Lithium and melatonin in pigmented eye rats: Effects of dose and time of day

Lithium has been suggested to exert sane of its theraputic effects by modifying the function of the retinal-hypothalamic pineal pathway that is essential for the chronobiology of an organism (Seggie et al., 1983). Previous work was done in Wistar rats, an Albino species which lacks the enzyme for synthesis of eye pigment. This pigment is important in the regulation of light cued rhythms. The present project investigated effects of lithium in a pigmented eye strain. Adult male Long Evans rats were maintained individually on a 12 hour light/12 hour dark schedule with free access to water and one of three diets: (1) normal laboratory chow; (2) a low lithium diet: lab chow supplemented with 30 mM/kg of lithium chloride and (3) a high lithium diet: lab chow supplemented with 50 mM/kg of lithium chloride. Body weight and water intake were measured after six weeks on the diets. In Experiment I, separate groups of rats were sacrificed by rapid decapitation every 4 hours in the light/dark cycle. In Experiment II, animals were sacrificed every 90 min. between 12:00 and 20:00 hours during the dark cycle. Blood and pineal glands were collected for lithium determination and assay of melatonin by RIA. In Experiment I, plasma lithium levels were 0.35 ± 0.01 and 0.57 ± 0.02 mmEq/1 for the low and high diets. Serum and pineal melatonin evidenced the expected diurnal rhythms. The diets had no effect on these parameters except at 14:00 hours. At this time, two hours after lights out, the high diet but not the low diet group evidenced significantly lower serum melatonin levels than the control group (F = 4.91, df 2, 10, p < .05 for means of 14.0 ± 3.9 pg/ml vs 34.7 ± 5.6 pg/ml). In Experiment II plasma lithium levels were 0.39 ± 0.01 and 0.62 ± 0.01 mEq/1 for the low and high diets. Pineal and serum melatonin rhythms were again as expected and the diets had no effect except at 13:30 hours. At this time pineal melatonin content was significantly reduced by both diets. Control values were 2100 ± 133 pg/pineal, low diet 1222 ± 299 pg/pineal and high diet 905 ± 251 pg/pineal. (Control vs low F = 6.89, df 1, 8, p < .05; control vs high F = 15.5 df 1, 9, p < .01). Body weight was significantly reduced by both diets. Water intake was increased by the high diet and not affected by the low diet. In view of the consistency of time for the present significant findings, it is suggested that lithium is exerting a subtle and short-lived effect on pineal melatonin levels during the period shortly after the switch to darkness and maybe influencing the characteristics of the “turn-on” effects of this important timing cue. In Albino rats, diet supplemented with 50 mM/Kg of lithium chloride affected the pattern of serum melatonin levels between 14:00 and 22:00 hours (Seggie et al., 1983). Thus, a strain effect of lithium cannot be ruled out.     

Studies on pineal melatonin levels in a diurnal species,the Eastern chipmunk (Tamias striatus): Effects of light at night,propranolol administration or superior cervical ganglionectomy

Summary Five experiments were carried out on the control of melatonin levels in the pineal gland of a diurnal species, the Eastern chipmunk (Tamias striatus). We confirmed that the exposure of chipmunks to fluorescent white light of 3,981–4,304 lux during the normal dark period does not prevent the rise in pineal melatonin levels normally associated with darkness. Also, the administration of propranolol (20mg/kg) at 8 p.m. did not block the rise in pineal melatonin in animals exposed to either dark or light at night. Similarly, if chipmunks received propranolol 4 hours into the dark phase, pineal melatonin levels were not depressed 2 hours later. When animals were superior cervical ganglionectomized, however, the pineal content of melatonin remained low regardless of whether the animals were exposed to darkness or light at night. The exposure of chipmunks acutely to light at midnight (4 hours after darkness onset) had only a slight depressive effect on pineal melatonin 30 min later; by comparison, when chipmunks were acutely exposed to light at 3 a.m. (7 hours after darkness onset) daytime pineal melatonin levels were reached within 15 min after light onset. These findings in a diurnal species, the Eastern chipmunk, differ markedly when compared to previously reported observations on nocturnal laboratory rodents.     

Changes in Pineal Indoleamines in Rats after Single Melatonin Injections: Evidence for a Diurnal Sensitivity to Melatonin

We recently determined that melatonin stimulated serotonin (5-HT) secretion from rat pineal glands by increasing 5-HT release from the pinealocytes (μM melatonin concentrations) and by inhibiting 5-HT uptake in the pineal sympathetic nerve endings (mM melatonin concentrations). The present study investigated whether a single melatonin injection could alter the content of indoleamines in the rat pineal gland, as well as its possible dependence on the daytime of administration. Melatonin (150 μg/kg) was i.p. injected at 8 time points (11.00 h, 14.00 h, 17.00 h, 20.00 h, 23.00 h, 02.00 h, 05.00 h and 08.00 h) to rats kept in 12:12 h light:dark cycle (lights on at 07.00 h). Melatonin injections in the afternoon (17:00 h) and late in the nighttime (02.00 h and 05.00 h) decreased pineal 5-HT content 90 min later. The levels of 5-hydroxyindoleacetic acid (5-HIAA) were also decreased 90 min after the melatonin treatment at 14.00 h, 17.00 h and 02.00 h. The effect of melatonin on 5-HT content was a long-lasting effect (still evident after 180 min) only when injected at 02.00 h, whereas 5-HIAA levels were found to be decreased 180 min after melatonin treatment at 14.00 h and 23.00 h. No changes in these compounds were detected 240 min after melatonin treatment. Moreover, melatonin did not change 5-hydroxytryptophan levels at any of the daytime points studied. By contrast, 90 min after the injection of melatonin at 20.00 h, an increased content of pineal N-acetylserotonin was observed. This effect of melatonin could be mediated through a phase alteration of the pineal N-acetyltransferase activity rhythm by acting on the suprachiasmatic clock, althought a direct melatonin effect on the pineal rhythmic function cannot be excluded. The effects of the hormone on 5-HT and 5-HIAA contents agree with previous findings on the inhibitory effect of pharmacological doses of melatonin on pineal 5-HT uptake, which presumably would result in a decreased intraneuronal content of 5-HT and its acid metabolite. These data point to an acute regulatory action of exogenous melatonin on the pineal melatonin synthesis pathway which seems to be limited to two daytime phases: the afternoon-early evening period and the second half of the night.     

Late evening brain activation patterns and their relation to the internal biological time, melatonin, and homeostatic sleep debt

Sleep propensity increases sharply at night. Some evidence implicates the pineal hormone melatonin in this process. Using functional magnetic resonance imaging, brain activation during a visual search task was examined at 22:00 h (when endogenous melatonin levels normally increase). The relationships between brain activation, endogenous melatonin (measured in saliva), and self-reported fatigue were assessed. Finally, the effects of exogenous melatonin administered at 22:00 h were studied in a double blind, placebo-controlled crossover manner. We show that brain activation patterns as well as the response to exogenous melatonin significantly differ at night from those seen in afternoon hours. Thus, activation in the rostro-medial and lateral aspects of the occipital cortex and the thalamus diminished at 22:00 h. Activation in the right parietal cortex increased at night and correlated with individual fatigue levels, whereas exogenous melatonin given at 22:00 h reduced activation in this area. The right dorsolateral prefrontal cortex, an area considered to reflect homeostatic sleep debt, demonstrated increased activation at 22:00 h. Surprisingly, this increase correlated with endogenous melatonin. These results demonstrate and partially differentiate circadian effects (whether mediated by melatonin or not) and homeostatic sleep debt modulation of human brain activity associated with everyday fatigue at night.     

Both pineal and lateral eyes are needed to sustain daily circulating melatonin rhythms in sea bass

This study investigates the role of the pineal organ and lateral eyes (the two most important sources of melatonin in vertebrate species) on daily melatonin rhythms of sea bass, a fish exhibiting reversed melatonin profiles, as well as their contribution to circulating melatonin levels. To this aim, the pineal and/or the eyes were surgically removed (Exp. 1), the optic nerve sectioned and retinal dopaminergic neurons damaged with injections of 6-hydroxydopamine (Exp. 2), and the pineal or the eyes covered with aluminium foil (Exp. 3). The results show that plasma and ocular melatonin display opposing profiles. In Experiment 1, pinealectomized fish displayed lower nightly plasma melatonin levels (66+/-22 pg/ml) than intact or sham-operated groups (131+/-14 pg/ml), as it occurred in ophthalmectomized fish (64+/-12 pg/ml). Fish that were both pinealectomized and ophthalmectomized showed a further decrease in plasma melatonin levels (1.4+/-0.4 pg/ml), which approached daytime levels. In Experiment 2, plasma melatonin levels in both optic nerve-sectioned and ophthalmectomized fish were lower than control levels, while injection of 6-hydroxydopamine did not modify plasma melatonin concentrations. In Experiment 3, covering only the pineal made melatonin drop after a light pulse at MD, and covering only the eyes had a similar effect. In conclusion, these findings suggest that even though sea bass eyes do not directly contribute to plasma melatonin, the pineal organ, which unlike that of mammals is a direct photoreceptor in fish, requires light information from the lateral eyes to normally secrete melatonin into the bloodstream.     

Melatonin synthesis in the pineal gland of the Richardson's ground squirrel (Spermophilus richardsonii): Influence of age and insulin-induced hypoglycemia

Summary The nocturnal rises in pineal N-acetyltransferase (NAT) activity and melatonin levels were compared in young (25–35 days old) and adult (at least 1 year old) Richardson's ground squirrels. When expressed as NAT activity per pineal gland, the nighttime rise in the activity of this enzyme was less in young than in the adult animals; conversely, the melatonin content of the pineal glands of young animals was higher at one point (4 a.m., 8 hours after darkness onset) when compared to that in adult squirrels. When data were expressed relative to total protein, the NAT and melatonin rhythms in the pineals of young and adult animals were very similar. The effect of insulin-induced hypoglycemia on both daytime and nighttime NAT and melatonin levels in the pineal gland of the Richardson's ground squirrel was also assessed. Low daytime levels of these constituents were not influenced by the administration of 10 units insulin, a treatment which caused a marked drop in circulating glucose levels. At night, when pineal NAT and melatonin levels were high insulin injection had a very modest stimulatory effect on NAT activity (one point was elevated above saline injected controls) while melatonin levels remained unchanged by the treatment. These findings in the ground squirrel in reference to insulin-induced hypoglycemia, and Stressors in general, appear to differ from those in the rat where stress can have a substantial influence or both low daytime and high nighttime levels of pineal NAT and melatonin.     

A chronobiological study of melatonin and cortisol secretion in depressed subjects: plasma melatonin, a biochemical marker in major depression

The temporal organization of plasma melatonin and cortisol secretion was examined in healthy rested controls and in depressed patients: 11 patients suffering from a primary affective disorder (10 female, 1 male) and 8 male controls were studied over a 24-hr period; blood was collected at 2-hr intervals during the day at 1-hr intervals at night. Plasma melatonin and cortisol levels were determined by radioimmunoassay. In addition, melatonin was determined in plasma sampled at 3 AM in older male controls (n = 8) and in females (n = 10) at ovulation. The controls showed low or undetectable (less than 5 pg/ml) diurnal plasma melatonin levels and a very marked nocturnal rhythm (acrophase: 2.27 AM, mesor: 34.4 pg/ml, amplitude: 58.7 pg/ml). For the three control groups, no significant difference was observed in the nocturnal melatonin peak at 3 AM. The depressed patients also showed a significant melatonin rhythm but with lower amplitude (14.5 pg/ml) and mesor (19.1 pg/ml). The latter rhythm was not significantly phase-advanced with respect to the controls (acrophase at 1.18 and 2.34 AM, respectively). In 9 of the 11 patients, nocturnal melatonin secretion was less marked and frequently associated with hypercortisolemia. An additional episodic melatonin secretion was observed in the late afternoon in only two patients. In depressed patients, there was an increase in the mean cortisol secretion level (mesor at 13.6 micrograms/100 ml against 9.1 micrograms/100 ml in the controls), but the amplitude and the acrophase were not significantly modified. These data are discussed in terms of both the hypothalamus-pituitary-adrenal-epiphysis and aminergic abnormalities.     

Lack of an effect of melatonin on the basal and L-Dopa stimulated growth hormone secretion in men

Summary The oral administration of melatonin to men has been reported to cause a rapid and significant elevation of serum GH, and to inhibit GH release after stimulation by L-Dopa. We studied the effect of melatonin i.v. on the basal and L-Dopa stimulated GH secretion in four young men. Each subject's control response to L-Dopa was first studied by an oral administration to 500 mg of L-Dopa under a placebo infusion and was followed 2 weeks later by a similar study under melatonin infusion, 2.1 mg/min (total dose of 500 mg). The infusion of melatonin was given for a 4-hour period, 2 hours before and 2 hours after the L-Dopa stimulation. Blood samples for GH were obtained at 30-min intervals. Basal values of serum GH did not rise under the melatonin infusion and peak GH values following L-Dopa stimulation during the control infusion and the melatonin infusion also did not differ (2± 0.5 to 22 ± 6 and 2 ± 0.8 to 25 ± 4 ng/ml respectively). Our data suggest that under an acute constant infusion melatonin does not stimulate GH secretion, nor does it interfere with the L-Dopa stimulated GH response in men.A preliminary report of this work was presented at the Sixth International Congress of Endocrinology, Melbourne, Australia, February 1980, and published as abstract #755 of the program book. This study was supported in part by Ford Foundation Grant 780-0348 and NIH Grants HD 13607 (to U.W.), and CRC Grant RR53.     

The effects of exogenous melatonin on the morphology of thyrocytes in pinealectomized and irradiated rats

Summary. We investigated the effects of exogenous melatonin on the thyrocytes morphology in gamma-irradiated rats under condition where the pineal gland, as a main physiological source of endogenous melatonin, was removed. Three months after pinealectomy animals were divided into two groups: one group of animals was treated with 0.5 ml of vehicle (ethanol diluted in water) and other group was injected intraperitoneally 2 mg/kg of melatonin dissolved in 0.5 ml of vehicle daily during the period of fourteen days. After this treatment all the animals were irradiated with a single dose of 8 Gy gamma rays. Ionising radiation induced apoptosis, hydropic swelling or/and necrosis in both groups of animals, however these changes were less discerned in the thyrocytes of melatonin-treated animals. Our findings demonstrate that administration of exogenous melatonin prior to irradiation reduces radiation-induced thyrocytes damage.     

Methylphenidate effects on blood serotonin and melatonin levels may help to synchronise biological rhythms in children with ADHD

The neuroendocrine mediators that may contribute to ADHD (Attention deficit and hyperactivity disorder), serotonin and melatonin, are both thought to regulate circadian rhythms, neurological function and stress response. The objective of this study was to determine the effect of the chronic administration of prolonged release methylphenidate (PRMPH) on daily variations in blood serotonin and melatonin and on the excretion of 6-sulphatoxy-melatonin. A total of 179 children (136 males, 42 females) between the ages of 5 and 14 (9.70 ± 2.55) years were enrolled in a controlled quasi-experimental open clinical study. Of the sample, there were 136 Children with ADHD (based on DSM-IV-TR criteria), who were further grouped into subtypes, and the 42 siblings of the participants who did not ADHD patients. Blood samples were taken at 20:00 and 09:00; urine was collected between 21:00 and 09:00. In the ADHD group, the study protocol was repeated after 4.61 ± 2.3 months of treatment. Measurements included melatonin and serotonin by RIA and urine 6-S-aMT by ELISA. Factorial analyses were conducted by STATA 12.0.     

Daily melatonin intake resets circadian rhythms of a sighted man with non-24-hour sleep-wake syndrome who lacks the nocturnal melatonin rise

Abstract Effects of daily melatonin intake on the circadian rhythms of sleep and wakefulness, rectal temperature and plasma cortisol were examined in a sighted man who had suffered from the non-24-hour sleep-wake syndrome. The subject lacked the nocturnal melatonin rise in plasma, but showed robust circadian rhythms in rectal temperature and plasma cortisol. The sleep-wake rhythm free-ran with a period longer than 24 hours. Daily melatonin intake at 21:00 h concentrated sleep episodes in the nocturnal period (24:00–8:00 h), and increased the length of the episodes. A single oral dose (3 mg) of melatonin increased plasma melatonin levels to about 1300 pg/mL within one hour and remained at pharmacological levels for approximately 6 hours. The trough of rectal temperature and the circadian rise of plasma cortisol were fixed to the early morning. A higher dose of melatonin (6 mg) did not improve the general feature. After the cessation of melatonin intake, the sleep-wake rhythm began to free-run together with the circadian rhythms in rectal temperature and plasma cortisol. It is concluded that daily intake of melatonin at early night time resets the circadian rhythms in a sighted man who lacked the nocturnal melatonin rise and showed free-running circadian rhythms in routine life.     

In vivo protection against kainate-induced apoptosis by the pineal hormone melatonin: effect of exogenous melatonin and circadian rhythm

We recently reported that the pineal hormone melatonin protected neuronal cultures from excitotoxicity mediated via kainate-sensitive glutamate receptors and from oxidative stress-induced apoptosis. It has been shown that in rats, a systemic administration of kainate induces apoptotic cell death in various brain regions. In this study, we assayed the extent of brain injury after intraperitoneal (i.p.) administration of 10 mg/kg kainate to rats, using the quantitative TUNEL technique and Nissl staining. We examined the role of melatonin on kainate-induced brain injury by (a) injecting melatonin (4 × 2.5 mg/kg i.p.) prior to and after kainate injection and (b) injecting kainate at the time of low circulating melatonin levels (day/light), and high melatonin levels (night/dark). The extent of kainate-triggered DNA damage and the loss of Nissl staining were lower in animals treated with melatonin, or when kainate was injected at night, i.e. in the presence of high endogenous levels of melatonin. Our results suggest that both the pharmacological use of melatonin and the circadian secretion of endogenous melatonin during the night may reduce the extent of excitotoxic brain injury. Further studies are needed to fully characterize the relevance of our findings for the treatment of progressive neurodegenerative processes which involve excitotoxicity and apoptotic neuronal death.     

The depression in rat pineal melatonin production after saline injection at night may be elicited by corticosterone

A hind-leg subcutaneous saline injection into rats at night elicits a decrease in N-acetyltransferase (NAT) activity and melatonin content of the pineal gland. The decrement in pineal melatonin production after saline injection is prevented by adrenalectomy. The present studies were undertaken to determine what factor(s) from the adrenal gland cause(s) the drop in pineal melatonin production after saline injection at night. In the first study, groups of intact and adrenal-demedullated male rats were given a saline injection at 23.10 h (3 h, 10 min after lights off) and their pineals were collected 15 or 30 min later. Pineal NAT activity was depressed in both intact and adrenal-demedullated rats at 15 min postinjection as compared to their respective control animals. Pineal melatonin levels exhibited a drop in intact animals at 15 min and in adrenal-demedullated rats at 30 min. In a second study, hypophysectomy was found to prevent the drop in nocturnal pineal NAT activity and melatonin levels normally associated with a hind leg injection of saline. Finally, in a third experiment, groups of hypophysectomized rats were injected i.p. with corticosterone at 23.10 h and killed 10, 25 or 40 min postinjection. Corticosterone injection in hypophysectomized rats produced a response similar to that caused by saline injection in intact animals: NAT activity was depressed at 10 min and melatonin content was lowered at 25 min. These results suggest that the adrenal-mediated depression in melatonin synthesis after saline injection at night in rats may be elicited by an adrenal cortical hormone (corticosterone) and apparently does not involve the release of factors from the adrenal medulla.     

Arginine vasotocin stimulates glucocorticoid secretion in male rats

1. 1. The endocrine effects of the putative pineal peptide arginine vasotocin (AVT) were compared with those of the pineal indoleamines melatonin and N-acetylserotonin and passive immunization against these indoleamines in male rats injected at 1200 hr or 2400 hr.

2. 2. A pharmacologie dose of AVT markedly enhanced both day and night serum gluoocorticoid levels but lowered prolactin levels at both time points.

3. 3. Melatonin decreased night prolactin and corticosterone levels whereas passive immunization against circulating melatonin and N-acetylserotonin elevated night levels of these hormones.

4. 4. Preliminary evidence that AVT (10 μM) produces a guanosine triphosphate (GTP)-dependent stimulation of adenylate cyclase activity in the hypothalamus, anterior pituitary and adrenal gland suggests that the endocrine effects of this peptide may involve alterations in cAMP levels at one or more of these sites.

Nocturnal headache associated with melatonin deficiency due to a pineal gland cyst

The cyclic nature of some of headache disorders is closely related to melatonin, which is secreted by the pineal gland. We report a 29-year-old male patient with a 2.5-year history of headaches that woke him in the middle of the night. These headaches were pulsatile and continued until sunrise. During these attacks he also suffered from allodynia over the scalp, bilateral conjunctival hyperemia, and nervousness. His brain MRI showed a 5mm by 4mm neuroepithelial cyst in the pineal gland. The peak plasma melatonin level that was measured at 2 am was 28 pg/mL. The patient underwent oral melatonin treatment (6 mg/day). After 1 month he experienced a 70% reduction in his symptoms. When the melatonin dosage was increased to 10mg/day he became headache-free, and 5 months after the treatment began, had no complaints. His 5-month follow-up plasma melatonin level at 2 am was 61 pg/mL. To our knowledge this is the first report of a patient with nocturnal headache associated with a low level of melatonin due to a neuroepithelial cyst in the pineal gland.     

Parathion (O,O-dimethyl-O-p-nitrophenyl phosphorothioate) induces pineal melatonin synthesis at night

The effects of parathion on male rat pineal N-acetyltransferase (NAT) activity, hydroxyindole-O-methyltransferase (HIOMT) activity and pineal and serum melatonin levels at the end of light period (2000 h) and at night (2300 h and 0100 h) were studied. Additionally, pineal levels of 5-hydroxytryptophan (5-HTP), serotonin (5-HT), and 5-hydroxyindole acetic acid (5-HIAA) were estimated. Parathion was administered intragastrically at total doses (over 6 days) of either 6.5 or 13 mg/kg. Control rats received vehicle (corn oil) only. During the study, the rats were exposed to light:dark cycles of 14:10 with light off at 2100 h. Pineal NAT activity was increased at 0100 h following parathion administration at both doses, but HIOMT activity was unaffected. Pineal and serum melatonin levels were increased at night (2300 h and 0100 h) after the 13 mg/kg dose of parathion while the lower dose increased pineal melatonin only at 0100 h. Also, both doses decreased 5-HTP at 2000 h while the lower dose increased it at 2300; 5-HT was significantly decreased at 2300 h and 5-HIAA levels were lower but only significantly so for the 13 mg/kg dose at 2000 h. The results indicate that parathion has significant effects on pineal melatonin synthesis by mechanisms which remain unknown.     

Effect of NMDA Receptor Blockade on Melatonin and Activity Rhythm Responses to a Light Pulse in Rats

The possible role of the excitatory amino acids as mediators of the acute suppression and subsequent delay by light of pineal melatonin production was studied in rats using the NMDA receptor antagonist MK-801. Saline or MK-801 in doses up to 3 mg/kg (IP), was administered 15 min before a 15-min light pulse (200 lx), 4 h after dark onset, and the excretion of 6-sulphatoxymelatonin (aMT.6S) determined. Under these conditions saline injected/light exposed animals exhibited an acute, total but transient suppression of urinary aMT.6S excretion and a delay in the onset of aMT.6S the following night of 1.5 ± 0.2 h. MK-801 failed to block either the acute or phase delaying effect of light (onset delayed by 2.2 ± 0.4 h). Pretreatment with MK-801 (3 mg/kg) failed to block the effects of shorter, less intense light pulses 15 min before the pulse (e.g., 1 min/2 lx; onset delayed by 2.0 ± 0.4 h following saline, 1.5 ± 0.1 h following MK-801) or 60 min before a short duration low intensity pulse. In other experiments MK-801 (1 and 3 mg/kg) failed to affect aMT.6S excretion when injected in the dark at the time of lights out or 4 h after dark onset. NMDA (10 and 30 mg/kg) injection at the time of lights out or 4 h after darkness did not mimic the effects of a light pulse by decreasing aMT.6S excretion or causing a delay in the onset of excretion the following night. Finally MK-801 (3 mg/kg) injected 4 h after dark failed to block the phase delaying effects of a 15 min light pulse (200 lx) on running activity in rats. These results do not support the hypothesis that excitatory amino acids in the retino-hypothalamic tract acting on the NMDA receptor subtype and terminating in the suprachiasmatic nucleus mediate the photic influences upon rat pineal melatonin and activity rhythms. Copyright © 1996 Elsevier Science Inc.     

Influence of light irradiance on hydroxyindole-O-methyltransferase activity, serotonin-N-acetyltransferase activity, and radioimmunoassayable melatonin levels in the pineal gland of the diurnally active Richardson's ground squirrel

When Richardson's ground squirrels were kept under light:dark cycles of 14:10 h there was no nocturnal rise in pineal hydroxyindole-O-methyltransferase (HIOMT) activity. Conversely, the 10 h dark period was associated with large nocturnal rises in both pineal serotonin-N-acetyltransferase (NAT) activity and radioimmunoassayable melatonin levels. The nighttime rises in pineal NAT and melatonin were not suppressed by the exposure of the animals to a light irradiance of 925 mu W/cm2 during the normal dark period. On the other hand, when the light irradiance was increased to 1850 mu W/cm2 the rise in pineal NAT activity was eliminated while the melatonin rise was greatly reduced. When ground squirrels were acutely exposed to a light irradiance of 1850 mu W/cm2 for 30 min beginning at 5.5 h after lights out, pineal NAT activity and melatonin levels were reduced to daytime values within 30 min. The half-time (t 1/2) for each constituent was less than 10 min. Exposure to a light irradiance of either 5 s or 5 min (beginning at 5.5 h into dark period) was equally as effective as 30 min light exposure in inhibiting pineal NAT activity and melatonin levels. When animals were returned to darkness after a 30 min exposure to a light irradiance of 1850 mu W/cm2 at night, both pineal NAT activity and melatonin levels were restored to high nighttime levels within 2 h of their return to darkness. The results indicate that the pineal gland of the wild-captured, diurnal Richardson's ground squirrel is 9000 X less sensitive to light at night than is the pineal gland of the laboratory raised, nocturnal Syrian hamster.     

Relation of nocturnal melatonin levels to serum matrix metalloproteinase-9 concentrations in patients with myocardial infarction

INTRODUCTION: The aims of the present study were to characterize the day/night variation of matrix metalloproteinase (MMP)-9 in patients who have developed ST-segment elevation myocardial infarction (STEMI), in response to light/dark differences in circulating melatonin and to assess whether melatonin, a day/night variation regulator, modulates the nocturnal inflammatory changes in patients who have STEMI. METHODS: The study included 75 patients diagnosed with STEMI and 75 control subjects. Each subject was studied under strictly controlled light/dark conditions. Blood samples for measurement of MMP-9 and melatonin were collected at 09:00 a.m. (light period) and 02:00 a.m. (dark period). RESULTS: In patients with STEMI, melatonin concentrations maintained a light/dark variation but the difference between nocturnal and diurnal levels was smaller than that in controls (p<0.001). In contrast to melatonin, serum MMP-9 concentrations showed no day/night variation in control subjects. MMP-9 concentrations were significantly higher in patients with STEMI than in control subjects. In the STEMI subjects, MMP-9 serum concentrations in the light period were significantly higher than those during the dark phase (291.1+/-59.5 vs. 261.8+/-57.8 ng/ml, p<0.01). Furthermore in the control subjects there was no correlation between MMP-9 and melatonin levels, while in the STEMI group there was a significant correlation between these parameters (Pearson's r=0.40, p<0.0004). CONCLUSIONS: Our results suggest that the light/dark variations in endogenous MMP-9 production in patients who have STEMI might be associated, at least in part, to the day/night variation of melatonin.     

Features of melatonin catabolism in chicks

Melatonin is a neurohormone that is involved in biorhythm synchronization that is produced by the pineal gland and retina. We have studied the production of the kynurenine products, N-acetyl-N-formyl-5-methoxykynurenamine (AFMK) and N-acetyl-5-methoxykynurenamine (AMK), during catabolism of melatonin in chicks (Gallus gallus). Analyses of blood serum samples, retinas, and pineal glands were performed using the HPLC method with fluorometric and electrochemical detection. Considerable amounts of melatonin were found in the pineal gland, retina, and blood serum, although endogenous AFMK and AMK were not found in these tissues. Nevertheless, after intraperitoneal injection of melatonin at a dose of 10 mg/kg, these compounds were found in the blood serum, and AMK was found in the retina. Analysis of AFMK and AMK pharmacodynamics in the blood serum showed that their peak concentrations were 4.8 ng/ml and 2 ng/ml, respectively; the peaks were observed within less than 5 minutes after melatonin injection for AFMK and in 20 minutes after melatonin injection for AMK. The amount of AMK in the retina also reached its peak (60 pg/retina) in 20 minutes. The concentration of both metabolites decreased to the detection limit within one hour. Thus, melatonin may be oxidized via the kynurenine pathway, and its oxidation products, AFMK and AMK, have short half lives. The total amount of both metabolites after melatonin injection was approximately 0.04% of its amount in the blood serum and retina, thus, this pathway of melatonin oxidation is not the major one.