The inferior colliculus (IC) is a critical site for induction of audiogenic
seizures in the genetically
epilepsy-prone rat (GEPR). Abnormal response properties observed in inferior colliculus neurons of that strain include a high incidence of onset-offset responses which may be a form of afterdischarge. These response abnormalities may involve altered actions of
neurotransmitters in that region of the brain.
GABA is implicated as a transmitter in endogenous sound-induced inhibition in the inferior colliculus. Endogenous inhibition and the actions of agents that affect
GABA receptors were examined in inferior colliculus neurons in epileptic and normal rats. The iontophoretic dose (current) of
GABA required to suppress neuronal firing in the
epilepsy-prone rat was significantly greater than that required in neurons of the normal rat. A form of endogenous (binaural) inhibition in inferior colliculus neurons, which is proposed to be
GABA-mediated, was also significantly reduced in the
epilepsy-prone rat as compared with the normal rat. A
benzodiazepine (
flurazepam) which enhances the action of
GABA in many brain sites including the inferior colliculus was significantly less effective than normal when applied iontophoretically onto the same neurons of the
epilepsy-prone rat. The GABAA antagonist,
bicuculline, which blocks the effect of
GABA on inferior colliculus neurons, frequently induced the onset-offset response in neurons not previously exhibiting this pattern. These findings suggest that the reduction of
GABA-mediated inhibition in the genetically
epilepsy-prone rat may result in the increased incidence of afterdischarges in neurons of the inferior colliculus and may serve as an important mechanism of epileptogenesis in audiogenic
seizures in this genetic form of
epilepsy.