Anatomical plasticity in the adult zebra finch song system

Abstract

In many songbirds, vocal learning-related cellular plasticity was thought to end following a developmental critical period. However, mounting evidence in one such species, the zebra finch, suggests that forms of plasticity common during song learning continue well into adulthood, including a reliance on auditory feedback for song maintenance. This reliance wanes with increasing age, in tandem with age-related increases in fine motor control. We investigated age-related morphological changes in the adult zebra finch song system by focusing on two cortical projection neuron types that a) share a common efferent target, b) are known to exhibit morphological and functional change during song learning, and c) exert opposing influences on song acoustic structure. Neurons in HVC (proper name) and the lateral magnocellular nucleus of the anterior nidopallium (LMAN) both project to the robust nucleus of the arcopallium (RA). During juvenile song learning and adult song maintenance, HVC promotes song syllable stereotypy while LMAN promotes learning and acoustic variability. Following retrograde labeling of these two cell types in adults, there were age-related increases in dendritic arbor in HVC-RA but not LMAN-RA neurons, resulting in an increase in the ratio of HVC-RA:LMAN-RA dendritic arbor. Differential growth of HVC relative to LMAN dendrites may relate to increases in song motor refinement, decreases in the reliance of song on auditory feedback, or both. Despite this differential growth with age, we also show that both cell types retain the capacity for experience-dependent growth. These results may provide insights on mechanisms that promote and constrain adult vocal plasticity.

Region-specific neuron recruitment in the hippocampus of brown-headed cowbirds Molothrus ater (Passeriformes: Icteridae)

“…In cowbirds, the species-and sex-dependent differences in spatial memory may be related to dispersion of existing neurons or new neuron recruitment. Examples include changes in dendritic arborization of existing neurons, addition of glial cells, and modification in the distance between neuron somata as well as new cell proliferation (Tronel et al 2010;McDonald & Kirn 2012). These mechanisms could possibly contribute to renovating the hippocampus in such a way that it can manage the high demand for spatial memory required by the female cowbirds, a generalist brood parasite that discovers nest locations of roughly 200 different host species (Davies 2000).…”

“…Structural modification of pre-existing Hp neurons, such as changes in soma volume or dendritic arborization, as well as neuron death and replacement may occur in the Hp (van Praag et al 2005;Leuner & Gould 2010;Tronel et al 2010;McDonald & Kirn 2012;Garthe & Kempermann 2013). While it is not clear whether new neuron recruitment in avian Hp can result in net growth as it does in some studies of mammals (Bayer et al 1982;Crespo et al 1986), recruitment clearly makes up for cell loss in the avian Hp (Kirn & Nottebohm 1993;Scharff 2000), and the rostral sub-region of the Hp exhibits distinct neuron replacement patterns (Barnea & Nottebohm 1994).…”

Genetic regulation of sex differences in songbirds and lizards

“…Area X and the lateral magnocellular nucleus of the anterior nidopallium (LMAN) are components of the anterior forebrain pathway and are more involved in song learning and plasticity [7,8]. The vast majority of research on mechanisms regulating sexual differentiation of the song circuit has been conducted in zebra finches.…”

Determinants and significance of corticosterone regulation in the songbird brain

“…They stand out for their extensive neuroplasticity that can be retained throughout adult life (Kirn, 2010; McDonald & Kirn, 2012; Tramontin and Brenowitz, 2000). Interestingly, many of the neural attributes that make songbirds unique are influenced by sex steroid hormones (Ball et al, 2002; Brenowitz, 2013; Brenowitz, 2014; Schlinger and Brenowitz, 2009; Schlinger and Soma et al, 2004; Tramontin et al, 2003).…”