6 d22 suggested that nt 6 was less potent that ngf in stimulating neurite outgrowth of chick drg this finding is indeed consistent with the results obtained in two separate studies gotz et al 1992 1994 which suggested that nt 6 was much less potent than xiphophorus ngf in promoting the survival of drg the concentration of xiphophorus ngf causing half maximal activity was 6 compared to 100 ng ml of nt 6 it was possible that in our experiments the concentration of the neurotrophin in cos cell supernatant was below the half maximal value of nt 6 but not ngf thus allowing for detectable response to ngf but not nt 6 in the drg nonetheless nt 6 d22 did exhibit the ability to induce weak trka phosphorylation neurotrophins mediate their actions via phosphorylation of the trk receptors in our studies the receptor specificity of nt 7 was determined by phosphorylation of chick trks ectopically expressed in fibroblasts as expected from the structural similarity to ngf nt 7 nt 7 d15 and nt 6 d22 specifically phosphorylated trka however the phosphorylation was considerably weaker than that induced by xiphophorus ngf the weaker phosphorylation of trka by nt 6 d22 or ngf nt 6 d22 was consistent with our hypothesis that nt 6 was less potent than xiphophorus ngf in activating drg neurons however nt 7 and nt 7 d15 despite their ability to activate drg in both neurite outgrowth and survival assay could only phosphorylate trka weakly.