Differential modulation of GABAA receptor function by Mel1a and Mel1b receptors

Potency of melatonin in living beings

Living beings are surrounded by various changes exhibiting periodical rhythms in environment. The environmental changes are imprinted in organisms in various pattern. The phenomena are believed to match the external signal with organisms in order to increase their survival rate. The signals are categorized into circadian, seasonal, and annual cycles. Among the cycles, the circadian rhythm is regarded as the most important factor because its periodicity is in harmony with the levels of melatonin secreted from pineal gland. Melatonin is produced by the absence of light and its presence displays darkness. Melatonin plays various roles in creatures. Therefore, this review is to introduce the diverse potential ability of melatonin in manifold aspects in living organism.

Sleep and circadian rhythm dysregulation in schizophrenia

Sleep-onset and maintenance insomnia is a common symptom in schizophrenic patients regardless of either their medication status (drug-naive or previously treated) or the phase of the clinical course (acute or chronic). Regarding sleep architecture, the majority of studies indicate that non-rapid eye movement (NREM), N3 sleep and REM sleep onset latency are reduced in schizophrenia, whereas REM sleep duration tends to remain unchanged. Many of these sleep disturbances in schizophrenia appear to be caused by abnormalities of the circadian system as indicated by misalignments of the endogenous circadian cycle and the sleep-wake cycle. Circadian disruption, sleep onset insomnia and difficulties in maintaining sleep in schizophrenic patients could be partly related to a presumed hyperactivity of the dopaminergic system and dysfunction of the GABAergic system, both associated with core features of schizophrenia and with signaling in sleep and wake promoting brain regions. Since multiple neurotransmitter systems within the CNS can be implicated in sleep disturbances in schizophrenia, the characterization of the neurotransmitter systems involved remains a challenging dilemma.

Phase shift in the 24-hour rhythm of hippocampal EEG spiking activity in a rat model of temporal lobe epilepsy

For over a century epileptic seizures have been known to cluster at specific times of the day. Recent studies have suggested that the circadian regulatory system may become permanently altered in epilepsy, but little is known about how this affects neural activity and the daily pattern of seizures. To investigate, we tracked long-term changes in the rate of spontaneous hippocampal EEG spikes (SPKs) in a rat model of temporal lobe epilepsy. In healthy animals, SPKs oscillated with near 24-h period; however, after injury by status epilepticus, a persistent phase shift of ∼12 h emerged in animals that later went on to develop chronic spontaneous seizures. Additional measurements showed that global 24-h rhythms, including core body temperature and theta state transitions, did not phase shift. Instead, we hypothesized that locally impaired circadian input to the hippocampus might be responsible for the SPK phase shift. This was investigated with a biophysical computer model in which we showed that subtle changes in the relative strengths of circadian input could produce a phase shift in hippocampal neural activity. MRI provided evidence that the medial septum, a putative circadian relay center for the hippocampus, exhibits signs of damage and therefore could contribute to local circadian impairment. Our results suggest that balanced circadian input is critical to maintaining natural circadian phase in the hippocampus and that damage to circadian relay centers, such as the medial septum, may disrupt this balance. We conclude by discussing how abnormal circadian regulation may contribute to the daily rhythms of epileptic seizures and related cognitive dysfunction.

Correlations between behavioural and oxidative parameters in a rat quinolinic acid model of Huntington's disease: protective effect of melatonin

The present study was designed to examine the correlations between behavioural and oxidative parameters in a quinolinic acid model of Huntington's disease in rats. The protective effect of melatonin against the excitotoxicity induced by quinolinic acid was investigated. Rats were pre-treated with melatonin (5 or 20mg/kg) before injection of quinolinic acid (240nmol/site; 1μl) into their right corpora striata. The locomotor and exploratory activities as well as the circling behaviour were recorded. The elevated body swing test was also performed. After behavioural experiments, biochemical determinations were carried out. Melatonin partially protected against the increase of circling behaviour caused by quinolinic acid injection. No alteration was found in the number of crossings and rearings of animals treated with melatonin and/or quinolinic acid. Melatonin decreased the percentage of contralateral biased swings induced by quinolinic acid. Melatonin protected against the increase in reactive species and protein carbonyl levels as well as the inhibition of superoxide dismutase activity resulting from quinolinic acid injection. Melatonin was partially effective against the inhibition of striatal catalase activity and a decrease of non-protein thiol levels induced by quinolinic acid. Melatonin was not effective against the inhibition of Na(+), K(+) ATPase activity caused by quinolinic acid injection. There were significant correlations between circling behaviour and oxidative parameters. The antioxidant property of melatonin is involved, at least in part, in its neuroprotective effect. The results reinforce the idea that melatonin could be useful in overwhelming neurotoxicity caused by quinolinic acid, a rat model of Huntington's disease.

Role of melatonin and its receptors in the vertebrate retina

Melatonin is a chemical signal of darkness that is produced by retinal photoreceptors and pinealocytes. In the retina, melatonin diffuses from the photoreceptors to bind to specific receptors on a variety of inner retinal neurons to modify their activity. Potential target cells for melatonin in the inner retina are amacrine cells, bipolar cells, horizontal cells, and ganglion cells. Melatonin inhibits the release of dopamine from amacrine cells and increases the light sensitivity of horizontal cells. Melatonin receptor subtypes show differential, cell-specific patterns of expression that are likely to underlie differential functional modulation of specific retinal pathways. Melatonin potentiates rod signals to ON-type bipolar cells, via activation of the melatonin MT2 (Mel1b) receptor, suggesting that melatonin modulates the function of specific retinal circuits based on the differential distribution of its receptors. The selective and differential expression of melatonin receptor subtypes in cone circuits suggest a conserved function for melatonin in enhancing transmission from rods to second-order neurons and thus promote dark adaptation.

Melatonin utility in neonates and children

Melatonin (N-acetyl-5-methoxytryptamine) is an endogenously produced indoleamine secreted by the pineal gland and the secretion is suppressed by light. Melatonin is a highly effective antioxidant, free radical scavenger, and has anti-inflammatory effect. Plenty of evidence supports the utility of melatonin in adults for cancer, neurodegenerative disorders, and aging. In children and neonates, melatonin has been used widely, including for respiratory distress syndrome, bronchopulmonary dysplasia, periventricular leukomalacia (PVL), hypoxia-ischemia encephalopathy and sepsis. In addition, melatonin can be used in childhood sleep and seizure disorders, and in neonates and children receiving surgery. This review article discusses the utility of melatonin in neonates and children.

Promotion of non-rapid eye movement sleep and activation of reticular thalamic neurons by a novel MT2 melatonin receptor ligand

Melatonin activates two brain G-protein coupled receptors, MT(1) and MT(2), whose differential roles in the sleep-wake cycle remain to be defined. The novel MT(2) receptor partial agonist, N-{2-[(3-methoxyphenyl) phenylamino] ethyl} acetamide (UCM765), is here shown to selectively promote non-rapid eye movement sleep (NREMS) in rats and mice. The enhancement of NREMS by UCM765 is nullified by the pharmacological blockade or genetic deletion of MT(2) receptors. MT(2), but not MT(1), knock-out mice show a decrease in NREMS compared to the wild strain. Immunohistochemical labeling reveals that MT(2) receptors are localized in sleep-related brain regions, and notably the reticular thalamic nucleus (Rt). Microinfusion of UCM765 in the Rt promotes NREMS, and its systemic administration induces an increase in firing and rhythmic burst activity of Rt neurons, which is blocked by the MT(2) antagonist 4-phenyl-2-propionamidotetralin. Since developing hypnotics that increase NREMS without altering sleep architecture remains a medical challenge, MT(2) receptors may represent a novel target for the treatment of sleep disorders.

The interaction of melatonin and agmatine on pentylenetetrazole-induced seizure threshold in mice

Melatonin, the major hormone produced by the pineal gland, has a number of functions in mammals, for example, its function as an anticonvulsant. Agmatine, a biogenic amine formed by decarboxylation of L-arginine by arginine decarboxylase, also has anticonvulsant effects. This study investigated the effect of the interaction of melatonin and agmatine on seizure susceptibility in the mouse model of pentylenetetrazole (PTZ)-induced clonic seizures. Further, the researchers investigated the involvement of melatonin receptors in this interaction using luzindole, a ML(1/2) receptor antagonist and prazosin, a ML(3) receptor antagonist. Melatonin, at 40 and 80 mg/kg, and agmatine, at 10 and 20mg/kg, exerted anticonvulsant effects. Luzindole, at 1.25 and 2.5mg/kg, or prazosin, at 0.5mg/kg, did not change the seizure threshold as compared with that of vehicle-treated mice. The anticonvulsant effect of melatonin (40 and 80 mg/kg) was prevented by luzindole (2.5mg/kg) (P<0.001) but not prazosin (0.5mg/kg), indicating the possible involvement of ML(1/2) receptors in the anticonvulsant effect of melatonin. Agmatine (5mg/kg) significantly increased the anticonvulsant effect of both the noneffective dose (20mg/kg) (P<0.05) and the effective dose (80 mg/kg) (P<0.001) of melatonin. Luzindole (2.5mg/kg), but not prazosin (0.5mg/kg), decreased the anticonvulsant effect of agmatine (20mg/kg) (P<0.05). Luzindole (2.5mg/kg), but not prazosin (0.5mg/kg), also decreased the seizure threshold when agmatine (5mg/kg) was administered before melatonin (20mg/kg); the decrease was significant compared with that of the group that received only agmatine and melatonin (P<0.001). In conclusion, melatonin and agmatine exhibit an additive effect in decreasing pentylenetetrazole-induced seizure threshold in mice, probably through ML(1/2) receptors.

Ethanol consumption and pineal melatonin daily profile in rats

It is well known that melatonin participates in the regulation of many important physiological functions such as sleep-wakefulness cycle, motor coordination and neural plasticity, and cognition. However, as there are contradictory results regarding the melatonin production diurnal profile under alcohol consumption, the aim of this paper was to study the phenomenology and mechanisms of the putative modifications on the daily profile of melatonin production in rats submitted to chronic alcohol intake. The present results show that rats receiving 10% ethanol in drinking water for 35 days display an altered daily profile of melatonin production, with a phase delay and a reduction in the nocturnal peak. This can be partially explained by a loss of the daily rhythm and the 25% reduction in tryptophan hydroxylase activity and, mainly, by a phase delay in arylalkylamine N-acetyltransferase gene expression and a 70% reduction in its peak activity. Upstream in the melatonin synthesis pathway, the results showed that noradrenergic signaling is impaired as well, with a decrease in β1 and α1 adrenergic receptors' mRNA contents and in vitro sustained loss of noradrenergic-stimulated melatonin production by glands from alcohol-treated rats. Together, these results confirm the alterations in the daily melatonin profile of alcoholic rats and suggest the possible mechanisms for the observed melatonin synthesis modification.

Melatonin in experimental seizures and epilepsy

Although melatonin is approved only for the treatment of jet-lag syndrome and some types of insomnia, clinical data suggest that it is effective in the adjunctive therapy of osteoporosis, cataract, sepsis, neurodegenerative diseases, hypertension, and even cancer. Melatonin also modulates the electrical activity of neurons by reducing glutamatergic and enhancing GABA-ergic neurotransmission. The indoleamine may also be metabolized to kynurenic acid, an endogenous anticonvulsant. Finally, the hormone and its metabolites act as free radical scavengers and antioxidants. The vast majority of experimental data indicates anticonvulsant properties of the hormone. Melatonin inhibited audiogenic and electrical seizures, as well as reduced convulsions induced by pentetrazole, pilocarpine, L-cysteine and kainate. Only a few studies have shown direct or indirect proconvulsant effects of melatonin. For instance, melatonin enhanced low Mg2+-induced epileptiform activity in the hippocampus, whereas melatonin antagonists delayed the onset of pilocarpine-induced seizures. However, the relatively high doses of melatonin required to inhibit experimental seizures can induce some undesired effects (e.g., cognitive and motor impairment and decreased body temperature). In humans, melatonin may attenuate seizures, and it is most effective in the treatment of juvenile intractable epilepsy. Its additional benefits include improved physical, emotional, cognitive, and social functions. On the other hand, melatonin has been shown to induce electroencephalographic abnormalities in patients with temporal lobe epilepsy and increase seizure activity in neurologically disabled children. The hormone showed very low toxicity in clinical practice. The reported adverse effects (nightmares, hypotension, and sleep disorders) were rare and mild. However, more placebo-controlled, double-blind randomized clinical trials are needed to establish the usefulness of melatonin in the adjunctive treatment of epilepsy.

A systematic, updated review on the antidepressant agomelatine focusing on its melatonergic modulation

Objective:

To present an updated, comprehensive review on clinical and pre-clinical studies on agomelatine.

Method:

A MEDLINE, Psycinfo and Web of Science search (1966-May 2009) was performed using the following keywords: agomelatine, melatonin, S20098, efficacy, safety, adverse effect, pharmacokinetic, pharmacodynamic, major depressive disorder, bipolar disorder, Seasonal Affective Disorder (SAD), Alzheimer, ADHD, Generalized Anxiety Disorder (GAD), Panic Disorder (PD), Obsessive-Compulsive Disorder (OCD), anxiety disorders and mood disorder.

Study collection and data extraction:

All articles in English identified by the data sources were evaluated. Randomized, controlled clinical trials involving humans were prioritized in the review. The physiological bases of melatonergic transmission were also examined to deepen the clinical comprehension of agomelatine’ melatonergic modulation.

Data synthesis:

Agomelatine, a melatonergic analogue drug acting as MT₁/MT₂ agonist and 5-HT_2C antagonist, has been reported to be an effective antidepressant therapy.

Conclusions:

Although a bias in properly assessing the “sleep core” of depression may still exist with current screening instruments, therefore making difficult to compare agomelatine’ efficacy to other antidepressant ones, comparative studies showed agomelatine to be an intriguing option for depression and, potentially, for other therapeutic targets as well.

Melatonin agonists in primary insomnia and depression-associated insomnia: are they superior to sedative-hypnotics?

Current pharmacological treatment of insomnia involves the use of sedative-hypnotic benzodiazepine and non-benzodiazepine drugs. Although benzodiazepines improve sleep, their multiple adverse effects hamper their application. Adverse effects include impairment of memory and cognitive functions, next-day hangover and dependence. Non-benzodiazepines are effective for initiating sleep but are not as effective as benzodiazepines for improving sleep quality or efficiency. Furthermore, their prolonged use produces adverse effects similar to those observed with benzodiazepines. Inasmuch as insomnia may be associated with decreased nocturnal melatonin, administration of melatonin is a strategy that has been increasingly used for treating insomnia. Melatonin can be effective for improving sleep quality without the adverse effects associated with hypnotic-sedatives. Ramelteon, a synthetic analog of melatonin which has a longer half life and a stronger affinity for MT1 and MT2 melatonergic receptors, has been reportedly effective for initiating and improving sleep in both adult and elderly insomniacs without showing hangover, dependence, or cognitive impairment. Insomnia is also a major complaint among patients suffering from depressive disorders and is often aggravated by conventional antidepressants especially the specific serotonin reuptake inhibitors. The novel antidepressant agomelatine, a dual action agent with affinity for melatonin MT1 and MT2 receptors and 5-HT2c antagonistic properties, constitutes a new approach to the treatment of major depressive disorders. Agomelatine ameliorates the symptoms of depression and improves the quality and efficiency of sleep. Taken together, the evidence indicates that MT1/MT2 receptor agonists like ramelteon or agomelatine may be valuable pharmacological tools for insomnia and for depression-associated insomnia.

Melatonin's protective action against ischemic neuronal damage is associated with up-regulation of the MT2 melatonin receptor

Melatonin is a potent free radical scavenger and antioxidant and has protective effects against ischemic damage. In the present study, we examined the relationship between the neuroprotective effects of melatonin and the activation of MT2 melatonin receptor in the hippocampal CA1 region (CA1) after transient cerebral ischemia. MT2 immunoreactivity and protein levels were increased in the CA1 after ischemic damage. Most of MT2-immunoreactive cells were colocalized with astrocytes, not microglia, in the ischemic CA1. In the melatonin-sham group, MT2 immunoreaction and protein levels were increased compared with the sham group, and MT2 immunoreactivity and its protein levels in the melatonin-ischemia group were similar to those in the melatonin-sham group. In addition, melatonin treatment attenuated the activation of astrocytes and microglia. These results indicate that MT2 are increased and expressed in astrocytes in the ischemic region after an ischemic insult. The activation of MT2 melatonin receptor in the CA1 after melatonin treatment may be involved in the neuroprotective effect associated with melatonin after ischemic injury.

Mutation screening of ASMT, the last enzyme of the melatonin pathway, in a large sample of patients with intellectual disability

Background

Intellectual disability (ID) is frequently associated with sleep disorders. Treatment with melatonin demonstrated efficacy, suggesting that, at least in a subgroup of patients, the endogenous melatonin level may not be sufficient to adequately set the sleep-wake cycles. Mutations in ASMT gene, coding the last enzyme of the melatonin pathway have been reported as a risk factor for autism spectrum disorders (ASD), which are often comorbid with ID. Thus the aim of the study was to ascertain the genetic variability of ASMT in a large cohort of patients with ID and controls.

Methods

Here, we sequenced all exons of ASMT in a sample of 361 patients with ID and 440 controls. We then measured the ASMT activity in B lymphoblastoid cell lines (BLCL) of patients with ID carrying an ASMT variant and compared it to controls.

Results

We could identify eleven variations modifying the protein sequence of ASMT (ID only: N13H, N17K, V171M, E288D; controls only: E61Q, D210G, K219R, P243L, C273S, R291Q; ID and controls: L298F) and two deleterious splice site mutations (IVS5+2T>C and IVS7+1G>T) only observed in patients with ID. We then ascertained ASMT activity in B lymphoblastoid cell lines from patients carrying the mutations and showed significantly lower enzyme activity in patients carrying mutations compared to controls (p = 0.004).

Conclusions

We could identify patients with deleterious ASMT mutations as well as decreased ASMT activity. However, this study does not support ASMT as a causative gene for ID since we observed no significant enrichment in the frequency of ASMT variants in ID compared to controls. Nevertheless, given the impact of sleep difficulties in patients with ID, melatonin supplementation might be of great benefit for a subgroup of patients with low melatonin synthesis.

Neuroprotection by melatonin after germinal matrix hemorrhage in neonatal rats

BACKGROUND

Germinal matrix hemorrhage (GMH) is a devastating neurological disorder of very low birth weight premature infants that leads to post-hemorrhagic hydrocephalus, cerebral palsy, and mental retardation. Melatonin is a potent antioxidant known to reverse free-radical mediated injury in the brain. This study investigated the effect of melatonin treatment after GMH injury.

METHODS

Clostridial collagenase was infused into the right germinal matrix region of neonatal rats with stereotaxic technique. Cognitive function, sensorimotor ability, cerebral, cardiac and splenic growths were measured in juvenile animals.

RESULTS

Systemic melatonin treatment ameliorated cognitive and sensorimotor dysfunction at the juvenile developmental stage. This hormone also normalized brain atrophy, splenomegaly, and cardiac hypertrophy consequences at 1 month after injury.

CONCLUSION

This study supports the role of free radicals in acute neonatal hemorrhagic brain injury. Melatonin is an effective antioxidant that can protect the infant's brain from the post-hemorrhagic consequences of mental retardation and cerebral palsy. Further mechanistic studies are warranted to determine the mechanisms behind these neuroprotective effects.

Jet lag, circadian rhythm sleep disturbances, and depression: the role of melatonin and its analogs

Traveling through several time zones results in a constellation of symptoms known as jet lag. These include reduced alertness, daytime fatigue, loss of appetite, reduced cognitive skills, and disruption of the sleep/wake cycle. In susceptible air travel passengers, jet lag may exacerbate affective illness and result in psychiatric morbidity. Dysregulation of circadian rhythms and melatonin secretion represent the common underlying factor in jet lag and other circadian disorders. Recent studies have established the effectiveness of strategically timed administration of melatonin and appropriate timed exposure to environmental schedules including light in counteracting the dysregulation (chronobiologic actions). With the introduction of melatonergic agonists such as ramelteon and tasimelteon, which have both a stronger affinity for MT₁ and MT₂ melatonin receptors and a longer half-life, new therapeutic options now exist for treating the sleep disturbances associated with jet lag. The melatonin analogs are unique inasmuch as they can also enhance daytime alertness. The recently introduced melatonergic antidepressant agomelatine, which has established its supremacy over other antidepressants in having a significant chronobiologic activity, represents a good choice for treating depressive symptoms that are associated with jet lag.

Signal transduction of receptor-mediated antiproliferative action of melatonin on human prostate epithelial cells involves dual activation of Gα(s) and Gα(q) proteins

Melatonin has been shown to inhibit the proliferation of malignant and transformed human prostate epithelial cells by transcriptional up-regulation of p27(Kip1) expression via MTNR1A receptor-mediated activation of protein kinase A (PKA) and protein kinase C (PKC) in parallel. Given that melatonin MTNR1A receptor is a G protein-coupled receptor, this study was conducted to identify the specific G proteins that mediate the antiproliferative action of melatonin on human prostate epithelial cells. In 22Rv1 and RWPE-1 cells, knockdown of either Gα(s) or Gα(q) , but not Gα(i2) expression by RNA interference, abrogated the effects of melatonin on p27(Kip1) and cell proliferation. Conversely, cellular overexpression of activated mutants of Gα(s) and Gα(q) in 22Rv1 and RWPE-1 cells mimicked the effects of melatonin on prostate epithelial cell antiproliferation by increasing p27(Kip1) expression through downstream activation of PKA and PKC in parallel. Moreover, melatonin or 2-iodomelatonin induced elevation of adenosine-3',5'-cyclic monophosphate (cAMP) in 22Rv1 and RWPE-1 cells. The effects of 2-iodomelatonin on cAMP were blocked by the nonselective MTNR1A/MTNR1B receptor antagonist luzindole but were not affected by the selective MTNR1B receptor antagonist 4-phenyl-2-propionamidotetraline (4-P-PDOT). Furthermore, knockdown of Gα(s) mitigated the stimulatory effects of 2-iodomelatonin on cAMP. Collectively, the data demonstrated, for the first time, functional coupling of MTNR1A receptor to Gα(s) in cancerous or transformed human cells expressing endogenous melatonin receptors. Our results also showed that dual activation of Gα(s) and Gα(q) proteins is involved in the signal transduction of MTNR1A receptor-mediated antiproliferative action of melatonin on human prostate epithelial cells.

Endogenous melatonin is not obligatory for the regulation of the rat sleep-wake cycle

STUDY OBJECTIVES

Though melatonin and melatonin receptor agonists are in clinical use and under development for treating insomnia, the role of endogenous melatonin in the regulation of the sleep-wake cycle remains uncertain. Some clinical case reports suggest that reduced nocturnal melatonin secretion is linked to sleep disruption, but pineal-gland removal in experimental animals has given variable results.

DESIGN

The present study examined the effects of pinealectomy on the diurnal sleep-wake cycle of rats implanted with a radiotransmitter to allow continuous measurement of cortical electroencephalogram, electromyogram, and core temperature (Tc) without restraint in their home cages.

MEASUREMENTS AND RESULTS

Tc was slightly (0.2 degrees C) but significantly lower after pineal removal. The total amount and diurnal distribution of locomotor activity, wake, non-rapid eye movement (NREM) sleep, and rapid eye movement (REM) sleep were unaltered in pinealectomized rats compared to sham-operated controls. Sleep consolidation measured by determining wake, NREM sleep, and REM sleep bout length and frequency was also unchanged. The EEG power spectrum during NREM sleep was unchanged, but a significant decrease in theta power (5-8 Hz) during REM sleep episodes was found.

CONCLUSIONS

Our data provide no evidence that endogenous circulating melatonin plays a role in regulating the sleep-wake cycle in rats. However, because cortical theta oscillations are generated in the CA1-3 layer of the hippocampus, neurons known to express melatonin receptors, this suggests that a lack of melatonin following pineal removal influences the function of these neurons and is consistent with previous work suggesting that endogenous melatonin is an important regulator of hippocampal physiology.

Protective effect of melatonin upon neuropathology, striatal function, and memory ability after intracerebral hemorrhage in rats

Since free radicals play a role in the mechanisms of brain injury after hemorrhagic stroke, the effect of melatonin (a potent antioxidant and free-radical scavenger) on outcomes was investigated after intracerebral hemorrhage (ICH) in rats. ICH was induced by clostridial collagenase infusion into the right caudate putamen, and several time points and doses of melatonin were studied. Brain edema and neurological function at 24 h were unchanged in comparison with vehicle-treated groups, in spite of oxidative stress reductions. Repeated treatment with the lower dose of melatonin (5 mg/kg) given at 1 h and every 24 h thereafter for 3 days after ICH, led to normalization of striatal function and memory ability over the course of 8 weeks, and less brain atrophy 2 weeks later. These results suggest that melatonin is safe for use after ICH, reduces oxidative stress, provides brain protection, and could be used for future investigations of free radical mechanisms after cerebral hemorrhage.

Light deprivation improves melatonin related suppression of hippocampal plasticity

In early postnatal life, sensory inputs deeply influence development as well as function of the brain. Plasticity of synaptic transmission including its experimentally induced form, long-term potentiation (LTP), is affected by sensory deprivation in neocortex. This study is devoted to assess if dark rearing and a dark phase synthesized hormone melatonin influence LTP in the hippocampus, an area of brain involved in learning and memory. In vivo experiments were carried out on two groups of 45-days-old male Wistar rats kept in standard 12-h light/dark condition [light reared (LR) tested during the light phase] or in complete darkness [dark reared (DR)] since birth to testing. Each group, in turn, was divided to two, vehicle- and melatonin-treated, groups. Stimulating the Schaffer collaterals of CA3 area of hippocampus extracellular postsynaptic potentials (EPSPs) were recorded in the CA1 area. Having the stable baseline responses to the test pulses, the hippocampus was perfused by either vehicle or 2 microg melatonin and EPSPs were recorded for 30 min. Then, for induction of LTP, the tetanus was applied to the Schaffer collaterals and the field potentials were pooled for 120-min post-tetanus. The light deprivation resulted in a significant augmentation in the amplitude of baseline responses. Also, we observed a melatonin-induced increase in amplitude of the baseline recordings in either LR or DR animals. Tetanic stimulation elicited LTP of EPSPs in both LR and DR groups, robustly in the former where it lasted for about 90 min. Generally, melatonin inhibited the production of LTP in the two groups especially in the LR animals leading to a noticeable depression. We concluded that higher level of neuronal activity in the DR rats gives rise to a lower level of LTP. Weaker effect of melatonin on blocking the potentiation of post-tetanus EPSPs in the DR rats may be the result of a desensitization of melatonin receptors due to chronically increased levels of this hormone in the visually deprived rats.

MT2 melatonin receptor immunoreactivity in neurons is very high in the aged hippocampal formation in gerbils

Melatonin exerts many physiological functions via its G protein-coupled receptors. In the present study, we investigated age-related changes in MT2 melatonin receptor immunoreactivity and its levels in the gerbil hippocampus during normal aging. In the postnatal month 1 (PM 1) group, MT2 immunoreaction was well observed in neurons in all subregions of the gerbil hippocampus. In the PM 3 and 6 groups, MT2 immunoreactivity in neurons was decreased compared to that in the PM 1 group. Thereafter, MT2 immunoreactivity in neurons was increased. In the PM 18 and 24 groups, MT2 immunoreactivity in neurons was strong in all subregions of the gerbil hippocampus. In addition, the number of MT2 immunoreactive cells was lowest at PM 3 and highest at PM 24. From western blot analysis, age-dependent change pattern in MT2 level in the gerbil hippocampus was similar to the immunohistochemical result. These results indicate that MT2 immunoreactivity and levels are altered in the gerbil hippocampus during normal aging; lowest at young adult stage and highest at aged stage.

Effects of preoperative oral melatonin medication on postoperative analgesia, sleep quality, and sedation in patients undergoing elective prostatectomy: a randomized clinical trial

PURPOSE

Our intention was to assess the effectiveness of preoperative oral melatonin medication on sedation, sleep quality, and postoperative analgesia in patients undergoing elective prostatectomy.

METHODS

Fifty-two ASA I-II patients undergoing elective prostatectomy were included in this study, randomly divided into two groups. Patients received an oral placebo (n = 26) or 6 mg melatonin (n = 26) the night before and 1 h before surgery. All patients received a standard anesthetic protocol. At the end of surgery, all patients received tramadol i.v. via a PCA device. Extubation time, intraoperative fentanyl consumption, and recovery time were assessed at the end of the operation. Pain scores, tramadol consumption, and sedation scores were assessed at 1, 2, 4, 6, 12, 18, and 24 h postoperatively, and sleep quality and subjective analgesic efficacy were assessed at 24 h after surgery.

RESULTS

There were no significant differences in demographic data between the groups. Extubation time and recovery time from anesthesia were significantly longer in the melatonin group (P < 0.05). Intraoperative fentanyl usage, pain scores, and tramadol consumption were significantly lower in the melatonin group (P < 0.05). The postoperative sleep quality of patients was significantly better in the melatonin group than in the control group (P < 0.05). Postoperative VAS of pain was significantly lower in the melatonin group compared with the control group at 1, 2, 4, 6, 12, 18, and 24 h postoperatively (P < 0.05). Subjective analgesic efficacy of patients was significantly different between groups (P < 0.05). The sedation scores were significantly higher in the melatonin group than in the control group at 1 h and 2 h after surgery (P < 0.05).

CONCLUSIONS

Preoperative oral melatonin administration decreased pain scores and tramadol consumption and enhanced sleep quality, sedation scores, and subjective analgesic efficacy during the postoperative period.

Reproductive and diurnal rhythms regulate vocal motor plasticity in a teleost fish

Seasonal and circadian rhythms control fundamental physiological processes including neural excitability and synaptic plasticity that can lead to the periodic modulation of motor behaviors like social vocalizations. Parental male midshipman fish produce three call types during the breeding season: long duration (min to >1 h) advertisement 'hums', frequency and amplitude modulated agonistic 'growls' (s), and very brief (ms) agonistic 'grunts' produced either singly or repetitively as ;grunt trains' for up to several minutes. Fictive grunts that establish the temporal properties of natural grunts are readily evoked and recorded in vivo from vocal occipital nerve roots at any time of day or year by electrical microstimulation in either the midbrain periaqueductal gray or a hindbrain vocal pre-pacemaker nucleus. Now, as shown here, the longer duration fictive growls and hums can also be elicited, but are restricted to the nocturnal reproductive season. A significant drop in call threshold accompanies the fictive growls and hums that are distinguished by their much longer duration and lower and more regular firing frequency. Lastly, the long duration fictive calls are dependent upon increased stimulation time and intensity and hence may result from activity-dependent changes in the vocal motor circuit that are themselves modulated by seasonal and circadian rhythms.

The end of a myth: cloning and characterization of the ovine melatonin MT(2) receptor

BACKGROUND AND PURPOSE

For many years, it was suspected that sheep expressed only one melatonin receptor (closely resembling MT(1) from other mammal species). Here we report the cloning of another melatonin receptor, MT(2), from sheep.

EXPERIMENTAL APPROACH

Using a thermo-resistant reverse transcriptase and polymerase chain reaction primer set homologous to the bovine MT(2) mRNA sequence, we have cloned and characterized MT(2) receptors from sheep retina.

KEY RESULTS

The ovine MT(2) receptor presents 96%, 72% and 67% identity with cattle, human and rat respectively. This MT(2) receptor stably expressed in CHO-K1 cells showed high-affinity 2[(125)I]-iodomelatonin binding (K(D)= 0.04 nM). The rank order of inhibition of 2[(125)I]-iodomelatonin binding by melatonin, 4-phenyl-2-propionamidotetralin and luzindole was similar to that exhibited by MT(2) receptors of other species (melatonin > 4-phenyl-2-propionamidotetralin > luzindole). However, its pharmacological profile was closer to that of rat, rather than human MT(2) receptors. Functionally, the ovine MT(2) receptors were coupled to G(i) proteins leading to inhibition of adenylyl cyclase, as the other melatonin receptors. In sheep brain, MT(2) mRNA was expressed in pars tuberalis, choroid plexus and retina, and moderately in mammillary bodies. Real-time polymerase chain reaction showed that in sheep pars tuberalis, premammillary hypothalamus and mammillary bodies, the temporal pattern of expression of MT(1) and MT(2) mRNA was not parallel in the three tissues.

CONCLUSION AND IMPLICATIONS

Co-expression of MT(1) and MT(2) receptors in all analysed sheep brain tissues suggests that MT(2) receptors may participate in melatonin regulation of seasonal anovulatory activity in ewes by modulating MT(1) receptor action.

Use of transdermal melatonin delivery to improve sleep maintenance during daytime

Oral melatonin (MEL) can improve daytime sleep, but the hormone's short elimination half-life limits its use as a hypnotic in shift workers and individuals with jet lag or other sleep problems. Here we show, in healthy subjects, that transdermal delivery of MEL during the daytime can elevate plasma MEL and reduce waking after sleep onset, by promoting sleep in the latter part of an 8-h sleep opportunity. Transdermal MEL may have advantages over fast-release oral MEL in improving sleep maintenance during adverse circadian phases.

Melatonin provides anxiolysis, enhances analgesia, decreases intraocular pressure, and promotes better operating conditions during cataract surgery under topical anesthesia

BACKGROUND

Melatonin has anxiolytic and potential analgesic effects. In this study, we assessed the effects of melatonin premedication on pain, anxiety, intraocular pressure (IOP), and operative conditions during cataract surgery under topical analgesia.

METHODS

Forty patients undergoing cataract surgery under topical anesthesia were randomly assigned into two groups (20 patients each) to receive either melatonin 10 mg tablet (melatonin group) or placebo tablet (control group) as oral premedication 90 min before surgery. Anxiety scores, verbal pain scores, heart rate, mean arterial blood pressure, and IOP were recorded. In addition, the surgeon was asked to rate operating conditions.

RESULTS

Melatonin significantly reduced the anxiety scores (median, interquartile range) from 5, 3.5-6 to 3, 2-3 after premedication and to 3, 2-3.5 during surgery (P = 0.04 and P = 0.005 compared with the placebo group, respectively). Perioperative verbal pain scores were significantly lower in the melatonin group with less intraoperative fentanyl requirement (median, interquartile range) compared with the control group, 0, 0-32.5 vs 47.5, 30-65 microg, respectively, P = 0.007. Melatonin also decreased IOP (mean +/- sd) significantly from 17.9 +/- 0.9 to 14.2 +/- 1.0 mm Hg after premedication and to 13.8 +/- 1.1 mm Hg during surgery (P < 0.001). It also provided better quality of operative conditions.

CONCLUSION

We concluded that oral melatonin premedication for patients undergoing cataract surgery under topical anesthesia provided anxiolytic effects, enhanced analgesia, and decreased IOP resulting in good operating conditions.

GABA synapses mediate neuroprotection after ischemic and epsilonPKC preconditioning in rat hippocampal slice cultures

Delayed neuroprotection against ischemic challenges is conferred by both ischemic preconditioning (IPC) and preconditioning by activation of the epsilon-isoform of protein kinase C (epsilonPKC-PC). In vivo, ischemic preconditioning enhances GABA release and ameliorates glutamate release during lethal cerebral ischemia. We tested the hypothesis that IPC and epsilonPKC-PC confer neuroprotection by GABA synapses in rat organotypic hippocampal slices. Ischemic preconditioning or epsilonPKC-PC was induced with 15 mins oxygen-glucose deprivation (OGD) or psiepsilonRACK, a selective epsilonPKC activator; and test ischemia consisted of 40 mins OGD. At the time of peak neuroprotection (48 h after preconditioning), we recorded GABA(A) receptor-mediated miniature postsynaptic currents (GABA mPSCs) in vulnerable CA1 pyramidal neurons using whole-cell voltage clamp techniques. The frequency and amplitude of GABA mPSCs significantly increased 48 h after IPC. In contrast, epsilonPKC-PC enhanced only the amplitude of GABA mPSCs with no effect on frequency. We next asked if neuroprotection depended on these changes in GABA synapses. Weak antagonism of the GABA(A) receptor with bicuculline (100 nmol/L) decreased the amplitude of GABA mPSCs by 20.9+/-6.1%. When applied during test ischemia, 100 nmol/L bicuculline abolished neuroprotection conferred by either IPC or epsilonPKC-PC. We conclude that neuroprotection conferred by preconditioning depends on functional modifications of GABA synapses.

Gene structures, biochemical characterization and distribution of rat melatonin receptors

G-protein coupled receptors for the pineal hormone melatonin have been partially cloned from rats. However, insufficient information about their cDNA sequences has hindered studies of their distribution and physiological responses to melatonin using rats as an animal model. We have cloned cDNAs of two rat membrane melatonin receptor subtypes, melatonin receptor 1a (MT1) and melatonin receptor 1b (MT2), using a rapid amplification of cDNA end (RACE) method. The rat MT1 and MT2 cDNAs encode proteins of 353 and 364 amino acids, respectively, and show 78-93% identities with the human and mouse counterparts. Stable expression of either rat MT1 or MT2 in NIH3T3 cells resulted in high affinity 2-[(125)I]-iodomelatonin ((125)I-Mel) binding (K (d) = 73.2 +/- 9.0 and 73.7 +/- 2.9 pM, respectively), and exhibited a similar rank order of inhibition of specific (125)I-Mel binding by five ligands (2-iodomelatonin > melatonin > 6-hydroxymelatonin > luzindole > N-acetyl-5-hydroxytryptamine). RT-PCR analysis showed that MT1 is highly expressed in the hypothalamus, lung, kidney, adrenal gland, stomach, and ovary, while MT2 is highly expressed in the hippocampus, kidney, and ovary. We also performed multi-cell RT-PCR to examine the expression of mRNAs encoding MT1 and MT2 in adult GnRH neurons. MT1 was weakly expressed in male GnRH neurons, and was less expressed in the female neurons. MT2 expression was undetectable in GnRH neurons from either sex. This study delineates the gene structures, fundamental properties, and distribution of both rat melatonin receptor subtypes, and may offer opportunities to assess the physiological significance of melatonin in rats.

Melatonin and its relevance to jet lag

Jet lag is a disorder in which body rhythms are out of phase with the environment because of rapid travel across time zones. Although it often produces minor symptoms it can cause serious problems in those who need to make rapid critical decisions including airline pilots and business travelers. In this article the authors review basic knowledge underlying the body clock, the suprachiasmatic nucleus (SCN) of the hypothalamus, and the manner in which it regulates the sleep/wake cycle. The regulation of melatonin by the SCN is described together with the role of the melatonin receptors which are integral to its function as the major hormonal output of the body clock. Several factors are known that help prevent and treat jet lag, including ensuring adequate sleep, appropriate timing of exposure to bright light and treatment with melatonin. Because travel can cross a variable number of time zones and in two different directions, recommendations for treatment are given that correspond with these different types of travel. In addition to use of bright light and melatonin, other factors including timed exercise, timed and selective diets and social stimuli deserve study as potential treatments. Moreover, new melatonin agonists are currently under investigation for treatment of jet lag.

Sexually dimorphic modulation of GABA(A) receptor currents by melatonin in rat gonadotropin-releasing hormone neurons

Gonadotropin-releasing hormone (GnRH) neurons represent the final output neurons in the central control of reproduction. gamma-Amino butyric acid (GABA), one of the major regulators of GnRH neurons, depolarizes GnRH neurons isolated from adult rats via GABA(A) receptors. The presence of GABA(A) receptors in GnRH neurons has also been demonstrated morphologically. Furthermore, the pineal hormone melatonin is involved in the regulation of reproductive function, including the timing of the luteinizing hormone surge. The suprachiasmatic nucleus and the GABAergic system in the medial preoptic area are considered as possible sites of the action of melatonin. Until now, however, a direct action of melatonin on GnRH neurons has not been reported. Therefore we examined the effect of melatonin on GABA(A) receptor currents in GnRH neurons isolated from GnRH-EGFP transgenic rats by means of perforated patch-clamp experiments. The GABA(A) receptor currents were modulated by melatonin in a sex-specific manner. In GnRH neurons from adult males, melatonin augmented these currents in 67% of the neurons examined, but attenuated the currents in only 19% of them. These modulations were blocked by the melatonin receptor antagonist luzindole, suggesting an involvement of melatonin receptors. The modulation by melatonin was not observed in GnRH neurons isolated from infantile rats. These findings indicate that GABA affects the excitability of GnRH neurons in adult rats through GABA(A) receptors, and that melatonin modifies this excitability via melatonin receptors in a sex-specific manner.

The possible interplay of synaptic and clock genes in autism spectrum disorders

Autism spectrum disorders (ASD) are complex neurodevelopmental conditions characterized by deficits in social communication, absence or delay in language, and stereotyped and repetitive behaviors. Results from genetic studies reveal one pathway associated with susceptibility to ASD, which includes the synaptic cell adhesion molecules NLGN3, NLGN4, and NRXN1 and a postsynaptic scaffolding protein SHANK3. This protein complex is crucial for the maintenance of functional synapses as well as the adequate balance between neuronal excitation and inhibition. Among the factors that could modulate this pathway are the genes controlling circadian rhythms. Indeed, sleep disorders and low melatonin levels are frequently observed in ASD. In this context, an alteration of both this synaptic pathway and the setting of the clock would greatly increase the risk of ASD. In this chapter, I report genetic and neurobiological findings that highlight the major role of synaptic and clock genes in the susceptibility to ASD. On the basis of these lines of evidence, I propose that future studies of ASD should investigate the circadian modulation of synaptic function as a focus for functional analyses and the development of new therapeutic strategies.

Melatonin receptors, heterodimerization, signal transduction and binding sites: what's new?

Melatonin is a neurohormone that has been claimed to be involved in a wide range of physiological functions. Nevertheless, for most of its effects, the mechanism of action is not really known. In mammals, two melatonin receptors, MT1 and MT2, have been cloned. They belong to the G-protein-coupled receptor (GPCR) superfamily. They share some specific short amino-acid sequences, which suggest that they represent a specific subfamily. Another receptor from the same subfamily, the melatonin-related receptor has been cloned in different species including humans. This orphan receptor also named GPR50 does not bind melatonin and its endogenous ligand is still unknown. Nevertheless, this receptor has been shown to behave as an antagonist of the MT1 receptor, which opens new pharmacological perspectives for GPR50 despite the lack of endogenous or synthetic ligands. Moreover, MT1 and MT2 interact together through the formation of heterodimers at least in cells transfected with the cDNA of these two receptors. Lastly, signalling complexes associated with MT1 and MT2 receptors are starting to be deciphered. A third melatonin-binding site has been purified and characterized as the enzyme quinone reductase 2 (QR2). Inhibition of QR2 by melatonin may explain melatonin's protective effect that has been reported in different animal models and that is generally associated with its well-documented antioxidant properties.

Melatonin, consciousness, and traumatic stress

Descartes intuitively anticipated the so-called 'binding problem' of consciousness and thought that the pineal gland enables spatio-temporal integration in cognitive processing. Recent findings indicate that a major role in the process of temporal integration and binding involve neurons in suprachiasmatic nuclei, specifically targeting the pineal gland and other structures, and control the neuroendocrine rhythms. Melatonin is an endocrine output signal of the clock and provides circadian information as an endogenous synchronizer which stabilizes and reinforces circadian rhythms. This integrative process occurs at the different levels of the circadian network via gene expression in some brain regions and peripheral structures that enables integration of circadian, hormonal, and metabolic information and creating temporal order of bodily and mental experience. This specific temporal order is reflected in associative sequentiality that is necessary for cognition, behavior and all processes of memory consolidation that must preserve all information in the temporal causal order and synchrony. In this context, recent findings suggest that melatonin could be a potential regulator in the processes that contribute to memory formation, long-term potentiation, and synaptic plasticity in the hippocampus and other brain regions. There is evidence that stress disrupts normal activity and memory consolidation in the hippocampus and prefrontal cortex, and this process leads to memories that are stored without a contextual or spatiotemporal frame. These findings emphasize a specific role of melatonin in mechanisms of consciousness, memory and stress and are also consistent with reported studies that indicate melatonin alterations under stressful conditions and in mental disorders.

Abnormal melatonin synthesis in autism spectrum disorders

Melatonin is produced in the dark by the pineal gland and is a key regulator of circadian and seasonal rhythms. A low melatonin level has been reported in individuals with autism spectrum disorders (ASD), but the underlying cause of this deficit was unknown. The ASMT gene, encoding the last enzyme of melatonin synthesis, is located on the pseudo-autosomal region 1 of the sex chromosomes, deleted in several individuals with ASD. In this study, we sequenced all ASMT exons and promoters in individuals with ASD (n=250) and compared the allelic frequencies with controls (n=255). Non-conservative variations of ASMT were identified, including a splicing mutation present in two families with ASD, but not in controls. Two polymorphisms located in the promoter (rs4446909 and rs5989681) were more frequent in ASD compared to controls (P=0.0006) and were associated with a dramatic decrease in ASMT transcripts in blood cell lines (P=2 x 10(-10)). Biochemical analyses performed on blood platelets and/or cultured cells revealed a highly significant decrease in ASMT activity (P=2 x 10(-12)) and melatonin level (P=3 x 10(-11)) in individuals with ASD. These results indicate that a low melatonin level, caused by a primary deficit in ASMT activity, is a risk factor for ASD. They also support ASMT as a susceptibility gene for ASD and highlight the crucial role of melatonin in human cognition and behavior.

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

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

Role of melatonin in the eye and ocular dysfunctions

Melatonin is a ubiquitous molecule and widely distributed in nature, with functional activity occurring in unicellular organisms, plants, fungi, and animals. Several studies have indicated that melatonin synthesis occurs in the retina of most vertebrates, including mammals. The retinal biosynthesis of melatonin and the mechanisms involved in the regulation of this process have been extensively studied. Circadian clocks located in the photoreceptors and retinal neurons regulate melatonin synthesis in the eye. Photoreceptors, dopaminergic amacrine neurons, and horizontal cells of the retina, corneal epithelium, stroma endothelium, and the sclera all have melatonin receptors, indicating a widespread ocular function for melatonin. In addition, melatonin is an effective antioxidant which scavenges free radicals and up-regulates several antioxidant enzymes. It also has a strong antiapoptotic signaling function, an effect that it exerts even during ischemia. Melatonin cytoprotective properties may have practical implications in the treatment of ocular diseases, like glaucoma and age-related macular degeneration.

Melatonin enhances the rewarding properties of morphine: involvement of the nitric oxidergic pathway

Melatonin has different interactions with opioids including the enhancement of the analgesic effects of morphine and also reversal of tolerance and dependence to morphine. The present study assessed the effect of melatonin on morphine reward in mice using a conditioned place preference (CPP) paradigm. Our data showed that subcutaneous administration of morphine (1-7.5 mg/kg) significantly increased the time spent in the drug-paired compartment in a dose-dependent manner. Intraperitoneal (i.p.) administration of melatonin (1-40 mg/kg) alone did not induce either CPP or conditioned place aversion (CPA), while the combination of melatonin (5-20 mg/kg) and sub-effective dose of morphine (0.5 mg/kg) led to rewarding effect. We further investigated the involvement of the nitric oxidergic pathway in the enhancing effect of melatonin on morphine CPP, by a general nitric oxide synthase inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME). L-NAME (1 and 5 mg/kg, i.p.) alone or in combination with morphine (0.5 mg/kg) did not show any significant CPP or CPA. Co-administration of L-NAME (5 mg/kg) with an ineffective combination of melatonin (1 mg/kg) plus morphine (0.5 mg/kg) produced significant CPP that may imply the similarity of action of melatonin and L-NAME and involvement of the nitric oxidergic pathway in this regard. Our results indicate that pretreatment of animals with melatonin enhances the rewarding properties of morphine via a mechanism which may involve the nitric oxidergic pathway.

Melatonin and anesthesia: a clinical perspective

The hypnotic, antinociceptive, and anticonvulsant properties of melatonin endow this neurohormone with the profile of a novel hypnotic-anesthetic agent. Sublingually or orally administered melatonin is an effective premedicant in adults and children. Melatonin premedication like midazolam is associated with sedation and preoperative anxiolysis, however, unlike midazolam these effects are not associated with impaired psychomotor skills or the quality of recovery. Melatonin administration also is associated with a tendency toward faster recovery and a lower incidence of postoperative excitement than midazolam. Oral premedication with 0.2 mg/kg melatonin significantly reduces the propofol and thiopental doses required for loss of responses to verbal commands and eyelash stimulation. In rats, melatonin and the more potent melatonin analogs 2-bromomelatonin and phenylmelatonin have been found to have anesthetic properties similar to those of thiopental and propofol, with the added advantage of providing potent antinociceptive effects. The exact mechanism(s) by which structurally diverse intravenous and volatile anesthetics produce general anesthesia is still largely unknown, but positive modulation of gamma-aminobutyric acid type A (GABAA) receptor function has been recognized as an important and common pathway underlying the depressant effects of many of these agents. Accumulating evidence indicates that there is interplay between the melatonergic and GABAergic systems, and it has been demonstrated that melatonin administration produces significant, dose-dependent increases in GABA concentrations in the central nervous system. Additional in vitro data suggest that melatonin alters GABAergic transmission by modulating GABAA receptor function. Of greater importance, data from in vivo studies suggest that the central anesthetic effects of melatonin are mediated, at least in part, via GABAergic system activation, as they can be blocked or reversed by GABAA receptor antagonists. Further work is needed to better understand the general anesthetic properties of melatonin at the molecular, cellular, and systems levels.

The Effects of Melatonin Premedication on Propofol and Thiopental Induction Dose–Response Curves: A Prospective, Randomized, Double-Blind Study

BACKGROUND

The effect of melatonin on the intraoperative requirements for i.v.anesthetics has not been documented. We studied the effect of melatonin premedication on the propofol and thiopental dose-response curves for abolition of responses to verbal commands and eyelash stimulation.

METHODS

This prospective, randomized, double-blind study included 200 adults with ASA physical status I. Patients received either 0.2 mg/kg melatonin or a placebo orally for premedication (n = 100 per group). Approximately 50 min later, subgroups of 10 melatonin and 10 placebo patients were administered various doses of propofol (0.5, 1.0, 1.5, 2.0, or 2.4 mg/kg) or thiopental (2.0, 3.0, 4.0, 5.0, or 6.0 mg/kg) for anesthetic induction. The ability of each patient to respond to the command, "open your eyes," and the disappearance of the eyelash reflex were assessed 60 s after the end of the injection of propofol or thiopental. Dose-response curves were determined by probit analysis.

RESULTS

Melatonin premedication decreased thiopental ED50 values for loss of response to verbal command and eyelash reflex from 3.4 mg/kg (95% confidence interval, 3.2-3.5 mg/kg) and 3.7 mg/kg (3.5-3.9 mg/kg) to 2.7 mg/kg (2.6-2.9 mg/kg) and 2.6 mg/kg (2.5-2.7 mg/kg), respectively (P < 0.05). Corresponding propofol ED50 values decreased from 1.5 mg/kg (1.4-1.6 mg/kg) and 1.6 mg/kg (1.5-1.7 mg/kg) to 0.9 mg/kg (0.8-0.96 mg/kg) and 0.9 mg/kg (0.8-0.95 mg/kg), respectively (P < 0.05).

CONCLUSIONS

Melatonin premedication significantly decreased the doses of both propofol and thiopental required to induce anesthesia.

Effect of chronic consumption of sodium valproate and melatonin on seizure activity in Krushinskii-Molodkina rats

Experiments on Krushinskii-Molodkina rats with hereditary predisposition to audiogenic seizures showed that chronic consumption of aqueous solution of melatonin (50 mg/liter) had no effect on the pattern of seizures induced by 20-fold acoustic stimulation. Sodium valproate (50 mg/liter) insignificantly decreased the seizure response. Combined treatment with sodium valproate and melatonin produced a potent anticonvulsant effect, i.e. increased the latency and decreased the severity of audiogenic seizures. However, myoclonus in animals receiving combined treatment with these drugs developed much more rapidly compared to rats receiving melatonin or sodium valproate monotherapy.