The hypothalamo-hypophysial system of hypophysectomized rats. II. Structure and ultrastructure of the median eminence

Evolution of oxytocin pathways in the brain of vertebrates

The central oxytocin system transformed tremendously during the evolution, thereby adapting to the expanding properties of species. In more basal vertebrates (paraphyletic taxon Anamnia, which includes agnathans, fish and amphibians), magnocellular neurosecretory neurons producing homologs of oxytocin reside in the wall of the third ventricle of the hypothalamus composing a single hypothalamic structure, the preoptic nucleus. This nucleus further diverged in advanced vertebrates (monophyletic taxon Amniota, which includes reptiles, birds, and mammals) into the paraventricular and supraoptic nuclei with accessory nuclei (AN) between them. The individual magnocellular neurons underwent a process of transformation from primitive uni- or bipolar neurons into highly differentiated neurons. Due to these microanatomical and cytological changes, the ancient release modes of oxytocin into the cerebrospinal fluid were largely replaced by vascular release. However, the most fascinating feature of the progressive transformations of the oxytocin system has been the expansion of oxytocin axonal projections to forebrain regions. In the present review we provide a background on these evolutionary advancements. Furthermore, we draw attention to the non-synaptic axonal release in small and defined brain regions with the aim to clearly distinguish this way of oxytocin action from the classical synaptic transmission on one side and from dendritic release followed by a global diffusion on the other side. Finally, we will summarize the effects of oxytocin and its homologs on pro-social reproductive behaviors in representatives of the phylogenetic tree and will propose anatomically plausible pathways of oxytocin release contributing to these behaviors in basal vertebrates and amniots.

The hypothalamo-hypophyseal system in Acipenseridae. XI. Morphological and immunohistochemical analysis of nonapeptidergic and corticoliberin-immunoreactive elements in hypophysectomized sterlet (Acipenser ruthenus L.)

The regeneration of the hypothalamo-hypophyseal system (HHS) has been studied in the sterlet following hypophysectomy (HypoX). Significant portions of nonapeptide and corticoliberinergic (CRH) neurosecretory cells (NSCs) underwent degeneration. Surviving NSCs had a great regenerative ability; damaged axons formed anew, uncharacteristicly for intact fishes, axovasal and axoventricular contacts. There are some specific features of the reorganization of the ANHyp in sterlet after HypoX: (a) blood capillaries from the meninges penetrate deep into the wall of the infundibular recess and form unusual axovasal contacts; (b) "neurovascular regeneratory complexes" appear; (c) single CRH-immunoreactive axons grow into the meninges. Morphological data show progressive appearance of dendroventricular contacts in the preoptic nucleus, formation of new axoventricular and axovasal contacts, and activation of surviving NSCs after HypoX. An ability to recognize HHS to release neurohormones into the CSF and bloodstream is suggested.

Meninges play a neurotrophic role in the regeneration of vasopressin nerves after hypophysectomy

Following hypophysectomy the regenerating fibers of magnocellular neurons are known to establish new neurohemal connections with reorganized vasculatures in the median eminence, which lead to establishment of a posterior pituitary-like structure. In order to examine the role of the meninges (the pia mater and the arachnoid) in this regeneration process, we implanted the meningeal tissues obtained from neonatal rat pups into the third ventricle of the adult rats, and then hypophysectomized the host animals. Ten days after hypophysectomy, the meningeal tissue grafts were found to be densely innervated by regenerating vasopressin-immunoreactive fibers. Such fibers had dots and frequently formed large punctuations. On the contrary, few vasopressin fibers were found within the cortical tissue grafts. Further, the exposure of primary hypothalamic cell cultures to the medium conditioned by meningeal cell cultures promoted not only the survival of vasopressin-immunoreactive neurons but also the outgrowth and aborization of the neurites. The survivals of cortical and cerebellum neurons in culture were also promoted by the conditioned medium. These findings raise the possibility that the meninges play an important role in the axonal regeneration process after hypophysectomy.

Regeneration of the neurohemal terminals for identified cerebral neurosecretory cells in an insect

The axons of specific neurosecretory cells, L-NSC III, in the brain of the tobacco hornworm, Manduca sexta, were transected during larval-pupal development to study the effects of this type of lesion on these peptidergic neurons and to begin to identify factors that may regulate their regeneration and growth. The two somata of these bilaterally paired neurons produce the prothoracicotropic hormone and are located in the pars intercerebralis. Their axons exit from the contralateral brain lobe via a retrocerebral nerve and pass through the corpus cardiacum before terminating at the glandular corpus allatum. At the corpus allatum, the L-NSC III axons arborize to form the terminal neurohemal organ for prothoracicotropic hormone release. The retrocerebral nerve was severed either in vitro followed by brain transplantation or in situ; in either protocol, the distal axon segments and corpus allatum were removed. The ability of the injured L-NSC III axons to regenerate was assessed immunocytologically by using a monoclonal antibody against the prothoracicotropic hormone. In both treatments, the proximal axon stumps exhibited regenerative growth as early as 1 day after axotomy, and, by the third day, neurites had extended. By the fifth day, the regenerating axons had branched to form terminal varicosities similar to those of a normal neurohemal organ. The regenerated neurohemal structure appeared to be functional, because larvae that had been bilaterally axotomized were able to metamorphose to pupae, a process requiring temporally precise periods of prothoracicotropic hormone release. In addition to the regeneration of the terminal axon structures, several other responses to axotomy and retrocerebral organ excision occurred. These included an apparent accumulation of prothoracicotropic hormone in the axons and regenerating neurohemal-like structure, sprouting of ectopic neurites from the axotomized somata, and a change in shape of the cell bodies from spherical to avoid.

Magnocellular neurosecretory axon regeneration into rat intrahypothalamic optic nerve allografts

To test the working hypothesis that neurosecretory neuronal regeneration is largely dependent on microenvironmental conditions at the lesion site, intact or predegenerated optic nerves were allografted intrahypothalamically into the hypothalamo-neurohypophysial tract. Neurosecretory axons regenerated consistently into all grafts. Early regeneration proceeded without glial cell association. At later stages of regeneration, however, neurosecretory axons were associated consistently with astrocytes that, within perivascular spaces, were surrounded by a basal lamina. Axons in contact with that basal lamina had the characteristics of terminals, suggesting functional recovery. It is postulated that it is the initial absence of a blood-brain barrier at the graft site that provides a microenvironment similar to that in the neural lobe and that induces neurosecretory axon regeneration.

CRF-containing neuron systems in the rat hypothalamus: retrograde tracing and immunohistochemical studies

By employing a combination of the immunohistochemistry for rat corticotropin-releasing factor (rCRF) and retrograde tracing with biotinylated wheat germ agglutinin (b-WGA) injected into the posterior pituitary (group 1) or into the middle portion of the median eminence (group 2), functionally different populations of CRF neurons were identified in the rat hypothalamus. In the group 1 animals, WGA-labeling was seen not only in the posterior lobe but also in the intermediate lobe, in which CRF fibers exist. In these animals, WGA-labeling occurred for almost all large neurons in the anterior commissural, paraventricular (PV), and supraoptic (SO) nuclei, some of the neurons showing a slight immunoreactivity for anti-rCRF. Conversely, CRF positive neurons appeared in large numbers, some being labeled with WGA, in the caudal periventricular region (CPR), and in the dorsomedial to lateral hypothalamic area (DLH), especially in the latter. In the group 2 animals, WGA was disparsed throughout the subependymal, internal, and external layers of the medial portion of the median eminence, and was taken up by many small cells in the PV, almost half of the cells being immunoreactive for CRF. Slight WGA-labeling further occurred in some large neurons of the PV and SO, and in some cells of the DLH. It is concluded that hypophysiotropic CRF existing in the external layer of the median eminence originates from small neurons located in the PV, whereas CRF distributed in the posterior and intermediate pituitary originates from the magnocellular PV and SO, and from some neurons in the DLH and CPR.

An immunohistochemical and fine-structural analysis of peptidergic hypothalamic neurosecretory axon regeneration into the leptomeninges of the rat

Regeneration of severed hypothalamic peptidergic neurosecretory axons into the ventral pia-arachnoid was observed in rats at the light microscopic and fine-structural levels. A temporal increase occurred in the number of neurophysin-positive axons regenerating into the leptomeninges for distances up to 3.3 mm by 40 days post-lesioning. A consistent pattern of parallel, meshed and clustered axons, occurring either singly or in bundles, was present within the connective tissue, while plexus and bundles were observed in association with leptomeningeal blood vessels. Axons were characterized by preterminal and terminal dilatations. Neurosecretory granulated vesicles occurred throughout axons. The presence of microvesicles at contact points with basal lamina suggests the possibility of hormone release. Most axons were arranged as fascicles associated closely with basal lamina-bounded support cells whose thin lamellar processes wrapped single axons or fascicles of axons. We conclude, therefore, that cellular and intercellular leptomeningeal microenvironments support and sustain the growth and regeneration of transected neurosecretory axons.

Regeneration of neurohypophyseal hormone-producing neurons in hypophysectomized immature rats

Age-related changes in the regeneration of vasopressin (AVP) and oxytocin (OXT) axons after hypophysectomy in rats was immunohistochemically examined. Rats were hypophysectomized at 9, 16, 23, 30 and 90 days of age, and sacrificed 10 days after the operation. AVP or OXT immunoreactivity in the external layer of the median eminence (ME) was generally stronger in hypophysectomized immature rats than in hypophysectomized adult rats, and the age-related difference in immunoreactivity was more conspicuous for AVP axons than OXT ones. The cell body size of AVP or OXT neurons in hypophysectomized adult rats was not significantly different from the value of unoperated or initial control rats. However, the neurons in immature rats became significantly larger after hypophysectomy, compared with those of initial controls. These results indicate that AVP- and OXT-producing neurons in immature rats, as early as at 9 days of age, are endowed with the capacity of axonal rearrangement to the external layer of the ME after hypophysectomy, and that the stronger immunoreactivity in the external layer of the ME in immature rats than in the adult may be due to the differences in the rate of synthesis of neurohypophyseal hormones and the regenerative potency of neurons.

Plasticity of vasopressin- and oxytocin-containing fibers in the median eminence in hypophysectomized young and old mice

Rearrangements of vasopressin- and oxytocin-containing fibers in the external layer of the median eminence after hypophysectomy were compared between young and old mice. In 3-month-old hypophysectomized mice, the increase in the number of fibers containing vasopressin was greater than that observed in 19-month-old hypophysectomized ones, suggesting a decrease in axonal plasticity in old mice. No difference with age was detected for the plasticity of fibers containing oxytocin.

Influence of histamine and pentobarbitone on plasma and CSF vasopressin levels of hypophysectomized rats

Histamine increased vasopressin levels, as measured by radioimmunoassay (RIA), in both cerebrospinal fluid (CSF) and plasma of hypophysectomized rats, while histamine enhanced plasma but not CSF levels of vasopressin in sham operated rats. Pentobarbitone increased CSF vasopressin levels in hypophysectomized rats and in sham operated animals. The present data demonstrate that the histamine induced elevation of vasopressin levels in the blood is only temporarily disturbed after hypophysectomy, while the effect of histamine on CSF vasopressin levels of hypophysectomized rats is of a more permanent nature.