In vitro brain slice studies of the rat's dorsal nucleus of the lateral lemniscus. III. synaptic pharmacology

Activity-dependent transmission and integration control the timescales of auditory processing at an inhibitory synapse

To capture the context of sensory information, neural networks must process input signals across multiple timescales. In the auditory system, a prominent change in temporal processing takes place at an inhibitory GABAergic synapse in the dorsal nucleus of the lateral lemniscus (DNLL). At this synapse, inhibition outlasts the stimulus by tens of milliseconds, such that it suppresses responses to lagging sounds, and is therefore implicated in echo suppression. Here, we untangle the cellular basis of this inhibition. We demonstrate with in vivo whole-cell patch-clamp recordings in Mongolian gerbils that the duration of inhibition increases with sound intensity. Activity-dependent spillover and asynchronous release translate the high presynaptic firing rates found in vivo into a prolonged synaptic output in acute slice recordings. A key mechanism controlling the inhibitory time course is the passive integration of the hyperpolarizing inhibitory conductance. This prolongation depends on the synaptic conductance amplitude. Computational modeling shows that this prolongation is a general mechanism and relies on a non-linear effect caused by synaptic conductance saturation when approaching the GABA reversal potential. The resulting hyperpolarization generates an efficient activity-dependent suppression of action potentials without affecting the threshold or gain of the input-output function. Taken together, the GABAergic inhibition in the DNLL is adjusted to the physiologically relevant duration by passive integration of inhibition with activity-dependent synaptic kinetics. This change in processing timescale combined with the reciprocal connectivity between the DNLLs implements a mechanism to suppress the distracting localization cues of echoes and helps to localize the initial sound source reliably.

NMDA currents modulate the synaptic input-output functions of neurons in the dorsal nucleus of the lateral lemniscus in Mongolian gerbils

Neurons in the dorsal nucleus of the lateral lemniscus (DNLL) receive excitatory and inhibitory inputs from the superior olivary complex (SOC) and convey GABAergic inhibition to the contralateral DNLL and the inferior colliculi. Unlike the fast glycinergic inhibition in the SOC, this GABAergic inhibition outlasts auditory stimulation by tens of milliseconds. Two mechanisms have been postulated to explain this persistent inhibition. One, an "integration-based" mechanism, suggests that postsynaptic excitatory integration in DNLL neurons generates prolonged activity, and the other favors the synaptic time course of the DNLL output itself. The feasibility of the integration-based mechanism was tested in vitro in DNLL neurons of Mongolian gerbils by quantifying the cellular excitability and synaptic input-output functions (IO-Fs). All neurons were sustained firing and generated a near monotonic IO-F on current injections. From synaptic stimulations, we estimate that activation of approximately five fibers, each on average liberating ∼18 vesicles, is sufficient to trigger a single postsynaptic action potential. A strong single pulse of afferent fiber stimulation triggered multiple postsynaptic action potentials. The steepness of the synaptic IO-F was dependent on the synaptic NMDA component. The synaptic NMDA receptor current defines the slope of the synaptic IO-F by enhancing the temporal and spatial EPSP summation. Blocking this NMDA-dependent amplification during postsynaptic integration of train stimulations resulted into a ∼20% reduction of the decay time course of the GABAergic inhibition. Thus, our data show that the NMDA-dependent amplification of the postsynaptic activity contributes to the GABAergic persistent inhibition generated by DNLL neurons.

Impaired presynaptic cytosolic and mitochondrial calcium dynamics in aged compared to young adult hippocampal CA1 synapses ameliorated by calcium chelation

Impaired regulation of presynaptic intracellular calcium is thought to adversely affect synaptic plasticity and cognition in the aged brain. We studied presynaptic cytosolic and mitochondrial calcium (Ca) dynamics using axonally loaded Calcium Green-AM and Rhod-2 AM fluorescence respectively in young (2-3 months) and aged (23-26 months) CA3 to CA1 Schaffer collateral excitatory synapses in hippocampal brain slices from Fisher 344 rats. After a tetanus (100 Hz, 200 ms), the presynaptic cytosolic Ca peaked at approximately 10 s in the young and approximately 12 s in the aged synapses. Administration of the membrane permeant Ca chelator, bis (O-aminophenoxy)-ethane-N,N,N,N-tetraacetic acid (BAPTA-AM), significantly attenuated the Ca response in the aged slices, but not in the young slices. The presynaptic mitochondrial Ca signal was much slower, peaking at approximately 90 s in both young and aged synapses, returning to baseline by 300 s. BAPTA-AM significantly attenuated the mitochondrial calcium signal only in the young synapses. Uncoupling mitochondrial respiration by carbonyl cyanide m-chlorophenylhydrazone (CCCP) application evoked a massive intracellular cytosolic Ca increase and a significant drop of mitochondrial Ca, especially in aged slices wherein the cytosolic Ca signal disappeared after approximately 150 s of washout and the mitochondrial Ca signal disappeared after 25 s of washout. These signals were preserved in aged slices by BAPTA-AM. Five minutes of oxygen glucose deprivation (OGD) was associated with a significant increase in cytosolic Ca in both young and aged synapses, which was irreversible in the aged synapses. These responses were significantly attenuated by BAPTA-AM in both the young and aged synapses. These results support the hypothesis that increasing intracellular calcium neuronal buffering in aged rats ameliorates age-related impaired presynaptic Ca regulation.

Response properties of single units in the dorsal nucleus of the lateral lemniscus of decerebrate cats

The dorsal nucleus of the lateral lemniscus (DNLL) receives afferent inputs from many brain stem nuclei and, in turn, is a major source of inhibitory inputs to the inferior colliculus (IC). The goal of this study was to characterize the monaural and binaural response properties of neurons in the DNLL of unanesthetized decerebrate cat. Monaural responses were classified according to the patterns of excitation and inhibition observed in contralateral and ipsilateral frequency response maps. Binaural classification was based on unit sensitivity to interaural level differences. The results show that units in the DNLL can be grouped into three distinct types. Type v units produce contralateral response maps that show a wide V-shaped excitatory area and no inhibition. These units receive ipsilateral excitation and exhibit binaural facilitation. The contralateral maps of type i units show a more restricted I-shaped region of excitation that is flanked by inhibition. Type o maps display an O-shaped island of excitation at low stimulus levels that is bounded by inhibition at higher levels. Both type i and type o units receive ipsilateral inhibition and exhibit binaural inhibition. Units that produce type v maps have a low best frequency (BF), whereas type i and type o units have high BFs. Type v and type i units give monotonic rate-level responses for both BF tones and broadband noise. Type o units are inhibited by tones at high levels, but are excited by high-level noise. These results show that the DNLL can exert strong, differential effects in the IC.

Unilateral cochlear ablation produces greater loss of inhibition in the contralateral inferior colliculus

Bilateral cochlear ablation leads to a profound weakening of synaptic inhibition within the inferior colliculus (IC) of gerbils [Vale & Sanes (2000) J. Neurosci., 20, 1912-1921]. To examine whether unilateral deafening leads to similar functional alterations, we studied the effect of unilateral cochlear ablation on inhibitory synaptic properties both ipsilateral and contralateral to the deafened ear. Lateral lemniscal and commissure of the IC-evoked inhibitory postsynaptic currents (IPSCs) were recorded in an IC brain slice preparation using whole-cell and gramicidin perforated-patch electrodes in the presence of kynurenic acid. Unilateral cochlear ablation led to a 23 mV depolarizing shift in the IPSC equilibrium potential for IC neurons contralateral to the deafened ear, but only a 10 mV depolarization in the ipsilateral IC. Lateral lemniscal-evoked inhibitory synaptic conductance declined significantly in the ipsilateral and contralateral IC, whereas commissural-evoked inhibitory synaptic conductance declined only contralateral to the ablated cochlea. An analysis of paired-pulse facilitation showed that inhibitory transmitter release was more affected ipsilateral to the ablated cochlea. Thus, unilateral cochlear ablation modifies inhibitory synapses in the inferior colliculus, but these changes appear to be dominated by postsynaptic alterations in the contralateral IC, and by presynaptic changes in the ipsilateral IC.

Excitatory and inhibitory intensity tuning in auditory cortex: evidence for multiple inhibitory mechanisms

The intensity tuning of excitatory and suppressive domain frequency response areas was investigated in 230 cat primary auditory cortical and 92 posterior auditory field neurons. Suppressive domains were explored using simultaneous 2-tone stimulation with one tone at the best excitatory frequency. The intensity tuning of excitatory and suppressive domains was negatively correlated, supporting the hypothesis that inhibitory sidebands are related to excitatory domain intensity tuning. To further test this hypothesis, we compared the slopes of the edges of suppressive bands to the intensity tuning of excitatory domains. Edges of suppressive bands next to excitatory domains had slopes significantly more slanted toward the excitatory area in neurons with intensity-tuned excitatory domains. This relationship was not observed for suppressive band edges not next to the excitatory domain (e.g., the lower edge of lower suppressive bands). This indicates that intensity tuning ultimately observed in the excitatory domain results from overlapping excitatory and inhibitory inputs. In combination with results using forward masking, our results suggest that there are separate early and late sources of inhibition contributing to cortical frequency response areas, and only the early-stage inhibition contributes to excitatory domain intensity tuning.

The effect of bilateral deafness on excitatory and inhibitory synaptic strength in the inferior colliculus

The consequences of deafness on the central auditory nervous system have been examined at many levels, from molecular to functional. However, there has never been a direct and selective measurement of excitatory synaptic function following total hearing loss. In the present study, gerbils were deafened at postnatal day 9, an age at which there is no deafferentation-induced cell death of ventral cochlear nucleus neurons. One to five days after bilateral cochlear ablation, the amplitude of evoked excitatory postsynaptic currents (EPSC) was measured with whole-cell voltage-clamp recordings in an inferior colliculus (IC) brain slice preparation in response to electrical stimulation of the ipsilateral lateral lemniscus (LL) or the commissure of the inferior colliculus (CIC). Deafness resulted in larger LL- and CIC-evoked EPSC amplitudes and durations. This result was observed at a depolarized holding potential. In addition, deafness caused a decrease in excitatory neurotransmitter release at the LL pathway, as assessed with a paired-pulse stimulation protocol. In contrast to its effect on excitatory synapses, bilateral cochlear ablation reduced inhibitory synaptic strength in IC neurons. The effects included a postsynaptic decrease in IPSC conductance, a 25-mV depolarization in the IPSC equilibrium potential and a decrease of neurotransmitter release. Thus normal innervation differentially affects excitatory and inhibitory synaptic strength in IC neurons, and these changes may contribute to alterations in auditory coding properties following sensory deprivation.

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.

Auditory gating processes and binaural inhibition in the inferior colliculus

Physiological/behavioral/perceptual responses to an auditory stimulus can be inhibited by another leading auditory stimulus at certain stimulus intervals, and have been considered useful models of auditory gating processes. Two typical examples of auditory gating are prepulse inhibition of the startle reflex and the precedence effect (echo suppression). This review summarizes studies of these two auditory gating processes with regard to their biological significance, cognitive modulation, binaural properties, and underlying neural mechanisms. Both prepulse inhibition and the precedence effect have gating functions of reducing the disruptive influence of the lagging sound, but prepulse inhibition has a much longer temporal window than the precedence effect. Attentional processes can modulate prepulse inhibition, and the listener's previous experience can modulate the precedence effect. Compared to monaural hearing, binaural hearing reduces prepulse inhibition but enhances the precedence effect. The inferior colliculus, the major structure of the auditory midbrain, plays an important role in mediating these two auditory gating processes, and inhibitory neural transmissions within the inferior colliculus may account for binaural inhibition observed in prepulse inhibition and lag suppression recorded in the inferior colliculus. The neural mechanisms underlying binaural inhibition in the inferior colliculus are also discussed.

Blocking GABAergic inhibition increases sensitivity to sound motion cues in the inferior colliculus

Responses of low-frequency neurons in the inferior colliculus (IC) of anesthetized guinea pigs were recorded to interaural phase modulation (IPM) before, during, and after iontophoresis of bicuculline, an antagonist to the inhibitory neurotransmitter GABA. Sensitivity to the direction of virtual motion resulting from IPM is an emergent property of neurons at the level of the IC. One model to account for this emergent sensitivity depends on GABAergic inhibition. Blocking GABAergic inhibition with bicuculline substantially increased neuronal discharge rates and increased the extent to which neurons were sensitive to the apparent-motion cues of IPM. The effect of GABA blockade is consistent with the hypothesis that sensitivity to the motion cues of IPM results from a process of adaptation-of-excitation whereby the magnitude of the recent response history influences subsequent neuronal responsiveness. These results indicate that GABAergic inhibition strongly influences the context-dependent processing of low-frequency binaural signals in the IC.

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

The contribution of N-methyl-D-aspartate (NMDA) and AMPA receptors to auditory responses in the rat's inferior colliculus was examined by recording single-unit activity before, during, and after local iontophoretic application of receptor-specific antagonists. Tone bursts and sinusoidal amplitude modulated sounds were presented to one ear, and recordings were made from the contralateral central nucleus of inferior colliculus (ICC). The receptor specific antagonists, (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) for NMDA receptors and 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide (NBQX) for AMPA receptors, were released at the recording site through a multi-barreled pipette. For most neurons, either CPP or NBQX alone resulted in a reversible reduction in the number of action potentials evoked by tonal stimulation. For neurons with an onset response pattern, NBQX either completely eliminated or greatly reduced the number of action potentials. CPP also reduced the number of action potentials but had a less pronounced effect than NBQX. For neurons with a sustained firing pattern, NBQX reduced the total number of action potentials, but had a preferential effect on the early part (first 10-20 ms) of the response. CPP also resulted in a reduction in the total number of action potentials, but had a more pronounced effect on the later part (>20 ms) of the response. These results indicate that both AMPA and NMDA receptors contribute to sound evoked excitatory responses in the ICC. They have a selective influence on early and late components of tone-evoked responses. Both receptor types are involved in generating excitatory responses across a wide range of sound pressure levels as indicated by rate level functions obtained before and during drug application. In addition, both CPP and NBQX reduced responses to sinusoidal amplitude modulated sounds. The synchrony of firing to the modulation envelope as measured by vector strength at different rates of modulation was not greatly affected by either CPP or NBQX in spite of the decrease in firing rate.

Developmental changes in physiological properties in the rat's dorsal nucleus of the lateral lemniscus

To better understand the development of the dorsal nucleus of the lateral lemniscus (DNLL), intrinsic membrane properties and synaptic responses of DNLL neurons in brain slice preparations were examined. Intracellular recordings were taken from DNLL neurons of rat pups at postnatal days 4-8 (early group), 10-12 (intermediate group) and 16-18 (late group). In response to positive current injection, neurons in the early group displayed firing with lower frequency and a longer action potential duration in comparison to the intermediate and late groups. The action potential amplitude of DNLL neurons increased during development. Postsynaptic potentials (PSPs), with excitatory predominance, were elicited by electrical stimulation of the lateral lemniscus and commissure of Probst throughout the three age groups. Neurons showed a longer latency and rise time of the PSPs in the early group in comparison with those in the intermediate and late groups. These results suggest that the early DNLL neurons display physiological characteristics associated with immature neurons, while the other two groups tend to illustrate mature-like neuronal properties. Furthermore, it seems that the neurons at day 10-12 are in a transitional period of development, which coincides with the onset of hearing.

Plasticity in the development of afferent patterns in the inferior colliculus of the rat after unilateral cochlear ablation

The central nucleus of the inferior colliculus (IC) is the site of convergence for nearly all ascending monaural and binaural projections. Several of these inputs, including inhibitory connections from the dorsal nucleus of the lateral lemniscus (DNLL), are highly ordered and organized into series of afferent bands or patches. Although inputs to the IC from the contralateral DNLL are present in the rat by birth [postnatal day 0 (P0)], the earliest indications of band formation are not evident until P4. Subsequently, the initially diffuse projection segregates into a pattern of bands and interband spaces, and by P12 adult-like, afferent-dense patches are established (Gabriele et al., 2000). To determine the role of the auditory periphery in the development of bands and patches before the onset of hearing (P12/P13), unilateral cochlear ablations were performed at P2 (before any evidence of banding). Rat pups were reared to P12, at which time glass pins coated with 1, 1'-dioctodecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate were placed in fixed tissue in the commissure of Probst where DNLL fibers cross the midline. The results indicate that a unilateral cochlear ablation disrupts the normal development of afferent patches in the IC. Although the crossed DNLL projections labeled via commissural dye placement always mirrored each other in P12 controls, ablation cases exhibited a consistent, bilateral asymmetry in pattern formation and relative density of the labeled projections. Possible developmental mechanisms likely to be involved in the establishment of afferent bands and patches before the onset of hearing are discussed.

AMPA receptor binding in adult guinea pig brain stem auditory nuclei after unilateral cochlear ablation

This study determined if an asymmetric hearing loss, due to unilateral cochlear ablation, could induce the regulation of intracellular AMPA receptors in brain stem auditory nuclei. In young adult guinea pigs, the high-affinity specific binding of [(3)H]AMPA was measured in the cochlear nucleus (CN), the superior olivary complex (SOC), and the auditory midbrain at 2-147 postlesion days. After correction for tissue shrinkage, changes in specific binding relative to that in age-matched unlesioned controls were interpreted as altered numbers and/or activity of intracellular AMPA receptors. In the CN, transient elevations and/or deficits in binding were evident in most regions, which usually recovered by 147 days. However, persistently deficient binding was evident ipsilaterally in the anterior part of the anteroventral CN (AVCNa). In the SOC, transient elevations in binding were evident at 2 days in the medial limb of the lateral superior olive (LSOmed) and the medial superior olive. Between 7 and 147 days, most SOC nuclei exhibited transient, temporally synchronized postlesion deficits in binding. However, late in the survival period, deficits persisted ipsilaterally in the LSOmed and the lateral (LSOlat) limb of the lateral superior olive. In the midbrain, transient elevations and/or deficits in binding were evident in the dorsal nucleus of the lateral lemniscus as well as in the central and dorsal nucleus of the inferior colliculus. A persistent deficit was evident in the intermediate nucleus of the lateral lemniscus. The findings implied that auditory neurons contain regulatory mechanisms that control the numbers and/or activity of intracellular AMPA receptors. Regulation was induced by cochlear nerve destruction and probably by changes in the excitation of glutamatergic neurons. Many of the regulatory changes were transient, except in the ipsilateral AVCNa and LSO, where postlesion downregulations were persistent. The downregulation in the ipsilateral AVCNa was probably induced directly by the loss of cochlear nerve endings. However, other regulatory changes may have been induced by signals carried on pathways emerging from the ipsilateral CN and on centrifugal auditory pathways.

The AMPA receptors of auditory neurons

The ionotropic glutamate receptor (GluR) subtype known as the AMPA receptor, which mediates rapid excitatory synaptic transmission in many regions of the nervous system, is composed of four different protein subunits, termed GluRs 1-4. The functional properties of each AMPA receptor are determined by the relative levels of GluRs 1-4 and by post-transcriptional modifications of these proteins through mRNA editing and alternative exon splicing. The present paper reviews the published evidence for (1) localization of mRNAs and immunoreactivity for GluRs 1-4 in the cochlea and subcortical central nervous system auditory pathways of mammals and birds, and (2) involvement of AMPA receptors in synaptic transmission in the auditory system. Recent biochemical and electrophysiological evidence concerning the specialized properties of AMPA receptors on brainstem auditory neurons is also reviewed, along with data concerning how these properties emerge during normal development.

NMDA and AMPA receptors in the dorsal nucleus of the lateral lemniscus shape binaural responses in rat inferior colliculus

Binaural responses of single neurons in the rat's central nucleus of the inferior colliculus (ICC) were recorded before and after local injection of excitatory amino acid receptor antagonists (either 1,2, 3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide disodium [NBQX], (+/-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid [CPP], 6-cyano-7-nitroquinoxaline-2,3-dione [CNQX], or (+/-)-2amino-5-phosphonovaleric acid [APV]) into the dorsal nucleus of the lateral lemniscus (DNLL). Responses were evoked by clicks delivered separately to the two ears at interaural time delays between -1.0 and +30 ms (positive values referring to ipsilateral leading contralateral click pairs). The neurons in our sample were excited by contralateral stimulation and inhibited by ipsilateral stimulation, and the probability of action potentials was reduced as the ipsilateral stimulus was advanced. Binaural inhibition resulted in response suppression that lasted up to 30 ms. Injection of excitatory amino acid antagonists into the DNLL contralateral to the recording site reduced the strength of binaural inhibition in the ICC. The alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist NBQX preferentially affected responses at small interaural time intervals (0-1.0 ms), whereas the N-methyl-D-aspartate (NMDA) antagonist CPP preferentially affected responses at longer intervals (1-30 ms). Both CNQX and APV produced a release from binaural inhibition, but neither drug was selective for specific intervals. The data support the idea that binaural inhibition in the rat ICC is influenced by both AMPA and NMDA receptor-mediated excitatory events in the contralateral DNLL. The results suggest that the AMPA receptors contribute selectively to the initial component of binaural inhibition and the NMDA receptors to a longer lasting component.

The commissure of probst as a source of GABAergic inhibition

Whole-cell patch-clamp recordings were made from neurons in the rat's dorsal nucleus of the lateral lemniscus (DNLL) in a brain slice preparation. Planes of section were chosen to preserve the integrity of fibers in the commissure of Probst (CP) and postsynaptic responses were evoked by electrical stimulation along its length. Results showed that the crossed projection to the DNLL through the CP is mainly, if not exclusively, inhibitory in the rat. Inhibitory postsynaptic responses (IPSPs) evoked by stimulation of the CP were blocked by the gamma-aminobutyric acid (GABA)(A) receptor antagonist bicuculline, but were unaffected by the glycine receptor antagonist strychnine, supporting the conclusion that the crossed inhibitory projection to DNLL from the contralateral DNLL is GABAergic. Stimulation of the CP close to the DNLL frequently evoked excitatory postsynaptic responses as well as IPSPs, but stimulation near the midline evoked IPSPs only. Thus, the excitatory responses probably originated from a pathway other than the projection to the DNLL from the contralateral DNLL through the CP.

Model calculations of steady state responses to binaural stimuli in the dorsal nucleus of the lateral lemniscus

Several studies have been performed in which both the time-dependent and steady state output of cells in the dorsal nucleus of the lateral lemniscus (DNLL) have been measured in response to binaural sound stimulation. In this paper, a mathematical and computational model for the steady state output of DNLL cells is formulated. The model includes ascending connections from both lateral and medial superior olives (LSO and MSO) as well connections from interneurons in the DNLL and connections from the contralateral DNLL through the commissure of Probst. Our intent is to understand how the steady state behavior arises from the cell properties in and connectional patterns from lower brainstem nuclei. In particular, we examine the connectional hypotheses put forward by Markovitz and Pollak (1994) to explain the observed behavior of EI, EI/F, EE/I and EE/FI cells. Using these connections (with minor modifications) and cells with simple input-output relations, we are able to account for the steady state behavior of these cell types. We are able to explain interesting features of the data not commented on before, for example, the initial dip in spike output for EE cells at low ipsilateral sound levels. The presence of an inhibitory interneuron in the DNLL is essential for facilitation. In addition, we examine the effects of the MSO and the commissure of Probst on DNLL output. Furthermore, we propose a simple mechanism by which the cells of the DNLL and LSO could create a topographic place map in the inferior colliculus.

Differential roles of NMDA and non-NMDA receptors in synaptic responses of neurons in nucleus tractus solitarii of the rat

The relative role of N-methyl-D-aspartate (NMDA) and non-NMDA receptors in synaptic responses of neurons in caudal nucleus tractus solitarii (cNTS) was delineated by immunohistochemical and electrophysiologic experiments in rats. Double immunohistochemical staining in in vivo experiments revealed that approximately 80% of cNTS neurons that showed Fos-like immunoreactivity induced by baroreceptor activation were generally also immunoreactive to non-NMDA receptor subunits GluR1 or GluR2. On the other hand, only 20% of Fos-labeled cNTS neurons showed immunoreactivity to NMDA receptor subunits NMDAR1 or NMDAR2. Stimulation of the ipsilateral solitary tract at suprathreshold intensity in slice preparations induced Fos expression in the cNTS and evoked either a single action potential or a complex synaptic response consisting of an initial action potential followed by a secondary slow depolarization. In a majority (70%) of cNTS neurons that exhibited the complex synaptic response, both the initial and secondary components were eliminated reversibly by 6-cyano-7-nitroquinoxaline-2,3-dione (20 microM). This non-NMDA antagonist also inhibited the single action potential manifested by the other population of cNTS neurons. On the other hand, only the secondary slow depolarization was blocked by D(-)-2-amino-5-phosphonopentanoic acid (250 microM) or potentiated by NMDA (1.7 microM). Our results suggested that NMDA and non-NMDA receptors are involved differentially in the synaptic responses of cNTS neurons. Non-NMDA receptors may be distributed predominantly on a majority of the second-order cNTS neurons that may receive primary baroreceptor afferent inputs. On the other hand, NMDA receptors are located primarily on higher-order neurons, which may be connected reciprocally with the second-order cNTS neurons.

Contribution of the commissure of Probst to binaural evoked responses in the rat's inferior colliculus: interaural time differences

Binaural evoked responses were recorded with glass micropipettes from the central nucleus of the rat's inferior colliculus (ICC) before and after transection of the commissure of Probst (CP) with a microsurgical knife. The peak-to-peak amplitude of the averaged evoked response was measured for binaural clicks with interaural time differences (ITDs) between -1.0 and +30.0 ms (positive values reflecting ipsilateral-leading-contralateral click pairs). Before transection, the amplitude of the evoked response decreased as the ITD was shifted in favor of larger ipsilateral lead times. After transection of the CP, acoustic stimulation of the ipsilateral ear was much less effective in reducing evoked response amplitude. Responses to both short (+/-1.0 ms) and long (1.0-30.0 ms) ITD intervals were affected. After recordings were made, both anterograde and retrograde tract tracing methods were used to verify that the CP was completely transected and that all crossed projections from the dorsal nucleus of the lateral lemniscus (DNLL) to ICC were destroyed. The surgery completely eliminated the retrograde transport of fluorogold from the ICC to the opposite DNLL and blocked the anterograde transport of biotinylated dextran to contralateral DNLL and ICC. The physiological consequences of CP transection are attributed to the complete destruction of decussating, inhibitory (GABAergic) efferent projections from the DNLL.

Synaptic excitation in the dorsal nucleus of the lateral lemniscus

The dorsal nucleus of the lateral lemniscus (DNLL) is a distinct auditory neuronal group located ventral to the inferior colliculus (IC). It receives excitatory and inhibitory afferent inputs from various structures of the auditory lower brainstem and sends GABAergic inhibitory efferents mainly to the contralateral DNLL and the bilateral IC. The synaptic excitation in DNLL neurons consists of two components, an early fast depolarization and a later long lasting one. Glutamate is the probable excitatory neurotransmitter for DNLL neurons. alpha-Amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors mediate the early part of the excitation while N-Methyl-D-aspartate (NMDA) receptors mediate the long lasting component. The long lasting NMDA receptor-mediated component in the DNLL may contribute to a prolonged inhibition in the IC. The DNLL is thought to be a structure for processing binaural information. Most DNLL neurons in rat and bat are sensitive to interaural intensity differences (IIDs). They are excited by stimulation of the contralateral ear and inhibited by stimulation of the ipsilateral ear, showing an excitatory/inhibitory (EI) binaural response pattern. The EI pattern can be attributed to synaptic inputs that originate from various structures in the lower auditory brainstem and impinge on the DNLL neurons. In cat some DNLL neurons are sensitive to IIDs and some are sensitive to interaural time differences. In addition, DNLL neurons exhibit different temporal response patterns to contralateral tonal stimulation and respond to amplitude modulated tones, implying that DNLL may contribute to processing temporally complex acoustic information. DNLL neurons shape binaural responses in the contralateral inferior colliculus and auditory cortex through their inhibitory brainstem projections and contribute to the accuracy with which animals localize sounds in space.

Glycine receptors in adult guinea pig brain stem auditory nuclei: regulation after unilateral cochlear ablation

In young adult guinea pigs, the effects of unilateral cochlear ablation were determined on the specific binding of [3H]strychnine measured in subdivisions of the cochlear nucleus (CN), the superior olivary complex, and the auditory midbrain, after 2, 7, 31, 60, and 147 postlesion days. Changes in binding relative to that in age-matched controls were interpreted as altered activity and/or expression of synaptic glycine receptors. Postlesion binding declined ipsilaterally in most of the ventral CN and in the lateral superior olive (LSO). Binding was modestly deficient in the ipsilateral dorsal CN and in the anterior part of the contralateral anteroventral CN. Binding was elevated in the contralateral LSO. Transient changes also occurred. Binding was elevated transiently, between 2 and 31 days, contralaterally in parts of the anteroventral CN, bilaterally in the medial superior olive (MSO), and bilaterally in most of the midbrain nuclei. Binding was deficient transiently, at 60 days, in most of the contralateral CN and bilaterally in the midbrain nuclei. The present findings, together with previously reported postlesion changes in glycine release, were consistent with persistently weakened glycinergic inhibitory transmission ipsilaterally in the ventral CN and the LSO and bilaterally in the dorsal CN. Glycinergic inhibitory transmission was strengthened in the contralateral LSO and transiently strengthened in the MSO bilaterally. A hypothetical model of the findings suggested that glycine receptor regulation may depend on excitatory and glycinergic input to auditory neurons. The present changes in glycine receptor activity may contribute to altered auditory functions, which often accompany hearing loss.

Discharge properties of single neurons in the dorsal nucleus of the lateral lemniscus of the rat

The aim of the present study was to characterize the discharge properties of single neurons in the dorsal nucleus of the lateral lemniscus (DNLL) of the rat. In the absence of acoustic stimulation, two types of spontaneous discharge patterns were observed: units tended to fire in a bursting or in a nonbursting mode. The distribution of units in the DNLL based on spontaneous firing rate followed a rostrocaudal gradient: units with high spontaneous rates were most commonly located in the rostral part of the DNLL, whereas in the caudal part units had lower spontaneous discharge rates. The most common response pattern of DNLL units to 200 ms binaural noise bursts contained a prominent onset response followed by a lower but steady-state response and an inhibitory response in the early-off period. Thresholds of response to noise bursts were on average higher for DNLL units than for units recorded in the inferior colliculus under the same experimental conditions. The DNLL units were arranged according to a mediolateral sensitivity gradient with the lowest threshold units in the most lateral part of the nucleus. In the rat, as in other mammals, the most common DNLL binaural input type was an excitatory response to contralateral ear stimulation and inhibitory response to ipsilateral ear stimulation (EI type). Pure tone bursts were in general a more effective stimulus compared to noise bursts. Best frequency (BF) was established for 97 DNLL units and plotted according to their spatial location. The DNLL exhibits a loose tonotopic organization, where there is a concentric pattern with high BF units located in the most dorsal and ventral parts of the DNLL and lower BF units in the middle part of the nucleus.

Monaural and binaural response properties of single neurons in the rat's dorsal nucleus of the lateral lemniscus

Extracellular recordings were made with microelectrodes from single neurons in the rat's dorsal nucleus of the lateral lemniscus (DNLL) and response characteristics were determined for monaural and binaural acoustic stimulation. The vast majority of DNLL neurons were narrowly tuned to sound frequency and their temporal responses to contralateral tone pulses fell into one of three broad categories: onset (57%), sustained (21%) or onset-pause-sustained (22%). Most DNLL neurons fired multiple action potentials to a single click delivered to the contralateral ear. The majority (77%) of DNLL neurons showed a monotonic increase in the number of spikes elicited by contralateral tone pulses of increasing sound pressure level; the remaining cells were weakly non-monotonic. No obvious tonotopic pattern was found in the distribution of characteristic frequency of neurons in DNLL. Most DNLL neurons exhibited either excitatory/inhibitory (74%) or excitatory/excitatory (9%) binaural response patterns. The remaining cells (17%) were monaural and driven exclusively by stimulation of the contralateral ear. The binaural neurons in DNLL were sensitive to both interaural intensity and interaural time differences as determined by presentation of dichotic tone bursts and clicks respectively. The responses of DNLL neurons could be distinguished on the basis of monaural and binaural response characteristics from those in surrounding areas including the sagulum, paralemniscal zone and the intermediate nucleus of the lateral lemniscus.

Plastic changes in glycine and GABA release and uptake in adult brain stem auditory nuclei after unilateral middle ear ossicle removal and cochlear ablation

[i] In young adult guinea pigs, the effects of unilateral ossicle removal and unilateral cochlear ablation were determined on [14C]glycine or [14C]GABA release and uptake measured in subdivisions of the cochlear nucleus (CN), the superior olivary complex, and the auditory midbrain, after 2 or 5, 59, and 145 postlesion days. Activities were compared to those of age-matched, unlesioned controls. [ii] [14C]Glycine release declined bilaterally in the anteroventral and dorsal CN after ossicle removal and in the dorsal CN after cochlear ablation. [iii] Transient elevations of release occurred at 59 days in the ipsilateral posteroventral CN ([14C]glycine) and bilaterally in the ventral nucleus of the lateral lemniscus ([14C]GABA) after ossicle removal, and bilaterally in the medial superior olive ([14C]glycine) after cochlear ablation. [iv] In the medial nucleus of the trapezoid body, [14C]GABA release was depressed bilaterally 5 days after ossicle removal, but was elevated at 145 days contralaterally after ossicle removal and ipsilaterally after cochlear ablation. [v] In the contralateral central nucleus of the inferior colliculus, [14C]GABA release was elevated persistently after ossicle removal. After cochlear ablation, release was elevated at 5 days, near the control at 59 days, and elevated again at 145 days. [vi] After both lesions, [14C]glycine uptake was elevated bilaterally in the CN and medial superior olive. [14C]GABA uptake became depressed by 59 or 145 days bilaterally in the auditory midbrain. [vii] These changes may stem from regulation and may contribute to mechanisms that generate symptoms such as loudness recruitment and tinnitus, which often accompany hearing loss.

Lack of topography in the ventral nucleus of the lateral lemniscus

In contrast to the ease of finding tonotopicity in other nuclei, both anatomical and electrophysiological methods have failed to demonstrate a clear and simple tonotopic map within the ventral nucleus of the lateral lemniscus (VLL). The present study was undertaken in cat with the hope that methods not used previously in studies of VLL might succeed in demonstrating an orderliness in its exiting fibers (i.e., efferents) or its incoming fibers (i.e., afferents). Since the same organization of ascending frequencies present in the cochlea is maintained in these fibers as well as in all main auditory nuclei, demonstration of a similar organization of frequencies in VLL would be evidence of the cochleo- or tono-topicity of this nucleus. Using triple injection of 3 different fluorescent dyes in inferior colliculus to study efferents, orderly and tonotopic cell-labeling is found in each of the brainstem auditory nuclei, with the notable exception of VLL. Instead, labeling of cell clusters, each cluster containing a small number of cells, is found randomly distributed throughout VLL in all 3 of its spatial dimensions. Using the 2-deoxyglucose (2-DG) method, during stimulation at 6 different frequencies, afferent orderliness, indeed, tonotopicity is found in all major brainstem auditory nuclei, again with the notable exception of VLL. Rather, each frequency evokes 2-DG label throughout VLL. In agreement with the results based on electrophysiological methods, therefore, the anatomical methods used here also yield no evidence of tonotopicity in VLL. Thus, if there is orderliness in VLL's efferents or afferents, it is based on an auditory dimension incommensurate with frequency.

Role of synaptic inhibition in processing of dynamic binaural level stimuli

We have recently discovered a paradoxical aftereffect associated with inhibition in the gerbil auditory midbrain. Single neurons in the inferior colliculus (IC) were assessed for sensitivity to a virtual motion stimulus produced by modulating the interaural level difference (ILD), a major cue for sound localization. The class of neuron studied was predominantly excited by contralateral stimulation and inhibited by ipsilateral stimulation. Sound pressure level was modulated trapezoidally at the ipsilateral "inhibitory" ear, whereas the contralateral "excitatory" level remained constant. When the inhibitory stimulus was decreased within a range of sound levels that maintained suppression under static conditions, an unexpected discharge was often elicited, apparently because of an aftereffect of synaptic inhibition. In contrast, when the inhibitory stimulus was increased within a range of sound levels that produced only modest suppression under static conditions, neuronal discharge was often profoundly suppressed. In many cases the "conditioned enhancement" or "conditioned suppression" persisted for several seconds after the modulation of ILD, and such conditioned responses were influenced by the modulation depth and rate. To test the effect of inhibition in the IC directly, glycine and GABA were pulsed from a glass recording pipette during a constant monaural excitatory stimulus. The acoustically elicited discharge rate was potentiated markedly if preceded immediately by the brief (0.5-10 sec) application of inhibitory transmitter. Collectively, these results revealed unusually long-lasting effects of inhibition that may establish a new range of acoustic cues to which the neuron responds best. This may have broad implications for processing ensuing auditory stimuli.

Projections from the superior olive and lateral lemniscus to tonotopic regions of the rat's inferior colliculus

The projections to physiologically defined tonotopic regions of the central nucleus of the inferior colliculus (ICC) from the adult rat's superior olivary complex (SOC) and lateral lemniscus were investigated using retrograde tract tracing methods. Iontophoretic injections of the retrograde tracers, Fluoro-Gold (FG) or horseradish peroxidase (HRP), were made into the ICC through a glass micropipette, which also served as a recording electrode to determine the frequency response at the injection site. Injections were made into frequency-specific regions based on the best responses of neurons to contralaterally presented tones between 2 25 kHz. In the dorsal nucleus of the lateral lemniscus (DNLL) neurons were labeled both ipsilaterally and contralaterally to the injection site with a larger proportion projecting to the contralateral side. The distribution of labeled cells was concentric, with high frequencies represented along the outer margin and low frequencies represented centrally within DNLL. The lateral superior olive (LSO) was labeled bilaterally, with high frequencies represented medially and low frequencies laterally along the nuclear axis. The projection from the medial superior olive (MSO) was ipsilateral, with high frequencies represented ventrally and low frequencies dorsally. The projection from the superior paraolivary nucleus (SPN) was also largely ipsilateral, with high frequencies represented medially and low frequencies laterally. The intermediate and ventral nuclei of the lateral lemniscus (INLL and VNLL) were also labeled ipsilaterally and exhibited a distribution of tracer that depended on the frequency of the injection site: the low frequency projection was banded but the high frequency projection was more evenly distributed.

Hyperpolarization-activated inward current in neurons of the rat's dorsal nucleus of the lateral lemniscus in vitro

Hyperpolarization-activated inward current in neurons of the rat's dorsal nucleus of the lateral lemniscus in vitro. J. Neurophysiol. 78: 2235-2245, 1997. The hyperpolarization-activated current (Ih) underlying inward rectification in neurons of the rat's dorsal nucleus of the lateral lemniscus (DNLL) was investigated using whole cell patch-clamp techniques. Patch recordings were made from DNLL neurons of young rats (21-30 days old) in 400 micro;m tissue slices. Under current clamp, injection of negative current produced a graded hyperpolarization of the cell membrane, often with a gradual sag in the membrane potential toward the resting value. The rate and magnitude of the sag depended on the amount of hyperpolarizing current. Larger current resulted in a larger and faster decay of the voltage. Under voltage clamp, hyperpolarizing voltage steps elicited a slowly activating inward current that was presumably responsible for the sag observed in the voltage response to a steady hyperpolarizing current recorded under current clamp. Activation of the inward current (Ih) was voltage and time dependent. The current just was seen at a membrane potential of -70 mV and was activated fully at -140 mV. The voltage value of half-maximal activation of Ih was -78.0 +/- 6.0 (SE) mV. The rate of Ih activation was best approximated by a single exponential function with a time constant that was voltage dependent, ranging from 276 +/- 27 ms at -100 mV to 186 +/- 11 ms at -140 mV. Reversal potential (Eh) of Ih current was more positive than the resting potential. Raising the extracellular potassium concentration shifted Eh to a more depolarized value, whereas lowering the extracellular sodium concentration shifted Eh in a more negative direction. Ih was sensitive to extracellular cesium but relatively insensitive to extracellular barium. The current amplitude near maximal-activation (about -140 mV) was reduced to 40% of control by 1 mM cesium but was reduced to only 71% of control by 2 mM barium. When the membrane potential was near the resting potential (about -60 mV), cesium had no effect on the membrane potential, current-evoked firing rate and input resistance but reduced the spontaneous firing. When the membrane potential was more negative than -70 mV, cesium hyperpolarized the cell, decreased current-evoked firing and increased the input resistance. Ih in DNLL neurons does not contribute to the normal resting potential but may enhance the extent of excitation, thereby making the DNLL a consistently powerful inhibitory source to upper levels of the auditory system.

Synaptic excitation in the dorsal nucleus of the lateral lemniscus: whole-cell patch-clamp recordings from rat brain slice

The synaptic events underlying the excitation of neurons in the rat's dorsal nucleus of the lateral lemniscus were studied by whole-cell patch-clamp recordings in a brain slice preparation of the auditory midbrain. Both current-clamp and voltage-clamp data were obtained with the brain slice submerged in artificial cerebrospinal fluid. The rats were between 21 and 35 days of age at the time the recordings were made. Synaptic responses were evoked by a bipolar stimulating electrode placed on the lateral lemniscus just ventral to the dorsal nucleus. To eliminate glycinergic inhibitory responses, all physiological data were gathered with 0.5 microM strychnine added to the saline bath. Under current-clamp conditions, excitatory postsynaptic potentials could be subdivided into early and late components. The early component produced a single, highly reliable, short-latency spike and the later component produced a more variable, long-latency spike or train of spikes. The non-N-methyl-D-aspartate antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione, completely blocked the early excitatory postsynaptic potential and its associated action potential. The N-methyl-D-aspartate antagonist, D,L-2-amino-5-phosphonovaleric acid, blocked the later excitatory postsynaptic potential and its action potentials. Typically, both early and late excitatory postsynaptic potentials could be recorded from the same cell, but the early excitatory postsynaptic potential was evoked at lower stimulus levels and had a larger amplitude than the later excitatory postsynaptic potential. Under voltage-clamp conditions, dorsal nucleus of the lateral lemniscus neurons responded to stimulation of the lateral lemniscus with excitatory postsynaptic currents. Outward excitatory postsynaptic currents were recorded with holding potentials that depolarized the cell membrane and inward currents were seen when the cell was hyperpolarized. The current-voltage (I-V) relation of the early peak portion of the excitatory postsynaptic current was nearly linear, whereas the I-V relation of the later excitatory postsynaptic current (12 ms after the peak) was non-linear over the range between -50 and - 100 mV. The outward excitatory postsynaptic current consisted of an early current that was selectively blocked by 6-cyano-7-nitroquinoxaline-2,3-dione and a later current that was blocked by D,L-2-amino-5-phosphonovaleric acid. In artificial cerebrospinal fluid with normal concentrations of Mg2+, the inward excitatory postsynaptic current was blocked by 6-cyano-7-nitroquinoxaline-2,3-dione, but was not affected by D,L-2-amino-5-phosphonovaleric acid. In Mg2+-free artificial cerebrospinal fluid. however, the early component of the inward excitatory postsynaptic current was selectively blocked by 6-cyano-7-nitroquinoxaline-2,3-dione and a later component was blocked by D,L-2-amino-5-phosphonovaleric acid. The results indicate that both N-methyl-D-aspartate and non-N-methyl-D-aspartate receptor-mediated synaptic responses are present in dorsal nucleus of the lateral lemniscus neurons of rats at 21-35 days of age. The N-methyl-D-aspartate component had a longer time-course and a higher threshold than the non-N-methyl-D-aspartate component, and was subject to a voltage-dependent Mg2+ block when the cell's membrane was hyperpolarized. The long-duration N-methyl-D-aspartate component is probably responsible for the prolonged inhibitory effect of dorsal nucleus of the lateral lemniscus neurons on physiological responses in the rat's inferior colliculus.

Two sources of inhibition affecting binaural evoked responses in the rat's inferior colliculus: the dorsal nucleus of the lateral lemniscus and the superior olivary complex

The present study was undertaken to determine the influence of two extrinsic sources of inhibition on auditory binaural evoked responses recorded from the rat's inferior colliculus. The first source, the dorsal nucleus of the lateral lemniscus (DNLL), is predominantly GABAergic and has both ipsi- and contralateral projections to the central nucleus of the inferior colliculus (ICC). The second, the superior olivary complex (SOC), has a large glycinergic projection from the lateral superior olive (LSO) to the ipsilateral ICC. Thus, both structures are candidates for imposing an inhibitory effect on responses in the ICC. Neural activity was experimentally blocked by local injection of the excitatory amino acids antagonist, kynurenic acid (KYNA), into either DNLL or SOC. Binaural evoked responses were recorded from the ICC as the intensity of the sound in the ipsilateral ear was increased. Interaural intensity difference functions based on the amplitude of the evoked responses were generated before and after the KYNA injection. An injection into the contralateral DNLL greatly reduced the response suppression produced by stimulation of the ipsilateral ear. Injection into the ipsilateral DNLL, however, had no effect. Injection into the ipsilateral SOC reduced the amount of binaural suppression but the effect was apparent only in cases with surgical transection of the contralateral lateral lemniscus at a level below the DNLL. These data support the conclusion that binaural responses in the rat's ICC are shaped by inhibitory projections from both contralateral DNLL and ipsilateral SOC.

Contribution of the dorsal nucleus of the lateral lemniscus to binaural responses in the inferior colliculus of the rat: interaural time delays

The contribution of the dorsal nucleus of the lateral lemniscus (DNLL) to binaural responses in the inferior colliculus of the rat was determined for a wide range of interaural time differences (ITDs). Single-unit action potentials were recorded from the inferior colliculus before and after local injection of the excitatory amino acid antagonist kynurenic acid into the DNLL. Binaural properties were determined by manipulating the time difference between paired clicks delivered to the ears ipsilateral and contralateral to the recording site. The probability of an action potential decreased as contralateral stimulation was delayed, relative to ipsilateral stimulation. These data generated a sigmoidal ITD curve for delays between -1.0 and + 1.0 msec. By extending the time intervals beyond 1 msec, it was possible to determine the trailing edge of the inhibition produced by ipsilateral stimulation. The duration of the inhibitory effect varied from cell to cell but lasted as long as 20 msec in some cases. Injection of kynurenic acid into the DNLL contralateral to the recording site reduced the extent of both short (0-1 msec) and long-lasting (1-20 msec) inhibition in the inferior colliculus. No effect was seen after injections ipsilateral to the recording site. The data demonstrate that the DNLL plays an important role in shaping ITD responses in the inferior colliculus and contributes to both the short and long-lasting inhibition produced by stimulation of the ipsilateral ear.