DEVELOPMENT OF FUNCTIONAL TOPOGRAPHY IN THE CORTICORUBRAL PROJECTION - AN IN-VIVO ASSESSMENT USING SYNAPTIC POTENTIALS RECORDED FROM FETAL AND NEWBORN CATS

In mammals, topographic maps emerge from initially diffuse projections during development. To gain insight into the mechanisms governing the transition from a diffuse projection to a topographic map, we studied topographic specificity of functional connections during development, using the cat corticorubral system as a model. In the adult cat, rubr ospinal neurons in the dorsomedial part of the red nucleus (RN) receiv e input primarily from the forelimb area of the sensorimotor cortex, w hereas those in the ventrolateral part receive input primarily from th e hindlimb area. During development, axons from the sensorimotor corte x arrive in the RN at embryonic day 50 (E50) (Song et al., 1995a) and are diffusely distributed in the RN until postnatal day 13 (P13) (Higa shi et al., 1990). Here, we studied the development of the pattern of functional cortical inputs to individual rubrospinal neurons, using sy naptic potentials recorded in vivo. The functional topography in each rubrospinal neuron in developing cats was examined and classified eith er as adult-like or nonadult-like by comparison with the adult pattern . In preterm kittens from E61 to E65, only about half of the recorded neurons (41%; n = 22) showed adult-like functional topography. This pe rcentage, however, increased to 82% (n = 56) in P1-P8 kittens and to 9 3% (n = 42) in P13-P28 kittens. These results, in conjunction with the above mentioned anatomical observations, suggest that corticorubral a xons make functional synapses nonselectively with rubrospinal neurons before birth. Furthermore, the functional topographic map developed ea rlier than the anatomical map (< P8 vs > P13), suggesting that there i s a developmental step of selective promotion of synapse formation and /or selective enhancement of synaptic efficacy in topographically appr opriate regions in the RN, before the emergence of the mature anatomic al map.