Synaptic depression in the localization of sound

Excitatory and feed-forward inhibitory hippocampal synapses work synergistically as an adaptive filter of natural spike trains

Short-term synaptic plasticity (STP) is an important mechanism for modifying neural circuits during computation. Although STP is much studied, its role in the processing of complex natural spike patterns is unknown. Here we analyze the responses of excitatory and inhibitory hippocampal synapses to natural spike trains at near-physiological temperatures. Our results show that excitatory and inhibitory synapses express complementary sets of STP components that selectively change synaptic strength during epochs of high-frequency discharge associated with hippocampal place fields. In both types of synapses, synaptic strength rapidly alternates between a near-constant level during low activity and another near-constant, but elevated (for excitatory synapses) or reduced (for inhibitory synapses) level during high-frequency epochs. These history-dependent changes in synaptic strength are largely independent of the particular temporal pattern within the discharges, and occur concomitantly in the two types of synapses. When excitatory and feed-forward inhibitory synapses are co-activated within the hippocampal feed-forward circuit unit, the net effect of their complementary STP is an additional increase in the gain of excitatory synapses during high-frequency discharges via selective disinhibition. Thus, excitatory and feed-forward inhibitory hippocampal synapses in vitro act synergistically as an adaptive filter that operates in a switch-like manner and is selective for high-frequency epochs.

Excitatory and inhibitory hippocampal synapses express complementary short term plasticity components that cooperate to amplify excitatory transmission in response to naturalistic stimulation.

Axonal site of spike initiation enhances auditory coincidence detection

Neurons initiate spikes in the axon initial segment or at the first node in the axon. However, it is not yet understood how the site of spike initiation affects neuronal activity and function. In nucleus laminaris of birds, neurons behave as coincidence detectors for sound source localization and encode interaural time differences (ITDs) separately at each characteristic frequency (CF). Here we show, in nucleus laminaris of the chick, that the site of spike initiation in the axon is arranged at a distance from the soma, so as to achieve the highest ITD sensitivity at each CF. Na+ channels were not found in the soma of high-CF (2.5-3.3 kHz) and middle-CF (1.0-2.5 kHz) neurons but were clustered within a short segment of the axon separated by 20-50 microm from the soma; in low-CF (0.4-1.0 kHz) neurons they were clustered in a longer stretch of the axon closer to the soma. Thus, neurons initiate spikes at a more remote site as the CF of neurons increases. Consequently, the somatic amplitudes of both orthodromic and antidromic spikes were small in high-CF and middle-CF neurons and were large in low-CF neurons. Computer simulation showed that the geometry of the initiation site was optimized to reduce the threshold of spike generation and to increase the ITD sensitivity at each CF. Especially in high-CF neurons, a distant localization of the spike initiation site improved the ITD sensitivity because of electrical isolation of the initiation site from the soma and dendrites, and because of reduction of Na+-channel inactivation by attenuating the temporal summation of synaptic potentials through the low-pass filtering along the axon.

Passive soma facilitates submillisecond coincidence detection in the owl's auditory system

Neurons of the avian nucleus laminaris (NL) compute the interaural time difference (ITD) by detecting coincident arrivals of binaural signals with submillisecond accuracy. The cellular mechanisms for this temporal precision have long been studied theoretically and experimentally. The myelinated axon initial segment in the owl's NL neuron and small somatic spikes observed in auditory coincidence detector neurons of various animals suggest that spikes in the NL neuron are generated at the first node of Ranvier and that the soma passively receives back-propagating spikes. To investigate the significance of the "passive soma" structure, we constructed a two-compartment NL neuron model, consisting of a cell body and a first node, and systematically changed the excitability of each compartment. Here, we show that a neuron with a less active soma achieves higher ITD sensitivity and higher noise tolerance with lower energy costs. We also investigate the biophysical mechanism of the computational advantage of the "passive soma" structure by performing sub- and suprathreshold analyses. Setting a spike initiation site with high sodium conductance, not in the large soma but in the small node, serves to amplify high-frequency input signals and to reduce the impact and the energy cost of spike generation. Our results indicate that the owl's NL neuron uses a "passive soma" design for computational and metabolic reasons.

MeCP2-dependent transcriptional repression regulates excitatory neurotransmission

Mutations in the transcriptional repressor, methyl-CpG binding protein 2 (MeCP2), result in a neurodevelopmental disorder called Rett Syndrome (RTT) . Based on the neurological phenotypes observed in Rett patients, we examined the potential role of MeCP2 in synaptic function. We compared elementary properties of synaptic transmission between cultured hippocampal neurons from MeCP2 knockout and wild-type littermate control mice and found a decrease in the frequency of spontaneous excitatory synaptic transmission (mEPSCs) in neurons lacking MeCP2. We also detected a significant increase in the rate of short-term synaptic depression. To explore whether these functional effects can be attributed to MeCP2's role as a transcriptional silencer, we treated cultures with a drug that impairs histone deacetylation and examined spontaneous synaptic transmission. Treatment with this compound induced a similar decrease in mEPSC frequency in wild-type control cultures, but this decrease was occluded in MeCP2-deficient neurons. Interestingly, neither the loss of MeCP2 nor the drug treatment resulted in changes in mIPSC properties. Finally, by means of a lentivirus expressing Cre recombinase, we show that loss of MeCP2 function after neurodevelopment and synaptogenesis was sufficient to mimic the decrease in mEPSC frequency seen in constitutive MeCP2 KO neurons. Taken together, these results suggest a role for MeCP2 in control of excitatory presynaptic function through regulation of gene expression.

Temporal measures and neural strategies for detection of tones in noise based on responses in anteroventral cochlear nucleus

To examine possible neural strategies for the detection of tones in broadband noise, single-neuron extracellular recordings were obtained from the anteroventral cochlear nucleus (AVCN) in anesthetized gerbils. Detection thresholds determined by average discharge rate and several temporal metrics were compared with previously reported psychophysical detection thresholds in cats (Costalupes 1985). Because of their limited dynamic range, the average discharge rates of single neurons failed to predict psychophysical detection thresholds for relatively high-level noise at all measured characteristic frequencies (CFs). However, temporal responses changed significantly when a tone was added to a noise, even for neurons with flat masked rate-level functions. Three specific temporal analyses were applied to neural responses to tones in noise. First, temporal reliability, a measure of discharge time consistency across stimulus repetitions, decreased with increasing tone level for most AVCN neurons at all measured CFs. Second, synchronization to the tone frequency, a measure of phase-locking to the tone, increased with tone level for low-CF neurons. Third, rapid fluctuations in the poststimulus time histograms (PSTHs) decreased with tone level for a number of neurons at all CFs. For each of the three temporal measures, some neurons had detection thresholds at or below psychophysical thresholds. A physiological model of a higher-stage auditory neuron that received simple excitatory and inhibitory inputs from AVCN neurons was able to extract the PSTH fluctuation information in a form of decreased rate with tone level.

Temperature-dependent shift of balance among the components of short-term plasticity in hippocampal synapses

Studies of short-term plasticity (STP) in the hippocampus, performed mostly at room temperature, have shown that small central synapses rapidly depress in response to high-frequency stimulation. This decrease in synaptic strength with synapse use places constraints on the use of STP as a dynamic filter for processing of natural high-frequency input. Here we report that, because of a strong but differential temperature dependence of STP components, the properties of STP in excitatory hippocampal synapses change dramatically with temperature. By separating the contributions of various STP processes during spike trains at different temperatures, we found a shift from dominating depression at 23 degrees C to prevailing facilitation and augmentation at 33-38 degrees C. This shift of balance among STP components resulted from a large increase in amplitudes of facilitation and augmentation (Q10 approximately 2.6 and approximately 5.1, respectively) and little change in the amplitude of depression (Q10 approximately 1.1) with temperature. These changes were accompanied by the accelerated decay of all three processes (Q10 = 3.2, 6.6, and 2.1, respectively). The balance of STP components achieved at higher temperatures greatly improved the maintenance of synaptic strength during prolonged synaptic use and had a strong effect on the processing of natural spike trains: a variable mixture of facilitated and depressed responses at 23 degrees C changed into a significantly more reproducible and depression-free filtering pattern at 33-38 degrees C. This filtering pattern was highly conserved among cells, slices, and animals, and under various physiological conditions, arguing for its physiological significance. Therefore, the fine balance among STP components, achieved only at near body temperatures, is required for the robust function of STP as a dynamic filter during natural stimulation.

Short-Term Plasticity at Primary Afferent Synapse in Rat Spinal Dorsal Horn and Its Biological Function

Short-term plasticity (STP) is an important element of information processing in neuronal networks. As the first synaptic relay between primary afferent fibers (PAFs) and central neurons, primary afferent synapses in spinal dorsal horn (DH) are essential to the initial processing of somatosensory information. In this research, we examined the STP between Adelta-PAFs and spinal DH neurons by patch-clamp recording. Our results showed that depression dominated the STP at primary afferent synapses. The curves of STP had no significant changes in the presence of bicuculline, CTZ or AP-5. Lowering extracellular Ca(2+) concentration ([Ca(2+)](o)) from 2.4 to 0.8 mM reduced the depression of synaptic responses at all stimulus rates, while raising [Ca(2+)](o) from 2.4 to 4.0 mM increased the synaptic depression. Increasing the bath temperature from 24 to 32 degrees C clearly reduced the depression of all responses. These results indicate that the observed STP is of presynaptic origin and depends on transmitter release. By fitting the experimental data recorded under different conditions, a model of STP was used to quantitatively characterize the observed STP and to analyze the possible mechanisms underlying the effects of [Ca(2+)](o) and temperature. Furthermore, using a model neuron receiving synaptic inputs, we found that with this form of STP, postsynaptic DH neurons could detect rate changes in both rapidly- and slowly-firing afferents with equal sensitivity. The present study links the intrinsic STP properties of primary afferent synapses with their role in processing neural information, and provides a basis for further research on the STP in spinal DH and its biological function under in vivo conditions.

Kv1.1-containing channels are critical for temporal precision during spike initiation

Low threshold, voltage-gated potassium currents (Ikl) are widely expressed in auditory neurons that can fire temporally precise action potentials (APs). In the medial nucleus of the trapezoid body (MNTB), channels containing the Kv1.1 subunit (encoded by the Kcna1 gene) underlie Ikl. Using pharmacology, genetics and whole cell patch-clamp recordings in mouse brain slices, we tested the role of Ikl in limiting AP latency-variability (jitter) in response to trains of single inputs at moderate to high stimulation rates. With dendrotoxin-K (DTX-K, a selective blocker of Kv1.1-containing channels), we blocked Ikl maximally (approximately 80% with 100 nM DTX-K) or partially (approximately 50% with 1-h incubation in 3 nM DTX-K). Ikl was similar in 3 nM DTX-K-treated cells and cells from Kcna1(-/-) mice, allowing a comparison of these two different methods of Ikl reduction. In response to current injection, Ikl reduction increased the temporal window for AP initiation and increased jitter in response to the smallest currents that were able to drive APs. While 100 nM DTX-K caused the largest increases, latency and jitter in Kcna1(-/-) cells and in 3 nM DTX-K-treated cells were similar to each other but increased compared with +/+. The near-phenocopy of the Kcna1(-/-) cells with 3 nM DTX-K shows that acute blockade of a subset of the Kv1.1-containing channels is functionally similar to the chronic elimination of all Kv1.1 subunits. During rapid stimulation (100-500 Hz), Ikl reduction increased jitter in response to both large and small inputs. These data show that Ikl is critical for maintaining AP temporal precision at physiologically relevant firing rates.

Spike-frequency adaptation separates transient communication signals from background oscillations

Spike-frequency adaptation is a prominent feature of many neurons. However, little is known about its computational role in processing behaviorally relevant natural stimuli beyond filtering out slow changes in stimulus intensity. Here, we present a more complex example in which we demonstrate how spike-frequency adaptation plays a key role in separating transient signals from slower oscillatory signals. We recorded in vivo from very rapidly adapting electroreceptor afferents of the weakly electric fish Apteronotus leptorhynchus. The firing-frequency response of electroreceptors to fast communication stimuli ("small chirps") is strongly enhanced compared with the response to slower oscillations ("beats") arising from interactions of same-sex conspecifics. We are able to accurately predict the electroreceptor afferent response to chirps and beats, using a recently proposed general model for spike-frequency adaptation. The parameters of the model are determined for each neuron individually from the responses to step stimuli. We conclude that the dynamics of the rapid spike-frequency adaptation is sufficient to explain the data. Analysis of additional data from step responses demonstrates that spike-frequency adaptation acts subtractively rather than divisively as expected from depressing synapses. Therefore, the adaptation dynamics is linear and creates a high-pass filter with a cutoff frequency of 23 Hz that separates fast signals from slower changes in input. A similar critical frequency is seen in behavioral data on the probability of a fish emitting chirps as a function of beat frequency. These results demonstrate how spike-frequency adaptation in general can facilitate extraction of signals of different time scales, specifically high-frequency signals embedded in slower oscillations.

Hyperpolarization-activated cyclic nucleotide-gated cation channels regulate auditory coincidence detection in nucleus laminaris of the chick

Coincidence detection of bilateral acoustic signals in nucleus laminaris (NL) is the first step in azimuthal sound source localization in birds. Here, we demonstrate graded expression of hyperpolarization-activated cyclic nucleotide-gated (HCN) cation channels along the tonotopic axis of NL and its role in the regulation of coincidence detection. Expression of HCN1 and HCN2, but not HCN3 or HCN4, was detected in NL. Based on measurement of both subtype mRNA and protein, HCN1 varied along the tonotopic axis and was minimal in high-characteristic frequency (CF) neurons. In contrast, HCN2 was evenly distributed. The resting conductance was larger and the steady-state activation curve of Ih was more positive in neurons of middle to low CF than those of high CF, consistent with the predominance of HCN1 channels in these neurons. Application of 8-Br-cAMP or noradrenaline generated a depolarizing shift of the Ih voltage activation curve. This shift was larger in neurons of high CF than in those of middle CF. The shift in the activation voltage of Ih depolarized the resting membrane, accelerated the EPSP time course, and significantly improved the coincidence detection in neurons of high CF, suggesting that Ih may improve the localization of sound sources.

Vesicle depletion and synaptic depression at a mammalian ribbon synapse

We estimated the size of the readily releasable pool (RRP) of vesicles at a ribbon synapse in the rat retina by making paired voltage-clamp recordings from presynaptic rod bipolar cells (RBCs) and postsynaptic AII amacrine cells in an in vitro retinal slice preparation. The RRP at each active zone was estimated to constitute seven vesicles, in the range of estimated RRP sizes at conventional synapses. During sustained presynaptic Ca(2+) entry, the RRP could be released with a time constant of about 4 ms. This ribbon synapse exhibited pronounced paired-pulse depression (PPD), which was attributable primarily to vesicle depletion. Recovery from PPD was slow (tau approximately 4 s) but could be accelerated by increasing the duration of the depressing stimulus. The small RRP and very high release probability likely contribute to the transient characteristics of neurotransmission at RBC synapses.

Neural plasticity of mushroom body-extrinsic neurons in the honeybee brain

Central interneurons exiting the alpha lobe of the mushroom bodies were studied with respect to their plasticity by electrically stimulating their presynaptic inputs, the Kenyon cells. Special attention was given to the analysis of a single, identified neuron, the PE1. Three stimulation protocols were tested: double pulses, tetanus (100 Hz for 1 s), and tetanus paired with intracellular de- or hyper-polarization of the recorded cell. Double-pulse stimulations revealed short-term facilitation and depression, tuning the responses of these interneurons to frequencies in the range of 20–40 Hz. The tetanus may lead to augmentation of responses to test stimuli lasting for several minutes, or to depression followed by augmentation. Associative long-term potentiation (LTP) was induced in the PE1 neuron by pairing a presynaptic tetanus with depolarization. This is the first time that associative LTP has been found in an interneuron of the insect nervous system. These data are discussed in the context of spike tuning in the output of the mushroom body, and the potential role of associative LTP in olfactory learning. It is concluded that the honeybee mushroom body output neurons are likely to contribute to the formation of olfactory memory.

Tonotopic specialization of auditory coincidence detection in nucleus laminaris of the chick

The interaural time difference (ITD) is a cue for localizing a sound source along the horizontal plane and is first determined in the nucleus laminaris (NL) in birds. Neurons in NL are tonotopically organized, such that ITDs are processed separately at each characteristic frequency (CF). Here, we investigated the excitability and coincidence detection of neurons along the tonotopic axis in NL, using a chick brainstem slice preparation. Systematic changes with CF were observed in morphological and electrophysiological properties of NL neurons. These properties included the length of dendrites, the input capacitance, the conductance of hyperpolarization-activated current, and the EPSC time course. In contrast to these gradients, the conductance of low-threshold K+ current and the expression of Kv1.2 channel protein were maximal in the central (middle-CF) region of NL. As a result, the middle-CF neuron had the smallest input resistance and membrane time constant, and consequently the fastest EPSP, and exhibited the most accurate coincidence detection. The specialization of middle-CF neurons as coincidence detectors may account for the high resolution of sound-source localization in the middle-frequency range observed in avians.

Target-Specific Short-Term Dynamics Are Important for the Function of Synapses in an Oscillatory Neural Network

Short-term dynamics such as facilitation and depression are present in most synapses and are often target-specific even for synapses from the same type of neuron. We examine the dynamics and possible functions of two synapses from the same presynaptic neuron in the rhythmically active pyloric network of the spiny lobster. Using simultaneous recordings, we show that the synapses from the lateral pyloric (LP) neuron to the pyloric dilator (PD; a member of the pyloric pacemaker ensemble) and the pyloric constrictor (PY) neurons both show short-term depression. However, the postsynaptic potentials produced by the LP-to-PD synapse are larger in amplitude, depress less, and recover faster than those produced by the LP-to-PY synapse. The main function of the LP-to-PD synapse is to slow down the pyloric rhythm. However, in some cases, it slows down the rhythm only when it is fast and has no effect or to speeds up when it is slow. In contrast, the LP-to-PY synapse functions to delay the activity of the PY neuron; this delay increases as the cycle period becomes longer. Using a computational model, we show that the short-term dynamics of synaptic depression observed for each of these synapses are tailored to their individual functions and that replacing the dynamics of either synapse with the other would disrupt these functions. Together, the experimental and modeling results suggest that the target-specific features of short-term synaptic depression are functionally important for synapses efferent from the same presynaptic neuron.

Contrasting short-term plasticity at two sides of the mitral-granule reciprocal synapse in the mammalian olfactory bulb

The mitral-granule reciprocal synapse shapes the response of the olfactory bulb to odour stimuli by mediating lateral and reciprocal inhibition. We investigated the short-term plasticity of both the mitral-to-granule excitatory synapse and the granule-to-mitral inhibitory synapse in rat olfactory bulb slices, using whole-cell patch clamp recordings. The granule-to-mitral inhibitory synapse invariably exhibited paired-pulse depression at interstimulus intervals of less than a second, while the mitral-to-granule excitatory synapse showed heterogeneous responses, which on average yielded a moderate facilitation. Trains of stimuli led to a much greater depression at the granule-to-mitral synapse than at the mitral-to-granule synapse. Since mitral cells commonly respond to odours by burst firing with each inhalation cycle, we used bursts of stimuli to study recovery from depression. We found that recovery from depression induced by fast trains of stimuli was more rapid at the mitral-to-granule synapse than at the granule-to-mitral synapse. In addition, depression was enhanced by higher calcium concentrations, suggesting at least partial contribution of presynaptic mechanisms to short-term depression. The observed short-term plasticity could enable mitral cells to overcome autoinhibition and increase action potential propagation along lateral dendrites by burst firing.

Target-specific regulation of synaptic amplitudes in the neocortex

In layers 2/3 in the rat visual cortex, glutamatergic synapses, between pyramidal neurons and GABAergic interneurons, show target-specific depression or facilitation. To study the mechanisms regulating these short-term synaptic modifications, we recorded from synaptically connected pyramidal neurons (presynaptic) and multipolar or bitufted interneurons (postsynaptic). Evoked AMPA receptor (AMPAR)- or NMDA receptor (NMDAR)-mediated EPSCs were pharmacologically isolated at these pyramidal-to-interneuron synapses while altering release probability (P(r)) by changing the extracellular Ca2+ concentration ([Ca2+]o). At the pyramidal-to-multipolar synapse, which shows paired-pulse depression, elevation of [Ca2+]o from physiological concentrations (2 mm) to 3 mm increased the amplitude of the initial AMPAR-mediated EPSC and enhanced paired-pulse depression. In contrast, the initial NMDAR-mediated EPSC did not change in amplitude with raised P(r) nor was paired-pulse depression altered. This lack of an increase of NMDAR-mediated currents is not a result of Ca2+-dependent effects on the NMDAR. Rather, at the pyramidal-to-multipolar synapse, raised P(r) increases the transient glutamate concentration at individual release sites, possibly reflecting multivesicular release. In contrast, at the pyramidal-to-bitufted synapse, which shows facilitation, AMPAR- and NMDAR-meditated EPSCs showed parallel increases in response to raised P(r). Thus, our results reveal differential recruitment of AMPA and NMDARs at depressing and facilitating synapses in layers 2/3 of the cortex and suggest that the mechanisms regulating dynamic aspects of synaptic transmission are target specific.

Dynamic synapses as archives of synaptic history: state-dependent redistribution of synaptic efficacy in the rat hippocampal CA1

Plastic modifications of synaptic strength are putative mechanisms underlying information processing in the brain, including memory storage, signal integration and filtering. Here we describe a dynamic interplay between short-term and long-term synaptic plasticity. At rat hippocampal CA1 synapses, induction of both long-term potentiation (LTP) and depression (LTD) was accompanied by changes in the profile of short-term plasticity, termed redistribution of synaptic efficacy (RSE). RSE was presynaptically expressed and associated in part with a persistent alteration in hyperpolarization-activated I(h) channel activity. Already potentiated synapses were still capable of showing RSE in response to additional LTP-triggering stimulation. Strikingly, RSE took place even after reversal of LTP or LTD, that is, the same synapse can display different levels of short-term plasticity without changing synaptic efficacy for the initial spike in burst presynaptic firing, thereby modulating spike transmission in a firing rate-dependent manner. Thus, the history of long-term synaptic plasticity is registered in the form of short-term plasticity, and RSE extends the information storage capacity of a synapse and adds another dimension of functional complexity to neuronal operations.

Synaptic depression as a timing device

A depressing synapse transforms a time interval into a voltage amplitude. The effect of that transformation on the output of the neuron and network depends on the kinetics of synaptic depression and properties of the postsynaptic neuron and network. Using as examples neural circuits that incorporate depressing synapses, we show how short-term depression can contribute to a surprising variety of time-dependent computational and behavioral tasks.

Target Cell-Dependent Normalization of Transmitter Release at Neocortical Synapses

The efficacy and short-term modification of neocortical synaptic connections vary with the type of target neuron. We investigated presynaptic Ca2+ and release probability at single synaptic contacts between pairs of neurons in layer 2/3 of the rat neocortex. The amplitude of Ca2+ signals in boutons of pyramids contacting bitufted or multipolar interneurons or other pyramids was dependent on the target cell type. Optical quantal analysis at single synaptic contacts suggested that release probabilities are also target cell-specific. Both the Ca2+ signal and the release probability of different boutons of a pyramid contacting the same target cell varied little. We propose that the mechanisms that regulate the functional properties of boutons of a pyramid normalize the presynaptic Ca2+ influx and release probability for all those boutons that innervate the same target cell.

Unmasking group III metabotropic glutamate autoreceptor function at excitatory synapses in the rat CNS

Presynaptic group III metabotropic glutamate receptor (mGluR) activation by exogenous agonists (such as L-2-amino-4-phosphonobutyrate (L-AP4)) potently inhibit transmitter release, but their autoreceptor function has been questioned because endogenous activation during high-frequency stimulation appears to have little impact on synaptic amplitude. We resolve this ambiguity by studying endogenous activation of mGluRs during trains of high-frequency synaptic stimuli at the calyx of Held. In vitro whole-cell patch recordings were made from medial nucleus of the trapezoid body (MNTB) neurones during 1 s excitatory postsynaptic current (EPSC) trains delivered at 200 Hz and at 37 degrees C. The group III mGluR antagonist (R,S)-cyclopropyl-4-phosphonophenylglycine (CPPG, 300 microm) had no effect on EPSC short-term depression, but accelerated subsequent recovery time course (tau: 4.6 +/- 0.8 s to 2.4 +/- 0.4 s, P = 0.02), and decreased paired pulse ratio from 1.18 +/- 0.06 to 0.97 +/- 0.03 (P = 0.01), indicating that mGluR activation reduced release probability (P). Modelling autoreceptor activation during repetitive stimulation revealed that as P declines, the readily releasable pool size (N) increases so that the net EPSC (NP) is unchanged and short-term depression proceeds with the same overall time course as in the absence of autoreceptor activation. Thus, autoreceptor action on the synaptic response is masked but the synapse is now in a different state (lower P, higher N). While vesicle replenishment clearly underlies much of the recovery from short-term depression, our results show that the recovery time course of P also contributes to the reduced response amplitude for 1-2 s. The results show that passive equilibration between N and P masks autoreceptor modulation of the EPSC and suggests that mGluR autoreceptors function to change the synaptic state and distribute metabolic demand, rather than to depress synaptic amplitude.

Coding interaural time differences at low best frequencies in the barn owl

In birds and mammals, precisely timed spikes encode the timing of acoustic stimuli, and interaural acoustic disparities propagate to binaural processing centers. The Jeffress model proposes that these projections act as delay lines to innervate an array of coincidence detectors, every element of which has a different relative delay between its ipsilateral and contralateral excitatory inputs. Thus, interaural time difference (ITD) is encoded into the position of the coincidence detector whose delay lines best cancel out the acoustic ITD. Neurons of the avian nucleus laminaris and mammalian MSO phase-lock to both monaural and binaural stimuli but respond maximally when phase-locked spikes from each side arrive simultaneously, i.e. when the difference in the conduction delays compensates for the ITD. McAlpine et al. [Nat. Neurosci. 4 (2001) 396] identified an apparent difference between avian and mammalian ITD coding. In the barn owl, the maximum firing rate appears to encode ITD. This may not be the case for the guinea pig, where the steepest region of the function relating discharge rate to interaural time delay (ITD) is close to midline for all neurons, irrespective of best frequency (BF). These data suggest that low BF ITD sensitivity in the guinea pig is mediated by detection of a change in slope of the ITD function, and not by maximum rate. We review coding of low best frequency ITDs in barn owls and mammals and discuss whether there may be differences in the code used to signal ITD in mammals and birds.

Synaptic computation

Neurons are often considered to be the computational engines of the brain, with synapses acting solely as conveyers of information. But the diverse types of synaptic plasticity and the range of timescales over which they operate suggest that synapses have a more active role in information processing. Long-term changes in the transmission properties of synapses provide a physiological substrate for learning and memory, whereas short-term changes support a variety of computations. By expressing several forms of synaptic plasticity, a single neuron can convey an array of different signals to the neural circuit in which it operates.

Calmodulin and Munc13 Form a Ca2+ Sensor/Effector Complex that Controls Short-Term Synaptic Plasticity

The efficacy of synaptic transmission between neurons can be altered transiently during neuronal network activity. This phenomenon of short-term plasticity is a key determinant of network properties; is involved in many physiological processes such as motor control, sound localization, or sensory adaptation; and is critically dependent on cytosolic [Ca2+]. However, the underlying molecular mechanisms and the identity of the Ca2+ sensor/effector complexes involved are unclear. We now identify a conserved calmodulin binding site in UNC-13/Munc13s, which are essential regulators of synaptic vesicle priming and synaptic efficacy. Ca2+ sensor/effector complexes consisting of calmodulin and Munc13s regulate synaptic vesicle priming and synaptic efficacy in response to a residual [Ca2+] signal and thus shape short-term plasticity characteristics during periods of sustained synaptic activity.

Dynamic influences on coincidence detection in neocortical pyramidal neurons

The firing rate of neocortical pyramidal neurons is believed to represent primarily the average arrival rate of synaptic inputs; however, it has also been found to vary somewhat depending on the degree of synchrony among synaptic inputs. We investigated the ability of pyramidal neurons to perform coincidence detection, that is, to represent input timing in their firing rate, and explored some factors that influence that representation. We injected computer-generated simulated synaptic inputs into pyramidal neurons during whole-cell recordings, systematically altering the phase delay between two groups of periodic simulated input events. We explored how input intensity, the synaptic time course, inhibitory synaptic conductance, and input jitter influenced the firing rate representation of input timing. In agreement with computer modeling studies, we found that input synchronization increases firing rate when intensity is low but reduces firing rate when intensity is high. At high intensity, the effect of synchrony on firing rate could be switched from reducing to increasing firing rate by shortening the simulated excitatory synaptic time course, adding inhibition (using the dynamic clamp technique), or introducing a small input jitter. These opposite effects of synchrony may serve different computational functions: as a means of increasing firing rate it may be useful for efficient recruitment or for computing a continuous parameter, whereas as a means of decreasing firing rate it may provide gain control, which would allow redundant or excessive input to be ignored. Modulation of dynamic input properties may allow neurons to perform different operations depending on the task at hand.

Structural contributions to short-term synaptic plasticity

Synaptic ultrastructure is critical to many basic hypotheses about synaptic transmission. Various aspects of synaptic ultrastructure have also been implicated in the mechanisms of short-term plasticity. These forms of plasticity can greatly affect synaptic strength during ongoing activity. We review the evidence for how synaptic ultrastructure may contribute to facilitation, depletion, saturation, and desensitization.

Synaptic dynamics mediate sensitivity to motion independent of stimulus details

Humans and other animals generally perceive motion independently of the cues that define the moving object. To understand the underlying mechanisms of this generalization of stimulus attributes, we have examined the cellular properties of avian wide-field tectal neurons that are sensitive to a variety of moving stimuli but not to static stationary stimuli. This in vitro study showed phasic signal transfer at the retinotectal synapse and binary dendritic responses to synaptic inputs that interact in a mutually exclusive manner in the postsynaptic tectal neuron. A model of the tectal circuitry predicts that these two cellular properties mediate sensitivity to a wide range of dynamic spatiotemporal stimuli, including moving stimuli, but not to static stationary stimuli in a tectal neuron. The computation that is independent of stimulus detail is initiated by tectal neurons and is completed by rotundal neurons that integrate outputs from multiple tectal neurons in a directionally selective manner.

Modulation of cochlear afferent response by the lateral olivocochlear system: activation via electrical stimulation of the inferior colliculus

The olivocochlear (OC) efferent innervation of the mammalian inner ear consists of two subdivisions, medial (MOC) and lateral (LOC), with different peripheral terminations on outer hair cells and cochlear afferent terminals, respectively. The cochlear effects of electrically activating MOC efferents are well known, i.e., response suppression effected by reducing outer hair cells' contribution to cochlear amplification. LOC peripheral effects are unknown, because their unmyelinated axons are difficult to electrically stimulate. Here, stimulating electrodes are placed in the inferior colliculus (IC) to indirectly activate the LOC system, while recording cochlear responses bilaterally from anesthetized guinea pigs. Shocks at some IC sites produced novel cochlear effects attributable to activation of the LOC system: long-lasting (5-20 min) enhancement or suppression of cochlear neural responses (compound action potentials and round window noise), without changes in cochlear responses dominated by outer hair cells (otoacoustic emissions and cochlear microphonics). These novel effects also differed from classic MOC effects in their lack of dependence on the level and frequency of the acoustic stimulus. These effects disappeared on sectioning the entire OC bundle, but not after selective lesioning of the MOC tracts or the cochlea's autonomic innervation. We conclude that the LOC pathway comprises two functional subdivisions, capable of inducing slow increases or decreases in response magnitudes in the auditory nerve. Such a system may be useful in maintaining accurate binaural comparisons necessary for sound localization in the face of slow changes in interaural sensitivity.

Two distinct types of corticothalamic EPSPs and their contribution to short-term synaptic plasticity

The lateral posterior nucleus (LPN) is innervated by two different morphological types of cortical terminals that originate from cortical layers V and VI. Here we describe two distinct types of excitatory postsynaptic potentials (EPSPs) that were recorded in the LPN after stimulation of corticothalamic fibers. These types of EPSPs differed in amplitude, latency, rise time, and response to increasing levels of stimulus intensity. The most frequently encountered EPSP, type I, displayed a longer latency and slower rise time than the less frequently encountered type II EPSP. Type I EPSPs also showed a graded increase in amplitude with increasing levels of stimulation, whereas type II EPSPs showed an all-or-none response. In response to repetitive stimulation (0.5-20 Hz), type I EPSPs displayed frequency-dependent facilitation, whereas type II EPSPs displayed frequency-dependent depression. Further details of these distinct forms of short-term synaptic plasticity were explored using paired-pulse stimuli. Pharmacology experiments revealed that both N-methyl-d-aspartate (NMDA) and non-NMDA glutamate receptors are involved in corticothalamic synaptic transmission in the LPN and contribute to both synaptic facilitation and depression. Taken together with the results of our previous anatomical studies, these results suggest that type I EPSPs arise from stimulation of layer VI afferents, whereas type II EPSPs arise from stimulation of layer V inputs. Moreover, type I and II EPSPs in the LPN may be functionally similar to corticogeniculate and retinogeniculate EPSPs, respectively.

Topography of interaural temporal disparity coding in projections of medial superior olive to inferior colliculus

Neurons in the medial superior olive encode interaural temporal disparity, and their receptive fields indicate the location of a sound source in the azimuthal plane. It is often assumed that the projections of these neurons transmit the receptive field information about azimuth from point to point, much like the projections of the retina to the brain transmit the position of a visual stimulus. Yet this assumption has never been verified. Here, we use physiological and anatomical methods to examine the projections of the medial superior olive to the inferior colliculus for evidence of a spatial topography that would support transmission of azimuthal receptive fields. The results show that this projection does not follow a simple point-to-point topographical map of receptive field location. Thus, the representation of sound location along the azimuth in the inferior colliculus most likely relies on a complex, nonlinear map.