Intracerebral microdialysis of awake rats was used to monitor the poss
ible release of neurotrophic factors from brain cells in response to i
njury and excitation. Perfusates were tested with ganglia bioassays an
d enzyme immunoassay. Trophic activity was released after implantation
of the microdialysis probe into the hippocampus but not into the stri
atum, as assessed by increased nerve fiber outgrowth from Remak's gang
lion. Kainic acid treatment significantly increased the release of tro
phic activity from hippocampal sites. These findings suggest that the
brain responds to mechanical injury as well as to certain excitatory s
timuli by regional extracellular release of neurotrophic activity that
is not identical to the actions of known neurotrophic factors.