The kynurenine pathway describes the steps that turn the amino acid tryptophan (the ingredient in turkey that might make you sleepy) into nicotinamide adenine dinucleotide. This pathway might be a connection between the immune system and neurotransmitters involved in schizophrenia.
A recent autopsy study by researcher Thomas Weickert and colleagues explored this link by determining that in the brains of people with schizophrenia and high levels of inflammation, messenger RNA for Kynurenine Aminotransferase II (KATII, a step on the kynurenine pathway) was elevated in the dorsolateral prefrontal cortex compared to the brains of people who died healthy and those with schizophrenia but low levels of inflammation.
The KATII mRNA levels also correlated with mRNA levels of inflammatory markers such as glial fibrillary acidic protein and interleukin-6.
Blood measures related to the kynurenine pathway also differentiated people with schizophrenia from healthy controls. People with schizophrenia had lower levels of tryptophan, kynurenine, and kynurenic acid in their blood. The low levels of kynurenic acid in the blood were correlated with deficits in working memory and smaller volume of the dorsolateral prefrontal cortex.
Weickert and colleagues suggest that blood levels of kynurenic acid might provide a measurable indicator of the degree to which people with schizophrenia are experiencing problems with executive functioning (planning and decision-making) and loss of brain volume.
Scientists have known for some time that heightened activity of dopaminergic neurons (neurons in the midbrain that contain the neurotransmitter dopamine) can make people vulnerable to depression. New research in animals suggests for the first time that noradrenergic neurons (those that contain the neurotransmitter norepinephrine) control the activity of dopaminergic neurons and that these noradrenergic neurons can make the difference between vulnerability to depression or resilience to stress. The research, published by Elsa Isingrini and colleagues in the journal Nature Neuroscience in 2015, showed that animals that cannot release norepinephrine are vulnerable to depression following chronic stress, but increasing the production of norepinephrine increases the animals’ resilience and reduces depression.
These findings may open up new avenues to treatment that target norepinephrine rather than or in addition to dopamine or serotonin, which is targeted by SSRI antidepressants, or selective serotonin reuptake inhibitors.
5-HT7 is a type of receptor activated by the neurotransmitter serotonin. Some of the most potent effects of lurasidone (Latuda), an atypical antipsychotic with antidepressant effects in bipolar depression, and vortioxetine (Brintellix), a unique antidepressant for unipolar depression that also has positive effects on cognition, occur through the blockade of 5-HT7 receptors. The atypical antipsychotics aripiprazole and sulpiride also act on 5-HT7 receptors.
Researcher Agnieszka Nikiforuk summarized the research to date on 5-HT7 receptors in the journal CNS Drugs in 2015.
The receptors play a role in regulating sleep and circadian rhythms, which may explain why drugs that target them can be helpful in depression. Drugs that target 5-HT7 receptors have also improved learning and memory.
One subject of research into 5-HT7 receptors is whether better results come from blocking the receptors or stimulating them.
Blockade of 5-HT7 receptors has improved depression-like symptoms in animals and enhances the effects of sub-therapeutic doses of antidepressants. In other animal studies, stimulation of the receptors has appeared promising for the prevention of age-related cognitive decline.
Cytokines are chemical messengers that send signals between immune cells and between the immune system and the central nervous system. Their levels in blood are considered a measure of inflammation, which has been implicated in depression and stress. A new study by Ghanshyam Pandey and colleagues reported increased levels of cytokines in the brains of people who committed suicide. In the prefrontal cortices of people who died by suicide, there were significantly elevated levels of the inflammatory cytokines IL-1 beta, IL-6 and TNF-alpha compared to the brains of normal controls. There were also lower levels of protein expression of the cytokine receptors IL-1R1, IL-1R2 and IL-1R antagonist (IL1RA) in the suicide brains compared to controls.
The researchers concluded that abnormalities in proinflammatory cytokines and their receptors are associated with the pathophysiology of depression and suicide. This research provides direct confirmation of the indirect measures of inflammation observed in the blood of depressed patients compared to controls.
Raphael Mechoulam, who first synthesized THC, the main ingredient in marijuana, gave the history of marijuana and its receptors in the central nervous system in a plenary talk at the 2014 meeting of the International College of Neuropsychopharmacology. In Syria hundreds of years ago the drug was named ganzigunnu, meaning “the drug that takes away the mind.” It has also been called azalla, meaning “hand of the ghost.” Among the 100 compounds in marijuana, the best-known ingredient is delta-9-tetrahydrocannabinol (delta-9 THC), which produces most of the actions of the drug. There is another active ingredient, cannabidiol (CBD), which has calming and anti-anxiety effects, but is present in very low levels.
The brain has cannabinoid receptors that respond to ingredients in marijuana in addition to other chemicals produced in the brain. They modulate calcium ions and decrease the release of many neurotransmitters.
THC acts at CB-1 receptors, producing the high. The CB-1 receptor is synthesized on demand, post-synaptically, and is transferred to the pre-synaptic terminal where it decreases calcium and transmitter release. Consistent with marijuana’s appetite-stimulating properties (“the munchies”), if the CB-1 receptor is blocked in animals, they lose their appetite and die of hunger.
There are also low levels of CB-2 receptors in the brain, whose activation does not cause a high, and whose levels may increase dramatically in pathological situations. Activation of the CB-2 receptor is anti-inflammatory and, in the same way that the immune system acts against foreign proteins, CB-2 acts as a protector against non-proteins.
CBD does not bind to any cannabinoid receptors, but its actions are blocked by cannabinoid antagonists.
There are two chemicals in the brain (endogenous ligands) that act at cannabinoid receptors—anandamide and 2-arachidonoylglycerol (2-AG). They are soluble only in lipids (not in water), and have never been given to people. In animals, 2-AG has neuroprotective effects, decreases the size of a stroke by 60%, and increases recovery from stroke.
Marijuana and CBD in particular have also had beneficial effects in people. Marijuana decreases the nausea and vomiting associated with chemotherapy in children, has anti-inflammatory effects in rheumatoid arthritis (decreasing inflammatory marker TNF alpha), and has anti-diabetes and anti-convulsant effects.
In 2012, researcher F. Markus Leweke and colleagues showed that CBD was about as effective as the atypical antipsychotic amisulpiride in alleviating the psychotic symptoms of schizophrenia. CBD’s other effects include reducing anxiety and improving psoriasis by increasing DNA methylation (Pucci et al. 2013).
It seems possible that some of these myriad effects of marijuana and endogenous ligands at CB receptors could be exploited for clinical therapeutics, as Mechoulam endorses, but when and how that will take place remains an unanswered question.
Editor’s Note: Despite all these potential positives of CBD, it should be noted that its levels are very low in marijuana, and that heavy smoking of marijuana has substantial adverse effects. These include low motivation, a doubling of the risk of psychosis, a hastening of the onset of bipolar disorder and schizophrenia, and cognitive impairment, as well as some changes in brain structure seen via magnetic resonance imaging (MRI). It may be becoming legal in many states, but is a bad idea for those at high risk for mood, anxiety, or bipolar disorders or for schizophrenia.
Exercise increases brain-derived neurotrophic factor (BDNF), a protein that protects neurons and is important for learning and memory. In a study of mice who were trained to find objects, sedentary mice could not discriminate between familiar object locations and novel ones 24 hours after receiving weak training, while mice who had voluntarily taken part in exercise over a 3-week period could easily distinguish between these locations after the weak training.
Mice who received sodium butyrate (NaB) after training behaved similarly well to those who had exercised. Sodium butyrate is a histone deacetylase (HDAC) inhibitor, meaning it helps keep acetyl groups on histones, around which DNA is wrapped, making the DNA easier to transcribe. In this case the easy transcription of DNA enables learning under conditions in which it might not usually take place.
Both sodium butyrate and exercise promote learning through their effects on BDNF in the hippocampus. They make the DNA for BDNF easier to transcribe, suggesting that exercise can put the brain in a state of readiness to create new or more lasting memories.
Sherman Brown of the University of Texas Southwestern reports that the neurosteroid allopregnanolone has positive effects in bipolar depression. Patients in Brown’s study received doses of 100mg capsules twice daily during the first week, then one capsule in the morning and two capsules in the evening during the second week, and two capsules in the morning and three capsules in the evening during the third week.
Neurosteroids can change the excitability of neurons through their interactions with the neurotransmitters that carry signals from neurons across synapses. Among the various types of neurotransmitters, GABA plays an inhibitory role, while glutamate is responsible for excitability. Allopregnanolone, which is naturally produced in the body, has positive effects on GABA receptors and inhibitory effects on glutamate NMDA receptors, so that it increases the balance of inhibition (GABA) over excitation (glutamate).
Research has shown a link between inflammation and mental illness. Inflammation leads to a series of chemical changes that can overexcite neurons and interfere with the protection of neurons.
Inflammation increases the production of indoleamine-pyrrole 2,3-dioxygenase (IDO), an enzyme that breaks down the amino acid tryptophan into kynurenic acid and quinolinic acid. They in turn increase glutamate, the main excitatory neurotransmitter, and decrease brain-derived neurotrophic factor (BDNF), which keeps neurons healthy.
Kynurenic acid stimulates microglia, which clean up the central nervous system as a form of immune defense, to produce inflammatory cytokine proteins.
Quinolinic acid directly stimulates glutamate receptors and encourages glutamate release from astrocytes. Quinolinic acid also blocks glutamate removal that would normally occur through reuptake into the astrocytes, leading to more stimulation of extrasynaptic glutamate receptors and decreases in BDNF.
Quinolinic acid’s effects are opposite to those of the antidepressant ketamine, which blocks glutamate NMDA receptors and increases BDNF. When people are given interferon protein for the treatment of cancers, quinolinic acid increases in cerebrospinal fluid, inducing depression. The severity of depression induced is correlated with the patient’s levels of quinolinic acid.
It appears that ketamine has indirect anti-inflammatory effects through its ability to block glutamate receptors and increase BDNF.
Oxidative stress has been implicated in a wide range of illnesses, but what is it exactly? Our bodies use the oxygen we breathe to burn the fuel we get from food, and while this is a natural process, it produces byproducts known as free radicals, which are unstable molecules that can strip electrons from other molecules in a process called oxidation. Antioxidants (such as vitamin C) act as a source of electrons, helping keep other cells stable and healthy. Oxidative stress refers to the stress on our bodies from the normal effects of free radicals combined with environmental stressors like tobacco smoke or radiation.
In work presented at the 2013 meeting of the Society of Biological Psychiatry, Anna Andreason showed that over-activity of neurons increases oxidative stress through the production of reactive oxygen species (ROS). These are a type of free radicals that can damage cells in two ways: nitrosylation of proteins (adding nitric oxide to a thiol molecule), and oxidation, which results in more lasting effects on synaptic structures. The chemical compound rotenone damages mitochondria by producing ROS, and Andreason found that lithium was able to reverse this production and reverse the adverse effects of oxidative stress.
Lithium Has an Amazing Array of Positive Effects
Editor’s Note: The ability of lithium to protect mitochondria (the energy storehouse of a cell) adds to an increasingly long list of lithium’s neurotropic and neuroprotective benefits. Lithium increases cell survival factors BDNF and Bcl-2, increases markers of neuronal integrity such as N-Acetylaspartic acid (NAA), increases the volume of the hippocampus and cortex, and now helps protect mitochondria from oxidative stress. Lithium also increases the length of telomeres, which cap the ends of chromosome and protect them from damage during the DNA replication that occurs each time a cell divides. Short telomeres are associated with many kinds of medical and psychiatric diseases, as well as shorter life spans. No wonder that in addition to preventing mania and depression it has other clinical benefits, such as preventing memory deterioration, medical mortality, and suicide.
Brooks R. Keeshin from a research group led by Frank Putnam presented a poster at the 2012 meeting of the American Academy of Child and Adolescent Psychiatry (AACAP) on neuroendocrine function in recently sexually abused adolescent girls with and without post-traumatic stress disorder (PTSD). Average age of the girls was 15 and they had experienced the sexual abuse six months to one year before the study. The researchers found that morning cortisol awakening response was flattened in the girls, and this was associated with PTSD severity and the severity of intrusive symptoms. Increased adversity prior to the sexual abuse experience was also associated with flattening of the cortisol awakening response.
The researchers suggest that alterations in the hypothalamic pituitary adrenal axis (HPA) appear around the time of the abuse and are associated with the severity of PTSD symptomatology in sexually abused adolescent girls.