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Depression is the most common psychiatric disorder in patients with epilepsy, and is associated with worse treatment outcomes and increases the risk of attempted suicide1. We have generated a rat model of epilepsy and depression comorbidity to study the underlying neurobiological relationship between these diseases. We previously found that rats selectively bred for low activity in the forced swim test (FST) model of depression (SwLo) were also more susceptible to seizures than their depression-resistant counterparts (SwHi rats). The ultimate goal of our research is to identify gene polymorphisms that contribute to depression- and epilepsy-like phenotypes in SwLo rats using Quantitative Trait Loci (QTL) analysis. This study represents the first step in the QTL mapping process. SwLo rats were crossed with SwHi rats to produce an F1 generation, which were then intercrossed to create F2 generation rats possessing unique combinations of SwHi and SwLo genes. As expected, the randomly inherited SwLo and SwHi genes in the F2 rats generated a wide spread of scores in the FST. Presumably, F2 rats with low FST scores are those that inherited mostly SwLo DNA at critical loci, while F2 rats with high FST scores are those that inherited mostly SwHi DNA at these loci. The F2 rats will next be tested for seizure susceptibility and then subjected to genotyping and QTL analyses. These studies will identify candidate genes underlying depression and epilepsy comorbidity.
Comorbidity:
About 3% of America’s population will develop epilepsy and 20-50% of those diagnosed will develop major depressive disorder (MDD)2.
Patients with epilepsy are 3-5 times more likely to suffer from depression3.
Patients with a history of MDD have a greater chance of developing epilepsy than the general population. This risk is quadrupled if history is accompanied by a suicide attempt4.
Rodent Model:
Two lines of rats were selectively bred based on FST scores. SwLo rats, a model of depression-like behavior, have low motor activity (high floating, low struggling). Their “depression-resistant” counterpart, the SwHi rats, were bred for high motor activity (high struggling, low floating)5.
Because they respond to chronic, but not acute, antidepressant treatment, SwLo rats have proved to be a useful model of depression for assessing antidepressant drug efficacy6.
SwLo rats have also been found to be more susceptible to seizures induced by kainic acid, pilocarpine, and electroshock, thus supporting the hypothesis that a neurobiological link exists between depression and epilepsy (reference 7 and unpublished data).
QTL:
Genome-wide markers that distinguish between SwLo and SwHi DNA will be used in F2 rats to find genomic regions contributing to the depression-like and epilepsy susceptibility phenotypes.
Generation of F2 Hybrid Rats (Figure 1):
1. SwHi and SwLo rats of the 50th generation of selective breeding were crossed, creating an F1 cohort of rats with half SwHi DNA and half SwLo DNA.
2. F1 males and females were then intercrossed to generate F2 rats with varying amounts of SwHi and SwLo genes due to random recombination events during meiosis in the F1 germlines.
Generation of F2 Hybrid Rats (Figure 1):
1. SwHi and SwLo rats of the 50th generation of selective breeding were crossed, creating an F1 cohort of rats with half SwHi DNA and half SwLo DNA.
2. F1 males and females were then intercrossed to generate F2 rats with varying amounts of SwHi and SwLo genes due to random recombination events during meiosis in the F1 germlines.
3. A tester then recorded the amount of time (seconds) the rat spent struggling (all 4 limbs in movement and forepaws breaking the surface) and floating (no limb movement) in a 15 minute time period
As a result of several generations of selective breeding, SwLo and SwHi rats had significantly different floating and struggling scores in the forced swim test.
Swim scores were similar between individual F1 rats (low variance), as expected given their genetic similarity (50% SwHi and 50% SwLo DNA).
Large differences in swim scores between individual F2 rats (high variance) were seen, as expected given their genetic variability (unique combinations of SwLo and SwHi DNA).
The lack of correlation (or negative correlation) between struggling and floating in F2 rats (Figure 6) suggests these behaviors may be controlled by genes at different loci.
Future Research
F2 Latency to Status Epilepticus
To assess the epilepsy-like phenotype of the F2 rats, latency to pilocarpine-induced status epilepticus (30 minutes of continuous motor seizure) will be measured at 12 weeks of age.
QTL Mapping
For sufficient statistical power, a total of about 400 F2 rats will be phenotyped and analyzed.
F2 rats will be genotyped at 140 loci (10 centimorgan intervals) across the genome to determine which regions contain SwLo DNA and which regions contain SwHi DNA. Swim scores and seizure latency will then be correlated with genotype to identify chromosomal regions controlling both depression- and epilepsy-like traits.
Expression microarrays and gene sequencing within identified QTL intervals will identify candidate genes underlying epilepsy and depression comorbidity.
This material is based upon work supported by the Student Inquiry and Research at Emory (SIRE) award from the Office of Undergraduate Studies, Emory College and grants from the National Institutes of Health (NS053444) and the Epilepsy Foundation (121961).
1. Kanner, A.M. Depression and Epilepsy: A New Perspective on Two Closely Related Disorders. Epilepsy Curr 6, 141-146 (2006).
2. Kanner, A.M. Depression in epilepsy: prevalence, clinical semiology, pathogenic mechanisms, and treatment. Biol Psychiatry 54, 388-398 (2003).
3. Kanner, A.M. & Balabanov, A. Depression and epilepsy: how closely related are they? Neurology 58, S27-39 (2002).
4. Harden, C.L. & Goldstein, M.A. Mood disorders in patients with epilepsy: epidemiology and management. CNS Drugs 16, 291-302 (2002).
5. Weiss, J.M., Cierpial, M.A. & West, C.H. Selective breeding of rats for high and low motor activity in a swim test: toward a new animal model of depression. Pharmacology, biochemistry, and behavior 61, 49-66 (1998).
6. West, C.H.K., Weiss, J.M. Effects of antidepressant drugs on rats bred for low activity in the swim test. Biochem Pharmacol 61, 67-79 (1998).
7. Tabb, K., Boss-Williams, K.A., Weiss, J.M. & Weinshenker, D. Rats bred for susceptibility to depression-like phenotypes have higher kainic acid-induced seizure mortality than their depression-resistant counterparts. Epilepsy Res 74, 140-146 (2007).
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