JC#3: Temporally Specific Burst in Cell Proliferation Increases Hippocampal

[DocToxyn]

Neurogenesis in Protracted Abstinence from Alcohol.

Nixon, K., Crews, F.T. Journal of Neuroscience 2004, 24(43):9714-9722

http://www.jneurosci.org/cgi/content/full/24/43/9714

Here's article number 3. My testing shows that it should be available freely to all, let me know if you have problems.

I selected this article for several reasons. First, it describes one of the most interesting revelations to come out of neuroscience in recent years. It has always been the dogma in neuroscience that once the brain is mature and fully developed then there are no new neurons formed and regeneration of injured neurons does not occur. What you got is what you get and there is no central nervous system neurogenesis once the plan is complete. This concept was so entrenched into every neuroscientist's brain that even when the initial experiments, performed by Altman in the 1960s, demonstrated neurogenesis in discrete regions of the adult rat brain, it was basically ignored for almost 40 years. There were scattered paper along the way that also showed adult neurogenesis, but it wasn't until the '90s that this field really came into its own. Thus, we currently know that there are specific regions of the brain where thousands of new neurons are created daily. This discovery of adult neural stem cells has dramatically changed the way neuroscientists think about learning and memory, disease states and numerous other brain specific processes. Now, just so I don't get too "neuro-centric" here, stem cells have been a hot topic for many years and continue to be a growing field in many different disciplines, not just neuroscience.

This brings me to my second reason, I am currently working outside the brain looking at the role of the aryl hydrocarbon receptor and environmental toxicants on hematopoietic stem cells (HSC). These HSCs reside in bone and produce all the blood cells and mature/precursor cells for the immune system. To say that they are important is an understatement, but no less so than the neuronal precursors in this discussion. In very general respects stem cells share common traits and characteristics, so I might learn something from this discussion that could ultimately prove important for my HSC work (don’t blame me for being selfish, Moonbear did the same thing).

The third reason I choose this paper, rather than one of the referenced papers that initially discovered adult neurogenesis, is that it goes on to examine adult neurogenesis in the context of a toxicological challenge, i.e., alcohol (ethanol) exposure. As alluded to above I am a toxicologist by training thus I thought I could bring a little of that particular flavor of science into our discussion as well. Having said that, I am not an "alcohol toxicologist" (maybe they're called intoxicologists); I study substances like dioxin, PCBs, and other halogenated aromatic hydrocarbons. While these do affect brain function, this paper gives me an opportunity to discuss (and learn about) another aspect of neurotoxicology that the general public is probably more familiar with.

By way of a brief introduction, the paper focuses mainly on the neuronal precursors in the hippocampus, a region of the brain believed to be the center of learning and memory. Originally it was thought that learning processes and memory consolidation was based in the plasticity of the resident neurons. Mechanisms such as increasing dendritic arborization and synaptic contacts were thought to play a role, however in addition to the mechanisms, it is now thought that the constant production of new neurons in this brain region also plays a significant role. So what's with the alcohol? We all know that it is bad from pregnant women to drink. One aspect of fetal alcohol syndrome is alterations in brain anatomy and function. Thus if exposing growing fetal neurons to alcohol is deemed bad, then it logically follows that exposing growing neurons in the adult could also have deleterious effects. Thus the researchers set out to study the process of new neuron growth, using an adult rat model that was exposed to alcohol, to determine if there was an impact on neurogenesis. Their previous work has established that alcohol intoxication alters hippocampal neurogenesis, thus the next question they sought to address was what happens when you stop drinking alcohol? Are the effects reversible or have you permanently altered the system? Read on and find out.

 

[Moonbear]

Sounds great! I'm able to access it from home without signing in through my library account, so it appears to be free.

Oh, I guess I better go read the one hypnagogue starts tomorrow!

 

[CosminaPrisma]

Nice to know that even just a few neurons regrow in people with alchoholism when they abstain. Further research into this might help come up with treatments to stimulate more neuron parts of the brain severely damaged by adult drinkers alchohol consumption.

 

[DocToxyn]

Here we go…..

We’ll start with a brief review of the areas of the brain where adult neurogenesis takes place and then focus in on the hippocampus (Hc) as our region of interest. I’ll intro some of the known effects of alcohol on the brain and then center in on the endpoints covered in the paper. Subsequent post will discuss the methods used and the results of the study.

As mentioned above, it is now understood that certain regions of the brain have the ability to generate new neurons on a regular basis. This is an ongoing process and is distinctly different from processes where new nerves may be regenerated or produced following some sort of injury or disease. The regions typically mentioned in the adult neurogenesis field are the subventricular zone (SVZ)/olfactory bulb and the subgranular zone (SGZ)/Hc. The stem cells reside and are active in one specific area and the neurons travel along a specific path from the proliferation/replication zones to their ultimate resting place all the while reacting to signals given off by other cells that direct their movement and differentiation. Much of this proliferation/differentiation/migration work has been performed in rodents as oppose to humans for obvious reasons. The limited amount of human data available suggests that it’s a little different in humans and there is indeed a population of SVZ stem cells but migration is limited if not absent. Once the neurons reach their destination they begin to make connections and act pretty much like normal, existing neurons. What determines if they survive or not is still under investigation, but it is most likely an issue of making proper connections and receiving proper stimuli, action potentials, growth factors, and other chemicals. Some things that have been demonstrated to enhance neurogenesis include exercise, learning tasks, and hormones. Such things as physiological/psychological stress, aging, pharmaceuticals and toxicants (alcohol remember?) cause reductions in adult neurogenesis.

So what can actually happen during alcohol exposure? The intro to the paper mentioned things like impairments in learning and memory. Many of these endpoints can be examined in humans via the application of tests similar to IQ tests you might be familiar with. In the rodent you can do even more and in addition, control the treatments and collection of data to eliminate some of the biases and problems that can arise in human testing. I’ll focus on one specific task to give you a feel for what it is like. The Morris water maze is basically a large pool filled with opaque water that has a platform hidden somewhere in the pool that is just under the water. Rats are typically used for this task because they are strong swimmers and they can withstand repeated dunking without losing too much body heat. So the rat is placed in the pool and swims around until it finds the platform, it will then crawl out because it really doesn’t like to swim. Thus the motivation is to get out of the water. The rat is trained to do this over a course of trial and learns the task by looking for visual cues that the researcher has set up around the room; this is the spatial part of the task. For the actual experimental trials one can either place the rat in the pool and see how long it takes to find the platform or you can remove the platform and see how much time the rat spends swimming where it “thinks” the platform should be. Either task will test how well the rat can orient itself in the pool by looking around the room and using those visual cues to tell it where the platform is or should be. Exposure of adults, adolescents and developing rats (exposure done while the rat is in the uterus) to alcohol all result in poor performance in this task. The important thing for this discussion is that previous studies have demonstrated that the Hc is very important in performing this task.

Another, more specific test, that can be done to look at Hc performance is an in vitro task based on electrophysiological responses in the HC. We all know about electrical signals in the brain and these correspond to numerous activities such as signal transduction and communication between brain regions. One fascinating aspect of Hc electrophysiology is the phenomena called long term potentiation (LTP). I don’t want to get too technical here, (we can in later posts if you wish) but this response is basically a measure of how well the Hc is doing its job. In brief, a slice of brain containing the HC is removed from the animal and kept “alive” in a perfusion apparatus while electrodes are placed on the tissue to record electrical signals. LTP has been correlated with learning and is sometimes classified as being able to “model learning in a dish”. By being able to experiment with something complex like learning in a simpler system like a brain slice allows the researcher to focus on very specific questions such as what receptors are involved. Alcohol causes reductions in the ability of brain slices from chronically exposed rats to initiate LTP. The effects of alcohol I mentioned above are only a few in the wide array of known effects. They will vary somewhat depending on amount of alcohol administered, length of exposure, age of the animal, type of animal… but in general I think we can all agree that in certain circumstances alcohol can be detrimental to health.

So the researchers know about adult neurogenesis in the Hc and they know about the effects of alcohol in the Hc, so now we arrive at their hypothesis. Their work, as well as other’s, previously demonstrated that alcohol reduces neurogenesis in the adult rat by decreasing the number of neurons formed and increasing the number that eventually die. This effect is specific to the Hc and is most likely mediated by an oxidative stress-mediated mechanism. Human use of alcohol can be quite different from one person to the next. The ones that are of most concern are heavy users that chronically abuse the drug. Sometimes this occurs as a “binge” exposure and then some period of abstinence. This is what the researchers are trying to model in this paper. They hypothesize that the effects of alcohol on neurogenesis may be different during and after binge alcohol use. My following post will discuss the methods they use to examine this issue.

 

[DocToxyn]

On to the methods...

Rats were used as the model and were dosed by putting a tube down their throats and pushing in a mixture of ethanol and Ensure, control rats were treated the same way except the ethanol was not included. The “isocaloric” adjustment in the control diet was basically done to even out the calories that the ethanol contributes to the treatment. No food or water was provided over the four day dosing regime to avoid any differences in adsorption or kinetics of the alcohol. The rats were examined to reveal signs of intoxication and blood alcohol was measured. The withdrawal period of 24 hours began after the four days of exposure.

Four hours prior to sacrifice the animals were given a “pulse” of bromodeoxyuridine (BrdU). Following the sacrifice, their brains were removed and prepared for histochemistry. Very thin slices of the brains were made using a device called a vibratome that is basically a razorblade that vibrates very rapidly and cuts through the tissue, think of your deli-slicer, but less appetizing.

BrdU is a chemical used to “birthdate” cells. Once in the animal, it is distributed throughout the body and incorporates itself into the DNA of any actively replicating cells. Once it is there it remains there for life of the cell, any remaining BrdU is rapidly broken down. This marker will only show up in cells that were “born” within a very small window of time (hours) so it is used here to track what cells were made within that 4 hour period prior to the animals death. These are sometimes called “pulse-chase” experiments because you pulse with the BrdU and then chase after the cells after some predetermined time period. The BrdU is visualized with a standard anti-BrdU antibody method. Other markers were also examined namely Ki-67, another marker of proliferating cells, doublecortin (DCX)- a marker that identifies newly-formed neurons (neuroblasts), glial fibrillary acidic protein (GFAP) – a marker of astrocytes, and neuronal specific nuclear protein (NeuN). Some of these may seem redundant, but in some cases the method drives what stains work best, i.e., fluorescent microscopy vs. light microscopy, or in other cases one stain is used to confirm the results of another (BrdU vs. Ki-67). Once the sections were stained with the appropriate agent, they were examined under a microscope and images were captured and analyzed.

You probably also noticed that some rats were treated with a drug called diazepam. This was done to reduce some of the negative symptoms of the withdrawal period. It is possible that stress and other factors secondary to the alcohol were mediating some of the effects so this was an attempt to control for this. BTW, don’t get any ideas about getting some diazepam, more commonly known as Valium, for your next drinking event. It’s only good for severe withdrawal symptoms like seizures and its only available by prescription.

 

[DocToxyn]

So, what did they find? Figure 1 really is the most important one of the whole paper. It shows first the reduction in neurogenesis one can obtain with alcohol exposure in the Day 0 bar. At Day 7 of withdrawal there is a statistically significant elevation in the number of cells that incorporated BrdU following the single pulse treatment when compared to the control rats. This means that more cells are being made when compared to normal rats, thus the neurogenesis not only came back but also surpassed the level of production observed in non-treated rats. By Day 14 and 28 the level of neurogenesis is back to normal control levels. The pictures included with the figure give you an idea of what the light microscopy staining for BrdU looks like.
Figure 2 demonstrates that the increase in neurogenesis observed at Day 7 with BrdU staining in comparable to that observed with another endogenous marker of proliferation Ki-67. Scientists like to back up their observations by applying a different method to examine the same endpoint. It makes their results and subsequent conclusions more believable.
Figure 3 demonstrates that burst in cell production on Day 7 as indicated with BrdU is followed by a burst in DCX expression in the same region at Day 14. Knowing the expression dynamics of DCX, the authors conclude that it is most likely neurons that are then formed from the cells that were tagged with BrdU seven days prior. However, if this were their only evidence to support this claim, their conclusions could be questioned. How do they really know that the same cells that they marked with BrdU are also the ones positive for DCX? They address this in Figure 4 where they did multiple stains to identify both neurons and glia and they looked for BrdU incorporation. Using confocal microscopy to “rebuild” the cell, the authors determined that in the ethanol-treated Hc sections, the NeuN marker co-localized (was found with, co-labeled) with 82.7% of the BrdU-positive cells and GFAP co-localized with 7.5% of the cells the BrdU cells. This was the same in the control animals, thus the differentiation characteristics of that brain regions were not altered by the ethanol. However, the total number of NeuN+/BrdU+ cells was elevated compared to controls (Figure 5), thus 35 days after the cessation of binge alcohol exposure, there were more neurons born in ethanol-treated brains than in controls.

The remainder of the results get a little more complicated, so I’ll give the major results and we can flesh out whatever sections you wish to in later posts. The researchers wished to know if any of the symptoms associated with the withdrawal were associated with the increased neurogenesis. They had been assessing the behavior of the rats all along the course of the study and scoring them for intoxication state during the alcohol exposure and for severity of withdrawal during the abstinence phase. The only measure found to have a positive correlation with the Day 7 neurogenesis burst was mean withdrawal score (again based on behavioral observation of the animals). Thus, they wished to test if they could alter the neurogenesis by pharmacologically manipulating the withdrawal symptoms. Here’s where the diazepam comes in. The bottom line is that even though diazepam treatment decreased the severity of the withdrawal symptoms, it did not alter the neurogenesis burst in the alcohol treated rats (Figs 6 and 7).

 

[DocToxyn]

In their discussion the authors delve into potential explanations of mechanisms behind the observed effects, comparisons to other models of alcohol exposure, and the possible functional role that the observed neurogenesis plays in alcohol withdrawal. This last point is, I think, the most important aspect of the work and one that I would liked to have seen them attempt to answer. If they could have performed some functional behavioral assessment of another set of animals both during the alcohol binge and during withdrawal perhaps one could get a picture of what, if any, compensatory function the burst performs. Something like the Morris water maze or LTP analysis could provide a great deal of information about what the ramifications of their cellular observations are. However, like they state in their conclusions, the role of neurogenesis in the normal, undisturbed state is still under investigation and bringing alcohol into the mix only makes it more difficult to make firm conclusions. Plus these techniques are time and money consuming and require special equipment and highly trained personnel to perform these tasks, so I can’t fault them too heavily for not doing such experiments. Despite these gaps in our knowledge, this paper was an interesting look into several aspects of neuroscience and how it impacts not only scientists and their work, but also the general population.

To follow up on previous posts encouraging other to contribute to the journal club...please give it a try . The previous JC submissions have been given by people who have obviously done them many time before, but that doesn't mean that you have to try to replicate the same. This club is an opportunity for us all to discuss and learn from each other all while discussing something we all enjoy. Don't feel intimidated by the first submissions. Do what you can, ask questions and it will be fun. Please volunteer today (sorry that was pretty corny ).

 

[DocToxyn]

Ouch! Either the paper was very boring or I did such a thorough discussion that no further comment is necessary. C'mon people...anything?

 

[Q_Goest]

Hi DocT'. I enjoyed your write up on the paper. In fact, I can't understand the paper but your explanation seems considerably more readable to someone not expert in the field, so I thank you for that. I'll use your explanation of the paper to discuss. I'll also break up my three separate questions so you have more posts on your paper

~

I didn't notice anywhere that explained how much alcohol was given to the rats, especially the amount that was given as compared to humans (ie: the rats were given the equivalent of X number of drinks). This seemed particularly important since this paper is obviously trying to make that connection. Also, it seems they must have been given a huge amount right away as opposed to slowly building up over months or years of increased exposure which would be a better comparison to humans. If the researchers had to administer some kind of "withdrawal" drug to the rats, it seems they really whacked them hard all at once with a huge amount of ethanol. People don't do that, they build up slowly over many years. I wonder also how that might affect the end result.

 

[Q_Goest]

Figure 1 really is the most important one of the whole paper. It shows first the reduction in neurogenesis one can obtain with alcohol exposure in the Day 0 bar. At Day 7 of withdrawal there is a statistically significant elevation in the number of cells that incorporated BrdU following the single pulse treatment when compared to the control rats. This means that more cells are being made when compared to normal rats, thus the neurogenesis not only came back but also surpassed the level of production observed in non-treated rats. By Day 14 and 28 the level of neurogenesis is back to normal control levels. . . .

However, the total number of NeuN+/BrdU+ cells was elevated compared to controls (Figure 5), thus 35 days after the cessation of binge alcohol exposure, there were more neurons born in ethanol-treated brains than in controls.

This is an amazing result I thought. You're saying that the rats that were given alcohol actually had MORE new brain cells? Is there any indication that the total number of brain cells is larger or smaller in the ethanol rats? Do they perform better on intelligence tests? What is the overall result of all these new brain cells?

 

[Q_Goest]

I really think the most intriguing thing about this whole paper has to do with new brain cells. You mentioned them here:

The stem cells reside and are active in one specific area and the neurons travel along a specific path from the proliferation/replication zones to their ultimate resting place all the while reacting to signals given off by other cells that direct their movement and differentiation. . . . Once the neurons reach their destination they begin to make connections and act pretty much like normal, existing neurons. What determines if they survive or not is still under investigation, but it is most likely an issue of making proper connections and receiving proper stimuli, action potentials, growth factors, and other chemicals. Some things that have been demonstrated to enhance neurogenesis include exercise, learning tasks, and hormones.

How do new brain cells 'find their way' along some specific path? What is affecting these brain cells, and is there any indication they are moving to their final site for a specific reason relating to brain function? If for example, the brain lost cells in that area due to alcohol exposure, they may migrate to this site to replace those cells. But how would they be guided to this site? It seems incredible that stem cells could find their way to some given site as if being cued.

I realize this question isn't really addressed by the paper but it seems like the more interesting aspect so any explanation would be appreciated.

 

[CosminaPrisma]

Thanks for taking the time out of your schedule to share this interesting paper with us.
I wouldn't have understood any of the paper if it weren't for your very lucid and concise explanation of their methodology and the concepts behind their research. It made the paper much clearer to me as a close-to-layperson.
I did have a couple of questions...why would they use Ensure as a nutritional supplement instead of something else like solid food? Are animals exposed to alcohol in utero for a short time able to regenerate new neurons at a faster pace than adults exposed to alcohol in an experiment like the one the paper is based on? I've been briefly introduced to FAS through my college coursework, but that is all. I'm also curious about answers the questions Q_Goest asked.

 

[GCT]

Their previous work has established that alcohol intoxication alters hippocampal neurogenesis, thus the next question they sought to address was what happens when you stop drinking alcohol? Are the effects reversible or have you permanently altered the system? Read on and find out.

hmmm, growing back some brain cells, or the speech impairment of alcohol...think I'll choose the alcohol.

 

[DocToxyn]

I didn't notice anywhere that explained how much alcohol was given to the rats, especially the amount that was given as compared to humans (ie: the rats were given the equivalent of X number of drinks). This seemed particularly important since this paper is obviously trying to make that connection. Also, it seems they must have been given a huge amount right away as opposed to slowly building up over months or years of increased exposure which would be a better comparison to humans. If the researchers had to administer some kind of "withdrawal" drug to the rats, it seems they really whacked them hard all at once with a huge amount of ethanol. People don't do that, they build up slowly over many years. I wonder also how that might affect the end result.

why would they use Ensure as a nutritional supplement instead of something else like solid food?

Q Goest, CosminaPrisma, thanks for the questions. Your questions about dosing and administration of the alcohol to the rats have touched on several issues that are critical in toxicology, those being, route of administration and relevance to the human condition. I’ll touch on those and hopefully at the same time answer your questions. The authors stated that the rats were given an initial “priming” dose of 5 gm/kg by oral gavage. The “gavage” term means that a tube or steel syringe was placed down the animal’s esophagus and the liquid was squeezed out into the stomach. This is a very common method of administering many different agents and does not cause a lot of stress on the animal. The amount of ethanol given was based on how heavy the animal was, so for a typical 300 gm rat, you would give it 1.5 gm (~1.5 mls) ethanol. They used the liquid Ensure as both a carrier for the alcohol (it dilutes it out to avoid stomach upset) and a nutritional supplement. As I mentioned in my discussion they eliminated access to solid foods to avoid any confounding issues of alcohol absorption due some rats eating more than others. However, they needed to feed them something so the liquid diet is easy to use. It’s both easy to control the amount that is given and thus adjust it so that each rat receives the same calories per feeding. Again alcohol has its own caloric content and if you didn’t balance that out across the treatment groups, the criticism could be made that the effects you are seeing are being influenced by the extra calories the alcohol rats would be receiving as compared to the controls.

As far as the amount of alcohol the rats received over the course of the experiment - that varied. The initial dose was 5 gm/kg, but after that the behavior of the rats was graded on a scale of intoxication (you can see this scale if you go to the supplementary methods section). Basically they looked to see how “drunk” that rats were and then continued to does them accordingly to keep them at a level of alcohol effect that they desired. Thus if a rat was not able to right itself, but could still blink its eyes it was given only 1 gm/kg, whereas a rat that was only mildly hypoactive/ataxic, was given 4 gm/kg. These observations/doses were performed every 8 hours for four days.

Now, on to relevance of the treatment. The scientists are ultimately trying to make assertions about the human condition by investigating what happens to the test animals. Well, they gave it by an oral route, which is how humans typically consume alcohol, and while few humans (other than some college students) have the alcohol physically forced into their stomachs, it’s as good as they can get given other alternatives. I have seen studies investigating the effects of alcohol on the brain that gave it via intraperitoneal injection (into the abdominal cavity), this seems way off in terms of relevance to humans, but these studies are still done. The researchers don’t try to make the connection between how much they gave the rats and how many drinks would equal that in humans. It’s difficult to make those types of relationships and since we all have different tolerance to alcohol, might not really be all that accurate. This does bring up another interesting issue in toxicology – extrapolation of data obtained in animals to humans. A lot of scientific experimentation is done in rodent models, but obviously rats, mice, guinea pigs, etc are not humans, so how can we make judgments for one based on another? Well this is a huge topic of debate and not one for here (we can open another thread if anyone wishes). Let’s just say that researchers are aware of this extrapolation problem and do their best to perform their experiments and draw their conclusions within its constraints.

To get back to the questions. The researchers were trying to model “binge” drinking. Now, while I’m not an expert in this field, I think this is defined as acute excessive use of alcohol followed by a period of relative abstinence. So, it would have been nice to have rats that have been chronic “users” of alcohol that could have then been used for the binge study, but this would be a very expensive and time consuming enterprise to keep that many rats going under those conditions. Plus, how much would you provide them, a little every day….a lot….just enough to get drunk on the weekends:uhh: ? It’s a difficult thing to try and model the human condition in a controlled and reproducible fashion in the lab, so we do things the best way we deem that we can. In many cases the researchers have to hit the animals with much higher doses than would normally be experienced by humans in order to see an observable effects in the limited numbers of animals they have allocated for the experiments. Again another topic for future discussion.

 

[DocToxyn]

This is an amazing result I thought. You're saying that the rats that were given alcohol actually had MORE new brain cells? Is there any indication that the total number of brain cells is larger or smaller in the ethanol rats? Do they perform better on intelligence tests? What is the overall result of all these new brain cells?

According to the data presented in figures 4 and 5 there are more neurons that were born on the Day 7 burst residing in the Hc of the ethanol rats than the controls. This doesn’t necessarily mean that the rats are better off. For one, they also went through a period of considerable neuronal loss during the intoxication stage. Plus there is some research that shows that more neurons do not always mean a better brain, bigger isn’t always better. Having said that, in other studies that just focus on neurogenesis, they have reported that the new neurons in the Hc are more responsive to LTP-inducing conditions. It still remains to be proven what is the true functional role of these new neurons and if they are better than the “old ones”. We can’t fully answer these questions yet, even in the unperturbed system. That’s why I suggested the application of some behavioral or electrophysiological studies to add to their study. Perhaps they would have observed some sort of functional recovery, partial or otherwise.

 

[DocToxyn]

How do new brain cells 'find their way' along some specific path? What is affecting these brain cells, and is there any indication they are moving to their final site for a specific reason relating to brain function? If for example, the brain lost cells in that area due to alcohol exposure, they may migrate to this site to replace those cells. But how would they be guided to this site? It seems incredible that stem cells could find their way to some given site as if being cued.

It's funny that you use the word cues, because that's more or less what is happening. Again much of this has been studied during development and the periods of initial formation of the brain, but there are many parrallels that can be said about how this process works in adult neurogenesis. It basically boils down to communication between the migrating/differentiating cells and their surroundings. The cells that have already found their "homes" and are doing their thing will release chemical signals that the newly formed cells will detect. There are many of these signals that have been identified and they sometimes get cool names like Sonic Hedgehog, Notch, Noggin, Prokineticin 2, Vasular Endothelial Growth Factor... These factors can also be neurotransmitters, hormones, and other substances. Anyway, the new cells will sense gradients that exist in the brain due to distant cells releasing these signals. This may cause them to move in one direction or express some new protein, or differentiate to become a dopaminergic neuron. There may also be surface proteins on certain cells that the migrating cells come into contact with that also guide/influence them. This process of releasing cues and receiving cues other means of cellular communication goes on in a complex, concerted fashion to ultimately result in the pattern that is set forth in the DNA and modified by current stimuli. It really is an incredible system that is "up there" and for as much as we do know about it, there remains a lot more to figure out.

 

[DocToxyn]

Are animals exposed to alcohol in utero for a short time able to regenerate new neurons at a faster pace than adults exposed to alcohol in an experiment like the one the paper is based on? I've been briefly introduced to FAS through my college coursework, but that is all. I'm also curious about answers the questions Q_Goest asked.

That's a very interesting question, and unfortunatley I don't think that I can directly answer it. As I keep saying (and it seems to get old sometimes) these are pretty recent studies and questions like yours that come up when reading the article are either currently under investigation or haven't be tackled yet. My feeling is that, for one, it would be hard to study because during development there is so much brain growth going on that you might not be able to differentiate the after-effects of the alcohol from the already "growth-crazy" (I wanted to write "neurogenically-permissive", but that seemed too technical or just wrong) environment that exists at that time. Plus the in utero state is a time of extreme sensitivity to many different types of insults and I would predict that there might be more nasty, persistent effects of alcohol when given at that time of life as compared to adult exposure.

 

[CosminaPrisma]

I definitely agree that an experiment to test that would be extremely difficult...I hadn't thought the necessary methodology.

 

[Q_Goest]

Thanks for the very clear response DocT.

If I'm not mistaken, the human body reacts to drinking by producing enzymes that break down alcohol. For people that drink consistantly, does this enzyme not get produced in greater quantities over time? It would seem that way as I've noticed people often develop a resistance to alcohol and are able to drink larger quantities than others, hence the phrase "I can drink you under the table". If these people are actually metabolizing alcohol more quickly or in greater quantities than others, are these people at less risk to brain damage?

~

According to the data presented in figures 4 and 5 there are more neurons that were born on the Day 7 burst residing in the Hc of the ethanol rats than the controls. This doesn’t necessarily mean that the rats are better off. For one, they also went through a period of considerable neuronal loss during the intoxication stage. Plus there is some research that shows that more neurons do not always mean a better brain, bigger isn’t always better. Having said that, in other studies that just focus on neurogenesis, they have reported that the new neurons in the Hc are more responsive to LTP-inducing conditions.

I'd agree there needs to be some correlation between drinking and brain function, and I'm sure there are numerous studies that do that. What do you mean by LTP-induced conditions, and what does you mean when you say "new neurons in the Hc are more responsive to LTP- induced conditions"?

 

[DocToxyn]

If I'm not mistaken, the human body reacts to drinking by producing enzymes that break down alcohol. For people that drink consistantly, does this enzyme not get produced in greater quantities over time? It would seem that way as I've noticed people often develop a resistance to alcohol and are able to drink larger quantities than others, hence the phrase "I can drink you under the table". If these people are actually metabolizing alcohol more quickly or in greater quantities than others, are these people at less risk to brain damage?

There appears to be several theoretical mechanisms for tolerance, one being increased production of alcohol dehydrogenase, another being neural insensitivity, other being behavioral adaptation to appear "less drunk". These may exist in some part, but the most convincing evidence I could find on the biochemical mechanism of tolerance has to do with the http://www.ncbi.nlm.nih.gov/entrez/...uids=15554233&query_hl=2&itool=pubmed_docsum" (MEOS). This system recruits inducible enzymes from the family of enzymes known for their metabolic capabilities, the cytochrome p450s. Apparently this is the major metabolic pathway induced by alcohol which leads to tolerance.
~

I'd agree there needs to be some correlation between drinking and brain function, and I'm sure there are numerous studies that do that. What do you mean by LTP-induced conditions, and what does you mean when you say "new neurons in the Hc are more responsive to LTP- induced conditions"?

Even though I stated that LTP is used as a model of learning and memory, it is still an artificial construct that is created by tetanic stimulation of specific regions in the Hc to result in a sustained elevation in cellular membrane potential. Basically if you shock the cells just right you can get them to be more excitable or more readily activated by the right stimuli. In the laboratory, LTP must be induced or initiated by the researcher under strictly-controlled conditions in order for it to be studied. Apparently the LTP is more readily induced in the new neurons as compared to older ones, thus they may be more adept at performing the processes required in Hc function and learning. I would add that LTP is not the perfect model for learning and memory (if ther is such a thing), there are some researchers with strong evidence that show LTP does not always correspond to learning processes. If anything, the types of synaptic changes and plasticity that may be modeled during Hc LTP are probably only part of what is required for true learning and memory processes to occur in the brain. Other recent posts in this section have demonstrated that memory is not solely relegated to the Hc.