Effectiveness of the GnRH agonist Deslorelin as a tool to decrease levels of circulating testosterone in zebra finches

- Testosterone was decreased within weeks after implant of the GnRH agonist Deslorelin.
- 5 mg/kg dose decreased testosterone without affecting health, testes or heart.
- Decreased testosterone did not affect performance of learned vocal behavior.
- Reduction of testosterone was reversible within 2 weeks following implant removal.

Songbirds are widely used in studies of the neurobiology underlying learning, memory and performance of the sounds used in vocal communication. Development and activity of neurons in many brain sites implicated in those behaviors are closely related to levels of circulating testosterone. Approaches to understand the effects of testosterone in songbirds are presently limited to testosterone implants, which elevate testosterone levels to supraphysiological values, or castration, which eliminates gonadal production of testosterone. Previous studies in mammals indicate that GnRH agonists may be an effective tool to reduce testosterone within that range of extremes and without invasive surgery. To evaluate the effectiveness of the GnRH agonist Deslorelin as a tool to modulate levels of testosterone in songbirds, we recorded the effects of Deslorelin in adult male zebra finches. We recorded songs, body mass and blood testosterone levels pre-treatment, then we gave each bird a small subcutaneous implant of Deslorelin. We measured blood plasma testosterone levels weekly and recorded song behavior and gross morphology of brain, testes and heart at the end of each experiment. Testosterone levels were reduced at the 5 mg/kg dose, and the very slight song changes we observed at that dose were like those reported for castrated zebra finches. As expected, there were no changes in the number of cells in androgen-sensitive brain structures. Suppression of testosterone at the 5 mg/kg dose was reversible through implant removal. Thus, Deslorelin is a new tool to transiently suppress testosterone levels without the invasiveness and undesirable aftereffects of surgical castration.