Midbrain-Level Neural Correlates of Behavioral Tone-in-Noise Detection: Dependence on Energy and Envelope Cues.

Hearing in noise is a problem often assumed to depend on encoding of energy level by channels tuned to target frequencies, but few studies have tested this hypothesis. The present study examined neural correlates of behavioral tone-in-noise (TIN) detection in budgerigars (Melopsittacus undulatus, either sex), a parakeet species with human-like behavioral sensitivity to many simple and complex sounds. Behavioral sensitivity to tones in band-limited noise was assessed using operant-conditioning procedures. Neural recordings were made in awake animals from midbrain-level neurons in the inferior colliculus, the first processing stage of the ascending auditory pathway with pronounced rate-based encoding of stimulus amplitude modulation. Budgerigar TIN detection thresholds were similar to human thresholds across the full range of frequencies (0.5-4 kHz) and noise levels (45-85 dB SPL) tested. Also as in humans, thresholds were minimally affected by a challenging roving-level condition with random variation in background-noise level. Many midbrain neurons showed a decreasing response rate as TIN signal-to-noise ratio (SNR) was increased by elevating the tone level, a pattern attributable to amplitude-modulation tuning in these cells and the fact that higher SNR tone-plus-noise stimuli have flatter amplitude envelopes. TIN thresholds of individual neurons were as sensitive as behavioral thresholds under most conditions, perhaps surprisingly even when the unit's characteristic frequency was tuned an octave or more away from the test frequency. A model that combined responses of two cell types enhanced TIN sensitivity in the roving-level condition. These results highlight the importance of midbrain-level envelope encoding and off-frequency neural channels for hearing in noise.SIGNIFICANCE STATEMENT Detection of target sounds in noise is often assumed to depend on energy-level encoding by neural processing channels tuned to the target frequency. In contrast, we found that tone-in-noise sensitivity in budgerigars was often greatest in midbrain neurons not tuned to the test frequency, underscoring the potential importance of off-frequency channels for perception. Furthermore, the results highlight the importance of envelope processing for hearing in noise, especially under challenging conditions with random variation in background noise level over time.

Cholinergic boutons are closely associated with excitatory cells and four subtypes of inhibitory cells in the inferior colliculus.

Acetylcholine (ACh) is a neuromodulator that has been implicated in multiple roles across the brain, including the central auditory system, where it sets neuronal excitability and gain and affects plasticity. In the cerebral cortex, subtypes of GABAergic interneurons are modulated by ACh in a subtype-specific manner. Subtypes of GABAergic neurons have also begun to be described in the inferior colliculus (IC), a midbrain hub of the auditory system. Here, we used male and female mice (Mus musculus) that express fluorescent protein in cholinergic cells, axons, and boutons to look at the association between ACh and four subtypes of GABAergic IC cells that differ in their associations with extracellular markers, their soma sizes, and their distribution within the IC. We found that most IC cells, including excitatory and inhibitory cells, have cholinergic boutons closely associated with their somas and proximal dendrites. We also found that similar proportions of each of four subtypes of GABAergic cells are closely associated with cholinergic boutons. Whether the different types of GABAergic cells in the IC are differentially regulated remains unclear, as the response of cells to ACh is dependent on which types of ACh receptors are present. Additionally, this study confirms the presence of these four subtypes of GABAergic cells in the mouse IC, as they had previously been identified only in guinea pigs. These results suggest that cholinergic projections to the IC modulate auditory processing via direct effects on a multitude of inhibitory circuits.

Inferior collicular cells that project to the auditory thalamus are increasingly surrounded by perineuronal nets with age.

The age-related loss of GABA in the inferior colliculus (IC) likely plays a role in the development of age-related hearing loss. Perineuronal nets (PNs), specialized aggregates of extracellular matrix, increase with age in the IC. PNs, associated with GABAergic neurotransmission, can stabilize synapses and inhibit structural plasticity. We sought to determine whether PN expression increased on GABAergic and non-GABAergic IC cells that project to the medial geniculate body (MG). We used retrograde tract-tracing in combination with immunohistochemistry for glutamic acid decarboxylase and Wisteria floribunda agglutinin across three age groups of Fischer Brown Norway rats. Results demonstrate that PNs increase with age on lemniscal and non-lemniscal IC-MG cells, however two key differences exist. First, PNs increased on non-lemniscal IC-MG cells during middle-age, but not until old age on lemniscal IC-MG cells. Second, increases of PNs on lemniscal IC-MG cells occurred on non-GABAergic cells rather than on GABAergic cells. These results suggest that synaptic stabilization and reduced plasticity likely occur at different ages on a subset of the IC-MG pathway.

Efferent feedback controls bilateral auditory spontaneous activity.

In the developing auditory system, spontaneous activity generated in the cochleae propagates into the central nervous system to promote circuit formation. The effects of peripheral firing patterns on spontaneous activity in the central auditory system are not well understood. Here, we describe wide-spread bilateral coupling of spontaneous activity that coincides with the period of transient efferent modulation of inner hair cells from the brainstem medial olivocochlear system. Knocking out α9/α10 nicotinic acetylcholine receptors, a requisite part of the efferent pathway, profoundly reduces bilateral correlations. Pharmacological and chemogenetic experiments confirm that the efferent system is necessary for normal bilateral coupling. Moreover, auditory sensitivity at hearing onset is reduced in the absence of pre-hearing efferent modulation. Together, these results demonstrate how afferent and efferent pathways collectively shape spontaneous activity patterns and reveal the important role of efferents in coordinating bilateral spontaneous activity and the emergence of functional responses during the prehearing period.

Cortical Stimulation Induces Excitatory Postsynaptic Potentials of Inferior Colliculus Neurons in a Frequency-Specific Manner.

Corticofugal modulation of auditory responses in subcortical nuclei has been extensively studied whereas corticofugal synaptic transmission must still be characterized. This study examined postsynaptic potentials of the corticocollicular system, i.e., the projections from the primary auditory cortex (AI) to the central nucleus of the inferior colliculus (ICc) of the midbrain, in anesthetized C57 mice. We used focal electrical stimulation at the microampere level to activate the AI (ESAI) and in vivo whole-cell current-clamp to record the membrane potentials of ICc neurons. Following the whole-cell patch-clamp recording of 88 ICc neurons, 42 ICc neurons showed ESAI-evoked changes in the membrane potentials. We found that the ESAI induced inhibitory postsynaptic potentials in 6 out of 42 ICc neurons but only when the stimulus current was 96 µA or higher. In the remaining 36 ICc neurons, excitatory postsynaptic potentials (EPSPs) were induced at a much lower stimulus current. The 36 ICc neurons exhibiting EPSPs were categorized into physiologically matched neurons (n = 12) when the characteristic frequencies of the stimulated AI and recorded ICc neurons were similar (≤1 kHz) and unmatched neurons (n = 24) when they were different (>1 kHz). Compared to unmatched neurons, matched neurons exhibited a significantly lower threshold of evoking noticeable EPSP, greater EPSP amplitude, and shorter EPSP latency. Our data allow us to propose that corticocollicular synaptic transmission is primarily excitatory and that synaptic efficacy is dependent on the relationship of the frequency tunings between AI and ICc neurons.

Amplitude modulation transfer functions reveal opposing populations within both the inferior colliculus and medial geniculate body.

Based on single-unit recordings of modulation transfer functions (MTFs) in the inferior colliculus (IC) and the medial geniculate body (MGB) of the unanesthetized rabbit, we identified two opposing populations: band-enhanced (BE) and band-suppressed (BS) neurons. In response to amplitude-modulated (AM) sounds, firing rates of BE and BS neurons were enhanced and suppressed, respectively, relative to their responses to an unmodulated noise with a one-octave bandwidth. We also identified a third population, designated hybrid neurons, whose firing rates were enhanced by some modulation frequencies and suppressed by others. Our finding suggests that perception of AM may be based on the co-occurrence of enhancement and suppression of responses of the opposing populations of neurons. Because AM carries an important part of the content of speech, progress in understanding auditory processing of AM sounds should lead to progress in understanding speech perception. Each of the BE, BS, and hybrid types of MTFs comprised approximately one-third of the total sample. Modulation envelopes having short duty cycles of 20-50% and raised-sine envelopes accentuated the degree of enhancement and suppression and sharpened tuning of the MTFs. With sinusoidal envelopes, peak modulation frequencies were centered around 32-64 Hz among IC BE neurons, whereas the MGB peak frequencies skewed toward lower frequencies, with a median of 16 Hz. We also tested an auditory-brainstem model and found that a simple circuit containing fast excitatory synapses and slow inhibitory synapses was able to reproduce salient features of the BE- and BS-type MTFs of IC neurons.NEW & NOTEWORTHY Opposing populations of neurons have been identified in the mammalian auditory midbrain and thalamus. In response to amplitude-modulated sounds, responses of one population (band-enhanced) increased whereas responses of another (band-suppressed) decreased relative to their responses to an unmodulated sound. These opposing auditory populations are analogous to the ON and OFF populations of the visual system and may improve transfer of information carried by the temporal envelopes of complex sounds such as speech.

Cholinergic Projections From the Pedunculopontine Tegmental Nucleus Contact Excitatory and Inhibitory Neurons in the Inferior Colliculus.

The inferior colliculus processes nearly all ascending auditory information. Most collicular cells respond to sound, and for a majority of these cells, the responses can be modulated by acetylcholine (ACh). The cholinergic effects are varied and, for the most part, the underlying mechanisms are unknown. The major source of cholinergic input to the inferior colliculus is the pedunculopontine tegmental nucleus (PPT), part of the pontomesencephalic tegmentum known for projections to the thalamus and roles in arousal and the sleep-wake cycle. Characterization of PPT inputs to the inferior colliculus has been complicated by the mixed neurotransmitter population within the PPT. Using selective viral-tract tracing techniques in a ChAT-Cre Long Evans rat, the present study characterizes the distribution and targets of cholinergic projections from PPT to the inferior colliculus. Following the deposit of viral vector in one PPT, cholinergic axons studded with boutons were present bilaterally in the inferior colliculus, with the greater density of axons and boutons ipsilateral to the injection site. On both sides, cholinergic axons were present throughout the inferior colliculus, distributing boutons to the central nucleus, lateral cortex, and dorsal cortex. In each inferior colliculus (IC) subdivision, the cholinergic PPT axons appear to contact both GABAergic and glutamatergic neurons. These findings suggest cholinergic projections from the PPT have a widespread influence over the IC, likely affecting many aspects of midbrain auditory processing. Moreover, the effects are likely to be mediated by direct cholinergic actions on both excitatory and inhibitory circuits in the inferior colliculus.

Circuit Mechanisms Underlying the Segregation and Integration of Parallel Processing Streams in the Inferior Colliculus.

The lateral cortex of the inferior colliculus (LCIC) forms a nexus between diverse multisensory, motor, and neuromodulatory streams. Like other integration hubs, it contains repeated neurochemical motifs with distinct inputs: GABA-rich modules are innervated by somatosensory structures, while auditory inputs to the LCIC target the surrounding extramodular matrix. To investigate potential mechanisms of convergence between these input streams, we used laser photostimulation circuit mapping to interrogate local LCIC circuits in adult mice of both sexes and found that input patterns are highly dependent on cell type (GABAergic/non-GABAergic) and location (module/matrix). At the circuit level, these inputs yield a directional flow of local information, primarily from the matrix to the modules. Further, the two compartments were found to project to distinct targets in the midbrain and thalamus. These data show that, while connectional modularity in the LCIC gives rise to segregated input-output channels, local circuits provide the architecture for integration between these two streams.SIGNIFICANCE STATEMENT Modularity is a widespread motif across the brain involving the segregation of structures into discrete subregions based on dichotomies in neurochemical expression or connectivity. The inferior colliculus is one such modular structure, containing auditory-recipient matrix regions and GABA-rich modules that are innervated by somatosensory inputs. While modularity suggests segregation of processing streams, here we show that local circuits in the inferior colliculus connect the module and matrix regions, providing an avenue for integration of information across compartments.

Serotonergic innervation of the auditory midbrain: dorsal raphe subregions differentially project to the auditory midbrain in male and female mice.

In the auditory inferior colliculus (IC), serotonin reflects features of context including the valence of social interactions, stressful events, and prior social experience. However, within the dorsal raphe nucleus (DRN; B6 + B7), the source of serotonergic projections to the IC has not been resolved at the level of DRN subregions. Additionally, few studies have investigated which DRN subregions are engaged during naturalistic, sensory-driven social behaviors. We employ traditional, retrograde tract-tracing approaches to comprehensively map the topographic extent of DRN-IC projection neurons in male and female mice. We combine this approach with immediate early gene (cFos) mapping in order to describe the functional properties of DRN subregions during contexts in which serotonin fluctuates within the IC. These approaches provide novel evidence that the dorsal (DRd) and lateral (DRl) B7 subregions are primarily responsible for serotonergic innervation of the IC; further, we show that this projection is larger in male than in female mice. Additionally, DRd and the ventral B7 (DRv) contained more transcriptionally active serotonergic neurons irrespective of behavioral context. Male mice had more active serotonergic neurons in DRd and DRv than females following sociosexual encounters. However, serotonergic activity was correlated with the expression of female but not male social behaviors. The topographic organization of the DRN-IC projection provides the anatomical framework to test a mechanism underlying context-dependent auditory processing. We further highlight the importance of including sex as a biological variable when describing the functional topography of DRN.

Postnatal exposure to an acoustically enriched environment alters the morphology of neurons in the adult rat auditory system.

The structure of neurons in the central auditory system is vulnerable to various kinds of acoustic exposures during the critical postnatal developmental period. Here we explored long-term effects of exposure to an acoustically enriched environment (AEE) during the third and fourth weeks of the postnatal period in rat pups. AEE consisted of a spectrally and temporally modulated sound of moderate intensity, reinforced by a behavioral paradigm. At the age of 3-6 months, a Golgi-Cox staining was used to evaluate the morphology of neurons in the inferior colliculus (IC), the medial geniculate body (MGB), and the auditory cortex (AC). Compared to controls, rats exposed to AEE showed an increased mean dendritic length and volume and the soma surface in the external cortex and the central nucleus of the IC. The spine density increased in both the ventral and dorsal divisions of the MGB. In the AC, the total length and volume of the basal dendritic segments of pyramidal neurons and the number and density of spines on these dendrites increased significantly. No differences were found on apical dendrites. We also found an elevated number of spines and spine density in non-pyramidal neurons. These results show that exposure to AEE during the critical developmental period can induce permanent changes in the structure of neurons in the central auditory system. These changes represent morphological correlates of the functional plasticity, such as an improvement in frequency tuning and synchronization with temporal parameters of acoustical stimuli.

Long-range Channelrhodopsin-assisted Circuit Mapping of Inferior Colliculus Neurons with Blue and Red-shifted Channelrhodopsins.

When investigating neural circuits, a standard limitation of the in vitro patch clamp approach is that axons from multiple sources are often intermixed, making it difficult to isolate inputs from individual sources with electrical stimulation. However, by using channelrhodopsin assisted circuit mapping (CRACM), this limitation can now be overcome. Here, we report a method to use CRACM to map ascending inputs from lower auditory brainstem nuclei and commissural inputs to an identified class of neurons in the inferior colliculus (IC), the midbrain nucleus of the auditory system. In the IC, local, commissural, ascending, and descending axons are heavily intertwined and therefore indistinguishable with electrical stimulation. By injecting a viral construct to drive expression of a channelrhodopsin in a presynaptic nucleus, followed by patch clamp recording to characterize the presence and physiology of channelrhodopsin-expressing synaptic inputs, projections from a specific source to a specific population of IC neurons can be mapped with cell type-specific accuracy. We show that this approach works with both Chronos, a blue light-activated channelrhodopsin, and ChrimsonR, a red-shifted channelrhodopsin. In contrast to previous reports from the forebrain, we find that ChrimsonR is robustly trafficked down the axons of dorsal cochlear nucleus principal neurons, indicating that ChrimsonR may be a useful tool for CRACM experiments in the brainstem. The protocol presented here includes detailed descriptions of the intracranial virus injection surgery, including stereotaxic coordinates for targeting injections to the dorsal cochlear nucleus and IC of mice, and how to combine whole cell patch clamp recording with channelrhodopsin activation to investigate long-range projections to IC neurons. Although this protocol is tailored to characterizing auditory inputs to the IC, it can be easily adapted to investigate other long-range projections in the auditory brainstem and beyond.

A Cortico-Collicular Amplification Mechanism for Gap Detection.

Auditory cortex (AC) is necessary for the detection of brief gaps in ongoing sounds, but not for the detection of longer gaps or other stimuli such as tones or noise. It remains unclear why this is so, and what is special about brief gaps in particular. Here, we used both optogenetic suppression and conventional lesions to show that the cortical dependence of brief gap detection hinges specifically on gap termination. We then identified a cortico-collicular gap detection circuit that amplifies cortical gap termination responses before projecting to inferior colliculus (IC) to impact behavior. We found that gaps evoked off-responses and on-responses in cortical neurons, which temporally overlapped for brief gaps, but not long gaps. This overlap specifically enhanced cortical responses to brief gaps, whereas IC neurons preferred longer gaps. Optogenetic suppression of AC reduced collicular responses specifically to brief gaps, indicating that under normal conditions, the enhanced cortical representation of brief gaps amplifies collicular gap responses. Together these mechanisms explain how and why AC contributes to the behavioral detection of brief gaps, which are critical cues for speech perception, perceptual grouping, and auditory scene analysis.

Integration of locomotion and auditory signals in the mouse inferior colliculus.

The inferior colliculus (IC) is the major midbrain auditory integration center, where virtually all ascending auditory inputs converge. Although the IC has been extensively studied for sound processing, little is known about the neural activity of the IC in moving subjects, as frequently happens in natural hearing conditions. Here, by recording neural activity in walking mice, we show that the activity of IC neurons is strongly modulated by locomotion, even in the absence of sound stimuli. Similar modulation was also found in hearing-impaired mice, demonstrating that IC neurons receive non-auditory, locomotion-related neural signals. Sound-evoked activity was attenuated during locomotion, and this attenuation increased frequency selectivity across the neuronal population, while maintaining preferred frequencies. Our results suggest that during behavior, integrating movement-related and auditory information is an essential aspect of sound processing in the IC.

Shaping of discrete auditory inputs to extramodular zones of the lateral cortex of the inferior colliculus.

The multimodal lateral cortex of the inferior colliculus (LCIC) exhibits a modular-extramodular micro-organization that is evident early in development. In addition to a set of neurochemical markers that reliably highlight its modular-extramodular organization (e.g. modules: GAD67-positive, extramodular zones: calretinin-positive, CR), mature projection patterns suggest that major LCIC afferents recognize and adhere to such a framework. In adult mice, distinct afferent projections appear segregated, with somatosensory inputs targeting LCIC modules and auditory inputs surrounding extramodular fields. Currently lacking is an understanding regarding the development and shaping of multimodal LCIC afferents with respect to its emerging modular-extramodular microarchitecture. Combining living slice tract-tracing and immunocytochemical approaches in GAD67-GFP knock-in mice, the present study characterizes the critical period of projection shaping for LCIC auditory afferents arising from its neighboring central nucleus (CNIC). Both crossed and uncrossed projection patterns exhibit LCIC extramodular mapping characteristics that emerge from initially diffuse distributions. Projection mismatch with GAD-defined modules and alignment with encompassing extramodular zones becomes increasingly clear over the early postnatal period (birth to postnatal day 12). CNIC inputs terminate almost exclusively in extramodular zones that express CR. These findings suggest multimodal LCIC inputs may initially be sparse and intermingle, prior to segregation into distinct processing streams. Future experiments are needed to determine the likely complex interactions and mechanisms (e.g. activity-dependent and independent) responsible for shaping early modality-specific LCIC circuits.

Differential cell-type dependent brain state modulations of sensory representations in the non-lemniscal mouse inferior colliculus.

Sensory responses of the neocortex are strongly influenced by brain state changes. However, it remains unclear whether and how the sensory responses of the midbrain are affected. Here we addressed this issue by using in vivo two-photon calcium imaging to monitor the spontaneous and sound-evoked activities in the mouse inferior colliculus (IC). We developed a method enabling us to image the first layer of non-lemniscal IC (IC shell L1) in awake behaving mice. Compared with the awake state, spectral tuning selectivity of excitatory neurons was decreased during isoflurane anesthesia. Calcium imaging in behaving animals revealed that activities of inhibitory neurons were highly correlated with locomotion. Compared with stationary periods, spectral tuning selectivity of excitatory neurons was increased during locomotion. Taken together, our studies reveal that neuronal activities in the IC shell L1 are brain state dependent, whereas the brain state modulates the excitatory and inhibitory neurons differentially.

Responses of neurons in the rat's inferior colliculus to a sound are affected by another sound in a space-dependent manner.

The perception of a sound can be influenced by another sound in a space-dependent manner. An understanding of this perceptual phenomenon depends on knowledge about how the spatial relationship between two sounds affects neural responses to the sounds. We used the rat as a model system and equal-probability two-tone sequences as stimuli to evaluate how spatial separation between two asynchronously recurring sounds affected responses to the sounds in midbrain auditory neurons. We found that responses elicited by two tone bursts when they were colocalized at the ear contralateral to the neuron were different from the responses elicited by the same sounds when they were separated with one at the contralateral ear while the other at another location. For neurons with transient sound-driven firing and not responsive to stimulation presented at the ipsilateral ear, the response to a sound with a fixed location at the contralateral ear was enhanced when the second sound was separated. These neurons were likely important for detecting a sound in the presence of a spatially separated competing sound. Our results suggest that mechanisms underlying effects of spatial separation on neural responses to sounds may include adaptation and long-lasting binaural excitatory/inhibitory interaction.

Neural coding and perception of auditory motion direction based on interaural time differences.

While motion is important for parsing a complex auditory scene into perceptual objects, how it is encoded in the auditory system is unclear. Perceptual studies suggest that the ability to identify the direction of motion is limited by the duration of the moving sound, yet we can detect changes in interaural differences at even shorter durations. To understand the source of these distinct temporal limits, we recorded from single units in the inferior colliculus (IC) of unanesthetized rabbits in response to noise stimuli containing a brief segment with linearly time-varying interaural time difference ("ITD sweep") temporally embedded in interaurally uncorrelated noise. We also tested the ability of human listeners to either detect the ITD sweeps or identify the motion direction. Using a point-process model to separate the contributions of stimulus dependence and spiking history to single-neuron responses, we found that the neurons respond primarily by following the instantaneous ITD rather than exhibiting true direction selectivity. Furthermore, using an optimal classifier to decode the single-neuron responses, we found that neural threshold durations of ITD sweeps for both direction identification and detection overlapped with human threshold durations even though the average response of the neurons could track the instantaneous ITD beyond psychophysical limits. Our results suggest that the IC does not explicitly encode motion direction, but internal neural noise may limit the speed at which we can identify the direction of motion.NEW & NOTEWORTHY Recognizing motion and identifying an object's trajectory are important for parsing a complex auditory scene, but how we do so is unclear. We show that neurons in the auditory midbrain do not exhibit direction selectivity as found in the visual system but instead follow the trajectory of the motion in their temporal firing patterns. Our results suggest that the inherent variability in neural firings may limit our ability to identify motion direction at short durations.

Near physiological spectral selectivity of cochlear optogenetics.

Cochlear implants (CIs) electrically stimulate spiral ganglion neurons (SGNs) and partially restore hearing to half a million CI users. However, wide current spread from intracochlear electrodes limits spatial selectivity (i.e. spectral resolution) of electrical CIs. Optogenetic stimulation might become an alternative, since light can be confined in space, promising artificial sound encoding with increased spectral selectivity. Here we compare spectral selectivity of optogenetic, electric, and acoustic stimulation by multi-channel recordings in the inferior colliculus (IC) of gerbils. When projecting light onto tonotopically distinct SGNs, we observe corresponding tonotopically ordered IC activity. An activity-based comparison reveals that spectral selectivity of optogenetic stimulation is indistinguishable from acoustic stimulation for modest intensities. Moreover, optogenetic stimulation outperforms bipolar electric stimulation at medium and high intensities and monopolar electric stimulation at all intensities. In conclusion, we demonstrate better spectral selectivity of optogenetic over electric SGN stimulation, suggesting the potential for improved hearing restoration by optical CIs.

Analysis of excitatory and inhibitory neuron types in the inferior colliculus based on Ih properties.

The inferior colliculus (IC) is a large midbrain nucleus that integrates inputs from many auditory brainstem and cortical structures. Despite its prominent role in auditory processing, the various cell types and their connections within the IC are not well characterized. To further separate GABAergic and non-GABAergic neuron types according to their physiological properties, we used a mouse model that expresses channelrhodopsin and enhanced yellow fluorescent protein in all GABAergic neurons and allows identification of GABAergic cells by light stimulation. Neuron types were classified upon electrophysiological measurements of the hyperpolarizing-activated current (Ih) in acute brain slices of young adult mice. All GABAergic neurons from our sample displayed slow-activating Ih with moderate amplitudes, whereas a subset of excitatory neurons showed fast-activating Ih with large amplitudes. This is in agreement with our finding that immunoreactivity against the fast-gating hyperpolarization-activated and cyclic-nucleotide-gated 1 (HCN1) channel was present around excitatory neurons, whereas the slow-gating HCN4 channel was found perisomatically around most inhibitory neurons. Ih properties and neurotransmitter types were correlated with firing patterns to depolarizing current pulses. All GABAergic neurons displayed adapting firing patterns very similar to the majority of glutamatergic neurons. About 15% of the glutamatergic neurons showed an onset spiking pattern, always in combination with large and fast Ih. We conclude that HCN channel subtypes are differentially distributed in IC neuron types and correlate with neurotransmitter type and firing pattern. In contrast to many other brain regions, membrane properties and firing patterns were similar in GABAergic neurons and about one-third of the excitatory neurons. NEW & NOTEWORTHY Neuron types in the central nucleus of the auditory midbrain are not well characterized regarding their transmitter type, ion channel composition, and firing pattern. The present study shows that GABAergic neurons have slowly activating hyperpolarizing-activated current (Ih) and an adaptive firing pattern whereas at least four types of glutamatergic neurons exist regarding their Ih properties and firing patterns. Many of the glutamatergic neurons were almost indistinguishable from the GABAergic neurons regarding Ih properties and firing pattern.

Characterization of the human central nucleus of the inferior colliculus.

The inferior colliculus (IC) is a major relay station for both ascending and descending auditory pathways. The IC is divided into three major regions, the external cortex (ECIC), the dorsal cortex (DCIC) and the central nucleus of the inferior colliculus (CNIC). While the ECIC and DCIC receive many non-auditory inputs, the CNIC receives predominantly auditory input ascending within the lateral lemniscus and descending input from the cerebral cortex. Recent work in animal models emphasizes the complexity of the CNIC and provides evidence for multiple ascending informational streams reaching this nucleus. Despite an abundance of research on the CNIC in laboratory animals, the microscopic anatomy and neurochemistry of the human CNIC is poorly understood. Herein, we utilize a combination of gross morphology, myelin staining, Nissl staining, histochemistry, immunohistochemistry and immunofluorescence to characterize the human CNIC. Our results indicate that the human CNIC occupies a volume of approximately 22.4 mm3 and includes over 420,000 neurons. The human CNIC is dominated by round/oval neurons arranged with their long axis parallel to fibrodendritic lamina. Additionally, the vast majority of CNIC neurons are associated with a perineuronal net, there is an abundance of tyrosine hydroxylase immunoreactive axons and puncta and neurons immunoreactive for glutamic acid decarboxylase. These results are largely consistent with observations in laboratory animals.