Direct evidence for loss and replacement of projection neurons in adult canary brain.

Normally occurring projection neuron loss and replacement were quantified over a 6 month period in the pathway from the high vocal center (HVC) to the robust nucleus of the archistriatum (RA) in adult male canaries. Fluorescent latex microspheres were injected into RA in April--a procedure resulting in long-term retrograde labeling of RA-projecting HVC neurons. Labeled cell densities were then obtained 4 and 20 d later in April and 195 d later in October. We found that 41-49% of the RA-projecting HVC neurons present the previous April were no longer present in October. Fluorogold injections in RA 3 d prior to death in April and October retrogradely labeled similar overall densities of RA-projecting HVC neurons, indicating that cells lost over this 6 month period were replaced by new RA-projecting HVC neurons. Newer cells were larger than older cells, suggesting that an age-dependent reduction in size might precede death. Over the same time interval, no loss was observed for neurons projecting from the lateral magnocellular nucleus of the anterior neostriatum to RA. Thus, loss was specific to the input from HVC to RA. These findings raise the possibility that much if not all of the pathway from HVC to RA is replaced within a year. The time period examined encompasses the yearly transition from stable song to song learning in the canary (Nottebohm et al., 1986, 1987). A pronounced loss and replacement of neurons implicated in vocal control during this period may relate to the canary's ability to modify song in adulthood.