A GABAergic, strongly inhibitory projection to a thalamic nucleus in the zebra finch song system

The anterior forebrain pathway (AFP) of the oscine song system is essential for song learning but not song production. Most cells recorded in this ser ially connected pathway show increased firing in response to song playback, suggesting largely excitatory connections among AFP nuclei. However, the n eurons forming a key projection in this pathway, from area X to the medial nucleus of the dorsolateral thalamus (DLM), express glutamic acid decarboxy lase in their somata and terminals, suggesting an inhibitory connection. To investigate the firing properties of DLM neurons and the functional influe nce of area X afferents in DLM, we made whole-cell recordings from DLM neur ons in brain slices from adult male zebra finches. Most cells had intrinsic properties closely resembling those of mammalian thalamocortical cells, in cluding a low-threshold Ca2+ spike and time-dependent, hyperpolarization-ac tivated inward rectification. Activation of afferents from area X evoked a strong, all-or-none IPSP whose amplitude and latency were unchanged by appl ication of glutamate antagonists,consistent with a monosynaptic contact. Th e 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 co mbine with the intrinsic properties of DLM neurons to allow signaling on di sinhibition. Our data are consistent with the hypothesis that the AFP corre sponds to the mammalian corticobasal ganglia-thalamocortical loop. The simi lar functional properties of avian and mammalian thalamic neurons suggest c onserved forebrain mechanisms of sensorimotor information processing across vertebrate taxa.