NERVE GROWTH-FACTOR ENHANCES THE DENDRITIC ARBORIZATION OF SYMPATHETIC-GANGLION CELLS UNDERGOING ATROPHY IN AGED RATS

We have investigated whether dendritic growth can be induced from symp athetic neurons of aged rats by the application of exogenous nerve gro wth factor to their target tissues. A previous study showed that signi ficant dendritic atrophy (19%) occurs during ageing in the sympathetic neurons innervating the middle cerebral artery and that dendritic atr ophy correlated with loss of axon colaterals in the same population of neurons. Using retrograde tracing in conjunction with intracellular i njection of fixed tissue and confocal microscopy, we now demonstrate t hat infusion of nerve growth factor over the peripheral processes of t he same neurons from aged rats induces significant dendritic (45%) and cell body (60%) growth. However, not all aspects of the dendritic arb orization were affected. Primary dendrites and branch points were not altered by nerve growth factor. In contrast, nerve growth factor induc ed a significant increase in the number of primary dendrites and branc h points (100%) of neurons from young adults. Our results demonstrate that sympathetic neurons undergoing dendritic atrophy during ageing ca n exhibit significant dendritic growth in response to the in vivo infu sion of nerve growth factor, although the lack of regenerative respons e displayed by some parts of the dendritic tree leads us to believe th at these neurons also show signs of reduced plasticity.