Ultrastructural evidence of a sexual dimorphism in the neuropil of the medial preoptic nucleus of the rat: a quantitative study

Steroids, sex and the cerebellar cortex: implications for human disease

Neurosteroids play an important role in the development of the cerebellum. In particular, estradiol and progesterone appear capable of inducing increases in dendritic spine density during development, and there is evidence that both are synthesized de novo in the cerebellum during critical developmental periods. In normal neonates and adults, there are few differences in the cerebellum between the sexes and most studies indicate that hormone and receptor levels also do not differ significantly during development. However, the sexes do differ significantly in risk of neuropsychological diseases associated with cerebellar pathology, and in animal models there are noticeable sex differences in the response to insult and genetic mutation. In both humans and animals, males tend to fare worse. Boys are more at risk for autism and Attention Deficit Hyperactivity Disorder than girls, and schizophrenia manifests at an earlier age in men. In rats males fare worse than females after perinatal exposure to polychlorinated biphenyls, and male mice heterozygous for the staggerer and reeler mutation show a more severe phenotype. Although very recent evidence suggests that differences in neurosteroid levels between the sexes in diseased animals may play a role in generating different disease phenotypes, the reason this hormonal difference occurs in diseased but not normal animals is currently unknown.

The neurobiology of sexual partner preferences in rams

The question of what causes a male animal to seek out and choose a female as opposed to another male mating partner is unresolved and remains an issue of considerable debate. The most developed biologic theory is the perinatal organizational hypothesis, which states that perinatal hormone exposure mediates sexual differentiation of the brain. Numerous animal experiments have assessed the contribution of perinatal testosterone and/or estradiol exposure to the development of a male-typical mate preference, but almost all have used hormonally manipulated animals. In contrast, variations in sexual partner preferences occur spontaneously in domestic rams, with as many as 8% of the population exhibiting a preference for same-sex mating partners (male-oriented rams). Thus, the domestic ram is an excellent experimental model to study possible links between fetal neuroendocrine programming of neural mechanisms and adult sexual partner preferences. In this review, we present an overview of sexual differentiation in relation to sexual partner preferences. We then summarize results that test the relevance of the organizational hypothesis to expression of same-sex sexual partner preferences in rams. Finally, we demonstrate that the sexual differentiation of brain and behavior in sheep does not depend critically on aromatization of testosterone to estradiol.

Perinatal exposure to androgen suppresses sexual dimorphism in nerve trunk diameter, axon number, and fiber size spectrum: a quantitative ultrastructural study of the adult rat mammary nerve

The adult mammary nerves (MNs) from female, male, and testosterone-androgenized female rats were studied by light and electron microscopy. The female MNs trunk has twice the diameter of that of the male. Morphometry showed a significantly more myelinated (307 +/- 6) and unmyelinated axons (1654 +/- 10) in the female MN than the male MN (278 +/- 6 and 1373 +/- 28, respectively). Perinatal exposure of the female to testosterone significantly reduced the number of both axon types in the MN in adulthood (244 +/- 6 myelinated and 1300 +/- 32, unmyelinated). Another sexual dimorphism is a distinct group of large (>7.0 microm in diameter) myelinated axons known to conduct sensory information (i.e., touch and vibration). Because the male and the perinatally-androgenized female MNs lack these fibers, it is concluded that gonadal sex hormones may promote the differentiation of specific sets of axons committed to transmission of sensory cues relevant to reproduction.

Induction of PGE2 by estradiol mediates developmental masculinization of sex behavior

Adult male sexual behavior in mammals requires the neuronal organizing effects of gonadal steroids during a sensitive perinatal period. During development, estradiol differentiates the rat preoptic area (POA), an essential brain region in the male copulatory circuit. Here we report that increases in prostaglandin-E(2) (PGE(2)), resulting from changes in cyclooxygenase-2 (COX-2) regulation induced by perinatal exposure to estradiol, are necessary and sufficient to organize the crucial neural substrate that mediates male sexual behavior. Briefly preventing prostaglandin synthesis in newborn males with the COX inhibitor indomethacin permanently downregulates markers of dendritic spines in the POA and severely impairs male sexual behavior. Developmental exposure to the COX inhibitor aspirin results in mild impairment of sexual behavior. Conversely, administration of PGE(2) to newborn females masculinizes the POA and leads to male sex behavior in adults, thereby highlighting the pathway of steroid-independent brain masculinization. Our findings show that PGE(2) functions as a downstream effector of estradiol to permanently masculinize the brain.

Neurons in the principal nucleus of the bed nuclei of the stria terminalis provide a sexually dimorphic GABAergic input to the anteroventral periventricular nucleus of the hypothalamus

Neurons of the principal nucleus of the bed nuclei of the stria terminalis (BSTp) process pheromonal and viscerosensory stimuli associated with reproduction and relay this information to preoptic and hypothalamic cell groups that regulate reproductive function. The anteroventral periventricular nucleus of the hypothalamus (AVPV), a nucleus involved in the regulation of gonadotropin secretory patterns, receives dense projections from BSTp neurons in males but not in females. By injecting the anterograde tracer, Phaseolus vulgaris leucoagglutinin (PHAL), into the BSTp of rats and immunohistochemically colocalizing the GABA synthetic enzyme, GAD65, to PHAL-immunoreactive fibers in the AVPV, we tested the hypothesis that these sex-specific projections arise from BSTp neurons that synthesize the inhibitory neurotransmitter GABA. Although dense GAD65-immunoreactive fiber terminals were observed in both the male and female AVPV, higher numbers of GAD65-labeled terminals were found in the male, and those localized to PHAL-immunoreactive fibers were seen almost exclusively in males. Treatment of newborn females with testosterone or neonatal orchidectomy of males reversed these sex differences, while GAD65-immunoreactivity in the AVPV was not altered in response to exogenous hormone treatments administered to peripubertal animals. Our results suggest that projections from BSTp neurons constitute a stable, sex-specific GABAergic input to the AVPV that is patterned permanently by perinatal hormone exposure.

A novel mechanism of dendritic spine plasticity involving estradiol induction of prostaglandin-E2

The mechanisms establishing and maintaining dendritic spines in the mammalian CNS remain primarily unknown. We report a novel mechanism of neuronal spine plasticity in the developing preoptic area in which estradiol induces prostaglandin-E2 (PGE2) synthesis that in turn increases the density of spine-like processes. Estradiol requires PGE2 synthesis, in vivo and in vitro, and upregulates the dendritic spine protein spinophilin, an effect attenuated by antagonism of the AMPA-kainate receptor. This signaling pathway may involve cross talk between neighboring neurons and astrocytes and appears specific to the preoptic area in that hippocampal neurons responded with an increase in spinophilin to estradiol but not PGE2. Regionally specific mechanisms of estradiol-mediated synaptic plasticity allow for epigenetic control of complex sex-typic behaviors.

Histological and ultrastructural characterization of interfascicular neurons in the rat anterior commissure

The histological, connectional, and ultrastructural characteristics of a peculiar neuron type in the rat anterior commissure (AC) are described. Since these cells are located among the axonal fascicles of the rostral and caudal parts of the AC, they are termed interfascicular neurons (IFN). In rapid-Golgi sections IFNs appeared in two forms: internuncial (i.e., short axon) and projection neurons (i.e., long axon). The axon of the internuncial neurons terminates upon neighboring IFNs. The projection neurons give rise to an axon which is either incorporated into commissural fibers or ramifies into 12-26 collaterals running laterally in opposite directions along commissural axons. Immunohistochemistry to microtubule-associated protein 2 combined with confocal microscopy showed that IFNs display short varicose dendrites which remain confined to the domain of the AC. The neuronal nature of IFNs was confirmed with the electron microscope on the basis of distinctive organelles and the presence of synaptic inputs. Small areas of neuropil surround some IFNs. These areas are composed of proximal dendrites, terminal axons, axo-shaft and axo-spinous synapses. Because IFNs with their afferents and efferents constitute sufficient elements to integrate neural inputs, it is proposed that they may be involved in processing nerve impulses proceeding from the bilateral cerebral structures connected by the AC.

Effects of testosterone on the synaptology of the medial preoptic nucleus of male Japanese quail

The medial preoptic nucleus (POM) of male Japanese quail is a sexually dimorphic testosterone-dependent structure that plays a key role in the activation of male sexual behavior. Both the total volume of the nucleus and the size of the dorsolateral neurons are decreased in gonadectomized males. Immunocytochemical studies have revealed a complex pattern of innervation: immunopositive fibers for several neuropeptides and neurotransmitters have been detected in the POM; some of them (e.g. vasotocin-immunoreactive fibers) are sexually dimorphic and testosterone-dependent To understand the anatomical bases of these testosterone-dependent neurochemical changes, we performed an ultrastructural study of the POM neuropil in intact sexually mature, gonadectomized, or testosterone-treated gonadectomized males. A complex synaptic organization of the POM neuropil was observed in intact male quail reflecting the heterogeneity of the neurotransmitters and neuropeptides present in this nucleus. Changes in this organization were observed after the endocrine manipulations. The number of axosomatic synapses per cell body decreased after gonadectomy and was restored to the level observed in the intact group after the administration of testosterone. By contrast, no significant change was observed in the density of axodendritic and axospinal synapses after hormonal manipulations which suggests that the total number of synapses in the nucleus should be affected by testosterone (constant density in a changing total volume). The cross-sectional area of synaptic boutons was also decreased by castration and restored to intact level by testosterone. The action of testosterone on the activation of male copulatory behavior in gonadectomized birds is hence paralleled by an extensive rearrangement of neuropil in the POM.

Perinatal administration of testosterone induces hypertrophy of the anterior commissure in adult male and female rats

A possible sex difference in the mean sagittal area of the anterior commissure (AC) was investigated in normal, newborn-castrated, and perinatally-androgenized rats. A second experiment included castrated adult rats from each sex exposed to testosterone twelve days before sacrifice. In normal rats, as well as in those exposed to testosterone as adults, no quantitative difference was found in the AC. However, perinatal exposure to testosterone induced a 20-25% increase in the mean area of the AC of rats from each sex. It is proposed that gonadal sex steroids may have a reciprocal influence upon the structure of central olfactory pathways, due to the influences of the main olfactory system upon gonadotropin secretion.

Galanin microinjected into the medial preoptic nucleus facilitates female- and male-typical sexual behaviors in the female rat

Galanin (GAL) microinjected within the sexually dimorphic medial preoptic nucleus (MPN) facilitates male-typical sexual behaviors in the male rat, a response that requires the presence of testosterone. As in the male, GAL-immunoreactive cells located within the MPN of the female also concentrate gonadal steroids and become less immunoreactive after gonadectomy. Thus, to investigate sexual behaviors in the female and to determine whether effects are comparable to those obtained in the male, GAL was microinjected unilaterally within the MPN of female rats. We report that GAL stimulated female-typical lordosis behavior after estrogen priming, and that the effect was not due to general arousal as measured by nonspecific locomotor activities. In a separate experiment, GAL microinjected within the MPN dose-responsively increased mount frequencies and decreased mount latencies in testosterone-primed females. A higher dose of testosterone was required in females for this stimulation of male-typical sexual behavior than required in a previous experiment in males.

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.

Sexually dimorphic contribution from the fornix to the ventromedial hypothalamic nucleus: a quantitative electron microscopic study

Quantitative electron microscopy confirmed that the neuropil of the ventrolateral part of the ventromedial hypothalamic nucleus (VL-VMHN) of the rat is sexually dimorphic with respect to the density of shaft and axo-spinous synapses, both of which are more numerous in the male. In addition, adult rats with complete interruption of the fornix displayed a sexually dimorphic input in the density of fornical synapses in the neuropil of the VL-VMHN, in which degenerating terminals were more numerous in the male. Perinatal exposure of the female to exogenous testosterone or castration of the newborn male 'inverted' these sex differences, demonstrating their hormonal dependence. It is concluded that (1) the fornix provides synaptic input to the VL-VMHN as proven by orthograde degeneration; (2) the number of fornical endings synapsing in the VL-VMHN is greater in the male than in the female; (3) this dimorphism depends of the organizational effect of gonadal sex steroids.

Gonadal hormones as promoters of structural synaptic plasticity: cellular mechanisms

It is now obvious that the CNS is capable of undergoing a variety of plastic changes at all stages of development. Although the magnitude and distribution of these changes may be more dramatic in the immature animal, the adult brain retains a remarkable capacity for undergoing morphological and functional modifications. Throughout development, as well as in the postpubertal animal, gonadal steroids exert an important influence over the architecture of specific sex steroid-responsive areas, resulting in sexual dimorphisms at both morphological and physiological levels. We are only now beginning to gain insight into the mechanisms involved in gonadal steroid-induced synaptic changes. The number of synaptic inputs to specific neuronal populations is sexually dimorphic and this can be modulated by changes in the sex steroid environment. These modifications can be correlated with other morphological changes, such as glial cell activation, that are occurring simultaneously in the same anatomical area. Indeed, the close physical relationship between glial cells and neuronal synaptic contacts makes them an ideal candidate for participating in this process. Interestingly, not only can the morphology and immunoreactivity of glial cells be modulated by gonadal steroids, but a close negative correlation between the number of synapses and the amount of glial ensheathing of a neuron has been demonstrated, suggesting an active participation of these cells in this process. Glia have sex steroid receptors, are capable of producing and metabolizing steroids, and can produce other neuronal trophic factors in response to sex steroids. Hence, their role in gonadal steroid-induced synaptic plasticity is becoming more apparent. In addition, there is recent evidence that this process may involve certain cell surface molecules, such as the N-CAMs, since a specific isoform of this molecule, previously referred to as the embryonic form, is found in those areas of the brain which maintain the capacity to undergo synaptic remodelling. However, there is much work to be done in order to fully understand this phenomenon and before bringing it into a clinical setting in hopes of treating neurodegenerative diseases or injuries to the nervous system.

Evidence of a direct projection of the vomeronasal organ to the medial preoptic nucleus and hypothalamus

In rodents the vomeronasal system plays an important role in modulating the hypothalamic control of gonadotropin secretion. It is accepted that the anatomical pathways by which the vomeronasal organ influences the neuroendocrine aspects of the reproductive behavior involves a polysynaptic system, including the accessory olfactory bulb, the 'vomeronasal amygdala', the bed nucleus of the stria terminalis and medial preoptic and ventromedial hypothalamic nuclei. We found that 72 h after unilateral transection of the vomeronasal nerves of an ipsilateral orthograde degeneration is seen in the neuropil of the medial preoptic, ventromedial, and arcuate hypothalamic nuclei. This represents an anatomical evidence of a monosynaptic link between the vomeronasal neuroepithelium and the medial preoptic nucleus, and hypothalamus.