Innervation of the avian pineal organ

The innervation of the pineal organ was studied in 26 avian species under particular consideration of comparative aspects. A population of nerve cells and their pinealofugal (afferent) fiber systems were stained by means of the acetylcholinesterase method, while catecholamine-containing pinealopetal (efferent) fibers were demonstrated with the use of the glyoxylic acid method. Afferent axons were mainly found in the postero-proximal portion of the organ, and the patterns of their distribution were classified into three groups according to the characteristic densities of the reaction product. The number of acetylcholinesterase-positive neurons in the avian pineal organs examined in this study varied extremely from species to species, ranging from 0 to 362. Catecholamine-containing nerve fibers penetrating the antero-lateral walls of the pineal follicles accompanied blood vessels and were arranged more densely in the distal portion of the organ, in contrast to the distribution of the acetylcholinesterase-positive nerve fibers. Three-dimensional reconstruction of the distributional patterns of both types of neural projections was performed for the pineal organ of every avian species examined. In avian species possessing a relatively conspicuous afferent projection, such as Passeriformes, Nycticorax, and Milvus, terminals of catecholamine-containing nerve fibers were observed exclusively in the interfollicular and perivascular tissues. In Galliformes, which display only few pineal afferents, catecholamine-containing fibers terminate not only in the interfollicular space, but also in the neuroepithelial parenchyma. The regional differences in the innervation in the avian pineal organ suggest that the pinealocytes ranging from more sensory-like to more secretory-like elements are arranged in a mosaic-like pattern.     

Neuronal degeneration in the pineal ganglion during the post-hatching development of the domestic fowl

Summary The frequency of pineal ganglia associated with the pineal tract, and the numbers of acetylcholinesterasepositive neurons in these ganglia were studied in the domestic fowl during the post-hatching period by means of the acetylcholinesterase method. Furthermore, the degeneration of nerve cells in pineal ganglia of 40-day-old domestic fowl was investigated in detail at the electron-microscopic level. The rate of pineal organs containing one or more ganglia was 50% in 2- to 13-day-old, 38% in 40-day-old, and only 10% in 1-year-old domestic fowl. In parallel, the number of acetylcholinesterase-reactive nerve cells that constitute individual pineal ganglia decreased after hatching. Various degrees of neuronal degeneration were found in the pineal ganglia: swelling of the endoplasmic reticulum, electron-dense degeneration of the cytoplasm, and pyknosis of the nerve cell nucleus. Clusters of macrophages containing numerous lysosomes filled with debris-like material were scattered in the ganglion. In addition, plasma cells were observed in association with degenerating nerve cells. These results confirm the suggestion that the loss of acetylcholinesterase-positive nerve cells in the pineal ganglia of the domestic fowl is due to naturally occurring, programmed neuronal cell death. This process is discussed with reference to phenomena of cell death observed in other components of central nervous system.Fellow of the Alexander von Humboldt Foundation, Bonn, Federal Republic of GermanyThe authors are indebted to Professor A. Oksche and Dr. H.-W. Korf (Giessen) for stimulating discussions     

Separation of the pineal vesicle from the wall of the third ventricle during the post-hatching development of the chicken

Summary According to light- and electron-microscopic observations the pineal organ of the 3-day-old chicken consists of a prominent end vesicle and a tapering parenchymal stalk. During this stage the pineal lumen is in open communication with the third ventricle. However, in the 40-day-old chicken, which still possesses a well-developed end vesicle, the proximal portion of the pineal stalk displays regressive changes leading to local fragmentation. At this stage the pineal stalk is reduced, and the pineal lumen is missing. In 1-year-old chickens the parenchyma of the proximal portion of the stalk is further diminished, and in 3-year-old domestic fowl is completely displaced by bundles of collagenous fibers, only some nerve fibers being present. This post-hatching pineal development may reflect the sequence of changes leading from pineal sense organs to pineal glands.This work was supported by a grant-in-aid for Scientific Research from the Ministry of Education, Science and Culture of Japan     

A pineal ganglion associated with the pineal tract in the domestic fowl

Summary A ganglion-like aggregate consisting of acetyl-cholinesterase-positive neurons was demonstrated in the pineal organ of the domestic fowl by means of light and electron microscopy. This ganglion is located in juxtaposition with the pineal tract at the posterior (caudal) aspect of the pineal stalk. Numerous large and small neurons formed the ganglion in 40-day-old domestic fowl. Some of these nerve cells established direct neuro-neuronal contacts, others were surrounded by satellite cells. These ganglion cells displayed axo-somatic and axo-dendritic synapses. The above-mentioned cluster of nerve cells may be considered as a pineal ganglion. Its central or peripheral nature is open to discussion. Send offprint requests to: Dr. K. Wake, Department of Anatomy, Faculty of Medicine, Tokyo Medical and Dental University, Yushima, Bunkyo-ku, 113, Japan     

Expression of neuron-specific enolase in the pineal organ of the domestic fowl during post-hatching development

Immunohistochemistry for neuron-specific enolase (NSE) revealed that NSE is localized in both a limited number of pinealocytes and intrinsic afferent neurons in the pineal organ of the domestic fowl. Furthermore, a computer-assisted three-dimensional imaging technique allowed to clarify the reverse distributional pattern of both elements: NSE-positive pinealocytes displayed a dense distribution especially in the vesicular portion of the gland, whereas NSE-immunoreactive nerve cells were mainly found in the pineal stalk. The number of NSE-positive intrinsic neurons in the pineal organ of chickens decreased rapidly after hatching, with a concentration of these elements in the basal portion (stalk) of the pineal organ. On the other hand, immunoreactive pinealocytes increased remarkably in the end-vesicle of the organ with age, followed by a gradual expansion toward the proximal portion. Thus, the spectacular increase in NSE-positive pinealocytes and the progressive reduction of reactive neurons occurred in parallel during the course of post-hatching development. NSE-immunoreactive pinealocytes displayed morphological characteristics of bipolar elements, endowed with an apical protrusion into the pineal lumen and a short basal process at younger stages, whereas multipolar types of NSE-positive pinealocytes were predominantly found in the adult domestic fowl. These results indicate that in the pineal organ of the domestic fowl (1) the ontogenetic expansion of NSE-immunoreactive pinealocytes is paralleled by a regressive afferent innervation, (2) the NSE-positive pinealocytes transform from a bipolar (columnar) type to a multipolar type during post-hatching development, and (3) these ontogenetic changes in the NSE-immunoreactivity and morphology of pinealocytes may reflect the development of a neurosecretory-like capacity of the organ.     

Immunohistochemical localization of vasoactive intestinal peptide (VIP) in the circumventricular organs of the rat

Summary The indirect peroxidase-antiperoxidase immunohistochemical technique was used to investigate the possible presence of vasoactive intestinal peptide (VIP) in the circumventricular organs of the rat. Considerable numbers of VIP-immunoreactive fibers were seen in the pineal gland. A moderate amount of VIP-immunoreactive fibers was present in the median eminence, the posterior lobe of the pituitary and the area postrema, but only few fibers were found in the organum vasculosum laminae terminalis. No immunoreactivity was observed in the subfornical organ or the subcommissural organ. The circumventricular organs investigated were completely free of VIP-immunoreactive perikarya. In the circumventricular organs, VIP-immunoreactive fibers were visible between the parenchymal cells and in the perivascular spaces. The presence of coarse VIP-immunoreactive terminals in apposition to the portal vessels in the external layer of the median eminence indicates that VIP may be secreted directly into the pituitary portal circulation, thus influencing the anterior pituitary cells. The presence of large VIP-immunoreactive boutons in the posterior lobe of the pituitary suggests a secretion of VIP directly into the systemic circulation. In the pineal gland, a dense innervation by VIP-immunoreactive fibers was found in the peripheral superficial part of organ, with fibers penetrating into its central portion where they mainly terminate near in vicinity of the capillaries. In the area postrema, VIP-immunoreactive material was mainly found at the ventral border of the organ. In addition to the secretion of VIP into the bloodstream via the circumventricular organs, this study provides evidence that VIP exerts specific influence on the cellular elements of these organs.     

Vergleichende Untersuchungen über acetylcholinesterase-haltige Neurone im Pinealorgan der Vögel

Zusammenfassung Im Pinealorgan der Vögel zeigen die acetylcholinesterase-positiven Strukturen speziesabhängige Unterschiede. Die passeriformen Arten Passer domesticus, Serinus canaria und Amadina faciata alexander besitzen acetylcholinesterase-positive Nervenzellen, deren Axone im Epiphysenstiel hirnwärts ziehen; außerdem kommen an den Blutgefäßen spezifisch tingierbare Nervenfasern vor, die offenbar zur Epiphyse verlaufen. Bei Columba livia sind diese Nervenstrukturen sehr stark reduziert. Dagegen zeigt das Parenchym der Taubenepiphyse lichtmikroskopisch eine schwache Anfärbung; diese Stellen manifestieren sich im elektronenmikroskopischen Bild als punktförmige interzelluläre Reaktionsorte zwischen einzelnen Pinealzellen. Ferner finden sich bei Passer domesticus elektronenmikroskopisch nachweisbare Reaktionsprodukte im endoplasmatischen Reticulum und im perinukleären Raum der Pinealocyten (=Pinealzellen) sowie am Plasmalemmüberzug (Außenmembran) der Perikaryen und der Axone der Nervenzellen. Die Befunde bei 3 Tage alten Küken (Gallus domesticus) und bei Excalfactoria chinensis ähneln denen bei Columba livia, während bei Coturnix coturnix japonica das Reaktionsprodukt vermehrt im basalen Abschnitt der Follikel liegt. Melopsittacus undulatus zeigt im Epiphysenstiel eine begrenzte Zahl von hirnwärts ziehenden Nervenfasern und nimmt zwischen den Passeriformes und der Taube etwa eine Mittelstellung ein. Die sensorische und sekretorische Kapazität des Pinealorgans von Passeriformes wird diskutiert.

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Comparative studies of neurons in the pineal organ of birds using the acetylcholinesterase method

Summary The acetylcholinesterase reaction of the pineal organ (epiphysis cerebri) varies among avian species. Acetylcholinesterase-positive neurons with axons running along the pineal stalk to unidentified brain centers are seen in the pineal organ of passeriform birds such as Passer domesticus, Serinus canaria, and Amadina faciata alexander. A different kind of acetylcholinesterase-positive fibers are observed running along the blood vessels to the pineal organ. The acetylcholinesterase-positive structures are much reduced in Columba livia. The pineal parenchyma in Columba livia shows a slightly brown color in the light microscope; spot-like deposits of the reaction product are seen between the pinealocytes in the electron microscope. In Passer domesticus, the reaction products are seen in the pinealocyte endoplasmic reticulum as well as at the outer membrane of the perikaryon and axon of the nerve cells. The results in three-day old chicken and in Excalfactoria chinensis are similar to those obtained in Columba livia. In Coturnix coturnix japonica, a heavy reaction is evident in the basal part of the follicles. A small number of nerve cells and fibers are seen in the pineal stalk of Melopsittacus undulatus. The results are discussed in respect of sensory and secretory functions of the pineal organs in passeriform birds.

Gastprofessor des Landes Hessen am Anatomischen Institut der Universität Gießen.     

Characteristic pattern of monoaminergic nerve fibers in the pineal organ of the monkey,Macaca fuscata

Summary Monoaminergic nerve fibers were studied in the pineal organ of the monkey, Macaca fuscata, by use of fluorescence and immunohistochemical procedures. Abundant formations of noradrenergic nerve fibers were observed in the pineal organ. They entered the parenchyma in the form of several coarse bundles via the capsule in the distal portion of the organ and spread throughout the organ after branching into smaller units. The density of the autonomic innervation decreased gradually toward the proximal portion of the organ. In the distal portion, numerous nerve fibers formed perivascular plexuses around the blood vessels and some fibers ran as bundles unrelated to the blood vessels in the stroma. Fine varicose fibers and bundles derived from these plexuses penetrated among the pinealocytes. However, only a few intraparenchymal fluorescent fibers were detected in the proximal third of the gland. With the use of serotonin antiserum serotonin-immunoreactive nerve fibers were clearly restricted to the ventroproximal part of the pineal organ. Although the somata of the pinealocytes showed intense immunoreactivity, their processes were not stained. In one exceptional case, clusters of pinealocytes displaying very intense immunoreactivity were found in an area extending from the distal margin of the ventral portion of the pineal stalk to the proximal portion of the pineal organ proper; these cells were bipolar or multipolar and endowed with well-stained processes.     

Autonomic nerves terminating on microvessels in the pineal organs of various submammalian vertebrates

In earlier works we have found that in the mammalian pineal organ, a part of autonomic nerves--generally thought to mediate light information from the retina--form vasomotor endings on smooth muscle cells of vessels. We supposed that they serve the vascular support for circadian and circannual periodic changes in the metabolic activity of the pineal tissue. In the present work, we investigated whether peripheral nerves present in the photoreceptive pineal organs of submammalians form similar terminals on microvessels. In the cyclostome, fish, amphibian, reptile and bird species investigated, autonomic nerves accompany vessels entering the arachnoidal capsule and interfollicular meningeal septa of the pineal organ. The autonomic nerves do not enter the pineal tissue proper but remain in the perivasal meningeal septa isolated by basal lamina. They are composed of unmyelinated and myelinated fibers and form terminals around arterioles, veins and capillaries. The terminals contain synaptic and granular vesicles. Comparing various vertebrates, more perivasal terminals were found in reptiles and birds than in the cyclostome, fish and amphibian pineal organs. Earlier, autonomic nerves of the pineal organs were predominantly investigated in connection with the innervation of pineal tissue. The perivasal terminals found in various submammalians show that a part of the pineal autonomic fibers are vasomotoric in nature, but the vasosensor function of some fibers cannot be excluded. We suppose that the vasomotor regulation of the pineal microvessels in the photosensory submamalian pineal--like in mammals--may serve the vascular support for circadian and circannual periodic changes in the metabolic activity of the pineal tissue. The higher number of perivasal terminals in reptiles and birds may correspond to the higher metabolic activity of the tissues in more differentiated species.     

Indications for the presence of two populations of serotonin-containing pinealocytes in the pineal complex of the golden hamster (Mesocricetus auratus)

Summary Serotonin-like immunoreactivity was investigated in the pineal complex of the golden hamster by use of the indirect immunohistochemical technique. The superficial and deep portions of the pineal gland, and also the pineal stalk exhibited an intense cellular immunoreaction for serotonin. In addition, perivascular serotonin-immunoreactive nerve fibers were observed. Some serotonin-immunoreactive processes of the pinealocytes terminated on the surface of the ventricular lumen in the pineal and suprapineal recesses, indicating a receptive or secretory function of these cells. Several serotonin-immunoreactive processes connected the deep pineal with the habenular area. One week after bilateral removal of both superior cervical ganglia the serotonin immunoreaction of the entire pineal complex was greatly decreased. However, some cells in the pineal complex, of which several exhibited a neuron-like morphology, remained intensively stained after ganglionectomy. This indicates that the indoleamine content of some cells in the pineal complex of the golden hamster is independent of the sympathetic innervation.Supported by a Grant from the Italian Society for Veterinary Sciences     

Photoreceptor-like outer segments in the pineal organ of the lovebird,Uroloncha domestica (Aves: Passeriformes)

Summary In the pineal organ of the lovebird, Uroloncha domestica, bulbous, cup-shaped and elongated outer segments of photoreceptor-like pinealocytes are demonstrated by scanning electron microscopy. These scarce outer segments, 4–11 m in length, extend into the pineal lumen. The present structural observations speak in favor of photosensitive pinealocytes in the pineal organ of Uroloncha domestica. The relation of the photoreceptor-like pinealocytes to acetylcholinesterase-positive nerve cells and a nervous connection between the pineal and the brain indicate that the pineal organ of this passeriform species may be the site of neuroendocrine and photoreceptive functions.Supported by a fellowship from the Japan Society for the Promotion of Science to M. UeckSupported by a grant from the Ministry of Education of Japan to K. Wake and by a grant of the Deutsche Forschungsgemeinschaft to M. Ueck     

Evidence for a nervous connection between the brain and the pineal organ in the guinea pig

Summary Following the injection of horseradish peroxidase into the pineal organ of the guinea pig, approximately 30 nerve fibers were demonstrated in the pineal stalk due to the retrograde transport of the tracer enzyme in these elements. Finely branched extensions of these nerve fibers are directed toward the distal portion of the pineal organ. This projection of central nervous elements enters the pineal organ via the habenular or posterior commissures. Neuronal perikarya projecting into the pineal organ are found in the region of the paraventricular nucleus near the border of the third ventricle.In partial fulfillment of the requirements for the degree of Dr. med., Faculty of Medicine, Justus Liebig University of Giessen     

Tyrosine hydroxylase- and neuropeptide Y-immunoreactive nerve fibers in the pineal complex of untreated rats and rats following removal of the superior cervical ganglia

Summary The distribution of tyrosine hydroxylase (TH)- and neuropeptide Y (NPY)-immunoreactive(IR) nerve fibers in the pineal complex was investigated in untreated rats and rats following bilateral removal of the superior cervical ganglia. In normal animals, a large number of TH- and NPY-IR nerve fibers were present in the pineal capsule, the perivascular spaces, and intraparenchymally between the pinealocytes throughout the superficial pineal and deep pineal gland. A small number of TH-IR and NPY-IR nerve fibers were found in the posterior and habenular commissures, a few fibers penetrating from the commissures into the deep pineal gland. To elucidate the origin of these fibers, the superior cervical ganglion was removed bilaterally in 10 animals, and the pineal complex was examined immunohistochemically. Two weeks after the ganglionectomy, the TH-IR and NPY-IR nerve fibers in the superficial pineal gland had almost completely disappeared. On the other hand, in the deep pineal and the pineal stalk, the TH-IR and NPY-IR fibers were still present after ganglionectomy. These data show that the deep pineal gland and the pineal stalk possess an extrasympathetic innervation by TH-IR and NPY-IR fibers. It is suggested that the extrasympathetic TH-IR and NPY-IR nerve fibers innervating the deep pineal and the pineal stalk originate from the brain.     

Development of the innervation in the human pineal organ

Summary This investigation is concerned with pineal organs of human embryos 60 to 150 days old. At every stage central nerve fibres enter the pineal organ by way of the habenular commissure, but are restricted to the pineal's proximal part. On about the 60th day of the development the sympathetic nervus conarii grows into the distal pole of the pineal organ from a dorso-caudal direction and plays the predominant part in the innervation of the pineal organ. After penetrating, it soon branches out and forms a network in the pineal tissue. Much later, not until the 5th embryonic month, sympathetic nerves appear accompanying the supplying vessels in the perivascular spaces. After a short time these nerves pierce the outer limiting basement membrane and penetrate the parenchyma. Towards the end of the 5th embryonic month the axons of the sympathetic nerves form varicosities containing clear and dense core vesicles. At this point large amounts of laminated granules appear primarily in cell processes, probably of pinealocytes. Isolated granules also occur in the varicosities of axons. The granules encountered here are most likely secretory granules.Dedicated to Professor Bargmann on his 65th birthday.     

Opsin-immunoreactive outer segments and acetylcholinesterase-positive neurons in the pineal complex of Phoxinus phoxinus (Teleostei,Cyprinidae)

Summary The pineal complex of the teleost, Phoxinus phoxinus L., was studied light-microscopically by the use of the indirect immunocytochemical antiopsin reaction and the histochemical acetylcholinersterase (AChE) method.Opsin-immunoreactive outer segments of photoreceptor cells were demonstrated in large numbers in all divisions of the pineal end-vesicle and in the pineal stalk. Moreover, they were found in the roof of the third ventricle, adjacent to the orifice of the pineal recess as well as scattered in the parapineal organ. These immunocytochemical observations provide direct evidence of the presence of an opsin associated with a photopigment in the photosensory cells of the pineal and parapineal organs of Phoxinus. By means of the AChE reaction (Karnovsky and Roots 1964) inner segments of pineal photoreceptors, intrinsic nerve cells, several intrapineal bundles of nerve fibers, and a prominent pineal tract were specifically marked. The pineal neurons can be divided into two types: one is located near the pineal lumen, the other near the basal lamina. The latter perikarya bear stained processes directed toward the photoreceptor layer. A rostral aggregation of two types of AChE-positive nerve cells occurs in the ventral wall of the pineal end-vesicle. The main portion of the AChE-positive pineal tract, which lies within the dorsal wall of the pineal stalk, can be followed to the posterior commissure where some of the nerve fibers course laterally. A few AChE-positive pineal nerve fibers run toward the lateral habenular nucleus via the habenular commissure. In the region of the subcommissural organ single AChE-positive neurons accompany the pineal tract. The nerve cells of the parapineal organ exhibit a moderate AChE activity.These findings extend the structural basis for the remarkable light-dependent activity of the pineal organ of Phoxinus phoxinus. To the memory of Professor Karl von Frisch, pioneer and master in the field of photoneuroendocrine systemsThis investigation was supported by grants from the Deutsche Forschungsgemeinschaft to A.O. (Ok 1/24; 1/25: Mechanismen biologischer Uhren) and to H.-W. K. (Ko 758/1; 758–2)On leave from the 2nd Department of Anatomy, SOTE, Budapest, Hungary     

Innervation of the mink pineal gland with neuropeptide Y (NPY)-containing nerve fibers

Summary An immunohistochemical investigation of the mink pineal gland was performed by use of antibodies raised in rabbits against neuropeptide Y (NPY) and Cys-NPY (32–36)-amide recognizing neuropeptide Y with an amidation at position 36 (NPYamide). NPY-immunoreactive nerve fibers were located predominantly in the rostral part of the pineal gland and in the pineal stalk. Immunoreactive nerve fibers were found throughout the pineal gland, but the number of fibers in the caudal part of the gland was low. The fibers were present both in the perivascular spaces and between the pinealocytes. Many NPY-immunoreactive fibers were also located in the posterior and habenular commissures; some of these fibers were connected with the fibers in the rostral part of the mink pineal gland, indicating that at least some of the NPY-immunoreactive nerve fibers are of central origin. The nerve fibers immunoreactive to amidated NPY were distributed in a similar manner. However, the number of fibers immunoreactive to NPYamide was lower than the number of fibers immunoreactive to NPY itself. After removal of the superior cervical ganglia bilaterally 22 days or 12 months before sacrifice, NPY-immunoreactive nerve fibers remained in the gland. This immunohistochemical study of the mink pineal gland therefore shows that the NPY/NPYamide-immunoreactive nerve fibers innervating the pineal gland in this spegcies are a component of the central innervation or originnate from extracerebral parasympathetic ganglia.     

Innervation of the cat pineal gland by neuropeptide Y-immunoreactive nerve fibers: an experimental immunohistochemical study

An immunohistochemical study of the cat pineal gland was performed using a rabbit polyclonal antibody directed against neuropeptide Y (NPY) and an antibody directed against the C-terminal flanking peptide of neuropeptide Y (CPON). Numerous NPY- and CPON-immunoreactive (IR) nerve fibers were demonstrated throughout the gland and in the pineal capsule. The number of IR nerve fibers in the capsule was high and from this location fibers were observed to penetrate into the gland proper via the pineal connective tissue septa, often following the blood vessels. From the connective tissue septa IR fibers intruded into the parenchyma between the pinealocytes. Many IR nerve fibers were observed in the pineal stalk and in the habenular as well as the posterior commissural areas. The number of NPY/CPON-IR nerve fibers in pineal glands from animals bilaterally ganglionectomized two weeks before sacrifice was low. The source of most of the extrasympathetic NPY/CPONergic nerve fibers is probably the brain from where they enter the pineal via the pineal stalk. However, an origin of some of the fibers from parasympathetic ganglia cannot be excluded due to the presence of a few IR fibers in the pineal capsule of ganglionectomized animals. It is concluded that the cat pineal is richly innervated with NPYergic nerve fibers mostly of sympathetic origin. The posttranslational processing of the NPY promolecule results in the presence of both NPY and CPON in intrapineal nerve fibers.     

Vascular permeability (problem of the blood-brain barrier) in the pineal organ of the rainbow trout,Salmo gairdneri

Summary The problem of the blood-brain barrier in the pineal organ of the rainbow trout, Salmo gairdneri, was investigated following intraperitoneal or intracardial injections of several tracers and dyes with different molecular weights. As demonstrated at the light-microscopic level, repeated injections of trypan blue or horseradish peroxidase (HRP) resulted in an accumulation of these substances in the pineal epithelium (parenchyma). By use of the electron microscope, HRP was found in electron-dense bodies, probably lysosomes, in (i) the endothelial cells and perivascular macrophages 4 h after intraperitoneal injection, (ii) the supporting cells and intrapineal or luminal macrophages 8 h after injection, and (iii) the receptor cells 24 h after injection of the tracer. Ferritin particles penetrated the fenestrated endothelium of pineal capillaries. They were confined to vesicles, vacuoles and the smooth endoplasmic reticulum of the supporting cells as well as to the synaptic vesicles and the smooth endoplasmic reticulum of the pineal photoreceptors. The intercellular passage of tannic acid mixed with the fixative was blocked at the luminal junctional complex separating the pineal lumen from the basal portion of the pineal epithelium. The passive intercellular transport of substances with high molecular weight from the bloodstream to the cerebrospinal-fluid compartment is thus prevented. However, no blood-brain barrier exists for exogenously administered proteins, which are rapidly taken up by pineal cells and actively transported in a transcellular manner.The findings on the blood-brain barrier of the pineal organ of the rainbow trout are discussed with particular reference to the endocrine capacity of pineal sensory organs.Fellow of the Alexander von Humboldt Foundation, Federal Republic of Germany.     

Acetylcholinesterase-containing nerve cells in the pineal complex and subcommissural area of the frogs,Rana ridibunda and Rana esculenta

Summary In Rana esculenta and Rana ridibunda the frontal organ and the pineal organ (epiphysis cerebri) form a pineal complex. Approximately 60 nerve cells of the frontal organ and 220–320 nerve cells of the pineal organ display a positive acetylcholinesterase reaction (Karnovsky and Roots, 1964). The dorsal wall of the pineal organ is considerably richer in acetylcholinesterase-positive neurons than the ventral wall (ratio 31); a group of unusually large-sized nerve cells occurs in the rostral portion of the frog pineal. Two different types of nerve cells were observed in the pineal complex: multipolar and pseudounipolar elements. The former are embedded in the pineal parenchyma and their processes penetrate radially into the plexiform layer, whereas the latter are distributed along the roots of the pineal tract near the basal lamina. The ratio of the multipolar to pseudounipolar neurons is 14 for the frontal organ and 35 for the pineal organ. The multipolar elements may be interneurons; the pseudouni-polar cells send one of their processes into the pineal tract. At the caudal end of the pineal organ 30–50 unipolar nerve cells are clustered in juxtaposition with the pineal tract, and other 30–50 unipolar neurons are scattered along the basis of the subcommissural organ. Some of these nerve cells emit their processes toward the mesencephalon and others toward the pineal organ via the pineal tract. The results are discussed with respect to previous physiological and morphological findings on the pineal complex of Anura.Supported by a fellowship from the Alexander von Humboldt Foundation, Federal Republic of Germany, to K. Wake. Completed November 22, 1973.Supported by the Deutsche Forschungsgemeinschaft.     

Neuropeptide Y: An Endogenous Inhibitor of Norepinephrine-Stimulated Melatonin Secretion in the Rat Pineal Gland

The biosynthesis of the hormone melatonin (MEL) by the mammalian pineal gland has been thought to be regulated strictly by stimulatory factors, most predominantly norepinephrine (NE), released from the sympathetic nerve fibers which heavily innervate the gland. Evidence from many investigators suggests that sympathetic fibers may colocalize other neuroactive factors in addition to NE. One of these factors is neuropeptide Y (NPY), which has been found in the nerve fibers of the pineal gland. The present study sought to explore potential interactions between NE and NPY in the regulation of pineal MEL secretion. Specific, saturable, and reversible binding of 125I-NPY to intact cultured pinealocytes was measured with an affinity constant of 1 nM and an NPY binding site density of 0.04 pmol/mg of protein. In addition, cell culture studies revealed that NPY represents a potent (IC50 of 0.4 nM) endogenous inhibitor of NE-stimulated MEL secretion. However, this inhibition is accompanied by only a modest reduction (35%) of cyclic AMP accumulation. These findings reinforce the view that the mammalian pineal gland, which appears to integrate both inhibitory as well as stimulatory signals, is an important model of autonomic function, particularly in the context of biological rhythmicity.