Protein synthesis inhibition in neocortical grafts evaluated by systemic amino acid uptake autoradiography

The temporal pattern of protein synthesis inhibition was examined in grafte d neocortical neurons using [H-3]valine in vivo autoradiography. Neuronal u ptake levels of systemically administered H-3-labeled amino acids which cro ss the blood-brain barrier (BBB) via endothelial cell neutral carriers have long been a hallmark in studies of experimental ischemic pathology; there is likely a strong correlation between persistent protein synthesis inhibit ion and the progression of cell damage. Because the grafting procedure invo lves the loss of blood flaw and the subsequent reperfusion of the donor tis sue there are, mechanistically, important similarities to reversible ischem ia models. The effects of ischemic injury on grafted CNS neurons are not fu lly understood. Quantitative analysis of grain distribution in individual g raft or control (adjacent host cortex) neurons indicated an initial breakdo wn of the amino acid barrier system, subsequent recovery, and progressive r eduction of amino acid uptake by 1 year. Up to 3 weeks after surgery grafts were flooded with the [H-3]valine tracer but individual neurons contained relatively few silver grains. After this time, the tracer was normally dist ributed within graft neurons but at significantly lower levels than in cont rols, Grain density gradually decreased over time such that 12-month grafte d neurons had approximately half that compared to control and only 58% of t hat in 2-month grafts; the 12-month levels were comparable to those observe d at early (10 days) postoperative times. Autoradiography of immunostained sections for MAP-2, SMI 311 (neurofilament marker), and neuron-specific eno lase showed reduced expression of these proteins in neurons coupled with we ak amino acid tracer uptake, The results further suggest that grafted neuro ns bear intriguing similarities to neurons placed at ischemic risk, particu larly "penumbral" neurons, which are affected by reduced blood flow and are metabolically weakened. The loss of Egg properties in early grafts may als o extend to the endothelial cell amino acid carrier system, and the delayed revascularization process could affect neuronal uptake mechanisms, (C) 200 0 Academic Press.