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REPORT
1The Bionic Ear Institute, East Melbourne, Victoria; 2Department of Otolaryngology, The University of Melbourne, East Melbourne, Victoria; and 3School of Psychological Science, LaTrobe University, Bundoora, Victoria, Australia
Submitted 16 February 2005; accepted in final form 1 April 2005
| ABSTRACT |
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| INTRODUCTION |
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| METHODS |
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37.5°C by a thermostatically controlled heating pad. At the end of the recording session, the animal was intra-cardially perfused with 10% formalin. Brains were removed, postfixed in 10% formalin, and sectioned on a freezing sledge microtome (100 µm sections). Sections were stained for Nissl substance using thionin, and standard histological procedures were used to reconstruct and verify the location of electrode tracks and recorded units (Paolini et al. 2001
Once the animal was deeply anesthetized, it was placed in a stereotaxic frame, and a pinhole craniotomy (area:
4 mm2) was made using stereotaxic coordinates (Paxinos and Watson 1998
) and skull suture landmarks as guides. Recording micropipettes were inserted dorsoventrally into the left hemisphere and traversed the cerebral cortex and IC before encountering the lateral lemniscus and its dorsal and ventral nuclei.
Intracellular neural responses were recorded using quartz glass micropipettes (1.0 mm OD, 0.7 mm ID, Sutter Instruments, Novato, CA), filled with 1 M potassium acetate and with impedances ranging from 30 to 70 M
. The amplified (Axoclamp 2B amplifier, Axon Instruments, Union City, CA) and filtered signal (output bandwidth = 1 kHz) from the microelectrode was played through a speaker and displayed on a Tektronix 465 storage oscilloscope (Beaverton, OR). The microelectrode was advanced remotely, using a motorized micro-drive (MPC-100; Sutter Instruments), in small steps (2 µm) once the VNLL was encountered, but occasionally intracellular recordings were made from the IC (n = 14).
Stimuli presented to both ears were 50 ms in duration with 5 ms rise-fall times and a 500 ms repetition interval; the left and right transducer outputs were offset by 200 ms. Responsive neurons were detected using a "search" stimulus of 80 dB noise bursts. Intracellular impalements were signaled by a sudden and stable drop (>25 mV) in the DC level and the presence of synaptic or large action potentials (>15 mV). Intracellular recordings typically lasted 2 min (maximum:
30 min); however, some recordings lasted for shorter periods, and only responses to noise bursts were obtained. This limitation was due to the difficulty in maintaining an in vivo impalement at recording depths of 58.5 mm from the brain surface. A Maclab 4-s data-acquisition system (AD Instruments, Sydney, Australia) was used to store membrane potential records (traces) at a bandwidth of 20 or 40 kHz. Once impaled, the neuron's CF and rate-level function at CF were determined. Other data (e.g., binaural response properties) were also sometimes collected as part of a larger study; however, such data are not presented in this report.
Acoustic stimuli were synthesized digitally and generated by either Beyer DT48 transducers (Beyerdynamic, Farmingdale, NY) or Tucker-Davis Technologies (TDT, Gainesville, FL) EC1 electrostatic speakers in concert with a TDT ED1 speaker driver. All transducers were controlled by a TDT signal generator (TDT System 2) and coupled to the end of each hollow earbar. The department's PC based "Neurophysiology Laboratory System" (program by R. E. Millard) was used to control all parameters of acoustic stimulation and data collection. The acoustic system was calibrated using a Brüel and Kjær (B&K) measuring amplifier (type 2606, Brüel & Kjær, Naerum, Denmark). Beyer transducers were calibrated with a B&K 0.5-in condenser microphone, coupled to a small probe tube positioned within the ear bar tube
3 mm from the tympanic membrane. The TDT speakers were calibrated using a 0.25-in B&K condenser microphone inserted into a custom built acoustic coupler designed to simulate the rat's ear canal at the end of the hollow earbar (designed and built by R. E. Millard). Both these methods allowed acoustic input to be measured in dB sound pressure level (SPL; referenced to 20 µPa). The noise bandwidth generated by TDT EC1 speakers was measured with a Stanford Research Systems dynamic signal analyzer (SR785, Sunnyvale, CA) and found to be spectrally flat at 80 ± 10 dB between 20 Hz and 60 kHz. The Beyer transducers had a nominal bandwidth of 45 Hz to 50 kHz with most of the energy <30 kHz; there was a gradual roll-off between 530 kHz of 10 dB/octave.
| RESULTS AND DISCUSSION |
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8 ms (Fig. 1C). The inhibition was also broadly tuned to tonal frequency (Fig. 1B).
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The amplitude and time course of the fast inhibition described in the preceding text is similar to that observed within the MNTB, a structure that also receives calyceal inputs (Awatramani et al. 2004
). Awatramani and colleagues (2004)
proposed that fast inhibition may control the excitatory drive from a calyx. However, in the VNLL the calyx appears to trigger fast inhibition in a local circuit that produces neural delays within the VNLL and therefore its output to the IC. The ramifications of this, and the reason that it occurs in only a subset of VNLL cells, are unknown but there are several possible outcomes at the IC. VNLL neurons receiving onset inhibition showed either a sustained or onset response to noise (Fig. 2b, 2 and 3, respectively). The response pattern of these VNLL neurons will determine the pattern of inhibition in the IC. Delayed inhibition from VNLL cells with a sustained response may create phasic responses in the IC (Fig. 2c1), while delayed inhibition from VNLL onset cells may mediate pauser IC responses (Fig. 2c2).
The proposed VNLL circuitry predicts the presence of fast inhibition preceding excitation in the IC (Fig. 2d) because the inhibitory projection within the VNLL (Fig. 2, gray axon) is a collateral of an axon projecting to the IC. Note that the VNLL neuron (Fig. 2b) and the IC neuron (Fig. 2d) receive the same pattern of inputs, and therefore they would both be expected to exhibit similar onset inhibition. This prediction is supported by our intracellular recordings from IC neurons (Fig. 2d1) and previous reports (Adams 1997
; Carney and Yin 1989
; Kuwada et al. 1997
; Smith et al. 1993
). This inhibitory pathway to the IC is presumably broadly tuned and would provide a source of onset-evoked wideband inhibition to narrowly tuned IC neurons (Fig. 2d). In the ventral CN, a similar arrangement exists, is mediated by glycinergic interneurons with onset responses, and results in the delay of the first AP to off-CF tones (Paolini et al. 2004
). These descriptions of fast, onset inhibition at several levels of the auditory pathway suggest that it may be a widespread means of controlling first spike timing.
| GRANTS |
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| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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Address for reprint requests and other correspondence: A.G. Paolini, School of Psychological Science, La Trobe University, Bundoora, Victoria 3086, Australia (E-mail: a.paolini{at}latrobe.edu.au)
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