Role of TNFα Induced Inflammation in Delay Eyeblink Conditioning in Young and Aged Rats

Tumor necrosis factor alpha (TNF-α) is a multifunctional proinflammatory cytokine, which is a critical inflammatory mediator involved in aging and neurodegenerative diseases of aging. Previous work has shown that diets enriched with antioxidants reduce levels of the cytokine TNF-α and improve classical eyeblink conditioning performance. Therefore we tested the hypothesis that the proinflamatory cytokine TNF-α may be a critical factor that modulates classical conditioning behavior. If increased levels of endogenous cerebellar TNF-α negatively affect performance on the eyeblink conditioning task in aged rats, then exogenous administration of TNF-α in young rats should result in an impaired acquisition and/or retention of eyeblink conditioning memory. On the other hand, the reduction or blockage of the age-related increase in cerebellar TNF-α levels in aged rats should result in an improvement in memory. Young (3 month old) F344 rats were pretreated with an intracerebellar injection of recombinant rat (rr)TNF-α or denatured (rr)TNF-α prior to eyeblink conditioning coupled to microdialysis. The results showed that young rats treated with rrTNF-α have a decreased rate of learning compared to the control group. Norepinephrine which has been shown to play a critical role in cerebellar learning tasks presented a shift on training day one of young rats resembling that observed in aged rats. In a second experiment aged (22 month old) F344 rats were pretreated with intracerebellar microinjection of anti-rat TNF-α three times a week for 4 weeks prior to eyeblink conditioning training couple to microdialysis. Aged rats showed a better performance in the conditioned responses when compared to controls. The release of norepinephrine in this group reached basal levels sooner than the control group but not as early as the young rats. The results of these experiments demonstrate a critical correlation between TNF-α and the rate of learning and the pattern of NE release during eyeblink conditioning.