Alteration in the relationship between tanycytes and gonadotrophin-releasing hormone neurosecretory terminals following long-term metabolic manipulation in the sheep

The activity of the hypothalamic-pituitary gonadal axis is influenced by energy reserves, such that an increase or a decrease in adiposity may perturb the secretion and action of gonadotrophin-releasing hormone (GnRH). This is considered to be a result of the signalling of hormones such as leptin, which act upon neuronal systems controlling GnRH secretion. Other work shows plasticity in the relationship between tanycytes and GnRH neurosecretory terminals in the median eminence across the oestrous cycle and we hypothesised that a similar plasticity may occur with altered metabolic status. We studied Lean, Normal and Fat ovariectomised ewes, which displayed differences in gonadotrophin status, and investigated the relationship between tanycytes and GnRH neuroterminals. Under both Lean and Fat conditions, an altered anatomical arrangement between these two elements was observed in the vicinity of the blood vessels of the primary plexus of the hypophysial portal blood system. These data suggest that such plasticity is an important determinant of the rate of secretion of GnRH in animals of differing metabolic status and that this also contributes to the relative hypogonadotrophic condition prevailing with metabolic extremes.

Gene expression of vascular endothelial growth factor-A in the pituitary during formation of the vascular system in the hypothalamic-pituitary axis of the rat

Techniques involving fluorescein-5-isothiocyanate-conjugated gelatin injection, immunohistochemistry, and in situ reverse transcription/polymerase chain reaction (RT-PCR) revealed a close relationship between vascular endothelial growth factor (VEGF)-A-expressing cells and microvessels in the hypothalamic-pituitary axis of the rat. In situ RT-PCR clearly indicated the presence of VEGF-A mRNA-expressing cells in the pars tuberalis and in the pars distalis both at embryonic day 15.5 (E15.5) and in later developmental stages. The primary capillaries extended along the developing pars tuberalis, whereas the portal vessels penetrated into the pars distalis at E15.5 and subsequently expanded into the lobe to connect with the secondary capillary plexus, emerging in the pars distalis. At the same time, several VEGF-A-positive cells appeared in the pars distalis. These VEGF-A-positive cells were found to correspond to a portion of adrenocorticotropin (ACTH) cells by dual-staining for in situ RT-PCR and immunohistochemistry, suggesting that some ACTH cells have the potential to produce VEGF-A. Thus, the present study suggests that VEGF-A is involved in the development of the primary capillaries and in the vascularization of the pars distalis, but not in the portal vessels since the formation of portal vessels begins at E13.5, before the appearance of VEGF-A in the rostral region of the pars distalis.

Hormonal cycle modulates arousal circuitry in women using functional magnetic resonance imaging

Sex-specific behaviors are in part based on hormonal regulation of brain physiology. This functional magnetic resonance imaging (fMRI) study demonstrated significant differences in activation of hypothalamic-pituitary-adrenal (HPA) circuitry in adult women with attenuation during ovulation and increased activation during early follicular phase. Twelve normal premenopausal women were scanned twice during the early follicular menstrual cycle phase compared with late follicular/midcycle, using negative valence/high arousal versus neutral visual stimuli, validated by concomitant electrodermal activity (EDA). Significantly greater magnitude of blood oxygenation level-dependent signal changes were found during early follicular compared with midcycle timing in central amygdala, paraventricular and ventromedial hypothalamic nuclei, hippocampus, orbitofrontal cortex (OFC), anterior cingulate gyrus (aCING), and peripeduncular nucleus of the brainstem, a network of regions implicated in the stress response. Arousal (EDA) correlated positively with brain activity in amygdala, OFC, and aCING during midcycle but not in early follicular, suggesting less cortical control of amygdala during early follicular, when arousal was increased. This is the first evidence suggesting that estrogen may likely attenuate arousal in women via cortical-subcortical control within HPA circuitry. Findings have important implications for normal sex-specific physiological functioning and may contribute to understanding higher rates of mood and anxiety disorders in women and differential sensitivity to trauma than men.

The hippocampus and depression

The effect of depression on the hippocampus has become the focus of a number of structural and functional neuroimaging studies. In the past two decades, advances in neuroimaging techniques now allow the examination of subtle changes in both regional structure and function that are associated with the pathophysiology of depression. Many studies using 3-dimensional magnetic resonance imaging (MRI) volumetric measurement have reported decreases in hippocampal volume among depressed subjects compared with controls, whereas other studies have not found any volume loss. Differences among studies have been discussed. In some studies, the volume loss appears to have functional significance including an association with memory loss. Furthermore, we have found a trend towards loss of 5-HT(2A) receptors in the hippocampus using positron emission tomography (PET) to detect regional changes in [18F]altanserin binding. Functional imaging extends the sensitivity and specificity of structural imaging and will lead to a better understanding of affective disorders.

Cocaine- and amphetamine-regulated transcript is present in hypothalamic neuroendocrine neurones and is released to the hypothalamic-pituitary portal circuit

Cocaine- and amphetamine-regulated transcript (CART) is present in a number of hypothalamic nuclei. Besides actions in circuits regulating feeding behaviour and stress responses, the hypothalamic functions of CART are largely unknown. We report that CART immunoreactivity is present in hypothalamic neuroendocrine neurones. Adult male rats received a systemic injection of the neuronal tracer Fluorogold (FG) 2 days before fixation, and subsequent double- and triple-labelling immunoflourescence analysis demonstrated that neuroendocrine CART-containing neurones were present in the anteroventral periventricular, supraoptic, paraventricular (PVN) and periventricular nuclei of the hypothalamus. In the PVN, CART-positive neuroendocrine neurones were found in all of cytoarchitectonically identified nuclei. In the periventricular nucleus, approximately one-third of somatostatin cells were also CART-immunoreactive. In the medial parvicellular subnucleus of the PVN, CART and FG coexisted with thyrotrophin-releasing hormone, whereas very few of the corticotrophin-releasing hormone containing cells were CART-immunoreactive. In the arcuate nucleus, CART was extensively colocalized with pro-opiomelanocortin in the ventrolateral part, but completely absent from neuroendocrine neurones of the dorsomedial part. To assess the possible role of CART as a hypothalamic-releasing factor, immunoreactive CART was measured in blood samples from the long portal vessels connecting the median eminence with the anterior pituitary gland. Adult male rats were anaesthetized and the infundibular stalk exposed via a transpharyngeal approach. The long portal vessels were transected and blood collected in 30-min periods (one prestimulatory and three poststimulatory periods). Compared to systemic venous plasma samples, baseline concentrations of immunoreactive CART were elevated in portal plasma. Exposure to sodium nitroprusside hypotension triggered a two-fold elevation of portal CART42-89 immunoreactivity throughout the 90-min stimulation period. In contrast, the concentration of portal plasma CART immunoreactivity dropped in the vehicle infused rats. The current study provides further evidence that CART is a neuroendocrine-releasing factor with a possible impact on anterior pituitary function during states of haemodynamic stress.

Effects of circulating tumor necrosis factor on the neuronal activity and expression of the genes encoding the tumor necrosis factor receptors (p55 and p75) in the rat brain: a view from the blood-brain barrier

Tumor necrosis factor is a potent activator of myeloid cells, which acts via two cell-surface receptors, the p55 and p75 tumor necrosis factor receptors. The present study describes the cellular distribution of both receptor messenger RNAs across the rat brain under basal conditions and in response to systemic injection with the bacterial endotoxin lipopolysaccharide and recombinant rat tumor necrosis factor-alpha. Time-related induction of the messenger RNA encoding c-fos, cyclo-oxygenase-2 enzyme and the inhibitory factor kappa B alpha was assayed as an index of activated neurons and cells of the microvasculature by intravenous tumor necrosis factor-alpha challenge. The effect of the proinflammatory cytokine on the hypothalamic-pituitary-adrenal axis was determined by measuring the transcriptional activity of corticotropin-releasing factor and plasma corticosterone levels. Constitutive expression of p55 messenger RNA was detected in the circumventricular organs, choroid plexus, leptomeninges, the ependymal lining cells of the ventricular walls and along the blood vessels, whereas p75 transcript was barely detectable in the brain under basal conditions. Immunogenic insults caused up-regulation of both tumor necrosis factor receptors in barrier-associated structures, as well as over the blood vessels, an event that was associated with a robust activation of the microvasculature. Indeed, intravenous tumor necrosis factor-alpha provoked a rapid and transient transcription of inhibitory factor kappa B alpha and cyclo-oxygenase-2 within cells of the blood-brain barrier, and a dual-labeling technique provided the anatomical evidence that the endothelium of the brain capillaries expressed inhibitory factor kappa B alpha. Circulating tumor necrosis factor-alpha also rapidly stimulated c-fos expression in nuclei involved in the autonomic control, including the bed nucleus of the stria terminalis, the paraventricular nucleus of the hypothalamus, the central nucleus of the amygdala, the nucleus of the solitary tract and the ventrolateral medulla. A delayed c-fos mRNA induction was detected in the circumventricular organs, organum vascularis of the lamina terminalis, the subfornical organ, the median eminence and the area postrema. The paraventricular nucleus of the hypothalamus exhibited expression of corticotropin-releasing factor primary transcript that was associated with a sharp increase in the plasma corticosterone levels 1h after intravenous tumor necrosis factor-alpha administration. Taken together, these data provide the evidence that p55 is the most abundant tumor necrosis factor receptor in the central nervous system and is expressed in barrier-associated structures. Circulating tumor necrosis factor has the ability to directly activate the endothelium of the brain's large blood vessels and small capillaries, which may produce soluble molecules (such as prostaglandins) to vehicle the signal through parenchymal elements. The pattern of c-fos-inducible nuclei suggests complex neuronal circuits solicited by the cytokine to activate neuroendocrine corticotropin-releasing factor and the corticotroph axis, a key physiological response for the appropriate control of the systemic inflammatory response.

Hypothalamic accessory nuclei and their relation to the angiotensinergic and vasopressinergic systems

The existence and colocalization of angiotensin II- and vasopressin-like immunoreactivity in individual magnocellular cell groups of the hypothalamus has been demonstrated by using immunocytochemical methods. These neurosecretory magnocellular groups consist of the paraventricular nucleus and the supraoptic nucleus, as well as different accessory cell groups. The fibers from the neurons of the accessory nuclei project directly to adjacent blood vessels and do not comigrate with the hypothalamo-neurohypophysial fiber pathway. On the basis of these findings it can be concluded that in the hypothalamus two different angiotensinergic and vasopressinergic neurosecretory systems exist: (1) an intrinsic hypothalamic and (2) a hypothalamo-neurohypophysial system. The distribution of the accessory cell groups in the hypothalamus is shown in a 3D reconstruction which includes the connection of these magnocellular nuclei with the vascular system in this area.

The inhibitory effect of hCG on counter current transfer of GnRH and the presence of LH/hCG receptors in the perihypophyseal cavernous sinus--carotid rete vascular complex of ewes

The existence of the hormone passage from venous blood into arterial blood in the area of the perihypophyseal vascular complex has been demonstrated in some mammals, but its mechanism has not been defined. To study the regulatory mechanism we infused hCG into perihypophyseal cavernous sinus of ovariectomized, conscious ewes to test if the hCG would affect putative LH/hCG receptors and inhibit counter-current transfer of GnRH from the venous cavernous sinus to the arterial carotid rete. The latter study was done on an isolated head model. Ewes were ovariectomized in mid-anestrus and, after 4 to 5 wk were used in the experiments. On the day of experiment ewes were treated intramuscularly with estradiol benzoate or oil vehicle, and 18 to 20 h later were infused either with a multielectrolyte solution or hCG for 2 h via the venae angularis oculi. Immediately thereafter the ewes were anesthetized and exanguinated, and subsequently decapitated. The isolated head was perfused with Dextran in multielectrolyte. The 125I-GnRH was infused into the cavernous sinus via the venae angularis oculi for 5 min; contemporaneous samples were taken from the carotid rete and both jugular veins at 1-min intervals. Transfer of 125I-GnRH from the cavernous sinus to the carotid rete was inhibited by hCG in ewes pretreated with estradiol benzoate but not with oil (P<0.005). We collected tissue samples from the vascular complex of the cavernous sinus and carotid rete of cyclic ewes to determine the presence of LH/hCG receptors. In situ hybridization showed the presence of LH/hCG receptor mRNA transcripts in the walls of both arterial and venous compartments of the cavernous sinus-carotid rete complex, and immunohistochemistry revealed the presence of receptor proteins. These novel findings confirm previously obtained data suggesting that LH is a modulatory factor for the counter-current transfer of neuropeptides from the venous blood of the cavernous sinus to the arterial blood supplying the brain and hypophysis. The LH could modulate 125I-GnRH transfer acting directly on the vascular smooth muscle.

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.

Adeno-pituitary hormones in human hypothalamic hypophysial blood

An in vivo technique for collecting blood from the pituitary stalk using transphenoidal microsurgery has recently been developed in men with nonfunctioning pituitary disease. To determine the origin of this blood and the direction of the stream, we measured contemporaneously the levels of LH, FSH, PRL, GH, TSH, and ACTH in hypothalamic-hypophysial blood (HHB) and peripheral blood (PB). Eleven patients with nonfunctioning pituitary adenomas entered the study. The surgical procedure used for collecting HHB consisted of periodically aspirating small amounts of blood using a microsuction apparatus, just after tumor removal, kept in the postero-superior corner of the sella turcica at the junction of the diaphragm with the dursum sellae. The data show clearly the existence of a dramatic concentration gap in HHB vs. PB in all adeno-pituitary hormones (P = 0.003). The HHB/PB ratio varied from 50-600 in the different hormones. The secretion of adeno-pituitary hormones in blood drawn at the pituitary stalk level in man was reported for the first time. The dramatic HHB/PB ratio of the hormone levels has been emphasized. The most likely explanation for the markedly elevated hormone concentration gradient between central and peripheral blood was sampling of peri- and/or suprapituitary blood. To consider the origin and direction of the HHB stream, two hypotheses have been further advanced: 1) a retrograde bloodflow from the pituitary, and 2) a central-hypothalamic secretion.

Corticotrophin-releasing factor and arginine vasopressin in the hypothalamo-hypophyseal portal blood of rats following high-dose glucocorticoid treatment and withdrawal

Hypothalamo-hypophyseal portal blood was obtained from rats treated with chronic, high-dose prednisolone in the drinking water and after subsequent withdrawal of the steroid for up to 7 days. Corticotrophin-releasing factor (CRF) immunoreactivity in portal blood was reduced by treatment with prednisolone but not completely abolished. There was a rapid recovery of CRF to control values within 3 days of withdrawal of prednisolone. There was no significant change in arginine vasopressin (AVP) in portal blood over the time-course of the experiment.

Antibodies in passive immunization studies: characteristics and consequences

Antibodies to neuropeptides or hormones are frequently used in passive immunization studies to unravel their physiological role in signal transfer. In such in vivo experiments antibodies are considered to bind and thereby to biologically inactivate the endogenous substance during its journey from its site of secretion to its site of action (signalling time). However, little is known about the mechanism of action and characteristics of antibodies that determine such biological activity. Since the signalling time in neuronal and hormonal communication is short, the kinetics of antibody binding is an important feature. Here, we present a theoretical framework to describe antibody binding kinetics which can contribute to the design of passive immunization protocols. The specific effects of variation in antibody concentration, dissociation constant, and on-rate constant on these binding kinetics are demonstrated. Simple methods are described to determine these parameters, which may guide the selection of antibodies for passive immunization studies. When time is limited, the on-rate constant and the local antibody concentration are the most important determinants. Several points are illustrated for CRF signal transfer in the rat. CRF signalling time in the hypothalamo-pituitary complex, as established from dye transport experiments, was 3-7 sec. Based on parameters measured for a rat monoclonal antibody to CRF (PFU 83), we computed that half-maximal and full blockades of ether-induced ACTH secretion were associated with approximately 85% and more than 99% binding of CRF, respectively. From the theoretical framework presented in this study we conclude that, in general, the kinetics of antigen binding are sufficiently fast for antibodies to interfere with hormonal and probably nonsynaptic neuronal signal transfer. However, interference with fast signalling processes (less than 10 msec), which may occur in the brain, is unlikely.

[Changes of the microcirculating blood volume in the reproductive axis during estrous cycle of the rats]

The microcirculating blood volume of the hypothalamic-pituitary-ovarian axis in the female rats was measured using radiobiomicrosphere method during the estrous cycle. The blood flow volume of the hypothalamus and the pituitary was not significantly different between each phase of the estrous cycle. The ovarian blood flow volume was the highest in metestrus and the lowest in estrus. The difference between them was significant (P less than 0.02). The uterine blood flow volume was the highest in metestrus and remained at higher level in diestrus and then changed to the lowest in estrus. The values in metestrus and diestrus were significantly higher than that in estrus (P less than 0.01, P less than 0.05, respectively). The blood flow volume of the oviduct in proestrus was significantly lower than that in estrus (P less than 0.05). The above results suggest that the cyclic changes of the blood volume of the ovary, uterus and oviduct are closely related to their functional state and the hormonal changes.

High concentrations of catecholamines in human hypothalamic-hypophysial blood

While the hypothalamic-hypophysial portal system has been extensively studied in laboratory animals, equivalent studies have not been performed in humans. Here, we present an experimental procedure for collecting suprapituitary blood in man. To solve the question on the origin of such blood we investigated specific markers of hypothalamic secretory activity: the catecholamines (CAs). We found (a) norepinephrine (NE), dopamine (DA), and epinephrine (E) concentrations from approximately 1.5 to 2.5, 3.5 to 4.5, and 6- to 10-fold higher, respectively, in suprapituitary than peripheral blood, (b) different NE/DA and NE/E ratios in favor of DA and E in suprapituitary blood, and (c), a complete (100%) group separation (suprapituitary vs. peripheral) when discriminant analysis included only DA and E. These data indicate that suprapituitary blood composition is different from that of the peripheral blood, and is particularly rich in CAs and claimed differences between DA and E release on one hand and NE release on the other in suprapituitary blood also are observed. We advance the hypothesis of a hypothalamic source of such amines draining via median eminence into portal vasculature, and name this blood "hypothalamic-hypophysial blood." Besides serving as "classical" neurotransmitters, CAs may also have a direct neurohormonal role in the regulation of the human hypothalamic-hypophysial function.

Hypothalamo-pituitary portal development in the ovine fetus

The ontogeny of the hypothalamo-pituitary portal system was studied in six ovine fetuses, 48-67 days of gestation (term = 147 days) after infusion of the vascular system with a silicone rubber compound. In all of these fetuses, a marked orange blush of silicone rubber could be seen, under the dissecting microscope, extending down the stalk from the hypothalamus to the pituitary. On microscopic examination, extensive and well-developed capillary connections between the median eminence, the pituitary stalk, and pituitary gland were seen in all fetuses. In an additional fetus, aged 45 days, which was not perfused with silicone rubber, portal capillary loops had penetrated the median eminence. These findings suggest that, in the ovine fetus, hypothalamic releasing factors can be transported directly via a portal vascular route to the pituitary gland as early as 45 days of gestation.

Hypothalamic-hypophyseal vascular connections in the fetal sheep

We have studied the development of the hypothalamic-pituitary vascular connections in the fetal sheep using ink-filling techniques. The presence of ink-filled vessels in components of the hypothalamic-pituitary axis is interpreted as indicating patency of the system. Ink-filled vessels were identified in the median eminence, the pituitary stalk and the pituitary gland as early as 45 days of gestation and at all subsequent ages investigated. These results suggest that the pituitary could potentially respond to hypothalamic releasing factors from as early as 45 days of gestation in the fetal sheep.

Regional neurohypophysial and hypothalamic blood flow in rats during hypercapnia

Regional cerebral blood flow (rCBF) was measured in the neurohypophysis and hypothalamus in normocapnic and hypercapnic rats using [14C]isopropyliodoamphetamine. Rats were surgically prepared using nitrous oxide and halothane and placed in plaster restraining casts. Hypercapnia was produced by increasing the fractional concentration of inspired CO2 (FICO2). rCBF in normocapnic rats was higher in the paraventricular nucleus, supraoptic nucleus, median eminence, and neural lobe than rates previously measured by use of diffusible tracers. During hypercapnia blood flow increased linearly with arterial PCO2 (PACO2) in all regions except the median eminence and neural lobe, which were not affected by hypercapnia. When rats were pretreated with phentolamine (1 mg/kg) to block the alpha-adrenergic receptors, blood flow in the median eminence and neural lobe increased significantly during hypercapnia. We conclude that blood flow in the cell bodies of the paraventricular nucleus and supraoptic nucleus is regulated differently during hypercapnia than blood flow in the nerve terminals in the median eminence and neural lobe. Furthermore, vasodilation produced by increased CO2 is offset by alpha-receptor stimulation in the median eminence and neural lobe.

Neurotransmitter modulation of corticotropin releasing factor secretion into the hypophysial-portal circulation

Secretion of corticotropin releasing factor (CRF) from the hypothalamic paraventricular nuclei into the hypophysial-portal circulation is modulated by a variety of neuronal afferents. Effects of intracerebroventricular acetylcholine (ACH), gamma-aminobutyric acid (GABA) and epinephrine (EPI) on immunoreactive (ir) CRF concentration in portal plasma were directly evaluated in urethanized male rats. ACH (0.1-10 nmole) administration was associated with a dose-dependent elevation of portal irCRF which was attenuated by pretreatment with either muscarinic or nicotinic receptor antagonists. GABA (0.1-10 nmole) also caused inhibition of irCRF concentration in the portal plasma which was prevented by bicuculline pretreatment. Finally, EPI (0.1-10 nmole) facilitated irCRF secretion via alpha 1- and beta-adrenergic mechanisms. These observations provide the first direct information on possible neurotransmitter actions on hypothalamic CRF secretion.

[Anatomical basis for vascular-pedicle transplantation of the hypothalamo-hypophyseal complex]

In 240 corpses of mature persons by means of complex morphological methods extraorganic arteries of the hypophysis, anterior hypothalamus have been investigated, as well as topography of the arterial peduncle of the hypothalamo-hypophyseal complex. Blood supply of the hypophysis and of the anterior hypothalamic part is performed from one common vascular pool--the arterial ring of the brain via multiple arterial branches, their amount and diameter varying considerably. Taking into account an important role of anastomotic vessels and numerous arterial plexuses, when the hypothalamo-hypophyseal complex is extirpated for investigation, it is necessary to include into its composition, besides the major arterial peduncle, the optic chiasm, the grey tuber and the system of cavernous sinuses. Owing to this, there is no need to separate the arterial peduncle of the transplant at the moment of obtaining the hypothalamo-hypophyseal complex. The topographic anatomical investigations performed make prerequisites for transplantation of the hypothalamo-hypophyseal complex on the vascular peduncle in clinic.

Hypothalamo-hypophysial blood flow: a novel control mechanism in pituitary function?

The flow rate of portal plasma was measured during hypothalamo-hypophysial portal sampling in the rat using a modified Worthington-Fink technique. In rats rendered hypothyroid with propylthiouracil, there was a significant 140% increase in portal plasma flow. Median eminence stimulation also increased portal plasma flow by 41%. Orchidectomy and adrenalectomy had no effects on plasma flow. Modification of flow rate in the hypothalamo-hypophysial vascular bed may represent a further mechanism involved in the control of pituitary function.

GnRH-associated peptide (GAP) is cosecreted with GnRH into the hypophyseal portal blood of ovariectomized sheep

The secretion of gonadotropin-releasing hormone (GnRH) and GnRH-associated peptide (GAP) into sheep hypothalamo-hypophyseal portal blood was investigated in ovariectomized ewes. GAP and GnRH were cosecreted into portal blood as determined in pooled 'peak' and 'trough' samples. The temporal pattern of GAP secretion into portal blood was coincidental with that of luteinizing hormone (LH) secretion into peripheral blood in three individual animals. The data provide the first evidence that GAP is a secretory product from mammalian hypothalamus and establish the temporal coexistence of the two peptides which appears to be of physiological significance in the regulation of pituitary function.

Vascularization of the hypothalamo-hypophysial complex in Japanese elasmobranchs: a scanning electron microscope study of blood vascular casts

To study in detail vascularization in the hypothalamo-hypophysial complex in some Japanese elasmobranchs, injection casts of blood vessels were observed by scanning electron microscopy. Materials used were a gummy shark (Mustelus manazo), a cloudy dogfish (Scyliorhinus torazame) and a skate (Raja porosa). The vascular supply to the hypothalamo-hypophysial region of the elasmobranchs is carried out by the inferior hypothalamic arteries and their branches that originate from the internal carotid artery. In the gummy shark, a pair of inferior hypothalamic arteries send several branches to the median eminence running along the anterolateral sides of the distal adenohypophysis. These branches form the capillary plexus, displaying anastomosis and convolution at the ventral surface of the median eminence. The plexus assumes capillary glomeruli at the lateral region of the posterior median eminence. Numerous branches derived from the plexus are directed backward through the pars distalis, join with the capillary net work (which develop in the neuro-intermediate lobe) and are finally gathered together to form a thick hypophysial vein. The ventral lobe receives several arterial branches from the internal carotid arteries and carotid artery to form a unique vascular bed. Therefore, it is assumed that in elasmobranchs the hypothalamic control of the ventral lobe is weaker than that of adenohypophysial components.

Morphometry of individual capillary beds in the hypothalamo-neurohypophysial system of rats

Unusually high capillary densities, volumes, and surface areas were revealed by morphometric analysis of the hypothalamic paraventricular nuclei (magnocellular and parvocellular divisions), supraoptic nuclei, and pituitary neural lobe of Long-Evans and Brattleboro rats. Capillaries of the magnocellular division of the paraventricular nucleus were greater in number, but smaller in diameter, than those in the parvocellular division. Expressed per unit area or volume, capillary dimensions in the hypothalamo-neurohypophysial system of Brattleboro rats were the same as in Long-Evans rats.

Effect of hypothalamic deafferentation on hypophysial and other endocrine gland blood flows

The effects of partial or complete deafferentation of the medical basal hypothalamus (MBH) on blood flow through endocrine glands was evaluated using a modification of Sapirstein's indicator fractionation method. Adult female Wistar rats were distributed into three groups consisting of normal control animals (n = 12), animals in which rostral deafferentation (Rd) of the MBH was performed (n = 11), and animals in which complete deafferentation (Cd) of the MBH was done (n = 6). Three weeks after the surgical procedures, Cd reduced adenohypophysial weight by 30% and raised its blood flow levels by 80% compared to those values in the Rd and control groups. In contrast to the anterior pituitary, both Rd and Cd led to weight reduction of 22% in posterior lobes, and Cd was also accompanied by a 50% net blood flow decrease. No statistically significant differences were noted in pineal gland weights or decreases in blood perfusion rates in adrenal and thyroid glands. There were no body weight changes in experimental animals compared to values in the control groups. These results suggest that there may be a loss of vasomotor tone in the primary vascular beds of the adenohypophysial portal systems after deafferentation which elicits the 80% increase in adenohypophysial blood flow. This increased circulation may play a role in pituitary function(s) and should be included along with diminished neuroendocrine inputs in interpretations of results obtained after deafferentation procedures.

Ependymal lining of infundibular recess in perinatal rats: relationships with portal capillaries and permeability

Structure and permeability of the ependymal lining the infundibular recess were studied in perinatal rats with silver impregnation, electron microscopy, radioautography, and tracer techniques. According to our data basal processes of ependymal cells reach the primary portal plexus linking the 3rd ventricle and the hypophysial portal system all through the perinatal period. After birth, some of the processes penetrate into the perivascular space of the primary portal plexus and abut there on the endothelium of capillaries. Ependymal cells of fetuses and neonates are joined by specialized junctions (tight junctions, gap junctions and zonulae adhaerentes). Intraventricularly injected ionic lanthanum crosses the ependymal lining of fetuses both trans- and extra-cellularly everywhere in the infundibular recess. By postnatal day 9 only the rostral portion of the recess remains readily permeable. Caudally, extracellular leakage becomes highly restricted, apparently due to the appearance of circumferential tight junctions. Finally, [3H]dopamine seems to penetrate through the ependymal lining in the same way as ionic lanthanum entering the portal capillaries. These findings suggest that the adenohypophysiotropic neurohormones can penetrate from the cerebrospinal fluid into the portal circulation from the very beginning of the establishment of the hypothalamo-hypophysial functional relationships during ontogenesis.

LH-RH in the rat and mouse hypothalamus and rat hypophysial portal blood: confirmation of identity by high performance liquid chromatography

The nature of the immunoreactive (IR) form of luteinizing hormone-releasing hormone (LH-RH) in rat hypophysial portal vessel blood and in extracts of hypothalamus from rats and normal and hypogonadal (hpg) mice was investigated using high performance liquid chromatography and two highly specific anti-LH-RH sera. In rat hypophysial portal blood and in hypothalamic extracts from rats and normal mice a single immunoreactive peak was present which corresponded in retention time to synthetic LH-RH. No LH-RH-IR was detected in hypothalamic extracts from the hpg strain of mouse.

Hypothalamic epinephrine is released into hypophysial portal blood during stress

The hypothalamic secretion of epinephrine (EPI), norepinephrine (NE), and dopamine (DA) into hypophysial portal blood was studied in adrenalectomized rats subjected to heat stress. Portal plasma EPI levels were increased 3-fold by heat stress whereas portal plasma concentrations of NE and DA were not altered by stress. These data suggest that EPI of hypothalamic origin may be involved directly in the modulation of anterior pituitary secretion during stress.

Somatostatin-28(1-12)-like immunoreactive substance is secreted into hypophysial portal vessel blood in the rat

Somatostatin-28(1-12)-like immunoreactivity was measured in extracts of rat hypophysial portal blood and peripheral blood. The concentration of somatostatin-28(1-12) was higher in portal than in peripheral extracts, and its release into portal vessel blood was increased 4- to 5-fold by electrical stimulation of the median eminence. These results show that somatostatin-28(1-12) may be a physiological neurohormone and/or neurotransmitter.

Dopamine in plasma of lateral and medial hypophysial portal vessels: evidence for regional variation in the release of hypothalamic dopamine into hypophysial portal blood

The plasma concentrations of dopamine in blood from hypophysial portal vessels in various locations on the pituitary stalk were evaluated in diestrous rats. It was found that the mean concentration of dopamine in blood from lateral hypophysial portal vessels, which contain the venous effluent of the lateral median eminence, was significantly less (P less than 0.005) than that in blood from medial portal vessels, which contain the venous effluent of the medial median eminence [1.59 +/- (SE) 0.23 ng/ml vs. 3.12 +/- 0.48 ng/ml]. The mean plasma concentration of dopamine in blood of lateral portal vessels and of medial portal vessels was at least 20-40 times greater than that in arterial blood of these animals. It was calculated that the rate of release of hypothalamic dopamine was 174 +/- 38 pg/h into a medial portal vessel and 73 +/- 15 pg/h into a lateral portal vessel. The mean plasma concentration of norepinephrine or epinephrine in blood from a medial portal vessel was not different from that from a lateral portal vessel. To address the issue of whether the rate of release of dopamine into a medial portal vessel and into a lateral portal vessel was correlated with the rate of synthesis of dopamine in discrete regions of the median eminence, the concentration of L-dihydroxyphenylalanine (DOPA), the precursor of dopamine, was evaluated in lateral and medial segments of the median eminence of diestrous rats treated with 3-hydroxybenzylhydrazine, an inhibitor of DOPA decarboxylase activity. The concentration of DOPA was similar in the medial and lateral segments of the median eminence, suggesting that the rate of synthesis of dopamine did not account for the difference in the rate of release of dopamine into portal blood. The finding of different concentrations of dopamine in blood from various hypophysial portal vessels may be important in view of the heterogenous perfusion of the pars distalis with hypophysial portal blood. We suggest that topographic differences may exist in the release of PRL by cells of the pituitary gland as a consequence of uneven concentrations of dopamine in portal blood perfusing the lactotropes.

The development of the hypothalamo-hypophysial portal system in human fetus

In the earliest stages of development of the nervous system, the hypophysial stalk has a horizontal disposition (having, above the hypothalamus and below the anterior lobe of the hypophysis, a group of cells placed in the sella turcica). The stalk becomes vertical in the VI--VII month of fetal life. Until the stage corresponding to the length of 92 mm vertex--coccis of the fetus, few blood vessels are to be found in this region; they follow the stalk's direction. These vessels originate from a vascular network which lies in the post-tuberal region being also considered the origin of a "special" kind of vessel observed very early in the hypophysis development. The veins can be found later at the 140 mm vertex-coccis fetus. Their structure is much like the structure of the portal vessels in the adult. At their proximal end their aspect can be referred to as an anastomotic arterio-venous system. After 7 months of development the vessels resemble their adult form, being however less numerous.

beta-Endorphin is present in high concentration in the hypophysial portal vessels of rats

beta-Endorphin-like immunoreactivity (beta-ELI) was measured in hypophysial portal and arterial blood of intact, adrenalectomized and hypophysectomized rats. beta-ELI levels were 61 times higher in the long portal vessels than in the general circulation. Circulating, and especially portal, levels of beta-ELI were significantly increased after adrenalectomy. After removal of the pituitary gland, the mean level of beta-ELI in portal blood was significantly lower than in intact rats. beta-ELI in portal blood displayed the same chromatographic properties as synthetic beta-endorphin.

Observations on the hypothalamo-hypophyseal portal vasculature in the developing human fetus

Human fetuses, 11.5 to 16.8 weeks' gestational age, were perfused with silicone rubber compound to study the hypothalamo-hypophyseal portal system. Impregnated vessels were observed throughout the median eminence, infundibular stalk, pars tuberalis and both lobes of the pituitary gland in all specimens. The data suggest that an intact portal system for the transport of hypothalamic releasing factors to the pituitary gland is established as early as 11.5 weeks' gestation.

[Interrelationship between tanycytes and capillaries of the primary plexus of the portal system in rats during the perinatal period]

The establishment of tanycyte connections with the primary plexus of capillaries during the perinatal period in rats was examined by light and electron microscopy. At this very time, basal processes of matrix cells and, later of differentiated tanycytes reach the primary plexus. Morphological features of the transport process between the basal process terminals and perivascular space have been noticed. The contact zone of basal processes with the external basal membrane of the primary plexus increased gradually. This may be due to the formation of preterminal ramification of the basal processes and to the flattening of some terminals. In addition, starting from the 20th day of the prenatal life, synaptoid contacts of monoaminnergic axons with tanycyte basal processes are noticed to appear.

Neural control of reproduction

The feedback loops involved in the neural control of reproductive cyclicity in primates are reviewed. A combination of morphological, endocrinological, and surgical experimental approaches was used to identify the different mechanisms involved. The main signal for pituitary secretion of gonadotropins is the estrogen secreted by the ovary. Estrogen probably affects the hypothalamic secretion of gonadotropin-releasing hormone (GnRH), which in turn stimulates the pituitary production of gonadotropins. These may in turn, through a short feedback loop, regulate the secretion of GnRH. Evidence for a direct effect of estrogen on the pituitary is presented. Future possible areas of research, including extrahypothalamic influences on reproductive mechanisms, are briefly outlined.

Pituitary-brain vascular relations: a new paradigm

Vascular casts of the pituitary gland have demonstrated a paucity of veins extending from the adenohypophysis to the systemic circulation and have suggested that some adenohypophyseal venous blood returns to the neurohypophysis. The neurohypophyseal capillary bed may function as a vascular switch and in this article a series of 14 questions are proposed regarding the vascular dynamics of the pituitary. Together these questions raise the larger question, namely, whether pituitary hormones are transported directly to the brain to modify brain function?

Regional and sexual differences in the size of the neuro-vascular contact surface of the rat median eminence and pituitary stalk

The size of the neuro-vascular contact surface was estimated along the median eminence (ME) proximal pituitary stalk (PS) of adult rats using serially cut 1 micrometer thick plastic sections. It was found 26% larger in female animals. The dorsal surface of the pars tuberalis was used as reference surface, and its size was estimated in the same way. The average rate of increase of the contact surface with respect to the reference surface was 3.05 and 2.98 in females and 2.34 and 2.35 in males. The reference surface was divided into small components and also the regional distribution of the rate of increase of the contact surface was analyzed. Very folded contact surface areas were found scattered in the rostral half of the ME. Folded areas made up the ME on both sides of the midline and the dorsal and ventral surfaces of the PS, whereas relatively smooth areas are localized along the 2 margins of the ME and the 2 sides of the PS. The results were correlated with the known termination sites of dopamine and various hypothalamic hormone containing pathways.

[Changes in the hypothalamo-hyophyseal neurosceretory system of mammals following pituitary stalk transection and hypophysectomy]

Hypophysectomy and pituitary stalk section result in dramatic morpho-functional changes in all parts of mammalian hypothalamo-hypophyseal neurosecretory system. Reorganization of the hypophyseal stalk consists of several interconnected but differing in time processes. Simultaneously with the developing traumatic changes (degeneration of the sectioned neurosecretory fibers, secretory disorders) proliferation of pituicytes with characteristic phagocytic activity is observed. A little bit later, intensive mitotic division of endothelial cells and capillary formation piercing the stalk periphery begins. At the same time, a new way for blood outflow from the capillaries of the primary portal plexus into the synuses of the brain pias is restored. Degenerated neurosecretory fibers are gradually substituted by regenerating fibers forming a dense network in heavily vascularizated stalk parts. As differentiation of endothelial cells and regeneration of neurosecretory fibers procede, axovasal contacts are gradually forming. At that time the hypophyseal stalk begins functioning as a neurohumoral organ but morpho-functionally less perfect than the posterior hypophyseal lobule. In the median eminence of the operated animals, unlike the intact ones, neurosecrete is accumulating around the capillaries of the portal plexus. Mechanical damage of neurosecretory fibers during the operation results in degeneration of a greater number of neurosecretory cells in the supraoptical and paraventricular nuclei. Preserved cells have an increased functional activity because of neurohormonal deficiency in the organism. As a result of the structural changes mentioned, diabetes mellitus develops, subsiding gradually with time course.

The neurohypophyseal capillary bed. II. Specializations within median eminence

Vascular casts of the pituitary gland, median eminence and hypothalamus from several mammalian species were examined by scanning electron microscopy. These observations were correlated with light microscopic studies of injected, cleared median eminence-pituitary specimens and with light microscopic examination of serial sections of injected hypothalamic, median eminence, and pituitary specimens employing reflected lighting or epi-illumination. Transmission electron microscopy was employed to study long portal vessels on the ventral surface of the rat median eminence. In each of the species examined, the median eminence (infundibular) capillary bed is subdivided into an external and an internal plexus. The external plexus (the neurohemal contact zone) receives an arterial supply from the superior hypophyseal arteries and is continuous with the capillary bed of the infundibular stem and process. Egress from the external plexus is possible via three vascular routes: (a) by fenestrated portal vessels and capillaries to the adenohypophysis, (b) by capillary connections to the medial basilar hypothalamus and, (c) by internal plexus capillaries to the ependyma of the median eminence. Median eminence vasculature is structurally organized to deliver (1) hypothalamic and neurohypophyseal peptides to the glandular pituitary via portal vessels, (2) hypothalamic and pituitary secretions to the medial basilar hypothalamus via capillaries, and (3) hypothalamic and pituitary secretions to distant brain sites through cerebrospinal fluid via ventricular and subarachnoid routes.

Pituitary secretes to brain. Experiments in sheep

Because vascular casts of the pituitary demonstrated that there are a few venous connections from the adenohypophysis to the juxtaposed cavernous sinus, it was predicted that some portal vessels must carry blood from the adenohypophysis back to the neurohypophysis. Physiological studies confirmed this prediction and verified earlier observations that blood-flow within the neurohypophysis can be towards the median eminence. In the present study, increased concentrations of prolactin and growth hormone were found in blood sampled from intracranial vessels (internal carotid artery and sagittal sinus). It was concluded that the neurohypophyseal capillary bed not only receives trophic hormones produced in the adenohypophysis but, under certain physiological circumstances, delivers those hormones directly to the brain.