Bilateral projections to the thalamus from individual neurons in the inferior colliculus

Cholinergic modulation in the vertebrate auditory pathway

Acetylcholine (ACh) is a prevalent neurotransmitter throughout the nervous system. In the brain, ACh is widely regarded as a potent neuromodulator. In neurons, ACh signals are conferred through a variety of receptors that influence a broad range of neurophysiological phenomena such as transmitter release or membrane excitability. In sensory circuitry, ACh modifies neural responses to stimuli and coordinates the activity of neurons across multiple levels of processing. These factors enable individual neurons or entire circuits to rapidly adapt to the dynamics of complex sensory stimuli, underscoring an essential role for ACh in sensory processing. In the auditory system, histological evidence shows that acetylcholine receptors (AChRs) are expressed at virtually every level of the ascending auditory pathway. Despite its apparent ubiquity in auditory circuitry, investigation of the roles of this cholinergic network has been mainly focused on the inner ear or forebrain structures, while less attention has been directed at regions between the cochlear nuclei and midbrain. In this review, we highlight what is known about cholinergic function throughout the auditory system from the ear to the cortex, but with a particular emphasis on brainstem and midbrain auditory centers. We will focus on receptor expression, mechanisms of modulation, and the functional implications of ACh for sound processing, with the broad goal of providing an overview of a newly emerging view of impactful cholinergic modulation throughout the auditory pathway.

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.

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

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

Convergence of bilateral auditory midbrain inputs on neurons in the auditory thalamus of chicken

In the avian ascending auditory pathway, the nucleus mesencephalicus lateralis pars dorsalis (MLd; the auditory midbrain center) receives inputs from virtually all lower brainstem auditory nuclei and sends outputs bilaterally to the nucleus ovoidalis (Ov; the auditory thalamic nucleus). Axons from part of the MLd terminate in a particular domain of Ov, thereby suggesting a formation of segregated pathways point-to-point from lower brainstem nuclei via MLd to the thalamus. However, it has not yet been demonstrated whether any spatial clustering of thalamic neurons that receive inputs from specific domains of MLd exists. Ov neurons receive input from bilateral MLds; however, the degree of laterality has not been reported yet. In this study, we injected a recombinant avian adeno-associated virus, a transsynaptic anterograde vector into the MLd of the chick, and analyzed the distribution of labeled postsynaptic neurons on both sides of the Ov. We found that fluorescent protein-labeled neurons on both sides of the Ov were clustered in domains corresponding to subregions of the MLd. The laterality of projections was calculated as the ratio of neurons labeled by comparing ipsilateral to contralateral projections from the MLd, and it was 1.86 on average, thereby indicating a slight ipsilateral projection dominance. Bilateral inputs from different subdomains of the MLd converged on several single Ov neurons, thereby implying a possibility of a de novo binaural processing of the auditory information in the Ov.

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.