ACCELERATION OF AXONAL OUTGROWTH IN MOTOR AXONS FROM MATURE AND OLD F344 RATS AFTER A CONDITIONING LESION

The conditioning lesion paradigm has proven to be a very useful model in which to examine the mechanisms of axonal outgrowth after injury. I n the present study, we have used the conditioning lesion model to exa mine the ability of motor axons from mature (6-8 months) and old (22-2 4 months) Fischer 344 rats to form new axonal sprouts. We show that af ter a single lesion (sham-conditioned axons followed by a testing lesi on), axonal outgrowth rates are slower at earlier vs longer postlesion times in mature rats: between 4 and 8 days postlesion, outgrowth rate s are 2.4 +/- 0.4 mm/day, whereas between 8 and 11 days postlesion out growth rates are 4.6 +/- 0.7 mm/day. After a single lesion in the old rat, at early postlesion times, the axonal outgrowth rate is 1.9 +/- 0 .4 mm/day but with increasing time after injury, outgrowth rates slow down to 1.1 +/- 0.8 mm/day. In conditioned motor axons from mature rat s, outgrowth rates increase from 3.1 +/- 0.4 mm/day at early postlesio n times to 5.2 +/- 0.6 mm/day at longer postlesion times. An even more dramatic increase in outgrowth rate is seen in conditioned axons from old rats: 2.4 +/- 0.4 mm/day at early postlesion times tc, 6.3 +/- 1. 0 mm/day at later times after lesion. There is no change in the initia l delay before sprouting under any condition. These data support the h ypothesis that axons from old animals can be stimulated to repair them selves at rates comparable to those seen in younger animals and sugges t that there may be an absolute maximum outgrowth rate attainable by n ewly forming axon sprouts. (C) 1998 Academic Press.