Synaptic Physiology in the Cochlear Nucleus Angularis of the Chick

Abstract

. Nucleus angularis (NA), one of the two cochlear nuclei in birds, is important for processing sound intensity for localization and most likely has role in sound recognition and other auditory tasks. Because the synaptic properties of auditory nerve inputs to the cochlear nuclei are fundamental to the transformation of auditory information, we studied the properties of these synapses onto NA neurons using whole cell patch-clamp recordings from auditory brain stem slices from embryonic chickens (E16 -E20). We measured spontaneous excitatory postsynaptic currents (EPSCs), and evoked EPSCs and excitatory postsynaptic potentials (EPSPs) by using extracellular stimulation of the auditory nerve. These excitatory EPSCs were mediated by AMPA and N-methyl-D-aspartate (NMDA) receptors. The spontaneous EPSCs mediated by AMPA receptors had submillisecond decay kinetics (556 s at E19), comparable with those of other auditory brain stem areas. The spontaneous EPSCs increased in amplitude and became faster with developmental age. Evoked EPSC and EPSP amplitudes were graded with stimulus intensity. The average amplitude of the EPSC evoked by minimal stimulation was twice as large as the average spontaneous EPSC amplitude (ϳ110 vs. ϳ55 pA), suggesting that single fibers make multiple contacts onto each postsynaptic NA neuron. Because of their small size, minimal EPSPs were subthreshold, and we estimate at least three to five inputs were required to reach threshold. In contrast to the fast EPSCs, EPSPs in NA had a decay time constant of ϳ12.5 ms, which was heavily influenced by the membrane time constant. Thus NA neurons spatially and temporally integrate auditory information arriving from multiple auditory nerve afferents. I N T R O D U C T I O NThe synaptic properties of auditory nerve inputs onto their postsynaptic targets in the cochlear nuclei are fundamental to the transformation of auditory information. In birds, acoustic cues for sound localization are segregated into parallel streams: in vivo recordings showed that cochlear nucleus magnocellularis (NM) encodes timing cues, whereas cochlear nucleus angularis (NA) encodes intensity cues (Konishi et al. 1985;Sullivan and Konishi 1984;Takahashi et al. 1984). Recent work studying the morphology, physiology, and auditory responses in NA suggest that this nucleus is also important for encoding sound for nonlocalization tasks, such as sound recognition and discrimination (Köppl and Carr 2003;.At the brain stem level, the cellular and synaptic specializations that allow temporal coding of sound phase for the computation of interaural time differences are well understood (Carr et al. 2001;Trussell 1999). Studies of nucleus magnocellularis neurons have revealed a suite of anatomical and physiological specializations that enable the precise encoding of the temporal properties of auditory nerve inputs, including large, calyceal synapses, very fast AMPA receptor (AMPAR)-mediated synaptic currents, short membrane time constants, and fast synaptic potentials (Carr et al. 2001;Jhaveri an...

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Development of NMDA R1 expression in chicken auditory brainstem

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Target-specific regulation of presynaptic release properties at auditory nerve terminals in the avian cochlear nucleus

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