Behavioral/Systems/Cognitive Spikes and Bursts in Two Types of Thalamic Projection Neurons Differentially Shape Sleep Patterns and Auditory Responses in a Songbird

Inmammals, the thalamusplays important roles for cortical processing, suchas relayof sensory informationand inductionof rhythmical firing during sleep. In neurons of the avian cerebrum, in analogy with cortical up and down states, complex patterns of regular-spiking anddense-burstingmodes are frequently observed during sleep.However, the roles of thalamic inputs for shaping these firingmodes are largely unknown.A suspectedkeyplayer is the avian thalamicnucleusuvaeformis (Uva).Uva is innervatedbypolysensory input, receives indirect cerebral feedback via the midbrain, and projects to the cerebrum via two distinct pathways. Using pharmacological manipula-tion, electrical stimulation, and extracellular recordings of Uva projection neurons, we study the involvement of Uva in zebra finches for thegenerationof spontaneousactivity andauditory responses inpremotor areaHVC(usedas apropername) and thedownstreamrobust nucleus of the arcopallium (RA). In awake and sleeping birds, we find that single Uva spikes suppress and spike bursts enhance sponta-neous and auditory-evoked bursts in HVC and RA neurons. Strong burst suppression is mediated mainly via tonically firing HVC-projecting Uva neurons, whereas a fast burst drive is mediated indirectly via Uva neurons projecting to the nucleus interface of the nidopallium. Our results reveal that cerebral sleep-burst epochs and arousal-related burst suppression are both shaped by sophisticated polysynaptic thalamic mechanisms