A GABAergic, Strongly Inhibitory Projection to a Thalamic Nucleus in the Zebra Finch Song System

Abstract

The anterior forebrain pathway (AFP) of the oscine song system is essential for song learning but not song production. Most cells recorded in this serially connected pathway show increased firing in response to song playback, suggesting largely excitatory connections among AFP nuclei. However, the neurons forming a key projection in this pathway, from area X to the medial nucleus of the dorsolateral thalamus (DLM), express glutamic acid decarboxylase in their somata and terminals, suggesting an inhibitory connection. To investigate the firing properties of DLM neurons and the functional influence of area X afferents in DLM, we made whole-cell recordings from DLM neurons in brain slices from adult male zebra finches. Most cells had intrinsic properties closely resembling those of mammalian thalamocortical cells, including a low-threshold Ca ²⁺ spike and time-dependent, hyperpolarization-activated inward rectification. Activation of afferents from area X evoked a strong, all-or-none IPSP whose amplitude and latency were unchanged by application of glutamate antagonists, consistent with a monosynaptic contact. The IPSP had a reversal potential near −70 mV and was blocked by the GABA _A receptor antagonist bicuculline methiodide. Post-inhibitory rebound firing occurred in DLM neurons with a delay near 50 msec. Strong inhibition can combine with the intrinsic properties of DLM neurons to allow signaling on disinhibition. Our data are consistent with the hypothesis that the AFP corresponds to the mammalian corticobasal ganglia–thalamocortical loop. The similar functional properties of avian and mammalian thalamic neurons suggest conserved forebrain mechanisms of sensorimotor information processing across vertebrate taxa.