LHRH in the ferret: pubertal decrease in the number of immunopositive arcuate neurons

Neuropeptidergic and Neuroendocrine Systems Underlying Eusociality and the Concomitant Social Regulation of Reproduction in Naked Mole-Rats: A Comparative Approach

The African mole-rat family (Bathyergidae) includes the first mammalian species identified as eusocial: naked mole-rats. Comparative studies of eusocial and solitary mole-rat species have identified differences in neuropeptidergic systems that may underlie the phenomenon of eusociality. These differences are found in the oxytocin, vasopressin and corticotrophin-releasing factor (CRF) systems within the nucleus accumbens, amygdala, bed nucleus of the stria terminalis and lateral septal nucleus. As a corollary of their eusociality, most naked mole-rats remain pre-pubertal throughout life because of the presence of the colony's only reproductive female, the queen. To elucidate the neuroendocrine mechanisms that mediate this social regulation of reproduction, research on the hypothalamo-pituitary-gonadal axis in naked mole-rats has identified differences between the many individuals that are reproductively suppressed and the few that are reproductively mature: the queen and her male consorts. These differences involve gonadal steroids, gonadotrophin-releasing hormone-1 (GnRH-1), kisspeptin, gonadotrophin-inhibitory hormone/RFamide-related peptide-3 (GnIH/RFRP-3) and prolactin. The comparative findings in eusocial and solitary mole-rat species are assessed with reference to a broad range of studies on other mammals.

A morning surge in plasma luteinizing hormone coincides with elevated Fos expression in gonadotropin-releasing hormone-immunoreactive neurons in the diurnal rodent, Arvicanthis niloticus

Arvicanthis niloticus is a diurnal murid rodent from sub-Saharan Africa. Here we report on processes associated with mating in this species in an attempt to elucidate how the neural mechanisms governing temporal organization differ in nocturnal and diurnal species. First, we systematically mapped the distribution of GnRH neurons in adult females. Second, we tested the hypothesis that Arvicanthis differ from nocturnal murid rodents with respect to the timing of the LH surge and the associated increase in Fos expression in GnRH-immunoreactive (IR) neurons. We examined these events around a postpartum estrus. When parturition occurred between zeitgeber time (ZT) 2 and 17 (lights on at ZT 0 and off at ZT 12; there are 24 ZT units a day, each equivalent to 1 standard hour), we collected blood and perfused females at ZT 17, 20, 23, or 2. A sharp peak in plasma LH occurred at ZT 20, and a 10-fold increase in the percentage of GnRH-IR neurons that expressed Fos-IR occurred between ZT 17 and 20. By contrast, this rise occurs in nocturnal rodents during the last few hours of the light period. This is the first indication of a difference between nocturnal and diurnal animals with respect to neural mechanisms associated with a precisely timed event of known significance.

GnRH mRNA increases with puberty in the male Syrian hamster brain

Puberty in the male Syrian hamster (Mesocricetus auratus) is characterized by decreased responsiveness to testosterone mediated negative feedback, but the neural mechanism for this change remains elusive. We hypothesized that decreased inhibition of the gonadotropin-releasing hormone (GnRH) system results in increased neurosecretory activity, which includes an increase in GnRH gene expression. This study examined GnRH mRNA in male hamsters before and after puberty, and sought to determine if any increase in mRNA was specific to particular subpopulations of GnRH neurones. Brains were collected from 21-day-old prepubertal males (n = 5) and 56-day-old postpubertal males (n = 5). Alternate 10 microm coronal sections from fresh-frozen brains were collected throughout the septo-hypothalamic region, and 25% of those sections were processed for in-situ hybridization histochemistry using an 35S-riboprobe complementary to hamster GnRH. No differences were observed in the number of GnRH mRNA expressing cells in any region, but in the diagonal band of Broca (DBB)/organum vasculosum of the lamina terminalis (OVLT) there was a significant increase in labelling intensity (defined as area of the cell occupied by silver grains) in postpubertal males. A second analysis compared the frequency distributions of cells based on labelling intensity between prepubertal and postpubertal males. This analysis revealed significant differences between the two frequency distributions in all areas analysed (DBB/OVLT, medial septum (MS), and preoptic area (POA)). Furthermore, examining the distribution of cells in these regions revealed a shift to the right in the postpubertal population of cells, which indicated an increased number of GnRH neurones with greater labelling intensity. These data clearly demonstrate increased GnRH mRNA during puberty. Furthermore, they suggest that the previous observation of brain region specific pubertal decreases in GnRH-immunoreactivity only within the DBB/OVLT and MS but not the POA are not due to differential levels of GnRH gene expression, but could indicate increased release from these neurones during puberty.

Changes in mediobasal hypothalamic gonadotropin-releasing hormone messenger ribonucleic acid levels induced by mating or ovariectomy in a reflex ovulator, the ferret

The ferret is a reflex-ovulating species in which receipt of an intromission induces a prolonged (+/- 12 h) preovulatory LH surge in the estrous female. This LH surge is probably stimulated by a large release of GnRH from the mediobasal hypothalamus (MBH). In Exp 1 we asked whether GnRH messenger RNA (mRNA) levels increase in response to mating so as to replenish the MBH GnRH stores needed to sustain the preovulatory LH surge. Estrous females were killed 0, 0.25, 0.5, 1, 3, 6, 14, or 24 h after the onset of a 10-min intromission from a male. Coronal brain sections ranging from the rostral preoptic area caudally to the posterior hypothalamus were processed for in situ hybridization using a 35S-labeled oligoprobe complementary to the human GnRH-coding region. We found no evidence of increased MBH GnRH mRNA levels during the ferret's mating-induced preovulatory LH surge. Instead, the number of GnRH mRNA-expressing cells dropped significantly in the arcuate region beginning 6 h after onset of intromission and remained low thereafter. Furthermore, cellular GnRH mRNA levels decreased in the arcuate region toward the end of the preovulatory LH surge. In Exp 2 we asked whether ovarian hormones regulate MBH GnRH mRNA levels in the female ferret. Ovariectomy of estrous females significantly reduced the number of GnRH mRNA-expressing cells in the arcuate region. This decrease was probably not due to the absence of circulating estradiol. Gonadally intact anestrous females had levels of MBH GnRH mRNA similar to those in estrous females even though plasma estradiol levels were equally low in anestrous females and ovariectomized females. Ovarian hormones other than estradiol may stimulate MBH GnRH mRNA levels in anestrous and estrous females.

Regional changes in GnRH immunoreactivity with puberty in the male Syrian hamster

Gonadotropin-releasing-hormone immunopositive (GnRH+) neurons were identified in juvenile and adult male Syrian hamsters. There were significantly fewer GnRH+ cells in the diagonal band of Broca/organum vasculosum of the lamina terminalis (DBB/OVLT) and medial septum (MS) in adults as compared to juvenile males, while no cell number difference was found in the preoptic area (POA). The decrease in cell number likely reflects reduced somal stores of GnRH in DBB/OVLT and MS, suggesting that these subpopulations promote increased GnRH release during pubertal maturation in male hamsters.

Distribution and morphology of luteinising hormone-releasing hormone neurones in a species of wild antelope, the springbok (Antidorcas marsupialis)

The distribution and morphology of luteinising hormone-releasing hormone (LHRH) neurones varies between species. The primary purpose of this study was to characterize the distribution and morphology of the LHRH system in a species of antelope, the springbok. This wild antelope has a well-defined social structure in which reproductive activity is confined to a few dominant, territorial rams. We also sought to determine whether social or reproductive status could be accounted for by differences in the distribution or morphology of hypothalamic LHRH neurones. Eleven anoestrous female, nine breeding territorial male (TM) and eight "bachelor" male (BM) springbok were obtained, and their reproductive and body conditions were assessed. By using standard immunocytochemical techniques, the LHRH system was visualised in the brains of four animals from each group. Immunoreactive neurones were located in a continuum from the septum to the arcuate nucleus, with the majority at the level of the organum vasculosum of the lamina terminalis. Neither the distribution nor the number of cells differed among the three groups. Furthermore, the area of LHRH perikarya was similar in both groups of males, suggesting that reproductive differences between TMs and BMs lie at another level of the neuroendocrine axis. The anoestrous females had significantly larger neurones than males (TM plus BM). This may reflect a sex difference in the LHRH system of this wild antelope. However, an alternative explanation is that the male/female difference is related to the comparatively inactive reproductive neuroendocrine state of the females.

Brain region-specific regulation of luteinizing hormone-releasing hormone messenger ribonucleic acid in the male ferret: interactions between pubertal maturation and testosterone

This study examined the regulation of LHRH messenger RNA (mRNA) during pubertal maturation and by testosterone in male ferrets. Prepubertal and postpubertal ferrets were either intact or were castrated and treated with daily injections of oil or 5 mg/kg testosterone propionate for 14 days. In situ hybridization for LHRH mRNA was performed using an 35S-labeled 48-base oligonucleotide complementary to the human LHRH-coding region. Computerized image analysis was performed on cells in the preoptic area, retrochiasmatic area, arcuate nucleus (ARC), and median eminence; cells were classified as labeled if the number of pixels representing silver grains over the cell was 5 or more times the number of background silver grain pixels. Both pubertal maturation of intact males and castration of prepubertal males resulted in an increase in the number of labeled cells in the ARC. These effects were not observed in any of the other three brain regions, suggesting that ARC LHRH-producing neurons are of primary importance in the presumed increase in LHRH release that occurs as a consequence of either pubertal maturation or castration of prepubertal males. Castration of adults did not increase the number of labeled cells in any brain area, but resulted in an increase in silver grains per labeled cell only in the preoptic area. Thus, LHRH mRNA is regulated during puberty primarily in the ARC, and the particular cell group in which LHRH mRNA is most strongly regulated by testosterone changes with pubertal maturation.

Distribution and morphology of immunoreactive gonadotropin-releasing hormone (GnRH) neurons in the basal forebrain of ponies

Recent reports have indicated that analysis of changes in the staining characteristics of gonadotropin-releasing hormone (GnRH) neurons and characterization of morphological plasticity of the related structural framework may help to elucidate the physiological mechanisms involved in neuroendocrine control of mammalian reproduction. Whether comparative studies will facilitate this process or simply elucidate species-specific mechanisms is not yet clear. The present study was performed in order to begin analysis of GnRH neurons in a seasonally breeding species that exhibits an unusually long ovulatory luteinizing hormone (LH) surge. To this end, light microscopy and image analysis were used to characterize distribution and morphology of GnRH neurons in 15 adult male and female ponies. Samples were collected in the middle of the normal ovulatory season. Unipolar, bipolar, and multipolar GnRH neurons were organized in a loosely defined continuum that extended from the medial septum to tuberoinfundibular areas in the medical basal hypothalamus (MBH). Most cells were bipolar, and the majority of neurons were located in the MBH. Fiber projections to the median eminence included presumptive pathways similar to those previously described in other species. Image analysis of cell size indicated that cells in the MBH were larger than those in preoptic areas and GnRH neurons in both of these locations were larger than neurons in rostral areas of the medial septum. Results from this experiment suggest that the large population of MBH GnRH neurons in the equine species is likely to be of primary importance to reproductive function, whereas cells in other areas are fewer and smaller. Further work is needed to characterize morphological characteristics that may be related to physiological fluctuations in reproductive function of the equine species.

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

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

Luteinizing Hormone-Releasing Hormone Neurons which are R-etrogradely Labeled After Peripheral Fluoro-Gold Administration in the Male Ferret

This study identified luteinizing hormone-releasing hormone (LHRH)-producing neurons which have access to fenestrated capillaries in prepubertal male European ferrets. Fluoro-Gold was injected intraperitoneally to retrogradely label neurons with terminals outside the blood-brain barrier. LHRH neurons were identified by immunofluorescence using a secondary antibody tagged with tetramethylrhodamine isothiocyanate. Cell bodies which demonstrated both tetramethylrhodamine isothiocyanate and Fluoro-Gold fluorescence were defined as LHRH-producing neurons with axon terminals in regions containing fenestrated capillaries. The total number and neuroanatomical distribution of immunopositive (LHRH +) cells concurred with previous studies in the ferret in which cell bodies were diffusely distributed from rostral forebrain through caudal diencephalon, with approximately 70% of the LHRH + cell bodies located in retrochiasmatic hypothalamus. In the present study, an average of 59.8% of all LHRH+ neuronal perikarya also contained Fluoro-Gold. The majority of Fluoro-Gold filled LHRH+ neurons demonstrated only faint to moderate amounts of Fluoro-Gold when compared to other Fluoro-Gold filled neurosecretory neurons. This limited uptake of Fluoro-Gold may be due to a relative inactivity of LHRH neurons projecting outside the blood-brain barrier. Double-labeled LHRH + neurons were dispersed throughout the entire population of LHRH+ cell bodies and no apparent nuclear groups of double-labeled neurons were found. This observation suggests that the LHRH+ neurons responsible for neurosecretion into the median eminence coexist with the LHRH+ neurons responsible for intracerebral neurotransmission or neuromodulation. One distinguishable population of LHRH + neurons was consistently observed in all the brains. Only 26% of total LHRH+ perikarya within the caudal arcuate nucleus contained Fluoro-Gold, while at least 50% of LHRH+ neurons in other structures, including the rostral arcuate nucleus, contained Fluoro-Gold. Thus, in the prepubertal male ferret, the majority of LHRH cell bodies located in the caudal arcuate nucleus may be differentially regulated and/or involved in non-neuroendocrine functions.