510 Degeneration and repair
Distinct effects of the serotonin–noradrenaline reuptake inhibitors milnacipran and venlafaxine on rat pineal monoamines Katsumasa Muneokaa, Makiko Kuwagatab, Tetsuo Ogawac and Seiji Shiodaa Monoamine systems are involved in the pathology and therapeutic mechanism of depression. The pineal gland contains large amounts of serotonin as a precursor for melatonin, and its activity is controlled by noradrenergic sympathetic nerves. Pineal diurnal activity and its release of melatonin are relevant to aberrant states observed in depression. We investigated the effects on pineal monoamines of serotonin–noradrenaline reuptake inhibitors, which are widely used antidepressants. Four days of milnacipran treatment led to an increase in noradrenaline and serotonin levels, whereas 4 days of venlafaxine treatment reduced 5-hydroxyindoleacetic acid levels; both agents induced an increase in dopamine levels. Our data suggest that milnacipran increases levels of the precursor for melatonin synthesis by facilitating the noradrenergic regulation of pineal activity and that venlafaxine inhibits
Introduction The pineal gland contains large amounts of serotonin that serve as a precursor for melatonin synthesis; diurnal changes in the production of melatonin are controlled by the noradrenergic sympathetic nerves that innervate the pineal gland (reviewed in Simonneaux and Ribelayga [1]). In addition, serotonin release from pinealocytes influences melatonin synthesis/release through autocrine/paracrine systems [2]. Depression is a common human disease in that is believed to involve monoamines such as serotonin, noradrenaline, and dopamine; the majority of pharmacological treatments for depression involve serotoninmediating and/or noradrenaline-mediating agents. Altered circulating melatonin levels have been reported in depressive patients [3]. Both melatonin (when used in combination with buspirone [4]) and melatonin receptor agonists [5] exert antidepressant actions. In addition, the pineal function that produces melatonin is related to many physiological states, especially sleep [1], metabolic activity [6], and pain perception [7], which are frequently disturbed in depressive patients. Hence, knowledge of the changes in monoamine metabolism in the pineal gland during antidepressive treatment is critical to understanding the physiological conditions under treatment as well as the therapeutic mechanism. In this study, we examined the effects of two serotonin–noradrenaline reuptake inhibitors (SNRIs), milnaciplan (MIL), and venlafaxine (VENL), on the pineal contents of noradrenaline, dopamine, serotonin, and 5-hydroxyindoleacetic acid (5-HIAA). MIL is a noradrenaline reuptake
serotonin reuptake into noradrenergic terminals on the pineal gland. NeuroReport 26:510–514 Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved. NeuroReport 2015, 26:510–514 Keywords: dopamine, milnacipran, noradrenaline, pineal gland, serotonin, venlafaxine a
Department of Anatomy I, Showa University School of Medicine, Tokyo, Laboratory of Pathology, Food and Drug Safety Center, Division of Toxicology, Hatano Research Institute, Kanagawa and cDepartment of Physiology, Saitama Medical University, Saitama, Japan b
Correspondence to Katsumasa Muneoka, MD, PhD, Department of Biochemistry, Showa University School of Medicine, 1-5-8 Hatanodai, Shinagawa-Ku, Tokyo 142-8555, Japan Tel: + 81 3 3784 8116; fax: + 81 3 3784 2346; e-mail: [email protected] Received 18 March 2015 accepted 17 April 2015
inhibition-dominant SNRI whereas VENL is a serotonin reuptake inhibition-dominant SNRI [8].
Materials and methods Animals and treatment
Six-week-old male Sprague–Dawley rats were purchased from JAPAN Clea Inc. (Tokyo, Japan). Animals were acclimatized in the animal facility for 1 week under a 12-h light/dark cycle, with lights on at 7 a.m. and lights off at 7 p. m. Rats that received antidepressant treatment were injected subcutaneously once daily with MIL hydrochloride (20 mg/kg) or VENL hydrochloride (10 mg/kg) dissolved in saline. Control groups were administered saline injections. We assigned an independent control group to each treatment group. Each group included six animals. The injection volume was 1 ml/kg for all injections. Antidepressant treatments were performed for 4 consecutive days. One hour after the last injection, animals were decapitated and their brains were removed. The pituitary gland was quickly isolated, washed with saline, frozen in dry ice, and stored at − 80°C. A single dose of each drug was chosen on the basis of other animal studies of the drugs’ antidepressive-like actions [8]. Injections were administered in the light phase and pituitary glands were obtained at 6 p.m. MIL hydrochloride and VENL hydrochloride were kindly supplied by Asahi-Kasei Co. (Tokyo, Japan) and Wyeth-Ayerst Research (Princeton, New Jersey, USA), respectively. The authors assert that all procedures contributing toward this work comply with
0959-4965 Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved.
Copyright © 2015 Wolters Kluwer Health, Inc. Unauthorized reproduction of this article is prohibited.
DOI: 10.1097/WNR.0000000000000379
Reuptake inhibitors on pineal monoamines Muneoka et al. 511
Pituitary glands were homogenized with ice-cold 50 mM tris(hydroxymethyl)aminomethane HCl buffer (pH 7.7 at 25°C) containing 5 mM ethylenediaminetetraacetic acid. An aliquot of the homogenate was used to assay protein concentrations by the method of Lowry et al. [9] The remaining homogenate was mixed with 0.1 M perchloric acid containing 1 mM ethylenediaminetetraacetic acid and 2 mM Na2S2O4. This mixture was washed with chloroform and centrifuged at 11 752g at 4°C for 30 min. The aqueous phase was decanted and subjected to reverse-phase high-performance liquid chromatography with electrochemical detection to measure noradrenaline, dopamine, serotonin, and 5-HIAA levels, as described previously [8]. This apparatus consisted of an ODS18 reverse-phase column (4.5 × 150 mm, Eicompak, MA-5ODS; Eicom, Kyoto, Japan) and an ECD-300 electrochemical detector with a carbon graphite working electrode (Eicom) maintained at + 0.7 V with respect to a Ag/AgCl reference electrode. The unit for monoamine levels was ng/mg protein. 5-HIAA/serotonin ratios were calculated as indices of the turnover rate of serotonin.
Tissue levels (ng/mg protein)
Tissue preparation and assay of noradrenaline, dopamine, serotonin, and 5-hydroxyindoleacetic acid in the pineal gland
Fig. 1
Noradrenaline
250 ∗∗ 200 150 100 50 0 Milnacipran
60
Venlafaxine Dopamine
70 Tissue levels (ng/mg protein)
the ethical standards of the relevant national and institutional guides on the care and use of laboratory animals.
∗∗ ∗∗
50 40 30 20 10 0 Milnacipran
Venlafaxine
Tissue levels (ng/mg protein) of noradrenaline and dopamine in the pineal gland. Data are mean ± SEM. Open bars indicate controls and hatched bars indicate groups treated with the indicated manipulation. **P < 0.01 compared with the controls (Student’s t-tests).
Statistical analysis
Between-group comparisons of monoamine levels were based on unpaired Student’s t-tests. Significance was assigned when P values were less than 0.05.
Results There was no significant difference in body weight between controls and drug-treated rats at the start of injections and on the last day of injections. Pineal noradrenaline content was significantly increased in rats treated with MIL compared with the controls, whereas no significant differences in noradrenaline content were detected between controls and the VENLtreated group (Fig. 1). Compared with the controls, pineal dopamine content was significantly increased in rats treated with MIL or VENL (Fig. 1). Pineal serotonin content was significantly increased in rats treated with MIL compared with the controls (Fig. 2). No significant differences were detected in serotonin contents between controls and the VENLtreated group (Fig. 2). Pineal 5-HIAA content was significantly lower in rats treated with VENL than in the controls, and no significant differences in 5-HIAA contents were found between the controls and the MILtreated group (Fig. 2). 5-HIAA/serotonin ratios were significantly decreased in rats treated with MIL or VENL compared with the controls (Fig. 2).
Discussion The current results indicate that treatments that include the SNRIs, MIL, and VENL alter pineal monoamine metabolism. MIL induced an increase in both noradrenaline and serotonin contents in pineal tissues, changes that were not observed in the VENL-treated group. Increased pineal noradrenaline levels likely reflect increased noradrenaline content in the noradrenergic sympathetic nerve terminals that innervate the pineal gland, suggesting an amplification of noradrenergic control over pineal activity. Increased serotonin levels likely indicate increases in the serotonin content in pinealocytes. This serotonin is a substrate for melatonin synthesis, as suggested by a previous observation that the serotonin levels in pineal tissues are much higher than the serotonin levels in brain regions that include serotonergic terminals [8]. Perhaps such noradrenergic and serotonergic changes enhance melatonin synthesis when the noradrenergic system is stimulated during the dark period. Interestingly, the MIL dose used in this study lacks sufficient potency to increase noradrenaline levels in any brain regions previously measured in rats [8]. Although it is conceivable that the increase in pineal noradrenaline content observed here is because of the blocking of noradrenaline reuptake by MIL, the mechanism by which MIL increases pineal serotonin levels is unclear. However, repeated treatments with MIL may
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512 NeuroReport 2015, Vol 26 No 9
Fig. 2
Serotonin
Tissue levels (ng/mg protein)
5000 4000
∗
3000 2000 1000 0 Venlafaxine
Tissue levels (ng/mg protein)
Milnacipran 5-HIAA
250 200 150
∗∗
100 50 0 Venlafaxine
Milnacipran 5-HIAA/serotonin
0.100 0.075 0.050
∗
∗∗
0.025 0.000 Milnacipran
Venlafaxine
Tissue levels (ng/mg protein) of serotonin and 5-HIAA and 5-HIAA/serotonin ratios in the pineal gland. Data are mean ± SEM. Open bars indicate controls and hatched bars indicate groups treated with the indicated manipulation. *P < 0.05 and **P < 0.01 indicate statistical significance compared with the controls (Student’s t-tests).
increase the capacity of melatonin synthesis by stimulating noradrenergic drive or MIL may more directly facilitate serotonin synthesis, for example by modulating activity of the synthetic enzyme tryptophan hydroxylase, as reported previously [10]. MIL may act more strongly on peripheral nerves than on the central nervous system as the sympathetic nerve system that innervates the pineal gland is located outside of the blood–brain barrier. VENL specifically reduced the 5-HIAA content of the pineal gland, indicating potent inhibition of serotonin reuptake into presynaptic serotonin nerve terminals. Thus, in addition to duloxetine, another SNRI, and sertraline, a selective serotonin reuptake inhibitor, VENL reduces 5-HIAA levels in various brain regions [8].
Neither serotonin reuptake sites nor an enzyme that converts serotonin into 5-HIAA seem to exist in pinealocytes [11]. Sympathetic nerve terminals have been reported to reuptake not only noradrenaline but also serotonin and dopamine, and the oxidative-deamination enzymes that convert serotonin into 5-HIAA are found in these terminals [11]. Hence, it is conceivable that the reduced levels of 5-HIAA observed here were because of inhibition of serotonin reuptake into noradrenergic sympathetic nerve terminals that innervate the pineal gland. Serotonin in the pineal gland is known to modulate melatonin synthesis/release through autocrine/paracrine systems [2]. Hence, the marked reduction in pineal 5-HIAA/serotonin ratios induced by VENL treatment may influence the production of pineal melatonin. MIL and VENL are SNRIs that inhibit the reuptake of both serotonin and noradrenaline, but these therapeutic agents show distinct specificity in blocking serotonin and noradrenaline transporters. The ratios of potency of noradrenaline/serotonin transporter blockade are 1/1.6 and 1/30 for MIL and VENL, respectively; of the current SNRIs, MIL most effectively inhibits noradrenaline reuptake, whereas VENL’s pharmacological profile is similar to that of a selective serotonin reuptake inhibitor at lower doses and may act like a SNRI at higher doses [12]. Moreover, an electrophysiological study previously suggested that MIL influences serotonin neuronal activity by noradrenaline modulation, rather than by the inhibition of serotonin reuptake [13]. Treatment with MIL at the dose used in the present study did not previously alter 5-HIAA levels, in contrast to obvious reductions after treatment with VENL, duloxetine, and sertraline [8]. Therefore, the present investigation supplies further evidence that MIL, but not VENL, influences serotonin metabolism through its influence on the noradrenergic nervous system. The pineal dopamine content measured in this study was much lower than the noradrenaline content, consistent with previous reports [14]. Here, treatment with MIL or VENL induced large increases in pineal dopamine content. In the rat pineal gland, dopamine concentrations show a daily rhythm [14], and dopamine is detected after superior cervical ganglionectomy [15]. These results suggest that central dopaminergic innervation with dopamine transporters is distinct from dopamine transporters in noradrenergic nerve terminals in rats, as observed in the bovine pineal gland [16]. However, MIL and VENL have no or weak affinity to dopamine transporters [12,17]. Short-term treatment with MIL previously increased dopamine content in the medial prefrontal cortex and treatment with VENL increased dopamine content in the nucleus accumbens and striatum [8]. Hence, MIL and VENL may induce an increase in pineal dopamine levels, affecting dopamine nuclei located outside of the pineal gland, although the origin of stored dopamine in the rat pineal gland is unknown.
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Reuptake inhibitors on pineal monoamines Muneoka et al. 513
The results from the present investigation suggest that repeated MIL treatment enhances melatonin secretion; VENL has already been reported to increase pineal melatonin levels [18]. Melatonin is known to have analgesic potency [7], and the effects of SNRIs (including MIL and VENL) on pain-related disorders and on animal models of pain have been reported [7]. Moreover, many reports have indicated that MIL is efficacious for fibromyalgia [19]. The pain-reducing effect of SNRIs may include an aspect of melatonin enhancement in addition to their analgesic actions through the modulation of monoaminergic nociceptive systems. Metabolic adverse effects such as body weight gain and an increased risk of diabetes mellitus [20] occur with the long-term use of antidepressants. MIL previously showed no clinically significant effects on body weight [21], whereas weight gain is a frequent adverse effect of treatment with VENL [22]. Melatonin is known to be involved in insulin secretion and it decreases plasma insulin levels in rats and humans [6]. In addition, administration of melatonin previously prevented atypical neuroleptic-induced weight gain [23]. A switch to mania is occasionally experienced during the treatment of depression with antidepressants [24]; the disturbance of systems that maintain circadian rhythms is a proposed risk factor for manic switching [25]. Moreover, the mechanism underlying seasonally switching mania may rely on a hypersensitivity to bright light that suppresses melatonin production [26]. Pharmacotherapy with agents that induce strong noradrenergic modification may facilitate switching to mania through an alteration in circulating melatonin levels or their changes in diurnal rhythms. Therefore, the present results may be important for evaluating the metabolic adverse effects of SNRIs or antidepressant-induced switching to mania.
Acknowledgements The authors thank Emeritus Professor of Kagoshima University Morikuni Takigawa for supporting this study. Conflicts of interest
There are no conflicts of interest.
References 1
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Conclusion The SNRI MIL increased noradrenaline and serotonin contents in the pineal gland, suggesting that MIL treatment enhances melatonin synthesis and its nocturnal release by facilitating the noradrenergic drive of pineal activity and by increasing the levels of a precursor required for melatonin synthesis (serotonin). Another SNRI, VENL, reduced 5-HIAA levels, likely by inhibiting serotonin reuptake into noradrenergic terminals, which affects serotonergic tone in the pineal gland. MIL and VENL induced an increase in pineal dopamine levels through an unknown mechanism. The present data indicate that the levels of pineal monoamines are critical to the activity of recently developed SNRIs. In addition, involvement of sex or aging on the effects of antidepressants on pineal monoamines should be investigated in a future study considering that risk of depression is influenced by sex, age, and life events.
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21
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Copyright © 2015 Wolters Kluwer Health, Inc. Unauthorized reproduction of this article is prohibited.
Distinct effects of the serotonin–noradrenaline reuptake inhibitors milnacipran and venlafaxine on rat pineal monoamines Katsumasa Muneokaa, Makiko Kuwagatab, Tetsuo Ogawac and Seiji Shiodaa Monoamine systems are involved in the pathology and therapeutic mechanism of depression. The pineal gland contains large amounts of serotonin as a precursor for melatonin, and its activity is controlled by noradrenergic sympathetic nerves. Pineal diurnal activity and its release of melatonin are relevant to aberrant states observed in depression. We investigated the effects on pineal monoamines of serotonin–noradrenaline reuptake inhibitors, which are widely used antidepressants. Four days of milnacipran treatment led to an increase in noradrenaline and serotonin levels, whereas 4 days of venlafaxine treatment reduced 5-hydroxyindoleacetic acid levels; both agents induced an increase in dopamine levels. Our data suggest that milnacipran increases levels of the precursor for melatonin synthesis by facilitating the noradrenergic regulation of pineal activity and that venlafaxine inhibits
Introduction The pineal gland contains large amounts of serotonin that serve as a precursor for melatonin synthesis; diurnal changes in the production of melatonin are controlled by the noradrenergic sympathetic nerves that innervate the pineal gland (reviewed in Simonneaux and Ribelayga [1]). In addition, serotonin release from pinealocytes influences melatonin synthesis/release through autocrine/paracrine systems [2]. Depression is a common human disease in that is believed to involve monoamines such as serotonin, noradrenaline, and dopamine; the majority of pharmacological treatments for depression involve serotoninmediating and/or noradrenaline-mediating agents. Altered circulating melatonin levels have been reported in depressive patients [3]. Both melatonin (when used in combination with buspirone [4]) and melatonin receptor agonists [5] exert antidepressant actions. In addition, the pineal function that produces melatonin is related to many physiological states, especially sleep [1], metabolic activity [6], and pain perception [7], which are frequently disturbed in depressive patients. Hence, knowledge of the changes in monoamine metabolism in the pineal gland during antidepressive treatment is critical to understanding the physiological conditions under treatment as well as the therapeutic mechanism. In this study, we examined the effects of two serotonin–noradrenaline reuptake inhibitors (SNRIs), milnaciplan (MIL), and venlafaxine (VENL), on the pineal contents of noradrenaline, dopamine, serotonin, and 5-hydroxyindoleacetic acid (5-HIAA). MIL is a noradrenaline reuptake
serotonin reuptake into noradrenergic terminals on the pineal gland. NeuroReport 26:510–514 Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved. NeuroReport 2015, 26:510–514 Keywords: dopamine, milnacipran, noradrenaline, pineal gland, serotonin, venlafaxine a
Department of Anatomy I, Showa University School of Medicine, Tokyo, Laboratory of Pathology, Food and Drug Safety Center, Division of Toxicology, Hatano Research Institute, Kanagawa and cDepartment of Physiology, Saitama Medical University, Saitama, Japan b
Correspondence to Katsumasa Muneoka, MD, PhD, Department of Biochemistry, Showa University School of Medicine, 1-5-8 Hatanodai, Shinagawa-Ku, Tokyo 142-8555, Japan Tel: + 81 3 3784 8116; fax: + 81 3 3784 2346; e-mail: [email protected] Received 18 March 2015 accepted 17 April 2015
inhibition-dominant SNRI whereas VENL is a serotonin reuptake inhibition-dominant SNRI [8].
Materials and methods Animals and treatment
Six-week-old male Sprague–Dawley rats were purchased from JAPAN Clea Inc. (Tokyo, Japan). Animals were acclimatized in the animal facility for 1 week under a 12-h light/dark cycle, with lights on at 7 a.m. and lights off at 7 p. m. Rats that received antidepressant treatment were injected subcutaneously once daily with MIL hydrochloride (20 mg/kg) or VENL hydrochloride (10 mg/kg) dissolved in saline. Control groups were administered saline injections. We assigned an independent control group to each treatment group. Each group included six animals. The injection volume was 1 ml/kg for all injections. Antidepressant treatments were performed for 4 consecutive days. One hour after the last injection, animals were decapitated and their brains were removed. The pituitary gland was quickly isolated, washed with saline, frozen in dry ice, and stored at − 80°C. A single dose of each drug was chosen on the basis of other animal studies of the drugs’ antidepressive-like actions [8]. Injections were administered in the light phase and pituitary glands were obtained at 6 p.m. MIL hydrochloride and VENL hydrochloride were kindly supplied by Asahi-Kasei Co. (Tokyo, Japan) and Wyeth-Ayerst Research (Princeton, New Jersey, USA), respectively. The authors assert that all procedures contributing toward this work comply with
0959-4965 Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved.
Copyright © 2015 Wolters Kluwer Health, Inc. Unauthorized reproduction of this article is prohibited.
DOI: 10.1097/WNR.0000000000000379
Reuptake inhibitors on pineal monoamines Muneoka et al. 511
Pituitary glands were homogenized with ice-cold 50 mM tris(hydroxymethyl)aminomethane HCl buffer (pH 7.7 at 25°C) containing 5 mM ethylenediaminetetraacetic acid. An aliquot of the homogenate was used to assay protein concentrations by the method of Lowry et al. [9] The remaining homogenate was mixed with 0.1 M perchloric acid containing 1 mM ethylenediaminetetraacetic acid and 2 mM Na2S2O4. This mixture was washed with chloroform and centrifuged at 11 752g at 4°C for 30 min. The aqueous phase was decanted and subjected to reverse-phase high-performance liquid chromatography with electrochemical detection to measure noradrenaline, dopamine, serotonin, and 5-HIAA levels, as described previously [8]. This apparatus consisted of an ODS18 reverse-phase column (4.5 × 150 mm, Eicompak, MA-5ODS; Eicom, Kyoto, Japan) and an ECD-300 electrochemical detector with a carbon graphite working electrode (Eicom) maintained at + 0.7 V with respect to a Ag/AgCl reference electrode. The unit for monoamine levels was ng/mg protein. 5-HIAA/serotonin ratios were calculated as indices of the turnover rate of serotonin.
Tissue levels (ng/mg protein)
Tissue preparation and assay of noradrenaline, dopamine, serotonin, and 5-hydroxyindoleacetic acid in the pineal gland
Fig. 1
Noradrenaline
250 ∗∗ 200 150 100 50 0 Milnacipran
60
Venlafaxine Dopamine
70 Tissue levels (ng/mg protein)
the ethical standards of the relevant national and institutional guides on the care and use of laboratory animals.
∗∗ ∗∗
50 40 30 20 10 0 Milnacipran
Venlafaxine
Tissue levels (ng/mg protein) of noradrenaline and dopamine in the pineal gland. Data are mean ± SEM. Open bars indicate controls and hatched bars indicate groups treated with the indicated manipulation. **P < 0.01 compared with the controls (Student’s t-tests).
Statistical analysis
Between-group comparisons of monoamine levels were based on unpaired Student’s t-tests. Significance was assigned when P values were less than 0.05.
Results There was no significant difference in body weight between controls and drug-treated rats at the start of injections and on the last day of injections. Pineal noradrenaline content was significantly increased in rats treated with MIL compared with the controls, whereas no significant differences in noradrenaline content were detected between controls and the VENLtreated group (Fig. 1). Compared with the controls, pineal dopamine content was significantly increased in rats treated with MIL or VENL (Fig. 1). Pineal serotonin content was significantly increased in rats treated with MIL compared with the controls (Fig. 2). No significant differences were detected in serotonin contents between controls and the VENLtreated group (Fig. 2). Pineal 5-HIAA content was significantly lower in rats treated with VENL than in the controls, and no significant differences in 5-HIAA contents were found between the controls and the MILtreated group (Fig. 2). 5-HIAA/serotonin ratios were significantly decreased in rats treated with MIL or VENL compared with the controls (Fig. 2).
Discussion The current results indicate that treatments that include the SNRIs, MIL, and VENL alter pineal monoamine metabolism. MIL induced an increase in both noradrenaline and serotonin contents in pineal tissues, changes that were not observed in the VENL-treated group. Increased pineal noradrenaline levels likely reflect increased noradrenaline content in the noradrenergic sympathetic nerve terminals that innervate the pineal gland, suggesting an amplification of noradrenergic control over pineal activity. Increased serotonin levels likely indicate increases in the serotonin content in pinealocytes. This serotonin is a substrate for melatonin synthesis, as suggested by a previous observation that the serotonin levels in pineal tissues are much higher than the serotonin levels in brain regions that include serotonergic terminals [8]. Perhaps such noradrenergic and serotonergic changes enhance melatonin synthesis when the noradrenergic system is stimulated during the dark period. Interestingly, the MIL dose used in this study lacks sufficient potency to increase noradrenaline levels in any brain regions previously measured in rats [8]. Although it is conceivable that the increase in pineal noradrenaline content observed here is because of the blocking of noradrenaline reuptake by MIL, the mechanism by which MIL increases pineal serotonin levels is unclear. However, repeated treatments with MIL may
Copyright © 2015 Wolters Kluwer Health, Inc. Unauthorized reproduction of this article is prohibited.
512 NeuroReport 2015, Vol 26 No 9
Fig. 2
Serotonin
Tissue levels (ng/mg protein)
5000 4000
∗
3000 2000 1000 0 Venlafaxine
Tissue levels (ng/mg protein)
Milnacipran 5-HIAA
250 200 150
∗∗
100 50 0 Venlafaxine
Milnacipran 5-HIAA/serotonin
0.100 0.075 0.050
∗
∗∗
0.025 0.000 Milnacipran
Venlafaxine
Tissue levels (ng/mg protein) of serotonin and 5-HIAA and 5-HIAA/serotonin ratios in the pineal gland. Data are mean ± SEM. Open bars indicate controls and hatched bars indicate groups treated with the indicated manipulation. *P < 0.05 and **P < 0.01 indicate statistical significance compared with the controls (Student’s t-tests).
increase the capacity of melatonin synthesis by stimulating noradrenergic drive or MIL may more directly facilitate serotonin synthesis, for example by modulating activity of the synthetic enzyme tryptophan hydroxylase, as reported previously [10]. MIL may act more strongly on peripheral nerves than on the central nervous system as the sympathetic nerve system that innervates the pineal gland is located outside of the blood–brain barrier. VENL specifically reduced the 5-HIAA content of the pineal gland, indicating potent inhibition of serotonin reuptake into presynaptic serotonin nerve terminals. Thus, in addition to duloxetine, another SNRI, and sertraline, a selective serotonin reuptake inhibitor, VENL reduces 5-HIAA levels in various brain regions [8].
Neither serotonin reuptake sites nor an enzyme that converts serotonin into 5-HIAA seem to exist in pinealocytes [11]. Sympathetic nerve terminals have been reported to reuptake not only noradrenaline but also serotonin and dopamine, and the oxidative-deamination enzymes that convert serotonin into 5-HIAA are found in these terminals [11]. Hence, it is conceivable that the reduced levels of 5-HIAA observed here were because of inhibition of serotonin reuptake into noradrenergic sympathetic nerve terminals that innervate the pineal gland. Serotonin in the pineal gland is known to modulate melatonin synthesis/release through autocrine/paracrine systems [2]. Hence, the marked reduction in pineal 5-HIAA/serotonin ratios induced by VENL treatment may influence the production of pineal melatonin. MIL and VENL are SNRIs that inhibit the reuptake of both serotonin and noradrenaline, but these therapeutic agents show distinct specificity in blocking serotonin and noradrenaline transporters. The ratios of potency of noradrenaline/serotonin transporter blockade are 1/1.6 and 1/30 for MIL and VENL, respectively; of the current SNRIs, MIL most effectively inhibits noradrenaline reuptake, whereas VENL’s pharmacological profile is similar to that of a selective serotonin reuptake inhibitor at lower doses and may act like a SNRI at higher doses [12]. Moreover, an electrophysiological study previously suggested that MIL influences serotonin neuronal activity by noradrenaline modulation, rather than by the inhibition of serotonin reuptake [13]. Treatment with MIL at the dose used in the present study did not previously alter 5-HIAA levels, in contrast to obvious reductions after treatment with VENL, duloxetine, and sertraline [8]. Therefore, the present investigation supplies further evidence that MIL, but not VENL, influences serotonin metabolism through its influence on the noradrenergic nervous system. The pineal dopamine content measured in this study was much lower than the noradrenaline content, consistent with previous reports [14]. Here, treatment with MIL or VENL induced large increases in pineal dopamine content. In the rat pineal gland, dopamine concentrations show a daily rhythm [14], and dopamine is detected after superior cervical ganglionectomy [15]. These results suggest that central dopaminergic innervation with dopamine transporters is distinct from dopamine transporters in noradrenergic nerve terminals in rats, as observed in the bovine pineal gland [16]. However, MIL and VENL have no or weak affinity to dopamine transporters [12,17]. Short-term treatment with MIL previously increased dopamine content in the medial prefrontal cortex and treatment with VENL increased dopamine content in the nucleus accumbens and striatum [8]. Hence, MIL and VENL may induce an increase in pineal dopamine levels, affecting dopamine nuclei located outside of the pineal gland, although the origin of stored dopamine in the rat pineal gland is unknown.
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Reuptake inhibitors on pineal monoamines Muneoka et al. 513
The results from the present investigation suggest that repeated MIL treatment enhances melatonin secretion; VENL has already been reported to increase pineal melatonin levels [18]. Melatonin is known to have analgesic potency [7], and the effects of SNRIs (including MIL and VENL) on pain-related disorders and on animal models of pain have been reported [7]. Moreover, many reports have indicated that MIL is efficacious for fibromyalgia [19]. The pain-reducing effect of SNRIs may include an aspect of melatonin enhancement in addition to their analgesic actions through the modulation of monoaminergic nociceptive systems. Metabolic adverse effects such as body weight gain and an increased risk of diabetes mellitus [20] occur with the long-term use of antidepressants. MIL previously showed no clinically significant effects on body weight [21], whereas weight gain is a frequent adverse effect of treatment with VENL [22]. Melatonin is known to be involved in insulin secretion and it decreases plasma insulin levels in rats and humans [6]. In addition, administration of melatonin previously prevented atypical neuroleptic-induced weight gain [23]. A switch to mania is occasionally experienced during the treatment of depression with antidepressants [24]; the disturbance of systems that maintain circadian rhythms is a proposed risk factor for manic switching [25]. Moreover, the mechanism underlying seasonally switching mania may rely on a hypersensitivity to bright light that suppresses melatonin production [26]. Pharmacotherapy with agents that induce strong noradrenergic modification may facilitate switching to mania through an alteration in circulating melatonin levels or their changes in diurnal rhythms. Therefore, the present results may be important for evaluating the metabolic adverse effects of SNRIs or antidepressant-induced switching to mania.
Acknowledgements The authors thank Emeritus Professor of Kagoshima University Morikuni Takigawa for supporting this study. Conflicts of interest
There are no conflicts of interest.
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Conclusion The SNRI MIL increased noradrenaline and serotonin contents in the pineal gland, suggesting that MIL treatment enhances melatonin synthesis and its nocturnal release by facilitating the noradrenergic drive of pineal activity and by increasing the levels of a precursor required for melatonin synthesis (serotonin). Another SNRI, VENL, reduced 5-HIAA levels, likely by inhibiting serotonin reuptake into noradrenergic terminals, which affects serotonergic tone in the pineal gland. MIL and VENL induced an increase in pineal dopamine levels through an unknown mechanism. The present data indicate that the levels of pineal monoamines are critical to the activity of recently developed SNRIs. In addition, involvement of sex or aging on the effects of antidepressants on pineal monoamines should be investigated in a future study considering that risk of depression is influenced by sex, age, and life events.
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