Biology subjects

Elgoyhen, A. B.

Publications and source records attributed to Elgoyhen, A. B..

10 recordsLinked to original sources

bioRxiv2026

Medial Olivocochlear Activity Modulates Central Auditory Synapse Structure and Function

In many mammals, the auditory system is immature at birth and undergoes activity-dependent refinement. The medial olivocochlear system (MOC) contributes to the maturation of central auditory connectivity by shaping spontaneous activity in the developing inner ear. Although altered MOC activity has been linked to electrical and synaptic dysfunctions in different central auditory nuclei, an in-depth analysis of the same synapse under conditions of absent versus enhanced MOC activity is lacking. In this work, we set out a physiological and structural analysis of the calyx of Held and the principal neurons of the medial nucleus of the trapezoid body (CH-MNTB) synapse at postnatal days 12-14 in three mouse models of either sex: wild-type (WT), 9 knock-in (9KI; with enhanced MOC activity), and 9 knock-out (9KO; which lacks MOC activity). Electrophysiological recordings in brain slices revealed a reduced synaptic strength efficacy in 9KI compared to WT, including smaller excitatory postsynaptic current (EPSC) amplitudes, stronger short-term depression during repetitive stimulation and a decrease readily releasable pool size. In contrast, 9KO mice showed minimal synaptic differences relative to WT. Serial block-face electron microscopy (SBEM) reconstructions demonstrated morphological alterations of the CH in both MOC-manipulated mouse models. However, 9KI mice exhibited the largest deviations, including fewer morphologically complex CHs and increased poly-innervation of MNTB cells. These results indicate that transient, well-regulated efferent control of cochlear activity is crucial for establishing accurate central auditory connectivity. Moreover, the enhancement of MOC activity drives more pronounced developmental changes in brainstem auditory circuitry than its absence. Significance StatementBefore hearing onset, inner hair cells in altricial mammals show spontaneous electrical activity crucial for proper auditory pathway development. This activity is finely regulated by descending efferent nerves from the central nervous system during a critical developmental period, ensuring precise auditory circuit formation. Our study shows that genetically manipulating efferent function --either by eliminating it or enhancing it-- leads to structural alterations and severe synaptic dysfunction within the auditory brainstem. These results indicate that transient, well-regulated efferent control of cochlear activity is crucial for establishing accurate central auditory connectivity. Importantly, enhancing efferent peripheral activity causes more pronounced central synaptic changes than its absence, highlighting the importance of balanced efferent modulation during early auditory system development for normal brainstem auditory function.

neuroscience↗

bioRxiv2025

Early Noise Exposure and Changes in Medial Olivocochlear Strength Alters Auditory Pathway Development

The early onset of peripheral deafness significantly alters the proper development of the auditory system. Likewise, exposure to loud noise during early development produces a similar disruptive effect. Before hearing onset in altricial mammals, cochlear inner hair cells exhibit spontaneous electrical activity that drives auditory circuit development. This activity is modulated by medial olivocochlear (MOC) efferent feedback through 910 nicotinic cholinergic receptors in inner hair cells. In adults, these receptors are restricted to outer hair cells, where they mediate MOC feedback to regulate cochlear amplification. Although the MOC systems protective role to prevent noise-induced hearing loss in adulthood is well-established, its influence during early developmental stages-especially in response to exposure to loud noise-remains largely unexplored. In this study, we investigated the role of MOC feedback during early postnatal development using 9 knockout (KO) and 9 knock-in (KI) mice of either sex, which respectively lack or exhibit enhanced cholinergic activity. Our findings reveal that both increased and absent olivocochlear activity result in altered auditory sensitivity at the onset of hearing, along with long- range alterations in the number and morphology of ribbon synapses. Early noise exposure caused lasting auditory damage in both wild-type and 9KO mice, with deficits persisting into adulthood. In contrast, 9KI mice were protected from noise-induced damage, with no long-term effects on auditory function. These results highlight the increased susceptibility of the auditory system during early postnatal development. Moreover, they indicate that an enhanced MOC feedback shields the auditory system from noise damage during this period. SIGNIFICANCE STATEMENTEarly development represents a sensitive window for shaping auditory function. We show that the medial olivocochlear system is critical for establishing normal ribbon synapse density and size; key features for proper hearing onset. We also show that the developing auditory system is especially vulnerable to loud noise, with early exposure causing more severe and lasting effects than similar noise later in life. Notably, enhancing 910 nAChR receptor activity during this early stage offers protection against noise-induced damage, revealing a time-sensitive opportunity to safeguard auditory development.

neuroscience↗

bioRxiv2024

Co-release of GABA and ACh from medial olivocochlear neurons fine tunes cochlear efferent inhibition

During development, inner hair cells (IHCs) in the mammalian cochlea are unresponsive to acoustic stimuli but instead exhibit spontaneous activity. During this same period, neurons originating from the medial olivocochlear complex (MOC) transiently innervate IHCs, regulating their firing pattern which is crucial for the correct development of the auditory pathway. Although the MOC-IHC is a cholinergic synapse, previous evidence indicates the widespread presence of gamma-aminobutyric acid (GABA) signaling markers, including presynaptic GABAB receptors (GABABR). In this study, we explore the source of GABA by optogenetically activating either cholinergic or GABAergic fibers. The optogenetic stimulation of MOC terminals from GAD;ChR2-eYFP and ChAT;ChR2-eYFP mice evoked synaptic currents in IHCs that were blocked by -bungarotoxin. This suggests that GABAergic fibers release ACh and activate 910 nicotinic acetylcholine receptors (nAChRs). Additionally, MOC cholinergic fibers release not only ACh but also GABA, as the effect of GABA on ACh response amplitude was prevented by applying the GABAB-R blocker (CGP 36216). Using optical neurotransmitter detection and calcium imaging techniques, we examined the extent of GABAergic modulation at the single synapse level. Our findings suggest heterogeneity in GABA modulation, as only 15 out of 31 recorded synaptic sites were modulated by applying the GABABR specific antagonist, CGP (100-200 {micro}M). In conclusion, we provide compelling evidence that GABA and ACh are co-released from at least a subset of MOC terminals. In this circuit, GABA functions as a negative feedback mechanism, locally regulating the extent of cholinergic inhibition at certain efferent-IHC synapses during an immature stage. Significance statementBefore hearing onset, the medial olivocochlear (MOC) efferent system of the mammalian cochlea regulates the pattern of IHC spontaneous firing rate through the activation of 910 nAChRs. However, GABA is also known to have a modulatory role at the MOC-IHC synapse. Our results show that GABA is co-released from at least a subset of MOC terminals, working as a precise regulatory mechanism for ACh release. Furthermore, we demonstrate that not all synaptic contacts within a single IHC are equally modulated by GABA.

physiology↗

bioRxiv2022

Cannabinoids Activate the Insulin Pathway to Modulate Mobilization of Cholesterol in C. elegans

The nematode Caenorhabditis elegans requires exogenous cholesterol to survive and its depletion leads to early development arrest. Thus, tight regulation of cholesterol storage and distribution within the organism is critical. Previously, we demonstrated that the endocannabinoid (eCB) 2-arachidonoylglycerol (2-AG) plays a key role in C. elegans modulating sterol mobilization, but the mechanism is unknown. Here we show that mutations in the ocr-2 and osm-9 genes coding for transient receptors potential V (TRPV) ion channels, dramatically reduces the effect of 2-AG in cholesterol mobilization. Through genetic analysis combined with the rescuing of larval arrest induced by sterol starvation we found that the insulin/IGF-1signaling (IIS) pathway and UNC-31/CAPS, a calcium-activated regulator of neural dense-core vesicles release, are essential for 2-AG-mediated stimulation of cholesterol mobilization. These findings indicate that 2-AG-dependent cholesterol trafficking requires the release of insulin peptides and signaling through the DAF-2 insulin receptor. These results suggest that 2-AG acts as an endogenous modulator of TRPV signal transduction to control intracellular sterol traffic through modulation of the IGF-1 signaling pathway. Author summaryAlthough cannabis extracts have been used in folklore medicine for centuries, the past few years have seen an increased interest in the medicinal uses of cannabinoids, the bioactive components of the cannabis plant, for treatment of many diseases of the nervous system. However, the human body naturally produces endocannabinoids that are similar to the cannabinoids present in Cannabis sativa. Our goal is to understand how endocannabinoids maintain cholesterol homeostasis in animals, underscoring the importance of cholesterol balance for healthy life. Both cholesterol excess and cholesterol deficiency can have detrimental effects on health, and a myriad of regulatory processes have thus evolved to control the metabolic pathways of sterol metabolism. The nematode C. elegans is auxotroph for sterols, that is; contrary to mammals they cannot synthesize sterols, therefore, dietary supply is essential for survival. The aim of our study was to elucidate the mechanism by which endocannabinoids abolish larval arrest of C. elegans induced by cholesterol depletion. We discovered that endocannabinoids stimulate the insulin pathway, which affects development, reproduction and life span, to modulate mobilization of cholesterol in C. elegans. Our studies have important implications for a better understanding of human pathological conditions associated with impaired cholesterol homeostasis.

neuroscience↗

bioRxiv2020

Loss of choline agonism in the inner ear hair cell nicotinic acetylcholine receptor linked to the α10 subunit

The 910 nicotinic acetylcholine receptor (nAChR) plays a fundamental role in inner ear physiology. It mediates synaptic transmission between efferent olivocochlear fibers that descend from the brainstem and hair cells of the auditory sensory epithelium. The 9 and 10 subunits have undergone a distinct evolutionary history within the family of nAChRs. Predominantly in mammalian vertebrates, the 910 receptor has accumulated changes at the protein level that may ultimately relate to the evolutionary history of the mammalian hearing organ. In the present work we investigated the responses of 910 nAChRs to choline, the metabolite of acetylcholine degradation at the synaptic cleft. Whereas choline is a full agonist of chicken 910 receptors it is a partial agonist of the rat receptor. Making use of the expression of 910 heterologous receptors, encompassing wild-type, heteromeric, homomeric, mutant, chimeric and hybrid receptors, and in silico molecular docking, we establish that the mammalian (rat) 10 nAChR subunit underscores the reduced efficacy of choline. Moreover, we show that whereas the complementary face of the 10 subunit does not play an important role in the activation of the receptor by ACh, it is strictly required for choline responses. Thus, we propose that the evolutionary changes acquired in the mammalian 910 nAChR resulted in the loss of choline acting as a full agonist at the efferent synapse, without affecting the triggering of ACh responses. This may have accompanied the fine-tuning of hair cell post-synaptic responses to the high frequency activity of efferent medial olivocochlear fibers that modulate the cochlear amplifier. Contribution to the Field StatementIn the inner ear of mammals, several evolutionary changes have occurred resulting in an expansion of the hearing range to higher sound frequencies. Fine tuning of cochlear synapses is required for sound enconding. The synapse between efferent olivocochlear fibers, that descend from the hindbrain, and sensory hair cells modulates sound amplification at the periphery and it has been proposed as a major player in the expansion of the hearing range. The 910 nicotinic acetylcholine receptor, which mediates synaptic neurotransmission at the efferent fiber-hair cell synapses, has accumulated a high number of amino acid substitutions in the mammalian lineage. We now show that these evolutionary acquired changes have led to a mammalian receptor with a lower efficacy for choline, the metabolite produced at the synaptic cleft by acetylcholine degradation. Making use of molecular, electrophysiological and in silico simulations techniques we show that it is the 10 subunit the one responsible for the loss of full choline agonism on the efferent receptor in mammals. This functional change may prove fundamental to faithfully reproduce the high frequency activity of efferent medial olivocochlear fibers and the modulation of the cochlear amplifier.

neuroscience↗

bioRxiv2020

Purinergic signaling controls spontaneous activityin the auditory system throughout early development

Spontaneous bursts of electrical activity in the developing auditory system arise within the cochlea prior to hearing onset and propagate through future sound processing circuits of the brain to promote maturation of auditory neurons. Studies in isolated cochleae revealed that this intrinsically generated activity is initiated by ATP release from inner supporting cells (ISCs), resulting in activation of purinergic autoreceptors, K+ efflux and subsequent depolarization of inner hair cells (IHCs). However, little is known about when this activity emerges or whether different mechanisms underlie distinct stages of development. Here we show that spontaneous electrical activity in mouse cochlea emerges within ISCs during the late embryonic period, preceding the onset of spontaneous correlated activity in IHCs and spiral ganglion neurons (SGNs), which begins at birth and follows a base to apex developmental gradient. At all developmental stages, pharmacological inhibition of P2Y1 metabotropic purinergic receptors dramatically reduced spontaneous activity in these three cell types. Moreover, in vivo imaging within the inferior colliculus of awake mice revealed that auditory neurons within future isofrequency zones exhibit coordinated neural activity at birth. The frequency of these discrete bursts increased progressively during the postnatal prehearing period, yet remained dependent on P2RY1. Analysis of mice with disrupted cholinergic signaling in the cochlea, indicate that this input modulates, rather than initiates, spontaneous activity before hearing onset. Thus, the auditory system uses a consistent mechanism involving ATP release from ISCs and activation of purinergic autoreceptors to elicit coordinated excitation of neurons that will process similar frequencies of sound. SIGNIFICANCE STATEMENTIn developing sensory systems, groups of neurons that will process information from similar sensory space exhibit highly correlated electrical activity that is critical for proper maturation and circuit refinement. Defining the period when this activity is present, the mechanisms responsible and the features of this activity are crucial for understanding how spontaneous activity influences circuit development. We show that, from birth to hearing onset, the auditory system relies on a consistent mechanism to elicit correlate firing of neurons that will process similar frequencies of sound. Targeted disruption of this activity will increase our understanding of how these early circuits mature and may provide insight into processes responsible for developmental disorders of the auditory system.

neuroscience↗

bioRxiv2020

Synaptic contributions to cochlear outer hair cell Ca2+ homeostasis

For normal cochlear function, outer hair cells (OHCs) require a precise regulation of intracellular Ca2+ levels. Influx of Ca2+ occurs both at the stereocillia tips and through the basolateral membrane. In this latter compartment, two different origins for Ca2+ influx have been poorly explored: voltage-gated Ca2+ channels (VGCC) at synapses with type II afferent neurons, and 910 cholinergic nicotinic receptors at synapses with medio-olivochlear complex (MOC) neurons. Using functional imaging in rodent OHCs, we report that these two Ca2+ entry sites are closely positioned, but present different regulation mechanisms. Ca2+ spread from MOC synapses is contained by cisternal Ca2+-ATPases. Considered a weak drive for transmitter release, we unexpectedly found that VGCC Ca2+ signals are comparable in size to those elicited by 910 and can be potentiated by ryanodine receptors. Finally, we showed that sorcin, a highly expressed gene product in OHCs with reported Ca2+ control function in cardiomy-ocytes, regulates basal Ca2+ levels and MOC synaptic activity in OHCs.

neuroscience↗

bioRxiv2019

Distinct evolutionary trajectories of neuronal and hair cell nicotinic acetylcholine receptors

The expansion and pruning of ion channel families has played a crucial role in the evolution of nervous systems. Remarkably, with a highly conserved vertebrate complement, nicotinic acetylcholine receptors (nAChRs) are unique among ligand-gated ion channels in that members of the family have distinct roles in synaptic transmission in non-overlapping domains, either in the nervous system, the inner ear hair cells or the neuromuscular junction. Here, we performed a comprehensive analysis of vertebrate nAChRs sequences, single cell expression patterns and comparative functional properties of receptors from three representative tetrapod species. We show that hair cell nAChRs underwent a distinct evolutionary trajectory to that of neuronal receptors. These were most likely shaped by different co-expression patterns and co-assembly rules of component subunits. Thus, neuronal nAChRs showed high degree of coding sequence conservation, coupled to greater co-expression variance and conservation of functional properties across tetrapod clades. In contrast, hair cell 910 nAChRs exhibited greater sequence divergence, narrow co-expression pattern and great variability of functional properties across species. These results point to differential substrates for random change within the family of gene paralogs that relate to the segregated roles of nAChRs in synaptic transmission.\n\nSignificance statementOur work exploits several peculiarities of the family of vertebrate nicotinic acetylcholine receptors (nAChRs) to explore the evolutionary trajectories of a ligand-gated ion channel family. By performing a comprehensive comparative analysis of nAChR subunits coding sequences, single cell expression patterns and functional properties we found a contrasting evolutionary history between nAChRs with widespread expression in the nervous system compared to those with isolated expression in the inner ear. Evolutionary changes were focused on differences in co-expression and co-assembly patterns for the former and coding sequences in the latter. This multidisciplinary approach provides further insight into the evolutionary processes that shaped the nervous and sensory systems of extant animals.

neuroscience↗

bioRxiv2018

Developmental synaptic changes at the transient olivocochlear-inner hair cell synapse

In the mature mammalian cochlea, inner hair cells (IHCs) are mainly innervated by afferent fibers that convey sound information to the central nervous system. During postnatal development, however, medial olivocochlear (MOC) efferent fibers transiently innervate the IHCs. The MOC-IHC synapse, functional from postnatal day (P)0 to hearing onset (P12), undergoes dramatic changes in the sensitivity to acetylcholine (ACh) and in the expression of key postsynaptic proteins. To evaluate whether there are associated changes in the properties of ACh release during this period, we used a cochlear preparation from mice at P4, P6-7 and P9-11 and monitored transmitter release from MOC terminals in voltage-clamped IHCs in the whole-cell configuration. The quantum content increased 5.6x from P4 to P9-11 due to increases in the size and replenishment rate of the readily releasable pool (RRP) of synaptic vesicles, without changes in their probability of release (Pvesicle) or quantum size. This strengthening in transmission was accompanied by changes in the short-term plasticity (STP) properties, which switched from facilitation at P4 to depression at P9-11. We have previously shown that at P9-11, ACh release is supported by P/Q and N-type voltage-gated calcium channels (VGCCs) and negatively regulated by BK potassium channels activated by Ca2+ influx through L-type VGCCs. We now show that at P4 and P6-7, release is mediated by P/Q-, R- and L-type VGCCs. Interestingly, L-type VGCCs have a dual role: they both support release and fuel BK channels, suggesting that at immature stages the presynaptic proteins involved in release are less compartmentalized.\n\nSignificance statementDuring postnatal development prior to the onset of hearing, cochlear IHCs present spontaneous Ca2+ action potentials which release glutamate at the first auditory synapse in the absence of sound stimulation. The IHC Ca2+ action potential frequency pattern, which is crucial for the correct establishment and function of the auditory system, is regulated by the efferent MOC system that transiently innervates IHCs during this period. We show short-term synaptic plasticity properties of the MOC-IHC synapse that tightly shape this critical developmental period.

neuroscience↗

bioRxiv2018

Strengthening of the efferent olivocochlear system leads to synaptic dysfunction and tonotopy disruption of a central auditory nucleus

The auditory system in many mammals is immature at birth but precisely organized in adults. Spontaneous activity in the inner ear plays a critical role in guiding this process. This is shaped by an efferent pathway that descends from the brainstem and makes transient direct synaptic contacts with inner hair cells (IHCs). In this work, we used an 9 cholinergic receptor knock-in mouse model (of either sex) with enhanced medial efferent activity (Chrna9L9T, L9T) to understand the role of the olivocochlear system in the correct establishment of auditory circuits. Wave III of auditory brainstem responses (which represents synchronized activity of synapses within the superior olivary complex) were smaller in L9T mice, suggesting a central dysfunction. The mechanism underlying this functional alteration was analysed in brain slices containing the medial nucleus of the trapezoid body (MNTB), where neurons are topographically organized along a medio-lateral axis. The topographic organization of MNTB physiological properties observed in WT mice was abolished in the L9T mice. Additionally, electrophysiological recordings in slices evidenced MNTB synaptic alterations, which were further supported by morphological alterations. The present results suggest that the transient cochlear efferent innervation to IHCs during the critical period before the onset of hearing is involved in the refinement of topographic maps as well as in setting the correct synaptic transmission at central auditory nuclei.\n\nSignificance StatementCochlear inner hair cells of altricial mammals display spontaneous electrical activity before hearing onset. The pattern and firing rate of these cells is crucial for the correct maturation of the central auditory pathway. A descending efferent innervation from the central nervous system contacts hair cells during this developmental window. The function of this transient efferent innervation remains an open question. The present work shows that the genetic enhancement of efferent function disrupts the orderly topographic distribution at the medial nucleus of the trapezoid body level and causes severe synaptic dysfunction. Thus, the transient efferent innervation to the cochlea is necessary for the correct establishment of the central auditory circuitry.

neuroscience↗