The dopamine reuptake inhibitor, Ritalin® (methylphenidate) has been used for almost 50 years to treat children with attention-deficit hyperactivity disorder (ADHD) and, more recently – and controversially, has been used by students to enhance academic performance and as a recreational drug. Although Ritalin® has been prescribed for millions of children, the mechanisms by which it modifies behavioural performance remain poorly understood. Researchers at the University of California, San Francisco have now shown, in animals at least, that Ritalin® improves ability to focus on tasks and directly enhances speed of learning by distinct dopamine receptor-mediated mechanisms.

By co-administering Ritalin® with either the dopamine D₁ receptor antagonist, SCH-23390, or the dopamine D₂ receptor antagonist, raclopride, the team were able to show the well-known benefit of improved focus was mediated through D₂ receptor-dependent mechanisms whereas learning efficiency was enhanced through D₁ receptor-dependent mechanisms. The study also established that Ritalin® strengthens synapses and enhances neuroplasticity. A better understanding of the way that Ritalin® improves focus and enhances learning could lead to the development of more targeted drugs for ADHD and learning enhancement.

The study is published in Nature Neuroscience.

Although the disorder is not very well known, narcolepsy is thought to affect 1 in 2000 individuals and this figure may be higher as a consequence of under-reporting and under-diagnosis. The most common symptom is excessive daytime sleepiness (EDS), which may be accompanied by sudden loss of muscular control (cataplexy) triggered by strong emotions. Narcoleptics may also experience sleep paralysis (short periods of paralysis when waking or falling asleep), hypnagogic or hypnopompic hallucinations (vivid images or sounds, respectively, when waking or falling asleep) or automatic behaviour (when routine activities are continued during a sleep episode).

For the last ten years it has been known that narcoleptics have a deficiency in hypocretin (orexin), a neurotransmitter involved in control of sleep/wakefulness. In parallel with the neurotransmitter deficiency there is a massive loss of hypothalamic neurons that produce hypocretin and it has been hypothesised that this results from an autoimmune response.

Swiss scientists have now identified autoantibodies to Tribbles homolog 2 (Trib2), an autoantigen previously identified in autoimmune uveitis, in narcolepsy patients. The team developed a transgenic mouse model to identify peptides enriched within hypocretin-producing neurons that could serve as potential autoimmune targets. Having identified enrichment of Trib2 in the mouse hypocretin neurons, the team went on to analyse sera from narcoleptics. Narcolepsy patients with cataplexy had higher Trib2-specific antibody titres compared with either normal controls or patients with other inflammatory neurological disorders. Trib2-specific antibody titres were highest early after narcolepsy onset, sharply decreased within 2–3 years, and then stabilized at levels substantially higher than that of controls for up to 30 years. Additionally, high Trib2-specific antibody titres correlated with the severity of cataplexy.

The study, published in the Journal of Clinical Investigation, provides the first evidence that narcolepsy is an autoimmune disorder.

Results from a phase II trial of the experimental drug Dimebon (latrepirdine) in people with Huntington’s disease have provided indications that it may improve cognition. The drug, being developed by Medivation, Inc., is also in Phase III trials for Alzheimer’s disease. In July 2009, Medivation and Pfizer, Inc. launched a Phase III clinical trial (HORIZON) of the drug for Huntington’s disease.

Huntington’s disease is a progressive neurodegenerative disorder that impacts movement, behaviour and cognition, generally resulting in death within 20 years of the disease’s onset. The disease steadily erodes memory and ability to think and learn. Over time, this cognitive impairment contributes to the loss of the ability to work and perform the activities of daily life. There are no treatments current available that effectively alter the course of the disease or improve cognition.

We have previously reported on the potential for Dimebon in Alzheimer’s disease (July 2008, July 2009), where the ability of the drug to stabilise and/or enhance mitochondrial function is believed to be of benefit. Mitochondria are also thought to play a role in the development of Huntington’s disease, suggesting that Dimebon could also have utility in this condition.

Karl Kieburtz, M.D., University of Rochester Medical Center neurologist and lead investigator on the Horizon trial, said:

“This is the first clinical trial that has focused on what is perhaps the most disabling aspect of the disease. While more investigation needs to be done, these results are encouraging and show, for the first time, a statistically significant benefit in terms of improved cognitive function in patients with Huntington’s disease.”

In the phase II study, the impact of the drug on 91 patients over a 90 day period was assessed. Half were given the drug and the other half a placebo. The patients were then evaluated using a cognitive tool called the Mini-Mental State Examination. This test – which is used by clinicians to evaluate the stage and severity of dementia and Alzheimer’s disease – consists of questions used to evaluate an individual’s orientation, memory, and ability to follow commands. The researchers found that the drug on average improved the scores of people taking the drug compared to those who received the placebo. Although the treatment had no significant impact on the Unified Huntington’s Disease Rating Scale (UHDRS) or the Alzheimer Disease Assessment Scale–cognitive subscale (ADAS-cog), the results support further investigation in Huntington’s disease.

Results of the study are published in the Archives of Neurology.

Roles have been suggested for brain-derived neurotrophic factor (BDNF) – which helps to support neurons and also stimulates and controls neurogenesis – in preventing or treating degenerative diseases such amyotrophic lateral sclerosis, Parkinson’s disease, and Alzheimer’s disease. The use of BDNF itself in therapy is limited by a poor pharmokinetic profile including rapid metabolism and poor CNS penetration. BDNF elicits at least some of its effects through binding to the high affinity tyrosine kinase receptor B, TrkB, and investigators at Emory University School of Medicine have now identified a small, high-affinity molecule that can also activate signalling through TrkB.

7,8-Dihydroxyflavone was shown to protect wild-type, but not TrkB-deficient, neurons from apoptosis. Following intraperitoneal administration, the compound was also found to activate TrkB in the brain and to be protective in animal models of seizure, stroke and Parkinson’s disease. The compound was also found to have low toxicity on chronic dosing. Although favonoids such as 7,8-dihydroxyflavone occur in a wide range of foodstuffs, levels obtained from a normal diet are believed to be insufficient for a sustained effect.

The study is published in the online early edition of PNAS.

Neurons are particularly sensitive to the toxic effects of misfolded proteins and the accumulation of such species has been associated with neurodegenerative diseases including Parkinson’s disease, amyotropic lateral sclerosis (ALS), Alzheimer’s disease and transmissible spongiform encephalopathies (prion diseases). Hereditary protein conformational disorders such as Huntington’s disease are characterised by trinucleotide repeats that result in the insertion of poly-glutamine (polyQ) stretches which adopt β-sheet structures and make the protein prone to incorrect folding and aggregation. The ability to stabilise native protein conformations would likely prevent the neurotoxicity linked to misfolding and scientists at Duke University Medical Center have now discovered compounds that may be able to achieve this.

Since increasing the levels of protein chaperones has been shown to suppress protein misfolding, the team focussed on identifying small molecule activators of heat shock transcription factor 1 (HSF1), the master regulator of protein chaperone gene transcription. HSF1A was discovered using a humanised yeast-based high throughput screen and shown to activate HSF1 in mammalian and fruit fly cells, to elevate protein chaperone expression, and to reduce protein misfolding. HSF1A was also shown to prevent cell death in polyQ-expressing neuronal precursor cells and to protect against cytotoxicity in a fruit fly model of polyQ-mediated neurodegeneration.

Previous screens that have identified activators of HSF1 have not been able to discriminate against compounds that promote HSF1 activation through the proteotoxic accumulation of unfolded proteins or through the inhibition of Hsp90, a central chaperone involved in cell growth, signalling, and proliferation. HSF1A is structurally distinct from other small molecule activators of HSF1 and, although the precise mechanism by which the compound activates human HSF1 is not yet understood, it could lead to new therapies for neurodegenerative diseases caused by protein misfolding.

The study is published in PLoS Biology.

Systemic infection and inflammation lead to release of cytokines, such as IL-1, which activate the brain’s stress response mechanisms, producing typical symptoms such as lethargy, fever, and lack of appetite. In response to inflammation or infection, the hypothalamus releases corticotropin-releasing factor which, in turn, stimulates the pituitary gland to secrete adrenocorticotropic hormone. This then causes the adrenal glands to increase production of glucocorticoids, which both mobilise energy reserves to cope with the inflammatory insult and also act as powerful immunosuppressants, preventing excessive cytokine production and immune cell proliferation. Since cytokines are not able to freely cross the blood-brain barrier, exactly how they initiate this cascade of events has not been clear but researchers at the Salk Institute for Biological Studies have now begun to unravel the process.

It had been suggested that cytokines might interact with epithelial cells in the brain’s vasculature to produce prostanoids which act as secondary messengers transmitting the signal onwards. Epithelial cells are ideally positioned to receive inflammatory signals from circulating blood but need a very strong signal to become activated. In contrast, perivascular macrophages, a subset of brain-resident macrophages, are much more sensitive to cytokines but are not in direct contact with the bloodstream. To clarify the roles of both cell types, liposomes containing clodronate, which specifically deplete macrophages, were injected into the lateral cerebral ventricles of rats. This procedure abolished responses to IL-1 which activates prostanoid synthesis only in perivascular cells, but enhanced responses to LPS which stimulates prostanoid synthesis by both perivascular cells and endothelial cells. Resident macrophages lined up along the blood-brain barrier thus play opposing roles in the transmission of immune signals to the brain depending on the nature of the stimulus.

As well as clarifying the cellular mechanisms of CNS responses to inflammatory insults, the team hope that a better understanding of how immune signals are transmitted across the blood-brain barrier may also lead ultimately to new treatments for chronic neurodegenerative diseases such as Amyotrophic Lateral Sclerosis, Parkinson’s disease, Alzheimer’s disease and prion diseases, in which inflammation is believed to play an important role.

The study is published in the January 14th issue of Neuron.

Prion diseases comprise the transmissible spongiform encephalopathies, including scrapie in sheep, bovine spongiform encephalopathy (BSE, “Mad Cow” disease) in cattle and Creutzfeldt-Jakob disease in humans. Central to these diseases is the conversion of normal cellular prion protein (PrP^c) into the abnormally folded, pathogenic species (PrP^Sc) in the brain. The misfolding results in prion protein with distinct biochemical properties compared to the normal protein, such as reduced solubility and decreased susceptibility to proteases. Aggregates of PrP^Sc accumulate in association with neurons in affected brain areas, which is thought to lead to the synapse degeneration and neuronal death observed in infected hosts.

Researchers in the UK and Italy have now shown that glimepiride, a sulfonyl urea approved for the treatment of non insulin dependent diabetes mellitus (NIDDM), is able to reduce PrP^Sc formation in cell culture. The rationale for the study was based on the knowledge that generation of PrP^Sc is dependent on the presence of PrP^c and that this appeared to require PrPc expressed at the cell surface. PrP^c is linked to the membrane by a glycosylphosphatidylinositol (GPI) anchor and can be released from the surface of cells by treatment with phosphatidylinositol-phospholipase C (PI-PLC). Consistent with the hypothesis that cell-surface PrP^c is required, treatment of prion-infected neuronal cells with PI-PLC reduced PrP^Sc formation.

Since glimepiride has been shown to stimulate the release of some GPI-anchored proteins in adipocytes (via stimulation of an endogenous GPI-PLC), the team explored the effects of the drug on PrP^c/PrP^Sc in neuronal cell culture. Similarly to PI-PLC, glimepiride reduced the amount of cell-surface PrP^c in primary cortical neurons and neuronal cell lines. In addition, glimepiride reduced formation of PrP^Sc in three prion-infected neuronal cell lines.

The study, published in PLoSone, also demonstrated that glimepiride treated neurons were resistant to the toxicity of a PrP-derived peptide, PrP82-146.

The team note that modulation of cell-surface PrP^c may also have application in Alzheimer’s disease since it is a receptor for β-amyloid oligomers. Whether glimepiride is sufficiently CNS-penetrant to be effective remains to be seen.

The body has no way to detect the absence of oxygen and feelings of suffocation are triggered instead by high levels of carbon dioxide. The ‘false-suffocation-alarm theory’ proposes that this alarm is triggered inappropriately in patients with panic disorder but, although several studies have linked high carbon dioxide levels with panic attacks in susceptible individuals, the reasons for this have not been clear. Researchers at the University of Iowa have now shown that inhaled carbon dioxide increases acidity in the brain and evokes fear by activating an acid-sensing ion channel (ASIC1a) in the amygdala. The amygdala plays a key role in the processing and memory of emotional reactions, including fear, but it was not known whether it also directly senses fear-evoking stimuli. ASICs are activated in vitro when extracellular pH falls, and although acidic pH modifies the activity of many receptors and proteins, few others are activated by extracellular acidosis, and few are as exquisitely pH sensitive as ASICs.

In tests in mice, increased carbon dioxide levels led to exaggerated innate and learned fear responses which could be blunted either by disrupting the ASIC1a gene or by pharmacological inhibition of ASIC1a using either the tarantula toxin, psalmotoxin, or A-317567.

The finding that ASIC1a channels in the amygdala act as chemosensors provides a molecular mechanism by which carbon dioxide can trigger fear and anxiety and suggests that targeting brain pH or ASIC channels could lead to new therapies for panic and anxiety disorders.

The study is published in the journal Cell.

Calcium signalling plays a vital role in the survival of brain neurons and increased intracellular calcium has been identified as an early event triggering neuronal death in age-related neurodegenerative disorders such as Alzheimer’s disease. Additionally, L-type voltage-gated calcium channels have been implicated in neuronal death during aging.

Researchers at Washington University, Missouri, have now evaluated antagonists of both L-type and T-type calcium channels using an in vitro neuronal culture model. In the model, configured to monitor long- and short-term survival, nimodipine, an L-type calcium channel blocker originally developed as an anti-hypertensive, was neuroprotective in both assays. The anti-epileptic T-type calcium channel blocker, trimethadione, and mibefradil, an antihypertensive T- and L-type channel blocker, were neuroprotective in the short-term but not the long-term assay.

The results, published in the journal Molecular Degeneration, suggest that more than one calcium signalling pathway may be involved in regulating neuronal survival. Clinical evaluation of patients receiving calcium channel blockers may provide better insight into their benefit in terms of cognitive function in neurodegenerative disorders. The absence of effective treatments for age-related neurodegeneration should encourage further studies to determine whether these established drug classes could have additional utility.

Amyloid plaques, which deposit around nerve cells, and neurofibrillary tangles, which build up inside the cells, are the primary hallmarks of Alzheimer’s disease. Many researchers believe that the plaques trigger a cascade of events leading to disease pathology but, although this hypothesis is supported by animal studies, it has not been conclusively proved in humans. Amyloid deposition has also been suggested to lead to neuroinflammation, creating a positive feedback loop resulting in further amyloid accumulation and chronic inflammation. Polymorphisms leading to increased levels of interleukin-6 (IL-6), a pro-inflammatory cytokine which activates microglial cells, have been linked to Alzheimer’s disease, adding weight to this hypothesis.

Scientists at the Mayo Clinic were carrying out experiments to demonstrate that activated microglia exacerbate neurodegeneration when they discovered, unexpectedly, that the microglia cleared the plaques from the brain. They had believed that the microglia would be unable to clear the plaques and that the resulting inflammation would worsen the disease. To examine the effect of IL-6 on amyloid processing and deposition, the team over-expressed murine IL-6 (mIL-6) in the brains of transgenic mice expressing mutated forms of the human amyloid precursor protein. Instead of creating a neurotoxic feedback loop that exacerbated amyloid pathology, IL-6 had no effect on amyloid processing and enhanced plaque clearance.

The study, which was published online on October 14^th in the FASEB Journal, is the first to examine in detail the effect of mIL-6 on amyloid deposition in vivo and suggests that the use of inflammatory mediators to manipulate the immune response could lead to new therapeutic approaches for the treatment of neurodegenerative diseases such as Alzheimer’s disease.

A study by researchers at the Universities of Florida and Frankfurt, published earlier this year in the journal Acta Neuropathologica, also addressed the role of microglia in Alzheimer’s disease. The study showed that microglia in amyloid-laden, degenerating regions of the brains of Alzheimer’s disease patients are not activated but, instead, are senescent and dystrophic. The team suggest that loss of microglia may contribute to neurodegeneration and that finding ways to keep microglia alive and healthy would be better than trying to inhibit their function with anti-inflammatory drugs.