Behavioral consequences of unilateral inferior colliculus lesions in the rat.

This study was carried out to determine the behavioral sensitivity to sound of rats with unilateral lesions of inferior colliculus (IC) located ipsilateral or contralateral to the projection pathway from one ear. Absolute thresholds for the detection of a broad-band noise burst were compared for rats with a profound conductive hearing loss in one ear and a lesion placed either ipsilateral or contralateral to the normally functioning ear. The rats were trained to make withdrawal responses to avoid a shock when they detected the presence of a noise burst. Sound pressure level was systematically lowered to obtain psychophysical curves from which absolute thresholds could be determined. Complete lesions of the contralateral IC resulted in substantial elevations in absolute threshold relative to normal whereas equivalent lesions of the ipsilateral IC produced relatively little elevation. In neither case did unilateral destruction of the IC produce a total inability to respond to sound. Contralateral IC lesions that included the dorsal nucleus of the lateral lemniscus (DNLL) produced a significantly greater elevation in behavioral thresholds than complete lesions limited to the IC. The results indicate a predominance of the contralateral over the ipsilateral pathway to IC for maintaining normal thresholds. They also indicate that other pathways that bypass the IC are likely involved in detecting the presence of a sound.

Changes in projections to the inferior colliculus following early hearing loss in rats.

The purpose of this study was to investigate the effects of early hearing loss on the anatomy of the central auditory system, specifically, the ascending projections to the inferior colliculus (IC). We compared normal animals with animals deafened during early development by administration of amikacin, an ototoxic antibiotic that is known to destroy the hair cells in the inner ear. The amikacin was injected subcutaneously every day from postnatal days P7 to P16. A retrograde tract tracer, Fluoro-Gold (FG), was then injected unilaterally directly into the IC at either 4 weeks of age or 12 weeks of age. After axonal transport the animals were sacrificed and their brains were prepared for histology. The FG labeled neurons in the cochlear nucleus (CN) and the dorsal nucleus of lateral lemniscus (DNLL) were counted for each of the animals in the two age groups. For deaf animals sacrificed at 4 weeks of age there was a significant reduction in the number of FG labeled neurons that was limited to the ventral CN ipsilateral to the tracer injection. For deaf animals sacrificed at 12 weeks of age, however, there was a significant decrease in the number of labeled cells in both dorsal and ventral CN on both sides of the brain. In DNLL there was no change in the number or pattern of labeled neurons. The results show that neonatal deafness reduces the number of labeled neurons projecting from the CN to the IC with the effect being more evident during later stages of deafness. In contrast, there are no significant changes in the projections from DNLL to IC.

Detection of sinusoidal amplitude modulated sounds: deficits after bilateral lesions of auditory cortex in the rat.

The ability of rats to detect the presence of sinusoidal amplitude modulation (AM) of a broadband noise carrier was determined before and after bilateral ablation of auditory cortex. The rats were trained to withdraw from a drinking spout to avoid a shock when they detected a modulation of the sound. Sensitivity was evaluated by testing the rats at progressively smaller depths of modulation. Psychophysical curves were produced to describe the limits of detection at modulation rates of 10, 100 and 1000Hz. Performance scores were based on the probability of withdrawal from the spout during AM (warning periods) relative to withdrawal during the un-modulated noise (safe periods). A threshold was defined as the depth of modulation that produced a score halfway between perfect avoidance and no avoidance (performance score=0.5). Bilateral auditory cortical lesions resulted in significant elevations in threshold for detection of AM at rates of 100 and 1000Hz. No significant shift was found at a modulation rate of 10Hz. The magnitude of the deficit for AM rates of 100 and 1000Hz was positively correlated with the size of the cortical lesion. Substantial deficits were found only in animals with lesions that included secondary as well as primary auditory cortical areas. The results show that the rat's auditory cortex is important for processing sinusoidal AM and that its contribution is most apparent at high modulation rates. The data suggest that the auditory cortex is a crucial structure for maintaining normal sensitivity to temporal modulation of an auditory stimulus.

Regulation of auditory responses in the central nucleus of the inferior colliculus by tetraethylammonium-sensitive potassium channels.

The role of potassium channels in regulating spontaneous firing and sound-evoked responses in the central nucleus of the inferior colliculus was studied by recording single-unit activity before and during iontophoretic application of a nonspecific potassium channel blocker, tetraethylammonium (TEA). Tone bursts and sinusoidal amplitude-modulated tones were used to evoke auditory responses. Our results show that release of TEA increased the width of spikes for all neurons tested. There was an increase in spontaneous firing for most of the neurons. There was also an increase in responses to tone bursts for most of the neurons, although in some cases there was a reduction in the evoked responses. TEA also increased the firing rate in responses to sinusoidal amplitude-modulated sounds in the majority of the neurons tested. For some neurons, the change in firing reduced the selectivity of responses for particular rates of modulation. There was also a reduction in the synchrony of action potentials to the modulation envelope in many cells. Our results show that potassium channels are important for regulating the strength of sound-evoked responses and the level of spontaneous activity, and determining the temporal properties of responses to amplitude-modulated sounds.

Glutamatergic and GABAergic regulation of neural responses in inferior colliculus to amplitude-modulated sounds.

Recordings were made from single neurons in the rat inferior colliculus in response to sinusoidally amplitude-modulated sounds (10-s duration) presented to the contralateral ear. Neural responses were determined for different rates of modulation (0.5 Hz to 1 kHz) at a depth of 100%, and modulation transfer functions were generated based on firing rate (MTFFR) and vector strength (MTFVS). The effects of AMPA, NMDA, and GABAA receptor antagonists were examined by releasing drugs iontophoretically through a multibarrel pipette attached to a single-barrel recording pipette. Both the AMPA receptor antagonist, 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide disodium (NBQX), and the NMDA receptor antagonist, (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) resulted in a decrease in firing rate, and the GABAA receptor antagonist, bicuculline, produced an increase in the firing rate in most of the cells examined. In some cases, the shape of the MTFFR was modified slightly by receptor antagonists, but in most cases, the peak firing rate that determined a neuron's best modulation frequency remained the same. Also there were no changes during delivery of either excitatory or inhibitory antagonists in the maximum response synchrony at the peak of the MTFVS although some changes were noticed at off-peak modulation rates particularly with the AMPA receptor antagonist, NBQX.