Paradoxical role of PKA inhibitor on amyloidβ-induced memory deficit

In spite of characterizing the role of protein kinase A (PKA) in activating biochemical mechanisms, few studies have investigated the effects of PKA inhibitors on memory functions. In the present study, we used Pavlovian fear conditioning paradigm to evaluate memory alterations caused by two doses of H89 (as a conditional inhibitor of PKA) alone and in combination with amyloid-β (Aβ) in rats. Moreover, we used the Western blotting method to investigate the alterations in markers of transcription, oxidative stress, inflammation, and apoptosis pathways involved in memory impairment. Stereotaxic surgery was done to inject Aβ (30ng/side) directly into the hippocampal CA1 area bilaterally and H89 (5 or 10μM) intracerebroventricular unilaterally. One series of rats were trained 7days after injections, then contextual and tone tests were conducted on days 8 and 9, respectively. Second series of rats were trained 14days after the injections and tests were carried out on days 15 and 16. Our behavioral results showed that H89 (5μM) not only has no destructive effect on memory, but also attenuates memory deficit caused by Aβ in combination groups. In contrast, H89 (10μM) has a reversible destructive effect on memory. Our molecular findings indicated that low dose of H89 increases CREB phosphorylation, Nrf2 and HO-1 which results in survival resistance to the stress. On the contrary, H89 with higher concentration leads to substantial increase of NF-κB and caspase-3 levels, which impair memory functions. In conclusion, our data suggest that H89 as a PKA inhibitor influences memory process through a dose and time dependent manner.