Minimal connectivity between neostriatal transplants and the host brain

The present study sought to determine if axonal connectivity is established between neostriatal transplants and the host brain during the first two months of graft development. Cell suspensions of embryonic neostriatum were transplanted into the adult rat neostriatum lesioned previously by kainic acid. After 1-2 months, injections of horseradish peroxidase conjugated with wheat germ agglutinin (HRP) were made either within the graft, into adjacent host neostriatum or the host ventral midbrain. In animals with HRP injection sites restricted to the graft no retrograde or anterograde label was found in the host brain. However, both anterograde axon label and retrogradely labelled neurons were found in areas within the transplant but distal to the injection site. Neither ventral midbrain nor host neostriatal HRP injections resulted in any significant anterograde or retrograde label within the graft. These results demonstrate a lack of connectivity between neostriatal grafts and the host brain 1-2 months post-transplantation but an ability of grafted neurons to project to different locations within the transplant. Therefore, transplanted neostriatal neurons develop for the first two months in the absence of normal neostriatal afferent and efferent connections.