Timing of brain-derived neurotrophic factor exposure affects life expectancy of new neurons

Abstract

The high vocal center (HVC) of adult male canaries, Serinus canaria, is necessary for the production of learned song. New neurons are added to HVC every day, where they replace older neurons that have died, but the length of their survival depends on the time of year when they are born. A great number of HVC neurons born in the fall, when adult canaries learn a new song, are still present 8 mo later, when this song is used during the breeding season. By contrast, most of the neurons born in HVC in the spring, when little song learning takes place, disappear much sooner. Here we show that infusion of brain-derived neurotrophic factor into HVC during days 14-20 after new HVC neurons are born in the spring confers on them a life expectancy comparable to that of fall-born neurons; this extension on life is not seen when infusion occurs 10 days earlier or later. We suggest that there is, in the adult HVC, a subset of neurons whose life expectancy is determined by brain-derived neurotrophic factor during a sensitive period soon after these neurons reach destination and start forming connections.

Fig. 1.

BDNF infusion into HVC. (a) Schematic diagram of the infusion system that delivered BDNF into HVC (yellow) as seen in a transverse section of canary brain; the plane of section is shown as a dotted blue line in the saggital representation of the brain, that also shows the position of two other song nuclei, the nucleus Robustus of the Archistriatum (RA) (green) and area X (pink) that receive projections from HVC. The bubble overlying the infusion needle represents the dental cement bonding the needle to the cannula and the cannula to the skull. The tubing used for drug infusion connects the needle to the Alzet pump. The cartoon shows the placement of the pump in the peritoneal cavity and its connection to the skull. Cresyl-stained (b) and Hu-stained (c) sections showing new neurons labeled with [³H]thymidine. (d) Protocol for infusion of BDNF into HVC during days 4-10, 14-20, or 24-30 and for the different survival times after the last [³H]thymidine injection.

Fig. 2.

The table shows the mean volume (±SD) of HVC in each of the experimental/survival groups. The underlying histograms show the distribution of ³H-labeled nuclear diameters of HVC neurons at 1-mo (A), 4-mo (B), and 8-mo (C) survival times after label injections.

Fig. 3.

Survival of new HVC neurons born in fall or spring; birds in these two groups received [³H]thymidine injections in October (black bars) or April (white bars), respectively, when ≈12- or 18-mo-old and were killed 0.5, 1, 4, or 8 mo after the last injection. Each group includes four to six birds, and the bars represent SD; ^*, P < 0.05; ^**, P < 0.01 for comparisons within a same survival time.

Fig. 4.

Effects of BDNF infusion on the longevity of new HVC neurons. (a) Percentage of new HVC neurons that survive up to 15 days after the last [³H]thymidine injection. There was no difference between untreated birds and birds that received BDNF during days 4-10. (b-d) Percentage of new HVC neurons at 1-, 4-, and 8-mo survivals, with BDNF infused on days 4-10, 14-20, or 24-30 after last [³H]thymidine injection. Notice that the life expectancy of the new neurons that received BDNF during days 14-20 is considerably higher than that of any of the other groups (error bars represent the SD).

Fig. 5.

Summary of results. Comparison of the timeline of survival of new HVC neurons born in untreated spring and fall birds and in birds that were treated at various times after [³H]thymidine injection with BDNF or saline. There is a striking resemblance in the survival profile of the spring neurons treated with BDNF 14-20 days after their birth and the new neurons of the untreated fall birds. The percentage of new neurons at 15-day survivals was established only for the three groups (asterisk) and assumed to be the same for the three others.

Fig. 6.

(a) Replicate in a different year of the 14-20 and 24-30 BDNF infusion groups at 1 mo and 4 mo after [³H]thymidine administration with Cresyl-violet staining. (b) Comparison of results obtained for new neuron percentage estimates with Cresyl-stained and Hu-stained material.