Superior Paraolivary Nucleus Neurons Research Articles

Anatomy of superior olivary complex and lateral lemniscus in Etruscan shrew

Based on the auditory periphery and the small head size, Etruscan shrews (Suncus etruscus) approximate ancestral mammalian conditions. The auditory brainstem in this insectivore has not been investigated. Using labelling techniques, we assessed the structures of their superior olivary complex (SOC) and the nuclei of the lateral lemniscus (NLL). There, we identified the position of the major nuclei, their input pattern, transmitter content, expression of calcium binding proteins (CaBPs) and two voltage-gated ion channels. The most prominent SOC structures were the medial nucleus of the trapezoid body (MNTB), the lateral nucleus of the trapezoid body (LNTB), the lateral superior olive (LSO) and the superior paraolivary nucleus (SPN). In the NLL, the ventral (VNLL), a specific ventrolateral VNLL (VNLLvl) cell population, the intermediate (INLL) and dorsal (DNLL) nucleus, as well as the inferior colliculus’s central aspect were discerned. INLL and VNLL were clearly separated by the differential distribution of various marker proteins. Most labelled proteins showed expression patterns comparable to rodents. However, SPN neurons were glycinergic and not GABAergic and the overall CaBPs expression was low. Next to the characterisation of the Etruscan shrew’s auditory brainstem, our work identifies conserved nuclei and indicates variable structures in a species that approximates ancestral conditions.

Structural and Functional Development of Inhibitory Connections from the Medial Nucleus of the Trapezoid Body to the Superior Paraolivary Nucleus.

The medial nucleus of the trapezoid body (MNTB) in the auditory brainstem is the principal source of synaptic inhibition to several functionally distinct auditory nuclei. Prominent projections of individual MNTB neurons comprise the major binaural nuclei that are involved in the early processing stages of sound localization as well as the superior paraolivary nucleus (SPON), which contains monaural neurons that extract rapid changes in sound intensity to detect sound gaps and rhythmic oscillations that commonly occur in animal calls and human speech. While the processes that guide the development and refinement of MNTB axon collaterals to the binaural nuclei have become increasingly understood, little is known about the development of MNTB collaterals to the monaural SPON. In this study, we investigated the development of MNTB-SPON connections in mice of both sexes from shortly after birth to three weeks of age, which encompasses the time before and after hearing onset. Individual axon reconstructions and electrophysiological analysis of MNTB-SPON connectivity demonstrate a dramatic increase in the number of MNTB axonal boutons in the SPON before hearing onset. However, this proliferation was not accompanied by changes in the strength of MNTB-SPON connections or by changes in the structural or functional topographic precision. However, following hearing onset, the spread of single-axon boutons along the tonotopic axis increased, indicating an unexpected decrease in the tonotopic precision of the MNTB-SPON pathway. These results provide new insight into the development and organization of inhibition to SPON neurons and the regulation of developmental plasticity in diverging inhibitory pathways.SIGNIFICANCE STATEMENT The superior paraolivary nucleus (SPON) is a prominent auditory brainstem nucleus involved in the early detection of sound gaps and rhythmic oscillations. The ability of SPON neurons to fire at the offset of sound depends on strong and precise synaptic inhibition provided by glycinergic neurons in the medial nucleus of the trapezoid body (MNTB). Here, we investigated the anatomic and physiological maturation of MNTB-LSO connectivity in mice before and after the onset of hearing. We observed a period of bouton proliferation without accompanying changes in topographic precision before hearing onset. This was followed by bouton elimination and an unexpected decrease in the tonotopic precision after hearing onset. These results provide new insight into the development of inhibition to the SPON.

Slow NMDA-Mediated Excitation Accelerates Offset-Response Latencies Generated via a Post-Inhibitory Rebound Mechanism.

In neural circuits, action potentials (spikes) are conventionally caused by excitatory inputs whereas inhibitory inputs reduce or modulate neuronal excitability. We previously showed that neurons in the superior paraolivary nucleus (SPN) require solely synaptic inhibition to generate their hallmark offset response, a burst of spikes at the end of a sound stimulus, via a post-inhibitory rebound mechanism. In addition SPN neurons receive excitatory inputs, but their functional significance is not yet known. Here we used mice of both sexes to demonstrate that in SPN neurons, the classical roles for excitation and inhibition are switched, with inhibitory inputs driving spike firing and excitatory inputs modulating this response. Hodgkin–Huxley modeling suggests that a slow, NMDA receptor (NMDAR)-mediated excitation would accelerate the offset response. We find corroborating evidence from in vitro and in vivo recordings that lack of excitation prolonged offset-response latencies and rendered them more variable to changing sound intensity levels. Our results reveal an unsuspected function for slow excitation in improving the timing of post-inhibitory rebound firing even when the firing itself does not depend on excitation. This shows the auditory system employs highly specialized mechanisms to encode timing-sensitive features of sound offsets which are crucial for sound-duration encoding and have profound biological importance for encoding the temporal structure of speech.

Octopus Cells in the Posteroventral Cochlear Nucleus Provide the Main Excitatory Input to the Superior Paraolivary Nucleus.

Auditory streaming enables perception and interpretation of complex acoustic environments that contain competing sound sources. At early stages of central processing, sounds are segregated into separate streams representing attributes that later merge into acoustic objects. Streaming of temporal cues is critical for perceiving vocal communication, such as human speech, but our understanding of circuits that underlie this process is lacking, particularly at subcortical levels. The superior paraolivary nucleus (SPON), a prominent group of inhibitory neurons in the mammalian brainstem, has been implicated in processing temporal information needed for the segmentation of ongoing complex sounds into discrete events. The SPON requires temporally precise and robust excitatory input(s) to convey information about the steep rise in sound amplitude that marks the onset of voiced sound elements. Unfortunately, the sources of excitation to the SPON and the impact of these inputs on the behavior of SPON neurons have yet to be resolved. Using anatomical tract tracing and immunohistochemistry, we identified octopus cells in the contralateral cochlear nucleus (CN) as the primary source of excitatory input to the SPON. Cluster analysis of miniature excitatory events also indicated that the majority of SPON neurons receive one type of excitatory input. Precise octopus cell-driven onset spiking coupled with transient offset spiking make SPON responses well-suited to signal transitions in sound energy contained in vocalizations. Targets of octopus cell projections, including the SPON, are strongly implicated in the processing of temporal sound features, which suggests a common pathway that conveys information critical for perception of complex natural sounds.

Modeling Responses in the Superior Paraolivary Nucleus: Implications for Forward Masking in the Inferior Colliculus

A phenomenological model of the responses of neurons in the superior paraolivary nucleus (SPON) of the rodent is presented in this study. Pure tones at the characteristic frequency (CF) and broadband noise stimuli evoke offset-type responses in these neurons. SPON neurons also phase-lock to the envelope of sinusoidally amplitude-modulated (SAM) stimuli for a range of modulation frequencies. Model SPON neuron received inhibitory input that was relayed by the ipsilateral medial nucleus of the trapezoid body from the contralateral model ventral cochlear nucleus neuron. The SPON model response was simulated by detecting the slope of its inhibitory postsynaptic potential. Responses of the proposed model to pure tones at CF and broadband noise were offset-type independent of the duration of the input stimulus. SPON model responses were also synchronized to the envelope of SAM stimuli with precise timing for a range of modulation frequencies. Modulation transfer functions (MTFs) obtained from the model response to SAM stimuli resemble the physiological MTFs. The output of the proposed SPON model provides an input for models of physiological responses at higher levels of the ascending auditory pathway and can also be utilized to infer possible mechanisms underlying gap detection and duration encoding as well as forward masking at the level of the auditory midbrain.

In-vivo intra-cellular characterization of “OFF” responses in the superior para-olivary nucleus

The superior para-olivary nucleus (SPN) is an auditory-brainstem OFF channel. Typically, SPN neurons are inhibited during sound presentation and fire at sound offset. Release from sound-induced inhibitory potentials is the hypothesized mechanism, but this has not previously been directly observed in vivo. We obtained in vivo axonal recordings from 33 SPN neurons in the chinchilla, and patch recordings from 17 SPN cell bodies in the Mongolian gerbil. We retrieved several cells anatomically by labeling with biocytin or neurobiotin. Labeled SPN neurons had large dendritic trees, and axons heading into the lateral lemniscus. Their responses typically showed offset spiking to contralateral, and sometimes to ipsilateral stimulation. Spikes in response to amplitude-modulated tones were highly synchronized, up to hundreds of Hz (vector strength >0.9). Some SPN neurons were ITD sensitive, either to envelope or low-frequency fine-structure components. Patch-clamp recording allowed us to study sub-threshold potentials. Large IPSPs were generated by contralateral, and often also by ipsilateral sounds, with few EPSPs. Individual phase-locked IPSPs were discernable to low-frequency tones. High-frequency tones elicited a large onset IPSP, followed by sustained inhibition. For amplitude-modulated tones, IPSPs were locked to the stimulus envelope, with spikes at the offset of inhibitory events.

Temporal processing capacity in auditory-deprived superior paraolivary neurons is rescued by sequential plasticity during early development

The leading treatments for severe hearing disabilities work on the principle of conveying electrical pulses to the auditory brainstem that enable perception of speech. It is currently not known how well the brainstem neurons specialized for decoding such coarse sound information develop when deprived of auditory input activity. Here, we used congenitally deaf α1D−/− mice, lacking activity in the auditory nerve, to investigate the superior paraolivary nucleus (SPON) – a prominent mammalian brainstem structure that responds selectively to sound pulses by rebound spiking. Whole-cell patch-clamp recordings from SPON neurons in the α1D−/− and control mice were obtained at equivalent pre- and post-hearing onset ages. The results show that SPON neurons in the α1D−/− display less precise, plateau-like rebound spiking compared to control neurons. However, the rebound spiking mechanism undergoes strong compensation with age in the α1D−/−. Voltage-activated Ca2+-currents lower the spike threshold, rescuing the capacity for spike initiation at pre-hearing onset ages. Gradual up-regulation of the inwardly rectifying h-current contributes to depolarize the membrane potential. Reduction of the membrane time constant and less recruitment of Ca2+-currents thereby normalize precise rebound spiking at post-hearing onset ages. We found the soluble form of the neurotrophic factor neuritin to be up-regulated in SPON of deaf mice, which may have promoted neuronal survival and prolonged plasticity of the SPON circuitry. A stereotyped timeline of compensation of rebound spiking in deaf SPON neurons indicates robust intrinsic regulation of the brainstem circuitry encoding sound rhythms. This may be a prerequisite for successful cochlear implants.

Development of glycinergic innervation to the murine LSO and SPN in the presence and absence of the MNTB.

Neurons in the superior olivary complex (SOC) integrate excitatory and inhibitory inputs to localize sounds in space. The majority of these inhibitory inputs have been thought to arise within the SOC from the medial nucleus of the trapezoid body (MNTB). However, recent work demonstrates that glycinergic innervation of the SOC persists in Egr2; En1CKO mice that lack MNTB neurons, suggesting that there are other sources of this innervation (Jalabi et al., 2013). To study the development of MNTB- and non-MNTB-derived glycinergic SOC innervation, we compared immunostaining patterns of glycine transporter 2 (GlyT2) at several postnatal ages in control and Egr2; En1CKO mice. GlyT2 immunostaining was present at birth (P0) in controls and reached adult levels by P7 in the superior paraolivary nucleus (SPN) and by P12 in the lateral superior olive (LSO). In Egr2; En1CKO mice, glycinergic innervation of the LSO developed at a similar rate but was delayed by one week in the SPN. Conversely, consistent reductions in the number of GlyT2+ boutons located on LSO somata were seen at all ages in Egr2; En1CKO mice, while these numbers reached control levels in the SPN by adulthood. Dendritic localization of GlyT2+ boutons was unaltered in both the LSO and SPN of adult Egr2; En1CKO mice. On the postsynaptic side, adult Egr2; En1CKO mice had reduced glycine receptor α1 (GlyRα1) expression in the LSO but normal levels in the SPN. GlyRα2 was not expressed by LSO or SPN neurons in either genotype. These findings contribute important information for understanding the development of MNTB- and non-MNTB-derived glycinergic pathways to the mouse SOC.

The superior paraolivary nucleus shapes temporal response properties of neurons in the inferior colliculus.

The mammalian superior paraolivary nucleus (SPON) is a major source of GABAergic inhibition to neurons in the inferior colliculus (IC), a well-studied midbrain nucleus that is the site of convergence and integration for the majority ascending auditory pathways en route to the cortex. Neurons in the SPON and IC exhibit highly precise responses to temporal sound features, which are important perceptual cues for naturally occurring sounds. To determine how inhibitory input from the SPON contributes to the encoding of temporal information in the IC, a reversible inactivation procedure was conducted to silence SPON neurons, while recording responses to amplitude-modulated tones and silent gaps between tones in the IC. The results show that SPON-derived inhibition shapes responses of onset and sustained units in the IC via different mechanisms. Onset neurons appear to be driven primarily by excitatory inputs and their responses are shaped indirectly by SPON-derived inhibition, whereas sustained neurons are heavily influenced directly by transient offset inhibition from the SPON. The findings also demonstrate that a more complete dissection of temporal processing pathways is critical for understanding how biologically important sounds are encoded by the brain.

Nitric oxide signaling modulates synaptic inhibition in the superior paraolivary nucleus (SPN) via cGMP-dependent suppression of KCC2.

Glycinergic inhibition plays a central role in the auditory brainstem circuitries involved in sound localization and in the encoding of temporal action potential firing patterns. Modulation of this inhibition has the potential to fine-tune information processing in these networks. Here we show that nitric oxide (NO) signaling in the auditory brainstem (where activity-dependent generation of NO is documented) modulates the strength of inhibition by changing the chloride equilibrium potential. Recent evidence demonstrates that large inhibitory postsynaptic currents (IPSCs) in neurons of the superior paraolivary nucleus (SPN) are enhanced by a very low intracellular chloride concentration, generated by the neuronal potassium chloride co-transporter (KCC2) expressed in the postsynaptic neurons. Our data show that modulation by NO caused a 15 mV depolarizing shift of the IPSC reversal potential, reducing the strength of inhibition in SPN neurons, without changing the threshold for action potential firing. Regulating inhibitory strength, through cGMP-dependent changes in the efficacy of KCC2 in the target neuron provides a postsynaptic mechanism for rapidly controlling the inhibitory drive, without altering the timing or pattern of the afferent spike train. Therefore, this NO-mediated suppression of KCC2 can modulate inhibition in one target nucleus (SPN), without influencing inhibitory strength of other target nuclei (MSO, LSO) even though they are each receiving collaterals from the same afferent nucleus (a projection from the medial nucleus of the trapezoid body, MNTB).

Development of on-off spiking in superior paraolivary nucleus neurons of the mouse

The superior paraolivary nucleus (SPON) is a prominent cell group in the auditory brain stem that has been increasingly implicated in representing temporal sound structure. Although SPON neurons selectively respond to acoustic signals important for sound periodicity, the underlying physiological specializations enabling these responses are poorly understood. We used in vitro and in vivo recordings to investigate how SPON neurons develop intrinsic cellular properties that make them well suited for encoding temporal sound features. In addition to their hallmark rebound spiking at the stimulus offset, SPON neurons were characterized by spiking patterns termed onset, adapting, and burst in response to depolarizing stimuli in vitro. Cells with burst spiking had some morphological differences compared with other SPON neurons and were localized to the dorsolateral region of the nucleus. Both membrane and spiking properties underwent strong developmental regulation, becoming more temporally precise with age for both onset and offset spiking. Single-unit recordings obtained in young mice demonstrated that SPON neurons respond with temporally precise onset spiking upon tone stimulation in vivo, in addition to the typical offset spiking. Taken together, the results of the present study demonstrate that SPON neurons develop sharp on-off spiking, which may confer sensitivity to sound amplitude modulations or abrupt sound transients. These findings are consistent with the proposed involvement of the SPON in the processing of temporal sound structure, relevant for encoding communication cues.

Sound Rhythms Are Encoded by Postinhibitory Rebound Spiking in the Superior Paraolivary Nucleus

The superior paraolivary nucleus (SPON) is a prominent structure in the auditory brainstem. In contrast to the principal superior olivary nuclei with identified roles in processing binaural sound localization cues, the role of the SPON in hearing is not well understood. A combined in vitro and in vivo approach was used to investigate the cellular properties of SPON neurons in the mouse. Patch-clamp recordings in brain slices revealed that brief and well timed postinhibitory rebound spiking, generated by the interaction of two subthreshold-activated ion currents, is a hallmark of SPON neurons. The I(h) current determines the timing of the rebound, whereas the T-type Ca(2+) current boosts the rebound to spike threshold. This precisely timed rebound spiking provides a physiological explanation for the sensitivity of SPON neurons to sinusoidally amplitude-modulated (SAM) tones in vivo, where peaks in the sound envelope drive inhibitory inputs and SPON neurons fire action potentials during the waveform troughs. Consistent with this notion, SPON neurons display intrinsic tuning to frequency-modulated sinusoidal currents (1-15Hz) in vitro and discharge with strong synchrony to SAMs with modulation frequencies between 1 and 20 Hz in vivo. The results of this study suggest that the SPON is particularly well suited to encode rhythmic sound patterns. Such temporal periodicity information is likely important for detection of communication cues, such as the acoustic envelopes of animal vocalizations and speech signals.

Connections of the superior paraolivary nucleus of the rat: projections to the inferior colliculus

GABAergic neurotransmission contributes to shaping the response properties of inferior colliculus (IC) neurons. In rodents, the superior paraolivary nucleus (SPON) is a prominent and well-defined cell group of the superior olivary complex that sends significant but often neglected GABAergic projections to the IC. To investigate the trajectory, distribution and morphology of these projections, we injected the neuroanatomical tracer biotinylated dextran amine into the SPON of albino rats. Our results demonstrate that: (1) the SPON innervates densely all three subdivisions of the ipsilateral IC: central nucleus (CNIC), dorsal cortex (DCIC) and external cortex (ECIC). The SPON also sends a sparse projection to the contralateral DCIC via the commissure of the IC. (2) SPON axons are relatively thick (diameter >1.2 μm), ascend to the midbrain tectum in the medial aspect of the lateral lemniscus, and, for the most part, do not innervate the nuclei of the lateral lemniscus. (3) SPON fibers ramify profusely within the IC and bear abundant en passant and terminal boutons. (4) The axons of neurons in discrete regions of the SPON form two laminar terminal plexuses in the ipsilateral IC: a medial plexus that spans the CNIC and DCIC parallel to the known fibrodendritic laminae of the CNIC, and a lateral plexus located in the ECIC and oriented more or less parallel to the surface of the IC. (5) The projection from SPON to the ipsilateral IC is topographic: medial SPON neurons innervate the ventromedial region of the CNIC and DCIC and the ventrolateral region of the ECIC, whereas more laterally situated SPON neurons innervate more dorsolateral regions of the CNIC and DCIC and more dorsomedial regions of the ECIC. Thus, SPON fibers follow a pattern of distribution within the IC similar to that previously reported for intracollicular and corticocollicular projections.

Encoding of temporal features of auditory stimuli in the medial nucleus of the trapezoid body and superior paraolivary nucleus of the rat

Neurons in the superior paraolivary nucleus (SPON) of the rat respond to the offset of pure tones with a brief burst of spikes. Medial nucleus of the trapezoid body (MNTB) neurons, which inhibit the SPON, produce a sustained pure tone response followed by an offset response characterized by a period of suppressed spontaneous activity. This MNTB offset response is duration dependent and critical to the formation of SPON offset spikes [Kadner A, Kulesza RJ Jr, Berrebi AS (2006) Neurons in the medial nucleus of the trapezoid body and superior paraolivary nucleus of the rat may play a role in sound duration coding. J Neurophysiol. 95:1499–1508; Kulesza RJ Jr, Kadner A, Berrebi AS (2007) Distinct roles for glycine and GABA in shaping the response properties of neurons in the superior paraolivary nucleus of the rat. J Neurophysiol 97:1610–1620]. Here we examine the temporal resolution of the rat's MNTB/SPON circuit by assessing its capability to i) detect gaps in tones, and ii) synchronize to sinusoidally amplitude modulated (SAM) tones. Gap detection was tested by presenting two identical pure tone markers interrupted by gaps ranging from 0 to 25 ms duration. SPON neurons responded to the offset of the leading marker even when the two markers were separated only by their ramps (i.e. a 0 ms gap); longer gap durations elicited progressively larger responses. MNTB neurons produced an offset response at gap durations of 2 ms or longer, with a subset of neurons responding to 0 ms gaps. SAM tone stimuli used the unit's characteristic frequency as a carrier, and modulation rates ranged from 40 to 1160 Hz. MNTB neurons synchronized to modulation rates up to ∼1 kHz, whereas spiking of SPON neurons decreased sharply at modulation rates ≥400 Hz. Modulation transfer functions based on spike count were all-pass for MNTB neurons and low-pass for SPON neurons; the modulation transfer functions based on vector strength were low-pass for both nuclei, with a steeper cutoff for SPON neurons. Thus, the MNTB/SPON circuit encodes episodes of low stimulus energy, such as gaps in pure tones and troughs in amplitude modulated tones. The output of this circuit consists of brief SPON spiking episodes; their potential effects on the auditory midbrain and forebrain are discussed.

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.

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.

Auditory response properties in the superior paraolivary nucleus of the gerbil.

The ascending auditory pathway is characterized by parallel processing. At the brain stem level, several structures are involved that are known to serve different well-defined functions. However, the function of one prominent brain stem nucleus, the rodent superior paraolivary nucleus (SPN) and its putative homologue in other mammals, the dorsomedial periolivary nucleus, is unknown. Based on extracellular recordings from anesthetized gerbils, we tested the role of the SPN in sound localization and temporal processing. First, the existence of binaural inputs indicates that the SPN might be involved in sound localization. Although almost half of the neurons exhibited binaural interactions (most of them excited from both sides), effects of interaural time and intensity differences (ITD; IID) were weak and ambiguous. Thus a straightforward function of SPN in sound localization appears to be implausible. Second, inputs from octopus and multipolar/stellate cells of the cochlear nucleus and from principal cells of the medial nucleus of the trapezoid body could relate to precise temporal processing in the SPN. Based on discharge types, two subpopulations of SPN cells were observed: about 60% of the neurons responded to pure tones with sustained discharges, with irregular spike patterns and no phase-locking. Only four neurons showed a regular spike pattern ("chopping"). About 40% of the neurons responded with phasic ON or OFF discharges. Average first spike latency observed in neurons with sustained discharges was significantly shorter than that of ON responders, but had a considerably higher trial-to-trial variation ("jitter"). A subpopulation of ON responders showed a jitter of less than +/-0.1 ms. Most neurons (66%) responded to sinusoidally amplitude-modulated sounds (SAM) with an ongoing response, phase-locked to the stimulus envelope. Again, ON responders showed a significantly higher temporal precision in the phase-locked discharge compared with the sustained responders. High variability was observed among spike-rate-based modulation transfer functions. Histologically, a massive concentration of cytochemical markers for glycinergic input to SPN cells was demonstrated. Application of glycine or its blockade revealed profound effects of glycinergic inhibition on the auditory responses of SPN neurons. The existence of at least two subpopulations of neurons is in line with different subsets of SPN cells that can be distinguished morphologically. One temporally less precise population might modulate the processing of its target structures by providing a rather diffuse inhibition. In contrast, precise ON responders might provide a short, initial inhibitory pulse to its targets.

Superior paraolivary nucleus of the rat is a GABAergic nucleus.

The presence of the inhibitory neurotransmitters glycine and GABA (gamma-amino butyric acid) and GAD (glutamic acid decarboxylase), the synthesizing enzyme for GABA, was examined by immunocytochemistry in the superior paraolivary nucleus (SPON) of the rat. Only rarely were SPON neurons observed to be glycine-immunoreactive, but the majority were GABA-immunoreactive. Using unbiased stereological counting methods, we estimated that this nucleus contains approximately 2500 neurons. Moreover, 90% of SPON neurons were immunolabeled by antisera directed against either the 65- or 67-kD isoform of GAD, or a third antiserum that recognizes both GAD isoforms. Morphometric analysis of GAD-immunolabeled neurons indicated that SPON neurons possess cell bodies and dendritic arbors that are elongated rostrocaudally and relatively flattened parasagittally. Abundant glycine-, GABA-, and GAD-immunoreactive punctate profiles-presumed to represent, for the most part, presynaptic axon terminals-were observed in apposition to SPON neurons. We conclude that the rat SPON contains a homogeneous population of multipolar GABAergic neurons that receive abundant GABAergic and glycinergic innervation. The vast majority of glycinergic inputs to SPON are presumed to originate in the ipsilateral medial nucleus of the trapezoid body, but the source(s) of its GABAergic innervation remains to be determined.

Anisotropic organization of the rat superior paraolivary nucleus.

In rodents, the superior paraolivary nucleus (SPON) is one of the major nuclei of the superior olivary complex that innervate the inferior colliculus. To analyze the intrinsic organization of the SPON and to gain further insight into its relationship with the inferior colliculus, the neuroanatomical tracers biotinylated dextran and horseradish peroxidase were unilaterally injected into different regions of the central nucleus of the inferior colliculus of adult albino rats. Both tracers resulted in retrograde labeling of SPON cell bodies. In addition, biotinylated dextran rendered excellent filling of dendritic and axonal processes within the nucleus. Our results confirm that the projection from the SPON to the central nucleus of the inferior colliculus is nearly exclusively ipsilateral and strictly topographic. Furthermore, our data show that virtually all SPON neurons participate in this projection. The labeling with biotinylated dextran reveals that typical SPON neurons are medium to large multipolar cells with four to seven thick, long, scarcely branched and smooth dendrites that extend over long distances within a nearly parasagittal plane and intermingle with similarly oriented axonal plexuses. Some of the neurons located ventrally within the nucleus possess dendrites that extend ventrally beyond the limits of the SPON to penetrate into the underlying ventral nucleus of the trapezoid body. The parallel arrangement of flattened dendritic and axonal fields within the SPON is reminiscent of the fibrodendritic laminae found in other mammalian auditory nuclei. This fact and the available data about the connectivity of the nucleus stress the similarities between the SPON and the principal nuclei of the superior olivary complex.