Circannual Transitions in Gene Expression: Lessons from Seasonal Adaptations - Current Topics in Developmental Biology - page 248

1997 ). In the anterior forebrain pathway, which plays an important role in
song learning and development, HVC projects to striatal Area X, which then
projects to the medial portion of the dorsal lateral nucleus (DLM) of the thal-
amus. DLM projects to the lateral magnocellular nucleus of the anterior
nidopallium (lMAN), which then projects both back to Area X and also to
RA to influence the song production pathway. Extensive seasonal plasticity
has been reported in these song nuclei, with a significant increase in nuclei size
during the breeding season when the singing behavior occurs ( Nottebohm,
1981; Smith, 1996; Smith, Brenowitz, Beecher, & Wingfield, 1997; Smith,
Brenowitz, Wingfield, & Baptista, 2004 ; Fig. 9.3 A). This increase in size is

A Song control system

Spring Winter

HVC

RA

Area X

B Hibernation

IBA Torpor

Figure 9.3 Seasonal plasticity in the brain. (A) Illustration of the representative changes
in song nuclei volume in the brains of songbirds from spring, when courtship singing
occurs, to winter, when singing behavior ceases. Model is based on data from Smith
(1996) . (B) Illustration of the plasticity and remodeling that occurs during hibernation
in the ground squirrel. Dendrites regress and synapses dissociate during hypothermic
torpor, but reconnect upon return to normothermic interbout arousals (IBAs). Model is
based on data from several sources ( Magariños, McEwen, Saboureau, & Pevet, 2006;
Popov, Bocharova, & Bragin, 1992; von der Ohe, Darian-Smith, Garner, & Heller, 2006;
von der Ohe, Garner, Darian-Smith, & Heller, 2007 ). HVC, used as proper name; RA,
robust nucleus of the arcopallium; IBA, interbout arousal.