Transient expression and transport of brain-derived neurotrophic factor in the male zebra finch's song system during vocal development

Abstract

The distribution of brain-derived neurotrophic factor (BDNF) in the song system of male zebra finches changes with posthatching age. At day 20, the hyperstriatum ventrale, pars caudale is the only song nucleus in which neurons showed BDNF immunoreactivity. At day 45, the staining in hyperstriatum ventrale, pars caudale was denser than at day 20 and the robust nucleus of the archistriatum, another song nucleus, showed BDNF labeling. By day 65, two additional song nuclei, area X and the lateral magnocellular nucleus of the anterior neostriatum, have become immunoreactive. In the adult, however, the amount of BDNF labeling in all of these brain nuclei is sharply reduced. These sequential events, the anatomical connections between these song nuclei, and the labeling of relevant axons and terminals suggest anterograde transport of BDNF. Furthermore, the timing of BDNF expression coincident with the development of singing behavior suggests that this neurotrophin may be directly involved with the differentiation of the song system.

Figure 1

Schematic diagram of the adult male zebra finch song system. The HVc projects to both RA and area X. Area X sends its afferents to DLM, which connects to LMAN. The LMAN makes afferent connections to RA, which sends some of its descending projections to the nXIIts that innervates the syringeal muscles used in singing. Anterior is to the right, dorsal side is up.

Figure 2

Sagittal sections of the caudal telencephalon stained for BDNF. A 20-day-old male has many BDNF containing cells, but little staining is seen in RA (Upper Left). By 45 days, BDNF staining is pronounced in HVc cells and neuropil, some of the axon bundles that connect HVc and RA, and in the RA neuropil only (Upper Right). In a 65-day-old male, BDNF staining within HVc is still heavy, but is mostly confined to the neuropil. The RA is also heavily labeled at this age (Lower Left). In the adult, however, BDNF immunostaining within these song nuclei is sharply reduced (Lower Right). (Bar = 500 μm.)

Figure 3

BDNF immunostaining of the male zebra finch HVc at different ages. A 20-day-old male already has many BDNF-containing neurons within HVc (Upper Left). At 45 days, staining specifically within HVc is very pronounced to both the cells and neuropil (Upper Right). Also seen at this age are several stained axon bundles that connect HVc to RA (arrows). A 65-day-old bird still shows heavy BDNF staining, but the labeling is largely confined to the neuropil (Lower Left). An adult bird shows little BDNF labeling (Lower Right). (Bar = 200 μm.)

Figure 4

BDNF immunostaining of LMAN and area X in a male zebra finch at different ages. A 20-day-old male shows little BDNF staining in either LMAN or area X (dotted outlines, Upper Left). At 45 days (Upper Right), the level of immunostaining within these areas is still very low, but is rapidly and specifically increased in the 65-day-old bird (Lower Left). The BDNF staining is much lower in the adult LMAN and area X (Lower Right), when compared with the 65-day-old male. Anterior is to the right, dorsal side is up. (Bar = 500 μm.)

Figure 5

The different immunostaining patterns within HVc and area X of the developing male zebra finch. At 20 days of age, HVc contains many BDNF stained neurons and fine processes (Upper Left). At 65 days, however, only a few HVc cell bodies are labeled, but most of the staining shifts to the neuropil and/or extracellular matrix surrounding the unstained cells (asterisk, Upper Right). Also at 65 days, BDNF labeling in area X is rapidly increased and appears clustered around unlabeled cell bodies (asterisk, Lower Left). In the adult, however, there are many widely spaced neurons in area X that contain BDNF surrounded by many small knob-like objects (Lower Right). (Bar = 20 μm.)

Figure 6

Quantification of the SMOD of BDNF in various song system nuclei as a function of age. In HVc, the moderate SMOD of a 20-day-old male is attributable to the many stained cells within the nucleus (Upper Left). The SMOD peaks in HVc at around 45 days of age. The other song system areas of RA, LMAN, and area X show peak levels of BDNF SMOD at a much later time of ≈65 days of age. The SMOD values for all of these song system areas drops sharply in the adult male (Lower Right).

Figure 7

Representation of an idealized timeline correlating the posthatching age with the relative stages of vocal development and the corresponding changes in BDNF expression in the male zebra finch song system.