Primordial rhythmic bursting in embryonic cochlear ganglion cells

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

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

Experience-Dependent Plasticity in Nucleus Laminaris of the Barn Owl

Barn owls experience increasing interaural time differences (ITDs) during development, because their head width more than doubles in the month after hatching. We therefore hypothesized that their ITD detection circuit might be modified by experience. To test this, we raised owls with unilateral ear inserts that delayed and attenuated the acoustic signal, then measured the ITD representation in the brainstem nucleus laminaris (NL) when they were adult. The ITD circuit is composed of delay line inputs to coincidence detectors, and we predicted that plastic changes would lead to shorter delays in the axons from the manipulated ear, and complementary shifts in ITD representation on the two sides. In owls that received ear inserts starting around P14, the maps of ITD shifted in the predicted direction, but only on the ipsilateral side, and only in those tonotopic regions that had not experienced auditory stimulation prior to insertion. The contralateral map did not change. Experience-dependent plasticity of the ITD circuit occurs in NL, and our data suggest that ipsilateral and contralateral delays are independently regulated. Thus, altered auditory input during development leads to long-lasting changes in the representation of ITD.

The Effect of Music on Livestock: Cattle, Poultry and Pigs

Simple Summary

In times of intensified livestock production, the search for methods that reduce stress, which has an adverse impact on the health and welfare of their animals, has become a challenge for breeders and producers. Therefore, the possibility of using various musical genres to alleviate stress in chickens, cattle or pigs was considered. It has turned out that choosing a musical item is extremely important, as it can positively affect the health and production performance of animals by increasing the feeling of relaxation. The time of exposure to sounds and their intensity are important as well, and some authors propose to also pay attention to the frequency of sound waves. Music therapy, which was previously more widely deployed among humans, is increasingly used for farm animals as an element of enriching their living environment. Current research shows the importance of sound waves’ influence in animal production. Proper selection of the music genre, music intensity and tempo can reduce the adverse effects of noise and, thus, reduce the level of stress. It should be remembered, however, that silence is equally important and necessary for the welfare of animals. The paper presents literature findings regarding the influence of music on cattle, poultry and pigs.

Abstract

The welfare of animals, especially those kept in intensive production systems, is a priority for modern agriculture. This stems from the desire to keep animals healthy, to obtain a good-quality final product, and to meet the demands of today’s consumers, who have been increasingly persuaded to buy organic products. As a result, new sound-based methods have been pursued to reduce external stress in livestock. Music therapy has been known for thousands of years, and sounds were believed to improve both body and spirit. Today, they are mostly used to distract patients from their pain, as well as to treat depression and cardiovascular disorders. However, recent studies have suggested that appropriately selected music can confer some health benefits, e.g., by increasing the level and activity of natural killer cells. For use in livestock, the choice of genre, the loudness of the music and the tempo are all important factors. Some music tracks promote relaxation (thus improving yields), while others have the opposite effect. However, there is no doubt that enriching the animals’ environment with music improves their welfare and may also convince consumers to buy products from intensively farmed animals. The present paper explores the effects of music on livestock (cattle, poultry and pigs) on the basis of the available literature.

Differential expression of microRNAs in the developing avian auditory hindbrain

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

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.

Ion channel mechanisms underlying frequency-firing patterns of the avian nucleus magnocellularis: A computational model

We have previously shown that late-developing avian nucleus magnocellularis (NM) neurons (embryonic [E] days 19–21) fire action potentials (APs) that resembles a band-pass filter in response to sinusoidal current injections of varying frequencies. NM neurons located in the mid- to high-frequency regions of the nucleus fire preferentially at 75 Hz, but only fire a single onset AP to frequency inputs greater than 200 Hz. Surprisingly, NM neurons do not fire APs to sinusoidal inputs less than 20 Hz regardless of the strength of the current injection. In the present study we evaluated intrinsic mechanisms that prevent AP generation to low frequency inputs. We constructed a computational model to simulate the frequency-firing patterns of NM neurons based on experimental data at both room and near physiologic temperatures. The results from our model confirm that the interaction among low- and high-voltage activated potassium channels (K_LVA and K_HVA, respectively) and voltage dependent sodium channels (Na_V) give rise to the frequency-firing patterns observed in vitro. In particular, we evaluated the regulatory role of K_LVA during low frequency sinusoidal stimulation. The model shows that, in response to low frequency stimuli, activation of large K_LVA current counterbalances the slow-depolarizing current injection, likely permitting Na_V closed-state inactivation and preventing the generation of APs. When the K_LVA current density was reduced, the model neuron fired multiple APs per sinusoidal cycle, indicating that K_LVA channels regulate low frequency AP firing of NM neurons. This intrinsic property of NM neurons may assist in optimizing response to different rates of synaptic inputs.

Developmental Profile of Ion Channel Specializations in the Avian Nucleus Magnocellularis

Ultrafast and temporally precise action potentials (APs) are biophysical specializations of auditory brainstem neurons; properties necessary for encoding sound localization and communication cues. Fundamental to these specializations are voltage dependent potassium (K_V) and sodium (Na_V) ion channels. Here, we characterized the functional development of these ion channels and quantified how they shape AP properties in the avian cochlear nucleus magnocellularis (NM). We report that late developing NM neurons (embryonic [E] days 19–21) generate fast APs that reliably phase lock to sinusoidal inputs at 75 Hz. In contrast, early developing neurons (<E12) have slower and less reliable APs that preferentially fire to lower frequencies (5–10 Hz). With development, the membrane time constant of NM neurons became faster, while input resistance and capacitance decreased. Change in input resistance was due to a 2-fold increase in K_V current from E10 to E21 and when high-voltage activated potassium (K⁺_HVA) channels were blocked, APs for all ages became significantly slower. This was most evident for early developing neurons where the ratio of K⁺_HVA current accounted for ~85% of the total K_V response. This ratio dropped to ~50% for late developing neurons, suggesting a developmental upregulation of low-voltage activated potassium (K⁺_LVA) channels. Indeed, blockade of K⁺_LVA eliminated remaining current and increased neural excitability for late developing neurons. We also report developmental changes in the amplitude, kinetics and voltage dependence of Na_V currents. For early developing neurons, increase in Na_V current amplitude was due to channel density while channel conductance dominated for late developing neurons. From E10 to E21, Na_V channel currents became faster but differed in their voltage dependence; early developing neurons (<E16) had similar Na_V channel inactivation voltages while late developing NM neurons (>E19) contained Na_V channels that inactivate at more negative voltages, suggesting alterations in Na_V channel subtypes. Taken together, our results indicate that the refinement of passive and active ion channel properties operate differentially in order to develop fast and reliable APs in the avian NM.

Reduced sensory stimulation alters the molecular make-up of glutamatergic hair cell synapses in the developing cochlea

Neural activity during early development is known to alter innervation pathways in the central and peripheral nervous systems. We sought to examine how reduced sound-induced sensory activity in the cochlea affected the consolidation of glutamatergic synapses between inner hair cells (IHC) and the primary auditory neurons as these synapses play a primary role in transmitting sound information to the brain. A unilateral conductive hearing loss was induced prior to the onset of sound-mediated stimulation of the sensory hair cells, by rupturing the tympanic membrane and dislocating the auditory ossicles in the left ear of P11 mice. Auditory brainstem responses at P15 and P21 showed a 40-50-dB increase in thresholds for frequencies 8-32kHz in the dislocated ear relative to the control ear. Immunohistochemistry and confocal microscopy were subsequently used to examine the effect of this attenuation of sound stimulation on the expression of RIBEYE, which comprises the presynaptic ribbons, Shank-1, a postsynaptic scaffolding protein, and the GluA2/3 and 4 subunits of postsynaptic AMPA receptors. Our results show that dislocation did not alter the number of pre- or postsynaptic protein puncta. However, dislocation did increase the size of RIBEYE, GluA4, GluA2/3 and Shank-1 puncta, with postsynaptic changes preceding presynaptic changes. Our data suggest that a reduction in sound stimulation during auditory development induces plasticity in the molecular make-up of IHC glutamatergic synapses, but does not affect the number of these synapses. Up-regulation of synaptic proteins with sound attenuation may facilitate a compensatory increase in synaptic transmission due to the reduced sensory stimulation of the IHC.

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.

Etiology of distinct membrane excitability in pre- and posthearing auditory neurons relies on activity of Cl- channel TMEM16A

The developmental rehearsal for the debut of hearing is marked by massive changes in the membrane properties of hair cells (HCs) and spiral ganglion neurons (SGNs). Whereas the underlying mechanisms for the developing HC transition to mature stage are understood in detail, the maturation of SGNs from hyperexcitable prehearing to quiescent posthearing neurons with broad dynamic range is unknown. Here, we demonstrated using pharmacological approaches, caged-Ca(2+) photolysis, and gramicidin patch recordings that the prehearing SGN uses Ca(2+)-activated Cl(-) conductance to depolarize the resting membrane potential and to prime the neurons in a hyperexcitable state. Immunostaining of the cochlea preparation revealed the identity and expression of the Ca(2+)-activated Cl(-) channel transmembrane member 16A (TMEM16A) in SGNs. Moreover, null deletion of TMEM16A reduced the Ca(2+)-activated Cl(-) currents and action potential firing in SGNs. To determine whether Cl(-) ions and TMEM16A are involved in the transition between pre- and posthearing features of SGNs we measured the intracellular Cl(-) concentration [Cl(-)]i in SGNs. Surprisingly, [Cl(-)]i in SGNs from prehearing mice was ∼90 mM, which was significantly higher than posthearing neurons, ∼20 mM, demonstrating discernible altered roles of Cl(-) channels in the developing neuron. The switch in [Cl(-)]i stems from delayed expression of the development of intracellular Cl(-) regulating mechanisms. Because the Cl(-) channel is the only active ion-selective conductance with a reversal potential that lies within the dynamic range of SGN action potentials, developmental alteration of [Cl(-)]i, and hence the equilibrium potential for Cl(-) (ECl), transforms pre- to posthearing phenotype.

Kölliker's organ and the development of spontaneous activity in the auditory system: implications for hearing dysfunction

Prior to the “onset of hearing,” developing cochlear inner hair cells (IHCs) and primary auditory neurons undergo experience-independent activity, which is thought to be important in retaining and refining neural connections in the absence of sound. One of the major hypotheses regarding the origin of such activity involves a group of columnar epithelial supporting cells forming Kölliker's organ, which is only present during this critical period of auditory development. There is strong evidence for a purinergic signalling mechanism underlying such activity. ATP released through connexin hemichannels may activate P2 purinergic receptors in both Kölliker's organ and the adjacent IHCs, leading to generation of electrical activity throughout the auditory system. However, recent work has suggested an alternative origin, by demonstrating the ability of IHCs to generate this spontaneous activity without activation by ATP. Regardless, developmental abnormalities of Kölliker's organ may lead to congenital hearing loss, considering that mutations in ion channels (hemichannels, gap junctions, and calcium channels) involved in Kölliker's organ activity share strong links with such types of deafness.

Are there really "epileptogenic" mechanisms or only corruptions of "normal" plasticity?

Plasticity in the nervous system, whether for establishing connections and networks during development, repairing networks after injury, or modifying connections based on experience, relies primarily on highly coordinated patterns of neural activity. Rhythmic, synchronized bursting of neuronal ensembles is a fundamental component of the activity-dependent plasticity responsible for the wiring and rewiring of neural circuits in the CNS. It is therefore not surprising that the architecture of the CNS supports the generation of highly synchronized bursts of neuronal activity in non-pathological conditions, even though the activity resembles the ictal and interictal events that are the hallmark symptoms of epilepsy. To prevent such natural epileptiform events from becoming pathological, multiple layers of homeostatic control operate on cellular and network levels. Many data on plastic changes that occur in different brain structures during the processes by which the epileptogenic aggregate is constituted have been accumulated but their role in counteracting or promoting such processes is still controversial. In this chapter we will review experimental and clinical evidence on the role of neural plasticity in the development of epilepsy. We will address questions such as: is epilepsy a progressive disorder? What do we know about mechanism(s) accounting for progression? Have we reliable biomarkers of epilepsy-related plastic processes? Do seizure-associated plastic changes protect against injury and aid in recovery? As a necessary premise we will consider the value of seizure-like activity in the context of normal neural development.

GABAA receptor-mediated tonic depolarization in developing neural circuits

The activation of GABAA receptors (the type A receptors for γ-aminobutyric acid) produces two distinct forms of responses, phasic (i.e., transient) and tonic (i.e., persistent), that are mediated by synaptic and extrasynaptic GABAA receptors, respectively. During development, the intracellular chloride levels are high so activation of these receptors causes a net outward flow of anions that leads to neuronal depolarization rather than hyperpolarization. Therefore, in developing neural circuits, tonic activation of GABAA receptors may provide persistent depolarization. Recently, it became evident that GABAA receptor-mediated tonic depolarization alters the structure of patterned spontaneous activity, a feature that is common in developing neural circuits and is important for neural circuit refinement. Thus, this persistent depolarization may lead to a long-lasting increase in intracellular calcium level that modulates network properties via calcium-dependent signaling cascades. This article highlights the features of GABAA receptor-mediated tonic depolarization, summarizes the principles for discovery, reviews the current findings in diverse developing circuits, examines the underlying molecular mechanisms and modulation systems, and discusses their functional specializations for each developing neural circuit.

Ephrin-B2 reverse signaling is required for topography but not pattern formation of lateral superior olivary inputs to the inferior colliculus

Graded and modular expressions of Eph-ephrins are known to provide positional information for the formation of topographic maps and patterning in the developing nervous system. Previously we have shown that ephrin-B2 is expressed in a continuous gradient across the tonotopic axis of the central nucleus of the inferior colliculus (CNIC), whereas patterns are discontinuous and modular in the lateral cortex of the IC (LCIC). The present study explores the involvement of ephrin-B2 signaling in the development of projections to the CNIC and LCIC arising from the lateral superior olivary nuclei (LSO) prior to hearing onset. Anterograde and retrograde fluorescent tracing methods in neonatal fixed tissue preparations were used to compare topographic mapping and the establishment of LSO layers/modules in wild-type and ephrin-B2(lacZ/+) mice (severely compromised reverse signaling). At birth, pioneer LSO axons occupy the ipsilateral IC in both groups but are delayed contralaterally in ephrin-B2(lacZ/+) mutants. By the onset of hearing, both wild-type and mutant projections form discernible layers bilaterally in the CNIC and modular arrangements within the ipsilateral LCIC. In contrast, ephrin-B2(lacZ/+) mice lack a reliable topography in LSO-IC projections, suggesting that fully functional ephrin-B2 reverse signaling is required for normal projection mapping. Taken together, these ephrin-B2 findings paired with known coexpression of EphA4 suggest the importance of these signaling proteins in establishing functional auditory circuits prior to experience.

The activity of spontaneous action potentials in developing hair cells is regulated by Ca(2+)-dependence of a transient K+ current

Spontaneous action potentials have been described in developing sensory systems. These rhythmic activities may have instructional roles for the functional development of synaptic connections. The importance of spontaneous action potentials in the developing auditory system is underpinned by the stark correlation between the time of auditory system functional maturity, and the cessation of spontaneous action potentials. A prominent K(+) current that regulates patterning of action potentials is I(A). This current undergoes marked changes in expression during chicken hair cell development. Although the properties of I(A) are not normally classified as Ca(2+)-dependent, we demonstrate that throughout the development of chicken hair cells, I(A) is greatly reduced by acute alterations of intracellular Ca(2+). As determinants of spike timing and firing frequency, intracellular Ca(2+) buffers shift the activation and inactivation properties of the current to more positive potentials. Our findings provide evidence to demonstrate that the kinetics and functional expression of I(A) are tightly regulated by intracellular Ca(2+). Such feedback mechanism between the functional expression of I(A) and intracellular Ca(2+) may shape the activity of spontaneous action potentials, thus potentially sculpting synaptic connections in an activity-dependent manner in the developing cochlea.

Developmental acquisition of a rapid calcium-regulated vesicle supply allows sustained high rates of exocytosis in auditory hair cells

Auditory hair cells (HCs) have the remarkable property to indefinitely sustain high rates of synaptic vesicle release during ongoing sound stimulation. The mechanisms of vesicle supply that allow such indefatigable exocytosis at the ribbon active zone remain largely unknown. To address this issue, we characterized the kinetics of vesicle recruitment and release in developing chick auditory HCs. Experiments were done using the intact chick basilar papilla from E10 (embryonic day 10) to P2 (two days post-hatch) by monitoring changes in membrane capacitance and Ca(2+) currents during various voltage stimulations. Compared to immature pre-hearing HCs (E10-E12), mature post-hearing HCs (E18-P2) can steadily mobilize a larger readily releasable pool (RRP) of vesicles with faster kinetics and higher Ca(2+) efficiency. As assessed by varying the inter-pulse interval of a 100 ms paired-pulse depolarization protocol, the kinetics of RRP replenishment were found much faster in mature HCs. Unlike mature HCs, exocytosis in immature HCs showed large depression during repetitive stimulations. Remarkably, when the intracellular concentration of EGTA was raised from 0.5 to 2 mM, the paired-pulse depression level remained unchanged in immature HCs but was drastically increased in mature HCs, indicating that the Ca(2+) sensitivity of the vesicle replenishment process increases during maturation. Concomitantly, the immunoreactivity of the calcium sensor otoferlin and the number of ribbons at the HC plasma membrane largely increased, reaching a maximum level at E18-P2. Our results suggest that the efficient Ca(2+)-dependent vesicle release and supply in mature HCs essentially rely on the concomitant engagement of synaptic ribbons and otoferlin at the plasma membrane.

EphA4 and ephrin-B2 expression patterns during inferior colliculus projection shaping prior to experience

Central processing of complex auditory tasks requires elaborate circuitry. The auditory midbrain, or inferior colliculus (IC), epitomizes such precise organization, where converging inputs form discrete, tonotopically-arranged axonal layers. Previously in rat, we established that shaping of multiple afferent patterns in the IC central nucleus (CNIC) occurs prior to experience. This study implicates an Eph receptor tyrosine kinase and a corresponding ephrin ligand in signaling this early topographic registry. We report that EphA4 and ephrin-B2 expression patterns in the neonatal rat and mouse IC correlate temporally and spatially with that of developing axonal layers. DiI-labeling confirms projections arising from the lateral superior olive (LSO) form frequency-specific layers within the ipsilateral and contralateral mouse CNIC, as has been described in other species. Immunohistochemistry (EphA4 and ephrin-B2) and ephrin-B2 lacZ histochemistry reveal clear gradients in expression across the tonotopic axis, with most concentrated labeling observed in high-frequency, ventromedial aspects of the CNIC. Discrete patches of labeling were also discernible in the external cortex of the IC (ECIC; EphA4 patches in rat, ephrin-B2 patches in mouse). Observed gradients in the CNIC and compartmentalized ECIC expression persisted through the first postnatal week, before becoming less intense and more homogeneously distributed by the functional onset of hearing. EphA4 and ephrin-B2-positive neurons were evident in several auditory brainstem nuclei known to send patterened inputs to the IC. These findings suggest the involvement of cell-cell EphA4 and ephrin-B2 signaling in establishing order in the developing IC.

ATP-induced morphological changes in supporting cells of the developing cochlea

The developing cochlea of mammals contains a large group of columnar-shaped cells, which together form a structure known as Kölliker's organ. Prior to the onset of hearing, these inner supporting cells periodically release adenosine 5'-triphosphate (ATP), which activates purinergic receptors in surrounding supporting cells, inner hair cells and the dendrites of primary auditory neurons. Recent studies indicate that purinergic signaling between inner supporting cells and inner hair cells initiates bursts of action potentials in auditory nerve fibers before the onset of hearing. ATP also induces prominent effects in inner supporting cells, including an increase in membrane conductance, a rise in intracellular Ca(2+), and dramatic changes in cell shape, although the importance of ATP signaling in non-sensory cells of the developing cochlea remains unknown. Here, we review current knowledge pertaining to purinergic signaling in supporting cells of Kölliker's organ and focus on the mechanisms by which ATP induces changes in their morphology. We show that these changes in cell shape are preceded by increases in cytoplasmic Ca(2+), and provide new evidence indicating that elevation of intracellular Ca(2+) and IP(3) are sufficient to initiate shape changes. In addition, we discuss the possibility that these ATP-mediated morphological changes reflect crenation following the activation of Ca(2+)-activated Cl(-) channels, and speculate about the possible functions of these changes in cell morphology for maturation of the cochlea.

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.

Suppression of spontaneous firing in inferior colliculus neurons during sound processing

Spontaneous activity is a well-known neural phenomenon that occurs throughout the brain and is essential for normal development of auditory circuits and for processing of sounds. Spontaneous activity could interfere with sound processing by reducing the signal-to-noise ratio. Multiple studies have reported that spontaneous activity in auditory neurons can be suppressed by sound stimuli. The goal of this study was to determine the stimulus conditions that cause this suppression and to identify possible underlying mechanisms. Experiments were conducted in the inferior colliculus (IC) of awake little brown bats using extracellular and intracellular recording techniques. The majority of IC neurons (82%) fired spontaneously, with a median spontaneous firing rate of 6 spikes/s. After offset of a 4 ms sound, more than half of these neurons exhibited suppression of spontaneous firing that lasted hundreds of milliseconds. The duration of suppression increased with sound level. Intracellular recordings showed that a short (<50 ms) membrane hyperpolarization was often present during the beginning of suppression, but it was never observed during the remainder of the suppression. Beyond the initial 50 ms period, the absence of significant changes in input resistance during suppression suggests that suppression is presynaptic in origin. Namely, it may occur on presynaptic terminals and/or elsewhere on presynaptic neurons. Suppression of spontaneous firing may serve as a mechanism for enhancing signal-to-noise ratios during signal processing.

Mechanisms underlying spontaneous patterned activity in developing neural circuits

Patterned, spontaneous activity occurs in many developing neural circuits, including the retina, the cochlea, the spinal cord, the cerebellum and the hippocampus, where it provides signals that are important for the development of neurons and their connections. Despite there being differences in adult architecture and output across these various circuits, the patterns of spontaneous network activity and the mechanisms that generate it are remarkably similar. The mechanisms can include a depolarizing action of GABA (gamma-aminobutyric acid), transient synaptic connections, extrasynaptic transmission, gap junction coupling and the presence of pacemaker-like neurons. Interestingly, spontaneous activity is robust; if one element of a circuit is disrupted another will generate similar activity. This research suggests that developing neural circuits exhibit transient and tunable features that maintain a source of correlated activity during crucial stages of development.

Theoretical models of spontaneous activity generation and propagation in the developing retina

Spontaneous neural activity is present in many parts of the developing nervous system, including visual, auditory and motor areas. In the developing retina, nearby neurons are spontaneously active and produce propagating patterns of activity, known as retinal waves. Such activity is thought to instruct the refinement of retinal axons. In this article we review several computational models used to help evaluate the mechanisms that might be responsible for the generation of retinal waves. We then discuss the models relative to the molecular mechanisms underlying wave activity, including gap junctions, neurotransmitters and second messenger systems. We examine how well the models represent these mechanisms and propose areas for future modelling research. The retinal wave models are also discussed in relation to models of spontaneous activity in other areas of the developing nervous system.

Dendritic calcium channels and their activation by synaptic signals in auditory coincidence detector neurons

The avian nucleus laminaris (NL) encodes the azimuthal location of low-frequency sound sources by detecting the coincidence of binaural signals. Accurate coincidence detection requires precise developmental regulation of the lengths of the fine, bitufted dendrites that characterize neurons in NL. Such regulation has been suggested to be driven by local, synaptically mediated, dendritic signals such as Ca(2+). We examined Ca(2+) signaling through patch clamp and ion imaging experiments in slices containing nucleus laminaris from embryonic chicks. Voltage-clamp recordings of neurons located in the NL showed the presence of large Ca(2+) currents of two types, a low voltage-activated, fast inactivating Ni(2+) sensitive channel resembling mammalian T-type channels, and a high voltage-activated, slowly inactivating Cd(2+) sensitive channel. Two-photon Ca(2+) imaging showed that both channel types were concentrated on dendrites, even at their distal tips. Single action potentials triggered synaptically or by somatic current injection immediately elevated Ca(2+) throughout the entire cell. Ca(2+) signals triggered by subthreshold synaptic activity were highly localized. Thus when electrical activity is suprathreshold, Ca(2+) channels ensure that Ca(2+) rises in all dendrites, even those that are synaptically inactive.

Patterning of multiple layered projections to the auditory midbrain prior to experience

The precise arrangement of patterned inputs into discrete functional domains is a common organizational feature of primary sensory structures. While the specific organization of patterned connections has been well documented in the visual and somatosensory systems, comparatively little is known about the arrangement of neighboring afferent patterns in the emerging auditory system. Here we report early projection specificity for multiple converging inputs to the rat central nucleus of the inferior colliculus (ICC). Afferents arising from the dorsal cochlear nucleus (DCN), the dorsal nucleus of the lateral lemniscus (DNLL), and the lateral superior olive (LSO) establish discernible axonal layers a week prior to experience. By hearing onset, contralateral DCN and contralateral LSO layers are clearly defined and segregated from contralateral DNLL terminal zones. Layering of the ipsilateral LSO projection, on the other hand, exhibits considerable spatial overlap with the contralateral DNLL pattern. This fine laminar structure of interdigitating and overlapping inputs likely underlies the complex signal processing performed in the auditory midbrain and may serve as a model system for examining competitive interactions between neighboring excitatory and inhibitory projections early in development.

Resting discharge patterns of macular primary afferents in otoconia-deficient mice

Vestibular primary afferents in the normal mammal are spontaneously active. The consensus hypothesis states that such discharge patterns are independent of stimulation and depend instead on excitation by vestibular hair cells due to background release of synaptic neurotransmitter. In the case of otoconial sensory receptors, it is difficult to test the independence of resting discharge from natural tonic stimulation by gravity. We examined this question by studying discharge patterns of single vestibular primary afferent neurons in the absence of gravity stimulation using two mutant strains of mice that lack otoconia (OTO-; head tilt, het-Nox3, and tilted, tlt-Otop1). Our findings demonstrated that macular primary afferent neurons exhibit robust resting discharge activity in OTO- mice. Spike interval coefficient of variation (CV = SD/mean spike interval) values reflected both regular and irregular discharge patterns in OTO- mice, and the range of values for rate-normalized CV was similar to mice and other mammals with intact otoconia although there were proportionately fewer irregular fibers. Mean discharge rates were slightly higher in otoconia-deficient strains even after accounting for proportionately fewer irregular fibers [OTO- = 75.4 +/- 31.1(113) vs OTO+ = 68.1 +/- 28.5(143) in sp/s]. These results confirm the hypothesis that resting activity in macular primary afferents occurs in the absence of ambient stimulation. The robust discharge rates are interesting in that they may reflect the presence of a functionally 'up-regulated' tonic excitatory process in the absence of natural sensory stimulation.

The origin of spontaneous activity in the developing auditory system

Spontaneous activity in the developing auditory system is required for neuronal survival as well as the refinement and maintenance of tonotopic maps in the brain. However, the mechanisms responsible for initiating auditory nerve firing in the absence of sound have not been determined. Here we show that supporting cells in the developing rat cochlea spontaneously release ATP, which causes nearby inner hair cells to depolarize and release glutamate, triggering discrete bursts of action potentials in primary auditory neurons. This endogenous, ATP-mediated signalling synchronizes the output of neighbouring inner hair cells, which may help refine tonotopic maps in the brain. Spontaneous ATP-dependent signalling rapidly subsides after the onset of hearing, thereby preventing this experience-independent activity from interfering with accurate encoding of sound. These data indicate that supporting cells in the organ of Corti initiate electrical activity in auditory nerves before hearing, pointing to an essential role for peripheral, non-sensory cells in the development of central auditory pathways.

Development and regeneration of hair cells share common functional features

The structural phenotype of neural connections in the auditory brainstem is sculpted by spontaneous and stimulus-induced neural activities during development. However, functional and molecular mechanisms of spontaneous action potentials (SAPs) in the developing cochlea are unknown. Additionally, it is unclear how regenerating hair cells establish their neural ranking in the constellation of neurons in the brainstem. We have demonstrated that a transient Ca(2+) current produced by the Ca(v)3.1 channel is expressed early in development to initiate spontaneous Ca(2+) spikes. Ca(v)1.3 currents, typical of mature hair cells, appeared later in development. Moreover, there is a surprising disappearance of the Ca(v)3.1 current that coincides with the attenuation of the transient Ca(2+) current as the electrical properties of hair cells transition to the mature phenotype. Remarkably, this process is recapitulated during hair-cell regeneration, suggesting that the transient expression of Ca(v)3.1 and the ensuing SAPs are signatures of hair cell development and regeneration.

Spontaneous discharge patterns in cochlear spiral ganglion cells before the onset of hearing in cats

Spontaneous neural activity has been recorded in the auditory nerve of cats as early as 2 days postnatal (P2), yet individual auditory neurons do not respond to ambient sound levels <90-100 dB SPL until about P10. Significant refinement of the central projections from the spiral ganglion to the cochlear nucleus occurs during this neonatal period. This refinement may be dependent on peripheral spontaneous discharge activity. We recorded from single spiral ganglion cells in kittens aged P3-P9. The spiral ganglion was accessed through the round window through the spiral lamina. A total of 112 ganglion cells were isolated for study in nine animals. Spike rates in neonates were very low, ranging from 0.06 to 56 spikes/s, with a mean of 3.09 +/- 8.24 spikes/s. Ganglion cells in neonatal kittens exhibited remarkable repetitive spontaneous bursting discharge patterns. The unusual patterns were evident in the large mean interval CV (CV(i) = 2.9 +/- 1.6) and burst index of 5.2 +/- 3.5 across ganglion cells. Spontaneous bursting patterns in these neonatal mammals were similar to those reported for cochlear ganglion cells of the embryonic chicken, suggesting this may be a general phenomenon that is common across animal classes. Rhythmic spontaneous discharge of retinal ganglion cells has been shown to be important in the development of central retinotopic projections and normal binocular vision. Bursting rhythms in cochlear ganglion cells may play a similar role in the auditory system during prehearing periods.

Early segregation of layered projections from the lateral superior olivary nucleus to the central nucleus of the inferior colliculus in the neonatal cat

The central nucleus of the inferior colliculus (IC) is a laminated structure that receives multiple converging afferent projections. These projections terminate in a layered arrangement and are aligned with dendritic arbors of the predominant disc-shaped neurons, forming fibrodendritic laminae. Within this structural framework, inputs terminate in a precise manner, establishing a mosaic of partially overlapping domains that likely define functional compartments. Although several of these patterned inputs have been described in the adult, relatively little is known about their organization prior to hearing onset. The present study used the lipophilic carbocyanine dyes DiI and DiD to examine the ipsilateral and contralateral projections from the lateral superior olivary (LSO) nucleus to the IC in a developmental series of paraformaldehyde-fixed kitten tissue. By birth, the crossed and uncrossed projections had reached the IC and were distributed across the frequency axis of the central nucleus. At this earliest postnatal stage, projections already exhibited a characteristic banded arrangement similar to that described in the adult. The heaviest terminal fields of the two inputs were always complementary in nature, with the ipsilateral input appearing slightly denser. This early arrangement of interdigitating ipsilateral and contralateral LSO axonal bands that occupy adjacent sublayers supports the idea that the initial establishment of this highly organized mosaic of inputs that defines distinct synaptic domains within the IC occurs largely in the absence of auditory experience. Potential developmental mechanisms that may shape these highly ordered inputs prior to hearing onset are discussed.

The role of spontaneous activity in development of the endbulb of Held synapse

In the mouse brainstem cochlear nucleus, the auditory nerve to bushy cell synapse (endbulb of Held) is specialised for rapid, high-fidelity transmission. Development of this synapse is modulated by auditory nerve activity. Here we investigate the role of spontaneous auditory nerve activity in synaptic transmission using deafness (dn/dn) mutant mice that have abnormal hair cells and lack spontaneous auditory nerve activity. Evoked and miniature alpha amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor-mediated excitatory post-synaptic currents (eEPSCs, mEPSCs) were compared in deafness and normal mice before the age of hearing onset (postnatal day 7-11: P7-11) using variance-mean, miniature event and tetanic depression analyses. Amplitudes were significantly greater in deafness mice for eEPSCs (2.1-fold), mEPSCs (1.4-fold) and quantal amplitudes (1.5-fold). eEPSCs in deafness mice decayed more rapidly with increasing age, indicating an input-independent transition in post-synaptic AMPA receptor properties. A comparison of normal mice before and after the onset of hearing showed a change in synaptic parameters with an increase in eEPSC (1.7-fold), mEPSC (1.6-fold) and quantal amplitude (1.7-fold) after hearing onset while release probability remained constant (0.5). Overall, the results in deafness mice suggest that synaptic strength is altered in the absence of spontaneous auditory nerve activity.

Metabotropic glutamate receptors in the lateral superior olive activate TRP-like channels: age- and experience-dependent regulation

The lateral superior olive (LSO) is the primary auditory nucleus for processing of interaural sound level differences, which is one of the major cues for sound localization. During development, survival and maturation of LSO neurons critically depend on synaptic activity and intracellular calcium signaling. Before hearing onset, glutamatergic synaptic inputs from the cochlear nucleus (CN) to the LSO activate group I metabotropic glutamate receptors (mGluRs), which leads to calcium release from intracellular stores and large calcium influx from the extracellular milieu. Here, we investigated the nature of the mGluR-activated membrane channel that mediates the influx of extracellular calcium. Using Fura-2 calcium imaging in brain stem slices of neonatal and juvenile mice, we found that this calcium channel is blocked by Ni(2+), La(3+), and 2-aminoethoxydiphenylborane (2-APB), known antagonists of transient receptor potential (TRP) channels. During postnatal development, the contribution of extracellular calcium influx to mGluR-mediated Ca(2+) responses gradually decreased and was almost abolished by the end of the third postnatal week. Over this period, the contribution of Ca(2+) release from internal stores remained unchanged. The developmental decrease of TRP-like channel-mediated calcium influx was significantly less in congenitally deaf waltzer mice, suggesting that early auditory experience is necessary for the normal age-dependent downregulation of functional TRP channels.

Unilateral cochlear ablation before hearing onset disrupts the maintenance of dorsal nucleus of the lateral lemniscus projection patterns in the rat inferior colliculus

During postnatal development, ascending and descending auditory inputs converge to form fibrodendritic layers within the central nucleus of the inferior colliculus (IC). Before the onset of hearing, specific combinations of inputs segregate into bands separated by interband spaces. These bands may define functional zones within the IC. Previous studies in our laboratory have shown that unilateral or bilateral cochlear ablation at postnatal day 2 (P2) disrupts the development of afferent bands from the dorsal nucleus of the lateral lemniscus (DNLL) to the IC. These results suggest that spontaneous activity propagated from the cochlea is required for the segregation of afferent bands within the developing IC. To test if spontaneous activity from the cochlea also may be required to maintain segregated bands of DNLL input, we performed cochlear ablations in rat pups at P9, after DNLL bands already are established. All animals were killed at P12 and glass pins coated with carbocyanine dye, DiI (1,1'-dioctodecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate), subsequently were placed in the commissure of Probst to label the crossed projections from both DNLLs. When compared with surgical controls, experimental results showed a similar pattern of DNLL bands in the IC contralateral to the ablated cochlea, but a disruption of DNLL bands in the IC ipsilateral to the cochlear ablation. The present results suggest that cochlear ablation after DNLL bands have formed may affect the maintenance of banded DNLL projections within the central nucleus of the IC.

Emergence of Hearing in the Chicken Embryo

It is commonly held that hearing generally begins on incubation day 12 (E12) in the chicken embryo ( Gallus domesticus). However, little is known about the response properties of cochlear ganglion neurons for ages younger than E18. We studied ganglion neurons innervating the basilar papilla of embryos (E12–E18) and hatchlings (P13–P15). We asked first, when do primary afferent neurons begin to encode sounds? Second, when do afferents evidence frequency selectivity? Third, what range of characteristic frequencies (CFs) is represented in the late embryo? Finally, how does sound transfer from air to the cochlea affect responses in the embryo and hatchling? Responses to airborne sound were compared with responses to direct columella footplate stimulation of the cochlea. Cochlear ganglion neurons exhibited a profound insensitivity to sound from E12 to E16 (stages 39–42). Responses to sound and frequency selectivity emerged at about E15. Frequency selectivity matured rapidly from E16 to E18 (stages 42 and 44) to reflect a mature range of CFs (170–4,478 Hz) and response sensitivity to footplate stimulation. Limited high-frequency sound transfer from air to the cochlea restricted the response to airborne sound in the late embryo. Two periods of ontogeny are proposed. First is a prehearing period (roughly E12–E16) of endogenous cochlear signaling that provides neurotrophic support and guides normal developmental refinements in central binaural processing pathways followed by a period (roughly E16–E19) wherein the cochlea begins to detect and encode sound.

Survey of inward ionic currents acquired by the cochleovestibular ganglion of the early-aged embryonic chick

The acquisition of ion channels is critical to the formation of neuronal pathways in the peripheral and central nervous systems. This study describes the different types of inward currents (Ii) recorded from the soma of isolated cochleovestibular ganglion (CVG) cells of the embryonic chicken, Gallus gallus. Cells were isolated for whole-cell tight-seal recording from embryonic day (ED) 3, an age when the CVG is a cell cluster, to ED 9, an age when the cochlear and vestibular ganglia (CG, VG) are distinct structures. Results show Na+ and Ca2+ currents (INa and ICa) are acquired by ED 3, although INa dominates with greater density levels that peak by ED 6-7 in VG neurons. In the CG, INa acquisition is slower, reaching peak values by ED 8-9. Isolation of ICa, using Ba2+ as the charge carrier, showed both transient (IBaT)- and sustained (IBaL)-type currents on ED 3. Unlike INa, IBa density varied with age and ganglion. Total IBa increased steadily, showing a decline only in CG cells on ED 8-9 as a result of a decrease in IBaT. IBaL density increased over time, reaching a maximum on ED 6-7 in VG cells, followed by a decline on ED 8-9. In comparison, IBaL in CG neurons, did not increase significantly beyond mean values measured on ED 5. The early onset of these currents and the variations in Ca2+ channel expression between the ganglia suggests that intracellular signals relevant to phenotypic differentiation begin within these early time frames.

Nicotinic cholinergic intercellular communication: implications for the developing auditory system

In this paper, research on the temporal and spatial distribution of cholinergic-related molecules in the lower auditory brainstem, with an emphasis on nicotinic acetylcholine receptors (nAChRs), is reviewed. The possible functions of acetylcholine (ACh) in driving selective auditory neurons before the onset of hearing, inducing glutamate receptor gene expression, synaptogenesis, differentiation, and cell survival are discussed. Experiments conducted in other neuronal and non-neuronal systems are drawn on extensively to discuss putative functions of ACh and nAChRs. Data from other systems may provide insight into the functions of ACh and nAChRs in auditory processing. The mismatch of presynaptic and postsynaptic markers and novel endogenous agonists of nAChRs are discussed in the context of non-classical interneuronal communication. The molecular mechanism that may underlie the many functions of ACh and its agonists is the regulation of intracellular calcium through nAChRs. The possible reorganization that may take place in the auditory system by the exposure to nicotine during critical developmental periods is also briefly considered.

Ion Channel Development, Spontaneous Activity, and Activity-Dependent Development in Nerve and Muscle Cells

At specific stages of development, nerve and muscle cells generate spontaneous electrical activity that is required for normal maturation of intrinsic excitability and synaptic connectivity. The patterns of this spontaneous activity are not simply immature versions of the mature activity, but rather are highly specialized to initiate and control many aspects of neuronal development. The configuration of voltage- and ligand-gated ion channels that are expressed early in development regulate the timing and waveform of this activity. They also regulate Ca2+influx during spontaneous activity, which is the first step in triggering activity-dependent developmental programs. For these reasons, the properties of voltage- and ligand-gated ion channels expressed by developing neurons and muscle cells often differ markedly from those of adult cells. When viewed from this perspective, the reasons for complex patterns of ion channel emergence and regression during development become much clearer.

Time course of embryonic midbrain and thalamic auditory connection development in mice as revealed by carbocyanine dye tracing

Central auditory connections develop in mice before the onset of hearing, around postnatal day 7. Two previous studies have investigated the development of auditory nuclei projections and lateral lemniscal nuclear projections in embryonic rats, respectively. Here, we provide detail for the first time of the initiation and progression of projections from the inferior colliculus (IC) to the medial geniculate body (MGB) and from the MGB to the auditory cortex (AC). Overall, the developmental progression of projections follows that of terminal mitoses in various nuclei, suggesting the consistent use of a developmental timetable at a given nucleus, independent of that of other nuclei. Our data further suggest that neurons project specifically and reciprocally from the MGB to the AC as early as embryonic day 14.5. These projections develop approximately a day before the reciprocal connections between the MGB and IC and before development of projections from the auditory nuclei to the IC. The development of IC projections is prolonged and progresses from rostral to caudal areas. Brainstem nuclear projections to the IC arrive first from the lateral lemniscus nuclei then the superior olive and finally the cochlear nuclei. Overall, the auditory connection development strongly suggests that most of the overall specificity of nuclear connections is set up at least 2 weeks before the onset of sound-mediated cochlea responses in mice and, thus, is likely governed predominantly by molecular genetic clues.

Sodium and calcium currents shape action potentials in immature mouse inner hair cells

Before the onset of hearing at postnatal day 12, mouse inner hair cells (IHCs) produce spontaneous and evoked action potentials. These spikes are likely to induce neurotransmitter release onto auditory nerve fibres. Since immature IHCs express both alpha1D (Cav1.3) Ca2+ and Na+ currents that activate near the resting potential, we examined whether these two conductances are involved in shaping the action potentials. Both had extremely rapid activation kinetics, followed by fast and complete voltage-dependent inactivation for the Na+ current, and slower, partially Ca2+-dependent inactivation for the Ca2+ current. Only the Ca2+ current is necessary for spontaneous and induced action potentials, and 29 % of cells lacked a Na+ current. The Na+ current does, however, shorten the time to reach the action-potential threshold, whereas the Ca2+ current is mainly involved, together with the K+ currents, in determining the speed and size of the spikes. Both currents increased in size up to the end of the first postnatal week. After this, the Ca2+ current reduced to about 30 % of its maximum size and persisted in mature IHCs. The Na+ current was downregulated around the onset of hearing, when the spiking is also known to disappear. Although the Na+ current was observed as early as embryonic day 16.5, its role in action-potential generation was only evident from just after birth, when the resting membrane potential became sufficiently negative to remove a sizeable fraction of the inactivation (half inactivation was at -71 mV). The size of both currents was positively correlated with the developmental change in action-potential frequency.

Statistical morphological analysis of hippocampal principal neurons indicates cell-specific repulsion of dendrites from their own cell

Traditionally, the sources of guidance cues for dendritic outgrowth are mainly associated with external bodies (A) rather than with the same neuron from which dendrites originate (B). To quantify the relationship between factors A and B as determinants of the adult dendritic shape, the morphology of 83 intracellularly characterized, stained, completely reconstructed, and digitized principal neurons of the rat hippocampus was statistically analyzed using Bayesian optimization. It was found that the dominant directional preference (tropism) manifested in dendritic turns is to grow away from the soma rather than toward the incoming fibers or in any other fixed direction; therefore, B is predominant. Results are robust and consistent for all examined morphological classes (dentate gyrus granule cells, basal and apical trees of CA3 and CA1 pyramidal cells). In addition, computer remodeling of neurons based on the measured parameters produced virtual structures consistent with real morphologies, as confirmed by measurement of several global emergent parameters. Thus, the simple description of dendritic shape based on dendrites' tendency to grow straight, away from their own soma, and with additional random deflections, proves remarkably accurate and complete. Although based on adult neurons, these results suggest that dendritic guidance during development may be associated primarily with the host cell. This possibility challenges the traditional concept of dendritic guidance: in that hippocampal cells are densely packed and have highly overlapping dendritic fields, the somatodendritic repulsion must be cell specific. Plausible mechanisms involving extracellular effects of spikes are discussed, together with feasible experimental tests and predicted results.

Distribution of alpha-amino-3-hydroxy-5-methyl-4 isoazolepropionic acid and N-methyl-D-aspartate receptor subunits in the vestibular and spiral ganglia of the mouse during early development

We investigated the distribution of the glutamate receptor subunits, alpha-amino-3-hydroxy-5-methyl-4 isoazolepropionic acid (AMPA) GluR2 and GluR2/R3, and N-methyl-D-aspartate (NMDA) NR1, and the timing of their appearance during early development of the mouse vestibular and spiral ganglia. NMDA NR1 was the first to be expressed, in the statoacoustic ganglion neurons on E11. GluR2/R3 immunoreactivity was detected in these neurons on E12. This signal probably corresponded exclusively to GluR3, as no signal was obtained for GluR2 alone at this stage. The appearance of these proteins began much earlier than previously reported. GluR2 staining was observed later, on E14 in the vestibular neurons and on E17 in the spiral neurons. The sequence in which these three glutamate receptors appeared suggested possible differences in their roles in the establishment of neuronal circuitry in the inner ear sensory epithelia. The production of NR1 and GluR2/R3 began during the early period of neuron growth and fasciculation. GluR2 appeared later and its expression paralleled synaptogenesis in the vestibular sensory epithelia and in the organ of Corti.

Development of the specialized AMPA receptors of auditory neurons

At maturity, the AMPA receptors of auditory neurons exhibit very rapid desensitization kinetics and high permeability to calcium, reflecting the predominance of GluR3 flop and GluR4 flop subunits and the paucity of GluR2. We used mRNA analysis and immunoblotting to contrast the development of AMPA receptor structure in the chick cochlear nucleus [nucleus magnocellularis (NM)] with that of the slowly desensitizing and calcium-impermeable AMPA receptors of brainstem motor neurons in the nucleus of the glossopharyngeal/vagal nerves. The relative abundance of transcripts for GluRs 1-4 changes substantially in auditory (but not motor) neurons after embryonic day (E)10, with large decreases in GluR2 and increases in GluR3 and GluR4. Relative to the motor neurons, NM neurons show a higher abundance of flop isoforms of GluRs 2-4 at E10, suggesting that auditory neurons are already biased toward expression of flop isoforms before the onset of synaptic function at E11. Immunoreactivities in NM show very distinct developmental patterns from E13 onward: GluR2 declines by >90%, GluR3 increases threefold, and GluR4 remains relatively constant. Our results show that there are a series of critical points during normal development, most occurring after the onset of function, when rapid changes in receptor structure (occurring via both transcriptional and post-transcriptional control mechanisms) produce the specialized AMPA receptor functions that enable auditory neurons to accurately encode acoustic information.

Physiological effects of sustained blockade of excitatory synaptic transmission on spontaneously active developing neuronal networks--an inquiry into the reciprocal linkage between intrinsic biorhythms and neuroplasticity in early ontogeny

Spontaneous bioelectric activity (SBA) taking the form of extracellularly recorded spike trains (SBA) has been quantitatively analyzed in organotypic neonatal rat visual cortex explants at different ages in vitro, and the effects investigated of both short- and long-term pharmacological suppression of glutamatergic synaptic transmission. In the presence of APV, a selective NMDA receptor blocker, 1-2- (but not 3-)week-old cultures recovered their previous SBA levels in a matter of hours, although in imitation of the acute effect of the GABAergic inhibitor picrotoxin (PTX), bursts of action potentials were abnormally short and intense. Cultures treated either overnight or chronically for 1-3 weeks with APV, the AMPA/kainate receptor blocker DNQX, or a combination of the two were found to display very different abnormalities in their firing patterns. NMDA receptor blockade for 3 weeks produced the most severe deviations from control SBA, consisting of greatly prolonged and intensified burst firing with a strong tendency to be broken up into trains of shorter spike clusters. This pattern was most closely approximated by acute GABAergic disinhibition in cultures of the same age, but this latter treatment also differed in several respects from the chronic-APV effect. In 2-week-old explants, in contrast, it was the APV+DNQX treated group which showed the most exaggerated spike bursts. Functional maturation of neocortical networks, therefore, may specifically require NMDA receptor activation (not merely a high level of neuronal firing) which initially is driven by endogenous rather than afferent evoked bioelectric activity. Putative cellular mechanisms are discussed in the context of a thorough review of the extensive but scattered literature relating activity-dependent brain development to spontaneous neuronal firing patterns.