28 Neurogenesis in the Adult Songbird: A Model for Inducible Striatal Neuronal Addition

The damaged brain has traditionally been thought to exhibit little significant structural repair after injury. In part, this appears to reflect the failure of the mature forebrain to generate new neurons, except for a few discrete, relatively archaic regions of the brain, the hippocampus and olfactory bulb (OB) (Altman and Das 1966; Bayer et al. 1982; for review, see Goldman 1998; Alvarez-Buylla and Garcia-Verdugo 2002; Gage 2002). The limitation on neuronal addition to the adult brain has clearly been selected, and thus comprises an adaptation of likely, if unclear, evolutionary benefit. Among other possibilities, the lack of persistent neurogenesis in most regions of the adult mammalian brain may be associated with the need to stabilize the retention of long-term memories and entrained behaviors (Rakic 2002). Perhaps as a result, the adult mammalian neocortex exhibits no constitutive neuronal addition, and little or none after injury, except for discrete experimental lesions of defined neuronal populations (Magavi et al. 2000). In contrast, the subcortical neostriatum retains the capacity to regenerate neurons after stroke and major traumatic injury (Arvidsson et al. 2002; Parent et al. 2002; Jin et al. 2003a; Parent 2003). However, the numbers of striatal neurons generated in response to stroke have thus far been described as comprising only a small fraction of the population lost to the ischemic insult and have not yet been demonstrated to contribute to functional recovery. In general terms, the lack of compensatory neuronal replacement in most adult brain regions has impeded not only the recovery of...