Discovery of MK-8768, a Potent and Selective mGluR2 Negative Allosteric Modulator.

Glutamate plays a key role in cognition and mood, and it has been shown that inhibiting ionotropic glutamate receptors disrupts cognition, while enhancing ionotropic receptor activity is pro-cognitive. One approach to elevating glutamatergic tone has been to antagonize presynaptic metabotropic glutamate receptor 2 (mGluR2). A desire for selectivity over the largely homologous mGluR3 motivated a strategy to achieve selectivity through the identification of mGluR2 negative allosteric modulators (NAMs). Extensive screening and optimization efforts led to the identification of a novel series of 4-arylquinoline-2-carboxamides. This series was optimized for mGluR2 NAM potency, clean off-target activity, and desirable physical properties, which resulted in the identification of improved C4 and C7 substituents. The initial lead compound from this series was Ames-positive in a single strain with metabolic activation, indicating that a reactive metabolite was likely responsible for the genetic toxicity. Metabolic profiling and Ames assessment across multiple analogs identified key structure-activity relationships associated with Ames positivity. Further optimization led to the Ames-negative mGluR2 negative allosteric modulator MK-8768.

Discovery of MK-8189, a Highly Potent and Selective PDE10A Inhibitor for the Treatment of Schizophrenia.

PDE10A is an important regulator of striatal signaling that, when inhibited, can normalize dysfunctional activity. Given the involvement of dysfunctional striatal activity with schizophrenia, PDE10A inhibition represents a potentially novel means for its treatment. With the goal of developing PDE10A inhibitors, early optimization of a fragment hit through rational design led to a series of potent pyrimidine PDE10A inhibitors that required further improvements in physicochemical properties, off-target activities, and pharmacokinetics. Herein we describe the discovery of an isomeric pyrimidine series that addresses the liabilities seen with earlier compounds and resulted in the invention of compound 18 (MK-8189), which is currently in Phase 2b clinical development for the treatment of schizophrenia.

Impact of acute nicotine exposure on monoaminergic systems in adolescent and adult male and female rats.

Adolescence is a period of risk for beginning tobacco addiction. Differential neural response to nicotine in adolescents vs. adults may help explain the increased vulnerability to nicotine self-administration seen with adolescent onset. We indexed the effects of acute nicotine ditartrate (0.4 mg/kg, salt weight) administration on dopamine (DA) and serotonin (5HT) as well as the DA metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in several brain regions (nucleus accumbens, striatum and frontal cortex) of 6-week old (adolescent) and 10-week old (young adult) Sprague-Dawley rats. When nicotine was administered DA concentrations in the accumbens were significantly higher in adults than in adolescents, whereas there was no age-related difference without nicotine. However neither age group showed a significant effect of nicotine vs. age-matched controls. DA turnover in the accumbens was significantly greater in adolescent females in response to nicotine, but adult females did not show this effect and neither did males of either age group. DA turnover in the striatum was significantly higher in adolescents than adults regardless of nicotine administration. In the frontal cortex, there was a more complex effect. Without nicotine, adult male rats had higher DA concentrations than adolescent males, whereas female rats did not differ from adolescent to adult ages. When given nicotine, the age effect was no longer seen in males. However, there was not a significant effect of nicotine vs. age-matched controls in either age group. No age or nicotine effects were seen in females. 5HT in the accumbens was significantly increased by nicotine administration in adults but not in adolescents. Altered neural responsivity of adolescents to nicotine-induced neural effects particularly in accumbens DA and 5HT may be related to the increased nicotine dose concentrations they self-administer.

Alzheimer's disease-associated U1 snRNP splicing dysfunction causes neuronal hyperexcitability and cognitive impairment.

Recent proteome and transcriptome profiling of Alzheimer's disease (AD) brains reveals RNA splicing dysfunction and U1 small nuclear ribonucleoprotein (snRNP) pathology containing U1-70K and its N-terminal 40-KDa fragment (N40K). Here we present a causative role of U1 snRNP dysfunction to neurodegeneration in primary neurons and transgenic mice (N40K-Tg), in which N40K expression exerts a dominant-negative effect to downregulate full-length U1-70K. N40K-Tg recapitulates N40K insolubility, erroneous splicing events, neuronal degeneration and cognitive impairment. Specifically, N40K-Tg shows the reduction of GABAergic synapse components (e.g., the GABA receptor subunit of GABRA2), and concomitant postsynaptic hyperexcitability that is rescued by a GABA receptor agonist. Crossing of N40K-Tg and the 5xFAD amyloidosis model indicates that the RNA splicing defect synergizes with the amyloid cascade to remodel the brain transcriptome and proteome, deregulate synaptic proteins, and accelerate cognitive decline. Thus, our results support the contribution of U1 snRNP-mediated splicing dysfunction to AD pathogenesis.

Identification of small molecules that mitigate vincristine-induced neurotoxicity while sensitizing leukemia cells to vincristine.

Vincristine (VCR) is one of the most widely prescribed medications for treating solid tumors and acute lymphoblastic leukemia (ALL) in children and adults. However, its major dose-limiting toxicity is peripheral neuropathy that can disrupt curative therapy. Peripheral neuropathy can also persist into adulthood, compromising quality of life of childhood cancer survivors. Reducing VCR-induced neurotoxicity without compromising its anticancer effects would be ideal. Here, we show that low expression of NHP2L1 is associated with increased sensitivity of primary leukemia cells to VCR, and that concomitant administration of VCR with inhibitors of NHP2L1 increases VCR cytotoxicity in leukemia cells, prolongs survival of ALL xenograft mice, but decreases VCR effects on human-induced pluripotent stem cell-derived neurons and mitigates neurotoxicity in mice. These findings offer a strategy for increasing VCR's antileukemic effects while reducing peripheral neuropathy in patients treated with this widely prescribed medication.

Indole acids as a novel PDE2 inhibitor chemotype that demonstrate pro-cognitive activity in multiple species.

An internal HTS effort identified a novel PDE2 inhibitor series that was subsequently optimized for improved PDE2 activity and off-target selectivity. The optimized lead, compound 4, improved cognitive performance in a rodent novel object recognition task as well as a non-human primate object retrieval task. In addition, co-crystallization studies of close analog of 4 in the PDE2 active site revealed unique binding interactions influencing the high PDE isoform selectivity.

Thalamic miR-338-3p mediates auditory thalamocortical disruption and its late onset in models of 22q11.2 microdeletion.

Although 22q11.2 deletion syndrome (22q11DS) is associated with early-life behavioral abnormalities, affected individuals are also at high risk for the development of schizophrenia symptoms, including psychosis, later in life. Auditory thalamocortical (TC) projections recently emerged as a neural circuit that is specifically disrupted in mouse models of 22q11DS (hereafter referred to as 22q11DS mice), in which haploinsufficiency of the microRNA (miRNA)-processing-factor-encoding gene Dgcr8 results in the elevation of the dopamine receptor Drd2 in the auditory thalamus, an abnormal sensitivity of thalamocortical projections to antipsychotics, and an abnormal acoustic-startle response. Here we show that these auditory TC phenotypes have a delayed onset in 22q11DS mice and are associated with an age-dependent reduction of miR-338-3p, a miRNA that targets Drd2 and is enriched in the thalamus of both humans and mice. Replenishing depleted miR-338-3p in mature 22q11DS mice rescued the TC abnormalities, and deletion of Mir338 (which encodes miR-338-3p) or reduction of miR-338-3p expression mimicked the TC and behavioral deficits and eliminated the age dependence of these deficits. Therefore, miR-338-3p depletion is necessary and sufficient to disrupt auditory TC signaling in 22q11DS mice, and it may mediate the pathogenic mechanism of 22q11DS-related psychosis and control its late onset.

Msh2 deficiency leads to dysmyelination of the corpus callosum, impaired locomotion, and altered sensory function in mice.

A feature in patients with constitutional DNA-mismatch repair deficiency is agenesis of the corpus callosum, the cause of which has not been established. Here we report a previously unrecognized consequence of deficiency in MSH2, a protein known primarily for its function in correcting nucleotide mismatches or insertions and deletions in duplex DNA caused by errors in DNA replication or recombination. We documented that Msh2 deficiency causes dysmyelination of the axonal projections in the corpus callosum. Evoked action potentials in the myelinated corpus callosum projections of Msh2-null mice were smaller than wild-type mice, whereas unmyelinated axons showed no difference. Msh2-null mice were also impaired in locomotive activity and had an abnormal response to heat. These findings reveal a novel pathogenic consequence of MSH2 deficiency, providing a new mechanistic hint to previously recognized neurological disorders in patients with inherited DNA-mismatch repair deficiency.

Specific disruption of thalamic inputs to the auditory cortex in schizophrenia models.

Auditory hallucinations in schizophrenia are alleviated by antipsychotic agents that inhibit D2 dopamine receptors (Drd2s). The defective neural circuits and mechanisms of their sensitivity to antipsychotics are unknown. We identified a specific disruption of synaptic transmission at thalamocortical glutamatergic projections in the auditory cortex in murine models of schizophrenia-associated 22q11 deletion syndrome (22q11DS). This deficit is caused by an aberrant elevation of Drd2 in the thalamus, which renders 22q11DS thalamocortical projections sensitive to antipsychotics and causes a deficient acoustic startle response similar to that observed in schizophrenic patients. Haploinsufficiency of the microRNA-processing gene Dgcr8 is responsible for the Drd2 elevation and hypersensitivity of auditory thalamocortical projections to antipsychotics. This suggests that Dgcr8-microRNA-Drd2-dependent thalamocortical disruption is a pathogenic event underlying schizophrenia-associated psychosis.

The relationship between glycine transporter 1 occupancy and the effects of the glycine transporter 1 inhibitor RG1678 or ORG25935 on object retrieval performance in scopolamine impaired rhesus monkey.

Reduced NMDA receptor functioning is hypothesized to underlie the cognitive and negative symptoms associated with schizophrenia. However, because direct activation of the NMDA receptor is accompanied by neurotoxicity, mechanisms that activate the glycine co-agonist site on the NMDA receptor could carry greater therapeutic potential. In the current study, the effects of two glycine transporter 1 (GlyT1) inhibitors, RG1678 and ORG25935, were characterized in the object-retrieval detour (ORD) task in scopolamine-impaired rhesus monkeys and, using positron emission tomography (PET), the GlyT1 occupancy to efficacy relationship of each compound was established. Scopolamine exerted a significant decrease in accuracy in the ORD task. Lower doses of RG1678 (0.3 and 1.0 mg/kg, p.o.) significantly attenuated the impact of scopolamine, whereas the highest dose tested (1.8 mg/kg) did not. The predicted GlyT1 occupancies of RG1678 at the effective doses were ~10 and 30 %. ORG25935 (0.1, 0.3, and 1 mg/kg, p.o.) also significantly attenuated the impact of scopolamine on the ORD task, whereas 3 mg/kg did not. The predicted GlyT1 occupancies of ORG25935 at the effective doses ranged from 16 to 80 %. These data suggest that GlyT1 inhibitors have the potential to improve performance on prefrontal cortex-dependent tests such as the ORD task, but that efficacy is lost when higher occupancies are achieved. Importantly, recent Ph2B data published by Roche suggests that low but not high doses of RG1678 improved negative symptoms in patients with schizophrenia, highlighting the potential translational nature of the current preclinical findings.

Orexin receptor antagonists differ from standard sleep drugs by promoting sleep at doses that do not disrupt cognition.

Current treatments for insomnia, such as zolpidem (Ambien) and eszopiclone (Lunesta), are γ-aminobutyric acid type A (GABAA)-positive allosteric modulators that carry a number of side effects including the potential to disrupt cognition. In an effort to develop better tolerated medicines, we have identified dual orexin 1 and 2 receptor antagonists (DORAs), which promote sleep in preclinical animal models and humans. We compare the effects of orally administered eszopiclone, zolpidem, and diazepam to the dual orexin receptor antagonist DORA-22 on sleep and the novel object recognition test in rat, and on sleep and two cognition tests (delayed match to sample and serial choice reaction time) in the rhesus monkey. Each compound's minimal dose that promoted sleep versus the minimal dose that exerted deficits in these cognitive tests was determined, and a therapeutic margin was established. We found that DORA-22 has a wider therapeutic margin for sleep versus cognitive impairment in rat and rhesus monkey compared to the other compounds tested. These data were further supported with the demonstration of a wider therapeutic margin for DORA-22 compared to the other compounds on sleep versus the expression of hippocampal activity-regulated cytoskeletal-associated protein (Arc), an immediate-early gene product involved in synaptic plasticity. These findings suggest that DORAs might provide an effective treatment for insomnia with a greater therapeutic margin for sleep versus cognitive disturbances compared to the GABAA-positive allosteric modulators currently in use.

The nicotinic α7 receptor agonist GTS-21 improves cognitive performance in ketamine impaired rhesus monkeys.

The cognitive deficits associated with schizophrenia are recognized as a core component of the disorder, yet there remain no available therapeutics to treat these symptoms of the disease. As a result, there is a need for establishing predictive preclinical models to identify the therapeutic potential of novel compounds. In the present study, rhesus monkeys were trained in the object retrieval-detour task, which is dependent on the prefrontal cortex, a brain region implicated in the cognitive deficits associated with schizophrenia. The NMDA receptor antagonist ketamine significantly impaired performance without affecting measures of motor or visuospatial abilities. Pre-treatment with the nicotinic α7 agonist GTS-21 (0.03 mg/kg) significantly attenuated the ketamine-induced impairment, consistent with reports from clinical trials suggesting that nicotinic α7 receptor agonism has pro-cognitive potential in clinical populations. In contrast, pretreatment with the acetylcholinesterase inhibitor donepezil failed to reverse the ketamine-induced impairment, consistent with studies showing a lack of pro-cognitive effects in patients with schizophrenia. These data suggest that the ketamine-impaired object retrieval-detour task could provide a model with improved predictive validity for drug development, and confirm the need for additional efforts in back-translation. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

The muscarinic M1 receptor positive allosteric modulator PQCA improves cognitive measures in rat, cynomolgus macaque, and rhesus macaque.

RATIONALE: The current standards of care for Alzheimer's disease, acetylcholinesterase inhibitors, have limited efficacy due to a host of mechanism-related side effects arising from indiscriminate activation of muscarinic and nicotinic receptors. The M1 muscarinic receptor is predominantly expressed in the brain in regions involved in cognition, and therefore selective activation of the M1 receptor would be expected to boost cognitive performance with reduced risk of peripheral side effects. OBJECTIVES: Here we investigated whether the selective M1 muscarinic receptor positive allosteric modulator, PQCA, improves cognitive performance and cerebral blood flow. RESULTS: PQCA attenuated a scopolamine-induced deficit in novel object recognition in rat, self-ordered spatial search in cynomolgus macaque, and the object retrieval detour task in rhesus macaque. Beneficial effects in each of these assays and species were observed at similar plasma drug concentrations. Furthermore, at similar drug concentrations that were effective in the behavioral studies, PQCA increased blood flow in the frontal cortex of mice, providing a translational biomarker that could be used to guide dose selection for clinical studies. CONCLUSIONS: These findings provide a framework for appropriately testing an M1 selective compound in patients with Alzheimer's disease.

The novel phosphodiesterase 10A inhibitor THPP-1 has antipsychotic-like effects in rat and improves cognition in rat and rhesus monkey.

Phosphodiesterase 10A (PDE10A) is a novel target for the treatment of schizophrenia that may address multiple symptomatic domains associated with this disorder. PDE10A is highly expressed in the brain and functions to metabolically inactivate the important second messengers cAMP and cGMP. Here we describe effects of a potent and orally bioavailable PDE10A inhibitor [2-(6-chloropyridin-3-yl)-4-(2-methoxyethoxy)-7,8-dihydropyrido[4,3-d]pyrimidin-6(5H)-yl](imidazo[1,5-a]pyridin-1-yl)methanone] (THPP-1) on striatal signaling pathways, in behavioral tests that predict antipsychotic potential, and assays that measure episodic-like memory in rat and executive function in rhesus monkey. THPP-1 exhibits nanomolar potency on the PDE10A enzyme, demonstrates excellent pharmacokinetic properties in multiple preclinical animal species, and is selective for PDE10A over other PDE families of enzymes. THPP-1 significantly increased phosphorylation of proteins in the striatum involved in synaptic plasticity, including the a-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid receptor (AMPA) GluR1 subunit, extracellular receptor kinase (ERK), and cAMP-response element binding protein (CREB). THPP-1 produced dose-dependent effects in preclinical assays predictive of antipsychotic activity including attenuation of MK-801-induced psychomotor activation and condition avoidance responding in rats. At similar plasma exposures, THPP-1 significantly increased object recognition memory in rat and attenuated a ketamine-induced deficit in the object retrieval detour task in rhesus monkey. These findings suggest that PDE10A inhibitors have the potential to impact multiple symptomatic domains of schizophrenia including positive symptoms and cognitive impairment. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

Hippocampal infusions of MARCKS peptides impair memory of rats on the radial-arm maze.

In vitro hippocampal studies by Gay et al. (2008) demonstrated that a myristoylated alanine-rich C kinase substrate (MARCKS) peptide comprising the phosphorylation site or effector domain of the protein acts as a powerful inhibitor of alpha7 nicotinic acetylcholine receptors (nAChRs), which are known to be critically involved in memory function. However, behavioral consequences of hippocampal MARCKS peptide infusions have not been investigated. The purpose of the current study was to determine if local infusions in the rat ventral hippocampus of long (comprising amino acids 151-175) and short (amino acids 159-165) forms of MARCKS peptides could affect memory performance in the 16-arm radial maze. Our results demonstrated a dramatic impairment of both working (changing) and reference (constant) memory with MARCKS(151-175) only. The shorter MARCKS peptide did not affect memory performance. This is in line with in vitro results reported by Gay et al. (2008) that long, but not short, MARCKS peptides inhibit alpha7 nAChRs. We also found that the effect of the MARCKS(151-175) peptide was dose-dependent, with a robust memory impairment at 10 microg/side, and smaller inconsistent effects at lower doses. Our present behavioral study, together with the earlier in vitro study by Gay et al. (2008), suggests that effector domain MARCKS peptides could play a significant role in memory regulation and impairment.

Zebrafish provide a sensitive model of persisting neurobehavioral effects of developmental chlorpyrifos exposure: comparison with nicotine and pilocarpine effects and relationship to dopamine deficits.

Chlorpyrifos (CPF) an organophosphate pesticide causes persisting behavioral dysfunction in rat models when exposure is during early development. In earlier work zebrafish were used as a complementary model to study mechanisms of CPF-induced neurotoxicity induced during early development. We found that developmental (first five days after fertilization) chlorpyrifos exposure significantly impaired learning in zebrafish. However, this testing was time and labor intensive. In the current study we tested the hypothesis that persisting effects of developmental chlorpyrifos could be detected with a brief automated assessment of startle response and that this behavioral index could be used to help determine the neurobehavioral mechanisms for persisting CPF effects. The swimming activity of adult zebrafish was assessed by a computerized video-tracking device after a sudden tap to the test arena. Ten consecutive trials (1/min) were run to determine startle response and its habituation. Additionally, habituation recovery trials were run at 8, 32 and 128 min after the end of the initial trial set. CPF-exposed fish showed a significantly (p<0.025) greater overall startle response during the 10-trial session compared to controls (group sizes: Control N=40, CPF N=24). During the initial recovery period (8 min) CPF-exposed fish showed a significantly (p<0.01) greater startle response compared to controls. To elucidate the contributions of nicotinic and muscarinic acetylcholine receptors to developmental CPF-mediated effects, the effects of developmental nicotine and pilocarpine exposure throughout the first five days after fertilization were determined. Developmental nicotine and pilocarpine exposure significantly increased startle response, though nicotine (group sizes: Control N=32, 15 mM N=12, 25 mM N=20) was much more potent than pilocarpine (group sizes: Control N=20, 100 microM N=16, 1000 microM N=12). Neither was as potent as CPF for developmental exposure increasing startle response in adulthood. Lastly, developmental CPF exposure decreased dopamine and serotonin levels and increased transmitter turnover in developing zebrafish larvae (N=4 batches of 50 embryos/treatment). Only the decline in dopamine concentrations persisted into adulthood (group sizes: Control N=14, CPF N=13). This study shows that a quick automated test of startle can detect persisting neurobehavioral impairments caused by developmental exposure to CPF. This may be helpful in screening for persisting neurobehavioral defects from a variety of toxicants.

Genetic aspects of behavioral neurotoxicology.

Considerable progress has been made over the past couple of decades concerning the molecular bases of neurobehavioral function and dysfunction. The field of neurobehavioral genetics is becoming mature. Genetic factors contributing to neurologic diseases such as Alzheimer's disease have been found and evidence for genetic factors contributing to other diseases such as schizophrenia and autism are likely. This genetic approach can also benefit the field of behavioral neurotoxicology. It is clear that there is substantial heterogeneity of response with behavioral impairments resulting from neurotoxicants. Many factors contribute to differential sensitivity, but it is likely that genetic variability plays a prominent role. Important discoveries concerning genetics and behavioral neurotoxicity are being made on a broad front from work with invertebrate and piscine mutant models to classic mouse knockout models and human epidemiologic studies of polymorphisms. Discovering genetic factors of susceptibility to neurobehavioral toxicity not only helps identify those at special risk, it also advances our understanding of the mechanisms by which toxicants impair neurobehavioral function in the larger population. This symposium organized by Edward Levin and Annette Kirshner, brought together researchers from the laboratories of Michael Aschner, Douglas Ruden, Ulrike Heberlein, Edward Levin and Kathleen Welsh-Bohmer conducting studies with Caenorhabditis elegans, Drosophila, fish, rodents and humans studies to determine the role of genetic factors in susceptibility to behavioral impairment from neurotoxic exposure.

Hippocampal infusions of apolipoprotein E peptides induce long-lasting cognitive impairment.

The inheritance of the varepsilon4 allele of apolipoprotein E (ApoE4) and cholinergic system dysfunction have long been associated with the pathology of Alzheimer's disease (AD). Recently, in vitro studies have established a direct link between ApoE and cholinergic function in that synthetic peptides containing segments of the ApoE protein (ApoE(133-149) and ApoE(141-148)) interact with alpha7 nicotinic acetylcholine receptors (nAChRs) in the hippocampus. This raises the possibility that ApoE peptides may contribute to cognitive impairment in AD in that the hippocampus plays a key role in cognitive functioning. To test this, we acutely infused ApoE peptides into the ventral hippocampus of female Sprague-Dawley rats and assessed the resultant effects on radial-arm maze choice accuracy over a period of weeks after the infusion. Local ventral hippocampal infusion of ApoE peptides caused significant cognitive impairment in radial-arm maze learning that persisted several weeks after the acute infusion. This persisting deficit may be an important model for understanding the relationship between ApoE protein-induced neurotoxicity and cognitive impairment as well as serve as a platform for the development of new therapies to avoid neurotoxicity and cognitive decline.

Nicotine effects on learning in zebrafish: the role of dopaminergic systems.

RATIONALE: Nicotine improves cognitive function in a number of animal models including rats, mice, monkeys, and recently, zebrafish. The zebrafish model allows higher throughput and ease in discovering mechanisms of cognitive improvement. MATERIALS AND METHODS: To further characterize the neural bases of nicotine effects on learning in zebrafish, we determined changes in dopaminergic systems that accompany nicotine-enhanced learning. RESULTS: Nicotine improved learning and increased brain levels of dihydroxyphenylacetic acid (DOPAC), the primary dopamine metabolite. There was a significant correlation between choice accuracy and DOPAC levels. The nicotinic antagonist mecamylamine blocked the nicotine-induced increase in DOPAC concentrations, in line with our previous finding that mecamylamine reversed nicotine-induced learning improvement. CONCLUSIONS: Dopamine systems are related to learning in zebrafish; nicotine exposure increases both learning rates and DOPAC levels; and nicotinic antagonist administration blocks nicotine-induced rises in DOPAC concentrations. Rapid cognitive assessment of drugs with zebrafish could serve as a useful screening tool for the development of new therapeutics for cognitive dysfunction.

Metallothionein in the central nervous system: Roles in protection, regeneration and cognition.

Metallothionein (MT) is an enigmatic protein, and its physiological role remains a matter of intense study and debate 50 years after its discovery. This is particularly true of its function in the central nervous system (CNS), where the challenge remains to link its known biochemical properties of metal binding and free radical scavenging to the intricate workings of brain. In this compilation of four reports, first delivered at the 11th International Neurotoxicology Association (INA-11) Meeting, June 2007, the authors present the work of their laboratories, each of which gives an important insight into the actions of MT in the brain. What emerges is that MT has the potential to contribute to a variety of processes, including neuroprotection, regeneration, and even cognitive functions. In this article, the properties and CNS expression of MT are briefly reviewed before Dr Hidalgo describes his pioneering work using transgenic models of MT expression to demonstrate how this protein plays a major role in the defence of the CNS against neurodegenerative disorders and other CNS injuries. His group's work leads to two further questions, what are the mechanisms at the cellular level by which MT acts, and does this protein influence higher order issues of architecture and cognition? These topics are addressed in the second and third sections of this review by Dr West, and Dr Levin and Dr Eddins, respectively. Finally, Dr Aschner examines the ability of MT to protect against a specific toxicant, methylmercury, in the CNS.