|Evidence for "direct" and "indirect" pathways through the song system basal ganglia
|Year of Publication
|Farries MA, Ding L, Perkel DJ
|J Comp Neurol
|0021-9967 (Print)0021-9967 (Linking)
|Animals, Basal Ganglia/*physiology, Electric Stimulation, Efferent Pathways/*physiology, Electrodes, Implanted, Electrophysiology, Male, Finches/*physiology, Fluorescent Dyes, Glutamic Acid/physiology, Receptors, GABA-A/physiology, Receptors, N-Methyl-D-Aspartate/physiology, Terminology as Topic, Washington
Song learning in oscine birds relies on a circuit known as the "anterior forebrain pathway," which includes a specialized region of the avian basal ganglia. This region, area X, is embedded within a telencephalic structure considered homologous to the striatum, the input structure of the mammalian basal ganglia. Area X has many features in common with the mammalian striatum, yet has distinctive traits, including largely aspiny projection neurons that directly innervate the thalamus and a cell type that physiologically resembles neurons recorded in the mammalian globus pallidus. We have proposed that area X is a mixture of striatum and globus pallidus and has the same functional organization as circuits in the mammalian basal ganglia. Using electrophysiological and anatomical approaches, we found that area X contains a functional analog of the "direct" striatopallidothalamic pathway of mammals: axons of the striatal spiny neurons make close contacts on the somata and dendrites of pallidal cells. A subset of pallidal neurons project directly to the thalamus. Surprisingly, we found evidence that many pallidal cells may not project to the thalamus, but rather participate in a functional analog of the mammalian "indirect" pathway, which may oppose the effects of the direct pathway. Our results deepen our understanding of how information flows through area X and provide more support for the notion that song learning in oscines employs physiological mechanisms similar to basal ganglia-dependent forms of motor learning in mammals.