Decrease in highly polysialylated neuronal cell adhesion molecules and in spatial learning during ageing are not correlated

Age-dependent spatial memory impairments have been related to a decline in hippocampal plasticity. Highly polysialylated neuronal cell adhesion molecules (PSA-NCAM) show a strong expression during adulthood within regions associated with neuroplastic events. Furthermore, NCAM molecules have been proposed to mediate neuronal plasticity during learning and memory. The aim of the present study was to examine the effect of ageing on the expression of PSA-NCAM within the hippocampus. To investigate whether age-dependent changes in expression of PSA-NCAM were accentuated in aged rats with learning impairment, animals were in a first step assessed for their cognitive abilities using a Morris water maze. Seven-month-old and 24-month-old rats were tested for their performance in the Morris water maze. The animals were sacrificed and brain sections were processed for PSA-NCAM immunohistochemistry. Ageing was accompanied by an overall decrease in PSA-NCAM-immunoreactivity (-IR) within the forebrain, presenting a important decrease of the number of PSA-NCAM-IR perikarya within the hippocampus. These results were confirmed by Western blot analysis. No difference in PSA-NCAM immunoreactivity was observed in aged rats with or without spatial learning impairment. It is concluded that although changes in PSA-NCAM accompanied the decrease in cognitive abilities, our data did not evidence a causal relationship between these two parameters.