Focal microinjection of gamma-acetylenic GABA into the rat entorhinal cortex: behavioral and electroencephalographic abnormalities and preferential neuron loss in layer III.

Neuron loss in layer III of the entorhinal cortex (EC) occurs in patients with temporal lobe epilepsy and in several animal models of the disease and may play a role in the development of spontaneously recurring seizures. This damage can be reproduced in rats by a focal microinjection of the indirect excitotoxin aminooxyacetic acid into the EC (Neurosci. Lett., 147: 185, 1992). We have now examined a similar but approximately 20 times more potent toxin, gamma-acetylenic GABA (GAG), for its ability to produce seizures and neurodegeneration in the rat EC. EEG activity was recorded continuously for 48 h after a focal injection of 4 micrograms GAG into the rat EC. Seizure episodes, spiking, and other irregularities occurred with a latency of 150 min. Behavioral abnormalities were observed in all animals and were always accompanied by EEG seizures. The behavioral changes subsided gradually, but EEG seizures continued up to 24 h after GAG treatment. Nissl and silver-stained tissue sections obtained 2-3 days after the injection of 4 micrograms GAG revealed neuron loss which preferentially affected the medial part of layer III of the EC, and caused a modest lesion in the hilar region of the ventral hippocampus. The neurodegenerative potency of GAG, in contrast to the effects of aminooxyacetic acid, was not influenced by the depth of anesthesia during surgery. A slight increase in the dose of GAG (to 5 micrograms) resulted in more severe behavioral seizures, causing generalized convulsions with salivation and loss of righting posture in 3 of 13 rats. These animals also showed a marked enlargement of the lesioned area, with substantial neuronal loss occurring in layer III of the EC, in the hilus of the dentate gyrus, and occasionally also in homotopic structures of the contralateral hemisphere. Seizure activity and lesions induced by 4 micrograms GAG were prevented by the NMDA receptor antagonist Dizolcipine (MK-801) (4 mg/kg, i.p., 10 min before and 12 h after GAG). These data support the notion of a close correlation between the occurrence of seizures and neuronal loss in layer III of the EC. Taken together, the study suggests that intraentorhinal injections of GAG may provide an advantageous model for the study of epileptogenic and epileptic mechanisms.