Early estrogen-induced nuclear changes in rat hypothalamic ventromedial neurons: an ultrastructural and morphometric analysis

Sex Differences in the Epigenome: A Cause or Consequence of Sexual Differentiation of the Brain?

Females and males display differences in neural activity patterns, behavioral responses, and incidence of psychiatric and neurological diseases. Sex differences in the brain appear throughout the animal kingdom and are largely a consequence of the physiological requirements necessary for the distinct roles of the two sexes in reproduction. As with the rest of the body, gonadal steroid hormones act to specify and regulate many of these differences. It is thought that transient hormonal signaling during brain development gives rise to persistent sex differences in gene expression via an epigenetic mechanism, leading to divergent neurodevelopmental trajectories that may underlie sex differences in disease susceptibility. However, few genes with a persistent sex difference in expression have been identified, and only a handful of studies have employed genome-wide approaches to assess sex differences in epigenomic modifications. To date, there are no confirmed examples of gene regulatory elements that direct sex differences in gene expression in the brain. Here, we review foundational studies in this field, describe transcriptional mechanisms that could act downstream of hormone receptors in the brain, and suggest future approaches for identification and validation of sex-typical gene programs. We propose that sexual differentiation of the brain involves self-perpetuating transcriptional states that canalize sex-specific development.

Effects of gonadotropin releasing hormone and estradiol on c-fos expression in the rat hippocampus

The effects of ovariectomy, estradiol, and gonadotropin releasing hormone (GnRH) on the protein synthetic activity of hippocampal neurons were studied with immunohistochemistry for the proto-oncogene c-fos. Ovariectomy caused a reduction in the number of c-fos-positive neurons to 16% in area CA(1) and to 25% in area CA(3) when compared to that in the intact control animal. The dentate gyrus was only slightly affected. The decline in the number of c-fos-immunoreactive neurons in the Ammon's horn was partially reversed by a single intravenous injection of estradiol which resulted in the expression of c-fos in 71% of the neurons in area CA(1) and 74% in CA(3) when compared to the numbers of positive cells in the control animals. Similarly, intracerebroventricular injections of 10 muM GnRH caused an increase in the number of c-fos-positive cells to 68% in area CA(1) and to 52% in area CA(3) compared to that in the control animals. The induction of c-fos synthesis after estradiol and GnRH was transient and reached a maximum after 1 to 2 h before it declined to pretreatment levels after 8 h. The results suggest that both estradiol and GnRH exert specific effects on protein synthesis in certain neurons of the hippocampus and that these effects of the hormones are, at least in part, mediated by c-fos.

Estrogen modulates the sexually dimorphic synaptic connectivity of the ventromedial nucleus

Neurons in the ventrolateral division of the hypothalamic ventromedial nucleus (VMNvl) display a remarkable estrogen-dependent functional and structural plasticity, which is likely to be mediated, in part at least, by neuronal afferents. The present study was designed to determine whether the number of synapses per neuron and the size of individual synapses in the VMNvl vary across the estrus cycle and, also, whether they differ between the sexes. To accomplish this, the VMNvl of adult female rats at proestrus or diestrus day 1 and of age-matched male rats was analyzed using electron microscopy. We found that a single VMNvl neuron receives around 7,000 synapses during diestrus and approximately 10,000 during proestrus. This estrus cycle-related variation is accounted for by increases in the number of all types of synapses. In males, the number of synapses received by each VMNvl neuron is similar to that of diestrus rats (approximately 7,500). However, in males the number of axodendritic and axospinous synapses is smaller than in proestrus rats, whereas the number of axosomatic synapses is higher than in diestrus rats. In addition, we found that the size of the postsynaptic densities of axospinous and axosomatic synapses is consistently larger in males than in females. Our results show that the synaptic organization of the VMNvl is sexually dimorphic, with females having more dendritic synapses and males more somatic synapses. They also show that the synaptic plasticity induced by estrogen in the VMNvl is characterized by changes in the number, but not the size, of the synapses.

Neuronal organelles and nuclear pores of hypothalamic ventromedial neurons are sexually dimorphic and change during the estrus cycle in the rat

Neurons in the ventrolateral division of the hypothalamic ventromedial nucleus (VMNvl) become hypertrophied when exposed to high estrogen levels, an effect that has been observed after estrogen treatment of ovariectomized rats as well as during the proestrus stage of the ovarian cycle. In an attempt to examine whether the neuronal hypertrophy noticed in these conditions reflects metabolic activation of the neurons we have examined, using quantitative methods, the cytoplasmic organelles involved in protein synthesis and the nuclear pores of VMNvl neurons from females on proestrus, when estrogen levels are high, and on diestrus, when estrogen levels are low. Because VMNvl neurons are sexually dimorphic with respect to their size we have performed, in parallel, similar analyses in neurons from age-matched male rats. Our results show that the volume and the surface area of the rough endoplasmic reticulum (RER) and Golgi apparatus are increased at proestrus. They also show that the density of nuclear pores is greater in males than in females whereas the volume and the surface area of the RER and Golgi apparatus are sexually dimorphic only at specific phases of the ovarian cycle: the male-female differences are notorious in the RER when females are on diestrus and in the Golgi apparatus when they are on proestrus. Given that the size of the RER and of the Golgi apparatus correlates with the level of neuronal protein synthesis, data obtained in this study suggest that the sex-related differences and the estrus cycle variations in neuronal size reflect corresponding differences and fluctuations in the metabolic activity of VMNvl neurons.

CD4-positive T cell-mediated neuroprotection requires dual compartment antigen presentation

Our laboratory discovered that CD4-positive (CD4+) T cells of the immune system convey transitory neuroprotection to injured mouse facial motoneurons (FMNs) (Serpe et al., 1999, 2000, 2003). A fundamental question in the mechanisms responsible for neuroprotection concerns the identity of the cell(s) that serves as the antigen-presenting cell (APC) to activate the CD4+ T cells. Here, we first establish that CD4+ T cells reactive to non-CNS antigen fail to support FMN survival and, second, demonstrate a two-compartment model of CD4+ T cell activation. Mouse bone marrow (BM) chimeras were developed that discriminate between resident antigen-presenting host cell and BM-derived antigen-presenting donor cell expression of major histocompatibility complex II within central and peripheral compartments, respectively. After facial nerve transection, neither compartment alone is sufficient to result in activated CD4+ T cell-mediated FMN survival. Rather, CD4+ T cell-mediated neuroprotection appears to depend on both resident microglial cells in the central compartment and a BM-derived APC in the peripheral compartment. This is the first in vivo report demonstrating a neuroprotective mechanism requiring APC functions by resident (i.e., parenchymal) microglial cells.

Natural killer cells do not mediate facial motoneuron survival after facial nerve transection

The goal of the current study was to determine if natural killer (NK) cells mediate facial motoneuron (FMN) survival following injury. Wild-type (WT), perforin/recombinase activating gene-2 knockout (pfp/RAG-2 KO), and common gamma-chain (gammac)/RAG-2 KO mice received a right facial nerve axotomy. In WT mice, FMN survival was 86+/-1.0% relative to the contralateral control side. In contrast, pfp/RAG-2 and gammac/RAG-2 KO mice exhibited significant decreases in FMN survival ( approximately 20% and approximately 30%, respectively), relative to WT. Reconstitution of pfp/RAG-2 and gammac/RAG-2 KO mice with normal NK cells alone, failed to restore FMN survival levels to those of WT, but did restore functional lytic activity against YAC-1 cells. Reconstitution of pfp/RAG-2 and gammac/RAG-2 KO mice with splenocytes, and pfp/RAG-2 KO mice with CD4+ T-lymphocytes alone or in combination with NK cells, restored FMN survival levels to those of WT. Thus, NK cells appear to not be a component of immune cell-mediated rescue of motoneurons from axotomy induced cell death.

CD4+ T, but not CD8+ or B, lymphocytes mediate facial motoneuron survival after facial nerve transection

The capacity of facial motor neurons (FMN) to survive injury and successfully regenerate is substantially compromised in immunodeficient mice, which lack T and B lymphocytes (). The goal of the present study was to determine which T cell subset (CD4+ and/or CD8+), and whether the B lymphocyte, is involved in FMN survival after nerve injury. All mice were subjected to a right facial nerve axotomy, with the left (uncut) side serving as an internal control. FMN survival, of the right (cut) side, was measured 4 weeks post-operative, and expressed as a percentage of the left (uncut) control side. FMN survival in wild-type mice was 86%+/-1.5. In contrast, FMN survival in CD4 KO mice was 60%+/-2.0. Reconstitution of either CD4 KO mice, or recombinase activating gene-2 knockout (RAG-2 KO) mice (which lack functional T and B cells) with CD4+ T cells alone restored FMN survival to wild-type levels (85%+/-1.2 and 84%+/-2.5, respectively). There was no difference in FMN survival between wild-type, CD8 KO and MmuMT (B cell deficient) mice. Reconstitution of RAG-2 KO mice with CD8+ T cells alone, or B cells alone, failed to restore FMN survival levels (65%+/-1.5 and 63%+/-1.0, respectively). It is concluded that, of the population of FMN that do not survive injury, CD4+ T lymphocytes, but not CD8+ T lymphocytes or B cells, mediate FMN survival after peripheral nerve injury.

Hormonal and genetic influences underlying arousal as it drives sex and aggression in animal and human brains

Estrogen treatment induces transcription and increases excitability and reproductive behavior. Estrogens provide the structural basis for increased synaptic activity and greater behavior-facilitating output. Administration of progesterone amplifies the effect of estrogens on mating behavior. The role of GnRH is to synchronize reproductive behavior with the ovulatory surge of LH. A causal connection can be charted from one individual gene to human social behavior, but only via six causal links. Glia, meninges and neurons may participate, under the influence of sex hormones, in the direction of sex behavior. Neural and genetic mechanisms for motivation may lead to biological understanding of functions that apply to the most primitive aspects of human mental functioning. With respect to aggression, besides testosterone and its metabolites, serotonergic projections to the forebrain play an important role.

Combination of gonadal steroid treatment and peripheral nerve grafting results in a peripheral motoneuron-like pattern of beta II-tubulin mRNA expression in axotomized hamster rubrospinal motoneurons

Rubrospinal motoneurons (RSMN) represent a population of androgen receptor-containing central motoneurons in rodents. In this study, the ability of testosterone propionate (TP), alone or in conjunction with a peripheral nerve graft (PNG), to alter the molecular program of injured RSMN was accomplished using betaII-tubulin cDNA probes and quantitative in situ hybridization (ISH). Initial fluoro-gold labeling experiments following a T1 hemisection established that, as in the rat, the hamster rubrospinal system is essentially crossed and that injured RSMN concentrate in the ventrolateral region of the red nucleus. In the second experimental series, adult gonadectomized male hamsters were subjected to a right T1 hemisection, with half of the operated animals immediately subcutaneously implanted with 1 10 mm TP Silastic capsule and the other half sham implanted. In a third experimental series, animals were subjected to T1 hemisection, followed by transplantation of a predegenerated autologous segment of peripheral nerve. Half of the animals in each group received TP implants at the time of spinal cord injury and PNG. Postoperative times were 2, 7, and 14 days (dpo). Quantitative ISH was performed using a betaII-tubulin-specific (33)P-labeled cDNA probe, emulsion autoradiography, and computerized image analysis for grain counting. Injury alone resulted in a short-lived increase in betaII-tubulin mRNA expression in the RSMN at 2 dpo, with a significant decline to well below control values at 7 and 14 dpo. TP treatment or PNG alone attenuated, but did not prevent, the down-regulation of betaII-tubulin mRNA. In contrast, the combination of TP with a PNG sustained the injury-induced increase in betaII-tubulin mRNA levels throughout the postoperative period of 2, 7, and 14 dpo. The synergistic effects of the two treatment strategies confirm the importance of targeting multiple aspects of the injury response for therapeutic intervention.

Testosterone treatment attenuates the effects of facial nerve transection on glial fibrillary acidic protein (GFAP) levels in the hamster facial motor nucleus

Testosterone propionate (TP) administration coincident with facial nerve injury accelerates the recovery rate from facial muscle paralysis in the hamster. One mechanism by which TP could augment peripheral nerve regeneration is through glial fibrillary acidic protein (GFAP) regulation in the facial motor nucleus. In a previous study, axotomy alone induces increases in GFAP mRNA. with TP significantly attenuating the axotomy-induced increases in GFAP mRNA. In the present study, immunoblotting techniques were used to extend our previous GFAP mRNA studies to the protein level. Castrated male hamsters were subjected to a right facial nerve transection, with half of the animals receiving subcutaneous implants of 100% crystalline TP. The left facial motor nucleus of each animal served as an internal control. Postoperative survival times include Days 4, 7, and 14. In non-TP-treated animals, facial nerve transections alone increased GFAP levels at all time points, relative to internal controls. As previously observed at the mRNA level, TP treatment attenuated but did not eliminate the axotomy-induced increase in GFAP levels at all time points tested. These results suggest that the regulatory actions of gonadal steroids on GFAP expression manifested in parallel at the mRNA/protein levels.

Oestrogen-dependent tracing in the rat CNS after pseudorabies virus infection

This study examines the hypothesis that neuronal infectivity and the spreading of the pseudorabies virus (PRV) through the synapses in the central nervous system (CNS) are influenced by the oestrogen levels. The arcuate nucleus (ARC) and the subfornical organ (SFO) were chosen as models for analysis; the neurons in both structures possess oestrogen receptors and are mutually connected. A genetically engineered pseudorabies virus (Ba-DupLac) was used as a transneuronal tract tracer. This virus is taken up preferably by axon terminals, and transported very specifically through the synapses in a retrograde manner. Ba-DupLac was injected into the ARC of rats, followed by monitoring of the PRV-immunoreactivity (PRV-IR) in the SFO 72 h following inoculation. We found no PRV immunolabelling in the SFO of ovariectomized (OVX) rats, or in those OVX animals that received oestrogen shortly (4 h) before PRV infection (OVX + E 4 h). In contrast, in those OVX animals that received oestrogen 12 h before PRV infection (OVX + E 12 h), and also in intact control animals, PRV-IR was demonstrated in the SFO in all cases. Surprisingly, a reverse labelling was observed in the OVX rats; PRV-IR appeared in the pyriform cortex, whereas PRV-IR could not be detected in the control and OVX + E 12 h animals. As far as we are aware, this is the first study to demonstrate that transneuronal PRV labelling depends on the effects of oestrogen on certain CNS structures and connections.

Differential effects of the aging process on the morphology of the hypothalamic ventromedial nucleus of male and female rats

We investigated the effects of aging on the neuroanatomical sex dimorphisms of the rat hypothalamic ventromedial nucleus (VMN). Stereological methods were used to estimate the volume of the VMN and the total number and size of its neurons, including their dendritic trees, in males and females aged 6 and 24 months. No cell loss was detected in aged rats. However, in aged females, the volume of the VMN and the somatic size of its neurons were increased, whereas in aged males the dendritic spine density was augmented. Due to these gender-specific effects, the male-female differences observed in the morphology of the VMN in adult rats were annulled in aging. Our findings support the notion that some structural sex dimorphisms in the brain are not stable throughout the life span.

Pregnancy and post partum: changes in cognition and mood

Steroidal hormones are increasingly recognized as highly relevant in multiple aspects of brain functioning. While basic science has actively worked to advance understanding of fundamental steroid mechanisms within the brain, investigation of the neurobehavioral outcomes of reproductive hormone actions on the human brain has received less attention. We argue that the dramatic steroidal hormone changes seen in human reproduction must be systematically studied and may provide novel explanations of cognitive and mood disorders associated with reproductive events. This chapter provides a review of current literature establishing a role for a variety of steroids on neuroactivity, and evidence from a variety of observational and experimental paradigms linking hormones and clinical aspects of cognition and mood in humans. The specific hormonal changes of pregnancy are described and discussed in relation to concomitant alterations in cognition and mood across the peri-natal period. A review of studies that have systematically observed cognitive and affective changes both during pregnancy and the post-partum period is presented, as well as new data that follow a small cohort of women for an extended period of time after delivery. We conclude that women may show specific areas of cognitive changes during and after pregnancy, notably deficits in verbal learning and memory. Mood appears to be impacted as well. While steroidal hormones show a pattern of associations with mood during and after pregnancy, no such pattern is evident for cognition. The embryonic state of our knowledge regarding reproductive hormones and neurobehavioral functioning is evident, as are the scientific and public health reasons to redress this lacuna.

Alterations of medial preoptic area neurons following pregnancy and pregnancy-like steroidal treatment in the rat

There is a marked increase in the maternal behavior displayed by a female rat following pregnancy-due primarily to exposure to the gonadal hormones progesterone and estradiol (P and E(2), respectively). We examined Golgi-Cox silver-stained, Vibratome-sectioned neurons visualized and traced using computerized microscopy and image analysis. In Part One, we examined the hormonal-neural concomitants in the medial preoptic area (mPOA), an area of the brain that regulates maternal behavior, by comparing cell body size (area in microm(2); also referred to as soma and perikaryon) in the mPOA and cortex of five groups (n = 4-6/group) of ovariectomized (OVX-minus), diestrous, sequential P and E(2)-treated (P+E(2)), late-pregnant, and lactating rats; for Part Two, we examined a subset of mPOA neurons, which were traced in their entirety, from these same subjects. In Part One, whereas there was no difference between OVX-minus and diestrous females, both had smaller somal areas compared to OVX+P+E(2)-treated and late-pregnant females. The area of the soma returned to diestrous/OVX-minus levels in the lactating females. We found no change among the five groups in area of cell body in cortical neurons, which generally lack steroid receptors. In Part Two, which included a more detailed morphometric analysis of mPOA neurons, we examined several additional measures of dendritic structure, including number of proximal dendritic branches (the largest proximal dendrite was defined as the one with the largest diameter leaving the soma); cumulative length of the largest proximal dendrite; area of the cell body; number of basal dendrites; cumulative basal dendritic length; number of basal dendritic branches; and branch-point (distance from cell body to first branch of largest proximal dendrite). Again, we found similar effects on cell body size as in Part One, together with effects on number of basal dendritic branches and cumulative basal dendritic length in pregnant and P+E(2)-treated groups compared to OVX, diestrous, and lactating. An increase in somal area denotes increased cellular activity, and stimulatory effects on additional neuronal variables represents modifications in information processing capacity. Pregnancy and its attendant hormonal exposure, therefore, may stimulate neurons in the mPOA, which then contribute (in an as yet undetermined manner) to the display of maternal behavior. During the postpartum lactational period, when cues from pups primarily maintain maternal attention, the neuronal soma appears to return to a pre-pregnancy, non-hormonally dependent state, whereas other aspects of the dendrite remain altered. Collectively, these data demonstrate a striking plasticity in the brains of females that may be reflected in modifications in behavior.

Influence of sex and estrus cycle on the sexual dimorphisms of the hypothalamic ventromedial nucleus: stereological evaluation and Golgi study

Neurons in the ventromedial nucleus of the hypothalamus (VMN) display structural and biochemical sex differences in response to estrogen. Despite this fact, reports on sex differences in the morphology of the VMN are restricted to its volume and synaptic patterning. The aim of this study was to characterize the neuroanatomical sexual dimorphisms in the VMN and to investigate whether endogenous changes in ovarian steroid secretion influence such dimorphisms. The VMN of adult male rats and intact, aged-matched female rats killed on proestrus and diestrus day 1 was examined by using stereological methods applied to conventionally stained sections and Golgi-impregnated material. The VMN contained 55,000 neurons in rats of both sexes, but its volume was, on average, 1.25 times larger in males than in females. The volume was greater in proestrus than in diestrus rats due to parallel changes in the neuronal somatic size. Unlike the dorsomedial division, neurons in the ventrolateral division had longer dendritic trees in proestrus than in diestrus females and males. The spine density was consistently higher in females than in males in both VMN divisions. In addition, in the ventrolateral part the magnitude of the sex differences varied across the estrus cycle, and reached the greatest value when females were in proestrus. The volume of the neuropil was significantly larger in males than in females, and was not affected by the estrus phase. Our results reveal that the magnitude of the neuroanatomical sex differences in the VMN vary across the estrus cycle due to the trophic influence of estrogen upon its neurons. They also show that the fundamental sex difference in the structure of the VMN is accounted for by the neuropil components.

Estrogen-induced remodeling of hypothalamic neural circuitry

For decades, sexual behavior has been a valuable model system for behavioral neuroscientists studying the neural basis of motivated behaviors. One striking example of a change in motivation is the binary switch in sexual receptivity that occurs during the estrous cycle in female rats. Investigations of the neural basis of this change in behavior have fundamentally advanced our understanding of both behaviorally relevant neural pathways and basic mechanisms of steroid action in the brain. These advances have made this behavioral model system a staple of neuroendocrinology. A challenge that remains before us, given our current understanding of the circuitry and chemistry, is to develop a coherent model of how neural plasticity in the hypothalamus contributes to the dependence of this behavior on motivational state. This review will focus on the ventromedial nucleus of the hypothalamus, especially its ventrolateral subdivision. First, the anatomical, neurochemical, and functional aspects of the macro- and microcircuitry of this brain region will be discussed, followed by a discussion of the likely mechanisms of estrogen action within the ventrolateral VMH. Then, the evidence for estrogen-induced neural plasticity will be considered, including a comparison with the effects of estrogen on synaptic organization in other brain regions. Finally, a working model of neural plasticity within the ventrolateral VMH microcircuitry will be presented as a starting point for future experiments to verify or, more likely, revise and expand.

Androgenic regulation of the central glia response following nerve damage

Current research on the effects of gonadal steroids on the brain and spinal cord indicates that these agents have profound trophic effects on many aspects of neuronal functioning, including cell survival, growth and metabolism, elaboration of processes, synaptogenesis, and neurotransmission (Jones et al., 1985; Luine, 1985; Nordeen et al., 1985; Matsumoto et al., 1988a,b; Gould et al., 1990). Since many of the aspects of normal neuronal functioning altered by gonadal steroids are affected by injury to the nervous system, we initiated a series of experiments designed to exploit the trophic capabilities of steroids as therapeutic agents in neuronal injury and repair (Kujawa et al., 1989, 1991; Kujawa and Jones, 1990). Three steroid-sensitive model systems were used for these studies: the hamster facial motoneuron, the rat sciatic motoneuron, and the hamster rubrospinal motoneuron. The results of our initial series of experiments suggest that androgens, and possibly estrogens, act either directly or indirectly on the injured motoneuron and enhance elements of the neuronal reparative response that are critical to successful recovery of function. Recently, we discovered that gonadal steroids may also modulate the central glia response to nerve damage. In this review, a summary of our data identifying a therapeutic role for androgens in enhancing the reparative response of motoneurons to injury is presented. This is followed by a discussion of the effects of androgens on the glial response to injury.

Effect of long-term castration and long-term androgen treatment on sexually dimorphic estrogen-inducible progesterone receptor mRNA levels in the ventromedial hypothalamus of whiptail lizards

In whiptail lizards, as in laboratory rodents, females will respond to exogenous estrogen by increasing progesterone receptor (PR) or PR mRNA in the ventromedial hypothalamus (VMH) while males show an attenuated response to the same treatment. In rodents, neonatal hormone manipulations affect the adult expression of this trait; however, few investigators have examined the effects of hormone treatment in adulthood. Therefore the current study was carried out to determine whether observed sex differences in the estrogen response in adulthood may be modified by steroid hormone manipulation. We castrated male whiptail lizards for 1 week (short term) or 6 weeks (long term). We also gonadectomized female whiptails and implanted them with either a Silastic capsule containing testosterone or an empty capsule. At the end of that time all implants were removed and the animals were injected with either estradiol benzoate (EB) or steroid suspension vehicle and their brains were assayed for PR mRNA expression using in situ hybridization. The results demonstrate that in male whiptail lizards, long-term castration increases sensitivity to estradiol as measured by induction of PR mRNA in the VMH; EB-injected long-term castrated males were not different from EB-injected females. However, long-term androgenization did not attenuate the estrogen response in females. This suggests that attenuation of the estrogen response in males requires activation by testicular secretions, but that females cannot be made to show a male phenotype via testosterone administration.

Responsiveness to depolarization of hypothalamic neurons secreting somatostatin under stress and estrous cycle conditions: involvement of GABAergic and steroidal interactions

We studied the sensitivity to a depolarizing stimulus of hypothalamic fragments dissected from cycling female donor rats exposed or not to 30-min stress at 4 degrees C. The neuronal response was estimated in terms of the ability of tissue to release somatostatin when stimulated with 40 mM K+. The data showed no differences in response to K+, regardless of the ovarian cycle of the female donors, whereas tissues dissected from ovariectomized or pregnant rats responded significantly to K+. However, when donors underwent previous cold stress, significant differences were noted at all stages of the cycle, except diestrus-1, compared with control rats. We tested whether GABA and/or neuroactive steroids could be involved in this phenomenon and observed no GABA inhibition of somatostatin release in vitro, but inhibition occurred in the presence of a neuroactive steroid, THDOC. The effect of GABA in vivo on somatostatin release was estrogen dependent because bicuculline modified the total amount of somatostatin secreted in estrus but not in diestrus II. Finally, in hypothalamic primary cultures, GABA inhibition of somatostatin release was only detected when steroids were present in the media throughout culture. Our results suggest that steroid-GABA-somatostatin interactions could explain the different responses of neurons to depolarization.

The magnocellular neurosecretory system of the hamster hypothalamus: an ultrastructural and morphometric study during lifetime

A quantitative study regarding the age-related changes occurring in the nucleus and the somatic organelles of neurosecretory magnocellular neurons of the hypothalamo neurohypophyseal system (HNS) was carried out in the hamster at six age-points during animal life. The magnocellular cells of both parts of the supraoptic nucleus (SON) and the paraventricular nucleus (PVN) of male Syrian hamsters between 3 and 30 months of age were examined ultrastructurally. Cells of all age groups present the same morphological ultrastructure. Standard manual morphometric techniques are used to calculate the following parameters related directly or indirectly with cellular activity: nuclear area, nucleolar area, nuclear invagination index and volumetric fractions of some intracellular structures (Golgi apparatus, mitochondria, rough endoplasmic reticulum and lipofuscin). With respect to the cell nucleus, the parameters are not modified during aging. No significant differences in the volume density of subcellular components, except lipofuscin, were detected at the age groups studies. However, there is a positive linear trend among all parameters and age except for the rough endoplasmic reticulum. Our results suggest maintenance of the synthetic activity of the magnocellular neurons in the hamster during aging but in no case an increase in their metabolic activity.

Distribution and hormonal regulation of androgen receptor immunoreactivity in the forebrain of the male European ferret

The distribution and hormonal regulation of androgen-receptor-immunoreactive (AR-ir) cells in the male European ferret forebrain were examined. AR-ir cells were found in many limbic and hypothalamic structures, and their distribution was similar to that reported for cells that either bind androgen or contain AR protein or mRNA in other species. Regulation of brain AR immunoreactivity by gonadal steroids was brain-region dependent. In most regions examined, including the preoptic area, amygdala, and several hypothalamic nuclei, castration reduced the density of AR-ir profiles and the intensity of immunocytochemical staining, and long-term (days) androgen, but not estrogen, replacement restored these parameters of AR immunoreactivity. Other areas, such as the bed nucleus of the stria terminalis, appeared to be relatively resistant to modulation of AR immunoreactivity by castration and long-term androgen treatment. The ability of testosterone to increase AR-ir profile density is not a simple consequence of translocation of AR from the cytoplasm to the nucleus, because short-term (hours) treatment with testosterone did not result in an increase in AR-ir profile density equivalent to that seen after 10 days of testosterone treatment. Thus, androgens appear to be able to increase AR levels within certain brain cell groups, thereby altering target tissue responsiveness to their own action.

Sex differences in androgen receptor mRNA levels and regulation in hamster facial motoneurons

We have previously shown that testosterone propionate augments hamster facial nerve regeneration to a greater extent in males than females. Further, sex differences in facial nerve regeneration have been observed. From those studies, we hypothesized that sex differences in nerve regeneration could be due to inherent differences in androgen receptor (AR) mRNA content within facial motor neurons (FMN) of male and female hamsters. In the present study, that hypothesis was tested using in situ hybridization, and computerized image analysis to quantify levels and regulation of AR mRNA in individual FMN of hamsters of both sexes. Intact and gonadectomized (gdx) male and female hamsters were used in the initial experiments in the study. In subsequent experiments, exogenous testosterone propionate (TP) was administered to the aforementioned groups of animals by subcutaneous implantation of one 10-mm Silastic capsule for 1, 2 or 7 days. FMN of intact females contained approximately 50% less AR mRNA than their male counterparts. Gonadectomy in males downregulated AR mRNA levels by approximately 50%, whereas no effects of gonadectomy were observed in females. Thus, in all paradigms where TP levels were low relative to the intact males, AR mRNA levels were approximately half of those in the intact male FMN. TP administration induced AR mRNA levels in gdx males within 1 day. Significant effects of TP were not detected in gdx females, and only after 7 days in the intact females. To our knowledge, the results of this study are the first quantitative demonstration of sex differences in steroid receptor mRNA content in a given neuronal population and substantiate the idea that sex differences in the effects of androgens on peripheral nerve regeneration are based on intrinsic sex differences in the levels and regulation of receptor mRNA in motor neurons.

Testosterone effects on the neuronal ultrastructure in the medial preoptic nucleus of male Japanese quail

Dorsolateral neurons of the medial preoptic nucleus (POM) of male Japanese quail are sensitive to the plasma levels of testosterone: their volume and optical density in Nissl-stained sections increase in castrated birds treated with testosterone. The present study was performed on castrated male quail treated or not with Silastic implants filled with testosterone to describe the ultrastructural variations induced by testosterone in these neurons. Gonadally intact male birds were included as controls. The ultrastructure of neurons, taken from the dorsolateral portion of the POM, was dramatically affected by the endocrine manipulations. Quantitative evaluations demonstrated a significant decrease in castrated birds of the rough endoplasmic reticulum (RER), of free polyribosomes, of Golgi complexes, and of dense bodies; these changes paralleled the decrease in cell size. The cell size and the percentage of volume occupied by the intracellular organelles in castrated birds treated with testosterone were comparable to values observed in controls. These ultrastructural changes are similar to those observed in neuronal targets for other gonadal hormones, supporting the idea that testosterone stimulates the development of cytoplasmic structures involved in protein synthesis and secretion. In addition, exposure to testosterone affects the synaptic inputs to POM. These ultrastructural changes are presumably related to the physiological effects (e.g., activation of male sexual behavior) exerted by testosterone on this preoptic region.

Chronic psychosocial stress induces morphological alterations in hippocampal pyramidal neurons of the tree shrew

The effect of sustained psychosocial stress on the morphology of hippocampal pyramidal neurons was analysed in male tree shrews after 14, 20, and 28 days of social confrontation. A variety of physiological changes such as constantly elevated levels of urinary cortisol and norepinephrine and reduced body weight, which are indicative of chronic stress were observed in the subordinate, but not in the dominant males. Light microscopic analysis of Nissl-stained hippocampal sections showed that the staining intensity of the nucleoplasm in the CA1 and CA3 pyramidal neurons was increased after prolonged psychosocial stress, indicating a change in the nuclear chromatin structure. These alterations were observed only in subordinate animals and increased in a time dependent manner in accordance with the length of the stress period. There was, however, neither a reduction in density nor a degeneration of pyramidal neurons in chronically stressed animals. Mechanisms which may possibly account for the observed alterations are discussed.

Androgenic enhancement of motor neuron regeneration

In conclusion, the available evidence to date suggests that many of the aspects of neuronal functioning affected by gonadal steroids under steady state conditions are also significantly affected by steroids under stress conditions such as axon disconnection. This argues toward a therapeutic usefulness of gonadal steroids in activating and/or accelerating the reparative response of neurons to injury, a concept that will be exciting to test in future clinical studies.

Hormonal regulation of hypothalamic gene expression: identification of multiple novel estrogen induced genes

Estrogen (E) has been shown to play a major role in hypothalamic function and is a prerequisite for progesterone (P) induced sexual behavior in female rats. In the course of studies in search of steroid induced hypothalamic genes, we discovered a surprisingly large number of E-induced genes (21 mRNAs in total). This is the largest number of E-induced genes ever identified in a single organ. Many of these mRNAs exhibit considerable magnitudes of induction and their levels were maintained typically during subsequent P treatment. Among the induced genes, several encode metabolic enzymes and may account for some of the morphological changes observed in hypothalamic neurons in response to E. Since E appears to play a major role in defining the pattern of hypothalamic gene expression in conjunction with its capacity for behavioral modulation, these newly identified cDNAs may serve as genetic markers for correlative studies of E-induced central nervous system behavior.

Oestrogen receptors in the preoptico-hypothalamic continuum: immunohistochemical study of the distribution and cell density during induced oestrous cycle in ovariectomized ewe

Oestrogen plays a key role in the regulation of the endocrine and behavioural events associated with the oestrous cycle. It is important, therefore, to know the location of neurones receptive to this steroid and to know whether their distribution varies with the oestrous cycle. We have undertaken experiments to identify the location of oestrogen receptors (ER) within the preoptico-hypothalamic continuum of ovariectomized ewes submitted to a variety of different hormone replacement regimes which mimic the different stages of the oestrous cycle. We used a monoclonal antibody to ER and detected receptors with immunohistological methods in the non-vascular part of the organum vasculosum of the lamina terminalis, the lateral septum, the medial preoptic area, the supraoptic, suprachiasmatic and arcuate (ARC) nuclei, the ventromedial hypothalamus (HVM) and in the region close to the mamillari recess. ER neurones were scarce or absent from the anterior hypothalamus and the paraventricular nucleus. The density of ER staining in the HVM, but in no other localization, was found to be higher, and in a more lateral position, during the induced luteal phase (progesterone treatment) than during the follicular phase (7 days of progesterone treatment followed by oestradiol) or in the ovariectomized female. In all areas studied, except for the ARC, the apparent surface area of the nucleus in ER immunoreactive cells varied with hormonal treatment. These data, and especially those in the HVM, contribute towards our understanding of how steroids may act in the ovine to control sexual behaviour.

Calcitonin gene-related peptide-like immunoreactivity in spinal motoneurons of the male mouse is affected by castration and genotype

Calcitonin gene-related peptide (CGRP) is found in motoneurons of the mammalian spinal cord, including motoneurons of the androgen-dependent spinal nucleus of the bulbocavernosus (SNB) of the mouse. Motoneurons of the SNB innervate the bulbocavernosus (BC), a striated muscle involved in penile reflexes. CGRP is though to be a trophic factor produced by motoneurons to regulate the expression of the acetylcholine receptor at the neuromuscular junction. In rats, the number of SNB motoneurons containing CGRP is increased by gonadal steroids. This regulation appears to rely on an activity-dependent factor produced by the BC muscle. The purpose of the present study was to examine, using immunohistochemistry, the steroid dependence of CGRP in the SNB of male house mice. Genotypic differences in the steroid regulation of CGRP immunoreactivity were examined in three strains of mice that differ in their behavioral sensitivity to castration. The results demonstrate that castration reduces the number of CGRP-positive SNB motoneurons in mice. The magnitude of the change in CGRP in response to castration and the length of time required following castration to alter CGRP were dependent on genotype. Interestingly, the effect of castration in mice, to reduce the number of CGRP-immunoreactive SNB motoneurons, is opposite in direction from the increase in CGRP SNB motoneurons observed in rats observed following castration. These experiments suggest that androgens may alter neuromuscular junction function of mouse SNB by regulating the production of CGRP in a species-specific, genotypically dependent fashion.

Estrous cycle-associated axosomatic synaptic plasticity upon estrogen receptive neurons in the rat preoptic area

This study examined the hypothesis that synaptic relationships change on a daily basis in the anterior preoptic area of the intact, cycling female rat. Specifically, the anteroventral periventricular nucleus (AVPv) was chosen for analysis due to its abundant estrogen- and progesterone-receptive neurons and its critical role in the control of gonadotropin secretion. Ultrastructural analysis revealed that in the 24 h between proestrus and estrus, there was a 39% increase in axosomatic synapses upon AVPv neurons. In the subsequent 24 h to metestrus, the number of synapses decreased by 22%. Additional data showed that ovariectomy resulted in more axosomatic synapses in the AVPv relative to proestrus (46%) and metestrus (35%). Another component of the study investigated synaptic relationships on estrogen receptor-immunoreactive (ER-ir) and non-ER-ir neurons across the estrous cycle. ER-ir neurons received significantly more synaptic input at proestrus and estrus than did non-ER-ir neurons. At metestrus and following ovariectomy, no significant differences were present. The present study indicates that estrous cycle-associated synaptic plasticity occurs in the AVPv and lends further support to the critical role of this nucleus in regulation of gonadotropin secretion. Moreover, it provides the first evidence that ER- and non-ER-ir neurons are differentially innervated.

Testosterone regulation of the regenerative properties of injured rat sciatic motor neurons

We have previously demonstrated that systemic administration of testosterone differentially regulates the regenerative properties of injured hamster facial motor neurons, which are androgen receptor-containing cranial motor neurons. In this investigation, the hypothesis that testosterone alters the regenerative properties of rat sciatic motor neurons, which are androgen receptor-containing spinal motor neurons, was tested using fast axonal transport of radioactively labeled proteins to assess sciatic nerve regeneration. Adult castrated male rats were subjected to crush axotomy of the sciatic nerve at the level of the gemelli tendons (mid-thigh). One-half of the axotomized animals received subcutaneous implants of testosterone propionate (TP), with the remainder of the animals sham implanted with blank capsules. The outgrowth distances of the leading axons were measured at 5, 6, 7, and 11 days postoperative. Linear regression analysis was accomplished, with the slope of the line representing the regeneration rate and the x-intercept the initial delay of sprout formation. Systemic administration of testosterone resulted in a 13% increase in the rate of regeneration, relative to the control, -TP group. Outgrowth distances were significantly increased in the +TP group only in the later stages of regeneration. However, TP did not shorten the delay in sprout formation in regenerating sciatic motor neurons, but instead produced a small prolongation in the delay time. This pattern of hormonal regulation of the regenerative properties of spinal motoneurons is similar to that previously found in cranial motoneurons. The prolongation of the initial delay may have been a factor in the lack of significant outgrowth distances during the early stages of regeneration.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential expression and gonadal hormone regulation of histone H1(0) in the developing and adult rat brain

The cellular distribution of histone H1(0) has been examined immunohistochemically in the rat brain. H1(0) accumulates in neurons and glial cells during postnatal development. In neurons, immunoreactivity increases progressively from about postnatal day 10, and reaches a distribution pattern similar to that of adult rats by postnatal day 20. Immunoreactivity in glial cells shows a prominent increase from postnatal day 20 to adult age. The accumulation of H1(0) during postnatal development appears to be correlated with terminal differentiation and maturation. Although immunoreactive neurons are widely distributed in all areas of the central nervous system, many neurons do not express immunoreactivity. For instance in the cerebellum, Purkinje neurons are negative. In females, the number of immunoreactive neurons in the arcuate area of the hypothalamus increases during postnatal development. In contrast, the percentage of immunoreactive neurons in males is low at all ages studied. The expression of H1(0) in the ventromedial part of the arcuate is reversibly and negatively regulated during the estrous cycle by the level of plasma estradiol. Ovariectomy increases the number of immunoreactive neurons while the restoration of the physiological levels of estradiol results in the opposite effect. Early postnatal androgenization of females suppresses the increment in the number of immunoreactive neurons in both the dorsolateral and the ventromedial parts of the arcuate during postnatal development, thus leading to permanently decreased levels of H1(0) immunoreactivity in postpuberal females.

Gonadal steroids as promoting factors in axonal regeneration

In this article, some of the trophic actions of gonadal steroids on receptor-concentrating neurons within the mammalian brain and spinal cord will be discussed. This will be followed by a summary of our recent data identifying a new role for gonadal steroids as therapeutic agents in neuronal injury and repair.

Fos-like expression and nuclear size in osmotically stimulated supraoptic nucleus neurons

This study has analysed by immunocytochemistry the pattern of expression of Fos-related proteins, as well as variations in nuclear size, after the osmotically induced activation of supraoptic nucleus neurons of the rat. In control rats most supraoptic nucleus neurons were Fos-like negative. After acute and chronic dehydration by salt-loading, the number of Fos-like positive neurons increased dramatically. The level of Fos-like immunoreactivity was higher in chronically stimulated rats, and also the neurons of the ventral region of the supraoptic nucleus were more intensely stained than those of the dorsal region. The karyometric analysis was made on electron micrographs. The mean nuclear profile area showed a significant increase in dehydrated rats with respect to the controls (73 +/- 16 microns 2 in those dehydrated for six days vs 54 +/- 13 in controls, mean +/- S.D.). However, no significant differences in this parameter were found when one-day and six-day dehydrated groups were compared. The invagination factor of the nuclear membrane, a nuclear shape indicator, decreased significantly in dehydrated rats, indicating a tendency towards spherical nuclei. It is noteworthy that the nuclear profile perimeter was constant, about 32 microns, in control and osmotically simulated rats. The higher nuclear accumulation of Fos-related antigens after six days of dehydration suggests that in chronically stimulated supraoptic nucleus neurons there is a sustained induction of cell-specific genes. Moreover, the transcription rate of the target genes containing the consensus DNA sequence TGAC/GTCA or c-AMP responsive elements recognition sites may depend upon the nuclear concentration of Fos-related antigens in supraoptic nucleus neurons. Our results also suggest that the initial Fos-related antigen expression and nuclear size increase are triggered concomitantly in supraoptic nucleus neurons after a short period of osmotic stimulation. On the other hand, we propose that nuclear envelope invaginations represent a reservoir of nuclear membrane which allows dynamic changes in nuclear size and shape depending on the metabolic status of the supraoptic nucleus neurons.

Role of local environmental factors in determining tissue-specific effects of estrogen: examination of uterine tissues transplanted to brain

Estrogen stimulates uterine epithelial cells to divide, but not estrogen-concentrating neurons in the adult brain. This effect correlates with recent evidence that estrogen can induce the expression of certain growth-related genes in uterus which are not directly induced by estrogen in the adult brain. The possibility that local diffusible factors play a major role in determining tissue-specific effects of estrogen was examined by transplanting uterine tissues into the brain, muscle and kidney of adult rats and then comparing the effects of estrogen on the incorporation of [3H]thymidine and the expression of Fos-, cdc2- and Rb-like immunoreactivity (IR) on native and transplanted uterine tissues, as well as in estrogen-concentrating regions of the brain adjacent to the uterine grafts. In native uteri, estrogen treatment stimulated Fos-, cdc2-, and Rb-like IR, as well as [3H]thymidine incorporation, within lumenal and glandular epithelial cells. All of these effects were estrogen responsive--no immunoreactive staining within uterine epithelial cells and no signs of epithelial cell proliferation were observed in the native uteri of non-estrogen-treated animals. When uterine tissues were transplanted to brain, Fos-, cdc2-, and Rb-like IR epithelial cells, as well as many [3H]thymidine-incorporating uterine epithelial cells, were observed in all estrogen-treated animals and in some non-estrogen-treated animals as well. Identical results were obtained when uterine tissues were transplanted to skeletal muscle, but not to kidney (in the kidney, transplanted epithelial cells expressed all four parameters but only in estrogen-treated animals, comparable to the native uterus). In contrast, estrogen did not stimulate cell division and did not induce Fos-, cdc2-, or Rb-like IR within estrogen-concentrating neuronal regions of the ventromedial hypothalamus. In addition, the presence of uterine tissue in the brain did not confer the ability of estrogen to stimulate any of these parameters within nearby, estrogen-concentrating regions. These data suggest that there are factors in brain and muscle which can allow uterine epithelial cells to divide in the absence of estrogen. There was no evidence of a diffusible factor in brain which inhibits uterine epithelial cell division, nor of a diffusible factor in uterus which can confer estrogenic stimulation of growth-related genes and cell division to central nervous system neurons. In addition, the data provide the first evidence for estrogen regulation of cdc2 and Rb expression in normal uterus.

Alterations in nuclear envelope invaginations in axotomized fetal and early postnatal hamster facial motoneurons

In this study, changes in the amount of nuclear envelope invaginations (NEI) were morphometrically assessed after axotomy during late fetal and early postnatal developmental stages in hamster facial motoneurons. These changes were expressed as boundary density or BA (length of nuclear envelope per unit area of nucleus). Axotomy-induced changes in nuclear area and perimeter were also quantitatively determined. At 17 h after axotomy in the fetal operative series, no changes in any of the parameters were seen. At 1 day postoperative (dpo) in newborn, 2 and 4 postnatal day animals, the boundary densities of the total and invaginated portion of the nuclear envelope increased significantly. No corresponding qualitative changes were observed. At 2 dpo in 4 and 7 postnatal day animals, there were significant increases in the boundary densities of both invaginated and total nuclear envelope and a decrease in nuclear area. These changes were not seen at 2 dpo in the 9-day operative series. At 4 dpo in 7 and 9 postnatal day animals, scalloping of the normally smooth nuclear profile, as well as a flattening and elongation in nuclear shape, occurred. These qualitative changes in the 7 and 9 day operated groups were also accompanied by significant changes in all the measured parameters. The boundary density of the invaginated, non-invaginated and total nuclear envelope increased; whereas, nuclear area and perimeter decreased. These results argue against the generally held hypothesis that an increase in nuclear envelope invaginations is indicative of an allied increase in cellular metabolism.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of estrogen and fimbria/fornix transection on p75NGFR and ChAT expression in the medial septum and diagonal band of Broca

NGF receptor-expressing cells located in the basal forebrain have recently been shown to contain estrogen (E) receptors (Toran-Allerand and MacLusky. 1989. Soc. Neurosci. Abstr. 15: 954). In the present study, we have examined the effects of E-treatment on p75NGFR and choline acetyltransferase (ChAT) expression by neurons in the medial septum (MS) and the vertical (VDB) and horizontal (HDB) limbs of the diagonal band of Broca using immunocytochemical and in situ hybridization techniques. First, since E-treatment has been shown to affect neuronal survival and to stimulate synaptic reorganization and growth within various regions of the brain, we hypothesized that E-treatment might attenuate the loss of p75NGFR immunoreactivity (IR) which occurs in the MS and VDB following transection of the fimbria/fornix. Contrary to our hypothesis, E-treatment did not attenuate the effects of fimbria/fornix transection. In fact, E-treatment alone produced a significant decrease in the number of p75NGFR-IR cells detected in the MS. Subsequent experiments confirmed that chronic E-treatment produces a down-regulation of both p75NGFR-IR and p75NGFR mRNA in the MS and VDB. In the MS, estrogen appeared to affect a subpopulation of p75NGFR-expressing neurons which were also affected by fimbria/fornix transection since the effects of these two treatments were not additive. In addition, effects of E-treatment on p75NGFR-IR were sex-specific (observed in females but not in males) and were reversible in the MS after 2 weeks, but not after 4 weeks (allowing 2 weeks recovery), of E-treatment. A time-course analysis revealed that effects of E-treatment on p75NGFR-IR were not observed until after 16 days (MS) or 30 days (VDB) of E-treatment and were preceded by a significant and transient increase in ChAT expression in both the MS and VDB. The data are consistent with the possibility that continuous, long-term exposure to gonadal steroids may contribute to a loss of p75NGFR-expressing neurons with age. In addition, the data suggest that p75NGFR expression may play a role in regulating the functioning of specific basal forebrain cholinergic neurons. Different mechanisms by which E-treatment might influence ChAT and p75NGFR expression in brain are discussed.

LHRH neurons: functions and development

Examination of the properties of developing LHRH neurons, by in situ hybridization procedures or LHRH immunocytochemistry, showed that these cells (1) are unique among neuroendocrine cells in their origin from the epithelium of the medial olfactory pit, and (2) express LHRH mRNA. LHRH neurons, visualized by either method, tended to be clustered when seen along the migration route in the nasal mesenchyme. Neural cell adhesion molecule (NCAM) is present on the central processes of the olfactory, vomeronasal and terminalis nerves, which form the scaffold along which LHRH neurons migrate into the brain. Injection of a small amount (1 microliter) of antiserum to NCAM into the olfactory pits of 10-day-old embryonic mice, while not sufficient to break up the NCAM scaffolding, appeared to decrease the number of LHRH-immunoreactive cells in the epithelium of the medial olfactory pit, and retarded their migration in the nasal mesenchyme. This suggest that NCAM is important for LHRH cell migration. Never found actually colocalized with LHRH in the same neurons, NCAM nevertheless may be required for the migration of LHRH-expressing cells.

Ultrastructural changes in hypothalamic supraoptic nucleus neurons of ovariectomized estrogen-deprived young rats

We report the occurrence of neuronal degeneration in the supraoptic nucleus of the hypothalamus of prepubertal female rats as a consequence of ovariectomy followed by estrogen deprivation (OVX-EB). In contrast, no degenerating neurons were observed in ovariectomized rats treated with estrogens (OVX + EB) or sham-operated animals. The altered neurons in the OVX-EB group presented a cytoplasm with dilation of the rough endoplasmic reticulum lumen. The nuclear envelope also appeared dilated. There was mitochondrial swelling with disintegrated cristae, while lysosomes appeared intact. The cell nucleus showed a pattern of chromatin condensation and a nucleolus hardly distinguishable from the nucleoplasm. The neuronal alterations reported here may be due to altered gene expression in the cell nucleus resulting from induced hormonal loss during early postnatal development.

Progesterone-facilitated sexual receptivity: a review of arguments supporting a nongenomic mechanism

Progesterone facilitates sexual receptivity in estradiol-primed female rats. While many experiments suggest a genomic mechanism for the behavioral action of progesterone, some results appear better explained by nongenomic mechanisms. Furthermore, the presence of membrane binding sites for progesterone and rapid modifications of neuronal excitability induced by this steroid suggest that a nongenomic action of progesterone could be membrane related. However, in spite of the discovery of such membrane-related actions of progesterone, their relation to progesterone-facilitated sexual behavior remains unclear.

Testosterone effects on ribosomal RNA levels in injured peripheral motor neurons: a preliminary report

We have previously demonstrated that administration of testosterone to hamsters during the early phases of axonal regeneration following facial nerve injury accelerates both the rate of regeneration of the fastest growing population of axons and the return of functional movement. We hypothesized from those studies that testosterone primes the neuronal cell body in such a way as to accelerate the "switch" from a normal to a reparative state. That hypothesis was tested in this study using ribosomal DNA (rDNA) probes in conjunction with in situ hybridization to map the molecular response of the polymerase I system to axotomy, with and without hormone exposure. Adult male hamsters were subjected to right facial nerve severance, with the left side serving as an internal control. Half the animals were administered testosterone propionate via subcutaneous implants. In situ hybridization using a genomic rDNA probe complementary to the 28S rRNA species was accomplished, and levels of rRNA in injured facial neurons assessed both qualitatively and quantitatively. Our initial findings indicate that testosterone markedly upregulates rRNA levels after injury, and support the hypothesis of an acceleration in the metabolic switch to a reparative state. This leads us, in turn, to suggest that this effect of testosterone on the ribosomal system is causally related to the increase in axonal regeneration rate and return of functional movement previously documented in this system.