RAPID ALTERATION OF THALAMOCORTICAL AXON MORPHOLOGY FOLLOWS PERIPHERAL DAMAGE IN THE NEONATAL RAT

The effect of day of birth (postnatal day 0; P0) infraorbital nerve se ction on the morphology of individual thalamocortical axons in rat som atosensory cortex was examined on P3. Thalamic fibers were labeled in fixed brains with the carbocyanine dye -dioctadecyl-3,3,3',3'-tetramet hylindocarbocyanine perchlorate, and individual photoconverted thalamo cortical fibers were reconstructed, In normal animals on P3, axon arbo r terminal formation within layer IV has commenced and terminal arbor width is comparable to that of a cortical ''barrel.'' After infraorbit al nerve section, the average width of thalamocortical terminal arbors is significantly greater than is the average arbor width of normal ra ts of the same age; however, neither the number of branches per termin al arbor nor total arbor length differs between groups, These observat ions suggest that the role of the periphery in guiding terminal arbor formation is exerted both very rapidly and at the level of the single thalamic axon, Further, these results indicate a close association bet ween individual axon terminal arbor morphology and pattern formation i n the rat somatosensory cortex.