Accelerated Development of the First-Order Central Auditory Neurons With Spontaneous Activity

Priming central sound processing circuits through induction of spontaneous activity in the cochlea before hearing onset

Sensory systems experience a period of intrinsically generated neural activity before maturation is complete and sensory transduction occurs. Here we review evidence describing the mechanisms and functions of this 'spontaneous' activity in the auditory system. Both ex vivo and in vivo studies indicate that this correlated activity is initiated by non-sensory supporting cells within the developing cochlea, which induce depolarization and burst firing of groups of nearby hair cells in the sensory epithelium, activity that is conveyed to auditory neurons that will later process similar sound features. This stereotyped neural burst firing promotes cellular maturation, synaptic refinement, acoustic sensitivity, and establishment of sound-responsive domains in the brain. While sensitive to perturbation, the developing auditory system exhibits remarkable homeostatic mechanisms to preserve periodic burst firing in deaf mice. Preservation of this early spontaneous activity in the context of deafness may enhance the efficacy of later interventions to restore hearing.

Biohybrid neural interfaces: improving the biological integration of neural implants

Implantable neural interfaces (NIs) have emerged in the clinic as outstanding tools for the management of a variety of neurological conditions caused by trauma or disease. However, the foreign body reaction triggered upon implantation remains one of the major challenges hindering the safety and longevity of NIs. The integration of tools and principles from biomaterial design and tissue engineering has been investigated as a promising strategy to develop NIs with enhanced functionality and performance. In this Feature Article, we highlight the main bioengineering approaches for the development of biohybrid NIs with an emphasis on relevant device design criteria. Technical and scientific challenges associated with the fabrication and functional assessment of technologies composed of both artificial and biological components are discussed. Lastly, we provide future perspectives related to engineering, regulatory, and neuroethical challenges to be addressed towards the realisation of the promise of biohybrid neurotechnology.

Upregulation of A-type potassium channels suppresses neuronal excitability in hypoxic neonatal mice

Neuronal excitability is a critical feature of central nervous system development, playing a fundamental role in the functional maturation of brain regions, including the hippocampus, cerebellum, auditory and visual systems. The present study aimed to determine the mechanism by which hypoxia causes brain dysfunction through perturbation of neuronal excitability in a hypoxic neonatal mouse model. Functional brain development was assessed in humans using the Gesell Development Diagnosis Scale. In mice, gene transcription was evaluated via mRNA sequencing and quantitative PCR; furthermore, patch clamp recordings assessed potassium currents. Clinical observations revealed disrupted functional brain development in 6- and 18-month-old hypoxic neonates, and those born with normal hearing screening unexpectedly exhibited impaired central auditory function at 3 months. In model mice, CA1 pyramidal neurons exhibited reduced spontaneous activity, largely induced by excitatory synaptic input suppression, despite the elevated membrane excitability of hypoxic neurons compared to that of control neurons. In hypoxic neurons, Kcnd3 gene transcription was upregulated, confirming upregulated hippocampal Kv 4.3 expression. A-type potassium currents were enhanced, and Kv 4.3 participated in blocking excitatory presynaptic inputs. Elevated Kv 4.3 activity in pyramidal neurons under hypoxic conditions inhibited excitatory presynaptic inputs and further decreased neuronal excitability, disrupting functional brain development in hypoxic neonates.

Bilirubin gates the TRPM2 channel as a direct agonist to exacerbate ischemic brain damage

Stroke prognosis is negatively associated with an elevation of serum bilirubin, but how bilirubin worsens outcomes remains mysterious. We report that post-, but not pre-, stroke bilirubin levels among inpatients scale with infarct volume. In mouse models, bilirubin increases neuronal excitability and ischemic infarct, whereas ischemic insults induce the release of endogenous bilirubin, all of which are attenuated by knockout of the TRPM2 channel or its antagonist A23. Independent of canonical TRPM2 intracellular agonists, bilirubin and its metabolic derivatives gate the channel opening, whereas A23 antagonizes it by binding to the same cavity. Knocking in a loss of binding point mutation for bilirubin, TRPM2-D1066A, effectively antagonizes ischemic neurotoxicity in mice. These findings suggest a vicious cycle of stroke injury in which initial ischemic insults trigger the release of endogenous bilirubin from injured cells, which potentially acts as a volume neurotransmitter to activate TRPM2 channels, aggravating Ca²⁺-dependent brain injury.

Differential expression of microRNAs in the developing avian auditory hindbrain

The avian auditory hindbrain is a longstanding model for studying neural circuit development. Information on gene regulatory network (GRN) components underlying this process, however, is scarce. Recently, the spatiotemporal expression of 12 microRNAs (miRNAs) was investigated in the mammalian auditory hindbrain. As a comparative study, we here investigated the spatiotemporal expression of the orthologous miRNAs during development of the chicken auditory hindbrain. All miRNAs were expressed both at E13, an immature stage, and P14, a mature stage of the auditory system. In most auditory nuclei, a homogeneous expression pattern was observed at both stages, like the mammalian system. An exception was the nucleus magnocellularis (NM). There, at E13, nine miRNAs showed a differential expression pattern along the cochleotopic axis with high expression at the rostromedial pole. One of them showed a gradient expression whereas eight showed a spatially selective expression at the rostral pole that reflected the different rhombomeric origins of this composite nucleus. The miRNA differential expression persisted in the NM to the mature stage, with the selective expression changed to linear gradients. Bioinformatics analysis predicted mRNA targets that are associated with neuronal developmental processes such as neurite and synapse organization, calcium and ephrin-Eph signaling, and neurotransmission. Overall, this first analysis of miRNAs in the chicken central auditory system reveals shared and strikingly distinct features between chicken and murine orthologues. The embryonic gradient expression of these GRN elements in the NM adds miRNA patterns to the list of cochleotopic and developmental gradients in the central auditory system.

Efferent feedback controls bilateral auditory spontaneous activity

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

HCN channels in the mammalian cochlea: Expression pattern, subcellular location, and age-dependent changes

Neuronal diversity in the cochlea is largely determined by ion channels. Among voltage‐gated channels, hyperpolarization‐activated cyclic nucleotide‐gated (HCN) channels open with hyperpolarization and depolarize the cell until the resting membrane potential. The functions for hearing are not well elucidated and knowledge about localization is controversial. We created a detailed map of subcellular location and co‐expression of all four HCN subunits across different mammalian species including CBA/J, C57Bl/6N, Ly5.1 mice, guinea pigs, cats, and human subjects. We correlated age‐related hearing deterioration in CBA/J and C57Bl/6N with expression levels of HCN1, −2, and −4 in individual auditory neurons from the same cohort. Spatiotemporal expression during murine postnatal development exposed HCN2 and HCN4 involvement in a critical phase of hair cell innervation. The huge diversity of subunit composition, but lack of relevant heteromeric pairing along the perisomatic membrane and axon initial segments, highlighted an active role for auditory neurons. Neuron clusters were found to be the hot spots of HCN1, −2, and −4 immunostaining. HCN channels were also located in afferent and efferent fibers of the sensory epithelium. Age‐related changes on HCN subtype expression were not uniform among mice and could not be directly correlated with audiometric data. The oldest mice groups revealed HCN channel up‐ or downregulation, depending on the mouse strain. The unexpected involvement of HCN channels in outer hair cell function where HCN3 overlaps prestin location emphasized the importance for auditory function. A better understanding may open up new possibilities to tune neuronal responses evoked through electrical stimulation by cochlear implants.

The Role of Emotional vs. Cognitive Intelligence in Economic Decision-Making Amongst Older Adults

The links between emotions, bio-regulatory processes, and economic decision-making are well-established in the context of age-related changes in fluid, real-time, decision competency. The objective of the research reported here is to assess the relative contributions, interactions, and impacts of affective and cognitive intelligence in economic, value-based decision-making amongst older adults. Additionally, we explored this decision-making competency in the context of the neurobiology of aging by examining the neuroanatomical correlates of intelligence and decision-making in an aging cohort. Thirty-nine, healthy, community dwelling older adults were administered the Iowa Gambling Task (IGT), an ecologically valid laboratory measure of complex, economic decision-making; along with standardized, performance-based measures of cognitive and emotional intelligence (EI). A smaller subset of this group underwent structural brain scans from which thicknesses of the frontal, parietal, temporal, occipital, cingulate cortices and their sub-sections, were computed. Fluid (online processing) aspects of Perceptual Reasoning cognitive intelligence predicted superior choices on the IGT. However, older adults with higher overall emotional intelligence (EI) and higher Experiential EI area/sub-scores learned faster to make better choices on the IGT, even after controlling for cognitive intelligence and its area scores. Thickness of the left rostral anterior cingulate (associated with fluid affective, processing) mediated the relationship between age and Experiential EI. Thickness of the right transverse temporal gyrus moderated the rate of learning on the IGT. In conclusion, our data suggest that fluid processing, which involves "online," bottom-up, cognitive processing, predicts value-based decision-making amongst older adults, while crystallized intelligence, which relies on "offline" previously acquired knowledge, does not. However, only emotional intelligence, especially its fluid "online" aspects of affective processing predicts the rate of learning in situations of complex choice, especially when there is a paucity of cues/information available to guide decision-making. Age-related effects on these cognitive, affective and decision mechanisms may have neuroanatomical correlates, especially in regions that form a subset of the human mirror-neuron and mentalizing systems. While superior decision-making may be stereotypically associated with "smarter people" (i.e., higher cognitive intelligence), our data indicate that emotional intelligence has a significant role to play in the economic decisions of older adults.

Ca2+-dependent recruitment of voltage-gated sodium channels underlies bilirubin-induced overexcitation and neurotoxicity

Neonatal jaundice is prevalent among newborns and can lead to severe neurological deficits, particularly sensorimotor dysfunction. Previous studies have shown that bilirubin (BIL) enhances the intrinsic excitability of central neurons and this can potentially contribute to their overexcitation, Ca²⁺ overload, and neurotoxicity. However, the cellular mechanisms underlying elevated neuronal excitability remain unknown. By performing patch-clamp recordings from neonatal neurons in the rat medial vestibular nucleus (MVN), a crucial relay station for locomotor and balance control, we found that BIL (3 μM) drastically increases the spontaneous firing rates by upregulating the current-mediated voltage-gated sodium channels (VGSCs), while shifting their voltage-dependent activation toward more hyperpolarized potentials. Immunofluorescence labeling and western immunoblotting with an anti-NaV1.1 antibody, revealed that BIL elevates the expression of VGSCs by promoting their recruitment to the membrane. Furthermore, we found that this VGSC-trafficking process is Ca²⁺ dependent because preloading MVN neurons with the Ca²⁺ buffer BAPTA-AM, or exocytosis inhibitor TAT-NSF700, prevents the effects of BIL, indicating the upregulated activity and density of functional VGSCs as the core mechanism accountable for the BIL-induced overexcitation of neonatal neurons. Most importantly, rectification of such overexcitation with a low dose of VGSC blocker lidocaine significantly attenuates BIL-induced cell death. We suggest that this enhancement of VGSC currents directly contributes to the vulnerability of neonatal brain to hyperbilirubinemia, implicating the activity and trafficking of NaV1.1 channels as a potential target for neuroprotection in cases of severe jaundice.

Functional Development of Principal Neurons in the Anteroventral Cochlear Nucleus Extends Beyond Hearing Onset

Sound information is transduced into graded receptor potential by cochlear hair cells and encoded as discrete action potentials of auditory nerve fibers. In the cochlear nucleus, auditory nerve fibers convey this information through morphologically distinct synaptic terminals onto bushy cells (BCs) and stellate cells (SCs) for processing of different sound features. With expanding use of transgenic mouse models, it is increasingly important to understand the in vivo functional development of these neurons in mice. We characterized the maturation of spontaneous and acoustically evoked activity in BCs and SCs by acquiring single-unit juxtacellular recordings between hearing onset (P12) and young adulthood (P30) of anesthetized CBA/J mice. In both cell types, hearing sensitivity and characteristic frequency (CF) range are mostly adult-like by P14, consistent with rapid maturation of the auditory periphery. In BCs, however, some physiological features like maximal firing rate, dynamic range, temporal response properties, recovery from post-stimulus depression, first spike latency (FSL) and encoding of sinusoid amplitude modulation undergo further maturation up to P18. In SCs, the development of excitatory responses is even more prolonged, indicated by a gradual increase in spontaneous and maximum firing rates up to P30. In the same cell type, broadly tuned acoustically evoked inhibition is immediately effective at hearing onset, covering the low- and high-frequency flanks of the excitatory response area. Together, these data suggest that maturation of auditory processing in the parallel ascending BC and SC streams engages distinct mechanisms at the first central synapses that may differently depend on the early auditory experience.