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

Abstract

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.

Keywords: behavior; juvenile; microdialysis; monoamine; striatum.

Figure 1

(A) Schematic representation of important song-related circuits in the zebra finch brain, including catecholaminergic projections (in purple and blue) to the anterior forebrain pathway (AFP), in red. HVC, letter-based proper name; RA, robust nucleus of the arcopallium; nXIIts, cranial nerve XII; LC, locus coeruleus; VTA/SNc, ventral tegmental area/substantia nigra pars compacta; DLM, dorsolateral nucleus of the medial thalamus; LMAN, lateral magnocellular nucleus of the anterior nidopallium. (B) Photomicrograph of representative coronal section of male zebra finch brain, illustrating location of microdialysis probe. Note that placement of probe avoids damage to LMAN. Image obtained on a Nikon Eclipse E800 microscope, with 2× objective and 10× ocular, captured with Zeiss AxioCamMRc5 and Axiovision 4.4 software. Scale bar = 1 mm. (C) Representative chromatogram for a single microdialysis sample collected from Area X of an adult zebra finch. Signal amplitude (in volts) is plotted on the y-axis and retention time (in minutes) is plotted on the x-axis. NE and DA peaks are designated with arrows. A clear separation from interfering peaks is apparent for both NE and DA.

Figure 2

Social interaction is associated with increases in extracellular dopamine levels. (A) Representative experimental time course (top panel) for an individual adult male zebra finch, illustrating extracellular DA levels (on the y-axis) at 15-min intervals throughout the course of one experiment, with histogram (bottom panel) illustrating amount of singing (in seconds, on the y-axis) produced during the same time intervals. Different experimental conditions [baseline state (‘alone’), social stimulation (‘female’) and post-stimulation (‘alone-post’)] are indicated on the x-axis. (B) Bar graph illustrating change in extracellular DA levels in adult male zebra finch Area X during courtship behavior. Group (n = 5) mean DA levels are expressed as normalized values on the y-axis, and the experimental conditions (as described in A) are indicated on the x-axis. The individual values of the five data points composing the group means are plotted within the bars for each condition. Error bars indicate SEM. (C) Representative experimental time course for an individual juvenile male zebra finch, illustrating extracellular DA levels at 10-min intervals throughout the course of one experiment, with histogram illustrating amount of singing (in seconds) produced by both the juvenile (black bars) and the adult tutor (gray bars) zebra finch. Different experimental conditions [baseline state (‘alone’), social stimulation (‘tutor’) and post-stimulation (‘alone-post’)] are indicated on the x-axis. Because of the duration of the experiment, every other interval is plotted. (D) Bar graph illustrating change in extracellular DA levels in juvenile male zebra finch Area X during observation of tutor. Group (n = 6) mean DA levels are expressed as normalized values on the y-axis, and the experimental conditions are indicated on the x-axis. The individual values of the six data points composing the group means are plotted within the bars for each condition. Error bars indicate SEM.

Figure 3

Extracellular DA levels can be pharmacologically manipulated by drugs that target dopaminergic and noradrenergic synapses. (A) Bar graph illustrating the effects of pharmacologic agents on Area X DA levels. The GABAergic agent baclofen (0.1 μL of a 0.1 μg/μL solution) was infused into VTA, and the dopaminergic [amphetamine (‘Amph’, 10 μm) and nomifensine (‘Nomi’, 10 μm)] and noradrenergic [reboxetine (‘Rebox’, 250 μm)] agents were reverse-dialysed into Area X. Bars represent normalized group mean DA values, and error bars indicate SEM. n = 3 for each pharmacologic agent. The individual values of the three data points composing the group means are plotted within the bars for each condition. The effect of reboxetine on NE is demonstrated with the inset, where bars represent normalized group mean DA values on the left and normalized group mean NE values on the right. Error bars indicate SEM (n = 3). (B) Representative experimental time course for an individual zebra finch, illustrating extracellular DA levels at 10-min intervals throughout the course of one experiment as exogenous NE (500 nm, indicated by the black box) was reverse-dialysed into Area X. (C) Bar graph illustrating the mean change in extracellular DA in Area X that occurred in the presence of NE (n = 3). Bars represent normalized group mean DA levels (y-axis) at baseline, in the first 50 min (‘early phase’), and last 50 min (‘late phase’) of NE reverse dialysis, as indicated on the x-axis. Error bars indicate SEM. The individual values of the three data points composing the group means are plotted within the bars for each condition.