Neuronal Tracer Injections

Once the mapping procedures were completed, the animal was put back on isoflurane anesthesia (∼2% in 100% O₂) for the injection of 4 neuronal tracers all within the M1 hand area. Based on previous results (Dancause, Barbay, Frost, Plautz, Popescu, et al. 2006), the M1 hand representation was arbitrarily subdivided into 4 quadrants (RM, RL, CM, and CL). One of 4 neuroanatomical tracers [biotinylated dextran amine (BDA; 3000 MW; Invitrogen), Fluoro-ruby (FR; 3000 MW; Invitrogen), Fast blue (FB; Dr Illig Plastics GmbH), or Fluoro-emerald (FE; 3000 MW; Invitrogen)] was injected in each quadrant. We alternated the tracer injected in the 4 quadrants of the M1 hand area among the 3 monkeys to minimize any bias that may be caused by the properties of the tracers. A 1-µL Hamilton syringe coupled to a microsyringe pump controller (Harvard Apparatus, Holliston, MA, USA) was used for all injections. To label the cortex through all layers of the gray matter, injections were made in 3 boluses at pre-determined depths (1800, 1500, and 1200 µm), creating a column labeling all layers of the gray matter. Following each bolus of injection, the syringe was kept in place for 5 min to reduce potential backflow. The amounts of tracer injected at each injection site are provided in Table 1.

Neuroanatomical tracers efficacy

BDA, biotinylated dextran amine; FB, Fast blue; FE, Fluoro-emerald; FR, Fluoro-ruby.