Development of cochlear potentials in rats

Adult Neurogenesis of the Medial Geniculate Body: In Vitro and Molecular Genetic Analyses Reflect the Neural Stem Cell Capacity of the Rat Auditory Thalamus over Time

Neural stem cells (NSCs) have been recently identified in the neonatal rat medial geniculate body (MGB). NSCs are characterized by three cardinal features: mitotic self-renewal, formation of progenitors, and differentiation into all neuroectodermal cell lineages. NSCs and the molecular factors affecting them are particularly interesting, as they present a potential target for treating neurologically based hearing disorders. It is unclear whether an NSC niche exists in the rat MGB up to the adult stage and which neurogenic factors are essential during maturation. The rat MGB was examined on postnatal days 8, 12, and 16, and at the adult stadium. The cardinal features of NSCs were detected in MGB cells of all age groups examined by neurosphere, passage, and differentiation assays. In addition, real-time quantitative polymerase chain reaction arrays were used to compare the mRNA levels of 84 genes relevant to NSCs and neurogenesis. In summary, cells of the MGB display the cardinal features of NSCs up to the adult stage with a decreasing NSC potential over time. Neurogenic factors with high importance for MGB neurogenesis were identified on the mRNA level. These findings should contribute to a better understanding of MGB neurogenesis and its regenerative capacity.

Bone conducted responses in the neonatal rat auditory cortex

Rats are born deaf and start hearing at the end of the second postnatal week, when the ear canals open and low-intensity sounds start to evoke responses in the auditory cortex. Here, using μECoG electrode arrays and intracortical silicon probe recordings, we found that bone-conducted (BC) sounds evoked biphasic responses in the auditory cortex starting from postnatal day (P) 8. The initial phase of these responses, generated by thalamocortical input, was followed by intracortical propagation within supragranular layers. BC-evoked responses co-localized with the responses evoked by electrical stimulation of the cochlea and the deepest layers of the inferior colliculus prior to onset of low-threshold hearing (P13), as well as with the responses evoked by high-frequency (30 kHz) low-intensity (70 dB) air-conducted sounds after that. Thus, BC signals reach high-frequency processing regions of the auditory cortex well before the onset of low-threshold hearing, reflecting early integrity of the auditory system.

Similarities and Differences Between Vestibular and Cochlear Systems - A Review of Clinical and Physiological Evidence

The evoked response to repeated brief stimuli, such as clicks or short tone bursts, is used for clinical evaluation of the function of both the auditory and vestibular systems. One auditory response is a neural potential - the Auditory Brainstem Response (ABR) - recorded by surface electrodes on the head. The clinical analogue for testing the otolithic response to abrupt sounds and vibration is the myogenic potential recorded from tensed muscles - the vestibular evoked myogenic potential (VEMP). VEMPs have provided clinicians with a long sought-after tool - a simple, clinically realistic indicator of the function of each of the 4 otolithic sensory regions. We review the basic neural evidence for VEMPs and discuss the similarities and differences between otolithic and cochlear receptors and afferents. VEMPs are probably initiated by sound or vibration selectively activating afferent neurons with irregular resting discharge originating from the unique type I receptors at a specialized region of the otolithic maculae (the striola). We review how changes in VEMP responses indicate the functional state of peripheral vestibular function and the likely transduction mechanisms allowing otolithic receptors and afferents to trigger such very short latency responses. In section "ELECTROPHYSIOLOGY" we show how cochlear and vestibular receptors and afferents have many similar electrophysiological characteristics [e.g., both generate microphonics, summating potentials, and compound action potentials (the vestibular evoked potential, VsEP)]. Recent electrophysiological evidence shows that the hydrodynamic changes in the labyrinth caused by increased fluid volume (endolymphatic hydrops), change the responses of utricular receptors and afferents in a way which mimics the changes in vestibular function attributed to endolymphatic hydrops in human patients. In section "MECHANICS OF OTOLITHS IN VEMPS TESTING" we show how the major VEMP results (latency and frequency response) follow from modeling the physical characteristics of the macula (dimensions, stiffness etc.). In particular, the structure and mechanical operation of the utricular macula explains the very fast response of the type I receptors and irregular afferents which is the very basis of VEMPs and these structural changes of the macula in Menière's Disease (MD) predict the upward shift of VEMP tuning in these patients.

CX3CR1 mutation alters synaptic and astrocytic protein expression, topographic gradients, and response latencies in the auditory brainstem

The precise and specialized circuitry in the auditory brainstem develops through adaptations of cellular and molecular signaling. We previously showed that elimination of microglia during development impairs synaptic pruning that leads to maturation of the calyx of Held, a large encapsulating synapse that terminates on neurons of the medial nucleus of the trapezoid body (MNTB). Microglia depletion also led to a decrease in glial fibrillary acidic protein (GFAP), a marker for mature astrocytes. Here, we investigated the role of signaling through the fractalkine receptor (CX3CR1), which is expressed by microglia and mediates communication with neurons. CX3CR1^-/- and wild-type mice were studied before and after hearing onset and at 9 weeks of age. Levels of GFAP were significantly increased in the MNTB in mutants at 9 weeks. Pruning was unaffected at the calyx of Held, but we found an increase in expression of glycinergic synaptic marker in mutant mice at P14, suggesting an effect on maturation of inhibitory inputs. We observed disrupted tonotopic gradients of neuron and calyx size in MNTB in mutant mice. Auditory brainstem recording (ABR) revealed that CX3CR1^-/- mice had normal thresholds and amplitudes but decreased latencies and interpeak latencies, particularly for the highest frequencies. These results demonstrate that disruption of fractalkine signaling has a significant effect on auditory brainstem development. Our findings highlight the importance of neuron-microglia-astrocyte communication in pruning of inhibitory synapses and establishment of tonotopic gradients early in postnatal development.

Endolymphatic Potential Measured From Developing and Adult Mouse Inner Ear

The mammalian inner ear has two major parts, the cochlea is responsible for hearing and the vestibular organ is responsible for balance. The cochlea and vestibular organs are connected by a series of canals in the temporal bone and two distinct extracellular fluids, endolymph and perilymph, fill different compartments of the inner ear. Stereocilia of mechanosensitive hair cells in the cochlea and vestibular end organs are bathed in the endolymph, which contains high K+ ions and possesses a positive potential termed endolymphatic potential (ELP). Compartmentalization of the fluids provides an electrochemical gradient for hair cell mechanotransduction. In this study, we measured ELP from adult and neonatal C57BL/6J mice to determine how ELP varies and develops in the cochlear and vestibular endolymph. We measured ELP and vestibular microphonic response from saccules of neonatal mice to determine when vestibular function is mature. We show that ELP varies considerably in the cochlear and vestibular endolymph of adult mice, ranging from +95 mV in the basal turn to +87 mV in the apical turn of the cochlea, +9 mV in the saccule and utricle, and +3 mV in the semicircular canal. This suggests that ELP is indeed a local potential, despite the fact that endolymph composition is similar. We further show that vestibular ELP reaches adult-like magnitude around post-natal day 6, ~12 days earlier than maturation of cochlear ELP (i.e., endocochlear potential). Maturation of vestibular ELP coincides with the maturation of vestibular microphonic response recorded from the saccular macula, suggesting that maturation of vestibular function occurs much earlier than maturation of hearing in mice.

Expression patterns of chloride transporters in the auditory brainstem of developing chicken

GABAergic transmission changes from depolarization to hyperpolarization in most vertebrate brain regions during development. By contrast, in the auditory brainstem of chicken a depolarizing effect of GABA persists after hatching. Since auditory brainstem neurons that receive GABAergic input have a Cl^- reversal potential above resting membrane potential, a specifically tuned activity of Cl^- transporters is likely. We here present a developmental study of the expression patterns of several members of the SLC12 family (NKCC1, NKCC2, KCC1, KCC2, KCC4, CCC6, CCC9) and of AE3 at developmental ages E7, E10, E12, E15, E17, and P1 with quantitative RT-PCR. NKCC2 and CCC9 were not detected in auditory brainstem (positive control: kidney). KCC1, CCC6 and AE3 were expressed, but not regulated, while NKCC1, KCC2 and KCC4 were regulated. The expression of the latter transporters increased, with KCC2 exhibiting the strongest expression at all time points. Biochemical analysis of the protein expression of NKCC1, KCC2 and KCC4 corroborated the findings on the mRNA level. All three transporters showed a localization at the outer rim of the cells, with NKCC1 and KCC2 expressed in neurons, and KCC4 predominantly in glia. The comparison of the published chloride reversal potential and expression of transporter proteins suggest strong differences in the efficiency of the three transporters. Further, the strong KCC2 expression could reflect a role in the structural development of auditory brainstem synapses that might lead to changes in the physiological properties.

Cross-Talk of Low-Level Sensory and High-Level Cognitive Processing: Development, Mechanisms, and Relevance for Cross-Modal Abilities of the Brain

The emergence of cross-modal learning capabilities requires the interaction of neural areas accounting for sensory and cognitive processing. Convergence of multiple sensory inputs is observed in low-level sensory cortices including primary somatosensory (S1), visual (V1), and auditory cortex (A1), as well as in high-level areas such as prefrontal cortex (PFC). Evidence shows that local neural activity and functional connectivity between sensory cortices participate in cross-modal processing. However, little is known about the functional interplay between neural areas underlying sensory and cognitive processing required for cross-modal learning capabilities across life. Here we review our current knowledge on the interdependence of low- and high-level cortices for the emergence of cross-modal processing in rodents. First, we summarize the mechanisms underlying the integration of multiple senses and how cross-modal processing in primary sensory cortices might be modified by top-down modulation of the PFC. Second, we examine the critical factors and developmental mechanisms that account for the interaction between neuronal networks involved in sensory and cognitive processing. Finally, we discuss the applicability and relevance of cross-modal processing for brain-inspired intelligent robotics. An in-depth understanding of the factors and mechanisms controlling cross-modal processing might inspire the refinement of robotic systems by better mimicking neural computations.

Anatomical Assessment of the Adult Skeleton of Zebrafish Reared Under Different Thyroid Hormone Profiles

Thyroid hormone (TH) directs the growth and maintenance of tissues throughout the body during development and into adulthood, and plays a particularly important role in proper ossification and homeostasis of the skeleton. To better understand the roles of TH in the skeletogenesis of a vertebrate model, and to define areas of the skeleton that are particularly sensitive to developmental TH, we examined the effects of hypo- and hyperthyroidism on skeletal development in zebrafish. Performing a bone-by-bone anatomical assessment on the entire skeleton of adult fish, we found that TH is required for proper ossification, growth, morphogenesis, and fusion of numerous bones. We showed that the pectoral girdle, dermatocranium, Weberian apparatus, and dentary are particularly sensitive to TH, and that TH affects development of skeletal element regardless of bone type and developmental origin. Indeed, the hormone does not universally promote ossification: we found that developmental TH prevents ectopic ossification in multiple thin bones and within connective tissue of the jaw. In all, we found that TH regulates proper morphogenesis and ossification in the majority of zebrafish bones, and that the requirement for the hormone extends across bone types and developmental profiles. Anat Rec, 302:1754-1769, 2019. © 2019 American Association for Anatomy.

Maturation of Spontaneous Firing Properties after Hearing Onset in Rat Auditory Nerve Fibers: Spontaneous Rates, Refractoriness, and Interfiber Correlations

Auditory nerve fibers (ANFs) exhibit a range of spontaneous firing rates (SRs) that are inversely correlated with threshold for sounds. To probe the underlying mechanisms and time course of SR differentiation during cochlear maturation, loose-patch extracellular recordings were made from ANF dendrites using acutely excised rat cochlear preparations of different ages after hearing onset. Diversification of SRs occurred mostly between the second and the third postnatal week. Statistical properties of ANF spike trains showed developmental changes that approach adult-like features in older preparations. Comparison with intracellularly recorded EPSCs revealed that most properties of ANF spike trains derive from the characteristics of presynaptic transmitter release. Pharmacological tests and waveform analysis showed that endogenous firing produces some fraction of ANF spikes, accounting for their unusual properties; the endogenous firing diminishes gradually during maturation. Paired recordings showed that ANFs contacting the same inner hair cell could have different SRs, with no correlation in their spike timing.

SIGNIFICANCE STATEMENT

The inner hair cell (IHC)/auditory nerve fiber (ANF) synapse is the first synapse of the auditory pathway. Remarkably, each IHC is the sole partner of 10-30 ANFs with a range of spontaneous firing rates (SRs). Low and high SR ANFs respond to sound differently, and both are important for encoding sound information across varying acoustical environments. Here we demonstrate SR diversification after hearing onset by afferent recordings in acutely excised rat cochlear preparations. We describe developmental changes in spike train statistics and endogenous firing in immature ANFs. Dual afferent recordings provide the first direct evidence that fibers with different SRs contact the same IHCs and do not show correlated spike timing at rest. These results lay the groundwork for understanding the differential sensitivity of ANFs to acoustic trauma.

In Ovo Electroporation in the Chicken Auditory Brainstem

Electroporation is a method that introduces genes of interest into biologically relevant organisms like the chicken embryo. It is long established that the chicken embryo is an effective research model for studying basic biological functions of auditory system development. More recently, the chicken embryo has become particularly valuable in studying gene expression, regulation and function associated with hearing. In ovo electroporation can be used to target auditory brainstem regions responsible for highly specialized auditory functions. These regions include the chicken nucleus magnocellularis (NM) and nucleus laminaris (NL). NM and NL neurons arise from distinct precursors of rhombomeres 5 and 6 (R5/R6). Here, we present in ovo electroporation of plasmid-encoded genes to study gene-related properties in these regions. We show a method for spatial and temporal control of gene expression that promote either gain or loss of functional phenotypes. By targeting auditory neural progenitor regions associated with R5/R6, we show plasmid transfection in NM and NL. Temporal regulation of gene expression can be achieved by adopting a tet-on vector system. This is a drug inducible procedure that expresses the genes of interest in the presence of doxycycline (Dox). The in ovo electroporation technique - together with either biochemical, pharmacological, and or in vivo functional assays - provides an innovative approach to study auditory neuron development and associated pathophysiological phenomena.

Autophagy is essential for hearing in mice

Hearing loss is the most frequent sensory disorder in humans. Auditory hair cells (HCs) are postmitotic at late-embryonic differentiation and postnatal stages, and their damage is the major cause of hearing loss. There is no measurable HC regeneration in the mammalian cochlea, and the maintenance of cell function is crucial for preservation of hearing. Here we generated mice deficient in autophagy-related 5 (Atg5), a gene essential for autophagy, in the HCs to investigate the effect of basal autophagy on hearing acuity. Deletion of Atg5 resulted in HC degeneration and profound congenital hearing loss. In autophagy-deficient HCs, polyubiquitinated proteins and p62/SQSTM1, an autophagy substrate, accumulated as inclusion bodies during the first postnatal week, and these aggregates increased in number. These findings revealed that basal autophagy has an important role in maintenance of HC morphology and hearing acuity.

Midbrain Frequency Representation following Moderately Intense Neonatal Sound Exposure in a Precocious Animal Model (Chinchilla laniger)

Auditory brain areas undergo reorganization resulting from abnormal sensory input during early postnatal development. This is evident from studies at the cortical level but it remains unclear whether there is reorganization in the auditory midbrain in a species similar to the human, that is, with early hearing onset. We have explored midbrain plasticity in the chinchilla, a precocious species that matches the human in terms of hearing development. Neonatal chinchillas were chronically exposed to a 2 kHz narrowband sound at 70 dB SPL for 4 weeks. Tonotopic maps in inferior colliculus (central nucleus) were defined based on single neuron characteristic frequency. We hypothesized an overrepresentation of the 2 kHz region of the maps. However, we observed a significant decrease in the proportion of neurons dedicated to the 2 kHz octave band and also away from the exposure frequency at 8 kHz. In addition, we report a significant increase in low frequency representation (<1 kHz), again a change to tonotopic mapping distant to the 2 kHz region. Thus in a precocious species, tonotopic maps in auditory midbrain are altered following abnormal stimulation during development. However, these changes are more complex than the overrepresentation of exposure related frequency regions that are often reported.

Postnatal maturation of auditory-nerve heterogeneity, as seen in spatial gradients of synapse morphology in the inner hair cell area

Auditory nerve fibers in the adult ear are divided into functional subgroups according to spontaneous rate (SR) and threshold sensitivity. The high-threshold, low-SR fibers are morphologically and spatially distinct from the low-threshold high-SR fibers at their synaptic contacts with inner hair cells. This distinction between SR groups in the adult ear is visible in confocal microscopy as complementary size gradients of presynaptic ribbons and post-synaptic glutamate receptor patches across the modiolar-pillar and habenular-cuticular axes in the inner hair cell area. The aim of the present study was to track the post-natal development of this morphological gradient, in mouse, to determine the earliest age at which this important aspect of cochlear organization is fully mature. Here we show, using morphometric analysis of the organ of Corti immunostained for pre- and post-synaptic markers of efferent and afferent innervation, that this SR-based morphological gradient is not fully established until postnatal day 28, well after other features, such as synaptic counts and efferent innervation density in both the inner and outer hair cell areas, appear fully mature.

Maturation of NaV and KV Channel Topographies in the Auditory Nerve Spike Initiator before and after Developmental Onset of Hearing Function

Auditory nerve excitation and thus hearing depend on spike-generating ion channels and their placement along the axons of auditory nerve fibers (ANFs). The developmental expression patterns and native axonal locations of voltage-gated ion channels in ANFs are unknown. Therefore, we examined the development of heminodes and nodes of Ranvier in the peripheral axons of type I ANFs in the rat cochlea with immunohistochemistry and confocal microscopy. Nodal structures presumably supporting presensory spiking formed between postnatal days 5 (P5) and P7, including Ankyrin-G, NaV1.6, and Caspr. These immature nodal structures lacked low-voltage-activated KV1.1 which was not enriched at juxtaparanodes until approximately P13, concurrent with the developmental onset of acoustic hearing function. Anatomical alignment of ANF spike-initiating heminodes relative to excitatory input from inner hair cell (IHC) ribbon synapses continued until approximately P30. High-voltage-activated KV3.1b and KV2.2 were expressed in mutually exclusive domains: KV3.1b was strictly localized to nodes and heminodes, whereas KV2.2 expression began at the juxtaparanodes and continued centrally along the first internode. At spike-initiating heminodes in the distal osseous spiral lamina, NaV1.1 partly overlapped NaV1.6 and ankyrin-G. ANFs displayed KV7.2 and KV7.3 at heminodes, nodes, internodes, and the unmyelinated synaptic terminal segments beneath IHCs in the organ of Corti. In response to sound, spikes are initiated at the heminode, which is tightly coupled to the IHC ribbon synapse ∼20-40 μm away. These results show that maturation of nodal alignment and ion channel content may underlie postnatal improvements of ANF excitability and discharge synchrony.

SIGNIFICANCE STATEMENT

Acoustic and electrical hearing depends on rapid, reliable, and precise spike generation in auditory nerve fibers. A limitation of current models and therapies is a lack of information on the identities and topographies of underlying ion channels. We report the developmental profile of the auditory nerve spike generator with a focus on NaV1.1, NaV1.6, KV1.1, KV2.2, KV3.1b, KV7.2, and KV7.3 in relation to the scaffold ankyrin-G. Molecular anatomy of the spike generator matures in the weeks after developmental onset of hearing function. Subcellular positioning of voltage-gated ion channels will enable multicompartmental modeling of auditory nerve responses elicited by afferent chemical neurotransmission from hair cells and modulated by efferent neurotransmitters or evoked by extracellular field stimulation from a cochlear implant.

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 development of spatial behaviour and the hippocampal neural representation of space

The role of the hippocampal formation in spatial cognition is thought to be supported by distinct classes of neurons whose firing is tuned to an organism's position and orientation in space. In this article, we review recent research focused on how and when this neural representation of space emerges during development: each class of spatially tuned neurons appears at a different age, and matures at a different rate, but all the main spatial responses tested so far are present by three weeks of age in the rat. We also summarize the development of spatial behaviour in the rat, describing how active exploration of space emerges during the third week of life, the first evidence of learning in formal tests of hippocampus-dependent spatial cognition is observed in the fourth week, whereas fully adult-like spatial cognitive abilities require another few weeks to be achieved. We argue that the development of spatially tuned neurons needs to be considered within the context of the development of spatial behaviour in order to achieve an integrated understanding of the emergence of hippocampal function and spatial cognition.

Targeting the somatostatin receptors as a therapeutic approach for the preservation and protection of the mammalian cochlea from excitotoxicity

The neuropeptide somatostatin (SST) is an important modulator of neurotransmission in the central nervous system (CNS) and binds to G-protein-coupled receptors (SSTR1-5) on target cells. Little is known about the expression and function of the somatostatinergic system in the mammalian cochlea. We analyzed the expression of SSTR1-SSTR5 in the immature mammalian cochlea. The peak in the expression of SSTR1 and SSTR2 at mRNA and protein level is around the onset of hearing to airborne sound, at postnatal day (P)14. This suggests their involvement in the maturation of the mammalian cochlea. We demonstrated that all five receptors are expressed in the inner hair cells (IHC) and outer hear cells (OHC) as well as in defined supporting cells of the organ of Corti (OC) in the adult mouse cochlea. A similar expression of the SSTRs in the IHC and OHC was found in cultivated P6 mouse OC explants as well as in neuroepithelial cell culture. In order to learn more about the regulation of SSTRs, we used mice with either a deletion of SSTR1, SSTR2 or SSTR1/SSTR2 double knock out (DKO). In DKO mice, SSTR5 was up-regulated and SSTR3 and SSTR4 were down regulated. These findings provide evidence of a compensatory regulation in the mammalian cochlea as a consequence of a receptor subtype deletion. In addition, we observed reduced levels of phospho-Akt and total-Akt in SSTR1 KO and DKO mice as compared to wild type (WT) mice. Akt is likely to be involved in hair cell survival. Most importantly, we found improved hair cell survival in somatostatin and octreotide treated OC explants that had been exposed to gentamicin compared to those explants exposed to gentamicin alone. These findings propose that the somatostatinergic system within the cochlea may have neuroprotective properties.

ATP-dependent intercellular Ca2+ signaling in the developing cochlea: facts, fantasies and perspectives

Hearing relies on a sensitive mechanoelectrical transduction process in the cochlea of the inner ear. The cochlea contains sensory, secretory, neural, supporting and epithelial cells which are all essential to the sound transduction process. It is well known that a complex extracellular purinergic signaling system contributes to cochlear homeostasis, altering cochlear sensitivity and neural output via ATP-gated ion channels (P2X receptors) and G protein-coupled P2Y receptors. This review focuses on the emerging roles of ATP that are currently under investigation in the developing sensory epithelium, with particular emphasis on the link between ATP release, Ca(2+) signaling, the expression and function of gap junction proteins connexin26 and connexin30, and the acquisition of hearing.

Somatostatin receptor types 1 and 2 in the developing mammalian cochlea

The neuropeptide somatostatin (SST) exerts several important physiological actions in the adult central nervous system through interactions with membrane-bound receptors. Transient expression of SST and its receptors has been described in several brain areas during early ontogeny. It is therefore believed that SST may play a role in neural maturation. The present study provides the first evidence for the developmental expression of SST receptors in the mammalian cochlea, emphasizing their possible roles in cochlear maturation. In the developing mouse cochlea, cells immunoreactive to somatostatin receptor 1 (SSTR1) and somatostatin receptor 2 (SSTR2) were located in the embryonic cochlear duct on Kolliker's organ as early as embryonic day (E) 14 (E14). At E17, the expression of both receptors was high and already located at the hair cells and supporting cells along the length of the cochlear duct, which have become arranged into the characteristic pattern for the organ of Corti (OC) at this stage. At birth, SSTR1- and SSTR2-containing cells were only localized in the OC. In general, immunoreactivity for both receptors increased in the mouse cochlea from postnatal day (P) 0 (P0) to P10; the majority of immunostained cells were inner hair cells, outer hair cells, and supporting cells. Finally, a peak in the mRNA and protein expression of both receptors is present near the time when they respond to physiological hearing (i.e., hearing of airborne sound) at P14. At P21, SSTR1 and SSTR2 levels decrease dramatically. A similar developmental pattern was observed for SSTR1 and SSTR2 mRNA, suggesting that the expression of the SSTR1 and SSTR2 genes is controlled at the transcriptional level throughout development. In addition, we observed reduced levels of phospho-Akt and total Akt in SSTR1 knockout and SSTR1/SSTR2 double-knockout mice compared with wild-type mice. We know from previous studies that Akt is involved in hair cell survival. Taken together, the dynamic nature of SSTR1 and SSTR2 expression at a time of major developmental changes in the cochlea suggests that SSTR1 and SSTR2 (and possibly other members of this family) are involved in the maturation of the mammalian cochlea.

The expression of nicotinic receptor alpha7 during cochlear development

Nicotinic acetylcholine receptor alpha7 expression was examined in the developing and adult auditory system using mice that were modified through homologous recombination to coexpress either GFP (alpha7GFP) or Cre (alpha7Cre), respectively. The expression of alpha7GFP is first detected at embryonic (E) day E13.5 in cells of the spiral prominence. By E14.5, sensory regions including the putative outer hair cells and Deiters' cells express alpha7GFP as do solitary efferent fibers. This pattern diminishes after E16.5 in a basal to apex progression, as Hensen's cells and cells of the spiral ligament acquire alpha7GFP expression. At birth and thereafter alpha7GFP also identifies a subset of spiral ganglion cells whose processes terminate on inner hair cells. Efferent fibers identified by peripherin or calcitonin gene-related protein do not coexpress alpha7GFP. In addition to cochlear structures, there is strong expression of alpha7GFP by cells of the central auditory pathways including the ventral posterior cochlear nucleus, lateral lemniscus, central inferior colliculus, and the medial geniculate nucleus. Our findings suggest that alpha7 expression by both neuronal and non-neuronal cells has the potential to impact multiple auditory functions through mechanisms that are not traditionally attributed to this receptor.

Developmental changes in short-term plasticity at the rat calyx of Held synapse

The calyx of Held synapse of the medial nucleus of the trapezoid body functions as a relay synapse in the auditory brainstem. In vivo recordings have shown that this synapse displays low release probability and that the average size of synaptic potentials does not depend on recent history. We used a ventral approach to make in vivo extracellular recordings from the calyx of Held synapse in rats aged postnatal day 4 (P4) to P29 to study the developmental changes that allow this synapse to function as a relay. Between P4 and P8, we observed evidence for the presence of large short-term depression, which was counteracted by short-term facilitation at short intervals. Major changes occurred in the last few days before the onset of hearing for air-borne sounds, which happened at P13. The bursting pattern changed into a primary-like pattern, the amount of depression and facilitation decreased strongly, and the decay of facilitation became much faster. Whereas short-term plasticity was the most important cause of variability in the size of the synaptic potentials in immature animals, its role became minor around hearing onset and afterward. Similar developmental changes were observed during stimulation experiments both in brain slices and in vivo following cochlear ablation. Our data suggest that the strong reduction in release probability and the speedup of the decay of synaptic facilitation that happen just before hearing onset are important events in the transformation of the calyx of Held synapse into an auditory relay synapse.

Between molecules and experience: role of early patterns of coordinated activity for the development of cortical maps and sensory abilities

Sensory systems processing information from the environment rely on precisely formed and refined neuronal networks that build maps of sensory receptor epithelia at different subcortical and cortical levels. These sensory maps share similar principles of function and emerge according to developmental processes common in visual, somatosensory and auditory systems. Whereas molecular cues set the coarse organization of cortico-subcortical topography, its refinement is known to succeed under the influence of experience-dependent electrical activity during critical periods. However, coordinated patterns of activity synchronize the cortico-subcortical networks long before the meaningful impact of environmental inputs on sensory maps. Recent studies elucidated the cellular and network mechanisms underlying the generation of these early patterns of activity and highlighted their similarities across species. Moreover, the experience-independent activity appears to act as a functional template for the maturation of sensory networks and cortico-subcortical maps. A major goal for future research will be to analyze how this early activity interacts with the molecular cues and to determine whether it is permissive or rather supporting for the establishment of sensory topography.

Frequency-dependent disynaptic inhibition in the pyramidal network: a ubiquitous pathway in the developing rat neocortex

The general structure of the mammalian neocortex is remarkably similar across different cortical areas. Despite certain cytoarchitectural specializations and deviations from the general blueprint, the principal organization of the neocortex is relatively uniform. It is not known, however, to what extent stereotypic synaptic pathways resemble each other between cortical areas, and how far they might reflect possible functional uniformity or specialization. Here, we show that frequency-dependent disynaptic inhibition (FDDI) is a generic circuit motif that is present in all neocortical areas we investigated (primary somatosensory, auditory and motor cortex, secondary visual cortex and medial prefrontal cortex of the developing rat). We did find, however, area-specific differences in occurrence and kinetics of FDDI and the short-term dynamics of monosynaptic connections between pyramidal cells (PCs). Connectivity between PCs, both monosynaptic and via FDDI, is higher in primary cortices. The long-term effectiveness of FDDI is likely to be limited by an activity-dependent attenuation of the PC-interneuron synaptic transmission. Our results suggest that the basic construction of neocortical synaptic pathways follows principles that are independent of modality or hierarchical order within the neocortex.

Neonatal maternal separation alters reward-related ultrasonic vocalizations in rat dams

We examined the effects of brief or long durations of repeated maternal separation (MS) on ultrasonic vocalizations (USVs) in rat dams. 50-kHz USVs putatively identified as maternal in origin were emitted only immediately after pups were returned following MS. Maternal USV emission also depended on separation duration and pup age. Given that 50-kHz USVs are emitted in response to rewarding stimuli, MS may induce duration-dependent effects on maternal positive affect.

Survival, migration, and differentiation of Sox1-GFP embryonic stem cells in coculture with an auditory brainstem slice preparation

The poor regeneration capability of the mammalian hearing organ has initiated different approaches to enhance its functionality after injury. To evaluate a potential neuronal repair paradigm in the inner ear and cochlear nerve we have previously used embryonic neuronal tissue and stem cells for implantation in vivo and in vitro. At present, we have used in vitro techniques to study the survival and differentiation of Sox1-green fluorescent protein (GFP) mouse embryonic stem (ES) cells as a monoculture or as a coculture with rat auditory brainstem slices. For the coculture, 300 microm-thick brainstem slices encompassing the cochlear nucleus and cochlear nerve were prepared from postnatal SD rats. The slices were propagated using the membrane interface method and the cochlear nuclei were prelabeled with DiI. After some days in culture a suspension of Sox1 cells was deposited next to the brainstem slice. Following deposition Sox1 cells migrated toward the brainstem and onto the cochlear nucleus. GFP was not detectable in undifferentiated ES cells but became evident during neural differentiation. Up to 2 weeks after transplantation the cocultures were fixed. The undifferentiated cells were evaluated with antibodies against progenitor cells whereas the differentiated cells were determined with neuronal and glial markers. The morphological and immunohistochemical data indicated that Sox1 cells in monoculture differentiated into a higher percentage of glial cells than neurons. However, when a coculture was used a significantly lower percentage of Sox1 cells differentiated into glial cells. The results demonstrate that a coculture of Sox1 cells and auditory brainstem present a useful model to study stem cell differentiation.

Dynamic development of the calyx of Held synapse

The calyx of Held is probably the largest synaptic terminal in the brain, forming a unique one-to-one connection in the auditory ventral brainstem. During early development, calyces have many collaterals, whose function is unknown. Using electrophysiological recordings and fast-calcium imaging in brain slices, we demonstrate that these collaterals are involved in synaptic transmission. We show evidence that the collaterals are pruned and that the pruning already begins 1 week before the onset of hearing. Using two-photon microscopy to image the calyx of Held in neonate rats, we report evidence that both axons and nascent calyces are structurally dynamic, showing the formation, elimination, extension, or retraction of up to 65% of their collaterals within 1 hour. The observed dynamic behavior of axons may add flexibility in the choice of postsynaptic partners and thereby contribute to ensuring that each principal cell eventually is contacted by a single calyx of Held.

Temporal bone study of development of the organ of Corti: correlation between auditory function and anatomical structure

OBJECTIVE

To study the development of the organ of Corti in the human cochlea, and to correlate our findings with the onset of auditory function.

MATERIAL AND METHODS

Step sections of 81 human fetal temporal bones were studied, from eight weeks of gestation to full term.

RESULTS

By the end of the 10th week, the tectorial membrane primordium could be traced even in the most apical turns. Individual hair cells became identifiable at the basal turn at 14 weeks. At the same time, a small but well formed oval space was observed between the inner and outer hair cells in the basal turn. This does not correspond to the tunnel of Corti, as is erroneously quoted in the literature, as the individual pillar cells develop at later stages. Between 14 and 15 weeks, Hensen's cells were recognised for the first time. Individual pillar cells were identifiable at 17 weeks and the tunnel of Corti opened at 20 weeks. By 25 weeks, the cochlea had reached its adult size, but continued to develop until full term.

DISCUSSION AND CONCLUSIONS

A temporal coincidence of different developmental events is responsible for early fetal audition at 20 weeks, including growth of pillar cells, opening of the tunnel of Corti and regression of Kollicker's organ, with the subsequent formation of the inner spiral sulcus and then separation of the tectorial membrane. The fine structures of the organ of Corti continue to develop well after the 25th week, and this may well alter the mechanical properties of the vibrating parts of the cochlea, which may in turn account for the frequency shift observed in preterm infants. These changes will have to be taken into account in the development of prenatal hearing screening tests.

Dynamic expression of FXYD6 in the inner ear suggests a role of the protein in endolymph homeostasis and neuronal activity

A key protein in the production and in the maintenance of the endocochlear potential is the Na,K-ATPase. Previously, we have shown that FXYD6 is a modulator of the Na,K-ATPase expressed in the inner ear (Delprat et al. [2007] J Biol Chem 282:7450-7456). To investigate the potential role of FXYD6 in inner ear function, we studied the developmental expression of FXYD6. Reverse transcriptase-polymerase chain reaction analysis demonstrates that FXYD6 is present as two splice variants. Both variants coimmunoprecipitate with Na,K-ATPase after expression in Xenopus oocytes. Immunohistochemistry of the cochlea (from birth to postnatal day 30) shows that FXYD6 is expressed in several epithelial cells important for endolymph homeostasis. Marked similarities were found in the developmental expression patterns of FXYD6 and Na,K-ATPase, suggesting functional cooperation between the two proteins in the generation and maintenance of the endocochlear potential and ion composition of the endolymph.

Branching of calyceal afferents during postnatal development in the rat auditory brainstem

Cells in the anteroventral cochlear nucleus (aVCN) send out calyceal axons that form large excitatory somatic terminals, the calyces of Held, onto principal cells of the contralateral medial nucleus of the trapezoid body (MNTB). It is unclear which fraction of these axons might form more than one calyx and whether this fraction changes during development. We combined in vitro anterograde tracing, stereological cell counts, analysis of apoptosis, and immunohistochemistry to study the development of calyceal afferents in rats of different postnatal ages. We found that some principal cells were contacted by multiple large axosomatic inputs, but these invariably originated from the same axon. Conversely, at least 18% of traced afferents branched to form multiple calyces, independently of age. Calyces from the same axon generally innervated nearby principal cells, and most of these branch points were <50 microm away from the synaptic terminals. Our results show that the projection from the aVCN to the MNTB is divergent, both when calyces have just been formed and in the adult. Cell counts did not provide evidence for principal cell loss during development, although analysis of apoptosis showed a large increase in nonneuronal cell death around the onset of hearing. Our data suggest that, once a calyceal synapse forms in the MNTB, it stays.

Hair Cells – Beyond the Transducer

OVERVIEW

This review considers the "tween twixt and twain" of hair cell physiology, specifically the signaling elements and membrane conductances which underpin forward and reverse transduction at the input stage of hair cell function and neurotransmitter release at the output stage. Other sections of this review series outline the advances which have been made in understanding the molecular physiology of mechanoelectrical transduction and outer hair cell electromotility. Here we outline the contributions of a considerable array of ion channels and receptor signaling pathways that define the biophysical status of the sensory hair cells, contributing to hair cell development and subsequently defining the operational condition of the hair cells across the broad dynamic range of physiological function.

Prenatal cocaine exposure accelerates morphological changes and transient expression of tyrosine hydroxylase in the cochlea of developing rats

Prenatal cocaine exposure causes alterations in auditory brainstem response in children and experimental animals and has adverse effects on auditory information processing and language skills in children. These effects may result from lesions in the cochlea since this organ is particularly sensitive to chemical insults during the development. We have thus studied here the effect of prenatal cocaine exposure on the maturation of the rat cochlea using the transient non-catecholaminergic expression of tyrosine hydroxylase in spiral ganglion neurons as an index of cochlear maturation and morphometry to evaluate the maturation of primary auditory neurons and the organ of Corti. We showed that prenatal cocaine exposure accelerated the cochlear maturation. In the basal coil of cochleas from PND8 cocaine-treated pups, the Kölliker's organ had disappeared, the tunnel of Corti was opened, and the stria vascularis no longer contained undifferentiated marginal cells. The maximum expression of tyrosine hydroxylase in type I primary auditory neurons occurred at PND8 instead of PND12 in pair-fed controls. On the other hand, the prenatal cocaine exposure had no effect on the width and height of the organ of Corti, spiral ganglion volume and number and size of primary auditory neurons. In conclusion, our data suggest that prenatal cocaine exposure, though not lethal to primary auditory neurons, accelerates aspects of the cochlear sensorineural maturation. This accelerated cochlear maturation in cocaine-treated rat pups could cause auditory dysfunctions by desynchronizing the development of the whole auditory pathway.

Transcriptional profiling of the developing rat brain reveals that the most dramatic regional differentiation in gene expression occurs postpartum

Neural development involves the expression of ensembles of regulatory genes that control the coordinate and region-specific expression of a host of other genes, resulting in the unique structure, connectivity, and function of each brain region. Although the role of some specific genes in neural development has been studied in detail, we have no global view of the orchestration of spatial and temporal aspects of gene expression across multiple regions of the developing brain. To this end, we used transcriptional profiling to examine expression levels of 9955 genes in the hypothalamus, hippocampus, and frontal cortex across seven stages of postnatal development and up to four stages of prenatal development in individual male rats (six per group). The results reveal dramatic changes across development in >97% of the neurally expressed genes. They also uncover a surprising degree of regional differentiation occurring after birth and through the first 2 weeks of life. Cluster analysis identifies 20 clusters of transcripts enriched in genes related to particular functions, such as DNA metabolism, nuclear function, synaptic vesicle transport, myelination, and neuropeptide hormone activity. Thus, groups of genes with related functions change in the brain at specific times, possibly marking critical periods for each function. These findings can broadly serve as a backdrop for studying the role of individual genes in neural development. They also underscore the importance of early postnatal life in the rat, which corresponds to late gestation in the human, as a critical late phase of neural organization and differentiation, even in subcortical regions.

The concentrations of calcium buffering proteins in mammalian cochlear hair cells

Calcium buffers are important for shaping and localizing cytoplasmic Ca2+ transients in neurons. We measured the concentrations of the four main calcium-buffering proteins (calbindin-D28k, calretinin, parvalbumin-alpha, and parvalbumin-beta) in rat cochlear hair cells in which Ca2+ signaling is a central element of fast transduction and synaptic transmission. The proteins were quantified by calibrating immunogold tissue counts against gels containing known amounts of each protein, and the method was verified by application to Purkinje cells in which independent estimates exist for some of the protein concentrations. The results showed that, in animals with fully developed hearing, inner hair cells had 110 of the proteinaceous calcium buffer of outer hair cells in which the cell body contained parvalbumin-beta (oncomodulin) and calbindin-D28k at levels equivalent to 5 mm calcium-binding sites. Both proteins were partially excluded from the hair bundles, which may permit fast unbuffered Ca2+ regulation of the mechanotransducer channels. The sum of the calcium buffer concentrations decreased in inner hair cells and increased in outer hair cells as the cells developed their adult properties during cochlear maturation. The results suggest that Ca2+ has distinct roles in the two types of hair cell, reflecting their different functions in auditory transduction. Ca2+ is used in inner hair cells primarily for fast phase-locked synaptic transmission, whereas Ca2+ may be involved in regulating the motor capability underlying cochlear amplification of the outer hair cell. The high concentration of calcium buffer in outer hair cells, similar only to skeletal muscle, may protect against deleterious consequences of Ca2+ loading after acoustic overstimulation.

Nicotinic acetylcholine receptor structure and function in the efferent auditory system

This article reviews and presents new data regarding the nicotinic acetylcholine receptor subunits alpha9 and alpha10. Although phylogentically ancient, these subunits have only recently been identified as critical components of the efferent auditory system and medial olivocochlear pathway. This pathway is important in auditory processing by modulating outer hair cell function to broadly tune the cochlea and improve signal detection in noise. Pharmacologic properties of the functionally expressed alpha9alpha10 receptor closely resemble the cholinergic response of outer hair cells. Molecular, immunohistochemical, and knockout mice studies have added further weight to the role this receptor plays in mediating the efferent auditory response. Alternate and complementary mechanisms of outer hair cell efferent activity might also be mediated through the nAChR alpha9alpha10, either through secondary calcium stores, second messengers, or direct protein-protein interactions. We investigated protein-protein interactions using a yeast-two-hybrid screen of the nAChR alpha10 intracellular loop against a rat cochlear cDNA library. Among the identified proteins was prosaposin, a precursor of saposins, which have been shown to act as neurotrophic factors in culture, can bind to a putative G0-coupled cell surface receptor, and may be involved in the prevention of cell death. This study and review suggest that nAChR alpha9alpha10 may represent a potential therapeutic target for a variety of ear disorders, including preventing or treating noise-induced hearing loss, or such debilitating disorders as vertigo or tinnitus.

Systematic variation of potassium current amplitudes across the tonotopic axis of the rat medial nucleus of the trapezoid body

Many central auditory nuclei preserve the tonotopic organization of their afferent inputs, generating a frequency "map" across the nucleus. In the medial nucleus of the trapezoid body (MNTB) the most medial neurons receive inputs corresponding to the highest frequency sounds and the most lateral neurons have the lowest characteristic frequencies. Whole-cell patch recording from MNTB principal neurons in rat brainstem slices demonstrates a corresponding tonotopic organization of voltage-gated outward potassium currents. Medial MNTB neurons had larger total outward K+ current amplitudes than lateral neurons and similar medial to-lateral gradients were observed for two K+ current subtypes distinguished by their low and high voltage activation thresholds. In contrast, a third K+ conductance with an intermediate voltage threshold and slower kinetics showed an inverse gradient (being smallest in medial MNTB). The orthogonal axes of MNTB did not exhibit potassium current gradients (dorsal-to-ventral, or rostral-to-caudal). The input resistance was unchanged across the MNTB, but a slow capacitative component was enhanced in lateral neurons. These data demonstrate that the intrinsic properties of rat MNTB neurons are tuned across the tonotopic axis so as to promote shorter action potentials, faster firing and therefore greater accuracy in transmission of auditory information in the high characteristic frequency regions.

Developmental regulation and adult maintenance of potassium channel proteins (Kv1.1 and Kv1.2) in the cochlear nucleus of the rat

The development and maintenance of the adult expression and distribution of Kv 1.1 and Kv 1.2, two voltage-dependent potassium channel subunits, were investigated in the anteroventral cochlear nucleus (AVCN) of the rat. Both Kv 1.1 and Kv 1.2 were found in AVCN neuronal cell bodies at birth, as detected by in situ hybridization and immunocytochemistry. However, Kv 1.1 and Kv 1.2 were not seen in axons until the end of the third postnatal week. From postnatal day 21 through adulthood, labeling for both potassium channels was in axonal processes, whereas the number of cell bodies labeled for Kv 1.1 decreased and there were no cell bodies labeled for Kv 1.2. Therefore, these two potassium channel proteins are targeted to their final subcellular destinations in axons well after hearing onset. Once the adult distribution pattern of Kv 1.1 and Kv 1.2 is attained, its maintenance does not depend on signals from auditory nerve synapses. Eliminating auditory nerve input to the cochlear nucleus by means of bilateral cochleotomy did not change Kv 1.1 or Kv 1.2 expression or distribution, as seen by in situ hybridization, immunocytochemistry and Western blot. Thus, normal excitatory synaptic input in adult animals is not a requirement to regulate the expression and cellular and subcellular distribution of these potassium channel proteins.

SK channels in excitability, pacemaking and synaptic integration

Small conductance calcium-activated potassium channels link elevations of intracellular calcium ions to membrane potential, exerting a hyperpolarizing influence when activated. The consequences of SK channel activity have been revealed by the specific blocker apamin, a peptide toxin from honeybee venom. Recent studies have revealed unexpected roles for SK channels in fine-tuning intrinsic cell firing properties and in responsiveness to synaptic input. They have also identified specific roles for different SK channel subtypes. A host of Ca2+ sources, including distinct subtypes of voltage-dependent calcium channels, intracellular Ca2+ stores and Ca2+-permeable ionotropic neurotransmitter receptors, activate SK channels. The macromolecular complex in which the Ca2+ source, SK channels and various modulators are assembled determines the kinetics and consequences of SK channel activation.

Transient Ca2+-permeable AMPA receptors in postnatal rat primary auditory neurons

Fast excitatory transmission in the nervous system is mostly mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors whose subunit composition governs physiological characteristics such as ligand affinity and ion conductance properties. Here, we report that AMPA receptors at inner hair cell (IHC) synapses lack the GluR2 subunit and are transiently Ca2+-permeable before hearing onset as evidenced using agonist-induced Co2+ accumulation, Western blots and GluR2 confocal microscopy in the rat cochlea. AMPA (100 microM) induced Co2+ accumulation in primary auditory neurons until postnatal day (PND) 10. This accumulation was concentration-dependent, strengthened by cyclothiazide (50 microM) and blocked by GYKI 52466 (80 microM) and Joro spider toxin (1 microM). It was unaffected by D-AP5 (50 microM), and it could not be elicited by 56 mM K+ or 1 mM NMDA + 10 microM glycine. Western blots showed that GluR1 immunoreactivity, present in homogenates of immature cochleas, had disappeared by PND12. GluR2 immunoreactivity was not detected until PND10 and GluR3 and GluR4 immunoreactivities were detected at all the ages examined. Confocal microscopy confirmed that the GluR2 immunofluorescence was not located postsynaptically to IHCs before PND10. In conclusion, AMPA receptors on maturing primary auditory neurons differ from those on adult neurons. They are probably composed of GluR1, GluR3 and GluR4 subunits and have a high Ca2+ permeability. The postsynaptic expression of GluR2 subunits may be continuously regulated by the presynaptic activity allowing for variations in the Ca2+ permeability and physiological properties of the receptor.

Differences between the negatively activating potassium conductances of Mammalian cochlear and vestibular hair cells

Cochlear and type I vestibular hair cells of mammals express negatively activating potassium (K(+)) conductances, called g(K,n) and g(K,L) respectively, which are important in setting the hair cells' resting potentials and input conductances. It has been suggested that the channels underlying both conductances include KCNQ4 subunits from the KCNQ family of K(+) channels. In whole-cell recordings from rat hair cells, we found substantial differences between g(K,n) and g(K,L) in voltage dependence, kinetics, ionic permeability, and stability during whole-cell recording. Relative to g(K,L), g(K,n) had a significantly broader and more negative voltage range of activation and activated with less delay and faster principal time constants over the negative part of the activation range. Deactivation of g(K,n) had an unusual sigmoidal time course, while g(K,L) deactivated with a double-exponential decay. g(K,L), but not g(K,n), had appreciable permeability to Cs(+). Unlike g(K,L), g(K,n)'s properties did not change ("wash out") during the replacement of cytoplasmic solution with pipette solution during ruptured-patch recordings. These differences in the functional expression of g(K,n) and g(K,L) channels suggest that there are substantial differences in their molecular structure as well.

A transiently expressed SK current sustains and modulates action potential activity in immature mouse inner hair cells

From just after birth, mouse inner hair cells (IHCs) expressed a Ca(2+)-activated K(+) current that was reduced by intracellular BAPTA at concentrations >or= 1 mM. The block of this current by nifedipine suggests the direct involvement of Ca(v)1.3 Ca(2+) channels in its activation. On the basis of its high sensitivity to apamin (K(D) 360 pM) it was identified as a small-conductance Ca(2+)-activated K(+) current (SK), probably SK2. A similar current was also found in outer hair cells (OHCs) from the beginning of the second postnatal week. In both cell types the appearance of the SK current coincided with their becoming responsive to acetylcholine (ACh), the main efferent neurotransmitter in the cochlea. The effect of ACh on IHCs was abolished when they were simultaneously superfused with strychnine, consistent with the presence of nicotinic ACh receptors (nAChRs). Extracellular Ca(2+) either potentiated or blocked the nAChR current depending on its concentration, as previously reported for the recombinant alpha9alpha10 nAChR. Outward currents activated by ACh were reduced by blocking the SK current with apamin or by preventing SK current activation with intracellular BAPTA (>or= 10 mM). The endogenous mobile Ca(2+) buffer concentration was estimated to be equivalent to about 1 mM BAPTA, suggesting that in physiological conditions the SK channel is significantly activated by Ca(2+) influx through both Ca(v)1.3 Ca(2+) channels and alpha9alpha10 nAChRs. Current clamp experiments showed that in IHCs the SK current is required for sustaining a train of action potentials and also modulates their frequency when activated by ACh.