Effects of steroids on beta-adrenergic binding sites in sheep pineal glands

As an initial step in investigations of putative differences between central nervous system light-sensitive mechanisms in seasonally shedding and non-shedding breeds of sheep, some beta-adrenoceptor characteristics of Merino sheep pineal glands were determined, using [3H]dihydroalprenolol as the labelled ligand. Overall, a dissociation constant of 17.2 +/- 2.6 nmoles/l and a daytime beta-receptor density of 1.6 +/- 0.3 pmoles/mg were determined at 37 degrees. The binding sites exhibited stereospecificity, saturability and apparent homogeneity. 17 beta-Estradiol and progesterone implants that provided hormone concentrations in the physiological range had no significant effect on pineal beta-receptors in male sheep castrated shortly after birth. Dexamethasone injections, on the other hand, in doses sufficient to loosen the attachment of wool fibres to the skin, resulted in decreased pineal beta-receptor density and increased receptor affinity for dihydroalprenolol. This effect was apparently not mediated by altered plasma catecholamine concentrations, since the glucocorticoid treatment did not affect jugular venous noradrenaline, adrenaline or dopamine levels. The possible involvement of glucocorticoids in the regulation of wool growth could thus have a central neuronal component, medicated via action on pineal beta-adrenoceptors in sheep; however, the existence of the putative gonadal steroid feedback on beta-adrenoceptor-mediated pineal function remains to be demonstrated in this species.     

Effect of venlafaxine on pineal melatonin and noradrenaline in the male rat

Studies in vitro indicate that the antidepressant drug, venlafaxine (VEN), inhibits the reuptake of both serotonin (5-hydroxytryptamine, 5-HT) and noradrenaline (NA) but has little activity on other neurotransmitter receptors. There are, however, few studies on the effects of VEN on monoamine neurotransmission in vivo. In the present study we examined the effect of VEN treatment on the melatonin content of the rat pineal gland because the synthesis of melatonin is regulated by the release of NA onto pinealocyte beta-adrenoceptors. Acute treatment with higher doses (15 mg/kg) of VEN significantly increased pineal melatonin and NA but this effect was attenuated by subchronic treatment. These data are consistent with in vitro data suggesting that VEN increases NA neurotransmission at higher doses and that repeated treatment can desensitize pinealocyte beta-adrenoceptors.     

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.     

Neuropharmacology of pineal secretions

A connection between pineal function and several psychiatric diseases has been shown recently. The diurnal and seasonal rhythmicity of melatonin production is associated with affective disorders and several types of endogenous depression. The cortisolmelatonin ratio was significantly higher in depressed individuals than in healthy controls. Alterations in melatonin secretion may also occur in non-affective psychiatric disorders, such as chronic schizophrenia. Antidepressants and other psychotropic drugs modify melatonin synthesis. In rodents, monoamine oxidase (MAO) inhibitors (e.g. pheniprazine, harmine or nialamide) increase pineal concentrations of the melatonin precursors, serotonin (5HT) and N-acetyl serotonin (NAS), by enhancing N-acetyl transferase activity. These drugs also increase melatonin, 5HT and NAS in the cerebrospinal fluid. Chronically administered tricyclic antidepressants reduce pineal and serum melatonin content in rodents. In humans, both the MAO-A selective inhibitor clorgyline and the non-selective inhibitor tranylcypromine increase serum melatonin levels. In contrast, serum melatonin remains unaltered by the MAO-B selective inhibitor L-deprenyl. The actions of other drugs on melatonin production, including lithium, propranolol, amphetamine and several monoamine precursors, are in accordance with their psychotropic effects and with their effect on monoamine functions.     

Subsensitive melatonin suppression by dim white light: possible biological marker of panic disorder

Light is involved in providing entrainment of circadian rhythms and the suppression of the pineal hormone melatonin. In patients with affective disorders, there have been indications of circadian as well as seasonal variation in illness, which may be reflected in melatonin production. Varying sensitivity to light has been noted within healthy individuals as well as in some patients with affective disorders. Recent evidence suggests that patients with panic disorder may have an altered and phase-delayed melatonin rhythm. The present study examined the nocturnal plasma melatonin rhythm in patients with panic disorder, and also examined their melatonin sensitivity to dim light. The melatonin rhythm was examined in 6 patients with panic disorder and 8 controls. The melatonin sensitivity to dim white light (200 lx) was examined in 8 patients with panic disorder and 63 controls and was compared to that of a group of 7 patients with other anxiety disorders. Patients with panic disorder demonstrated a trend towards higher and delayed peak melatonin levels compared to controls. Patients with panic disorder also had a subsensitive melatonin suppression by dim white light, compared to controls and patients with other anxiety disorders (p<0.005). The phase-delayed circadian rhythm observed in patients with panic disorder may be secondary to the subsensitivity of the melatonin response to light. It is hypothesized that the subsensitivity may be due to abnormal neurotransmitter/receptor systems involved in regulation of melatonin suppression and circadian rhythmicity, and may lead to phase- delayed circadian rhythms. The melatonin subsensitivity to light may be used as a biological marker of panic disorder.     

Chronopharmacological actions of the pineal gland

In all mammalian species studied to date, the pineal gland shows a pronounced circadian rhythm with high activity at night and very low activity in daytime, as reflected in the output of its hormone, melatonin. The pineal is one of the few organs which can synthesize melatonin and, under normal circumstances, is virtually the only source of circulating melatonin. Pineal activity is tightly controlled by the light/dark cycle, in such a manner that melatonin can convey information to the organism on the length of the dark phase of the photoperiod. In seasonal breeding species, the 24 hour pattern of secretion of melatonin shows seasonal changes which are crucial for determining the timing of seasonal changes in bodily function. In man, alterations in pineal function are of significance in jet-lag, shift work and in affective disorder.     

Comparative experimental study of the effect of melatonin and 5-methoxytryptamine on the thyroid gland of rats

The authors studied the reactivity of the pituitary-thyroid system as influenced by melatonin and mexamine at varying times of photoperiod. It is concluded that the drugs under study had uncertain effects on rat thyroid function. It is assumed that different components of the hypothalamus-pituitary-thyroid system experience seasonal alterations in the sensitivity to the action of pineal gland methoxyindoles.     

Melatoninergic receptor agonists and antagonists: pharmacological aspects and therapeutic perspective

Melatonin, or N-acetyl 5-methoxytryptamine, a neurohormone produced in the pineal gland during periods of darkness, plays a key role in the regulation of circadian and seasonal biological rhythms. In mammals, specific MT1 and MT2 receptors are located in the central nervous system, mainly in suprachiasmatic nuclei, and also in a number of peripheral sites. Besides its chronobiotic action on light-dependant functions, such as sleep/waking alternance or seasonal depression, melatonin exerts modulatory effects on immune, endocrine and metabolic functions. However, its short half-life and extensive metabolism lead to a poor bioavailability. This prompted to search for metabolically stable analogs displaying new and innovative properties. The S 20098 compound, a melatoninergic agonist, has proven potent antidepressive and anxiolytic actions. The S 20928 compound, a melatonin antagonist, was shown to enhance basal metabolism and reduce weight gain. Thus, both of these melatonin derivatives open perspectives for the development of innovative therapeutic agents in the fields of depression and obesity.     

5-Methoxyindoles in pineal gland of cow,pig, sheep and rat

Summary The occurrence and concentration of the four 5-methoxyindoles: 5-methoxytryptamine, 5-methoxytryptophol, 5-methoxyindole-3-acetic acid and melatonin in the pineal gland of pig, cow, sheep and rat was investigated. The analytical method involved the use of deuterated analogues as internal standards and capillary column gas chromatography — mass spectrometry. The analyses of pineal glands obtained during the morning hours revealed the presence of 5-methoxyindole-3-acetic acid and melatonin in nmoles/g and 5-methoxytryptophol and 5-methoxytryptamine in pmoles/g amounts in pig, cow and sheep. In Wistar and Sprague-Dawley strain of rat, melatonin was present at a concentration of about 0.5 pmoles/pineal, a level which was elevated more than five times by treatment with a monoamine oxidase inhibitor. 5-methoxytryptamine was found at a concentration of about 0.03 pmoles/pineal, and was elevated by monoamine oxidase inhibition.     

Potential therapeutic use of melatonin in migraine and other headache disorders

There is increasing evidence that headache disorders are connected with melatonin secretion and pineal function. Some headaches have a clearcut seasonal and circadian pattern, such as cluster and hypnic headaches. Melatonin levels have been found to be decreased in both migraine and cluster headaches. Melatonin mechanisms are related to headache pathophysiology in many ways, including its anti-inflammatory effect, toxic free radical scavenging, reduction of pro-inflammatory cytokine upregulation, nitric oxide synthase activity and dopamine release inhibition, membrane stabilisation, GABA and opioid analgesia potentitation, glutamate neurotoxicity protection, neurovascular regulation, 5-HT modulation and the similarity in chemical structure to indometacin. The treatment of headache disorders with melatonin and other chronobiotic agents, such as melatonin agonists (ramelteon and agomelatin), is promising and there is a great potential for their use in headache treatment.     

Potential therapeutic use of melatonin in migraine and other headache disorders

There is increasing evidence that headache disorders are connected with melatonin secretion and pineal function. Some headaches have a clearcut seasonal and circadian pattern, such as cluster and hypnic headaches. Melatonin levels have been found to be decreased in both migraine and cluster headaches. Melatonin mechanisms are related to headache pathophysiology in many ways, including its anti-inflammatory effect, toxic free radical scavenging, reduction of pro-inflammatory cytokine upregulation, nitric oxide synthase activity and dopamine release inhibition, membrane stabilisation, GABA and opioid analgesia potentitation, glutamate neurotoxicity protection, neurovascular regulation, 5-HT modulation and the similarity in chemical structure to indometacin. The treatment of headache disorders with melatonin and other chronobiotic agents, such as melatonin agonists (ramelteon and agomelatin), is promising and there is a great potential for their use in headache treatment.     

Antiinflammatory activity of melatonin in central nervous system

Melatonin is mainly produced in the mammalian pineal gland during the dark phase. Its secretion from the pineal gland has been classically associated with circadian and circanual rhythm regulation. However, melatonin production is not confined exclusively to the pineal gland, but other tissues including retina, Harderian glands, gut, ovary, testes, bone marrow and lens also produce it. Several studies have shown that melatonin reduces chronic and acute inflammation. The immunomodulatory properties of melatonin are well known; it acts on the immune system by regulating cytokine production of immunocompetent cells. Experimental and clinical data showing that melatonin reduces adhesion molecules and pro-inflammatory cytokines and modifies serum inflammatory parameters. As a consequence, melatonin improves the clinical course of illnesses which have an inflammatory etiology. Moreover, experimental evidence supports its actions as a direct and indirect antioxidant, scavenging free radicals, stimulating antioxidant enzymes, enhancing the activities of other antioxidants or protecting other antioxidant enzymes from oxidative damage. Several encouraging clinical studies suggest that melatonin is a neuroprotective molecule in neurodegenerative disorders where brain oxidative damage has been implicated as a common link. In this review, the authors examine the effect of melatonin on several neurological diseases with inflammatory components, including dementia, Alzheimer disease, Parkinson disease, multiple sclerosis, stroke, and brain ischemia/reperfusion but also in traumatic CNS injuries (traumatic brain and spinal cord injury).     

Melatonin sensitivity to dim white light in affective disorders.

Both dim and bright light has been shown to suppress the nocturnal secretion of the pineal hormone melatonin. Early reports suggests that an abnormal response to light occurs in patients with bipolar affective disorder, where as patients with major depressive disorder respond similarly to controls. It has been suggested that this abnormal sensitivity of the melatonin response to light could be a trait marker of bipolar affective disorder. However reports lack consistency. Hence, we investigated the melatonin suppression by dim light (200 lux) in patients with bipolar affective disorder, seasonal affective disorder and major depressive disorder. Results suggest that a supersensitive melatonin suppression to light in bipolar affective disorder (p < .005), and seasonal affective disorder (p < .05), whereas patients with major depressive disorder display similar suppression to controls. The supersensitivity may be a mechanism where by phase-delayed rhythms, are resynchronised to a new circadian position. Conversely, an abnormality may exist in the pathway from the retina to the suprachiamatic nucleus.     

The effect of chronic antidepressant administration on beta-adrenoceptor function of the rat pineal

1 The β-adrenoceptor agonist, isoprenaline (1.5-3.0 mg kg^-1 intravenously), produced a dose-related increase in rat pineal melatonin content. This increase was prevented by pretreatment with the selective β₁-adrenoceptor antagonist, atenolol (2 mg kg^-1), but not by the β₂-adrenoceptor antagonist, butoxamine (2 mg kg^-1). The β₂-adrenoceptor agonist, terbutaline (5.0 mg kg^-1), produced a moderate increase in pineal melatonin content.

2 Repeated daily administration of desmethylimipramine (10 mg kg^-1 for 10 days) and maprotiline (10 mg kg^-1 for 10 days), antidepressants predominantly inhibiting noradrenaline (NA) uptake, reduced the isoprenaline-induced increase in pineal melatonin content. Amitriptyline (20 mg kg^-1 for 14 days), a drug which inhibits both NA and 5-hydroxytryptamine (5-HT) uptake, had a similar effect. The β-adrenoceptor agonist, clenbuterol (5 mg kg^-1 for 14 days), also attenuated the increase in pineal melatonin produced by isoprenaline.

3 In contrast, chronic administration of the selective 5-HT uptake inhibitor, fluoxetine (10 mg kg^-1 for 10 days), or the antidepressants, iprindole and mianserin (both 20 mg kg^-1 for 14 days), which do not inhibit monoamine uptake, failed to reduce the increase in pineal melatonin following isoprenaline. Repeated electroconvulsive shock was similarly without effect.

4 Ten hours after the final dose of desmethylimipramine (10 mg kg^-1) once daily for 10 days there was no change in the usual dark phase increase in pineal melatonin.

5 The data suggest that repeated administration of certain antidepressant drugs results in reduced pineal β-adrenoceptor sensitivity. However the lack of change in the dark phase increase in pineal melatonin following repeated desmethylimipramine, implies that the reduced ß-adrenoceptor sensitivity may be part of an adaptive process which maintains normal pineal function. Therefore the decrease in β-adrenoceptor number in the brain reported after chronic antidepressant administration may not be associated with a change in overall synaptic function.

     

Effects of P-chlorophenylalanine on pineal and endocrine function in the rat

This study attempted to determine whether brain serotonin (5-HT), which is altered by melatonin administration, is involved in mediating the effects of melatonin on basal endocrine function. Pineal melatonin levels, serum N-acetylserotonin (NAS) levels, adrenocortical activity, and other endocrine parameters were measured following 5-HT depletion by p-chlorophenylalanine (p-CPA) together with either pineal stimulation by blinding or blinding plus pinealectomy. Blinding increased pineal melatonin levels in both saline and p-CPA treated animals. P-CPA treatment increased adrenal weights and morning plasma corticosterone levels in both blinded and blinded-pinealectomized animals. Conversely, p-CPA depressed pineal melatonin levels and serum NAS but elevated morning plasma corticosterone levels in sighted controls. P-CPA also decreased plasma prolactin and growth hormone levels in intact animals. These findings suggest that 5-HT inhibits morning corticosterone secretion and stimulates prolactin and growth hormone release. In addition, melatonin and serotonin may function independently in regulating adrenocortical function, while melatonin's effect is superceded by that of serotonin.     

Effect of psychotropic drugs on the contents of melatonin, serotonin and N-acetylserotonin in rat pineal gland

In the dark phase, the effects of the psychotropic drugs on the contents of melatonin, serotonin (5-HT) and N-acetylserotonin (NAS) in rat pineal gland were examined. The pineal gland was removed at a certain period of time after subcutaneous injection of the drugs. 5-HT, NAS and melatonin contents in the pineal gland were determined by high performance liquid chromatography with fluorometric detection. A dose-dependent decrease was observed for melatonin content in the administration of diazepam (DZP), hydroxyzine (HYZ), chlorpromazine (CPZ) or haloperidol (HPD). When imipramine (IPM) or amitriptyline (APL) was given to rats, pineal 5-HT content was significantly decreased. On the other hand, the administration of IPM or APL caused increases in pineal NAS and melatonin. Furthermore, the administration of phenytoin (PYT) revealed no changes in the content of pineal indoleamines. These results suggest that the psychotropic drugs widely used in clinical applications could cause significant changes in pineal indoleamine content.     

Scotophobin A causes dark avoidance in goldfish by elevating pineal N-acetylserotonin

We had shown that synthetic rat scotophobin A caused several effects upon goldfish, apparently mediated by the pineal gland. Here we report that norepinephrine decreased goldfish dark avoidance in a manner that was blocked by scotophobin or pinealectomy. Increased dark avoidance was caused by either propranolol or scotophobin alone. Certain components of the pineal melatonin pathway also affected goldfish light-dark preference: serotonin, and especially N-acetylserotonin, increased dark avoidance, as did the hydroxyindole-O-methyl-transferase (HIOMT) product inhibitor, S-adenosyl-homocysteine. Melatonin and S-adenosyl-methionine were without effect in this regard. Pinealectomy prevented the dark avoidance increase caused by serotonin and N-acetylserotonin. These data suggested that increased dark avoidance behavior in goldfish was correlated with N-acetylserotonin buildup in the pineal, and that scotophobin could cause this, if it were to inhibit pineal HIOMT. To test this hypothesis the effect of various agents upon pineal melatonin levels was determined. Scotophobin was found to both reduce pineal melatonin and to block the melatonin-increasing effect of N-acetylserotonin. This led to the discovery that, indeed, scotophobin was an effective inhibitor (KI50, 6 x 10(-7) M) of purified bovine HIOMT.     

The effect of age and pre-light melatonin concentration on the melatonin sensitivity to dim light.

The hormone melatonin is secreted at night from the pineal gland, with light being a potent inhibitor of its secretion. Age related decreases in plasma melatonin concentrations have indicated that this may be related to pineal calcification with aging. Recently, it was shown that the melatonin sensitivity to light may be a biological marker of bipolar disorder. However, on average, patients were older than the control group in most studies, and it is not known if age has an effect on the melatonin suppression by light. To test this hypothesis, the present study investigated the effect of age on the melatonin sensitivity to dim light (200 lux). Participants were grouped into three age groups. On the testing night, they were placed in a dark room from 21.00 h to 02.30 h. Light exposure was for an hour from midnight to 01.00 h. Blood samples were collected at regular intervals for measurement of plasma melatonin. No significant differences were found in the percentage suppression of melatonin within the age groups defined in the present study (P > 0.5). No correlation was also found between age and percentage suppression of melatonin (r2 = 0.007; P > 0.1). Our results suggest that the melatonin suppression by light (200 lux) is not affected by age.     

Chronic diazepam administration differentially affects melatonin synthesis in rat pineal and Harderian glands

RATIONALE: Pineal and Harderian gland melatonin production as well as plasma melatonin levels were investigated in male Wistar rats (12 weeks old) after administration of diazepam, a benzodiazepine widely used as anxiolytic. OBJECTIVE: The present study investigates the effects of a chronic administration of diazepam on pineal and Harderian gland melatonin contents. METHODS: Diazepam was administered subcutaneously, for 21 days, at a dosage of 3 mg/kg body weight per day, 1 h before the onset of darkness. RESULTS: Diazepam clearly affected pineal melatonin biosynthesis and plasma melatonin levels. Diazepam reduced the pineal melatonin content (by a factor of 2) and the activity of N-acetyltransferase (NAT) (by a factor of 3.5), as well as plasma melatonin levels (by a factor of 1.5), but had no effects on pineal hydroxyindole-O-methyltransferase activity. By contrast to the pineal gland, diazepam failed to affect the Harderian gland melatonin content. CONCLUSIONS: Our results suggest that the inhibition of melatonin production induced by diazepam in vivo may be due to a direct action of this benzodiazepine on the pineal gland, through its action on NAT, the key enzyme of melatonin synthesis, and that the control of melatonin production in the Harderian glands may be different from that observed in the pineal gland.     

Effect of sodium valproate on nocturnal melatonin sensitivity to light in healthy volunteers.

Sensitivity of the pineal hormone melatonin to bright light at night has been proposed as a putative marker of bipolar affective disorder. Patients with bipolar disorder have a super-sensitive melatonin response to light. No studies have investigated whether super-sensitivity is due to agents used to treat the illness or is associated with the disorder per se. We investigated the effect of valproate on this phenomenon. Melatonin sensitivity to light was determined on two nights in 12 healthy volunteers (5M, 7F). Between testing nights participants received 200 mg of valproate b.d. for 5 days. Valproate significantly decreased the sensitivity of melatonin to light. On the other hand, valproate had no effect on overall melatonin secretion or dim light melatonin onset. The ability of valproate to decrease the sensitivity of melatonin to light may relate to its therapeutic effect in bipolar disorder--an ability to lengthen circadian period similar to that of lithium.