Learning deficits in rats with early neurotoxic lesions to the globus pallidus, substantia nigra, median raphe or pontine reticular formation.

Previous studies have shown that weanling rats with electrolytic lesions of the globus pallidus (GP), substantia nigra (SN), median raphe (MR) or pontine reticular formation (PRF) are deficient in learning a wide variety of laboratory tasks. The current study was designed to investigate whether this nonspecific learning deficiency is due to destruction of cells intrinsic to these subcortical regions or to fibers passing through these regions. Accordingly, neurotoxic lesions of the GP, SN, MR or PRF were made in weanling rats using ibotenic acid. Rats were subsequently required to learn a visual discrimination, a 3-cul maze and a nonvisual (incline plane) discrimination. While those groups with GP, SN or MR lesions showed significant deficits on all three problems, only the animals with GP lesions exhibited deficits comparable in magnitude to those associated with corresponding electrolytic lesions. Animals with lesions to the PRF were impaired only on the nonvisual discrimination. These results suggest that while destruction of neurons alone within the GP, SN or MR can produce a nonspecific learning impairment, the combined destruction of neurons and fibers of passage within the SN, MR or PRF produces a more profound learning deficit.