Disrupted retention of the classically conditioned nictitating membrane response in rabbits with aluminum-induced neurofibrillary degeneration

Rabbits underwent classical conditioning of the nictitating membrane response (NMR) to a tone conditioned stimulus and an air puff unconditioned stimulus until they emitted 90 percent or greater conditioned responses (CRs) for two consecutive days. They then received intraventricular injection of aluminum chloride, hydrochloric acid or saline. Ten days post injection they were tested for retention of the conditioned response. Animals injected with saline or hydrochloric acid emitted an average of greater than 90 percent CRs. Aluminum injected rabbits, in contrast, emitted an average of only about 40 percent CRs. This disruption of retention of conditioning in aluminum treated rabbits could not be attributed to deficits in sensory or motor processes or to illness. Neuropathological analysis revealed widespread neurofibrillary tangle formation in the aluminum treated animals. Furthermore, the degree of neurofibrillary degeneration was significantly negatively correlated with retention of the CR. The results are considered in the context of using the rabbit NMR preparation as a model system for studying age-related conditioning disorders.