Early testosterone exposure during development advances myelination and affects neurogenesis of the vocal control motor path in male zebra finches ( <i>Taeniopygia guttata</i> )

Birdsong learning and production is mediated through a vocal control circuit that exhibits ubiquitous and profound sex differences. In zebra finches (Taeniopygia guttata) only males sing, and sex differences in both neuroanatomy and myelination of this circuit emerge during the first 3 months of life as song learning is taking place. Song crystallization occurs at the onset of sexual maturity, at a time when neuron recruitment to the vocal control region HVC is reduced and the myelination of the projection from HVC to the motor nucleus RA (robust nucleus of arcopallium) rapidly develops. Prior work demonstrated that experimental testosterone treatment early in song development disrupted song learning, potentially by leading to premature song crystallization, but the effects of testosterone on neurogenesis and myelination of the vocal control system are little studied. We implanted male zebra finches with blank or testosterone pellets around Day 35 at the onset of the sensorimotor phase of song development. We examined the effects of early testosterone treatment on song development, myelination, and neurogenesis (doublecortin labeled cells) of the motor path of the vocal control circuit (HVC to RA). We also quantified singing consistency at two ages in adulthood. Testosterone treatment accelerated changes in myelin within HVC and in the projection from HVC to RA and accelerated age-related changes in doublecortin-labeled cells in HVC. Song and syllable stereotypy increased with age, but we did not detect an effect of hormone treatment. These results are consistent with the hypothesis that the testosterone exposure during development initiates processes that normally occur at sexual maturity, including changes in neurogenesis and myelination of the motor control path of the vocal control system.