Glial cells of the nervous system directly influence neuronal and synaptic activities by releasing transmitters. However, the physiological consequences of this glial transmitter release on brain information processing remain poorly understood. We demonstrate here in hippocampal slices of 2-to 5-week-old rats that glutamate released from glial cells generates slow transient currents (STCs) mediated by the activation of NMDA receptors in pyramidal cells. STCs persist in the absence of neuronal and synaptic activity, indicating a nonsynaptic origin of the source of glutamate. Indeed, STCs occur spontaneously but can also be induced by pharmacological tools known to activate astrocytes and by the selective mechanical stimulation of single nearby glial cells. Bath application of the inhibitor of the glutamate uptake DL-threo-␤-benzyloxyaspartate increases both the frequency of STCs and the amplitude of a tonic conductance mediated by NMDA receptors and probably also originated from glial glutamate release. By using dual recordings, we observed synchronized STCs in pyramidal cells having their soma distant by Ͻ100 m. The degree of precision (Ͻ100 msec) of this synchronization rules out the involvement of calcium waves spreading through the glial network. It also indicates that single glial cells release glutamate onto adjacent neuronal processes, thereby controlling simultaneously the excitability of several neighboring pyramidal cells. In conclusion, our results show that the glial glutamate release occurs spontaneously and synchronizes the neuronal activity in the hippocampus.

Fig. 3. Decision tree for hybrid sequencing strategy. For organisms with a small genome size (Ͻ3 Mb) and͞or a small number of gaps and͞or high levels of repetitive structure inducing physical ends, we found 8ϫ Sanger sequencing to be the most cost-effective approach. For organisms with a large genome size, many sequencing gaps, and͞or hard stops, we found initial sequencing of 5.3ϫ Sanger data followed by the addition of two 454 runs to be the most cost-effective approach.

High-level neurons processing complex, behaviorally relevant signals are sensitive to conjunctions of features. Characterizing the receptive fields of such neurons is difficult with standard statistical tools, however, and the principles governing their organization remain poorly understood. Here, we demonstrate multiple distinct receptivefield features in individual high-level auditory neurons in a songbird, European starling, in response to natural vocal signals (songs). We then show that receptive fields with similar characteristics can be reproduced by an unsupervised neural network trained to represent starling songs with a single learning rule that enforces sparseness and divisive normalization. We conclude that central auditory neurons have composite receptive fields that can arise through a combination of sparseness and normalization in neural circuits. Our results, along with descriptions of random, discontinuous receptive fields in the central olfactory neurons in mammals and insects, suggest general principles of neural computation across sensory systems and animal classes. is an important question in sensory neuroscience. Dimensionality reduction involves extracting a hierarchy of features to obtain a selective and invariant (categorical) representation useful for behavior. To understand better the principles underlying this process in the central auditory system, we characterized receptive fields of neurons in the caudo-medial nidopallium (NCM) of the European starling (Sturnus vulgaris), a songbird with an acoustically rich vocal repertoire (1). The NCM, a secondary auditory cortexlike region in songbirds, receives convergent inputs from the primary thalamorecipient region, Field L, and other secondary auditory regions (2) and contains neurons selectively tuned to birdsong, a behaviorally relevant natural stimulus (3-5).We recorded action potentials extracellularly from individual well-isolated NCM neurons during the playback of starling songs and estimated the structure of the neurons' receptive fields using the Maximum Noise Entropy (MNE) method (6). Statistical inference methods in this class (7,8) maximize the noise entropy of the conditional response distribution to produce models that are constrained by a given set of stimulus-response correlations but that are otherwise as random, and therefore as unbiased, as possible. Unlike the spike-triggered covariance (STC) method (9), MNE works well with natural stimuli; in contrast to the maximally informative dimensions (MID) method (10), MNE can identify any number of relevant receptive-field features. Results Single NCM Neurons Respond to Multiple Distinct Features of StarlingSong. We recorded neuronal responses to six different 1-minutelong songs, each repeated 30 times. These songs were recorded from three male starlings, and together they contained over 200 motifs, brief segments of starling song that are perceived as distinct auditory objects (11). An NCM neuron usually responds to a variety of motifs (4, 12) (Fig. 1), and NCM neurons display ...

The hair cell's mechanoreceptive organelle, the hair bundle, is highly sensitive because its transduction channels open over a very narrow range of displacements. The synchronous gating of transduction channels also underlies the active hair-bundle motility that amplifies and tunes responsiveness. The extent to which the gating of independent transduction channels is coordinated depends on how tightly individual stereocilia are constrained to move as a unit. Using dual-beam interferometry in the bullfrog's sacculus, we found that thermal movements of stereocilia located as far apart as a bundle's opposite edges display high coherence and negligible phase lag. Because the mechanical degrees of freedom of stereocilia are strongly constrained, a force applied anywhere in the hair bundle deflects the structure as a unit. This feature assures the concerted gating of transduction channels that maximizes the sensitivity of mechanoelectrical transduction and enhances the hair bundle's capacity to amplify its inputs.The high sensitivity of sensory systems requires an efficient use of the energy in stimuli to bias the open probability of ion channels. For a hair cell of the inner ear, mechanical forces directly gate transduction channels atop stereocilia, the rod-like constituents of the mechanosensitive hair bundle 1 . A hair bundle's sensitivity is determined by the relation between the applied force and the number of channels opened: the narrower the force range over which gating occurs, the greater the sensitivity. The coordinated gating of transduction channels is also thought to underlie active hair-bundle motility, a component of the active process that amplifies and tunes the responses of hair cells 2 .Because mechanical stimuli are ordinarily applied at the tall edge of a hair bundle, channel gating depends upon the propagation of mechanical force across the array of stereocilia. Each stereocilium possesses a basal rootlet of actin filaments that tends to hold the process upright; as measured at the hair bundle's tip, the combined stiffness of these stereociliary pivots is about 200 μN·m −1 (ref.3). In addition, the successive stereocilia in each file are joined by tip links that are thought to represent the gating springs attached to transduction channels at one or both ends. For large bundle deflections and in the presence of a physiological concentration of Ca 2+ , the combined stiffness of these gating springs is typically 1000 μN·m −1 (ref. HHMI Author Manuscript HHMI Author Manuscript HHMI Author Manuscriptsprings-the phenomenon of gating compliance-to a value comparable to that of the pivots, or even lower 4,5 .The stereocilia of a hair bundle appear at first glance to be connected in a series-parallel configuration such that a force applied to the tallest stereocilium in each file would first deflect that process alone (Fig. 1a). Movement of the tallest stereocilium would then tighten the tip link and perhaps other filaments connecting it to the second, deflecting that process; the second stereoc...

The detection of sound begins when energy derived from an acoustic stimulus deflects the hair bundles atop hair cells1. As hair bundles move, the viscous friction between stereocilia and the surrounding liquid poses a fundamental physical challenge to the ear’fs high sensitivity and sharp frequency selectivity. Part of the solution to this problem lies in the active process that uses energy for frequency-selective sound amplification2,3. Here we demonstrate that a complementary part of the solution involves the fluid-structure interaction between stereocilia and the liquid within the hair bundle. Using force measurement on a dynamically scaled model, finite-element analysis, analytical estimation of hydrodynamic forces, stochastic simulation, and high-resolution interferometric measurement of hair bundles, we characterize the origin and magnitude of the forces between individual stereocilia during small hair-bundle deflections. We find that the close apposition of stereocilia effectively immobilizes the liquid between them, which reduces the drag and suppresses the relative squeezing but not the sliding mode of stereociliary motion. The obliquely oriented tip links couple the mechanotransduction channels to this least dissipative coherent mode, whereas the elastic horizontal top connectors that stabilize the structure further reduce the drag. As measured from the distortion products associated with channel gating at physiological stimulation amplitudes of tens of nanometres, the balance of viscous and elastic forces in a hair bundle permits a relative mode of motion between adjacent stereocilia that encompasses only a fraction of a nanometre. A combination of high-resolution experiments and detailed numerical modelling of fluid-structure interactions reveals the physical principles behind the basic structural features of hair bundles and shows quantitatively how these organelles are adapted to the needs of sensitive mechanotransduction.

Voltage-dependent activity around the resting potential is determinant in neuronal physiology and participates in the definition of the firing pattern. Low-voltage-activated T-type Ca2 + channels directly affect the membrane potential and control a number of secondary Ca2 + -dependent permeabilities. We have studied the ability of the cloned T-type channels (alpha1G,H,I) to carry Ca2 + currents in response to mock action potentials. The relationship between the spike duration and the current amplitude is specific for each of the T-type channels, reflecting their individual kinetic properties. Typically the charge transfer increases with spike broadening, but the total Ca2 + entry saturates at different spike durations according to the channel type: 4 ms for alpha1G; 7 ms for alpha1H; and > 10 ms for alpha1I channels. During bursts, currents are inhibited and/or transiently potentiated according to the alpha1 channel type, with larger effects at higher frequency. The inhibition may be induced by voltage-independent transitions toward inactivated states and/or channel inactivation through intermediate closed states. The potentiation is explained by an acceleration in the channel activation kinetics. Relatively fast inactivation and slow recovery limit the ability of alpha1G and alpha1H channels to respond to high frequency stimulation ( > 20 Hz). In contrast, the slow inactivation of alpha1I subunits allows these channels to continue participating in high frequency bursts (100 Hz). The biophysical properties of alpha1G, H and I channels will therefore dramatically modulate the effect of neuronal activities on Ca2 + signalling.

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The frequency sensitivity of auditory hair cells in the inner ear varies with their longitudinal position in the sensory epithelium. Among the factors that determine the differential cellular response to sound is the resonance of a hair cell's transmembrane electrical potential, whose frequency correlates with the kinetic properties of the high-conductance Ca 2؉ -activated K ؉ (BK) channels encoded by a Slo (kcnma1) gene. It has been proposed that the inclusion of specific alternative axons in the Slo transcripts along the cochlea underlies the gradient of BK-channel kinetics. By analyzing the complete sequences of chicken Slo gene (cSlo) cDNAs from the chicken's cochlea, we show that most transcripts lack alternative exons. Transcripts with more than one alternative exon constitute only 10% of the total. Although the fraction of transcripts containing alternative exons increases from the cochlear base to the apex, the combination of alternative exons is not regulated. There is also a clear increase in the expression of BK transcripts with long carboxyl termini toward the apex. When long and short BK transcripts are expressed in HEK-293 cells, the kinetics of single-channel currents differ only slightly, but they are substantially slowed when the channels are coexpressed with the auxiliary ␤ subunit that occurs more widely at the apex. These results argue that the tonotopic gradient is not established by the selective inclusion of highly specific cSlo exons. Instead, a gradient in the expression of ␤ subunits slows BK channels toward the low-frequency apex of the cochlea.The auditory system maps continuous sensory variables, such as the frequency of a sound or the spatial location of its source, onto cellular detector arrays whose individual elements are narrowly tuned. How sensory maps of any kind are established and maintained is an open question. In only a single case, the tonotopic frequency map in the inner ear, is the physiological mechanism responsible for the individual cell's differential tuning understood at the molecular level.In the process of electrical resonance, a combination of voltage-and ion-dependent conductances operates in conjunction with the passive electrical properties of a hair cell's membrane to accentuate cellular responsiveness to a particular range of frequencies while attenuating that to frequencies outside this range. A dynamic interaction between two types of ion channels mediates this band-pass behavior. Voltage-sensitive Ca 2ϩ channels activated by the current through mechanotransduction channels depolarize the hair cell's membrane and increase the intracellular concentration of Ca 2ϩ . With some delay, these ions activate the Ca 2ϩ -sensitive high-conductance Ca 2ϩ -activated K ϩ (BK) channels that repolarize the membrane, thereby closing the Ca 2ϩ channels, decreasing the intracellular concentration of Ca 2ϩ , and deactivating the K ϩ channels. How rapidly this dynamic system progresses through the cycle depends on several factors, most notably on the kinetic properties of th...