Introduction to the chemical neuroanatomy of birdsong

Dopamine physiology in the basal ganglia of male zebra finches during social stimulation

Accumulating evidence suggests that dopamine (DA) is involved in altering neural activity and gene expression in a zebra finch cortical–basal ganglia circuit specialized for singing, upon the shift between solitary singing and singing as a part of courtship. Our objective here was to sample changes in the extracellular concentrations of DA in Area X of adult and juvenile birds, to test the hypothesis that DA levels would change similarly during presentation of a socially salient stimulus in both age groups. We used microdialysis to sample the extracellular milieu of Area X in awake, behaving adult and juvenile male zebra finches, and analysed the dialysate using high-performance liquid chromatography coupled with electrochemical detection. The extracellular levels of DA in Area X increased significantly during both female presentation to adult males and tutor presentation to juvenile males. DA levels were not correlated with the time spent singing. We also reverse-dialysed Area X with pharmacologic agents that act either on DA systems directly or on norepinephrine, and found that all of these agents significantly increased DA levels (3- to 10-fold) in Area X. These findings suggest that changes in extracellular DA levels can be stimulated similarly by very different social contexts (courtship and interaction with tutor), and influenced potently by dopaminergic and noradrenergic drugs. These results raise the possibility that the arousal level or attentional state of the subject (rather than singing behavior) is the common feature eliciting changes in extracellular DA concentration.

A Bird's Eye View of Sleep-Dependent Memory Consolidation

How new experiences are solidified into long-lasting memories is a central question in the study of brain and behavior. One of the most intriguing discoveries in memory research is that brain activity during sleep helps to transform newly learned information and skills into robust memories. Though the first experimental work linking sleep and memory was conducted 90 years ago by Jenkins and Dallenbach, the case for sleep-dependent memory consolidation has only garnered strong support in the last decade. Recent studies in humans provide extensive behavioral, imaging, and polysomnographic data supporting sleep consolidation of a broad range of memory tasks. Likewise, studies in a few animal model systems have elucidated potential mechanisms contributing to sleep consolidation such as neural reactivation and synaptic homeostasis. Here, we present an overview of sleep-dependent memory consolidation, focusing on how investigations of sleep and learning in birds have complemented the progress made in mammalian systems by emphasizing a strong connection between behavior and physiology. We begin by describing the behavioral approach that has been utilized to demonstrate sleep consolidation in humans. We then address neural reactivation in the rodent hippocampal system as a putative mechanism of sleep consolidation. Next, we discuss the role of sleep in the learning and maintenance of song in zebra finches. We note that while both the rodent and zebra finch systems provide evidence for sleep-dependent memory changes in physiology and behavior, neither duplicates the pattern of changes most commonly observed in humans. Finally, we present a recently developed model of sleep consolidation involving auditory classification learning in European starlings , which has the potential to connect behavioral evidence of sleep consolidation as developed in humans with underlying neural mechanisms observable in animals.

Behavioral state-dependent reconfiguration of song-related network activity and cholinergic systems

The song system of oscine songbirds mediates multiple complex perceptive and productive behaviors. These discrete behaviors are modulated according to external variables such as social context, directed attention and other forms of experience. In addition, sleep has been implicated in song learning and song maintenance. Changes in behavioral state are associated with complex changes in auditory responsiveness and tonic/bursting properties of song system neurons. Cholinergic input, principally from the basal forebrain has been implicated in some of these state-dependent properties. Cholinergic modulation may affect numerous song system nuclei, with in vivo and in vitro studies indicating that a major target of cholinergic input is the forebrain nucleus HVC. Within HVC, a muscarinic cholinergic system has strong regulatory effects on most neurons, and may serve to couple and uncouple circuitry within HVC projecting along the premotor pathway with circuitry within HVC projecting along the cortico-basal ganglia pathway. These observations begin to describe how neuromodulatory regulation in the song system may contribute to learning phenomena.