Dopamine Modulates Excitability of Spiny Neurons in the Avian Basal Ganglia

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.