Dopamine modulates excitability of spiny neurons in the avian basal ganglia

Abstract

The neural substrate of vocal learning in songbirds is an accessible system for studying motor learning and motor control in vertebrates. In the so-called song system, the anterior forebrain pathway (AFP), which is essential for song learning, resembles the mammalian basal ganglia-thalamocortical loop in its macroscopic organization, neuronal intrinsic properties, and microcircuitry. Area X, the first station in the AFP, and the surrounding lobus parolfactorius (LPO), are both parts of the avian basal ganglia. Like their mammalian counterparts, they receive dense dopaminergic innervation from the midbrain, but the physiological functions of this projection remain unclear. In this study, we investigated the effect of dopamine (DA) on excitability of spiny neurons in area X and LPO. We recorded from neurons in brain slices of adult zebra finches and Bengalese finches, using whole-cell and perforated-patch recording techniques in current-clamp configuration. We found that DA modulates excitability in spiny neurons; activation of D1- and D2-like DA receptors enhances and reduces excitability, respectively. These effects are similar to those observed in the mammalian neostriatum, with the main difference being that D1-like DA receptor activation enhances excitability in avian spiny neurons at hyperpolarized states. Our findings also indicate that some spiny neurons express both receptor types and suggest that receptor colocalization in the entire population can account for the spectrum of DA actions. The diversity of DA actions enables the DA system to fine-tune the dynamics of the song system and allows flexible control over song learning and production.

Fig. 1.

The song system and an example of a spiny neuron in area X. A, A simplified diagram of the oscine song system. The song system consists of three major pathways. The nucleus interfacialis (NIf) provides the auditory inputs to the song system. The motor pathway starts with nucleus HVc (used as a proper name). HVc projects to the robust nucleus of the archistriatum (RA), which innervates several brainstem nuclei for control of respiration and vocalization. The AFP starts with the projection from HVc to area X, a paleostriatal nucleus surrounded by lobus parolfactorius (LPO). Area X projects to the medial portion of the dorsolateral anterior thalamic nucleus (DLM), which sends its output to the lateral portion of the magnocellular nucleus of the anterior neostriatum (lMAN), which projects to RA. The AFP is essential for normal song learning but is not required for song production in adults. Area X and LPO receive dense dopaminergic inputs from the ventral area of Tsai (AVT). Thegray area represents the paleostriatal complex.B, Voltage responses of a spiny neuron in area X to current pulse injections. Current pulse intensities: +0.22, +0.15, +0.12, −0.02, −0.04, −0.12, and −0.14 nA. C, A montage of photomicrograph images of the neuron in B. The images were taken at different focal planes and locations. They were then aligned, using the soma and processes of the neuron as landmarks, onto a uniformly gray background. The cell has spiny dendrites and small soma (diameter ∼10 μm). Scale bar, 20 μm.

Fig. 2.

DA had opposite effects on two spiny neurons.A, DA (40 μm) reversibly reduced evoked firing in a spiny neuron. DA was applied twice sequentially. The number of spikes in response to a current pulse (+0.14 nA) was plotted.B_1–3, Example traces from the experiment shown in A. The numbers shown in A indicate the timing of each example trace. Resting membrane potential: −74 mV. C, DA (50 μm) increased evoked firing in a spiny neuron.D_1–3, Example traces from the experiment shown in C. Resting membrane potential: −74 mV. E, Distribution of the effects of DA on excitability in spiny neurons, shown as percentage change of evoked firing (n = 15).

Fig. 3.

The D1-like DA receptor agonist SKF-38393 (10 μm) reversibly enhanced excitability in a spiny neuron in area X. A, The number of spikes evoked by suprathreshold current pulses (+0.08 nA). B_1–3, Example traces from the experiment shown in A. The numbers shown in A indicate the timing of each example trace. Resting membrane potential: −74 mV.

Fig. 4.

Voltage dependence of the effect of SKF-38393 (n = 14). The percentage change in firing was plotted as a function of the baseline membrane potential of each cell measured before current pulses.

Fig. 5.

The D1-like DA receptor antagonist SCH-23390 blocked the excitatory effect of SKF-38393 and DA. All data were collected from the same spiny neuron. A, SKF-38393 (10 μm) enhanced excitability. B, SCH-23390 (20 μm) blocked the effect of SKF-38393 (10 μm). C, DA (50 μm) enhanced excitability.D, In the presence of SCH-23390 (20 μm), DA (50 μm) reduced evoked firing.

Fig. 6.

The D2-like DA receptor agonist quinpirole (10 μm) significantly reduced evoked firing in a spiny neuron. A, The number of spikes evoked by suprathreshold current pulses (+0.10 nA). B_1–3, Example traces from the experiment shown in A. Thenumbers shown in A indicate the exact timing of each example trace. Resting membrane potential: −83 mV.

Fig. 7.

The effect of quinpirole is not voltage dependent (n = 16). The percentage change in firing is plotted as a function of the baseline membrane potential of each cell measured before current pulse injections. The data points connected by a line are from the same cell.

Fig. 8.

The D2-like DA receptor antagonist sulpiride (10 μm) blocked the effect of quinpirole (10 μm). A, the number of spikes evoked by suprathreshold current pulse injections (+0.10 nA).B_1–3, Example traces from the experiment shown in A. The numbers shown in A indicate the timing of each example trace. Resting membrane potential: −88 mV.

Fig. 9.

Distribution of the effects on cell excitability of DA (n = 15) (○, cells obtained with perforated-patch technique), SKF-38393 (n = 14), quinpirole (n = 16) (▵, cells obtained with perforated-patch technique), DA in the presence of SCH-23390 (n = 2), DA in the presence of sulpiride (n = 4), SKF-38393 in the presence of SCH-23390 (n = 4), and quinpirole in the presence of sulpiride (n = 4). A horizontal lineindicates the median value for each distribution. Dashed horizontal line indicates 0%. The distribution of the effect of DA is identical to that shown in Figure 2E. For statistical analysis, see Results.

Fig. 10.

The slope of the initial ramp is modulated by quinpirole, SKF-38393, and TTX. A, The percentage change in the slope of the initial ramp as a function of the percentage change in evoked firing induced by quinpirole (n = 15) (median value, −28.6%; the slope was not measured in one cell held at −62 mV, which fired action potentials with very short latency). Solid line, Linear regression,r² = 0.738; dotted line, 95% confidence interval of the regression line.Horizontal dashed line indicates 0%. Data points in shaded box were obtained in the presence of 1 μm TTX (n = 7; median value −12.5%). B, The percentage change in the slope of the initial ramp as a function of the percentage change in evoked firing induced by SKF-38393 (n = 14; median value 3.3%). Solid line, Linear regression,r² = 0.531; dotted line, 95% confidence interval of the regression line. Note: the rightmost data point was included for all analyses except the linear regression. Horizontal dashed line indicates 0%.Data points in shaded box were obtained in the presence of 1 μm TTX (n = 5; median value −1.0%). C, Example traces from a spiny neuron. The baseline voltage response (thin line) had a steeper ramp before action potential and an earlier onset of the first spike, compared with the response in the presence of quinpirole (thick line). Membrane potential: −65 mV.D, TTX reduced the slope of the ramp but did not block the quinpirole-induced decrease. Example traces from another spiny neuron. Quinpirole (10 μm) was applied in the presence of 1 μm TTX. Membrane potential: −88 mV. The control trace (PRE) is shifted by −1.9 mV to facilitate comparison. The TTX and QUIN traces shown are averages of three traces each.

Fig. 11.

Proposed model to account for the spectrum of DA actions on excitability in avian spiny neurons in area X and LPO. The effect of DA on excitability in a given spiny neuron depends on its relative expression of D1- and D2-like DA receptors. DA reduces evoked firing in cells where D2-like DA receptors dominate and enhances firing in cells where D1-like DA receptors dominate. DA has no apparent effect in cells with no DA receptors at all (not shown) or in cells with equal influence by the two receptor types.