Supraoptico-hypophysial Tract Research Articles

60 YEARS OF NEUROENDOCRINOLOGY: The posterior pituitary, from Geoffrey Harris to our present understanding.

Geoffrey Harris pioneered our understanding of the posterior pituitary, mainly with experiments that involved the electrical stimulation of the supraoptico-hypophysial tract. In the present essay, we explain how his observations included clues to the pulsatile nature of the oxytocin signal - clues that were followed up by subsequent workers, including his students and their students. These studies ultimately led to our present understanding of the milk-ejection reflex and of the role of oxytocin in parturition. Discoveries of wide significance followed, including: the recognition of the importance of pulsatile hormone secretion; the recognition of the importance of stimulus-secretion coupling mechanisms in interpreting the patterned electrical activity of neurons; the physiological importance of peptide release in the brain; the recognition that peptide release comes substantially from dendrites and can be regulated independently of nerve terminal secretion; and the importance of dynamic morphological changes to neuronal function in the hypothalamus. All of these discoveries followed from the drive to understand the milk-ejection reflex. We also reflect on Harris's observations on vasopressin secretion, on the effects of stress, and on oxytocin secretion during sexual activity.

Oxytocin and vasopressin in rat hypophysial portal blood: experimental studies in normal and Brattleboro rats

Oxytocin (OT) and vasopressin (VP) were measured by radioimmunoassay in hypophysial portal and peripheral blood from male Wistar rats and heterozygous and homozygous Brattleboro rats anaesthetized with urethane. In Wistar rats the concentrations of OT and VP were about 50 times greater than the concentrations in peripheral blood, whether or not the pituitary gland was left in situ during collection, and also considerably greater than the reported concentrations of the peptides in the cerebrospinal fluid. The release of both peptides was increased significantly by a lesion of the supraoptico-hypophysial tract that led to diabetes insipidus, but which left intact the external layer of the median eminence (ME). Concentrations of VP were undetectable in plasma from homozygous Brattleboro rats, but the portal plasma concentrations of VP in heterozygous Brattleboro rats were not significantly lower than in Wistar rats. The concentrations of OT in portal plasma from both types of Brattleboro rat were significantly higher than in Wistar rats. The output of VP and OT into hypophysial portal blood of Wistar rats was not significantly affected by electrical stimulation of the suprachiasmatic, supraoptic or paraventricular nuclei or the ME using two types of stimuli, one of which produced an increase in peripheral plasma concentrations of VP and OT in intact rats and a significant increase in the release of LH-releasing hormone into hypophysial portal blood. The output of VP and OT into portal blood was also not significantly affected by either adrenalectomy with or without injection of dexamethasone or the injection of either the 5-hydroxytryptamine (5-HT) synthesis blocker, parachlorophenylalanine, or the 5-HT uptake blockers, alaproclate or zimelidine. These results show that large amounts of OT as well as VP are released into hypophysial portal blood from fibres of the hypothalamo-neurohypophysial system that terminate in the external layer of the ME. Although distinct from the fibres that terminate in the pars nervosa (PN), the findings in Brattleboro rats show that the VP fibres of the ME system originate in neurones with a genomic mechanism for VP synthesis similar to that of the VP neurones that project to the PN. The lack of effect of adrenalectomy and the administration of 5-HT synthesis and uptake blockers must be interpreted with caution since the results obtained with electrical stimulation suggest that when the pituitary stalk is cut the release of OT and VP into portal blood approaches a maximum and may therefore be difficult to alter by experimental manipulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunocytochemical localization of neuropeptides in the ventral hypothalamus of the cat.

The localization of immunoreactive neuropeptides such as arginin vasopressin (AVP), oxytocin (OT), luteinizing hormone-releasing hormone (LHRH), somatostatin (SOM), corticotropin-releasing factor (CRF), thyrotropin-releasing hormone (TRH), met-enkephalin (ENK) and substance P (SP) was studied immunocytochemically in the ventral hypothalamus of the cat. Each peptidergic neuron system represents its own peculiar distributional pattern in the hypothalamus, while some nuclei in the ventral hypothalamus contain many kinds of neuropeptide-producing neurons, which show an overlapped or mosaic-like distribution. The overlapped distribution of different peptidergic systems suggests their functional correlations through the information exchange within the central nervous system. In the median eminence, many kinds of peptide-containing fibers show a mosaic-like distribution. In the internal zone, AVP- and OT-fibers pass to the neural lobe via supraoptico-hypophysial tract. AVP-, CRF-, TRH-, and ENK- immunoreactive fibers are distributed exclusively in the external layer of the anterior part of the median eminence and terminate on the basement membrane of the external surface of the median eminence in intimate contact with the anterior primary capillary plexus of the portal vessels. LHRH- and SOM-immunoreactive fibers are more numerous in the external layer of the posterior part of the median eminence. The intrinsic distribution of each peptide-containing fiber in the median eminence suggests the regional control mechanisms of the pars distalis by the discharge of each peptide into the respective portal vessels.

The supraoptic nucleus and the supraopticohypophysial tract in the monkey (Macaca mulatta).

The observations here on Golgi material are based on the study of fortunate impregnations in one horizontally sectioned and one frontally sectioned series found in the collection of Golgi preparations of the adult monkey brains (Macaca mulatta) available in this laboratory. The soma of the large supraoptic neurons have jagged protrusions and irregular crevices which give their surfaces a craggy appearance. Also they have a few somatic spines. Their dendrites, which usually arise from two or three dendritic trunks, emerging from the cell body, are moderately branched and have occasional dendritic spines. The axon emerges from a conical elevation either on the soma or a dendrite and is directed towards the supraopticohypophysial tract. In the present material there are a few neurons of the supraoptic nucleus impregnated that are definitely smaller than the larger neurons. They have short beaded axons, ending in the supraoptic nucleus a short distance from their point of origin. In the horizontal sections the supraopticohypophysial tract fibers can be seen passing over the posterior aspect of the optic tract. In the frontal sections, this fan-shaped tract, wide above and constricted below, can be followed into the median eminence. Some of these axons in their intrahypothalamic course have short beaded collaterals. Electron micrographs reveal: (1) endings with spherical synaptic vesicles and endings with flattened synaptic vesicles synapsing on neurosecretory axons in the supraoptic nucleus; (2) a few myelinated fibers containing neurosecretory granules in the intrahypothalamic portion of the supraopticohypophysial tract. As can be seen in both Golgi and electron microscopic preparations the presence of fibrous astrocytes in the supraoptic nucleus and their abundance in the hypothalamic portion of the supraopticohypophysial tract is characteristic of this neurosecretory system. In the nucleus and in the tract processes of the fibrous astrocytes are intimately associated with neurosecretory fibers.

Ultrastructural studies on the localization of neurohypophysial hormones and their carrier proteins.

Neurophysin, vasopressin and oxytocin were localized in different portions of the supraopticohypophysial tract (SHT) using the unlabeled antibody enzyme technique at the ultrastructural level. In vasopressin-positive supraoptic perikarya, vasopressin and neurophysin were present in all neurosecretory granules. Within the zona interna of the median eminence, vasopressin and neurophysin were present in two populations of axons, one with granules of 1300-1500 A and one with granules of 900-1300 A. Following exposure of thin sections of median eminence to antiserum to neurophysin, reaction products were present in granules and in the extragranular cytoplasm in the axons with larger granules; in all other cases reaction product was confined to the granules. Vasopressin-positive fibers were also presented in large numbers of the zona externa of the median eminence and many terminated on the pituitary primary portal plexus. A few oxytocin fibers were present on the portal capillaries in the infundibular stalk. In the posterior pituitary all axon profiles were neurophysin positive. Neurophysin was present as both a granular and cytoplasmic pool. Vasopressin-containing axons account for 90% of the neuronal elements in the posterior pituitary and oxytocin for the remaining 10%. Findings on the subcellular distribution of these peptides are related to current theories on transport and release of neurohormones.

The hypothalamic magnocellular neurosecretory system of the guinea pig. I. Immunohistochemical localization of neurophysin in the adult.

With the use of the unlabeled antibody enzyme technique and antiserum against bovine neurophysin I, neurophysin was localized in the hypothalamic magnocellular neurosecretory system of the adult guinea pig. Immunoreactive deposits were found in the perikarya of the supraoptic and paraventricular nuclei, their fiber projections and terminals in the posterior pituitary. No parvicellular neurophysin-positive components were observed. In the median eminence neurophysin was seen in the zona interna where axons of the supraoptico-hypophysial tract pass on their way to the neural lobe. The peptide was also present in axons projecting into zona externa which terminate on the primary portal plexus.

Stimulation of the supraopticohypophysial tract in the rat during suckling: failure to alter the inherent periodicity of reflex oxytocin release.

A.R.C. Institute of Animal Physiology, Babraham, Cambridge, CB2 4AT (Received 14 April 1975) One explanation for the intermittent milk ejection (ME) in the rat is that the activation of paraventricular (PV) and supraoptic (SO) cells during oxytocin release (Wakerley & Lincoln, 1973; Lincoln & Wakerley, 1974) causes a temporary block of further transmission of the suckling stimulus to the hypothalamus by inhibitory feedback on to afferent pathways of the milk-ejection reflex. Since oxytocin injections have no effect on endogenous hormone release (Lincoln, 1974), this feedback is unlikely to be humorally mediated. However, there still remains the possibility of a direct neural feedback, especially since PV and SO cells appear to possess recurrent collateral axons (Koizumi & Yamashita, 1972). In the present study we investigated the existence of such a neural feedback by attempting to alter the inherent periodicity of reflex oxytocin release with stimulation of the supraopticohypophysial (SOH) tract. Lactating

Formation of secondary neurohemal organs in the median eminence of the White-crowned Sparrow and Japanese quail

Midline electrolytic or radio frequency lesions were placed in the basal hypothalamus of adult male White-crowned Sparrows and Japanese quail. In White-crowned Sparrows, destruction of the neurohemal organs of the stainable hypothalamo-hypophysial neurosecretory system in the neural lobe (NL) and/or palisade layer of the anterior division of the median eminence (AME) resulted in the formation of large aggregations of stainable neurosecretory material (NSM) surrounding hypertrophied capillaries. Very few of these occurred in quail. These structures, herein referred to as secondary neurohemal organs, were located in the AME dorsal to that region in which NSM is usually found. In normal birds, this region was relatively devoid of capillaries. Interruption of fibers of the supraopticohypophysial tract dorsal to the median eminence was not conducive to formation of these structures. Damage to the stainable neurosecretory system of quail was apparently permanent, as practically no indication of repair was seen. It is suggested that secondary neurohemal organs in the White-crowned Sparrow may represent neurovascular organs of the NL and/or AME, and may function accordingly.

A study of the parameters of electrical stimulation of unmyelinated fibres in the pituitary stalk.

1. The milk-ejection response in lactating rabbits has been used to study the effect of electrical stimuli of different types applied to the supraopticohypophysial tract in the pituitary stalk.2. Sine-wave alternating-current pulses were compared with balanced biphasic square-wave pulses of the same frequency and peak-to-peak current strength. At a pulse duration of 2-4 msec the square-wave stimulation was less effective than the sine wave, but at a pulse duration of 8 msec and over, more effective.3. Above threshold levels of 0.12 mA for the current strength, and of 0.5 msec for the pulse duration, the response increased with increasing current strengths to 2.4 mA and increasing pulse durations to 10 msec.4. With constant and effective strength and duration of the pulse a slight diminution of the response was seen as the frequency was diminished from 100/sec to 50/sec. Further diminution in the frequency revealed that at some point between 50 and 10 c/s a sudden abrupt diminution in the magnitude of the responses occurred over a small range of frequency. This was a reversible phenomenon and it is suggested that it may be related to neurosecretory events occurring in the nerve terminals.5. Stimuli with parameters within the ranges mentioned above are effective in eliciting an oxytocic response on the uterus in the rabbit and ovulation responses (when applied to the hypothalamus) in rats and rabbits. It appears likely that stimuli with such parameters are suitable for experiments concerned with stimulation of the hypothalamus.

The role of the median eminence in control of photoperiodically induced testicular growth in the white-crowned sparrow. Zonotrichia leucophrys gambelii.

Midline electrolytic lesions were created in the basal hypothalamus of adult white-crowned sparrows, Zonotrichia leucophrys gambelii, in order to explore further the neuroendocrine basis of photoperiodic control of testicular growth. Lesions that destroyed the anterior and posterior divisions of the median eminence (AME and PME) prevented photoperiodically induced testicular growth, confirming the previously reported experiments of Wilson (1967) on this species. Destruction of the PME alone also abolished photoperiodic testicular growth. Destruction of the AME usually had no inhibitory effect. Moreover, extraeminential lesions that interrupted a majority of the fibers of the supraoptico-hypophysial tract thereby causing a marked depletion of stainable neurosecretory material from the AME had, in all but two birds, no noticeable effect on testicular growth. On the other hand, destruction of selected regions of the infundibular nucleus, without involvement of the median eminence, suppressed the photoperiodic testicular response. Destruction of the posterior region of the infundibular nucleus, causing antegrade degeneration of tubero-hypophysial fibers into the PME, also suppressed the testicular response. These results suggest that the infundibular nucleus may be composed of separate cellular populations functioning in the control of photoperiodically induced testicular growth and that only one of these may contribute neuronal elements to the neurohemal organs of the PME.

Hypothalamic pathways for independent release of vasopressin and oxytocin.

By means of a simple method for extraction of blood samples, reliable recoveries of vasopressin and oxytocin were obtained. This method was used in anaesthetized cats for estimation of these hormones, released into external jugular venous blood, after localized electrical stimulation in the hypothalamus. Stimulation of the supraoptic nucleus or supraoptico-hypophysial tract released large amounts of vasopressin without oxytocin. Stimulation of the paraventricular nucleus or tractus paraventricular is cinereus (Greving) released moderate amounts of vasopressin alone. Stimulation at points in the tuberal region of the hypothalamus released oxytocin with or without vasopressin. These could lie on the caudal tract from the paraventricular nucleus (Laqueur).

Observations on the uptake of 35-sulfur by the hypothalamo-hypophysial system of the white-crowned sparrow (Zonotrichia leucophrys gambelii) following intraventricular injection of 35-S DL-cysteine.

By use of a stereotaxic instrument it has been consistently possible to introduce 35S DL-cysteine into the third ventricle of the White-crowned Sparrow, Zonotrichia leucophrys gambelii. Such injections are followed within six hours by a reproducible pattern of accumulation of the isotope in the hypothalamic nuclei around the third ventricle. The nucleus supraopticus, the nucleus paraventricularis magnocellularis, the lamina terminalis, and the area ependymalis vasculosa consistently showed the greatest accumulation of the isotope. In the supraoptic and paraventricular nuclei, conspicuous accumulation of the isotope was consistently observed in association with cells containing aldehyde-fuchsin positive material. The supraoptico-hypophysial tract, the pars nervosa, and the palisade layer of the zona externa of the median eminence showed pronounced accumulations of the isotope. These accumulations were, to a great extent, associated with aldehyde-fuchsin positive neurosecretory material.

Effects of water deprivation on the hypothalamo-hypophysial neurosecretory system of the grass parakeet, Melopsittacus undulatus

The nuclear diameters of hypothalamic neurosecretory cells of the grass parakeet, Melopsittacus undulatus, do not change significantly after water deprivation for 3 or 5 days. The amount of neurosecretory material in the pars nervosa, the palisade layer of the median eminence, and the supraoptico-hypophysial tract in the median eminence is not decreased. Body weight is not decreased significantly in parakeets after 7 days of water deprivation.

A Possible Mechanism of Neurosecretion: Release of Vasopressin by Depolarization and its Dependence on Calcium

IT is recognized that the various influences, nervous and osmotic, which bring about release of the hormones vasopressin and oxytocin from the neuroseeretory terminals in the posterior lobe of the pituitary gland act through the parent cells in the supra-optic and paraventricular nuclei. Such a scheme demands that these cell bodies in the hypothalamus regulate release of hormone from their terminals a considerable distance away and it is simplest to suppose that this regulation is effected by action potentials propagated down the supra-opticohypophysial tract. In this view the cells of the supra-opticohypophysial system possess the characteristic property of neurones—the propagation of action potentials–as well as the characteristic property of endocrine organs–the liberation of hormones into the blood stream1,2.

The hypothalamo-hypophysial neurosecretory system of the zebra finch. Taeniopygia castanotis.

The hypothalamo-hypophysial neurosecretory system of the zebra finch. Taeniopygia castanotis.

Neurosecretion in birds

Morphologically the hypothalamic neurosecretory system of birds includes (1) two rather diffuse neurosecretory nuclei, the paraventricular nucleus and supraoptic nucleus which, according to the species concerned, consist of several more or less distinct divisions; (2) two hypothalamo-hypophysial neurosecretory tracts, the paraventriculo-hypophysial tract and the supraoptico-hypophysial tract, which are not separately identifiable in many species; (3) the neurosecretion-rich infundibular branches of the hypothalamo-hypophysial neurosecretory tract in the median eminece; and (4) the neurohypophysis and the neurosecretory fibers leading thereto. Specific functions cannot be assigned as yet to the individual neurosecretory nuclei or to their divisions. For the distal parts of the system, functions may be divided into those of the neurohypophysial and eminential components. The former is identified primarily with the regulation of water balance and apparently also with the ecbolic function in ovulation. The hormone involved is probably arginine vasotocin which is probably produced by hypothalamic neurosecretory cells and transported in association with the stainable neurosecretory material via the hypothalamo-hypophysial neurosecretory tract to the posterior lobe. Although oxytocin occurs in the posterior pituitary its function is unknown. The eminential component, whose radial neurosecretory fibers emanate from the eminential plexus and extend outwardly into juxtaposition with the primary capillaries of the hypophysial portal system, is primarily involved in control of the function of the adenohypophysis. This component is best known from the studies on species in which the gonadotropic function of the adenohypophysis is photoperiodically controlled. To us, the best hypothesis of the general functional mechanism of the eminential component is that the “releasing factor” or “releasing factors” are formed in hypothalamic neurosecretory cells where they become associated with neurosecretory material, which is usually stainable with aldehyde-fuchsin or chromalum hematoxylin, and which passes via neurosecretory fibers into the radial fibers of the zona externa of the median eminence. It seems evident that the control of the adenohypophysis is exerted primarily through the transfer of such “releasing factors” from these fibers into the primary capillaries of the portal system. It is possible that this transfer involves a functional relationship between the neurosecretory fibers and fibers from the nonneurosecretory tubero-hypophysial tract. The very fragmentary data available suggest that the gonadotropic function of the adenohypophysis is almost completely dependent on such “releasing factors” although the gland does have basic adrenocorticotropic and thyrotropic functions that are independent of the median eminence. However, higher levels of thyrotropic and adrenocorticotropic activity by the adenohypophysis apparently require the appropriate “releasing factors”.

The possible role of oxytocin in stimulating the release of prolactin.

Hypothalamic lesions in the supraoptico-hypophysial tract of lactating rats produceda block of both milk ejection and milk secretion. The defect in milk secretion was correlated with the deficiency in endogenous oxytocin secretion, but there was no correlation between polydipsia and milk ejection or milk secretion. The effective lesions interrupted the supraoptico-hypophysial tract either in the median eminence or in front of this structure. The defect in milk secretion was seen in rats where median eminence, hypophysial portal vessels, and anterior lobe of the pituitary appeared to be essentially normal on histological examination. Pitocin and synthetic oxytocin delayed mammary involution in lactating rats. Pitressin produced a smaller effect. The effect of Pitocin and synthetic oxytocin persisted in rats with hypothalamic lesions. Since a defectin endogenous oxytocin release led to a defect in milk secretion, whereas oxytocin delayed mammaryinvolution, it was postulated that oxytocin may be the neurohum...

On the mechanism of labour in the rabbit.

SUMMARY The parturitional behaviour of full-term rabbits injected with 100 or 200 mu. oxytocin is described. Young were born at rates varying from twelve in 7 min to three in 20 min. Newborn pups suckled the doe while she was still in labour. Labour was induced in full-term does under sodium pentobarbitone anaesthesia by injection of 50–200 mu. oxytocin or by electrical stimulation of the supraoptico-hypophysial tract. Abdominal contractions, milk ejection and delivery of young were recorded kymographically. The delivery of every pup was assisted by reflex straining movements of the doe. Though commonly labour was completed without the secretion of additional oxytocin from the neurohypophysis (as shown by the milk-ejection record), occasionally a reflex release of oxytocin did occur in amounts sufficient to influence the course of labour. In many cases labour appeared to be as efficient as in the conscious animal. Suppression of abdominal contractions by spinal anaesthesia did not prevent effective delivery of young after injection of oxytocin. However, the time taken to expel individual pups from the vagina tended to increase, and the last of the litter was generally retained in the vagina. The physiological mechanisms involved in parturition in the rabbit are discussed in the light of these and earlier findings.

The neurosecretory substance in the hypothalamic-hypophysial system of the horse.

A good deal of hypothetical evidence has been presented for the view that the antidiuretic and oxytocic hormones are not produced in the posterior lobe of the hypophysis. These hormones appear to originate from a neurosecretory process in the neurosecretory nerve cells of the supraoptic and paraventricular nuclei in the hypothalamus and to migrate within the nerve fibers of the supraoptico-hypophysial tract and into the neurohypophysis (Hild, 1951, 1954 a, b and Hild & Zetler, 1951, 1952, 1953 a, b). Here they are stored and if necessary released. Neurosecretory neurons are described in many species of the animal kingdom from invertebrates to man. The product of the neurosecretory cells contains a stainable carrier substance in addition to the above mentioned hormones (Scharrer & Scharrer, 1954, and Scharrer, 1956). The product of the hypothalamic neurosecretory cells can be demonstrated histologically by certain methods. The chrome-alum-hematoxylin-phloxin-method (Gomori, 1941), first used to stain

PITUITARY-HYPOTHALAMIC MECHANISMS

I. THE NEUROHYPOPHYSIS THE NEUROHYPOPHYSIS and the adrenal medulla are the only two endocrine glands dependent on a secretory nerve supply for their functional activity. It is only within relatively recent years that the mechanisms controlling the secretion in the neurohypophysis have been worked out. Observations of Ramon y Cajal, Pines, and Greving defined the tract of unmyelinated nerve fibers running from the paraventricular and supraoptic nuclei to the neurohypophysis. Ranson and his colleagues observed that fibers comprising the tuberohypophysial tract, of uncertain origin, ran in the dorsal wall of the hypothalamohypophysial tract, most of whose fibers came from the supraoptic nucleus. Lesions of the supraopticohypophysial tract made with the Horsley-Clarke stereotaxic instrument in cats were shown by Fisher, Ingram, and Ranson to lead to diabetes insipidus controllable by posterior pituitary extracts. Morphologically, atrophy of the neurohypophysis occurred, and the median eminence and infundibular stem and process became shrunken and