Possible Role of Dynorphins in Alzheimer's Disease and Age-Related Cognitive Deficits

<b><i>Background/Aims:</i></b> Expression of dynorphin, an endogenous opioid peptide, increases with age and has been associated with cognitive deficits in rodents. Elevated dynorphin levels have been reported in postmortem samples from Alzheimer's disease (AD) patients, and prodynorphin (PDYN) gene polymorphisms might be linked to cognitive function in the elderly. Activation of &#954;-opioid receptors by dynorphins has been associated with stress-related memory impairments. Interestingly, these peptides can also modulate glutamate neurotransmission and may affect synaptic plasticity underlying memory formation. N-methyl-<smlcap>D</smlcap>-aspartate (NMDA) and a-amino-3-hydroxy-5-methyl-4-isoxazol-propionate (AMPA) ionotropic glutamate receptor levels generally decrease with aging, and their function is impaired in AD. <b><i>Methods:</i></b> Here, we compared the impact of aging on ionotropic glutamate receptor levels in the hippocampal formation of wild-type (WT) and Pdyn knock-out (KO) mice. <b><i>Results:</i></b> We observed a significant reduction in GluR1 and GluR2 AMPA receptor subunits in the hippocampal formation of 18- to 25-month-old WT mice in comparison with 6-month-old mice. Conversely, the GluR1 protein level was maintained in old Pdyn KO mice, and the NMDA NR2B subunit level was increased by 42% when compared to old WT animals. <b><i>Conclusions:</i></b> These results suggest that elevated dynorphin expression occurring during aging and AD may mediate cognitive deficits by altering the glutamatergic system integrity.