Light microscopic study of GluR1 and calbindin expression in interneurons of neocortical microgyral malformations

Rat neocortex that has been injured on the first or second postnatal day (P0-1) develops an epileptogenic, aberrantly layered malformation called a microgyrus. To investigate the effects of this developmental plasticity on inhibitory interneurons, we studied a sub-population of GABAergic cells that co-express the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor GluR1 subunit and the calcium-binding protein, calbindin (CB). Both malformed and control cortex of adult (P40-60) animals contained numerous interneurons double-stained for CB and GluR1. Immunoreactivity (IR) for CB was up-regulated in perikarya of interneurons within supragranular layers of control cortex between P12 and P40. However, in malformed adult (P40) cortex, CB-IR levels were significantly lower than in adult controls, and fell midway between levels in immature and adult control animals. Between P12 and P40, GluR1-IR was down-regulated in perikarya of interneurons in control cortex. Somatic GluR1-IR levels in malformed adult (P40) cortex were not different from adult controls. These neurons formed a dense plexus of highly GluR1-positive spiny dendrites within layer II. The dendritic plexus in the malformation was more intensely GluR1-immunoreactive than that in layer II of control cortex. This was due to apparent changes in thickness and length of dendrites, rather than to significant changes in the number of interneuronal perikarya in the microgyral cortex. Results indicate that the population of GluR1/CB-containing interneurons is spared in malformed microgyral cortex, but that these cells sustain lasting decreases in their somatic expression of calbindin and alterations of dendritic structure. Potential functional implications of these findings are discussed.