Anatomical changes of the GABAergic system in the inferior colliculus of the genetically epilepsy-prone rat

The number of GABAergic neurons as determined by GAD immunocytochemistry and total neurons as determined from Nissl preparations were counted and classified at the light microscopic level in the inferior colliculus (IC) of the genetically epilepsy prone rat (GEPR) and the non-epileptic Sprague-Dawley (SD) strain of rat. GAD-positive neurons are abundant in the IC and a significant increase in the number of GAD-positive neurons occurs in the GEPR as compared to the SD in all three subdivisions. However, the most pronounced difference occurs in the ventral lateral portion of the central nucleus, where there is a selective increase in the small (200%) and medium-sized (90%) GABAergic somata (10-15 microns in diameter and 15-25 microns in diameter, respectively). As determined from Nissl preparations an increase in total numbers of neurons also occurs. Thus, a 100% increase in the number of small neurons and a 30% increase in the number of medium-sized neurons occur in the adult GEPR as compared to the SD rat. A statistically significant increase in the numbers of small neurons also occurred in the IC of the young GEPR. At 4 days of age, a 55% increase in the number of small neurons was found, and at 10 days of age this increase was 105%. The numbers of the medium and large neurons were similar in the older group of rats. These data suggest that the increase in cell number observed in the adult GEPR is not compensatory to the seizure activity, but may either be genetically programmed or be a failure of cell death. Based on other studies of genetic models of epilepsy, we propose that the additional GABAergic neurons may disinhibit excitatory projection neurons in the IC.