The presence of opioidergic pinealocytes in the pineal gland of the European hamster (Cricetus cricetus): an immunocytochemical study

Melatonin and morphine: potential beneficial effects of co-use

Morphine is a potent analgesic agent used to control acute or chronic pain. Chronic administration of morphine results in analgesic tolerance, hyperalgesia, and other side effects including dependence, addiction, respiratory depression, and constipation, which limit its clinical usage. Therefore, identifying the new analgesics with fewer side effects which could increase the effect of morphine and reduce its side effects is crucial. Melatonin, a multifunctional molecule produced in the body, is known to play an important role in pain regulation. The strong anti-inflammatory effect of melatonin is suggested to be involved in the attenuation of the pain associated with inflammation. Melatonin also increases the anti-nociceptive actions of opioids, such as morphine, and reverses their tolerance through regulating several cellular signaling pathways. In this review, published articles evaluating the effect of the co-consumption of melatonin and morphine in different conditions were investigated. Our results show that melatonin has pain-killing properties when administered alone or in combination with other anti-nociceptive drugs. Melatonin decreases morphine consumption in different pathologies. Furthermore, attenuation of morphine intake can be accompanied by reduction of morphine-associated side-effects, including physical dependence, morphine tolerance, and morphine-related hyperalgesia. Therefore, it is reasonable to believe that the combination of melatonin with morphine could reduce morphine-induced tolerance and hyperalgesia, which may result from anti-inflammatory and antioxidant properties of melatonin. Overall, we underscore that, to further ameliorate patients' life quality and control their pain in various pathological conditions, melatonin deserves to be used with morphine by anesthesiologists in clinical practice.

Characterization of human pineal gland proteome

The pineal gland is a neuroendocrine gland located at the center of the brain. It is known to regulate various physiological functions in the body through secretion of the neurohormone melatonin. Comprehensive characterization of the human pineal gland proteome has not been undertaken to date. We employed a high-resolution mass spectrometry-based approach to characterize the proteome of the human pineal gland. A total of 5874 proteins were identified from the human pineal gland in this study. Of these, 5820 proteins were identified from the human pineal gland for the first time. Interestingly, 1136 proteins from the human pineal gland were found to contain a signal peptide domain, which indicates the secretory nature of these proteins. An unbiased global proteomic profile of this biomedically important organ should benefit molecular research to unravel the role of the pineal gland in neuropsychiatric and neurodegenerative diseases.

Developmental and diurnal expression of the synaptosomal-associated protein 25 (Snap25) in the rat pineal gland

Snap25 (synaptosomal-associated protein) is a 25 kDa protein, belonging to the SNARE-family (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) of proteins, essential for synaptic and secretory vesicle exocytosis. Snap25 has by immunohistochemistry been demonstrated in the rat pineal gland but the biological importance of this is unknown. In this study, we demonstrate a high expression of mRNA encoding Snap25 in all parts of the rat pineal complex, the superficial-, and deep-pineal gland, as well as in the pineal stalk. Snap25 showed a low pineal expression during embryonic stages with a strong increase in expression levels just after birth. The expression showed no day/night variations. Neither removal of the sympathetic input to the pineal gland by superior cervical ganglionectomy nor bilateral decentralization of the superior cervical ganglia significantly affected the expression of Snap25 in the gland. The pineal expression levels of Snap25 were not changed following intraperitoneal injection of isoproterenol. The strong expression of Snap25 in the pineal gland suggests the presence of secretory granules and microvesicles in the rat pinealocyte supporting the concept of a vesicular release. At the transcriptional level, this Snap25-based release mechanism does not exhibit any diurnal rhythmicity and is regulated independently of the sympathetic nervous input to the gland.

Melatonin: a hormone that modulates pain

AIMS

Melatonin is a hormone synthesized principally in the pineal gland that has been classically associated with endocrine actions. However, several lines of evidence suggest that melatonin plays a role in pain modulation. This paper reviews the available evidence on melatonin's analgesic effects in animals and human beings.

MAIN METHODS

A medline search was performed using the terms "melatonin", "inflammatory pain", "neuropathic pain", "functional pain", "rats", "mice", "human", "receptors", "opioid" and "free radicals" in combinations.

KEY FINDINGS

The antinociceptive effect of melatonin has been evaluated in diverse pain models, and several findings show that melatonin receptors modulate pain mechanisms as activation induces an antinociceptive effect at spinal and supraspinal levels under conditions of acute and inflammatory pain. More recently, melatonin induced-antinociception has been extended to neuropathic pain states. This effect agrees with the localization of melatonin receptors in thalamus, hypothalamus, dorsal horn of the spinal cord, spinal trigeminal tract, and trigeminal nucleus. The effects of melatonin result from activation of MT(1) and MT(2) melatonin receptors, which leads to reduced cyclic AMP formation and reduced nociception. In addition, melatonin is able to activate opioid receptors indirectly, to open several K(+) channels and to inhibit expression of 5-lipoxygenase and cyclooxygenase 2. This hormone also inhibits the production of pro-inflammatory cytokines, modulates GABA(A) receptor function and acts as a free radical scavenger.

SIGNIFICANCE

Melatonin receptors constitute attractive targets for developing analgesic drugs, and their activation may prove to be a useful strategy to generate analgesics with a novel mechanism of action.

Melatonin enhances the anticonvulsant and proconvulsant effects of morphine in mice: Role for nitric oxide signaling pathway

Melatonin has different interactions with opioids including enhancing their analgesic effect and reversal of opioid tolerance and dependence. Opioids are known to exert dose-dependent anti- and proconvulsant effects in different experimental seizure paradigms. This study investigated the effect of melatonin on biphasic modulation of seizure susceptibility by morphine, in mouse model of pentylenetetrazole (PTZ)-induced clonic seizures. We further investigated the involvement of the nitric oxidergic pathway in this interaction, using a nitric oxide synthase inhibitor, NG-nitro-L-arginine-methyl-ester (L-NAME). Melatonin exerted anticonvulsant effect with doses as high as 40-80 mg/kg, but with a dose far bellow that amount (10 mg/kg), it potentiated both the anticonvulsant and proconvulsant effects of morphine on the PTZ-induced clonic seizures. Possible pharmacokinetic interaction of melatonin and morphine cannot be ruled out in the enhancement of two opposing effects of morphine on seizure threshold. L-NAME (1 mg/kg) reversed the anticonvulsant property of the combination of melatonin (10 mg/kg) plus morphine (0.5 mg/kg). Moreover, L-NAME (5 mg/kg) blocked the enhancing effect of melatonin (10 mg/kg) on proconvulsant activity of morphine (60 mg/kg). Our results indicate that co-administration of melatonin enhances both anti- and proconvulsant effects of morphine via a mechanism that may involve the nitric oxidergic pathway.

Proteomic analysis of day–night variations in protein levels in the rat pineal gland

The pineal gland secretes the hormone melatonin. This secretion exhibits a circadian rhythm with a zenith during night and a nadir during day. We have performed proteome analysis of the superficial pineal gland in rats during daytime and nighttime. The proteins were extracted and subjected to 2-DE. Of 1747 protein spots revealed by electrophoresis, densitometric analysis showed the up-regulation of 25 proteins during nighttime and of 35 proteins during daytime. Thirty-seven of the proteins were identified by MALDI-TOF MS. The proteins up-regulated during the night are involved in the Krebs cycle, energy transduction, calcium binding, and intracellular transport. During the daytime, enzymes involved in glycolysis, electron transport, and also the Krebs cycle were up-regulated as well as proteins taking part in RNA binding and RNA processing. Our data show a prominent day-night variation of the protein levels in the rat pineal gland. Some proteins are up-regulated during the night concomitant with the melatonin secretion of the gland. Other proteins are up-regulated during the day indicating a pineal metabolism not related to the melatonin synthesis.

Immunocytochemical Characterization of Delta-Opioid and Mu-Opioid Receptor Protein in the Bovine Pineal Gland

Opioidergic innervation has been identified in the mammalian pineal gland. Recently, opioid receptors in bovine pineal glands have been characterized; the activation of these receptors leads to the stimulation of melatonin synthesis. In this study, the precise localization of opioid receptors in bovine pineal glands was determined by an immunohistochemical technique using antibodies raised against delta-opioid and mu-opioid receptors. Immunoreactivity of these two receptors was present at a moderate level in pinealocytes. A double-labeling study has shown that delta-opioid receptors are localized predominantly with mu-opioid receptors in the same pinealocytes. These immunopositive pinealocytes are often located in a group; however, some of them are dispersed individually. In addition, both types of receptors were found in glial cells and processes. A small number of delta-receptor-immunoreactive nerve fibers were observed in the perivascular space and intraparenchyma of the pineal gland. Mu-opioid receptor immunoreactivity was found in a number of nerve fibers throughout the gland, and in terminal-like dots on pinealocytes. There was immunocolocalization between delta-opioid receptors or mu-opioid receptors and leu-enkephalin in some nerve fibers. The results of this study indicate that the modulatory effect of the opioid system on melatonin secretion in pineal glands might act via opioid receptors on pinealocytes, whereas receptors located on nerve fibers might modulate the release of opioid peptides.

Survival mechanisms in a physiological oxidative stress model

The Syrian hamster Harderian gland has as the remarkable feature of an extraordinary rate of porphyrin production, even higher than the liver. The low activity of the last enzyme of the route gives rise to the accumulation of the uncomplex porphyrins in the female glands. Moreover, due to the localization of the Harderian gland, porphyrins exposed to light produce reactive oxygen species and, thus, the gland presents a physiological oxidative stress, with a great number of sings of degeneration, but without compromising the gland integrity. The appearance of abnormal features in this gland was largely described in the past, but the significance is interpreted for the first time in this study. We have found that autophagic processes are the first result of an elevated porphyrin metabolism, as it is observed in both sexes. This mechanism is considered, in this case, as a constant renovation system that allows the normal gland activity to be sustained. Furthermore, there is a second procedure, invasive processes toward connective tissue, which even occasionally reach blood vessels with intravasation of damaged gland components into the bloodstream. This effect is a consequence of a strong oxidative stress environment that is mainly observed in the female gland, resembling to tumoral progression. Both mechanisms, autophagy and invasive processes, have to be implied in the maintenance of the gland integrity.

Melatonin exerts its analgesic actions not by binding to opioid receptor subtypes but by increasing the release of beta-endorphin an endogenous opioid

The occurrence of systematic diurnal variations in pain thresholds has been demonstrated in human. Salivary melatonin levels change following acute pain when other factors that could explain the change have been removed or controlled. Melatonin-induced analgesia is blocked by naloxone or pinealectomy. By using selective radioligands [3H]-DAMGO, [3H]-DPDPE, [3-U69593, and 3H]-nociceptin, we have shown that the bovine pinealocytes contain delta and mu, but not kappa or ORL1 opioid receptor subtypes. In the present study, by using melatonin receptor agonists (6-chloromelatonin or 2-iodo-N-butanoyl-5-methoxytryptamine) or melatonin receptor antagonist (2-phenylmelatonin), we have shown that these agents do not compete with opioid receptor subtypes. However, we observed a time-dependent release of beta-endorphin an endogenous opioid peptide, by melatonin from mouse pituitary cells in culture. Hence, it is suggested that melatonin exerts its analgesic actions not by binding to opioid receptor subtypes but by binding to its own receptors and increasing the release of beta-endorphin.

The stimulatory effect of mu- and delta-opioid receptors on bovine pinealocyte melatonin synthesis

Mammalian pinealocytes synthesize and secrete melatonin. The synthesis of melatonin is regulated by several biogenic amine, amino acid and peptide transmitters. In our previous study, the delta- and mu-opioid receptors have been identified and characterized in bovine pinealocytes. In order to elaborate the function of different types of opioid receptors in regulating melatonin synthesis, we used a selective mu-opioid receptor agonist, Tyr-[D-Ala(2), N-methyl-phe(4), glycol(5)] (DAMGO), a selective delta-opioid receptor agonist, Enkephalin [D-Pen(2), D-Pen(5)], (DPDPE) and a selective kappa-opioid receptor agonist, ((+)-(5alpha, 7alpha, 8beta)-N-methyl-N-[7- (1-pyrrolidinyl)-1-oxaspiro [4,5] dec-8-yl]-benzene acetamide) (U69593) to investigate the activity of N-acetyltransferase (NAT) activity and melatonin secretion. The results of the present study show that both DAMGO and DPDPE stimulated NAT activity and increased the level of melatonin in cultured bovine pinealocytes. These stimulatory effects were blocked by naloxone, an opioid receptor antagonist. However, the kappa-opioid receptor agonist U69593 was unable to alter either the activity of NAT or the level of melatonin. In order to clarify the mechanism of how the activation of mu- and delta-opioid receptors in bovine pinealocytes leads to an increase in NAT activity, cyclic AMP levels were measured after bovine pinealocytes were treated with morphine, DAMGO and DPDPE. The results indicated that these stimulatory effects acted via induction of cAMP production. This study reveals that the stimulatory effect of opioid receptor on melatonin synthesis is mediated via the activation of adenylate cyclase system.

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.

The presence of delta and mu-, but not kappa or ORL(1) receptors in bovine pinealocytes

Physicians have noted since antiquity that their patients complained of less pain and required fewer analgesics at night-time. In humans, the circulating levels of melatonin, a pineal substance with analgesic and hypnotic properties, exhibit a pronounced circadian rhythm with serum levels being high at night and low during day-time. Moreover, pinealectomy abolishes the analgesic effects of melatonin, and naloxone disrupts the day-night rhythm of nociception. In this study, we have attempted to identify and characterize the nature and types of opioid receptor in bovine pinealocyte membranes, using a radioligand binding technique with the selective radioligands [3H]DAMGO, [3H]DPDPE, [3H]U69593 and [3H]orphanin-FQ (OFQ) for identifying mu (mu)-, delta (delta)-, kappa (kappa)- and opioid receptor-like (ORL(1)) receptors, respectively. The saturation experiments on bovine pinealocyte membranes for [3H]DPDPE binding provided B(max) and K(d) values of 553+/-24 fmol/mg protein and 1.3+/-0.6 nM; and for [3H]DAMGO binding provided B(max) and K(d) values of 6.3+/-1.3 fmol/mg protein and 1.2+/-0.4 nM, respectively. On the other hand, the specific radioligands ([3H]U69593 and [3H]OFQ) binding of kappa and ORL(1) receptors were undetectable in bovine pinealocyte membranes. Furthermore, competitive experiments with opioid agonist and antagonist and related compounds confirmed the presence of mu- and delta-opioid binding sites in bovine pinealocyte membranes. These results indicate that neither kappa nor ORL(1) receptors are present on the pinealocytes, and the majority of opioid receptors found in the bovine pineal gland are delta (possibly, both delta(1) and delta(2)) types, with a minority being mu type, and that both are primarily located on the bovine pinealocyte membranes. These opioid receptors, by stimulating the activity of N-acetyltransferase, enhance the synthesis of melatonin.

Somatostatin and somatostatin receptors in the pig pineal gland during postnatal development: an immunocytochemical study

An immunohistochemical study of the pineal gland of the domestic pig was carried out using rabbit antisera raised against synthetic peptide fragments corresponding to different amino acid sequences of the prosomatostatin, the somatostatin-14, and the somatostatin-28 molecule. The study was supplemented by immunohistochemical staining with rabbit antisera raised against five subtypes of somatostatin receptors. The pineal glands were taken from the newborn, 21-day-old and 7-month-old pigs. Immunoreactive nerve fibers and cells were observed in the pineal gland with all the antisera against somatostatin and prosomatostatin. The nerve fibers were located throughout the pineal gland-in the capsule, connective septa, and parenchyma-with the highest density in proximo-ventral part of the gland. The somatostatin positive fibers were also found in the habenular and posterior commissurae areas. Somatostatin-immunoreactive cell bodies were observed mostly in the central part of the gland. These results point to the existence of two somatostatin sources in the pig pineal gland: 1) nerve fibers, probably of central origin; and 2) cells that may represent intrapineal neurons or specialised pinealocytes. A clear difference in the immunoreactivity between newborn, 21-day-old, and 7-month-old pigs was found. Generally, the density of nerve fibers was lower in adult than young animals. The number of the cells also decreased with age. By using the antisera against the five somatostatin receptors, only sst3 - receptor immunoreactivity could be detected. The receptor-immunoreactivity was confined to varicose and smooth fibers and some cells. The sst(3)-receptor positive structures were localised in all parts of the gland and their number was higher in younger pigs.

Gene expressions of opioid receptors and G-proteins in pineal glands

In our previous studies, the opioid receptors located on pinealocytes have been identified and characterized, and these receptors have been found to play a stimulatory role in melatonin synthesis by activating the rate limiting enzyme, N-acetyltransferase (NAT). In the present study, by using reverse transcriptase polymerase chain reaction (RT-PCR) followed by nested-PCR, segments of delta and mu opioid receptors have been amplified from mRNA of rat pineal gland and cerebral cortex. In addition, segments of delta and mu opioid receptors have also been amplified from mRNA of human pineal gland. Furthermore, G(alphai/o)- and G(beta)-protein-coupled receptor mRNAs have been amplified and identified from rat pineal gland. The regulatory effects of morphine on G(alphai/o) and G(beta) mRNA levels have been semiquantitatively analyzed. Acute morphine administration caused significant increase in G(alphai/o), and G(beta), mRNA levels in rat pineal gland, but not in other brain regions. Further studies are needed in order to elaborate the mechanisms of these opioid receptors in regulating G-protein expression in pineal gland.

Cholinergic innervation and function in the mammalian pineal gland

Besides the noradrenergic sympathetic system originating from the superior cervical ganglion, a cholinergic innervation of the mammalian pineal gland has been studied over the past three decades. In 1961, it was shown that lesion of the parasympathetic greater superficial petrosal nerve of the monkey resulted in degeneration of nerve fibers in the pineal gland. This was supported by ultrastructural studies of nerve terminals within the pineal gland, demonstrating the presence of cholinergic terminals containing small clear transmitter vesicles. Biochemical studies further showed the presence of the enzyme acetylcholinesterase in several mammalian species. During the last decade, several advanced and more elaborate technologies have been developed, allowing pinealogists to establish the presence of cholinergic fibers and their receptors. Thus, choline acetyltransferase was shown in bovine pineal by immunohistochemistry. Muscarinic and nicotinic receptors were identified, characterized, and localized. Gene expression of receptors was visualized, and the receptor-mediated effector systems and functions were elucidated. Taken together, the present data suggest the presence of a cholinergic innervation of the mammalian pineal gland originating in peripheral parasympathetic ganglia. However, some of the neuronal projections to the pineal gland with origin in the brain (the central innervation) might also be cholinergic. The cholinergic nerve fibers enter the gland, where they are located both in the perivascular spaces and between the pinealocytes. Some of the terminals make synapses on pinealocytes or intrapineal neurons. The released acetylcholine from the terminals interacts with the receptors, then alters the cascade of receptor-mediated events, which results in decreased N-acetyltransferase enzyme activity, thus leading to decreased melatonin synthesis. This counterbalance mechanism between the sympathetic noradrenergic and the cholinergic systems maintains the homeostasis of pineal functions.

Stress-reactive response of the gerbil pineal gland: concretion genesis

The reaction of pinealocytes and glia cells to an acute immobilization stress and their poststress recovery was studied in gerbils. Pinealocytes responded to immobilization with an increased peptidergic activity and formation of new concretions, whereas glia cells with an increased growth of interstitial concretions. The occurrence of degenerating pinealocytes indicated deleterious actions of immobilization stress on functionally stimulated cells. The pyroantimonate method to detect Ca2+ demonstrated enlarged crystalline profiles (Ca2+ crystallization into hydroxyapatite) in functionally stimulated pinealocytes and the accumulation of Ca2+ in the interstitial concretion. The pinealocyte concretions did not show the Ca2+ accumulation. The pineal gland poststress recovery was manifested by a reduced functionally stimulated pinealocyte activity and a protracted increase in glia cell activity. It is suggested that the physiological relevance of the crystallization of Ca2+ into hydroxyapatite is to maintain a noradrenalin-stimulated Ca2+ influx at an optimal level during attentuated pinealocyte turnover. The interstitial concretions may lower the extracellular Ca2+ concentrations and thereby stimulate pinealocytes and restrict an increased Ca2+ influx.

The chemical neuroanatomy of the mammalian pineal gland: neuropeptides

The mammalian pineal gland contains multiple afferent peptidergic nerve fibres. Sympathetic nerve fibres, with their origin in the superior cervical ganglia, contain neuropeptide Y colocalized with norepinephrine. Other pinealopetal nerve fibres, probably originating in the pterygopalatine ganglion, contain vasoactive intestinal peptide and peptide histidine isoleucine. Fibres containing substance P and calcitonin gene-related peptide have also been demonstrated in pinealopetal nerve fibres. These fibres might originate in the trigeminal ganglion. The neurotransmitter content of the fibres of the central innervation, innervating the gland from the brain via the pineal stalk, has not been elucidated. However, strong indications for the presence of neuropeptide Y, substance P, somatostatin, and vasopressin in these fibres have been presented. Recent immunohistochemical studies have further shown the presence of subtypes of pinealocytes containing neuropeptides. Thus, pinealocytes containing beta-endorphin, leu-enkephalin, and somatostatin have been demonstrated in the gland. Immunohistochemistry at the electron microscopical level has shown, that in some species, leu-enkephalin containing pinealocytes make synaptic contacts with other pinealocytes indicating of paracrine regulation of the pineal gland. It must however be emphasized that large interspecies variations exist with regard to the peptidergic pineal innervation and its content of peptidergic cells.