Post-coital ultrastructural changes in neurons of the suprachiasmatic nucleus of the rabbit

Localization of Neuroendocrine Functions Within the Hypothalamus

The results of lesion, stimulation, deafferentation, implantatión and transplantation studies employed in the identification of hypophysiotrophic control areas in the hypothalamus to date suggest the following probable locations: corticotropic releasing factor (CRF) is formed in a diffuse area along the base of the median eminence, if not the base of the entire hypothalamus. Follicle stimulating hormone releasing factor (FSHRF) is elaborated in the paraventricular-suprachiasmatic areas but its cyclic control may reside in the anterior hypothalamic area. Luteinizing hormone (LH) is controlled by luteinizing hormone releasing factor (LHRF) formed in the suprachiasmatic area: its cyclic control may be in the preoptic area. Prolactin is controlled by prolactin inhibiting factor (PIF) localized in a diffuse area comprising the ventromedial, dorsomedial, arcuate and paraventricular nuclei. The hypothalamic area involved in thyroid control is also rather large, since thyrotropin stimulating hormone releasing factor (TSHRF) has been found in an area including the supraoptic and chiasmatic nuclei. Growth hormone releasing factor (GHRF) is elaborated in a rather narrow zone, the ventromedial hypothalamic nuclei.

Ultrastructure of Hypothalamic Neurons and of the Median Eminence

Our light, and electron microscopic (EM) findings within the hypothalamic supraoptic (SO) and paraventricular (PV) nuclei of the normal female rabbit are in agreement with those reported earlier by other investigators for the same nuclei of the dog and rat. The neurons of these nuclei are the hypothalamic synthesis sites of the neurohypophyseal hormones.

With the exception of the arcuate nucleus, none of the hypothalamic nuclei associated with the control of adenohypohpyseal function have been studied extensively with the electron microscope. On the basis of our EM findings within the female rabbit hypothalamus, all neurons observed within the preoptic (PO) and suprachiasmatic (SCH) nuclei of the non-mated control animal were morphologically identical to the conventional neuron as described by Peters, Palay and Webster (1970). However, following coitus, castration and laparotomy, many neurons of these nuclei showed subcellular changes that have been repeatedly associated with enhanced protein synthesis. These large ‘neurosecretory’ neurons were usually located near capillaries and characterized by their well developed Rough endoplasmic reticulum (RER) and Golgi profiles, dense populations of mitochondria and lysosomes and by the presence of a homogeneous population of densecore vesicles (DCV) showing a peak distribution of 120-140 nm. Since similar neurons were not observed within the PO and SCH of the normal control rabbit it is suggested that we were observing functional states of the same type of neuron and that these ultrastructural changes occur in response to endocrine manipulation.

Two types of neurons described as ‘pale’ and ‘dark’ were observed within the arcuate nucleus of both the control and experimental female rabbit. Ultrastructurally, these neuron types were identical to those described by other investigators for the rat. It has been suggested that the ‘pale’ and ‘dark’ neurons of this hypothalamic nucleus represent functional states of the same type of cell. However, increases in the ratio of ‘dark’ to ‘pale’ neurons as observed within the arcuate nucleus of the rat following castration, were not seen in the rabbit. Similar findings were also not evident within the arcuate nucleus of the female rabbit following coitus.

As far as could be determined, all neurons of the ventromedial (VMN) nuclei of both the control and experimental rabbit were morphologically identical to the smaller, conventional type neuron. Certainly, ultrastructural changes similar to those observed within the PO and SCH nuclei of the female rabbit following coitus, castration or laparotomy, were never observed.

The basic zonation and subcellular organization of the female rabbit Median Eminence (ME) is similar to that described for other mammalian species. Our EM findings within the external layer of the rabbit ME, however, are not entirely in agreement with the earlier study of Duffy and Menefeef 1965). These investigators reported only one population of DCV within the axon terminals of the rabbit ME external layer. We feel that we have ultrastructural evidence for the presence of at least two distinct populations of DCV within this layer of the rabbit ME. Furthermore, since these vesicle populations occurred within separate axon profiles and terminals, differences in their content and origin are suggested.

Certainly, the relationship between releasing factors (RF) and the various populations of DCV observed within the external layer of the mammalian ME is not well established. The smaller (90 nm - 100 nm) DCV we have observed probably contain the catecholamines, while those of larger (120 nm - 140 nm) diameters may well represent the carriers of the RF associated with gonadotropic activity. The latter view is based primarily on our finding or numerous ‘vesicle ghosts’ within the axon terminals abutting the perivascular space (PVS) of portal capillaries of rabbits sacrificed at 10 minutes post-coitus. The mean diameters of 137±14 nm obtained for these ghosts strongly supports the suggested depletion of only the larger of the two DCV populations. Similar changes were not apparent within the axon terminals containing homogenous populations of only the smaller DCV.

Unquestionably, the precise hypothalamic synthesis sites for the RF associated with control of adenohypophyseal function, continues to provoke comment. From the results obtained from countless studies that have employed a variety of neuroendocrinilogical techniques, two main hypothalamic centers of RF synthesis have been suggested: a) the medial basal hypothalamus (MBH) or hypophysiotropic area (HTA) and b) the anterior hypothalamus. The ultrastructural studies carried out to date within this laboratoiy are in favour of the latter for the following reasons:

1) — the presence of large DCV and ‘vesicle ghosts’ within the external layer of the rabbit ME with diameters similar to those of the large (120-150 nm) DCV synthesized within the PO and SCH nuclei of the same animal in response to coitus, castration and laparotomy.

2) — the absence of evidence for the storage of these large DCV within the somata of PO and SCH nuclei, suggesting their immediate transport toward the ME.

3) — the absence of any ultrastructural changes within neuron somata of the rabbit arcuate nuclei which might reflect enhanced neurosecretory activity in response to coitus and/or castration.

These ultrastructural findings within the rabbit hypothalamus may, therefore, provide the first evidence of a morphological nature for the actual release of RF from their ME storage sites, as well as their synthesis within certain neurons of the anterior hypothalamus.

Scanning and Transmission Electron Microscopy of the Ependymal Lining of the Third Ventricle

In its simplest form, the ependyma of the third ventricle consists of a single layer of cuboidal cells. Although these typical mural cells constitute the greater part of the lining of the ventricle, a specialized variety of ependymal cell (the tanycyte) can also be distinguished within circumscribed areas of the ventricular wall. Although such cells are found scattered throughout the dorsoventral extent of the third ventricle, they are particularly numerous along the ventrolateral walls and floor. The regional variation in the surface morphology of the ventricle walls as evident with the scanning electron microscope is consistent with this pattern of tanycyte distribution. Ultrastructural studies have established that the tanycyte is a fundamentally distinct cell with a long basal process extending into the subjacent neuropil and frequently directed toward a capillary wall. This unique morphology conforms closely to its three-dimensional appearance as demonstrated with the scanning electron microscope. The significance of ependymal tanycytes particularly of the third ventricle derives largely from the connections they establish between the ventricular lumen and vasculature of the median eminence. This intriguing structural relationship has led to the suggestion that ependymal cells and cerebrospinal fluid of the third ventricle may be involved in the regulation of adenohypophysial activity. Evidence indicating the functional involvement of specialized ependymal cells in the neuroendocrine control of pituitary activity is reviewed.

Sexual differentiation of the human hypothalamus

Functional sex differences in reproduction, gender and sexual orientation and in the incidence of neurological and psychiatric diseases are presumed to be based on structural and functional differences in the hypothalamus and other limbic structures. Factors influencing gender, i.e., the feeling to be male or female, are prenatal hormones and compounds that change the levels of these hormones, such as anticonvulsants, while the influence of postnatal social factors is controversial. Genetic factors and prenatal hormone levels are factors in the determination of sexual orientation, i.e. heterosexuality, bisexuality or homosexuality. There is no convincing evidence for postnatal social factors involved in the determination of sexual orientation. The period of overt sexual differentiation of the human hypothalamus occurs between approximately four years of age and adulthood, thus much later than is generally presumed, although the late sexual differentiation may of course be based upon processes that have already been programmed in mid-pregnancy or during the neonatal period. The recently reported differences in a number of structures in the human hypothalamus and adjacent structures depend strongly on age. Replication of these data is certainly necessary. Since the size of brain structures may be influenced by premortem factors (e.g. agonal state) and postmortem factors (e.g. fixation time), one should not only perform volume measurements, but also estimate a parameter that is not dependent on such factors as, i.e., total cell number of the brain structure in question. In addition, functional differences that depend on the levels of circulating hormones in adulthood have been observed in several hypothalamic and other brain structures. The mechanisms causing sexual differentiation of hypothalamic nuclei, the pre- and postnatal factors influencing this process, and the exact functional consequences of the morphological and functional hypothalamic differences await further elucidation.

Sex differences in the hypothalamus in the different stages of human life

Quite a number of structural and functional sex differences have been reported in the human hypothalamus and adjacent structures that may be related to not only reproduction, sexual orientation and gender identity, but also to the often pronounced sex differences in prevalence of psychiatric and neurological diseases. One of the recent focuses of interest in this respect is the possible beneficial effect of sex hormones on cognition in Alzheimer patients. The immunocytochemical localization of estrogen receptors (ER) alpha, beta and androgen receptors has shown that there are indeed numerous targets for sex hormones in the adult human brain. Observations in the infundibular nucleus have, however, indicated that in this brain area the hyperactivity resulting from a lack of estrogens in the menopause seems to protect females against Alzheimer changes, in contrast to males. It is thus quite possible that estrogen replacement therapy may, in these brain areas, lead to inhibition of neuronal metabolism and thus to the same proportion of Alzheimer changes as are observed in men. Knowledge about the functional sex differences in the brain and the effect of sex hormones on neuronal metabolism may thus provide clues not only for the possible beneficial effects of these hormones (e.g., on cognition or hypertension), but also on possible central side effects of estrogen replacement therapy.

Biological rhythms in the human life cycle and their relationship to functional changes in the suprachiasmatic nucleus

Biological rhythms play a prominent role in the human life cycle. The endogenous rhythms are entrained by the environment and have an astronomical counterpart which is obvious for daily, monthly, and yearly rhythms, and may possibly also be present in weekly rhythms. Circadian rhythms are present in, e.g. testosterone levels, spontaneous birth, strokes, and death from cardiovascular causes. Circaseptan rhythms are present in, e.g. spontaneous birth, 17-ketosteroid levels, myocardial infarctions, and strokes. The relationship of these rhythms with the suprachiasmatic nucleus (SCN) has not yet been established. Circatrigintan rhythms, such as the menstrual cycle, have so far not been associated with the SCN. Circannual rhythms are present in, e.g. mood, suicides, reproduction, birth weight, sleep and season of birth of psychiatric patients. The human SCN shows strong circadian and circannual fluctuations in the number of neurons expressing vasopressin. The vasopressin and VIP cell population of the SCN develop late, i.e. for a major part postnatally. After the age of 50 the amplitudes of circadian and circannual fluctuations of the vasopressin cell numbers are reduced whereas the number of vasopressin expressing neurons decreases after the age of 80 and do so even more and earlier in Alzheimer's disease. Sex differences are present in the shape of the vasopressin subnucleus of the SCN and in the vasoactive intestinal polypeptide (VIP) cell number. The sex differences in the SCN, the doubling of the number of vasopressin neurons in the SCN of homosexual men, and a variety of animal experimental observations indicate that the SCN is involved in sexual behavior and reproduction. The exact role of the SCN in these processes is subject to current research.

Increased number of vasopressin neurons in the suprachiasmatic nucleus (SCN) of 'bisexual' adult male rats following perinatal treatment with the aromatase blocker ATD

In an earlier article an enlarged subpopulation of vasopressin containing neurons was found in the suprachiasmatic nucleus (SCN) of homosexual men as compared to heterosexuals. The present study investigates the possibility that the number of vasopressin neurons in the SCN and sexual partner preference behavior in male rats are both influenced by sex hormones during brain development. For this purpose, we studied groups of adult male rats that had been treated either prenatally or pre- and postnatally with the aromatase inhibitor ATD (1,4,6-androstatriene-3,17-dione) which blocks the aromatization of testosterone to estradiol. Rats treated with ATD in both pre- and postnatal periods showed 'bisexual' partner preference behavior and appeared to have 59% more vasopressin-expressing neurons in the SCN than the controls. The prenatally treated rats did not differ from the controls. This observation supports the hypothesis that the increased number of vasopressin neurons found earlier in the SCN of adult homosexual men might reflect differences that took place in the interaction between sex hormones and the brain early in development.

Development of vasoactive intestinal polypeptide neurons in the human suprachiasmatic nucleus in relation to birth and sex

The development of vasoactive intestinal polypeptide (VIP) neurons was determined in the human suprachiasmatic nucleus (SCN) of 43 subjects ranging from 27 weeks of gestation to 30 years of age using immunocytochemistry and morphometry. VIP neurons were first observed at 31 weeks of gestation in the ventrolateral part of the SCN. From 3 months postnatally onwards, VIP positive neurons were observed in some subjects in the centromedial part of the SCN. The centromedial type of VIP staining became a constant finding only at 19 years of age, at term the SCN was still very immature. Only in a few subjects some VIP neurons stained in the ventrolateral SCN and their number and nuclear diameter was small. Postnatally the number of VIP neurons increased gradually until around 3 years of age adult values were reached. After the age of 10 a clear sex difference in the number of VIP neurons was found: males having on average twice as many VIP neurons in the SCN as females. The adult VIP cell numbers in the SCN amounted only 35% of those found earlier for vasopressin. The present data do not support a particular role for VIP neurons in those rhythms that are already present in early development, e.g., of the temperature rhythm in prematures of around 30 weeks gestational age. Our observations in this and earlier papers as well as animal studies do suggest though a possible role for VIP neurons in the SCN in sexual dimorphic functions such as reproduction and sexual behavior.

Neuronal associations in the rat suprachiasmatic nucleus demonstrated by immunoelectron microscopy

The synaptic associations of neurons in the suprachiasmatic nucleus (SCN) of rats were examined by single immunolabeling for somatostatin (SRIH) and arginine vasopressin (AVP), and double immunolabeling for SRIH plus AVP and vasoactive intestinal polypeptide (VIP) plus AVP. Single immunolabeling showed that SRIH neurons, which displayed some somatic and dendritic spines, formed synaptic contacts with immunonegative and positive axon terminals. AVP neurons also formed synaptic contacts with both immunonegative and positive axon terminals. The immunonegative terminals contained small, spherical clear vesicles or flattened clear vesicles. A few immunopositive AVP fibers made synapses with immunonegative somatic or dendritic spines. Double immunolabeling showed synaptic associations between SRIH axons and AVP cell bodies or dendritic processes, and between AVP axons and the somata or dendrites of SRIH neurons. These findings suggest a reciprocal relation between the two types of neurons. Synaptic contacts between AVP neurons and VIP axon terminals were also demonstrated. Previously, we found synapses between SRIH axons and VIP neurons. Thus SRIH neurons appeared to regulate AVP and VIP neurons. On the basis of these findings, two possible oscillation systems of the SCN are proposed.

A route for direct retinal input to the preoptic hypothalamus: dendritic projections into the optic chiasm

With the use of Golgi, horseradish peroxidase, and electron microscopic techniques, neurons within a broad region of the preoptic hypothalamus of the mouse were shown to have dendrites that projected well into the depths of the optic chiasm. Further experimental and ultrastructural investigation demonstrated synapses between these dendrites and retinal axonal boutons within the chiasm. All synapses located in the chiasm were classified as Gray's type I. The possible function of these dendritic projections is discussed.

Neuronal and non-neuronal control of the neurosecretory caudo-dorsal cells of the freshwater snail Lymnaea stagnalis (L.)

The cerebral ganglia of the freshwater snail Lymnaea stagnalis contain two clusters of neurosecretory Caudo-Dorsal Cells (CDC). These cells produce a neurohormone which stimulates ovulation. Ganglion transplantation and quantitative electron microscopy show that neuronal isolation of the cerebral ganglia complex (CCC) results in an activation of the CDC. It was, therefore, concluded that the CDC are controlled by an inhibitory neuronal input originating outside the cerebral ganglia. Ultrastructural studies on synaptic degeneration in the CCC suggest that this input reaches the CDC via a special type of synapse-like structure, the type C-SLS. Furthermore, transplantation of CCC into acceptor snails leads to a reduced release and an increased intracellular brekdown of neurohormone in the CDC of the nervous system of the acceptors. It is supposed that these phenomena are caused by the release of an (unknown) factor from the transplanted CCC. Special attention was given to the formation and degradation of a peculiar type of neurohormone granule, the large electron dense granule. The physiological significance of the neuronal and non-neuronal control mechanisms which regulate CDC activity is discussed.

Retinohypothalamic projection in the mouse: electron microscopic and iontophoretic investigations of hypothalamic and optic centers

The problem of the direct retinohypothalamic projection in mammals (Moore, 1973) was reinvestigated in the laboratory mouse by electron microscopy and cobalt chloride-iontophoresis. The time-course of the axonal degeneration in the suprachiasmatic nucleus was studied 3, 6 and 12 h, 1, 2, 4, 6, 9 and 12 days after unilateral retinectomy. Specificity of the degenerative changes was controlled by investigation of the superficial layers of the superior colliculus. The ratio of crossed to uncrossed optic fibers could could be determined by counting degenerating structures (axons and terminals) in the optic chiasma and the ipsilateral and contralateral areas of the optic tract, the suprachiasmatic nucleus, and the superior colliculus. The number of degenerating axons in the suprachiasmatic nucleus showed a maximum one day after unilateral retinectomy and was, at all stages studied, two to three times higher in the contralateral than in the ipsilateral nuclear area. In the optic tract and in the superior colliculus the number of degenerating profiles was three times higher in the contralateral than in the ipsilateral area. Retinohypothalamic connections and crossing pattern of retinal fibers were studied light microscopically using impregnation with cobalt sulfide in whole mounts of brains. Most of the optic fibers in the laboratory mouse are crossed crossed (70-80%). A bundle of predominantly crossed optic fibers runs to the suprachiasmatic nucleus.

Atlas of estrogen-concentrating cells in the central nervous system of the squirrel monkey

Estrogen is concentrated within cellular nuclei in discrete regions of the monkey brain 30 and 60 minutes following intravenous injection of [3H] estradiol. Chromatographic data is provided to suggest that most of the localized estrogen is in the form of estradiol with lesser amounts of estrone and estriol. Three "major" areas of estrogen accumulation include: (1) preopticostrial accumulation: n. preopticus medialis--n. interstitialis striae terminalis, (2) basal hypothalamic accumulation: n. infundibularis--n. ventromedialis--n. premammillaris ventralis, and (3) the amygdaloid accumulation. Several "minor" areas of estrogen accumulation include the tuberculum olfactorium, insulae Calleja, n. triangularis septi, a. hypothalamica anterior, n. anterior hypothalami, n. paraventricularis, n. supraopticus, n. periventricularis and the substantia grisea centralis. The neocortex, rhombencephalon and spinal cord are essentially unlabeled. The major areas of accumulation are similar in several other mammalian and avian species while these, and some minor areas of accumulation, have been shown in neuroanatomical studies to be interconnected by several pathways, especially the stria terminalis. Lesion, implant, stimulation, recording and morphometric studies, in several species, support the concept that this arrangement provides a neuroanatomical substrate which would allow the integration of the various facets of the neuroendocrine reproductive response.