Electrophysiological properties of embryonic neocortex transplants replacing the primary visual cortex of adult rats

Solid pieces of the occipital neocortex derived from 17-day rat fetuses were placed in a cavity formed by complete unilateral aspiration of the primary visual cortex in adult rats. Vital labeling of the brain with bisbenzimide was used to differentiate grafts from the host brain tissue. 2 to 10 months after operation electrophysiological experiments were performed in which neuronal activity and field potentials in transplants were recorded in response to sensory and electrical stimulation of the host brain. This study shows that in a large portion of the transplants (14 out of 25): (1) the majority of neurons (183/270) are controlled by visual stimuli and many of them respond to electrical stimulation of the lateral geniculate body (53/62) and the homotopic sites of the contralateral neocortex (28/62); latencies of these responses are within the ranges typical of the normal visual cortex; (2) there is a topical representation of the visual field on the transplants; (3) receptive field sizes, the preference to stationary flashes or to moving visual stimuli and the temporal response pattern of the grafted neurons are similar to those of the primary visual cortex. However, the field potentials evoked visually were recorded only in part of the transplants (8/14) which revealed clear neuronal visual responses, and field potential depth profile differed from that in visual cortex in situ. The functional organization of the transplants remained unchanged throughout the long-time testing. Taken together, these results suggest that after primary visual cortex removal, fetal neocortex transplants may be able to replace functionally the damaged neural circuitries of the host brain.