Androgens and estrogens synergistically regulate the expression of doublecortin and enhance neuronal recruitment in the song system of adult female canaries.

Vocal control nuclei in songbirds display seasonal changes in volume that are regulated by testosterone (T) and its androgenic (5α-dihydrotestosterone; DHT) or estrogenic metabolites (17β-estradiol; E2). In male canaries, T regulates expression of the microtubule-associated protein doublecortin (DCX), a marker of neurogenesis. We examined the effect of T and its two metabolites alone or in combination on DCX expression in adult female canaries. Treatment with T or with DHT+E2 increased HVC volume and neuron numbers as well as the total numbers of fusiform (migrating) and round (differentiating) DCX neurons in the nucleus but generally not in adjacent areas. DHT or E2 alone did not increase these measures but increased the density of fusiform DCX cells per section. Similar results were observed in Area X although some effects did not reach significance presumably because plasticity in X is mediated transynaptically and follows HVC changes with some delay. There was no effect of any treatment on the total number of neurons in Area X and no change in DCX cell densities was detected in other parts of the nidopallium nor in LMAN. DHT and E2 by themselves thus increase density of DCX cells migrating through HVC but are not sufficient in isolation to induce the recruitment of these newborn neurons in the nucleus. These effects are generally not observed in the rest of the nidopallium implying that steroids only act on the attraction and recruitment of new neurons in HVC without having any major effects on their production at the ventricle wall.