Long-range GABAergic projection in a circuit essential for vocal learning

The anterior forebrain pathway (AFP) in the passerine song system is essential for song learning but not for song production. Several lines of evidence suggest that area X, a major nucleus in the AFP, forms part of the avian striatum. A key feature of striatal projection neurons is that they use the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). Some area X neurons express GABA-like immunoreactivity, but the neurotransmitter phenotype of the projection neurons is largely unknown. To determine whether area X projection neurons are GABAergic, we used immunocytochemistry and confocal microscopy to examine whether these neurons in adult male zebra finches express the GABA synthetic enzyme glutamic acid decarboxylase (GAD). We observed numerous large and small GAD+ somata in area X, and dense GAD+ terminals, but no GAD+ somata in the target of area X, the medial nucleus of the dorsolateral thalamus (DLM). The density of GAD+ terminals in DLM was strongly reduced by ibotenic acid lesions of area X. After tracer injection into the DLM, all of the retrogradely labeled neurons in area X were GAD+. After tracer injection into area X, the vast majority of anterogradely labeled terminals in DLM were GAD+. We conclude that area X neurons projecting to DLM express GAD and are thus likely GABAergic. If this projection is indeed inhibitory, information processing in the AFP is substantially more complicated than previously realized. Moreover, because a GABAergic projection to a thalamic target is reminiscent of pallidal rather than of striatal circuitry, area X may contain both striatal and pallidal components.