I Know Why the Caged Rat Runs

(source)

I know what the caged rat feels, I say, 
when the moon shines bright upon the brush
when the sunset sends out its last ray
with the earth still pulsing the warmth of day.
When the spring comes quietly, in no rush
and the seeds emerge as delicious meals,
I know how the caged rat feels

I know why the caged rat builds its nest
when the bedding will just be changed,
its work its effort its strength invest
in a task that requires its all, its best
each times its world gets rearranged,
it goes back to work with little rest
I know why the caged rat builds its nest

I know why the cages rat runs, I know

paws beating, pounding on the wheel

thinking, knowing it has a place to go

a goal to reach, something to show

it stops for moments to let wounds heal

then resumes its race for its reasons

I know why the caged rat runs.

 

  © TheCellularScale

Based on Paul Lawrence Dunbar's Sympathy

Fire Together Wire Together

Fire together Wire together (source)

Synaptic plasticity, the strengthening and weakening of neuronal connections, is thought to be the cellular correlate of learning and memory. Neurons that are active around the same time (fire together) will generally strengthen the connection between them (wire together). 

One theory is that neurons strengthen their synapses based on the specific timing between receiving a signal and firing an action potential. This is called Spike Timing Dependent Plasticity (STDP). The basic premise is that if neuron A fires first, and then neuron B fires, neuron A is probably partly responsible for neuron B firing. If this is the case then the signal from A to B probably contains meaningful information.

Neurons Connecting (modified from here)

The action potential in B coming right after a signal from A is a trigger for the cell to strengthen that synapse. The most common hypothesis is that it does this by causing a calcium surge inside the dendrite.

And the opposite? What if neuron B fires before neuron A? It could be a meaningful signal or it could just be random noise. Maybe neuron A fired for no good reason (or even just a little vesicle of glutamate popping out untriggered). Many studies show that when B fires before A, the connection was actually weakened.

So the saying perhaps should be 'neurons that fire one right after the other wire together' ... But that just doesn't flow off the tongue the way 'fire together wire together' does.

STDP is incredibly complex and even though this A before B = strengthening and B before A = weakening explanation makes intuitive sense, there is an exception for every rule.  Some studies find just the opposite! And some studies find that both directions strengthen the synapse (A-B and B-A). 

Dan and Poo (2006) have a nice table explaining these exceptions and the parts of the brain where they are found.
© TheCellularScale


Bi G, & Poo M (1999). Distributed synaptic modification in neural networks induced by patterned stimulation. Nature, 401 (6755), 792-6 PMID: 10548104

Dan Y, & Poo MM (2006). Spike timing-dependent plasticity: from synapse to perception. Physiological reviews, 86 (3), 1033-48 PMID: 16816145

Do small men think like big women?

Endless research has been conducted on the neurological differences between women and men. However, a study out of the University of Florida explains that almost all of the anatomical differences previously reported can be accounted for simply by adjusting for total brain size.

(Lady Gaga is an excellent source of exaggerated imagery)

Leonard et al., (2008) recruited 100 men and 100 women and imaged their brains. They showed that men generally have larger brains that women (not surprising, men generally have larger bodies than women).

Leonard et al., 2008 Figure 2

But what is fascinating is that when comparing specific regions, the gender of the brain mattered less than the size of the whole brain. 

In other words if you had a small male brain, it would look almost indistinguishable from a large female brain.  (See their Figure 3)

What I find most interesting in this paper is that it refutes the much purported "Corpus Callosum Myth".

Corpus Callosum (source)

The Corpus Callosum is main white matter connection between the two hemispheres of the brain. The "Corpus Callosum Myth" is that female brains have larger corpus callosa than male brains.

I have to admit that I am not immune from gender bias.  When I first heard that women had larger corpus callosa than men, my immediate thoughts were towards how that could make sense.  I thought "ah, well then maybe that is why women are better at seeing the big picture or at multi-tasking" and other thoughts along those lines.  

What I definitely did NOT think was "I bet that was a small, poorly controlled study which did not even reach statistical significance."  Well as it turns out, I should have.  DeLacoste-Utamsing and Holloway (1982) analyzed only 14 brains (9 male and 5 female), and found that

"The average area of the posterior fifth of the corpus callosum was larger in females than in males (p=0.08)" DeLacoste-Utamsing and Holloway (1982) p. 1431

A result hardly worth speculating upon.

Leonard et al., 2008 also found some corpus callosum differences between the genders, but when they graphed the size of the corpus callosum against the size of the whole brain...

Figure 3B (female brains white circles, male brains filled squares)

They found a continuum. The difference in size between the female and male corpus callosum is entirely due to the difference in size of the female and male brain as a whole. 

As with Von Economo neurons, maybe brains of different sizes work similarly, but have to be shaped differently to do so.

So rather than wildly speculating that women are better at this or that because they have stronger connections between their hemispheres, we should put our efforts into discovering evolutionary reasons why small men would be better multi-taskers that large men.


© TheCellularScale
 

UPDATE (7/23/12): I just want to be perfectly clear. I don't actually think that small men think like women.  The whole point of this post is to show that popular studies explaining that 'men and women's brains are different' may sound like they make sense, but there is often another explanation. In this case: if you are going to claim that the size of the corpus callosum means that women are better multi-taskers, then you have to ALSO claim that small men are better multi-taskers. And that large women are worse multi-taskers.  (These seem like totally ridiculous claims to me, but feel free to construct an experiment to test these hypotheses). 

For more on gender and gender differences (or lack thereof) in the brain, see my previous posts:

Neurosexism and Delusions of Gender

4 Reasons why All Women should play Mass Effect

An Attitude Adjustment for Women

In Defense of Pink Microscopes

DeLacoste-Utamsing C, & Holloway RL (1982). Sexual dimorphism in the human corpus callosum. Science (New York, N.Y.), 216 (4553), 1431-2 PMID: 7089533

Leonard CM, Towler S, Welcome S, Halderman LK, Otto R, Eckert MA, & Chiarello C (2008). Size matters: cerebral volume influences sex differences in neuroanatomy. Cerebral cortex (New York, N.Y. : 1991), 18 (12), 2920-31 PMID: 18440950

The shape of a memory

Luna Moth (Source)

If an animal changes shape, do its memories change shape as well?
Blackiston et al., (2008) from Georgetown University designed an experiment to test exactly that question.

They exposed caterpillars to a specific smell and then gave them an electric shock. They then tested the caterpillar's aversion to the smell by letting it run around in a Y shaped structure.  One arm had the 'scary smell' and the other just had normal air.

Blackiston et al. 2008 figure 1

After the caterpillars had learned that the smell predicted an electric shock, they preferred the ambient air arm compared to the 'scary smell' arm. Specifically 78% of the caterpillars spent more time in the ambient air arm. 


So, great, caterpillars can learn to avoid a smell.  Not super exciting on its own.  The real test was to train the creature as a caterpillar and then test the creature as a moth.

And indeed, the moths remembered.  80% of adult moths chose the ambient air over the 'scary smell' air.  Interestingly, the moths only remembered if they were trained late in their caterpillar life, but not if they were trained as very young caterpillars.

So what does this mean? Well, like most fascinating scientific findings, it raises many questions.  In particular it makes me wonder what exactly is happening in the brain during metamorphosis?
Are the neurons even firing? Do they go into some kind of paused-frozen state?

I haven't heard of anyone recording the electrical signals from neurons or imaging the calcium dynamics of pupal moths or butterflies, but I think this would be a great experiment.

Clearly the caterpillar isn't completely destroyed and rebuilt, some components persist. The specific synaptic connections that encode the connection between the smell and the scariness must be maintained.

Metamorphosis (source)

And of course this answers the longstanding literary question of how exactly Gregor Samsa can remember who he is after he transforms into a cockroach.

© TheCellularScale


Blackiston DJ, Silva Casey E, & Weiss MR (2008). Retention of memory through metamorphosis: can a moth remember what it learned as a caterpillar? PloS one, 3 (3) PMID: 18320055

A Pain in the Hippocampus

Neuropathic Pain (source)

Pain is usually a helpful sign that something is wrong with a part of your body. Heat-pain will cause you to pull your hand back from something hot before it burns you. The pain of a cut will draw your attention to it, so you can clean it.

However damage to the central or peripheral nervous system can result in chronic neuropathic pain, which is not helpful form of pain. Neuropathic pain is basically some mis-firing or mis-connected pain neurons sending meaningless, but persistant pain signals to the brain. And as bad as that sounds, chronic pain can also apparantly wreak havoc on your brain.

A recent study by Mutso et al., (2012) shows that in both humans and experimental animals, the brain is actually re-organized in response to chronic pain.  Specifically, they look at pain-related changes in the hippocampus, the part of the brain most strongly implicated in memory encoding. 

They compared human patients with chronic back pain, complex regional pain syndrome, and osteoarthritis to people with no pain-related condition, and found that the people with both chronic back pain and with complex regional pain syndrome both had reduced hippocampal volume when compared with the normal control group. The osteoarthritis patients showed a trend toward reduced hippocampal volume, but the result was not statistically significant. 

Hippocampus (source)

So what does this mean? If you have chronic pain you have a smaller hippocampus? We've covered this kind of study before, basketball players had larger striatums that non-basketball players, but it is never really clear what the volume of a brain region tells us. 

Does the volume of a brain structure mean more neurons, more blood flow to that region, more glia cells, or differently shaped neurons?

It is very difficult to draw any conclusions about the effect of pain on the hippocampus simply by learning that the hippocampi of people with chronic pain are smaller than the hippocampi of normal people. 

Luckily the study did not end there. Mutso et al. also investigated the effects of chronic pain on the cellular level. 

Hippocampal Neurons (source)

They found that in mice with chronic pain, the hippocampus has fewer 'new' cells. By staining for two specific markers DCX and BrdU, you can tell which neurons are new.  The hippocampi of control (normal) mice had around 40 new cells, while the chronic pain mice had only 14.  This is an indication that neurogenesis is much reduced in response to chronic pain, and suggests that the reduction in hippocampal volume could be related to fewer new neurons being generated (though it does not show this conclusively).

Unfortunately, chronic pain is bad for your hippocampus, and a cure for both the pain and the collateral brain re-organization are still illusive. 

© TheCellularScale


Mutso AA, Radzicki D, Baliki MN, Huang L, Banisadr G, Centeno MV, Radulovic J, Martina M, Miller RJ, & Apkarian AV (2012). Abnormalities in hippocampal functioning with persistent pain. The Journal of neuroscience : the official journal of the Society for Neuroscience, 32 (17), 5747-56 PMID: 22539837

Beer Yeast and Zoloft

Beer Sampler (I took this picture)

Yeast is an amazing organism that converts sugar into ethanol, or in other words barley into beer. It is used to ferment beer and is then usually filtered out.  (The leftmost beer sample in the picture above is an unfiltered beer and is cloudy because of the yeast still floating in it). 

Aside from providing proof that god loves us and wants us to be happy, yeast also provides a fascinating model in which scientists can study specific cellular processes. Because it is a simple eukaryote and can be easily cultured and easily mutated, yeast has long been used to test the effects of genetic manipulation on eukaryotic intracellular workings.

(source)

However, it's not often thought that yeast would be a good model for studying processes specific to the brain. But a recent paper published in PLoS One uses yeast to test the cellular actions of anti-depressants.  Specifically, they apply Zoloft to yeast cells.

Zoloft (like Prozac and other highly prescribed anti-depressants) works as a selective serotonin reuptake inhibitor (SSRI), inhibiting the uptake of serotonin after it has been released into the synapse, functionally allowing more serotonin to remain in the synaptic cleft. 

But Yeast don't have serotonin receptors and they don't have synapses. So what on earth could Zoloft do in these cells?

("Zoloft Does Everything" from Hipstercards.com)

Well here's a possibility: maybe Zoloft doesn't just alleviate depression by inhibiting the re-uptake of serotonin.  Maybe it does something else too. And if you are looking for non-serotonergic actions of Zoloft, yeast becomes the perfect organism to experiment on.

One reason to think there might be other (non-serotonergic) effects is that Zoloft takes a while to start 'working'.  That is, when Zoloft is taken, the enhancement of serotonin is almost immediate, but the actual effect on depressive symptoms can take weeks to appear.

So the question is: Are there effects of Zoloft which take a while to appear and do not specifically involve serotonin? 


Well, yes, there are.  Zoloft actually accumulates in the membrane of yeast cells, often killing them.  Which... doesn't sound promising. But Chen et al., 2012 shows that this membrane accumulation doesn't always kill the yeast cell and that in some select situations the membrane accumulation could have a protective effect by triggering cell-repair activities.  At "sub-lethal" doses, Zoloft can partially rescue stunted growth in certain yeast mutants.


This work seems to support the neurotrophic hypothesis of depression which says that neurons die depression, and that anti-depressants actually "reduce neuronal atrophy"
Does this paper show that Zoloft prevents neuronal death? No, not at all. It is investigating yeast, and shows that Zoloft could in some situations trigger cell repair.  But it doesn't say that Zoloft acts this way in neurons.


Obviously a lot more work needs to be done to really understand the actions of Zoloft and other anti-depressants. This study in yeast is a first step, but the findings need to be extended actual neurons before the trophic mechanism of Zoloft becomes anywhere close to as convincing as the SSRI mechanism. 

© TheCellularScale

Chen J, Korostyshevsky D, Lee S, & Perlstein EO (2012). Accumulation of an antidepressant in vesiculogenic membranes of yeast cells triggers autophagy. PloS one, 7 (4) PMID: 22529904

Advances in Neuronal Destruction

Destroying neurons is not difficult.  Destroying specific neurons, but leaving others intact is another story. Ablating specific neurons usually involves fancy genetic trickery, but it can also be accomplished with fancy mechanical lasers!

Laser near cell (source)

A new study published in PNAS (Hayes et al., 2012) uses the cells own rhythm generating properties to target the neurons for destruction.

Specifically, Hayes et al. is investigating the breathing neurons. These neurons are in the Pre-Botzinger Complex (preBotC) of the Medulla and they control the inhalation phase of breathing.  They work together as a complex to generate rhythms even in a brain slice. 

Using a calcium-sensitive dye, Hayes et al. could tell which neurons were participating in the rhythm generation. The breathing neurons show specific calcium patterns, increasing and decreasing with a frequency of 0.15-0.5Hz. 

The breathing neurons are located and the specific spatial coordinates of each neuron is saved.  A mechanically controlled laser can then automatically target each specific neuron for destruction (red dots in figure below). 

Hayes et al., 2012 Figure 1

Because silencing the neurons (NK1R-containing) in the preBotC completely stops breathing, they wanted to see how many neurons could be destroyed before the rhythm stopped.  And they wanted to see how it stopped.  Is there some magic number of cells that are needed to maintain the rhythmic output? or does the rhythm slowly decrease in amplitude? 

So measuring the XII nerve for output, they began randomly destroying the rhythmic cells one by one. They found that destroying these neurons one by one caused a decrease in amplitude and frequency of the XII nerve output and eventually stopped it entirely.  It took about 120 neurons to completely stop the rhythm, but the weird thing is that even after destroying 120 neurons, the rhythm continued for about half an hour. 

The mechanisms underlying this delay are not completely clear, but the authors attribute it to the slow effects of a decrease in mGluR stimulation. 

This new technique is pretty exciting because it allows the sequential deletion of specific cells.  Even the study erasing memories cell by cell didn't actually delete the cells one at a time.

This technique is especially interesting for investigating the way that a collection of individual cells create emergent network properties.  Now questions like 'how many cells are needed to form or maintain a functional network?' and 'which cells are necessary for the network's function?' can be answered. 

© TheCellularScale

Hayes JA, Wang X, & Del Negro CA (2012). Cumulative lesioning of respiratory interneurons disrupts and precludes motor rhythms in vitro. Proceedings of the National Academy of Sciences of the United States of America, 109 (21), 8286-91 PMID: 22566628

Neurons are like Fireworks

Neuron Firework (source)

In honor of American Independence, today here are some beautiful pictures of neurons that look sort of like fireworks.

(source)

And some actual neuroscience studies using light-up cells.  One example, I've already covered is the imaging of neurons as they fire. Since neurons fire really fast, you can see them light up under the microscope.



Computational neuroscience can lead to fireworks as well.  When neurons fire in an artificial network, they light up in beautiful patterns:  (It really sparkles at 2:00)




 © TheCellularScale

A little stress goes a long way

.... toward preventing PTSD symptoms.

Post Traumatic Stress Disorder

This may surprise you as the S in PTSD stands for STRESS.  How on earth could stress prevent it? But you heard correctly. A new paper by Rao et al., (2012) from Biological Psychiatry shows that a little stress in the form of glucocorticoids, prior to an acute stress event actually prevents PTSD-like symptoms in rats.

First of all how do you tell if a rat has PTSD?
This study uses two measures: one behavioral and one cellular.

To test anxiety in a rat, you can put in on an Elevated Plus Maze (EPM). Rats don't love heights, and they do love dark corners. But, they are also somewhat naturally curious. The EPM makes use of these rat characteristics to test how anxious the rat is.

Elevated Plus Maze (source)

The EPM has four arms, two are open (but far enough off the ground that the rat can't just step off the maze) and two are enclosed with walls. Normal rats tend to explore all the arms of the maze roughly equally, but anxious rats tend to strongly avoid the open arms. The amount of time spent in the open arm area is a generally accepted measure of how anxious the rat is.

An earlier paper from the same lab, found that rats who had undergone the single stress event were more anxious (spent less time in the open arms of the EPM) 10 days after the event, but NOT 1 day after the event.  The single event stress and the delay of symptom onset are why this study is more relevant for PTSD than for chronic stress. 

Rao et al., 2012 Fig 4B

As interesting as the behavioral experiments are, the cellular level experiments are where it gets really cool (The Cellular Scale is not biased or anything). They used the Golgi stain to visualize neurons in the Amygdala. They measured how long the dendrites were and also how many spines they had on them. (Spines are the little protrusions that come of dendrites to receive synaptic inputs).


They found that the stressed rats had more dendritic spines on the amygdala neurons than the non-stressed rats.  Not only that, but this increase in spine density was apparent 10 days after the stress event, but not 1 day after.  


You might think dendritic spine growth is a good thing, and likely signifies synaptic plasticity and pathway strengthening... but remember this is the amygdala, a structure critical for FEAR learning, more spines here may not be beneficial. Stronger pathways to these amygdala neurons likely means that they fire more easily.


Now that we understand how PTSD is measured in a rat, we can move on to how they 'cured' it in this paper.  

Rao et al found that when they injected vehicle (a fancy science term for 'nothing' or 'placebo' or 'saline') into the rat 30 minutes before the 2 hour stress event, the rat no longer showed either the increased in anxiety (fewer open arm entries on the EPM) or the increase in dendritic spine density.

Pretty weird, considering they were injecting vehicle prior to the stress event.  How could inactive saline (essentially nothing) cure PTSD symptoms?

They figured out that the actual injection process was stressing the rat out a little bit. When animals (including humans) are stressed, they release a hormone called cortisol.

Rao et al., 2012 Fig 1C,D,E

They found that the 2 hour stress event caused a huge rise in corticosterone (right and left panels), while the injection (vehicle) alone caused a small rise (middle panel). 

Because they were injecting nothing, they hypothesized that the corticosterone produced by the small stress of being injected was somehow protecting against the large 2 hour stress event.

The rest of their paper is basically confirming this. They add corticosterone to the water of the rats and this also prevents the PTSD-like symptoms.  They find that all their manipulations isolating the corticosterone confirm that this is what is protecting the rats from the delayed impact of the stress event.  

Interestingly there is evidence that 'small stress' can help prevent 'big stress' in humans too. They cite clinical studies reporting that intensive care unit (ICU) patients who receive injections of stress-level cortisol during treatment are less likely to develop ICU-related PTSD symptoms.

It is a puzzling paradox at the moment, but the next step is to figure out how exactly this little stress can reduce big stress.


Epilogue: 

I was lucky enough to see Dr. Chattarji, the principle investigator of this study, give a talk at a conference a few months ago.  And one interesting piece of information that you can get from a talk, but will never read in a paper is how the scientists originally stumbled upon their finding.  In this case, Chattarji's lab didn't start their study by injecting vehicle. They were actually testing a real drug that they thought might help alleviate PTSD.  They had a beautiful result showing that when you injected "drug X" before the 2 hour stress event, you eliminated the PTSD symptoms. The natural conclusion is to think that "drug X" is a new cure for PTSD.

 But therein lies the importance of the control group. To control for any effects of simply injecting the rat, they injected vehicle. When they saw that the vehicle prevented the PTSD symptoms just like the actual drug, they were crushed! This is the ultimate demise of an experiment.  The control group shows the same thing as the drug group, which means that the drug does not work! Luckily they were flexible and smart enough to investigate what they did see, that the injection alone could protect against the PTSD symptoms.

Also, if someone would like to explain the difference between cortisol and corticosterone, please do. I clearly do not have a full understanding here.

© TheCellularScale




Rao RP, Anilkumar S, McEwen BS, & Chattarji S (2012). Glucocorticoids Protect Against the Delayed Behavioral and Cellular Effects of Acute Stress on the Amygdala. Biological psychiatry PMID: 22572034

Mitra R, Jadhav S, McEwen BS, Vyas A, & Chattarji S (2005). Stress duration modulates the spatiotemporal patterns of spine formation in the basolateral amygdala. Proceedings of the National Academy of Sciences of the United States of America, 102 (26), 9371-6 PMID: 15967994