Physiological actions of dopamine in an avian basal ganglia nucleus essential for vocal learning

Vocal behavior in songbirds exemplifies a rich integration of motor, cognitive and social functions that are shared among vertebrates. As a part of the underlying neural substrate, the song system, the anterior forebrain pathway (AFP) is required for song learning and maintenance. The AFP resembles the mammalian basal ganglia-thalamo-cortical loop in its macroscopic organization, neuronal intrinsic properties and microcircuitry. Area X, the first station in the AFP, is a part of the basal ganglia essential for vocal learning. It receives glutamatergic inputs from pallial structures and sends gamma-aminobutyric acid (GABA)ergic outputs to thalamic structures. It also receives dense dopaminergic innervation from the midbrain. The role of this innervation is largely unknown. Here we provide evidence that DA is well placed to modulate the neural activity in area X. With a focus on the principle cell type in area X, the spiny neuron, my thesis research shows that DA modulates excitability of spiny neurons and their glutamatergic inputs in slice preparations. More specifically, activation of D1- and D2-like DA receptors enhances and reduces excitability, respectively. Activation of D1-like DA receptors induces presynaptic depression of the glutamatergic inputs. In addition, we demonstrate, for the first time, the presence of activity-dependent synaptic plasticity in the AFP of both juvenile and adult birds. Long-term potentiation can be induced at the glutamatergic synapses onto spiny neurons in area X. The potentiation also requires activation of D1-like DA receptors. These results indicate that DA can influence information processing in area X and may exert fine control over vocal learning.