Differential modulation of monoamine levels and turnover rates by estrogen and/or androgen in hypothalamic and vocal control nuclei of male zebra finches

Sexual behaviors, including singing, in male zebra finches are under the combined control of androgens and estrogens. Treating castrates with androstenedione (AE), a hormone which provides both androgenic and estrogenic metabolites, restores catecholamine (CA; i.e., norepinephrine (NE) and dopamine (DA)) function in brain areas known to be involved in the control of these behaviors. Since these behaviors can only be activated by the combined action of androgens and estrogens, the present study determined whether estrogen alone, androgen alone, or only their combination would restore CA function in these brain areas. Males were castrated for at least three weeks and then received one of three hormone implants: (1) estradiol (E) alone, (2) dihydrotestosterone (DHT) alone, or (3) E + DHT. Each male was then housed with a female for at least one week. Levels of NE, DA, epinephrine, serotonin and the latter's primary metabolite, 5-hydroxyindoleacetic acid, were measured in 3 hypothalamic, one auditory and 6 vocal control areas in their brains. NE and DA turnover in these areas were also estimated using α-methylparatyrosine. CA function proved to be hormone sensitive in 64% of the comparisons in the three hypothalamic and six vocal control nuclei examined. As expected, the treatment which provided both estrogenic + androgenic stimulation (E + DHT) restored CA function in all of these cases. Although E + DHT restored CA function in these areas, only 22% required both estrogenic + androgenic 3timulation to restore normal CA function. For 57% of the comparisons, treatment with E alone also restored normal function, while in the remaining 22%, DHT alone was sufficient. All hormone-induced changes in noradrenergic neurotransmission were estrogen dependent; while modulation of DA function was much more variable, and frequently androgen dependent. To the best of our knowledge, this is the first time that these types of functional differences in the role of estrogen versus androgen in the modulation of neurotransmitter systems has been demonstrated.