Ultrastructural changes in the hypothalamic ventromedial nucleus of ovariectomized rats after estrogen treatment

Neural mechanisms of persistent aggression

While aggression is often conceptualized as a highly stereotyped, innate behavior, individuals within a species exhibit a surprising amount of variability in the frequency, intensity, and targets of their aggression. While differences in genetics are a source of some of this variation across individuals (estimates place the heritability of behavior at around 25-30%), a critical driver of variability is previous life experience. A wide variety of social experiences, including sexual, parental, and housing experiences can facilitate "persistent" aggressive states, suggesting that these experiences engage a common set of synaptic and molecular mechanisms that act on dedicated neural circuits for aggression. It has long been known that sex steroid hormones are powerful modulators of behavior, and also, that levels of these hormones are themselves modulated by experience. Several recent studies have started to unravel how experience-dependent hormonal changes during adulthood can create a cascade of molecular, synaptic, and circuit changes that enable behavioral persistence through circuit level remodeling. Here, we propose that sex steroid hormones facilitate persistent aggressive states by changing the relationship between neural activity and an aggression "threshold".

Neural basis for estrous cycle-dependent control of female behaviors

Females display changes in distinct behaviors along the estrous cycle. Levels of circulating ovarian sex steroid hormones peak around ovulation, which occur around estrus phase of the cycle. This increase of sex hormones is thought to be important for changes in behaviors, however, neural circuit mechanisms of periodic behavioral changes in females are not understood well. Different lines of research indicate sex hormonal effects on several forms of neuronal plasticity. This review provides an overview of behavioral and plastic changes that occur in an estrous cycle-dependent manner and explores the current research linking these changes to understand neural circuit mechanisms that control female behaviors.

The Structural and Electrophysiological Properties of Progesterone Receptor-Expressing Neurons Vary along the Anterior-Posterior Axis of the Ventromedial Hypothalamus and Undergo Local Changes across the Reproductive Cycle

Sex hormone levels continuously fluctuate across the reproductive cycle, changing the activity of neuronal circuits to coordinate female behavior and reproductive capacity. The ventrolateral division of the ventromedial hypothalamus (VMHvl) contains neurons expressing receptors for sex hormones and its function is intimately linked to female sexual receptivity. However, recent findings suggest that the VMHvl is functionally heterogeneous. Here, we used whole recordings and intracellular labeling to characterize the electrophysiological and morphologic properties of individual VMHvl neurons in naturally cycling females and report the existence of multiple electrophysiological phenotypes within the VMHvl. We found that the properties of progesterone receptor expressing (PR+) neurons, but not PR– neurons, depended systematically on the neuron’s location along the anterior-posterior (AP) axis of the VMHvl and the phase within the reproductive cycle. Prominent among this, the resting membrane potential of anterior PR+ neurons decreased during the receptive phase, while the excitability of medial PR+ neurons increased during the non-receptive phase. During the receptive phase of the cycle, posterior PR+ neurons simultaneously showed an increase in dendritic complexity and a decrease in spine density. These findings reveal an extensive diversity of local rules driving structural and physiological changes in response to fluctuating levels of sex hormones, supporting the anatomic and functional subdivision of the VMHvl and its possible role in the orchestration of different aspects of female socio-sexual behavior.

The Complex Interplay between Endocannabinoid System and the Estrogen System in Central Nervous System and Periphery

The endocannabinoid system (ECS) is a lipid cell signaling system involved in the physiology and homeostasis of the brain and peripheral tissues. Synaptic plasticity, neuroendocrine functions, reproduction, and immune response among others all require the activity of functional ECS, with the onset of disease in case of ECS impairment. Estrogens, classically considered as female steroid hormones, regulate growth, differentiation, and many other functions in a broad range of target tissues and both sexes through the activation of nuclear and membrane estrogen receptors (ERs), which leads to genomic and non-genomic cell responses. Since ECS function overlaps or integrates with many other cell signaling systems, this review aims at updating the knowledge about the possible crosstalk between ECS and estrogen system (ES) at both central and peripheral level, with focuses on the central nervous system, reproduction, and cancer.

Gender-Affirming Hormone Therapy Modifies the CpG Methylation Pattern of the ESR1 Gene Promoter After Six Months of Treatment in Transmen

BACKGROUND

Brain sexual differentiation is a process that results from the effects of sex steroids on the developing brain. Evidence shows that epigenetics plays a main role in the formation of enduring brain sex differences and that the estrogen receptor α (ESR1) is one of the implicated genes.

AIM

To analyze whether the methylation of region III (RIII) of the ESR1 promoter is involved in the biological basis of gender dysphoria.

METHODS

We carried out a prospective study of the CpG methylation profile of RIII (-1,188 to -790 bp) of the ESR1 promoter using bisulfite genomic sequencing in a cisgender population (10 men and 10 women) and in a transgender population (10 trans men and 10 trans women), before and after 6 months of gender-affirming hormone treatment. Cisgender and transgender populations were matched by geographical origin, age, and sex. DNAs were treated with bisulfite, amplified, cloned, and sequenced. At least 10 clones per individual from independent polymerase chain reactions were sequenced. The analysis of 671 bisulfite sequences was carried out with the QUMA (QUantification tool for Methylation Analysis) program.

OUTCOMES

The main outcome of this study was RIII analysis using bisulfite genomic sequencing.

RESULTS

We found sex differences in RIII methylation profiles in cisgender and transgender populations. Cismen showed a higher methylation degree than ciswomen at CpG sites 297, 306, 509, and at the total fragment (P ≤ .003, P ≤ .026, P ≤ .001, P ≤ .006). Transmen showed a lower methylation level than trans women at sites 306, 372, and at the total fragment (P ≤ .0001, P ≤ .018, P ≤ .0107). Before the hormone treatment, transmen showed the lowest methylation level with respect to cisgender and transgender populations, whereas transwomen reached an intermediate methylation level between both the cisgender groups. After the hormone treatment, transmen showed a statistically significant methylation increase, whereas transwomen showed a non-significant methylation decrease. After the hormone treatment, the RIII methylation differences between transmen and transwomen disappeared, and both transgender groups reached an intermediate methylation level between both the cisgender groups.

CLINICAL IMPLICATIONS

Clinical implications in the hormonal treatment of trans people.

STRENGTHS & LIMITATIONS

Increasing the number of regions analyzed in the ESR1 promoter and increasing the number of tissues analyzed would provide a better understanding of the variation in the methylation pattern.

CONCLUSIONS

Our data showed sex differences in RIII methylation patterns in cisgender and transgender populations before the hormone treatment. Furthermore, before the hormone treatment, transwomen and transmen showed a characteristic methylation profile, different from both the cisgender groups. But the hormonal treatment modified RIII methylation in trans populations, which are now more similar to their gender. Therefore, our results suggest that the methylation of RIII could be involved in gender dysphoria. Fernández R, Ramírez K, Gómez-Gil E, et al. Gender-Affirming Hormone Therapy Modifies the CpG Methylation Pattern of the ESR1 Gene Promoter After Six Months of Treatment in Transmen. J Sex Med 2020;17:1795-1806.

Sex steroid hormones as neuroprotective elements in ischemia models

Among sex steroid hormones, progesterone and estradiol have a wide diversity of physiological activities that target the nervous system. Not only are they carried by the blood stream, but also they are locally synthesized in the brain and for this reason, estradiol and progesterone are considered 'neurosteroids'. The physiological actions of both hormones range from brain development and neurotransmission to aging, illustrating the importance of a deep understanding of their mechanisms of action. In this review, we summarize key roles that estradiol and progesterone play in the brain. As numerous reports have confirmed a substantial neuroprotective role for estradiol in models of neurodegenerative disease, we focus this review on traumatic brain injury and stroke models. We describe updated data from receptor and signaling events triggered by both hormones, with an emphasis on the mechanisms that have been reported as 'rapid' or 'cytoplasmic actions'. Data showing the therapeutic effects of the hormones, used alone or in combination, are also summarized, with a focus on rodent models of middle cerebral artery occlusion (MCAO). Finally, we draw attention to evidence that neuroprotection by both hormones might be due to a combination of 'cytoplasmic' and 'nuclear' signaling.

Estrogens and cognition: Friends or foes?: An evaluation of the opposing effects of estrogens on learning and memory

This article is part of a Special Issue "Estradiol and cognition". Estrogens are becoming well known for their robust enhancement on cognition particularly for learning and memory that relies upon functioning of the hippocampus and related neural systems. What is also emerging is that estrogen modulation of cognition is not uniform, at times enhancing yet at other times impairing learning. This review explores the bidirectional effects of estrogens on learning from a multiple memory systems view, focusing on the hippocampus and striatum, whereby modulation by estrogens sorts according to task attributes and neural systems engaged during cognition. We highlight our findings showing that the ability to solve hippocampus-sensitive tasks typically improves under relatively high estrogen status while the ability to solve striatum-sensitive tasks degrades with estrogen exposures. Though constrained by dose and timing of exposure, these opposing enhancements and impairments of cognition can be observed following treatments with different estrogenic compounds including the hormone estradiol, the isoflavone genistein found in soybeans, and agonists that are selective for specific estrogen receptors, suggesting that activation of a single receptor type is sufficient to produce the observed shifts in learning strategies. Using this multi-dimensional framework will allow us to extend our thinking of the relationship between estrogens and cognition to other brain regions and cognitive functions.

The role of oxytocin in male and female reproductive behavior

Oxytocin (OT) is a nonapeptide with an impressive variety of physiological functions. Among them, the 'prosocial' effects have been discussed in several recent reviews, but the direct effects on male and female sexual behavior did receive much less attention so far. As our contribution to honor the lifelong interest of Berend Olivier in the control mechanisms of sexual behavior, we decided to explore the role of OT in the present review. In the successive sections, some physiological mechanisms and the 'pair-bonding' effects of OT will be discussed, followed by sections about desire, female appetitive and copulatory behavior, including lordosis and orgasm. At the male side, the effects on erection and ejaculation are reviewed, followed by a section about 'premature ejaculation' and a possible role of OT in its treatment. In addition to OT, serotonin receives some attention as one of the main mechanisms controlling the effects of OT. In the succeeding sections, the importance of OT for 'the fruits of labor' is discussed, as it plays an important role in both maternal and paternal behavior. Finally, we pay attention to an intriguing brain area, the ventrolateral part of the ventromedial hypothalamic nucleus (VMHvl), apparently functioning in both sexual and aggressive behavior, which are at first view completely opposite behavioral systems.

Ovarian hormone action in the hypothalamic ventromedial nucleus: remodelling to regulate reproduction

The ventromedial nucleus of the hypothalamus (VMH) is a major site for the control of female sexual behaviour by ovarian steroid hormones. This review explores recent details that have emerged regarding the ovarian hormone-induced remodelling of neural circuits within the VMH in adult female rats, with the goal of refining the model of the VMH neural circuit. VMH neurones exhibit simple dendritic arbours, with a single long primary dendrite (LPD) and several short primary dendrites. We recently found that ovarian hormones have unanticipated differential effects on the length of the LPDs, suggesting an intricate synaptic reorganisation. LPDs extend into the lateral fibre plexus where they contact oxytocin-labelled terminals. Oestradiol treatment rearranges this oxytocin innervation, in particular by withdrawing some of the LPDs and intensifying the oxytocin input to the remaining dendrites. These changes are reversed with concomitant progesterone treatment. Incorporating these new results, we have updated our working model of hormone-induced synaptic reorganisation in the VMH, emphasising the rebalancing of local versus extrinsic connectivity. The new working model synthesises the recent evidence for rewiring with insights from electrophysiological and behavioural pharmacological studies that pertain to the roles of oxytocin and glutamate in VMH neural activity and mating behaviour.

Sex differences in the neural circuit that mediates female sexual receptivity

Female sexual behavior in rodents, typified by the lordosis posture, is hormone-dependent and sex-specific. Ovarian hormones control this behavior via receptors in the hypothalamic ventromedial nucleus (VMH). This review considers the sex differences in the morphology, neurochemistry and neural circuitry of the VMH to gain insights into the mechanisms that control lordosis. The VMH is larger in males compared with females, due to more synaptic connections. Another sex difference is the responsiveness to estradiol, with males exhibiting muted, and in some cases reverse, effects compared with females. The lack of lordosis in males may be explained by differences in synaptic organization or estrogen responsiveness, or both, in the VMH. However, given that damage to other brain regions unmasks lordosis behavior in males, a male-typical VMH is unlikely the main factor that prevents lordosis. In females, key questions remain regarding the mechanisms whereby ovarian hormones modulate VMH function to promote lordosis.

Ovarian hormone-induced reorganization of oxytocin-labeled dendrites and synapses lateral to the hypothalamic ventromedial nucleus in female rats

Central oxytocin (OT) modulates many social behaviors, including female rat sexual receptivity, quantified as the copulatory stance known as lordosis. The expression of the lordosis response is modulated by OT action in the ventromedial nucleus of the hypothalamus (VMH), as demonstrated by behavioral pharmacology experiments. However, the subcellular localization of OT in this brain region has been unclear. We tested the hypothesis that ovarian hormones reorganize OT-labeled pre- or postsynaptic elements in the fiber complex lateral to the VMH by using immunoelectron microscopy. OT immunolabeling occurred in axonal boutons identified by the presence of small, clear synaptic vesicles and double labeling with the presynaptic markers synaptophysin and vesicular glutamate transporter 2. OT immunoreactivity also was observed in dendritic profiles, verified with double labeling for the dendrite-specific marker microtubule-associated protein 2. Ovarian hormones did not alter the density of axonal boutons; however, estradiol treatment reduced the density of dendritic profiles by 34%. This effect was reversed when progesterone was given subsequent to estradiol. The effect of estradiol treatment was specific to dendrites that lacked OT immunostaining; the density of OT-labeled dendritic profiles remained constant during estradiol treatment. With the estradiol-induced exit of non-OT-labeled dendritic profiles, the remaining OT-labeled dendritic profiles experienced an increase in their number of synaptic contacts. Thus, hormone treatments that mimic the 4-day rat estrous cycle provoke a chemically coded reorganization of dendrite innervation in the fiber plexus lateral to the VMH that may underlie the hormone-specific effect of OT on reproductive behavior.

Stress, sex, and neural adaptation to a changing environment: mechanisms of neuronal remodeling

The adult brain is much more resilient and adaptable than previously believed, and adaptive structural plasticity involves growth and shrinkage of dendritic trees, turnover of synapses, and limited amounts of neurogenesis in the forebrain, especially the dentate gyrus of the hippocampal formation. Stress and sex hormones help to mediate adaptive structural plasticity, which has been extensively investigated in the hippocampus and to a lesser extent in the prefrontal cortex and amygdala, all brain regions that are involved in cognitive and emotional functions. Stress and sex hormones exert their effects on brain structural remodeling through both classical genomic as well as non-genomic mechanisms, and they do so in collaboration with neurotransmitters and other intra- and extracellular mediators. This review will illustrate the actions of estrogen on synapse formation in the hippocampus and the process of stress-induced remodeling of dendrites and synapses in the hippocampus, amygdala, and prefrontal cortex. The influence of early developmental epigenetic events, such as early life stress and brain sexual differentiation, is noted along with the interactions between sex hormones and the effects of stress on the brain. Because hormones influence brain structure and function and because hormone secretion is governed by the brain, applied molecular neuroscience techniques can begin to reveal the role of hormones in brain-related disorders and the treatment of these diseases. A better understanding of hormone-brain interactions should promote more flexible approaches to the treatment of psychiatric disorders, as well as their prevention through both behavioral and pharmaceutical interventions.

Influence of substitutive ovarian steroids in the nuclear and cell body volumes of neurons in the posterodorsal medial amygdala of adult ovariectomized female rats

The volumes of the neuronal nucleus and the cell body in the left posterodorsal medial amygdala (MePD) of adult ovariectomized (OVX) female rats submitted to different hormonal therapies were studied here, aiming to reveal possible influence of substitutive sex steroids in these morphological parameters. One week following ovariectomy and at the end of treatments, brains were cut to semi-thin sections (1 microm) and stained with 1% toluidine blue for stereological estimations, carried out using the Cavalieri method and the technique of point counting. Both the volume of the neuronal nucleus and the soma showed a statistically significant difference when comparing the data among OVX females treated with vehicle (V), estradiol (EB) alone, EB plus progesterone (EB+P) or P alone [n=5 rats in each group; one-way ANOVA test, P<0.01 in both cases]. The Tukey test showed that OVX and EB+P treated females had higher mean neuronal nucleus and somatic volumes when compared to V (P<0.01) or EB alone (P<0.01). Also, OVX females treated with P alone showed larger mean neuronal nucleus and somatic volumes when compared to V (P<0.05). These results suggest that the neuronal nucleus and the somatic volumes can be modulated by substitutive ovarian hormones administered to OVX females, for which P can lead to higher results. These findings reveal additional epigenetic actions of the sex steroids in the MePD and new neuronal morphological features in adult female rats.

Reverse engineering the lordosis behavior circuit

Reverse engineering takes the facts we know about a device or a process and reasons backwards to infer the principles underlying the structure-function relations. The goal of this review is to apply this approach to a well-studied hormone-controlled behavior, namely the reproductive stance of female rodents, lordosis. We first provide a brief overview on the considerable amount of progress in the analysis of female reproductive behavior. Then, we propose an analysis of the mechanisms of this behavior from a reverse-engineering perspective with the goal of generating novel hypotheses about the properties of the circuitry elements. In particular, the previously proposed neuronal circuit modules, feedback signals, and genomic mechanisms are considered to make predictions in this manner. The lordosis behavior itself appears to proceed ballistically once initiated, but negative and positive hormonal feedback relations are evident in its endocrine controls. Both rapid membrane-initiated and slow genomic hormone effects contribute to the behavior's control. We propose that the value of the reverse-engineering approach is based on its ability to provide testable, mechanistic hypotheses that do not emerge from either traditional evolutionary or simple reductionistic perspectives, and several are proposed in this review. These novel hypotheses may generalize to brain functions beyond female reproductive behavior. In this way, the reverse-engineering perspective can further develop our conceptual frameworks for behavioral and systems neuroscience.

Estradiol induces hypothalamic dendritic spines by enhancing glutamate release: a mechanism for organizational sex differences

The naturally occurring sex difference in dendritic spine number on hypothalamic neurons offers a unique opportunity to investigate mechanisms establishing synaptic patterning during perinatal sensitive periods. A major advantage of the rat as a model of sexual differentiation is that treatment of neonatal females with estradiol will permanently induce the male phenotype. During the development of other systems, exuberant innervation is followed by activity-dependent pruning necessary for elimination of spurious synapses. In contrast, we demonstrate that estradiol-induced organization in the hypothalamus involves the induction of new synapses on dendritic spines. Activation of estrogen receptors by estradiol triggers a nongenomic activation of PI3 kinase that results in enhanced glutamate release from presynaptic neurons. Subsequent activation of ionotropic glutamate receptors activates MAP kinases, thereby inducing dendritic spine formation. These results reveal a transneuronal mechanism by which estradiol acts during a sensitive period to establish a profound and lasting sex difference in hypothalamic synaptic patterning.

Postpubertal decrease in hippocampal dendritic spines of female rats

Hippocampal dendritic spine and synapse numbers in female rats vary across the estrous cycle and following experimental manipulation of hormone levels in adulthood. Based on behavioral studies demonstrating that learning patterns are altered following puberty, we hypothesized that dendritic spine number in rat hippocampal CA1 region would change postpubertally. Female Sprague-Dawley rats were divided into prepubertal (postnatal day (P) 22), peripubertal (P35) and postpubertal (P49) groups, with the progression of puberty evaluated by vaginal opening, and estrous cyclicity subsequently assessed by daily vaginal smears. Spinophilin immunoreactivity in dendritic spines was used as an index of spinogenesis in area CA1 stratum radiatum (CA1sr) of hippocampus. First, electron microscopy analyses confirmed the presence of spinophilin specifically in dendritic spines of CA1sr, supporting spinophilin as a reliable marker of hippocampal spines in young female rats. Second, stereologic analysis was performed to assess the total number of spinophilin-immunoreactive puncta (i.e. spines) and CA1sr volume in developing rats. Our results indicated that the number of spinophilin-immunoreactive spines in CA1sr was decreased 46% in the postpubertal group compared to the two younger groups, whereas the volume of the hippocampus underwent an overall increase during this same developmental time frame. Third, to determine a potential role of estradiol in this process, an additional group of rats was ovariectomized (OVX) prepubertally at P22, then treated with estradiol or vehicle at P35, and spinophilin quantified as above in rats perfused on P49. No difference in spinophilin puncta number was found in OVX rats between the two hormone groups, suggesting that this developmental decrease is independent of peripheral estradiol. These changes in spine density coincident with puberty may be related to altered hippocampal plasticity and synaptic consolidation at this phase of maturity.

Steroid effects on neuronal activity: when is the genome involved?

For over four decades steroids have been regarded first as facilitators of enzymic reactions and subsequently as activators of genomic activity. The brain, long studied in terms of its bioelectric properties and anatomical connectivity, has now been recognized as a complex target tissue for genomic effects of steroid hormones, which bring about long-lasting alterations in brain structure and neurochemistry as well as changes in behaviour and neuroendocrine function. Studies of steroid effects on brain bioelectric activity have also shown rapid effects which are difficult to explain by a strictly genomic mechanism. One way to distinguish between genomic and non-genomic effects is by the time course, with extremely rapid effects being non-genomic and delayed effects being genomic. Effects with onset latencies of minutes to an hour may be due to either mechanism. Examples illustrating genomic actions include delayed effects of oestrogen which alter oxytocin and GABAA receptors and induce spines on dendrites and delayed glucocorticoid effects on neuronal survival. There are also examples of apparent genomic effects of oestradiol which interact with rapid and apparently non-genomic effects of progesterone: progesterone rapidly promotes spread of oestrogen-induced oxytocin receptors in ventromedial hypothalamus and rapidly modifies oestrogen-regulated GABAA receptor density in hypothalamus. The former effect is one produced by progesterone itself whereas the latter effect may be related to the ability of progesterone metabolites to interact with the chloride channel of the GABAA-benzodiazepine receptor complex.

Sexual behaviour induces the expression of activity-regulated cytoskeletal protein and modifies neuronal morphology in the female rat ventromedial hypothalamus

Female sexual behaviour activates a distributed network within the brain, including the ventrolateral subdivision of the hypothalamic ventromedial nucleus (vlVMH), as demonstrated by behavioural studies performed in conjunction with the neuroanatomical analysis of immediate early gene (IEG) expression. However, it has been difficult to interpret mating-induced IEG expression because the precise function of many IEGs remains poorly defined. One possible function for genomic activation of the vlVMH during mating behaviour is to establish synaptic remodelling. The present experiments tested the hypothesis that sexual behaviour rapidly induces the expression of a structural protein associated with synaptic plasticity and ultimately causes morphological changes in the vlVMH. First, the expression of activity-regulated cytoskeletal protein (Arc), an IEG associated with neural plasticity, was assayed immunohistochemically in females after approximately 1 h of mating. The number of Arc-labelled neurones in the vlVMH was greater in mated females compared to unmated controls. Second, VMH neurones were biolistically labelled for morphological measurements, including soma size, dendrite number and length and dendritic spine density. Dendritic spine density in the vlVMH was significantly reduced 5 days after mating in experienced females compared to sexually naïve females. There were no differences between these groups in soma size, dendrite length or dendrite number. Collectively, these studies suggest that mating behaviour produces short-term changes in structural proteins and long-term, selective changes in dendrite morphology, which then may influence future behaviours and/or physiology.

CNS arousal mechanisms bearing on sex and other biologically regulated behaviors

It now seems possible to move beyond analyzing only the mechanisms for specific sexual behaviors to the analysis of 'generalized arousal' that underlies all motivated behaviors. Our science has advanced sufficiently to attack mechanisms linking specific motivations to these general arousal mechanisms that intrinsically activate all biologically-regulated behaviors including ingestive behaviors. Learning from the well-developed reproductive behavior paradigm, we know that sex hormone effects on hypothalamic neurons have been studied to a point where receptor mechanisms are relatively well understood, a neural circuit for a sex steroid-dependent behavior has been worked out, and several functional genomic regulations have been discovered. Here we focus for the first time on three chemical systems that signal 'generalized arousal' and which impact hormone-dependent hypothalamic neurons of importance to sexual arousal: histamine, norepinephrine and enkephalin. Progress in linking generalized arousal to specific motivational mechanisms is reviewed.

Functional genomics of sex hormone-dependent neuroendocrine systems: specific and generalized actions in the CNS

Sex hormone effects on hypothalamic neurons have been worked out to a point where receptor mechanisms are relatively well understood, a neural circuit for a sex steroid-dependent behavior has been determined, and several functional genomic regulations have been discovered and conceptualized. With that knowledge in hand, we approach deeper problems of explaining sexual arousal and generalized CNS arousal. After a brief summary of arousal mechanisms, we focus on three chemical systems which signal generalized arousal and impact hormone-dependent hypothalamic neurons of behavioral importance: histamine, norepinephrine and enkephalin.

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.

Stability of hippocampal place cell activity across the rat estrous cycle

Findings from both in vitro and in vivo studies have shown that estrogen exerts pronounced effects on hippocampal morphology and physiology. The degree to which these molecular findings influence hippocampal processing in freely behaving animals is unclear. The present study assessed the effect of the estrous cycle on hippocampal place cells in naturally cycling rats during two behavioral states. Female Sprague-Dawley rats were trained to alternate on a U-shaped runway for food reinforcement. Single-unit recordings of hippocampal CA1 cells were conducted under two conditions: (1) at rest on a holder, and (2) running on the maze. Spatial firing characteristics of the cells were examined at different stages of the estrous cycle (i.e., diestrus, proestrus, and estrus). Specifically, information was collected on (1) mean firing rates; (2) basic place field parameters; and (3) changes in the firing dynamics of these cells (e.g., burst properties). The findings showed a decrease in mean firing rate on the maze during proestrus. However, other basic measures of spatial tuning and burst properties were unchanged. The current study suggests that there is relative stability of hippocampal place cells across the estrous cycle during a well-trained task.

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.

Inhibition of tyrosine kinase receptor type B synthesis blocks axogenic effect of estradiol on rat hypothalamic neurones in vitro

17-beta-estradiol (E2) increases axonal growth and tyrosine kinase receptor (Trk)B levels of male-derived hypothalamic neurones in vitro. To investigate whether the axogenic response depends on the upregulation of TrkB, we analysed neuritic growth and neuronal polarization in cultures treated with an antisense oligonucleotide against TrkB mRNA. In cultures without E2, treatment with 7.5 or 10 micro m antisense reduced TrkB levels and the percentage of neurones showing an identifiable axon; the number and length of minor processes were increased. In cultures treated with 5 micro m antisense, morphometric parameters were normal although total TrkB levels were reduced. The same dose prevented the E2-dependent increase of TrkB levels and suppressed the axogenic effect of E2. These results indicate that TrkB is necessary for normal neuronal growth and maturation and further suggest that an increase in TrkB is necessary for E2 to exert its axogenic effect in male-derived neurones.

Hormonal symphony: steroid orchestration of gene modules for sociosexual behaviors

Genes induced by estrogens in the mammalian forebrain influence a variety of neural functions. Among them, reproductive behavior mechanisms are very well understood. Their functional genomics provide a theoretical paradigm for linking genes to neural circuits to behavior. We propose that estrogen-induced genes are organized in modules: Growth of hypothalamic neurons; Amplification of the estrogen effect by progesterone; Preparative behaviors; Permissive actions on sex behavior circuitry; and Synchronization of mating behavior with ovulation. These modules may represent mechanistic routes for CNS management of successful reproduction. Moreover, new microarray results add estrogen-dependent genes, including some expressed in glia, suggesting possible hormone-dependent neuronal/glial coordination.

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.

Axonal sprouting of a brainstem-spinal pathway after estrogen administration in the adult female rhesus monkey

The nucleus retroambiguus (NRA) is located in the caudal medulla oblongata and contains premotor neurons that project to motoneuronal cell groups in the brainstem and spinal cord. NRA projections to the lumbosacral cord are species specific and might be involved in mating behavior. In the female cat, this behavior is estrogen dependent, and estrogen induces axonal sprouting in the NRA-lumbosacral pathway. Because female receptive behavior in primates is not fully dependent on estrogen, the question arises as to whether the capacity of estrogen-induced sprouting is preserved in primates. The effect of estrogen was studied on the NRA-lumbosacral projection with the use of wheat germ agglutinin conjugated to horseradish peroxidase as a tracer in six adult ovariectomized rhesus monkeys with or without estrogen priming (three controls and three treated with 20 microg/day of estradiol benzoate subcutaneously for 14 days). Light microscopy showed that the density of arborizing labeled NRA axons in the lumbosacral cord was greater in estrogen-treated than in control animals. Ultrastructurally, labeled NRA terminal profiles were quantified in motoneuron pools that supply muscles of the abdominal wall, axial, and pelvic floor. After estrogen treatment, the average number of labeled terminal profiles per area of the abdominal wall, axial, and pelvic floor motoneuron pool increased 1.5-, 3.3-, and 2.8-fold, respectively. In the estrogen-treated cases, 8.9% of labeled terminal profiles showed characteristics of growth cones. In controls, such profiles were rarely observed. The results showed that estrogen induces axonal sprouting in a brainstem-spinal pathway in the adult female rhesus monkey. These findings supported the concept that the NRA-lumbosacral pathway may be involved in sexual behavior. Moreover, they demonstrated that a long descending brainstem-spinal tract in adult nonhuman primates retains the capacity for axonal sprouting.

Estrogen-induced dendritic spine elimination on female rat ventromedial hypothalamic neurons that project to the periaqueductal gray

Neurons of the ventromedial hypothalamic nucleus (VMH) that project to the periaqueductal gray (PAG) form a crucial segment of the motor pathway that produces the lordosis posture, the hallmark of female rat sexual behavior. One suggested mechanism through which estrogen facilitates lordosis is by remodeling synaptic connectivity within the VMH. For instance, estrogen alters VMH dendritic spine density. Little is known, however, about the local VMH microcircuitry governing lordosis nor how estrogen alters synaptic connectivity within this local circuit to facilitate sexual behavior. The goal of this study was to define better the neuron types within the VMH microcircuitry and to examine whether estrogen alters synaptic connectivity, as measured by dendritic spine density, on VMH projection neurons. A retrograde tracer was injected into the PAG of ovariectomized rats treated with vehicle or estradiol. Retrogradely labeled VMH neurons were filled with Lucifer yellow, then immunostained for estrogen receptor-alpha (ER alpha). VMH neurons that project to the PAG had more dendrites than functionally unidentified neurons. Additionally, VMH projection neurons could be subdivided into those located within the cluster of ER alpha-containing neurons and those medial to the cluster. Estrogen decreased spine density by 57% on the long primary dendrites of VMH projection neurons located within the ER alpha cluster but not on projection neurons medial to the cluster. Only 4% of the VMH projection neurons expressed ER alpha. These results suggest that estrogen may facilitate sexual behavior by decreasing spines selectively, via an indirect mechanism, on a subset of VMH neurons that project to the PAG.

Oxytocin-secreting neurons: A physiological model of morphological neuronal and glial plasticity in the adult hypothalamus

Oxytocin-secreting neurons of the hypothalamoneurohypophysial system undergo reversible morphological changes whenever they are strongly stimulated. In the hypothalamus, such structural plasticity is represented by modifications in the size and shape of their somata and dendrites, in the extent to which their surfaces are covered by glia, and in the density of their synapses. In the neurohypophysis, there is a parallel reduction in glial (pituicyte) coverage of their axons together, with retraction of pituicyte processes from the perivascular basal lamina and an increase in the number and size of their terminals. These changes occur rapidly, within a few hours. On the other hand, the system returns to its prestimulated condition on arrest of stimulation at a rate that depends on the length of time it has remained activated. Such neuronal-glial changes have several functional consequences. In the hypothalamic nuclei, reduction in astrocytic coverage of oxytocinergic neurons and their synapses modifies extracellular ionic homeostasis and glutamate clearance and, therefore, their overall excitability. Since it results in extensive dendritic bundling, it may also lead to ephaptic interactions and may facilitate dendritic electrotonic coupling. A most important indirect effect may be to permit synaptic remodeling that occurs concomitantly and that results in significant increases in the number of excitatory and inhibitory synapses driving their activity. In the stimulated neurohypophysis, glial retraction results in increased levels of extracellular K+ which can enhance neurohormone release while an enlarged neurovascular contact zone may facilitate diffusion of neurohormone into the circulation. Ongoing work aims to unravel the cell mechanisms and factors underlying such plasticity and has revealed that neurons and glia of the hypothalamoneurohypophysial system continue to express juvenile molecular features associated with similar neuronglial interactions and synaptic events during development and regeneration. They include strong expression of cell surface adhesion molecules like F3/contactin and polysialylated neural cell adhesion molecule, extracellular matrix glycoproteins like tenascin C, and cytoskeletal proteins like vimentin and microtubule-associated protein 1D. Some of these molecules reach the cell surface constitutively while others follow the activity-dependent regulated pathway. We consider many of these molecular features permissive, allowing oxytocin neurons and their glia to undergo morphological remodeling throughout life, provided the proper stimulus intervenes. In the hypothalamic nuclei, one such stimulus is centrally released oxytocin; in the neurohypophysis, an adrenergic, cAMP-mediated mechanism appears responsible.

Estrogen effects on object memory and cholinergic receptors in young and old female mice

We investigated whether object recognition memory is modulated by estrogen in young (5 month) and aged (24 month) female C57Bl/6J mice, and if cholinergic muscarinic receptors might contribute to this response. Mice that were ovariectomized, or ovariectomized plus estradiol-treated three weeks before behavioral testing or quantitative autoradiography were compared to intact mice. Memory for a previously encountered object deteriorated significantly between 3 and 6h after initial exposure, regardless of animal age. In both young and aged mice, estradiol-treated mice showed significantly greater recall than did ovariectomized mice. In both age groups, the apparent number of [(3)H]pirenzepine/M(1)-like and [(3)H]AFDX384/M(2)-like muscarinic receptor binding sites was reduced in the basal forebrain as well as its projection areas following ovariectomy, but this decrease was not alleviated by estrogen. Aging poorly affected object memory, but reduced muscarinic binding in some cortical subregions and in the caudate nucleus. These findings suggest that estrogen effects on memory in C57Bl/6J mice are not due to changes in the number of muscarinic receptors.

Invited review: Estrogens effects on the brain: multiple sites and molecular mechanisms

Besides their well-established actions on reproductive functions, estrogens exert a variety of actions on many regions of the nervous system that influence higher cognitive function, pain mechanisms, fine motor skills, mood, and susceptibility to seizures; they also appear to have neuroprotective actions in relation to stroke damage and Alzheimer's disease. Estrogen actions are now recognized to occur via two different intracellular estrogen receptors, ER-alpha and ER-beta, that reside in the cell nuclei of some nerve cells, as well as by some less well-characterized mechanisms. In the hippocampus, such nerve cells are sparse in number and yet appear to exert a powerful influence on synapse formation by neurons that do not have high levels of nuclear estrogen receptors. However, we also find nonnuclear estrogen receptors outside of the cell nuclei in dendrites, presynaptic terminals, and glial cells, where estrogen receptors may couple to second messenger systems to regulate a variety of cellular events and signal to the nuclear via transcriptional regulators such as CREB. Sex differences exist in many of the actions of estrogens in the brain, and the process of sexual differentiation appears to affect many brain regions outside of the traditional brain areas involved in reproductive functions. Finally, the aging brain is responsive to actions of estrogens, which have neuroprotective effects both in vivo and in vitro. However, in an animal model, the actions of estrogens on the hippocampus appear to be somewhat attenuated with age. In the future, estrogen actions over puberty and in pregnancy and lactation should be further explored and should be studied in both the hypothalamus and the extrahypothalamic regions.

The synaptic organization of VMH neurons that mediate the effects of estrogen on sexual behavior

Estrogen acts in the hypothalamic ventromedial nucleus (VMH) of female rats to promote sexual behavior, as typified by the lordosis response. Morphological changes in the VMH, such as increased synaptic profiles and increased dendritic spines, suggest that estrogen may modulate behavior by altering VMH synaptic organization. To understand the significance of these changes, this laboratory has been investigating the functional classes of lordosis-relevant neurons and their local connectivity. A neurotropic virus, pseudorabies virus (PRV), was used to transneuronally label the CNS network that controls the lordosis-producing muscles. When PRV was placed in the lumbar epaxial muscles, it was sequentially detected in the lumbar ventral horn, the medullary reticular formation, the periaqueductal gray, and finally the VMH. Subsequent analysis showed that the population of VMH neurons that were initially infected with PRV largely resided beyond the cluster of estrogen receptor-containing neurons. In a separate study, VMH neurons were visualized with Lucifer yellow, and their morphology was analyzed using confocal microscopy. Such analysis confirmed that estrogen treatment increased dendritic spines in the VMH. The particular VMH neurons in this study did not express nuclear estrogen receptor, which suggests that estrogen can increase spine density indirectly. These results represent initial steps toward unraveling the local circuit that mediates estrogenic action on a specific reproductive behavior.

Neurotrophic and neuroprotective actions of estrogens and their therapeutic implications

Originally known for its regulation of reproductive functions, estradiol, a lipophilic hormone that can easily cross plasma membranes as well as the blood-brain barrier, maintains brain systems subserving arousal, attention, mood, and cognition. In addition, both synthetic and natural estrogens exert neurotrophic and neuroprotective effects. There is increasing evidence that estrogen actions are mediated by nongenomic as well as direct and indirect genomic pathways. Although in vitro models have provided the most extensive evidence for neurotrophic and neuroprotective actions to date, there are also in vivo studies that support these actions.

Ovariectomy up-regulates neuronal neurofilament light chain mRNA expression with regional and temporal specificity

Estrogens can influence the survival, plasticity and function of many adult neurons. Many of these effects, such as neurite outgrowth and increased dendritic spine density, are mediated by changes in neuronal cytoskeletal architecture. Since neurofilament proteins play a key role in the maintenance and remodeling of the neuronal cytoskeleton, we postulated that changes in neurofilament light chain mRNA may parallel some of the alterations in neuronal architecture which follow bilateral ovariectomy. We measured neurofilament light chain mRNA levels using a ribonuclease protection assay at two time-points after ovariectomy in mature female rats. One week after ovariectomy, neurofilament light chain mRNA levels (corrected for glucose-6-phosphate dehydrogenase mRNA) did not differ from sham-operated animals in the five brain regions examined (hypothalamus, striatum, hippocampus, frontal cortex and occipital cortex). Four months after ovariectomy, neurofilament light chain mRNA levels were similarly unchanged in the hypothalamus and striatum. In contrast, statistically significant increases in neurofilament light chain mRNA expression were observed in the three regions receiving basal forebrain projections (hippocampus, frontal cortex and occipital cortex). In situ hybridization demonstrated increases in neurofilament light chain mRNA expression involving subpopulations of smaller medial septal neurons. There also appeared to be an increased number of larger septal neurons following long-term ovariectomy. We propose that atrophic changes involving basal forebrain projection fibers are followed by compensatory axonal growth by other 'intact' basal forebrain neurons. Increased neurofilament light chain mRNA expression and somatic hypertrophy in medial septal neurons may both be reflective of the need to sustain an axonal network which is larger and more complex. In contrast, increased neurofilament light chain mRNA expression observed in basal forebrain targets following long-term ovariectomy may be reflective of compensatory changes taking place in local neurons.

Tracking the estrogen receptor in neurons: implications for estrogen-induced synapse formation

Estrogens (E) and progestins regulate synaptogenesis in the CA1 region of the dorsal hippocampus during the estrous cycle of the female rat, and the functional consequences include changes in neurotransmission and memory. Synapse formation has been demonstrated by using the Golgi technique, dye filling of cells, electron microscopy, and radioimmunocytochemistry. N-methyl-d-aspartate (NMDA) receptor activation is required, and inhibitory interneurons play a pivotal role as they express nuclear estrogen receptor alpha (ERalpha) and show E-induced decreases of GABAergic activity. Although global decreases in inhibitory tone may be important, a more local role for E in CA1 neurons seems likely. The rat hippocampus expresses both ERalpha and ERbeta mRNA. At the light microscopic level, autoradiography shows cell nuclear [3H]estrogen and [125I]estrogen uptake according to a distribution that primarily reflects the localization of ERalpha-immunoreactive interneurons in the hippocampus. However, recent ultrastructural studies have revealed extranuclear ERalpha immunoreactivity (IR) within select dendritic spines on hippocampal principal cells, axon terminals, and glial processes, localizations that would not be detectable by using standard light microscopic methods. Based on recent studies showing that both types of ER are expressed in a form that activates second messenger systems, these findings support a testable model in which local, non-genomic regulation by estrogen participates along with genomic actions of estrogens in the regulation of synapse formation.

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.

A confocal microscopic study of synaptic inputs to gonadotropin-releasing hormone cells in mouse brain: regional differences and enhancement by estrogen

Even though the cells producing gonadotropin-releasing hormone (GnRH) are scattered in the basal forebrain, a large proportion of them is present in the organum vasculosum of the lamina terminalis (OVLT) and in the preoptic area. The present studies were undertaken to investigate whether there is any difference in the number of synaptic inputs between GnRH cells located in the OVLT and those located at more anterior levels of the brain. Immunohistochemical staining for the synaptic marker synaptophysin coupled with confocal microscopy was employed to analyze synaptic inputs to GnRH cells located at the two levels examined. The results indicate that GnRH cells in the OVLT region receive a greater number of synaptophysin-immunoreactive appositions as compared with those located in the anterior septum. This supports the existence of subsets among the GnRH cells located in the basal forebrain. The effect of estradiol on the number of synaptophysin-immunoreactive appositions onto GnRH cells was also studied. Treatment of ovariectomized mice with estradiol significantly enhanced the number of synaptophysin-immunoreactive appositions to GnRH cells located at both levels examined. Thus the effect of estrogen on GnRH cells may be mediated in part by changes in the number of synaptic contacts.

Enlarging effects of estradiol on the nuclear volume of neurons in the hypothalamus during aging

Neuronal nuclear volumes (NNVs) were measured in the medial preoptic nucleus (MPN), anterior hypothalamic area (AHA) and arcuate nucleus (ARN) of young adult, middle-aged, and old rats of both sexes. The NNVs in the darkly stained sexual-dimorphic nucleus of the preoptic area (SDN-POA) and the lighter staining surrounding area (non-SDN-POA) within the MPN were measured separately. Intact young and middle-aged female rats had larger NNVs than those of the males in SDN-POA, non-SDN-POA and AHA but not in ARN. During aging, only intact old female rats manifested significant NNV shrinkage in all the measured areas. Long-term treatment with estradiol benzoate (EB) caused a significant enlargement of the NNVs in non-SDN-POA and ARN of middle-aged and old male rats as well as the NNVs in SDN-POA, non-SDN-POA and ARN of old female rats. The enlarging effect of EB on NNVs in both SDN-POA and non-SDN-POA of female rats could be prevented by ovariectomy. Furthermore, NNVs in SDN-POA and non-SDN-POA of ovariectomized female rats were even smaller than those of the age-matched intact female rats. These results indicate that: (1) the NNVs of MPN and ARN in male and female rats were enlarged after long-term exposure of physiological dose of estradiol; (2) the enlarging effects of EB on NNV in MPN can explain why the NNV of intact female rats is larger than that of males, and (3) during aging, the sex-specific shrinkage of NNVs in MPN, AHA and ARN of female rats may be due to an intrinsic aging process rather than long-term effects of EB.

Differential effect of oestradiol and astroglia-conditioned media on the growth of hypothalamic neurons from male and female rat brains

To determine whether soluble products from different CNS regions differ in their ability to support oestrogen-stimulated neurite growth, hypothalamic neurons from sexually segregated embryos were cultured with astroglia-conditioned medium (CM) derived from cortex, striatum and mesencephalon, with or without 17-beta-oestradiol 100 nM added to the medium. After 48 h in vitro, neurite outgrowth was quantified by morphometric analysis. Astroglia-CM from mesencephalon (a target for the axons of hypothalamic neurons) induced the greatest axogenic response in males and in this case only a neuritogenic effect could be demonstrated for oestradiol. On the other hand, astroglia-CM from regions that do not receive projections from ventromedial hypothalamus inhibited axon growth. A sexual difference in the response of hypothalamic neurons to astroglia-CM and oestradiol was found; growth of neurons from female foetuses was increased by astroglia-CM from mesencephalon, but no neuritogenic effect could be demonstrated for oestradiol in these cultures. Blot immunobinding demonstrated the presence of receptors for neurotrophic factors in cultures of hypothalamic neurons; Western blot analysis of these cultures demonstrated that oestradiol increased the concentration of trkB and IGF-I Rbeta, whereas trkA was not detected and the concentration of trkC was not modified. These results support the hypothesis that target regions produce some factor(s) that stimulate the growth of axons from projecting neurons and further indicate that in the case of males this effect is modulated by oestradiol, perhaps mediated through the upregulation of trkB and IGF-I receptors.

Pineal influence on annual nuclear volume changes in ventromedial hypothalamic nucleus (VMH) neurons of the male Wistar rat

The ventromedial hypothalamic nucleus (VMH) regulates various autonomic, endocrine, and behavioral activities. These activities show annual changes, and the pineal gland is involved in their adjustment to environmental cues. Therefore, this study investigated whether the VMH belongs to the effector structures of the pineal gland. To abolish the rhythmic melatonin release, male Wistar rats were subjected to pinealectomy (PX) or ganglionectomy (sympathetic denervation of the pineal gland, GX) regularly at the beginning of any of the four seasons. Brains from animals of PX-, GX-, and sham-operated control groups were prepared 3 months later for measurement of the nuclear volume, which changes according to the general gene activity. At each of the four seasons, 2000 nuclei of VMH neurons stemming from 18 animals per group were measured to obtain both seasonal daily mean values and annual mean values, respectively, as well as to calculate annual curves of the nuclear volume using empirical regression and locally adjusted polynomial approximation. The major findings are the following. First, inactivation of the pineal function influences the nuclear activity of VMH neurons, (2) PX and GX mainly depress the nuclear activity, indicating that the pineal influence on the VMH may predominantly be a stimulatory one. Third, size and direction of the changes caused by PX and GX vary in a seasonally dependent manner. Fourth, the annual rhythm of the nuclear activity of the VMH is modified by PX and GX. To explain how the pineal effects on the VMH may be mediated, a possible inhibitory influence of the suprachiasmatic nucleus (SCN), which has been activated in the same animals following both PX and GX, is discussed. In conclusion, the results confirm that the nuclear activity of VMH neurons underlies pineal influences. This also indicates an involvement of the pineal gland in many VMH-regulated functions.

Inhibition of dendritic spine induction on hippocampal CA1 pyramidal neurons by a nonsteroidal estrogen antagonist in female rats

Estrogens regulate the formation of excitatory synaptic connections in the hippocampus of female rats. Because the adult hippocampus has a very low concentration of intracellular estrogen receptors, it is unclear whether a conventional genomic mechanism is involved. Nonsteroidal estrogen antagonists are useful tools to study estrogen action because they can provide pharmacological data in favor of a particular pathway of estrogen action and evidence against other pathways. To investigate the role of intracellular estrogen receptors in the estrogen induction of synapse formation, we took advantage of previous studies in which we had shown that an estrogen antagonist, CI-628, enters the brain and blocks estrogen induction of progestin receptors to study whether the same antagonist would either mimic or block effects of estradiol to induce excitatory spine synapses. Using silver impregnation of neurons by the single section Golgi technique and morphometric analysis, we found that CI-628 effectively prevented estrogen induction of spines on CA1 pyramidal neurons, without having any agonist effects of its own. This result is consistent with an action of estradiol via intracellular estrogen receptors that are known to be expressed by interneurons within the hippocampus.

Estrogen-mediated structural and functional synaptic plasticity in the female rat hippocampus

Light and electron microscopic studies have shown that ovarian steroids regulate the density and number of excitatory synaptic inputs to hippocampal pyramidal cells in the adult female rat; elevated levels of estradiol are associated with a higher density of dendritic spine synapses on CA1 pyramidal cells. Electrophysiological analyses indicate that these hormone-induced synapses increase hippocampal excitability as well as the potential for synaptic plasticity. Importantly, correlation of dendritic spine density and sensitivity to synaptic input of individual CA1 pyramidal cells from estradiol-treated and control animals suggests that synapses induced by estradiol may be a specialized subpopulation that contains primarily the NMDA subtype of glutamate receptor. The apparent NMDA receptor specificity of these synapses may be key to understanding their functional significance. Currently, the behavioral consequences of additional spine synapses are unknown. Numerous studies have aimed at correlating hormone-induced changes in hippocampal connectivity with differences in hippocampus-dependent spatial learning ability in mazes, but the results of these efforts have been equivocal. Anatomical, electrophysiological, and behavioral studies of estradiol-mediated hippocampal plasticity are reviewed. In conclusion, it is suggested that standard behavioral tests of hippocampal function are not sufficient to reveal the behavioral consequences of hormone-induced hippocampal plasticity. Rather, understanding the behavioral consequences of estradiol and progesterone effects on hippocampal connectivity may require analysis of the hippocampus' cognitive and spatial information processing functions in relation to alternative biologically relevant behaviors. A (nonexclusive) proposal that hormone-induced hippocampal plasticity may facilitate appropriate prepartum/maternal behavior is discussed.

Sensory and motor components of reproductive behavior: pathways and plasticity

Reproductive behavior in most mammalian species consists of a highly stereotyped pattern of movements, is elicited by specific sensory stimuli and is sex steroid dependent. The present paper describes a concept of the pathways in the midbrain, brainstem and spinal cord which control the receptive posture of the female cat. The midbrain periaqueductal gray (PAG), which is an important structure in the Emotional Motor System (EMS), receives direct input from a distinct group of neurons in the dorsal horn of the lumbosacral cord. This cell group overlaps with the location of pelvic and to lesser extent, pudendal nerve primary afferents, which convey information from the pelvic viscera and sex organs to the central nervous system. The PAG, in turn, controls various motor components of female receptive behavior using different pathways. For example, immobility, which is one of the characteristics of receptive behavior, might be mediated by a diffuse pathway from the PAG, via the ventral part of the medial medullary tegmentum, to all parts of the spinal ventral horn. More specific components, such as hindlimb treading, lateral deviation of the tail and elevation of the lower back, are thought to be controlled by a circumscribed projection from the PAG to the nucleus retroambiguus (NRA). The NRA is a group of interneurons at the transition between brainstem and spinal cord and projects directly to distinct lumbosacral motoneuronal cell groups, which innervate muscles that are likely to be involved in the female receptive posture. Estrogen induces axonal sprouting of the NRA-lumbosacral pathway in adult female cats, which explains why female cats only display receptive behavior when estrogen levels are high.