Prodynorphin knockout mice demonstrate diminished age-associated impairment in spatial water maze performance

Dynorphins, endogenous kappa-opioid agonists widely expressed in the central nervous system, have been reported to increase following diverse pathophysiological processes, including excitotoxicity, chronic inflammation, and traumatic injury. These peptides have been implicated in cognitive impairment, especially that associated with aging. To determine whether absence of dynorphin confers any beneficial effect on spatial learning and memory, knockout mice lacking the coding exons of the gene encoding its precursor prodynorphin (Pdyn) were tested in a water maze task. Learning and memory assessment using a 3-day water maze protocol demonstrated that aged Pdyn knockout mice (13-17 months) perform comparatively better than similarly aged wild-type (WT) mice, based on acquisition and retention probe trial indices. There was no genotype effect on performance in the cued version of the swim task nor on average swim speed, suggesting the observed genotype effects are likely attributable to differences in cognitive rather than motor function. Young (3-6 months) mice performed significantly better than aged mice, but in young mice, no genotype difference was observed. To investigate the relationship between aging and brain dynorphin expression in mice, we examined dynorphin peptide levels at varying ages in hippocampus and frontal cortex of WT 129SvEv mice. Quantitative radioimmunoassay demonstrated that dynorphin A levels in frontal cortex, but not hippocampus, of 12- and 24-month mice were significantly elevated compared to 3-month mice. Although the underlying mechanisms have yet to be elucidated, the results suggest that chronic increases in endogenous dynorphin expression with age, especially in frontal cortex, may adversely affect learning and memory.