Estradiol modulates brainstem catecholaminergic cell groups and projections to the auditory forebrain in a female songbird

In songbirds, hearing conspecific song induces robust expression of the immediate early gene zenk in the auditory forebrain. This genomic response to song is well characterized in males and females of many species, and is highly selective for behaviorally relevant song. In white-throated sparrows, the selectivity of the zenk response requires breeding levels of estradiol; we previously showed that in non-breeding females with low levels of plasma estradiol, the zenk response to hearing song is no different than the response to hearing frequency-matched tones. Here, we investigated the role of brainstem catecholaminergic cells groups, which project to the forebrain, in estradiol-dependent selectivity. First, we hypothesized that estradiol treatment affects catecholaminergic innervation of the auditory forebrain as well as its possible sources in the brainstem. Immunohistochemical staining of tyrosine hydroxylase revealed that estradiol treatment significantly increased the density of catecholaminergic innervation of the auditory forebrain as well as the number of catecholaminergic cells in the locus coeruleus (A6) and the ventral tegmental area (A10), both of which are known to contain estrogen receptors in songbirds. Second, we hypothesized that during song perception, catecholaminergic cell groups of the brainstem actively participate in auditory selectivity via estrogen-dependent changes in activity. We found that hearing songs did not induce the expression of zenk, a putative marker of activity, within catecholaminergic neurons in any of the cell groups quantified. Together, our results suggest that estradiol induces changes in brainstem catecholaminergic cell groups that may play a neuromodulatory role in behavioral and auditory selectivity.