Melatonin inhibits amphetamine-induced increase in alpha-synuclein and decrease in phosphorylated tyrosine hydroxylase in SK-N-SH cells.

alpha-Synuclein is an abundant presynaptic protein implicated in neuronal plasticity and neurodegeneration disorders. Understanding alpha-synuclein function in dopaminergic cells could add to our knowledge of this key protein which is implicated in Parkinson's disease. Chronic or intermittent amphetamine (AMPH) abuse may create temporary or permanent disturbances in the dopaminergic system of the brain that may predispose individuals to Parkinsonism. Our previous studies showed that neurotoxicity induced by AMPH was mediated by enhanced oxidative stress and these effects were abolished by melatonin, a main secretory product of pineal gland. The present study was conducted to investigate the effect of AMPH on alpha-synuclein in regulating tyrosine hydroxylase (TH), a rate limiting enzyme for dopamine synthesis, in cultured human dopaminergic SK-N-SH cells. Of these, phosphorylation of Ser40 (pSer40) contributes significantly to TH activation and dopamine synthesis. Our data indicated that AMPH significantly increased the level of alpha-synuclein to 183% of the control value while reducing the levels of phosphorylated TH (TH-pSer40) enzyme and mitochondrial complex I to 78 and 52.9% of the control values, respectively and these effects were attenuated by melatonin. Further studies are needed to explore the mechanism by which alpha-synuclein contributes to TH-pSer40 dephosphorylation and the mechanism by which melatonin contributes to this interaction.

Melatonin protects SK-N-SH neuroblastoma cells from amphetamine-induced neurotoxicity.

Several hypotheses regarding the mechanism underlying amphetamine-induced neurotoxicity have been proposed. One of them is based on the observation of free radical formation and oxidative stress produced by auto-oxidation of dopamine (DA). The formation of DA-related reactive oxygen species (ROS) such as superoxide and hydroxyl radicals appears to play an important role in amphetamine-induced neurotoxicity. Melatonin, the main secretory product of pineal gland, is well known for its protective effects that are currently attributed mainly to its radical scavenging and antioxidant properties. The present study was conducted to investigate the protective effects of melatonin on d-amphetamine (AMPH)-induced neurotoxicity in cultured human dopaminergic neuroblastoma SK-N-SH cells. Our data indicate that AMPH significantly reduces cell viability, induces oxidative stress (enhances ROS production and malondialdehyde levels), up-regulates alpha-synuclein expression and decreases intracellular ATP levels. However, pretreatment of SK-N-SH cells with melatonin prevents AMPH-induced loss of cell viability and induction of oxidative stress, while reducing alpha-synuclein expression and increasing ATP production. These results suggest that the antioxidant properties of melatonin may provide a protective mechanism against AMPH-induced neuronal degeneration.

Attenuation of cocaine and methamphetamine neurotoxicity by coenzyme Q10.

The neurotoxic effects of cocaine and methamphetamine (METH) were studied in mice brain with a primary objective to determine the neuroprotective potential of coenzyme Q10 (CoQ10) in drug addiction. Repeated treatment of cocaine or METH induced significant reduction in the striatal dopamine and CoQ10 in mice. Cocaine or METH-treated mice exhibited increased thiobarbituric acid reactive substances (TBARs) in the striatum and cerebral cortex without any significant change in the cerebellum. Complex I immunoreactivity was inhibited in both cocaine and METH-treated mice, whereas tyrosine hydroxylase (TH) immunoreactivity was decreased in METH-treated mice and increased in cocaine-treated mice. Neither cocaine nor METH could induce significant change in alpha-synuclein expression at the doses and duration we have used in the present study. CoQ10 treatment attenuated cocaine and METH-induced inhibition in the striatal 18F-DOPA uptake as determined by high-resolution microPET neuroimaging. Hence exogenous administration of CoQ10 may provide neuroprotection in drug addiction.

Melatonin exerts its analgesic actions not by binding to opioid receptor subtypes but by increasing the release of beta-endorphin an endogenous opioid.

The occurrence of systematic diurnal variations in pain thresholds has been demonstrated in human. Salivary melatonin levels change following acute pain when other factors that could explain the change have been removed or controlled. Melatonin-induced analgesia is blocked by naloxone or pinealectomy. By using selective radioligands [3H]-DAMGO, [3H]-DPDPE, [3-U69593, and 3H]-nociceptin, we have shown that the bovine pinealocytes contain delta and mu, but not kappa or ORL1 opioid receptor subtypes. In the present study, by using melatonin receptor agonists (6-chloromelatonin or 2-iodo-N-butanoyl-5-methoxytryptamine) or melatonin receptor antagonist (2-phenylmelatonin), we have shown that these agents do not compete with opioid receptor subtypes. However, we observed a time-dependent release of beta-endorphin an endogenous opioid peptide, by melatonin from mouse pituitary cells in culture. Hence, it is suggested that melatonin exerts its analgesic actions not by binding to opioid receptor subtypes but by binding to its own receptors and increasing the release of beta-endorphin.