Regional variations in glia and neuropil within the hypothalamic ventromedial nucleus

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.

Special relationship of gamma-aminobutyric acid to the ventromedial nucleus of the hypothalamus during embryonic development

The ventromedial nucleus of the hypothalamus (VMH) is a key nucleus for regulating homeostatic, neuroendocrine, and behavioral functions. We conducted immunocytochemical analyses by using antisera directed against gamma-aminobutyric acid (GABA), its synthetic enzyme glutamic acid decarboxylase (GAD67), GABA-A receptor subunits (alpha2, beta3, epsilon), estrogen receptor-alpha, and Neuropeptide Y (NPY) in the region of the VMH in embryonic mice to identify potential patterning elements for VMH formation. Cells and fibers containing GABA and GAD67 encircled the primordial VMH as early as embryonic day 13 (E13) when the cytoarchitecture of the VMH was not recognizable by Nissl stain. At E16-17 the cytoarchitecture of the VMH became recognizable by Nissl stain as GABAergic fibers invaded the nucleus, continued postnatally, and by adulthood the density of GABAergic fibers was greater inside than outside the VMH. GABA-A receptor subunit expression (beta3 by E13 and alpha2 by E15) within the primordial VMH suggested potential sensitivity to the surrounding GABA signal. Brain slices were used to test whether fibers from distal or proximal sites influenced VMH development. Coronal Vibratome slices were prepared and maintained in vitro for 0-3 days. Nissl stain analyses showed a uniform distribution of cells in the region of the VMH on the day of plating (E15). After 3 days in vitro, cellular aggregation suggesting VMH formation was seen. Nuclear formation in vitro suggests that key factors resided locally within the coronal plane of the slices. It is suggested that either GABA intrinsic to the region nearby the VMH directly influences the development and organization of the VMH, or along with other markers provides an early indicator of pattern determination that precedes the cellular organization of the VMH.

Müller glial cells of the tree shrew retina

The tree shrew is one of the few mammalian species whose retinae are strongly cone dominated, which is usually the case in reptilian and avian retinae. Müller cells of the tree shrew (Tupaia belangeri) retina were studied by transmission electron microscopy of tissue sections and freeze-fracture replicas, by immunolabeling of the intermediate filament protein vimentin in radial paraffin sections and in whole retinae, as well as by intracellular dye injection in slices of retinae. In addition, enzymatically isolated cells were stained by Pappenheim's panoptic staining method. The cells showed an ultrastructure that is similar to other mammalian Müller cells with two exceptions: Due to the extensive lateral fins of cone inner segments, the apical microvilli of Müller cells are arranged in peculiar palisades, and the basket-like Müller cell sheaths around neuronal somata in both nuclear layers consist of unusual multilayered membrane lamellae. Unlike Müller cells in other mammalian species studied thus far, but similar to reptilian and avian Müller cells, those of tree shrews commonly have two or more vitread processes rather than one main trunk. Müller cell densities range between some 13,000 mm-2 in the periphery and about 20,000 mm-2 in the retinal center. Neuron:(Müller)glial cell ratios were estimated to be 7.9:1 in the center and 6.2:1 in the periphery. For each Müller cell, about 1.5 (cone) photoreceptor cells, four or five interneurons of the inner nuclear layer, and about one cell of the ganglion cell layer were counted. This is a much lower number of neurons per Müller cell than in most other mammals studied.

Postnatal development of the hypothalamic ventromedial nucleus: neurons and synapses

1. In this report the postnatal differentiation of the hypothalamic ventromedial nucleus (VMN) was studied. The main maturational changes detected at the fine structural level occurred between 10 and 20 days of postnatal life. 2. In 5-day-old rats the majority of neurons was undifferentiated, with rudimentary cytoplasmic organelles. Dendritic profiles presented an empty appearance due to an electron-lucent matrix and scarce content of organelles. 3. At 10 days there was a significant proliferation of cytoplasmic organelles in the perikaryon, mainly of those involved in protein biosynthesis as the rough endoplasmic reticulum (RER) and the Golgi complex. 4. After 20 days of age the VMN neurons acquired the cytological appearance of adult neurons, with well-organized RER, Golgi complexes, and pleomorphic mitochondria. Concurrent with these changes, there was a marked development of other organelles in the neuropil, which was accompanied by an increase in synaptic density and differentiation of their subsynaptic structures.

Stereological analysis of the hypothalamic ventromedial nucleus. II. Hormone-induced changes in the synaptogenic pattern

Stereological electron microscopic analysis of the rat hypothalamic ventromedial nucleus (VMN) throughout postnatal development revealed that synaptogenesis takes place up to day 45. Our results disclosed a sexual dimorphism in the synaptic organization of the neuropil of the ventrolateral (VL) subdivision of the VMN. The numerical densities of spine and shaft synapses in the adult male were higher than in the female. A dimorphic pattern in the numerical density of spine synapses occurred as early as day 5, and was present throughout postnatal life, even though in the adult rats both spine and shaft synapses were sexually dimorphic. Neonatal treatment of female rats with testosterone increased the numerical density of axodendritic synapses, inducing a pattern similar to the adult male. On the other hand, administration of tamoxifen to newborn male rats significantly reduced the numerical density of spine synapses to levels comparable to normal female rats.

Synaptogenic action of sex steroids in developing and adult neuroendocrine brain

Sex steroids exert potent influences on modulating neural development and neural circuit formation in both developing and adult sex steroid-sensitive neuroendocrine brain. During development, estrogen or aromatizable androgen can act as a neurotropic factor on neural tissues, stimulating axonal and dendritic growth and synapse formation. The development of sexual dimorphic synaptic organization may reflect sex steroid-modulating synaptogenesis in the hypothalamus and limbic system during the perinatal period. The onset of puberty also may be due, at least in part, to stimulation of synapse formation by estrogen in the hypothalamus. In adulthood, estrogen has a facilitatory effect on synapse formation in neural structures such as septum, hypothalamus and midbrain with or without brain lesions, and androgen plays a significant role in regulating synaptic remodeling in the androgen-sensitive spinal motoneuron pools. Thus, sex steroids seems to be critical from the developmental period to adulthood for organizing and reorganizing the neuronal circuitry driving neuroendocrine and behavioral functions.

Synaptic plasticity to estrogen in the lateral septum of the adult male and female rats

There is no sex difference in the number of axodendritic synapses in the lateral septum of adult rats. However, the treatment of adult females with estradiol (E2) for 4 weeks increases significantly the number of synapses, whereas E2 treatment fails to increase the synaptic number in males, suggesting a possible sexually dimorphic synaptic response to estrogen.

Projections of oestrogen-sensitive neurones from the ventromedial hypothalamic nucleus of the female rat

Stimulation of the mesencephalic central grey matter caused antidromic activation of 199 neurones in the hypothalamic ventromedial nucleus of forty-six urethane-anaesthetized female rats, of which twenty-two (ten ovariectomized and twelve ovariectomized and oestrogen treated) had the lateral projection of the nucleus disrupted by a small parasagittal knife-cut. The remaining twenty-four rats (ten ovariectomized and fourteen ovariectomized and oestrogen treated) had a cut in the frontal plane to interrupt caudal projection of the nucleus. In eight animals with the lateral knife-cut, eighteen cells were antidromically activated from the central grey matter as well as from the dorsal longitudinal fascicle. Collision between the potentials activated from these two sites revealed that seventeen of them descended to the central grey matter by way of the periventricular system, one cell having a bifurcated axon to these sites. Likewise, eleven cells in another group of eight animals with the posterior cut were found to project to the central grey matter via the perifornical region. Pre-treatment with oestrogen significantly lowered the activation thresholds and absolute refractory periods of neurones antidromically stimulated from the central grey in ovariectomized animals bearing posterior but not lateral knife-cuts. These results suggest that the ventromedial hypothalamic neurones responsible for oestrogen-dependent autonomic and behavioural functions project to the central grey via a lateral rather than a posterior pathway.

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

Alterations in the fine structure of nuclei of ventromedial hypothalamic neurons in ovariectomized (OVX) rats after either a 2-hour exposure to estradiol (E2) or a discontinuous exposure (2 hours E2/7 hours off/2 hours E2), previously shown sufficient for female rat sexual behavior (Parsons et al., '82a), were examined with the electron microscope. Morphometric measurements of nucleolar, nuclear, and somal areas, and nuclear shape and perimeter were accomplished at the light microscope level. After 2 hours of E2, the appearance of the nucleoplasm was altered, with a decrease in the small, scattered clumps of heterochromatin. Nuclear shape appeared dramatically altered from nonspherical, invaginated profiles toward spherical, smooth profiles. Nucleolar, nuclear, and somal hypertrophy were evident. In addition, stacked rough endoplasmic reticulum was present more frequently in E2-treated than control OVX neurons. After the discontinuous (2 hours/7 hours/2 hours) E2 treatment, progressive loss of small clumps of heterochromatin along the nuclear envelope as well as in the nucleoplasm had occurred. Decrease in a large heterochromatin clump along the nuclear envelope was correlated with an increase in nucleolus-associated chromatin. As determined by a distribution analysis, these estrogen-induced nuclear changes co-occurred more frequently than predicted from mutual independence. These findings, the marked co-occurrence of E2-induced changes in 30% or more of the cells, and the differences between the 2-hour E2 and the 2-hour/7-hour/2-hour group are consistent with a cascade of cell nuclear changes in the first few hours after estrogen onset.

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

The changes produced in the hypothalamic ventromedial nucleus (VMN) of ovariectomized rats after administration of 100 microgram estradiol benzoate/kg body weight were studied using light and electron microscopy. Quantitative morphometric studies included number and size of VMN neurons and nuclei, size and density of terminals and synaptic contacts, spine-to-shaft ratio of postsynaptic elements and relative frequency of two types of synaptic vesicles. Evidence was obtained favoring the concept of heterogeneous composition of the VMN: in ovariectomized animals many cells appeared in a state of quiescence, but other neurons showed no major alterations. Estrogen administration to ovariectomized rats produced evidence of metabolic stimulation such as increase in rough surfaced endoplasmic reticulum, condensation of nucleolar material, enlarged Golgi and presence of pleomorphic mitochondria. The number of neurons in the VMN was not modified by estrogen treatment; however, neuron soma and nuclei were larger. In the ventrolateral division of the VMN terminals and synaptic contacts per unit area were increased after estrogen treatment, but synaptic contact length, terminal size and spine-to-shaft ratio were not modified. The possibility that the differences observed may be consequent to changes in synaptic organization of the VMN related to its estrogen-dependent functions is discussed.

D-glucose infusions into the basal ventromedial hypothalamus and feeding

Following symmetrical bilateral infusion of D-glucose into the basal ventromedial hypothalamus (BVMH), using Alzet minipumps (1 microliter/h of 10% D-glucose for 6 days), average daily food intake was reduced by 27% for the period of treatment. Symmetrical bilateral infusions into the posterior medial hypothalamus had a transitory effect on feeding, as did asymmetrical and unilateral infusions. Infusion of L-glucose into the BVMH did not yield a chronic reduction in food intake. Included in the infused solutions were tracer amounts of [14C]glucose, [14C]proline or [14C]leucine to permit radioautographic estimations of infusate dispersal in the brain and axonal transport patterns from the infusion sites. Infusions were generally well-restricted to 1-1.5 mm of the cannulae tips, and descending transport of amino acids was highest in substantia nigra and midline structures of the mesencephalon and pons including central gray, ventral tegmental area, raphe and ventral tegmental nuclei. These results provide evidence for an energy intake regulatory mechanism situated in the BVMH whose outputs may modulate activity in the substantia nigra and midline brain stem areas.

Insulin binding sites localized to nerve terminals in rat median eminence and arcuate nucleus

Specific binding sites for blood-borne insulin were determined to be selectively localized on axons and axon terminals in the external median eminence and the hypothalamic arcuate nucleus by means of quantitative fine structural radioautography. This localization suggests that discrete populations of hypothalamic nerve terminals are potential targets for the direct effects of insulin and that insulin may act through synaptic mechanisms to influence hypothalamic circuits regulating energy balance and hypophyseal function.

Changes in the fine structure of tanycytes during the annual reproductive cycle of the male little brown bat Myotis lucifugus lucifugus

The hypothalamic arcuate nucleus in the male little brown bat Myotis lucifugus lucifugus was studied with the electron microscope. Animals were killed by intracardial perfusion at each season throughout the year so that the arcuate nucleus could be examined for seasonal variations in morphology. Striking seasonal changes in the fine structure of ependymal tanycytes lining the arcuate nucleus were observed. Tanycytes in animals collected in the fall and early winter exhibited pale processes characterized by a scant internal framework of microtubules and fine filaments. These processes, which were found throughout the arcuate nucleus, exhibited simple irregular shapes. In animals collected between January and June, tanycyte processes contained dense accumulations of fine filaments intermingled with microtubules, and projected long attenuated extensions that often formed multilamellar sheets around axodendritic terminals or other neuronal elements. Tanycyte processes of animals collected in July and August were densely packed with microtubules and fine filaments. The processes radiated elaborate multilamellar extensions that encapsulated axons, dendrites and even entire neuronal perikarya. Multilamellar sheets consisted of as many as 10 or 12 closely spaced gyres. The seasonal variations in tanycyte structure are suggestive of astrocyte-like behaviour. These changes are discussed with respect to seasonal changes in hypothalamic neuroendocrine activity.

The effects of oestrogen on hypothalamic tissue

Oestrogens exert potent influences on hypothalamic differentiation and on sexual function throughout life. During the period in which the hypothalamus differentiates, the availability of oestrogens (or aromatizable androgens) to the hypothalamus results in a male pattern of gonadotropin secretion and in male sexual behaviour in the adult. Lack of oestrogens during this period results in a cyclic schedule of gonadotropin secretion and in female sexual behaviour. Oestrogens or their metabolites determine future sex-specific activity by facilitating synapse formation in developing target circuits. The onset of puberty may also in part reflect oestrogen-stimulated synapse formation in the hypothalamus. In contrast, oestrogen given to an adult female rat causes specific multifocal lesions in the lateral arcuate nucleus. Such animals exhibit persistent oestrus and small multicystic ovaries. Since the same anovulatory syndrome can be produced by mechanical disruption of neuronal pathways from more anterior regions to the arcuate nucleus, it would appear that oestrogen could effect a 'disconnection' of the circuit responsible for cyclic drive of gonadotropin secretion. Furthermore, old female rats spontaneously exhibit the same type of reproductive derangement as well as the arcuate lesion. Thus direct effects of oestrogens on neuronal circuitry within the hypothalamus may be responsible for (1) sexual differentiation, (2) the onset of puberty and sexual activity, and (3) cessation of reproductive function in the female rat.