Extracellular Molecular Markers and Soma Size of Inhibitory Neurons: Evidence for Four Subtypes of GABAergic Cells in the Inferior Colliculus

Exploring the relationship between larmor-frequency electrical conductivity, diffusivity, and tissue volume in the aging brain

Background

The aging brain undergoes various microstructural changes that influence its electrical properties. Conductivity, a measure of ion mobility, is particularly sensitive to these changes and can be assessed non-invasively using magnetic resonance electrical properties tomography (MREPT). Despite advancements in imaging techniques, the relationship between brain conductivity, diffusivity, and tissue volume in the context of aging and neurodegeneration remains incompletely understood. This study explores the relationships between electrical conductivity, diffusivity, and brain tissue volume in the aging brain, which is crucial for early diagnosis and monitoring of neurodegenerative diseases such as Alzheimer's, where these parameters could serve as potential biomarkers for disease progression.

Methods

In this cross-sectional, prospective study, 77 patients were assessed brain MREPT and diffusion tensor imaging with multiple shells and gradient directions (b=0, 800, and 2,000 s/mm2). High-frequency conductivity (HFC) was calculated and separated into extra-neurite (EC) and intra-neurite conductivities (IC). We analyzed correlations between these conductivity indices and other magnetic resonance imaging (MRI) metrics, controlling for age, and explored the relationship between conductivity, diffusion, and Mini-Mental State Examination (MMSE) scores using multiple regression analysis.

Results

EC within the insular region negatively correlated with MMSE scores (r=-0.3027, P=0.0079). HFC in the hippocampus was positively associated with mean diffusivity (MD; β=192.4, P=0.008) and radial diffusivity (RD; β=207.6, P=0.004). HFC in the insula was positively associated with axial diffusivity (AxD; β=356.9, P=0.0004), MD (β=314.4, P=0.004), RD (β=275.5, P=0.012). EC in the hippocampus was positively associated with AxD (β=309.3, P=0.0001), MD (β=333.7, P<0.001), RD (β=341.8, P<0.001). EC in the insular was positively associated with AxD (β=324.1, P=0.0009) and MD (β=270.4, P=0.01). IC was positively correlated with intra-neurite diffusivity (ID) in the amygdala, thalamus, and insula.

Conclusions

These findings suggest that increased conductivity is associated with altered diffusivity and reduced cognitive performance, suggesting the use of MREPT to differentiate between conductivity changes due to ion mobility versus proton density, and how this approach contributes to understanding the aging brain and neurodegeneration. MREPT-derived measurements primarily reflect ion mobility and caution that clinical interpretations should consider the direct relationships between conductivity and diffusion changes.

Distinct Neuron Types Contribute to Hybrid Auditory Spatial Coding

Neural decoding is a tool for understanding how activities from a population of neurons inside the brain relate to the outside world and for engineering applications such as brain-machine interfaces. However, neural decoding studies mainly focused on different decoding algorithms rather than different neuron types which could use different coding strategies. In this study, we used two-photon calcium imaging to assess three auditory spatial decoders (space map, opponent channel, and population pattern) in excitatory and inhibitory neurons in the dorsal inferior colliculus of male and female mice. Our findings revealed a clustering of excitatory neurons that prefer similar interaural level difference (ILD), the primary spatial cues in mice, while inhibitory neurons showed random local ILD organization. We found that inhibitory neurons displayed lower decoding variability under the opponent channel decoder, while excitatory neurons achieved higher decoding accuracy under the space map and population pattern decoders. Further analysis revealed that the inhibitory neurons' preference for ILD off the midline and the excitatory neurons' heterogeneous ILD tuning account for their decoding differences. Additionally, we discovered a sharper ILD tuning in the inhibitory neurons. Our computational model, linking this to increased presynaptic inhibitory inputs, was corroborated using monaural and binaural stimuli. Overall, this study provides experimental and computational insight into how excitatory and inhibitory neurons uniquely contribute to the coding of sound locations.

Ototoxicity-related changes in GABA immunolabeling within the rat inferior colliculus

Several studies suggest that hearing loss results in changes in the balance between inhibition and excitation in the inferior colliculus (IC). The IC is an integral nucleus within the auditory brainstem. The majority of ascending pathways from the lateral lemniscus (LL), superior olivary complex (SOC), and cochlear nucleus (CN) synapse in the IC before projecting to the thalamus and cortex. Many of these ascending projections provide inhibitory innervation to neurons within the IC. However, the nature and the distribution of this inhibitory input have only been partially elucidated in the rat. The inhibitory neurotransmitter, gamma aminobutyric acid (GABA), from the ventral nucleus of the lateral lemniscus (VNLL), provides the primary inhibitory input to the IC of the rat with GABA from other lemniscal and SOC nuclei providing lesser, but prominent innervation. There is evidence that hearing related conditions can result in dysfunction of IC neurons. These changes may be mediated in part by changes in GABA inputs to IC neurons. We have previously used gene micro-arrays in a study of deafness-related changes in gene expression in the IC and found significant changes in GAD as well as the GABA transporters and GABA receptors (Holt 2005). This is consistent with reports of age and trauma related changes in GABA (Bledsoe et al., 1995; Mossop et al., 2000; Salvi et al., 2000). Ototoxic lesions of the cochlea produced a permanent threshold shift. The number, intensity, and density of GABA positive axon terminals in the IC were compared in normal hearing and deafened rats. While the number of GABA immunolabeled puncta was only minimally different between groups, the intensity of labeling was significantly reduced. The ultrastructural localization and distribution of labeling was also examined. In deafened animals, the number of immuno gold particles was reduced by 78 % in axodendritic and 82 % in axosomatic GABAergic puncta. The affected puncta were primarily associated with small IC neurons. These results suggest that reduced inhibition to IC neurons contribute to the increased neuronal excitability observed in the IC following noise or drug induced hearing loss. Whether these deafness diminished inhibitory inputs originate from intrinsic or extrinsic CNIC sources awaits further study.

Reduction of prolonged excitatory neuron swelling after spinal cord injury improves locomotor recovery in mice

Spinal cord injury (SCI) results in acute damage and triggers secondary injury responses with sustained neuronal loss and dysfunction. However, the underlying mechanisms for these delayed neuronal pathologies are not entirely understood. SCI results in the swelling of spinal neurons, but the contribution of cell swelling to neuronal loss and functional deficits after SCI has not been systematically characterized. In this study, we devised a three-dimensional image analysis pipeline to evaluate spinal neurons, examining their types, quantities, volumes, and spatial distribution in a double-lateral hemisection SCI mouse model. We found that both excitatory and inhibitory neurons swell and are lost, albeit with distinct temporal patterns. Inhibitory neurons demonstrated marked swelling and decline in number on day 2 after SCI, which resolved by day 14. In contrast, excitatory neurons maintained persistent swelling and continued cell loss for at least 35 days after SCI in mice. Excitatory neurons exhibited sustained expression of the Na+-K+-Cl- cotransporter 1 (NKCC1), whereas inhibitory neurons down-regulated the protein by day 14 after SCI. Treatment with a Food and Drug Administration-approved NKCC1 inhibitor, bumetanide, mitigated swelling of excitatory neurons and reduced their loss in the secondary injury phase after SCI. The administration of bumetanide after SCI in mouse improved locomotor recovery, with functional benefits persisting for at least 4 weeks after treatment cessation. This study advances our understanding of SCI-related pathology and introduces bumetanide as a potential treatment to mitigate sustained neuronal swelling and enhance recovery after SCI.

Hearing loss-related altered neuronal activity in the inferior colliculus

Hearing loss is well known to cause plastic changes in the central auditory system and pathological changes such as tinnitus and hyperacusis. Impairment of inner ear functions is the main cause of hearing loss. In aged individuals, not only inner ear dysfunction but also senescence of the central nervous system is the cause of malfunction of the auditory system. In most cases of hearing loss, the activity of the auditory nerve is reduced, but that of the successive auditory centers is increased in a compensatory way. It has been reported that activity changes occur in the inferior colliculus (IC), a critical nexus of the auditory pathway. The IC integrates the inputs from the brainstem and drives the higher auditory centers. Since abnormal activity in the IC is likely to affect auditory perception, it is crucial to elucidate the neuronal mechanism to induce the activity changes of IC neurons with hearing loss. This review outlines recent findings on hearing-loss-induced plastic changes in the IC and brainstem auditory neuronal circuits and discusses what neuronal mechanisms underlie hearing-loss-induced changes in the activity of IC neurons. Considering the different causes of hearing loss, we discuss age-related hearing loss separately from other forms of hearing loss (non-age-related hearing loss). In general, the main plastic change of IC neurons caused by both age-related and non-age-related hearing loss is increased central gain. However, plastic changes in the IC caused by age-related hearing loss seem to be more complex than those caused by non-age-related hearing loss.

Lineage-tracing reveals an expanded population of NPY neurons in the inferior colliculus

Growing evidence suggests that neuropeptide signaling shapes auditory computations. We previously showed that neuropeptide Y (NPY) is expressed in the inferior colliculus (IC) by a population of GABAergic stellate neurons and that NPY regulates the strength of local excitatory circuits in the IC. NPY neurons were initially characterized using the NPY-hrGFP mouse, in which humanized renilla green fluorescent protein (hrGFP) expression indicates NPY expression at the time of assay, i.e., an expression-tracking approach. However, studies in other brain regions have shown that NPY expression can vary based on several factors, suggesting that the NPY-hrGFP mouse might miss NPY neurons not expressing NPY on the experiment date. Here, we hypothesized that neurons with the ability to express NPY represent a larger population of IC GABAergic neurons than previously reported. To test this hypothesis, we used a lineage-tracing approach to irreversibly tag neurons that expressed NPY at any point prior to the experiment date. We then compared the physiological and anatomical features of neurons labeled with this lineage-tracing approach to our prior data set, revealing a larger population of NPY neurons than previously found. In addition, we used optogenetics to test the local connectivity of NPY neurons and found that NPY neurons provide inhibitory synaptic input to other neurons in the ipsilateral IC. Together, our data expand the definition of NPY neurons in the IC, suggest that NPY expression might be dynamically regulated in the IC, and provide functional evidence that NPY neurons form local inhibitory circuits in the IC.NEW & NOTEWORTHY Across brain regions, neuropeptide Y (NPY) expression is dynamic and influenced by extrinsic and intrinsic factors. We previously showed that NPY is expressed by a class of inhibitory neurons in the auditory midbrain. Here, we find that this neuron class also includes neurons that previously expressed NPY, suggesting that NPY expression is dynamically regulated in the auditory midbrain. We also provide functional evidence that NPY neurons contribute to local inhibitory circuits in the auditory midbrain.

Two Prediction Error Systems in the Nonlemniscal Inferior Colliculus: "Spectral" and "Nonspectral"

According to the predictive processing framework, perception emerges from the reciprocal exchange of predictions and prediction errors (PEs) between hierarchically organized neural circuits. The nonlemniscal division of the inferior colliculus (IC) is the earliest source of auditory PE signals, but their neuronal generators, properties, and functional relevance have remained mostly undefined. We recorded single-unit mismatch responses to auditory oddball stimulation at different intensities, together with activity evoked by two sequences of alternating tones to control frequency-specific effects. Our results reveal a differential treatment of the unpredictable "many-standards" control and the predictable "cascade" control by lemniscal and nonlemniscal IC neurons that is not present in the auditory thalamus or cortex. Furthermore, we found that frequency response areas of nonlemniscal IC neurons reflect their role in subcortical predictive processing, distinguishing three hierarchical levels: (1) nonlemniscal neurons with sharply tuned receptive fields exhibit mild repetition suppression without signaling PEs, thereby constituting the input level of the local predictive processing circuitry. (2) Neurons with broadly tuned receptive fields form the main, "spectral" PE signaling system, which provides dynamic gain compensation to near-threshold unexpected sounds. This early enhancement of saliency reliant on spectral features was not observed in the auditory thalamus or cortex. (3) Untuned neurons form an accessory, "nonspectral" PE signaling system, which reports all surprising auditory deviances in a robust and consistent manner, resembling nonlemniscal neurons in the auditory cortex. These nonlemniscal IC neurons show unstructured and unstable receptive fields that could result from inhibitory input controlled by corticofugal projections conveying top-down predictions.

Research progress of the inferior colliculus: from Neuron, neural circuit to auditory disease

The auditory midbrain, also known as the inferior colliculus (IC), serves as a crucial hub in the auditory pathway. Comprising diverse cell types, the IC plays a pivotal role in various auditory functions, including sound localization, auditory plasticity, sound detection, and sound-induced behaviors. Notably, the IC is implicated in several auditory central disorders, such as tinnitus, age-related hearing loss, autism and Fragile X syndrome. Accurate classification of IC neurons is vital for comprehending both normal and dysfunctional aspects of IC function. Various parameters, including dendritic morphology, neurotransmitter synthesis, potassium currents, biomarkers, and axonal targets, have been employed to identify distinct neuron types within the IC. However, the challenge persists in effectively classifying IC neurons into functional categories due to the limited clustering capabilities of most parameters. Recent studies utilizing advanced neuroscience technologies have begun to shed light on biomarker-based approaches in the IC, providing insights into specific cellular properties and offering a potential avenue for understanding IC functions. This review focuses on recent advancements in IC research, spanning from neurons and neural circuits to aspects related to auditory diseases.

Lineage-tracing reveals an expanded population of NPY neurons in the inferior colliculus

Growing evidence suggests that neuropeptide signaling shapes auditory computations. We previously showed that neuropeptide Y (NPY) is expressed in the inferior colliculus (IC) by a population of GABAergic stellate neurons and that NPY regulates the strength of local excitatory circuits in the IC. NPY neurons were initially characterized using the NPY-hrGFP reporter mouse, in which hrGFP expression indicates NPY expression at the time of assay, i.e., an expression-tracking approach. However, studies in other brain regions have shown that NPY expression can vary based on a range of factors, suggesting that the NPY-hrGFP mouse might miss NPY neurons not expressing NPY proximal to the experiment date. Here, we hypothesized that neurons with the ability to express NPY represent a larger population of IC GABAergic neurons than previously reported. To test this hypothesis, we used a lineage-tracing approach to irreversibly tag neurons that expressed NPY at any point prior to the experiment date. We then compared the physiological and anatomical features of neurons labeled with this lineage-tracing approach to our prior data set, revealing a larger population of NPY neurons than previously found. In addition, we used optogenetics to test the local connectivity of NPY neurons and found that NPY neurons routinely provide inhibitory synaptic input to other neurons in the ipsilateral IC. Together, our data expand the definition of NPY neurons in the IC, suggest that NPY expression might be dynamically regulated in the IC, and provide functional evidence that NPY neurons form local inhibitory circuits in the IC.

Parvalbumin and Somatostatin: Biomarkers for Two Parallel Tectothalamic Pathways in the Auditory Midbrain

The inferior colliculus (IC) represents a crucial relay station in the auditory pathway, located in the midbrain's tectum and primarily projecting to the thalamus. Despite the identification of distinct cell classes based on various biomarkers in the IC, their specific contributions to the organization of auditory tectothalamic pathways have remained poorly understood. In this study, we demonstrate that IC neurons expressing parvalbumin (ICPV+) or somatostatin (ICSOM+) represent two minimally overlapping cell classes throughout the three IC subdivisions in mice of both sexes. Strikingly, regardless of their location within the IC, these neurons predominantly project to the primary and secondary auditory thalamic nuclei, respectively. Cell class-specific input tracing suggested that ICPV+ neurons primarily receive auditory inputs, whereas ICSOM+ neurons receive significantly more inputs from the periaqueductal gray and the superior colliculus (SC), which are sensorimotor regions critically involved in innate behaviors. Furthermore, ICPV+ neurons exhibit significant heterogeneity in both intrinsic electrophysiological properties and presynaptic terminal size compared with ICSOM+ neurons. Notably, approximately one-quarter of ICPV+ neurons are inhibitory neurons, whereas all ICSOM+ neurons are excitatory neurons. Collectively, our findings suggest that parvalbumin and somatostatin expression in the IC can serve as biomarkers for two functionally distinct, parallel tectothalamic pathways. This discovery suggests an alternative way to define tectothalamic pathways and highlights the potential usefulness of Cre mice in understanding the multifaceted roles of the IC at the circuit level.

Identifying neuron types and circuit mechanisms in the auditory midbrain

The inferior colliculus (IC) is a critical computational hub in the central auditory pathway. From its position in the midbrain, the IC receives nearly all the ascending output from the lower auditory brainstem and provides the main source of auditory information to the thalamocortical system. In addition to being a crossroads for auditory circuits, the IC is rich with local circuits and contains more than five times as many neurons as the nuclei of the lower auditory brainstem combined. These results hint at the enormous computational power of the IC, and indeed, systems-level studies have identified numerous important transformations in sound coding that occur in the IC. However, despite decades of effort, the cellular mechanisms underlying IC computations and how these computations change following hearing loss have remained largely impenetrable. In this review, we argue that this challenge persists due to the surprisingly difficult problem of identifying the neuron types and circuit motifs that comprise the IC. After summarizing the extensive evidence pointing to a diversity of neuron types in the IC, we highlight the successes of recent efforts to parse this complexity using molecular markers to define neuron types. We conclude by arguing that the discovery of molecularly identifiable neuron types ushers in a new era for IC research marked by molecularly targeted recordings and manipulations. We propose that the ability to reproducibly investigate IC circuits at the neuronal level will lead to rapid advances in understanding the fundamental mechanisms driving IC computations and how these mechanisms shift following hearing loss.

Age-related upregulation of perineuronal nets on inferior collicular cells that project to the cochlear nucleus

Introduction

Disruptions to the balance of excitation and inhibition in the inferior colliculus (IC) occur during aging and underlie various aspects of hearing loss. Specifically, the age-related alteration to GABAergic neurotransmission in the IC likely contributes to the poorer temporal precision characteristic of presbycusis. Perineuronal nets (PNs), a specialized form of the extracellular matrix, maintain excitatory/inhibitory synaptic environments and reduce structural plasticity. We sought to determine whether PNs increasingly surround cell populations in the aged IC that comprise excitatory descending projections to the cochlear nucleus.

Method

We combined Wisteria floribunda agglutinin (WFA) staining for PNs with retrograde tract-tracing in three age groups of Fischer Brown Norway (FBN) rats.

Results

The data demonstrate that the percentage of IC-CN cells with a PN doubles from ~10% at young age to ~20% at old age. This was true in both lemniscal and non-lemniscal IC.

Discussion

Furthermore, the increase of PNs occurred on IC cells that make both ipsilateral and contralateral descending projections to the CN. These results indicate that reduced structural plasticity in the elderly IC-CN pathway, affecting excitatory/inhibitory balance and, potentially, may lead to reduced temporal precision associated with presbycusis.

Neuropeptide Y Signaling Regulates Recurrent Excitation in the Auditory Midbrain

Neuropeptides play key roles in shaping the organization and function of neuronal circuits. In the inferior colliculus (IC), which is in the auditory midbrain, Neuropeptide Y (NPY) is expressed by a class of GABAergic neurons that project locally and outside the IC. Most neurons in the IC have local axon collaterals; however, the organization and function of local circuits in the IC remain unknown. We previously found that excitatory neurons in the IC can express the NPY Y1 receptor (Y1R+) and application of the Y1R agonist, [Leu31, Pro34]-NPY (LP-NPY), decreases the excitability of Y1R+ neurons. As NPY signaling regulates recurrent excitation in other brain regions, we hypothesized that Y1R+ neurons form interconnected local circuits in the IC and that NPY decreases the strength of recurrent excitation in these circuits. To test this hypothesis, we used optogenetics to activate Y1R+ neurons in mice of both sexes while recording from other neurons in the ipsilateral IC. We found that nearly 80% of glutamatergic IC neurons express the Y1 receptor, providing extensive opportunities for NPY signaling to regulate local circuits. Additionally, Y1R+ neuron synapses exhibited modest short-term synaptic plasticity, suggesting that local excitatory circuits maintain their influence over computations during sustained stimuli. We further found that application of LP-NPY decreased recurrent excitation in the IC, suggesting that NPY signaling strongly regulates local circuit function in the auditory midbrain. Our findings show that Y1R+ excitatory neurons form interconnected local circuits in the IC, and their influence over local circuits is regulated by NPY signaling.SIGNIFICANCE STATEMENT Local networks play fundamental roles in shaping neuronal computations in the brain. The IC, localized in the auditory midbrain, plays an essential role in sound processing, but the organization of local circuits in the IC is largely unknown. Here, we show that IC neurons that express the Neuropeptide Y1 receptor (Y1R+ neurons) make up most of the excitatory neurons in the IC and form interconnected local circuits. Additionally, we found that NPY, which is a powerful neuromodulator known to shape neuronal activity in other brain regions, decreases the extensive recurrent excitation mediated by Y1R+ neurons in local IC circuits. Thus, our results suggest that local NPY signaling is a key regulator of auditory computations in the IC.

Differential optogenetic activation of the auditory midbrain in freely moving behaving mice

Introduction

In patients with severe auditory impairment, partial hearing restoration can be achieved by sensory prostheses for the electrical stimulation of the central nervous system. However, these state-of-the-art approaches suffer from limited spectral resolution: electrical field spread depends on the impedance of the surrounding medium, impeding spatially focused electrical stimulation in neural tissue. To overcome these limitations, optogenetic activation could be applied in such prostheses to achieve enhanced resolution through precise and differential stimulation of nearby neuronal ensembles. Previous experiments have provided a first proof for behavioral detectability of optogenetic activation in the rodent auditory system, but little is known about the generation of complex and behaviorally relevant sensory patterns involving differential activation.

Methods

In this study, we developed and behaviorally tested an optogenetic implant to excite two spatially separated points along the tonotopy of the murine inferior colliculus (ICc).

Results

Using a reward based operant Go/No-Go paradigm, we show that differential optogenetic activation of a sub-cortical sensory pathway is possible and efficient. We demonstrate how animals which were previously trained in a frequency discrimination paradigm (a) rapidly respond to either sound or optogenetic stimulation, (b) generally detect optogenetic stimulation of two different neuronal ensembles, and (c) discriminate between them.

Discussion

Our results demonstrate that optogenetic excitatory stimulation at different points of the ICc tonotopy elicits a stable response behavior over time periods of several months. With this study, we provide the first proof of principle for sub-cortical differential stimulation of sensory systems using complex artificial cues in freely moving animals.

The role of GABAB receptors in the subcortical pathways of the mammalian auditory system

GABAB receptors are G-protein coupled receptors for the inhibitory neurotransmitter GABA. Functional GABAB receptors are formed as heteromers of GABAB1 and GABAB2 subunits, which further associate with various regulatory and signaling proteins to provide receptor complexes with distinct pharmacological and physiological properties. GABAB receptors are widely distributed in nervous tissue, where they are involved in a number of processes and in turn are subject to a number of regulatory mechanisms. In this review, we summarize current knowledge of the cellular distribution and function of the receptors in the inner ear and auditory pathway of the mammalian brainstem and midbrain. The findings suggest that in these regions, GABAB receptors are involved in processes essential for proper auditory function, such as cochlear amplifier modulation, regulation of spontaneous activity, binaural and temporal information processing, and predictive coding. Since impaired GABAergic inhibition has been found to be associated with various forms of hearing loss, GABAB dysfunction could also play a role in some pathologies of the auditory system.

Recurrent Circuits Amplify Corticofugal Signals and Drive Feedforward Inhibition in the Inferior Colliculus

The inferior colliculus (IC) is a midbrain hub critical for perceiving complex sounds, such as speech. In addition to processing ascending inputs from most auditory brainstem nuclei, the IC receives descending inputs from auditory cortex that control IC neuron feature selectivity, plasticity, and certain forms of perceptual learning. Although corticofugal synapses primarily release the excitatory transmitter glutamate, many physiology studies show that auditory cortical activity has a net inhibitory effect on IC neuron spiking. Perplexingly, anatomy studies imply that corticofugal axons primarily target glutamatergic IC neurons while only sparsely innervating IC GABA neurons. Corticofugal inhibition of the IC may thus occur largely independently of feedforward activation of local GABA neurons. We shed light on this paradox using in vitro electrophysiology in acute IC slices from fluorescent reporter mice of either sex. Using optogenetic stimulation of corticofugal axons, we find that excitation evoked with single light flashes is indeed stronger in presumptive glutamatergic neurons compared with GABAergic neurons. However, many IC GABA neurons fire tonically at rest, such that sparse and weak excitation suffices to significantly increase their spike rates. Furthermore, a subset of glutamatergic IC neurons fire spikes during repetitive corticofugal activity, leading to polysynaptic excitation in IC GABA neurons owing to a dense intracollicular connectivity. Consequently, recurrent excitation amplifies corticofugal activity, drives spikes in IC GABA neurons, and generates substantial local inhibition in the IC. Thus, descending signals engage intracollicular inhibitory circuits despite apparent constraints of monosynaptic connectivity between auditory cortex and IC GABA neurons.SIGNIFICANCE STATEMENT Descending "corticofugal" projections are ubiquitous across mammalian sensory systems, and enable the neocortex to control subcortical activity in a predictive or feedback manner. Although corticofugal neurons are glutamatergic, neocortical activity often inhibits subcortical neuron spiking. How does an excitatory pathway generate inhibition? Here we study the corticofugal pathway from auditory cortex to inferior colliculus (IC), a midbrain hub important for complex sound perception. Surprisingly, cortico-collicular transmission was stronger onto IC glutamatergic compared with GABAergic neurons. However, corticofugal activity triggered spikes in IC glutamate neurons with local axons, thereby generating strong polysynaptic excitation and feedforward spiking of GABAergic neurons. Our results thus reveal a novel mechanism that recruits local inhibition despite limited monosynaptic convergence onto inhibitory networks.

Neuropeptide Y signaling regulates recurrent excitation in the auditory midbrain

Neuropeptides play key roles in shaping the organization and function of neuronal circuits. In the inferior colliculus (IC), which is located in the auditory midbrain, Neuropeptide Y (NPY) is expressed by a large class of GABAergic neurons that project locally as well as outside the IC. The IC integrates information from numerous auditory nuclei making the IC an important hub for sound processing. Most neurons in the IC have local axon collaterals, however the organization and function of local circuits in the IC remains largely unknown. We previously found that neurons in the IC can express the NPY Y1 receptor (Y 1 R + ) and application of the Y 1 R agonist, [Leu 31 , Pro 34 ]-NPY (LP-NPY), decreases the excitability of Y 1 R + neurons. To investigate how Y 1 R + neurons and NPY signaling contribute to local IC networks, we used optogenetics to activate Y 1 R + neurons while recording from other neurons in the ipsilateral IC. Here, we show that 78.4% of glutamatergic neurons in the IC express the Y1 receptor, providing extensive opportunities for NPY signaling to regulate excitation in local IC circuits. Additionally, Y 1 R + neuron synapses exhibit modest short-term synaptic plasticity, suggesting that local excitatory circuits maintain their influence over computations during sustained stimuli. We further found that application of LP-NPY decreases recurrent excitation in the IC, suggesting that NPY signaling strongly regulates local circuit function in the auditory midbrain. Together, our data show that excitatory neurons are highly interconnected in the local IC and their influence over local circuits is tightly regulated by NPY signaling.

Age-related Changes of GAD1 mRNA Expression in the Central Inferior Colliculus

Encoding sounds with a high degree of temporal precision is an essential task for the inferior colliculus (IC) to perform and maintain the accurate processing of sounds and speech. However, the age-related reduction of GABAergic neurotransmission in the IC interrupts temporal precision and likely contributes to presbycusis. As presbycusis often manifests at high or low frequencies specifically, we sought to determine if the expression of mRNA for glutamic decarboxylase 1 (GAD1) is downregulated non-uniformly across the tonotopic axis or cell size range in the aging IC. Using single molecule in situ fluorescent hybridization across young, middle age and old Fisher Brown Norway rats (an aging model that acquires low frequency presbycusis) we quantified individual GAD1 mRNA in small, medium and large GABAergic cells. Our results demonstrate that small GABAergic cells in low frequency regions had ~58% less GAD1 in middle age and continued to decline into old age. In contrast, the amount of GAD1 mRNA in large cells in low frequency regions significantly increased with age. As several studies have shown that downregulation of GAD1 decreases the release of GABA, we interpret our results in two ways. First, the onset of presbycusis may be driven by small GABAergic cells downregulating GAD1. Second, as previous studies demonstrate that GAD67 expression is broadly downregulated in the old IC, perhaps the translation of GAD1 to GAD67 is interrupted in large GABAergic IC cells during aging. These results point to a potential genetic mechanism explaining reduced temporal precision in the aging IC, and in turn, presbycusis.

Age-related ultrastructural changes in the lateral cortex of the inferior colliculus

Temporal precision, a key component of sound and speech processing in the inferior colliculus (IC), depends on a balance of inhibition and excitation, and this balance degrades during aging. The cause of disrupted excitatory-inhibitory balance in aging is unknown, however changes at the synapse are a likely candidate. We sought to determine whether synaptic changes occur in the lateral cortex of the IC (IClc), a multimodal nucleus that processes lemniscal, intrinsic, somatosensory, and descending auditory input. Using electron microscopic techniques across young, middle age and old Fisher Brown Norway rats, our results demonstrate minimal loss of synapses in middle age, but significant (∼28%) loss during old age. However, in middle age, targeting of GABAergic dendrites by GABAergic synapses is increased and the active zones of excitatory synapses (that predominantly target GABA-negative dendrites) are lengthened. These synaptic changes likely result in a net increase of excitation in the IClc during middle age. Thus, disruption of excitatory-inhibitory balance in the aging IClc may be due to synaptic changes that begin in middle age.

Neurotransmitter phenotype and axonal projection patterns of VIP-expressing neurons in the inferior colliculus

Neurons in the inferior colliculus (IC), the midbrain hub of the central auditory pathway, send ascending and descending projections to other auditory brain regions, as well as projections to other sensory and non-sensory brain regions. However, the axonal projection patterns of individual classes of IC neurons remain largely unknown. Vasoactive intestinal polypeptide (VIP) is a neuropeptide expressed by subsets of neurons in many brain regions. We recently identified a class of IC stellate neurons that we called VIP neurons because they are labeled by tdTomato (tdT) expression in VIP-IRES-Cre x Ai14 mice. Here, using fluorescence in situ hybridization, we found that tdT+ neurons in VIP-IRES-Cre x Ai14 mice express Vglut2, a marker of glutamatergic neurons, and VIP, suggesting that VIP neurons use both glutamatergic and VIPergic signaling to influence their postsynaptic targets. Next, using viral transfections with a Cre-dependent eGFP construct, we labeled the axonal projections of VIP neurons. As a group, VIP neurons project intrinsically, within the ipsilateral and contralateral IC, and extrinsically to all the major targets of the IC. Within the auditory system, VIP neurons sent axons and formed axonal boutons in higher centers, including the medial geniculate nucleus and the nucleus of the brachium of the IC. Less dense projections terminated in lower centers, including the nuclei of the lateral lemniscus, superior olivary complex, and dorsal cochlear nucleus. VIP neurons also project to several non-auditory brain regions, including the superior colliculus, periaqueductal gray, and cuneiform nucleus. The diversity of VIP projections compared to the homogeneity of VIP neuron intrinsic properties suggests that VIP neurons play a conserved role at the microcircuit level, likely involving neuromodulation through glutamatergic and VIPergic signaling, but support diverse functions at the systems level through their participation in different projection pathways.

Inhibitory NPY Neurons Provide a Large and Heterotopic Commissural Projection in the Inferior Colliculus

Located in the midbrain, the inferior colliculus (IC) plays an essential role in many auditory computations, including speech processing and sound localization. The right and left sides of the IC are interconnected by a dense fiber tract, the commissure of the IC (CoIC), that provides each IC with one of its largest sources of input (i.e., the contralateral IC). Despite its prominence, the CoIC remains poorly understood. Previous studies using anterograde and retrograde tract-tracing showed that IC commissural projections are predominately homotopic and tonotopic, targeting mirror-image locations in the same frequency region in the contralateral IC. However, it is unknown whether specific classes of neurons, particularly inhibitory neurons which constitute ~10%–40% of the commissural projection, follow this pattern. We, therefore, examined the commissural projections of Neuropeptide Y (NPY) neurons, the first molecularly identifiable class of GABAergic neurons in the IC. Using retrograde tracing with Retrobeads (RB) in NPY-hrGFP mice of both sexes, we found that NPY neurons comprise ~11% of the commissural projection. Moreover, focal injections of Retrobeads showed that NPY neurons in the central nucleus of the IC exhibit a more divergent and heterotopic commissural projection pattern than non-NPY neurons. Thus, commissural NPY neurons are positioned to provide lateral inhibition to the contralateral IC. Through this circuit, sounds that drive activity in limited regions on one side of the IC likely suppress activity across a broader region in the contralateral IC.

Juxtacellular Labeling of Stellate, Disk and Basket Neurons in the Central Nucleus of the Guinea Pig Inferior Colliculus

We reconstructed the intrinsic axons of 32 neurons in the guinea pig inferior colliculus (IC) following juxtacellular labeling. Biocytin was injected into cells in vivo, after first analyzing physiological response properties. Based on axonal morphology there were two classes of neuron: (1) laminar cells (14/32, 44%) with an intrinsic axon and flattened dendrites confined to a single fibrodendritic lamina and (2) translaminar cells (18/32, 56%) with axons that terminated in two or more laminae in the central nucleus (ICc) or the surrounding cortex. There was also one small, low-frequency cell with bushy-like dendrites that was very sensitive to interaural timing differences. The translaminar cells were subdivided into three groups of cells with: (a) stellate dendrites that crossed at least two laminae (8/32, 25%); (b) flattened dendrites confined to one lamina and that had mainly en passant axonal swellings (7/32, 22%) and (c) short, flattened dendrites and axons with distinctive clusters of large terminal boutons in the ICc (3/32, 9%). These terminal clusters were similar to those of cortical basket cells. The 14 laminar cells all had sustained responses apart from one offset response. Almost half the non-basket type translaminar cells (7/15) had onset responses while the others had sustained responses. The basket cells were the only ones to have short-latency (7–9 ms), chopper responses and this distinctive temporal response should allow them to be studied in more detail in future. This is the first description of basket cells in the auditory brainstem, but more work is required to confirm their neurotransmitter and precise post-synaptic targets.

Dense cholinergic projections to auditory and multisensory nuclei of the intercollicular midbrain

Cholinergic axons from the pedunculopontine tegmental nucleus (PPT) innervate the inferior colliculus where they are positioned to modulate both excitatory and inhibitory circuits across the central nucleus and adjacent cortical regions. More rostral regions of the auditory midbrain include the nucleus of the brachium of the inferior colliculus (NBIC), the intercollicular tegmentum (ICt) and the rostral pole of the inferior colliculus (ICrp). These regions appear especially important for multisensory integration and contribute to orienting behavior and many aspects of auditory perception. These regions appear to receive cholinergic innervation but little is known about the distribution of cholinergic axons in these regions or the cells that they contact. The present study used immunostaining to examine the distribution of cholinergic axons and then used chemically-specific viral tracing to examine cholinergic projections from the PPT to the intercollicular areas in male and female transgenic rats. Staining with antibodies against vesicular acetylcholine transporter revealed dense cholinergic innervation throughout the NBIC, ICt and ICrp. Deposits of viral vector into the PPT labeled cholinergic axons bilaterally in the NBIC, ICt and ICrp. In each area, the projections were denser on the ipsilateral side. The axons appeared morphologically similar across the three areas. In each area, en passant and terminal boutons from these axons appeared in the neuropil and also in close apposition to cell bodies. Immunostaining with a marker for GABAergic cells suggested that the cholinergic axons likely contact both GABAergic and non-GABAergic cells in the NBIC, ICt and ICrp. Thus, the cholinergic axons could affect multisensory processing by modulating excitatory and inhibitory circuits in the NBIC, ICt and ICrp. The similarity of axons and their targets suggests there may be a common function for cholinergic innervation across the three areas. Given what is known about the PPT, such functions could be associated with arousal, sleep-wake cycle, reward and plasticity.

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.

Mapping complex cell morphology in the grey matter with double diffusion encoding MR: A simulation study

This paper investigates the impact of cell body (namely soma) size and branching of cellular projections on diffusion MR imaging (dMRI) and spectroscopy (dMRS) signals for both standard single diffusion encoding (SDE) and more advanced double diffusion encoding (DDE) measurements using numerical simulations. The aim is to investigate the ability of dMRI/dMRS to characterize the complex morphology of brain cells focusing on these two distinctive features of brain grey matter. To this end, we employ a recently developed computational framework to create three dimensional meshes of neuron-like structures for Monte Carlo simulations, using diffusion coefficients typical of water and brain metabolites. Modelling the cellular structure as realistically connected spherical soma and cylindrical cellular projections, we cover a wide range of combinations of sphere radii and branching order of cellular projections, characteristic of various grey matter cells. We assess the impact of spherical soma size and branching order on the b-value dependence of the SDE signal as well as the time dependence of the mean diffusivity (MD) and mean kurtosis (MK). Moreover, we also assess the impact of spherical soma size and branching order on the angular modulation of DDE signal at different mixing times, together with the mixing time dependence of the apparent microscopic anisotropy (μA), a promising contrast derived from DDE measurements. The SDE results show that spherical soma size has a measurable impact on both the b-value dependence of the SDE signal and the MD and MK diffusion time dependence for both water and metabolites. On the other hand, we show that branching order has little impact on either, especially for water. In contrast, the DDE results show that spherical soma size has a measurable impact on the DDE signal's angular modulation at short mixing times and the branching order of cellular projections significantly impacts the mixing time dependence of the DDE signal's angular modulation as well as of the derived μA, for both water and metabolites. Our results confirm that SDE based techniques may be sensitive to spherical soma size, and most importantly, show for the first time that DDE measurements may be more sensitive to the dendritic tree complexity (as parametrized by the branching order of cellular projections), paving the way for new ways of characterizing grey matter morphology, non-invasively using dMRS and potentially dMRI.

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.

On the potential for mapping apparent neural soma density via a clinically viable diffusion MRI protocol

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B-tensor encoding enables estimation of spherical cellular structures in the brain.

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Spherical compartments may provide markers for apparent neural soma density.

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Model parameters can be estimated in a fast and robust way using deep learning.

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Practical acquisition times are achievable on widely available clinical scanners.

Diffusion MRI is a valuable tool for probing tissue microstructure in the brain noninvasively. Today, model-based techniques are widely available and used for white matter characterisation where their development is relatively mature. Conversely, tissue modelling in grey matter is more challenging, and no generally accepted models exist. With advances in measurement technology and modelling efforts, a clinically viable technique that reveals salient features of grey matter microstructure, such as the density of quasi-spherical cell bodies and quasi-cylindrical cell projections, is an exciting prospect. As a step towards capturing the microscopic architecture of grey matter in clinically feasible settings, this work uses a biophysical model that is designed to disentangle the diffusion signatures of spherical and cylindrical structures in the presence of orientation heterogeneity, and takes advantage of B-tensor encoding measurements, which provide additional sensitivity compared to standard single diffusion encoding sequences. For the fast and robust estimation of microstructural parameters, we leverage recent advances in machine learning and replace conventional fitting techniques with an artificial neural network that fits complex biophysical models within seconds. Our results demonstrate apparent markers of spherical and cylindrical geometries in healthy human subjects, and in particular an increased volume fraction of spherical compartments in grey matter compared to white matter. We evaluate the extent to which spherical and cylindrical geometries may be interpreted as correlates of neural soma and neural projections, respectively, and quantify parameter estimation errors in the presence of various departures from the modelling assumptions. While further work is necessary to translate the ideas presented in this work to the clinic, we suggest that biomarkers focussing on quasi-spherical cellular geometries may be valuable for the enhanced assessment of neurodevelopmental disorders and neurodegenerative diseases.

Microglial peri-somatic abutments classify two novel types of GABAergic neuron in the inferior colliculus

Emerging evidence suggests functional roles for microglia in the healthy, mature nervous system. However, we know little of the cellular density and ramified morphology of microglia in sensory systems, and even less of their inter-relationship with inhibitory neurons. We therefore conducted fluorescent multi-channel immunohistochemistry and confocal microscopy in guinea pigs of both sexes for Iba1, GAD67, GFAP, calbindin, and calretinin. We explored these markers in the inferior colliculi (IC), which contain sub-regions specialized for different aspects of auditory processing. First, we found that while the density of Iba1+ somata is similar throughout the IC parenchyma, Iba1+ microglia in dorsal cortex are significantly more ramified than those in the central nucleus or lateral cortex. Conversely, Iba1+ ramifications in ventral central nucleus, a region with the highest density of GAD67+ (putative GABAergic) neurons in IC, are longer with fewer ramifications. Second, we observed extensive abutments of ramified Iba1+ processes onto GAD67+ somata throughout the whole IC and developed novel measures to quantify these. Cluster analyses revealed two novel sub-types of GAD67+ neuron that differ in the quantity of Iba1+ somatic abutments they receive. Unlike previous classification schemes for GAD67+ neurons in IC, these clusters are not related to GAD67+ soma size. Taken together, these data demonstrate that microglial ramifications vary between IC sub-regions in the healthy, adult IC, possibly related to the ongoing demands of their niche. Furthermore, Iba1+ abutments onto neuronal somata are a novel means by which GAD67+ neurons can be classified.

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.

Neuropeptide Y Expression Defines a Novel Class of GABAergic Projection Neuron in the Inferior Colliculus

Located in the midbrain, the inferior colliculus (IC) integrates information from numerous auditory nuclei and is an important hub for sound processing. Despite its importance, little is known about the molecular identity and functional roles of defined neuron types in the IC. Using a multifaceted approach in mice of both sexes, we found that neuropeptide Y (NPY) expression identifies a major class of inhibitory neurons, accounting for approximately one-third of GABAergic neurons in the IC. Retrograde tracing showed that NPY neurons are principal neurons that can project to the medial geniculate nucleus. In brain slice recordings, many NPY neurons fired spontaneously, suggesting that NPY neurons may drive tonic inhibition onto postsynaptic targets. Morphologic reconstructions showed that NPY neurons are stellate cells, and the dendrites of NPY neurons in the tonotopically organized central nucleus of the IC cross isofrequency laminae. Immunostaining confirmed that NPY neurons express NPY, and we therefore hypothesized that NPY signaling regulates activity in the IC. In crosses between Npy1r^cre and Ai14 Cre-reporter mice, we found that NPY Y₁ receptor (Y₁R)-expressing neurons are glutamatergic and were broadly distributed throughout the rostrocaudal extent of the IC. In whole-cell recordings, application of a high-affinity Y₁R agonist led to hyperpolarization in most Y₁R-expressing IC neurons. Thus, NPY neurons represent a novel class of inhibitory principal neurons that are well poised to use GABAergic and NPY signaling to regulate the excitability of circuits in the IC and auditory thalamus.SIGNIFICANCE STATEMENT The identification of neuron types is a fundamental question in neuroscience. In the inferior colliculus (IC), the hub of the central auditory pathway, molecular markers for distinct classes of inhibitory neurons have remained unknown. We found that neuropeptide Y (NPY) expression identifies a class of GABAergic principal neurons that constitute one-third of the inhibitory neurons in the IC. NPY neurons fire spontaneously, have a stellate morphology, and project to the auditory thalamus. Additionally, we found that NPY signaling hyperpolarized the membrane potential of a subset of excitatory IC neurons that express the NPY Y₁ receptor. Thus, NPY neurons are a novel class of inhibitory neurons that use GABA and NPY signaling to regulate activity in the IC and auditory thalamus.

Perineuronal nets and subtypes of GABAergic cells differentiate auditory and multisensory nuclei in the intercollicular area of the midbrain

The intercollicular region, which lies between the inferior and superior colliculi in the midbrain, contains neurons that respond to auditory, visual, and somatosensory stimuli. Golgi studies have been used to parse this region into three distinct nuclei: the intercollicular tegmentum (ICt), the rostral pole of the inferior colliculus (ICrp), and the nucleus of the brachium of the IC (NBIC). Few reports have focused on these nuclei, especially the ICt and the ICrp, possibly due to lack of a marker that distinguishes these areas and is compatible with modern methods. Here, we found that staining for GABAergic cells and perineuronal nets differentiates these intercollicular nuclei in guinea pigs. Further, we found that the proportions of four subtypes of GABAergic cells differentiate intercollicular nuclei from each other and from adjacent inferior collicular subdivisions. Our results support earlier studies that suggest distinct morphology and functions for intercollicular nuclei, and provide staining methods that differentiate intercollicular nuclei and are compatible with most modern techniques. We hope that this will help future studies to further characterize the intercollicular region.

The Density of Perineuronal Nets Increases With Age in the Inferior Colliculus in the Fischer Brown Norway Rat

Age-related hearing loss, one of the most frequently diagnosed disabilities in industrialized countries, may result from declining levels of GABA in the aging inferior colliculus (IC). However, the mechanisms of aging and subsequent disruptions of temporal processing in elderly hearing abilities are still being investigated. Perineuronal nets (PNs) are a specialized form of the extracellular matrix and have been linked to GABAergic neurotransmission and to the regulation of structural and synaptic plasticity. We sought to determine whether the density of PNs in the IC changes with age. We combined Wisteria floribunda agglutinin (WFA) staining with immunohistochemistry to glutamic acid decarboxylase in three age groups of Fischer Brown Norway (FBN) rats. The density of PNs on GABAergic and non-GABAergic cells in the three major subdivisions of the IC was quantified. Results first demonstrate that the density of PNs in the FBN IC increase with age. The greatest increases of PN density from young to old age occurred in the central IC (67% increase) and dorsal IC (117% increase). Second, in the young IC, PNs surround non-GABAergic and GABAergic cells with the majority of PNs surrounding the former. The increase of PNs with age in the IC occurred on both non-GABAergic and GABAergic populations. The average density of PN-surrounded non-GABAergic cells increased from 84.9 PNs/mm² in the young to 134.2 PNs/mm² in the old. While the density of PN-surrounded GABAergic cells increased from 26 PNs/mm² in the young to 40.6 PNs/mm² in the old. The causality is unclear, but increases in PN density in old age may play a role in altered auditory processing in the elderly, or may lead to further changes in IC plasticity.

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.

A novel class of inferior colliculus principal neurons labeled in vasoactive intestinal peptide-Cre mice

Located in the midbrain, the inferior colliculus (IC) is the hub of the central auditory system. Although the IC plays important roles in speech processing, sound localization, and other auditory computations, the organization of the IC microcircuitry remains largely unknown. Using a multifaceted approach in mice, we have identified vasoactive intestinal peptide (VIP) neurons as a novel class of IC principal neurons. VIP neurons are glutamatergic stellate cells with sustained firing patterns. Their extensive axons project to long-range targets including the auditory thalamus, auditory brainstem, superior colliculus, and periaqueductal gray. Using optogenetic circuit mapping, we found that VIP neurons integrate input from the contralateral IC and the dorsal cochlear nucleus. The dorsal cochlear nucleus also drove feedforward inhibition to VIP neurons, indicating that inhibitory circuits within the IC shape the temporal integration of ascending inputs. Thus, VIP neurons are well-positioned to influence auditory computations in a number of brain regions.

Analysis of excitatory and inhibitory neuron types in the inferior colliculus based on 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 (I_h) in acute brain slices of young adult mice. All GABAergic neurons from our sample displayed slow-activating I_h with moderate amplitudes, whereas a subset of excitatory neurons showed fast-activating I_h 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. I_h 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 I_h. 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 (I_h) and an adaptive firing pattern whereas at least four types of glutamatergic neurons exist regarding their I_h properties and firing patterns. Many of the glutamatergic neurons were almost indistinguishable from the GABAergic neurons regarding I_h properties and firing pattern.

Inhibitory Neural Circuits in the Mammalian Auditory Midbrain

The auditory midbrain is the critical integration center in the auditory pathway of vertebrates. Synaptic inhibition plays a key role during information processing in the auditory midbrain, and these inhibitory neural circuits are seen in all vertebrates and are likely essential for hearing. Here, we review the structure and function of the inhibitory neural circuits of the auditory midbrain. First, we provide an overview on how these inhibitory circuits are organized within different clades of vertebrates. Next, we focus on recent findings in the mammalian auditory midbrain, the most studied of the vertebrates, and discuss how the mammalian auditory midbrain is functionally coordinated.

Inhibitory Projections from the Inferior Colliculus to the Medial Geniculate body Originate from Four Subtypes of GABAergic Cells

GABAergic cells constitute 20-40% of the cells that project from the inferior colliculus [(IC) a midbrain auditory hub] to the medial geniculate body [(MG) the main auditory nucleus of the thalamus]. Four subtypes of GABAergic IC cells have been identified based on their association with perineuronal nets (PNs) and dense rings of axosomatic terminals expressing vesicular glutamate transporter 2 (VGLUT2 rings). These subtypes differ in their soma size and distribution within the IC. Based on previous work emphasizing large GABAergic cells as the origin of GABAergic IC-MG projections, we hypothesized that GABAergic IC cells surrounded by PNs and VGLUT2 rings, which tend to have larger somas, were more likely to project to the MG than smaller cells lacking these extracellular markers. Here, we injected retrograde tract tracers into the MG of guinea pigs of either sex and analyzed retrogradely labeled GABAergic cells in the ipsilateral IC for soma size and association with PNs and/or VGLUT2 rings. We found a range of GABAergic soma sizes present within the IC-MG pathway, which were reflective of the full range of GABAergic soma sizes present within the IC. Further, we found that all four subtypes of GABAergic IC cells participate in the IC-MG pathway, and that GABAergic cells lacking PNs and VGLUT2 rings were more prevalent within the pathway than would be expected based on their overall prevalence in the IC. These results may provide an anatomical substrate for the multiple roles of inhibition in the IC-MG pathway, which have emerged in electrophysiological studies.

Cell Densities in the Mouse Brain: A Systematic Review

The mouse brain is the most extensively studied brain of all species. We performed an exhaustive review of the literature to establish our current state of knowledge on cell numbers in mouse brain regions, arguably the most fundamental property to measure when attempting to understand a brain. The synthesized information, collected in one place, can be used by both theorists and experimentalists. Although for commonly-studied regions cell densities could be obtained for principal cell types, overall we know very little about how many cells are present in most brain regions and even less about cell-type specific densities. There is also substantial variation in cell density values obtained from different sources. This suggests that we need a new approach to obtain cell density datasets for the mouse brain.

Subtypes of GABAergic cells in the inferior colliculus

The inferior colliculus occupies a central position in ascending and descending auditory pathways. A substantial proportion of its neurons are GABAergic, and these neurons contribute to intracollicular circuits as well as to extrinsic projections to numerous targets. A variety of types of evidence - morphology, physiology, molecular markers - indicate that the GABAergic cells can be divided into at least four subtypes that serve different functions. However, there has yet to emerge a unified scheme for distinguishing these subtypes. The present review discusses these criteria and, where possible, relates the different properties. In contrast to GABAergic cells in cerebral cortex, where subtypes are much more thoroughly characterized, those in the inferior colliculus contribute substantially to numerous long range extrinsic projections. At present, the best characterized subtype is a GABAergic cell with a large soma, dense perisomatic synaptic inputs and a large axon that provides rapid auditory input to the thalamus. This large GABAergic subtype projects to additional targets, and other subtypes also project to the thalamus. The eventual characterization of these subtypes can be expected to reveal multiple functions of these inhibitory cells and the many circuits to which they contribute.

Electrical stimulation of the midbrain excites the auditory cortex asymmetrically

BACKGROUND

Auditory midbrain implant users cannot achieve open speech perception and have limited frequency resolution. It remains unclear whether the spread of excitation contributes to this issue and how much it can be compensated by current-focusing, which is an effective approach in cochlear implants.

OBJECTIVE

The present study examined the spread of excitation in the cortex elicited by electric midbrain stimulation. We further tested whether current-focusing via bipolar and tripolar stimulation is effective with electric midbrain stimulation and whether these modes hold any advantage over monopolar stimulation also in conditions when the stimulation electrodes are in direct contact with the target tissue.

METHODS

Using penetrating multielectrode arrays, we recorded cortical population responses to single pulse electric midbrain stimulation in 10 ketamine/xylazine anesthetized mice. We compared monopolar, bipolar, and tripolar stimulation configurations with regard to the spread of excitation and the characteristic frequency difference between the stimulation/recording electrodes.

RESULTS

The cortical responses were distributed asymmetrically around the characteristic frequency of the stimulated midbrain region with a strong activation in regions tuned up to one octave higher. We found no significant differences between monopolar, bipolar, and tripolar stimulation in threshold, evoked firing rate, or dynamic range.

CONCLUSION

The cortical responses to electric midbrain stimulation are biased towards higher tonotopic frequencies. Current-focusing is not effective in direct contact electrical stimulation. Electrode maps should account for the asymmetrical spread of excitation when fitting auditory midbrain implants by shifting the frequency-bands downward and stimulating as dorsally as possible.

Neuronal Organization in the Inferior Colliculus Revisited with Cell-Type-Dependent Monosynaptic Tracing

The inferior colliculus (IC) is a critical integration center in the auditory pathway. However, because the inputs to the IC have typically been studied by the use of conventional anterograde and retrograde tracers, the neuronal organization and cell-type-specific connections in the IC are poorly understood. Here, we used monosynaptic rabies tracing and in situ hybridization combined with excitatory and inhibitory Cre transgenic mouse lines of both sexes to characterize the brainwide and cell-type-specific inputs to specific neuron types within the lemniscal IC core and nonlemniscal IC shell. We observed that both excitatory and inhibitory neurons of the IC shell predominantly received ascending inputs rather than descending or core inputs. Correlation and clustering analyses revealed two groups of excitatory neurons in the shell: one received inputs from a combination of ascending nuclei, and the other received inputs from a combination of descending nuclei, neuromodulatory nuclei, and the contralateral IC. In contrast, inhibitory neurons in the core received inputs from the same combination of all nuclei. After normalizing the extrinsic inputs, we found that core inhibitory neurons received a higher proportion of inhibitory inputs from the ventral nucleus of the lateral lemniscus than excitatory neurons. Furthermore, the inhibitory neurons preferentially received inhibitory inputs from the contralateral IC shell. Because IC inhibitory neurons innervate the thalamus and contralateral IC, the inhibitory inputs we uncovered here suggest two long-range disinhibitory circuits. In summary, we found: (1) dominant ascending inputs to the shell, (2) two subpopulations of shell excitatory neurons, and (3) two disinhibitory circuits.SIGNIFICANCE STATEMENT Sound undergoes extensive processing in the brainstem. The inferior colliculus (IC) core is classically viewed as the integration center for ascending auditory information, whereas the IC shell integrates descending feedback information. Here, we demonstrate that ascending inputs predominated in the IC shell but appeared to be separated from the descending inputs. The presence of inhibitory projection neurons is a unique feature of the auditory ascending pathways, but the connections of these neurons are poorly understood. Interestingly, we also found that inhibitory neurons in the IC core and shell preferentially received inhibitory inputs from ascending nuclei and contralateral IC, respectively. Therefore, our results suggest a bipartite domain in the IC shell and disinhibitory circuits in the IC.

Perineuronal nets in subcortical auditory nuclei of four rodent species with differing hearing ranges

Perineuronal nets (PNs) are aggregates of extracellular matrix molecules that surround some neurons in the brain. While PNs occur widely across many cortical areas, subcortical PNs are especially associated with motor and auditory systems. The auditory system has recently been suggested as an ideal model system for studying PNs and their functions. However, descriptions of PNs in subcortical auditory areas vary, and it is unclear whether the variation reflects species differences or differences in staining techniques. Here, we used two staining techniques (one lectin stain and one antibody stain) to examine PN distribution in the subcortical auditory system of four different species: guinea pigs (Cavia porcellus), mice (Mus musculus, CBA/CaJ strain), Long-Evans rats (Rattus norvegicus), and naked mole-rats (Heterocephalus glaber). We found that some auditory nuclei exhibit dramatic differences in PN distribution among species while other nuclei have consistent PN distributions. We also found that PNs exhibit molecular heterogeneity, and can stain with either marker individually or with both. PNs within a given nucleus can be heterogeneous or homogenous in their staining patterns. We compared PN staining across the frequency axes of tonotopically organized nuclei and among species with different hearing ranges. PNs were distributed non-uniformly across some nuclei, but only rarely did this appear related to the tonotopic axis. PNs were prominent in all four species; we found no systematic relationship between the hearing range and the number, staining patterns or distribution of PNs in the auditory nuclei.

A Sweet Talk: The Molecular Systems of Perineuronal Nets in Controlling Neuronal Communication

Perineuronal nets (PNNs) are mesh-like structures, composed of a hierarchical assembly of extracellular matrix molecules in the central nervous system (CNS), ensheathing neurons and regulating plasticity. The mechanism of interactions between PNNs and neurons remain uncharacterized. In this review, we pose the question: how do PNNs regulate communication to and from neurons? We provide an overview of the current knowledge on PNNs with a focus on the cellular interactions. PNNs ensheath a subset of the neuronal population with distinct molecular aspects in different areas of the CNS. PNNs control neuronal communication through molecular interactions involving specific components of the PNNs. This review proposes that the PNNs are an integral part of neurons, crucial for the regulation of plasticity in the CNS.

Identified GABAergic and Glutamatergic Neurons in the Mouse Inferior Colliculus Share Similar Response Properties

GABAergic neurons in the inferior colliculus (IC) play a critical role in auditory information processing, yet their responses to sound are unknown. Here, we used optogenetic methods to characterize the response properties of GABAergic and presumed glutamatergic neurons to sound in the IC. We found that responses to pure tones of both inhibitory and excitatory classes of neurons were similar in their thresholds, response latencies, rate-level functions, and frequency tuning, but GABAergic neurons may have higher spontaneous firing rates. In contrast to their responses to pure tones, the inhibitory and excitatory neurons differed in their ability to follow amplitude modulations. The responses of both cell classes were affected by their location regardless of the cell type, especially in terms of their frequency tuning. These results show that the synaptic domain, a unique organization of local neural circuits in the IC, may interact with all types of neurons to produce their ultimate response to sound.SIGNIFICANCE STATEMENT Although the inferior colliculus (IC) in the auditory midbrain is composed of different types of neurons, little is known about how these specific types of neurons respond to sound. Here, for the first time, we characterized the response properties of GABAergic and glutamatergic neurons in the IC. Both classes of neurons had diverse response properties to tones but were overall similar, except for the spontaneous activity and their ability to follow amplitude-modulated sound. Both classes of neurons may compose a basic local circuit that is replicated throughout the IC. Within each local circuit, the inputs to the local circuit may have a greater influence in determining the response properties to sound than the specific neuron types.

Effects of noise-induced hearing loss on parvalbumin and perineuronal net expression in the mouse primary auditory cortex

Noise induced hearing loss is associated with increased excitability in the central auditory system but the cellular correlates of such changes remain to be characterized. Here we tested the hypothesis that noise-induced hearing loss causes deterioration of perineuronal nets (PNNs) in the auditory cortex of mice. PNNs are specialized extracellular matrix components that commonly enwrap cortical parvalbumin (PV) containing GABAergic interneurons. Compared to somatosensory and visual cortex, relatively less is known about PV/PNN expression patterns in the primary auditory cortex (A1). Whether changes to cortical PNNs follow acoustic trauma remains unclear. The first aim of this study was to characterize PV/PNN expression in A1 of adult mice. PNNs increase excitability of PV+ inhibitory neurons and confer protection to these neurons against oxidative stress. Decreased PV/PNN expression may therefore lead to a reduction in cortical inhibition. The second aim of this study was to examine PV/PNN expression in superficial (I-IV) and deep cortical layers (V-VI) following noise trauma. Exposing mice to loud noise caused an increase in hearing threshold that lasted at least 30 days. PV and PNN expression in A1 was analyzed at 1, 10 and 30 days following the exposure. No significant changes were observed in the density of PV+, PNN+, or PV/PNN co-localized cells following hearing loss. However, a significant layer- and cell type-specific decrease in PNN intensity was seen following hearing loss. Some changes were present even at 1 day following noise exposure. Attenuation of PNN may contribute to changes in excitability in cortex following noise trauma. The regulation of PNN may open up a temporal window for altered excitability in the adult brain that is then stabilized at a new and potentially pathological level such as in tinnitus.

Age-Related Deterioration of Perineuronal Nets in the Primary Auditory Cortex of Mice

Age-related changes in inhibitory neurotransmission in sensory cortex may underlie deficits in sensory function. Perineuronal nets (PNNs) are extracellular matrix components that ensheath some inhibitory neurons, particularly parvalbumin positive (PV+) interneurons. PNNs may protect PV+ cells from oxidative stress and help establish their rapid spiking properties. Although PNN expression has been well characterized during development, possible changes in aging sensory cortex have not been investigated. Here we tested the hypothesis that PNN+, PV+ and PV/PNN co-localized cell densities decline with age in the primary auditory cortex (A1). This hypothesis was tested using immunohistochemistry in two strains of mice (C57BL/6 and CBA/CaJ) with different susceptibility to age-related hearing loss and at three different age ranges (1–3, 6–8 and 14–24 months old). We report that PNN+ and PV/PNN co-localized cell densities decline significantly with age in A1 in both mouse strains. In the PNN+ cells that remain in the old group, the intensity of PNN staining is reduced in the C57 strain, but not the CBA strain. PV+ cell density also declines only in the C57, but not the CBA, mouse suggesting a potential exacerbation of age-effects by hearing loss in the PV/PNN system. Taken together, these data suggest that PNN deterioration may be a key component of altered inhibition in the aging sensory cortex, that may lead to altered synaptic function, susceptibility to oxidative stress and processing deficits.

Late postnatal shifts of parvalbumin and nitric oxide synthase expression within the GABAergic and glutamatergic phenotypes of inferior colliculus neurons

The inferior colliculus (IC) is partitioned into three subdivisions: the dorsal and lateral cortices (DC and LC) and the central nucleus (ICC), and serves as an integration center of auditory information. Recent studies indicate that a certain population of IC neurons may represent the non-GABAergic phenotype, while they express well-established cortical/hippocampal GABAergic neuron markers. In this study we used the optical disector to investigate the phenotype of IC neurons expressing parvalbumin (PV) and/or nitric oxide synthase (NOS) in C57BL/6J mice during the late postnatal period. Four major types of IC neurons were defined by the presence (+) or absence (-) of PV, NOS, and glutamic acid decarboxylase 67 (GAD67): PV⁺ /NOS^- /GAD67⁺ , PV⁺ /NOS⁺ /GAD67⁺ , PV⁺ /NOS^- /GAD67^- , and PV^- /NOS⁺ /GAD67^- . Fluorescent in situ hybridization for vesicular glutamate transporter 2 mRNA indicated that almost all GAD67^- IC neurons represented the glutamatergic phenotype. The numerical densities (NDs) of total GAD67⁺ IC neurons remained unchanged in all subdivisions. The NDs of PV⁺ /NOS^- /GAD67⁺ neurons and PV^- /NOS⁺ /GAD67^- neurons were reduced with age in the ICC, while they remained unchanged in the DC and LC. By contrast, the NDs of PV⁺ /NOS⁺ /GAD67⁺ neurons and PV⁺ /NOS^- /GAD67^- neurons were increased with age in the ICC, although there were no changes in the DC and LC. The cell body size of GAD67⁺ IC neurons did not vary according to the expression of PV with or without NOS. The present findings indicate that the expression of PV and NOS may shift with age within the GABAergic and glutamatergic phenotypes of IC neurons during the late postnatal period. J. Comp. Neurol. 525:868-884, 2017. © 2016 Wiley Periodicals, Inc.