The specific estrogen receptor antagonist ICI 182,780 masculinizes development of the zebra finch syrinx

Intramuscular antagonism of the G-protein coupled estrogen receptor 1 partially affects dimorphic characteristics of the syrinx, but is ineffective within the neural song circuit of zebra finches

Within the zebra finch song system, robust sex differences exist that enable singing behavior in males, but not females. Estradiol is a potent contributor to this process, but how and through which receptor(s) it acts is not clear. Historically, pharmacological manipulations of nuclear estrogen receptors have yielded conflicting results possibly due to method of drug delivery. More recently, the membrane bound G-protein coupled estrogen receptor 1 (GPER1) has also been identified as a potential candidate, but its function has not been fully described. To further investigate the role of GPER1, and the importance of the route of drug administration, a specific antagonist (G-15) was intramuscularly administered to zebra finches for 25 days, starting on the day of hatching. G-15 significantly decreased muscle fiber sizes of ventralis and dorsalis in the syrinx of males only. Dimorphic characteristics of the neural song system were unaffected by this manipulation in either sex. These results contrast with a study in which G-15 was intracranially delivered. In males, select song nuclei were decreased in volume, and in females, syrinx muscle fiber size was increased. Together, these results support the hypothesis that estrogens acting through GPER1 influence dimorphic development of the song system, and that method of drug administration is important in this species.

Intracranial administration of the G-protein coupled estrogen receptor 1 antagonist, G-15, selectively affects dimorphic characteristics of the song system in zebra finches (Taeniopygia guttata)

In zebra finches (Taeniopygia guttata), estradiol contributes to sexual differentiation of the song system but the receptor(s) underlying its action are not exactly known. Whereas mRNA and/or protein for nuclear estrogen receptors ERα and ERβ are minimally expressed, G-protein coupled estrogen receptor 1 (GPER1) has a much greater distribution within neural song regions and the syrinx. At present, however, it is unclear if this receptor contributes to dimorphic development of the song system. To test this, the specific GPER1 antagonist, G-15, was intracranially administered to zebra finches for 25 days beginning on the day of hatching. In males, G-15 significantly decreased nuclear volumes of HVC and Area X. It also decreased the muscle fiber sizes of ventralis and dorsalis in the syrinx. In females, G-15 had no effect on measures within the brain, but did increase fiber sizes of both muscle groups. In sum, these data suggest that GPER1 can have selective and opposing influences on dimorphisms within the song system, but since not all features were affected additional factors are likely involved. © 2018 Wiley Periodicals, Inc. Develop Neurobiol, 2018.

Treatment with the specific estrogen receptor antagonist ICI 182,780 demasculinizes neuron soma size in the developing zebra finch brain

In zebra finches, many features of the neural song system are more pronounced in males compared to females. The exact mechanism(s) responsible for these differences are unknown, but may involve steroid hormones. More specifically, estrogens are most effective in masculinizing the female brain. Attempts to prevent masculine development through various estrogen receptor antagonists have been relatively ineffective, possibly due to partial agonistic activity of the compounds tested. To further investigate the role of estrogens in dimorphic development we utilized a more potent estrogen receptor blocker, ICI 182,780. Animals were treated during the first 25 days post-hatching. Daily intracranial injections significantly decreased neuron soma size in RA and HVC of both sexes. A similar effect was noted in LMAN. Treatment also appeared to decrease the volume of several song control nuclei. Together, these data support the hypothesis that ICI is an effective estrogen receptor antagonist in the zebra finch brain and that estrogens may influence sexually dimorphic development of the zebra finch song circuit. However, reported inconsistencies about sex differences in estrogen exposure and/or utilization exist, suggesting that complete sexual differentiation and development likely involve additional factors.