Reduction of the amplitude of preovulatory LH and FSH surges and of the amplitude of the in vitro GnRH-induced LH release by substance P. Reversal of the effect by RP 67580

Rfamide-related peptide-3 suppresses the substance P-induced promotion of the reproductive performance in female rats modulating hypothalamic Kisspeptin expression

RFamide-related peptide-3 (RFRP-3) has been postulated as the suppressor of the reproductive axis at hypothalamic, pituitary and gonadal levels. Considering the hypothalamic level, RFRP-3 can suppress the activity of gonadotropin-releasing hormone (GnRH) neurons and their upstream neuronal stimulator, namely; the kisspeptin neurons. The effects of the RFRP-3 on the other regulators of GnRH neurons, however, are not completely investigated. Furthermore, substance P (SP) has been known as one of the coordinators of GnRH/ luteinizing hormone (LH) and the kisspeptin/G protein-coupled receptor 54 (GPR54) systems. The present study was aimed at investigating the impacts of RFRP-3 on the effects of SP on the reproductive performance in ovariectomized female rats. After intracerebroventricular (ICV) cannulation, the rats were subjected to the ICV injection of either SP or RFRP-3 and simultaneous injection of them and their selective antagonists. Blood and hypothalamic samplings and also sexual behavioral test were carried out on two main groups of rats. The analyses of the results of LH radioimmunoassay, gene expression assay for hypothalamic Gnrh1, Kisspeptin and Gpr54 accompanied by sexual behavioral examination revealed that the SP administration promotes reproductive behavior and GnRH/LH system and upregulates Kisspeptin expression. The RFRP-3 administration suppressed reproductive behavior, GnRH / LH system and Kisspeptin expression; however, the simultaneous injection of SP and RFRP-3 was devoid of significant alterations in the assessed parameters. The results showed that RFRP-3 can modulates the impacts of SP on the reproductive performance in ovariectomized female rats in part through adjusting Kisspeptin expression.

Sexual Dimorphic Distribution of Hypothalamic Tachykinin1 Cells and Their Innervations to GnRH Neurons in the Zebrafish

Substance P (SP) and neurokinin A (NKA), encoded by TAC1/Tac1 gene are members of the tachykinin family, which exert their neuromodulatory roles in vertebrate reproduction. In mammals, SP and NKA have been shown to regulate gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) secretion via kisspeptin neurons. On the other hand, the role of SP/NKA in the regulation of reproduction in non-mammalian vertebrates is not well known. In the present study, we first localized expression of tac1 mRNA in the brain of male and female zebrafish, Danio rerio. Next, using an antibody against zebrafish tachykinin1 (Tac1), we examined the neural association of SP/NKA neural processes with GnRH3 neurons, and with kisspeptin (kiss2) neurons, in the brains of male and female zebrafish. In situ hybridization showed an apparent male-dominant tac1 expression in the ventral telencephalic area, the anterior and posterior parts of the parvocellular preoptic nucleus, and the suprachiasmatic nucleus. On the other hand, there was female-dominant tac1 expression in the ventral periventricular hypothalamus. Confocal images of double-labeled zebrafish Tac1 and GnRH3 showed associations between Tac1-immunoreactive processes and GnRH3 neurons in the ventral telencephalic area. In contrast, there was no apparent proximity of Tac1 processes to kiss2 mRNA-expressing neurons in the hypothalamus. Lastly, to elucidate possible direct action of SP/NKA on GnRH3 or Kiss2 neurons, expression of SP/NKA receptor, tacr1a mRNA was examined in regions containing GnRH3 or Kiss2 neurons by in situ hybridization. Expression of tacr1a mRNA was seen in several brain regions including the olfactory bulb, preoptic area and hypothalamus, where GnRH3 and Kiss2 cells are present. These results suggest that unlike in mammals, Tac1 may be involved in male reproductive functions via direct action on GnRH3 neurons but independent of kisspeptin in the zebrafish.

Expanding the Role of Tachykinins in the Neuroendocrine Control of Reproduction

Reproductive function is driven by the hormonal interplay between the gonads and brain-pituitary axis. Gonadotropin-releasing hormone (GnRH) is released in a pulsatile manner, which is critical for the attainment and maintenance of fertility, however, GnRH neurons lack the ability to directly respond to most regulatory factors, and a hierarchical upstream neuronal network governs its secretion. We and others proposed a model in which Kiss1 neurons in the arcuate nucleus (ARC), so called KNDy neurons, release kisspeptin (a potent GnRH secretagogue) in a pulsatile manner to drive GnRH pulses under the coordinated autosynaptic action of its cotransmitters, the tachykinin neurokinin B (NKB, stimulatory) and dynorphin (inhibitory). Numerous genetic and pharmacological studies support this model; however, additional regulatory mechanisms (upstream of KNDy neurons) and alternative pathways of GnRH secretion (kisspeptin-independent) exist, but remain ill defined. In this aspect, attention to other members of the tachykinin family, namely substance P (SP) and neurokinin A (NKA), has recently been rekindled. Even though there are still major gaps in our knowledge about the functional significance of these systems, substantial evidence, as discussed below, is placing tachykinin signaling as an important pathway for the awakening of the reproductive axis and the onset of puberty to physiological GnRH secretion and maintenance of fertility in adulthood.

Administration of an antagonist of neurokinin receptors 1, 2, and 3 results in reproductive tract changes in beagle dogs, but not rats

SCH 206272, an antagonist of neurokinin receptors 1, 2, and 3, was administered orally by gavage for 1 month to 8- to 10-month-old dogs at doses of 0, 15, 30, or 60 mg/kg, and to 6-week-old rats at doses of 0, 30, 100, or 300 mg/kg. The most important changes occurred in the reproductive tract of the dogs at all doses. Absolute and relative group mean organ weights for the testes, prostate gland, epididymides, ovaries, and uterus were 33-86% lower than concurrent controls in groups receiving SCH 206272. Organ weight changes were not dose-related. Microscopic changes that correlated with the organ weight changes occurred in all groups receiving SCH 206272. For males, they included minimal to severe atrophy of the testes, epididymides, and prostate gland. In addition, the epididymides exhibited severe oligospermia or aspermia, minimal epithelial apoptosis and mild epithelial vacuolation. In female dogs, the ovaries and uteri appeared immature. Microscopic changes were similar in incidence and severity in dogs receiving 30 or 60 mg/kg, but were slightly less in dogs receiving 15 mg/kg. In contrast, similar findings were not observed in the reproductive tract of male or female rats, despite overlapping systemic, hypothalamic, and pituitary gland concentrations of SCH 206272.

Three-dimensional representation of the neurotransmitter systems of the human hypothalamus: inputs of the gonadotrophin hormone-releasing hormone neuronal system

The gonadotrophin-releasing hormone (GnRH) represents the final common pathway of a neuronal network that integrates multiple external and internal factors to control fertility. Among the many inputs GnRH neurones receive, oestrogens play the most important role. In females, oestrogen, in addition to the negative feedback, also exhibits a positive feedback influence upon the activity and output of GnRH neurones to generate the preovulatory luteinising hormone surge and ovulation. Until recently, the belief has been that the GnRH neurones do not contain oestrogen receptors and that the action of oestrogen upon GnRH neurones is indirect, involving several, oestrogen-sensitive neurotransmitter and neuromodulator systems that trans-synaptically regulate the activity of the GnRH neurones. Although this concept still holds for humans, recent studies indicate that oestrogen receptor-beta is expressed in GnRH neurones of the rat. This review provides three dimensional stereoscopic images of GnRH-immunoreactive (IR) and some peptidergic (neuropeptide Y-, substance P-, beta-endorphin-, leu-enkaphalin-, corticotrophin hormone-releasing- and galanin-IR) and catecholaminergic neurones and the communication of these potential oestrogen-sensitive neuronal systems with GnRH neurones in the human hypothalamus. Because the post-mortem human tissue does not allow the electron microscopic identification of synapses on GnRH neurones, the data presented here are based on light microscopic immunocytochemical experiments using high magnification with oil immersion, semithin sections or confocal microscopy.

Peptides interact in gonadotrophin regulation

The regulation of luteinizing hormone (LH) activity is vital to normal reproductive functioning of the female. Although gonadotrophin-releasing hormone (GnRH) has a prominent role in the regulation of LH it is now believed that other peptides are also involved. Among these peptides is oxytocin. The addition of oxytocin to cultures of pituitary cells from female rats elicited a concentration-dependent secretion of LH. This secretion was enhanced in an oestrogenised environment and was inhibited by progesterone and testosterone. Oxytocin administered to female rats at pro-oestrus advanced the endogenous LH surge that occurs on the evening of pro-oestrus. Conversely oxytocin receptor antagonist suppressed the production of the LH surge in a dose-dependent manner, indicating that endogenous oxytocin is a crucial component of LH regulation. In the human female, oxytocin administered during the late follicular phase advanced the onset of the midcycle LH surge. Oxytocin added to rat pituitary cells in vitro induced LH synthesis. Furthermore rats administered oxytocin on pro-oestrus had higher LH pituitary content following development of the LH surge than did rats administered saline. Thus oxytocin promoted synthesis and replacement in the pituitary of LH released into the circulation. Incubation of pituitary pieces with oxytocin plus GnRH induced secretion of amounts of LH greater than the sum of the amounts released by oxytocin and GnRH separately. Additionally the increased LH levels observed in the peripheral circulation of pentobarbitone-anaesthetised rats administered GnRH were enhanced if the rats received oxytocin prior to the GnRH. Thus oxytocin synergised with GnRH in stimulating LH release. Addition of diBucAMP reduced the oxytocin-mediated augmentation and dideoxyadenosine enhanced the augmentation, suggesting that oxytocin worked most efficiently in a milieu low in cAMP activity. The use of a cell immunoblot assay revealed that individual cells responded differently to oxytocin and to GnRH and that the two peptides could act on the same cell. Perifusion studies performed on hemipituitaries demonstrated that a LH response could be determined by the presence of three peptides, oxytocin, neuropeptide Y and GnRH. Hence oxytocin is potentially involved also in multiple interactions during the process of LH regulation. LH regulation is therefore apparently the result of a community of peptides acting in a co-operative network.

Substance P downregulates basal and gonadotropin-releasing hormone-induced gonadotropin in vitro secretion by pituitary gland of crested newt, Triturus carnifex

The possible role of Substance P (SP) was studied in the modulation of basal and gonadotopin-releasing hormone (GnRH)-induced gonadotropin secretion in the urodele crested newt, Triturus carnifex. During prereproduction, reproduction (noncourtship and courtship), refractory, recovery and aestivation, male and female pituitaries were incubated with medium-alone, GnRH, SP, GnRH receptor antagonist (antide), and SP receptor antagonist (L-703606). Since antisera raised against gonadotropins are not available for this species, we measured these hormones indirectly through their effects on the secretion of testicular androgens and ovarian progesterone from gonads superfused with the preincubated pituitaries. Pituitaries of both sexes preincubated with medium-alone, GnRH, GnRH plus L-703606, and GnRH plus SP plus L-703606 increased steroid secretion during prereproduction, noncourtship, courtship, and recovery; the increase induced by the pituitaries incubated with medium-alone was lower during prereproduction, noncourtship, and recovery. Pituitaries preincubated with SP, GnRH plus SP, GnRH plus SP plus antide, and SP plus antide did not change basal steroid secretion in any of the reproductive phases considered. Antide, L-703606, GnRH plus antide, GnRH plus SP plus antide plus L-703606, SP plus L-703606, and antide plus L-703606 experimental groups showed the same results as those with medium-alone. These results suggest that SP downregulates gonadotropin release in both Triturus carnifex sexes. In addition, an antagonist role, through receptor-independent mechanisms, exists between GnRH (upregulation) and SP (downregulation) in the modulation of pituitary.

Modulation of gonadotropin levels by peptides acting at the anterior pituitary gland

There are several lines of evidence that point to peptides participating in the regulation of LH and/or FSH levels by action at the pituitary. This evidence includes altered secretion of gonadotropins from the anterior pituitary cells or tissue in vitro when exposed to the peptide. Additionally, modification of GnRH-stimulated LH/FSH secretion has been observed. Furthermore, there is potential for a separately modulated interaction with the primed response. Another potential of action is by interaction among non-GnRH peptides on gonadotropin-regulating processes, although there are no good data available on this aspect. Other observations, consistent with a pituitary role for the peptides in modulation of LH, include detection of the peptides in portal blood, detection of high-affinity receptors or receptor mRNA in the pituitary, and detection of intrapituitary peptide or peptide mRNA in the pituitary. The modulation by steroids of both concentrations and type of activities provides a further level of physiological refinement. There is, however, some confusion regarding the involvement of these peptides in gonadotropin control. The reasons can be seen by considering aspects of investigations. There are experimental variations such as 1) species studied, e.g., NPY has been reported to have an effect on LH secretion from rat cells (168) but not on sheep anterior pituitary tissue (64), and substance P inhibits GnRH-stimulated release from rat cells (182) but potentiates the response in prepubertal porcine cells (92); 2) the steroidal conditions under which the study is performed, e.g., NPY has opposite effects in certain endocrine environments, augmenting GnRH-stimulated LH release in proestrus-like conditions (168), and inhibiting in metestrus-like environment (66); 3) the type of cell preparation, e.g., responsiveness to substance P might depend on whether cells in overnight culture were in separated or clustered state (91); 4) the time course considered, e.g., oxytocin that might induce marked LH release from pituitary cells after a longer length of incubation than GnRH requires (68); 5) length of exposure to peptide, e.g., endothelin that augmented or inhibited GnRH-stimulated LH release (50); 6) In addition, it is possible that the traditional endpoint selected in such studies, namely, observation of gonadotropin secretion, is not necessarily the most important for these peptides (56, 81, 117). Unfortunately, at this stage a definitive answer to the question "What do the peptides actually do?" cannot be provided and we remain tantalized by the glimpses of potential roles. Perhaps in a few years an updated review will be able to include a more complete answer. It is necessary for the full understanding of LH control that not only the properties of the peptides in isolation be characterized but also their interactions.