Dopaminergic Input to the Inferior Colliculus in Mice

Neural Correlates of Perceptual Plasticity in the Auditory Midbrain and Thalamus

Hearing is an active process in which listeners must detect and identify sounds, segregate and discriminate stimulus features, and extract their behavioral relevance. Adaptive changes in sound detection can emerge rapidly, during sudden shifts in acoustic or environmental context, or more slowly as a result of practice. Although we know that context- and learning-dependent changes in the sensitivity of auditory cortical (ACX) neurons support many aspects of perceptual plasticity, the contribution of subcortical auditory regions to this process is less understood. Here, we recorded single- and multiunit activity from the central nucleus of the inferior colliculus (ICC) and the ventral subdivision of the medial geniculate nucleus (MGV) of male and female Mongolian gerbils under two different behavioral contexts: as animals performed an amplitude modulation (AM) detection task and as they were passively exposed to AM sounds. Using a signal detection framework to estimate neurometric sensitivity, we found that neural thresholds in both regions improve during task performance, and this improvement is largely driven by changes in the firing rate rather than phase locking. We also found that ICC and MGV neurometric thresholds improve as animals learn to detect small AM depths during a multiday perceptual training paradigm. Finally, we revealed that in the MGV, but not the ICC, context-dependent enhancements in AM sensitivity grow stronger during perceptual training, mirroring prior observations in the ACX. Together, our results suggest that the auditory midbrain and thalamus contribute to changes in sound processing and perception over rapid and slow timescales.

The multifaceted role of the inferior colliculus in sensory prediction, reward processing, and decision-making

The inferior colliculus (IC) has traditionally been regarded as an important relay in the auditory pathway, primarily involved in relaying auditory information from the brainstem to the thalamus. However, this study uncovers the multifaceted role of the IC in bridging auditory processing, sensory prediction, and reward prediction. Through extracellular recordings in monkeys engaged in a sound duration-based deviation detection task, we observed a 'climbing effect' in neuronal firing rates, indicative of an enhanced response over sound sequences linked to sensory prediction rather than reward anticipation. Moreover, our findings demonstrate reward prediction errors within the IC, highlighting its complex integration in auditory and reward processing. Further analysis revealed a direct correlation between IC neuronal activity and behavioral choices, suggesting its involvement in decision-making processes. This research highlights a more complex role for the IC than traditionally understood, showcasing its integral role in cognitive and sensory processing and emphasizing its importance in integrated brain functions.

Neural correlates of flexible sound perception in the auditory midbrain and thalamus

Hearing is an active process in which listeners must detect and identify sounds, segregate and discriminate stimulus features, and extract their behavioral relevance. Adaptive changes in sound detection can emerge rapidly, during sudden shifts in acoustic or environmental context, or more slowly as a result of practice. Although we know that context- and learning-dependent changes in the spectral and temporal sensitivity of auditory cortical neurons support many aspects of flexible listening, the contribution of subcortical auditory regions to this process is less understood. Here, we recorded single- and multi-unit activity from the central nucleus of the inferior colliculus (ICC) and the ventral subdivision of the medial geniculate nucleus (MGV) of Mongolian gerbils under two different behavioral contexts: as animals performed an amplitude modulation (AM) detection task and as they were passively exposed to AM sounds. Using a signal detection framework to estimate neurometric sensitivity, we found that neural thresholds in both regions improved during task performance, and this improvement was driven by changes in firing rate rather than phase locking. We also found that ICC and MGV neurometric thresholds improved and correlated with behavioral performance as animals learn to detect small AM depths during a multi-day perceptual training paradigm. Finally, we reveal that in the MGV, but not the ICC, context-dependent enhancements in AM sensitivity grow stronger during perceptual training, mirroring prior observations in the auditory cortex. Together, our results suggest that the auditory midbrain and thalamus contribute to flexible sound processing and perception over rapid and slow timescales.

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.

Hearing, touching, and multisensory integration during mate choice

Mate choice is a potent generator of diversity and a fundamental pillar for sexual selection and evolution. Mate choice is a multistage affair, where complex sensory information and elaborate actions are used to identify, scrutinize, and evaluate potential mating partners. While widely accepted that communication during mate assessment relies on multimodal cues, most studies investigating the mechanisms controlling this fundamental behavior have restricted their focus to the dominant sensory modality used by the species under examination, such as vision in humans and smell in rodents. However, despite their undeniable importance for the initial recognition, attraction, and approach towards a potential mate, other modalities gain relevance as the interaction progresses, amongst which are touch and audition. In this review, we will: (1) focus on recent findings of how touch and audition can contribute to the evaluation and choice of mating partners, and (2) outline our current knowledge regarding the neuronal circuits processing touch and audition (amongst others) in the context of mate choice and ask (3) how these neural circuits are connected to areas that have been studied in the light of multisensory integration.

Early auditory responses to speech sounds in Parkinson's disease: preliminary data

Parkinson’s disease (PD), as a manifestation of basal ganglia dysfunction, is associated with a number of speech deficits, including reduced voice modulation and vocal output. Interestingly, previous work has shown that participants with PD show an increased feedback-driven motor response to unexpected fundamental frequency perturbations during speech production, and a heightened ability to detect differences in vocal pitch relative to control participants. Here, we explored one possible contributor to these enhanced responses. We recorded the frequency-following auditory brainstem response (FFR) to repetitions of the speech syllable [da] in PD and control participants. Participants with PD displayed a larger amplitude FFR related to the fundamental frequency of speech stimuli relative to the control group. The current preliminary results suggest the dysfunction of the basal ganglia in PD contributes to the early stage of auditory processing and may reflect one component of a broader sensorimotor processing impairment associated with the disease.

5-HT1A Receptors Alter Temporal Responses to Broadband Vocalizations in the Mouse Inferior Colliculus Through Response Suppression

Neuromodulatory systems may provide information on social context to auditory brain regions, but relatively few studies have assessed the effects of neuromodulation on auditory responses to acoustic social signals. To address this issue, we measured the influence of the serotonergic system on the responses of neurons in a mouse auditory midbrain nucleus, the inferior colliculus (IC), to vocal signals. Broadband vocalizations (BBVs) are human-audible signals produced by mice in distress as well as by female mice in opposite-sex interactions. The production of BBVs is context-dependent in that they are produced both at early stages of interactions as females physically reject males and at later stages as males mount females. Serotonin in the IC of males corresponds to these events, and is elevated more in males that experience less female rejection. We measured the responses of single IC neurons to five recorded examples of BBVs in anesthetized mice. We then locally activated the 5-HT1A receptor through iontophoretic application of 8-OH-DPAT. IC neurons showed little selectivity for different BBVs, but spike trains were characterized by local regions of high spike probability, which we called "response features." Response features varied across neurons and also across calls for individual neurons, ranging from 1 to 7 response features for responses of single neurons to single calls. 8-OH-DPAT suppressed spikes and also reduced the numbers of response features. The weakest response features were the most likely to disappear, suggestive of an "iceberg"-like effect in which activation of the 5-HT1A receptor suppressed weakly suprathreshold response features below the spiking threshold. Because serotonin in the IC is more likely to be elevated for mounting-associated BBVs than for rejection-associated BBVs, these effects of the 5-HT1A receptor could contribute to the differential auditory processing of BBVs in different behavioral subcontexts.

Dopamine in Auditory Nuclei and Lemniscal Projections is Poised to Influence Acoustic Integration in the Inferior Colliculus

Dopamine (DA) modulates the activity of nuclei within the ascending and descending auditory pathway. Previous studies have identified neurons and fibers in the inferior colliculus (IC) which are positively labeled for tyrosine hydroxylase (TH), a key enzyme in the synthesis of dopamine. However, the origins of the tyrosine hydroxylase positive projections to the inferior colliculus have not been fully explored. The lateral lemniscus (LL) provides a robust inhibitory projection to the inferior colliculus and plays a role in the temporal processing of sound. In the present study, immunoreactivity for tyrosine hydroxylase was examined in animals with and without 6-hydroxydopamine (6-OHDA) lesions. Lesioning, with 6-OHDA placed in the inferior colliculus, led to a significant reduction in tyrosine hydroxylase immuno-positive labeling in the lateral lemniscus and inferior colliculus. Immunolabeling for dopamine beta-hydroxylase (DBH) and phenylethanolamine N-methyltransferase (PNMT), enzymes responsible for the synthesis of norepinephrine (NE) and epinephrine (E), respectively, were evaluated. Very little immunoreactivity for DBH and no immunoreactivity for PNMT was found within the cell bodies of the dorsal, intermediate, or ventral nucleus of the lateral lemniscus. The results indicate that catecholaminergic neurons of the lateral lemniscus are likely dopaminergic and not noradrenergic or adrenergic. Next, high-pressure liquid chromatography (HPLC) analysis was used to confirm that dopamine is present in the inferior colliculus and nuclei that send projections to the inferior colliculus, including the cochlear nucleus (CN), superior olivary complex (SOC), lateral lemniscus, and auditory cortex (AC). Finally, fluorogold, a retrograde tracer, was injected into the inferior colliculus of adult rats. Each subdivision of the lateral lemniscus contained fluorogold within the somata, with the dorsal nucleus of the lateral lemniscus showing the most robust projections to the inferior colliculus. Fluorogold-tyrosine hydroxylase colocalization within the lateral lemniscus was assessed. The dorsal and intermediate nuclei neurons exhibiting similar degrees of colocalization, while neurons of the ventral nucleus had significantly fewer colocalized fluorogold-tyrosine hydroxylase labeled neurons. These results suggest that several auditory nuclei that project to the inferior colliculus contain dopamine, dopaminergic neurons in the lateral lemniscus project to the inferior colliculus and that dopaminergic neurotransmission is poised to play a pivotal role in the function of the inferior colliculus.

Distribution and co-expression of adrenergic receptor-encoding mRNA in the mouse inferior colliculus

Adrenergic receptors are mediators of adrenergic and noradrenergic modulation throughout the brain. Previous studies have provided evidence for the expression of adrenergic receptors in the midbrain auditory nucleus, the inferior colliculus (IC), but have not examined the cellular patterns of expression in detail. Here, we utilize multichannel fluorescent in situ hybridization to detect the expression of adrenergic receptor-encoding mRNA in the inferior colliculus of male and female mice. We found expression of α₁ , α_2A , and β₂ receptor-encoding mRNA throughout all areas of the IC. While we observed similar levels of expression of α₁ receptor-encoding mRNA across the subregions of the IC, α_2A and β₂ receptor-encoding mRNA was expressed differentially. To account for developmental changes in noradrenergic receptor expression, we measured expression levels in mice aged P15, P20, and P60. We observed little change in levels of expression across these ages. To ascertain the modulatory potential of multiple adrenergic receptor subtypes in a single IC cell, we measured co-expression of α₁ , α_2A , and β₂ receptor-encoding mRNA. We found greater proportions of cells in the IC that expressed no adrenergic receptor-encoding mRNA, α1 and α2A adrenergic receptor-encoding mRNA, and α1, α2A, and β2 receptor-encoding mRNA than would be predicted by independent expression of each receptor subtype. These data suggest a coordinated pattern of adrenergic receptor expression in the IC and provide the first evidence for adrenergic receptor expression and co-expression in the subregions of the mouse auditory midbrain.

Dopamine modulates subcortical responses to surprising sounds

Dopamine guides behavior and learning through pleasure, according to classic understanding. Dopaminergic neurons are traditionally thought to signal positive or negative prediction errors (PEs) when reward expectations are, respectively, exceeded or not matched. These signed PEs are quite different from the unsigned PEs, which report surprise during sensory processing. But mounting theoretical accounts from the predictive processing framework postulate that dopamine, as a neuromodulator, could potentially regulate the postsynaptic gain of sensory neurons, thereby scaling unsigned PEs according to their expected precision or confidence. Despite ample modeling work, the physiological effects of dopamine on the processing of surprising sensory information are yet to be addressed experimentally. In this study, we tested how dopamine modulates midbrain processing of unexpected tones. We recorded extracellular responses from the rat inferior colliculus to oddball and cascade sequences, before, during, and after the microiontophoretic application of dopamine or eticlopride (a D₂-like receptor antagonist). Results demonstrate that dopamine reduces the net neuronal responsiveness exclusively to unexpected sensory input without significantly altering the processing of expected input. We conclude that dopaminergic projections from the thalamic subparafascicular nucleus to the inferior colliculus could encode the expected precision of unsigned PEs, attenuating via D₂-like receptors the postsynaptic gain of sensory inputs forwarded by the auditory midbrain neurons. This direct dopaminergic modulation of sensory PE signaling has profound implications for both the predictive coding framework and the understanding of dopamine function.

Information about unexpected stimuli is encoded in the form of prediction error signals. The earliest prediction error signals identified in the auditory brain emerge subcortically in the inferior colliculus. This study reveals the essential role of dopamine in encoding the precision of prediction errors at the auditory midbrain.

The Relationship Between Speech Perceptual Discrimination and Speech Production in Parkinson's Disease

Purpose We recently demonstrated that individuals with Parkinson's disease (PD) respond differentially to specific altered auditory feedback parameters during speech production. Participants with PD respond more robustly to pitch and less robustly to formant manipulations compared to control participants. In this study, we investigated whether differences in perceptual processing may in part underlie these compensatory differences in speech production. Methods Pitch and formant feedback manipulations were presented under 2 conditions: production and listening. In the production condition, 15 participants with PD and 15 age- and gender-matched healthy control participants judged whether their own speech output was manipulated in real time. During the listening task, participants judged whether paired tokens of their previously recorded speech samples were the same or different. Results Under listening, 1st formant manipulation discrimination was significantly reduced for the PD group compared to the control group. There was a trend toward better discrimination of pitch in the PD group, but the group difference was not significant. Under the production condition, the ability of participants with PD to identify pitch manipulations was greater than that of the controls. Conclusion The findings suggest perceptual processing differences associated with acoustic parameters of fundamental frequency and 1st formant perturbations in PD. These findings extend our previous results, indicating that different patterns of compensation to pitch and 1st formant shifts may reflect a combination of sensory and motor mechanisms that are differentially influenced by basal ganglia dysfunction.

Dopamine Acts via D2-Like Receptors to Modulate Auditory Responses in the Inferior Colliculus

The ability to understand speech relies on accurate auditory processing of complex sounds. Individuals with Parkinson's disease suffer from speech perception deficits, suggesting that dopamine is involved in the encoding of complex sounds. Recent studies have demonstrated that dopamine has heterogeneous effects on the responses of many neurons in the inferior colliculus (IC) of mice, although the strongest effect is to suppress neural activity. However, it was previously unknown which dopamine receptors are involved in modulating neuronal responses, and whether the observed preponderance of depressive effects reflects the endogenous dopamine system in the IC. In this study, we tested whether dopamine acts via D1- and/or D2-like receptors to alter responses of IC neurons in female and male mice. We also tested the effect of optogenetically induced dopamine release on auditory responses in the IC. We found that the effects of dopamine in the IC occur via D2-like receptors. In iontophoretic and freely behaving experiments, the single-unit and multi-unit effects of dopamine and a D2-like agonist were heterogeneous as both either increased or decreased responses of IC neurons to tones, while a D2-like antagonist had opposite effects. We also found that optogenetic activation of the endogenous dopamine system in the IC alters responses of auditory neurons. Similar to the effects of exogenous dopamine application, optogenetic induction of endogenous dopamine release heterogeneously altered auditory responses in the majority of cells in mice expressing channelrhodopsin-2 (ChR2). Understanding how dopamine modulates auditory processing will ultimately inform therapies targeting mechanisms underlying auditory-related communication disorders.

Forebrain Dopamine System Regulates Inner Ear Auditory Sensitivity to Socially Relevant Acoustic Signals

Dopamine is integral to attentional and motivational processes, but studies are largely restricted to the central nervous system. In mammals [1, 2] and fishes [3, 4], central dopaminergic neurons project to the inner ear and could modulate acoustic signals at the earliest stages of processing. Studies in rodents show dopamine inhibits cochlear afferent neurons and protects against noise-induced acoustic injury [5-10]. However, other functions for inner ear dopamine have not been investigated, and the effect of dopamine on peripheral auditory processing in non-mammalians remains unknown [11, 12]. Insights could be gained by studies conducted in the context of intraspecific acoustic communication. We present evidence from a vocal fish linking reproductive-state-dependent changes in auditory sensitivity with seasonal changes in the dopaminergic efferent system in the saccule, their primary organ of hearing. Plainfin midshipman (Porichthys notatus) migrate from deep-water winter habitats to the intertidal zone in the summer to breed. Nesting males produce nocturnal vocalizations to attract females [13]. Both sexes undergo seasonal enhancement of hearing sensitivity at the level of the hair cell [14-16], increasing the likelihood of detecting conspecific signals [17, 18]. Importantly, reproductive females concurrently have reduced dopaminergic input to the saccule [19]. Here, we show that dopamine decreases saccule auditory sensitivity via a D2-like receptor. Saccule D2a receptor expression is reduced in the summer and correlates with sensitivity within and across seasons. We propose that reproductive-state-dependent changes to the dopaminergic efferent system provide a release of inhibition in the saccule, enhancing peripheral encoding of social-acoustic signals.

Alignment of EphA4 and ephrin-B2 expression patterns with developing modularity in the lateral cortex of the inferior colliculus

In the multimodal lateral cortex of the inferior colliculus (LCIC), there are two neurochemically and connectionally distinct compartments, termed modular and extramodular zones. Modular fields span LCIC layer 2 and are recipients of somatosensory afferents, while encompassing extramodular domains receive auditory inputs. Recently, in developing mice, we identified several markers (among them glutamic acid decarboxylase, GAD) that consistently label the same modular set, and a reliable extramodular marker, calretinin, (CR). Previous reports from our lab show similar modular-extramodular patterns for certain Eph-ephrin guidance members, although their precise alignment with the developing LCIC neurochemical framework has yet to be addressed. Here we confirm in the nascent LCIC complementary GAD/CR-positive compartments, and characterize the registry of EphA4 and ephrin-B2 expression patterns with respect to its emerging modular-extramodular organization. Immunocytochemical approaches in GAD67-GFP knock-in mice reveal patchy EphA4 and ephrin-B2 domains that precisely align with GAD-positive LCIC modules, and are complementary to CR-defined extramodular zones. Such patterning was detectable neonatally, yielding discrete compartments prior to hearing onset. A dense plexus of EphA4-positive fibers filled modules, surrounding labeled ephrin-B2 and GAD cell populations. The majority of observed GABAergic neurons within modular boundaries were also positive for ephrin-B2. These results suggest an early compartmentalization of the LCIC that is likely instructed in part through Eph-ephrin guidance mechanisms. The overlap of developing LCIC neurochemical and guidance patterns is discussed in the context of its seemingly segregated multimodal input-output streams.

Stimulation of the subparafascicular thalamic nucleus modulates dopamine release in the inferior colliculus of rats

Although dopamine is commonly studied for its role in incentive motivation, cognition, and various neuropsychiatric disorders, evidence from Parkinson's disease (PD) patients that present auditory deficits suggest that dopamine is also involved in central auditory processing. It has been recently discovered that the subparafascicular thalamic nucleus (SPF) sends dopaminergic projections to the inferior colliculus (IC), an important convergence hub for the ascending and descending auditory pathways. In the present study, our aim was to provide neurochemical evidence that activation of SPF neurons evokes dopamine release in the IC of anesthetized rats using fast-scan cyclic and paired pulse voltammetry in combination with carbon fiber microelectrodes. Electrical stimulation of the SPF (60 and 90 Hz) evoked dopamine release in the IC in a frequency-dependent manner, with higher frequencies evoking greater amplitude dopamine responses. Optogenetic-evoked dopamine responses were similar to the effects of electrical stimulation suggesting that electrical stimulation-evoked dopamine release was not due to nonspecific activation of fibers of passage, but rather to activation of SPF cells projecting to the IC. Selective dopamine reuptake blockade enhanced the evoked dopamine response, while selective blockade of serotonin did not, confirming the selectivity of the neurochemical recordings to dopamine. Therefore, the SPF neuronal pathway functionally mediates dopamine release in the IC and thus may be involved in auditory processing deficits associated with PD.

Monoaminergic Neuromodulation of Sensory Processing

All neuronal circuits are subject to neuromodulation. Modulatory effects on neuronal processing and resulting behavioral changes are most commonly reported for higher order cognitive brain functions. Comparatively little is known about how neuromodulators shape processing in sensory brain areas that provide the signals for downstream regions to operate on. In this article, we review the current knowledge about how the monoamine neuromodulators serotonin, dopamine and noradrenaline influence the representation of sensory stimuli in the mammalian sensory system. We review the functional organization of the monoaminergic brainstem neuromodulatory systems in relation to their role for sensory processing and summarize recent neurophysiological evidence showing that monoamines have diverse effects on early sensory processing, including changes in gain and in the precision of neuronal responses to sensory inputs. We also highlight the substantial evidence for complementarity between these neuromodulatory systems with different patterns of innervation across brain areas and cortical layers as well as distinct neuromodulatory actions. Studying the effects of neuromodulators at various target sites is a crucial step in the development of a mechanistic understanding of neuronal information processing in the healthy brain and in the generation and maintenance of mental diseases.

Putting it in Context: Linking Auditory Processing with Social Behavior Circuits in the Vertebrate Brain

Context is critical to the adaptive value of communication. Sensory systems such as the auditory system represent an important juncture at which information on physiological state or social valence can be added to communicative information. However, the neural pathways that convey context to the auditory system are not well understood. The serotonergic system offers an excellent model to address these types of questions. Serotonin fluctuates in the mouse inferior colliculus (IC), an auditory midbrain region important for species-specific vocalizations, during specific social and non-social contexts. Furthermore, serotonin is an indicator of the valence of event-based changes within individual social interactions. We propose a model in which the brain's social behavior network serves as an afferent effector of the serotonergic dorsal raphe nucleus in order to gate contextual release of serotonin in the IC. Specifically, discrete vasopressinergic nuclei within the hypothalamus and extended amygdala that project to the dorsal raphe are functionally engaged during contexts in which serotonin fluctuates in the IC. Since serotonin strongly influences the responses of IC neurons to social vocalizations, this pathway could serve as a feedback loop whereby integrative social centers modulate their own sources of input. The end result of this feedback would be to produce a process that is geared, from sensory input to motor output, toward responding appropriately to a dynamic external world.

Projections of the Diencephalospinal Dopaminergic System to Peripheral Sense Organs in Larval Zebrafish (Danio rerio)

Dopaminergic neurons of the descending diencephalospinal system are located in the posterior tuberculum (PT) in zebrafish (Danio rerio), and correspond in mammals to the A11 group in hypothalamus and thalamus. In the larval zebrafish, they are likely the only source of central dopaminergic projections to the periphery. Here, we characterized posterior tubercular dopaminergic fibers projecting to peripheral sense organs, with a focus on the lateral line neuromasts. We labeled and identified catecholaminergic neurons and their projections by combining two immunofluorescence techniques, (i) using an antibody against Tyrosine hydroxylase, and (ii) using an antibody against GFP in transgenic zebrafish expressing in catecholaminergic neurons either membrane-anchored GFP to track fibers, or a Synaptophysin-GFP fusion to visualize putative synapses. We applied the CLARITY method to 6 days old whole zebrafish larvae to stain and analyze dopaminergic projections by confocal microscopy. We found that all lateral line neuromasts receive direct innervation by posterior tubercular dopaminergic neurons, and tracked these projections in detail. In addition, we found dopaminergic fibers projecting to the anterior and posterior lateral line ganglia, and extensive central dopaminergic arborizations around the terminal projection field of the lateral line afferent neurons in the hindbrain medial octavolateralis nucleus (MON). Therefore, dopaminergic innervation may affect lateral line sense information at different processing stages. Additional dopaminergic fibers innervate the trigeminal ganglion, and we observed fine catecholaminergic fibers in the skin with arborization patterns similar to free sensory nerve endings. We also detected potentially dopaminergic fibers innervating inner ear sensory epithelia. Therefore, the diencephalospinal A11-type dopaminergic system may broadly modulate peripheral senses. We also briefly report peripheral sympathetic catecholaminergic projections labeled in our experiments, and their innervation of the developing intestine, swim bladder and abdominal organs.

Temporal Expression of Wnt1 Defines the Competency State and Terminal Identity of Progenitors in the Developing Cochlear Nucleus and Inferior Colliculus

The auditory system contains a diverse array of interconnected anatomical structures that mediate the perception of sound. The cochlear nucleus of the hindbrain serves as the initial site of convergence for auditory stimuli, while the inferior colliculus of the midbrain serves as an integration and relay station for all ascending auditory information. We used Genetic Inducible Fate Mapping (GIFM) to determine how the timing of Wnt1 expression is related to the competency states of auditory neuron progenitors. We demonstrate that the Wnt1 lineage defines progenitor pools of auditory neurons in the developing midbrain and hindbrain. The timing of Wnt1 expression specifies unique cell types during embryogenesis and follows a mixed model encompassing a brief epoch of de novo expression followed by rapid and progressive lineage restriction to shape the inferior colliculus. In contrast, Wnt1 fate mapping of the embryonic hindbrain revealed de novo induction of Wnt1 in auditory hindbrain progenitors, which is related to the development of biochemically distinct neurons in the cochlear nucleus. Thus, we uncovered two modes of lineage allocation that explain the relationship between the timing of Wnt1 expression and the development of the cochlear nucleus and the inferior colliculus. Finally, our analysis of Wnt1^sw/sw mutant mice demonstrated a functional requirement of Wnt1 for the development of auditory midbrain and hindbrain neurons. Collectively, our study provides a deeper understanding of Wnt1 lineage allocation and function in mammalian brain development.

Dopaminergic projections of the subparafascicular thalamic nucleus to the auditory brainstem

Neuromodulators can alter the response properties of sensory neurons, including those in the auditory system. Dopamine, which plays a major role in reward and movement, has been shown to alter neural responses in the auditory brainstem and midbrain. Recently we identified the subparafascicular thalamic nucleus (SPF), part of the A11 dopaminergic cell group, as the source of dopamine to the inferior colliculus (IC). The superior olivary complex (SOC) is also a likely target of dopaminergic projections from the SPF because it receives projections from the SPF and contains fibers and terminals immunoreactive for tyrosine hydroxylase, the rate limiting enzyme in dopamine synthesis. However, it is unknown if the projections from the SPF to SOC are dopaminergic, and if single neurons in the SPF project to both the IC and SOC. Using anterograde tracing combined with fluorescent immunohistochemistry, we found that the SPF sends dopaminergic projections to the superior paraolivary nucleus and the medial nucleus of the trapezoid body, but not the lateral superior olive. We confirmed these projections using a retrograde tracer. By making dual retrograde deposits in the IC and SOC, we found that individual dopaminergic cells innervate both the IC and SOC. These results suggest dopaminergic innervation, likely released in a context dependent manner, occurs at multiple levels of the auditory pathway.

Descending projections from the inferior colliculus to the dorsal cochlear nucleus are excitatory

Ascending projections of the dorsal cochlear nucleus (DCN) target primarily the contralateral inferior colliculus (IC). In turn, the IC sends bilateral descending projections back to the DCN. We sought to determine the nature of these descending axons in order to infer circuit mechanisms of signal processing at one of the earliest stages of the central auditory pathway. An anterograde tracer was injected in the IC of CBA/Ca mice to reveal terminal characteristics of the descending axons. Retrograde tracer deposits were made in the DCN of CBA/Ca and transgenic GAD67-EGFP mice to investigate the cells giving rise to these projections. A multiunit best frequency was determined for each injection site. Brains were processed by using standard histologic methods for visualization and examined by fluorescent, brightfield, and electron microscopy. Descending projections from the IC were inferred to be excitatory because the cell bodies of retrogradely labeled neurons did not colabel with EGFP expression in neurons of GAD67-EGFP mice. Furthermore, additional experiments yielded no glycinergic or cholinergic positive cells in the IC, and descending projections to the DCN were colabeled with antibodies against VGluT2, a glutamate transporter. Anterogradely labeled endings in the DCN formed asymmetric postsynaptic densities, a feature of excitatory neurotransmission. These descending projections to the DCN from the IC were topographic and suggest a feedback pathway that could underlie a frequency-specific enhancement of some acoustic signals and suppression of others. The involvement of this IC-DCN circuit is especially noteworthy when considering the gating of ascending signal streams for auditory processing. J. Comp. Neurol. 525:773-793, 2017. © 2016 Wiley Periodicals, Inc.