Lateral superior olive function in congenital deafness

Modeling the Effect of Temperature on Membrane Response of Light Stimulation in Optogenetically-Targeted Neurons

Optogenetics is revolutionizing Neuroscience, but an often neglected effect of light stimulation of the brain is the generation of heat. In extreme cases, light-generated heat kills neurons, but mild temperature changes alter neuronal function. To date, most in vivo experiments rely on light stimulation of neural tissue using fiber-coupled lasers of various wavelengths. Brain tissue is irradiated with high light power that can be deleterious to neuronal function. Furthermore, absorbed light generates heat that can lead to permanent tissue damage and affect neuronal excitability. Thus, light alone can generate effects in neuronal function that are unrelated to the genuine "optogenetic effect." In this work, we perform a theoretical analysis to investigate the effects of heat transfer in rodent brain tissue for standard optogenetic protocols. More precisely, we first use the Kubelka-Munk model for light propagation in brain tissue to observe the absorption phenomenon. Then, we model the optothermal effect considering the common laser wavelengths (473 and 593 nm) used in optogenetic experiments approaching the time/space numerical solution of Pennes' bio-heat equation with the Finite Element Method. Finally, we then modeled channelrhodopsin-2 in a single and spontaneous-firing neuron to explore the effect of heat in light stimulated neurons. We found that, at commonly used light intensities, laser radiation considerably increases the temperature in the surrounding tissue. This effect alters action potential size and shape and causes an increase in spontaneous firing frequency in a neuron model. However, the shortening of activation time constants generated by heat in the single firing neuron model produces action potential failures in response to light stimulation. We also found changes in the power spectrum density and a reduction in the time required for synchronization in an interneuron network model of gamma oscillations. Our findings indicate that light stimulation with intensities used in optogenetic experiments may affect neuronal function not only by direct excitation of light sensitive ion channels and/or pumps but also by generating heat. This approach serves as a guide to design optogenetic experiments that minimize the role of tissue heating in the experimental outcome.

Intrinsic and Miniature Postsynaptic Current Changes in Rat Principal Neurons of the Lateral Superior Olive after Unilateral Auditory Deprivation at an Early Age

Unilateral auditory deprivation results in lateralization changes in the central auditory system, interfering with the integration of binaural information and thereby leading to a decrease in binaural auditory functions such as sound localization. Principal neurons of the lateral superior olive (LSO) are responsible for computing the interaural intensity differences that are critical for sound localization in the horizontal plane. To investigate changes caused by unilateral auditory deprivation, electrophysiological activity was recorded from LSO principal neurons in control rats and rats with unilateral cochlear ablation. At one week after unilateral cochlear ablation, the excitability of LSO principal neurons on the side ipsilateral to the ablation (the ablated side) was greater than that on the side contralateral to the ablation (the intact side); however, the input resistance increased on both sides. Furthermore, by analysing the miniature inhibitory postsynaptic currents and miniature excitatory postsynaptic currents, we found that unilateral auditory deprivation weakened the inhibitory driving force on the intact side, whereas it strengthened the excitatory driving force on the ablated side. In summary, asymmetric changes in the electrophysiological activity of LSO principal neurons were found on both sides at postnatal day 19, one week after unilateral cochlear ablation.

Topographic map refinement and synaptic strengthening of a sound localization circuit require spontaneous peripheral activity

KEY POINTS

Loss of the calcium sensor otoferlin disrupts neurotransmission from inner hair cells. Central auditory nuclei are functionally denervated in otoferlin knockout mice (Otof KOs) via gene ablation confined to the periphery. We employed juvenile and young adult Otof KO mice (postnatal days (P)10-12 and P27-49) as a model for lacking spontaneous activity and deafness, respectively. We studied the impact of peripheral activity on synaptic refinement in the sound localization circuit from the medial nucleus of the trapezoid body (MNTB) to the lateral superior olive (LSO). MNTB in vivo recordings demonstrated drastically reduced spontaneous spiking and deafness in Otof KOs. Juvenile KOs showed impaired synapse elimination and strengthening, manifested by broader MNTB-LSO inputs, imprecise MNTB-LSO topography and weaker MNTB-LSO fibres. The impairments persisted into young adulthood. Further functional refinement after hearing onset was undetected in young adult wild-types. Collectively, activity deprivation confined to peripheral protein loss impairs functional MNTB-LSO refinement during a critical prehearing period.

ABSTRACT

Circuit refinement is critical for the developing sound localization pathways in the auditory brainstem. In prehearing mice (hearing onset around postnatal day (P)12), spontaneous activity propagates from the periphery to central auditory nuclei. At the glycinergic projection from the medial nucleus of the trapezoid body (MNTB) to the lateral superior olive (LSO) of neonatal mice, super-numerous MNTB fibres innervate a given LSO neuron. Between P4 and P9, MNTB fibres are functionally eliminated, whereas the remaining fibres are strengthened. Little is known about MNTB-LSO circuit refinement after P20. Moreover, MNTB-LSO refinement upon activity deprivation confined to the periphery is largely unexplored. This leaves a considerable knowledge gap, as deprivation often occurs in patients with congenital deafness, e.g. upon mutations in the otoferlin gene (OTOF). Here, we analysed juvenile (P10-12) and young adult (P27-49) otoferlin knockout (Otof KO) mice with respect to MNTB-LSO refinement. MNTB in vivo recordings revealed drastically reduced spontaneous activity and deafness in knockouts (KOs), confirming deprivation. As RNA sequencing revealed Otof absence in the MNTB and LSO of wild-types, Otof loss in KOs is specific to the periphery. Functional denervation impaired MNTB-LSO synapse elimination and strengthening, which was assessed by glutamate uncaging and electrical stimulation. Impaired elimination led to imprecise MNTB-LSO topography. Impaired strengthening was associated with lower quantal content per MNTB fibre. In young adult KOs, the MNTB-LSO circuit remained unrefined. Further functional refinement after P12 appeared absent in wild-types. Collectively, we provide novel insights into functional MNTB-LSO circuit maturation governed by a cochlea-specific protein. The central malfunctions in Otof KOs may have implications for patients with sensorineuronal hearing loss.

Monaural or binaural sound deprivation in postlingual hearing loss: Cochlear implant in the worse ear

OBJECTIVES/HYPOTHESIS

To determine whether speech recognition scores (SRS) differ between adults with long-term auditory deprivation in the implanted ear and adults who received cochlear implant (CI) in the nonsound-deprived ear, either for hearing aid-assisted or due to rapidly deteriorating hearing loss.

STUDY DESIGN

Retrospective study.

METHODS

Speech recognition scores at evaluations (3 and 14 months postimplantation) conducted with CI alone at 60-dB sound pressure level intensity were compared in 15 patients (4 with bilateral severe hearing loss; 11 with asymmetric hearing loss, 7 of which had contralateral hearing aid), all with long-term auditory deprivation (mean duration 16.9 years) (group A), and in 15 other patients with postlingual hearing loss (10 symmetric, 5 asymmetric with bimodal stimulation) (controls, group B).

RESULTS

Comparison of mean percentage of correctly recognized words on speech audiometry at 3 and 14 months showed improvement within each group (P < 0.05). Between-group comparison showed no significant difference at 3 (P = 0.17) or 14 months (P = 0.46). Comparison of SRSs in group A (bimodal stimulation [n = 7] and binaural sound deprivation [n = 4]) versus group B showed no significant differences at 3 (bimodal stimulation P = 0.16; binaural sound deprivation P = 0.19) or 14 months (bimodal stimulation P = 0.14; binaural sound deprivation P = 0.82).

CONCLUSIONS

Speech recognition scores in monaural and binaural sound-deprived ears did not significantly differ from ears with unilateral cochlear implantation in nonsound-deprived ears when tested with CI alone. Improvement in the implanted worse ear indicates that it could be a potential candidate ear for cochlear implantation even when sound deprived.

LEVEL OF EVIDENCE

4. Laryngoscope, 126:1905-1910, 2016.

Spontaneous activity in the developing auditory system

Spontaneous electrical activity is a common feature of sensory systems during early development. This sensory-independent neuronal activity has been implicated in promoting their survival and maturation, as well as growth and refinement of their projections to yield circuits that can rapidly extract information about the external world. Periodic bursts of action potentials occur in auditory neurons of mammals before hearing onset. This activity is induced by inner hair cells (IHCs) within the developing cochlea, which establish functional connections with spiral ganglion neurons (SGNs) several weeks before they are capable of detecting external sounds. During this pre-hearing period, IHCs fire periodic bursts of Ca(2+) action potentials that excite SGNs, triggering brief but intense periods of activity that pass through auditory centers of the brain. Although spontaneous activity requires input from IHCs, there is ongoing debate about whether IHCs are intrinsically active and their firing periodically interrupted by external inhibitory input (IHC-inhibition model), or are intrinsically silent and their firing periodically promoted by an external excitatory stimulus (IHC-excitation model). There is accumulating evidence that inner supporting cells in Kölliker's organ spontaneously release ATP during this time, which can induce bursts of Ca(2+) spikes in IHCs that recapitulate many features of auditory neuron activity observed in vivo. Nevertheless, the role of supporting cells in this process remains to be established in vivo. A greater understanding of the molecular mechanisms responsible for generating IHC activity in the developing cochlea will help reveal how these events contribute to the maturation of nascent auditory circuits.

The precise temporal pattern of prehearing spontaneous activity is necessary for tonotopic map refinement

Patterned spontaneous activity is a hallmark of developing sensory systems. In the auditory system, rhythmic bursts of spontaneous activity are generated in cochlear hair cells and propagated along central auditory pathways. The role of these activity patterns in the development of central auditory circuits has remained speculative. Here we demonstrate that blocking efferent cholinergic neurotransmission to developing hair cells in mice that lack the α9 subunit of nicotinic acetylcholine receptors (α9 KO mice) altered the temporal fine structure of spontaneous activity without changing activity levels. KO mice showed a severe impairment in the functional and structural sharpening of an inhibitory tonotopic map, as evidenced by deficits in synaptic strengthening and silencing of connections and an absence in axonal pruning. These results provide evidence that the precise temporal pattern of spontaneous activity before hearing onset is crucial for the establishment of precise tonotopy, the major organizing principle of central auditory pathways.

Functional localization of neurotransmitter receptors and synaptic inputs to mature neurons of the medial superior olive

Neurons of the medial superior olive (MSO) code for the azimuthal location of low-frequency sound sources via a binaural coincidence detection system operating on microsecond time scales. These neurons are morphologically simple and stereotyped, and anatomical studies have indicated a functional segregation of excitatory and inhibitory inputs between cellular compartments. It is thought that this morphological arrangement holds important implications for the computational task of these cells. To date, however, there has been no functional investigation into synaptic input sites or functional receptor distributions on mature neurons of the MSO. Here, functional neurotransmitter receptor maps for amino-3-hydroxyl-5-methyl-4-isoxazole propionate (AMPA), N-methyl-d-aspartate (NMDA), glycine (Gly), and ionotropic γ-aminobutyric acid (GABAA) receptors (Rs) were compared and complemented by their corresponding synaptic input map. We find in MSO neurons from postnatal day 20–35 gerbils that AMPARs and their excitatory inputs target the soma and dendrites. Functional GlyRs and their inhibitory inputs are predominantly refined to the somata, although a pool of functional GlyRs is present extrasynaptically on MSO dendrites. GABAAR responses are present throughout the cell but lack direct synaptic contact indicating an involvement in volume transmission. NMDARs are present both synaptically and extrasynaptically with an overall distribution similar to GlyRs. Interestingly, even at physiological temperatures these functional NMDARs can be potentiated by synaptically released Gly. The functional receptor and synaptic input maps produced here led to the identification of a cross talk between transmitter systems and raises the possibility that extrasynaptic receptors could be modulating leak conductances as a homeostatic mechanism.

Cav1.3 calcium channels are required for normal development of the auditory brainstem

Within the Ca(v)1 family of voltage-gated calcium channels, Ca(v)1.2 and Ca(v)1.3 channels are the predominant subtypes in the brain. Whereas specific functions for each subtype were described in the adult brain, their role in brain development is poorly understood. Here we assess the role of Ca(v)1.3 subunits in the activity-dependent development of the auditory brainstem. We used Ca(v)1.3-deficient (Ca(v)1.3(-/-)) mice because these mice lack cochlea-driven activity that deprives the auditory centers from peripheral input. We found a drastically reduced volume in all auditory brainstem centers (range 25-59%, total 35%), which was manifest before hearing onset. A reduction was not obvious outside the auditory system. The lateral superior olive (LSO) was strikingly malformed in Ca(v)1.3(-/-) mice and had fewer neurons (1/3 less). The remaining LSO neurons displayed normal dendritic trees and received functional glutamatergic input, yet they fired action potentials predominantly with a multiple pattern upon depolarization, in contrast to the single firing pattern prevalent in controls. The latter finding appears to be due to a reduction of dendrototoxin-sensitive potassium conductances, presumably mediated through the K(v)1.2 subtype. Fura2 imaging provided evidence for functional Ca(v)1.3 channels in the LSO of wild-type mice. Our results imply that Ca(v)1.3 channels are indispensable for the development of the central auditory system. We propose that the unique LSO phenotype in Ca(v)1.3(-/-) mice, which hitherto was not described in other hereditary deafness models, is caused by the synergistic contribution of two factors: on-site loss of Ca(v)1.3 channels in the neurons plus lack of peripheral input.