Diurnal variations in melatonin binding sites in the hamster brain: impact of melatonin

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.

Nap and melatonin-induced changes in hippocampal activation and their role in verbal memory consolidation

Overnight sleep contributes to memory consolidation; even a short nap improves memory performance. Such improvement has been linked to hippocampal activity during sleep. Melatonin has been shown to affect the human hippocampus and to induce 'sleep like' changes in brain activation. We therefore conducted and compared two functional magnetic resonance imaging studies: the first study assessed the effect of a 2-hr mid-day nap versus an equal amount of wakefulness on a verbal memory task (unrelated word pair association); the second assessed the effect of melatonin versus placebo (both conditions without nap) on a similar task. We report that following a nap relative to wakefulness, successful retrieval-related activation in the parahippocampus is decreased. A smaller decrease is seen in wakefulness with melatonin but not placebo. In parallel, an improvement in verbal memory recall was found after a nap compared with wakefulness but not with melatonin during wakefulness compared with placebo. Our findings demonstrate effects of melatonin that resemble those of sleep on verbal memory processing in the hippocampus thus suggesting that melatonin, like sleep, can initiate offline plastic changes underlying memory consolidation; they also suggest that concomitant rest without interferences is necessary for enhanced performance.

Melatonin and the human hippocampus, a time dependent interplay

Melatonin serves as a signal of darkness and participates in sleep/wake regulation. Animal studies demonstrated effects of melatonin in the hippocampus, particularly suggesting involvement in synaptic plasticity. We used functional magnetic resonance imaging to identify and investigate effects of melatonin in the human hippocampus. Activity in the hippocampal complex during a memory task was examined at 22:00 hr (when endogenous melatonin levels are normally increasing) and compared with 16:00 hr (when endogenous melatonin levels are minimal). The relationship between observed activation patterns and endogenous melatonin was assessed. Finally, the effects of exogenous melatonin administered at 22:00 hr were studied in a double-blind, placebo-controlled crossover manner. Our findings indicate that activation in the left hippocampus at 22:00 hr is significantly reduced compared with afternoon hours compatible with diurnal variation in hippocampal activity. Exogenous melatonin further reduced activation in this region, only in subjects who already crossed the melatonin onset phase at this hour and in correlation with endogenous melatonin levels. As such an effect was not demonstrated with afternoon administration of melatonin, a time depended effect is suggested. Contrary, activation in the left para-hippocampus at 22:00 hr was higher in subjects that crossed the melatonin onset phase. Parahippocampal activation correlated with individual endogenous melatonin levels and was not further affected by exogenous melatonin. These results demonstrate that memory related activation in the hippocampus and para-hippocampus are affected by time of day and melatonin in a differential manner and may implicate the circadian clock and melatonin in human memory processing during the night.

Factors regulating the influence of melatonin on hippocampal evoked potentials: comparative studies on different strains of mice

Factors regulating the influence of melatonin on the hippocampal glutamergic system in mouse hippocampal slices were evaluated. The sensitivity of hippocampal pyramidal neurons to melatonin (Sigma) was highest at 2 h following slice preparation and then declined with time. This pattern of sensitivity to melatonin correlated well with a reduced binding of melatonin to its receptors. The slices obtained from older animals remained sensitive to melatonin through the entire incubation period. Most of the experiments evaluating the influence of melatonin on hippocampal evoked potentials were performed within 2 h following slice preparation. The effect of melatonin was biphasic: an initial depression of the potential was followed by a recovery/amplification phase. The recovery phase was not a result of melatonin decomposition. The effect of melatonin was similar in three different strains of mice tested: CD-1, C57J/B6, and Swiss Webster. While the melatonin from another vendor (Regis) gave similar results, it was effective at much lower concentrations. In slices obtained from CD-1 light-deprived mice, the sensitivity to melatonin was significantly reduced. Thus, it appears that melatonin may control the hippocampal glutamergic system in a complex manner, which may be regulated by the circadian rhythm. This may influence memory formation in the hippocampus.

African trypanosomiasis in the rat alters melatonin secretion and melatonin receptor binding in the suprachiasmatic nucleus

Rats infected with Trypanosoma brucei brucei, a subspecies of the extracellular parasites that cause African sleeping sickness, were examined for disturbances in the circadian rhythms of melatonin secretion (evaluated by determination of the excretion of melatonin in the urine) and the binding of melatonin to its receptor in the suprachiasmatic nuclei of the anterior hypothalamus. In normal and infected rats, Cosinor analysis showed a significant nocturnal peak. The amplitude of this peak was, however, significantly decreased in the infected rats. The peak of melatonin receptor binding in the suprachiasmatic nuclei showed a 4-h phase advance in the infected rats, compared with the controls (0400 and 0800, respectively). These data point to a disturbance in the circadian rhythm of the melatonin-generating systems in the pathogenesis of African sleeping sickness.

Modulation of 2[125I]iodomelatonin binding sites in the guinea pig spleen by melatonin injection is dependent on the dose and period but not the time

Effects of dose, time and period of melatonin injection on 2[125I]iodomelatonin binding sites in the guinea pig spleen were studied. Guinea pigs (Dunkin-hartley), kept under 12h light/12 h darkness, were given daily intraperitoneal injections of either vehicle or 0.01, 0.1 or 1 mg melatonin/kg body weight in either early (1 hour after onset of light period) or late light period (1 hour before offset of light period) for 2 or 7 days. To study the effect of opioid antagonist on the binding, intraperitoneal injections of 2 or 20 mg naltrexone/kg body weight alone or together with 0.1 mg melatonin/kg body weight was given daily in late light period for 2 days. 2[125I]Iodomelatonin binding assays were performed on spleen membrane preparations and radioimmunoassays of melatonin levels were carried out in serum and pineal glands collected during mid-light. High dose (1 mg/kg body weight) of melatonin injection elevated the mid-light serum melatonin levels without affecting pineal melatonin levels. Early light injection group had a higher mid-light serum melatonin level. Melatonin injection for 2 days at either time points caused a dose-dependent decrease in Bmax and increase in Kd of 2[125I]iodomelatonin binding sites in the spleen. The response was independent of the time of injection. A greater suppression of binding was achieved by injecting melatonin for 7 days. Naltrexone did not affect the binding by itself and was not able to reverse the melatonin-induced suppression of binding in the spleen. The modulation of the splenic 2[125I]iodomelatonin binding sites by exogenous melatonin suggests that melatonin may act directly on the immune system to affect its function.

Putative melatonin receptors in the male guinea pig kidney

The direct action of pineal melatonin on the renal system is supported by our demonstration of 2-[125I]iodomelatonin binding sites in the male guinea pig kidney. Scatchard analyses and Hill coefficients revealed a single type of binding site with an equilibrium dissociation constant (Kd) of 22.3 +/- 1.6 pmol/l and a maximum binding density (Bmax) of 0.99 +/- 0.03 fmol/mg protein (n = 7) at mid-light. There was no significant difference in the Kd and Bmax values between kidney tissues collected at the middle of light and dark periods. The pharmacological profile of these 2-[125I]iodomelatonin binding sites indicated high specificity for melatonin, 2-iodomelatonin and 6-chloromelatonin while kinetic studies generated a Kd value of 28.4 +/- 7.3 pmol/l (n = 5) which was comparable to that determined from Scatchard transformations. Our results suggest that these binding sites are stable, reversible, saturable, specific, and of high affinity. Regional distribution study showed that specific binding of 2-[125I]iodomelatonin was 8-fold higher in the cortical region than that in the medullary region. Studies of subcellular distribution showed that 59.3% of binding sites were localized in crude nuclear fractions followed by crude mitochondrial fractions (22.3%) and crude microsomal fractions (18.3%) with no detectable binding in cytosolic fractions. Our present findings suggest the presence of putative melatonin receptors in the guinea pig kidney, which support the hypotheses of melatonin-regulated renin secretion together with renal excretory functions via melatonin receptors.

Regulation of melatonin-sensitivity and firing-rate rhythms of hamster suprachiasmatic nucleus neurons: constant light effects

Rhythms of spontaneous firing rate and of responsiveness to pressure ejection of melatonin were recorded from neurons in the Syrian hamster suprachiasmatic nuclei (SCN) in a slice preparation. In animals taken from light-dark cycles (LD 14:10), SCN cells had high firing rates during the projected day and lower rates during the projected night. The proportion of melatonin-suppressed cells (35% overall) was also high during the day and fell during the night, while melatonin activated approximately 23% of cells at all phases. To assess the source of the melatonin-responsiveness rhythm, hamsters were exposed for approximately 48 h to constant illumination (LL) to suppress melatonin secretion. LL exposure before slice preparation altered both firing-rate and melatonin-responsiveness rhythms. Firing rates failed to show a morning peak and remained at low levels, with no indication of daily rhythmicity. Melatonin responsiveness also failed to show the usual rhythm and even tended to rise at night. Overall melatonin responsiveness rose after LL exposure so that 50% of cells were suppressed and 21% activated. LL exposure also increased the proportions of cells which showed regular baseline firing rates. Control studies indicated that pressure artifacts did not account for either suppression or activation by melatonin, while the composition of the saline vehicle appeared to be responsible for the activations recorded. The results indicate that brief LL exposure alters SCN sensitivity to melatonin and SCN rhythmicity in Syrian hamsters, perhaps as a result of the loss of the daily melatonin secretion rhythm. Physiological melatonin patterns may have important effects on the rodent circadian pacemaker.

Modification by oxazepam of the diurnal variations in brain 125I-melatonin binding sites in sham-operated and pinealectomized rats

Sham-operated and pinealectomized male rats were maintained at 14 h light:10 h dark cycles (lights-on 5.00 h) and injected daily, for 14 days, with oxazepam or vehicle. 125I-melatonin binding was recorded in synaptosomes prepared at 10.00, 18.00, and 24.00 h from the hypothalamus, hippocampus and medulla-pons of the rats. In the sham-operated, vehicle treated rats, specific 125I-melatonin binding in all brain areas studied was higher at 18.00 h, whereas in the oxazepam-treated animals, binding was higher at 24.00 h than at the other times tested. In the pinealectomized, vehicle-treated rats, the binding recorded at 18.00 h in all three brain areas, was lower than at the other times of day tested. Oxazepam treatment decreased 125I-melatonin binding at 24.00 h in the hippocampus and medulla-pons of the pinealectomized rats and did not significantly affect the binding in the hypothalamus. These results indicate the ability of oxazepam, pinealectomy and their combination, to manipulate the diurnal variations in 125I-melatonin binding sites in the rat brain.

Serum melatonin levels in schizophrenic and schizoaffective hospitalized patients

Conflicting results on changes of the diurnal melatonin rhythms of patients with affective disorders have been reported in the literature. The heterogeneous data may derive from the great discrepancy in the diagnostic criteria of different authors. A study of 12 schizoaffective and chronic schizophrenic psychotic patients found a constant pattern of an obliterated nocturnal melatonin rise only in the latter group. The presence or absence of the nocturnal melatonin rise was determined in drug-free hospitalized patients and remained unchanged despite 2 months of drug treatment including large doses of neuroleptics. This finding, when confirmed in a larger number of patients, could possibly serve as a marker for the type of mental disorder, drug to be applied and response expected.

Melatonin binding proteins identified in the rat brain by affinity labeling

N-Bromoacetyl-2-iodo-5-methoxytryptamine (BIM), a novel derivative of the biologically active melatonin analog, 2-iodomelatonin, was prepared and used to identify melatonin binding proteins in rat brain synaptosomes. Incubation of the synaptosomes with BIM resulted in a time and concentration dependent, irreversible inhibition of 2-[125I]iodomelatonin binding. In parallel, the radioactive form of BIM, N-bromoacetyl-2-[125I]iodo-5-methoxytryptamine ([125I]BIM) became incorporated into the synaptosomes. The incorporation of [125I]BIM was inhibited by BIM, 2-iodomelatonin and melatonin but not by 5-methoxytryptamine or N-acetyl serotonin. [125I]BIM became covalently attached to three polypeptides with apparent molecular weight values of 92, 55 and 45 kDa; the labeling of all three proteins was markedly inhibited by melatonin. These results indicate that the 92, 55 and 45 kDa polypeptides are melatonin binding proteins.

Affinity labeling of melatonin binding sites in the hamster brain

N-Bromoacetyl-2-iodo-5-methoxytryptamine (BIM), a novel derivative of the biologically active melatonin analog, 2-iodomelatonin, was used to identify melatonin binding proteins in synaptosomes from Syrian hamster brain. Incubation of the synaptosomes with BIM resulted in a concentration dependent, irreversible inhibition of 2-125I-iodomelatonin binding. The radioactive form of BIM, N-Bromoacetyl-2-125I-iodo-5-methoxytryptamine (125I-BIM), became covalently attached to three proteins in the synaptosomes, in a concentration dependent manner. These proteins had apparent molecular weight values of 92, 55 and 45 kilodaltons. The incorporation of 125I-BIM into all three proteins was inhibited by BIM greater than 2-iodomelatonin greater than melatonin whereas the melatonin antagonist N-(1,4 dinitrophenyl)- 5-methoxytryptamine (ML-23) selectively inhibited the labeling of the 45 kDa protein. These results indicate that the 92, 55 and 45 KDa polypeptides are melatonin binding proteins.

Castration affects brain iodomelatonin binding in hamsters maintained in long but not short days

The effects of castration on 2-[125I]iodomelatonin ([125I]melatonin) binding sites in discrete brain areas were investigated in male Syrian hamsters exposed to long and short days. In hamsters maintained in long days (14 h light: 10 h darkness), castration produced a marked decrease in [125I]melatonin binding in the brain, particularly in the medulla-pons hypothalamus and hippocampus. Maximal response in the medulla-pons and hypothalamus was observed at 3 days; specific [125I]melatonin binding subsequently increased to reach control levels within 30 days after castration. In the hippocampus, the decrease in [125I]melatonin binding was still evident at 90 days after castration and could be reversed by testosterone. Exposure to short days (8 h light: 16 h darkness) did not affect [125I]melatonin binding in the various brain areas of the intact hamsters; even after 90 days when circulating testosterone decreased to castrated levels, the binding remained as in intact, long-day-housed controls. Moreover, [125I]melatonin binding in the various brain areas of hamsters exposed to short days was unaffected by castration. The results clearly indicate that the regulation by testosterone of melatonin receptors in the medulla-pons, hypothalamus and hippocampus of the male hamster depends on the prevailing photoperiod.

Presynaptic effects of melatonin on norepinephrine release and uptake in rat pineal gland

The effect of melatonin injection on norepinephrine (NE) turnover rate in rat pineal gland was estimated from the decline of tissue NE levels after the injection of the tyrosine hydroxylase inhibitor alpha-methyl-p-tyrosine. The administration of a single injection of 300 micrograms/Kg of melatonin at the beginning of the scotophase induced, 3 hr later, a significant decrease of pineal NE turnover. The possible direct effect of melatonin on pineal NE release was examined in vitro. Exposure of rat pineal explants previously loaded with 3H-NE to 10(-8)-10(-6) M melatonin decreased significantly 3H-NE release triggered by 60 mM K+. This activity of melatonin was revealed only in pineals excised at night (0000 and 0400, i.e., at the fourth or eighth hours of darkness) and not in those excised in the middle (1400) or late light phase of the daily photoperiod (2000). Melatonin did not modify the spontaneous pineal 3H-NE efflux. Melatonin decreased 3H-NE uptake at a low NE concentration (0.5 microM) in a dose-dependent manner (IC50 identical to 10(-10) M). A kinetic analysis of the pineal NE uptake process indicated that melatonin augmented both Vmax and Km of transmitter uptake. These results suggest that endogenously released melatonin may be a regulatory signal for rat pineal sympathetic synapses.