest (P < .05) in March for stallions, but in June for mares. Mean half-maximal dose, which was assumed to be proportional to the dopaminergic input to the pituitary, was lowest (P < .05) in June and greatest in September. Experiment 2 used the same approach to determine whether the stimulatory effect of estradiol pretreatment on prolactin secretion was associated with an alteration of the half-maximal response. Geldings (n = 6/group) were administered 100 mg of estradiol cypionate in oil, or oil alone, on day 0 (October 3) and increasing doses of l-sulpiride starting on day 6. Estradiol treatment increased (P < .08) the prolactin response to l-sulpiride at 0.41 μg/kg body weight and all higher doses (P < .05); mean half-maximal dose did not differ (P > .1) between groups. We conclude that dopaminergic input to the adenohypophysis of mares and stallions varies with season and that the stimulatory effect of estradiol on prolactin secretion is not associated with a decrease in dopaminergic input to the adenohypophysis.

Annual Changes in Day-length, Temperature, and Circulating Reproductive Hormones in Thoroughbred Stallions and Geldings.

Changes in follicle-stimulating hormone (FSH), luteinizing hormone (LH), prolactin, immunoreactive(ir)-inhibin, testosterone, estradiol-17β, and insulin-like growth factor (IGF)-I in Thoroughbred stallions along with changes in prolactin secretion in geldings were studied. The correlations of day-length with changes in the concentrations of these hormones were also studied. Five stallions and thirteen geldings were employed to draw blood samples in monthly basis and radioimmunoassay was performed to measure these hormones. All hormones showed a seasonal pattern, the levels being highest during the breeding season and lowest during the winter months. Most of the hormones were at their highest concentration during the month of April, the mid of spring in northern hemisphere. The concentration of circulating IGF-I also demonstrated seasonality, the peak lying on the month of April. The plasma concentration of prolactin also increased during the breeding season. This phenomenon was similar both in stallions and geldings although geldings had lower concentration than that of stallions. The changes in concentration of prolactin in stallions and geldings correlated more towards the day-length than towards the temperature. These results clearly indicate the seasonality of pituitary and gonadal hormones of Thoroughbred stallions, the activity being highest during the month of April and May of the breeding season.

Effects of manganese exposure on dopamine and prolactin production in rat

Although manganese (Mn) has been shown to increase prolactin (PRL) by decreasing dopamine (DA) in the hypothalamus, the mechanism of Mn-induced regulation of the hypothalamic-hypophyseal-pituitary axis is unclear. We assessed the effects of inhaled Mn on hypothalamic DA and pituitary PRL production and evaluated the role of pituitary-specific transacting factor 1 (Pit-1), a transacting factor of PRL gene, in Mn-induced changes in PRL secretion in the rat brain. Male rats exposed to Mn for 4 or 13 weeks (1.5 mg/m3, 6 h/day, 5 days/week) showed a progressive and significant decrease in hypothalamic DA, whereas PRL and Pit-1 mRNA levels increased in response to Mn exposure. These results suggest that exposure to Mn decreases hypothalamic DA and promotes the production of PRL in the pituitary and that Pit-1 might be a regulator of DA and PRL.

Male pheromones initiate prolactin-induced neurogenesis and advance maternal behavior in female mice

Prolactin is required for rapid onset of maternal behavior after parturition, inducing adaptive changes in the maternal brain including enhanced neurogenesis in the subventricular zone during pregnancy. The resultant increase in olfactory interneurons may be required for altered processing of olfactory cues during the establishment of maternal behavior. Pheromones act through olfactory pathways to exert powerful effects on behavior in rodents and also affect prolactin secretion. Hence, this study aimed to investigate the effect of male pheromones on neurogenesis and maternal behavior in female mice. Virgin female mice were housed individually or in split-cages where they had pheromonal but not physical contact with a male. Maternal behavior was assessed in a foster pup retrieval paradigm. Some mice were injected with bromodeoxyuridine, and the labeled cells visualized using immunohistochemistry. The data show that exposure to male pheromones, for a duration equivalent to a murine pregnancy, advanced maternal behavior in both virgin and postpartum female mice. The pheromone action was dependent on prolactin and ovarian steroids, and was associated with increased cell proliferation in the subventricular zone and subsequent increases in new neurons in the olfactory bulb. Moreover, the effect of pheromones on both cell proliferation and maternal behavior could be induced solely through administration of exogenous prolactin to mimic the pheromone-induced changes in prolactin secretion. The data suggest that male pheromones induce a prolactin-mediated increase in neurogenesis in female mice, resulting in advanced maternal behavior.

Reproductive Experience and Expression of Dopamine D2 Receptor mRNA: A Possible Mechanism for Reduced Prolactin Secretion in Primiparous Rats

Reproductive experience (i.e. pregnancy and lactation) leads to reduced levels of circulating prolactin in both women and rats. Stimulation of prolactin secretion by dopamine antagonists is also blunted following reproductive experience in both species. Whereas a parity-induced reduction in haloperidol-stimulated prolactin secretion is evident in ovariectomised rats, it is unknown whether a similar attenuation of prolactin secretion is present in reproductively experienced, cycling pro-oestrous rats. The present study examined this possibility. Moreover, to determine possible mechanisms involved in parity-mediated changes in prolactin secretion, both dopamine utilisation within the arcuate nucleus/median eminence and expression of dopamine D(2) receptor mRNA (short and long forms) in the anterior pituitary were measured across the afternoon of pro-oestrous in reproductively experience and inexperienced females. Prolactin secretion was lower on the afternoon of pro-oestrous in primiparous females compared to age-matched, nulliparous controls. In addition, haloperidol-stimulated prolactin secretion was reduced in ovariectomised, reproductively experienced females. Although no differences in dopamine utilisation were observed as a function of reproductive experience, parity did affect the expression of both forms of D(2) receptor mRNA in the anterior pituitary. Compared with nulliparous controls, primiparous females had increased D(2 long) mRNA expression at 12.00 h on pro-oestrous as well as increased D(2 short) mRNA expression at 14.00 h. Because the ratio of D(2 long)/D(2 short) can significantly effect lactotroph proliferation and prolactin secretion, a shift in relative expression of the two D(2) receptor isoforms within the anterior pituitary of parous females may help account for the reduction in prolactin secretion that occurs following reproductive experience.

Melatonin induces gene‐specific effects on rhythmic mRNA expression in the pars tuberalis of the Siberian hamster (Phodopus sungorus)

In mammals, circadian and photoperiodic information is encoded in the pineal melatonin signal. The pars tuberalis (PT) of the pituitary is a melatonin target tissue, which transduces photoperiodic changes and drives seasonal changes in prolactin secretion from distal lactotroph cells. Measurement of photoperiodic time in the PT is believed to occur through melatonin dependent changes in clock gene expression, although it is unclear whether the PT should be considered a melatonin sensitive peripheral oscillator. We tested this hypothesis in the Siberian hamster (Phodopus sungorus) firstly by investigating the effects of melatonin injection, and secondly by determining whether temporal variation in gene expression within the PT persists in the absence of a rhythmic melatonin signal. Hamsters preconditioned to long days were treated with melatonin during the late light phase, to advance the timing of the nocturnal melatonin peak, or placed in constant light for one 24 h cycle, thereby suppressing endogenous melatonin secretion. Gene expression in the PT was measured by in situ hybridization. We show that melatonin rapidly induces cry1 mRNA expression without the need for a prolonged melatonin-free interval, acutely inhibits mt1 expression, advances the timing of peak rev-erb alpha expression and modulates per1 expression. With the exception of cry1, these genes continue to show temporal changes in expression over a first cycle in the absence of a melatonin signal. Our data are consistent with the hypothesis that the hamster PT contains a damped endogenous circadian oscillator, which requires a rhythmic melatonin signal for long-term synchronization.

Neuroendocrine regulation of prolactin secretion in adult female rhesus monkeys during different phases of the menstrual cycle: role of neuroexcitatory amino acid (NMA)

The present study attempts to examine the role of N-methyl-D, L-aspartate (NMDA) receptors in the central regulation of prolactin (PRL) secretion, which may be involved in ovarian function and its alteration by glutamate in various phases of the menstrual cycle of female rhesus monkeys (Macaca mulatta). The results suggest that the glutaminergic component of the control system, which governs PRL secretion by utilizing NMDA receptors, may have an important role in regulating changes in PRL secretion. The response of PRL during the luteal phase of the cycle was different from that observed in follicular and menstrual phases. Steroids may influence the NMDA-dependent drive to release PRL. N-methyl-D-aspartic acid (NMA) involvement in the regulation of PRL secretion may occur through activation of the PRL-stimulating system depending on the physiological state or steroidal milieu. It is possible, therefore, that the NMA-induced release of PRL-releasing factors (PRF) and PRL are enhanced in the presence of ovarian feedback.

Reproductive patterns of stray male dogs in the tropics

Two studies were performed to determine annual reproductive patterns in stray male dogs in the tropics. In Study 1, four dogs housed individually outdoors were monitored once monthly for 12 months, including collection and assessment of semen, measurements of scrotal width, and determination of serum testosterone and prolactin concentrations. In Study 2 (conducted concurrently), a single blood sample (for serum testosterone concentration) was collected from 220 clinically healthy dogs, and after euthanasia, scrotal width and morphology of epididymal sperm were determined. The year was divided into three seasons: warm–dry (March to June); warm–humid (July to October) and fresh–humid (November to February). In Study 1, scrotal width, ejaculate volume, sperm count and motility were significantly lower during the fresh–humid season and sperm midpiece abnormalities were significantly more common during the warm–humid and fresh–humid seasons. Serum testosterone concentrations remained constant during the year. Prolactin concentrations did not differ significantly among seasons, but had a well-defined increase from the beginning of March to the end of August. In Study 2, sperm morphology was similar to in Study 1 and serum testosterone concentrations varied nonsignificantly during the year. Environmental factors, e.g. daylength may have influenced circannual changes in prolactin secretion. Seasonal variations in some reproductive tract and seminal traits were significant but of small magnitude and the percentage of morphologically normal sperm did not vary significantly among seasons. In conclusion, healthy male dogs constantly produced sperm and were apparently fertile throughout the year.

Photoperiodic Regulation of Seasonal Breeding in Birds

Day length-dependent breeding in birds commonly occurs in spring and summer, but may occur after exposure to complex changes in day length, as for example in transequatorial migrants. More rarely, some photoperiodic birds breed when day lengths are decreasing or are short. The flexibility of avian photoperiodic breeding strategies may reflect modifications to a common reproductive photoperiodic neuroendocrine system. This involves an extraretinal photoreceptor and a biological clock, which generates a circadian rhythm of photoinducibility to measure photoperiodic time. The pineal gland is not essential for the reproductive photoperiodic response. The current model of the avian photoperiodic response has been modified to accommodate short day breeders, by incorporating a role for seasonal changes in prolactin secretion in the termination of breeding. Analysis of the sites of expression of clock genes suggests that the biological clock for reproductive photoperiodic time measurement is in the medial basal hypothalamus. Photoperiodic signal transduction may involve a clock-dependent local conversion of thyroxine to triiodothyronine (T(3)) in the medial basal hypothalamus mediated by increased expression of the gene encoding type 2 iodothyronine deiodinase. This photoinduced increase in T(3) may stimulate the release of gonadotrophin-releasing hormone (GnRH) through thyroid hormone receptors in the median eminence. These may mediate retraction of glial cell end-feet ensheathing GnRH nerve terminals abutting onto the hypophysial portal vasculature, allowing GnRH to be released to stimulate gonadotrophin secretion.

Relationship between MT1 melatonin receptor gene polymorphism and seasonal physiological responses in Île-de-France ewes

The gene encoding the MT1 melatonin receptor in sheep has a restriction fragment length polymorphism (RFLP) site to the MnlI enzyme whose incidence is associated to the expression of seasonality in several breeds. The aim of this study was to examine the relationship between this genetic marker and the physiological effects of MT1 receptor gene polymorphism on several seasonal functions in Ile-de-France ewes. The study was performed using 12 pairs of half-sib adult Ile-de-France ewes. Within each pair, ewes were selected on the basis of their genotype at the MnlI RFLP site: group +/+ and -/- (presence and absence of MnlI restriction site, respectively). No difference in the dates of the beginning, the end or the length of the breeding season was observed between groups during the two-year study. The seasonal changes in prolactin secretion were not different between groups. Similarly, wool growth rate and primary follicle activity, measured for one year, varied with the time of the year in the same way in the two groups. Our study therefore failed to show any relationship between MT1 polymorphism and reproductive seasonality in Ile-de-France ewes. This suggests that the influence of this polymorphism on the regulation of seasonal function is dependent upon the breed and/or environmental conditions. The MT1 polymorphism can explain only a small part of the genetic variability of seasonal functions and the implication of other genes must be investigated.

Genistein, a Phytoestrogen, Effectively Modulates Luteinizing Hormone and Prolactin Secretion in Ovariectomized Ewes during Seasonal Anestrus

Through binding with estrogen receptors, phytoestrogens, plant-derived estrogen-like compounds, affect numerous reproductive functions. It is not known whether these compounds are capable of evoking effective changes in luteinizing hormone (LH) and prolactin (PRL) secretion in ewes by acting directly within the central nervous system (CNS). The hypothesis studied was that genistein, infused for several hours into the third ventricle, could immediately affect LH and PRL secretion in ovariectomized (OVX) ewes during seasonal anestrus. Two doses of genistein, 1 µg/100 µl/h (total 4 µg, n = 7) and 10 µg/100 µl/h (total 40 µg, n = 7), were infused intracerebroventricularly from 12.00 to 16.00 h and blood samples were collected from 8.00 to 20.00 h at 10-min intervals. Randomly selected ewes were infused with a vehicle (control, n = 5). The mean plasma LH concentration in control ewes was significantly (p < 0.01) higher during infusion of the vehicle than before the infusion. It remained on an insignificantly changed level after the infusion. The frequency of LH pulses in control ewes did not differ significantly before, during, or after vehicle infusion. In ewes infused with a lower dose of genistein, plasma LH concentrations decreased significantly (p < 0.001) after the infusion, as compared with the values noted before and during genistein infusion. Only a tendency towards a decrease in LH pulse frequency occurred after infusion of a lower dose of genistein. In ewes infused with a higher dose of genistein, the plasma LH concentration decreased significantly (p < 0.01) after phytoestrogen administration as compared with the values noted before and during infusion. The frequency of LH pulses was also significantly (p < 0.01) lower after genistein administration. Because the changes in PRL secretion were more dynamic in response to genistein infusion, the statistical analysis included 2-hour periods. The mean plasma PRL concentration in control animals was significantly enhanced (p < 0.01) only during the first 2-hour period of sampling. After that it decreased and remained on an unchanged level up to the end of sampling. Similar changes in PRL secretion were observed in both experimental groups before genistein infusion. In contrast, significant (p < 0.01 to p < 0.001) increases in PRL concentration were noted regularly during and shortly after the genistein infusion in either low-dose or high-dose genistein-infused ewes, compared with the concentrations noted before genistein treatment. Plasma PRL concentrations during and after genistein infusion in both experimental groups were also significantly higher than the control (p < 0.01 to p < 0.001). The presented data demonstrate that genistein, a phytoestrogen, may effectively modulate LH and PRL secretion in OVX ewes by acting within the CNS.

Clock genes and the long-term regulation of prolactin secretion: evidence for a photoperiod/circannual timer in the pars tuberalis.

Prolactin secretion is regulated by photoperiod through changes in the 24-h melatonin profile and displays circannual rhythmicity under constant photoperiod. These two processes appear to occur principally within the pituitary gland, controlled by the pars tuberalis. This is evident because: (i) hypothalamic-pituitary disconnected (HPD) sheep show marked changes in prolactin secretion in response to switches in photoperiod and manipulations of melatonin, similar to brain-intact controls; (ii) HPD sheep also show photoperiod-specific, long-term cycles in prolactin secretion under constant long or short days, with the timing maintained even when prolactin secretion is blocked for 2-3 months; and (iii) pars tuberalis cells, but not lactotrophs, express high concentrations of melatonin (MT1) receptor, and exhibit a duration-sensitive, cAMP-dependant, inhibitory response to physiological concentrations of melatonin. This suggests the existence of an intrinsic, reversible photoperiod-circannual timer in pars tuberalis cells. A full complement of clock genes (Bmal1, Clock, Per1, Per2, Cry1 and Cry2) are expressed in the ovine pars tuberalis, and undergo 24-h cyclical expression as observed in a cell autonomous, circadian clock. Activation of Per genes occurs in the early day (melatonin off-set), while activation of Cry genes occurs in the early night (melatonin on-set). This temporal association is evident under both long and short days, thus the Per-Cry interval varies directly with photoperiod. Because, PER : CRY, protein : protein interactions affect stability, nuclear entry and gene transcription based on rodent data, the change in phasing of Per/Cry expression provides a potential mechanism for decoding the long day/short day melatonin signal. A speculative, but testable, extension of this hypothesis is that intrinsically regulated changes in the phase of Per/Cry rhythms, regulates both photorefractoriness and the generation of circannual rhythms in prolactin secretion.