What can invertebrates tell us about our brains?

With its hundred billion neurons and quadrillion synapses, the human central nervous system(CNS) can seem intractably complex. Fortunately, there is a class of animals whose nervous systems and behaviors are much more easily understood.  Invertebrates, such as sea slugs and worms, have on the order of only hundreds or thousands of neurons and their connections are extremely well stereotyped. This simplicity makes them amenable to experimentation and modeling, and has allowed scientists to understand the structure and function of their neural circuits.

In his review, Allen I. Selverston, Professor Emeritus at UCSD, asks if information gained from the study of invertebrates can be translated to our understanding of the human CNS.  He focuses on a particularly well characterized type of circuit called Central Pattern Generators (CPG).  CPGs are networks of neurons which produce rhythmic outputs in the absence of sensory feedback, and often control simple motor actions such as feeding or swimming. CPGs are not only found in invertebrates but vertebrates as well, where they control certain low level functions.  An example of a CPG is the leech heartbeat network which is shown in the diagram below.

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Leech heartbeat neuronal network

The study CPGs using electrical and chemical manipulation of their constituent neurons has led to three primary types of discoveries.  First, it has revealed how a complex array of ion channels contributes to the distinct activity properties of individual neurons. Second, it has shed light on the types of synapses and how they are modulated and third, how circuits produce functional outputs.

Selverston uses these three types of analysis to explain how many different CPGs from the invertebrate world work. Unfortunately, he concludes that there are very few general principles for the design of these circuits that are transferable from model to model. Each CPG has its own evolutionary history that has crafted it into a bespoke circuit for the unique function that it serves. Moreover, the experimental methods used to study CPGs are unlikely to be effective in more complicated vertebrate systems because they cannot be probed with single cell techniques. This means that while the cellular and synapse level data may broadly applicable, the further study of invertebrate CPGs is unlikely to give us much insight into the human CNS.

Selverston’s review can be found here.

Leo Breston is a first year student in the Neuroscience Graduate Program. He is currently rotating in the Navlaka lab. 

 

How to make a schizophrenic mouse

Dopamine is perhaps the best known neurotransmitter, almost certainly due to its association with the idea of reward. It’s often brought up to explain why we like the things we do, and how people can develop addictions to different types of rewards. However, dopamine isn’t just a reward chemical; it’s very important for a wide variety of brain processes, including voluntary movement, attention, sensory gating, evaluating the salience of a stimulus, decision making, and motivation. Given all this chemical does, it’s not too surprising that changes in dopamine signaling have been implicated in mental disorders, like schizophrenia, ADHD, and depression. But how, then, does a healthy brain regulate dopamine? And how does this system go wrong?

Larry Zweifel’s lab at the University of Washington studies these questions. Soden et al. examined the effect of a mutation in a gene called KCNN3 that was discovered in a schizophrenia patient (Bowen et al., 2001). This gene codes for an ion channel called SK3 that is activated when calcium is inside the cell and then lets potassium out of the neuron, reducing its excitability. The mutated form, however, has an early stop codon due to a frame shift, and therefore only produces a small fragment of the original protein. Interestingly, this mutation was found to be dominant in cell culture, needing only one copy to exert its full effect and suppress SK3 currents in neurons, likely because the protein fragments bind to and inactivate SK3 channels (Miller et al., 2001).

Since SK3 is expressed in dopamine neurons and was mutated in a schizophrenia patient, it seems a promising candidate for a gene regulating dopamine function. Soden et al. tested the effect of this mutation in a mouse model by adding the mutated gene into the genome of dopamine neurons using a viral vector and the Cre-lox system. Indeed, they found that dopamine neurons in the mice with the mutant gene were more excitable and fired less regularly than usual, making them more prone to firing bursts of action potentials.

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These bursts are thought to be a functionally different form of dopamine signaling than the neurons’ regular spiking, causing different effects in dopamine-responsive brain regions, so this altered neuronal function should correspond to altered behavior in tasks where dopamine is important. Since dopamine is involved in sensory gating, meaning the brain’s filtering of irrelevant stimuli, the researchers tested this ability in the mutant mice. In their task, the mice were presented with two sounds, one which was was always followed by a reward (a sugary pellet), and one which was rarely followed by a reward. The mice learned to look for the pellet quickly after the more predictive sound, but not after the other. Once the mice had learned to distinguish the sounds, the researchers flashed a light at the same time the reward-predictive sound was played. The normal mice became distracted, but the mutant mice paid no attention to the novel stimulus and still proceeded quickly to the pellet, indicating that their sensory gating was altered.

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The researchers also tested their mice on prepulse inhibition (PPI), a neurological process by which the startle response of an animal to a sudden, high amplitude stimulus, such as a loud sound, is reduced if the strong stimulus is preceded by a weaker one. This phenomenon occurs in both mice and humans, is affected by dopamine-modulating drugs, and is reduced in people with schizophrenia. Indeed, the control mice showed prepulse inhibition, while the mutant mice did not.

soden-3

This paper is significant in that the authors were able to demonstrate a link across multiple levels of biology, from disrupted gene function to neuronal function to behavior. As the KCNN3 gene is in a chromosomal region (1q21) that is associated with schizophrenia, it’s possible that this gene, and pathological processes similar to that shown here by the author, are at play in more cases of schizophrenia. The ability understand how the brain is disrupted across different scales in psychiatric illness is crucial to developing better, targeted treatments for these conditions.

Bowen, T. et al. Mutation screening of the KCNN3 gene reveals a rare frameshift mutation. Mol. Psychiatry 6, 259–260 (2001).
Miller, M. J. Nuclear Localization and Dominant-negative Suppression by a Mutant SKCa3 N-terminal Channel Fragment Identified in a Patient with Schizorphrenia. Journal of Biological Chemistry 276, 27753–27756 (2001).
Soden, M. E. et al. Disruption of Dopamine Neuron Activity Pattern Regulation through Selective Expression of a Human KCNN3 Mutation. Neuron 80, 997–1009 (2013).
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Jacob Garrett is a first-year PhD student in the neurosciences program. He has not yet narrowed his interests enough to provide any sort of useful description here.

Let’s Talk About Neural Time Travel.

Unfortunately Foster has not developed a functional time machine. Instead, he is currently working on elucidating the underlying mechanisms of neural time travel in rats and its role in goal directed navigation. Neural time travel, or chronesthesia, was first proposed by Endel Tulving in the 1980s. It refers to the ability to perceive the difference between perceived, remembered, known, and imagined time. At first glance, this appears to be a thought experiment that will leave you with a headache and not much to show for it but fear not. The basic principle of neural time travel, is the ability to distinguish now from then, whether it is a time in the past or the future. To make a grievous oversimplification, neural time travel is akin to locating memories, thoughts and imaginings in temporal space.

Now, what does this have to do with hippocampal place cells which by definition are interested in physical space?

Pyramidal place cell activity in the hippocampus encodes the spatial relationships between landmarks in the environment. The cells have spatial receptive fields which fire when the animal is in a specific location in the local environment. Each environment is independently represented by place cell activity and the relationship between the spatial fields is unique to each local environment. The firing sequences of the place cells encode the navigation of the animal as it moves through individual receptive fields, in a predictable manner. In addition to place cell activity in navigating animals, these networks of cells additionally exhibit oscillatory activity, hippocampal short wave ripple (SWR)-associated place cell sequences, in sleeping and stationary animals. This feature is the focus of David Foster’s most recent work. The SWR-associated place cell events are also referred to as “replay,” during which time the relative sequence of place cell firing generated by a navigating animal is repeated in a rapid burst.

The hippocampus of rats was implanted with forty tetrodes, to allow monitoring of network activity while the rat navigated through an open field in search of a reward in either a known location or a random location. From this, it was possible to accurately determine the location of the place cell receptive fields making it possible to determine the position and trajectory of the rat from the firing sequence. Foster and colleagues identified many brief increases in population activity in stationary animals. The firing sequence (trajectory event) of many of these events was not random, but encoded a trajectory through two dimensional space that was temporally compressed. Surprisingly, these trajectory events were not simple replay of the animal’s most recent path.

Capture

The end point of these trajectory events was more likely to be the known location of a reward than anywhere else, suggesting that the events are goal directed. Foster also demonstrated that the trajectory represented in these SWR-associated events predicted the actual navigational trajectory of the rat, the prediction was improved if the end point was the location of a known reward, suggesting the events are a reflection of future behavior. By analyzing the trajectory events when the animal was forced to learn a new location for the reward, Foster discovered that initially the trajectory events emphasize novel combinations of start and end points. This is additional evidence that this is not replay of previous firing sequences.

So now you can pretend to understand why the rats are running around in such a seemingly random manner, but how is this neural time travel? The presence of predictive neural firing suggests that the rats are able to utilize episodic memories to facilitate a goal directed future action. Establishing a trajectory from a novel start to a known reward location suggests that the rat is able to extract information from multiple other pathways and string them together in order to get somewhere in the future. There are many implications for how this may allow us to study episodic memory in the future, but I’m pretty sure that the most important finding is that your rats likely know that you were late to feed them yesterday and are probably coming up with a plan to do something about it when you get in to lab tomorrow.

Alex Smirnov is a first year student in the neuroscience graduate program currently rotating with Gentry Patrick. She is a big fan of electrophysiology and unproductive thought experiments.

References

  • Nyberg, L., Kim, A. S. N., Habib, R., Levine, B., & Tulving, E. (2010). Consciousness of subjective time in the brain. Proceedings of the National Academy of Sciences, 107(51), 22356–22359. doi:10.1073/pnas.1016823108
  • Pfeiffer, B. E., & Foster, D. J. (2015). Autoassociative dynamics in the generation of sequences of hippocampal place cells. Science, 349(6244), 180–183. doi:10.1126/science.aaa9633·
  • Pfeiffer, B. E., & Foster, D. J. (2013). Hippocampal place-cell sequences depict future paths to remembered goals. Nature, 497(7447), 74–79. doi:10.1038/nature12112

Exploring the Dichotomous Consciousness

“One individual studied well, and thoughtfully, might enable you to draw conclusions that apply to the entire human species.”

-David Roberts, Professor of Surgery and Neurology at Dartmouth-Hitchcock Medical Center

The fascinating story of the split-brain patient dates back to the 1940’s. You might rightfully ask: “What is a split-brain patient?”

brain-split

Split-brain patients are individuals who have been plagued by intractable epilepsy — so much so that they were willing to undergo split-brain surgery, which is essentially a procedure that severs the connections between the left and right hemispheres of the brain. This surgical procedure was meant to prevent the spread of seizure activity from its site of origin, thereby controlling the occurrence of debilitating epileptic seizures. The procedure is also known as a corpus callosotomy because the anatomical structure that connects the two hemispheres of our brains is called the corpus callosum, the so-called highway system of information transfer in the brain.

“It was a total shot in the dark.”

– Michael Gazzaniga

The first group that investigated these patients in the 1940’s claimed that there were no significant cognitive or behavioral impairments as a result of split-brain surgery. Fast forward to the 1960’s and along came Michael Gazzaniga, a driven young student at Dartmouth. During his junior year, a Scientific American article on how nerves grow piqued Gazzaniga’s interest, so he wrote a letter to the author, the one and only Roger Sperry, one of the biggest names in neurobiology. In his letter, Gazzaniga inquired about research opportunities — a move he now refers to as a “shot in the dark” — and landed an NSF summer fellowship at Caltech.

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Gazzaniga as a student at Caltech in 1963

Sperry’s group at Caltech had been studying split-brain rats, cats, and monkeys for some time, and were observing dramatic effects on behavior, which raised a huge question mark in their minds about why earlier assessments of split-brain humans had not revealed significant post-surgical differences. They hypothesized that surgeries done in the 1940’s had not severed the corpus callosum and anterior commissure completely. Gazzaniga was thus tasked with coming up with novel and better ways of testing split-brain patients. So he did…

 

And his findings introduced the notion of functional lateralization to the field of neuroscience:gazz3

 

When split-brain patients were presented with visual information (such as an object or a word) in their right visual field, they were able to verbally identify the stimulus. Interestingly, if visual information was presented in their left visual field, patients were unable to do so — in fact they would typically say, “I don’t know.” To understand this phenomenon we must recall the following generalizations:

  1. Information in the right visual field is known to be processed by the left hemisphere, and information in the left visual field is known to be processed by the right hemisphere (see above figure).
  2. Certain aspects of language are known to predominantly reside in the left hemisphere of the brain.

From his observations, Gazzaniga came to the conclusion that split-brain patients were unable to verbally identify stimuli presented in their left visual field because, though the information would travel to the right hemisphere, it would not be transferred to the left hemisphere where ‘language resides’ due to the severed connection between the two hemispheres.

There is a twist however. Patients who stated that they “did not know” what the stimuli presented in their right visual field was were able to draw what they saw with their left hand. These observations along with many many follow-up studies testing for part-whole relations, apparent motion detection, mental rotation, mirror image discrimination, etc. led to the idea that there is perhaps a right hemisphere dominance for visuospatial processing. These ideas are not meant to be mutually exclusive for one hemisphere or the other. In fact, one patient clearly demonstrates that certain aspects of language, such as spelling, can also reside in the right hemisphere: P.S., a teenager split-brain patient, was asked “Who is your favorite girlfriend?” with the word ‘girlfriend’ flashing only in his left visual field. He was unable to answer the question verbally because the information remained in his right hemisphere; however his left hand (controlled by the right hemisphere) was able to select Scrabble letters and align them to spell’L-I-Z.’

gazz4.jpg

Split-brain patients were the key to studying the functions of the two hemispheres independently, and Gazzaniga recognized the value in capitalizing on what this unique patient population had to offer to the advancement of neuroscience. Among his many accomplishments are serving on the President’s Council on Bioethics between 2001-09, basically founding the field of cognitive neuroscience with fellow psychologist/linguist George A. Miller, and being awarded the Guggenheim Fellowship for Natural Sciences. Come hear him talk on lessons learned from split-brain research this Tuesday, January 12 at 4 pm.

Ege A. Yalcinbas is a first-year student in the neurosciences graduate program currently rotating in Dr. Chalasani’s lab. Michael Gazzaniga was one of the first neuroscientists she read about in high school so she is excited to fangirl him at his talk on Tuesday.

References: