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.