Local inhibition of GABA affects precedence effect in the inferior colliculus

The precedence effect is a prerequisite for faithful sound localization in a complex auditory environment, and is a physiological phenomenon in which the auditory system selectively suppresses the directional information from echoes. Here we investigated how neurons in the inferior colliculus respond to the paired sounds that produce precedence-effect illusions, and whether their firing behavior can be modulated through inhibition with gamma-aminobutyric acid （GABA）. We recorded extracellularly from 36 neurons in rat inferior colliculus under three conditions： no injection, injection with saline, and injection with gamma-aminobutyric acid. The paired sounds that produced precedence effects were two identical 4-ms noise bursts, which were delivered contralaterally or ipsilaterally to the recording site. The normalized neural responses were measured as a function of different inter-stimulus delays and half-maximal interstimulus delays were acquired. Neuronal responses to the lagging sounds were weak when the inter-stimulus delay was short, but increased gradually as the delay was lengthened. Saline injection produced no changes in neural responses, but after local gamma-arninobutyric acid application, responses to the lagging stimulus were suppressed. Application of gamma-aminobutyric acid affected the normalized response to lagging sounds, independently of whether they or the paired sounds were contralateral or ipsilateral to the recording site. These observations suggest that local inhibition by gamma-aminobutyric acid in the rat inferior colliculus shapes the neural responses to lagging sounds, and modulates the precedence effect.

Responses from two firing patterns in inferior colliculus neurons to stimulation of the lateral lemniscus dorsal nucleus

The γ-aminobutyric acid neurons（GABAergic neurons） in the inferior colliculus are classified into various patterns based on their intrinsic electrical properties to a constant current injection. Although this classification is associated with physiological function, the exact role for neurons with various firing patterns in acoustic processing remains poorly understood. In the present study, we analyzed characteristics of inferior colliculus neurons in vitro, and recorded responses to stimulation of the dorsal nucleus of the lateral lemniscus using the wholecell patch clamp technique. Seven inferior colliculus neurons were tested and were classified into two firing patterns： sustained-regular（n = 4） and sustained-adapting firing patterns（n = 3）. The majority of inferior colliculus neurons exhibited slight changes in response to stimulation and bicuculline. The responses of one neuron with a sustained-adapting firing pattern were suppressed after stimulation, but recovered to normal levels following application of the γ-aminobutyric acid receptor antagonist. One neuron with a sustained-regular pattern showed suppressed stimulation responses, which were not affected by bicuculline. Results suggest that GABAergic neurons in the inferior colliculus exhibit sustained-regular or sustained-adapting firing patterns. Additionally, GABAergic projections from the dorsal nucleus of the lateral lemniscus to the inferior colliculus are associated with sound localization. The different neuronal responses of various firing patterns suggest a role in sound localization. A better understanding of these mechanisms and functions will provide better clinical treatment paradigms for hearing deficiencies.

PERIPHERAL HEARING LOSS CAUSES HYPEREXCITABILITY OF THE INFERIOR COLLICULUS

Growing evidence has been found to suggest that early development of the central auditory system is dependent on acoustic stimuli. Peripheral damage caused by noise exposure and ototoxic drugs can induce functional and anatomical changes along the auditory pathways. The inferior colliculus (IC) is a unique structure in the auditory system located between the primary auditory nuclei of the brainstem and the thala-mus. Damage to the IC inhibitory circuitry may affect central auditory processing and sound perception. Here, we review some of the striking electrophysiological changes in the IC that occur after noise exposure and ototoxic drug treatment. A common occurrence that emerges in the IC after peripheral damage is hyper-excitability of sound-evoked response. The hyperexcitability of the IC is likely related with reduced inhibi-tory response that requires normal peripheral inputs. Early age hearing loss can result in a long lasting in-creased susceptibility to audiogenic seizure which is related to hyperactivity in the IC evoked by loud sounds. Our studies suggest that hearing loss can cause increased IC neuron responsiveness which may be related to tinnitus, hyperacusis, and audiogenic seizure.

FIRING PROPERTY OF INFERIOR COLLICULUS NEURONS AFFECTED BY FMR1 GENE MUTATION

Fragile X syndrome is the most common form of inherited mental retardation affecting up to 1 in 4000 individuals. The syn- drome is induced by a mutation in the FMR1 gene, causing a deficiency in its gene by-product FMRP. Impairment in the nor- mal functioning of FMRP leads to learning and memory deficits and heightened sensitivity to sensory stimuli, including sound （hyperacusis）. The molecular basis of fragile X syndrome is thoroughly understood; however, the neural mechanisms underly- ing hyperacusis have not yet been determined. As the inferior colliculus （IC） is the principal midbrain nucleus of the auditory pathway, the current study addresses the questions underlying the neural mechanism of hyperacusis within the IC of fragile X mice. Acute experiments were performed in which electrophysiological recordings of the IC in FMR1-KO and WT mice were measured. Results showed that Q-values for WT were significantly larger than that of FMR-1 KO mice, indicating that WT mice exhibit sharper tuning curves than FMR1-KO mice. We also found the ratio of the monotonic neurons in the KO mice was much higher than the WT mice. These results suggest that lack of FMRP in the auditory system affects the developmental maturation and function of structures within the auditory pathway, and in this case specifically the IC. The dysfunction ob- served within the auditory neural pathway and in particular the IC may be related to the increased susceptibility to sound as seen in individuals with fragile X syndrome. Our study may help on understanding the mechanisms of the fragile X syndrome and hyperacusis.

Local inhibition mediated by γ-aminobutyric acid-A receptor and duration tuning in inferior colliculus in guinea pigs

Duration is a salient feature of acoustic signals including speech. Duration tuning was first reported in frogs and later in echolocating bats. More recently, duration tuning has been reported in non-echolocating mammals and appears to be a fundamental encoding mechanism throughout the animal kingdom. However, the duration tuning reported in these non-echolocating mammals appears to be much weaker than that in the previous studies on bats. In contrast to this finding, our recent study reported that duration tuning in the IC in guinea pigs appeared to be strong when it was measured using an appropriate temporal window. With such a temporal window, duration tuning was found to be compatible with that of echo-locating bats. In the present report, we further demonstrate that duration tuning in the IC of this species is established by interaction between excitation and GABAergic inhibition. In addition to overall increase in responsiveness, application of bicuculline(BIC), a GABA-A receptor antagonist, was found to significantly reduce or eliminate duration selectivity in 44 out of the 67 neurons that showed clear duration tuning from a sample of 340 neurons.

Cross-talk between NMDA and GABA_A receptors in cultured neurons of the rat inferior colliculus

Neuronal ion channels of different types often do not function independently but will inhibit or potentiate the activity of other types of channels,a process called cross-talk.The N-methyl-D-aspartate receptor (NMDA receptor) and the γ-aminobutyric acid type A receptor (GABAA receptor) are important excitatory and inhibitory receptors in the central nervous system,respectively.Currently,cross-talk between the NMDA receptor and the GABAA receptor,particularly in the central auditory system,is not well understood.In the present study,we investigated functional interactions between the NMDA receptor and the GABAA receptor using whole-cell patch-clamp techniques in cultured neurons from the inferior colliculus,which is an important nucleus in the central auditory system.We found that the currents induced by aspartate at 100 μmol L-1 were suppressed by the pre-perfusion of GABA at 100 μmol L-1,indicating cross-inhibition of NMDA receptors by activation of GABAA receptors.Moreover,we found that the currents induced by GABA at 100 μmol L-1 (IGABA) were not suppressed by the pre-perfusion of 100 μmol L-1 aspartate,but those induced by GABA at 3 μmol L-1 were suppressed,indicating concentration-dependent cross-inhibition of GABAA receptors by activation of NMDA receptors.In addition,inhibition of IGABA by aspartate was not affected by blockade of voltage-dependent Ca2+ channels with CdCl2 in a solution that contained Ca2+,however,CdCl2 effectively attenuated the inhibition of IGABA by aspartate when it was perfused in a solution that contained Ba2+ instead of Ca2+ or a solution that contained Ca2+ and 10 mmol L-1 BAPTA,a membrane-permeable Ca2+ chelator,suggesting that this inhibition is mediated by Ca2+ influx through NMDA receptors,rather than voltage-dependent Ca2+ channels.Finally,KN-62,a potent inhibitor of Ca2+/calmodulin-dependent protein kinase II (CaMKII),reduced the inhibition of IGABA by aspartate,indicating the involvement of CaMKII in this cross-inhibition.Our study demonstrates a functional interaction between NMDA and GABAA receptors in the inferior colliculus of rats.The presence of cross-talk between these receptors suggests that the mechanisms underlying information processing in the central auditory system may be more complex than previously believed.

Neural modulation in inferior colliculus and central auditory plasticity

The neural modulation in central auditory system plays an important role in perception and processing of sound signal and auditory cognition.The inferior colliculus(IC)is both a relay station in central auditory pathway and a sub-cortical auditory center doing the sound signal processing.IC is also modulated by the descending projections from the cortex and auditory thalamus,medial geniculate body,and these neural modulations not only can affect ongoing sound signal processing but can also induce plastic changes in IC.

Modelling envelope and temporal fine structure components of frequency-following responses in rat inferior colliculus

In studies of auditory perception, a dichotomy between envelope and temporal fine structure(TFS) has been emphasized. It has been shown that frequency-following responses(FFRs) in the rat inferior colliculus can be divided into the envelope component(FFREnv)and the temporal fine structure component(FFRTFS). However, the existing FFR models cannot successfully separate FFREnv and FFRTFS. This study was to develop a new FFR model to effectively distinguish FFREnv from FFRTFS by both combining the advantages of the two existing FFR models and simultaneously adding cellular properties of inferior colliculus neurons. To evaluate the validity of the present model, correlations between simulated FFRs and experimental data from the rat inferior colliculus were calculated. Different model parameters were tested, FFRs were calculated, and the parameters with highest prediction were chosen to establish an ideal FFR model. The results indicate that the new FFR model can provide reliable predictions for experimentally obtained FFREnv and FFRTFS.

Masking effect of different durations of forward masker sound on acoustical responses of mouse inferior collicular neurons to probe sound

To study the effects of different durations of forward masker sound on neuronal firing and rate-intensity function(RIF)of mouse inferior collicular(IC)neurons,a tone relative to 5 dB above the minimum threshold(re MT+5 dB)of the best frequency of recorded neurons was used as forward masker sound under free field stimulation condition.The masker durations used were 40,60,80,and 100 ms.Results showed that as masker duration was increased,inhibition in neuronal firing was enhanced(P<0.0001,n=41)and the latency of neurons was lengthened(P<0.01,n=41).In addition,among 41 inhibited IC neurons,90.2%(37/41)exhibited narrowed dynamic range(DR)when masker sound duration was increased(P<0.0001),whereas the DR of 9.8%(4/41)became wider.These data suggest that masking effects of different durations of forward masker sound might be correlated with the amplitude and duration of inhibitory input to IC neurons elicited by the masker sound.

Latency represents sound frequency in mouse IC

Frequency is one of the fundamental parameters of sound.The frequency of an acoustic stimulus can be represented by a neural response such as spike rate,and/or first spike latency(FSL)of a given neuron.The spike rates/frequency function of most neurons changes with different acoustic ampli-tudes,whereas FSL/frequency function is highly stable.This implies that FSL might represent the fre-quency of a sound stimulus more efficiently than spike rate.This study involved representations of acoustic frequency by spike rate and FSL of central inferior colliculus(IC)neurons responding to free-field pure-tone stimuli.We found that the FSLs of neurons responding to characteristic frequency(CF)of sound stimulus were usually the shortest,regardless of sound intensity,and that spike rates of most neurons showed a variety of function according to sound frequency,especially at high intensities.These results strongly suggest that FSL of auditory IC neurons can represent sound frequency more precisely than spike rate.

The adaptive value of increasing pulse repetition rate during hunting by echolocating bats

During hunting, bats of suborder Microchiropetra emit intense ultrasonic pulses and analyze the weak returning echoes with their highly developed auditory system to extract the information about insects or obstacles. These bats progressively shorten the duration, lower the frequency, decrease the intensity and increase the repetition rate of emitted pulses as they search, approach, and finally intercept insects or negotiate obstacles. This dynamic variation in multiple parameters of emitted pulses predicts that analysis of an echo parameter by the bat would be inevitably affected by other co-varying echo parameters. The progressive increase in the pulse repetition rate throughout the entire course of hunting would presumably enable the bat to extract maximal information from the increasing number of echoes about the rapid changes in the target or obstacle position for successful hunting. However, the increase in pulse repetition rate may make it difficult to produce intense short pulse at high repetition rate at the end of long-held breath. The increase in pulse repetition rate may also make it difficult to produce high frequency pulse due to the inability of the bat laryngeal muscles to reach its full extent of each contraction and relaxation cycle at a high repetition rate. In addition, the increase in pulse repetition rate increases the minimum threshold （i.e. decrease auditory sensitivity） and the response latency of auditory neurons. In spite of these seemingly physiological disadvantages in pulse emission and auditory sensitivity, these bats do progressively increase pulse repetition rate throughout a target approaching sequence. Then, what is the adaptive value of increasing pulse repetition rate during echolocation？ What are the underlying mechanisms for obtaining maximal information about the target features during increasing pulse repetition rate？ This article reviews the electrophysiological studies of the effect of pulse repetition rate on multiple- parametric selectivity of neurons in the central nucleus of the inferior colliculus of the big brown bat, Eptesicusfuscus using single repetitive sound pulses and temporally patterned trains of sound pulses. These studies show that increasing pulse repetition rate improves multiple-parametric selectivity of inferior collicular neurons. Conceivably, this improvement of multiple-parametric selectivity of collicular neurons with increasing pulse repetition rate may serve as the underlying mechanisms for obtaining maximal information about the prey features for successful hunting by bats.