AMPA and NMDA receptors regulate responses of neurons in the rat's inferior colliculus

GluN2C/D-containing NMDA receptors enhance temporal summation and increase sound-evoked and spontaneous firing in the inferior colliculus

Along the ascending auditory pathway, there is a broad shift from temporal coding, which is common in the lower auditory brainstem, to rate coding, which predominates in auditory cortex. This temporal-to-rate transition is particularly prominent in the inferior colliculus (IC), the midbrain hub of the auditory system, but the mechanisms that govern how individual IC neurons integrate information across time remain largely unknown. Here, we report the widespread expression of Glun2c and Glun2d mRNA in IC neurons. GluN2C/D-containing NMDA receptors are relatively insensitive to voltage-dependent Mg2+ blockade, and thus can conduct current at resting membrane potential. Using in situ hybridization and pharmacology, we show that vasoactive intestinal peptide neurons in the IC express GluN2D-containing NMDA receptors that are activatable by commissural inputs from the contralateral IC. In addition, GluN2C/D-containing receptors have much slower kinetics than other NMDA receptors, and we found that GluN2D-containing receptors facilitate temporal summation of synaptic inputs in vasoactive intestinal peptide neurons. In a model neuron, we show that a GluN2C/D-like conductance interacts with the passive membrane properties of the neuron to alter temporal and rate coding of stimulus trains. Consistent with this, we show in vivo that blocking GluN2C/D-containing receptors decreases both the spontaneous firing rate and the overall firing rate elicited by amplitude-modulated sounds in many IC neurons. These results suggest that GluN2C/D-containing NMDA receptors influence rate coding for auditory stimuli in the IC by facilitating the temporal integration of synaptic inputs. KEY POINTS: NMDA receptors are critical components of most glutamatergic circuits in the brain, and the diversity of NMDA receptor subtypes yields receptors with a variety of functions. We found that many neurons in the auditory midbrain express GluN2C and/or GluN2D NMDA receptor subunits, which are less sensitive to Mg2+ blockade than the more commonly expressed GluN2A/B subunits. We show that GluN2C/D-containing receptors conducted current at resting membrane potential and enhanced temporal summation of synaptic inputs. In a model, we show that GluN2C/D-containing receptors provide additive gain for input-output functions driven by trains of synaptic inputs. In line with this, we found that blocking GluN2C/D-containing NMDA receptors in vivo decreased both spontaneous firing rates and firing evoked by amplitude-modulated sounds.

GluN2C/D-containing NMDA receptors enhance temporal summation and increase sound-evoked and spontaneous firing in the inferior colliculus

Along the ascending auditory pathway, there is a broad shift from temporal coding, which is common in the lower auditory brainstem, to rate coding, which predominates in auditory cortex. This temporal-to-rate transition is particularly prominent in the inferior colliculus (IC), the midbrain hub of the auditory system, but the mechanisms that govern how individual IC neurons integrate information across time remain largely unknown. Here, we report the widespread expression of Glun2c and Glun2d mRNA in IC neurons. GluN2C/D-containing NMDA receptors are relatively insensitive to voltage-dependent Mg2+ block, and thus can conduct current at resting membrane potential. Using in situ hybridization and pharmacology, we show that VIP neurons in the IC express GluN2D-containing NMDA receptors that are activatable by commissural inputs from the contralateral IC. In addition, GluN2C/D-containing receptors have much slower kinetics than other NMDA receptors, and we found that GluN2D-containing receptors facilitate temporal summation of synaptic inputs in VIP neurons. In a model neuron, we show that a GluN2C/D-like conductance interacts with the passive membrane properties of the neuron to alter temporal and rate coding of stimulus trains. Consistent with this, we show in vivo that blocking GluN2C/D-containing receptors decreases both the spontaneous firing rate and the overall firing rate elicited by amplitude-modulated (AM) sounds in many IC neurons. These results suggest that GluN2C/D-containing NMDA receptors influence rate coding for auditory stimuli in the IC by facilitating the temporal integration of synaptic inputs.

Time-dependent effects of acoustic trauma and tinnitus on extracellular levels of amino acids in the inferior colliculus of rats

Chronic tinnitus is a debilitating condition with very few management options. Acoustic trauma that causes tinnitus has been shown to induce neuronal hyperactivity in multiple brain areas in the auditory pathway, including the inferior colliculus. This neuronal hyperactivity could be attributed to an imbalance between excitatory and inhibitory neurotransmission. However, it is not clear how the levels of neurotransmitters, especially neurotransmitters in the extracellular space, change over time following acoustic trauma and the development of tinnitus. In the present study, a range of amino acids were measured in the inferior colliculus of rats during acoustic trauma as well as at 1 week and 5 months post-trauma using in vivo microdialysis and high-performance liquid chromatography. Amino acid levels in response to sound stimulation were also measured at 1 week and 5 months post-trauma. It was found that unilateral exposure to a 16 kHz pure tone at 115 dB SPL for 1 h caused immediate hearing loss in all the animals and chronic tinnitus in 58 % of the animals. Comparing to the sham condition, extracellular levels of GABA were significantly increased at both the acute and 1 week time points after acoustic trauma. However, there was no significant difference in any of the amino acid levels measured between sham, tinnitus positive and tinnitus negative animals at 5 months post-trauma. There was also no clear pattern in the relationship between neurochemical changes and sound frequency/acoustic trauma/tinnitus status, which might be due to the relatively poorer temporal resolution of the microdialysis compared to electrophysiological responses.

Propagation of temporal and rate signals in cultured multilayer networks

Analyses of idealized feedforward networks suggest that several conditions have to be satisfied in order for activity to propagate faithfully across layers. Verifying these concepts experimentally has been difficult owing to the vast number of variables that must be controlled. Here, we cultured cortical neurons in a chamber with sequentially connected compartments, optogenetically stimulated individual neurons in the first layer with high spatiotemporal resolution, and then monitored the subthreshold and suprathreshold potentials in subsequent layers. Brief stimuli delivered to the first layer evoked a short-latency transient response followed by sustained activity. Rate signals, carried by the sustained component, propagated reliably through 4 layers, unlike idealized feedforward networks, which tended strongly towards synchrony. Moreover, temporal jitter in the stimulus was transformed into a rate code and transmitted to the last layer. This novel mode of propagation occurred in the balanced excitatory-inhibitory regime and is mediated by NMDA-mediated receptors and recurrent activity.

Puncta of Neuronal Nitric Oxide Synthase (nNOS) Mediate NMDA Receptor Signaling in the Auditory Midbrain

Nitric oxide (NO) is a neurotransmitter synthesized in the brain by neuronal nitric oxide synthase (nNOS). Using immunohistochemistry and confocal imaging in the inferior colliculus (IC, auditory midbrain) of the guinea pig (Cavia porcellus, male and female), we show that nNOS occurs in two distinct cellular distributions. We confirm that, in the cortices of the IC, a subset of neurons show cytoplasmic labeling for nNOS, whereas in the central nucleus (ICc), such neurons are not present. However, we demonstrate that all neurons in the ICc do in fact express nNOS in the form of discrete puncta found at the cell membrane. Our multi-labeling studies reveal that nNOS puncta form multiprotein complexes with NMDA receptors, soluble guanylyl cyclase (sGC), and PSD95. These complexes are found apposed to glutamatergic terminals, which is indicative of synaptic function. Interestingly, these glutamatergic terminals express both vesicular glutamate transporters 1 and 2 denoting a specific source of brainstem inputs. With in vivo electrophysiological recordings of multiunit activity in the ICc, we found that local application of NMDA enhances sound-driven activity in a concentration-dependent and reversible fashion. This response is abolished by blockade of nNOS or sGC, indicating that the NMDA effect is mediated solely via the NO and cGMP signaling pathway. This discovery of a ubiquitous, but highly localized, expression of nNOS throughout the ICc and demonstration of the dramatic influence of the NMDA activated NO pathway on sound-driven neuronal activity imply a key role for NO signaling in auditory processing.SIGNIFICANCE STATEMENT We show that neuronal nitric oxide synthase (nNOS), the enzyme that synthesizes nitric oxide (NO), occurs as puncta in apparently all neurons in the central nucleus of the inferior colliculus (ICc) in the auditory midbrain. Punctate nNOS appears at glutamatergic synapses in a complex with glutamate NMDA receptors (NMDA-Rs), soluble guanylyl cyclase (sGC, the NO receptor), and PSD95 (a protein that anchors receptors and enzymes at the postsynaptic density). We show that NMDA-R modulation of sound-driven activity in the ICc is solely mediated by activation of nNOS and sGC. The presence of nNOS throughout this sensory nucleus argues for a major role of NO in hearing. Furthermore, this punctate form of nNOS expression may exist and have gone unnoticed in other brain regions.

Electrogenic N-methyl-D-aspartate receptor signaling enhances binaural responses in the adult brainstem

In sensory systems, the neuronal representation of external stimuli is enhanced along the sensory pathway. In the auditory system, strong enhancement of binaural information takes place between the brainstem and the midbrain; however, the underlying cellular mechanisms are unknown. Here we investigated the transformation of binaural information in the dorsal nucleus of the lateral lemniscus (DNLL), a nucleus that connects the binaural nuclei in the brainstem and the inferior colliculus in the midbrain. We used in vitro and in vivo electrophysiology in adult Mongolian gerbils to show that N-methyl-D-aspartate receptor (NMDARs) play a critical role in neuronal encoding of stimulus properties in the DNLL. While NMDARs increase firing rates, the timing and the accuracy of the neuronal responses remain unchanged. NMDAR-mediated excitation increases the information about the acoustic stimulus. Taken together, our results show that NMDARs in the DNLL enhance the auditory information content in adult mammal brainstem.

Synaptic and circuit mechanisms promoting broadband transmission of olfactory stimulus dynamics

Sensory stimuli fluctuate on many timescales. However, short-term plasticity causes synapses to act as temporal filters, limiting the range of frequencies that they can transmit. How synapses in vivo might transmit a range of frequencies in spite of short-term plasticity is poorly understood. The first synapse in the Drosophila olfactory system exhibits short-term depression, but can transmit broadband signals. Here we describe two mechanisms that broaden the frequency characteristics of this synapse. First, two distinct excitatory postsynaptic currents transmit signals on different timescales. Second, presynaptic inhibition dynamically updates synaptic properties to promote accurate transmission of signals across a wide range of frequencies. Inhibition is transient, but grows slowly, and simulations reveal that these two features of inhibition promote broadband synaptic transmission. Dynamic inhibition is often thought to restrict the temporal patterns that a neuron responds to, but our results illustrate a different idea: inhibition can expand the bandwidth of neural coding.

Asymmetric temporal interactions of sound-evoked excitatory and inhibitory inputs in the mouse auditory midbrain

In the auditory midbrain, synaptic mechanisms responsible for the precise temporal coding of inputs in the brainstem are absent. Instead, in the inferior colliculus (IC), the diverse temporal firing patterns must be coded by other synaptic mechanisms, about which little is known. Here, we demonstrate the temporal characteristics of sound-evoked excitatory and inhibitory postsynaptic currents (seEPSCs and seIPSCs, respectively) in vivo in response to long-duration tones. The seEPSCs and seIPSCs differ in the variability of their temporal properties. The seEPSCs have either early or late current peaks, and the early-peaked currents may be either transient or sustained varieties. The seIPSCs have only early-peaked sustained responses but often have offset responses. When measured in a single neuron, the seIPSC peaks usually follow early, transient seEPSCs, but the seIPSCs precede latest-peaking seEPSCs. A model of the firing produced by the integration of asymmetric seEPSCs and seIPSCs showed that the temporal pattern of the early-peaked sustained neurons was easily modified by changing the parameters of the seIPSC. These results suggest that the considerable variability in the peak time and duration of the seEPSCs shapes the overall time course of firing and often precedes or follows the less variable seIPSC. Despite this, the inhibitory currents are potent in modifying the firing patterns, and the inhibitory response to sound offset appears to be one area where the integration of excitatory and inhibitory synaptic currents is lacking. Thus, the integration of sound-evoked activity in the IC often employs the asymmetric temporal interaction of excitatory and inhibitory synaptic currents to shape the firing pattern of the neuron.

Temporally advanced dynamic change of receptive field of lateral geniculate neurons during brief visual stimulation: Effects of brainstem peribrachial stimulation

Processing of visual information in the brain seems to proceed from initial fast but coarse to subsequent detailed processing. Such coarse-to-fine changes appear also in the response of single neurons in the visual pathway. In the dorsal lateral geniculate nucleus (dLGN), there is a dynamic change in the receptive field (RF) properties of neurons during visual stimulation. During a stimulus flash centered on the RF, the width of the RF-center, presumably related to spatial resolution, changes rapidly from large to small in an initial transient response component. In a subsequent sustained component, the RF-center width is rather stable apart from an initial slight widening. Several brainstem nuclei modulate the geniculocortical transmission in a state-dependent manner. Thus, modulatory input from cholinergic neurons in the peribrachial brainstem region (PBR) enhances the geniculocortical transmission during arousal. We studied whether such input also influences the dynamic RF-changes during visual stimulation. We compared dynamic changes of RF-center width of dLGN neurons during brief stimulus presentation in a control condition, with changes during combined presentation of the visual stimulus and electrical PBR-stimulation. The major finding was that PBR-stimulation gave an advancement of the dynamic change of the RF-center width such that the different response components occurred earlier. Consistent with previous studies, we also found that PBR-stimulation increased the gain of firing rate during the sustained response component. However, this increase of gain was particularly strong in the transition from the transient to the sustained component at the time when the center width was minimal. The results suggest that increased modulatory PBR-input not only increase the gain of the geniculocortical transmission, but also contributes to faster dynamics of transmission. We discuss implications for possible effects on visual spatial resolution.

Duration tuning across vertebrates

Signal duration is important for identifying sound sources and determining signal meaning. Duration-tuned neurons (DTNs) respond preferentially to a range of stimulus durations and maximally to a best duration (BD). Duration-tuned neurons are found in the auditory midbrain of many vertebrates, although studied most extensively in bats. Studies of DTNs across vertebrates have identified cells with BDs and temporal response bandwidths that mirror the range of species-specific vocalizations. Neural tuning to stimulus duration appears to be universal among hearing vertebrates. Herein, we test the hypothesis that neural mechanisms underlying duration selectivity may be similar across vertebrates. We instantiated theoretical mechanisms of duration tuning in computational models to systematically explore the roles of excitatory and inhibitory receptor strengths, input latencies, and membrane time constant on duration tuning response profiles. We demonstrate that models of duration tuning with similar neural circuitry can be tuned with species-specific parameters to reproduce the responses of in vivo DTNs from the auditory midbrain. To relate and validate model output to in vivo responses, we collected electrophysiological data from the inferior colliculus of the awake big brown bat, Eptesicus fuscus, and present similar in vivo data from the published literature on DTNs in rats, mice, and frogs. Our results support the hypothesis that neural mechanisms of duration tuning may be shared across vertebrates despite species-specific differences in duration selectivity. Finally, we discuss how the underlying mechanisms of duration selectivity relate to other auditory feature detectors arising from the interaction of neural excitation and inhibition.

Effects of ketamine on response properties of neurons in the superior paraolivary nucleus of the mouse

The superior paraolivary nucleus (SPON; alternative abbreviation: SPN for the same nucleus in certain species) is a prominent brainstem structure that provides strong inhibitory input to the auditory midbrain. Previous studies established that SPON neurons encode temporal sound features with high precision. These earlier characterizations of SPON responses were recorded under the influence of ketamine, a dissociative anesthetic agent and known antagonist of N-methyl-d-aspartate glutamate (NMDA) receptors. Because NMDA alters neural responses from the auditory brainstem, single unit extracellular recordings of SPON neurons were performed in the presence and absence of ketamine. In doing so, this study represents the first in vivo examination of the SPON of the mouse. Herein, independent data sets of SPON neurons are characterized that did or did not receive ketamine, as well as neurons that were recorded both prior to and following ketamine administration. In all conditions, SPON neurons exhibited contralaterally driven spikes triggered by the offset of pure tone stimuli. Ketamine lowered both evoked and spontaneous spiking, decreased the sharpness of frequency tuning, and increased auditory thresholds and first-spike latencies. In addition, ketamine limited the range of modulation frequencies to which neurons phase-locked to sinusoidally amplitude-modulated tones.

Time dependence of binaural responses in the rat's central nucleus of the inferior colliculus

Recordings were made from single neurons in the rat's central nucleus of the inferior colliculus. Excitatory/inhibitory binaural interactions and interaural-level difference curves were determined for responses to 100 ms dichotic tone bursts presented to the left and right ears simultaneously. Most neurons with sustained responses to tone bursts had the same binaural response type throughout the 100 ms stimulus period. However, some neurons (39% of our sample) showed qualitatively different binaural response types during the early and late parts of the stimulus (the first 20 ms versus the last 80 ms of the tone burst). Also, for many neurons with consistent early and late binaural response patterns, the strength of binaural interaction was different during the early and late periods. For example, for neurons excited by the contralateral ear and inhibited by the ipsilateral ear during the entire 100 ms period (the most common binaural response type), the degree of inhibition was generally greater during the later part of a stimulus. This change in the strength and/or quality of binaural interaction during dichotic stimulation likely reflects a complex pattern of converging excitatory and inhibitory inputs to the inferior colliculus from lower brainstem structures as well as the time course of local synaptic events. The temporal properties of binaural interaction may influence how sound source location is represented in the central auditory system.

Metabotropic glutamate receptors modulate glutamatergic and GABAergic synaptic transmission in the central nucleus of the inferior colliculus

Fast glutamatergic and GABAergic transmission in the central nucleus of the inferior colliculus (ICC), a major auditory midbrain structure, is mediated respectively by alpha-amino-3-hydroxy-5-methylisoxazole-4 propionic acid (AMPA) and gamma-aminobutyric acid (GABA)(A) receptors. In this study, we used whole-cell patch clamp recordings in brain slices to investigate the effects of activation of metabotropic glutamate receptors (mGluRs) on synaptic responses mediated by AMPA and GABA(A) receptors in ICC neurons of young rats. Excitatory and inhibitory postsynaptic currents (EPSCs and IPSCs) mediated respectively by AMPA and GABA(A) receptors were elicited by stimulation of the lateral lemniscus, the major afferent pathway to the ICC. The agonists for groups I and II mGluRs, (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (ACPD), and for group III mGluRs, L-2-amino-3-hydroxypropanoic acid 3-phosphate (L-SOP), did not affect intrinsic membrane properties of the ICC neurons. The agonist for group II mGluRs, (1R,4R,5S,6R)-4-amino-2-oxabicyclo[3.1.0] hexane-4,6-dicarboxylic acid (LY379268), significantly reduced the AMPA receptor-mediated EPSCs and GABA(A) receptor-mediated IPSCs. The effects were reversed by the group II mGluR antagonist, (2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid (LY341495). The agonists for groups I and III, (RS)-3,5-dihydroxyphenylglycine (DHPG) and L-SOP, respectively, did not affect AMPA or GABA(A) receptor-mediated responses. The reduction of the synaptic responses by LY379268 was accompanied by a substantial increase in a ratio of the second to the first AMPA receptor-mediated EPSCs and GABA(A) receptor-mediated IPSCs to paired-pulse stimulation. The results suggest that group II mGluRs regulate both fast glutamatergic and GABAergic synaptic transmission in the ICC, probably through a presynaptic mechanism due to reduction of transmitter release.

Time-dependent effects of ipsilateral stimulation on contralaterally elicited responses in the rat's central nucleus of the inferior colliculus

Recordings were made from single neurons in the rat's central nucleus of the inferior colliculus (ICc). Binaural responses were studied when dichotic tone bursts with various interaural-level differences were presented simultaneously or with a contralateral delay. These dichotic tone bursts allowed us to probe temporal changes in the effect produced by an ipsilateral sound on a contralaterally elicited response. Most of the neurons in the rat's ICc were excited by contralateral and inhibited by ipsilateral stimulation. For the majority of neurons with excitatory/inhibitory interactions, the early part of an ipsilateral stimulus caused stronger inhibition than the late part. The ipsilateral stimulus frequently produced an excitatory or inhibitory "offset" effect that was apparent soon after cessation of the stimulus. For many neurons, this aftereffect substantially changed the strength and temporal firing pattern of the response elicited by a lagging contralateral stimulus. Our results suggest that there are time-dependent changes in the effect of ipsilateral stimulation on the pattern and strength of responses to contralateral stimulation. These effects frequently outlast the duration of a leading ipsilateral stimulus. These characteristics of binaural interaction likely reflect the time courses of converging excitatory and inhibitory synaptic inputs to ICc neurons as well as the intrinsic membrane properties of those neurons.

Glycinergic "inhibition" mediates selective excitatory responses to combinations of sounds

In the mustached bat's inferior colliculus (IC), combination-sensitive neurons display time-sensitive facilitatory interactions between inputs tuned to distinct spectral elements in sonar or social vocalizations. Here we compare roles of ionotropic receptors to glutamate (iGluRs), glycine (GlyRs), and GABA (GABA(A)Rs) in facilitatory combination-sensitive interactions. Facilitatory responses to 36 single IC neurons were recorded before, during, and after local application of antagonists to these receptors. The NMDA receptor antagonist CPP [(+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid], alone (n = 14) or combined with AMPA receptor antagonist NBQX (n = 22), significantly reduced or eliminated responses to best frequency (BF) sounds across a broad range of sound levels, but did not eliminate combination-sensitive facilitation. In a subset of neurons, GABA(A)R blockers bicuculline or gabazine were applied in addition to iGluR blockers. GABA(A)R blockers did not "uncover" residual iGluR-mediated excitation, and only rarely eliminated facilitation. In nearly all neurons for which the GlyR antagonist strychnine was applied in addition to iGluR blockade (22 of 23 neurons, with or without GABA(A)R blockade), facilitatory interactions were eliminated. Thus, neither glutamate nor GABA neurotransmission are required for facilitatory combination-sensitive interactions in IC. Instead, facilitation may depend entirely on glycinergic inputs that are presumed to be inhibitory. We propose that glycinergic inputs tuned to two distinct spectral elements in vocal signals each activate postinhibitory rebound excitation. When rebound excitations from two spectral elements coincide, the neuron discharges. Excitation from glutamatergic inputs, tuned to the BF of the neuron, is superimposed onto this facilitatory interaction, presumably mediating responses to a broader range of acoustic signals.

Local inhibition shapes duration tuning in the inferior colliculus of guinea pigs

Neural tuning to sound durations is a useful filter for the identification of a variety of sounds. Previous studies have shown that the interaction between excitatory and inhibitory inputs plays a role in duration selectivity in echolocating bats. However, this has not been investigated in non-echolocating mammals. In the inferior colliculus (IC) of these mammals, it is recognized that the excitatory responses to sounds are mediated through AMPA and NMDA receptors while the inhibitory input is mediated through gamma-aminobutyric acid (GABA) and glycine receptors. The present study explores the potential interplay between inhibitory and excitatory inputs and its role in the duration selectivity of IC neurons in guinea pigs. It was found that the application of bicuculline (BIC, a GABA A blocker) and/or strychnine (STRY, a glycine blocker) eliminated or reduced duration tuning in most units that were duration tuned (32 out of 39 for BIC, 50 out of 64 for STRY, respectively). The inhibitory input (either by GABA or by glycine) appeared to have a stronger regulating effect on the early excitation mediated by AMPA than on later excitation by NMDA. This is more distinguishable in neurons that show duration selectivity. In conclusion, the inhibitory effect on the early responses appears to be the main contributor for the duration selectivity of the IC in guinea pigs; potential mechanisms for this duration selectivity are also discussed.

Comparison of responses of neurons in the mouse inferior colliculus to current injections, tones of different durations, and sinusoidal amplitude-modulated tones

We made in vivo whole cell patch-clamp recordings from the inferior colliculus of young-adult, anesthetized C57/Bl6 mice to compare the responses to constant-current injections with the responses to tones of different duration or to sinusoidal amplitude-modulated (SAM) tones. We observed that voltage-dependent ion channels contributed in several ways to the response to tones. A sustained response to long tones was observed only in cells showing little accommodation during current injection. Cells showing burst-onset firing during current injection showed a small response to SAM tones, whereas burst-sustained cells showed a good response to SAM tones. The hyperpolarization-activated nonselective cation channel I(h) had a special role in shaping the responses: I(h) was associated with an increased excitability, with chopper and pauser responses, and with an afterhyperpolarization following tones. Synaptic properties were more important in determining the responses to tones of different durations. A short-latency inhibitory response appeared to contribute to the long-pass response in some cells and short-pass and band-pass neurons were characterized by their slow recovery from synaptic adaptation. Cells that recovered slowly from synaptic adaptation showed a relatively small response to SAM tones. Our results show an important role for both intrinsic membrane properties -- most notably the presence of I(h) and the extent of accommodation -- and synaptic adaptation in shaping the response to tones in the inferior colliculus.

Changes in firing pattern of lateral geniculate neurons caused by membrane potential dependent modulation of retinal input through NMDA receptors

An optimal visual stimulus flashed on the receptive field of a retinal ganglion cell typically evokes a strong transient response followed by weaker sustained firing. Thalamocortical (TC) neurons in the dorsal lateral geniculate nucleus, which receive their sensory input from retina, respond similarly except that the gain, in particular of the sustained component, changes with level of arousal. Several lines of evidence suggest that retinal input to TC neurons through NMDA receptors plays a key role in generation of the sustained response, but the mechanisms for the state-dependent variation in this component are unclear. We used a slice preparation to study responses of TC neurons evoked by trains of electrical pulses to the retinal afferents at frequencies in the range of visual responses in vivo. Despite synaptic depression, the pharmacologically isolated NMDA component gave a pronounced build-up of depolarization through temporal summation of the NMDA receptor mediated EPSPs. This depolarization could provide sustained firing, the frequency of which depended on the holding potential. We suggest that the variation of sustained response in vivo is caused mainly by the state-dependent modulation of the membrane potential of TC neurons which shifts the NMDA receptor mediated depolarization closer to or further away from the firing threshold. The pharmacologically isolated AMPA receptor EPSPs were rather ineffective in spike generation. However, together with the depolarization evoked by the NMDA component, the AMPA component contributed significantly to spike generation, and was necessary for the precise timing of the generated spikes.

Contribution of NMDA and AMPA receptors to temporal patterning of auditory responses in the inferior colliculus

Although NMDA receptors (NMDARs) are associated with synaptic plasticity, they form an essential part of responses to sensory stimuli. We compared contributions of glutamatergic NMDARs and AMPA receptors (AMPARs) to auditory responses in the inferior colliculus (IC) of awake, adult mustached bats. We examined the magnitude and temporal pattern of responses to tonal signals in single units before, during, and after local micro-iontophoretic application of selective antagonists to AMPARs [NBQX (1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide)] and NMDARs [CPP ((+/-)3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid)]. Combined blockade of AMPARs and NMDARs eliminated excitatory responses in nearly all neurons, whereas separate blockade of each receptor was quantitatively similar, causing substantial (> 50%) spike reductions in approximately 75% of units. The major result was that effects of receptor blockade were most closely related to the first-spike latency of a unit. Thus, AMPAR blockade substantially reduced spikes in all short-latency units (< 12 ms) but never in long-latency units (> or = 12 ms). NMDAR blockade had variable effects on short-latency units but reduced spikes substantially for all long-latency units. There were no distinct contributions of AMPARs and NMDARs to early and late elements of responses. Thus, AMPAR blockade reduced early (onset) spikes somewhat more effectively than NMDAR blockade in short-latency units, but NMDAR blockade reduced onset spikes more effectively in long-latency units. AMPAR and NMDAR blockade were equally effective in reducing later elements of sustained responses in short-latency units, whereas NMDAR blockade was much more effective in long-latency units. These results indicate that NMDARs play multiple roles for signal processing in adult IC neurons.

Intrinsic membrane properties and synaptic response characteristics of neurons in the rat's external cortex of the inferior colliculus

The inferior colliculus (IC) can be divided into three anatomical subdivisions: the central nucleus (ICc), the dorsal cortex (ICd) and the external cortex (ICx). ICx receives its primary auditory inputs from ICc and auditory cerebral cortical areas, and non-auditory inputs from regions of motor and other sensory systems. This wide array of projections makes the ICx a distinct structure within the auditory brainstem. The purpose of the current study was to comprehensively characterize the neuronal population of ICx, by intrinsic and synaptic response properties. Visual whole-cell patch clamp recordings were taken from ICx neurons (N=129) from rats between postnatal days 8 to 12. Neurons displayed various types of firing patterns in response to current injection, including regular, adapting, pauser and bursting. The regular cells constitute the majority (66%), followed by adapting (18%), pauser (13%) and bursting cells (2%). In response to hyperpolarizing current injection, many neurons illustrated a pronounced sag in the membrane potential, representing a hyperpolarization-activated current (I(h)). Some neurons (25%) displayed a Ca(2+)-dependent rebound depolarization following negative current injection. In response to depolarizing current injection, 70% of ICx neurons displayed a Ca(2+)-mediated potential expressed as Ca(2+) spikes/humps, uncovered when Na(+) and K(+) currents were removed. Also, spikes displayed an undershoot which was in part mediated by Ca(2+). Stimulation of the ICc elicited graded synaptic responses, which displayed a combination of excitatory and/or inhibitory potentials, with excitation being predominant across firing patterns. Neurons displayed temporal summation in response to repetitive stimulation at 20 Hz and higher. The results indicate a relatively modest diversity in firing pattern and in intrinsic membrane properties, making this subnucleus distinct from its counterparts within the IC. The data show that ICx receives major excitatory input from ICc, supporting its role in integrating signals from brainstem and directing information to higher brain centers.

Stimulus selectivity is enhanced by voltage-dependent conductances in combination-sensitive neurons

Central sensory neurons often respond selectively to particular combinations of stimulus attributes, but we know little about the underlying cellular mechanisms. The weakly electric fish Gymnarchus discriminates the sign of the frequency difference (Df) between a neighbor's electric organ discharge (EOD) and its own EOD by comparing temporal patterns of amplitude modulation (AM) and phase modulation (PM). Sign-selective neurons in the midbrain respond preferentially to either positive frequency differences (Df >0 selective) or negative frequency differences (Df <0 selective). To study the mechanisms of combination sensitivity, we made whole cell intracellular recordings from sign-selective midbrain neurons in vivo and recorded postsynaptic potential (PSP) responses to AM, PM, Df >0, and Df <0. Responses to AM and PM consisted of alternating excitatory and inhibitory PSPs. These alternating responses were in phase for the preferred sign of Df and offset for the nonpreferred sign of Df. Therefore a certain degree of sign selectivity was predicted by a linear sum of the responses to AM and PM. Responses to the nonpreferred sign of Df, but not the preferred sign of Df, were substantially weaker than linear predictions, causing a significant increase in the actual degree of sign selectivity. By using various levels of current clamp and comparing our results to simple models of synaptic integration, we demonstrate that this decreased response to the nonpreferred sign of Df is caused by a reduction in voltage-dependent excitatory conductances. This finding reveals that nonlinear decoders, in the form of voltage-dependent conductances, can enhance the selectivity of single neurons for particular combinations of stimulus attributes.

GABAB receptor-mediated presynaptic inhibition of glutamatergic transmission in the inferior colliculus

Whole-cell patch clamp recordings were made from ICC neurons in brain slices of 9-16 day old rats. Postsynaptic currents were evoked by electrical stimulation of the lemniscal inputs. Excitatory postsynaptic currents (EPSCs) were isolated pharmacologically by blocking GABA(A) and glycine receptors. EPSCs were further dissected into AMPA and NMDA receptor-mediated responses by adding the receptor antagonists, APV and CNQX, respectively. The internal solution in the recording electrodes contained CsF and TEA to block K(+) channels that might be activated by postsynaptic GABA(B) receptors. The modulatory effects of GABA(B) receptors on EPSCs in ICC neurons were examined by bath application of the GABA(B) receptor agonist, baclofen, and the antagonist, CGP 35348. The amplitudes of EPSCs in ICC neurons were reduced to 34.4+/-3.2% of the control by baclofen (5-10 microM). The suppressive effect by baclofen was concentration-dependent. The reduction of the EPSC amplitude was reversed by CGP35348. The ratio of the 2nd to 1st EPSCs evoked by paired-pulse stimulation was significantly increased after application of baclofen. These results suggest that glutamatergic excitation in the ICC can be modulated by presynaptic GABA(B) receptors. In addition, baclofen reduced NMDA EPSCs more than AMPA EPSCs. The GABA(B) receptor-mediated modulation of glutamatergic excitation in the ICC provides a likely mechanism for preventing overstimulation and/or regulating the balance of excitation and inhibition involved in processing auditory information.

The inferior colliculus of the rat: quantitative immunocytochemical study of GABA and glycine

Both GABA and glycine (Gly) containing neurons send inhibitory projections to the inferior colliculus (IC), whereas inhibitory neurons within the IC are primarily GABAergic. To date, however, a quantitative description of the topographic distribution of GABAergic neurons in the rat's IC and their GABAergic or glycinergic inputs is lacking. Accordingly, here we present detailed maps of GABAergic and glycinergic neurons and terminals in the rat's IC. Semithin serial sections of the IC were obtained and stained for GABA and Gly. Images of the tissue were digitized and used for a quantitative densitometric analysis of GABA immunostaining. The optical density, perimeter, and number of GABA- and Gly immunoreactive boutons apposed to the somata were measured. Data analysis included comparisons across IC subdivisions and across frequency regions within the central nucleus of the IC. The results show that: 1) 25% of the IC neurons are GABAergic; 2) there are more GABAergic neurons in the central nucleus of the IC than previously estimated; 3) GABAergic neurons are larger than non-GABAergic; 4) GABAergic neurons receive less GABA and glycine puncta than non-GABAergic; 5) differences across frequency regions are minor, except that the non-GABAergic neurons from high frequency regions are larger than their counterparts in low frequency regions; 6) differences within the laminae are greater along the dorsomedial-ventrolateral axis than along the rostrocaudal axis; 7) GABA and non-GABAergic neurons receive different numbers of puncta in different IC subdivisions; and 8) GABAergic puncta are both apposed to the somata and in the neuropil, glycinergic puncta are mostly confined to the neuropil.

Serotonin shifts first-spike latencies of inferior colliculus neurons

Many studies of neuromodulators have focused on changes in the magnitudes of neural responses, but fewer studies have examined neuromodulator effects on response latency. Across sensory systems, response latency is important for encoding not only the temporal structure but also the identity of stimuli. In the auditory system, latency is a fundamental response property that varies with many features of sound, including intensity, frequency, and duration. To determine the extent of neuromodulatory regulation of latency within the inferior colliculus (IC), a midbrain auditory nexus, the effects of iontophoretically applied serotonin on first-spike latencies were characterized in the IC of the Mexican free-tailed bat. Serotonin significantly altered the first-spike latencies in response to tones in 24% of IC neurons, usually increasing, but sometimes decreasing, latency. Serotonin-evoked changes in latency and spike count were not always correlated but sometimes occurred independently within individual neurons. Furthermore, in some neurons, the size of serotonin-evoked latency shifts depended on the frequency or intensity of the stimulus, as reported previously for serotonin-evoked changes in spike count. These results support the general conclusion that changes in latency are an important part of the neuromodulatory repertoire of serotonin within the auditory system and show that serotonin can change latency either in conjunction with broad changes in other aspects of neuronal excitability or in highly specific ways.

Neuronal responses to lemniscal stimulation in laminar brain slices of the inferior colliculus

The central nucleus of the inferior colliculus (ICC) receives inputs from all parts of the auditory brainstem and transmits the information to the forebrain. Fibrodendritic laminae of the ICC provide a structural basis for a tonotopic organization, and the interaction of inputs within a single layer is important for ICC processing. Transverse slice planes of the ICC sever the layers and many of the ascending axons that enter through the lateral lemniscus. Consequently, the activity initiated within a lamina by a pure lemniscal stimulus is not well characterized. Here, we use a slice plane that maintains the integrity of the laminae in ICC and allows the axons in the lateral lemniscus to be stimulated at a distance from the ICC. We examined both the postsynaptic currents and potentials of the same neurons to lemniscal stimuli in this laminar brain slice. Our main finding is that lemniscal stimulation evokes prolonged synaptic potentials in ICC neurons. Synaptic potential amplitudes and durations increase with lemniscal shock strength. In approximately 50% of ICC neurons, the postsynaptic potential is equal in duration to the postsynaptic current, whereas in the remaining neurons it is three to four times longer. Synaptic responses to single shocks or shock trains exhibit plateau potentials that enable sustained firing in ICC neurons. Plateau potentials are evoked by N-methyl-D-aspartate (NMDA) receptor activation, and their amplitudes and durations are regulated by both NMDA-R and gamma-aminobutyric acid A (GABAA)-R activation. These data suggest that in the intact laminae of the ICC, lemniscal inputs initiate sustained firing through monosynaptic and polysynaptic NMDA-mediated synapses regulated by GABAA synapses.

Intercollicular commissural projections modulate neuronal responses in the inferior colliculus

The right and left inferior colliculi (ICs) in the auditory midbrain are connected to one another by a bundle of fibres, the commissure of the IC. Previous studies show that this commissural projection connects corresponding frequency regions in the two sides and originates mainly from excitatory neurons, although some studies suggest a smaller number of GABAergic inhibitory neurons may also project via the commissure. Although the commissure of the IC is a major pathway connecting the most important nuclei of the auditory tectum, little is known about its functional significance. To investigate its role in auditory processing in the rat, we recorded sound-evoked responses of single neurons in one IC while injecting kynurenic acid into a corresponding region of the opposite IC. This procedure enabled us to block reversibly excitation of commissural projections to the recorded IC. The changes in the neural responses when input from the opposite IC was blocked are consistent with the commissural projection exerting both an excitatory and an inhibitory influence. The inhibition could be accounted for by monosynaptic or disynaptic connections. The responses to both monaural and binaural stimulation were affected, and the effects were proportionately greater at near-threshold sound levels. The results suggest that one function of the commissure of the IC may be to modulate the response gain of IC neurons to acoustic stimulation.

Contribution of AMPA, NMDA, and GABA(A) receptors to temporal pattern of postsynaptic responses in the inferior colliculus of the rat

The central nucleus of the inferior colliculus (ICC) is a major site of synaptic interaction in the central auditory system. To understand how ICC neurons integrate excitatory and inhibitory inputs for processing temporal information, we examined postsynaptic responses of ICC neurons to repetitive stimulation of the lateral lemniscus at 10-100 Hz in rat brain slices. The excitatory synaptic currents mediated by AMPA and NMDA receptors and the inhibitory current mediated by GABA(A) receptors were pharmacologically isolated and recorded by whole-cell patch-clamp techniques. The response kinetics of AMPA receptor-mediated EPSCs and GABA(A) receptor-mediated IPSCs were similar and much faster than those of NMDA receptor-mediated EPSCs. AMPA EPSCs could follow each pulse of stimulation at a rate of 10-100 Hz but showed response depression during the course of repetitive stimulation. GABA(A) IPSCs could also follow stimulus pulses over this frequency range but showed depression at low rates and facilitation at higher rates. NMDA EPSCs showed facilitation and temporal summation in response to repetitive stimulation, which was most pronounced at higher rates of stimulation. GABA(A) inhibition suppressed activation of NMDA receptors and reduced both the degree of AMPA EPSC depression and the extent of temporal summation of NMDA EPSCs. The results indicate that GABA(A) receptor-mediated inhibition plays a crucial role in maintaining the balance of excitation and inhibition and in allowing ICC neurons to process temporal information more precisely.

Regulation of auditory responses in the central nucleus of the inferior colliculus by tetraethylammonium-sensitive potassium channels

The role of potassium channels in regulating spontaneous firing and sound-evoked responses in the central nucleus of the inferior colliculus was studied by recording single-unit activity before and during iontophoretic application of a nonspecific potassium channel blocker, tetraethylammonium (TEA). Tone bursts and sinusoidal amplitude-modulated tones were used to evoke auditory responses. Our results show that release of TEA increased the width of spikes for all neurons tested. There was an increase in spontaneous firing for most of the neurons. There was also an increase in responses to tone bursts for most of the neurons, although in some cases there was a reduction in the evoked responses. TEA also increased the firing rate in responses to sinusoidal amplitude-modulated sounds in the majority of the neurons tested. For some neurons, the change in firing reduced the selectivity of responses for particular rates of modulation. There was also a reduction in the synchrony of action potentials to the modulation envelope in many cells. Our results show that potassium channels are important for regulating the strength of sound-evoked responses and the level of spontaneous activity, and determining the temporal properties of responses to amplitude-modulated sounds.

Neural processing of amplitude-modulated sounds

Amplitude modulation (AM) is a temporal feature of most natural acoustic signals. A long psychophysical tradition has shown that AM is important in a variety of perceptual tasks, over a range of time scales. Technical possibilities in stimulus synthesis have reinvigorated this field and brought the modulation dimension back into focus. We address the question whether specialized neural mechanisms exist to extract AM information, and thus whether consideration of the modulation domain is essential in understanding the neural architecture of the auditory system. The available evidence suggests that this is the case. Peripheral neural structures not only transmit envelope information in the form of neural activity synchronized to the modulation waveform but are often tuned so that they only respond over a limited range of modulation frequencies. Ascending the auditory neuraxis, AM tuning persists but increasingly takes the form of tuning in average firing rate, rather than synchronization, to modulation frequency. There is a decrease in the highest modulation frequencies that influence the neural response, either in average rate or synchronization, as one records at higher and higher levels along the neuraxis. In parallel, there is an increasing tolerance of modulation tuning for other stimulus parameters such as sound pressure level, modulation depth, and type of carrier. At several anatomical levels, consideration of modulation response properties assists the prediction of neural responses to complex natural stimuli. Finally, some evidence exists for a topographic ordering of neurons according to modulation tuning. The picture that emerges is that temporal modulations are a critical stimulus attribute that assists us in the detection, discrimination, identification, parsing, and localization of acoustic sources and that this wide-ranging role is reflected in dedicated physiological properties at different anatomical levels.

Integrative Neurobiology of the Alcohol Withdrawal Syndrome???From Anxiety to Seizures

This article represents the proceedings of a symposium presented at the 2003 Research Society on Alcoholism meeting in Ft. Lauderdale, Florida, organized and chaired by Carl L. Faingold. The presentations were (1) Overview, by Carl L. Faingold; (2) Stress, Multiple Alcohol Withdrawals, and Anxiety, by Darin Knapp; (3) Relationship Between Genetic Differences in Alcohol Drinking and Alcohol Withdrawal, by Julia Chester; (4) Neuronal Mechanisms in the Network for Alcohol Withdrawal Seizures: Modulation by Excitatory Amino Acid Receptors, by Carl L. Faingold; and (5) Treatment of Acute Alcohol Withdrawal and Long-Lasting Alterations in Hippocampal Neuronal Networks, by Larry P. Gonzalez. The presentations emphasized the importance of using intact behaving animals to advance the understanding of the human alcohol withdrawal syndrome. This involves applying and amplifying the neurophysiological and neurotransmitter findings observed in vitro to the network-based neurobiological mechanisms that are involved in several important aspects of the specific behaviors observed clinically. The symposium provided evidence that the organizational aspects of neuronal networks in the intact nervous system add another nexus for the action of alcohol and drugs to treat alcohol withdrawal that may not be readily studied in isolated neural elements used in in vitro approaches.

AGS-induced expression of Narp is concomitant with expression of AMPA receptor subunits GluR1 and GluR2 in hippocampus but not inferior colliculus of P77PMC rats

To explore mechanisms of epileptogenesis in audiogenic seizures (AGS), we examined the expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazoleopropionic acid (AMPA) receptor subunits GluR1 and GluR2 and of the GluR-associated protein Narp in the hippocampus and the inferior colliculus (IC) from AGS-susceptible P77PMC rats after a single AGS and audiogenic kindling. Western blotting and immunohistochemistry showed that Narp was rapidly induced in both the hippocampus and the IC by AGS. In the hippocampus, up-regulation of Narp was concomitant with GluR1 and GluR2 under both conditions of a single AGS and AGS kindling. In the IC, however, Narp was up-regulated, GluR2 down-regulated, and GluR1 unchanged after kindling. In comparison with kindling, neither GluR1 nor GluR2 was changed, while Narp significantly increased in the IC following a single AGS. These findings suggest that down-regulation of AMPA receptor GluR2 subunit in the IC may contribute to AGS-mediated epileptogenesis, and up-regulation of Narp in the IC may be involved in audiogenic seizures.

Hyperpolarization-activated current (Ih) in the inferior colliculus: distribution and contribution to temporal processing

Neurons in the inferior colliculus (IC) process acoustic information converging from inputs from almost all nuclei of the auditory brain stem. Despite its importance in auditory processing, little is known about the distribution of ion currents in IC neurons, namely the hyperpolarization-activated current Ih. This current, as shown in neurons of the auditory brain stem, contributes to the precise analysis of temporal information. Distribution and properties of the Ih current and its contribution to membrane properties and synaptic integration were examined by current- and voltage-clamp recordings obtained from IC neurons in acute slices of rats (P17-P19). Based on firing patterns to positive current injection, three basic response types were distinguished: onset, adapting, and sustained firing neurons. Onset and adapting cells showed an Ih-dependent depolarizing sag and had a more depolarized resting membrane potential and lower input resistance than sustained neurons. Ih amplitudes were largest in onset, medium in adapting, and small in sustained neurons. Ih activation kinetics was voltage dependent in all neurons and faster in onset and adapting compared with sustained neurons. Injecting trains of simulated synaptic currents into the neurons or evoking inhibitory postsynaptic potentials (IPSPs) by stimulating the lemniscal tract showed that Ih reduced temporal summation of excitatory and inhibitory potentials in onset but not in sustained neurons. Blocking Ih also abolished afterhyperpolarization and rebound spiking. These results suggest that, in a large proportion of IC cells, namely the onset and adapting neurons, Ih improves precise temporal processing and contributes to the temporal analysis of input patterns.

The commissure of the inferior colliculus shapes frequency response areas in rat: an in vivo study using reversible blockade with microinjection of kynurenic acid

The commissure of the inferior colliculus (CoIC) interconnects corresponding frequency-band laminae in the two inferior colliculi (ICs). Although the CoIC has been studied neurophysiologically in vitro, the effect of the CoIC on the responses of IC neurons to physiological stimuli has not been addressed. In this study, we injected the glutamate receptor blocker kynurenic acid into one IC while recording the frequency response areas (FRAs) of neurons in the other, to test the hypothesis that frequency response properties of IC neurons are influenced by commissural inputs from the contralateral IC. Following blockade of the commissure, 10 of 12 neurons tested exhibited an increase or a decrease in their FRAs. In most neurons (9/12) the response area changed in the same direction, irrespective of whether the neuron was stimulated monaurally (at the ear contralateral to the recorded IC) or binaurally. In one neuron, blockade of the CoIC resulted in an expansion of the response area under binaural stimulation and a contraction under monaural stimulation. In the remaining two units, no effect was observed. Changes in response areas that exceeded the criterion ranged between 17 and 80% of control values with monaural stimulation, and 35 and 77% with binaural stimulation. Area changes could also be accompanied by changes in spike rate and monotonicity. From our observation that FRAs contract following commissure block, we infer that the commissure contains excitatory fibres. The expansion of response areas in other cases, however, suggests that the commissure also contains inhibitory fibres, or that its effects are mediated by disynaptic as well as monosynaptic circuits. The small sample size precludes a definitive conclusion as to which effect predominates. We conclude that inputs from the contralateral IC projecting via the CoIC influence the spectral selectivity and response gain of neurons in the IC.

Glutamatergic and GABAergic regulation of neural responses in inferior colliculus to amplitude-modulated sounds

Recordings were made from single neurons in the rat inferior colliculus in response to sinusoidally amplitude-modulated sounds (10-s duration) presented to the contralateral ear. Neural responses were determined for different rates of modulation (0.5 Hz to 1 kHz) at a depth of 100%, and modulation transfer functions were generated based on firing rate (MTFFR) and vector strength (MTFVS). The effects of AMPA, NMDA, and GABAA receptor antagonists were examined by releasing drugs iontophoretically through a multibarrel pipette attached to a single-barrel recording pipette. Both the AMPA receptor antagonist, 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide disodium (NBQX), and the NMDA receptor antagonist, (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) resulted in a decrease in firing rate, and the GABAA receptor antagonist, bicuculline, produced an increase in the firing rate in most of the cells examined. In some cases, the shape of the MTFFR was modified slightly by receptor antagonists, but in most cases, the peak firing rate that determined a neuron's best modulation frequency remained the same. Also there were no changes during delivery of either excitatory or inhibitory antagonists in the maximum response synchrony at the peak of the MTFVS although some changes were noticed at off-peak modulation rates particularly with the AMPA receptor antagonist, NBQX.

Early postnatal sound exposure induces lasting neuronal changes in the inferior colliculus of senescence accelerated mice (SAMP8): a morphometric study on GABAergic neurons and NMDA expression

Senescence-acceleration-prone mice (SAMP8) provide a model to study the influence of early postnatal sound exposure upon the aging auditory midbrain. SAMP8 were exposed to a 9-kHz monotone of either 53- or 65-dB sound pressure level during the first 30 postnatal days, the neurons in the auditory midbrain responding selectively to 9 kHz were localized by c-fos immunohistochemistry and the following parameters were compared to control SAMP8 not exposed to sound: mortality after sound exposure, dendritic spine density, and quantitative neurochemical alterations in this 9-kHz isofrequency lamina. For morphometric analysis, animals were examined at 1, 4, and 8 months of age. Serial sections of the inferior colliculus were Golgi impregnated or stained immunohistochemically for the expression of epsilon1 subunit of NMDA receptor or GABA. Mortality after exposure to 53 dB was the same as in controls, but was markedly increased from 7 months of age onward after postnatal exposure to 65 dB. No gross morphological alterations were observed in the auditory midbrain after sound exposure. However, sound exposure to 53 or 65 dB significantly reduced dendritic spine density by 11% at 4 months or by 11-17% both at 1 and 4 months of age, respectively. The effect of sound exposure upon neurons expressing the NMDAepsilon1 subunit was dose-dependent. Increasing with age until 4 months in control mice and remaining essentially stable thereafter, the percentage of NMDAepsilon1-immunoreactive neurons was significantly elevated by 40-66% in 1- and 8-month-old SAMP8 exposed to 53 dB, whereas no significant effect of 65 dB was apparent. The proportion of GABAergic cells declined with age in controls. It was significantly decreased at 1 month after 53 and 65 dB sound exposure. In contrast, it was elevated at later stages, being significantly increased at 4 months after exposure to 53 dB and at 8 months after exposure to 65 dB. The total cell number in the 9-kHz isofrequency lamina of SAMP8 decreased with age, but was not affected by exposure to either 53 or 65 dB. The present results indicate that early postnatal exposure to a monotone of mild intensity has long-term effects upon the aging auditory brain stem. Some of the changes induced by sound exposure, e.g., decline in spine density, are interpreted as accelerations of the normal aging process, whereas other effects, e.g., increased NMDAepsilon1 expression after 53 dB and elevated GABA expression after both 53 and 65 dB, are not merely explicable by accelerated aging.

In vivo approach to the cellular mechanisms for sensory processing in sleep and wakefulness

1. The present review analyzes sensory processing during sleep and wakefulness from a single neuronal viewpoint. Our premises are that processing changes throughout the sleep-wakefulness cycle may be at least partially evidenced in single neurons by (a) changes in the phase locking of the response to the hippocampal theta rhythm, (b) changes in the discharge rate and firing pattern of the response to sound, and (c) changes in the effects of the neurotransmitters involved in the afferent and efferent pathways. 2. The first part of our report is based on the hypothesis that the encoding of sensory information needs a timer in order to be processed and stored, and that the hippocampal theta rhythm could contribute to the temporal organization. We have demonstrated that the guinea pig's auditory and visual neuronal discharge exhibits a temporal relationship (phase locking) to the hippocampal theta waves during wakefulness and sleep phases. 3. The concept that the neural network organization during sleep versus wakefulness is different and can be modulated by sensory signals and vice versa, and that the sensory input may be influenced by the CNS state, i.e., asleep or awake, is introduced. During sleep the evoked firing of auditory units increases, decreases, or remains similar to that observed during quiet wakefulness. However, there has been no auditory unit yet that stops firing as the guinea pig enters sleep. Approximately half of the cortical neurons studied did not change firing rate when passing into sleep while others increased or decreased. Thus, the system is continuously aware of the environment. We postulate that those neurons that changed their evoked firing during sleep are also related to still unknown sleep processes. 4. Excitatory amino acid neurotransmitters participate in the synaptic transmission of the afferent and efferent pathways in the auditory system. In the inferior colliculus, however, the effects of glutamate's mediating the response to sound and the efferent excitation evoked by cortical stimulation failed to show differences in sleep and wakefulness. 5. Considering that neonates and also infants spend most of the time asleep, the continuous arrival of sensory information to the brain during both sleep phases may serve to "sculpt" the brain by activity-dependent mechanisms of neural development, as has been postulated for wakefulness.

Presynaptic modulation of GABAergic inhibition by GABA(B) receptors in the rat's inferior colliculus

Whole-cell patch clamp recordings were made from neurons in a brain slice preparation of the inferior colliculus in 11-15-day-old rat pups. Synaptic responses were elicited by applying a current pulse to the lateral lemniscus just below the central nucleus of the inferior colliculus. To examine GABAergic inhibition in the inferior colliculus all excitatory postsynaptic potentials and glycinergic inhibitory postsynaptic potentials were blocked by bath application of their respective antagonists and the contribution of GABA(B) receptors was determined for the remaining inhibitory postsynaptic potentials. For most cells the isolated inhibitory postsynaptic potential was completely blocked by the GABA(A) receptor antagonist, bicuculline, but was unaffected by the GABA(B) receptor antagonist, phaclofen. The GABA(B) receptor agonist, baclofen (10-20 microM), decreased the amplitude of the inhibitory postsynaptic potentials. This effect was completely blocked by phaclofen. Baclofen did not increase the cell membrane conductance or alter the rate of firing produced by depolarization of the cell membrane. In contrast, muscimol, a GABA(A) receptor agonist, greatly increased membrane conductance and lowered the firing rate produced by depolarization. Our results indicate that GABAergic inhibition in the auditory midbrain can be reduced by the activation of GABA(B) receptors and suggest that the effects are presynaptic.

Role of GABA abnormalities in the inferior colliculus pathophysiology - audiogenic seizures

gamma-Aminobutyric acid (GABA), acting at GABA(A) receptors, mediates inhibition in inferior colliculus (IC) central nucleus (ICc) neurons and plays a prominent role in mediating acoustically evoked non-monotonicity, offset inhibition, and binaural inhibition, and is also important in tonic inhibition. The IC plays an important role in a number of pathophysiological conditions that involve hearing, including tinnitus, age-related hearing loss, and audiogenic seizures (AGS). AGS are a major form of rodent neurological disorder that can be genetically mediated and can also be readily induced in both young and mature animals. A deficit in GABA-mediated inhibition in IC neurons has been shown to be a critical mechanism in genetic and induced forms of AGS. Thus, both endogenously evoked GABA-mediated inhibition and exogenously applied GABA are reduced in efficacy in IC neurons of rats that are susceptible to AGS. GABA-mediated inhibition in IC neurons is significantly more easily blocked by a GABA(A) antagonist in genetic and induced forms of AGS in vivo and in vitro. AGS can be induced in normal animals by treatments that reduce the effectiveness of GABA in the IC. Glutamate-mediated excitation is a critical element of neurotransmission in IC neurons, and excessive activation of glutamate receptors in the IC is also strongly implicated as the other major mechanism in the pathophysiology of AGS. These neurotransmitter abnormalities result in excessive firing of ICc neurons that acts as the critical initiation mechanism for triggering seizures in response to intense acoustic stimuli.

Synaptic modification in neurons of the central nucleus of the inferior colliculus

Whole-cell patch clamp recordings were made from neurons in the central nucleus of the inferior colliculus (ICC) in brain slices from rat (8-13 days old). ICC neurons were classified by their discharge pattern in response to depolarizing and hyperpolarizing current injection. Excitatory postsynaptic currents (EPSCs) were elicited by stimulation of synaptic inputs under the condition that the synaptic inhibition was suppressed by strychnine and picrotoxin. EPSCs in all tested types of ICC neurons showed posttetanic, long-term potentiation (LTP) and long-term depression with tetanic stimulation. The potentiated EPSCs consisted of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor and NMDA receptor mediated components. The magnitude of LTP was larger when the intracellular concentration of the calcium buffer ethylene glycol-bis (beta-aminoethyl ether)-N,N,N',N'-tetracetic acid (EGTA) was lower and stimulation frequency was higher in cells with rebound firing patterns. Blocking N-methyl-D-aspartate (NMDA) receptors in rebound cells prevented generation of LTP. These results suggest that excitatory synaptic transmission in ICC neurons can be modified. LTP in the auditory midbrain may be important for activity-dependent, adaptive changes in response to normal and pathological stimulus conditions.

AMPA and NMDA receptors mediate synaptic excitation in the rat's inferior colliculus

The synaptic mechanisms underlying excitation in the rat's central nucleus of the inferior colliculus (ICC) were examined by making whole-cell patch clamp recordings in brain slice preparations of the auditory midbrain. Responses were elicited by current pulse stimulation of the lateral lemniscus and recordings were made in ICC using either current clamp or voltage clamp methods. The excitatory postsynaptic responses in either current or voltage clamp mode consisted of two distinct components, an early component that could be blocked by bath application of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonists, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide (NBQX), and a later component that could be blocked by application of the N-methyl-D-aspartate (NMDA) receptor antagonists, (+/-)-2-amino-5-phosphonovaleric acid (APV) or (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP). Both AMPA and NMDA receptor-mediated responses were present at resting potential and could be isolated pharmacologically by application of receptor antagonists. Voltage clamp experiments revealed that the NMDA receptor-mediated current was voltage-dependent and increased in magnitude as the cell membrane was depolarized. This NMDA receptor-mediated response was enhanced at resting potential when Mg(2+) was eliminated from the bath solution. The ratio of response amplitudes associated with the late and early components, an estimate of the relative contribution of NMDA and AMPA receptor types, changed with age. There was a progressive decline in the ratio between 9 and 13 days of age, but no further reduction between days 13 and 16. The data show that both AMPA and NMDA receptors are important for determining excitatory responses in the ICC and that both receptor types probably play a role in auditory processing after the onset of hearing.

Effects of excitatory amino acid antagonists on the activity of inferior colliculus neurons during sleep and wakefulness

The contribution of N-methyl-D-aspartate to the response to sound of guinea pig inferior colliculus neurons was analyzed by recording single-unit activity before and after iontophoretic injection of a receptor specific antagonist, 2-amino-5-phosphonovaleric acid (AP5), during the sleep-waking cycle. The AP5 produced a significant firing decrease in most of the units recorded, while some neurons exhibited a particular decrease in the later part of the response. A latency reduction in one out of three units in paradoxical sleep was observed. A low proportion of them exhibited a significant firing increase. These actions were observed in wakefulness (W) as well as during sleep phases. We compared the action of kynurenic acid (Kyn) and the electrical stimulation of the auditory cortex on the same inferior colliculus neuron in anesthetized animals and during W. Both Kyn iontophoresis and cortical stimulation evoked similar changes, decreased firing rate in most inferior colliculus units, whereas a low proportion of them increased their discharge, in anesthetized guinea pigs and in W. Ascending as well as descending - efferent - glutamatergic fibers impinging on inferior colliculus neurons contribute to sound-evoked responses. The enhanced unitary activity observed in some neurons with after glutamatergic receptor blocking may indicate that polysynaptic pathways involving inhibitory neurons decreased their activity. These effects were observed in anesthetized and in behaving animals.

Contribution of AMPA and NMDA receptors to excitatory responses in the inferior colliculus

Brain slice studies of neurons in the central nucleus of the inferior colliculus (ICC) indicate that excitatory responses evoked by electrical stimulation of the lateral lemniscus consist of two components, an early, rapid response mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and a later, a slower one mediated by N-methyl-D-aspartate (NMDA) receptors. The early response can be selectively blocked by AMPA receptor antagonists (1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide disodium [NBQX]; or 6-cyano-7-nitroquinoxaline-2,3-dione) [CNQX], and the later one by NMDA receptor antagonists ((+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid [CPP]; or (+/-)-2-amino-5-phosphonovaleric acid) [APV]. Both AMPA and NMDA receptor-mediated responses can be elicited at resting potential, although the NMDA response is voltage dependent and makes a greater contribution when the cell membrane is depolarized. In vivo studies indicate that both AMPA and NMDA receptors contribute to sound-evoked responses. Both AMPA and NMDA receptor antagonists reduce the firing rate of single neurons in the ICC to contralaterally presented tones. Both classes of antagonist lower evoked activity over a wide range of sound intensities from threshold to maximum sound pressure levels. Thus, both NMDA and AMPA receptors contribute to responses over the full dynamic range of auditory sensitivity. The AMPA receptor antagonist, NBQX, is more effective than the NMDA receptor antagonist, CPP, in blocking responses of onset cells. Furthermore, NBQX and CPP have preferential effects in blocking the early or late responses of neurons that exhibited sustain activity to a 100 ms tone. Excitatory responses to sinusoidally amplitude-modulated stimuli are also reduced by application of either AMPA or NMDA antagonists. However, the synchrony of firing of action potentials to the modulation period (vector strength) is largely unaffected. The data suggest that the synchrony of firing of neurons in the inferior colliculus is determined primarily by the pattern of activity at lower levels of the auditory pathway and/or the local intrinsic properties of the cells.