A recent study found that mice that ate more cinnamon got better and faster at learning. In the study, published in the Journal of Neuroimmune Pharmacology in 2016, separated mice into good learners and poor learners based on how easily they navigated a maze to find food. After the poor learners were fed cinnamon for a month, they could find the food more than twice as quickly as before.
The benefits of cinnamon come from sodium benzoate, a chemical produced as the body breaks down the cinnamon. Sodium benzoate enters the brain and allows the hippocampus to create new neurons.
Feeding cinnamon to the poor learning mice normalized their levels of receptors for the neurotransmitter GABA, closing the gap with good learners. Sodium benzoate also improved the structural integrity of some brain cells. Cinnamon also can help sensitize insulin receptors.
Doctors hope these findings may eventually contribute to treatment research on Alzheimer’s and Parkinson’s diseases.
Cinnamon should be consumed in moderate quantities because the Chinese variety most commonly found in North American supermarkets has high levels of coumarin, a compound that can be toxic to the liver when consumed in large quantities. Ceylon (Sri Lankan) cinnamon has lower levels of coumarin.
Stress is a risk factor for depression and other mental health disorders. Researchers are currently working to clarify how stress leads to depression, anxiety, and post-traumatic stress disorder, and why trauma early in life has lasting consequences.
Two recent studies in mice examined whether just witnessing a stressful event leads to depression-like behaviors. In one, adult female mice watched a male mouse as it was repeatedly attacked by a larger mouse. After ten days of this, the female mice were socially withdrawn, had lost interest in drinking sucrose, and gave up more easily during a physical challenge. They also lost weight and showed higher levels of the stress hormone corticosterone in their blood. The researchers, led by Sergio Iniguez, believe their study clarifies how witnessing traumatic events can lead to stress-induced mood disorders.
In the other study, by Carlos Bolanos-Guzman, adolescent male mice witnessed another mouse being attacked. Both the mice that went through the physical stress of being attacked and the mice that went through the emotional stress of watching the attacks occur showed similar depressive behaviors to the mice in the previous study—social withdrawal, loss of interest in sucrose, decreased food intake and exploration of the environment, and decreased motivation in physical challenges. These behaviors persisted into adulthood. Both groups of mice also had increased levels of corticosterone and reduced expression of a particular protein in the ventral tegmental area, a part of the brain linked to stress response. Bolanos-Guzman suggests that both physical and emotional stress have lifelong consequences in mice.
The studies were presented at a scientific meeting in December.
In 2015 Claudia Chauvet and colleagues reported in the journal Neuropsychopharmacology that the brain-penetrating statins simvastatin and Atorvastatin reduced cocaine seeking behaviors in mice that were taught to self-administer cocaine and then were denied access to it for 21 days compared to pravastatin, a statin that does not penetrate the brain as thoroughly. The researchers found that the brain-penetrating statins also reduced nicotine seeking, but not food reward seeking. The statins also worked in mice that had stopped seeking cocaine but relapsed due to stress, allowing them to abstain from cocaine seeking again.
Statins are considered a very safe treatment in humans. The ability of statins to prevent relapse to addictions in mice may mean that one day they could be used to treat addictions in people as well. A review article by Cassie Redlich and colleagues in the journal BMC Psychiatry in 2014 indicated that statins may reduce recurrence of depression in people. The researchers found that simvastatin had a protective effect while Atorvastatin was associated with increased risk of depression, so the choice of statins may be important for both depression and addiction.
In a 2015 article in Nature Neuroscience, Stefan Bonn and André Fischer reported that when mice were prompted to use their long-term memory to recognize a specific environment, epigenetic changes occurred in their neurons and glia. Epigenetic changes refer to chemical alterations in DNA or histones (which give DNA structure) that increase or decrease the expression of certain genes. Sometimes environmental factors lead to a methyl or acetyl group joining a strand of DNA or histones, changing how easily the genes are turned on or off.
When the mice used their long-term memory, the main change that occurred was DNA methylation in their neurons. There were also changes to histones that were linked to memory acquisition but resulted in few changes in gene expression. The DNA methylation changes, on the other hand, changed neural pathways, leading to “rewiring” of the brain.
Epigenetics is the process by which environmental factors affect the way a person’s genes are transcribed. These changes, which may include the addition or subtraction of methyl groups from DNA, change the DNA’s structure (how tightly it is wound around the histones that give it shape) but not its sequence. These structural changes, which affect how easily the DNA is transcribed, can then be passed on to future generations. A new study by Ulrike Stadlbauer and colleagues presented at the Society of Biological Psychiatry explored a particular pathway by which an infection in a pregnant mouse can lead to behavioral changes in three following generations of mice.
Pregnant mice were given injections that produced an infection. A first generation of offspring were interbred to create a second generation of offspring, and these were interbred to create a third generation of offspring. The first generation of offspring had epigenetic changes in methylation and hydroxymethylation to promoter regions of two enzymes that regulate synthesis of the neurotransmitter GABA, and these epigenetic changes were associated with reduced mRNA expression of these two genes.
All three generations of offspring had deficits in social interaction, short-term memory, and cued fear conditioning. Interestingly, the second and third offspring generations also exhibited depression-like behavior that had not been present in the original mothers or the first generation of offspring.
Editor’s Note: This is another fascinating demonstration of how environmental occurrences, which can include stressors, exposure to drugs, and now immune challenges, can have effects across generations, likely through epigenetic changes that persist in ova or sperm. Amazingly, it turns out that the environment can change traits in future generations, not by inducing changes to gene sequences, but through epigenetic changes to the structure of DNA or histones that persist across generations.
Events like surgery or heart attacks that cause inflammation can lead to cognitive deficits or depression for months or years afterward, even though the direct effects of inflammation wear off within weeks. In a recent study, Natalie Tronson and colleagues subjected mice to surgical heart attack, sham surgery, or no operation, and observed how well they absorbed new learning eight weeks later.
Both male and female mice had impairments in fear learning following surgical heart attacks. Female mice that received sham surgery also showed deficits in fear learning. When the researchers dissected the mice, analyzing their blood and hippocampi after the eight-week period, inflammatory cytokine measures had normalized as expected, but the researchers found other abnormalities.
Intracellular signaling was dysregulated, and there had been epigenetic changes in cells of the hippocampus. (Epigenetic changes refer to those that change the structure of DNA, such as how tightly it is wound, rather than its sequence. For example, the addition of acetyl groups to DNA or the histones around which it is wound.) The researchers observed increased histone acetylation and phospho-acetylation following the heart attacks.
The researchers concluded that a systemic inflammatory event, such as heart attack or surgery, can cause long-term memory impairment and changes in mood through epigenetic mechanisms. They compared the findings to those of other studies in which normal aging and memory-impairing treatments such as chemotherapy had also been associated with increases in histone acetylation or decreases in histone deacetylase activity.
Stressors in early life can contribute to the risk of developing mood disorders. Given that many treatments for mood disorders work by blocking the serotonin 5-HT transporter, Nicole Baganz and colleagues designed a study to see whether an early life stressor, in this case maternal separation, would affect immune processes that in turn affect serotonin signaling.
In this study as in many before it, mice that were removed from their mothers exhibited behaviors that resembled human anxiety and depression. They were also found to have elevated messenger RNA for several inflammatory cytokines (including IL-1beta and IL-6) in their brain and blood. Mice that had a gene for the interleukin-1 receptor (IL-1R) removed exhibited neither the depressive behavioral effects nor the changes in cytokine levels following maternal separation, showing that the IL-1R gene plays a necessary role in the signaling process that leads to this type of depression. Levels of the stress hormone corticosterone in the blood did not differ in the mice with and without the IL-1R gene.
The researchers concluded that early life stressors can cause lifelong changes in inflammatory cytokine levels in mice.
While it can sometimes take weeks for the effects of antidepressant treatments to appear, intravenous ketamine can produce antidepressant effects in as little as two hours. However, ketamine’s effects fade after three to five days. New animal research by Chi-Tso Chiu et al. explores whether adding lithium to ketamine treatment can produce more sustained antidepressant effects.
Mice who are restrained by being placed in a tube for several hours (chronic restraint stress) exhibit a behavioral and neurochemical profile that resembles human depression. When Chiu and colleagues pretreated these stressed mice with sub-therapeutic doses of lithium (600 mg/L) in their drinking water for several weeks, a sub-therapeutic dose of ketamine (2.5 mg/kg of body weight) was enough to produce robust antidepressant effects in the mice, while neither drug alone was effective at these doses.
The combination of ketamine and lithium also restored the density of spines on the dendrites of neurons in the medial prefrontal cortex. Post-treatment with lithium (1200 mg/L) for several weeks was also successful in extending the effects of a single (50 mg/kg) ketamine injection.
Both lithium and ketamine affect the intracellular signaling pathway mTOR. Ketamine activates the pathway, increasing levels of synaptic proteins and dendritic spine density. It also increases brain-derived neurotrophic factor (BDNF) and the BDNF receptor TrkB. BDNF is important for learning and memory.
When lithium was added to the treatment of the mice with ketamine, the mTOR and BNDF pathways were further activated. Lithium also inhibits the receptor GSK-3, supporting ketamine’s rapid-acting antidepressant effects.
Ketamine treatment can produce oxidative stress, in which toxic free radicals can endanger cells, and the addition of low doses of lithium also completely prevented this neurochemical side effect.
Chiu and colleagues hope that the findings of this study in mice can eventually be applied to research in humans in the hopes of finding a clinical option that would sustain the rapid-onset antidepressant effects of ketamine for the long term.
The antidepressant agomelatine (which is available in many countries, but not the US) and the anti-insomnia drug ramelteon (Rozerem) both act as agonists at melatonin M1 and M2 receptors. New research is clarifying the role of these receptors in sleep.
In new research from Stefano Comai et al., mice who were genetically altered to have no M1 receptor (MT1KO knockout mice) showed a decrease in rapid eye movement (REM) sleep, which is linked to dreaming, and an increase in slow wave sleep. Mice who were missing the M2 receptor (MT2KO knockout mice) showed a decrease in slow wave sleep. The effects of knocking out a particular gene like M1 or M2 end up being opposite to the effect of stimulating the corresponding receptor.
The researchers concluded that MT1 receptors are responsible for REM sleep (increasing it while decreasing slow wave sleep), and MT2 receptors are responsible for slow wave non-REM sleep.
The new information about these melatonin receptors may explain why oral melatonin supplements can make a patient fall asleep faster, but do not affect the duration of non-REM sleep. The authors suggest that targeting MT2 receptors could lead to longer sleep by increasing slow wave sleep, potentially helping patients with insomnia.
Epidemiological studies have linked methamphetamine use to risk of Parkinson’s disease, and animal studies of the illicit drug have shown that it harms dopamine neurons. A 2014 study by Sara Ares-Santos et al. in the journal Neuropsychopharmacology compared the effects of repeated low or medium doses to those of a single high dose on mice. Loss of dopaminergic terminals, where dopamine is released, was greatest after three injections of 10mg/kg given at three-hour intervals, followed by three injections of 5 mg/kg at three-hour intervals, and a one-time dose of 30mg/kg. All of the dosages produced similar rates of degeneration of dopamine neurons via necrosis (cell destruction) and apoptosis (cell suicide) in the substantia nigra pars compacta (the part of the brain that degenerates in Parkinson’s disease) and the striata.