Discharge patterns of neurons in the ventral nucleus of the lateral lemniscus of the unanesthetized rabbit

Information Processing by Onset Neurons in the Cat Auditory Brainstem

Octopus cells in the ventral cochlear nucleus (VCN) have been difficult to study because of the very features that distinguish them from other VCN neurons. We performed in vivo recordings in cats on well-isolated units, some of which were intracellularly labeled and histologically reconstructed. We found that responses to low-frequency tones with frequencies < 1 kHz reveal higher levels of neural synchrony and entrainment to the stimulus than the auditory nerve. In responses to higher frequency tones, the neural discharges occur mostly near the stimulus onset. These neurons also respond in a unique way to 100 % amplitude-modulated (AM) tones with discharges exhibiting a bandpass tuning. Responses to frequency-modulated sounds (FM) are unusual: Octopus cells react more vigorously during the ascending than the descending parts of the FM stimulus. We examined responses of neurons in the ventral nucleus of the lateral lemniscus (VNLL) whose discharges to tones and AM sounds are similar to octopus cells. Repeated stimulation with short tone pips of VCN and VNLL onset neurons evokes trains of action potentials with gradual shifts toward later times in their first spike latency. This behavior parallels short-term post-synaptic depression observed by other authors in in vitro VCN recordings of octopus cells. VCN and VNLL onset units in cats respond to frozen noise stimuli with gaps as narrow as 1 ms with a robust discharge near the stimulus onset following the gap. This finding suggests that VCN and VNLL onset cells play a role in gap detection, which is of great importance to speech perception.

Heterogeneity of Intrinsic and Synaptic Properties of Neurons in the Ventral and Dorsal Parts of the Ventral Nucleus of the Lateral Lemniscus

The ventral nucleus of the lateral lemniscus (VNLL) provides a major inhibitory projection to the inferior colliculus (IC). Neurons in the VNLL respond with various firing patterns and different temporal precision to acoustic stimulation. The present study investigates the underlying intrinsic and synaptic properties of various cell types in different regions of the VNLL, using in vitro electrophysiological recordings from acute brain slices of mice and immunohistochemistry. We show that the biophysical membrane properties and excitatory input characteristics differed between dorsal and ventral VNLL neurons. Neurons in the ventral VNLL displayed an onset-type firing pattern and little hyperpolarization-activated current (I_h). Stimulation of lemniscal inputs evoked a large all-or-none excitatory response similar to Calyx of Held synapses in neurons in the lateral part of the ventral VNLL. Neurons that were located within the fiber tract of the lateral lemniscus, received several and weak excitatory input fibers. In the dorsal VNLL onset-type and sustained firing neurons were intermingled. These neurons showed large I_h and were strongly immunopositive for the hyperpolarization-activated cyclic nucleotide-gated channel 1 (HCN1) subunit. Both neuron types received several excitatory inputs that were weaker and slower compared to ventrolateral VNLL neurons. Using a mouse model that expresses channelrhodopsin under the promotor of the vesicular GABA transporter (VGAT) suggests that dorsal and ventral neurons were inhibitory since they were all depolarized by light stimulation. The diverse membrane and input properties in dorsal and ventral VNLL neurons suggest differential roles of these neurons for sound processing.

Acoustic signal characteristic detection by neurons in ventral nucleus of the lateral lemniscus in mice

Under free field conditions, we used single unit extracellular recording to study the detection of acoustic signals by neurons in the ventral nucleus of the lateral lemniscus (VNLL) in Kunming mouse (Mus musculus). The results indicate two types of firing patterns in VNLL neurons: onset and sustained. The first spike latency (FSL) of onset neurons was shorter than that of sustained neurons. With increasing sound intensity, the FSL of onset neurons remained stable and that of sustained neurons was shortened, indicating that onset neurons are characterized by precise timing. By comparing the values of Q10 and Q30 of the frequency tuning curve, no differences between onset and sustained neurons were found, suggesting that firing pattern and frequency tuning are not correlated. Among the three types of rate-intensity function (RIF) found in VNLL neurons, the proportion of monotonic RIF is the largest, followed by saturated RIF, and non-monotonic RIF. The dynamic range (DR) in onset neurons was shorter than in sustained neurons, indicating different capabilities in intensity tuning of different firing patterns and that these differences are correlated with the type of RIF. Our results also show that the best frequency of VNLL neurons was negatively correlated with depth, supporting the view point that the VNLL has frequency topologic organization.

Temporal properties of responses to sound in the ventral nucleus of the lateral lemniscus

Besides the rapid fluctuations in pressure that constitute the "fine structure" of a sound stimulus, slower fluctuations in the sound's envelope represent an important temporal feature. At various stages in the auditory system, neurons exhibit tuning to envelope frequency and have been described as modulation filters. We examine such tuning in the ventral nucleus of the lateral lemniscus (VNLL) of the pentobarbital-anesthetized cat. The VNLL is a large but poorly accessible auditory structure that provides a massive inhibitory input to the inferior colliculus. We test whether envelope filtering effectively applies to the envelope spectrum when multiple envelope components are simultaneously present. We find two broad classes of response with often complementary properties. The firing rate of onset neurons is tuned to a band of modulation frequencies, over which they also synchronize strongly to the envelope waveform. Although most sustained neurons show little firing rate dependence on modulation frequency, some of them are weakly tuned. The latter neurons are usually band-pass or low-pass tuned in synchronization, and a reverse-correlation approach demonstrates that their modulation tuning is preserved to nonperiodic, noisy envelope modulations of a tonal carrier. Modulation tuning to this type of stimulus is weaker for onset neurons. In response to broadband noise, sustained and onset neurons tend to filter out envelope components over a frequency range consistent with their modulation tuning to periodically modulated tones. The results support a role for VNLL in providing temporal reference signals to the auditory midbrain.

Local neuronal circuits that may shape the discharge patterns of inferior collicular neurons

The discharge patterns of neurons in auditory centers encode information about sounds. However, few studies have focused on the synaptic mechanisms underlying the shaping of discharge patterns using intracellular recording techniques. Here, we investigated the discharge patterns of inferior collicular (IC) neurons using intracellular recordings to further elucidate the mechanisms underlying the shaping of discharge patterns. Under in vivo intracellular recording conditions, recordings were obtained from 66 IC neurons in 18 healthy adult mice (Mus musculus, Km) under free field-stimulation. Fifty-eight of these neurons fi red bursts of action potentials (APs) to auditory stimuli and the remaining eight just generated local responses such as excitatory (n = 4) or inhibitory (n = 4) postsynaptic potentials. Based on the APs and subthreshold responses, the discharge patterns were classified into seven types: phasic (24/58, 41.4%), phasic burst (8/58,13.8%), pauser (4/58, 6.9%), phasic-pauser (1/58, 1.7%), chopper (2/58, 3.4%), primary-like tonic (14/58, 24.1%) and sound-induced inhibitory (5/58,8.6%). We concluded that (1) IC neurons exhibit at least seven distinct discharge patterns; (2) inhibition participates in shaping the discharge pattern of most IC neurons and plays a role in sculpting the pattern, except for the primary-like tonic pattern which was not shaped by inhibition; and (3) local neural circuits are the likely structural basis that shapes the discharge patterns of IC neurons and can be formed either in the IC or in lower-level auditory structures.

Distinct neural firing mechanisms to tonal stimuli offset in the inferior colliculus of mice in vivo

Offset neurons, which fire at the termination of sound, likely encode sound duration and serve to process temporal information. Offset neurons are found in most ascending auditory nuclei; however, the neural mechanisms that evoke offset responses are not well understood. In this study, we examined offset neural responses to tonal stimuli in the inferior colliculus (IC) in vivo with extracellular and intracellular recording techniques in mice. Based on peristimulus time histogram (PSTH) patterns, we classified extracellular offset responses into four types: Offset, Onset-Offset, Onset-Sustained-Offset and Inhibition-Offset types. Moreover, using in vivo whole-cell recording techniques, we found that offset responses were generated in most cells through the excitatory and inhibitory synaptic inputs. However, in a small number of cells, the offset responses were generated as a rebound to hyperpolarization during tonal stimulation. Many offset neurons fired robustly at a preferred duration of tonal stimulus, which corresponded with the timing of rich excitatory synaptic inputs. We concluded that most IC offset neurons encode the termination of the tone stimulus by responding to inherited ascending synaptic information, which is tuned to sound duration. The remainder generates offset spikes de novo through a post-inhibitory rebound mechanism.

Spectral and temporal modulation tradeoff in the inferior colliculus

The cochlea encodes sounds through frequency-selective channels that exhibit low-pass modulation sensitivity. Unlike the cochlea, neurons in the auditory midbrain are tuned for spectral and temporal modulations found in natural sounds, yet the role of this transformation is not known. We report a distinct tradeoff in modulation sensitivity and tuning that is topographically ordered within the central nucleus of the inferior colliculus (CNIC). Spectrotemporal receptive fields (STRFs) were obtained with 16-channel electrodes inserted orthogonal to the isofrequency lamina. Surprisingly, temporal and spectral characteristics exhibited an opposing relationship along the tonotopic axis. For low best frequencies (BFs), units were selective for fast temporal and broad spectral modulations. A systematic progression was observed toward slower temporal and finer spectral modulation sensitivity at high BF. This tradeoff was strongly reflected in the arrangement of excitation and inhibition and, consequently, in the modulation tuning characteristics. Comparisons with auditory nerve fibers show that these trends oppose the pattern imposed by the peripheral filters. These results suggest that spectrotemporal preferences are reordered within the tonotopic axis of the CNIC. This topographic organization has profound implications for the coding of spectrotemporal features in natural sounds and could underlie a number of perceptual phenomena.

Glycinergic inhibition creates a form of auditory spectral integration in nuclei of the lateral lemniscus

For analyses of complex sounds, many neurons integrate information across different spectral elements via suppressive effects that are distant from the neurons' excitatory tuning. In the mustached bat, suppression evoked by sounds within the first sonar harmonic (23-30 kHz) or in the subsonar band (<23 kHz) alters responsiveness to the higher best frequencies of many neurons. This study examined features and mechanisms associated with low-frequency (LF) suppression among neurons of the lateral lemniscal nuclei (NLL). We obtained extracellular recordings from neurons in the intermediate and ventral nuclei of the lateral lemniscus, observing different forms of LF suppression related to the two above-cited frequency bands. To understand the mechanisms underlying this suppression in NLL neurons, we examined the roles of glycinergic and GABAergic input through local microiontophoretic application of strychnine, an antagonist to glycine receptors (GlyRs), or bicuculline, an antagonist to gamma-aminobutyric acid type A receptors (GABA(A)Rs). With blockade of GABA(A)Rs, neurons showed an increase in firing rate to best frequency (BF) and/or LF tones but retained LF suppression of BF sounds. For neurons that displayed LF suppression tuned to 23-30 kHz, the suppression was eliminated or nearly eliminated by GlyR blockade. In contrast, GABA(A)R blockade did not eliminate nor had any consistent effect on suppression tuned to these frequencies. We conclude that LF suppression tuned in the 23- to 30-kHz range results from neuronal inhibition within the NLL via glycinergic inputs. For neurons displaying suppression tuned <23 kHz, neither GlyR nor GABAR blockade altered LF suppression. We conclude that such suppression originates at a lower auditory level, perhaps a result of cochlear mechanisms. These findings demonstrate that neuronal interactions within NLL create a particular form of LF suppression that contributes to the analysis of complex acoustic signals.

Temporal features of spectral integration in the inferior colliculus: effects of stimulus duration and rise time

This report examines temporal features of facilitation and suppression that underlie spectrally integrative responses to complex vocal signals. Auditory responses were recorded from 160 neurons in the inferior colliculus (IC) of awake mustached bats. Sixty-two neurons showed combination-sensitive facilitation: responses to best frequency (BF) signals were facilitated by well-timed signals at least an octave lower in frequency, in the range 16-31 kHz. Temporal features and strength of facilitation were generally unaffected by changes in duration of facilitating signals from 4 to 31 ms. Changes in stimulus rise time from 0.5 to 5.0 ms had little effect on facilitatory strength. These results suggest that low frequency facilitating inputs to high BF neurons have phasic-on temporal patterns and are responsive to stimulus rise times over the tested range. We also recorded from 98 neurons showing low-frequency (11-32 kHz) suppression of higher BF responses. Effects of changing duration were related to the frequency of suppressive signals. Signals<23 kHz usually evoked suppression sustained throughout signal duration. This and other features of such suppression are consistent with a cochlear origin that results in masking of responses to higher, near-BF signal frequencies. Signals in the 23- to 30-kHz range-frequencies in the first sonar harmonic-generally evoked phasic suppression of BF responses. This may result from neural inhibitory interactions within and below IC. In many neurons, we observed two or more forms of the spectral interactions described here. Thus IC neurons display temporally and spectrally complex responses to sound that result from multiple spectral interactions at different levels of the ascending auditory pathway.

The ventral nucleus of the lateral lemniscus of the gerbil (Meriones unguiculatus): organization of connections with the cochlear nucleus and the inferior colliculus

The spatial organization of projections from the ventral cochlear nucleus (VCN) to the ventral nucleus of the lateral lemniscus (VNLL) and from the VNLL to the central nucleus of the inferior colliculus (CNIC) was investigated by using neuroanatomical tracing methods in the gerbil. In order to label cells in the VNLL that project to the CNIC, focal injections of biotinylated dextran amine (BDA) were made into different CNIC regions. Retrogradely labeled cells were distributed throughout the dorsal-to-ventral axis of the VNLL in all cases. In contrast, the distribution of labeled cells across the lateral-to-medial dimension of the VNLL was related to the location of the injection site along the dorsolateral to ventromedial (frequency) axis of the CNIC. Cells projecting to dorsolateral (low-frequency) regions of the CNIC were located peripherally in the VNLL, mainly laterally and caudally, whereas those projecting to ventromedial (high-frequency) regions of the CNIC tended to be clustered centrally. Projections to the VNLL were labeled anterogradely following injections of BDA in the VCN. The distribution of terminal fields in the VNLL closely paralleled the topographic arrangement of cells projecting to the CNIC; projections from ventrolateral (low-frequency) areas of the VCN terminated mainly along the lateral and caudal borders of the VNLL, whereas projections from dorsomedial (high-frequency) areas terminated in more central regions. The results demonstrate a topographic organization of the major afferent and efferent connections of the gerbil VNLL.

Intracellular responses and morphology of rat ventral complex of the lateral lemniscus neurons in vivo

The function of the ventral and intermediate nuclei of the lateral lemniscus (VNLL and INLL), collectively termed ventral complex of the lateral lemniscus (VCLL), is unclear. Several studies have suggested that it plays a role in coding the temporal aspects of sound. In our study, a sample (n = 161) of intracellular responses to dichotically presented noise or tone bursts was collected from the VCLL of urethane-anesthetized rats in vivo. Intracellular recordings revealed six distinct response types to tones, distinguished by their synaptic and membrane characteristics as well as firing pattern. Three of these response types were correlated with distinct cellular morphologies revealed by intracellular injection of neurobiotin. 3D reconstructions of recorded neurons within the VCLL showed the spatial distribution of various response properties, including response type, laterality, characteristic frequency (CF), and binaural influences. Cells that responded to monaural (55%) or binaural (45%) stimulation were distributed throughout the VCLL. Almost all VCLL units were responsive to contralateral stimulation (97%). Most neurons were excited by contralateral stimulation (83%), many exclusively (43%), and some in conjunction with ipsilateral inhibition (28%) or excitation (12%). The INLL contained mostly binaural neurons (65%), typically with ipsilateral inhibition and contralateral excitation. These results indicate that the VCLL is not a monaural structure and there is a dorsal-ventral segregation of binaural and monaural cells. 3D reconstructions of intracellular CFs did not reveal the presence of any tonotopic arrangement within the VCLL. Presumably, the proposed timing role of this structure does not require a systematic representation of tonal frequency.

A neural circuit transforming temporal periodicity information into a rate-based representation in the mammalian auditory system

Periodic amplitude modulations (AMs) of an acoustic stimulus are presumed to be encoded in temporal activity patterns of neurons in the cochlear nucleus. Physiological recordings indicate that this temporal AM code is transformed into a rate-based periodicity code along the ascending auditory pathway. The present study suggests a neural circuit for the transformation from the temporal to the rate-based code. Due to the neural connectivity of the circuit, bandpass shaped rate modulation transfer functions are obtained that correspond to recorded functions of inferior colliculus (IC) neurons. In contrast to previous modeling studies, the present circuit does not employ a continuously changing temporal parameter to obtain different best modulation frequencies (BMFs) of the IC bandpass units. Instead, different BMFs are yielded from varying the number of input units projecting onto different bandpass units. In order to investigate the compatibility of the neural circuit with a linear modulation filterbank analysis as proposed in psychophysical studies, complex stimuli such as tones modulated by the sum of two sinusoids, narrowband noise, and iterated rippled noise were processed by the model. The model accounts for the encoding of AM depth over a large dynamic range and for modulation frequency selective processing of complex sounds.

Responses of Neurons in the Rat's Ventral Nucleus of the Lateral Lemniscus to Amplitude-Modulated Tones

Recordings were made from single neurons in the rat's ventral nucleus of the lateral lemniscus (VNLL) to determine responses to amplitude-modulated (AM) tones. The neurons were first characterized on the basis of their response to tone bursts presented to the contralateral ear and a distinction was made between those with transient onset responses and those with sustained responses. Sinusoidal AM tones were then presented to the contralateral ear with a carrier that matched the neuron's characteristic frequency (CF). Modulation transfer functions were generated on the basis of firing rate (MTFFR) and vector strength (MTFVS). Ninety-two percent of onset neurons that responded continuously to AM tones had band-pass MTFFRs with best modulation frequencies from 10 to 300 Hz. Fifty-four percent of sustained neurons had band-pass MTFFRs with best modulation frequencies from 10 to 500 Hz; other neurons had band-suppressed, all-pass, low-pass, or high-pass functions. Most neurons showed either band-pass or low-pass MTFVS. Responses were well synchronized to the modulation cycle with maximum vector strengths ranging from 0.37 to 0.98 for sustained neurons and 0.78 to 0.99 for onset neurons. The upper frequency limit for response synchrony was higher than that reported for inferior colliculus, but lower than that seen in more peripheral structures. Results suggest that VNLL neurons, especially those with onset responses to tone bursts, are sensitive to temporal features of sounds and narrowly tuned to different modulation rates. However, there was no evidence of a topographic relation between dorsoventral position along the length of VNLL and best modulation frequency as determined by either firing rate or vector strength.

Distinct roles for glycine and GABA in shaping the response properties of neurons in the superior paraolivary nucleus of the rat

The superior paraolivary nucleus (SPON) is a prominent periolivary cell group of the superior olivary complex. SPON neurons use gamma-aminobutyric acid (GABA) as their neurotransmitter and are contacted by large numbers of glycinergic and GABAergic punctate profiles, representing a dense inhibitory innervation from the medial nucleus of the trapezoid body (MNTB) and from collaterals of SPON axons, respectively. SPON neurons have low rates of spontaneous activity, respond preferentially to the offset of pure tones, and phase-lock to amplitude-modulated tones. To determine the roles of glycine and GABA in shaping SPON responses, we recorded from single units in the SPON of anesthetized rats before, during, and after application of the glycine receptor antagonist strychnine, the GABA(A) receptor antagonist bicuculline, or both drugs applied simultaneously. Strychnine caused a major increase in spike counts during the stimulus presentation, followed by the disappearance of offset spikes. In half of the recorded units, bicuculline caused moderately increased firing during the stimulus. However, in 86% of units bicuculline also caused a large increase in the magnitude of the offset response. Application of the drug cocktail caused increased spontaneous activity, dramatically increased spike counts during the stimulus presentation, and eliminated the offset response in most units. We conclude that glycinergic inhibition from the MNTB suppresses SPON spiking during sound stimulation and is essential in generating offset responses. GABAergic inhibition, presumably from intrinsic SPON collaterals, plays a subtler role, contributing in some cells to suppression of firing during the stimulus and in most cells to restrict firing after stimulus offset.

Responses of neurons in the ventral nucleus of the lateral lemniscus to sinusoidally amplitude modulated tones

Fluctuations in the amplitude of a sound play an important role in our perception of pitch and acoustic space, but their neural analysis has not been fully elucidated. The ventral nucleus of the lateral lemniscus (VNLL) has been implicated in the processing of such temporal features of a sound. This study examines responses of neurons in the VNLL of unanesthetized rabbits to sinusoidally amplitude modulated tones, a type of stimulus that has often been used to investigate encoding of temporal information. Modulation transfer functions of responses were calculated in two ways: based on discharge rates (rMTFs) and on synchronization to the envelope (tMTFs). Among the variety of rMTFs, two types were readily identifiable: flat and band-pass. The responses of neurons exhibiting these types of rMTF differed in several ways. Neurons with flat rMTFs typically had moderate rates of spontaneous activity, sustained responses to short tone bursts, and low-pass or band-pass tMTFs. Neurons with band-pass rMTFs typically had low spontaneous activity, onset responses to short tone bursts, and flat tMTFs. The vast majority synchronized strongly to the modulation envelope. The best modulation frequencies of neurons with band-pass rMTFs extended from 14 to 283 Hz. The presence of neurons with band-pass rMTFs in the VNLL suggests that this nucleus plays a role in converting the temporal code for modulation frequency used in lower structures into a rate-based code for use higher in the auditory pathway. The substantial number of neurons with more complex modulation transfer functions indicates that the VNLL has other functions.

Responses of neurons in the rat's ventral nucleus of the lateral lemniscus to monaural and binaural tone bursts

Responses to monaural and binaural tone bursts were recorded from neurons in the rat's ventral nucleus of the lateral lemniscus (VNLL). Most of the neurons (55%) had V- or U-shaped frequency-tuning curves with a single clearly defined characteristic frequency (CF). However, many neurons had more complex, multipeaked tuning curves (37%), or other patterns (8%). Temporal firing patterns included both onset and sustained responses to contralateral tone bursts. Onset and sustained responses were distributed along the dorsoventral length of VNLL with no indication of segregation into different regions. Onset neurons had shorter average first-spike latencies than neurons with sustained responses (means, 8.3 vs. 14.8 ms). They also had less jitter, as reflected in the SD of first-spike latencies, than neurons with sustained responses (means, 0.59 and 4.2 ms, respectively). The extent of jitter decreased with an increase in stimulus intensity for neurons with sustained responses, but remained unchanged for onset neurons tested over the same range. Many neurons had binaural responses, primarily of the excitatory/inhibitory (EI) type, widely distributed along the dorsoventral extent of VNLL. Local application of the AMPA receptor antagonist NBQX reduced excitatory responses, indicating that responses were dependent on synaptic activity and not recorded from passing fibers. The results show that many neurons in VNLL have a precision of timing that is well suited for processing auditory temporal information. In the rat, these neurons are intermingled among cells with less precise temporal response features and include cells with binaural as well as monaural responses.

Phase-locked responses to pure tones in the inferior colliculus

In the auditory system, some ascending pathways preserve the precise timing information present in a temporal code of frequency. This can be measured by studying responses that are phase-locked to the stimulus waveform. At each stage along a pathway, there is a reduction in the upper frequency limit of the phase-locking and an increase in the steady-state latency. In the guinea pig, phase-locked responses to pure tones have been described at various levels from auditory nerve to neocortex but not in the inferior colliculus (IC). Therefore we made recordings from 161 single units in guinea pig IC. Of these single units, 68% (110/161) showed phase-locked responses. Cells that phase-locked were mainly located in the central nucleus but also occurred in the dorsal cortex and external nucleus. The upper limiting frequency of phase-locking varied greatly between units (80-1,034 Hz) and between anatomical divisions. The upper limits in the three divisions were central nucleus, >1,000 Hz; dorsal cortex, 700 Hz; external nucleus, 320 Hz. The mean latencies also varied and were central nucleus, 8.2 +/- 2.8 (SD) ms; dorsal cortex, 17.2 ms; external nucleus, 13.3 ms. We conclude that many cells in the central nucleus receive direct inputs from the brain stem, whereas cells in the external and dorsal divisions receive input from other structures that may include the forebrain.

Early Appearance of Inhibitory Input to the MNTB Supports Binaural Processing During Development

Despite the peripheral and central immaturities that limit auditory processing in juvenile animals, they are able to lateralize sounds using binaural cues. This study explores a central mechanism that may compensate for these limitations during development. Interaural time and level difference processing by neurons in the superior olivary complex depends on synaptic inhibition from the medial nucleus of the trapezoid body (MNTB), a group of inhibitory neurons that is activated by contralateral sound stimuli. In this study, we examined the maturation of coding properties of MNTB neurons and found that they receive an inhibitory influence from the ipsilateral ear that is modified during the course of postnatal development. Single neuron recordings were obtained from the MNTB in juvenile (postnatal day 15–19) and adult gerbils. Approximately 50% of all recorded MNTB neurons were inhibited by ipsilateral sound stimuli, but juvenile neurons displayed a much greater suppression of firing as compared with those in adults. A comparison of the prepotential and postsynaptic action potential indicated that inhibition occurred at the presynaptic level, likely within the cochlear nucleus. A simple linear model of level difference detection by lateral superior olivary neurons that receive input from MNTB suggested that inhibition of the MNTB may expand the response of LSO neurons to physiologically realistic level differences, particularly in juvenile animals, at a time when these cues are reduced.

Synaptic transmission mediated by ionotropic glutamate, glycine and GABA receptors in the rat’s ventral nucleus of the lateral lemniscus

The synaptic pharmacology of the ventral nucleus of the lateral lemniscus (VNLL) was investigated in brain slices obtained from rats of 14-37 days old using intracellular recording techniques. Excitatory and inhibitory synaptic potentials (EPSPs and IPSPs) were elicited by electrical stimulation of the lemniscal pathway and recorded from neurons with five types of intrinsic firing patterns (onset, pause, adapting, regular and bursting types). Synaptic receptors that mediated the EPSPs and IPSPs were identified using AMPA, NMDA, GABA(A) and glycine receptor antagonists. The early/short EPSPs were mediated by AMPA receptors. The late/long EPSPs, encountered only in neurons of younger animals, were mediated by NMDA receptors. The IPSPs in most neurons were mediated by glycine receptors. In some neurons the IPSPs were mediated by GABA(A) receptors or both glycine and GABA(A) receptors. The temporal dynamics of fast AMPA EPSPs and glycinergic IPSPs were very similar. AMPA EPSPs and glycinergic (and/or GABAergic) IPSPs could be encountered in a single neuron. The results suggest that the VNLL not only relays incoming signals rapidly from the lower brainstem to the inferior colliculus, but also integrates excitatory and inhibitory inputs to modify and process auditory information.

Powerful, onset inhibition in the ventral nucleus of the lateral lemniscus

The function of the ventral nucleus of the lateral lemniscus (VNLL), a secondary processing site within the auditory brain stem, is unclear. It is known to be a major source of inhibition to the inferior colliculus (IC). It is also thought to play a role in coding the temporal aspects of sound, such as onsets and the periodic components of complex stimuli. In vivo intracellular recordings from VNLL neurons (n = 56) in urethane anesthetized rats revealed the presence of large-amplitude, short-duration, onset inhibition in a subset of neurons (14.3%). This inhibition occurred before the first action potential (AP) elicited by noise or tone bursts, was broadly tuned to tonal frequency and was shown to delay the first AP. Our data suggest it is a result of an intrinsic circuit activated by the octopus cell pathway originating in the contralateral cochlear nucleus; this pathway is known to convey exquisitely timed and broadly tuned onset information. This powerful inhibition within the VNLL appears to control the timing of this structure's inhibitory output to higher centers, which has important auditory processing outcomes. The circuit also provides a pathway for fast, broadly tuned, onset inhibition to the IC.

Physiological response properties of neurons in the superior paraolivary nucleus of the rat

The superior paraolivary nucleus (SPON) is a prominent nucleus of the superior olivary complex. In rats, this nucleus is composed of a morphologically homogeneous population of GABAergic neurons that receive excitatory input from the contralateral cochlear nucleus and inhibitory input from the ipsilateral medial nucleus of the trapezoid body. SPON neurons provide a dense projection to the ipsilateral inferior colliculus and are thereby capable of exerting profound modulatory influence on collicular neurons. Despite recent interest in the structural and connectional features of SPON, little is presently known concerning the physiological response properties of this cell group or its functional role in auditory processing. We utilized extracellular, in vivo recording methods to study responses of SPON neurons to broad band noise, pure tone, and amplitude-modulated pure tone stimuli. Localization of recording sites within the SPON provides evidence for a medial (high frequency) to lateral (low frequency) tonotopic representation of frequencies within the nucleus. Best frequencies of SPON neurons spanned the audible range of the rat and receptive fields were narrow with V-shaped regions near threshold. Nearly all SPON neurons responded at the offset of broad band noise and pure tone stimuli. The vast majority of SPON neurons displayed very low rates of spontaneous activity and only responded to stimuli presented to the contralateral ear, although a small population showed binaural facilitation. Most SPON neurons also generated spike activity that was synchronized to sinusoidally amplitude-modulated tones. Taken together, these data suggest that SPON neurons may serve to encode temporal features of complex sounds, such as those contained in species-specific vocalizations.

Processing of interaural temporal disparities in the medial division of the ventral nucleus of the lateral lemniscus

The medial division of the ventral nucleus of the lateral lemniscus (VNLLm) contains a specialized population of neurons that is sensitive to interaural temporal disparities (ITDs), a potent cue for sound localization along the azimuth. Unlike many ITD-sensitive neurons elsewhere in the auditory system, neurons in the VNLLm respond only at the onset of tones. An onset response may be significant for behavior because, under echoic conditions, tones require sharp onsets for accurate localization. In contrast, noise can generally be localized even with gradual onsets, presumably because transients occur at random intervals in noise. We recorded responses of neurons in the VNLLm to tones and noise in unanesthetized rabbits. We found that although tones elicited a transient response, noise elicited a sustained response as if it was a sequence of transients. The responses to tones indicate that these neurons represent a secondary stage in the processing of ITDs. The onset response to tones was only weakly synchronized to the phase of the tone, indicating that neurons in the VNLLm inherit their sensitivity to ITDs from their inputs. The latencies were short (~8 ms), implying that the ITD sensitivity is derived from ascending inputs. Most neurons in the VNLLm discharged maximally at the same ITD at all frequencies, a characteristic shared with neurons of the medial superior olive. However, the latency of neurons in the VNLLm to interaurally delayed stimuli is linked strongly to the timing of the contralateral stimulus. This suggests that these neurons receive a suprathreshold, contralateral input that is modulated by a subthreshold input conveying information about ITDs. Other stations in the auditory pathway contain a subset of neurons that respond transiently to tones and are sensitive to ITDs. These neurons may represent a novel pathway that assists in localizing sounds in the presence of reflections.

Monaural and binaural processing in the ventral nucleus of the lateral lemniscus: a major source of inhibition to the inferior colliculus

The ventral nucleus of the lateral lemniscus (VNLL) is a major source of input to the inferior colliculus. This paper reviews recent studies of neural responses in the VNLL of the unanesthetized rabbit. The VNLL has generally been viewed as a monaural nucleus, with its neurons responding primarily to stimulation of the contralateral ear. In the rabbit, the VNLL is divided into a medial division (VNLLm) comprising neurons intercalated in the medial limb of the lemniscus, a compact lateral division (VNLLl), and a dorsal division. The VNLLm contains an abundance of neurons sensitive to interaural temporal disparities (ITDs), one of the major binaural cues for sound localization. These neurons respond only at the onset of tones, and therefore appear to encode the ITDs of transients. Even in the VNLLl, many neurons are sensitive to binaural stimulation. The VNLLl contains a variety of neurons with different discharge patterns, the two most common of which are sustained and onset. The discharge patterns, frequency-tuning and dynamic ranges of VNLLl neurons indicate that this division is able to supply the inferior colliculus with a variety of inputs, each serving a different function in the analysis of sound.

Morphology and physiology of neurons in the ventral nucleus of the lateral lemniscus in rat brain slices

The ventral nucleus of the lateral lemniscus (VNLL) is a prominent neuronal group that lies within the auditory pathway connecting the auditory lower brainstem and midbrain. Previous physiologic studies showed that VNLL neurons respond mainly to contralaterally presented sounds and display various firing patterns. To understand better the role that VNLL neurons play in transmitting and processing of auditory information, we examined the morphology of VNLL neurons and their cellular physiology in young rat brain slices. We made whole-cell patch-clamp recordings and labeled cells intracellularly with neurobiotin to investigate the relation between morphologic neuronal types, intrinsic membrane properties, and postsynaptic responses. VNLL neurons fell into two distinct morphologic groups, i.e., bushy cells and stellate cells, based on their dendritic patterns. Stellate cells were grouped further into stellate I, II, and elongate cells according to soma shape, dendritic branches, and orientation. Bushy cells showed an onset firing pattern and a nonlinear current-voltage relationship. All three subtypes of stellate cells had a linear current-voltage relationship, but exhibited different firing patterns. Stellate I cells showed regular and onset-pause firing patterns, whereas stellate II cells showed adapting and elongate cells showed burst firing patterns. Bushy cells and stellate cells responded to stimulation of the lateral lemniscus with excitatory and/or inhibitory synaptic potentials. These results suggest that the VNLL is a heterogeneous neuronal group and that it contains many channels for processing different kinds of auditory information. Neuronal morphology and intrinsic membrane properties contribute to the behavior of individual neurons.

Colocalization of GABA and glycine in the ventral nucleus of the lateral lemniscus in rat: an in situ hybridization and semiquantitative immunocytochemical study

We have studied by in situ hybridization for GAD65 mRNA in thick sections and by semiquantitative postembedding immunocytochemistry in consecutive semithin sections, the expression of gamma-aminobutyric acid (GABA) and glycine in cell bodies and axosomatic puncta of the rat ventral nucleus of the lateral lemniscus (VNLL), a prominent monaural brainstem auditory structure. The in situ hybridization and the densitometric analysis of the immunostaining suggest that the rat VNLL contains two main populations of neurons. Approximately one-third of neurons are unstained with either technique and are presumably excitatory; their cell bodies are enveloped by a large number of glycine-immunoreactive puncta. Most if not all of the remaining two-thirds colocalize GABA and glycine and are assumed to be inhibitory. These two populations show a complementary distribution within the VNLL, with inhibitory neurons located mainly ventrally and excitatory neurons dorsally. In scatterplots of gray values measured from cell bodies, the double-labeled cells appear to form a single cluster in terms of their staining intensities for the two transmitter candidates. However, this cluster may have to be further subdivided because cells with extreme GABA/glycine ratios differ from those with average ratios with respect to location or size. The VNLL seems unique among auditory structures by its large number of neurons that colocalize GABA and glycine. Although the functional significance of this colocalization remains unknown, our results suggest that the VNLL exerts convergent excitatory and inhibitory influences over the inferior colliculus, which may underlie the timing processing in the auditory midbrain.

Latency as a function of intensity in auditory neurons: influences of central processing

The response latencies of sensory neurons typically shorten with increases in stimulus intensity. In the central auditory system this phenomenon should have a significant impact on a number of auditory functions that depend critically on an integration of precisely timed neural inputs. Evidence from previous studies suggests that the auditory system not only copes with the potential problems associated with intensity-dependent latency change, but that it also modifies latency change to shape the response properties of many cells for specific functions. This observation suggests that intensity-dependent latency change may undergo functional transformations along the auditory neuraxis. The goal of our study was to explore these transformations by making a direct, quantitative comparison of intensity-dependent latency change among a number of auditory centers from the lower brainstem to the thalamus. We found two main ways in which intensity-dependent latency change transformed along the neuraxis: (1) the range of latency change increased substantially and (2) one particular type of latency change, which has been suggested to be associated with sensitivity to temporally segregated stimulus components, occurred only at the highest centers tested, the midbrain and thalamus. Additional testing in the midbrain (inferior colliculus) indicated that inhibitory inputs are involved in shaping latency change. Our findings demonstrate that the central auditory system modifies intensity-dependent latency changes. We suggest that these changes may be functionally incorporated, actively enhanced, or modified to suit specific functions of the auditory system.