Slowly emerging glycinergic transmission enhances inhibition in the sound 1 localization pathway of the avian auditory system 2 3 Glycinergic transmission in the avian brainstem 4 5

32 Localization of low frequency acoustic stimuli is processed in dedicated neural 33 pathways where coincidence detecting neurons compare the arrival time of sound stimuli 34 at the two ears, or interaural time disparity (ITD). ITDs occur in the submillisecond 35 range and vertebrates have evolved specialized excitatory and inhibitory circuitry to 36 compute these differences. Glycinergic inhibition is a computationally significant and 37 prominent component of the mammalian ITD pathway. However, evidence for 38 glycinergic transmission is limited in birds where GABAergic inhibition has been 39 thought to be the dominant or exclusive inhibitory transmitter. Indeed, previous work 40 showed that GABA antagonists completely eliminate inhibition in avian nuclei 41 specialized for processing temporal features of sound, nucleus magnocellularis (NM) and 42 nucleus laminaris (NL). However, more recent work shows that glycine is co-expressed 43 with GABA in synaptic terminals apposed to neurons in both nuclei (Kuo et al. 2009; 44 Coleman et al. 2011). Here we show complementary evidence of functional glycine 45 receptor (GlyR) expression in NM and NL. Additionally, we show that glycinergic input 46 can be evoked under particular stimulus conditions. Stimulation at high but 47 physiologically relevant rates evokes a slowly emerging glycinergic response in NM and 48 NL that builds over the course of the stimulus. Glycinergic response magnitude was 49 stimulus rate dependent, representing 18% and 7% of the total inhibitory current in NM 50 and NL, respectively, at the end of the 50-pulse, 200Hz stimulus. Finally, we show that 51 the glycinergic component is functionally relevant as its elimination reduced inhibition of 52 discharges evoked by current injection into NM neurons. 53 54 Fischl et al. J Neurophys. 3 Introduction 55 Animals use differences in the arrival time of sound at each ear, or interaural time 56 disparities (ITDs), to compute the location of low frequency sound sources (Rayleigh 57 1907). In vertebrates, ITDs are computed by binaural coincidence detecting neurons in 58 the brainstem. Coincidence detecting neurons reside in nucleus laminaris (NL) in birds 59 (Parks and Rubel 1975; Sullivan and Konishi 1986; Carr and Konishi 1990; Pena et al. 6