Sex Differences in the Expression of Sex Steroid Receptor mRNA in the Quail Brain

In Japanese quail, males will readily exhibit the full sequence of male‐typical sexual behaviors but females never show this response, even after ovariectomy and treatment with male‐typical concentrations of exogenous testosterone. Testosterone aromatisation plays a key‐limiting role in the activation of this behavior but the higher aromatase activity in the brain of males compared to females is not sufficient to explain the behavioural sex difference. The cellular and molecular bases of this prominent sex difference in the functional consequences of testosterone have not been identified so far. We hypothesised that the differential expression of sex steroid receptors in specific brain areas could mediate this behavioural sex difference. Therefore, using radioactive in situ hybridisation histochemistry, we quantified the expression of the mRNA coding for the androgen receptor (AR) and the oestrogen receptors (ER) of the α and β subtypes. All three receptors were expressed in an anatomically discrete manner in various nuclei of the hypothalamus and limbic system and, at usually lower densities, in a few other brain areas. In both sexes, the intensity of the hybridisation signal for all steroid receptors was highest in the medial preoptic nucleus (POM), a major site of testosterone action that is related to the activation of male sexual behaviour. Although no sex difference in the optical density of the AR hybridisation signal could be found in POM, the area covered by AR mRNA was significantly larger in males than in females, indicating a higher overall degree of AR expression in this region in males. By contrast, females tended to have significantly higher levels of AR expression than males in the lateral septum. ERα was more densely expressed in females than males throughout the medial preoptic and hypothalamic areas (including the POM and the medio‐basal hypothalamus), an area implicated in the control of female receptivity) and in the mesencephalic nucleus intercollicularis. ERβ was more densely expressed in the medio‐basal hypothalamus of females but a difference in the reverse direction (males > females) was observed in the nucleus taeniae of the amygdala. These data suggest that a differential expression of steroid receptors in specific brain areas could mediate at least certain aspects of the sex differences in behavioural responses to testosterone, although they do not appear to be sufficient to explain the complete lack of activation by testosterone of male‐typical copulatory behaviour in females.     

Sex difference in the bed nucleus of the stria terminalis of the human brain.

A quantitative analysis of the volume of the darkly staining region of the posteromedial bed nucleus of the stria terminalis was performed on the brains of 26 age-matched male and female human subjects. We suggest the term "darkly staining posteromedial" component of the bed nucleus of the stria terminalis (BNST-dspm) to describe this sexually dimorphic region of the human brain. The volume of the BNST-dspm was 2.47 times greater in males than in females. This region in humans appears to correspond to an area of the bed nucleus of the stria terminalis in laboratory animals that exhibits volumetric and neurochemical sexual dimorphisms, concentrates gonadal steroids, and is anatomically connected to several other sexually dimorphic nuclei. Furthermore, the bed nucleus of the stria terminalis is involved in sexually dimorphic functions, including aggressive behavior, sexual behavior, and gonadotropin secretion, which are also influenced by gonadal steroids. Therefore, it is possible that in human beings as well, gonadal hormones influence the sexual dimorphism in the BNST-dspm and that this morphological difference, in part, underlies sexually dimorphic function.     

Sex and region-specific regulation of oestrogen receptor beta in the rat hypothalamus

Sexual dimorphism in the expression of oestrogen receptor (ER)beta mRNA and protein was characterized in the rostral forebrain of the rat and its dependence on the neonatal endocrine environment was revealed. We present novel data demonstrating, in gonadectomized adult rats, that the amount of oestrogen caused a significant reduction in the number of ERbeta messages and protein in the ventromedial nucleus in both sexes, but no such effects were detected in the preoptic area or the amygdala. In gonadectomized females, more so than in males, the ventromedial nucleus of the adult rat contained a significantly larger number of ERbeta-positive neurones both in terms of ERbeta mRNA and protein. In the juvenile rat on day 14, sex difference in ERbeta expression was already observed in the ventromedial nucleus. Treatment of neonatal females with oestrogen from days 1-10 or neonatal orchidectomy of males reversed the sex difference in the ventromedial nucleus when observed on day 14, showing that the neonatal presence of oestrogen had caused irreversible masculinization of this structure. Our results suggest that sex-specific expression of ERbeta is patterned by perinatal hormone exposure: down-regulation of ERbeta caused by oestrogen in a region-specific manner.     

Ovarian Steroid Regulation of Estrogen and Progesterone Receptor Messenger Ribonucleic Acid in the Anteroventral Periventricular Nucleus of the Rat

The anteroventral periventricular nucleus of the preoptic region (AVPV) represents a key site for hormonal feedback on gonadotropin secretion. It plays a critical role in the neural control of luteinizing hormone secretion and contains high densities of neurons that express receptors for estrogen and progesterone. In this study in situ hybridization was used to examine the expression of mRNAs encoding the estrogen (ER) and progesterone (PR) receptors in the AVPV during the estrous cycle. ER gene expression fluctuated during the cycle with the lowest levels of ER mRNA observed in animals killed on the afternoon of proestrus, and the highest levels present in animals killed during metestrus. This apparent inverse relationship between circulating levels of estradiol (E2) and ER mRNA levels in AVPV neurons was supported by the observation that treatment of ovariectomized rats with E2 suppressed expression of ER mRNA in the AVPV. The influence of progesterone (P4) on ER expression was less pronounced, but a significant increase in ER mRNA in the AVPV was detected 3 h after treatment with P4. In contrast, PR mRNA levels were highest in the AVPV during diestrus and lowest on the morning of proestrus suggesting that PR expression in the AVPV is regulated in a complex manner that may reflect the combined regulatory effects of E2 and P4. E2 treatment caused a dramatic induction of PR mRNA in the AVPV, but P4 did not affect PR mRNA expression acutely, although PR mRNA appears to be attenuated in the AVPV 27 h after P4 treatment. These findings suggest that ovarian steroid hormones regulate ER and PR gene expression in the AVPV during the estrous cycle, which may represent molecular events that contribute to cyclic changes in the responsiveness of AVPV neurons to steroid hormones.     

Neurotensin-lmmunoreactive Neurons in the Rat Medial Preoptic Area are Oestrogen-Receptive

The identity of neurons influenced by oestrogen is critical for the understanding of ovarian steroid actions in the brain. The medial preoptic area (MPOA) contains one of the largest oestrogen-receptive cell populations in the rat brain and participates in several oestrogen-dependent functions, including the regulation of luteinizing hormone (LH) secretion and sexual behaviour. Using double immunostaining procedures with antibodies specific for the oestrogen receptor and neurotensin, a neuropeptide implicated in the regulation of LH secretion within this area, we found that approximately half of the neurotensin-immunoreactive neurons in the MPOA also displayed immunoreactivity for the oestrogen receptor. We estimate that oestrogen-receptive neurotensin neurons represent 5% to 12% of all oestrogen receptor-positive cells in the MPOA. Our results provide morphological evidence that neurotensin mediates oestrogen-dependent mechanisms within the brain and suggest that oestrogen may act through preoptic neurotensin neurons to aid in the generation of the LH surge.     

The organization of neural inputs to the medial preoptic nucleus of the rat

There is general agreement that the medial preoptic nucleus (MPN) receives projections from widespread regions of the brain, although there are significant discrepancies in the literature with regard to certain specific inputs. Therefore, we have reexamined the inputs to this nucleus with both retrograde and anterograde axonal transport techniques. First, injections of the retrograde tracers true blue, SITS, or wheat germ agglutinin were made into the region of the MPN and the distribution of retrogradely labeled cells was charted. Then, autoradiographic material was used to confirm the results of the retrograde studies, to identify the route taken by fibers projecting to the MPN, and to describe the distribution of projections with respect to the three cytoarchitectonic subdivisions of the nucleus. The results indicate that the MPN receives inputs from widely distributed areas in both the forebrain and brainstem, and that these inputs appear to be distributed topographically within the three cytoarchitectonic subdivisions of the nucleus. Direct inputs to the MPN arise from all major areas of the hypothalamus (except for the median and magnocellular preoptic nuclei, the supraoptic and suprachiasmatic nuclei, and the medial and lateral mammillary nuclei). Projections from nuclei within the periventricular zone of the hypothalamus end primarily in the medial part of the MPN, while inputs from the lateral zone are mainly confined to the lateral part of the nucleus, as are projections from the nuclei within the medial zone, except for those from the anterior and ventromedial nuclei, which appear to be more widespread. The MPN receives major inputs from limbic regions including the amygdala, ventral subiculum, and ventral lateral septal nucleus, all of which end preferentially in the lateral part of the MPN. In contrast, the projection from the encapsulated part of the bed nucleus of the stria terminalis appears to end preferentially in the central part of the MPN and in immediately adjacent regions of the medial subdivision. In addition, the MPN may receive relatively sparse inputs from infralimbic and insular cortical areas, the nucleus accumbens, and the substantia innominata. Finally, ascending serotoninergic projections from the raphe nuclei appear to terminate principally in the lateral part of the MPN, whereas inputs from regions containing noradrenergic cell groups are chiefly distributed to the central and medial parts of the nucleus. Other brainstem regions that appear to provide modest inputs include the ventral tegmental area, central tegmental field, periaqueductal gray, pedunculopontine nucleus, and the peripeduncular nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)     

Fos Expression in the Rat Brain Following Vaginal-Cervical Stimulation by Mating and Manual Probing

Vaginal-cervical stimulation (VCS), provided by mating or manual probing, induces many reproductive behavioral and endocrine changes in female rats. These changes include an increase in lordosis duration, heat termination and pseudopregnancy. Electro-physiological and [¹⁴C]2-deoxy-D-glucose studies collectively show that neurons in the medial preoptic area, ventromedial hypothalamus and midbrain central gray respond to manual VCS. In the present study we immunocytochemically labeled brain sections for Fos, the protein product of the immediate early gene c-fos, to detect VCS-responsive neurons in hormone-primed animals receiving VCS by mating or manual probing. In Experiment 1, females receiving mounts and intromissions were compared to: 1) vaginally-masked females receiving mounts but no VCS, 2) females exposed to an intact anesthetized male or 3) females not exposed to males or the testing arena. Those animals receiving VCS showed a dramatic increase in the number of Fos-immunoreactive cells in the medial preoptic area, posterodorsal portion of the medial amygdala and bed nucleus of the stria terminalis, as well as the dorsomedial hypothalamus, ventromedial hypothalamus and midbrain central gray. These effects of VCS were confirmed in Experiment 2 in animals receiving manual vaginal-cervical probing. These findings extend previous electrophysiological and [¹⁴C]2-deoxy-D-glucose studies by providing evidence that additional brain areas respond to VCS by mating, as well as manual probing.     

Hormonal Control of Sex Differences in the Brain, Behavior and Accessory Sex Structures of Whiptail Lizards (Cnemidophorus Species)

The effects of steroid hormones on sexual dimorphisms in the brain, behavior and accessory sex structures were investigated in two species of whiptail lizards. The studies were conducted both in adults and hatchlings of a sexually reproducing species (Cnemidophorus inornatus) and an all-female species (C. uniparens) which displays ‘sexual’ behaviors typical of males and females. Adults were gonadectomized and approximately 3 months later given either a Silastic capsule filled with sex steroid or an empty capsule. Young animals of both species were left intact and given a capsule on the day of hatching. An additional group of C. uniparens was ovariectomized on the day of hatching. Following treatment, measures of oviduct (estrogen-dependent), renal sex segment (androgen-dependent) and wolffian duct (androgen-dependent) hypertrophy were taken in some experiments. Animals were also tested for sexual behavior in some of the studies. The volumes of the anterior hypothalamus-preoptic area and ventromedial hypothalamus were measured in each individual. Estrogen, testosterone and dihydrotestosterone stimulated peripheral structures at both time periods in both sexes and species. The hormones also stimulated courtship and copulatory behaviors in many of the adult animals. However, testosterone in the anterior hypothalamus-preoptic area of male C. inornatus was the only treatment which produced parallel effects on the volume of a brain area and the behaviors which it controls. These data add whiptail lizards to the list of species in which steroid hormones affect the volume of brain regions in adulthood, but suggest that such changes in morphology are not necessarily predictive of functional differences.     

Induction of Substance P-immunoreactivity by Estrogen in Neurons Containing Estrogen Receptors in the Anterovental Periventricular Nucleus of Female but not Male Rats

Effects of gonadal steroids on numbers of neurons containing estrogen receptor (ER) and/or substance P (SP) were examined in the anteroventral periventricular nucleus (AVPV) of female and male rats by double-labeling immunohistochemistry employing antibodies specific for ER and SP. Animals were gonadectomized and received subcutaneously either oil alone (Control group), sequential injections of estradiol benzoate and oil (EB + Oil group), or those of EB and progesterone (EB + P group). In the female control rat, a large population of ER-immunoreactive (IR) cells were found clustered throughout the AVPV. They were counted more than 2,000 in total of 4 sections in this nucleus. On the contrary, SP-IR neurons were scarcely observed in the same area of this group. Administration of estrogen to female animals decreased the total number of ER-IR cells to 67% of the control group. In contrast to the supressive effect of estrogen to its own receptor, it induced SP-IR neurons in the AVPV of the female. Approximately 50–80 SP-IR neurons were counted in the 4 sections, and 59% of these neurons expressed ER-IR material in their nuclei. In the female EB + P group, the number of ER-IR neurons also decreased to 79% of the control group. Although the number of SP-IR neurons in this group decreased to 32% of that in the EB + Oil group, a ratio of coexistence of ER-IR material in these neurons increased to 75%. The male control group contained a smaller population of ER-IR cells relative to the female control (1497 vs 2143). SP-IR neurons were rarely observed as were in the female control. Administration of estrogen to the male also decreased the number of ER-IR cells in a manner similar to that in the female. However, unlike the female, the steroid failed to induce the SP-IR neurons in the male. These results demonstrate sexual dimorphism in the AVPV not only in the number of ER-IR neurons but also in the responsiveness of SP neurons to estrogen. They further provide anatomical evidence that a subset of SP neurons are regulated by estradiol in estrogen sensitive neurons in the female rat. The data also suggest that this peptide is involved in mechanisms of luteinizing hormone surge by mediating actions of gonadal steroids in the AVPV.     

Formation of Neurons in the Sexually Dimorphic Anteroventral Periventricular Nucleus of the Preoptic Area of the Rat: Effects of Prenatal Treatment with Testosterone Propionate

An examination was made of neurogenesis in the anteroventral periventricular nucleus (AVPv) of the preoptic area of the rat using bromodeoxyuridine (BrdU), a thymidine analog, and a BrdU-specific antibody. Cells in the AVPv of adult rats were labeled with the antibody when BrdU was injected into pregnant rats once during day 13 to 18 of gestation, but not during day 10 to 12 nor 19 to 20 of gestation nor on postnatal day 1, indicating that neurogenesis of the AVPv occurs during a limited period from day 13 to 18 of gestation. Next, to examine the effects of androgen on neurogenesis, BrdU was injected once on day 15 into pregnant rats that also received injections of testosterone propionate (TP). The number of BrdU-labeled cells in the AVPv was similar in control female and male fetuses and female fetuses from pregnant rats that received daily injections of TP during days 14 to 16, when fetuses were examined on day 17 of gestation. These results suggest that the neurogenesis that was recognized by labeling with BrdU was not affected by the treatment with TP. On day 21 of gestation, BrdU-labeled cells in the AVPv of control male fetuses and female fetuses that received TP during days 14 to 18 were fewer in number than those in female fetuses of the control group, whereas treatments with TP during days 14 to 16 and during days 17 to 18 did not cause any significant decrease in number of BrdU-labeled cells. These findings support the hypothesis that elimination of a population of cells, for example, by cell death as described previously, is enhanced in male fetuses and in female fetuses treated with TP repetitively.     

The sex hormone-dependent development of opiate receptors in the rat medial preoptic area

The opiate receptor content of the sexually dimorphic medial preoptic area (MPOA) was examined in newborn and 5-day-old (D6) male and female rats. A significant increase of [³H]naloxone binding was observed in and around the sexually dimorphic nucleus of the preoptic area (SDN-POA) in D6 female rats, relative to newborn females. Opiate receptor labeling did not increase over this period in males, nor was labeling different between males and females at birth. This dramatic alteration of MPOA opiate receptor content was observed to occur in either sex in the absence of testosterone postnatally; that is, neonatally-castrated males exhibited the same increase of labeling by D6 as did normal females. Conversely, daily postnatal testosterone treatment of females from birth to D6 resulted in the development of male-like MPOA opiate receptor pattern. The sex hormone-dependence of MPOA opiate receptor development is discussed in relation to the sex hormone-dependent ontogeny of SDN-POA structure. The overlap of critical periods for the development of these structural and chemical sexual dimorphisms suggests a role for endogenous opioids in modulating MPOA development.     

Neurotransmitter specificity of cells and fibers in the medial preoptic nucleus: an immunohistochemical study in the rat

The medial preoptic nucleus (MPN) is a sexually dimorphic complex with three major subdivisions. The cell-dense central (MPNc) and medial (MPNm) subdivisions are larger in male rats, while the cell-sparse lateral subdivision (MPNl) occupies a majority of the nucleus in females. In the present study we evaluated the distribution of possible monoaminergic and peptidergic cells and fibers within the MPN, as well as in adjacent regions of the medial preoptic area of the adult male rat. For this, we used an indirect immunohistochemical method with antisera to serotonin (5HT), dopamine beta-hydroxylase (DBH), tyrosine hydroxylase (TH), neuropeptide Y (NPY), cholecystokinin (CCK), vasoactive intestinal polypeptide (VIP), substance P (SP), neurotensin (NT), corticotropin-releasing factor (CRF), luteotropin-releasing hormone (LRH), somatostatin (SS), thyrotropin-releasing hormone (TRH), oxytocin (OXY), vasopressin (VAS), adrenocorticotropic hormone (1-24; ACTH), alpha-melanocyte-stimulating hormone (alpha-MSH), leucine-enkephalin (L-ENK), and calcitonin gene-related peptide (CGRP). The results suggest that cell bodies and/or fibers crossreacting with all of these putative neurotransmitters are differentially distributed within the MPN. Within the MPNm, the densest plexuses of fibers were stained with antisera to SP and NPY, while moderate densities of fibers were stained with anti-DBH, SS, CCK, CGRP, ACTH, and alpha-MSH, and only a few fibers were stained with anti-5HT, TH, NT, VAS, and L-ENK. Moderate numbers of SP- and L-ENK-immunoreactive cell bodies, and a few SS-, NT-, CRF-, and TRH-stained cell bodies were also found within the MPNm. The MPNc contained a dense plexus of CCK-immunoreactive fibers, as well as a few CRF-immunoreactive fibers. Both fiber types were localized almost exclusively to this subdivision, while most of the others studied here appeared to avoid it selectively. This suggests that there are relatively few inputs to the MPNc, and that they tend to avoid other parts of the nucleus, although moderate densities of DBH- and NPY-immunoreactive fibers were found in both the MPNm and MPNc. The MPNc contained several CCK-immunoreactive cell bodies as well as a moderate number of TRH-stained cell bodies. Both cell types were nearly completely localized to the MPNc. The major inputs to the MPNl studied here appear to be stained with antisera to 5HT and L-ENK, although moderate numbers of NT- and CRF- immunoreactive fibers were also found in this part of the nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)     

Estrogen configures sexual dimorphism in the preoptic area of C57BL/6J and ddN strains of mice

Immunohistochemistry using a calbindin D28k antibody revealed a marked sex difference in neuronal distribution in the central portion of the medial preoptic area in C57BL/6J and ddN strains of mice when the animals were sacrificed on D65 (D1 = the day of birth). Male mice had a distinct ellipsoidal cell aggregate, whereas females lacked such a structure. This sex difference was not observed in Nissl‐stained sections. Co‐localization of calbindin D28k and the neuron‐specific nuclear protein NeuN confrmed that the cells in the aggregate were neurons. The aggregates were larger in males than in females in both strains. When observed on D65, males orchidectomized on D1 had smaller aggregates. However, daily injections of 2 μg estradiol benzoate through D1–D5 as well as a single injection of 100 μg testosterone propionate on D1 enlarged the aggregates in females, but a single injection of 100 μg dihydrotestosterone on D1 had no effect on the female phenotype. Similar endocrine manipulations had no effects in adult animals of both sexes. Thus, the calbindin‐immunoreactive cell aggregates in the preoptic area of C57BL/6J and ddN mice are homologous to the sexually dimorphic nucleus of the rat preoptic area in terms of the morphology and sex steroid‐dependent organization. J. Comp. Neurol. 518:3618–3629, 2010. © 2010 Wiley‐Liss, Inc.     

Effects of Paced Mating on c-fos Gene Expression in the Female Rat Brain

When estrous female rats control or pace (P) their sexual contacts with males, several neuroendocrine and behavioral responses to mating occur that are not observed or are greatly attenuated after nonpaced mating. The present study examined whether the distribution and amount of FOS immunoreactivity (FOS-IR) induced in brain by mating would be altered in females receiving paced rather than nonpaced mating stimulation. In the first experiment, females received 5 or 15 intromissions during paced mating tests (5P and 15P), 5 or 15 intromissions during nonpaced mating tests (5NP and 15NP), 15 mounts-without-intromission (MO) or remained in their homecages (HC). Selective increases 1 h after paced mating stimulation were observed in the posterodorsal medial amygdala (MePD), where significantly more FOS-IR cells were present in the 5P and 15P groups than in the respective NP groups. The 5P, 5NP and 15NP had significantly more FOS-IR than the HC, MO, and 5NP groups, and the 5P group had levels of FOS-IR which were equivalent to that seen in the 15NP group. In the posteromedial portion of the bed nucleus of the stria terminalis (BNSTpm) and the ventrolateral portion of the ventromedial nucleus of the hypothalamus (VMHvl), paced mating induced significantly greater numbers of FOS-IR cells than did either MO or HC treatments; increases induced by nonpaced mating were not statistically greater than HC controls. No differences between groups were seen in the medial preoptic area (mPOA). In the second experiment, experimentally lengthening the interintromission interval (III) as well as increasing the intromission duration to mimic the characteristics of paced mating, resulted in significant increases in FOS-IR in the MePD but not in the other three brain regions. These results demonstrate that paced mating is more effective in inducing c-fos expression than nonpaced mating, and that the MePD is particularly sensitive to differing characteristics of the mating stimuli received.     

Mating-lnduced FOS-Like Immunoreactivity in the Rat Forebrain: A Sex Comparison and a Dimorphic Effect of Pelvic Nerve Transection

Previous research has shown that mating induces the expression of the immediate-early gene, c-fos, as detected by the increased presence of nuclear FOS-like immunoreactivity (FOS-IR), in specific forebrain regions of both male and female rats. In the male both olfactory/vomeronasal (O/V) and genital/somatosensory (G/S) inputs appear to contribute to the neural FOS response to mating whereas in the female G/S input carried by the pelvic nerves appears to mediate the forebrain FOS response. To date, however, no direct sex comparison of the mating-induced forebrain FOS response has been made in rats maintained under the same steroidal conditions nor has the contribution of afferent sensory input from the pelvic nerves been assessed in males. We first compared the level of FOS-IR in brain regions of mated and unpaired gonadectomized male and female rats given 5μg/kg estradiol benzoate (EB) for 7 days and 500 μg progesterone (P) 4 h prior to testing. One h after experiencing 1 ejaculation, both sexes showed increased FOS-IR in the medial preoptic area (MPOA), bed nucleus of the stria terminalis (BNST), medial amygdala, the ventro-lateral portion of the ventromedial nucleus of the hypothalamus (VMN), and the midbrain central tegmental field (CTF). This increase was significantly greater in the MPOA and medial amygdala of mated females than of males. Bilateral transection of the pelvic nerves significantly attenuated the increase in FOS-IR after mating in the CTF of male rats and in the MPOA, BNST, VMN, medial amygdala and CTF of females. Thus, following mating there is no sex difference in the brain regions which express c-fos, but there is a dimorphism in the contribution of afferent information conveyed by the pelvic nerves to the mating-induced FOS response. The neural FOS response of the female to mating is heavily dependent upon the G/S afferent inputs carried by the pelvic nerves whereas the male's neural c-fos response may depend on O/V input plus G/S input conveyed via other afferent pathways such as the pudendal nerves.     

The sexually dimorphic region of the preoptic area in rats contains denser opiate receptor binding sites in females

Quantitative autoradiographic analysis of opiate receptor binding using [³H]naloxone shows higher levels in the sexually dimorphic region of the medial preoptic area in female rats than in males. Opiate receptor density varies across the estrous cycle being densest in diestrous females. The sexually dimorphic nucleus of the preoptic are lies within the opiate receptor-rich region. Endogenous opiates in the medial preoptic region acting at opiate receptors which are of differential density in males and females could influence sex-specific behavior mediated by the region.     

Cytoarchitectonic analysis of the SDN-POA of the intact and gonadectomized rat

The densely staining group of cells referred to as the sexually dimorphic nucleus of the preoptic area (SDN-POA) is greater in volume in the male than in the female rat. Because we and others have reported absolute volumes that have been consistent within individual studies but that vary considerably, we characterized the SDN-POA by describing its morphology with respect to the cytoarchitectonic divisions of the medial preoptic nucleus (MPN) in intact and gonadectomized rats. We report three major findings: the SDN-POA is heterogeneous and is composed of cells belonging to three distinct cytoarchitectonic divisions; the cytoarchitecture of the MPN and its medial and lateral divisions (MPNm and MPNl, respectively) in male rats appear to be influenced by the hormonal status in adulthood; and a small anteroventral division of the MPN (MPNav) is present in males but virtually absent in females. Specifically, the SDN-POA is located within the MPNm, but consists of subcomponents located within the central division of the MPN (MPNc), the MPNav, and part of the MPNm-exclusive of the MPNc and MPNav. The percentage of the total SDN-POA located within the MPNc and MPNav. The percentage of the total SDN-POA located within the MPNc and MPNav was greater in males, and that in the MPNm-exclusive of the MPNc and MPNav was greater in females, indicating that the SDN-POA has a different cytoarchitectonic composition in the two sexes. Gonadectomy produced no significant differences in SDN-POA volume, but the MPN, MPNl, and MPNm were significantly reduced in gonadectomized versus intact males, suggesting an activational effect of testicular hormones on these structures.