Inferior and Superior Colliculi

GluN2C/D-containing NMDA receptors enhance temporal summation and increase sound-evoked and spontaneous firing in the inferior colliculus

Along the ascending auditory pathway, there is a broad shift from temporal coding, which is common in the lower auditory brainstem, to rate coding, which predominates in auditory cortex. This temporal-to-rate transition is particularly prominent in the inferior colliculus (IC), the midbrain hub of the auditory system, but the mechanisms that govern how individual IC neurons integrate information across time remain largely unknown. Here, we report the widespread expression of Glun2c and Glun2d mRNA in IC neurons. GluN2C/D-containing NMDA receptors are relatively insensitive to voltage-dependent Mg2+ block, and thus can conduct current at resting membrane potential. Using in situ hybridization and pharmacology, we show that VIP neurons in the IC express GluN2D-containing NMDA receptors that are activatable by commissural inputs from the contralateral IC. In addition, GluN2C/D-containing receptors have much slower kinetics than other NMDA receptors, and we found that GluN2D-containing receptors facilitate temporal summation of synaptic inputs in VIP neurons. In a model neuron, we show that a GluN2C/D-like conductance interacts with the passive membrane properties of the neuron to alter temporal and rate coding of stimulus trains. Consistent with this, we show in vivo that blocking GluN2C/D-containing receptors decreases both the spontaneous firing rate and the overall firing rate elicited by amplitude-modulated (AM) sounds in many IC neurons. These results suggest that GluN2C/D-containing NMDA receptors influence rate coding for auditory stimuli in the IC by facilitating the temporal integration of synaptic inputs.

Paradoxical kinesia may no longer be a paradox waiting for 100 years to be unraveled

Parkinson's disease (PD) is a progressive neurodegenerative disorder mainly characterized by bradykinesia and akinesia. Interestingly, these motor disabilities can depend on the patient emotional state. Disabled PD patients remain able to produce normal motor responses in the context of urgent or externally driven situations or even when exposed to appetitive cues such as music. To describe this phenomenon Souques coined the term "paradoxical kinesia" a century ago. Since then, the mechanisms underlying paradoxical kinesia are still unknown due to a paucity of valid animal models that replicate this phenomenon. To overcome this limitation, we established two animal models of paradoxical kinesia. Using these models, we investigated the neural mechanisms of paradoxical kinesia, with the results pointing to the inferior colliculus (IC) as a key structure. Intracollicular electrical deep brain stimulation, glutamatergic and GABAergic mechanisms may be involved in the elaboration of paradoxical kinesia. Since paradoxical kinesia might work by activation of some alternative pathway bypassing basal ganglia, we suggest the IC as a candidate to be part of this pathway.

Hierarchical differences in the encoding of sound and choice in the subcortical auditory system

Detection of sounds is a fundamental function of the auditory system. Although studies of auditory cortex have gained substantial insight into detection performance using behaving animals, previous subcortical studies have mostly taken place under anesthesia, in passively listening animals, or have not measured performance at threshold. These limitations preclude direct comparisons between neuronal responses and behavior. To address this, we simultaneously measured auditory detection performance and single-unit activity in the inferior colliculus (IC) and cochlear nucleus (CN) in macaques. The spontaneous activity and response variability of CN neurons were higher than those observed for IC neurons. Signal detection theoretic methods revealed that the magnitude of responses of IC neurons provided more reliable estimates of psychometric threshold and slope compared with the responses of single CN neurons. However, pooling small populations of CN neurons provided reliable estimates of psychometric threshold and slope, suggesting sufficient information in CN population activity. Trial-by-trial correlations between spike count and behavioral response emerged 50-75 ms after sound onset for most IC neurons, but for few neurons in the CN. These results highlight hierarchical differences between neurometric-psychometric correlations in CN and IC and have important implications for how subcortical information could be decoded.NEW & NOTEWORTHY The cerebral cortex is widely recognized to play a role in sensory processing and decision-making. Accounts of the neural basis of auditory perception and its dysfunction are based on this idea. However, significantly less attention has been paid to midbrain and brainstem structures in this regard. Here, we find that subcortical auditory neurons represent stimulus information sufficient for detection and predict behavioral choice on a trial-by-trial basis.

Excitatory cholecystokinin neurons of the midbrain integrate diverse temporal responses and drive auditory thalamic subdomains

Our ability to identify sounds and understand communication signals depends upon our brains’ capacity to combine information about diverse sound features, including temporal patterns. The central nucleus of the inferior colliculus (ICC) performs an initial stage of this integration, but a circuit-based understanding of these processes has been hampered by difficulties in separating clearly defined functional cell types. Here we identify and characterize a major excitatory projection neuron of the ICC. These neurons show uniform intrinsic firing patterns and tuning to frequency, but strikingly diverse temporal responses to sound. Our results suggest that diversity in temporal coding is represented even within a single cell class and is likely primarily driven by differences in circuit connectivity.

The central nucleus of the inferior colliculus (ICC) integrates information about different features of sound and then distributes this information to thalamocortical circuits. However, the lack of clear definitions of circuit elements in the ICC has limited our understanding of the nature of these circuit transformations. Here, we combine virus-based genetic access with electrophysiological and optogenetic approaches to identify a large family of excitatory, cholecystokinin-expressing thalamic projection neurons in the ICC of the Mongolian gerbil. We show that these neurons form a distinct cell type, displaying uniform morphology and intrinsic firing features, and provide powerful, spatially restricted excitation exclusively to the ventral auditory thalamus. In vivo, these neurons consistently exhibit V-shaped receptive field properties but strikingly diverse temporal responses to sound. Our results indicate that temporal response diversity is maintained within this population of otherwise uniform cells in the ICC and then relayed to cortex through spatially restricted thalamic subdomains.

Eye-movement patterns to social and non-social cues in early deaf adults

Previous research on covert orienting to the periphery suggested that early profound deaf adults were less susceptible to uninformative gaze-cues, though were equally or more affected by non-social arrow-cues. The aim of this work was to investigate whether spontaneous eye movement behaviour helps explain the reduced impact of the social cue in deaf adults. We tracked the gaze of 25 early profound deaf and 25 age-matched hearing observers performing a peripheral discrimination task with uninformative central cues (gaze vs arrow), stimulus-onset asynchrony (250 vs 750 ms), and cue validity (valid vs invalid) as within-subject factors. In both groups, the cue effect on reaction time (RT) was comparable for the two cues, although deaf observers responded significantly slower than hearing controls. While deaf and hearing observers' eye movement pattern looked similar when the cue was presented in isolation, deaf participants made significantly more eye movements than hearing controls once the discrimination target appeared. Notably, further analysis of eye movements in the deaf group revealed that independent of the cue type, cue validity affected saccade landing position, while latency was not modulated by these factors. Saccade landing position was also strongly related to the magnitude of the validity effect on RT, such that the greater the difference in saccade landing position between invalid and valid trials, the greater the difference in manual RT between invalid and valid trials. This work suggests that the contribution of overt selection in central cueing of attention is more prominent in deaf adults and helps determine the manual performance, irrespective of the cue type.

Foreground stimuli and task engagement enhance neuronal adaptation to background noise in the inferior colliculus of macaques

Auditory neuronal responses are modified by background noise. Inferior colliculus (IC) neuronal responses adapt to the most frequent sound level within an acoustic scene (adaptation to stimulus statistics), a mechanism that may preserve neuronal and behavioral thresholds for signal detection. However, it is still unclear whether the presence of foreground stimuli and/or task involvement can modify neuronal adaptation. To investigate how task engagement interacts with this mechanism, we compared the response of IC neurons to background noise, which caused adaptation to stimulus statistics, while macaque monkeys performed a masked tone detection task (task-driven condition) with responses recorded when the same background noise was presented alone (passive listening condition). In the task-dependent condition, monkeys performed a Go/No-Go task while 50-ms tones were embedded within an adaptation-inducing continuous background noise whose levels changed every 50 ms and were drawn from a probability distribution. The adaptation to noise stimulus statistics in IC neuronal responses was significantly enhanced in the task-driven condition compared with the passive listening condition, showing that foreground stimuli and/or task-engagement can modify IC neuronal responses. Additionally, the response of IC neurons to noise was significantly affected by the preceding sensory information (history effect) regardless of task involvement. These studies show that dynamic range adaptation in IC preserves behavioral and neurometric thresholds irrespective of noise type and a dependence of neuronal activity on task-related factors at subcortical levels of processing.NEW & NOTEWORTHY Auditory neuronal responses are influenced by maskers and distractors. However, it is still unclear whether the neuronal sensitivity to the masker stimulus is influenced by task-dependent factors. Our study represents one of the first attempts to investigate how task involvement influences the neural representation of background sounds in the subcortical, midbrain auditory neurons of behaving animals.

From Near-Optimal Bayesian Integration to Neuromorphic Hardware: A Neural Network Model of Multisensory Integration

While interacting with the world our senses and nervous system are constantly challenged to identify the origin and coherence of sensory input signals of various intensities. This problem becomes apparent when stimuli from different modalities need to be combined, e.g., to find out whether an auditory stimulus and a visual stimulus belong to the same object. To cope with this problem, humans and most other animal species are equipped with complex neural circuits to enable fast and reliable combination of signals from various sensory organs. This multisensory integration starts in the brain stem to facilitate unconscious reflexes and continues on ascending pathways to cortical areas for further processing. To investigate the underlying mechanisms in detail, we developed a canonical neural network model for multisensory integration that resembles neurophysiological findings. For example, the model comprises multisensory integration neurons that receive excitatory and inhibitory inputs from unimodal auditory and visual neurons, respectively, as well as feedback from cortex. Such feedback projections facilitate multisensory response enhancement and lead to the commonly observed inverse effectiveness of neural activity in multisensory neurons. Two versions of the model are implemented, a rate-based neural network model for qualitative analysis and a variant that employs spiking neurons for deployment on a neuromorphic processing. This dual approach allows to create an evaluation environment with the ability to test model performances with real world inputs. As a platform for deployment we chose IBM's neurosynaptic chip TrueNorth. Behavioral studies in humans indicate that temporal and spatial offsets as well as reliability of stimuli are critical parameters for integrating signals from different modalities. The model reproduces such behavior in experiments with different sets of stimuli. In particular, model performance for stimuli with varying spatial offset is tested. In addition, we demonstrate that due to the emergent properties of network dynamics model performance is close to optimal Bayesian inference for integration of multimodal sensory signals. Furthermore, the implementation of the model on a neuromorphic processing chip enables a complete neuromorphic processing cascade from sensory perception to multisensory integration and the evaluation of model performance for real world inputs.

Human Frequency Following Response Correlates of Spatial Release From Masking

Purpose Speech recognition in complex listening environments is enhanced by the extent of spatial separation between the speech source and background competing sources, an effect known as spatial release from masking (SRM). The aim of this study was to investigate whether the phase-locked neural activity in the central auditory pathways, reflected in the frequency following response (FFR), exhibits SRM. Method Eighteen normal-hearing adults (8 men and 10 women, ranging in age from 20 to 42 years) with no known neurological disorders participated in this study. FFRs were recorded from the participants in response to a target vowel /u/ presented with spatially colocated and separated competing talkers at 3 ranges of signal-to-noise ratios (SNRs), with median SNRs of -5.4, 0.5, and 6.8 dB and for different attentional conditions (attention and no attention). Results Amplitude of the FFR at the fundamental frequency was significantly larger in the spatially separated condition as compared to the colocated condition for only the lowest (< -2.4 dB SNR) of the 3 SNR ranges tested. A significant effect of attention was found when subjects were actively focusing on the target stimuli. No significant interaction effects were found between spatial separation and attention. Conclusions The enhanced representation of the target stimulus in the separated condition suggests that the temporal pattern of phase-locked brainstem neural activity generating the FFR may contain information relevant to the binaural processes underlying SRM but only in challenging listening environments. Attention may modulate FFR fundamental frequency amplitude but does not seem to modulate spatial processing at the level of generating the FFR. Supplemental Material https://doi.org/10.23641/asha.9992597.

Neuronal adaptation to sound statistics in the inferior colliculus of behaving macaques does not reduce the effectiveness of the masking noise

The detectability of target sounds embedded within noisy backgrounds is affected by the regularities that summarize acoustic sceneries. Previous studies suggested that the dynamic range of neurons in the inferior colliculus (IC) of anesthetized guinea pigs shifts toward the mean sound pressure level in irregular acoustic environments. Yet, it is unclear how this neuronal adaptation processes may influence the effectiveness of sounds as a masker, both behaviorally and in terms of neuronal encoding. To answer this question, we measured the neural response of IC neurons while macaque monkeys performed a Go/No-Go tone detection task. Macaques detected a 50-ms tone that was either simultaneously gated with a burst of noise or embedded within a continuous noise background, whose levels were randomly sampled (every 50 ms) from a probability distribution. The mean of the distribution matched the level of the gated burst of noise. Psychometric and IC neurometric thresholds to tones did not differ between the two masking conditions. However, the neuronal firing rate versus level function was significantly affected by the temporal characteristics of the noise masker. Simultaneously gated noise caused higher baseline responses and greater dynamic range compression compared with noise distribution. The slopes of psychometric and neurometric functions were significantly shallower for higher variance distributions, suggesting that neuronal sensitivity might change with the variability of the sound. Our results suggest that the adaptive response of IC neurons to sound regularities does not affect the effectiveness of the noise-masking signal, which remains invariant to surrounding noise amplitudes. NEW & NOTEWORTHY Auditory neurons adapt to the statistics of sound levels in the acoustic scene. However, it is still unclear to what extent such adaptation influences the effectiveness of the stimulus as a masker. Our study represents the first attempt to investigate how the adaptation to the statistics of masking stimuli may be related to the effectiveness of masking, and to the single-unit encoding of the midbrain auditory neurons in behaving animals.

GABAergic and non-GABAergic projections to the superior colliculus from the auditory brainstem

The superior colliculus (SC) contains an auditory space map that is shaped by projections from several subcortical auditory nuclei. Both GABAergic (inhibitory) and excitatory cells contribute to these inputs, but there are contradictory reports regarding the sources of these inputs. We used retrograde tracing techniques in guinea pigs to identify cells in the auditory brainstem that project to the SC. We combined retrograde tracing with immunohistochemistry for glutamic acid decarboxylase (GAD) to identify putative GABAergic cells that participate in this pathway. Following a tracer injection in the SC, the nucleus of the brachium of the inferior colliculus (NBIC) contained the most labeled cells, followed by the inferior colliculus (IC). Smaller populations were observed in the sagulum, paralemniscal area, periolivary nuclei and ventrolateral tegmental nucleus. Overall, only 10% of the retrogradely labeled cells were GAD immunopositive. The presumptive inhibitory cells were observed in the NBIC, IC, superior paraolivary nucleus, sagulum and paralemniscal area. We conclude that the guinea pig SC receives input from a diverse set of auditory brainstem nuclei, some of which provide GABAergic input. These diverse origins of input to the SC likely represent a variety of functions. Inputs from the NBIC and IC likely provide spatial information for guiding orienting behaviors. Inputs from subcollicular nuclei are less likely to provide spatial information; rather, they may provide a shorter route for auditory information to reach the SC, and could generate avoidance or escape responses to an external threat.

The distribution of calbindin-D28k, parvalbumin, and calretinin immunoreactivity in the inferior colliculus of circling mouse

The circling mice with tmie gene mutation are known as an animal deafness model, which showed hyperactive circling movement. Recently, the reinvestigation of circling mouse was performed to check the inner ear pathology as a main lesion of early hearing loss. In this trial, the inner ear organs were not so damaged to cause the hearing deficit of circling (cir/cir) mouse at 18 postnatal day (P18) though auditory brainstem response data indicated hearing loss of cir/cir mice at P18. Thus, another mechanism may be correlated with the early hearing loss of cir/cir mice at P18. Hearing loss in the early life can disrupt the ascending and descending information to inferior colliculus (IC) as integration site. There were many reports that hearing loss could result in the changes in Ca²⁺ concentration by either cochlear ablation or genetic defect. However, little was known to be reported about the correlation between the pathology of IC and Ca²⁺ changes in circling mice. Therefore, the present study investigated the distribution of calcium-binding proteins (CaBPs), calbindin-D28k, parvalbumin, and calretinin immunoreactivity (IR) in the IC to compare among wild-type (+/+), heterozygous (+/cir), and homozygous (cir/cir) mice by immunohistochemistry. The decreases of CaBPs IR in cir/cir were statistically significant in the neurons as well as neuropil of IC. Thus, this study proposed overall distributional alteration of CaBPs IR in the IC caused by early hearing defect and might be helpful to elucidate the pathology of central auditory disorder related with Ca²⁺ metabolism.

An Integrated Circuit for Simultaneous Extracellular Electrophysiology Recording and Optogenetic Neural Manipulation

OBJECTIVE

The ability to record and to control action potential firing in neuronal circuits is critical to understand how the brain functions. The objective of this study is to develop a monolithic integrated circuit (IC) to record action potentials and simultaneously control action potential firing using optogenetics.

METHODS

A low-noise and high input impedance (or low input capacitance) neural recording amplifier is combined with a high current laser/light-emitting diode (LED) driver in a single IC.

RESULTS

The low input capacitance of the amplifier (9.7 pF) was achieved by adding a dedicated unity gain stage optimized for high impedance metal electrodes. The input referred noise of the amplifier is [Formula: see text], which is lower than the estimated thermal noise of the metal electrode. Thus, the action potentials originating from a single neuron can be recorded with a signal-to-noise ratio of at least 6.6. The LED/laser current driver delivers a maximum current of 330 mA, which is adequate for optogenetic control. The functionality of the IC was tested with an anesthetized Mongolian gerbil and auditory stimulated action potentials were recorded from the inferior colliculus. Spontaneous firings of fifth (trigeminal) nerve fibers were also inhibited using the optogenetic protein Halorhodopsin. Moreover, a noise model of the system was derived to guide the design.

SIGNIFICANCE

A single IC to measure and control action potentials using optogenetic proteins is realized so that more complicated behavioral neuroscience research and the translational neural disorder treatments become possible in the future.

The organization of frequency and binaural cues in the gerbil inferior colliculus

The inferior colliculus (IC) is the common target of separate pathways that transmit different types of auditory information. Beyond tonotopy, little is known about the organization of response properties within the 3-dimensional layout of the auditory midbrain in most species. Through study of interaural time difference (ITD) processing, the functional properties of neurons can be readily characterized and related to specific pathways. To characterize the representation of ITDs relative to the frequency and hodological organization of the IC, the properties of neurons were recorded and the sites recovered histologically. Subdivisions of the IC were identified based on cytochrome oxidase (CO) histochemistry. The results were plotted within a framework formed by an MRI atlas of the gerbil brain. The central nucleus was composed of two parts, and lateral and dorsal cortical areas were identified. The lateral part of the central nucleus had the highest CO activity in the IC and a high proportion of neurons sensitive to ITDs. The medial portion had lower CO activity and fewer ITD-sensitive neurons. A common tonotopy with a dorsolateral to ventromedial gradient of low to high frequencies spanned the two regions. The distribution of physiological responses was in close agreement with known patterns of ascending inputs. An understanding of the 3-dimensional organization of the IC is needed to specify how the single tonotopic representation in the IC central nucleus leads to the multiple tonotopic representations in core areas of the auditory cortex.

Effect of background noise on neuronal coding of interaural level difference cues in rat inferior colliculus

Humans can accurately localize sounds even in unfavourable signal-to-noise conditions. To investigate the neural mechanisms underlying this, we studied the effect of background wide-band noise on neural sensitivity to variations in interaural level difference (ILD), the predominant cue for sound localization in azimuth for high-frequency sounds, at the characteristic frequency of cells in rat inferior colliculus (IC). Binaural noise at high levels generally resulted in suppression of responses (55.8%), but at lower levels resulted in enhancement (34.8%) as well as suppression (30.3%). When recording conditions permitted, we then examined if any binaural noise effects were related to selective noise effects at each of the two ears, which we interpreted in light of well-known differences in input type (excitation and inhibition) from each ear shaping particular forms of ILD sensitivity in the IC. At high signal-to-noise ratios (SNR), in most ILD functions (41%), the effect of background noise appeared to be due to effects on inputs from both ears, while for a large percentage (35.8%) appeared to be accounted for by effects on excitatory input. However, as SNR decreased, change in excitation became the dominant contributor to the change due to binaural background noise (63.6%). These novel findings shed light on the IC neural mechanisms for sound localization in the presence of continuous background noise. They also suggest that some effects of background noise on encoding of sound location reported to be emergent in upstream auditory areas can also be observed at the level of the midbrain.

A review on auditory space adaptations to altered head-related cues

In this article we present a review of current literature on adaptations to altered head-related auditory localization cues. Localization cues can be altered through ear blocks, ear molds, electronic hearing devices, and altered head-related transfer functions (HRTFs). Three main methods have been used to induce auditory space adaptation: sound exposure, training with feedback, and explicit training. Adaptations induced by training, rather than exposure, are consistently faster. Studies on localization with altered head-related cues have reported poor initial localization, but improved accuracy and discriminability with training. Also, studies that displaced the auditory space by altering cue values reported adaptations in perceived source position to compensate for such displacements. Auditory space adaptations can last for a few months even without further contact with the learned cues. In most studies, localization with the subject's own unaltered cues remained intact despite the adaptation to a second set of cues. Generalization is observed from trained to untrained sound source positions, but there is mixed evidence regarding cross-frequency generalization. Multiple brain areas might be involved in auditory space adaptation processes, but the auditory cortex (AC) may play a critical role. Auditory space plasticity may involve context-dependent cue reweighting.

Alteration of glycine receptor immunoreactivity in the auditory brainstem of mice following three months of exposure to radiofrequency radiation at SAR 4.0 W/kg

The increasing use of mobile communication has triggered an interest in its possible effects on the regulation of neurotransmitter signals. Due to the close proximity of mobile phones to hearing-related brain regions during usage, its use may lead to a decrease in the ability to segregate sounds, leading to serious auditory dysfunction caused by the prolonged exposure to radiofrequency (RF) radiation. The interplay among auditory processing, excitation and inhibitory molecule interactions plays a major role in auditory function. In particular, inhibitory molecules, such a glycine, are predominantly localized in the auditory brainstem. However, the effects of exposure to RF radiation on auditory function have not been reported to date. Thus, the aim of the present study was to investigate the effects of exposure to RF radiation on glycine receptor (GlyR) immunoreactivity (IR) in the auditory brainstem region at 835 MHz with a specific absorption rate of 4.0 W/kg for three months using free-floating immunohistochemistry. Compared with the sham control (SC) group, a significant loss of staining intensity of neuropils and cells in the different subdivisions of the auditory brainstem regions was observed in the mice exposed to RF radiation (E4 group). A decrease in the number of GlyR immunoreactive cells was also noted in the cochlear nuclear complex [anteroventral cochlear nucleus (AVCN), 31.09%; dorsal cochlear nucleus (DCN), 14.08%; posteroventral cochlear nucleus (PVCN), 32.79%] and the superior olivary complex (SOC) [lateral superior olivary nucleus (LSO), 36.85%; superior paraolivary nucleus (SPN), 24.33%, medial superior olivary nucleus (MSO), 23.23%; medial nucleus of the trapezoid body (MNTB), 10.15%] of the mice in the E4 group. Auditory brainstem response (ABR) analysis also revealed a significant threshold elevation of in the exposed (E4) group, which may be associated with auditory dysfunction. The present study suggests that the auditory brainstem region is susceptible to chronic exposure to RF radiation, which may affect the function of the central auditory system.

Subcortical auditory structures in the Mongolian gerbil: I. Golgi architecture

By means of the Golgi-Cox and Nissl methods we investigated the cyto- and fiberarchitecture as well as the morphology of neurons in the subcortical auditory structures of the Mongolian gerbil (Meriones unguiculatus), a frequently used animal model in auditory neuroscience. We describe the divisions and subdivisions of the auditory thalamus including the medial geniculate body, suprageniculate nucleus, and reticular thalamic nucleus, as well as of the inferior colliculi, nuclei of the lateral lemniscus, superior olivary complex, and cochlear nuclear complex. In this study, we 1) confirm previous results about the organization of the gerbil's subcortical auditory pathway using other anatomical staining methods (e.g., Budinger et al. [2000] Eur J Neurosci 12:2452-2474); 2) add substantially to the knowledge about the laminar and cellular organization of the gerbil's subcortical auditory structures, in particular about the orientation of their fibrodendritic laminae and about the morphology of their most distinctive neuron types; and 3) demonstrate that the cellular organization of these structures, as seen by the Golgi technique, corresponds generally to that of other mammalian species, in particular to that of rodents.

Ephrin-B2 reverse signaling is required for topography but not pattern formation of lateral superior olivary inputs to the inferior colliculus

Graded and modular expressions of Eph-ephrins are known to provide positional information for the formation of topographic maps and patterning in the developing nervous system. Previously we have shown that ephrin-B2 is expressed in a continuous gradient across the tonotopic axis of the central nucleus of the inferior colliculus (CNIC), whereas patterns are discontinuous and modular in the lateral cortex of the IC (LCIC). The present study explores the involvement of ephrin-B2 signaling in the development of projections to the CNIC and LCIC arising from the lateral superior olivary nuclei (LSO) prior to hearing onset. Anterograde and retrograde fluorescent tracing methods in neonatal fixed tissue preparations were used to compare topographic mapping and the establishment of LSO layers/modules in wild-type and ephrin-B2(lacZ/+) mice (severely compromised reverse signaling). At birth, pioneer LSO axons occupy the ipsilateral IC in both groups but are delayed contralaterally in ephrin-B2(lacZ/+) mutants. By the onset of hearing, both wild-type and mutant projections form discernible layers bilaterally in the CNIC and modular arrangements within the ipsilateral LCIC. In contrast, ephrin-B2(lacZ/+) mice lack a reliable topography in LSO-IC projections, suggesting that fully functional ephrin-B2 reverse signaling is required for normal projection mapping. Taken together, these ephrin-B2 findings paired with known coexpression of EphA4 suggest the importance of these signaling proteins in establishing functional auditory circuits prior to experience.

Classification of frequency response areas in the inferior colliculus reveals continua not discrete classes

A differential response to sound frequency is a fundamental property of auditory neurons. Frequency analysis in the cochlea gives rise to V-shaped tuning functions in auditory nerve fibres, but by the level of the inferior colliculus (IC), the midbrain nucleus of the auditory pathway, neuronal receptive fields display diverse shapes that reflect the interplay of excitation and inhibition. The origin and nature of these frequency receptive field types is still open to question. One proposed hypothesis is that the frequency response class of any given neuron in the IC is predominantly inherited from one of three major afferent pathways projecting to the IC, giving rise to three distinct receptive field classes. Here, we applied subjective classification, principal component analysis, cluster analysis, and other objective statistical measures, to a large population (2826) of frequency response areas from single neurons recorded in the IC of the anaesthetised guinea pig. Subjectively, we recognised seven frequency response classes (V-shaped, non-monotonic Vs, narrow, closed, tilt down, tilt up and double-peaked), that were represented at all frequencies. We could identify similar classes using our objective classification tools. Importantly, however, many neurons exhibited properties intermediate between these classes, and none of the objective methods used here showed evidence of discrete response classes. Thus receptive field shapes in the IC form continua rather than discrete classes, a finding consistent with the integration of afferent inputs in the generation of frequency response areas. The frequency disposition of inhibition in the response areas of some neurons suggests that across-frequency inputs originating at or below the level of the IC are involved in their generation.

Identification of inputs to olivocochlear neurons using transneuronal labeling with pseudorabies virus (PRV)

Olivocochlear (OC) neurons respond to sound and provide descending input that controls processing in the cochlea. The identities of neurons in the pathways providing inputs to OC neurons are incompletely understood. To explore these pathways, the retrograde transneuronal tracer pseudorabies virus (Bartha strain, expressing green fluorescent protein) was used to label OC neurons and their inputs in guinea pigs. Labeling of OC neurons began 1 day after injection into the cochlea. On day 2 (and for longer survival times), transneuronal labeling spread to the cochlear nucleus, inferior colliculus, and other brainstem areas. There was a correlation between the numbers of these transneuronally labeled neurons and the number of labeled medial (M) OC neurons, suggesting that the spread of labeling proceeds mainly via synapses on MOC neurons. In the cochlear nucleus, the transneuronally labeled neurons were multipolar cells including the subtype known as planar cells. In the central nucleus of the inferior colliculus, transneuronally labeled neurons were of two principal types: neurons with disc-shaped dendritic fields and neurons with dendrites in a stellate pattern. Transneuronal labeling was also observed in pyramidal cells in the auditory cortex and in centers not typically associated with the auditory pathway such as the pontine reticular formation, subcoerulean nucleus, and the pontine dorsal raphe. These data provide information on the identity of neurons providing input to OC neurons, which are located in auditory as well as non-auditory centers.

On the classification of pathways in the auditory midbrain, thalamus, and cortex

Auditory forebrain pathways exhibit several morphological and physiological properties that underlie their specific neurobiological roles in auditory processing. Anatomically, such projections can be distinguished by their terminal size, arborization patterns, and postsynaptic dendritic locations. These structural features correlate with several postsynaptic physiological properties, such as EPSP amplitude, short-term plasticity, and postsynaptic receptor types. Altogether, these synaptic properties segregate into two main classes that are associated with either primarily information-bearing (Class 1) or modulatory (Class 2) roles, and have been used to delineate the principle routes of information flow through the auditory midbrain, thalamus, and cortex. Moreover, these synaptic properties engender as yet unexplored issues regarding the neuronal processing of auditory information, such as the convergent integration and long-term plasticity of auditory forebrain inputs.

Topography and physiology of ascending streams in the auditory tectothalamic pathway

Auditory information is relayed from the cochlea along parallel pathways and reaches the inferior colliculus (IC) and the medial geniculate body (MGB) en route to the cortex. Although the ascending tectothalamic pathway to the ventral division of the MGB is regarded as a high-fidelity information-bearing channel, the roles of the pathways to the dorsal and medial divisions are more opaque. Here, we show fundamental differences between these ascending pathways using an in vitro slice preparation. Using photostimulation, we found three main patterns of input (excitatory, inhibitory, and mixed) that differed in each pathway. Furthermore, electrical stimulation of the central nucleus of the IC evoked a depressing response in the MGB with no metabotropic glutamate (mGlu) receptor component, whereas stimulation of the lateral cortex of the IC evoked a facilitating response with an mGlu receptor component. These data suggest that the ascending tectothalamic pathways are functionally distinct from one another.

A classification of NOergic neurons in the inferior colliculus of rat according to co-existence with classical amino acid transmitters

Since the localization of nitric oxide synthase (NOS) can be identified by enzyme histochemistry for NADPH-diaphorse (NADPH-d), this method has been used widely for mapping NOS-containing (presumably NOergic) neurons in the central nervous system. So far several studies suggest that NADPH-d is present in distinct neuronal populations in the inferior colliculus (IC), a major processing center for both the ascending and descending auditory pathway, and NO may play an important role in audition. On one hand, there is evidence from several lines of research that the IC makes extensive use of the neuroactive amino acids, in particular the inhibitory transmitter g-aminobutyric acid (GABA) and the excitatory amino acid glutamate (GLU). However, lacking is a description of the distribution of NOergic neurons to which traditional neurotransmitters may be linked. The present research utilized NADPH-d enzyme histochemistry in combination with immunocytochemistry to determine if NO may colocalize with either or both GABA and glutamate in distinct subpopulations of IC neurons. The NADPH-d positive neurons were predominantly found in two main subdivisions of the IC: the external cortex (ECIC) and the dorsal cortex (DCIC). The large numbers of these NADPH-d positive neurons appeared immunostained for GLU while only a small number, seemed to belong to the small cells (somatic area < 100 microm2) similarity to stellate cells group was positive for GABA throughout the cortex of the IC. Owing to no coexistence between GABA and GLU in the same NADPH-d positive neuron in the pairs of adjacent sections of the IC by the mirror-image technique, the present results consequently support that NOergic neurons could be subdivided into at least three distinct populations with a large proportion of about 77% being GLUergic, much lower frequency of about 11% being GABAergic and the remaining 12% expressing non-GABA and non-GLU. In summary, the existence of two functionally distinct populations of NO/GABAergic and NO/GLUergic neurons in the NOergic neurons of IC suggest that at least two differential pattern of GLU-mediated excitatory NO transmission and GABA-mediated inhibitory NO transmission are involved in the networking of auditory communication in the cortex of IC.

Inhibitory neurotransmission, plasticity and aging in the mammalian central auditory system

Aging and acoustic trauma may result in partial peripheral deafferentation in the central auditory pathway of the mammalian brain. In accord with homeostatic plasticity, loss of sensory input results in a change in pre- and postsynaptic GABAergic and glycinergic inhibitory neurotransmission. As seen in development, age-related changes may be activity dependent. Age-related presynaptic changes in the cochlear nucleus include reduced glycine levels, while in the auditory midbrain and cortex, GABA synthesis and release are altered. Presumably, in response to age-related decreases in presynaptic release of inhibitory neurotransmitters, there are age-related postsynaptic subunit changes in the composition of the glycine (GlyR) and GABA(A) (GABA(A)R) receptors. Age-related changes in the subunit makeup of inhibitory pentameric receptor constructs result in altered pharmacological and physiological responses consistent with a net down-regulation of functional inhibition. Age-related functional changes associated with glycine neurotransmission in dorsal cochlear nucleus (DCN) include altered intensity and temporal coding by DCN projection neurons. Loss of synaptic inhibition in the superior olivary complex (SOC) and the inferior colliculus (IC) likely affect the ability of aged animals to localize sounds in their natural environment. Age-related postsynaptic GABA(A)R changes in IC and primary auditory cortex (A1) involve changes in the subunit makeup of GABA(A)Rs. In turn, these changes cause age-related changes in the pharmacology and response properties of neurons in IC and A1 circuits, which collectively may affect temporal processing and response reliability. Findings of age-related inhibitory changes within mammalian auditory circuits are similar to age and deafferentation plasticity changes observed in other sensory systems. Although few studies have examined sensory aging in the wild, these age-related changes would likely compromise an animal's ability to avoid predation or to be a successful predator in their natural environment.

Neural representation of spectral and temporal information in speech

Speech is the most interesting and one of the most complex sounds dealt with by the auditory system. The neural representation of speech needs to capture those features of the signal on which the brain depends in language communication. Here we describe the representation of speech in the auditory nerve and in a few sites in the central nervous system from the perspective of the neural coding of important aspects of the signal. The representation is tonotopic, meaning that the speech signal is decomposed by frequency and different frequency components are represented in different populations of neurons. Essential to the representation are the properties of frequency tuning and nonlinear suppression. Tuning creates the decomposition of the signal by frequency, and nonlinear suppression is essential for maintaining the representation across sound levels. The representation changes in central auditory neurons by becoming more robust against changes in stimulus intensity and more transient. However, it is probable that the form of the representation at the auditory cortex is fundamentally different from that at lower levels, in that stimulus features other than the distribution of energy across frequency are analysed.

Somatosensory nuclei of the manatee brainstem and thalamus

Florida manatees have an extensive, well-developed system of vibrissae distributed over their entire bodies and especially concentrated on the face. Although behavioral and anatomical assessments support the manatee's reliance on somatosensation, a systematic analysis of the manatee thalamus and brainstem areas dedicated to tactile input has never been completed. Using histochemical and histological techniques (including stains for myelin, Nissl, cytochrome oxidase, and acetylcholinesterase), we characterized the relative size, extent, and specializations of somatosensory regions of the brainstem and thalamus. The principal somatosensory regions of the brainstem (trigeminal, cuneate, gracile, and Bischoff's nucleus) and the thalamus (ventroposterior nucleus) were disproportionately large relative to nuclei dedicated to other sensory modalities, providing neuroanatomical evidence that supports the manatee's reliance on somatosensation. In fact, areas of the thalamus related to somatosensation (the ventroposterior and posterior nuclei) and audition (the medial geniculate nucleus) appeared to displace the lateral geniculate nucleus dedicated to the subordinate visual modality. Furthermore, it is noteworthy that, although the manatee cortex contains Rindenkerne (barrel-like cortical nuclei located in layer VI), no corresponding cell clusters were located in the brainstem ("barrelettes") or thalamus ("barreloids").

Intrinsic membrane properties and synaptic response characteristics of neurons in the rat's external cortex of the inferior colliculus

The inferior colliculus (IC) can be divided into three anatomical subdivisions: the central nucleus (ICc), the dorsal cortex (ICd) and the external cortex (ICx). ICx receives its primary auditory inputs from ICc and auditory cerebral cortical areas, and non-auditory inputs from regions of motor and other sensory systems. This wide array of projections makes the ICx a distinct structure within the auditory brainstem. The purpose of the current study was to comprehensively characterize the neuronal population of ICx, by intrinsic and synaptic response properties. Visual whole-cell patch clamp recordings were taken from ICx neurons (N=129) from rats between postnatal days 8 to 12. Neurons displayed various types of firing patterns in response to current injection, including regular, adapting, pauser and bursting. The regular cells constitute the majority (66%), followed by adapting (18%), pauser (13%) and bursting cells (2%). In response to hyperpolarizing current injection, many neurons illustrated a pronounced sag in the membrane potential, representing a hyperpolarization-activated current (I(h)). Some neurons (25%) displayed a Ca(2+)-dependent rebound depolarization following negative current injection. In response to depolarizing current injection, 70% of ICx neurons displayed a Ca(2+)-mediated potential expressed as Ca(2+) spikes/humps, uncovered when Na(+) and K(+) currents were removed. Also, spikes displayed an undershoot which was in part mediated by Ca(2+). Stimulation of the ICc elicited graded synaptic responses, which displayed a combination of excitatory and/or inhibitory potentials, with excitation being predominant across firing patterns. Neurons displayed temporal summation in response to repetitive stimulation at 20 Hz and higher. The results indicate a relatively modest diversity in firing pattern and in intrinsic membrane properties, making this subnucleus distinct from its counterparts within the IC. The data show that ICx receives major excitatory input from ICc, supporting its role in integrating signals from brainstem and directing information to higher brain centers.

Inhibitory and excitatory response areas of neurons in the central nucleus of the inferior colliculus in unanesthetized chinchillas

In unanesthetized chinchillas, we determined excitatory and inhibitory response regions of neurons in the central nucleus of the inferior colliculus (ICc). The responses of 250 multiunits and 47 single units in the ICc to one- and two-tone stimuli were measured by extracellular recordings. The one-tone excitatory response area of ICc neurons from awake chinchillas was classified as either narrow with a steep high-frequency slope >140 dB/oct (type 1), broad with a high-frequency slope <140 dB/oct (type 2), or complex with a negative high-frequency slope (type 3). One-tone inhibition was prominent only in units with a high spontaneous firing rate. As revealed with two-tone stimuli, inhibition in the ICc of awake chinchillas and its relation to excitatory response regions was different from what is reported in anesthetized animals. The two-tone inhibitory responses were classified as follows: (1) inhibitory regions of equal strength on both sides of the characteristic frequency; (2) asymmetrical inhibitory regions, more prominent at the high-frequency side of the characteristic frequency; (3) strong inhibitory regions overlying most of the one-tone excitatory response region; (4) inhibitory response regions lying only within the one-tone excitatory response region; and (5) neurons without clear two-tone inhibition. One-tone and two-tone inhibitory regions of the same unit were markedly different in 66% of the units with a high spontaneous rate. The neural response to frequencies within the inhibitory regions often was an onset response followed by inhibition. Excitatory and inhibitory response properties were similar over considerable penetration distances (600-1,000 microm) in a particular dorso-ventral recording track.

Neuronal responses to lemniscal stimulation in laminar brain slices of the inferior colliculus

The central nucleus of the inferior colliculus (ICC) receives inputs from all parts of the auditory brainstem and transmits the information to the forebrain. Fibrodendritic laminae of the ICC provide a structural basis for a tonotopic organization, and the interaction of inputs within a single layer is important for ICC processing. Transverse slice planes of the ICC sever the layers and many of the ascending axons that enter through the lateral lemniscus. Consequently, the activity initiated within a lamina by a pure lemniscal stimulus is not well characterized. Here, we use a slice plane that maintains the integrity of the laminae in ICC and allows the axons in the lateral lemniscus to be stimulated at a distance from the ICC. We examined both the postsynaptic currents and potentials of the same neurons to lemniscal stimuli in this laminar brain slice. Our main finding is that lemniscal stimulation evokes prolonged synaptic potentials in ICC neurons. Synaptic potential amplitudes and durations increase with lemniscal shock strength. In approximately 50% of ICC neurons, the postsynaptic potential is equal in duration to the postsynaptic current, whereas in the remaining neurons it is three to four times longer. Synaptic responses to single shocks or shock trains exhibit plateau potentials that enable sustained firing in ICC neurons. Plateau potentials are evoked by N-methyl-D-aspartate (NMDA) receptor activation, and their amplitudes and durations are regulated by both NMDA-R and gamma-aminobutyric acid A (GABAA)-R activation. These data suggest that in the intact laminae of the ICC, lemniscal inputs initiate sustained firing through monosynaptic and polysynaptic NMDA-mediated synapses regulated by GABAA synapses.

Descending projections from the auditory cortex to the inferior colliculus in the gerbil, Meriones unguiculatus

Corticofugal projections to the auditory midbrain, the inferior colliculus (IC), influence the way in which specific sets of IC neurons process acoustic signals. We used retrograde tracer (Fluorogold, Fluororuby, microbeads) injections in the IC to study the morphology and location of cortico-collicular projecting neurons and anterograde tracer (dextran biotin) injections in auditory cortical fields to describe the distribution of terminals in the IC. Nissl staining, cytochrome oxidase activity, and neurofilament SMI32 immunostaining were used to delimit the different auditory areas. We defined a primary or "core" auditory cortex and a secondary "caudal" auditory area containing layer V pyramidal neurons that project to the IC. These projections target the central nucleus of the IC (CNIC) ipsilaterally and the IC cortices bilaterally, with the ipsilateral component predominant. Other secondary auditory areas, dorsal and ventral to the core, do not directly participate in this projection. The ventral secondary cortex targets midbrain periaqueductal gray. The projection from the core cortex originates from two classes of layer V pyramidal cells. Cells presenting a tufted apical dendrite in layer I have dense terminal fields in the IC cortices. Pyramids lacking layer I dendritic tufts target the CNIC in a less dense but tonotopic manner. The caudal cortex projection originates from smaller layer V pyramids and targets the IC cortices with dense terminal fields. Descending auditory inputs from the core and caudal areas converge in the dorsal and external cortices of the IC. Descending connections to the gerbil IC form a segregated system in which multiple descending channels originating from different neuronal subpopulations may modulate specific aspects of ascending auditory information.

Development of the projection from the nucleus of the brachium of the inferior colliculus to the superior colliculus in the ferret

Neurons in the deeper layers of the superior colliculus (SC) have spatially tuned receptive fields that are arranged to form a map of auditory space. The spatial tuning of these neurons emerges gradually in an experience-dependent manner after the onset of hearing, but the relative contributions of peripheral and central factors in this process of maturation are unknown. We have studied the postnatal development of the projection to the ferret SC from the nucleus of the brachium of the inferior colliculus (nBIC), its main source of auditory input, to determine whether the emergence of auditory map topography can be attributed to anatomical rewiring of this projection. The pattern of retrograde labeling produced by injections of fluorescent microspheres in the SC on postnatal day (P) 0 and just after the age of hearing onset (P29), showed that the nBIC-SC projection is topographically organized in the rostrocaudal axis, along which sound azimuth is represented, from birth. Injections of biotinylated dextran amine-fluorescein into the nBIC at different ages (P30, 60, and 90) labeled axons with numerous terminals and en passant boutons throughout the deeper layers of the SC. This labeling covered the entire mediolateral extent of the SC, but, in keeping with the pattern of retrograde labeling following microsphere injections in the SC, was more restricted rostrocaudally. No systematic changes were observed with age. The stability of the nBIC-SC projection over this period suggests that developmental changes in auditory spatial tuning involve other processes, rather than a gross refinement of the projection from the nBIC.

Quality of pronase dissociation of mature inferior colliculus neurons

One major advantage of acutely dissociated inferior colliculus (IC) neurons in electrophysiological investigations is their complete isolation from the surrounding cellular network. In this way, patch-clamp recordings can be performed under controlled conditions to study membrane properties of IC neurons in more detail. The aim of the present study was to adapt a dissociation method for immature IC neurons to the highly sensitive, fragile and vulnerable mature IC neurons of mammals (mice). The modification of a pronase-based dissociation protocol with respect to concentration, incubation time and handling (trituration) of the cells yielded intact, live IC neurons with a clean cell surface so that they were well suited for further electrophysiological investigations in our study. The largely modified dissociation protocol is described in detail and critically discussed.

Serotonin modulates responses to species-specific vocalizations in the inferior colliculus

Neuromodulators such as serotonin are capable of altering the neural processing of stimuli across many sensory modalities. In the inferior colliculus, a major midbrain auditory gateway, serotonin alters the way that individual neurons respond to simple tone bursts and linear frequency modulated sweeps. The effects of serotonin are complex, and vary among neurons. How serotonin transforms the responses to spectrotemporally complex sounds of the type normally heard in natural settings has been poorly examined. To explore this issue further, the effects of iontophoretically applied serotonin on the responses of individual inferior colliculus neurons to a variety of recorded species-specific vocalizations were examined. These experiments were performed in the Mexican free-tailed bat, a species that uses a rich repertoire of vocalizations for the purposes of communication as well as echolocation. Serotonin frequently changed the number of recorded calls that were capable of evoking a response from individual neurons, sometimes increasing (15% of serotonin-responsive neurons), but usually decreasing (62% of serotonin-responsive neurons), this number. A functional consequence of these serotonin-evoked changes would be to change the population response to species-specific vocalizations.

Degradation of temporal resolution in the auditory midbrain after prolonged deafness is reversed by electrical stimulation of the cochlea

In an animal model of prelingual deafness, we examined the anatomical and physiological effects of prolonged deafness and chronic electrical stimulation on temporal resolution in the adult central auditory system. Maximum following frequencies (Fmax) and first spike latencies of single neurons responding to electrical pulse trains were evaluated in the inferior colliculus of two groups of neonatally deafened cats after prolonged periods of deafness (>2.5 yr): the first group was implanted with an intracochlear electrode and studied acutely (long-deafened unstimulated, LDU); the second group (LDS) received a chronic implant and several weeks of electrical stimulation (pulse rates > or =300 pps). Acutely deafened and implanted adult cats served as controls. Spiral ganglion cell density in all long-deafened animals was markedly reduced (mean <5.8% of normal). Both long-term deafness and chronic electrical stimulation altered temporal resolution of neurons in the central nucleus (ICC) but not in the external nucleus. Specifically, LDU animals exhibited significantly poorer temporal resolution of ICC neurons (lower Fmax, longer response latencies) as compared with control animals. In contrast, chronic stimulation in LDS animals led to a significant increase in temporal resolution. Changes in temporal resolution after long-term deafness and chronic stimulation occurred broadly across the entire ICC and were not correlated with its tonotopic gradient. These results indicate that chronic electrical stimulation can reverse the degradation in temporal resolution in the auditory midbrain after long-term deafness and suggest the importance of factors other than peripheral pathology on plastic changes in the temporal processing capabilities of the central auditory system.

Organization of binaural excitatory and inhibitory inputs to the inferior colliculus from the superior olive

The major excitatory, binaural inputs to the central nucleus of the inferior colliculus (ICC) are from two groups of neurons with different functions-the ipsilateral medial superior olive (MSO) and the contralateral lateral superior olive (LSO). A major inhibitory, binaural input emerges from glycinergic neurons in the ipsilateral LSO. To determine whether these inputs converge on the same postsynaptic targets in the ICC, two different anterograde tracers were injected in tonotopically matched areas of the MSO and the LSO on the opposite side in the same animal. The main findings were that the boutons from MSO axons terminated primarily in the central and caudal parts of the ICC laminae but that contralateral LSO terminals were concentrated more rostrally and on the ventral margins of the MSO inputs. In contrast, the ipsilateral LSO projection converged with the MSO inputs and was denser than the contralateral LSO projection. Consistent with this finding, retrograde transport experiments showed that the very low-frequency areas of the ICC with dense MSO inputs also received inputs from greater numbers of ipsilateral LSO neurons than from contralateral LSO neurons. The results suggest that different binaural pathways through the low-frequency ICC may be formed by the segregation of excitatory inputs to ICC from the MSO and the contralateral LSO. At the same time, the ipsilateral LSO is a major inhibitory influence in the target region of the MSO. These data support the concept that each frequency-band lamina in the ICC may comprise several functional modules with different combinations of inputs.

Effects of restricted cochlear lesions in adult cats on the frequency organization of the inferior colliculus

Restricted cochlear lesions in adult animals result in plastic changes in the representation of the lesioned cochlea, and thus in the frequency map, in the contralateral auditory cortex and thalamus. To examine the contribution of subthalamic changes to this reorganization, the effects of unilateral mechanical cochlear lesions on the frequency organization of the central nucleus of the inferior colliculus (ICC) were examined in adult cats. Lesions typically resulted in a broad high-frequency hearing loss extending from a frequency in the range 15-22 kHz. After recovery periods of 2.5-18 months, the frequency organization of ICC contralateral to the lesioned cochlea was determined separately for the onset and late components of multiunit responses to tone-burst stimuli. For the late response component in all but one penetration through the ICC, and for the onset response component in more than half of the penetrations, changes in frequency organization in the lesion projection zone were explicable as the residue of prelesion responses. In half of the penetrations exhibiting nonresidue type changes in onset-response frequency organization, the changes appeared to reflect the unmasking of normally inhibited inputs. In the other half it was unclear whether the changes reflected unmasking or a dynamic process of reorganization. Thus, most of the observed changes were explicable as passive consequences of the lesion, and there was limited evidence for plasticity in the ICC. The implications of the data with respect to the primary locus of the changes and to the manner in which they contribute to thalamocortical reorganization are considered.

Dendritic orientation and laminar architecture in the rabbit auditory thalamus

A laminar organization composed of the dendritic fields of principal neurons and afferent axonal arbors has been proposed as the anatomical substrate for the frequency map at several levels of the mammalian central auditory system, including the inferior colliculus and medial geniculate body (MGB). In contrast to the auditory thalamus in most mammals, the ventral division of the rabbit medial geniculate body (MGV) has cellular laminae visible in routine Nissl stains, allowing a direct comparison of the laminar organization with the dendritic architecture and frequency organization. In total 30 presumptive relay neurons in the MGV were labeled with the juxtacellular recording method, and their dendritic arbors were fully reconstructed from serial sections with the aid of a computer microscope. The spatial organization of MGV dendritic fields was analyzed using the dendritic prism, dendritic stick, and fan-in projection methods. Quantitative spatial analyses revealed that, for MGV neurons in the central pars lateralis subdivision, the major axis of the dendritic fields (approximately 29 degrees relative to the horizontal plane) was closely aligned with that of the Nissl laminae (approximately 25 degrees). Both were oriented orthogonally to the tonotopic axis. In contrast, cells in the pars ovoidea had their major axis of orientation parallel to the anteroposterior axis of the brain. Although a bitufted dendritic field was the norm, it was not uncommon for MGV neurons to have pronounced spatial asymmetries in their dendritic fields. A model is presented that incorporates cellular laminae and oriented dendritic growth to form frequency-related slabs within the MGV.

Presynaptic modulation of GABAergic inhibition by GABA(B) receptors in the rat's inferior colliculus

Whole-cell patch clamp recordings were made from neurons in a brain slice preparation of the inferior colliculus in 11-15-day-old rat pups. Synaptic responses were elicited by applying a current pulse to the lateral lemniscus just below the central nucleus of the inferior colliculus. To examine GABAergic inhibition in the inferior colliculus all excitatory postsynaptic potentials and glycinergic inhibitory postsynaptic potentials were blocked by bath application of their respective antagonists and the contribution of GABA(B) receptors was determined for the remaining inhibitory postsynaptic potentials. For most cells the isolated inhibitory postsynaptic potential was completely blocked by the GABA(A) receptor antagonist, bicuculline, but was unaffected by the GABA(B) receptor antagonist, phaclofen. The GABA(B) receptor agonist, baclofen (10-20 microM), decreased the amplitude of the inhibitory postsynaptic potentials. This effect was completely blocked by phaclofen. Baclofen did not increase the cell membrane conductance or alter the rate of firing produced by depolarization of the cell membrane. In contrast, muscimol, a GABA(A) receptor agonist, greatly increased membrane conductance and lowered the firing rate produced by depolarization. Our results indicate that GABAergic inhibition in the auditory midbrain can be reduced by the activation of GABA(B) receptors and suggest that the effects are presynaptic.

Synaptic modification in neurons of the central nucleus of the inferior colliculus

Whole-cell patch clamp recordings were made from neurons in the central nucleus of the inferior colliculus (ICC) in brain slices from rat (8-13 days old). ICC neurons were classified by their discharge pattern in response to depolarizing and hyperpolarizing current injection. Excitatory postsynaptic currents (EPSCs) were elicited by stimulation of synaptic inputs under the condition that the synaptic inhibition was suppressed by strychnine and picrotoxin. EPSCs in all tested types of ICC neurons showed posttetanic, long-term potentiation (LTP) and long-term depression with tetanic stimulation. The potentiated EPSCs consisted of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor and NMDA receptor mediated components. The magnitude of LTP was larger when the intracellular concentration of the calcium buffer ethylene glycol-bis (beta-aminoethyl ether)-N,N,N',N'-tetracetic acid (EGTA) was lower and stimulation frequency was higher in cells with rebound firing patterns. Blocking N-methyl-D-aspartate (NMDA) receptors in rebound cells prevented generation of LTP. These results suggest that excitatory synaptic transmission in ICC neurons can be modified. LTP in the auditory midbrain may be important for activity-dependent, adaptive changes in response to normal and pathological stimulus conditions.

AMPA and NMDA receptors mediate synaptic excitation in the rat's inferior colliculus

The synaptic mechanisms underlying excitation in the rat's central nucleus of the inferior colliculus (ICC) were examined by making whole-cell patch clamp recordings in brain slice preparations of the auditory midbrain. Responses were elicited by current pulse stimulation of the lateral lemniscus and recordings were made in ICC using either current clamp or voltage clamp methods. The excitatory postsynaptic responses in either current or voltage clamp mode consisted of two distinct components, an early component that could be blocked by bath application of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonists, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide (NBQX), and a later component that could be blocked by application of the N-methyl-D-aspartate (NMDA) receptor antagonists, (+/-)-2-amino-5-phosphonovaleric acid (APV) or (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP). Both AMPA and NMDA receptor-mediated responses were present at resting potential and could be isolated pharmacologically by application of receptor antagonists. Voltage clamp experiments revealed that the NMDA receptor-mediated current was voltage-dependent and increased in magnitude as the cell membrane was depolarized. This NMDA receptor-mediated response was enhanced at resting potential when Mg(2+) was eliminated from the bath solution. The ratio of response amplitudes associated with the late and early components, an estimate of the relative contribution of NMDA and AMPA receptor types, changed with age. There was a progressive decline in the ratio between 9 and 13 days of age, but no further reduction between days 13 and 16. The data show that both AMPA and NMDA receptors are important for determining excitatory responses in the ICC and that both receptor types probably play a role in auditory processing after the onset of hearing.

Serotonin in the inferior colliculus

It has been recognized for some time that serotonin fibers originating in raphe nuclei are present in the inferior colliculi of all mammalian species studied. More recently, serotonin has been found to modulate the responses of single inferior colliculus neurons to many types of auditory stimuli, ranging from simple tone bursts to complex species-specific vocalizations. The effects of serotonin are often quite strong, and for some neurons are also highly specific. A dramatic illustration of this is that serotonin can change the selectivity of some neurons for sounds, including species-specific vocalizations. These results are discussed in light of several theories on the function of serotonin in the IC, and of outstanding issues that remain to be addressed.

Contribution of AMPA and NMDA receptors to excitatory responses in the inferior colliculus

Brain slice studies of neurons in the central nucleus of the inferior colliculus (ICC) indicate that excitatory responses evoked by electrical stimulation of the lateral lemniscus consist of two components, an early, rapid response mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and a later, a slower one mediated by N-methyl-D-aspartate (NMDA) receptors. The early response can be selectively blocked by AMPA receptor antagonists (1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide disodium [NBQX]; or 6-cyano-7-nitroquinoxaline-2,3-dione) [CNQX], and the later one by NMDA receptor antagonists ((+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid [CPP]; or (+/-)-2-amino-5-phosphonovaleric acid) [APV]. Both AMPA and NMDA receptor-mediated responses can be elicited at resting potential, although the NMDA response is voltage dependent and makes a greater contribution when the cell membrane is depolarized. In vivo studies indicate that both AMPA and NMDA receptors contribute to sound-evoked responses. Both AMPA and NMDA receptor antagonists reduce the firing rate of single neurons in the ICC to contralaterally presented tones. Both classes of antagonist lower evoked activity over a wide range of sound intensities from threshold to maximum sound pressure levels. Thus, both NMDA and AMPA receptors contribute to responses over the full dynamic range of auditory sensitivity. The AMPA receptor antagonist, NBQX, is more effective than the NMDA receptor antagonist, CPP, in blocking responses of onset cells. Furthermore, NBQX and CPP have preferential effects in blocking the early or late responses of neurons that exhibited sustain activity to a 100 ms tone. Excitatory responses to sinusoidally amplitude-modulated stimuli are also reduced by application of either AMPA or NMDA antagonists. However, the synchrony of firing of action potentials to the modulation period (vector strength) is largely unaffected. The data suggest that the synchrony of firing of neurons in the inferior colliculus is determined primarily by the pattern of activity at lower levels of the auditory pathway and/or the local intrinsic properties of the cells.