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.
Genetic variation in L-type calcium channel genes have been linked to bipolar disorder. Since calcium plays an important role in circadian rhythms, abnormalities in the calcium channel in bipolar disorder could explain some of the circadian rhythm disturbances patients with bipolar disorder exhibit. New research by Michael McCarthy and colleagues shows that calcium channels in general, and the gene CACNA1C in particular, affect signaling pathways that regulate circadian rhythms in both human and animal cells. The researchers also found that calcium channels affected how lithium changes circadian rhythms, suggesting a mechanism by which the treatment may work. They suggest that drugs that affect the L-type calcium channel may be promising treatments for bipolar disorder.
Editor’s Note: The L-type calcium channel blocker nimodipine has had antidepressant, antimanic, and anticycling effects in some patients with bipolar disorder in small studies both by Peggy Pazzaglia and colleagues (including this author Robert Post) and a larger randomized study by Haroon R. Chaudhry.
The clinical effects of nimodipine results thus align with studies linking the CACNA1C gene to bipolar illness and its early onset, increased expression of the gene in the brain of bipolar patients in autopsy studies, increased levels of calcium in white cells of bipolar patients, and a variety of other neurobiological phenomena observed in normal controls carrying the risk gene.
The new link found between CACNA1C and circadian rhythms further links the L-type calcium channel abnormality and bipolar disorder, as well as the therapeutic effects of the L-type calcium channel blocker nimodipine. This drug deserves further study, especially in those with the genetic variation in CACNA1C that has been linked to bipolar disorder.
Both bipolar disorder and unipolar depression often begin in childhood or adolescence, but it can be difficult to distinguish the two using symptoms only. People with bipolar illness may go a decade without receiving a correct diagnosis. Researcher Jorge Almeida and colleagues recently performed a meta-analysis of previous studies to determine what neural activity is typical of children with bipolar disorder versus children with unipolar depression while processing images of facial emotion. They found that youth with bipolar disorder were more likely to show limbic hyperactivity and cortical hypoactivity during emotional face processing than youth with unipolar depression. Almeida and colleagues hope that this type of data may eventually be used to diagnose these disorders or to measure whether treatment has been successful.
Researchers hope to map out the neurocircuitry by which stress leads to compulsive drug taking. A recent study by Klaus Miczek and colleagues examined different rodents’ responses to the stress of being repeatedly placed in the cage of a larger, more aggressive rodent, developing what is known as defeat stress, a set of behaviors that mimic human depression. Mice and rats showed increases in the stress hormone corticosterone that did not diminish over repeated run-ins with a larger animal. Rodents who were exposed to this stress became sensitized to cocaine or amphetamine, showing hyperactivity that increased each time they accessed the drug (the opposite of a tolerance response). Some also “binged” on cocaine, which they were able to self-administrate by pushing a lever to receive infusions. The mice and rats that went through the social defeat showed elevated levels of dopamine in the nucleus accumbens, the brain’s reward center. Levels were related to the severity of their stressful experience.
Later the rodents had a choice between water and a 20% alcohol solution. The researchers determined what type of stress led the rodents to consume the alcohol solution instead of the water. The maximal effect was seen in two types of mice that suffered an attack of less than five minutes that resulted in a moderate number of attack bites (30); this resulted in the mice consuming large amounts (15–30 g/kg/day) of the alcohol solution. Earlier sensitization to cocaine or amphetamine did not predict later alcohol or cocaine self-administration.
When the researchers injected the rodents with antagonists of the receptors for corticotropin-releasing factor, a hormone and neurotransmitter important in stress response, prior to each episode of social defeat, the rodents did not escalate their cocaine or alcohol self-administration, indicating that CRF plays an essential role in the process by which stress makes animals prone to using substances.
In related research by Camilla Karlsson and colleagues, IL-1R1 and TNF-1R, the receptors for two inflammatory cytokines, mediated the effects of social stress on escalated alcohol use in mice.
People and animals can rapidly learn to associate environmental stimuli with positive or negative outcomes, learning what to approach or avoid as they go through daily life. The amygdala plays a role in this type of emotional learning, which can be disrupted by mood disorders. In new research, Praneeth Namburi and colleagues determined that activity at the synapses in the basolateral amygdala reveals differences in the creation of fear memories and reward memories.
In animals trained with reward and fear conditioning tasks, photostimulation of neurons that then travel from the basolateral amygdala complex to the nucleus accumbens (the brain’s reward center) is positively reinforcing, while photostimulation of neurons that will travel from the basolateral amygdala complex to the centromedial nucleus of the amygdala causes aversion. There are genetic differences between the two types of neurons, including a difference in the gene for the neurotensin-1 receptor. The researchers found that neurotensin, a neuropeptide, modulates glutamate’s effect on neurons, causing some to project to the nucleus accumbens and some to project to the centromedial nucleus of the amygdala.
The researchers wrote that the results “provide a mechanistic explanation, on both a synaptic and circuit level, for how positive and negative associations can be rapidly formed, represented, and expressed within the amygdala.”
Editor’s Note: The amygdala’s creation of opposing outputs may provide clues to the mechanisms behind mania (involving the nucleus accumbens) and depression (involving the centromedial nucleus of the amygdala).
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.
A new technology is making it possible to view the mammalian brain’s structure and connectivity for the first time. Karl Deisseroth discussed the technology, called CLARITY, at a plenary lecture at the 2014 meeting of the International College of Neuropsychopharmacology.
The way CLARITY works is by replacing lipids in the brain with a hydrogel substance. This preserves the structure of the brain’s neural networks, leaves proteins and nucleic acids intact, but allows for observation by rendering the brain transparent. This can be done in a system as large as the entire adult mouse brain. Early attempts took a whole day, but Deisseroth eventually found a way to render a mouse’s brain transparent in a matter of minutes.
The pictures are truly amazing, allowing for the visualization of previously microscropic neurons, dendrites, axons and connections in life-sized images. Pictures and details are available at www.clarityresourcecenter.org.
Deisseroth and colleagues have used CLARITY imaging to determine where neurons fire during different social activities. By placing photosensitive fibers in selected neurons using a virally based gene insertion technique, Deisseroth and colleagues were able to selectively fire dopamine neurons in the ventral tegmental area, part of the brain’s reward system, and thus increase or decrease the social interaction of mice by increasing or decreasing firing. The effects were selective to social interaction; the firing did not affect locomotor activity or exploration of an inanimate object.
The ventral tegmental area contains neurons that project to several locations in the brain, and Deisseroth and colleagues hoped to observe which were important to social interaction. Stimulating the ventral tegmental area to drive the medial prefrontal cortex caused anxiety in the mice and made them averse to social interaction. However, when the ventral tegmental area was used to selectively drive the nucleus accumbens, another part of the brain’s reward system, social interaction increased.
Deisseroth wanted to know if the nucleus accumbens was also involved in normal spontaneous social interactions. The researchers used a virus to insert an opsin-sensitive calcium gene that could give an ongoing readout of neural activity. (Opsin is a light-sensitive receptor found in cells in the retina.) The team found that the nucleus accumbens was implicated in social interaction with another mouse, but not in exploration of a novel object. Based on CLARITY imaging of the structure of ion channels (which are so small they cannot even be seen with an electron microscope), Deisseroth was able to selectively alter ion fluxes and turn neuronal firing on or off at will.
In the last 50 years, the brain and its billions of neurons and hundreds of trillions of synapses have gone from complete inaccessibility toward increasing clarity.
Researcher Tony Pitts presented a study at the 2014 meeting of the International College of Neuropsychopharmacology (CINP) that described the neurobiology of an animal model of depression in rodents. In animal models, researchers provoke depression-like symptoms in animals with the hopes of finding neurobiological clues to human depression. Pitts’ studies explored the effects of acute stressors as well as more chronic long-term stressors such as learned helplessness.
In the rodents, acute stressors caused increased cell firing in the hippocampus, which caused increases in burst firing and an increase in the number of cells firing in the ventral tegmental area, which then led to increased activity in the nucleus accumbens (the brain’s reward center). However, after the stressor was over, there was an opponent process that resulted in a much more prolonged period of inhibition in the nucleus accumbens, with associated decreases in psychomotor activity and reward seeking. The rodents lost their preference for sucrose and engaged in less intracranial self-stimulation, pressing a bar to stimulate the brain pleasurably. These and other effects suggest an analogy to anhedonia (loss of pleasure in activities that were previously enjoyed), which is a key component of human depression.
In related studies, after experiencing periods of inescapable shocks, rodents developed learned helplessness, failing to avoid the area where shocks were delivered even when an exit was readily available. Rodents who had learned helplessness showed inhibited firing of cells in the ventral tegmental area, less activity in the nucleus accumbens, and apparent anhedonia. This inhibition was mediated via messages from the infralimbic prefrontal cortex (the equivalent to the subgenual cingulate cortex in humans, important for motivation) to the amygdala and then the GABAergic ventral pallidum, which decreased the number of dopaminergic cells firing in the ventral tegmental area. Blocking the amygdala input to this inhibitory pathway reversed the low dopamine firing and the anhedonia-like behaviors.
The anesthetic ketamine (which has rapid-acting antidepressant effects in humans) produces an immediate reversal of the learned helpless behavior in the rodents and increases the number of dopamine cells firing in the ventral tegmental area. Ketamine administered directly into the nucleus accumbens induces long-term potentiation (enhanced synaptic responsivity) and reverses helpless behavior and the long-term depression of neural firing that is associated with it.
Thus, when an acute stressor is over and the opponent process emerges, or following long-term chronic stressors such as learned helplessness, the excitatory path to the ventral tegmental area is absent, while the inhibitory path to the ventral tegmental area (via the infralimbic prefrontal cortex, amygdala, and ventral pallidum) predominates. Ketamine is able to re-activate the activating pathway and increase activity in the ventral tegmental area and the nucleus accumbens, changes that are associated with the reversal of learned helplessness and anhedonia.
Editor’s Note: In the previous BNN, we reported researcher Scott Russo’s findings that input from the intralaminar nucleus of the thalamus was critical to the depression-like behaviors seen in a different animal model of depression, social defeat stress, where repeated exposure to defeat by a larger, more aggressive animal produces behaviors that resemble human depression. Here in Pitts’ research, learned helplessness is induced by inescapable shocks. Both models share the finding that firing decreases in the reward area of the brain (the nucleus accumbens). However, the key part of the brain driving the low levels of activity in the nucleus accumbens and the associated depression-like behavior appear to be different in these two different models. The intralaminar nucleus of the thalamus plays a key role in the social defeat stress model, while the infralimbic cortex and the amygdala play key roles in the learned helplessness model. These data together suggest that part of the reason depression differs from person to person may be because the illness can be driven by different brain areas as a result of different kinds of stressors.
To study depression in humans, researchers look to rodents to learn more about behavior. Rodents who are repeatedly defeated by more aggressive animals often begin to exhibit behavior that resembles depression. At the 2014 meeting of the International College of Neuropsychopharmacology (CINP), researcher Andre Der-Avakian reported that in a recent study, repeated experiences of social defeat led to depressive behavior in a subgroup of animals (which he calls susceptible), but not in others (which he calls resilient). Among many biological differences, the resilient animals showed increases in neurogenesis in the dentate gyrus of the hippocampus.
Chronic treatment of the susceptible animals with the selective serotonin reuptake inhibitor (SSRI) antidepressant fluoxetine or the tricyclic antidepressant desipramine, which both increase neurogenesis, also reversed the depressive behavior in about half of the animals. A single injection of the anesthetic ketamine (which has rapid-acting antidepressant effects in humans) reversed social avoidance behavior in about 25% of the animals. One depression-like symptom was anhedonia (loss of pleasure from previously enjoyed activities), which researchers measured by observing to what extent the animals engaged in intracranial self-stimulation, pressing a bar to stimulate the brain pleasurably. The effectiveness of the drugs in inducing resilient behavior was related to the degree of anhedonia seen in the animals. The drugs worked less well in the more anhedonic animals (those who gave up the intracranial stimulation more easily, indicating that they experienced less reward from it.)
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.