Reduced laminin immunoreactivity in the blood vessel wall of ageing rats correlates with reduced innervation in vivo and following transplantation.

Changes in extracellular matrix composition and/or organization, and in particular in the ratio of axonal growth-promoting components such as laminin to growth-inhibiting molecules, could contribute to the degenerative changes observed in the innervation of some peripheral tissues in old age. We have investigated this issue by evaluating laminin content 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 immunoreactivity by a densitometric application of confocal microscopy, because more conventional biochemical techniques would have been unable to distinguish specific, localized changes in laminin at sites accessible to nerves from heterogeneous changes in other areas of the vessel wall, such as the endothelial basal lamina. We found that in 24-month-old rats laminin immunoreactivity is decreased by 50% at the medial-adventitial border in association with the outer layer of smooth muscle cells, where a parallel decrease is observed in innervation density. Axonal terminals were shown to have access to laminin in this region of the blood vessel wall by double staining with laminin and a general neuronal marker. Changes in laminin immunoreactivity were region-specific on the same blood vessel, thus excluding the possibility of a generalized decrease in immunoreactivity in old age. For example, in the basilar artery intensity of laminin immunoreactivity decreased in old age at the medial-adventitial border, but showed no change in endothelial cell basal lamina and in the adventitia. Moreover, we performed in oculo transplants of blood vessels displaying differences in laminin immunoreactivity 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 old age.