PRIOR INSTRUMENTAL-CONDITIONING IMPROVES SPATIAL COGNITION AND ATTENUATES CHANGES IN HIPPOCAMPAL FUNCTION IN AGED RATS

These experiments examined the effects of long-term instrumental train ing on subsequent radial arm maze performance and synaptic transmissio n within the hippocampal formation. In the first experiment, young (3 mo) and aged (18 mo) male mts underwent 12 weeks of appetitive instrum ental conditioning; half were continually reinforced and the other hal f alternated between reinforcement and extinction. Afterward, spatial cognition was evaluated using an eight-arm radial maze. Subjects under going instrumental training performed at rates superior to untrained ( control) animals regardless of age or training condition; age-related differences did not exist in the trained groups. In the second experim ent, subjects underwent 12 weeks of instrumental training with continu ous reinforcement, and excitability of the hippocampus was examined by paired-impulse stimulation of the perforant path. Training enhanced m aximal facilitation of population spikes evoked in the granule cell la yer of the dentate gyrus of aged subjects to the degree that no statis tical difference existed between young and aged animals. Data from unt rained control animals indicated a robust decline in paired-impulse ex citability in aged subjects. These findings suggest that learning-indu ced plasticity may attenuate both behavioral and neurobiological chang es observed in aged subjects. It is postulated that disuse may underli e some of the cognitive changes exhibited across the life span.