HCN channels

Researchers find that the anesthetic and fast-acting antidepressant switches natural patterns of neuronal activity in the cortex of mice. Ketamine, an established anesthetic and increasingly popular antidepressant, dramatically reorganizes activity in the brain, as if a switch had been flipped on its active circuits, according to a new study by Penn Medicine researchers. In a Nature Neuroscience paper released this month, the team described starkly changed neuronal activity patterns in the cerebral cortex of animal models after ketamine administration -- observing normally active neurons that were silenced and another set that were normally quiet suddenly springing to action. This ketamine-induced activity switch in key brain regions tied to depression may impact our understanding of ketamine's treatment effects and future research in the field of neuropsychiatry. "Our surprising results reveal two distinct populations of cortical neurons, one engaged in normal awake brain function, the other linked to the ketamine-induced brain state," said the co-lead and co-senior author Joseph Cichon, MD, PhD, an assistant professor of Anesthesiology and Critical Care and Neuroscience in the Perelman School of Medicine at the University of Pennsylvania. "It's possible that this new network induced by ketamine enables dreams, hypnosis, or some type of unconscious state. And if that is determined to be true, this could also signal that it is the place where ketamine's therapeutic effects take place." Anesthesiologists routinely deliver anesthetic drugs before surgeries to reversibly alter activity in the brain so that it enters its unconscious state. Since its synthesis in the 1960s, ketamine has been a mainstay in anesthesia practice because of its reliable physiological effects and safety profile. One of ketamine's signature characteristics is that it maintains some activity states across the surface of the brain (the cortex). This contrasts with most anesthetics, which work by totally suppressing brain activity. It is these preserved neuronal activities that are thought to be important for ketamine's antidepressant effects in key brain areas related to depression. But, to date, how ketamine exerts these clinical effects remains mysterious. In their new study, the researchers analyzed mouse behaviors before and after they were administered ketamine, comparing them to control mice who received placebo saline. One key observation was that those given ketamine, within minutes of injection, exhibited behavioral changes consistent with what is seen in humans on the drug, including reduced mobility, impaired responses to sensory stimuli, which are collectively termed "dissociation." "We were hoping to pinpoint exactly what parts of the brain circuit ketamine affects when it's administered so that we might open the door to better study of it and, down the road, more beneficial therapeutic use of it," said co-lead and co-senior author Alex Proekt, MD, PhD, an associate professor of Anesthesiology and Critical Care at Penn. Two-photon microscopy was used to image cortical brain tissue before and after ketamine treatment. By following individual neurons and their activity, they found that ketamine turned on silent cells and turned off previously active neurons. The neuronal activity observed was traced to ketamine's ability to block the activity of synaptic receptors -- the junction between neurons -- called NMDA receptors and ion channels called HCN channels. The researchers found that they could recreate ketamine's effects without the medications by simply inhibiting these specific receptors and channels in the cortex. The scientists showed that ketamine weakens several sets of inhibitory cortical neurons that normally suppress other neurons. This allowed the normally quiet neurons, the ones usually being suppressed when ketamine wasn't present, to become active. The study showed that this dropout in inhibition was necessary for the activity switch in excitatory neurons -- the neurons forming communication highways, and the main target of commonly prescribed antidepressant medications. More work will need to be undertaken to determine whether the ketamine-driven effects in excitatory and inhibitory neurons are the ones behind ketamine's rapid antidepressant effects. "While our study directly pertains to basic neuroscience, it does point at the greater potential of ketamine as a quick-acting antidepressant, among other applications," said co-author Max Kelz, MD, PhD,a distinguished professor of Anesthesiology and vice chair of research in Anesthesiology and Critical Care. "Further research is needed to fully explore this, but the neuronal switch we found also underlies dissociated, hallucinatory states caused by some psychiatric illnesses." Support for the study was provided by the Foundation for Anesthesia Education and Research, and the National Institutes of Health (T32NS091006, R01GM124023-01A1, R01GM088156-08, R01 EY020765).

The mechanism by which fat-related molecules called lipids regulate pacemaker ion channel proteins, which help control the heart rhythm, has been revealed in a new study. The mechanism by which fat-related molecules called lipids regulate pacemaker ion channel proteins, which help control the heart rhythm, has been revealed in a study from researchers at Weill Cornell Medicine. In the study, published Nov. 9 in Nature Structural & Molecular Biology, the researchers used advanced methods including cryogenic electron microscopy (cryo-EM) to show in high-resolution detail how certain lipids interact with pacemaker ion channels to enhance their activity. In principle, modulating this lipid interaction with a drug could be a good strategy for treating cardiac arrhythmias and other conditions. "Ion channels have been notoriously difficult to target with drugs, because of the challenge of identifying drug binding sites that are specific to particular channels, but this work reveals sites that might be highly specific and thus viable drug targets," said senior author Dr. Crina Nimigean, professor of physiology and biophysics in anesthesiology at Weill Cornell Medicine. The study's first author is Dr. Philipp Schmidpeter, a research associate in the Nimigean Lab Laboratory in the Department of Anesthesiology at Weill Cornell Medicine. Ion channels are tubelike protein structures that reside within the membranes of a cells, enabling and regulating flows of charged molecules of potassium, sodium and other electrolytes into and out of the cell. These flows of charged molecules, or ions, are key determinants of cell behavior. Specialized ion channels called pacemaker channels are particularly important for the rhythmic activities of heart cells and neurons. Understanding precisely how these pacemaker channels work could thus lead to better treatments for cardiac arrhythmias, chronic pain, epilepsy and other conditions. Arrhythmias alone are estimated to affect millions of people in the United States. Scientists have known that lipids, the major constituents of cell membranes, are involved in modulating the activities of pacemaker channels. But they haven't known much about how these interactions work. In general, methods for studying lipids have been less well developed than methods for studying proteins and other biological molecules. Moreover, ion channels function while embedded in the cell membrane, which is made of two thin layers of lipids -- and that membrane and its lipid components have been hard to replicate in a way that allows detailed experimental manipulation. Drs. Schmidpeter and Nimigean nevertheless were able to obtain important clues about lipid-pacemaker channel interactions by first examining a bacterial pacemaker channel, SthK. In prior studies, they had developed an experimental platform for studying SthK, including a membrane-like environment that they could alter experimentally. SthK is a useful model for pacemaker channels found in humans -- known as HCN channels -- because the two channels, despite the evolutionary gulf between them, have many important similarities, they said. The SthK studies revealed that two lipids, phosphatidyl-glycerol and cardiolipin, can bind to the channel in a way that disrupts a specific molecular connection, known as a salt bridge, which normally tends to clamp the channel shut. When the salt bridge was disrupted by the lipids, the channel became more open and active. Similarly, removing the salt bridge by other means dramatically increased channel activity and abolished the ability of these lipids to affect that activity. The same salt bridge is found in HCN channels, and experiments suggested that in the latter the same lipid-modulation mechanism is at work: Removing the salt bridge had the same effect on channel activity as in the SthK experiments, although for HCN channels the key lipid binding partner was a different lipid, phosphatidic acid. The experiments included cryo-EM imaging of lipids binding to SthK -- imaging that offers clues to how a future drug might disrupt or enhance this lipid interaction to modulate pacemaker channel function. The researchers hope that in follow-on work they can illuminate the role of this lipid-pacemaker interaction in abnormal conditions such as arrhythmias or cancers. "Various conditions can affect the lipid composition of the heart and other tissues, so it wouldn't be surprising to see this pacemaker mechanism altered in diseases," Dr. Schmidpeter said.

Social avoidance is a common symptom of PTSD, and scientists working to better understand why have laboratory evidence that while stress hormone levels consistently increase in the immediate aftermath of a traumatic event, there can be polar opposite consequences in parts of the brain down the line. Social avoidance is a common symptom of PTSD, and scientists working to better understand why have laboratory evidence that while stress hormone levels consistently increase in the immediate aftermath of a traumatic event, there can be polar opposite consequences in parts of the brain down the line. In response to a significant stressor and a subsequent surge of stress hormones, some rodent models experience the expected short-term increase in the excitability of neurons in areas of their brain key to memory and to how they see their environment, as part of the natural instinct to fight or flee. Other genetically identical mice instead experience a decrease in neuron excitability in this key area called the dorsal hippocampus, Dr. Chung Sub Kim, neuroscientist at the Medical College of Georgia at Augusta University, and his colleagues report in the journal Molecular Psychiatry. Too little neuronal activity in the hippocampus has been linked to PTSD in humans; and detailed brain imaging of people with PTSD indicates structural and functional changes in key brain areas, like the hippocampus. Glucocorticoid receptors for the stress hormone cortisol are highly expressed in the hippocampus and have been shown to be more highly expressed in PTSD patients than controls when they reexperience stressful situations. "We are trying to answer the question as to why hippocampal activity is decreased in PTSD or depressed patients," Kim says. "We know it happens, but the mechanism we don't know." One of the things they are finding is that, like humans, some mice just seem more susceptible to a lasting impact from a major and/or chronic stressor and that both their behavior and internal molecular response to stress are distinctive from their more resilient peers. "One was affected directly by stress, and another not so much," Kim says. To mimic stressful scenarios like a bullied child or armed robbery, the scientists created a scenario where a male mouse, which is naturally aggressive, established his territory, then repeatedly attacked another mouse who ventured into that territory. Again, somewhat like human victims, some of the mice did not seem phased after the attack, rather were still naturally inquisitive about the other mouse; while the susceptible mouse clearly avoided The aggressor. In the "susceptible" mice, Kim and his colleagues found increased expression of receptors for stress hormones on neurons in the CA1 region of the dorsal hippocampus of their brain. Those plentiful receptors appeared in turn -- and perhaps counterintuitively -- to enable elevated expression of the protein HCN1, a natural modulator of neuron activity and connectivity already found in naturally high levels in the hippocampus. HCN1 is a major research focus for Kim, who has evidence that even a single episode of significant stress can further increase HCN1 expression in the CA1 region of the dorsal hippocampus and bring neuron excitability down. Also increased in the susceptible rodents wasthe protein TRIP8b, which regulates HCN channel levels. "Stress changes everything," Kim says. The scientists found that this cascade resulted in an increase as well in another natural tamping down mechanism, called hyperpolarization-activated current, which was known to be increased by stress but just how was unknown. Again, the changes were specific to the dorsal -- in humans the back part -- portion of the hippocampus. Even months later, these levels which drove down neuron excitability remained high, and the susceptible mice continued to avoid contact with the aggressive male mouse. The reduced neuron excitability did not change even in response to direct application of a stress hormone to the neurons, which again should increase neuron excitability. The susceptible mice also experienced impaired spatial working memory, which for humans is basically trouble remembering where you left your car keys and how to get to work. The clearly different expression of the HCN1 protein in this region of the hippocampus may be the molecular mechanism driving susceptibility to social avoidance, Kim and his colleagues write. "They have some malfunction in hippocampal information processing," he says. Whether those changes are permanent is not certain but at three months, a long time in mouse years, they were still present: The average mouse lives maybe two to three years, while the average human in the U.S. lives into their late 70s. But in the "resilient" mice, expression of the stress hormone receptor and HCN channel did not increase, but neuron excitability did, in the immediate aftermath of stress. "There are clearly physical differences in the response to stress in the two mice that correlate with their behavior," Kim says, even though you would not suspect the differences in these genetically identical rodents. More work still needs to be done to understand exactly why some mice are resilient and others are susceptible to emotional trauma, the scientists write. In the mouse, the dorsal hippocampus is more linked to learning and memory while the ventral hippocampus is linked to emotion-related reactions like anxiety, Kim and his colleagues write. Comparatively speaking, the dorsal hippocampus has less neuron excitability and is clearly the most reactive to chronic stress. HCN channels are involved in a variety of physiologic processes like sleep and wake states, taste and fear learning. Work by Kim and others has found evidence of a link between HCN channels and mental diseases, including depression and anxiety. The adrenal gland releases cortisol and adrenaline as well in response to a fearful situation like someone threatening you. The increase helps prepare the body for the so-called protective flight or fight response, by making adjustments like increasing levels of glucose, which your body uses as fuel, while suppressing functions like digestion and reproduction, which are not considered essential in that moment. PTSD has also been shown to produce changes in the amygdala, which helps perceive and store memories of emotions like anger, fear and sadness and recognize threat; and the medial prefrontal cortex, which is thought to be important to cognitive functions like attention, habit formation and long-term memory.