Sex difference in the effect of mating on c-fos expression in luteinizing hormone-releasing hormone neurons of the ferret forebrain

Ovulation mechanism in South American Camelids: The active role of β-NGF as the chemical signal eliciting ovulation in llamas and alpacas

The ovulation-inducing effect of seminal plasma was first suggested in Bactrian camels over 30 years ago, initiating a long search to identify the 'ovulation-inducing factor' (OIF) present in camelids semen. During the last decade, primarily in llamas and alpacas, this molecule has been intensively studied characterizing its biological and chemical properties and ultimately identifying it as β-Nerve Growth Factor (β-NGF). The high concentration of OIF/β-NGF in seminal plasma of llamas and alpacas, and the striking effects of seminal fluid on ovarian function strongly support the notion of an endocrine mode of action. Also, have challenged the dogma of mating induced ovulation in camelid species, questioning the classical definition of reflex ovulators, which at the light of new evidence should be revised and updated. On the other hand, the presence of OIF/β-NGF and its ovulatory effect in camelids confirm the notion that seminal plasma is not only a transport and survival medium for sperm but also, a signaling agent targeting female tissues after insemination, generating relevant physiological and reproductive consequences. The presence of this molecule, conserved among induced as well as spontaneous ovulating species, clearly suggests that the potential impacts of this reproductive feature extend beyond the camelid species and may have broad implications in mammalian fertility. The aim of the present review is to provide a brief summary of all research efforts undertaken to isolate and identify the ovulation inducing factor present in the seminal plasma of camelids. Also to give an update of the current understanding of the mechanism of action of seminal β-NGF, at central and ovarian level; finally suggesting possible brain targets for this molecule.

Courtship interactions stimulate rapid changes in GnRH synthesis in male ring doves

Many birds and mammals show changes in the hypothalamo-pituitary-gonadal (HPG) axis in response to social or sexual interactions between breeding partners. While alterations in GnRH neuronal activity play an important role in stimulating these changes, it remains unclear if acute behaviorally-induced alterations in GnRH release are accompanied by parallel changes in GnRH synthesis. To investigate this relationship, we examined changes in the activity of GnRH neurons in the brains of male ring doves following brief periods of courtship interactions with females. Such interactions have been previously shown to increase plasma LH in courting male doves at 24 h, but not at 1 h, after pairing with females. In the first study, males allowed to court females for 2 h had 60% more cells that showed immunocytochemical labeling for GnRH-I in the preoptic area (POA) of the hypothalamus than did control males that remained isolated from females. To determine whether an increase in GnRH gene expression preceded this increase in GnRH immunoreactivity in the POA, changes in the number of cells with detectable GnRH-I mRNA in the POA were measured by in situ hybridization following a 1 h period of courtship interactions with females. In this second study, courting males exhibited 40% more cells with GnRH-I in this region than did isolated control males. GnRH-immunoreactive neurons in two other diencephalic regions failed to show these courtship-induced changes. Plasma LH was not elevated after 1 or 2 h of courtship. These results demonstrate that the release of GnRH-I in the POA that is presumably responsible for courtship-induced pituitary and gonadal activation is accompanied by a rapid increase in GnRH synthesis that occurs before plasma LH levels increase. We suggest that this increase in GnRH synthesis is necessary to support the extended period of HPG axis activation that is seen in this species during the 5-10 day period of courtship and nest building activity.

Functional internal complexity of amygdala: focus on gene activity mapping after behavioral training and drugs of abuse

The amygdala is a heterogeneous brain structure implicated in processing of emotions and storing the emotional aspects of memories. Gene activity markers such as c-Fos have been shown to reflect both neuronal activation and neuronal plasticity. Herein, we analyze the expression patterns of gene activity markers in the amygdala in response to either behavioral training or treatment with drugs of abuse and then we confront the results with data on other approaches to internal complexity of the amygdala. c-Fos has been the most often studied in the amygdala, showing specific expression patterns in response to various treatments, most probably reflecting functional specializations among amygdala subdivisions. In the basolateral amygdala, c-Fos expression appears to be consistent with the proposed role of this nucleus in a plasticity of the current stimulus-value associations. Within the medial part of the central amygdala, c-Fos correlates with acquisition of alimentary/gustatory behaviors. On the other hand, in the lateral subdivision of the central amygdala, c-Fos expression relates to attention and vigilance. In the medial amygdala, c-Fos appears to be evoked by emotional novelty of the experimental situation. The data on the other major subdivisions of the amygdala are scarce. In conclusion, the studies on the gene activity markers, confronted with other approaches involving neuroanatomy, physiology, and the lesion method, have revealed novel aspects of the amygdala, especially pointing to functional heterogeneity of this brain region that does not fit very well into contemporarily active debate on serial versus parallel information processing within the amygdala.

Reproductive activation of pine voles (Microtus pinetorum): examination of physiological markers

We tested the hypothesis that the presence of an opposite-sex conspecific will result in time-related changes in measures of reproductive activation. We housed male-female pairs of pine voles together for 0, 2, 6, 12, or 24 h before collecting blood, reproductive organs and brains for immunocytochemical analysis of LHRH and c-fos. Control animals were never exposed to an opposite-sex conspecific. Following exposure to a male, there was a significant increase in uterine weight but not in LH levels. In males, there were no changes in peripheral indices of activation, i.e. LH levels, testes and seminal vesicle weights were not altered. Consistent with no change in circulating levels of LH, there was no change in LHRH immunoreactivity at any time. However, c-fos immunoreactivity was significantly greater in both males and females in the cingulate cortex and rostral bed nucleus of the stria terminalis (BNST) at 2 h, and in the caudal BNST at 2, 6 and 12 h. Similarly, c-fos immunoreactivity was increased in the rostral MPOA in both males and females at 2 and 6 h. However, in the caudal MPOA, there was a significant interaction between sex and time due to increased c-fos immunoreactivity in females only at 6 h. These results indicate that, in both male and female pine voles, exposure to an opposite-sex conspecific is sufficient to produce rapid, neural activation in brain areas known to be involved in reproductive activation and sexual behavior. This early activation did not occur in LHRH neurons. It is not known if this activation, particularly at early times, is due to reproductive activation or to the formation of pair bonds.

Sexually dimorphic activation of galanin neurones in the ferret's dorsomedial preoptic area/anterior hypothalamus after mating

Male ferrets in breeding condition possess three times as many galanin-immunoreactive (IR) neurones as oestrous females in the sexually dimorphic dorsomedial preoptic area/anterior hypothalamus (dmPOA/AH). Using Fos-IR as a marker of activation, we investigated whether mating with intromission differentially activates this sexually dimorphic group of galanin-IR neurones in male and female ferrets. Male ferrets that intromitted had a significantly greater percentage of galanin-IR neurones in the dmPOA/AH that were colabelled with nuclear Fos-IR than oestrous females that received an intromission. Intromissive stimulation augmented Fos-IR in an equal percentage of galanin-IR neurones in both sexes in the medial amygdala (MA) and bed nucleus of the stria terminalis (BNST). Peripheral anosmia induced by bilateral occlusion of males' nares did not reduce the mating-induced activation of galanin-IR neurones in the dmPOA/AH, and there was a significant correlation among individual males between intromission duration and the percentage of dmPOA/AH galanin-IR neurones colabelled with Fos-IR. Exposure of castrated, testosterone propionate-treated male ferrets to either soiled bedding or to volatile odours from oestrous females failed to induce nuclear Fos-IR in galanin-IR neurones located in the dmPOA/AH, BNST or MA, suggesting that the mating-induced activation of galanin-IR forebrain neurones in male ferrets depends more on genital-somatosensory than on olfactory inputs. The observed sex dimorphism in the mating-induced activation of galanin-IR neurones in the dmPOA/AH raises the possibility that these neurones perform a mating-dependent function that occurs only in males.

Vomeronasal organ: electrical stimulation activates Fos in mating pathways and in GnRH neurons

Electrical stimulation of the vomeronasal organ in male hamsters activated Fos expression in neurons of the chemosensory pathways, as in experiments where animals were stimulated with female chemical stimuli. Fos was also activated in gonadotropin hormone releasing hormone (GnRH, or LHRH) neurons in the rostral medial preoptic region of the brain, a possible substrate for GnRH influence on chemosensory-dependent reproductive behavior.

The ferret's vomeronasal organ and accessory olfactory bulb: effect of hormone manipulation in adult males and females

The male ferret, a carnivore, was recently shown to possess a vomeronasal organ (VNO). We compared the morphology of the VNO and its associated accessory olfactory bulb (AOB) in male and female ferrets that were killed in adulthood. The volume and surface area of the VNO neuroepithelium were similar in adult gonadectomized male and female ferrets regardless of whether they were treated with testosterone propionate (TP) or oil vehicle. An AOB was localized bilaterally in the medial caudal part of the olfactory bulbs of adult ferrets using soybean agglutin binding and immunostaining for luteinizing hormone-releasing hormone and tyrosine hydroxylase as well as Nissl staining of coronal, horizontal, and sagittal brain sections. There was no effect of sex or TP treatment on AOB cell layer volume in adult gonadectomized animals. We found the ferret's AOB to be more medially located and much smaller than previously reported in this species, thus highlighting the importance of using several histochemical markers to characterize this structure in any previously unexamined species. Adult male and female ferrets both have a VNO and an associated AOB. More research is needed to determine what role, if any, this accessory olfactory system plays in mediating behavioral and neuroendocrine responses to pheromones in ferrets of either sex.

Mating induces gonadotropin-releasing hormone neuronal activation in anosmic female ferrets

In females of both spontaneously and induced ovulating species, pheromones from male conspecifics can directly stimulate GnRH neuronal activity, thereby inducing pituitary LH secretion and stimulating the onset of estrus. However, whether pheromones contribute to the steroid- or mating-induced preovulatory activation of GnRH neurons is less clear. Previous studies in the ferret, an induced ovulator, raised the possibility that olfactory cues contribute to the ability of genital-somatosensory stimulation to activate GnRH neurons in the mediobasal hypothalamus (MBH). In the present study the percentage of GnRH neurons colabeled with Fos-immunoreactivity (IR), used as a marker for neuronal activation, was investigated in the MBH of mated gonadectomized, estradiol-treated female ferrets in which both nares were occluded. In addition, the percentage of GnRH neurons colabeled with Fos-IR was examined in the MBH of gonadectomized, estradiol-treated female ferrets exposed to male bedding. Bilateral nares occlusion successfully blocked mating or odor-induced increments in Fos-IR in central olfactory regions, including the cortical and medial amygdala. By contrast, the percentage of GnRH neurons expressing Fos-IR did not differ between mated nares- and sham-occluded females. Exposure to male bedding alone failed to induce Fos-IR in MBH GnRH neurons. Thus, the mating-induced preovulatory activation of GnRH neurons in the female ferret's MBH appears to rely solely on genital-somatosensory as opposed to olfactory inputs.

Neuroendocrine regulation of GnRH release in induced ovulators

GnRH is the key neuropeptide controlling reproductive function in all vertebrate species. Two different neuroendocrine mechanisms have evolved among female mammals to regulate the mediobasal hypothalamic (MBH) release of GnRH leading to the preovulatory secretion of LH by the anterior pituitary gland. In females of spontaneously ovulating species, including rats, mice, guinea pigs, sheep, monkeys, and women, ovarian steroids secreted by maturing ovarian follicles induce a pulsatile pattern of GnRH release in the median eminence that, in turn, stimulates a preovulatory LH surge. In females of induced ovulating species, including rabbits, ferrets, cats, and camels, the preovulatory release of GnRH, and the resultant preovulatory LH surge, is induced by the receipt of genital somatosensory stimuli during mating. Induced ovulators generally do not show "spontaneous" steroid-induced LH surges during their reproductive cycles, suggesting that the positive feedback actions of steroid hormones on GnRH release are reduced or absent in these species. By contrast, mating-induced preovulatory surges occasionally occur in some spontaneously ovulating species. Most research in the field of GnRH neurobiology has been performed using spontaneous ovulators including rat, guinea pig, sheep, and rhesus monkey. This review summarizes the literature concerning the neuroendocrine mechanisms controlling GnRH biosynthesis and release in females of several induced ovulating species, and whenever possible it contrasts the results with those obtained for spontaneously ovulating species. It also considers the adaptive, evolutionary benefits and disadvantages of each type of ovulatory control mechanism. In females of induced ovulating species estradiol acts in the brain to induce aspects of proceptive and receptive sexual behavior. The primary mechanism involved in the preovulatory release of GnRH among induced ovulators involves the activation of midbrain and brainstem noradrenergic neurons in response to genital-somatosensory signals generated by receipt of an intromission from a male during mating. These noradrenergic neurons project to the MBH and, when activated, promote the release of GnRH from nerve terminals in the median eminence. In contrast to spontaneous ovulators, there is little evidence that endogenous opioid peptides normally inhibit MBH GnRH release among induced ovulators. Instead, the neural signals that induce a preovulatory LH surge in these species seem to be primarily excitatory. A complete understanding of the neuroendocrine control of ovulation will only be achieved in the future by comparative studies of several animal model systems in which mating-induced as well as spontaneous, hormonally stimulated activation of GnRH neurons drives the preovulatory LH surge.

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.

Effects of sexual interactions with a male on fos-like immunoreactivity in the female quail brain

Sexual interactions can cause changes in plasma hormone levels and activate immediate early genes within the mammalian brain. There are marked anatomical differences between the regions activated that relate directly to the sexual specific behaviour and neuroendocrinology of each sex. The aim of this study was to determine if such a sexual dimorphism exists in birds by examining the brain regions stimulated in adult virgin female Japanese quail (Coturnix japonica) during sexual behaviour and comparing this to previously reported data concerning males. Female quail were allowed to freely interact with adult males and both female and male sexual behaviour was recorded. Contrary to previous findings in male quail, no significant induction of Fos-like immunoreactive (FLI) cells was observed following sexual interactions in the preoptic area of females; this area is fundamentally involved in the control of male-type copulatory behaviour. Sexual interactions significantly induced FLI cells in the hyperstriatum ventrale, the part of the archistriatum just lateral to the anterior commissure, and the nucleus intercollicularis. Moreover, prominent activation was detected throughout most of the ventromedial nucleus of the hypothalamus, a region reported to be rich in oestrogen receptors. FLI induction was not a consequence of sexual behaviour induced changes in luteinizing hormone (LH) as plasma LH levels were unaltered. Instead, brain activation must be a consequence of copulation-associated somatosensory inputs or direct stimuli originating from the male. Male quail, like the majority of other birds, lack an intromittant organ (penis) so that the somatosensory inputs to the female are rather different from those in mammals; the precise nature of these inputs is yet to be determined.

Prenatal testosterone masculinizes synaptic input to gonadotropin-releasing hormone neurons in sheep

In sheep, the control of tonic and surge GnRH secretion is sexually differentiated by testosterone in utero. However, GnRH neurons are not sexually dimorphic with respect to number, distribution, or gross morphology. Therefore, this study tested the hypothesis that prenatal steroids influence synaptic input to GnRH neurons. We compared the number of synapses on GnRH neurons from male, female, and androgenized female lambs (n = 5 each). Androgenized females were exposed to testosterone during mid-gestation. Yearling lambs were perfused, and GnRH neurons were visualized using the LR-1 antibody. Five to seven GnRH neurons from the rostral preoptic area in each animal were viewed at the ultrastructural level. Afferent synapses and glial ensheathment on each neuron were counted in a single section through the plane of the nucleus. GnRH neurons from females received approximately twice as many contacts (3.6 +/- 0.7 synapses/100 microm plasma membrane) as those from male lambs (1.6 +/- 0.3; p < 0.05), similar to previous reports in rats. In addition, the number of synapses on GnRH neurons from androgenized female lambs (1.5 +/- 0.5) was similar to that from male lambs, suggesting that prenatal steroids give rise to sex differences in synaptic input to GnRH neurons.

Fos localization in cytosolic and nuclear compartments in neurones of the frog, Rana esculenta, brain: an analysis carried out in parallel with GnRH molecular forms

C-fos activity was determined in the brain of the frog, Rana esculenta, during the annual sexual cycle. The localization of GnRH molecular forms (mammalian- and chicken-GnRHII) was also carried out to determine whether or not the proto-oncogene and the peptides showed a functional relationship. Northern blot analysis of total RNA revealed the presence of a single strong signal of c-fos like mRNA of 1.9 Kb during February and April. This was followed by expression of c-Fos protein (Fos) in several brain areas during March and July shown by immunocytochemistry. In particular, the olfactory region, the lateral and medial pallium, the nucleus lateralis septi, the ventral striatum, the caudal region of the anterior preoptic area, the suprachiasmatic nucleus, the ventral thalamus, tori semicircularis and ependymal layers of the tectum were immunostained. There was no overlap between Fos immunoreactive perikarya and GnRH immunoreactive perikarya (e.g. gonadotrophin-releasing hormone (GnRH) in the rostral part and Fos in the caudal region of the anterior preoptic area). Interestingly, a cytoplasmic localization of Fos was also observed by immunocytochemistry and gel retardation experiments supported this observation. Cytoplasmic extracts from September-October animals bound the AP1 oligonucleotide. The complex was not available in the nuclear extracts from the same preparation, suggesting that, besides Fos, Jun products were also present. Conversely, nuclear but not cytosolic binding was detected in the brain of animals collected in July. In conclusion, we show that Fos and GnRH activity does not correlate in the frog brain and, for the first time in a vertebrate species, we give evidence of a cytoplasmic AP1 complex in neuronal cells.

Protein synthesis in the medial preoptic area is important for the mating-induced decrease in estrus duration in hamsters

Sexual receptivity in female hamsters potentially lasts for about 16 h. However, vaginal cervical stimulation (VCS) from a male during mating eventually reduces receptivity and can shorten the duration of behavioral estrus. The process by which this change in response to the male takes place is unknown. Recently, detection of the Fos protein has indicated that the medial preoptic area (POA) is one of the brain regions particularly responsive to VCS. Additionally, the POA may have an inhibitory effect on sexual receptivity. To determine if protein synthesis in the POA is required to initiate the VCS-induced decrease in estrus duration, a protein synthesis inhibitor (anisomycin, 0.50 microg) or a control substance (cholesterol) was applied bilaterally to the POA of steroid-primed ovariectomized female hamsters. Females were tested with a sexually active male at five time points following the initial test for sexual receptivity (hour 1, 2, 6, 12, and 24). Half of the females tested were allowed to receive VCS from a male, while half were fitted with vaginal masks to prevent penile intromission. Each group receiving VCS showed a significant decrease in lordosis duration evident between hour 2 and hour 6, except the group which received anisomycin in the POA. In this respect the POA anisomycin group was similar to animals which did not receive VCS. Hamsters with vaginal masks and the anisomycin/POA animals allowed to receive VCS exhibited their first decrease in lordosis duration between hour 6 and hour 12. These results indicate that protein synthesis is important for VCS-induced decrease in estrus duration in the POA.

Sex difference and testosterone modulation of pheromone-induced NeuronalFos in the Ferret's main olfactory bulb and hypothalamus

A carnivore, the ferret possesses a vomeronasal organ--accessory olfactory bulb (VNO-AOB) projection to the hypothalamus; however, little is known about its function. Pheromones in soiled bedding from estrous female ferrets or an artificial peppermint odor significantly augmented nuclear Fos protein immunoreactivity (Fos-IR), a marker of neural activation, in several main olfactory bulb (MOB) sites but not in the AOB of gonadectomized male and females. Testosterone propionate (TP) significantly augmented the MOB's neuronal Fos responses to estrous females' pheromones, but not to peppermint. Estrous odors, but not peppermint, also augmented neuronal Fos-IR in the medial preoptic area (mPOA) of female, but not male, subjects. Pheromones in soiled bedding from breeding male ferrets significantly augmented neuronal Fos-IR in the MOB and in the medial amygdala of gonadectomized, TP-treated male and female subjects. Again, male pheromones failed to influence neuronal Fos-IR in the AOB of either sex, and only females showed significant increases in neuronal Fos-IR in the lateral aspect of the ventromedial nucleus and mPOA. These results point to an essential role among higher mammals of the main olfactory epithelium-MOB projection to the hypothalamus in detecting and processing pheromones. Gonadectomized ferrets showed significant increases in sniffing behavior when placed on either female or male bedding. This occurred regardless of whether they had received TP or oil vehicle, suggesting that testosterone's facilitation of neuronal Fos responses to estrous females' odors in the MOB of both sexes cannot be attributed to increased scent gathering. Androgen receptor-IR was present in the MOB granule cell layer of male and female ferrets, raising the possibility that testosterone acts directly on these cells to augment their responsiveness to pheromones.

Induction of fos-like immunoreactivity in musk shrews after mating

In many mammalian species the neuroendocrine regulation of male and female reproductive behavior is sexually dimorphic. By contrast, many features of female sexual behavior in the musk shrew (Suncus murinus) more closely resemble those of males than of females of other species. Female musk shrews require testosterone (T), which is neurally aromatized to estrogen, to induce sexual behavior. Aromatization occurs in the medial preoptic area (MPOA), and this region is critical for the expression of female receptivity. To compare neural responses to sexual behavior in females and males, we compared the number of Fos-like immunoreactive (Fos-ir) neurons after mating in musk shrews. In both males and females the number of Fos-ir neurons was increased by mating activity in the granule layer of the accessory olfactory bulb (gr-AOB), the bed nucleus of the stria terminalis (BNST), MPOA, the medial amygdala (MeA), and the region corresponding to the midbrain central tegmental field (CTF). Although Fos was induced by mating in several regions, this response was only dimorphic in the ventral medial nucleus of the hypothalamus (VMN), where mating significantly increased Fos-ir in females, but not in males. In both sexes, only the gr-AOB displayed an increase in Fos-ir after exposure to chemosensory cues alone. Thus, the pattern of Fos expression in the brain after mating is only sexually dimorphic in one region, the VMN. Further, in spite of past behavioral studies done in this species, which show a role for pheromones in induction of receptivity, these data show that exposure to pheromones does not induce Fos in structures caudal to the olfactory bulbs.

Anatomical interrelationships of the medial preoptic area and other brain regions activated following male sexual behavior: a combined fos and tract-tracing study

The medial preoptic nucleus (MPN) is an essential site for the regulation of male sexual behavior. Previous studies using c-fos as a marker for neural activation have shown that copulation increased c-fos expression in the MPN. Neural activation was also present in brain regions that are connected with the MPN and are involved in male sexual behavior, including the posteromedial bed nucleus of the stria terminalis (BNSTpm), posterodorsal preoptic nucleus (PD), posterodorsal medial amygdala (MEApd), and parvocellular subparafascicular thalamic nucleus (SPFp). The present study investigated whether the copulation-induced, activated neurons in these brain regions are involved in the bidirectional connections with the MPN. Therefore, mating-induced Fos expression was combined with application of anterograde (biotinylated dextran amine) or retrograde (cholera toxin B subunit) tracers in the MPN. The results demonstrated that neurons in the BNSTpm, PD, MEApd, and SPFp that project to the MPN were activated following copulation. However, in males that displayed sexual behavior but did not achieve ejaculation, few double-labeled neurons were evident, although both retrogradely labeled neurons and Fos-immunoreactive cells were present. In addition, retrograde neurons that expressed Fos were located in discrete subdivisions within the brain regions studied, where Fos is induced after ejaculation. Likewise, anterogradely labeled fibers originating from the MPN were not distributed homogeneously but were particularly dense in these discrete subdivisions. These results demonstrate that copulation-induced Fos-positive neurons in specific subdivisions of the BNSTpm, PD, MEApd, and SPFp have bidirectional connections with the MPN. Taken together with previous findings, this supports the existence of a discrete subcircuit within a larger neural network underlying male sexual behavior.

The GnRH system of seasonal breeders: anatomy and plasticity

Seasonal breeders, such as sheep and hamsters, by virtue of their annual cycles of reproduction, represent valuable models for the study of plasticity in the adult mammalian neuroendocrine brain. A major factor responsible for the occurrence of seasonal reproductive transitions is a striking change in the responsiveness of gonadotropin-releasing hormone (GnRH) neurons to the inhibitory effects of gonadal steroids. However, the neural circuitry mediating these seasonal changes is still relatively unexplored. In this article, we review recent findings that have begun to define that circuitry and its plasticity in a well-studied seasonal breeder, the ewe. Tract tracing studies and immunocytochemical analyses using Fos and FRAs as markers of activation point to a subset of neuroendocrine GnRH neurons in the MBH as potential mediators of pulsatile GnRH secretion. Because the vast majority of GnRH neurons lack estrogen receptors, seasonal changes in responsiveness to estradiol are most probably conveyed by afferents. Two possible mediators of this influence are dopaminergic cells in the A14/A15 cell groups of the hypothalamus, and estrogen receptor-containing cells in the arcuate nucleus that project to the median eminence. The importance of GnRH afferents in the regulation of season breeding is underscored by observations of seasonal changes in the density of synaptic inputs onto GnRH neurons. Thyroid hormones may participate in this remodeling, because they are important in seasonal reproduction, influence the morphology of other brain systems, and thyroid hormone receptors are expressed within GnRH neurons. Finally, in the hamster, neonatal hypothyroidism affects the number of caudally placed GnRH neurons in the adult brain, suggesting that thyroid hormones may influence development of the GnRH system as well as its reproductive functions in the adult brain.

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.

Sexually dimorphic processing of somatosensory and chemosensory inputs to forebrain luteinizing hormone-releasing hormone neurons in mated ferrets

The ferret is a reflexively ovulating species in which mating induces a preovulatory LH surge in the estrous female but significantly decreases LH secretion in the breeding male. This sexually dimorphic hormonal response is reflected in a sex difference in Fos-like immunoreactivity (Fos-IR) in forebrain LHRH and non-LHRH neurons after mating. We used dual immunocytochemistry for Fos and LHRH to determine whether the sex dimorphism occurs in the initial detection and transmission or in the central processing of sensory stimuli associated with mating? We also assessed the ability of chemosensory cues alone to augment neuronal Fos-IR in the ferret forebrain. Breeding male and female ferrets were paired, whereupon the male partner achieved an intromission lasting for 16-90 min. Mated male and female subjects were always perfused 90 min after the onset of the male's intromission. Additional male and female subjects were placed alone in a cage in which an opposite sex ferret in breeding condition had been housed for 48 h. Other control ferrets were placed alone in a clean cage. Chemosensory-stimulated and unpaired control subjects were perfused 90 min after being placed in their respective cages. In both sexes mating augmented neuronal Fos-IR in the granular layer of the main olfactory bulb, the caudal thalamic central tegmental field, and the medial amygdala, regions situated early in the putative input pathway to mediobasal hypothalamic LHRH neurons. Neuronal Fos-IR was also increased in these same forebrain regions (the central tegmental field excluded) in both sexes after exposure to chemosensory cues alone. However, more central components of this input pathway, including the preoptic area, the bed nucleus of the stria terminalis, and the ventrolateral portion of the ventromedial hypothalamus as well as the mediobasal hypothalamic LHRH neurons themselves were activated by mating only in the female. In estrous females, exposure only to chemosensory stimuli from a breeding male augmented Fos-IR in the preoptic area and the ventrolateral portion of the ventromedial hypothalamus, but not in the bed nucleus of the stria terminalis or mediobasal hypothalamic LHRH neurons. In breeding males, exposure only to chemosensory cues from an estrous female failed to affect Fos-IR in any of these proximal components of the input pathway or in LHRH neurons themselves. These results suggest that the sex dimorphism in mating-induced LH secretion reflects a sex difference in the central processing of genital-somatosensory stimuli and possibly of chemosensory inputs as well.

Fos immunoreactivity in the rat brain following consummatory elements of sexual behavior: a sex comparison

In the present study a comparison was made between the distribution of Fos immunoreactivity in the brain of female and male rats following successive elements of sexual behavior. The distribution of Fos immunoreactivity following either mounting, eight intromissions or one or two ejaculations was compared with that in control animals. In both females, Fos immunoreactivity was induced in the medial preoptic nucleus, posteromedial part of the bed nucleus of the stria terminalis, posterodorsal part of the medial amygdala, and the parvicellular part of the subparafascicular thalamic nucleus. In addition, Fos immunoreactivity in females was induced in the ventrolateral part and the most caudoventral part of the ventromedial nucleus of the hypothalamus and in the premammillary nucleus. Differences between females and males were detected in the phases of sexual activity that resulted in Fos immunoreactivity in these brain areas, allowing more insight in the nature of the sensory and hormonal stimuli leading to the induction of Fos immunoreactivity. The posteromedial bed nucleus of the stria terminalis appears to be involved in chemosensory investigation, while specific distinct subregions are only activated following ejaculation. In addition, the parvicellular subparafascicular nucleus and the lateral part of the posterodorsal medial amygdala appear to be involved in the integration of viscero-sensory input. The neural circuitries underlying sexual behavior in males and females appear to be similar in terms of integration of sensory information. In males the medial preoptic nucleus may be regarded as the brain area where the integration of sensory and hormonal stimulation leads to the onset of male sexual behavior, while in females the ventrolateral part of the ventromedial hypothalamic nucleus appears to have this function. In addition, Fos immunoreactivity was distributed in distinct clusters in subregions with various brain areas in males and females. This was observed especially in the posteromedial bed nucleus of the stria terminalis and posterodorsal medial amygdala, but also in the parvicellular subparafascicular nucleus, ventromedial hypothalamic nucleus and ventral premammillary nucleus. It appears that relatively small subunits within these nuclei seem to be concerned with the integration of sensory and hormonal information and may play a critical role in sexual behavior.

FOS expression in grafted gonadotropin-releasing hormone neurons in hypogonadal mouse: mating and steroid induction

We used FOS expression, widely accepted as a marker for neuronal activation, to evaluate physiologically induced activation of gonadotropin-releasing hormone (GnRH) neurons within intraventricular preoptic area grafts in hypogonadal (hpg) female mice. Hpg mice lack endogenous GnRH due to a mutated gene, but can respond to grafted GnRH neurons with reproductive development. The purpose of this study was to determine the degree to which the host brain regulates grafted GnRH neurons. FOS expression in grafted GnRH neurons was induced in progesterone-primed female mice paired with sexually active males. The degree of sexual activity did not affect the outcome, with 40.9 +/- 12.2% of the grafted GnRH cells expressing FOS when male partners performed intromissions, and 47.5 +/- 10.2% when they also ejaculated. There was little or no FOS expression in the grafts of unprimed hpg mice paired with sexually active males, in unpaired mice primed with progesterone or sequential estradiol benzoate and progesterone, or in controls. The pattern of FOS expression in the brains of the female hpg mice engaged in mating behavior was similar to that reported in other species, with moderate to high expression in the medial preoptic area, ventromedial nucleus, and medial amygdala in females paired with males that ejaculated. The present results support the hypothesis that host-derived activation of grafted GnRH neurons underlies aspects of reproductive responses seen in hpg mice with grafts, and further, that at least a portion of the host-graft connectivity is steroid sensitive.

Hormonal and neurotransmitter regulation of GnRH gene expression and related reproductive behaviors

Gonadotropin-releasing hormone (GnRH), having a highly conserved structure across mammalian species, plays a pivotal role in the control of the neuroendocrine events and the inherent sexual behaviors essential for reproductive function. Recent advances in molecular genetic technology have contributed greatly to the investigation of several aspects of GnRH physiology, particularly steroid hormone and neurotransmitter regulation of GnRH gene expression. Behavioral studies have focused on the actions of GnRH in steroid-sensitive brain regions to understand better its role in the facilitation of mating behavior. To date, however, there are no published reports which directly correlate GnRH gene expression and reproductive behavior. The intent of this article is to review the current understanding of the way in which changes in GnRH gene expression, and modifications of GnRH neuronal activity, may ultimately influence reproductive behavior.

Sexual stimulation activates c-fos within estrogen-concentrating regions of the female rat forebrain

Regions of the brain that concentrate estrogen and progesterone are thought to regulate female sexual behavior by altering gene expression and neural sensitivity to afferent stimulation. We used immunocytochemistry and in situ hybridization to examine c-fos gene expression within estrogen-concentrating regions of the forebrain following various types of sexual stimulation with or without hormone treatment. Ovariectomized rats received injections of estradiol benzoate 48 h and progesterone 4 h before testing. Control rats that had been ovariectomized at least 5 months before testing did not receive hormone treatment. Rats were then either placed into bilevel testing chambers with sexually vigorous males, received manual stimulation of the flanks, received vaginocervical stimulation with a glass rod, or were left in their home cages. Copulation with intromission and ejaculation in hormone-treated rats, or stimulation of the vaginal cervix in both hormone-treated and control rats, produced a dramatic induction of c-fos mRNA and Fos-like immunoreactivity in estrogen-concentrating regions, such as the lateral septum, medial preoptic area, bed nucleus of the stria terminalis, paraventricular nucleus of the hypothalamus, ventromedial hypothalamus, lateral habenula, and medial amygdala, in addition to regions that do not readily concentrate estrogen, such as the neocortex, thalamus, and striatum. Mechanical stimulation of the flanks produced a smaller induction of Fos in these rats, whereas hormone treatment alone had no effect. These data demonstrate that afferent sensory stimulation, but not estrogen or progesterone, regulates c-fos gene expression within different estrogen-concentrating and non-concentrating regions of the female rat forebrain.

Sexual dimorphism in the effects of mating on the in vitro release of LHRH from the ferret mediobasal hypothalamus

A sexually dimorphic pattern in the secretion of luteinizing hormone (LH) has previously been shown to occur in response to mating in an induced ovulating species, the ferret, with mating augmenting the secretion of LH in females but not in males. The aim of this study was to determine whether this dimorphic pattern in the postcoital secretion of LH reflects a dimorphic effect of mating on the neural release of luteinizing hormone releasing hormone (LHRH). The effect of mating on the in vitro release of LHRH from mediobasal hypothalami (MBH) collected from breeding male and female ferrets was studied. Luteinizing hormone releasing hormone release and content were significantly reduced in tissues from estrous females sacrificed 0.25 h after mating compared to unpaired estrous females and estrous females sacrificed 1 or 2.6 h after the mating stimulus. By contrast, the release of LHRH from MBH fragments and LHRH tissue content were equivalent in breeding males that were sacrificed 0.25 h after mating and in breeding males that were left unpaired. These data suggest that the postcoital surge of LH in the female ferret is preceded by a release of LHRH that initially depletes neuronal terminals within the MBH, whereas LHRH release, like pituitary LH secretion, is minimally affected by mating in males.