2 dimensions of value: Dopamine neurons represent reward but not aversiveness

To make decisions, we need to estimate the value of sensory stimuli and motor actions, their “goodness” and “badness.” We can imagine that good and bad are two ends of a single continuum, or dimension, of value. This would be analogous to the single dimension of light intensity, which ranges from dark on one end to bright light on the other, with many shades of gray in between. Past models of behavior and learning have been based on a single continuum of value, and it has been proposed that a particular group of neurons (brain cells) that use dopamine as a neurotransmitter (chemical messenger) represent the single dimension of value, signaling both good and bad.

The experiments reported here show that dopamine neurons are sensitive to the value of reward but not punishment (like the aversiveness of a bitter taste). This demonstrates that reward and aversiveness are represented as two discrete dimensions (or categories) in the brain. “Reward” refers to the category of good things (food, water, sex, money, etc.), and “punishment” to the category of bad things (stimuli associated with harm to the body and that cause pain or other unpleasant sensations or emotions).

Rather than having one neurotransmitter (dopamine) to represent a single dimension of value, the present results imply the existence of four neurotransmitters to represent two dimensions of value. Dopamine signals evidence for reward (“gains”) and some other neurotransmitter presumably signals evidence against reward (“losses”). Likewise, there should be a neurotransmitter for evidence of danger and another for evidence of safety. It is interesting that there are three other neurotransmitters that are analogous to dopamine in many respects (serotonin, norepinephrine, and acetylcholine), and it is possible that they could represent the other three value signals.

A turbocharger for nerve cells

Locating a car that’s blowing its horn in heavy traffic, channel-hopping between football and a thriller on TV without losing the plot, and not forgetting the start of a sentence by the time we have read to the end – we consider all of these to be normal everyday functions. They enable us to react to fast-changing circumstances and to carry out even complex activities correctly. For this to work, the neuron circuits in our brain have to be very flexible. Scientists working under the leadership of neurobiologists Nils Brose and Erwin Neher at the Max Planck Institutes of Experimental Medicine and Biophysical Chemistry in Göttingen have now discovered an important molecular mechanism that turns neurons into true masters of adaptation.

Neurons communicate with each other by means of specialised cell-to-cell contacts called synapses. First, an emitting neuron is excited and discharges chemical messengers known as neurotransmitters. These signal molecules then reach the receiving cell and influence its activation state. The transmitter discharge process is highly complex and strongly regulated. Its protagonists are synaptic vesicles, small blisters surrounded by a membrane, which are loaded with neurotransmitters and release them by fusing with the cell membrane. In order to be able to respond to stimulation at any time by releasing transmitters, a neuron must have a certain amount of vesicles ready to go at each of its synapses. Brose has been studying the molecular foundations of this stockpiling for years.

The problem is not merely academic. “The number of immediately releasable vesicles at a synapse determines its reliability,” explains Brose. “If there are too few and they are replenished too slowly, the corresponding synapse becomes tired very quickly in conditions of repeated activation. The opposite applies when a synapse can quickly top up its immediately available vesicles under pressure. In fact, such a synapse may even improve with constant activation.”

This synaptic adaptability can be observed in practically all neurons. It is known as short-term plasticity and is indispensable for a large number of extremely important brain processes. Without it, we would not be able to localise sounds, mental maths would be impossible, and the speed and flexibility with which we can alter our behaviour and turn our attention to new goals would be lost.

Some years ago, Brose and his team discovered a protein with the cryptic name of Munc13. Not only is this protein indispensable for the replenishment of vesicles for immediate release at synapses; neuron activity regulates it in such a way that the fresh supply of vesicles can be adjusted in line with demand. This regulation occurs by means of a complex consisting of the signal protein calmodulin and calcium ions that build up in the synapses during intense neuron activity.

“Our earlier work on individual neurons in culture dishes showed that the calcium-calmodulin complex activates Munc13 and consequently ensures that immediately releasable vesicles are replenished faster,” says Noa Lipstein, an Israeli guest scientist in Brose’s lab. “But many colleagues were not convinced that this process also played a role in neurons in the intact brain.”

So Lipstein and her Japanese colleague Takeshi Sakaba created a mutant mouse with genetically altered Munc13 proteins that could not be activated by calcium-calmodulin complexes. The two neurophysiologists first studied the effects of this genetic manipulation on synapses involved in the localisation of sound, which are typically activated several hundred times every second. “Our study shows that the sustained efficiency of synapses in intact neuron networks is critically dependent on the activation of Munc13 by calcium-calmodulin complexes,” explains Lipstein.

The Göttingen-based scientists are convinced of the significance of their study. After all, leading neuroscientists of the past described the calcium sensor responsible for synaptic short-term plasticity and its target protein as the Holy Grail. “I am confident that we have discovered a key molecular mechanism of short-term plasticity that plays a role in all synapses in the brain, and not only in cultivated neurons, as many colleagues believed,” affirms Lipstein. And if she is, in fact, proved right about the interpretation of her findings, Munc13 could even be an ideal pharmacological target for drugs that influence brain function.

Scientists map the wiring of the biological clock

The World Health Organization lists shift work as a potential carcinogen, says Erik Herzog, PhD, Professor of Biology in Arts & Sciences at Washington University in St. Louis. And that’s just one example among many of the troubles we cause ourselves when we override the biological clocks in our brains and pay attention instead to the mechanical clocks on our wrists.

In the June 5 issue of Neuron, Herzog and his colleagues report the discovery of a crucial part of the biological clock: the wiring that sets its accuracy to within a few minutes out of the 1440 minutes per day. This wiring uses the neurotransmitter, GABA, to connect the individual cells of the biological clock in a fast network that changes strength with time of day.

Daily rhythms of sleep and metabolism are driven by a biological clock in the suprachiasmatic nucleus (SCN), a structure in the brain made up of 20,000 neurons, all of which can keep daily (circadian) time individually.

If the SCN is to be a robust, but sensitive, timing system, the neurons must synchronize precisely with one another and adjust their rhythms to those of the environment.

Herzog’s lab has discovered a push-pull system in the SCN that does both. In 2005 they reported that the neurons in the clock network communicate by means of a neuropeptide (VIP) that pushes them to synchronize with one another. And, as they now report in Neuron, these neurons also communicate with GABA that pulls on them weakly, so they are not too tightly coupled.

Together these two networks (VIP and GABA) ensure the clock runs as coordinated, precise timepiece but one that can still adjust its timing to synchronize with the environment.

“We think the neurotransmitter network is there to introduce enough jitter into the system to allow the neurons to resynchronize when environmental cues change, as they do with the seasons,” Herzog says. But, he says, since this biological ‘reset button’ evolved long before mechanical clocks, artificial lights, and high-speed travel, it doesn’t introduce enough jitter to allow us to adjust quickly to the extreme time shifts of modern life, such as flying “backward” (east) through several time zones.

Understanding the push-pull system in the SCN has enormous implications for public health, bearing, as it does, on daylight saving times, shift work, school starting times, medical intern schedules, truck driver hours, and many other issues where the clock in the brain is pitted against the clock in the hand.

Synchronizing the cellular clocks
The “clock” inside each SCN neuron depends on the cyclic expression of a family of genes such as the Period (PER) genes. The expression of these genes and the neuron’s firing rate typically peak at mid-day and fall at night. The gene activity is like the cogs in a clock, and the electrical activity like the hands on the clock.

Each neuron in the SCN keeps time, but because they’re different cells, they have slightly different rhythms. Some run a little bit fast and others a bit slow. If the SCN as a whole is to function as a clock, its neurons need to synchronize with one another.

The goal of the recent work in the Herzog lab has been to figure out how the clock cells are connected to each other. “It wasn’t clear, for example, if each neuron communicated with just a few of its neighbors or with all of them,” Herzog says.

Mark Freeman, a graduate student in the lab, developed a method for recording the firing rate of about 100 neurons simultanously on a multi-electrode array. “You float the SCN neurons down gently,” Herzog says, “and the neurons will attach to the electrodes, creating a clock in a dish that will tick away for weeks or months.”

Using these electrode arrays, his lab demonstrated that the neurons in the SCN are synchronized by the exchange of the neuropeptide VIP (vasoactive intestinal polypeptide), which alters the expression of PER to speed up or slow down neurons until they are all in synch.

These synchronized networks are very precise, says Herzog. If you let them free-run in constant darkness they will lose or gain only a few minutes out of the 1,440 minutes in a day. So they’re accurate to within 1 or 2 percent.

But they’re ever so slightly off the 24-hour cycle tied to one turn of the planet on its axis. Over time they would drift far enough off that cycle to be of little use to us, unless they also had some means of synchronizing to local time.

Resetting the cellular clocks
In the article published in Neuron, Herzog and his colleagues report on a second network in the biological clock.

In this network the connections are made by the neurotransmitter GABA (γ-amino-butyric acid). “We proved we had found a GABAergic network by applying drugs that block GABA receptors on the cells,” Herzog says. “All of the connections we had mapped between neurons dropped out.”

Remarkably, when the network drops out, the clock becomes more precise. So the GABAergic network destabilizes the clock; it jiggles it a little.

Herzog points out that the GABAergic network, is sparse, weak and fast (much faster than the VIP network, which relies on the slower action of a neuropeptide), as you might expect a jitter-generator to be.

“We think the GABAergic network is there to let our clocks adjust to environmental cues, such as gradual, seasonal changes in sunrise and sunset,” says Herzog. 

It’s a bit like whacking an old television set that has lost vertical synch to get it to resynch with the broadcast signal.

But there isn’t enough jitter in the clock to allow it to make abrupt adjustments, such as the one-hour forward jump when Daylight Savings Time starts. That “spring forward” has been statistically shown to increase the likelihood of heart attacks and car accidents, Herzog says.

Some sleep aids, such as benzodiazepines, that activate the GABA receptors may make the circadian clock a little more jittery, helping people adjust to big time jumps, such as flying across time zones. “But we don’t yet know whether they can improve jetlag; if they do, we want to know if it is because they help you sleep on the long flight or because they help the biological clock adjust to the new time zone,” Herzog cautions.

In any case, it is clear that if people repeatedly force the clock to reset, they throw off more than sleep. The biological clock regulates metabolism and cell division as well as sleep/wake cycles. So shift work, for example, is associated both with metabolic disorders, such as diabetes, and with the unregulated cell division that characterizes cancer.

Fighting our biological clocks does a lot more than make us crabby coffee drinkers.

Gene switches make prairie voles fall in love

Epigenetic changes affect neurotransmitters that lead to pair-bond formation.

Love really does change your brain — at least, if you’re a prairie vole. Researchers have shown for the first time that the act of mating induces permanent chemical modifications in the chromosomes, affecting the expression of genes that regulate sexual and monogamous behaviour. The study is published today in Nature Neuroscience.

Prairie voles (Microtus ochrogaster) have long been of interest to neuroscientists and endocrinologists who study the social behaviour of animals, in part because this species forms monogamous pair bonds — essentially mating for life. The voles’ pair bonding, sharing of parental roles and egalitarian nest building in couples makes them a good model for understanding the biology of monogamy and mating in humans.

Previous studies have shown that the neurotransmitters oxytocin and vasopressin play a major part in inducing and regulating the formation of the pair bond. Monogamous prairie voles are known to have higher levels of receptors for these neurotransmitters than do voles who have yet to mate; and when otherwise promiscuous montane voles (M. montanus) are dosed with oxytocin and vasopressin, they adopt the monogamous behaviour of their prairie cousins.

Because behaviour seemed to play an active part in changing the neurobiology of the animals, scientists suspected that epigenetic factors were involved. These are chemical modifications to the chromosomes that affect how genes are transcribed or suppressed, as opposed to changes in the gene sequences themselves.

Love potion

To look for clues of epigenetic agents at play in monogamous behaviour, neuroscientist Mohamed Kabbaj and his team at Florida State University in Tallahassee took voles which had been housed together for 6 hours but had not mated. The researchers injected drugs into the voles’ brains near a region called the nucleus accumbens, which is closely associated with the reinforcement of reward and pleasure. The drugs blocked the activity of an enzyme that normally keeps DNA tightly wound up and thus prevents the expression of genes.

The team found that the genes for the vasopressin and oxytocin receptors had been transcribed, and as a result the nucleus accumbens of the animals bore high levels of these receptors. Animals that had been permitted to mate also had high levels of vasopressin and oxytocin receptors, confirming the link between bond formation and gene activity.

“Mating activates this brain area which leads to partner preference — we can induce this same change in the brain with this drug,” Kabbaj explains.

Interestingly, the injection alone cannot induce the partner preference. “The drug by itself won’t do all these molecular changes — you need the context: it’s the drug plus the six hours of cohabitation,” says Kabbaj.

“This is a study I myself wanted to do years ago,” says Thomas Insel, who heads the US National Institute of Mental Health in Bethesda, Maryland. “If mating causes the release of the neuropeptide, how does this kick into a higher gear for the rest of the animal’s life? This study for me really is the first experimental demonstration that the epigenetic change would be necessary for the long-term change in behaviour.”

“This paper really shows that there is an epigenetic mechanism underlying pair bonds — we ourselves have looked for that and not found it,” says Alaine Keebaugh of Emory University in Atlanta, Georgia, who also studies the neuroscience of prairie voles.

Kabbaj says he hopes that the work could ultimately lead to an enhanced understanding of how epigenetic factors affect social behaviour in humans — not only in monogamy and pair bonding, but also in conditions such as autism and schizophrenia, which affect social interactions.

Newly understood circuits add finesse to nerve signals

An unusual kind of circuit fine-tunes the brain’s control over movement and incoming sensory information, and without relying on conventional nerve pathways, according to a study published this week in the journal Neuron.

Researchers at the University of Alabama at Birmingham (UAB) discovered new details of a mechanism operating in the cerebellum, the brain region that processes nerve signals coming in from the spinal cord and cortex.

“Our results explain a second layer of nerve signal transmission that depends, not on whether a nerve cell is wired into a defined signaling pathway circuit, but instead on how close it is to the pathway,” said Jacques Wadiche, Ph.D., assistant professor in the Department of Neurobiology within the UAB School of Medicine, investigator in the Evelyn McKnight Brain Institute at UAB and senior study author. “It has become clear that this kind of nerve circuit is intimately linked with autism and certain movement disorders, and we hope the mechanisms detailed here contribute to the design of new treatments.”

Beyond nerve pathways

Nerve cells are known to occur in defined pathways that transmit messages in one direction. This pathway-specific view of nerve signaling has been reinforced by high-tech imaging studies yielding detailed connectivity maps. Along these lines, the Obama Administration will soon ask Congress for $100 million in research funding to further improve such maps.

Within nerve pathways, each nerve cell sends an electric pulse down an extension of itself called an axon until it reaches a synapse, a gap between itself and the next cell in line. When it reaches an axon’s end, the pulse triggers the release of chemicals called neurotransmitters that float across the gap, where they either cause the downstream nerve cell to “fire” and pass on the message, or stop the message. In this way, each synapse between nerve cells in a pathway “decides” whether or not a message continues on.

In recent years, studies have found that neurotransmitters also spill into tissue surrounding axons in a type signaling not restricted to synaptic connections. With the term itself implying a mess, “spillover” was thought to degrade the capacity of nerve cells to precisely pass on signals.

The current study adds to recent evidence arguing that spillover may instead enhance message transmission, with the results revolving around three nerve cell types in the cerebellum: climbing fibers, Purkinje cells and interneurons.

Climbing fibers, which carry information from the brainstem into the cerebellum, play key roles in motor timing and sensory processing. Within these fibers, nerve cells release the excitatory neurotransmitter glutamate into synapses that then strive to pass messages deeper into the cerebellum. Purkinje cells are paired with climbing fibers and intent on inhibiting their signals.

When excited by glutamate from climbing fibers at one end, Purkinje cells release another neurotransmitter called GABA at their downstream synapse to stop the message. An excitatory signal triggers an inhibitory one as a counter-balance, a form of feedback critical to the function of the central nervous system. Lack of inhibition, for instance, causes circuits to seize, seizures and the death of Purkinje cells, the latter of which has been linked by post mortem studies to a higher incidence of autism spectrum disorders.

Previously, researchers thought that incoming signals from climbing fibers caused a single, strong response in the cerebellum: the activation of Purkinje cells that released GABA. The current study argues that such signals also trigger the firing of interneurons, nearby inhibitory middlemen that connect sets of nerve cells.

Interneurons within, and outside of, the glutamate spill zone around climbing fibers may have different effects on the other interneurons and Purkinje cells they connect to, according to the current finding. The interactions either inhibit or excite many Purkinje cells surrounding an active climbing fiber and refine its messages in a feedback system more sophisticated than once thought.

Glutamate has its effect by fitting into AMPA and NMDA receptor proteins, like a key into a lock, on the surfaces of nerve cells it signals to. The consensus has been that glutamate receptors occur only within synapses. Finding them on nerve cells outside of synapse-defined pathways represents “a fundamental shift in understanding,” said Wadiche, and may result in longer-lasting inhibition within key signaling pathways.

“A 2007 study published in Nature Neuroscience found that many climbing fibers signal to interneurons in the outer layer of the cerebellum outside nerve pathways and exclusively through glutamate spillover,” said Luke Coddington, a graduate student in Wadiche’s lab and study author. “Our team built on that observation to show how spillover affects the function of interneurons, Purkinje cells, and ultimately, the entire cerebellum. Spillover-mediated signaling recruits local microcircuits to extend the reach and finesse of climbing fiber signaling.”

Ketamine Shows Significant Therapeutic Benefit in People with Treatment-Resistant Depression

Patients with treatment-resistant major depression saw dramatic improvement in their illness after treatment with ketamine, an anesthetic, according to the largest ketamine clinical trial to-date led by researchers from the Icahn School of Medicine at Mount Sinai. The antidepressant benefits of ketamine were seen within 24 hours, whereas traditional antidepressants can take days or weeks to demonstrate a reduction in depression.

The research will be discussed at the American Psychiatric Association meeting on Monday, May 20, 2013 at 12:30 pm in the Press Briefing Room at the Moscone Center in San Franscico.

Led by Dan Iosifescu, MD, Associate Professor of Psychiatry at Mount Sinai; Sanjay Mathew, MD, Associate Professor of Psychiatry at Baylor College of Medicine; and James Murrough, MD Assistant Professor of Psychiatry at Mount Sinai, the research team evaluated 72 people with treatment-resistant depression—meaning their depression has failed to respond to two or more medications—who were administered a single intravenous infusion of ketamine for 40 minutes or an active placebo of midazolam, another type of anesthetic without antidepressant properties. Patients were interviewed after 24 hours and again after seven days. After 24 hours, the response rate was 63.8 percent in the ketamine group compared to 28 percent in the placebo group. The response to ketamine was durable after seven days, with a 45.7 percent response in the ketamine group versus 18.2 percent in the placebo group. Both drugs were well tolerated.

“Using midazolam as an active placebo allowed us to independently assess the antidepressant benefit of ketamine, excluding any anesthetic effects,” said Dr. Murrough, who is first author on the new report. “Ketamine continues to show significant promise as a new treatment option for patients with severe and refractory forms of depression.”

Major depression is caused by a breakdown in communication between nerve cells in the brain, a process that is controlled by chemicals called neurotransmitters. Traditional antidepressants such as selective serotonin reuptake inhibitors (SSRIs) influence the activity of the neurotransmitters serotonin and noreprenephrine to reduce depression. In these medicines, response is often significantly delayed and up to 60 percent of people do not respond to treatment, according to the U.S Department of Health and Human Services. Ketamine works differently than traditional antidepressants in that it influences the activity of the glutamine neurotransmitter to help restore the dysfunctional communication between nerve cells in the depressed brain, and much more quickly than traditional antidepressants.

Future studies are needed to investigate the longer term safety and efficacy of a course of ketamine in refractory depression. Dr. Murrough recently published a preliminary report in the journal Biological Psychiatry on the safety and efficacy of ketamine given three times weekly for two weeks in patients with treatment-resistant depression.

“We found that ketamine was safe and well tolerated and that patients who demonstrated a rapid antidepressant effect after starting ketamine were able to maintain the response throughout the course of the study,” Dr. Murrough said. “Larger placebo-controlled studies will be required to more fully determine the safety and efficacy profile of ketamine in depression.”

The potential of ketamine was discovered by Dennis S. Charney, MD, Anne and Joel Ehrenkranz Dean of the Icahn School of Medicine at Mount Sinai, and Executive Vice President for Academic Affairs of The Mount Sinai Medical Center, in collaboration with John H. Krystal, MD, Chair of the Department of Psychiatry at Yale University.

“Major depression is one of the most prevalent and costly illnesses in the world, and yet currently available treatments fall far short of alleviating this burden,” said Dr. Charney. “There is an urgent need for new, fast-acting therapies, and ketamine shows important potential in filling that void.”

Dr. Murrough will present his research on Sunday, May 19, 2013 from 1:00 pm to 3:00 pm in the Moscone exhibit hall at the APA meeting.

Longer Days Bring ‘Winter Blues’—For Rats, Not Humans

Most of us are familiar with the “winter blues,” the depression-like symptoms known as “seasonal affective disorder,” or SAD, that occurs when the shorter days of winter limit our exposure to natural light and make us more lethargic, irritable and anxious. But for rats it’s just the opposite.

Biologists at UC San Diego have found that rats experience more anxiety and depression when the days grow longer. More importantly, they discovered that the rat’s brain cells adopt a new chemical code when subjected to large changes in the day and night cycle, flipping a switch to allow an entirely different neurotransmitter to stimulate the same part of the brain.

Their surprising discovery, detailed in the April 26 issue of Science, demonstrates that the adult mammalian brain is much more malleable than was once thought by neurobiologists. Because rat brains are very similar to human brains, their finding also provides a greater insight into the behavioral changes in our brain linked to light reception. And it opens the door for new ways to treat brain disorders such as Parkinson’s, caused by the death of dopamine-generating cells in the brain.

The neuroscientists discovered that rats exposed for one week to 19 hours of darkness and five hours of light every day had more nerve cells making dopamine, which made them less stressed and anxious when measured using standardized behavioral tests. Meanwhile, rats exposed for a week with the reverse—19 hours of light and five hours of darkness—had more neurons synthesizing the neurotransmitter somatostatin, making them more stressed and anxious.

“We’re diurnal and rats are nocturnal,” said Nicholas Spitzer, a professor of biology at UC San Diego and director of the Kavli Institute for Brain and Mind. “So for a rat, it’s the longer days that produce stress, while for us it’s the longer nights that create stress.”

Because rats explore and search for food at night, while humans evolved as creatures who hunt and forage during the daylight hours, such differences in brain chemistry and behavior make sense. Evolutionary changes presumably favored humans who were more active gatherers of food during the longer days of summer and saved their energy during the shorter days of winter.

“Light is what wakes us up and if we feel depressed we go for a walk outside,” said Davide Dulcis, a research scientist in Spitzer’s laboratory and the first author of the study. “When it’s spring, I feel more motivation to do the things I like to do because the days are longer. But for the rat, it’s just the opposite. Because rats are nocturnal, they’re less stressed at night, which is good because that’s when they can spend more time foraging or eating.”

But how did our brains change when humans evolved millions of years ago from small nocturnal rodents to diurnal creatures to accommodate those behavioral changes?

“We think that somewhere in the brain there’s been a change,” said Spitzer. “Sometime in the evolution from rat to human there’s been an evolutionary adjustment of circuitry to allow switching of neurotransmitters in the opposite direction in response to the same exposure to a balance of light and dark.”

A study published earlier this month in the American Journal of Preventive Medicine found some correlation to the light-dark cycle in rats and stress in humans, at least when it comes to people searching on the internet for information in the winter versus the summer about mental illness. Using Google’s search data from 2006 to 2010, a team of researchers led by John Ayers of San Diego State University found that mental health searches on Google were, in general, 14 percent higher in the winter in the United States and 11 percent higher in the Australian winter.

“Now that we know that day length can switch transmitters and change behavior, there may be a connection,” said Spitzer.

In their rat experiments, the UC San Diego neuroscientists found that the switch in transmitter synthesis in the rat’s brain cells from dopamine to somatostatin or back again was not due to the growth of new neurons, but to the ability of the same neurons there to produce different neurotransmitters.

Rats exposed to 19 hours of darkness every 24 hours during the week showed higher numbers of dopamine neurons within their brains and were more likely, the researchers found, to explore the open end of an elevated maze, a behavioral test showing they were less anxious. These rats were also more willing to swim, another laboratory test that showed they were less stressed.

“Because rats are nocturnal animals, they like to explore during the night and dopamine is a key part of our and their reward system,” said Spitzer. “It’s part of what allows them to be confident and reduce anxiety.”

The researchers said they don’t know precisely how this neurotransmitter switch works. Nor do they know what proportion of light and darkness or stress triggers this switch in brain chemistry. “Is it 50-50? Or 80 percent light versus dark and 20 percent stress? We don’t know,” added Spitzer. “If we just stressed the animal and didn’t change their photoperiod, would that lead to changes in transmitter identity? We don’t know, but those are all doable experiments.”

But as they learn more about this trigger mechanism, they said one promising avenue for human application might be to use this neurotransmitter switch to deliver dopamine effectively to parts of the brain that no longer receive dopamine in Parkinson’s patients.

“We could switch to a parallel pathway to put dopamine where it’s needed with fewer side effects than pharmacological agents,” said Dulcis.

First steps of synapse building captured in live zebra fish embryos

Using spinning disk microscopy on barely day-old zebra fish embryos, University of Oregon scientists have gained a new window on how synapse-building components move to worksites in the central nervous system.

What researchers captured in these see-through embryos — in what may be one of the first views of early glutamate-driven synapse formation in a living vertebrate — were orderly movements of protein-carrying packets along axons to a specific site where a synapse would be formed.

Washbourne addresses:

► The basic importance of the findings

► The connection to diseases, including autism

The discovery, in research funded by the National Institutes of Health, is described in a paper placed online ahead of publication in the April 25 issue of the open-access journal Cell Reports. It is noteworthy because most synapses formed in vertebrates use glutamate as a neurotransmitter, and breakdowns in the process have been tied to conditions such as autism, schizophrenia and mental retardation.

The zebra fish has become one of the leading research models for studying early development, in general, and human-disease states.

In this case, researchers used immunofluorescence labeling to highlight the area they put under the microscopes. The embryos they studied were barely 24-hours old and a millimeter in length, but neurons in their spinal cord were already forming connections called synapses. Images were taken every 30 seconds over two hours.

"If we zoom out a bit and look at development in the human, the majority of synapse formation occurs in the cortex after birth and continues for the first two years in a baby’s life," said Philip Washbourne, a professor of biology and member of the UO’s Institute of Neuroscience.

Previous studies, done in vitro, contradicted each other, with one, in 2000, identifying a single packet of building blocks arriving at a pre-synaptic terminal. The other, in 2004, identified two protein packets. After watching the process unfold live, with imaging over long time spans, Washbourne said: “We now see at least three, and maybe more, such deliveries.”

"Axons are long processes — think of them as highways — of neurons. In humans, these can be a meter long, from spinal cord to your big toe," he said. It’s in the cell body where all the proteins are made, and they have to be transported out. Is it done by a single bus or by several cars? These results point to additional layers of complexity in the established mechanisms of synaptogenesis."

The new research also showed that sequence also is crucial. Two different pre-synaptic packages of molecules repeatedly arrived in the same order. A key building block — the protein synapsin — always arrived third. As these delivery vehicles traveled the axonal highway, another protein, a cyclin-dependent kinase known as Cdk5, acts as a stoplight at the synapse-construction site, where phosphorylation occurs. More research is needed on Cdk5, Washbourne said.

"Understanding how all this happens will inform us to what’s going wrong in neurodevelopment that leads to diseases," Washbourne said. "We have indications that the glue that gets all this going includes a gene that has been linked to autism, so knowing how these molecules start the process of synapse formation — and what goes wrong in people with mutations in these genes — might allow for a therapeutic targeting to correct the mutations and manipulate the stop signs."