Adult Neurogenesis in Songbirds A Tale of Two Neurons

the HVC neurons that project to RA (HVC→RA) and those that project to X (HVC→X) are intermingled within HVC, they are quite distinct from each other in terms of neuro-genesis: HVC→RA neurons are primarily born during posthatch development, whereas HVC→X neurons are born during embryogenesis. Furthermore, HVC→RA HVC→X neurons are not. Thus, it would be advantageous to separately manipulate these two types of projection neurons in order to study the regulation of adult neurogenesis in HVC. Scharff et al. (2000) did just that, Neurogenesis has long been left off the list of tools available to the adult brain for plasticity. Reports of which let them examine the kind of cell death that leads to addition of new neurons in adult HVC. adult neurogenesis are increasing, however, with recent descriptions in macaques (Gould et al., 1999) and hu-To selectively lesion these projection neurons, Scharff et al. (2000) used a targeted photolysis technique (Madi-mans (Eriksson et al., 1998). In contrast, it has long been known that the avian telencephalon receives new son and Macklis, 1993). One group of HVC projection neurons was retrogradely labeled with a compound that, neurons throughout life. Adult neurogenesis has been enthusiastically studied in songbirds, which are espe-when photoactivated with a laser, resulted in apoptosis. They found that induced cell death actually resulted in cially fascinating given that new neurons are incorporated into circuits important for song behavior (reviewed increased incorporation of new neurons into HVC. This shows that cell death can trigger neuron addition, which by Alvarez-Buylla and Kirn, 1997). The high vocal center (HVC), a nucleus critical for song production, has regular had been previously suggested by an observation that cell death preceded new neuron addition (Kirn et al., neuronal addition in adult zebra finches. How these adults maintain their stereotyped song in the face of 1994). Specifically, Scharff et al. (2000) found that induced death of HVC→RA neurons in adult birds led ongoing neuron addition is a great mystery and indicates that the incorporation of these new neurons must be to an increase in new HVC→RA neurons. In contrast, induced death of HVC→X neurons did not result in in-tightly regulated. The paper by Scharff et al. (2000) in this issue of Neuron steps toward an understanding of creased neuron addition. In juvenile birds, however, this precise lesion resulted in incorporation of new HVC→RA the regulation of neuronal addition into a fully mature brain. Also, because …