Inhibition of Trk receptors before exercise blocks exercise-induced growth and synapsin I mRNA increase. (A)L4–5 DRGs were pretreated with K252A (gray columns) or CytC (white columns) before exercise as described in Materials and Methods. After a 3-day exercise period, L5 DRGs were cultured for 20 h. Mean neurite length is presented (error bars = 2 × SEM). The difference between CytC-exercised DRG vs. K252A-exercised DRG is statistically significant (**, P < 0.0001 by Student's t test). Although mean neurite length in the K252A exercised DRG is closer to that of the sedentary conditions, the difference between exercise and sedentary K252A-treated DRGs is statistically significant (*, P < 0.01 by Student's t test). (B and C) RNAs from the L5 DRGs of the sedentary and exercised animals from A were used as a template for real-time RT-PCR to evaluate levels of synapsin I (B) and GAP43 (C) mRNAs. Signals were normalized to GAPDH levels and are expressed as the relative to the CytC-injected, sedentary signals (error bars = SD for six animals per condition). Synapsin I showed statistically significant differences when comparing the exercise CytC with the exercise K252A samples (*, P < 0.01 by ANOVA). Differences in GAP43 mRNA levels between CytC and K252A exercise samples were not statistically significant (P > 0.05).