Role of GABAergic Inhibition in the Coding of Interaural Time Differences of Low-Frequency Sounds in the Inferior Colliculus

Abstract

. A major cue for the localization of sound in space is the interaural time difference (ITD). We examined the role of inhibition in the shaping of ITD responses in the inferior colliculus (IC) by iontophoretically ejecting ␥-aminobutyric acid (GABA) antagonists and GABA itself using a multibarrel pipette. The GABA antagonists block inhibition, whereas the applied GABA provides a constant level of inhibition. The effects on ITD responses were evaluated before, during and after the application of the drugs. If GABA-mediated inhibition is involved in shaping ITD tuning in IC neurons, then applying additional amounts of this inhibitory transmitter should alter ITD tuning. Indeed, for almost all neurons tested, applying GABA reduced the firing rate and consequently sharpened ITD tuning. Conversely, blocking GABA-mediated inhibition increased the activity of IC neurons, often reduced the signal-to-noise ratio and often broadened ITD tuning. Blocking GABA could also alter the shape of the ITD function and shift its peak suggesting that the role of inhibition is multifaceted. These effects indicate that GABAergic inhibition at the level of the IC is important for ITD coding.

Spatial tuning to sound-source azimuth in the inferior colliculus of unanesthetized rabbit

“…Binaural processing in the form of tuning to interaural time differences became sharper under barbiturate anesthesia in the IC of the rabbit . Furthermore, tuning to interaural time differences became broader when GABA was blocked and sharper when GABA was applied (D'Angelo et al 2005). This may explain why the median half-width of Delgutte et al (1999) (64°), who used barbiturate anesthesia, was less than one-half that of ours (143°) at comparable stimulus levels (10 -20 dB).…”

Monaural spectral processing differs between the lateral superior olive and the inferior colliculus: Physiological evidence for an acoustic chiasm

“…It is known that MSO units and low-frequency ICC neurons play an important role in the encoding of stimulus location based on interaural time differences (ITDs; Batra et al, 1997; Kuwada et al, 1987; Palmer et al, 1990; Yin and Kuwada, 1983a,b), and that ITD tuning curves are sharper in the ICC than in the MSO (Fitzpatrick et al, 1997; Spitzer and Semple, 1998). Evidence is equivocal for a role of GABAergic inhibition in sharpening ITD curves in the ICC (D'Angelo et al, 2005; Ingham and McAlpine, 2005); perhaps glycinergic inhibition from the low-frequency LSO plays this role. Consistent with this possibility, low-BF LSO units have been shown to exhibit ITD sensitivity in rabbits and rodents (Batra et al, 1997; Finlayson and Caspary, 1991).…”

Stimulus-specific adaptation in the inferior colliculus: The role of excitatory, inhibitory and modulatory inputs

“…The GABAergic-mediated inhibition acts on GABA A and GABA B receptors expressed across IC neurons. The pharmacological manipulation of the GABA A receptors significantly affects soundevoked responses (Faingold, Boersma Anderson, & Caspary, 1991;Yang, Pollak, & Resler, 1992), modifying different response properties including frequency tuning (Le Beau, Rees, & Malmierca, 1996;LeBeau, Malmierca, & Rees, 2001;Palombi & Caspary, 1996;Yang et al, 1992), response to sound intensity (Sivaramakrishnan, Sterbing-D'Angelo, Filipovic, D'Angelo, Oliver, & Kuwada, 2004), coding of interaural time and level differences (D'Angelo, Sterbing, Ostapoff, & Kuwada, 2005;Fujita & Konishi, 1991;Vater, Habbicht, Kossl, & Grothe, 1992) as well as IC responsiveness to binaural motion cues (McAlpine & Palmer, 2002).…”