Neuropeptide Y in the mammalian pineal gland

An ultrastructural study of the deep pineal gland of the Sprague Dawley rat using transmission and serial block face scanning electron microscopy: cell types, barriers, and innervation

The morphology of the deep pineal gland of the Sprague Dawley rat was investigated by serial block face scanning electron microscopy. Cells were three-dimensionally (3-D) reconstructed using the software Fiji TrackEM. The deep pineal gland consisted of 2-5 layers of electron-lucent pinealocytes, with a euchromatic nucleus, endowed with one or two processes. Laterally, the deep pineal merged with the habenula and the stria medullaris thalami, via an intermediate area containing cells with more electron-dense cytoplasm and an indented nucleus with heterochromatin. Neither nerve terminals nor capillaries were observed in the deep pineal itself but present in the intermediate parts of the gland. The deep pineal was in contact with the third ventricle via the pineal and suprahabenular recesses. The ependymal lining in these recesses was an epithelium connected by tight junctions between their lateral cell membranes. Several intraventricular nerve terminals were in contact with the ependyma. 3-D reconstructions showed the ependymal cells endowed with long slender process penetrating the underlying pineal parenchyma. Few "tanocyte-like" ependymal cells, endowed with a process, reaching the subarachnoid space on the inferior surface of the deep pineal were observed. In addition, pinealocyte and astrocyte processes, often connected by gap junctions, bordered the inferior surface. In summary, the rat deep pineal gland is a neuroendocrine structure connected to the habenula. We here report specialized ependymal cells that might transmit signals from the cerebrospinal fluid to the deep pineal parenchyma and a "trans-pineal tanocyte-like cell" that connects the ventricular system with the subarachnoid space.

Thymus-Pineal Gland Axis: Revisiting Its Role in Human Life and Ageing

For years the thymus gland (TG) and the pineal gland (PG) have been subject of increasingly in-depth studies, but only recently a link that can associate the activities of the two organs has been identified. Considering, on the one hand, the well-known immune activity of thymus and, on the other, the increasingly emerging immunological roles of circadian oscillators and the rhythmically secreted main pineal product, melatonin, many studies aimed to analyse the possible existence of an interaction between these two systems. Moreover, data confirmed that the immune system is functionally associated with the nervous and endocrine systems determining an integrated dynamic network. In addition, recent researches showed a similar, characteristic involution process both in TG and PG. Since the second half of the 20th century, evidence led to the definition of an effectively interacting thymus-pineal axis (TG-PG axis), but much has to be done. In this sense, the aim of this review is to summarize what is actually known about this topic, focusing on the impact of the TG-PG axis on human life and ageing. We would like to give more emphasis to the implications of this dynamical interaction in a possible therapeutic strategy for human health. Moreover, we focused on all the products of TG and PG in order to collect what is known about the role of peptides other than melatonin. The results available today are often unclear and not linear. These peptides have not been well studied and defined over the years. In this review we hope to awake the interest of the scientific community in them and in their future pharmacological applications.

Neuropeptide Y as a presynaptic modulator of norepinephrine release from the sympathetic nerve fibers in the pig pineal gland

Norepinephrine (NE) released from the sympathetic nerve endings is the main neurotransmitter controlling melatonin synthesis in the mammalian pineal gland. Although neuropeptide Y (NPY) co-exists with NE in the pineal sympathetic nerve fibers it also occurs in a population of non-adrenergic nerve fibers located in this gland. The role of NPY in pineal physiology is still enigmatic. The present study characterizes the effect of NPY on the depolarization-evoked 3H-NE release from the pig pineal explants. The explants of the pig pineal gland were loaded with 3H-NE in the presence of pargyline and superfused with Tyrode medium. They were exposed twice to the modified Tyrode medium containing 60 mM of K+ to evoke the 3H-NE release via depolarization. NPY, specific agonists of Y1- and Y2- receptors and pharmacologically active ligands of α2-adrenoceptors were added to the medium before and during the second depolarization. The radioactivity was measured in medium fractions collected every 2 minutes during the superfusion. NPY (0.1-10 μM) significantly decreased the depolarization-induced 3H-NE release. Similar effect was observed after the treatment with Y2-agonist: NPY13-36, but not with Y1-agonist: [Leu31,Pro34]-NPY. The tritium overflow was lower in the explants exposed to the 5 μM NPY and 1 μM rauwolscine than to rauwolscine only. The effects of 5 μM NPY and 0.05 μM UK 14,304 on the depolarization-evoked 3H-NE release were additive. The results show that NPY is involved in the regulation of NE release from the sympathetic terminals in the pig pineal gland, inhibiting this process via Y2-receptors.

Circadian dynamics of the cone-rod homeobox (CRX) transcription factor in the rat pineal gland and its role in regulation of arylalkylamine N-acetyltransferase (AANAT)

The cone-rod homeobox (Crx) gene encodes a transcription factor in the retina and pineal gland. Crx deficiency influences the pineal transcriptome, including a reduced expression of arylalkylamine N-acetyltransferase (Aanat), a key enzyme in nocturnal pineal melatonin production. However, previous functional studies on pineal Crx have been performed in melatonin-deficient mice. In this study, we have investigated the role of Crx in the melatonin-proficient rat pineal gland. The current study shows that pineal Crx transcript levels exhibit a circadian rhythm with a peak in the middle of the night, which is transferred into daily changes in CRX protein. The study further shows that the sympathetic innervation of the pineal gland controls the Crx rhythm. By use of adenovirus-mediated short hairpin RNA gene knockdown targeting Crx mRNA in primary rat pinealocyte cell culture, we here show that intact levels of Crx mRNA are required to obtain high levels of Aanat expression, whereas overexpression of Crx induces Aanat transcription in vitro. This regulatory function of Crx is further supported by circadian analysis of Aanat in the pineal gland of the Crx-knockout mouse. Our data indicate that the rhythmic nature of pineal CRX protein may directly modulate the daily profile of Aanat expression by inducing nighttime expression of this enzyme, thus facilitating nocturnal melatonin synthesis in addition to its role in ensuring a correct tissue distribution of Aanat expression.

Neuropeptide Y in the adult and fetal human pineal gland

Neuropeptide Y was isolated from the porcine brain in 1982 and shown to be colocalized with noradrenaline in sympathetic nerve terminals. The peptide has been demonstrated to be present in sympathetic nerve fibers innervating the pineal gland in many mammalian species. In this investigation, we show by use of immunohistochemistry that neuropeptide Y is present in nerve fibers of the adult human pineal gland. The fibers are classical neuropeptidergic fibers endowed with large boutons en passage and primarily located in a perifollicular position with some fibers entering the pineal parenchyma inside the follicle. The distance from the immunoreactive terminals to the pinealocytes indicates a modulatory function of neuropeptide Y for pineal physiology. Some of the immunoreactive fibers might originate from neurons located in the brain and be a part of the central innervation of the pineal gland. In a series of human fetuses, neuropeptide Y-containing nerve fibers was present and could be detected as early as in the pineal of four- to five-month-old fetuses. This early innervation of the human pineal is different from most rodents, where the innervation starts postnatally.

Peptidergic and Nitrergic Innervation of the Pineal Gland in the Domestic Pig: An Immunohistochemical Study

The presence and co-localization of vasoactive intestinal polypeptide (VIP), peptide N-terminal histidine C-terminal isoleucine (PHI), pituitary adenylate cyclase-activating peptide (PACAP), somatostatin (SOM), calcitonin gene-related peptide (CGRP), substance P (SP) and the neuronal isoform of nitric oxide synthase (NOS) were studied in neuronal structures of the pig pineal gland. Paraformaldehyde-fixed pineals of 3-month-old gilts were sliced into serial cryostat sections, which were subjected to a set of double immunofluorescence stainings. Based on the co-existence patterns of neuropeptides, five populations of nerve fibres supplying the pig pineal were distinguished: (1) PHI-positive, (2) PACAP-positive, (3) SOM-positive, (4) SP/CGRP-positive and (5) SP-positive/CGRP-negative. Only a subpopulation of PHI-positive fibres contained VIP at the level detectable by immunofluorescence. NOS was found in some intrapineal PHI- and VIP-positive fibres. PHI-, VIP- and NOS-positive nerve fibres were more numerous in the peripheral than in the central part of the pineal. PACAP-positive fibres were equally distributed within the gland. The density of SOM-positive fibres was higher in the ventro-proximal than in the dorso-distal part of the pineal. SOM was also detected in some neuronal-like cells or specialized pinealocytes situated in the central region of the gland. Two populations of fibres containing SP were found: CGRP-positive, present in the distal and central parts of the pineal as well as CGRP-negative, localized in the proximal compartment of the gland.

Influence of sympathectomy in humans on the rhythmicity of 6-sulphatoxymelatonin urinary excretion

The amount of 6-sulphatoxymelatonin, the chief metabolite of melatonin, in the urine was measured in nine patients, who were subjected to bilateral sympathectomy at the second thoracic ganglionic level for treatment of hyperhidrosis of the palms. All patients showed before surgery a normal 6-sulphatoxymelatonin excretion with a peak in the excretion during the night time. After the sympathectomy, the high night time excretion was clearly abolished in five patients but remained high in four patients. This indicates that the segmental locations of the preganglionic sympathetic perikarya in the spinal cord, stimulating the melatonin secretion in the pineal gland in humans, vary between individuals. An increase in daytime melatonin excretion was observed in the patients responding to the sympathectomy with an abolished 6-sulphatoxymelatonin rhythm. This increase could indicate that the final sympathetic neurons innervating the pineal gland might have a both stimulatory and inhibitory function.

Tachykinins and tachykinin-receptors in the rat pineal gland

High-pressure liquid chromatography of extracts of rat pineal glands, followed by radio immunological analysis with antibodies against tachykinins, demonstrated the presence of substance P, neurokinin A and neurokinin B in the superficial rat pineal gland. Immunohistochemistry on perfusion-fixed rat brain sections showed substance P and neurokinin A to be present in nerve fibers located both in the perivascular spaces as well as intraparenchymally between the pinealocytes. After extracting total RNA, followed by reverse transcription and polymerase chain reaction amplification with primers specific for NK1-, NK2- and NK3-receptors, agarose gel analysis of the reaction products showed the presence of mRNA encoding all three neurokinin receptors. Immunohistochemical analysis showed NK1 receptor to be located in the interstitial cells of the gland. This location was confirmed by use of in situ hybridization using radioactively labeled antisense oligonucleotide probes. Double immunohistochemical stainings showed that the NK1-immunoreactive cells were not a part of the macrophages or antigen-presenting cells of the gland. Our study suggests that tachykinins, after release from intrapineal nerve fibers, are involved in an up to now unknown function, different from that of melatonin synthesis.

PACAP-containing intrapineal nerve fibers originate predominantly in the trigeminal ganglion: a combined retrograde tracing- and immunohistochemical study of the rat

Pituitary adenylate-cyclase activating polypeptide (PACAP) is a neuropeptide originally isolated from the hypothalamus and located in many neuronal systems in both the central and peripheral nervous system. PACAP is also found in nerve fibers innervating the pineal gland, where it stimulates the secretion of the pineal hormone, melatonin, by binding to specific PACAP-receptors located on the cell membrane of the pinealocyte. In this study we have investigated the origin of PACAP-containing nerve fibers innervating the rat pineal gland by combined retrograde tracing with Fluorogold and immunohistochemistry for PACAP. A solution of 2% Fluorogold was injected iontophoretically into the superficial pineal gland of Wistar rats, and the animals were allowed to survive for 1 week. After perfusion fixation of the rats, the location of the tracer was investigated in the brain and the sphenopalatine, otic, and trigeminal ganglia. The tracer was found in all the investigated ganglia. However, colocalization with PACAP was predominantly found in the trigeminal ganglion and only occasionally in the sphenopalatine and otic ganglia. Due to the stimulatory function of PACAP on pineal melatonin secretion, the PACAP-containing neurons of this ganglion could be considered a subset of the parasympathetic nervous system. The presence of neurons with a parasympathetic function in a ganglion that has been considered a purely sensory ganglion, is a new concept in neuroanatomy.

Generation of the melatonin endocrine message in mammals: a review of the complex regulation of melatonin synthesis by norepinephrine, peptides, and other pineal transmitters

Melatonin, the major hormone produced by the pineal gland, displays characteristic daily and seasonal patterns of secretion. These robust and predictable rhythms in circulating melatonin are strong synchronizers for the expression of numerous physiological processes in photoperiodic species. In mammals, the nighttime production of melatonin is mainly driven by the circadian clock, situated in the suprachiasmatic nucleus of the hypothalamus, which controls the release of norepinephrine from the dense pineal sympathetic afferents. The pivotal role of norepinephrine in the nocturnal stimulation of melatonin synthesis has been extensively dissected at the cellular and molecular levels. Besides the noradrenergic input, the presence of numerous other transmitters originating from various sources has been reported in the pineal gland. Many of these are neuropeptides and appear to contribute to the regulation of melatonin synthesis by modulating the effects of norepinephrine on pineal biochemistry. The aim of this review is firstly to update our knowledge of the cellular and molecular events underlying the noradrenergic control of melatonin synthesis; and secondly to gather together early and recent data on the effects of the nonadrenergic transmitters on modulation of melatonin synthesis. This information reveals the variety of inputs that can be integrated by the pineal gland; what elements are crucial to deliver the very precise timing information to the organism. This also clarifies the role of these various inputs in the seasonal variation of melatonin synthesis and their subsequent physiological function.

Melatonin enters the cerebrospinal fluid through the pineal recess

The pineal recess (PR), a third ventricle (IIIV) evagination penetrating into the pineal gland, could constitute a site of melatonin passage to the cerebrospinal fluid (CSF) and explain the high concentrations of melatonin in this fluid. To test this hypothesis, we characterized melatonin distribution in the IIIV of sheep by CSF collection in the ventral part of IIIV (vIIIV) and in PR. At 30 microl/min collection rate, melatonin concentrations were much higher in PR than in vIIIV (19,934 +/- 6,388 vs. 178 +/- 70 pg/ml, mean +/- SEM, respectively, P < 0.005), and they increased in vIIIV when CSF collection stopped in the PR (P < 0.05). At 6 microl/min, levels increased to 1,682 +/- 585 pg/ml in vIIIV and were not influenced by CSF collection in the PR. This concentration difference between sites and the influence of PR collection on vIIIV levels suggest that melatonin reaches the PR and then diffuses to the IIIV. To confirm the role of PR, we demonstrated that its surgical sealing off decreased IIIV melatonin levels (1,020 +/- 305 pg/ml, compared with 5,984 +/- 1,706 and 6,917 +/- 1,601 pg/ml in shams or animals with a failed sealing off, respectively, P < 0.01) without changes in blood levels. Therefore, this study identified the localization of the main site of penetration of melatonin into the CSF, the pineal recess.