CONNECTIVITY OF FETAL NEOCORTICAL BLOCK TRANSPLANTS IN THE EXCITOTOXICALLY ABLATED CORTEX OF ADULT-RATS

Fetal neocortical block grafts placed into newborn recipients are able to exchange axonal projections with the host central nervous system, as shown in several previous experiments. The present study examined t he connectivity of fetal neocortical block transplants placed into the excitotoxically ablated cortex of adult rats. Young adult rats receiv ed injections of the excitotoxic amino acid N-methyl-D-aspartate into the sensorimotor cortex area 1 week prior to receiving a fetal (E14-15 ) neocortical transplant. Afferent and efferent connections of these g rafts were examined 3-6 months after transplantation by injecting the transplants with the fluorescent retrograde tracers fast blue and diam idino yellow or with the anterograde tracer Phaseolus vulgaris leucoag glutinin. Retrogradely labeled neurons were observed within several ho st brain regions including the ipsilateral neocortex, several thalamic nuclei, subcortical areas such as claustrum and lateral hypothalamus, nucleus basalis, dorsal raphe nuclei and locus coeruleus. Fibers labe led with Phaseolus vulgaris leucoagglutinin were found extending throu ghout the transplants, but with rare exceptions fibers were not observ ed within the host brain. The experiments showed that neocortical bloc k grafts placed into the excitotoxically ablated neocortex receive aff erent input from areas in the host brain that normally innervate the s ensorimotor cortex. The extensive Phaseolus vulgaris leucoagglutinin-p ositive axonal labeling found within the grafts demonstrated the abili ty of the grafted neurons to establish extensive intrinsic graft conne ctions. Their failure to project out of the grafts suggests that the m ature host brain does not provide a permissive environment for neurite extension.