Emerging drugs for major depressive disorder

Quercetin reverses chronic unpredictable mild stress-induced depression-like behavior in vivo by involving nuclear factor-E2-related factor 2

Quercetin is a flavonoid compound rich in many natural plants with a wide range of pharmacological effects and nutritional value. Although previous studies have initially shown the antidepressant effect of quercetin in some models. However, the exact mechanism of the antidepressant effect of quercetin on the depression model induced by chronic unpredictable mild stress (CUMS) is still unclear or has not been clearly elucidated. The present study aimed to investigate the antidepressant effect of quercetin in vivo on a CUMS-induced depression model that is closest to human depression, and to explore its mechanism of action around nuclear factor-E2-related factor 2 (Nrf2) related signaling pathways, for the first time. Our results demonstrated that CUMS for 21 consecutive days caused significant decreases in the sucrose preference, and the horizontal score and vertical score in the open field test of mice respectively by 22.6%, 34.4%, and 66.6% (all P < 0.01), and a significant increase in the immobility time during the forced swimming test by 110.5% (P < 0.01), but fortunately, after chronic oral administration of high dose quercetin at 40 mg/kg, the abnormalities of the above indicators were significantly reversed by 26.2%, 40.1%, 152.7%, 43.5% (all P < 0.01). Further western blot analysis showed that CUMS caused the phosphorylation or expression levels of phosphoinositide 3-kinase (PI3K), protein kinase B (Akt), Nrf2 and heme oxygenase-1 (HO-1) proteins in the hippocampus of mice to significantly down-regulate by 60.0%, 72.1%, 90.0% and 50.1% (all P < 0.01), while after chronic oral administration of high dose quercetin at 40 mg/kg, the abnormalities of these proteins were significantly up-regulated by 85.8%, 182.0%, 325.1% and 60.3% (all P < 0.01). In addition, CUMS also caused significant reduction in the levels of antioxidants including superoxide dismutase (SOD) and glutathione-s transferase (GST) in the mice hippocampus by 51.3%, 40.3% (both P < 0.01), while after chronic oral administration of high dose quercetin at 40 mg/kg, the abnormalities of the above indicators were significantly reversed by 69.2% and 49.5% (both P < 0.01), as well as significant elevation in the levels of lipid peroxide malondialdehyde (MDA), inflammation medium nitric oxide (NO) and inducible nitric oxide synthase (iNOS) by 156.4%, 255.4% and 72.7% (all P < 0.01), while after chronic oral administration of high dose quercetin at 40 mg/kg, the abnormalities of the above indicators were significantly reversed by 45.9%, 26.8% and 55.2% (all P < 0.01). The medium dose of quercetin (20 mg/kg) only reversed some of the above indicators, while the low dose of quercetin (10 mg/kg) had no reversal effect on the above indicators. Collectively, the present study confirmed for the first time that quercetin weakened CUMS-induced depression in vivo, and its mechanism was at least partially attributable to the upregulation of hippocampal Nrf2 and the inhibition of iNOS, thereby correcting the central inflammatory response, and the imbalance between oxidation and antioxidant.

Phencynonate hydrochloride exerts antidepressant effects by regulating the dendritic spine density and altering glutamate receptor expression

Phencynonate hydrochloride (PCH) is a drug that crosses the blood-brain barrier. Cellular experiments confirmed that PCH protects against glutamate toxicity and causes only weak central inhibition and limited side effects. As shown in our previous studies, PCH alleviates depression-like behaviours induced by chronic unpredictable mild stress (CUMS). Here we administered PCH at three different doses (4, 8 and 16 mg/kg) to male rats for two continuous days after CUMS and conducted behavioural tests to assess the dose-dependent antidepressant effects of PCH and its effects on the neuroplasticity in the hippocampus and medial prefrontal cortex (mPFC). Meanwhile, we measured the spine density and expression of related proteins to illustrate the mechanism of PCH. PCH treatment (8 mg/kg) significantly alleviated depression-like behaviours induced by CUMS. All doses of PCH treatment reversed the spine loss in prelimbic and CA3 regions induced by CUMS. Kalirin-7 expression was decreased in the hippocampus and mPFC of the CUMS group. The expression of the NR1 and NR2B subunits in the hippocampus, and NR2B in mPFC are increased by CUMS. PCH treatment (8 and 16 mg/kg) reversed all of these changes of Kalirin-7 in PFC and hippocampus, as well as NR1 and NR2B expression in the hippocampus. PCH is expected to be developed as a new type of rapid antidepressant. Its antidepressant effect may be closely related to the modulation of dendritic spine density in the prelimbic and CA3 regions and the regulation of Kalilin-7 and N-methyl-D-aspartic acid receptor levels in the hippocampus.

Perilla aldehyde attenuates CUMS-induced depressive-like behaviors via regulating TXNIP/TRX/NLRP3 pathway in rats

BACKGROUND

Current evidence supports that inflammatory reaction in the hippocampus is a potential cause of major depressive disorder (MDD). Perilla aldehyde (PAH), a major constituent from Perilla frutescens, has been reported to have anti-inflammatory and anti-oxidant activity. The aim of this study is to explore the antidepressant-like effect and the underlying mechanism of PAH on the rats model induced by chronic unpredictable mild stress (CUMS).

METHODS

CUMS rats model was built to tested their depressive-like behaviors. The levels of pro-inflammatory cytokines were tested. Proteins were analyzed by Western blot and Immunohistochemistry.

RESULTS

We found that treatment with PAH (20, 40 mg/kg) and fluoxetine (FLU, 10 mg/kg) significantly improved the sucrose consumption, immobility time in forced swim test (FST), as well as locomotor activity in open-field test (OFT). The levels of pro-inflammatory cytokines in hippocampus were also suppressed effectively by PAH and FLU administration. Western blot analysis showed the up-regulated levels of TXNIP, NLRP3, Cleaved caspase-1 and p-NF-κB p65 in the hippocampus in rats exposed to CUMS paradigm, while different degrees of down-regulation in their expression were detected after PAH (20, 40 mg/kg) and FLU (10 mg/kg) treatment respectively. The results from histopathological examination further demonstrated that PAH (20, 40 mg/kg) and FLU (10 mg/kg) treatment reversed the alteration of TRX, NLRP3 and Cleaved caspase-1 induced by CUMS procedure.

CONCLUSIONS

Our results demonstrated that PAH exhibited antidepressant-like effect in CUMS-induced rats model of depression, which might be mediated by TXNIP/TRX/NLRP3 pathway.

Elevated dopamine D1 receptor availability in striatum of Göttingen minipigs after electroconvulsive therapy

Electroconvulsive therapy (ECT), a direct form of brain stimulation, is an effective antidepressant. We hypothesized that the beneficial effects of ECT are mediated by increased dopaminergic neurotransmission, in which the baseline activity of D1 receptors may predict the response to ECT. We established a novel model of brain stimulation in Göttingen minipigs based on the protocol of ECT applied in humans. With positron emission tomography (PET), we determined a measure of dopaminergic neurotransmission with the dopamine D1 receptor antagonist [¹¹C]SCH23390. Seven minipigs were anesthetized and completed PET at baseline, prior to the onset of ECT treatment, and at 24-48 h and 8-10 days after the end of a clinical course of ECT, consisting of 10 ECT sessions over a 3.5-week period. In all pigs, the binding of [¹¹C]SCH23390 to striatal D1 receptors had increased by 24-48 h after ECT, and in most, binding returned towards baseline at 8-10 days. Increased binding was observed in inverse proportion to baseline binding rates. Increased binding to dopamine D1 receptors suggests facilitation of dopaminergic neurotransmission, which may contribute to the therapeutic effects of ECT. Importantly, the baseline binding capacity of D1 receptors predicts the magnitude of increased binding, up to a maximum binding capacity.

Changes of TSPO-mediated mitophagy signaling pathway in learned helplessness mice

Low response rate was witnessed with the present monoaminergic based antidepressants, urging a need for new therapeutic target identification. Accumulated evidences strongly suggest that mitochondrial deficit is implicated in major depression and 18kDa translocator protein (TSPO) plays an important role in regulating mitochondrial function. However the changes of TSPO and TSPO mediated mitophagy pathway in the depressive brain is unclear. In present study, a well validated animal model of depression, learned helplessness (LH), was employed to investigate the relevant changes. Significant behavioral changes were observed in the LH mice. Results showed that TSPO and other mitophagy related proteins, such as VDAC1, Pink1 and Beclin1 were significantly decreased by LH challenge. Moreover, KIFC2, relevant to the mitochondrial transport and Snap25, relevant to neurotransmitter vesicle release, were also obviously down-regulated in the LH mice, which further rendered supportive evidence for the existing mitochondrial dysfunction in LH mice. Present results demonstrated that LH induced depressive symptoms and affected TSPO-mediated mitophagy pathway, indicating a potential target candidate for depression treatment.

Effect of memantine combination therapy on symptoms in patients with moderate-to-severe depressive disorder: randomized, double-blind, placebo-controlled study

WHAT IS KNOWN AND OBJECTIVE

Current treatments for depressive disorders are far from optimum. This study was planned to evaluate possible antidepressant effects and safety of memantine, a selective N-methyl-d-aspartate receptor antagonist, in humans.

METHODS

Sixty-six outpatients with the diagnosis of moderate-to-severe major depressive disorder, based on DSM-V diagnostic criteria, were recruited to participate in a parallel, randomized, controlled trial. Sixty-two participants completed 6 weeks of treatment with either memantine (20 mg/day) plus sertraline (200 mg/day) or placebo plus sertraline (200 mg/day). Patients were evaluated using the Hamilton Depression Rating Scale (HDRS) at baseline and at weeks 2, 4 and 6. Comparison of treatment efficacy in improving depressive symptoms between the two groups was the principal outcome measure.

RESULTS AND DISCUSSION

A repeated-measures analysis demonstrated significant time × treatment interaction on HDRS score [F (2·09, 125·67) = 5·09, P = 0·007]. Significantly greater improvement was seen at all three follow-up sessions as well as significantly greater response rates at weeks 4 and 6 (P = 0·018 and P < 0·001, respectively) in the memantine group. Significantly more early improvers and more rapid response to treatment were observed in the memantine group (P = 0·001 and P < 0·001, respectively). A significant reduction was observed in HDRS score from baseline to the study endpoint in both memantine (P < 0·001, Cohen's d = 12·71) and placebo groups (P < 0·001, Cohen's d = 5·13). No serious adverse event occurred. No significantly greater remission rate was seen in the adjunctive memantine therapy.

WHAT IS NEW AND CONCLUSION

A 6-week course of treatment with memantine as adjunct to sertraline showed a favourable safety and efficacy profile in patients with major depressive disorder. Nonetheless, larger controlled studies of longer duration are necessary to assess long-term safety, efficacy and optimal dosing.

Wuling powder prevents the depression-like behavior in learned helplessness mice model through improving the TSPO mediated-mitophagy

ETHNOPHARMACOLOGICAL RELEVANCE

Wuling powder (trade name: Wuling capsule), a traditional Chinese medicine (TCM), was extracted from mycelia of precious Xylaria Nigripes (Kl.) Sacc by modern fermentation technology, and has been claimed to be fully potent in improving the signs of insomnia and cognitive deficits. Moreover, Wuling capsule was effective in treating post-stroke and orther co-cormbid depression both in clinical and in basic research. In order to clarify the molecular mechanisms of the antidepressant effect of Wuling powder, we established learned helplessness (LH) depression animal model and focused on 18kDa translocator protein (TSPO) mediated-mitophagy pathway.

MATERIALS AND METHODS

Mice were exposed to the inescapable e-shock (IS) once a day for three consecutive days to establish the LH model. Then mice were orally administered Wuling powder for 2 weeks. For the behavioral assessment, Shuttle box test, novelty suppressed feeding test (NSF) and forced swimming test (FST) were performed. Following the behavioral assessment, we assessed the protein expression level that were related to TSPO-mediated mitophagy signaling pathway by Western blotting analysis. Finally, immunohistochemistry method was used to assess the neuroprotective effects of Wuling powder.

RESULTS

Compared with mice that were subjected to inescapable e-shock, Wuling powder exhibited antidepressant effect in the multiple behavioral tests. In addition, Wuling powder altered the expression level of multiple proteins related to TSPO-mediated mitophagy signaling pathway.

CONCLUSIONS

Our results suggested that Wuling powder exhibited an obvious antidepressant effect, which could be due to the improvement of TSPO-mediated mitophagy signaling pathway.

Treadmill exercise after social isolation increases the levels of NGF, BDNF, and synapsin I to induce survival of neurons in the hippocampus, and improves depression-like behavior

[Purpose]

We investigated the effects of 8 weeks of treadmill exercise on nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and synapsin I protein expression and on the number of 5-bromo-2'-deoxyuridine-5'-mono-phosphate (BrdU)-positive cells in the dentate gyrus of the hippocampus in socially isolated rats. Additionally, we examined the effects of exercise on the number of serotonin (5-HT)- and tryptophan hydroxylase (TPH)-positive cells in the raphe nuclei and on depression behaviors induced by social isolation.

[Methods]

Forty male Sprague-Dawley rats were divided into four groups: (1) group housing and control group (GCG, n = 10); (2) group housing and exercise group (GEG, n = 10); (3) isolated housing and control group (ICG, n = 10); and (4) isolated housing and exercise group (IEG, n = 10). After 1 week of housing under the normal condition of 3 animals per cage, rats were socially isolated via transfer to individual cages for 8 weeks. Rats were then subjected to treadmill exercise for 5 days per week for 8 weeks during which time the speed of the treadmill was gradually increased.

[Results]

Compared to the GCG, levels of NGF, BDNF, and synapsin I were significantly decreased in the ICG and significantly increased in the IEG (p < 0.001 respectively). Significantly more BrdU-positive cells in the GEG were present as compared to the GCG and ICG, and more BrdU-positive cells were found in the IEG as compared to the ICG (p < 0.001). 5-HT-positive cells in the GEG were significantly increased compared to the GCG and ICG, and more of these cells were found in the IEG as compared to the ICG (p < 0.01). TPH-positive cells in the GEG were significantly increased compared to those in the GCG and ICG (p < 0.05). In the forced swim test, immobility time was significantly increased in the ICG and significantly decreased in the IEG as compared to the ICG (p < 0.01).

[Conclusion]

These results showed that regular treadmill exercise following social isolation not only increased the levels of NGF, BDNF, and synapsin I to induce survival of neurons in the hippocampus but also improved depression by increasing the number of serotonergic cells in the raphe nuclei.

An intricate relationship between pain and depression: clinical correlates, coactivation factors and therapeutic targets

INTRODUCTION

An apparent clinical relationship between pain and depression has long been recognized, which makes an enormous impact on the individual health care. At present, the practical implication of such overlapping symptomatology between pain and depression is not clear, but the prevalence estimates for depression are substantially inflated among patients with chronic pain and vice versa. This interaction has been labeled as the depression-pain syndrome or depression-pain dyad.

AREAS COVERED

This article discusses the neurobiological substrates and neuroanatomical pathways involved in pain-depression dyad along with newer therapeutic targets.

EXPERT OPINION

Several key themes emerged from our review of the relationship between depression and pain. First, the diagnosis of depression in pain or vice versa is particularly challenging, and the development of better diagnostic framework that involves both pain and depression is particularly required. Secondly, the entwined relationship between pain and depression supports the possibility of common coactivating factors that results in their neurophysiological overlap. A broad understanding of the role played by the central nervous system (CNS) in the processing of pain and depression may eventually lead to the introduction of triple reuptake inhibitors, agomelatine, vilazodone and ketamine with novel mechanism of action, hence appear to be of promising potential for pain with depression.

Effects of lurasidone in behavioral models of depression. Role of the 5-HT₇ receptor subtype

Major depression is a common psychiatric disorder associated with high symptomatic and functional burdens. Pharmacological treatment is often effective, but there remain substantial unmet needs in the form of non-responders, delayed onset of clinical effect, and side effects. Recent studies have positioned the serotonin 5-HT₇ receptor as a new target for the treatment of depression. Preclinical studies have shown that antagonists induce an antidepressant-like response, a phenotype that can also be observed in mice lacking the receptor. Lurasidone is a new atypical antipsychotic agent with very high affinity for the 5-HT₇ receptor. Patients in clinical trials have reported improved scores in depression ratings. We have tested lurasidone in both acute and chronic mouse models of depression. In the tail suspension and forced swim tests lurasidone decreased immobility, an antidepressant-like response. The effect required functional 5-HT₇ receptors as it was absent in mice lacking the receptor. In the repeated open-space swim test lurasidone was able to reverse the despair induced by repeated swims in a manner similar to the commonly used antidepressant citalopram. The results provide evidence that lurasidone can act as a 5-HT₇ receptor antagonist and provide a possible explanation for the antidepressant effect data currently emerging from lurasidone clinical trials. Additionally, the results give further support for targeting the 5-HT₇ receptor in the treatment of depression. It will be of interest to clinically evaluate lurasidone as an antidepressant either as monotherapy or as an adjunctive therapy to available drugs.

[Integral Care Guide for Early Detection and Diagnosis of Depressive Episodes and Recurrent Depressive Disorder in Adults. Integral Attention of Adults with a Diagnosis of Depressive Episodes and Recurrent Depressive Disorder: Part II: General Aspects of Treatment, Management of the Acute Phase, Continuation and Maintenance of Patients with a Depression Diagnosis]

INTRODUCTION

This article presents recommendations based on evidence gathered to answer a series of clinical questions concerning the depressive episode and the recurrent depressive disorder, with emphasis on general treatment aspects, treatment in the acute phase and management of the continuation/maintenance, all intended to grant health care parameters based on the best and more updated available evidence for achieving minimum quality standards with adult patients thus diagnosed.

METHODOLOGY

A practical clinical guide was elaborated according to standards of the Methodological Guide of the Ministry of Social Protection. Recommendation from NICE90 and CANMAT guides were adopted and updated so as to answer the questions posed while de novo questions were developed.

RESULTS

Recommendations 5-22 corresponding to management of depression are presented.

Inflammatory biomarkers in depression: an opportunity for novel therapeutic interventions

Currently available antidepressants are effective in less than two thirds of depressed patients, with even lower remission rates in the context of co-morbid medical illness. A rapidly expanding evidence base suggests that maladaptive inflammatory immune responses may be a common pathophysiology underlying depression, particularly in the presence of a general medical condition. The inflammatory hypothesis of depression marks a significant shift away from monoamine-based approaches and is a major step towards developing novel treatments that directly target causal factors of depression. Many antidepressants exert anti-inflammatory effects and there is an emerging literature documenting the efficacy of anti-inflammatory agents as adjunctive treatments for depression. Identification of inflammatory biomarkers in depression will require a re-conceptualization of both the diagnostic phenomenology and the experimental approaches to studying multi-determined psychiatric disorders. In addition to their application in diagnosis, predicting prognosis, and monitoring severity and response to treatment, inflammatory biomarkers may serve as novel therapeutic targets in the treatment of depression.

The keys to improving depression outcomes

The heterogeneity of symptoms within major depressive disorder poses significant challenges for treatment and it is likely that current pharmacotherapies do not target all symptoms equally, although they have similar efficacy rates. While there is still continuing interest in understanding monoamine interactions and consequent downstream effects, the limited efficacy and tolerability achieved with classical antidepressants provides a compelling argument to move beyond the monoamines. Several lines of biological research in depression exploring immune function, neurotrophins, amino acid and neuropeptide neurotransmitters, neuroanatomical function and circadian rhythms, may lead to novel therapeutic targets and enhance depression outcomes. This review will evaluate the evidence for emerging treatments as well as recommendations from current international guidelines regarding antidepressant management.

Strategies to achieve clinical effectiveness: refining existing therapies and pursuing emerging targets

BACKGROUND

Clinical effectiveness reflects a balance between efficacy and tolerability as well as patient satisfaction and overall improvement in quality of life and function. This is of particular importance when considering the long term use of antidepressant therapies for relapse prevention.

METHODS

The purpose of this review is to explore methods to enhance the modest efficacy and effectiveness outcomes reported with current antidepressant strategies. Two strategies are addressed: a) Doing better with existing treatments and b) pursuing novel targets beyond the monoamine system for new antidepressant drug development.

RESULTS

In the first instance, it is important to consider the balance between antidepressant efficacy and tolerability for individual patients and also be aware of evidence supporting superiority of one agent over others. Both sequential and concurrent combination therapies with existing antidepressants are also reviewed. The second approach involves a review of emerging novel pharmacological treatments based on biomarker research. Unique targets where antidepressant treatments appear effective include the melatonergic, glutamatergic, neurotrophic, cytokine, and neuropeptide systems.

CONCLUSIONS

While agomelatine represents an example of a clinically available antidepressant that targets melatonin receptors, drugs that act on other candidate systems are still in the development phase.

A 5-HT(2C) receptor antagonist potentiates a low dose amphetamine-induced conditioned place preference

This study was designed to determine whether a 5-HT(2C) receptor antagonist could induce a conditioned place preference indicative of reward and/or abuse potential. Here, we present the first evidence that a selective 5-HT(2C) receptor antagonist, 6-chloro-5-ethoxy-N-(pyridin-2-yl)indoline-1-carboxamide hydrochloride (CEPC), can potentiate a low dose (0.5 mg/kg) amphetamine-induced positive conditioned place preference (CPP). CEPC did not produce any CPP given alone at doses of either 2.0 or 4.0 mg/kg, whereas low dose amphetamine alone produced only a slight, but statistically nonsignificant, place preference. These studies suggest that 5-HT(2C) receptor antagonists can indirectly potentiate the rewarding effects of amphetamine, and perhaps other psychostimulants. If the results can be translated to man, putative 5-HT(2C) receptor antagonist treatments for anxiety or depression may enhance or potentiate the rewarding effects of drugs of abuse such as amphetamine, which release dopamine.

In vivo brain microdialysis: advances in neuropsychopharmacology and drug discovery

INTRODUCTION: Microdialysis is an important in vivo sampling technique, useful in the assay of extracellular tissue fluid. The technique has both pre-clinical and clinical applications but is most widely used in neuroscience. The in vivo microdialysis technique allows measurement of neurotransmitters such as acetycholine (ACh), the biogenic amines including dopamine (DA), norepinephrine (NE) and serotonin (5-HT), amino acids such as glutamate (Glu) and gamma aminobutyric acid (GABA), as well as the metabolites of the aforementioned neurotransmitters, and neuropeptides in neuronal extracellular fluid in discrete brain regions of laboratory animals such as rodents and non-human primates. AREAS COVERED: In this review we present a brief overview of the principles and procedures related to in vivo microdialysis and detail the use of this technique in the pre-clinical measurement of drugs designed to be used in the treatment of chemical addiction, neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD) and as well as psychiatric disorders such as attention-deficit/hyperactivity disorder (ADHD) and schizophrenia. This review offers insight into the tremendous utility and versatility of this technique in pursuing neuropharmacological investigations as well its significant potential in rational drug discovery. EXPERT OPINION: In vivo microdialysis is an extremely versatile technique, routinely used in the neuropharmacological investigation of drugs used for the treatment of neurological disorders. This technique has been a boon in the elucidation of the neurochemical profile and mechanism of action of several classes of drugs especially their effects on neurotransmitter systems. The exploitation and development of this technique for drug discovery in the near future will enable investigational new drug candidates to be rapidly moved into the clinical trial stages and to market thus providing new successful therapies for neurological diseases that are currently in demand.

Electroconvulsive therapy alters dopamine signaling in the striatum of non-human primates

Electroconvulsive therapy (ECT) is one of the most effective therapies for depression and has beneficial motor effects in parkinsonian patients. However, little is known about the mechanisms of therapeutic action of ECT for either condition. The aim of this work was to explore the impact of ECT on dopaminergic function in the striatum of non-human primates. Rhesus monkeys underwent a course of six ECT treatments under a human clinical protocol. Longitudinal effects on the dopaminergic nigrostriatal system were studied over 6 weeks using the in vivo capabilities of positron emission tomography (PET). PET scans were performed prior to the onset of ECT treatments and at 24-48 h, 8-10 days, and 6 weeks after the final ECT treatment. Early increases in dopamine transporter and vesicular monoamine transporter 2 binding returned to baseline levels by 6 weeks post-ECT. Transient increases in D1 receptor binding were also observed, whereas the binding potential to D2 receptors was unaltered. The increase in dopaminergic neurotransmission suggested by our results may account in part for the therapeutic effect of ECT in mood disorders and Parkinson's disease.

Neurotoxins: free radical mechanisms and melatonin protection

Toxins that pass through the blood-brain barrier put neurons and glia in peril. The damage inflicted is usually a consequence of the ability of these toxic agents to induce free radical generation within cells but especially at the level of the mitochondria. The elevated production of oxygen and nitrogen-based radicals and related non-radical products leads to the oxidation of essential macromolecules including lipids, proteins and DNA. The resultant damage is referred to as oxidative and nitrosative stress and, when the molecular destruction is sufficiently severe, it causes apoptosis or necrosis of neurons and glia. Loss of brain cells compromises the functions of the central nervous system expressed as motor, sensory and cognitive deficits and psychological alterations. In this survey we summarize the publications related to the following neurotoxins and the protective actions of melatonin: aminolevulinic acid, cyanide, domoic acid, kainic acid, metals, methamphetamine, polychlorinated biphenyls, rotenone, toluene and 6-hydroxydopamine. Given the potent direct free radical scavenging activities of melatonin and its metabolites, their ability to indirectly stimulate antioxidative enzymes and their efficacy in reducing electron leakage from mitochondria, it would be expected that these molecules would protect the brain from oxidative and nitrosative molecular mutilation. The studies summarized in this review indicate that this is indeed the case, an action that is obviously assisted by the fact that melatonin readily crosses the blood brain barrier.