Abstract
Aging is a multi-factorial process and leads to cognitive decline. Such age-related cognitive decline is associated with dysfunction of the hippocampus and prefrontal cortex (PFC). Polyamines putrescine, spermidine and spermine are the down-stream metabolites of L-arginine, and have important roles in maintaining normal cellular function, regulating neurotransmitter receptors and modulating learning and memory. There is evidence suggesting an important role of putrescine in neurogenesis in the dentate gyrus (DG) of the hippocampus that is severely impaired during aging. The present thesis systematically investigated: (i) the effects of aging on behavioural function (including learning and memory) and polyamines and their main biosynthesis and regulatory enzymes in the sub-regions of the hippocampus and PFC (Experiments 1 and 2); and (ii) the behavioural effects of acute and chronic depletion of putrescine by alpha-difluoromethylornithine (DFMO) (Experiments 3 and 4), through combined behavioural, neurochemical and molecular biological approaches.
Experiment 1 quantified the tissue concentrations of polyamines in the CA1, CA2/3 and DG sub-regions of the hippocampus and PFC from two sets of 4 (young), 12 (middle-aged) and 24 (aged) months old Sprague-Dawley (SD) rats without (Set I) and with (Set II) behavioural tests in the elevated plus maze, open field, water maze and object recognition memory tasks. There were reduced anxiety level and exploratory activity, and impaired spatial learning and memory with aging. Putrescine levels were significantly decreased in the CA1, DG and PFC, but increased in the CA2/3, with age, whereas there were significantly increased levels of spermidine in the CA1, CA2/3 and PFC, and spermine in the PFC, with age. Animals’ behavioral experience had little influence on the overall pattern of age-related changes in polyamines.
It has been documented that behavioural performance and lifespan vary across different strains of rats. Hence Experiment 2 investigated the effects of aging on behavioural performance and polyamines and their main biosynthesis and regulatory enzymes in Long-Evans (LE) rats using the experimental protocols for Set II animals in Experiment 1. Although there were slightly reduced anxiety level and exploratory and locomotor activities with aging, no marked age-related spatial learning and memory deficits were observed, which are different from SD rats. The levels of putrescine, spermidine and spermine in the sub-regions of the hippocampus and PFC were altered with age in a region-specific manner, however with different patterns when compared to SD rats. Western blot revealed increased protein expression of ornithine decarboxylase (ODC, the key biosynthesis enzyme of putrescine) and antizyme inhibitor (an enzyme that regulates ODC activity indirectly) with age in a region-specific manner, with no age-related changes in antizyme (an enzyme that inhibits ODC). Collectively, the results of Experiments 1 and 2 demonstrate altered polyamine system in the hippocampus and PFC during aging, which may contribute to age-related cognitive decline to a certain extent, although there are marked differences between SD and LE rats.
DFMO is a potent and irreversible inhibitor of ODC, and mainly depletes putrescine among three polyamines. Experiments 3 and 4 systematically investigated the behavioural and neurochemical effects of acute intracerebroventricular administration and chronic oral delivery of DFMO, respectively. Acute central delivery of DFMO depleted putrescine with very mild effects on spermidine, produced anxiety-like behavior and impaired memory for the object displacement without affecting animals’ locomotor and exploratory activities and spatial learning and memory. By contrast, chronic delivery of DFMO in drinking water depleted putrescine and significantly decreased spermidine levels in the hippocampus and PFC, inhibited growth, and impaired spatial learning and memory in the reference memory version of the water maze specifically. Collectively, these findings suggest that physiological levels of endogenous putrescine, and spermidine in particular, may be important for behavioural function, including learning and memory.
In summary, this thesis demonstrates altered behavioural function and polyamine system in the hippocampus and PFC during aging in strain- and region-specific manners, provides evidence for the involvement of endogenous polyamines in learning and memory. Further studies are required to fully understand the functional significance of altered polyamine system in learning and memory during aging. It is of interest to emphasize that the spermidine/spermine ratio displays a consistent trend of increase with age irrespective of strain, and that lowering this ratio by chronic DFMO treatment has an impact on spatial learning and memory in the reference memory version of the water maze. These findings suggest increased spermidine/spermine ratio with age may be one of the compensatory mechanisms against age-related cognitive decline. Given the modulatory role of spermidine in learning and memory and its potential as a new longevity drug, future research will explore the therapeutic potential for age-associated cognitive decline by targeting polyamines.