Animal studies of BDNF have looked specifically at the relation between epilepsy and depression using a novel "double hit" design. After chronic stress exposure, measurement of BDNF levels allowed identification of 2 sub-groups: a vulnerable population and non-vulnerable population. A "second hit" of kainic acid induced status epilepticus (SE) was then applied to both the vulnerable and non-vulnerable populations. Only the vulnerable population exposed to SE developed a depression-like profile.