哺乳动物季节性繁殖的神经内分泌调节机制

动物的季节性繁殖,是指其繁殖活动从静止到复苏的一个年周期性循环。研究显示,kisspeptin和RFRP对繁殖的季节性变化具有重要作用。非繁殖期最显著的特点是雌激素对GnRH分泌的负反馈效应的增加,而雌激素的这种效应是由kisspeptin神经元传导的。因此,kisspeptin是影响繁殖活动的一个重要因子。RFRP的表达依赖于褪黑激素的分泌并呈现出季节性变化,在非繁殖期对繁殖活动的抑制作用非常明显。此外,甲状腺激素在繁殖期的终止上发挥着至关重要的作用,而多巴胺能神经元A14/A15也促进了雌激素负反馈效应的季节性变化。这些神经元系统通过协同作用一起调节了生殖功能随光周期的季节性转变。文章对繁殖的季节性和这4个神经内分泌系统之间的关系进行了系统的阐述。     

哺乳动物季节性繁殖的神经内分泌调节机制

动物的季节性繁殖, 是指其繁殖活动从静止到复苏的一个年周期性循环。研究显示, kisspeptin和RFRP对繁殖的季节性变化具有重要作用。非繁殖期最显著的特点是雌激素对GnRH分泌的负反馈效应的增加, 而雌激素的这种效应是由kisspeptin神经元传导的。因此, kisspeptin是影响繁殖活动的一个重要因子。RFRP的表达依赖于褪黑激素的分泌并呈现出季节性变化, 在非繁殖期对繁殖活动的抑制作用非常明显。此外, 甲状腺激素在繁殖期的终止上发挥着至关重要的作用, 而多巴胺能神经元A14/A15也促进了雌激素负反馈效应的季节性变化。这些神经元系统通过协同作用一起调节了生殖功能随光周期的季节性转变。文章对繁殖的季节性和这4个神经内分泌系统之间的关系进行了系统的阐述。     

Kisspeptin Signaling Is Required for the Luteinizing Hormone Response in Anestrous Ewes following the Introduction of Males

The introduction of a novel male stimulates the hypothalamic-pituitary-gonadal axis of female sheep during seasonal anestrus, leading to the resumption of follicle maturation and ovulation. How this pheromone cue activates pulsatile secretion of gonadotropin releasing hormone (GnRH)/luteinizing hormone (LH) is unknown. We hypothesised that pheromones activate kisspeptin neurons, the product of which is critical for the stimulation of GnRH neurons and fertility. During the non-breeding season, female sheep were exposed to novel males and blood samples collected for analysis of plasma LH profiles. Females without exposure to males served as controls. In addition, one hour before male exposure, a kisspeptin antagonist (P-271) or vehicle was infused into the lateral ventricle and continued for the entire period of male exposure. Introduction of a male led to elevated mean LH levels, due to increased LH pulse amplitude and pulse frequency in females, when compared to females not exposed to a male. Infusion of P-271 abolished this effect of male exposure. Brains were collected after the male effect stimulus and we observed an increase in the percentage of kisspeptin neurons co-expressing Fos, by immunohistochemistry. In addition, the per-cell expression of Kiss1 mRNA was increased in the rostral and mid (but not the caudal) arcuate nucleus (ARC) after male exposure in both aCSF and P-271 treated ewes, but the per-cell content of neurokinin B mRNA was decreased. There was also a generalized increase in Fos positive cells in the rostral and mid ARC as well as the ventromedial hypothalamus of females exposed to males. We conclude that introduction of male sheep to seasonally anestrous female sheep activates kisspeptin neurons and other cells in the hypothalamus, leading to increased GnRH/LH secretion.     

Sex steroid control of hypothalamic Kiss1 expression in sheep and rodents: comparative aspects

In recent years, the Kiss1 gene has been cast into the reproductive spotlight. In the short period since the discovered link between kisspeptins, the encoded peptides of Kiss1, and fertility, these peptides are now known to be critical for the neuroendocrine control of reproduction. Kisspeptin producing cells in the hypothalamus are poised to become the 'missing link' in the sex steroid feedback control of GnRH secretion. These cells contain all the necessary components to relay information of the sex steroid environment to GnRH neurons, which possess the kisspeptin receptor, GPR54. Sex steroids regulate Kiss1 mRNA, and kisspeptin expression in the hypothalamus, in a manner consistent with both negative and positive feedback control of GnRH. The precise nature of sex steroid effects, in particular those of estrogen, on Kiss1 expression have been extensively studied in the female rodent and ewe. In the arcuate nucleus (ARC) of both species, kisspeptin cells appear to forward signals pertinent to negative feedback regulation of GnRH, although in the ewe it appears this population of Kiss1 cell is also responsible for positive feedback regulation of GnRH at the time of the preovulatory GnRH/LH surge. In rodents, these positive feedback signals appear to be mediated by kisspeptin cells exclusively within the anteroventral periventricular nucleus (AVPV). There are no Kiss1 cells in the ovine AVPV, but there is a population in the preoptic area. The role these preoptic area cells play in the sex steroid feedback regulation of GnRH secretion, if any, is yet to be revealed.     

Physiological role of metastin/kisspeptin in regulating gonadotropin-releasing hormone (GnRH) secretion in female rats

Various studies have attempted to unravel the physiological role of metastin/kisspeptin in the control of gonadotropin-releasing hormone (GnRH) release. A number of evidences suggested that the population of metastin/kisspeptin neurons in the anteroventral periventricular nucleus (AVPV) is involved in generating a GnRH surge to induce ovulation in rodents, and thus the target of estrogen positive feedback. Females have an obvious metastin/kisspeptin neuronal population in the AVPV, but males have only a few cell bodies in the nucleus, suggesting that the absence of the surge-generating mechanism or positive feedback action in males is due to the limited AVPV metastin/kisspeptin neuronal population. On the other hand, the arcuate nucleus (ARC) metastin/kisspeptin neuronal population is considered to be involved in the regulation of tonic GnRH release. The ARC metastin/kisspeptin neurons show no sex difference in their expression, which is suppressed by gonadal steroids in both sexes. Thus, the ARC population of metastin/kisspeptin neurons is a target of estrogen negative feedback action on tonic GnRH release. The lactating rat model provided further evidence indicating that ARC metastin/kisspeptin neurons are involved in GnRH pulse generation, because pulsatile release of luteinizing hormone (LH) is profoundly suppressed by suckling stimulus and the LH pulse suppression is well associated with the suppression of ARC metastin/kisspeptin and KiSS-1 gene expression in lactating rats.     

LH responses to single doses of exogenous GnRH in the Cape mole rat (Georychus capensis): the pituitary potential for opportunistic breeding

The Cape mole rat Georychus capensis is a solitary mole rat that inhabits the winter rainfall region of the Western Cape Province of South Africa. Circulating basal concentrations of luteinizing hormone (LH) were found to be significantly higher in the breeding season in both sexes. During both the breeding and non-breeding season, administration of exogenous gonadotropin-releasing hormone (GnRH) increased circulatory LH levels. The magnitude of the LH response to an overdose of exogenous GnRH both in and out of the breeding season in males and females was not significantly different. Typically, seasonally breeding species exhibit a down-regulation of the pituitary and reproductive functions out of the breeding season. It appears that there is no down-regulation of GnRH receptors at the level of the pituitary out of the breeding season, because the pituitary responds to an exogenous GnRH challenge equally both in and out of the breeding period. The Cape mole rat exhibits the potential for opportunistic breeding out of the breeding period, provided that environmental factors are favourable. This finding questions whether this mole rat is actually a seasonal breeder or whether reproduction is hindered by the ecological constraint of the lack of opportunities to burrow and find mates at certain times of the year.     

Expression of a functional g protein-coupled receptor 54-kisspeptin autoregulatory system in hypothalamic gonadotropin-releasing hormone neurons

The G protein-coupled receptor 54 (GPR54) and its endogenous ligand, kisspeptin, are essential for activation and regulation of the hypothalamic-pituitary-gonadal axis. Analysis of RNA extracts from individually identified hypothalamic GnRH neurons with primers for GnRH, kisspeptin-1, and GPR54 revealed expression of all three gene products. Also, constitutive and GnRH agonist-induced bioluminescence resonance energy transfer between Renilla luciferase-tagged GnRH receptor and GPR54 tagged with green fluorescent protein, expressed in human embryonic kidney 293 cells, revealed heterooligomerization of the two receptors. Whole cell patch-clamp recordings from identified GnRH neurons showed initial depolarizing effects of kisspeptin on membrane potential, followed by increased action potential firing. In perifusion studies, treatment of GT1-7 neuronal cells with kisspeptin-10 increased GnRH peak amplitude and duration. The production and secretion of kisspeptin in cultured hypothalamic neurons and GT1-7 cells were detected by a specific RIA and was significantly reduced by treatment with GnRH. The expression of kisspeptin and GPR54 mRNAs in identified hypothalamic GnRH neurons, as well as kisspeptin secretion, indicate that kisspeptins may act as paracrine and/or autocrine regulators of the GnRH neuron. Stimulation of GnRH secretion by kisspeptin and the opposing effects of GnRH on kisspeptin secretion indicate that GnRH receptor/GnRH and GPR54/kisspeptin autoregulatory systems are integrated by negative feedback to regulate GnRH and kisspeptin secretion from GnRH neurons.     

Voltage-gated potassium currents are targets of diurnal changes in estradiol feedback regulation and kisspeptin action on gonadotropin-releasing hormone neurons in mice

Estradiol has both negative and positive feedback actions upon gonadotropin-releasing hormone (GnRH) release; the latter actions trigger the preovulatory GnRH surge. Although neurobiological mechanisms of the transitions between feedback modes are becoming better understood, the roles of voltage-gated potassium currents, major contributors to neuronal excitability, are unknown. Estradiol alters two components of potassium currents in these cells: a transient current, I(A), and a sustained current, I(K). Kisspeptin is a potential mediator between estradiol and GnRH neurons and can act directly on GnRH neurons. We examined how estradiol, time of day, and kisspeptin interact to regulate these conductances in a mouse model exhibiting daily switches between estradiol negative (morning) and positive feedback (evening). Whole-cell voltage clamp recordings were made from GnRH neurons in brain slices from ovariectomized (OVX) mice and from OVX mice treated with estradiol (OVX+E). There were no diurnal changes in either I(A) or I(K) in GnRH neurons from OVX mice. In contrast, in GnRH neurons from OVX+E mice, I(A) and I(K) were greater during the morning when GnRH neuron activity is low and smaller in the evening when GnRH neuron activity is high. Estradiol increased I(A) in the morning and decreased it in the evening, relative to that in cells from OVX mice. Exogenously applied kisspeptin reduced I(A) regardless of time of day or estradiol status. Estradiol, interacting with time of day, and kisspeptin both depolarized I(A) activation. These findings extend our understanding of both the neurobiological mechanisms of estradiol negative vs. positive regulation of GnRH neurons and of kisspeptin action on these cells.     

Reproductive Hormone-Dependent and -Independent Contributions to Developmental Changes in Kisspeptin in GnRH-Deficient Hypogonadal Mice

Kisspeptin is a potent activator of GnRH-induced gonadotropin secretion and is a proposed central regulator of pubertal onset. In mice, there is a neuroanatomical separation of two discrete kisspeptin neuronal populations, which are sexually dimorphic and are believed to make distinct contributions to reproductive physiology. Within these kisspeptin neuron populations, Kiss1 expression is directly regulated by sex hormones, thereby confounding the roles of sex differences and early activational events that drive the establishment of kisspeptin neurons. In order to better understand sex steroid hormone-dependent and -independent effects on the maturation of kisspeptin neurons, hypogonadal (hpg) mice deficient in GnRH and its downstream effectors were used to determine changes in the developmental kisspeptin expression. In hpg mice, sex differences in Kiss1 mRNA levels and kisspeptin immunoreactivity, typically present at 30 days of age, were absent in the anteroventral periventricular nucleus (AVPV). Although immunoreactive kisspeptin increased from 10 to 30 days of age to levels intermediate between wild type (WT) females and males, corresponding increases in Kiss1 mRNA were not detected. In contrast, the hpg arcuate nucleus (ARC) demonstrated a 10-fold increase in Kiss1 mRNA between 10 and 30 days in both females and males, suggesting that the ARC is a significant center for sex steroid-independent pubertal kisspeptin expression. Interestingly, the normal positive feedback response of AVPV kisspeptin neurons to estrogen observed in WT mice was lost in hpg females, suggesting that exposure to reproductive hormones during development may contribute to the establishment of the ovulatory gonadotropin surge mechanism. Overall, these studies suggest that the onset of pubertal kisspeptin expression is not dependent on reproductive hormones, but that gonadal sex steroids critically shape the hypothalamic kisspeptin neuronal subpopulations to make distinct contributions to the activation and control of the reproductive hormone cascade at the time of puberty.     

Seasonal variations in daily sperm production rate of rhesus and bonnet monkeys

Daily sperm production (DSP) rate was estimated in adult male rhesus and bonnet monkeys to evaluate seasonal changes in the gametogenic activity of the testes. Three monkeys of each species were castrated during breeding and non-breeding seasons and DSP rate was estimated by enumerating the homogenization-resistant spermatid nuclei of steps 13 and 14. Results indicated a significant reduction in the DSP rate per testis during the non-breeding season in two species, along with a marked decline in the testis weight. However, the gametogenic capacity of seminiferous tubules did not appear to be markedly affected during non-breeding season, as the DSP rate per gram parenchyma of testis was only marginally reduced. The seasonal changes in DSP were much more pronounced in the rhesus than in the bonnet monkey. The feasibility of circanual rhythm in DSP of sub-human primates to form a baseline for the study of reproductive function in male is discussed.     

Kisspeptin and the seasonal control of reproduction in hamsters

Reproduction is a complex and energy demanding function. When internal and external conditions might impair reproductive success (negative energy balance, stress, harsh season) reproductive activity has to be repressed. Recent evidence suggests that these inhibitory mechanisms operate on Kiss1-expressing neurons, which were recently shown to be implicated in the regulation of GnRH release. Hamsters are seasonal rodents which are sexually active in long photoperiod and quiescent in short photoperiod. The photoperiodic information is transmitted to the reproductive system by melatonin, a pineal hormone whose secretion is adjusted to night length. The photoperiodic variation in circulating melatonin has been shown to synchronize reproductive activity with seasons, but the mechanisms involved in this effect of melatonin were so far unknown. Recently we have observed that Kiss1 mRNA level in the arcuate nucleus of the Syrian hamster is lower in short photoperiod, when animals are sexually quiescent. Notably, intracerebroventricular infusion of Kiss1 gene product, kisspeptin, in hamsters kept in short photoperiod is able to override the inhibitory photoperiod and to reactivate sexual activity. The inhibition of Kiss1 expression in short photoperiod is driven by melatonin because pinealectomy prevents decrease in Kiss1 mRNA level in short photoperiod and melatonin injection in long photoperiod down regulates Kiss1 expression. Whether melatonin acts directly on arcuate Kiss1 expressing neurons or mediates its action via interneurons is the subject of the current investigations.     

Changes in ovarian morphology and hormone concentrations associated with reproductive seasonality in wild large Japanese field mice (Apodemus speciosus)

Wild large Japanese field mice (Apodemus speciosus) responses to cyclic seasonal changes are associated with physiological and behavioral changes. However, the detailed regulation of oogenesis in the ovary during the seasonal reproductive cycle in wild large Japanese field mice has not been studied. We assessed the dynamics and changes in ovarian morphology and hormone concentrations associated with reproductive seasonality throughout the year. The stages of the ovarian morphological breeding cycle of wild large Japanese field mice were classified as breeding, transition, and non-breeding periods during the annual reproductive cycle. Measurement of blood estradiol concentrations throughout the year showed that the levels in September and October were higher than those in other months. It is presumed that follicle development starts from a blood estradiol concentration of 38.4 ± 27.1 pg/mL, which marks a shift from the transitional season to the breeding season, followed by the transition to the non-breeding season at 26.1 ± 11.6 pg/mL. These results suggest that seasonal follicle development in wild rodents is correlated with estradiol regulation. We consider this species to be an alternative animal model for studying seasonal reproductive changes and the effects of environmental changes.     

Circulating LH levels and the response to exogenous GnRH in the common mole-rat: implications for reproductive regulation in this social, seasonal breeding species

The effects of breeding season and reproductive status on male and female reproduction were investigated in the common mole-rat, Cryptomys hottentotus hottentotus, a cooperatively breeding rodent which exhibits a unique combination of seasonal breeding and a reproductive division of labor. Pituitary function was examined by measuring the luteinizing hormone (LH) responses to single doses of 2 microg exogenous gonadotrophin-releasing hormone (GnRH) and physiological saline in 69 males and 58 females from 35 wild caught colonies. Neither males nor females exhibited any apparent manifestation of season on basal LH concentrations or on pituitary sensitivity to stimulation by exogenous GnRH. The continuance of reproductive function during the nonbreeding period is essential in common mole-rat males and females, as this period coincides with the period of maximal dispersal opportunity in the winter rainfall area they inhabit. Normal circulating levels of reproductive hormones in dispersing animals may aid intersexual recognition, assist pairbond formation, and thus prime animals for independent reproduction. Circulating basal concentrations of LH as well as LH levels measured in response to a single exogenous GnRH challenge were not significantly different between the reproductive and non-reproductive groups of either sex, suggest the absence of a physiologically well-defined suppression of reproduction in subordinate common mole-rats.     

Pituitary luteinizing hormone responses to single doses of exogenous GnRH in female social Cape ground squirrels exhibiting low reproductive skew

The Cape ground squirrel Xerus inauris is unusual among social mammals as it exhibits a low reproductive skew, being a facultative plural breeder with not all females breeding within a group. We investigated pituitary function to assess whether there was reproductive inhibition at the level of the pituitary and potentially the hypothalamus in breeding and non-breeding female Cape ground squirrels. We did so during the summer and winter periods by measuring luteinizing hormone (LH) responses to single doses of 2 g exogenous gonadotropin-releasing hormone (GnRH) and physiological saline administered to 42 females from 11 colonies. Basal LH concentrations of females increased in response to the GnRH challenge. Basal plasma LH concentrations were greater during winter, when most oestrus events are observed. However, we found no differences in plasma LH concentrations between breeding and non-breeding females. We showed that the anterior pituitary of non-breeding female ground squirrels is no less sensitive to exogenously administered GnRH than that of breeding females. We therefore concluded that the pituitary is no more active in breeding than non-breeding females. The lack of differentiation in response to GnRH suggests that either non-breeding females have ovaries that are less sensitive to LH or that they refrain from sexual activity with males through an alternative mechanism of self-restraint.     

Inducing ovulation and artificial insemination in the European brown hare (Lepus europaeus Pallas, 1778)

The aim of the study was to show whether it is possible to induce ovulation in the hare by GnRH analogue administration and to carry out an effective artificial insemination (AI). The research was carried out during the breeding and non-breeding season. During the breeding season, plasma progesterone concentrations increased on the 4th day after intramuscular injection of GnRH analogue (buserelin), indicating induced ovulation and corpus luteum development. Prostaglandin F(2)alpha (dinoprost) was an effective luteolytic agent on day 9. During the non-breeding season, the GnRH analogue injection does not cause an increase of progesterone. The 17beta-estradiol concentrations during the breeding and non-breeding season were similar. It was shown that after GnRH analogue administration and artificial insemination with semen diluted in Tris buffer extender 80% females delivered live young (39-43 days after artificial insemination), which proves the effectiveness of inducing ovulation in the hare by means of hormonal stimulation.     

Effects of kisspeptin1 on electrical activity of an extrahypothalamic population of gonadotropin-releasing hormone neurons in medaka (Oryzias latipes)

Kisspeptin (product of the kiss1 gene) is the most potent known activator of the hypothalamo-pituitary-gonadal axis. Both kiss1 and the kisspeptin receptor are highly expressed in the hypothalamus of vertebrates, and low doses of kisspeptin have a robust and long-lasting stimulatory effect on the rate of action potential firing of hypophysiotropic gonadotropin releasing hormone-1 (GnRH1) neurons in mice. Fish have multiple populations of GnRH neurons distinguished by their location in the brain and the GnRH gene that they express. GnRH3 neurons located in the terminal nerve (TN) associated with the olfactory bulb are neuromodulatory and do not play a direct role in regulating pituitary-gonadal function. In medaka fish, the electrical activity of TN-GnRH3 neurons is modulated by visual cues from conspecifics, and is thought to act as a transmitter of information from the external environment to the central nervous system. TN-GnRH3 neurons also play a role in sexual motivation and arousal states, making them an important population of neurons to study for understanding coordination of complex behaviors. We investigated the role of kisspeptin in regulating electrical activity of TN-GnRH3 neurons in adult medaka. Using electrophysiology in an intact brain preparation, we show that a relatively brief treatment with 100 nM of kisspeptin had a long-lasting stimulatory effect on the electrical activity of an extrahypothalamic population of GnRH neurons. Dose-response analysis suggests a relatively narrow activational range of this neuropeptide. Further, blocking action potential firing with tetrodotoxin and blocking synaptic transmission with a low Ca(2+)/high Mg(2+) solution inhibited the stimulatory action of kisspeptin on electrical activity, indicating that kisspeptin is acting indirectly through synaptic regulation to excite TN-GnRH3 neurons. Our findings provide a new perspective on kisspeptin's broader functions within the central nervous system, through its regulation of an extrahypothalamic population of GnRH neurons involved in multiple neuromodulatory functions.     

Brain androgen receptor expression correlates with seasonal changes in the behavior of a weakly electric fish,Brachyhypopomus gauderio

Seasonal breeders are superb models for understanding natural relationships between reproductive behavior and its neural bases. We investigated the cellular bases of hormone effects in a weakly pulse-type electric fish with well-defined hormone-sensitive communication signals. Brachyhypopomus gauderio males emit social electric signals (SESs) consisting of rate modulations of the electric organ discharge during the breeding season. This discharge is commanded by a medullary pacemaker nucleus (PN), composed of pacemaker and relay neurons. We analyzed the contribution of androgen receptor (AR) expression to the seasonal generation of SESs, by examining the presence of ARs in the PN in different experimental groups: breeding, non-breeding, and testosterone (T)-implanted non-breeding males. AR presence and distribution in the CNS was assessed through western blotting and immunohistochemistry using the PG-21 antibody, which was raised against the human AR. We found AR immunoreactivity, for the first time in a pulse-type Gymnotiform, in several regions throughout the brain. In particular, this is the first report to reveal the presence of AR in both pacemaker and relay neurons within the Gymnotiform PN. The AR immunoreactivity was present in breeding males and could be induced in T-implanted non-breeding males. This seasonal and T-induced AR expression in the PN suggests that androgens may play an important role in the generation of SESs by modulating intrinsic electrophysiological properties of pacemaker and relay neurons.     

Neuroanatomical Evidence That Kisspeptin Directly Regulates Isotocin and Vasotocin Neurons

Neuropeptide kisspeptin has been suggested to be an essential central regulator of reproduction in response to changes in serum gonadal steroid concentrations. However, in spite of wide kisspeptin receptor distribution in the brain, especially in the preoptic area and hypothalamus, the research focus has mostly been confined to the kisspeptin regulation on GnRH neurons. Here, by using medaka whose kisspeptin (kiss1) neurons have been clearly demonstrated to be regulated by sex steroids, we analyzed the anatomical distribution of kisspeptin receptors Gpr54-1 and Gpr54-2. Because the both receptors were shown to be activated by kisspeptins (Kiss1 and Kiss2), we analyzed the anatomical distribution of the both receptors by in situ hybridization. They were mainly expressed in the ventral telencephalon, preoptic area, and hypothalamus, which have been suggested to be involved in homeostatic functions including reproduction. First, we found gpr54-2 mRNA expression in nucleus preopticus pars magnocellularis and demonstrated that vasotocin and isotocin (Vasopressin and Oxytocin ortholog, respectively) neurons express gpr54-2 by dual in situ hybridization. Given that kisspeptin administration increases serum oxytocin and vasopressin concentration in mammals, the present finding are likely to be vertebrate-wide phenomenon, although direct regulation has not yet been demonstrated in mammals. We then analyzed co-expression of kisspeptin receptors in three types of GnRH neurons. It was clearly demonstrated that gpr54-expressing cells were located adjacent to GnRH1 neurons, although they were not GnRH1 neurons themselves. In contrast, there was no gpr54-expressing cell in the vicinities of neuromodulatory GnRH2 or GnRH3 neurons. From these results, we suggest that medaka kisspeptin neurons directly regulate some behavioral and neuroendocrine functions via vasotocin/isotocin neurons, whereas they do not regulate hypophysiotropic GnRH1 neurons at least in a direct manner. Thus, direct kisspeptin regulation of GnRH1 neurons proposed in mammals may not be the universal feature of vertebrate kisspeptin system in general.     

Transient receptor potential (TRP) channel function in the reproductive axis

Transient receptor potential (TRP) channels play important functional roles in the signal transduction machinery of hormone-secreting cells and have recently been implicated in reproductive physiology. While expression studies have demonstrated TRP channel expression at all levels of the hypothalamic–pituitary–gonadal (hpg) axis, functional details about TRP channel action at the level of the individual cells controlling reproduction are just beginning to emerge. Canonical TRP (TRPC) channels are prominently expressed in the reproductive center of the neuroendocrine brain, i.e. in kisspeptin and gonadotropin-releasing hormone (GnRH) neurons. Kisspeptin neurons are depolarized by leptin via activation of TRPC channels and kisspeptin depolarizes GnRH neurons through TRPC4 activation. Recent studies have functionally identified TRPC channels also in gonadotrope cells in the anterior pituitary gland, which secrete gonadotropins in response to GnRH and thus regulate gonadal function. TRP channel expression in these cells exhibits remarkable plasticity and depends on the hormonal status of the animal. Subsequent functional analyses have demonstrated that TRPC5 in gonadotropes contributes to depolarization of the plasma membrane upon GnRH stimulation and increases the intracellular Ca²⁺ concentration via its own Ca²⁺ permeability and via the activation of voltage-gated Ca²⁺ channels. However, conditional gene targeting experiments will be needed to unambiguously dissect the physiological role of TRPC channels in the different cell types of the reproductive axis in vivo.     

Seasonal variation in the feedback of sex steroid hormones on serum LH concentrations in the male horse

The possibility of seasonal variation in the feedback effect of testosterone or oestradiol was investigated by giving replacement treatment to geldings for 2-3 weeks during breeding and non-breeding seasons. In the non-breeding season, testosterone suppressed LH values (mean +/- s.e.m., ng/ml) in all geldings (before treatment, 7.5 +/- 2.3; final treatment week, 1.8 +/- 0.2; P less than 0.05), whereas early in the breeding season, testosterone caused a prolonged rise in LH (before, 6.8 +/- 2.3; final week, 18.9 +/- 6.4; P less than 0.05). In all testosterone experiments, LH returned to pretreatment levels within 2 weeks after treatment. Oestradiol treatment caused a prolonged increase (P less than 0.05) in LH concentrations (mean +/- s.e.m., ng/ml) in both seasons (breeding: before 5.2 +/- 1.1; final week, 16.2 +/- 4.8; non-breeding before, 10.9, 20.1 +/- 5.2). We conclude that in geldings the feedback effect of testosterone varies with season and, further, that testosterone replacement may be able to restore to geldings the stallion's seasonal pattern of LH secretion. The results suggest that, in male horses, testosterone and possibly oestradiol, are important components in the neuroendocrine pathway controlling seasonal breeding and, moreover, are essential for the generation of a positive signal for LH secretion in the breeding season.