A primary objective of my research is to study the ways in which dietary choline levels affect an individual’s risk for and outcomes in psychological disorders, particularly ones for which stress is likely to be a contributing factor. The basic hypothesis that I am pursuing is that dietary choline alters emotional reactivity thus diminishing the potential for stressful situations to exert negative effects on brain and behavior and to trigger psychopathology. Presently I am funded through Maine’s IDeA Network of Biomedical Research Excellence (which gets it’s funding from the National Institutes of Health and the National Institute of General Medical Science) to study choline and depression in a rat model. We have lots of intriguing new data emerging on that front that I’ll be sure to shamelessly promote in an upcoming post. Today, however, I’d like to talk about a paper we recently published about choline and schizophrenia.
Burning question #1: How do you study schizophrenia in rats? Excellent question. With great difficulty, caution, and attention to etiology and outcome. But first let me give you some background. The first author on this paper is Jenn Corriveau. Jenn walked into my office my first official day on the job at Colby in 2007. She was an art major (of all things) at the time but with a keen interest in neuroscience and an even keener interest in schizophrenia. She was a newly risen sophomore on that day and shortly after joined the lab and worked with me until the day of her graduation in May 2010. She is the first student of mine to first-author a paper and she set the bar very high for all subsequent students (lucky for me I have many great contenders jumping at that bar).
After Jenn had been in the lab for about a year doing lots of other things, most having to do with choline and memory, we decided to plan our first study on choline and schizophrenia. So back to our first burning question: how do we study schizophrenia in rats? Well, we knew we wanted to focus on the cognitive deficits in schizophrenia, mainly because choline supplementation during development enhances adult attention and memory abilities. Jenn’s plan was to try to induce memory deficits by replicating features of hypothesized etiology and neuropathology. Enter the two-hit model of schizophrenia. In its simplest incarnation it posits that something happens early in life, hit one, and later in life something else happens, hit two, that triggers the full expression of the disorder. Either hit alone will not trigger the disorder: it is the combined effect of the two that is key. The nature of each hit is under investigation in many labs and there are some “hits”, like early life infection or hypoxia, that may be particularly important in the story. Another important part of this story is stress. We (try to) study stress and we think choline may have effects on the neural systems that regulate stress and emotional reactions, so this was a logical target for us.
Jenn’s plan was to subject a group of rats to two “hits”: one early in life and one in adulthood. The first hit was implemented by exposing developing rat pups to elevated levels of the stress hormone, corticosterone, by stressing the pregnant dam. Our stress manipulation was relatively mild and not variable. We needed to make sure that this “hit” by itself would not profoundly affect memory in adult male rats. The second hit was a pretty large but one-time dose of a drug that blocks excitatory neurotransmission in the brain (NMDA anatagonist, MK-801). Again, even though it was a lot of the drug, we wanted to ensure that rats’ memory abilities would not be too adversely impacted by it and so we waited a week to assess memory. A group of rats received just the first hit and no second hit; another group received just the second hit and not the first.
I’ll cut to the chase: it worked! These methods were novel in the field and it was exceptionally critical that the combination be worse than either hit alone. And it was. In fact, it worked out really well because all the rats that received just one of the two hits showed no memory deficits at all! And the group that had both hits showed a big memory deficit.
Burning question #2: What about choline? Great question! First let me tell you a little about choline. It’s an essential nutrient that does all manner of nifty biological things in the central nervous system. Noted among its host of terrific tasks is being a key ingredient in the recipe for the neurotransmitter, acetylcholine. This is an important and abundant neurotransmitter in parts of the brain that make substantial contributions to attention and memory functions. Another notable task of choline is to affect gene transcription in a bunch of different and probably very complicated ways (that lots of people including us are trying very hard to understand and which is a pivotal goal of my funding). Choline shows up in lots of different types of food and some common sources are as follows: red meat, eggs, wheat germ, dark green leafy veggies, nuts, and legumes. (yes, eat up)
Back to the second burning question and how choline fits into our experiment. We feed the rats these terribly expensive though awesome synthetic diets where the only thing that differs is the level of choline. To test our hypothesis that choline may combat memory deficits in schizophrenia we decided to insert our choline manipulation in between the two hits. From my past work we knew that prenatal choline exerts the biggest effects on behavior, but choline given over the period of adolescence also had measurable effects.
Here is the experiment in a nutshell:
Some rats were prenatally stressed and some were not. They were born, reared, and weaned. Starting a few days after weaning we began the diet manipulation. We took each group of stressed and unstressed rats (all males) and divided them into three: one-third were fed a standard diet with decent though perfectly ordinary amounts of choline, one-third were fed a diet with nearly 5 times the amount of choline than the standard diet, and the remaining third of each group were fed a diet with zero choline in it. These diets continued until just after adolescence. At this point we have 6 groups of rats (stressed with each of the 3 diets, unstressed with each of the 3 diets). But, ya know, that’s not nearly enough, particularly for an undergraduate with a full course load, who, I should point out, was now a psych major with a concentration in neuroscience (sayonara art). When the rats were fully adult, about 25 days later, we took those 6 groups and divided them all into 2 (not perfectly evenly because honestly we were running out of rats as we’d had a rather female heavy breeding this time): most of each group received the second hit–the drug– while the few remaining in each group received a saline injection as a control. Then we waited a week and tested everyone’s memory. In case anyone’s not yet confused, let me also say that we tested baseline memory performance in all the rats a week before we gave the drug and saline injections. We just wanted to check to see what effect the diet and stress had first. Hopefully, that’s clear…
Burning question #3: why the no choline group? Well, choline supplementation may protect, but choline deficiency could make things worse. We wanted to know. And the results? Very cool findings if we do say so ourselves. Choline supplementation did indeed completely prevent the memory deficits brought on by both hits. Choline deficiency did indeed make things worse. In fact choline deficiency alone was not so bad but if it was combined with just one of either of the hits then memory suffered.
I include a picture at the bottom to show you our memory testing situation. We use a test of object recognition that takes advantage of rats natural propensity to explore new objects they encounter in their environment. Rats first enter a box like the one shown but there are two identical objects in it. They have 5 minutes to freely explore the box and the objects. They’ve been in the box several times before so they really like it when the objects show up and check them out. We then take them out of the box and put them in a holding cage for about 20 minutes. Then they go back into the box where they now find one object from before and one new one (as pictured below). Most normal rats will explore the new object more than the old one and from this we infer memory for the old object. So when I refer to a memory deficit, those rats did not show a preference for the novel object and explore both objects about the same, just as they do with the identical objects during the study phase when both are equivalently familiar to them.
This concludes my post of shameless self-promotion. Again, look for the paper here, here, or here. Please leave us any comments or questions you may have. And really this is less shameless self-promotion and a lot of not-so-shameless Jenn promotion! She worked her butt of in the lab when she was an undergraduate and then worked her butt off on this paper while juggling her new responsibilities as a graduate student at UConn. Jenn has a bunch of posts on this blog and to this day she is the #1 comment-leaver. Look for her on Bourbon Street in New Orleans in October. She’ll be the slightly inebriated woman with pink hair handing out copies of a paper about choline and schizophrenia.