Effect of transplantation of embryonic anterior hypothalamic tissue from spontaneously hypertensive rats to normotensive Wistar rats on circadian rhythms of systolic arterial pressure and heart rate

The anterior hypothalamus (AH) participates in the regulation of arterial pressure. The suprachiasmatic nuclei (SCN) of the AH are a major circadian oscillator necessary for the generation and/or the entrainment of circadian rhythms. Circadian rhythms of systolic arterial pressure (SAP) and heart rate (HR) were investigated in spontaneously hypertensive rats (SHR) and in normotensive Wistar rats (NWI) with intact SCN, grafted with SHR embryonic AH tissue containing the SCN. Prominent circadian rhythms for SAP and HR in both NWI and SHR with acrophases during dark were found. The elevation of the MESOR (midline-estimated statistic of rhythm) of the SAP in normotensive rats grafted with AH embryonic tissue obtained from SHR was accompanied by disappearance of the circadian rhythm of SAP. This result suggests an interaction between the grafted tissue containing the SCN on the one hand, and the host SCN on the other hand. Our data ascribe a role for the SCN in the entrainment of the circadian rhythm of arterial pressure. The circadian rhythm of HR was not eliminated by the SCN graft in spite of the amplitude decrease and the phase delay observed. It seems that the entrainment of the circadian rhythm of HR is probably not crucially dependent on the SCN in rats. The circadian rhythms of SAP and HR in rats were differently affected by the grafts, thus suggesting a multioscillatory system for circadian regulation in rats.

Fiber outgrowth from anterior hypothalamic and cortical xenografts in the third ventricle

Fetal grafts of the anterior hypothalamus (SCN/AH) containing the suprachiasmatic nucleus (SCN) restore circadian rhythms to SCN-lesioned host hamsters and rats following implantation into the third ventricle. Previous studies suggest that intraventricular SCN/AH grafts are variable in their attachment sites, the extent of their outgrowth, and the precise targets innervated in the host brain. However, the use of different methods to analyze graft outgrowth in this model has previously led to inconsistent results. We have reevaluated the outgrowth of fetal rat SCN/AH grafts implanted in the third ventricle of hamsters by using two methods: the carbocyanine dye, 1,1'dioctadecyl-3,3'-tetramethylindocarbocyanine percholate (DiI), was placed directly onto grafted tissue; and a donor-specific neurofilament marker was used in conjunction with xenografts. We examined the specificity of outgrowth by comparing SCN/AH xenografts with that of control cortical (CTX) xenografts. To evaluate whether SCN/AH graft efferents arise from the donor SCN, we used micropunch grafts that contained minimal extra-SCN tissue. The results show that the use of a donor-specific neurofilament marker reveals more extensive SCN/AH graft outgrowth than DiI. SCN/AH graft efferents project into areas normally innervated by the intact SCN. However, this outgrowth is variable among graft recipients, is not specific to SCN/AH tissue, and does not necessarily derive from the donor SCN. The precise functional role of neural efferents arising from SCN/AH grafts in the restoration of circadian clock function and the extent of SCN-derived efferents remain to be determined.

Restoration of circadian behavior by anterior hypothalamic heterografts

The suprachiasmatic nucleus (SCN) of the anterior hypothalamus (AH) is a circadian oscillator and an important component of the mammalian circadian system. To determine whether the SCN is the dominant circadian pacemaker responsible for generating a species-typical characteristic of circadian rhythms [i.e., period length (tau)], neural transplantation was conducted using fetal AH donors of different species and SCN-lesioned (SCNx) hosts. The circadian behavior of each of the three donor species is clearly distinguishable by its species-typical tau. The extent of SCN pacemaker autonomy was assessed by noting whether the period of the restored circadian rhythm following heterograft transplantation was characteristic of the donor or the host, or whether an atypical circadian period was established. Hamsters rendered arhythmic by SCN ablation were implanted with AH tissue from fetal hamsters (E13-E14, homograft controls) or fetal mice or rats (E15-E17). The AH homografts restored circadian activity rhythms with a tau similar to that of intact hamsters, and fetal mouse AH heterografts restored circadian rhythmicity with a tau similar to that of the donor mouse strain. However, fetal rat AH tissue implanted into SCNx hamsters renewed circadian rhythmicity with a period significantly shorter than either the species-typical tau of the rat donor or the hamster host. In both the mouse and rat AH heterograft experiments, immunocytochemical analysis performed with species-specific monoclonal antibodies revealed extensive fiber outgrowth from the implant into the host hypothalamus, evident up to 7 months postimplantation. The rat implants were consistently larger, more fully vascularized and exhibited less necrosis than the implanted mouse tissue. The histological appearance of the grafts, thus, provides no explantation for the difference in efficacy of the grafts to restore species-typical behavior. However, several interpretations are considered that are consistent with the combined behavioral results observed.

Behavioral and morphological studies of fetal neural transplants into SCN-lesioned rats

We have studied the effects of fetal neuronal grafts on the temporal pattern of drinking behavior of suprachiasmatic nuclei (SCN)-lesioned adult rats. Additionally, in an independent set of animals, the immunohistochemical staining for vasopressin, vasoactive intestinal polypeptide, and neuropeptide Y and the retinal connections to the hypothalamus were studied. The behavioral experiments indicate that anterior hypothalamic transplants induced reorganization of the temporal pattern of drinking behavior when placed in the third ventricle of adult hosts bearing complete SCN lesions, but not when placed in a cavity in the occipital cortex. Such rhythmicity persists only when the animals were recorded under constant darkness but not under constant light, indicating that the restored rhythmicity was generated endogenously but that the oscillator was extremely sensitive to light. Fetal occipital cortex induced reorganization of the temporal pattern of previously arrhythmic hosts, but it disappeared when the animals were recorded under constant light or constant darkness. It is clear that this rhythmicity was exogenous. In contrast to the cortical transplants, the hypothalamic transplants showed a morphological organization similar to that found in the normal hypothalamus regardless of their placement in the host brain. From these observations it is concluded that development of neocortex is more affected by environmental factors than that of the hypothalamus. Both hypothalamic and cortical transplants induced sprouting of retinal fibers into the anterior hypothalamus and the grafted tissue. It is possible that such fibers could be the neuroanatomical substrate by which rhythmicity is induced by cortical tissue.

Morphological and functional development of the suprachiasmatic nucleus in transplanted fetal hypothalamus

The development of the suprachiasmatic nucleus (SCN) in fetal rat hypothalamus transplanted to the adult brain was studied using morphological and functional methods. Anterior hypothalamic tissue was transplanted into the third ventricle, lateral ventricle or subarachnoid space of intact, adult hosts from E17 fetuses. These transplants developed the cytoarchitectonic and immunohistochemical staining characteristics of SCN, clusters of parvocellular neurons expressing vasopressin- and vasoactive intestinal polypeptide-like immunoreactivity in adjacent cellular populations, irrespective of the exact location of the transplanted tissue in the host brain. The functional status of the transplants placed in the rostral third ventricle and the foramen of Monroe was analyzed and compared to host SCN using in vitro recording of neuronal firing rate and measurement of metabolism using the 2-deoxyglucose (2-DG) technique. During subjective day, neuronal firing rates and 2-DG uptake were high in discrete cell groups within the transplants which were subsequently demonstrated to exhibit the cytoarchitectonic and immunohistochemical characteristics of SCN. The firing rates and 2-DG uptake in these areas were lower during the subjective night. This pattern of activity closely resembles that of the intact SCN. In contrast, neither transplanted anterior hypothalamic area, lacking an identifiable SCN-like structure, nor posterior hypothalamic area showed day-night differences in firing rate or 2-DG uptake. These observations indicate that SCN transplanted into intact adult hosts exhibits morphological and functional differentiation nearly identical to the host and that the transplanted SCN maintains circadian function which is probably entrained to the host SCN.

[Microsurgical anatomy of extraorganic arteries of the hypothalamo-hypophyseal complex and the rational technic of its removal for transplantation]

The authors ascertained the existence of numerous anastomoses between the right and left hypophyseal arteries on the level of both the interlobar sulcus and the base of the hypophyseal funnel in the form of an inferior and superior hypophyseal anastomotic rings, as well as a system of arterial connections between the anterosuperior and anteroinferior subgroups of the hypothalamic arteries located in front of and lateral to the chiasma as prechiasmal networks. This creates anatomo-physiological preconditions for adequate circulation in the hypothalamo-hypophyseal graft after its transplantation and inclusion in the blood flow of the internal carotid artery or one of its terminal branches. Bearing in mind the close connection of the hypophyseal and hypothalamic arteries with the cavernous sinuses, chiasma, and components of the sella turcica, the hypothalamo-hypophyseal graft must be removed in a complex with these structures.

Correction of genetic diabetes insipidus by adult hypothalamic grafts

Brattleboro rats manifest chronic diabetes insipidus as a result of the genetic deficiency of hypothalamic vasopressin. When basal hypothalamic tissue derived from adult F344 rats was implanted as cell suspensions or tissue blocks in the supraoptic regions of these animals, concentration of urine together with reduced urine output and water intake was observed in some animals. Histologic examination of the grafted brains from the responding animals revealed neuronal cells at the implant sites and vasopressin-staining fibers in the median eminence. This study demonstrates the feasibility of the grafting of adult cerebral tissues to correct a genetic hormonal deficiency.

Differentiation of the suprachiasmatic nucleus in fetal rat anterior hypothalamic transplants in oculo

The capacity of the rat anterior hypothalamus, and particularly the suprachiasmatic nucleus (SCN), to develop and differentiate when removed from its normal environment was examined in this study using light and electron microscopy. The hypothalamus from fetuses ranging in age from embryonic day 12 (E 12) to E 16 was transplanted to the anterior chamber of the eye of adult rats. In initial experiments, we found that transplants from E 15 fetuses and older routinely differentiated into fields of neurons with extensive neuropil with an appearance similar to the anterior hypothalamic area. Groups of small, compactly organized neurons were observed only occasionally in this tissue. Ultrastructural analysis of these transplants typically revealed well-differentiated neuronal perikarya and neuropil with a complex synaptic organization similar in appearance to the normal rat anterior hypothalamic area. Occasionally both mature and immature tissue coexisted in some of the transplants. Tissue from young embryos (E 12-14) frequently showed development of a compact, small neuron nucleus with the cytoarchitectonic appearance of the SCN. At least 45 days were required after transplantation for the successful differentiation to occur in this situation. The SCN in these transplants displayed vasoactive intestinal polypeptide-immunoreactive cells and fibers surrounded by vasopressin-immunoreactive cells and fibers, similar to the pattern observed in the normal adult SCN. Our results indicate that the anterior hypothalamus will differentiate normally in oculo and that the phenotypic specification of the SCN occurs prior to the birthdate of its component neurons.

Fetal hypothalamic transplants in diabetic rats analysed by scanning electron microscopy

Adult male Brattleboro rats with chronic diabetes insipidus underwent stereotaxic surgery wherein minced fragments of anterior hypothalamus from fetal rats, 17 days post-coitus, were stereotaxically positioned into the lumen of the host third cerebral ventricle. Host rats with fetal donor tissue were killed at various times following surgery and were prepared for correlative scanning-transmission electron microscopy. Examination with this technique revealed the presence of large neurografts which grew to occupy the entire lumen of the host third ventricle. Grafts were well vascularized and in addition exhibited remarkable numbers of supraependymal, cerebrospinal fluid-contacting neurons. The physical emergence of this cell line in proximity to viable grafts is discussed with respect to the biochemical influences that a neuropeptide producing fetal transplant has upon a peptide-deficient host.

A Golgi study of hypothalamic transplants in young and old host rats

The supraoptic nucleus of the F344 rat shows an age-related dendritic regression. In order to determine whether this previously observed dendritic regression may have been related to extrinsic (to the cell) hormonal, neurotoxic, or other circulating factors unique to the hypothalamus of older brains, we conducted a quantitative Golgi study of F344 embryonic anterior hypothalamic transplants into the third ventricle of young adult (5 months) and older (25 months) male F344 rats. Three months following transplantation there were no qualitative effects of host age on neuronal morphology, nor were there quantitative effects on transplant size, dendritic length or branching frequency within the transplanted tissue. These results suggest that either (a) there were no age-related changes in factors in the host brain which were sufficient to significantly affect dendritic extent or, (b) intrinsic connections or other properties within the transplant may be important in moderating the effect of the milieu of the aged brain on the transplanted tissue.

Vasopressin neuron survival in neonatal Brattleboro rats; critical factors in graft development and innervation of the host brain

Previously it was found that grafts of supraoptic plus paraventricular areas from 19-day-old foetal normal rats survived in the third ventricle of the brain of 4- to 6-day-old, vasopressin-deficient Brattleboro pups, but could not alleviate their polyuria. In the present series, factors important in graft development were analysed. Again using day-19 fetuses as donors, anterohypothalamus grafts as well as grafts placed near a crushed median eminence survived relatively poorly, but showed the presence of vasopressin neurons immunocytochemically one month post-grafting. Homotopic grafting in the supraoptic nucleus, however, even failed to show surviving vasopressin neurons. Graft survival was improved by the use of donor tissue of fetuses younger than day 19. Parvocellular vasopressin cells were frequently seen, organized into clusters resembling the normal suprachiasmatic nucleus. However, magnocellular neurons, as normally seen in supraoptic and paraventricular nuclei, only survived grafting when taken between days 11 and 15 of fetal age. It was concluded that only immature vasopressin neurons survived grafting under the condition employed. Magnocellular neurons had a limited fiber outgrowth into the host brain and median eminence. Most large neurons only stained with non-specific neurophysin antiserum, not with specific vasopressin-associated neurophysin antiserum. Thin fibers of the parvocellular vasopressin neurons provided only occasional and sparse innervation of the host median eminence and lateral septum (one case), but several examples of massive fiber bundles running dorsally from graft into host brain were observed. These fibers terminated in the thalamic periventricular area, a nucleus that is normally innervated by the vasopressin neurons of the suprachiasmatic nucleus. The failure of the grafts to provide adequate vasopressinergic innervation of the host median eminence probably explains why none of the nearly 200 Brattleboro neonates operated upon showed any sign of relief of their diabetes insipidus. It suggests, however, that the present procedures might be useful in restoring central vasopressinergic functions in the developing Brattleboro rat.

In vitro and in vivo studies on development and regeneration of vasopressin neurons

The present paper reviews recent work conducted in our laboratory on vasopressin neurons either grown in culture or transplanted into vasopressin deficient rats. The in vitro model of reaggregated cell culture used the anterior hypothalamus, including vasopressin neurons of the SON from normal timed-pregnant LE rats of similar ages used in our in vivo model. Various cells were co-cultured with their known target tissue, the posterior pituitary to analyze further the influence of the target tissue on hormone production. At a designated end point, cultured cells were fixed and stained immunocytochemically for vasopressin and neurophysin. Radioimmunoassay of the samples was performed for vasopressin quantification. Hypothalamic cells from all ages produced vasopressin (VP). The co-culturing of hypothalamus with posterior pituitary produced a significant increase in VP. Correlative transplantation studies were conducted using timed-pregnant Long-Evans (LE) rats at various days post coitus (dpc) and neonatal tissue from 0- and 5-day old rat pups. Animals survived about 40 days then were perfused and their brains processed for vasopressin and neurophysin thick-section immunocytochemistry. The results showed that the capability for survival of younger grafts was much greater than that of older tissue. With this paper, we have shown that the reaggregation of anterior hypothalamic cells in a culture system can be used for microassay of neurosecretory activity. These data suggest a close correlation between the ability of a neuron to survive transplantation and its stage of development. With the present studies, we have shown that neurons not fully differentiated maintain a greater degree of plasticity than older tissue and are better able to survive the rigors of transplantation and that various manipulations of environmental factors have an effect on brain development at critical times.

Fetal hypothalamic transplants in the third ventricle of the adult rat brain. Correlative scanning and transmission electron microscopy

Blocks of anterior hypothalamus were transplanted from 19 day-old fetuses of Wistar/Lewis rats into the third ventricle of adult male Brattleboro rats. Physiological changes in graft recipients and in sham-operated animals were monitored daily. Twenty days after surgery, the graft recipients and sham-operated animals were killed and their brains examined by correlative scanning and transmission electron microscopy. Host animals that exhibited both decreased polydipsia and increased urine concentration were found to have viable grafts within the third ventricle. The observed physiological changes suggested that synthesis and release of vasopressin occurred in the transplanted neurons. Grafts were well vascularized by vessels arising from the host hypothalamus. Neurons, with perikarya ranging from 8 to 30 micron in diameter, glial cells, and neurites were located throughout the transplants. A neurohemal contact zone, similar to that normally seen in the median eminence, could not be demonstrated in the grafts. The absence of complete glial and ependymal barriers indicates a relatively close association between cells in the transplants and the cerebrospinal fluid. A large increase in supraependymal neurons and their processes, including an eruption of neurons through the floor of the third ventricle in one animal, was observed in graft recipients but not in sham-operated animals.