Ibotenic acid-induced neuronal degeneration: a morphological and neurochemical study

Possible neurotoxic actions of intracerebral injections of ibotenic acid, a conformationally restricted analogue of glutamic acid, have been evaluated in rat brain and compared with those of kainic acid. Light microscopical analysis revealed that ibotenic acid produced a marked disappearance of nerve cells in all areas studied, namely striatum, the hippocampal formation, substantia nigra and piriform cortex. Lesions in areas distant to the injection site were not seen. Axons of passage and nerve terminals of extrinsic origin did not seem to be damaged, since, e.g., no apparent degeneration of the dopaminergic terminals in the neostriatum was observed except for a small area surrounding the cannula. In the neostriatum, enkephalin immunoreactive neuronal cell bodies as well as nerve terminals disappeared after injection of ibotenic acid into this nucleus. After injection into the substantia nigra tyrosine hydroxylase immunoreactive cell bodies in the zona compacta disappeared, whereas no certain effect could be seen on the enkephalin immunoreactive nerve fibers. In vitro experiments, conducted with striatal synaptosomal and membrane preparations, showed that ibotenic acid differed from kainic acid by being devoid of a significant inhibitory effect on high affinity glutamate uptake and by having a low affinity for 3H-kainic acid binding sites. Furthermore, ibotenic acid did not interfere with the binding of a number of radioligands for other transmitter receptors. As compared to kainic acid, ibotenic acid has the advantage of being less toxic to the animals and of producing more discrete lesions, possibly due to faster metabolism and/or other fundamental biochemical differences. Because of these special features, ibotenic acid seems to represent a valuable new tool in the morphological and functional analysis of central neuronal systems.