ACTIVITY, AND PROPERTIES OF THE y-AMINOBUTYRIC ACID POSTSYNAPTIC RECEPTOR IN COBALT EPILEPSY IN THE RAT1

Crude mitochondrial synaptosomal (Pz) fractions were used to measure L-glutamate l-decarboxylase (GAD) activity, and crude synaptic membranes were isolated from rat brains and used to determine y-aminobutyric acid (GABA) concentration and postsynaptic GABA receptor binding characteristics in rats with cobalt, copper, or glass implanted in right and left cerebral cortices. Copper was employed as a positive metal control because it elicits a morphological profile similar to that of cobalt but is non-epileptogenic. From tissue adjacent to the lesion, GAD activity was assessed by counting trapped 14C02 liberated from [‘4C]glutamate and was reduced maximally to 25% of glass controls 7 days following cobalt insult, a period of peak seizure incidence. No reduction in GAD activity was observed 1 or 21 days after cobalt treatment or at any time period in copperor glasstreated animals. A radioligand [3H]GABA receptor assay was utilized to determine GABA levels, postsynaptic receptor number (B,,,), and the affinity of the postsynaptic receptor for the ligand (KU) in tissue surrounding the lesion. GABA concentration was reduced maximally to 47% of glass controls 7 days following cobalt implantation. Scatchard plot analysis of tissue adjacent to the cobalt lesion revealed a significant increase in apparent receptor density (B,,,) to 200% of glass controls 7 days after bilateral cobalt implantation (B,,, = 3.97 + 0.83 pmol/mg of protein, cobalt versus 1.36 & 0.17, glass control). Moreover, at 7 days, no change in kinetic parameters was noted after copper treatment. From days 7 to 21, the density (B,,, ) of postsynaptic GABA receptors in cobalt-treated tissue appears to return slowly to glass control values. Results from the present study suggest that degeneration of the GABA pathway in the frontal cortex of the cobalt-epileptic rat occurs and, coupled with the increased B,,,, may represent a “denervation supersensitivity” phenomenon. The chronic model of epilepsy produced in the rat by administration of cobalt to the cerebral cortices has been widely studied and appears to be a suitable model for the study of neurochemical changes associated with a chronic seizure disorder. In addition to its chronicity and paroxysmal nature, characteristic electroencephalogram (EEG) changes, and increased susceptibility to central nervous system (CNS) convulsants, there are also other similarities between cobalt epilepsy and human epilepsy. Certain anticonvulsants are able to attenuate seizures in both (Van Duijn and Visser, 1972; Craig et al., 1976a). Craig and Hartman (1973) also showed that the same amino acids found to be decreased in the brain of humans with epilepsy (Van Gelder et al., 1972) were decreased in the brains of cobaltepileptic rats. ’ This work was supported by the West Virginia University Medical Corp., National Institutes of Health Biomedical Research Grant 5-SO7RR05433-18, and National Institute of General Medical Sciences Predoctoral Training Grant 5 T32 GM 07039-05. A preliminary report of this work has appeared previously (Ross, S. M., and C. R. Craig (1981) Toxicologist 1: 50). Several studies have shown an altered neurochemistry in this experimental seizure model. Hoover et al. (1977a) demonstrated that acetylcholine was depressed at the time of peak seizure activity, extending the observation of Goldberg et al. (1972) that both the synthetic enzyme, choline acetyltransferase, and the degradative enzyme, acetylcholinesterase, were depressed 7 days following cobalt implantation. ’ Present address: Albert Einstein College of Medicine, Institute of Neurotoxicology, Bronx, NY 10461. ’ To whom correspondence should be addressed. Several investigators (Van Gelder, 1974; Van Gelder and Courtois, 1972; Koyama, 1972; Altamura et al., 1978) have been concerned with changes in y-aminobutyric acid (GABA) and/or the synthetic enzyme, L-glutamate 1-decarboxylase (GAD), in cobalt epilepsy. Emson and Joseph (1975) showed that GABA, glutamate, and asparThe Journal of Neuroscience GABA Mechanisms in Cobalt Experimental Epilepsy 1389 tate were reduced in the area of the lesion but that no change occurred in glycine or glutamine levels. They also studied the activation of the transmitter-related enzymes, glutamate decarboxylase, acetylcholinesterase, choline acetyltransferase, and aromatic amino acid decarboxylase, and showed significant reductions 4 to 8 days following cobalt implantation with a subsequent recovery to normal levels by 24 days. Emson and Joseph concluded that cobalt-induced epilepsy in the rat is associated with a relatively selective loss of neuronal tissue. This selectivity is the reason for using this model in an attempt to gain a better understanding of seizure processes in general. Materials and Methods