In the fully differentiated adult central nervous system (CNS) neuronal processes need to maintain a fine balance between stability and plasticity. In addition to being very plastic to adapt to the changing environmental stimulation, neuronal wiring diagrams must be also sufficiently stable to accomplish specific functional tasks on which they are tuned. In this dynamic status, the neuronal cytoskeleton and its components (actin filaments, neurofilaments and microtubules together with the respective associated proteins) are reported to play a major and critical role. MAP2 is selectively located at the somatodendritic compartment of neurones where it stabilizes polymerized tubulin and participates in the regulation of both microtubule spacing and actin filaments crosslinking, thus contributing significantly to the stabilization of dendritic processes. By quantitative immunohistochemical analysis, MAP2 levels and distribution were measured in the hippocampus and olfactory bulb of rats of different ages to seek age-related changes in dendritic structural dynamics and to test the reliability of MAP2 quantitative immunohistochemistry as a laboratory procedure to estimate the actual CNS plastic condition. Immunocytochemical localization of MAP2 was performed by the avidin-biotin peroxidase complex method. Optical density of MAP2 immunoreactivity (OD), the ratio between the MAP2 stained area/total test area (area fraction: AF), the total length of MAP2 labeled profiles (TL), the ratio between the perimeter/area of the immunostained profiles (pleomorphism index: PI) were the parameters measured in 15 fields/animal yielding a total area of about 300 mm2. In the dentate gyrus molecular layer and CA1 stratum radiatum as well as in the granular layer of the olfactory bulb of old rats OD and AF values were significantly lower than in young and adult rats, on the contrary the PI value that was significantly higher. TL was significantly deceased in the olfactory bulb of old rats, while in both the hippocampal areas showed a not significant reduction. Taken together, the present findings show that dendritic structural dynamics are significantly impaired in aging. Considering that both the hippocampus and the olfactory bulb are reported to be very plastic zones of the CNS, i.e. they are capable of consistent rearrangements during the individual's lifespan, these data lend further support to the many converging results on the higher vulnerability to aging of the CNS areas featuring higher plasticity. Although the present findings should be compared with data from studies conducted in CNS zones reported to be less plastic and less sensitive to age-related changes before proposing a tenable conclusion, the marked changes (e.g. the several fold decrease of OD and AF values) of all the parameters measured in the present investigation support that MAP2 immunohistochemistry may serve as a reliable method to test the actual plastic status of discrete CNS zones in different experimental conditions.
Dendritic structural dynamics