ERα and GnRH co-localize in the hypothalamic neurons of the South American plains vizcacha, Lagostomus maximus (Rodentia, Caviomorpha)

Effect of estrogens on apoptosis in the pituitary of viscachas (Lagostomus maximus maximus)

Estrogens, acting through their receptors (ERα and ERβ), regulate cell turnover in the pituitary gland, influencing cell proliferation and apoptosis across various species. However, their role in pituitary processes in seasonally reproducing animals remains poorly understood. This study aims to investigate the influence of estrogens, through the expression of their specific receptors, on the apoptosis of PD cells in relation to sexual maturity, the reproductive cycle, and pregnancy in a seasonal reproductive rodent (Lagostomus maximus maximus). ERα and caspase-3-cleaved (CASP3c) immunoreactive (-ir) cells were identified through immunohistochemistry. Apoptotic cells were detected using the TUNEL technique, with quantitative analysis facilitated by image analysis software, alongside measurement of serum estradiol levels using radioimmunoassay The immunostaining pattern for ERα included nuclear (ERαn) and cytoplasmic (ERαc) staining. In male viscachas, ERα expression significantly increases from immature to adult animals, correlating with the rise in serum estradiol levels and a decrease in the percentage of apoptotic cells. During the gonadal regression period in adult males, a decrease in the number of ER-ir cells and serum levels of estradiol corresponds with an increase in the number of apoptotic cells. In females, serum levels of estradiol peaked during mid-pregnancy, coinciding with a significant decrease in the number of apoptotic cells in the PD. Simultaneously, the percentage of ERαn-ir cells reaches its maximum value during late pregnancy, indicating the need to maintain the protective action of this gonadal hormone throughout the extensive pregnancy in these rodents. Regional ERα receptor expression and apoptotic cells appear to be associated with distinct PD cell populations and their hormonal responses. Finally, elevated estradiol levels coincide with diminished apoptotic cells in the male reproductive cycle and during pregnancy, suggesting an antiapoptotic role of estradiol in this species.

Achieving full-term pregnancy in the vizcacha relies on a reboot of luteal steroidogenesis in mid-gestation (Lagostomus maximus, Rodentia)

Reactivation of the hypothalamic-pituitary-ovarian (HPO) axis triggered by the decline in serum progesterone in mid-gestation is an uncommon trait that distinguishes the vizcacha from most mammals. Accessory corpora lutea (aCL) developed upon this event have been proposed as guarantors of the restoration of the progesterone levels necessary to mantain gestation. Therefore, the steroidogenic input of primary CL (pCL) vs aCL was evaluated before and after HPO axis-reactivation (BP and AP respectively) and in term pregnancy (TP). Nonpregnant-ovulated females (NP) were considered as the pCL-starting point group. In BP, the ovaries mainly showed pCL, whose LH receptor (LHR), StAR, 3β-HSD, 20α-HSD, and VEGF immunoexpressions were similar or lower than those of NP. In AP, luteal reactivity increased significantly compared to the previous stages, and the pool of aCL developed in this stage represented 20% of the ovarian structures, equaling the percentage of pCL. Both pCL and aCL luteal cells shared similar histological features consistent with secretory activity. Although pCL and aCL showed equivalent labeling intensity for the luteotropic markers, pCL were significantly larger than aCL. Towards TP, both showed structural disorganization and loss of secretory characteristics. No significant DNA fragmentation was detected in luteal cells throughout gestation. Our findings indicate that the LH surge derived from HPO axis-reactivation targets the pCL and boost luteal steroidogenesis and thus progesterone production. Because there are many LHR-expressing antral follicles in BP, they also respond to the LH stimuli and luteinize without extruding the oocyte. These aCL certainly contribute but it is the steroidogenic restart of the pCL that is the main force that restores progesterone levels, ensuring that gestation is carried to term. Most importantly, the results of this work propose luteal steroidogenesis reboot as a key event in the modulation of vizcacha pregnancy and depict yet another distinctive aspect of its reproductive endocrinology.

Mutual Shaping of Circadian Body-Wide Synchronization by the Suprachiasmatic Nucleus and Circulating Steroids

Background

Steroids are lipid hormones that reach bodily tissues through the systemic circulation, and play a major role in reproduction, metabolism, and homeostasis. All of these functions and steroids themselves are under the regulation of the circadian timing system (CTS) and its cellular/molecular underpinnings. In health, cells throughout the body coordinate their daily activities to optimize responses to signals from the CTS and steroids. Misalignment of responses to these signals produces dysfunction and underlies many pathologies.

Questions Addressed

To explore relationships between the CTS and circulating steroids, we examine the brain clock located in the suprachiasmatic nucleus (SCN), the daily fluctuations in plasma steroids, the mechanisms producing regularly recurring fluctuations, and the actions of steroids on their receptors within the SCN. The goal is to understand the relationship between temporal control of steroid secretion and how rhythmic changes in steroids impact the SCN, which in turn modulate behavior and physiology.

Evidence Surveyed

The CTS is a multi-level organization producing recurrent feedback loops that operate on several time scales. We review the evidence showing that the CTS modulates the timing of secretions from the level of the hypothalamus to the steroidogenic gonadal and adrenal glands, and at specific sites within steroidogenic pathways. The SCN determines the timing of steroid hormones that then act on their cognate receptors within the brain clock. In addition, some compartments of the body-wide CTS are impacted by signals derived from food, stress, exercise etc. These in turn act on steroidogenesis to either align or misalign CTS oscillators. Finally this review provides a comprehensive exploration of the broad contribution of steroid receptors in the SCN and how these receptors in turn impact peripheral responses.

Conclusion

The hypothesis emerging from the recognition of steroid receptors in the SCN is that mutual shaping of responses occurs between the brain clock and fluctuating plasma steroid levels.

Distribution of kisspeptin system and its relation with gonadotropin-releasing hormone in the hypothalamus of the South American plains vizcacha, Lagostomus maximus

Kisspeptin (KISS), a key hormone involved in the regulation of the hypothalamic-pituitary-ovarian (HPO) axis, has been localized in the anteroventral periventricular (AVPV) nucleus and the neighboring rostral periventricular nucleus (PeVN), and in the arcuate (ARC) nucleus of the mammalian hypothalamus. In the ARC, the KISS neurons that co-express neurokinin B (NKB) and dynorphin A (Dyn) are named KNDy cells. The South American plains vizcacha is a rodent with peculiar reproductive traits. Around mid-pregnancy, vizcacha shows the reactivation of its HPO axis with the pulsatile release of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH), an essential event for the success of gestation. Considering the role of KISS system in GnRH modulation, the aim of this work was to study their neuroanatomical distribution in adult vizcachas. AVPV showed sexual dimorphism with a significant smaller area in males (t-Test, p < 0.05), and KISS immunoreactivity was detected in somas and varicosities homogenously distributed in the AVPV with a concordant sex-related expression pattern. NKB and Dyn expression was also observed in cytoplasm of neurons scattered in the AVPV. Three subpopulations of neurons were detected in the AVPV: neurons expressing Dyn and NKB (DyNK cells), neurons expressing KISS and NKB (KiNK cells), and single NKB expressing neurons. Strikingly, KISS and Dyn were always expressed in different cells. In addition, in the ARC nucleus, KNDy cells were detected. On the other hand, KISS and GnRH expression was detected in different subpopulations of neurons, GnRH cells showed KISS receptor (KISSR or GPR-54) expression, and KISS immunoreactive afferent contacts were detected making close appositions onto somas and dendrites of GnRH cells. These results show similarities and differences between the KISS system in the hypothalamus of the vizcacha and other mammals, and constitute crucial observations about KISS and GnRH relation. Considering the peculiarity of HPO axis regulation in this species, the present work provides a neuroanatomical framework for the further elucidation of molecular mechanisms underlying GnRH expression and secretion.

Identification and functional characterization of gonadotropin -releasing hormone in pompano (Trachinotus ovatus)

Gonadotropin-releasing hormone (GnRH) is an important neuropeptide in the reproductive system. Although GnRH analogues have been used to artificially spawn pompano (Trachinotus sp.), the native forms of GnRH have not been described in this species. In this study three GnRH subtypes [sea bream GnRH (sbGnRH), chicken GnRH-Ⅱ (cGnRH-Ⅱ) and salmon GnRH (sGnRH)] were identified in pompano (Trachinotus ovatus). cgnrh-Ⅱ and sgnrh were mainly expressed in the brain of male and female fish, showing a tissue-specific expression pattern, while sbgnrh was expressed at different transcriptional levels in all tested tissues. In vivo injection experiment showed that sbGnRH significantly increased fsh and lh genes expression in a dose-dependent manner, but a high concentration of sbGnRH could desensitize the expression of lh. High concentrations of cGnRH-Ⅱ and sGnRH could induce the expression of fsh and lh. In addition, the results of in vitro incubation experiments showed that the high concentration of sbGnRH peptide could induce the expression of fsh and lh, while cGnRH-Ⅱ and sGnRH peptides could only induce the expression of fsh. 17β-estradiol (E₂) and 17α-methyltestosterone (MT) significantly inhibited sbgnrh mRNA expression in a dose-dependent manner, but did not affect the expression of cgnrh-Ⅱ and sgnrh mRNA. sbGnRH is the main GnRH subtype in pompano. E₂ and MT can play a negative role in the regulation of sbgnrh. This study provides a theoretical basis for the reproductive endocrinology of pompano.

Melatonin is involved in the modulation of the hypothalamic and pituitary activity in the South American plains vizcacha, Lagostomus maximus

Melatonin, the key messenger of photoperiodic information, is synthesized in the pineal gland by arylalkylamine N-acetyltransferase enzyme (AANAT). It binds to specific receptors MT₁ and MT₂ located in the hypothalamus and pituitary gland. Melatonin can modulate the reproductive axis affecting the secretion of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH). The South American plains vizcacha, Lagostomus maximus, shows natural poliovulation of up to 800 oocytes per estrous cycle, a 154-day long pregnancy, and reactivation of the reproductive axis at mid-gestation with pre-ovulatory follicular recruitment, presence of active corpora lutea, and variations of the endocrine status. Here we analyzed the involvement of melatonin in the modulation of the hypothalamic and pituitary gland physiology of vizcacha thorough several approaches, including histological localization of melatoninergic system components, assessment of melatoninergic components expression throughout the reproductive cycle, and evaluation of the effect of melatonin on hypothalamic and pituitary activities during the follicular and luteal phases of the estrous cycle. AANAT and melatonin receptors were localized in the pineal gland and preoptic area of the hypothalamus. Increase in pineal AANAT and serum melatonin expression was observed as pregnancy progressed, with the lowest hypothalamic MT₁ and MT₂ levels at mid-pregnancy. Pulsatility assays demonstrated that melatonin induces GnRH and LH secretion at luteal phase. The melatoninergic system effects on hypothalamic and pituitary gland hormones secretion during pregnancy pinpoint to melatonin as a potential key factor underlying the reactivation of the reproductive axis activity at mid-gestation.

Neocortical Anatomy in the South American Plains Vizcacha, Lagostomus maximus, Reveals Different Strategies in Encephalic Development among Hystricomorpha and Myomorpha Rodents

Depending on the presence or absence of sulci and convolutions, the brains of mammals are classified as gyrencephalic or lissencephalic. We analyzed the encephalic anatomy of the hystricomorph rodent Lagostomus maximus in comparison with other evolutionarily related species. The encephalization quotient (EQ), gyrencephaly index (GI), and minimum cortical thickness (MCT) were calculated for the plains vizcacha as well as for other myomorph and hystricomorph rodents. The vizcacha showed a gyrencephalic brain with a sagittal longitudinal fissure that divides both hemispheres, and 3 pairs of sulci with bilateral symmetry; that is, lateral-rostral, intraparietal, and transverse sulci. The EQ had one of the lowest values among Hystricomorpha, while GI was one of the highest. Besides, the MCT was close to the mean value for the suborder. The comparison of EQ, GI, and MCT values between hystricomorph and myomorph species allowed the detection of significant variations. Both EQ and GI showed a significant increase in Hystricomorpha compared to Myomorpha, whereas a Pearson's analysis between EQ and GI depicted an inverse correlation pattern for Hystricomorpha. Furthermore, the ratio between MCT and GI also showed a negative correlation for Hystricomorpha and Myomorpha. Our phylogenetic analyses showed that Hystricomorpha and Myomorpha do not differ in their allometric patterning between the brain and body mass, GI and brain mass, and MCT and GI. In conclusion, gyrencephalic neuroanatomy in the vizcacha could have developed from the balance between the brain size, the presence of invaginations, and the cortical thickness, which resulted in a mixed encephalization strategy for the species. Gyrencephaly in the vizcacha, as well as in other Hystricomorpha, advocates in favor of the proposal that in the more recently evolved Myomorpha lissencephaly would have arisen from a phenotype reversal process.

Identification, functional characterization, and estrogen regulation on gonadotropin-releasing hormone in the spotted scat, Scatophagus argus

Gonadotropin-releasing hormone (GnRH) is a key neuropeptide of the reproductive system. However, little is known about the role of GnRH in the spotted scat (Scatophagus argus). Here, three GnRH subtypes (cGnRH-II, sGnRH, and sbGnRH) were identified in the spotted scat. cGnRH-II and sGnRH were only expressed in the brains and gonads of both male and female fish, exhibiting a tissue-specific expression pattern, while sbGnRH was expressed at different transcription levels in all examined tissues. During ovarian maturation, hypothalamus-associated sbGnRH was upregulated, while the expression of sGnRH was variable and cGnRH-II first increased and then decreased. In vivo experiments showed that sbGnRH significantly promoted the expression of fsh and lh genes in a dose-dependent manner and exhibited a desensitization effect on lh expression at high concentrations. For sGnRH and cGnRH-II, only high concentrations could induce fsh and lh expression. Furthermore, treatment with highly concentrated sbGnRH peptide also induced fsh and lh expression, whereas the sGnRH and cGnRH-II peptides only induced fsh expression in vitro. 17β-Estradiol (E₂) significantly inhibited the expression of sbGnRH mRNA in a dose-dependent manner and did not impact sGnRH and cGnRH-II mRNA levels in vivo or in vitro. The inhibitory effect of E₂ on sbGnRH expression was attenuated by the estrogen receptor (ER) broad-spectrum antagonist (fulvestrant) and the ERα-specific antagonist (methyl-piperidinopyrazole), respectively, implying that the feedback regulation on sbGnRH is mediated via ERα. This study provides a theoretical basis for the reproductive endocrinology of the spotted scat by studying GnRH.

Mammary gland-specific regulation of GNRH and GNRH-receptor gene expression is likely part of a local autoregulatory system in female vizcachas (Rodentia: Chinchillidae)

In addition to key mammotrophic hormones such as the pituitary prolactin (PRL) and the ovarian steroids progesterone and estradiol, there are local factors that modulate the tissue dynamics of the mammary glands during pregnancy and lactation. By immunohistochemistry and RT-PCR, we found local transcription and translation of gonadotropin-releasing hormone (GNRH), GNRH receptor (GNRHR), PRL and PRL receptor (PRLR) in mammary glands of adult vizcachas during pregnancy and lactation. Both GNRH and GNRHR showed a lag between protein expression and gene transcription throughout the gestational period: while the highest transcription levels of these genes were recorded at early-pregnancy, the epithelial immunoexpressions of both showed their maximum during lactation. RIA results corroborated the presence of GNRH in mammary glands at all the analyzed stages and confirmed the maximum amount of this peptide in the lactating group. Significant amounts of GNRH were detected in milk samples as well. Conversely, PRL and PRLR shared similar protein and gene expression profiles, all exhibiting maximum values during lactation. GNRH peptide content in mammary glands of females with sulpiride-induced hyperprolactinemia (HP) was significantly lower than that of control females (CT). Although PRL mRNA levels remained unchanged, there was a marked increase in theα-lactalbumin (LALBA) transcription in mammary glands of HP- vs CT-females. These results suggest that after targeting mammary glands, PRL stimulates the expression of milk protein genes, but also, tempers the local expression of GNRH. Mammary gland-explantssupplemented with a GNRH analogue (GN-explants) had no differences in terms of PRLR orLALBA transcription levels compared to CT-explants, so the mammary PRLR signaling would not appear to be modulated by GNRH. Yet, mRNA expression levels of both GNRH and the GNRHR-downstream factor, EGR1, were significantly higher in GN-explants compared to that of CT which would point to a GNRH-positive feedback mechanism. In summary, the local coupled expression of GNRH, GNRHR and EGR1 in the mammary gland throughout pregnancy of vizcachas, the PRL-dependent mammary GNRH secretion as well as the GNRH positive feedback on its own transcription suggest an autocrine-paracrine regulatory mechanism and propose an active role for GNRH in mammary gland tissue remodeling.

The key action of estradiol and progesterone enables GnRH delivery during gestation in the South American plains vizcacha, Lagostomus maximus

The South American plains vizcacha, Lagostomus maximus, is the only mammal described so far that shows expression of estrogen receptors (ERs) and progesterone receptors (PRs) in gonadotropin-releasing hormone (GnRH) neurons. This animal therefore constitutes an exceptional model for the study of the effect of steroid hormones on the modulation of the hypothalamic-pituitary-ovarian (HPO) axis. By using both in vivo and ex vivo approaches, we have found that pharmacological doses of progesterone (P4) and estradiol (E2) produced an inhibition in the expression of hypothalamic GnRH, while physiological doses produced a differential effect on the pulsatile release frequency or genomic expression of GnRH. Our ex vivo experiment indicates that a short-term effect of E2 modulates the frequency of GnRH release pattern that would be associated with membrane ERs. On the other hand, our in vivo approach suggests that a long-term effect of E2, acting through the classical nuclear ERs-PRs pathway, would produce the modification of GnRH mRNA expression during the GnRH pre-ovulatory surge. Particularly, P4 induced a rise in GnRH mRNA expression and protein release with a decrease in its release frequency. These results suggest different levels of action of steroid hormones on GnRH modulation. We conclude that the fine action of E2 and P4 constitute the key factor to enable the hypothalamic activity during the pregnancy of this mammal.

Structural organization, GABAergic and tyrosine hydroxylase expression in the striatum and globus pallidus of the South American plains vizcacha, Lagostomus maximus (Rodentia, Caviomorpha)

The striatum is an essential component of the basal ganglia that regulatessensory processing, motor, cognition, and behavior. Depending on the species, the striatum shows a unique structure called caudate-putamen as in mice, or its separation into two regions called caudate and lenticular nuclei, the latter formed by putamen and globus pallidus areas, as in primates. These structures have two compartments, striosome and matrix. We investigated the structural organization, GABAergic and tyrosine hydroxylase (TH) expression in the striatum and globus pallidus of the South American plains vizcacha, Lagostomus maximus. Its striatum showed regionalization arising from the presence of an internal capsule, and a similar organization to a striosome-matrix compartmentalization. GABAergic neurons in the matrix of caudate exhibited parvalbumin, calretinin, calbindin, GAD65, and NADPH-d-immunoreactivity. These were also expressed in cells of the putamen with the exception of calretinin showing neurofibers localization. Globus pallidus showed parvalbumin- and GAD65-immunoreactive cells, and calretinin- and calbindin-immunoreactive neuropil, plus GABA-A-immunoreactive neurofibers. NADPH-d-, GAD65- and GABA-A-immunoreactive neurons were larger than parvalbumin-, calretinin-, and calbindin-immunoreactive cells, whereas calbindin-immunoreactive cells were the most abundant. In addition, TH-immunoreactive neuropil was observed in the matrix of the striatum. A significant larger TH-immunoreactive area and neuron number was found in females compared to males. The presence of an internal capsule suggests an adaptive advantage concerning motor and cognitive abilities favoring reaction time in response to predators. In an anatomy-evolutive perspective, the striatum of vizcacha seems to be closer to that of humans than to that of laboratory traditional models such as mouse.

Multiple failures in the lutenising hormone surge generating system in GABAB1KO female mice

Female mice lacking GABAB receptors, GABAB1KO, show disrupted oestrous cycles, reduced pregnancies and increased hypothalamic Gnrh1 mRNA expression, whereas anteroventral periventricular/periventricular preoptic nucleus (AVPV/PeN) Kiss1 mRNA was not affected. In the present study, we characterise the important components of the gonadotrophic preovulatory surge, aiming to unravel the origin of this reproductive impairment. In GABAB1KO and wild-type (WT) females, we determined: (i) hypothalamic oestrogen receptor (ER)α and β and aromatase mRNA and protein expression; (ii) ovulation index and oestrus serum follicle-stimulating hormone (FSH) and pituitary Gnrh1r expression; (iii) in ovariectomised-oestradiol valerate-treated mice, we evaluated ex vivo hypothalamic gonadotrophin-releasing hormone (GnRH) pulsatility in the presence/absence of kisspeptin (Kiss-10, constant or pulsatile) and oestradiol (constant); and (iv) in ovariectomised-oestradiol silastic capsule-treated mice (proestrous-like environment), we evaluated morning and evening kisspeptin neurone activation (c-Fos+) and serum luteinising homrone (LH). In the medial basal hypothalamus of oestrus GABAB1KOs, aromatase and ERα mRNA and protein were increased, whereas ERβ was decreased. In GABAB1KOs, the ovulation index was decreased together with decreased first oestrus serum FSH and increased pituitary Gnrh1r mRNA. Under constant Kiss-10 stimulation, hypothalamic GnRH pulse frequency did not vary, although GnRH mass/pulse was increased in GABAB1KOs. In WTs, pulsatile Kiss-10 together with constant oestradiol significantly increased GnRH pulsatility, whereas, in GABAB1KOs, oestradiol alone increased GnRH pulsatility and this was reversed by pulsatile Kiss-10 addition. In GABAB1KOs AVPV/PeN kisspeptin neurones were similarly activated (c-Fos+) in the morning and evening, whereas WTs showed the expected, marked evening stimulation. LH correlated with activated kisspeptin cells in WT mice, whereas GABAB1KO mice showed high, similar LH levels both in the morning and evening. Taken together, all of these alterations point to impairment in the trigger of the preovulatory GnRH surge that entails the reproductive alterations described.

Pituitary estrogen receptor alpha is involved in luteinizing hormone pulsatility at mid-gestation in the South American plains vizcacha, Lagostomus maximus (Rodentia, Caviomorpha)

The South American plains vizcacha, Lagostomus maximus, is a caviomorph rodent native from Argentina, Bolivia and Paraguay. It shows peculiar reproductive features like pre-ovulatory follicle recruitment during pregnancy with an ovulatory process at around mid-gestation. We have described the activation of the hypothalamic - pituitary - ovarian (HPO) axis during pregnancy. A progressive decrease of progesterone (P4) at mid-pregnancy elicits the delivery of gonadotropin-releasing hormone (GnRH) with the consequent secretion of follicle stimulating hormone (FSH) and estradiol (E₂) followed by luteinizing hormone (LH) release resulting in follicular luteinization and the P4 concentration recover. Pituitary gland is the central regulator of the HPO axis being E₂ a key hormone involved in the regulation of its activity. In this work we analyzed the action of E₂ on the pituitary response to the GnRH wave as well as its involvement on LH secretion at mid-gestation in L. maximus. The expression of GnRHR at the pituitary pars distalis showed a significant decrease at mid-pregnancy compared to early- and term-gestating females. ERα showed a significant increment from mid-gestation whereas ERβ did not show variations throughout pregnancy; whereas the LH expression in the pituitary pars distalis showed a significant increase at mid-gestation, concordantly with serum LH, which was followed by a decrease at term-gestation with similar values than at early-pregnancy. The number of cells with co-localization of ERα and GnRHR showed a decline at mid-pregnancy related to early- and term-gestation, whereas the cells with co-localization of ERα and LH increased at mid- and term-pregnancy. On the other hand, ex vivo measuring of LH pulsatility showed a significant increment in the total mass of LH delivered at mid-pregnancy followed by a decrease at term-gestation. The stimulation of ERα with the PPT specific agonist induced a significant increment in the total mass of LH released, whereas no changes were determined when ERβ was stimulated with its specific agonist MPP. These results suggest that LH pulsatility rise at mid-pregnancy would be enabled by the increase of E₂ acting through ERα.

Prolactin Is a Strong Candidate for the Regulation of Luteal Steroidogenesis in Vizcachas (Lagostomus maximus)

Prolactin (PRL) is essential for the maintenance of the corpora lutea and the production of progesterone (P4) during gestation of mice and rats, which makes it a key factor for their successful reproduction. Unlike these rodents and the vast majority of mammals, female vizcachas (Lagostomus maximus) have a peculiar reproductive biology characterized by an ovulatory event during pregnancy that generates secondary corpora lutea with a consequent increment of the circulating P4. We found that, although the expression of pituitary PRL increased steadily during pregnancy, its ovarian receptor (PRLR) reached its maximum in midpregnancy and drastically decreased at term pregnancy. The luteinizing hormone receptor (LHR) exhibited a similar profile than PRLR. Maximum P4 and LH blood levels were recorded at midpregnancy as well. Remarkably, the P4-sinthesizing enzyme 3β-HSD accompanied the expression pattern of PRLR/LHR throughout gestation. Instead, the luteolytic enzyme 20α-HSD showed low expression at early and midpregnancy, but reached its maximum at the end of gestation, when PRLR/LHR/3ß-HSD expressions and circulating P4 were minimal. In conclusion, both the PRLR and LHR expressions in the ovary would define the success of gestation in vizcachas by modulating the levels of 20α-HSD and 3ß-HSD, which ultimately determine the level of serum P4 throughout gestation.

Local production of neurostradiol affects gonadotropin-releasing hormone (GnRH) secretion at mid-gestation in Lagostomus maximus (Rodentia, Caviomorpha)

Females of the South American plains vizcacha, Lagostomus maximus, show peculiar reproductive features such as massive polyovulation up to 800 oocytes per estrous cycle and an ovulatory process around mid‐gestation arising from the reactivation of the hypothalamic–hypophyseal–ovary (H.H.O.) axis. Estradiol (E₂) regulates gonadotropin‐releasing hormone (GnRH) expression. Biosynthesis of estrogens results from the aromatization of androgens by aromatase, which mainly occurs in the gonads, but has also been described in the hypothalamus. The recently described correlation between GnRH and ERα expression patterns in the hypothalamus of the vizcacha during pregnancy, with coexpression in the same neurons of the medial preoptic area, suggests that hypothalamic synthesis of E₂ may affect GnRH neurons and contribute with systemic E₂ to modulate GnRH delivery during the gestation. To elucidate this hypothesis, hypothalamic expression and the action of aromatase on GnRH release were evaluated in female vizcachas throughout pregnancy. Aromatase and GnRH expression was increased significantly in mid‐pregnant and term‐pregnant vizcachas compared to early‐pregnant and nonpregnant females. In addition, aromatase and GnRH were colocalized in neurons of the medial preoptic area of the hypothalamus throughout gestation. The blockage of the negative feedback of E₂ induced by the inhibition of aromatase resulted in a significant increment of GnRH‐secreted mass by hypothalamic explants. E₂ produced in the same neurons as GnRH may drive intracellular E₂ to higher levels than those obtained from systemic circulation alone. This may trigger for a prompt GnRH availability enabling H.H.O. activity at mid‐gestation with ovulation and formation of accessory corpora lutea with steroidogenic activity that produce the necessary progesterone to maintain gestation to term and guarantee the reproductive success.

Hormonal behavior correlates with follicular recruitment at mid-gestation in the South American plains vizcacha, Lagostomus maximus (Rodentia, Caviomorpha)

In mammals, hormonal regulation during gestation is crucial for embryo implantation and pregnancy success. This regulation is controlled through the level of progesterone (P4) that blocks the activity of the hypothalamic-hypophyseal-gonadal (HHG) axis. Previous studies in the pregnant South American plains vizcacha, Lagostomus maximus, have shown that the HHG axis activates around mid-gestation, promoting pre-ovulatory follicle formation. However, the characterization of the hormonal dynamics throughout gestation and its ovarian correlation has not been studied in depth. We studied the ovarian dynamics of L. maximus and its correlation with the hormonal profile during gestation, analyzing serum levels of P4, 17β-estradiol (E2), 4Δ-androstenedione (A4), luteinizing hormone (LH) and follicle stimulating hormone (FSH) as well as the ovarian distribution and expression of their receptors. Additionally, we have analyzed the folliculogenesis and accessory corpora lutea (ACL) formation. P4 showed two concentration peaks reaching its highest level at mid-gestation decreasing at 91-100days post-coitum. P4 decrease is followed by an increase of circulating levels of A4, E2, FSH and LH and with an elevated number of antral/pre-ovulatory follicles which express PGR, ESR1, ESR2, AR, LHR and FSHR. In addition, ACL with oocyte retention and cytoplasmic lipid droplets in luteal cells were detected at this time point. These results show that in L. maximus the decrease of P4 level from mid-gestation enables follicular recruitment until pre-ovulatory stage and the development of functional ACL.