Neuronal response abnormalities in the inferior colliculus of the genetically epilepsy-prone rat.

The genetically epilepsy-prone (GEP) rat is susceptible to seizure induction by acoustic stimuli. The inferior colliculus (IC) has been implicated as being critically important in audiogenic seizure susceptibility based on lesion, electrical stimulation, and focal implantation experiments. The current study determined that GEP rats were most susceptible to seizure induction by pure tone bursts at 100 dB at a frequency of 12 kHz. IC neurons in the GEP rat exhibited a significantly elevated incidence of a particular response pattern at 12 kHz and at characteristic frequency. This pattern consisted of a peak at the beginning and end of the stimulus (onset-offset response). This response pattern only occurred with high intensity stimuli approximating those which induce seizures and may represent an afterdischarge phenomenon. The response threshold was significantly elevated and tuning characteristics were also significantly altered in IC neurons of GEP rats as compared to normal IC neurons. The latter two findings may be related to the deficit of hearing which is reported in the GEP rat. The increased incidence of onset-offset responses may be due to a decreased efficacy of inhibition in the GEP rat neurons as compared to normal rat neurons.