The Pituitary Portal System

Anatomical organization of the melanin-concentrating hormone peptide family in the mammalian brain

More than 20 years ago, melanin-concentrating hormone (MCH) and its peptide family members - neuropeptide EI (NEI) and neuropeptide GE (NGE) - were described in various species, including mammals (rodents, humans, and non-human primates). Since then, most studies have focused on the role of MCH as an orexigenic peptide, as well as on its participation in learning, spatial memory, neuroendocrine control, and sleep. It has been shown that MCH mRNA or the neuropeptide MCH are present in neurons of the prosencephalon, hypothalamus and brainstem. However, most of the neurons containing MCH/NEI are within the incerto-hypothalamic and lateral hypothalamic areas. In addition, the terminals of those neurons are distributed widely throughout the central nervous system. In this review, we will discuss the relationship between those territories and the roles played by MCH/NEI, as well as the importance of MCH receptor 1 in the respective terminal fields. Certain neurochemical features of MCH- and NEI-immunoreactive (MCH-ir and NEI-ir) neurons will also be discussed. The overarching theme is the anatomical organization of an inhibitory neuropeptide colocalized with an inhibitory neurotransmitter in integrative territories of the central nervous system, such as the IHy and LHA. Although these territories have connections to few brain regions, the regions to which they are connected are relevant, being responsible for the organization of motivated behaviors. All available information on this peptidergic system (anatomical, neurochemical, hodological, physiological, pharmacological and behavioral data) suggests that MCH is intimately involved in arousal and the initiation of motivated behaviors.

Co-localization of neuroactive substances in the endocrine hypothalamus

In addition to their characterizing secretory products, both magnocellular and parvocellular neurosecretory neurons are now known to express other neuroactive substances. Parvocellular neurons that make corticotropin-releasing factor (CRF) for example are capable of synthesizing at least seven neuropeptides. Some of these, like arginine vasopressin (AVP), interact with CRF at the level of the anterior pituitary to promote corticotropin secretion, and, like CRF, are regulated negatively by glucocorticoids and positively by at least some stressors. others are inert in these two contexts but are responsive to various challenges. Magnocellular neurosecretory oxytocin- and AVP-containing neurons are capable of producing similarly broad and distinctive complements of neuroactive principles. These are typically expressed at levels far lower than those of the nonapeptides, suggesting local modulatory effects on oxytocin and/or AVP secretion at the level of the posterior lobe. Differential regulation of coexisting molecules within magnocellular neurons by systemic challenges and steroid hormones has also been described. Secretory products of magnocellular neurons may gain access to the anterior pituitary via exocytotic release at the level of the median eminence or through vascular links between the posterior and anterior lobes, suggesting another form of 'co-localization' by which the two neurosecretory cell types may interact in the control of stress and perhaps other pituitary-mediated responses.

Immunocytochemical distribution of catecholamine-synthesizing neurons in the hypothalamus and pituitary gland of pigs: tyrosine hydroxylase and dopamine-beta-hydroxylase

This study describes the distribution of catecholaminergic neurons in the hypothalamus and the pituitary gland of the domestic pig, Sus scrofa, an animal that is widely used as an experimental model of human physiology in addition to its worldwide agricultural importance. Hypothalamic catecholamine neurons were identified by immunocytochemical staining for the presence of the catecholamine synthesizing enzymes, tyrosine hydroxylase and dopamine-beta-hydroxylase. Tyrosine hydroxylase-immunoreactive perikarya were observed in the periventricular region throughout the extent of the third ventricle, the anterior and retrochiasmatic divisions of the supraoptic nucleus, the suprachiasmatic nucleus, the ventral and dorsolateral regions of the paraventricular nucleus and adjacent dorsal hypothalamus, the ventrolateral arcuate nucleus, and the posterior hypothalamus. Perikarya ranged from parvicellular (10-15 microns) to magnocellular (25-50 microns) and were of multiple shapes (rounded, fusiform, triangular, or multipolar) and generally had two to five processes with branched arborization. No dopamine-beta-hydroxylase immunoreactive perikarya were observed within the hypothalamus or in the adjacent basal forebrain structures. Both tyrosine hydroxylase- and dopamine-beta-hydroxylase-immunoreactive fibers and punctate varicosities were observed throughout areas containing tyrosine hydroxylase perikarya, but dopamine-beta-hydroxylase immunoreactivity was very sparse within the median eminence. Within the pituitary gland, only tyrosine hydroxylase fibers, and not dopamine-beta-hydroxylase immunoreactive fibers, were located throughout the neurohypophyseal tract and within the posterior pituitary in both pars intermedia and pars nervosa regions. Generally, the location and patterns of both catecholamine-synthesizing enzymes were similar to those reported for other mammalian species except for the absence of the A15 dorsal group and the very sparse dopamine-beta-hydroxylase immunoreactive fibers and varicosities in the median eminence in the pig. These findings provide an initial framework for elucidating behavioral and neuroendocrine species differences with regard to catecholamine neurotransmitters.

Local tetrodotoxin blocks chronic stress effects on corticotropin-releasing factor and vasopressin messenger ribonucleic acids in hypophysiotropic neurons

To test the hypothesis that synaptic inputs to the paraventricular nucleus mediate stress-induced increases in corticotropin-releasing peptide expression in the paraventricular nucleus of the hypothalamus (PVH), relative levels of the mRNAs encoding corticotropin-releasing factor (CRF) and arginine vasopressin (AVP) were followed, in situ, in animals subjected to chronic footshock stress and concurrent local administration of tetrodotoxin or vehicle. Consistent with previous findings, a 7-day exposure to chronic footshock resulted in a 2.1-fold increase in CRF mRNA levels in the parvocellular division of the PVH. The footshock paradigm also resulted in at least a 41% increase in AVP transcripts in this same region; this effect was localized predominantly to CRF-immunoreactive neurons. The stressor did not significantly alter AVP mRNA levels in the magnocellular division of the PVH. Tetrodotoxin, administered to the PVH via osmotic minipump, blocked the stress-induced rise in CRF and AVP mRNAs, but had no significant effect on basal levels of these transcripts. The results support the view that maintenance of the enhanced central drive on pituitary-adrenal activity seen in response to chronic stress is mediated via neural inputs to the PVH.

Computed three-dimensional reconstruction of median-eminence capillary modules: image alignment and correlation

Image alignment is an absolute requirement for three-dimensional (3-D) reconstruction from serial sections, and Fourier correlation is the most powerful way to compute alignments. The rotational and translational components of misalignment can be corrected by an iterative correlation procedure, but for images having significant differences, alignment can fail with a likelihood proportional to the extent of the differences. We found that translational correction was determined much more reliably when low-pass filters were applied to the product transforms from which the correlations were calculated. Rotational corrections based on polar analyses of the auto-correlations of the images instead of on the images directly contributed to more accurate alignments. These methods were used to generate 3-D reconstructions of brain capillary modules from serial-section mosaics of digitized transmission electron micrographs.

Vasoactive intestinal peptide (VIP) in magnocellular neurons of the hypothalamo-neurohypophysial system of the mink (Mustela vision) is co-localized with vasopressin or oxytocin

The distribution of vasoactive intestinal peptide (VIP) was analysed in perikarya of the mink hypothalamus with immunohistochemistry and, surprisingly, a large population of magnocellular VIP-immunoreactive neurons was present in the paraventricular and supraoptic nuclei as well as in accessory hypothalamic nuclei. From perikarya in the paraventricular as well as supraoptic nuclei, a large number of VIP immunoreactive nerve fibers was observed to enter the hypothalamo-neurohypophysial tract. Within the median eminence, a high density of VIP-immunoreactive nerve fibers was present in the external and internal zones. Fibers in the external zone of the median eminence were endowed with varicosities and perivascular terminals, while fibers in the internal zone were smooth and without terminal specializations. From the internal zone of the median eminence, fibers coursed via the infundibular stalk to terminate in perivascularly situated terminals in the neurohypophysis. In addition, a substantial number of small VIP-immunoreactive perikarya was observed within the suprachiasmatic nucleus. These perikarya were immunoreactive to neither vasopressin nor neurophysin. To elucidate the co-existence of VIP-immunoreactivity with vasopressin, oxytocin or neurophysin, a sequential double immunoperoxidase procedure to localize antigens with diaminobenzidine and benzidine dihydrochloride as chromagens was performed. From these experiments it was evident that VIP in nearly all magnocellular hypothalamo-neurohypophysial neurons co-existed with neurophysin. Based on a semi-quantitative estimate, half the VIP-immunoreactive magnocellular perikarya co-stored vasopressin, while another half co-stored oxytoxin. The present study describes the presence of a large population of VIP-containing neurons in the hypothalamo-neurohypophysial system of the mink. These findings raise evidence that within the mink, VIP may be involved in neurohypophysial physiology.

The melanin-concentrating hormone system of the rat brain: an immuno- and hybridization histochemical characterization

In addition to a nonadecapeptide homologous to the teleost melanin-concentrating hormone (MCH), the amino acid sequence predicted from a rat prepro-MCH (ppMCH) cDNA suggested that at least one (neuropeptide EI, or NEI), and possibly a second (NGE), additional neuropeptide may be encoded by this precursor. Cross-reactivity with epitopes of NEI or NGE can account for reported localization of alpha-MSH, rat CRF, and human GRF in rat dorsolateral hypothalamic neurons. We have used antisera raised against rat MCH and NEI in immunohistochemical studies at the light and electron microscopic levels, along with hybridization histochemical localization of ppMCH mRNA, to define the organization of this system. As expected, ppMCH mRNA is prominently expressed in cells in the lateral hypothalamic area and zona incerta. The MCH and NEI peptides were extensively colocalized in neurons in both of these areas. In addition, smaller cell groups in the olfactory tubercle and pontine tegmentum were also positively hybridized for ppMCH mRNA and immunostained for MCH and NEI. Fibers stained for MCH and NEI were similarly, and very broadly, distributed throughout the central nervous system in patterns that generally conformed with known projection fields of the lateral hypothalamic area and zona incerta. A differential distribution was seen in at least one region, the interanterodorsal nucleus of the thalamus, which contained a prominent terminal field stained for MCH but not NEI. At the electron microscopic level, MCH-stained perikarya displayed a prominent staining associated with the Golgi apparatus; this was not encountered in NEI-stained cells. Both peptides were distributed similarly in terminals in the lateral hypothalamic area and median eminence, with staining associated principally with dense-cored vesicles. The results suggest that ppMCH-derived peptides may serve as neurotransmitters or modulators of prominence in a surprisingly expansive projection field of incerto-hypothalamic neurons. The terminal distributions of this system seem most compatible with functional roles in generalized arousal and sensorimotor integration, processes previously implicated as being subject to modulation by the lateral hypothalamic area.

Computed alignment of dissimilar images for three-dimensional reconstructions

Three-dimensional reconstructions from serial section images require the accurate registration of those images. Image correlation is the most powerful computed alignment method and its performance on identical images, or parts thereof, has been thoroughly studied. Correlation alignments of complex, dissimilar images can fail, however, with a likelihood proportional to the magnitude of the differences. We report that alignments can be computed more reliably and more accurately (higher-valued correlation coefficients) by the combined use of lowpass-filtered product transforms (from which the correlation functions are formed), autocorrelation correction of rotational misalignment, and covariance correction of translation misalignment. A simple rule is proposed for the lowpass filter cutoff radius depending on measures of the images' differences. These methods are demonstrated with a reconstruction of a capillary loop in the median eminence of the hypothalamus.

Blood flow in portal systems with special reference to the rat pituitary gland

Regional pituitary blood flow has been studied in adult female Fischer 344 rats by [14C]iodoantipyrine autoradiography. A general mathematical solution has been derived to allow the calculation of blood flow in the second compartment of a portal system and the proportion of blood "shunted" through the first compartment without exposure to tissue uptake from a knowledge of (a) the volume ratios of the two compartments, (b) the tissue tracer uptakes of the two compartments, and (c) the arterial tracer concentration with respect to time of a freely diffusible tracer. Significant diffusion limitation and/or arteriovenous shunting has been demonstrated in the neurohypophysis, suggesting that the majority of incoming blood is "shunted" unchanged to the adenohypophysis. The mean value of the shunt is 89% (range of 84-93%) for the median eminence and lies between 72% (range of 52-82%) and 73% (range of 59-81%) for the posterior pituitary. Neurohypophysial flow rates of 1.20 (range of 0.99-1.55) ml g-1 min-1 for the median eminence and 1.68 (range of 0.83-3.53) ml g-1 min-1 for the posterior pituitary were measured. These values represent "tissue-available" (nonshunted) flow; estimated mean total (shunted plus nonshunted) neurohypophysial flow rates were 11.7 (range of 9.5-17.5) ml g-1 min-1 for the median eminence and 6.1 (range of 3.1-8.9) ml g-1 min-1 (minimum) for the posterior pituitary. Adenohypophysial blood flow is heterogeneous. In the long portal territory, the flow rate was 1.18 (range of 0.95-1.75) ml g-1 min-1 but short portal territory flow calculation is complicated by an unquantifiable nonportal venous drainage; using the natural limits of zero and 100% gives a minimum adenohypophysial flow rate of 1.42 (range of 0.76-2.07) ml g-1 min-1 and a maximum value of 1.97 (range of 1.03-2.82) ml g-1 min-1.

Microanatomy and possible clinical significance of anastomoses among hypothalamic arteries

We examined anastomoses among the hypothalamic arteries in 14 human brains using an injection technique, microdissection, and a stereoscopic microscope. Five to 22 anastomoses (mean 10.1) were found in all 14 brains on each side, varying from 20 to 280 (mean 71) microns in diameter and from 0.1 to 5.3 (mean 1.52) mm long. A single hypothalamic artery may be connected to other vessels by one to 10 anastomoses. The anastomoses were channel-like or plexiform; both types may be ipsilateral or right-left. They connected the hypothalamic arteries "end-to-end," "end-to-side," and "side-to-side." The interconnected arteries ranged from 30 to 1,900 (mean 148) microns in diameter. Anastomoses were most frequent among the commissural arteries and in the distribution of the superior hypophyseal arteries and the tuberoinfundibular branches of the posterior communicating artery. The largest anastomoses were found among the tuberoinfundibular branches of the posterior communicating and internal carotid arteries, as well as among the premamillary arteries and the mamillary branches. We discuss the neurologic, neuroendocrinologic, and neurosurgical significance of the described anastomoses.

Prenatal development of the gonadotropin-releasing hormone-containing systems in rat brain

Pre-embedding immunofluorescence was used to study the development of the gonadotropin-releasing hormone (GnRH)-containing neuronal systems in the brain of fetal and newborn rats. Immunoreactive cell bodies are first seen in association with the ventral olfactory bulb at the fetal embryonic (E) day 15. At day E 17, single GnRH-containing neurons are found in the nasal system, along the olfactory nerve, in the ganglion terminale, olfactory bulb, septum-diagonal band complex, olfactory tubercle and in the ventrolateral hypothalamus. A major spurt in the development of the brain GnRH-containing systems occurs between days E 18 and E 19. At day E 19, immunoreactive cell bodies are present in all areas as in the adult animals, however, in smaller number. GnRH-containing fibers are first detected in the nasal system at day E 17. At day E 18, GnRH positive projections are present in the nervus terminalis, in the developing organum vasculosum of the lamina terminalis and ventral hypothalamus. By day E 19, GnRH containing connections are established with most of the final target areas. These areas include the caudal olfactory bulb, lateral septum, stria terminalis, fimbria hippocampi, habenula, supramammillary commissure and central gray. In the hypothalamus, 3 major pathways are recognized: a ventral projection which runs in and beneath the optic chiasm toward the median eminence; a ventrolateral tract which is located lateral to the optic chiasm and which projects along the optic tract toward the lateral thalamus as well as to the median eminence; a periventricular network which projects to the habenula and, in a caudal direction, to the mediobasal hypothalamus including the median eminence. The results of the present study suggest that the GnRH neuronal systems develop considerably earlier than previously reported and that the intracerebral GnRH-containing fiber connections are established several days before birth.

A zonal pattern of neuroglial cells during the development of the intermediate lobe of the rat pituitary

In this paper the development of the intermediate lobe (IL) of the pituitary of the rat is described using alkaline phosphatase (AP) (EC 3.1.3.1.) enzyme histochemistry and stage-specific embryonic antigen 1 (SSEA-1) immunocytochemistry. SSEA-1 and AP are co-localized during late development and reveal the existence of two cytochemically different cell types within the IL, i.e. SSEA-1/AP-positive and SSEA-1/AP-negative cells. The SSEA-1/AP-positive cells are initially arranged along the hypophyseal lumen, in a number of longitudinally oriented zones. alpha-Melanocyte-stimulating hormone (alpha-MSH) immunoreactivity is expressed in the SSEA-1/AP-negative cells from E20 onwards. Eventually the SSEA-1/AP-positive cells develop into a layer of cells covering the luminal surface of the IL lobules. These cells represent the glio-epithelial or neuroglial cells of the IL.

Some blood vessels in the rat median eminence are surrounded by a dense plexus of vasoactive intestinal polypeptide/peptide histidine isoleucine (VIP/PHI) immunoreactive nerves

Using immunohistochemistry at the light and electron microscopic level some blood vessels along the median eminence were shown to be surrounded by dense networks of vasoactive intestinal polypeptide/peptide histidine isoleucine (VIP/PHI)-positive fibers. VIP and PHI released from these fibers may contribute to the elevated levels of these two peptides measured in portal blood as compared to peripheral blood by radioimmunoassay. VIP and PHI may also be important in the control of blood flow through the median eminence.