Trends in Neurosciences
ReviewSeasonal plasticity in the adult brain
Section snippets
The volumes of song control nuclei change seasonally
Song is a learned behavior used by birds to attract mates and to defend breeding territories21. In oscine passerines (songbirds), song behavior is controlled by a network of discrete interconnected brain regions or nuclei (Fig. 1). Song behavior and the brain regions that control it are seasonally plastic in every species of seasonally breeding adult songbird that has been systematically studied36, 37, 38, 39. The entire volumes of several song nuclei, including the neostriatal region HVc
Neuron number changes seasonally in HVc
Seasonal changes in the morphology of HVc were first demonstrated in male canaries by Nottebohm. He used Nissl-stained tissue to show that HVc volume3 and neuron number60 were twice as large during the spring than during autumn (Table 3). The reliability of Nissl criteria to define the borders of song nuclei, especially HVc, was subsequently called into question (see Box 1). A wide series of studies have since shown, however, that the Nissl-defined borders of HVc coincide with those defined by
Dendritic growth and synaptogenesis in RA
The cellular basis of the volumetric growth of RA differs from that observed in HVc. Neuron number does not change seasonally in RA, but neuron size and spacing in this nucleus are both greater during the breeding season37, 39, 44, 49, 61, 62, 75. The increase in neuronal spacing is associated with a vernal increase in the dendritic arborizations of RA neurons60. Synaptic morphology also varies seasonally in RA, such that the sizes of presynaptic and postsynaptic profiles are largest during the
Testosterone mediates seasonal changes in the song system
Several lines of evidence strongly suggest that testosterone (or its active metabolites) is the primary physiological cue that mediates the cyclical anatomical changes in the song circuitry. Several song nuclei, including HVc, RA, the medial and lateral portions of the magnocellular nucleus of the anterior neostriatum (mMAN and lMAN), the tracheosyringeal portion of the hypoglossal nucleus (nXIIts), and the dorsolateral nucleus of the medial thalamus (DLM) contain gonadal steroid receptors79, 80
Non-steroidal cues contribute to seasonal plasticity
The seasonal plasticity in the song control system is not governed solely by gonadal steroids. Studies from two laboratories have indicated that manipulations of day length can induce changes in the song nuclei of castrated animals. Long days increased the volume of HVc slightly in castrated white-crowned sparrows44 and starlings95. Another study suggested that photostimulation increased the volumes of HVc, RA and area X in castrated American tree sparrows (Spizella arborea)46. These
Why does the structure of the song system change seasonally? An hypothesis
The sustained peak performance of a seasonally predictable behavioral task is often preceded by hypertrophy of the organs or tissues, that support that task101. For example, the size of the gonads and other reproductive structures increases dramatically in preparation for the annual breeding season and these organs regress when the breeding season is terminated47. Long-distance migration in birds is preceded by a host of physiological changes including elevated fat stores and increased flight
Concluding remarks and prospects for the future
Seasonal plasticity of the song system can serve as a model to address the following questions, which are of fundamental importance to the study of neural plasticity in general.
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What are the mechanisms that mediate hormone action in the adult brain? Does hormone metabolism contribute to adult brain changes? How does trans-synaptic support from afferent and efferent targets support or influence, hormone action in a given brain region?
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What growth factors and trophic agents influence adult brain
Acknowledgements
The authors’ research is supported by the Virginia Merrill Bloedel Hearing Research Center, grant MH53032 from the NIH, and grant DGE-9616736AM02 from the NSF.
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