Elevated dynorphin in the hippocampal formation of aged rats: relation to cognitive impairment on a spatial learning task.

Radioimmunoassay revealed increased dynorphin A(1-8)-like immunoreactivity [dynA(1-8)LI] in the aged rat brain. Among a number of brain regions examined, an age-related dynA(1-8)LI elevation was found only in the hippocampal formation and frontal cortex. Moreover, the increase in dynA(1-8)LI in the aged hippocampus was associated with a decline in spatial learning ability: dynA(1-8)LI distinguished aged rats that were behaviorally impaired from aged cohorts that learned the spatial task as rapidly as younger animals. Northern blot hybridization using a 32P-labeled complementary RNA probe encoding rat prodynorphin indicated that the abundance of prodynorphin mRNA was also significantly increased in the hippocampal formation of aged rats with identified spatial learning impairments.