Testosterone metabolism in the avian hypothalamus

Area of Science:

• Neuroendocrinology
• Comparative Endocrinology
• Behavioral Neuroscience

Background:

• Testosterone (T) exerts central effects through metabolites like 5 alpha-dihydrotestosterone (5 alpha-DHT) and estradiol (E2).
• Avian brains metabolize T into 5 beta-DHT, a metabolite with unclear reproductive roles.
• Understanding steroid metabolism in the brain is crucial for comprehending T's diverse actions.

Purpose of the Study:

• To analyze the distribution, sex differences, and steroid regulation of key T-metabolizing enzymes (aromatase, 5 alpha-reductase, 5 beta-reductase) in the Japanese quail and zebra finch brains.
• To investigate differential regulation of these enzymes between the hypothalamus and steroid-sensitive telencephalic nuclei involved in vocalization (HVc, RA).
• To localize aromatase-immunoreactive neurons and assess the subcellular localization of aromatase.

Main Methods:

• In vitro product-formation assays to measure enzyme activities.
• Analysis of enzyme activity in gonadally intact and gonadectomized birds, with and without T treatment.
• Immunocytochemistry (ICC) to localize aromatase-expressing neurons and electron microscopy to examine subcellular localization.

Main Results:

• Hypothalamic aromatase activity is higher in males, suppressed by castration, and induced by T. 5 alpha-reductase activity is not sexually differentiated or T-dependent. 5 beta-reductase activity is higher in females but normalized after gonadectomy.
• In zebra finch vocalization nuclei (HVc, RA), aromatase activity is higher in females and unaffected by castration or T. 5 alpha-reductase is higher in males, and 5 beta-reductase is higher in females, with both showing sex differences independent of gonadectomy.
• Aromatase-immunoreactive neurons are localized in the preoptic area and hypothalamus, with immunoreactive material found in synaptic boutons, suggesting a direct role for locally produced estrogens.

Conclusions:

• Steroid metabolism enzyme regulation differs significantly between the hypothalamus and vocalization control nuclei in songbirds.
• Hypothalamic enzyme regulation is largely dependent on gonadal steroids and T, while telencephalic nuclei exhibit organizational sex differences potentially established by neonatal steroids.
• The presence of aromatase in synaptic boutons indicates a local action of estrogen within specific neural circuits, impacting brain function and behavior.