Studies on Selective Neuroregeneration in Lower Vertebrates

The specificity with which terminal axons of the peripheral and central nervous systems functionally innervate foreign fields was studied in frog and goldfish. The effects of competitive reinnervation by left and right cutaneous nerves were carried out in frog in an effort to test for the existence of left-right biochemical differentiation of neural tissue. The dorsal cutaneous nerves were cut, transposed and allowed to regenerate under different conditions of denervation and competition and the resultant reinnervation and reflex patterns were determined by behavioral and electrophysiological mapping techniques. There was no indication that growth patterns and functional reconnections were influenced by the specific laterality of the fibers. In goldfish, the course of functional recovery of vision during the compression of the retinotectal projection that follows hemitectal ablations was investigated with fish maintained postsurgically in diurnal and continuous-light environments. Behavioral and electrophysiological mapping methods indicated that vision was restored throughout the temporal half-field, originally blinded by the tectal lesion regardless of the postoperative lighting conditions. The scotoma diminished in an orderly anteriorposterior fashion and color discriminability was concommitant with visual recovery. Functional topographic reorganization of the retinotectal projection implies that locus-specific affinities between retinal and tectal neurons, which may play a prominent role in the direction of nerve growth and formation of synapses under more normal circumstances, are not permanently fixed even in mature goldfish. The results of the present investigations indicate that factors such as availability of terminal sites and the tendency of fibers to seek terminal connections seem to override any qualifications imposed by the existence of lateral or locus specificity on the formation of terminal connections. The return of color vision in goldfish, however, indicates prespecification of retinal and tectal cells for color, and selective reconnection of neurons according to their specificities which survived the compensatory developmental pressures created by the retinotectal size disparity.