Changes in the density of the calretinin-immunoreactive interneurons in control and epileptic human hippocampi

Tóth Kinga, Erőss Loránd, Czirják Sándor, Vajda János, Halász Péter, Freund Tamás, Maglóczky Zsófia

Calretinin (CR) is a member of the calmodulin superfamily. This calcium-binding protein is exclusively expressed in interneurons in the hippocampus. The human CR-containing interneurons are considerably different from the neurochemically similar cell-type found in rodents. In rats the CRpositive cells with smooth dendrites in the CA1 region are interneuron-specific inhibitory cells, while in humans CR was found in dendritic inhibitory cells as well, forming a functionally heterogenous cell population. The CR-immunopositive cells are morphologically more heterogenous: besides the cells which are present also in rats, there is a group of small neurons in the dentate gyrus (DG), and there is a multipolar cell-type at the border of the stratum radiatum and lacunosum-moleculare. CR-containing cells are particularly sensitive to ischemia and epilepsy in animal models. Therefore we aimed to reveal the fate of this cell type in human epilepsy. We examined the surgically removed hippocampi of drug-resistant temporal lobe epilepsy patients and compared them with control samples with different post mortem delay. The samples were immunostained for CR and the changes in the distribution and density of CR-immunopositive cells were analysed. The epileptic cases were classified and grouped into three types according to the degree of principal cell death. Longer post mortem delay resulted in a reduced number of immunopositive cells. The number of CR-positive cells in the epileptic tissue is considerably decreased in parallel with the severity of principal cell loss. The largest cell loss was found in the hilus and CA3. The number of the cells at the upper border of the stratum moleculare also decreased. However, a moderate number of the multipolar cells in the stratum lacunosum-moleculare and radiatum of the CA1 region are still detectable, but their dendrites are segmented and shortened. Our results suggest that CR-containing interneurons are also sensitive for epilepsy in humans similar to that seen in rat models, thus, interneuron specific inhibition might be decreased in human epileptic hippocampi. Post mortem delays longer than 6 hours of control samples can also cause significant loss of immunostaining, therefore examination of changes of the number and distribution of CRimmunopositive cells can be carried out only in very carefully selected human samples with short post mortem delay and good quality fixation.