Expression of functional TrkB by MM: binding and internalization of ligand. (A) Binding of [¹²⁵I]-BDNF. Dilutions of unlabeled BDNF were assayed for their ability to displace [¹²⁵I]-BDNF from JJN3 HMCL. Each condition was assessed in triplicate. Results were corrected for nonspecific binding, determined in parallel incubations with 500-fold excess BDNF, and are presented as the mean ± standard deviation. The K_d for binding of [¹²⁵I]-BDNF to JJN3 was determined as 0.1 nM, using the equation of Cheng and Prusoff.²⁶  (B) Internalization of [¹²⁵I]-BDNF. JJN3 HMCL was incubated with 0.1 nM [¹²⁵I]-BDNF at either 4°C (♦) or 37°C (▪). Internalization was determined as counts remaining after acid stripping of surface-bound BDNF. Each condition was assessed in triplicate, and the results are presented as the mean ± standard deviation.