Low doses of psychedelic drug erases conditioned fear in mice

Low doses of a psychedelic drug erased the conditioned fear response in mice, suggesting that the agent may be a treatment for post-traumatic stress disorder and related conditions, a new study by University of South Florida researchers found.

The unexpected finding was made by a USF team studying the effects of the compound psilocybin on the birth of new neurons in the brain and on learning and short-term memory formation. Their study appeared online June 2 in the journal Experimental Brain Research, in advance of print publication.

Psilocybin belongs to a class of compounds that stimulate select serotonin receptors in the brain.  It occurs naturally in certain mushrooms that have been used for thousands of years by non-Western cultures in their religious ceremonies.

While past studies indicate psilocybin may alter perception and thinking and elevate mood, the psychoactive substance rarely causes hallucinations in the sense of seeing or hearing things that are not there, particularly in lower to moderate doses.

There has been recent renewed interest in medicine to explore the potential clinical benefit of psilocybin, MDMA and some other psychedelic drugs through carefully monitored, evidence-based research.

“Researchers want to find out if, at lower doses, these drugs could be safe and effective additions to psychotherapy for treatment-resistant psychiatric disorders or adjunct treatments for certain neurological conditions,” said Juan Sanchez-Ramos, MD, PhD, professor of neurology and Helen Ellis Endowed Chair for Parkinson’s Disease Research at the USF Health Morsani College of Medicine.

Dr. Sanchez-Ramos and his colleagues wondered about psilocybin’s role in the formation of short-term memories, since the agent binds to a serotonin receptor in the hippocampus, a region of the brain that gives rise to new neurons. Lead author for this study was neuroscientist Briony Catlow, a former PhD student in Dr. Sanchez-Ramos’ USF laboratory who has since joined the Lieber Institute for Brain Development, a translational neuroscience research center located in the Johns Hopkins Bioscience Park.

The USF researchers investigated how psilocybin affected the formation of memories in mice using a classical conditioning experiment. They expected that psilocybin might help the mice learn more quickly to associate a neutral stimulus with an unpleasant environmental cue.

To test the hypothesis, they played an auditory tone, followed by a silent pause before delivering a brief shock similar to static electricity. The mice eventually learned to link the tone with the shock and would freeze, a fear response, whenever they heard the sound.

Later in the study, the researchers played the sound without shocking the mice after each silent pause. They assessed how many times it took for the mice to resume their normal movements, without freezing in anticipation of the shock.

Regardless of the doses administered, neither psilocybin nor ketanserin, a serotonin inhibitor, made a difference in how quickly the mice learned the conditioned fear response.  However, mice receiving low doses of psilocybin lost their fearful response to the sound associated with the unpleasant shock significantly more quickly than mice getting either ketanserin or saline (control group). In addition, only low doses of psilocybin tended to increase the growth of neurons in the hippocampus.

“Psilocybin enhanced forgetting of the unpleasant memory associated with the tone,” Dr. Sanchez-Ramos said. “The mice more quickly dissociated the shock from the stimulus that triggered the fear response and resumed their normal behavior.”

The result suggests that psilocybin or similar compounds may be useful in treating post-traumatic stress disorder or related conditions in which environmental cues trigger debilitating behavior like anxiety or addiction, Dr. Sanchez-Ramos said.

Getting a grip on sleep

All mammals sleep, as do birds and some insects. However, how this basic function is regulated by the brain remains unclear. According to a new study by researchers from the RIKEN Brain Science Institute, a brain region called the lateral habenula plays a central role in the regulation of REM sleep. In an article published today in the Journal of Neuroscience, the team shows that the lateral habenula maintains and regulates REM sleep in rats through regulation of the serotonin system. This study is the first to show a role of the lateral habenula in linking serotonin metabolism and sleep.

The lateral habenula is a region of the brain known to regulate the metabolism of the neurotransmitter serotonin in the brain and to play a key role in cognitive functions.

“Serotonin plays a central role in the pathophysiology of depression, however, it is not clear how abnormalities in regulation of serotonin metabolism in the brain lead to symptoms such as insomnia in depression,” explain Dr. Hidenori Aizawa and Dr. Hitoshi Okamoto who led the study.

Since animals with increased serotonergic activity at the synapse experienced less REM sleep, the researchers hypothesized that the lateral habenula, which regulates serotonergic activity in the brain, must modulate the duration of REM sleep.

They show that removing the lateral habenula in rats results in a reduction of theta rhythm, an oscillatory activity that appears during REM sleep, in the hippocampus, and shortens the rats’ REM sleep periods. However, this inhibitory effect of the lateral habenular lesion on REM sleep disappears when the serotonergic neurons in the midbrain are lesioned.

The team recorded neural activity simultaneously in the lateral habenula and hippocampus in a sleeping rat. They find that the lateral habenular neurons, which fire persistently during non-REM sleep, begin to fire rhythmically in accordance with the theta rhythm in the hippocampus when the animal is in REM sleep.

“Our results indicate that the lateral habenula is essential for maintaining theta rhythms in the hippocampus, which characterize REM sleep in the rat, and that this is done via serotonergic modulation,” concludes Dr Aizawa.

“This study reveals a novel role of the lateral habenula, linking serotonin and REM sleep, which suggests that an hyperactive habenula in patients with depression may cause altered REM sleep,” add the authors.

Serotonin Mediates Exercise-Induced Generation of New Neurons

Mice that exercise in running wheels exhibit increased neurogenesis in the brain. Crucial to this process is serotonin signaling. These are the findings of a study by Dr. Friederike Klempin, Daniel Beis and Dr. Natalia Alenina from the research group led by Professor Michael Bader at the Max Delbrück Center (MDC) Berlin-Buch. Surprisingly, mice lacking brain serotonin due to a genetic mutation exhibited normal baseline neurogenesis. However, in these serotonin-deficient mice, activity-induced proliferation was impaired, and wheel running did not induce increased generation of new neurons. (Journal of Neuroscience)

Scientists have known for some time that exercise induces neurogenesis in a specific brain region, the hippocampus. However, until this study, the underlying mechanism was not fully understood. The hippocampus plays an important role in learning and in memory and is one of the brain regions where new neurons are generated throughout life.

Serotonin facilitates precursor cell maturation

The researchers demonstrated that mice with the ability to produce serotonin are likely to release more of this hormone during exercise, which in turn increases cell proliferation of precursor cells in the hippocampus. Furthermore, serotonin seems to facilitate the transition of stem to progenitor cells that become neurons in the adult mouse brain.

For Dr. Klempin and Dr. Alenina it was surprising that normal baseline neurogenesis occurs in mice that, due to a genetic mutation, cannot produce serotonin in the brain. However, they noted that some of the stem cells in serotonin-deficient mice either die or fail to become neurons.

Yet, these animals seem to have a mechanism that allows compensation for the deficit, in that progenitor cells, an intermediate stage in the development from a stem cell to a neuron, divide more frequently. According to the researchers, this is to maintain the pool of these cells.

However, the group of wheel-running mice that do not produce serotonin did not exhibit an exercise-induced increase in neurogenesis. The compensatory mechanism failed following running. The researchers concluded: “Serotonin is not necessarily required for baseline generation of new neurons in the adult brain, but is essential for exercise-induced hippocampal neurogenesis.”

Hope for new approaches to treat depression and memory loss in the elderly

Deficiency in serotonin, popularly known as the “molecule of happiness”, has been considered in the context of theories linking major depression to declining neurogenesis in the adult brain. “Our findings could potentially help to develop new approaches to prevent and treat depression as well as age-related decline in learning and memory,” said Dr. Klempin and Dr. Alenina.

How Serotonin Receptors Can Shape Drug Effects from LSD to Migraine Medication

A team including scientists from The Scripps Research Institute (TSRI), the University of North Carolina at Chapel Hill and the Chinese Academy of Sciences has determined and analyzed the high-resolution atomic structures of two kinds of human serotonin receptor. The new findings help explain why some drugs that interact with these receptors have had unexpectedly complex and sometimes harmful effects.

“Understanding the structure-function of these receptors allows us to discover new biology of serotonin signaling and also gives us better ideas about what biological questions to probe in a more intelligent manner,” said TSRI Professor Raymond Stevens, who was a senior investigator for the new research. The studies were published in two papers on March 21, 2013 in Science Express [1 , 2], the advance online version of the journal Science.

Pioneering Important Molecular Structures

Stevens’s laboratory at TSRI has pioneered the development of techniques for determining the 3D atomic structures of cellular receptors—particularly the large receptor class known as G protein-coupled receptors (GPCRs). GPCRs sit in the cell membrane and sense various molecules outside cells. When certain molecules bind to them, the receptor’s respond in a way to transmit a signal inside the cell.

“Because G protein-coupled receptors are the targets of nearly 50 percent of medicines, they are the focus of several major National Institutes of Health (NIH) initiatives,” said Jean Chin of the NIH’s National Institute of General Medical Sciences, which partly funded the work through the Protein Structure Initiative. “These detailed molecular structures of two serotonin receptor subfamilies bound to antimigraines, antipsychotics, antidepressants or appetite suppressants will help us understand how normal cellular signaling is affected by these drugs and will offer a valuable framework for designing safer and more effective medicines.”

In the past several years, using X-ray crystallography, the Stevens laboratory has determined the high-resolution structures of 10 of the most important GPCRs for human health—including the β2 adrenergic receptor, the A2a adenosine receptor (the target of caffeine), HIV related CXCR4 receptor, the pain-mediating nociceptin receptor, S1P1 receptor important for inflammatory diseases, H1 histamine receptor (antihistamine medications) and the D3 dopamine receptor which is involved in mood, motivation and addiction.

Serotonin receptors are no less important. “Nearly all psychiatric drugs affect serotonin receptors to some extent, and these receptors also mediate a host of effects outside the brain, for example on blood coagulation, smooth muscle contraction and heart valve growth,” said Bryan Roth, a collaborator on both studies who is professor of pharmacology at the University of North Carolina (UNC).

Untangling Two Serotonin Receptors

Roth’s laboratory teamed up with Stevens’s as part of the National Institute of General Medical Sciences (NIGMS) Protein Structure Initiative. For this project the two labs also worked with the laboratories of Professors Eric Xu and Hualiang Jiang at the Shanghai Institute of Materia Medica, part of the Chinese Academy of Sciences. “By collaborating with the Chinese teams we were able to complete a much more thorough study and get the most out of our fundamental structural results,” said Stevens.

In the first of the new studies, co-lead author Chong Wang, a graduate student in the Stevens laboratory, and his colleagues determined the structure of the serotonin receptor subtype 5-HT_1B, the principal target of several drug classes. (5-HT, or 5-hydroxytryptamine, is a technical term for serotonin.) The team produced the 5-HT_1B receptor while it was bound by either ergotamine or dihydroergotamine—two old-line anti-migraine drugs that work in part by activating 5-HT_1B receptors.

With the help of the special fusion protein, nicknamed BRIL (apocytochrome b562RIL), Wang and colleagues were able to stabilize these structures and coax them to line up in a regular ordering known as a crystal. X-ray crystallography revealed, at high resolution, an atomic structure of 5-HT_1B with a main binding pocket and a separate, extended binding pocket.

Harmful Off-Target Effects

In the second study, TSRI graduate student and lead author Daniel Wacker and colleagues used similar techniques to determine the structure of the 5-HT_2B receptor bound to ergotamine. The 5-HT_2B receptor was chiefly of interest because drug developers want to avoid activating it.

“Drugs that are meant to target other serotonin receptors in the brain can have harmful off-target effects on 5-HT_2B receptors, which are found abundantly on heart valves, for example,” said Roth. The weight-loss drug fenfluramine and closely related dexfenfluramine were withdrawn from the US market in 1997 after being linked to heart valve disease. Roth’s laboratory later showed that this side effect was mediated by heart valve 5-HT_2B receptors.

Analyses of the 5-HT_1B and 5-HT_2B receptor structures revealed a subtle difference between them. “Although their main binding pockets look very similar, their extended binding pockets are not as similar—the one for 5-HT_2B is narrower and in a slightly different position,” said Wang.

With the two receptor structures in hand, the Xu and Jiang team simulated the bindings of various drugs. They showed, for example, that anti-migraine drugs called triptans should bind well to 5-HT_1B receptors but poorly to 5-HT_2B receptor structures, in which the extended binding pocket is less accessible. Similarly, the team’s calculations confirmed that fenfluramine’s active metabolite should bind very tightly to the 5-HT_2B receptor.

Delving Deeper

In the second study, the researchers used the 5-HT_2B and 5-HT_1B structural data to better understand a recently discovered GPCR signaling pathway.

When a neurotransmitter such as serotonin binds to its GPCR receptor and triggers the primary, G protein-mediated activation signal, it also usually triggers another signal, often mediated by a protein called β-arrestin. This second signaling cascade may simply have the effect of “arresting” or inhibiting the primary, G protein-mediated signaling. But it can also have other effects on the cell, and although most molecules bind to their target GPCRs in a way that activates these primary and secondary signals equally, others preferentially activate one or the other. “Such functional selectivity, as we call it, adds another layer of complexity to drug effects on GPCRs,” said Roth, a co-senior author of the study.

Roth’s laboratory produced several 5-HT receptor subtypes in test cells, and compared the strength of G-protein and β-arrestin signaling when these receptors were bound by ergotamine or various other drugs, including the ergotamine-derived hallucinogen LSD (lysergic acid diethylamide). Most of the tested drugs showed no bias. However, ergotamine, LSD and some of their relatives turned out to be clearly biased in favor of β-arrestin signaling at the 5-HT_2B receptor. Comparison of the ergotamine-bound 5-HT_2B structure with the ergotamine-bound 5-HT_1B structure revealed the likely reason. “We could see that when ergotamine is bound to the 5-HT_2B receptor it stabilizes the receptor structure in a conformation that interferes with G protein signaling,” said Wacker.

The findings allow scientists to start probing this arrestin-mediated signaling pathway and its downstream effects in a more targeted manner. “These structural data are teaching us to ask better questions about receptor biology,” said Stevens.

Depression stems from miscommunication between brain cells

A new study from the University of Maryland School of Medicine suggests that depression results from a disturbance in the ability of brain cells to communicate with each other. The study indicates a major shift in our understanding of how depression is caused and how it should be treated. Instead of focusing on the levels of hormone-like chemicals in the brain, such as serotonin, the scientists found that the transmission of excitatory signals between cells becomes abnormal in depression. The research, by senior author Scott M. Thompson, Ph.D., Professor and Interim Chair of the Department of Physiology at the University of Maryland School of Medicine, was published online in the March 17 issue of Nature Neuroscience.

"Dr. Thompson’s groundbreaking research could alter the field of psychiatric medicine, changing how we understand the crippling public health problem of depression and other mental illness," says E. Albert Reece, M.D., Ph.D., M.B.A., Vice President for Medical Affairs at the University of Maryland and John Z. and Akiko K. Bowers Distinguished Professor and Dean at the University of Maryland School of Medicine. "This is the type of cutting-edge science that we strive toward at the University of Maryland, where discoveries made in the laboratory can impact the clinical practice of medicine."

The first major finding of the study was the discovery that serotonin has a previously unknown ability to strengthen the communication between brain cells. “Like speaking louder to your companion at a noisy cocktail party, serotonin amplifies excitatory interactions in brain regions important for emotional and cognitive function and apparently helps to make sure that crucial conversations between neurons get heard,” says Dr. Thompson. “Then we asked, does this action of serotonin play any role in the therapeutic action of drugs like Prozac?”

To understand what might be wrong in the brains of patients with depression and how elevating serotonin might relieve their symptoms, the study team examined the brains of rats and mice that had been repeatedly exposed to various mildly stressful conditions, comparable to the types of psychological stressors that can trigger depression in people.

The researchers could tell that their animals became depressed because they lost their preference for things that are normally pleasurable. For example, normal animals given a choice of drinking plain water or sugar water strongly prefer the sugary solution. Study animals exposed to repeated stress, however, lost their preference for the sugar water, indicating that they no longer found it rewarding. This depression-like behavior strongly mimics one hallmark of human depression, called anhedonia, in which patients no longer feel rewarded by the pleasures of a nice meal or a good movie, the love of their friends and family, and countless other daily interactions.

A comparison of the activity of the animals’ brain cells in normal and stressed rats revealed that stress had no effect on the levels of serotonin in the ‘depressed’ brains. Instead, it was the excitatory connections that responded to serotonin in strikingly different manner. These changes could be reversed by treating the stressed animals with antidepressants until their normal behavior was restored.

"In the depressed brain, serotonin appears to be trying hard to amplify that cocktail party conversation, but the message still doesn’t get through," says Dr. Thompson. Using specially engineered mice created by collaborators at Johns Hopkins University School of Medicine, the study also revealed that the ability of serotonin to strengthen excitatory connections was required for drugs like antidepressants to work.

Sustained enhancement of communication between brain cells is considered one of the major processes underlying memory and learning. The team’s observations that excitatory brain cell function is altered in models of depression could explain why people with depression often have difficulty concentrating, remembering details, or making decisions. Additionally, the findings suggest that the search for new and better antidepressant compounds should be shifted from drugs that elevate serotonin to drugs that strengthen excitatory connections.

"Although more work is needed, we believe that a malfunction of excitatory connections is fundamental to the origins of depression and that restoring normal communication in the brain, something that serotonin apparently does in successfully treated patients, is critical to relieving the symptoms of this devastating disease," Dr. Thompson explains.

(Image: McGovern Institute, MIT)

Why your brain tires when exercising

A marathon runner approaches the finishing line, but suddenly the sweaty athlete collapses to the ground. Everyone probably assumes that this is because he has expended all energy in his muscles. What few people know is that it might also be a braking mechanism in the brain which swings into effect and makes us too tired to continue. What may be occurring is what is referred to as ‘central fatigue’.

"Our discovery is helping to shed light on the paradox which has long been the subject of discussion by researchers. We have always known that the neurotransmitter serotonin is released when you exercise, and indeed, it helps us to keep going. However, the answer to what role the substance plays in relation to the fact that we also feel so exhausted we have to stop has been eluding us for years. We can now see it is actually a surplus of serotonin that triggers a braking mechanism in the brain. In other words, serotonin functions as an accelerator but also as a brake when the strain becomes excessive," says Associate Professor Jean-François Perrier from the Department of Neuroscience and Pharmacology, who has spearheaded the new research.

Help in the battle against doping

Jean-François Perrier hopes that mapping the mechanism that prompts central fatigue will be useful in several ways. Central fatigue is a phenomenon which has been known for about 80 years; it is a sort of tiredness which, instead of affecting the muscles, hits the brain and nervous system. By conducting scientific experiments, it is possible to observe and measure that the brain sends insufficient signals to the muscles to keep going, which in turn means that we are unable to keep performing. This makes the mechanism behind central fatigue an interesting area in the battle against doping, and it is for this reason that Anti Doping Danmark has also helped fund the group’s research.

"In combating the use of doping, it is crucial to identify which methods athletes can use to prevent central fatigue and thereby continue to perform beyond what is naturally possible. And the best way of doing so is to understand the underlying mechanism," says Jean-François Perrier.

Developing better drugs

The brain communicates with our muscles using so-called motoneurons. In several diseases, motoneurons are hyperactive. This is true, for example, of people suffering from spasticity and cerebral palsy, who are unable to control their movements. Jean-François Perrier therefore hopes that, in the long term, this new knowledge can also be used to help develop drugs against these symptoms and to find out more about the effects of antidepressants.

"This new discovery brings us a step closer to finding ways of controlling serotonin. In other words, whether it will have an activating effect or trigger central fatigue. It is all about selectively activating the receptors which serotonin attaches to," explains Jean-François Perrier.

"For selective serotonin re-uptake inhibitor (SSRI) drugs which are used as antidepressants, we can possibly help explain why those who take the drugs often feel more tired and also become slightly clumsier than other people. What we now know can help us develop better drugs," concludes Jean-François Perrier.

(Image credit)