Estrogens and non‐estrogenic ovarian influences combine to promote the recruitment and decrease the turnover of new neurons in the adult female canary brain

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Abstract
The higher vocal center (HVC) of the songbird forebrain exhibits persistent neurogenesis in adulthood, particularly in a region of the mediocaudal neostriatum that is associated with a subventricular layer of estrogen receptive cells. We asked whether estrogens might influence adult neurogenesis, by assessing the effect of ovariectomy on HVC neuronal production in the adult female canary. Fifteen 1‐year‐old females were separated into groups of ovariectomized, estradiol‐replaced ovariectomized, and gonadally intact birds. To label dividing cells and their progency, the birds were given [3H]thymidine for 8 days, killed 32 days later, and their brains autoradiographed. A significant rise was noted in the number of HVC neurons per section in estradiol‐treated birds relative to the untreated control birds. The number of [3H]‐thymidine‐labeled HVC neurons was also higher in the estrogen‐treated birds; however, the neuronal labeling index (LI) did not vary as a function of estradiol replacement, as the total number of HVC neurons rose in parallel with the added new neurons. In contrast, the neuronal LI did rise as a result of ovariectomy, and this ovariectomy‐associated increase in the LI was not reversed by estradiol. Among non‐neuronal cell types, the endothelial LI was higher in estrogen‐treated birds than in their untreated counterparts, suggesting estrogen‐associated angiogenesis. Radioimmunoassay confirmed that serum estradiol was reduced in the castrated birds. Since estrogen appeared to promote the survival of [3H]thymidine+ neurons, we next sought to determine whether estrogen acted directly on the newly generated neurons, or rather indirectly through an intermediary cell population. To this end, we asked whether the new neurons or their precursors expressed estrogen receptor immunoreactivity (ER‐IR). Five adult male canaries were given [3H]thymidine for periods ranging from 2 to 28 days, killed at varying times up to 3 weeks therafter, then probed for ER‐IR and autoradiographed. [3H]thymidine+ cells displayed no detectable ER‐IR within their first 4 weeks of postmitotic life. Rather, during migration from the ventricular zone (VZ), the new neurons traversed a layer of mitotically quiescent, ER+ subventricular cells. Double labeling for ER‐IR and cell‐type selective antigens confirmed that these ER+ cells were neurons. These results indicate that the early survival of new neurons in the adult songbird HVC is promoted by estrogen, and may be mediated by the estrogen‐stimulated paracrine release of neurotrophic agents by ER‐IR subventricular neurons. Our data suggest that estrogen's promotion of neuronal survival may operate concurrently with an estrogen‐independent ovarian suppression of neuronal mitogenesis.