The hypothalamo-hypophysial system of hypophysectomized rats. I. Ultrastructure of nerve fibres in "intact" and dehydrated animals

Age-dependent increase in Thy-1 protein in the rat supraoptic nucleus

Mature mammalian CNS neurons often do not recover successfully following injury. To this point, unilateral lesion of the hypothalamo-neurohypophysial tract results in collateral sprouting from uninjured axons of the supraoptic nucleus (SON) in 35-day-old but not in 125-day-old rats. Thus, it appears that there are age-related changes within the SON that preclude the older rat from recovering following axotomy. We hypothesize that the intrinsic capacity for axon reorganization may depend, in part, on age-related alterations in cell adhesion molecules that allow normal astrocyte-neuron interactions in the SON. In support of our hypothesis, numerous reports have shown that Thy-1 is increased in neurons at the cessation of axon outgrowth. Therefore, we compared protein levels of Thy-1 and the Thy-1 interacting integrin subunits, alpha-v (α_v), beta-3 (ß₃), and beta-5 (ß₅), in 35- and 125-day-old SON using western blot analysis. Our results demonstrated that there was significantly more Thy-1 protein in the 125-day-old SON compared to 35-day-old SON, but no change in the protein levels of the integrin subunits. Furthermore, we localized Thy-1-, α_v integrin-, ß₃ integrin-, and ß₅ integrin-immunoreactivity to both neurons and astrocytes in the SON. Altogether, our results suggest that the observed increase in Thy-1 protein levels in the SON with age may contribute to an environment that prevents collateral axonal sprouting in the SON of the 125-day-old rat.

Absence of axonal sprouting following unilateral lesion in 125-day-old rat supraoptic nucleus may be due to age-dependent decrease in protein levels of ciliary neurotrophic factor receptor alpha

Within the supraoptic nucleus (SON) of a 35-day-old rat, we previously demonstrated a collateral sprouting response that reinnervates the partially denervated neural lobe (NL) after unilateral lesion of the hypothalamo-neurohypophysial tract. Others have shown a decreased propensity for axonal sprouting in an aged brain; therefore, to see if the SON exhibits a decreased propensity for axonal sprouting as the animal ages, we performed a unilateral lesion in the 125-day-old rat SON. Ultrastructural analysis of axon profiles in the NL of the 125-day-old rat demonstrated an absence of axonal sprouting following injury. We previously demonstrated that ciliary neurotrophic factor (CNTF) promotes process outgrowth from injured magnocellular neuron axons in vitro. Thus, we hypothesized that the lack of axonal sprouting in the 125-day-old rat SON may be due to a reduction in CNTF or the CNTF receptor components. To this point, we found that as the rat ages there is significantly less CNTF receptor alpha (CNTFRα) protein in the uninjured, 125-day-old rat compared to the uninjured, 35-day-old rat. We also observed that protein levels of CNTF and the CNTF receptor components were increased in the SON and NL following injury in the 35-day-old rat, but there was no difference in the protein levels in the 125-day-old rat. Altogether, the results presented herein demonstrate that the plasticity within the SON is highly dependent on the age of the rat, and that a decrease in CNTFRα protein levels in the 125-day-old rat may contribute to the loss of axonal sprouting following axotomy.

Regeneration of neurosecretory axons into various types of intrahypothalamic grafts is promoted by the absence of blood brain barrier: fine structural analysis

Isogenous grafts of neural lobe and optic nerve and autologous grafts of sciatic nerve were placed into contact with the intrahypothalamically transected hypothalamo-neurohypophysial tract, and their fine structural characteristics examined at various time periods thereafter. The vascular bed of neural lobe grafts is composed primarily of fenestrated capillaries, that are permeable to blood-borne HRP throughout the entire experimental period. The microvasculature of sciatic nerve grafts consists of continuous, as well as fenestrated capillaries, which are similarly permeable to HRP. Fenestrated capillaries and HRP leakage in optic nerve grafts are observed at 10 days, but only in grafts located ventrally in the hypothalamus at 30 days. Neurosecretory axon regeneration is seen only in grafts or adjacent hypothalamus where the blood-brain barrier is breached. Regenerating axons are closely associated with the specific glial cells of the respective graft. Based on these observations, we conclude that blood-borne factors are necessary to initiate and sustain regeneration of transected neurosecretory axons, and that such regeneration occurs only in the presence of glial cells.

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.

Regeneration of neurosecretory axons into various types of intrahypothalamic graft is promoted by the absence of the blood-brain barrier: a neurophysin-immunohistochemical and horseradish peroxidase-histochemical study

In order to test the hypothesis that neurosecretory axon regeneration occurs only in the presence of specific vascular, perivascular, and glial microenvironments, isografts of neural lobe and optic nerve and autografts of sciatic nerve were transplanted into the hypothalamo-neurohypophysial tract at the lateral retrochiasmatic area of adult male rats. The integrity of the blood-brain barrier (BBB) to intravenously administered horseradish peroxidase (HRP), the regenerative process of neurosecretory axons, and functional recovery from lesion-induced diabetes insipidus were analyzed at 18 hr, 36 hr, 10 days, 30 days, and 80 days postsurgery. Neurophysin-positive axons invaded all grafts, as well as perivascular spaces of the adjacent hypothalamus. Wherever neurosecretory axon regeneration occurred, the BBB was breached. Reestablishment of the BBB was paralleled by a decrease in both density and staining intensity of regenerated neurophysin-positive axons. These observations illustrate that neurosecretory axon regeneration is tributary of the absence of BBB. It is speculated that blood-borne factors, provided when the BBB is breached, initiate and sustain neurosecretory axon regeneration. In addition, products of glial elements may enhance or complement the above stimulatory processes.

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.

Development of neural lobe-like neurovascular contact regions after intrahypothalamic transection of the hypothalamo-neurohypophysial tract

Fifteen days after bilateral transection of the hypothalamo-neurohypophysial tract at the level of the lateral retrochiasmatic area, neurovascular contact regions had developed proximal to 66% of the lesions. Contact regions developed in every case when neural lobe explants were placed into the lesions, and near approximately half of the lesions into which small pieces of sciatic or optic nerve were transplanted. Neurovascular contact regions were characterized by microvascular networks surrounded by dense neurophysin-immunoreactive plexuses. At the fine structural level, the organization of such regions resembled that of the neural lobe, with the single exception that capillaries were not fenestrated. Numerous neurosecretory axons were present, and palisades of neurosecretory axon terminals abutted perivascular basal laminae. Lamellopodia from glial cells partially ensheathed regenerating neurosecretory axons and often lay between terminals and the perivascular basal lamina. Terminals with many microvesicles and few neurosecretory granulated vesicles provided morphological evidence of hormone release.

Compensatory sprouting of uninjured magnocellular neurosecretory axons in the rat neural lobe following unilateral hypothalamic lesion

Axonal sprouting of intact neurons of the magnocellular neurosecretory system was investigated using a unilateral hypothalamic knife cut of the hypothalamoneurohypophysial tract to partially denervate the rat neural lobe (NL). Densitometric, morphometric, ultrastructural, and metabolic measures were utilized to demonstrate the compensatory response to denervation in this system. Densitometric analysis revealed a transient reduction in the intensity of vasopressin staining in the NL at 10 days postsurgery (PS) with a subsequent recovery by 20 days PS. There was a comparable initial reduction in the cross-sectional area of the NL followed by a more gradual recovery to normal by 90 days PS. Ultrastructural investigation revealed a reduction in total axon number in the NL at 10 days PS similar to the declines in vasopressin immunoreactivity and size of the NL. A subsequent partial recovery of axon number occurred, paralleling the return to normal NL size between 30 and 90 days PS. Hypertrophy of both somata and cell nuclei of magnocellular neurons in the supraoptic and paraventricular hypothalamic nuclei contralateral to the lesion was also apparent during this period. Daily measurements of urine osmolality revealed an initial transient hypoosmolality followed by a chronic hyperosmolality which persisted throughout the 90 day postsurgical period. There was a concomitant chronic decrease in both daily drinking and urine excretion volumes which began immediately following surgery. These results suggest that intact, contralateral magnocellular vasopressinergic efferents undergo compensatory sprouting as a result of partial denervation of the NL in the absence of a functional deficit in vasopressin.

Regrowth of damaged neurosecretory axons to fenestrated vessels of implanted peripheral tissues

A major target of neurosecretory axons (NSA) is the basal lamina around fenestrated blood vessels (FBV) in the neural lobe of the pituitary gland. We have posed the question of whether there is neurovascular specificity. Do mature, regenerating NSA terminate selectively on the FBV of the neural lobe compared with the FBV of other tissues that normally are not innervated by NSA? Three types of tissue were transplanted between inbred Fisher rats. Fragments, about 1 mm3, of pineal, adrenal medulla, and neural lobe were grafted bilaterally to the hypothalamic, retro-chiasmatic area, which includes bundles of NSA from supraoptic and paraventricular neurosecretory nuclei exclusively, but no FBV. Two and 4 weeks later, the grafts were prepared for the immunohistochemical localization of NSA and for electron microscopy. NSA-FBV proximity was measured, and the number of NSA, FBV, and of NSA-FBV associations was counted per surface area of each graft. Regenerating NSA can associate as closely with FBV of other tissues as they can with the FBV of the neural lobe. There does not appear to be specificity with respect to the closeness of association between neurosecretory terminals and fenestrated capillaries. However, the number of these associations is greater in neural lobe grafts than in adrenal or pineal grafts at 4 weeks. The number of FBV is also greatest in neural lobe grafts at this time, an increase that would provide a greater opportunity for NSA-FBV associations.

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.

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.

Changes of vasopressin- and oxytocin-immunoreactive neurons after hypophysectomy in young and old mice

Changes of magnocellular neurons after hypophysectomy were immunohistochemically studied using antisera to arginine vasopressin (AVP) and oxytocin (OXT) in young and old female mice of the C57BL/Tw strain. AVP-immunoreactive neurons in the supraoptic and paraventricular nuclei of intact 19-month-old mice showed a marked reduction in number and immunoreactivity as compared with those of intact 3-month-old mice. Age difference of OXT-immunoreactive neurons was less pronounced than that of AVP-immunoreactive neurons. After hypophysectomy, both AVP- and OXT-immunoreactive neurons showed an intense stainability 10 days after the operation regardless of ages. However, the rate of reduction in number of immunoreactive neurons after hypophysectomy was less marked in 19-month-old than 3-month-old mice.

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

The median eminence (ME) of hypophysectomized rats was studied by means of light and electron microscopy. Paraldehyde-fuchsin (PAF)-positive material is seen in the external zone (EZ) of the ME 2--5 days after the operation. Its amount gradually increases especially in the caudal part of the ME during the following few days. Some PAF-positive fibers make contact with the subependymally located blood capillaries. In the most caudal region of the recessus infundibuli they penetrate into the third ventricle. PAF-positive material decreases markedly from the ME of rats two months after hypophysectomy and exposure to a 1% salt load. Fibers of types A1, A2 and B containing granules of 120--220 nm, 100--150 nm and 80--100 nm in diameter, respectively, are seen in the EZ of the ME in hypophysectomized rats, although almost exclusively A2- and B-type structures make contact with the primary portal capillaries in intact animals. All types of neurosecretory fibers establish contact with the subependymal nonfenestrated blood capillaries and penetrate the recessus infundibuli. Some neurosecretory terminals of different types make direct contact with the glandular cells of the pars tuberalis or are separated from them by a thin basal lamina. It is assumed that mainly neurosecretory fibers of types A2 and B are permanently connected with the primary portal capillaries in the EZ of the ME in intact mammals, while the overwhelming majority of fibers of A1-type shows ingrowth during the course of postoperative reparation. The possible physiological significance of the described changes is discussed.

Section of the pituitary stalk in the rhesus monkey: morphological and endocrine observations

The effect of sectioning the pituitary stalk on the hypothalamic magnocellular system of the rhesus monkey was studied by specific immunocytochemical techniques. Regeneration of the proximal stump with reconstitution of a "new" neurohypophysis occurred as early as three weeks after operation, illustrating the remarkable regenerative ability of the system. The functional capacity of the newly formed neurovascular units was also tested, and the results indicate that they not only resume basal secretion of posterior pituitary hormones, but are also able to increase their output in response to a variety of challenges.

Regeneration of the magnocellular system of the rhesus monkey following hypothalamic lesions

The hypothalamic magnocellular system of the rhesus monkey was studied with specific immunocytochemical techniques in animals that had undergone hypothalamic lesions. The results indicate that this system maintains a regenerative capacity even when its tracts are interrupted within the hypothalamus. New neurohemal units are reconstituted from newly formed vessels within the scar as well as from preexistent blood vessels, such as perforating and pial arterioles, and the vessels of the pars tuberalis of the pituitary gland, which normally do not contain neurosecretory terminals.

The hypothalamo-hypophysial system in Acipenseridae. V. Ecological-histophysiological analysis of the neurohypophysis of the female sturgeon Acipenser güldenstädti Brandt during up-stream migration and after spawning

Neurohypophyses from sexually mature female sturgeons, Acipenser guldenstadti Brandt, killed before, soon after spawning, and during down-stream migration were studied light and electron microscopically. Ovaries were examined only under a light microscope. A large amount of neurosecretory material (3.5-4.5 arbitrary units) is found in the neurohypophysial "roots" during up-stream migration to spawning grounds. Neurosecretory fibres and their terminals are replete with elementary neurosecretory granules. Side by side with the latter some terminals contain single residual granules and few "synaptic" vesicles. Soon after spawning the amount of neurosecretory material decreases markedly in most individuals (content up to 1-3 units), and elementary granules are few in the fibres and their terminals. The number of residual granules increases, and "synaptic" vesicles are especially numerous. Granulated, disintegrating neurosecretory granules and granule-shadows occur in the preterminal parts of the fibres. The number of pituicytes increases becuase some "light" tanycytes seem to migrate from the ependymal into the subependymal layer. The restoration of neurosecretory material (reaching again up to 3.5-4.5 units) occurs within a month after spawning. The number of elementary granules increases in fibres and terminals while that of residual granules and "synaptic" vesicles decreases. These changes in the neurohypophysis of females after spawning are due to discharge of great amounts of peptide neurohorsmones into the general circulation after spawning. Spawning is regarded as a "physiological" stress for these females.