REDUCED LAMININ IMMUNOREACTIVITY IN THE BLOOD-VESSEL WALL OF AGING RATS CORRELATES WITH REDUCED INNERVATION IN-VIVO AND FOLLOWING TRANSPLANTATION

Changes in extracellular matrix composition and/or organisation, and i n particular in the ratio of axonal growth-promoting components such a s laminin to growth-inhibiting molecules, could contribute to the dege nerative changes observed in the innervation of some peripheral tissue s in old age. We have investigated this issue by evaluating laminin co ntent or accessibility at various locations on blood vessels where we had previously studied age-related alterations in innervation density. We have employed a morphological approach, measuring laminin immunore activity by a densitometric application of confocal microscopy, becaus e more conventional biochemical techniques would have been un able to distinguish specific, localized changes in laminin at sites accessible to nerves from heterogeneous changes in other areas of the vessel wal l, such as the endothelial basal lamina. We found that in 24-month-old rats laminin immunoreactivity is decreased by 50% at the medial-adven titial border in association with the outer layer of smooth muscle cel ls, where a parallel decrease is observed in innervation density. Axon al terminals were shown to have access to laminin in this region of th e blood vessel wall by double staining with laminin and a general neur onal marker. Changes in laminin immunoreactivity were region-specific on the same blood vessel, thus excluding the possibility of a generali zed decrease in immunoreactivity in old age. For example, in the basil ar artery intensity of laminin immunoreactivity decreased in old age a t the medial-adventitial border, but showed no change in endothelial c ell basal lamina and in the adventitia. Moreover, we performed in ocul o transplants of blood vessels displaying differences in laminin immun oreactivity and found that the density of innervation correlated with the intensity of laminin staining, thus lending further support to the hypothesis that laminin might play a role in nerve fibre atrophy in o ld age.