Melatonin receptors in rat hippocampus: molecular and functional investigations

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.

Effects of monomethyltin and dimethyltin compounds on heterologously expressed neuronal ion channels (Xenopus oocytes) and synaptic transmission (hippocampal slices)

The aim of this study was to investigate the effects of monomethyltin trichloride (MMT) and dimethyltin dichloride (DMT) on various neuronal ion channels heterologously expressed in Xenopus oocytes and on synaptic transmission in hippocampal slices of young (14-21 days old) and adult (2-4 months old) rats. The Xenopus oocyte expression system was chosen to allow direct assessment of the effects of MMT and DMT both on glutamate receptors sensitive to AMPA and NMDA and on various voltage-operated potassium and sodium channels. Hippocampal slices were used to analyze the effects of MMT and DMT on synaptic potentials generated by the important excitatory Schaffer collateral-CA1 synapse. In general, MMT and DMT were found to have no effect either on voltage-operated sodium and potassium channels or on the metabotropic glutamate receptor but they did differentially affect the functions of ionotropic glutamate receptors and glutamatergic synaptic transmission. MMT (100 microM) significantly reduced NMDA-mediated ion currents by up to 32%, but had no effect on ion currents through AMPA receptors. In slices of adult rats, MMT had no effect on the amplitudes of evoked fEPSPs and brought about a 35% reduction in the LTP amplitudes. In contrast, in slices of young rats MMT evoked a reversible 30% increase in the amplitudes of fEPSPs but had no effect on LTP induction. DMT (100 microM) reduced ion currents through NMDA-receptor ion channels by up to 29% and those through AMPA-receptor ion channels by up to 7%. In hippocampal slices 100 microM DMT reduced the amplitudes of fEPSPs (adults: 50%; young rats: 70%) and LTP (adults: 40%; young rats: 55%). Neither of the organotins affected the paired-pulse facilitation at this synapse, indicating that the organotins exert their effects at the postsynaptic site. The action of MMT and DMT may contribute to the organotin-induced impairment of behavior patterns in connection with learning and memory.

Effects of melatonin on synaptic transmission and long-term potentiation in two areas of mouse hippocampus

We have examined the effects of melatonin on synaptic transmission and long-term potentiation (LTP) in the Schaffer Collateral-CA1 and the mossy fiber-CA3 pathways in mouse hippocampus brain slices. Melatonin (0.1-1 mM) application had different actions on both the field excitatory postsynaptic potentials (fEPSPS) and LTP in the CA1 as compared to the CA3. In CA1, 0.1 mM melatonin blocked LTP, while 1 mM melatonin also depressed the fEPSP. In CA3, neither 0.1 nor 1 mM melatonin altered the fEPSP, whereas both concentrations only slightly reduced LTP. These results demonstrate that melatonin significantly alters synaptic transmission and LTP in the CA1 but has only modest actions in CA3.

Chronic stress decreases the expression of sympathetic markers in the pineal gland and increases plasma melatonin concentration in rats

Chronic stress affects brain areas involved in learning and emotional responses. Although most studies have concentrated on the effect of stress on limbic-related brain structures, in this study we investigated whether chronic stress might induce impairments in diencephalic structures associated with limbic components of the stress response. Specifically, we analyzed the effect of chronic immobilization stress on the expression of sympathetic markers in the rat epithalamic pineal gland by immunohistochemistry and western blot, whereas the plasma melatonin concentration was determined by radioimmunoassay. We found that chronic stress decreased the expression of three sympathetic markers in the pineal gland, tyrosine hydroxylase, the p75 neurotrophin receptor and alpha-tubulin, while the same treatment did not affect the expression of the non-specific sympathetic markers Erk1 and Erk2, and glyceraldehyde-3-phosphate dehydrogenase. Furthermore, these results were correlated with a significant increase in plasma melatonin concentration in stressed rats when compared with control animals. Our findings indicate that stress may impair pineal sympathetic inputs, leading to an abnormal melatonin release that may contribute to environmental maladaptation. In addition, we propose that the pineal gland is a target of glucocorticoid damage during stress.

Chronic stress induces upregulation of brain-derived neurotrophic factor (BDNF) mRNA and integrin alpha5 expression in the rat pineal gland

Chronic stress affects brain areas involved in learning and emotional responses. These alterations have been related with the development of cognitive deficits in major depression. Moreover, stress induces deleterious actions on the epithalamic pineal organ, a gland involved in a wide range of physiological functions. The aim of this study was to investigate whether the stress effects on the pineal gland are related with changes in the expression of neurotrophic factors and cell adhesion molecules. Using reverse transcription-polymerase chain reaction (RT-PCR) and Western blot, we analyzed the effect of chronic immobilization stress on the BDNF mRNA and integrin alpha5 expression in the rat pineal gland. We found that BDNF is produced in situ in the pineal gland. Chronic immobilization stress induced upregulation of BDNF mRNA and integrin alpha5 expression in the rat pineal gland but did not produce changes in beta-actin mRNA or in GAPDH expression. Stressed animals also evidenced an increase in anxiety-like behavior and acute gastric lesions. These results suggest that BDNF and integrin alpha5 may have a counteracting effect to the deleterious actions of immobilization stress on functionally stimulated pinealocytes. Furthermore, this study proposes that the pineal gland may be a target of glucocorticoid damage during stress.

Effects of arsenite on long-term potentiation in hippocampal slices from young and adult rats

The effects of trivalent arsenite were tested at the Schaffer collateral-CA1 synapse of adult (2-4 month) and young (14-21 days) rats. Exposure of 100micromol/l arsenite led to a slight and reversible reduction of the amplitudes of evoked excitatory postsynaptic field potentials in adult and young rats, while exposure of 0.1 and 1micromol/l arsenite had no effects. The long-term potentiation (LTP) was significantly inhibited by arsenite in adult but not in young rats. Exposure of 0.1, 1 and 100micromol/l arsenite to slices of adult rats before and during the LTP stimulus led to a significant reduction in the potentiated amplitudes amounting to a maximum of 50% of the control values. In young animals, however, exposure of 1micromol/l arsenite showed no effect on the LTP potentiated amplitudes, while exposure of 100micromol/l arsenite led initially to a significant reduction in the amplitudes, compared to the control level, which was completely reversible 20min after washout. Exposure of 100micromol/l arsenite did not affect the paired-pulse facilitation, indicating that arsenite does not exert its effects by influencing presynaptic transmitter release mechanisms.

Short photoperiods impair spatial learning and alter hippocampal dendritic morphology in adult male white-footed mice (Peromyscus leucopus)

Although seasonal changes in brain morphology and function are well established in songbirds, seasonal plasticity of brain structure and function remain less well documented in mammals. Nontropical animals display many adaptations to reduce energy use to survive winter, including cessation of reproductive activities. Because of the high energetic costs of brain tissue, we hypothesized that male white-footed mice (Peromyscus leucopus) would reduce brain size in response to short days as well as regress their reproductive systems. Because short days may decrease hippocampal volume and impair spatial learning and memory in rodents and because of the potential for seasonal plasticity in the hippocampus, we hypothesized that photoperiod alters hippocampal morphology to affect spatial learning and memory. Mice housed in either long or short days for 10 weeks were examined for performance in a water maze; brains were then removed and weighed, and hippocampal volumes were determined. We also measured dendritic morphology and spine density in the CA1, CA3, and dentate gyrus. Short days decreased brain mass and hippocampal volume compared with long days. Short days also impaired long-term spatial learning and memory relative to long days but did not affect sensory discrimination or other types of memory. Short days decreased apical (stratum lacunosum-moleculare) CA1 spine density, as well as increased basilar (stratum oriens) CA3 spine density. Results from this study suggest that photoperiod alters brain size and morphology, as well as cognitive function. Understanding the mechanisms mediating these photoperiod-induced alterations may provide insight for treatment of seasonal cognitive and affective disorders.

Pinealectomy does not affect diurnal PER2 expression in the rat limbic forebrain

A role for the pineal hormone, melatonin, in the regulation of the rhythmic expression of circadian clock genes is suggested by the finding that surgical removal of the pineal gland abolishes the rhythm of expression of clock genes such as Per1 in several neural and endocrine tissues in rodents, including the caudate-putamen (CP) and nucleus accumbens, the hypophyseal pars tuberalis and adrenal cortex. Pinealectomy has no effect on clock gene rhythms in the suprachiasmatic nucleus (SCN), the master circadian clock, as well as in the eyes and heart, indicating that the effect of melatonin on clock gene rhythms is tissue specific. To further study the role of melatonin in the regulation of the rhythm of clock genes, we assessed in rats the effect of pinealectomy on the rhythm of expression of the clock protein, PER2, in a number of key limbic forebrain structures, the oval nucleus of the bed nucleus of the stria terminalis (BNST-OV), the central nucleus of the amygdala (CEA) and the hippocampus (HIPP). Despite previous evidence showing that these regions are sensitive to melatonin, pinealectomy had no effect on the daily rhythm of expression of PER2 within these structures, further supporting the view that the role of endogenous melatonin in the regulation of clock gene expression is tissue specific.

Melatonin receptors in humans: biological role and clinical relevance

In addition to its antioxidative effects melatonin acts through specific nuclear and plasma membrane receptors. To date, two G-protein coupled melatonin membrane receptors, MT(1) and MT(2), have been cloned in mammals, while the newly purified MT(3) protein belongs to the family of quinone reductases. Screening studies have shown that various tissues of rodents express MT(1) and/or MT(2) melatonin receptors. In humans, melatonin receptors were also detected in several organs, including brain and retina, cardiovascular system, liver and gallbladder, intestine, kidney, immune cells, adipocytes, prostate and breast epithelial cells, ovary/granulosa cells, myometrium, and skin. This review summarizes the data published so far about MT(1) and MT(2) receptors in human tissues and human cells. Established and putative functions of melatonin after receptor activation as well as the clinical relevance of these findings will be discussed.

Melatonin inhibits hippocampal long-term potentiation

The goal of this study is to investigate the effect of the hormone melatonin on long-term potentiation and excitability measured by stimulating the Schaffer collaterals and recording the field excitatory postsynaptic potential from the CA1 dendritic layer in hippocampal brain slices from mice. Application of melatonin produced a concentration-dependent inhibition of the induction of long-term potentiation, with a concentration of 100 nm producing an approximately 50% inhibition of long-term potentiation magnitude. Long-duration melatonin treatments of 6 h were also effective at reducing the magnitude of long-term potentiation. Melatonin (100 nm) did not alter baseline evoked responses or paired-pulse facilitation recorded at this synapse. The inhibitory actions of melatonin were prevented by application of the melatonin (MT) receptor antagonist luzindole as well as the MT2 receptor subtype antagonist 4-phenyl-2-propionamidotetraline. These inhibitory actions of melatonin were lost in mice deficient in MT2 receptors but not those deficient in MT1 receptors. In addition, application of the protein kinase A inhibitor H-89 both mimicked the effects of melatonin and precluded further inhibition by melatonin. Finally, the application an activator of adenylyl cyclase, forskolin, overcame the inhibitory effects of melatonin on LTP without affecting the induction of long-term potentiation on its own. These results suggest that hippocampal synaptic plasticity may be constrained by melatonin through a mechanism involving MT2-receptor-mediated regulation of the adenylyl cyclase-protein kinase A pathway.

Sustained activation of Akt by melatonin contributes to the protection against kainic acid-induced neuronal death in hippocampus

In the present study, the underlying protective mechanism of melatonin on kainic acid (KA)-induced excitotoxicity was examined in the hippocampus of mice. KA, administered intracerebroventricularly (i.c.v.), induced marked neuronal cell death with concurrent microglial activation and subsequent induction of inducible nitric oxide synthase (iNOS) in the hippocampus. Histopathological analysis demonstrated that melatonin (10 mg/kg), administered 1 hr prior to KA, attenuated KA-induced death of pyramidal neurons in the CA3 region. Melatonin obviously suppressed KA-induced microglial activation and consequent iNOS expression that were determined by increased immunoreactivities of microglial marker OX-6 and iNOS, respectively. Increased phosphorylation of Akt in pyramidal neurons was observed as early as 2 hr after administration of melatonin. Further, melatonin resulted in increased expression of astroglial glial cell line-derived neurotrophic factor (GDNF), which started to appear approximately 6 hr after administration of melatonin. The results of the present study demonstrate that melatonin exerts its neuroprotective action against KA-induced excitotoxicity both through the activation of neuronal Akt and via the direct action on hippocampal neurons and through the increased expression of astroglial GDNF, which subsequently activates neuronal PI3K/Akt pathway. Therefore, the present study suggests that melatonin, pineal secretory product, is potentially useful in the treatment of acute brain pathologies associated with excitotoxic neuronal damage such as epilepsy, stroke, and traumatic brain injury.

Removal of G(ialpha1) constraints on adenylyl cyclase in the hippocampus enhances LTP and impairs memory formation

Stimulation of adenylyl cyclase in the hippocampus is critical for memory formation. However, generation of cAMP signals within an optimal range for memory may require a balance between stimulatory and inhibitory mechanisms. The role of adenylyl cyclase inhibitory mechanisms for memory has not been addressed. One of the mechanisms for inhibition of adenylyl cyclase is through activation of G(i)-coupled receptors, a mechanism that could serve as a constraint on memory formation. Here we report that ablation of G(ialpha1) by gene disruption increases hippocampal adenylyl cyclase activity and enhances LTP in area CA1. Furthermore, gene ablation of G(ialpha1) or antisense oligonucleotide-mediated depletion of G(ialpha1) disrupted hippocampus-dependent memory. We conclude that G(ialpha1) provides a critical mechanism for tonic inhibition of adenylyl cyclase activity in the hippocampus. We hypothesize that loss of G(ialpha1) amplifies the responsiveness of CA1 postsynaptic neurons to stimuli that strengthen synaptic efficacy, thereby diminishing synapse-specific plasticity required for new memory formation.

Circadian regulation of hippocampal long-term potentiation

The goal of this study is to investigate the possible circadian regulation of hippocampal excitability and long-term potentiation (LTP) measured by stimulating the Schaffer collaterals (SC) and recording the field excitatory postsynaptic potential (fEPSP) from the CA1 dendritic layer or the population spike (PS) from the soma in brain slices of C3H and C57 mice. These 2 strains of mice were of interest because the C3H mice secrete melatonin rhythmically while the C57 mice do not. The authors found that the magnitude of the enhancement of the PS was significantly greater in LTP recorded from night slices compared to day slices of both C3H and C57 mice. They also found significant diurnal variation in the decay of LTP measured with fEPSPs, with the decay slower during the night in both strains of mice. There was evidence for a diurnal rhythm in the input/output function of pyramidal neurons measured at the soma in C57 but not C3H mice. Furthermore, LTP in the PS, measured in slices prepared during the day but recorded during the night, had a profile remarkably similar to the night group. Finally, PS recordings were carried out in slices from C3H mice maintained in constant darkness prior to experimentation. Again, the authors found that the magnitude of the enhancement of the PS was significantly greater in LTP recorded from subjective night slices compared to subjective day slices. These results provide the 1st evidence that an endogenous circadian oscillator modulates synaptic plasticity in the hippocampus.

Blockade of glutamatergic and GABAergic receptor channels by trimethyltin chloride

1. Organotin compounds such as trimethyltin chloride (TMT) are among the most toxic of the organometallics. As their main target for toxicity is the central nervous system, the aim of the present study was to investigate the effects of TMT on receptor channels involved in various processes of synaptic transmission. 2. The Xenopus oocyte expression system was chosen for direct assessment of TMT effects on voltage-operated potassium channels and glutamatergic and GABAergic receptors, and hippocampal slices from rat brain for analyzing TMT effects on identified synaptic sites. 3. TMT was found to be ineffective, at 100 micromol l(-1), against several potassium- and sodium-operated ion channel functions as well as the metabotropic glutamate receptor. 4. The functions of the ionotropic glutamate and the GABA(A) receptor channels were inhibited by TMT in micromolar concentrations. Thus, at a maximum concentration of 100 micromol l(-1), around 20-30% of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and GABA(A) receptor-mediated ion currents and 35% of the N-methyl-D-aspartate receptor-mediated ion currents were blocked. 5. In the hippocampal slice model, the inhibitory effects of TMT were much stronger than expected from the results on the ion channels. Bath application of TMT significantly reduced the amplitudes of evoked excitatory postsynaptic field potentials in a concentration-dependent and nonreversible manner. 6. Induction of long-term potentiation, recorded from the CA1 dendritic region, was inhibited by TMT and failed completely at a concentration of 10 micromol l(-1). 7. In general, TMT affects the excitatory and inhibitory synaptic processes in a receptor specific manner and is able to disturb the activity within a neuronal network.

Hippocampal melatonin receptors modulate seizure threshold

PURPOSE

The pineal hormone melatonin has been shown to enhance hippocampal excitability. We therefore investigated whether inactivation of hippocampal melatonin receptors affects behavioral seizures.

METHODS

Intrahippocampal infusions were performed in rats to study the effect of different melatonin receptor antagonists on behavioral activity, EEG, and seizure susceptibility. Experiments were conducted at 2 times of the day that coincided with the peak and trough of the daily melatonin rhythm.

RESULTS

Local infusion of the Mel(1b) receptor antagonist 4-phenyl-2-propionamidotetralin (4-P-PDOT) into the hippocampus, but not the overlying neocortex, significantly increased seizure latency and in some cases provided complete protection against seizure development. In addition, 4-P-PDOT suppressed open field activity and hippocampal EEG amplitude. The mixed Mel(1a)/Mel(1b) receptor antagonist luzindole also increased seizure latency but to a lesser degree than 4-P-PDOT. The behavioral effects of Mel(1b) receptor inhibition were comparable to those of the gamma-aminobutyric acid (GABA)(A) receptor agonist muscimol and were observed during the dark phase (2400-0200 h) but not the light phase (1200-1400 h) of the daily photocycle. The anticonvulsant effect of intrahippocampal infusion of 4P-P-DOT was blocked by coadministration of the GABA(A) antagonist bicuculline.

CONCLUSIONS

Our results suggest that nocturnal activation of hippocampal Mel(1b) receptors depresses GABA(A) receptor function in the hippocampus and enhances seizure susceptibility.

Melatonin affects the temporal organization of the song of the zebra finch

In birds and mammals, including humans, melatonin-binding sites are abundant in brain areas that have no known clock function. Although the role of such binding sites is still unclear, it is assumed that these sites link neural functions to circadian or circannual demands of neuroendocrine homeostasis and reproduction. To investigate a possible direct role of melatonin in motor control, we studied the song and neural song system of the zebra finch. Neurons of two sensory-motor areas of the descending song control circuit that are crucial for the organization of the song pattern, the HVC and RA, express the melatonin-1B receptor (Mel1B), while the hypoglossal motor neurons of the song circuit express melatonin-1C receptors (Mel1C). Application of melatonin to brain slices decreases the firing-rate of RA-neurons. Systemic administration of a Mel1B antagonist at the beginning of the night shortens the song and motif length and affects the song syllable lengths produced the next day. The temporal pattern of the song, however, does not undergo daily changes. Thus, melatonin is likely to affect a non-circadian motor pattern by local modulation of song control neurons and in consequence alters a sexual signal, the song of the zebra finch.

The actions of a charged melatonin receptor ligand, TMEPI, and an irreversible MT2 receptor agonist, BMNEP, on mouse hippocampal evoked potentials in vitro

We have previously determined that melatonin modulates hippocampal synaptic transmission in a biphasic way: an initial depression was followed by a recovery/amplification phase. Here we describe the influence of two novel melatonin receptor ligands, BMNEP (N-bromoacetyl-2-iodo-5-methoxytryptamine) and TMPEI (N-[2-(2-Trimethylammoniumethyleneoxy-7-methoxy)ethyl]propionamide iodide), on the population spike (PS) and excitatory postsynaptic potentials (EPSP) recorded from mouse hippocampal slices. BMNEP, which specifically alkylates and constitutively activates the MT2 melatonin receptor, mimicked the first phase of melatonin's action by irreversibly depressing both the PS and EPSP. TMPEI, a charged ligand of plasma membrane melatonin receptors, amplified those potentials in a manner similar to the effect of melatonin observed during the second, recovery phase. Melatonin had no influence on the potentials amplified by the action of TMPEI. Our results suggest that the biphasic, receptor-dependent action of melatonin and its analogs modulates the efficiency of the hippocampal glutamergic synapse and is most likely mediated through two different, sequentially occurring mechanisms.

Pro-oxidant activity of aluminum in the rat hippocampus: gene expression of antioxidant enzymes after melatonin administration

Aluminum (Al)-induced pro-oxidant activity and the protective role of exogenous melatonin, as well as the mRNA levels of some antioxidant enzymes, were determined in the hippocampi of rats following administration of Al and/or melatonin. Two groups of male rats were intraperitoneally injected with Al (as Al lactate) or melatonin only, at doses of 7 and 10 mg/kg/day, respectively, for 11 weeks. During this period, a third group of animals received Al (7 mg/kg/day) plus melatonin (10 mg/kg/day). At the end of the treatment, hippocampus was removed and processed to examine the following oxidative stress markers: glutathione transferase (GST), reduced glutathione (GSH), oxidized glutathione (GSSG), superoxide dismutase (SOD), glutathione reductase (GR), glutathione peroxidase (GPx), catalase (CAT), thiobarbituric acid reactive substances (TBARS), as well as protein content. Gene expression of Cu-ZnSOD, MnSOD, GPx, and CAT was evaluated by real-time RT-PCR. On the other hand, Al, Fe, Mn, Cu, and Zn concentrations in hippocampus were also determined. The results show that Al exposure promotes oxidative stress in the rat hippocampus, with an increase in Al concentrations. The biochemical changes observed in this tissue indicate that Al acts as pro-oxidant agent, while melatonin exerts antioxidant action by increasing the mRNA levels of the antioxidant enzymes evaluated. The protective effects of melatonin, together with its low toxicity and its capacity to increase mRNA levels of antioxidant enzymes, suggest that this hormone might be administered as a potential supplement in the treatment of neurological disorders in which oxidative stress is involved.

Reduced hippocampal MT2 melatonin receptor expression in Alzheimer's disease

The aim of the present study was to identify the distribution of the second melatonin receptor (MT2) in the human hippocampus of elderly controls and Alzheimer's disease (AD) patients. This is the first report of immunohistochemical MT2 localization in the human hippocampus both in control and AD cases. The specificity of the MT2 antibody was ascertained by fluorescence microscopy using the anti-MT2 antibody in HEK 293 cells expressing recombinant MT2, in immunoblot experiments on membranes from MT2 expressing cells, and, finally, by immunoprecipitation experiments of the native MT2. MT2 immunoreactivity was studied in the hippocampus of 16 elderly control and 16 AD cases. In controls, MT2 was localized in pyramidal neurons of the hippocampal subfields CA1-4 and in some granular neurons of the stratum granulosum. The overall intensity of the MT2 staining was distinctly decreased in AD cases. The results indicate that MT2 may be involved in mediating the effects of melatonin in the human hippocampus, and this mechanism may be heavily impaired in AD.

Expression of membrane and nuclear melatonin receptors in mouse peripheral organs

Previous studies have shown that melatonin acts through specific receptors, including MT(1) and MT(2) membrane receptors as well as a nuclear receptor belonging to the orphan nuclear receptor family. Therefore, the goal of this study was to determine whether melatonin receptors mRNA is expressed in mouse peripheral tissues. To study the different receptors subtype expression, we have used a reverse-transcription polymerase chain reaction (RT-PCR) procedure followed by Southern hybridization with specific digoxigenin-labeled probes. RT-PCR studies revealed the presence of both MT(1) membrane receptors and ROR(alpha)1 nuclear receptors in all the peripheral tissues examined (brain, heart, lung, liver and kidney). Moreover, the expression of ROR(alpha)1 nuclear receptors was also demonstrated by Western-blot. In contrast, expression of MT(2) membrane receptors was only observed in brain and lung. These results suggest that melatonin, acting through its different subtypes receptors, plays a role in the neuroendocrine regulation of peripheral tissues function.

Effect of melatonin on the oxidative stress in N1E-115 cells is not mediated by mt1 receptors

To explore if protective effect of melatonin on oxidative stress induced by okadaic acid, an inhibitor of protein phosphatases PP1 and PP2A, is mediated by membrane receptors subtype mt1, we used an in vitro model with N1E-115 neuroblastoma cells. We demonstrated that exposure of cells to 50 nM okadaic acid for 2 h induces a reduction in the activity of antioxidative enzymes, and an increase of lipid peroxidation products, while melatonin prevents the effect of okadaic acid. On the other hand, the presence of luzindole, 20 min before adding melatonin, did not cause changes on the effect of the melatonin on oxidative stress. These results seem to indicate that protective effect of melatonin is not mediated by mt1 receptors.

Diurnal actions of melatonin on epileptic activity in hippocampal slices of rats

Since melatonin receptors have been found in the hippocampus of mammals it has been suggested that melatonin can modulate neuronal functions of hippocampal cells. The effect of melatonin (10 nM/l and 1 microM/l) on frequency and amplitude of epileptiform field potentials (EFP) elicited by low Mg(2+) or by bicuculline was tested in the CA1 region of hippocampal slices of rats. In the low Mg(2+) model, melatonin, applied in a near physiological concentration of 10 nM/l, exerts no effect on EFP in slices prepared at night or during the day. In a concentration of 1 microM/l, however, melatonin enhances the frequency of EFP to approximately 140% in slices prepared during the day. This effect was suppressed through simultaneous administration of the melatonin receptor antagonist luzindole (10 microM/l). In contrast, melatonin did not affect epileptic activity in slices prepared at night. Epileptiform discharges elicited by blocking the GABAergic inhibition (bicuculline model) were not affected by melatonin, either during the day or at night. The results indicate that melatonin affects epileptic activity in a diurnal manner and that the action of melatonin is different in relation to the epilepsy model.

Melatonin receptor signaling in pregnant and nonpregnant rat uterine myocytes as probed by large conductance Ca2+-activated K+ channel activity

The mRNAs of MT1 and MT2 melatonin receptors are present in cells from nonpregnant (NPM) and pregnant (PM) rat myometrium. To investigate the coupling of melatonin receptors to Gq- and Gi-type of heterotrimeric G proteins, we analyzed the activity of large-conductance Ca2+-activated K+ (BKCa) channels, the expression of which in the uterus is confined to smooth muscle cells. The melatonin receptor agonist 2-iodomelatonin induced a pertussis toxin (PTX)-insensitive increase in channel open probability that was blocked by the nonselective antagonist luzindole. The 2-iodomelatonin effect on channel open probability was suppressed by overexpression of the Gqalpha-inactivating protein RGS16 and the phospholipase C inhibitor U-73122. The activity of BKCa channels is differentially regulated by protein kinase A (PKA) in NPM and PM cells. Thus, the beta-adrenoceptor agonist isoprenaline inhibited the BKCa channel conducted whole-cell outward current (Iout) in NPM cells and enhanced Iout in PM cells. Additional application of 2-iodomelatonin antagonized the isoprenaline effect on Iout in NPM cells but enhanced Iout in PM cells. All 2-iodomelatonin effects on Iout were sensitive to PTX treatment and the PKA inhibitor H-89. We therefore conclude that melatonin activates both the PTX-insensitive Gq/phospholipase C/Ca2+ and the PTX-sensitive Gi/cAMP/PKA signaling pathway in rat myometrium.

Gene expression and functional characterization of melatonin receptors in the spinal cord of the rat: implications for pain modulation

Recently, a species-dependent distribution of melatonin binding sites have been found in lamina I-V and lamina X of the spinal cord. In order to learn more about the function of spinal melatonin receptors, we investigated (i) the gene expression for melatonin receptor subtypes in lumbar and thoracal spinal cord tissue by means of the reverse-transcriptase polymerase chain reaction (RT-PCR) technique, and (ii) the electrophysiological and pharmacological properties of melatonin receptors heterologously expressed in Xenopus oocytes after injection of spinal cord mRNA by means of the voltage clamp technique. Because ample evidence indicates an antinociceptive effect of melatonin, (iii) the role of spinal melatonin receptors for maintaining mechanical and thermal hyperalgesia was studied in a rat model for postoperative pain. The RT-PCR data revealed that transcripts for MT1 and MT2 melatonin receptors are present in the dorsal and ventral horn of lumbar and thoracal spinal cord tissue. Injection of mRNA from lumbar spinal cord tissue into Xenopus oocytes led to the functional reconstitution of melatonin receptors which activate calcium-dependent chloride inward currents. Melatonin responses were abolished by simultaneous administration of the antagonists, 2-phenylmelatonin and luzindole and were unaffected by the MT2 antagonist 4-phenyl-2-propionamidotetralin. Intrathecal administration of different melatonin doses (10-100 nmol) did not inhibit mechanical or thermal hyperalgesia. However, intrathecal application of a low dose of morphine together with melatonin caused a brief antinociceptive effect suggesting an enhanced morphine analgesia by melatonin. In conclusion, the present study demonstrated for the first time the presence of transcripts of MT1 and MT2 receptors located in the dorsal and ventral horn of the spinal cord. Furthermore, spinal melatonin enhanced the antinociceptive effect of morphine indicating that melatonin acts as a neuromodulator in the spinal cord.

Melatonin regulates neuronal plasticity in the hippocampus

The influence of melatonin on hippocampal evoked potentials initiated by low- and high-frequency electrical stimulations and by two pulses applied in rapid succession was investigated. In confirmation of our previous studies, melatonin attenuated the population spike triggered by low-frequency stimulation (0.03 Hz). High-frequency stimulation (HFS; 100 Hz for 1 sec, three times every 10 sec), which in control slices permanently facilitated neuronal excitability (347% +/- 32%), was also able to amplify the melatonin-depressed potential (467.8% +/- 59.6%). Because melatonin is a hydrophobic molecule, it was dissolved and applied in ethanol. Ethanol (0.4%) by itself reduced the magnitude of HFS-induced potentiation (233.5% +/- 16.8%). The slices stimulated with two pulses separated with a delay longer than 15 msec demonstrated a facilitation of the response to the second stimuli (paired-pulse facilitation; PPF). The influence of melatonin (100 microM) on PPF was biphasic: Shortly after addition of melatonin, PPF was briefly (5-10 min) reversed to paired-pulse inhibition (PPI), which gradually returned to a stable PPF. Ethanol (0.4%) applied without melatonin exerted only a marginal, facilitatory effect on PPF. The delay between two successively applied pulses, shorter than 13 msec, resulted in attenuation of the response to the second stimuli (PPI). Melatonin (100 microM) reversed the attenuation of the second potential within 15-20 min following its application. Ethanol applied by itself at the concentration of 0.4% temporarily (5-10 min), but significantly, depressed the second potential. These results demonstrate the ability of melatonin to modulate specific forms of plasticity in hippocampal pyramidal neurons.