Serotonergic modulation of synapses in the developing gerbil lateral superior olive

Brainstem serotonin neurons selectively gate retinal information flow to thalamus

Retinal ganglion cell (RGC) types relay parallel streams of visual feature information. We hypothesized that neuromodulators might efficiently control which visual information streams reach the cortex by selectively gating transmission from specific RGC axons in the thalamus. Using fiber photometry recordings, we found that optogenetic stimulation of serotonergic axons in primary visual thalamus of awake mice suppressed ongoing and visually evoked calcium activity and glutamate release from RGC boutons. Two-photon calcium imaging revealed that serotonin axon stimulation suppressed RGC boutons that responded strongly to global changes in luminance more than those responding only to local visual stimuli, while the converse was true for suppression induced by increases in arousal. Converging evidence suggests that differential expression of the 5-HT1B receptor on RGC presynaptic terminals, but not differential density of nearby serotonin axons, may contribute to the selective serotonergic gating of specific visual information streams before they can activate thalamocortical neurons.

Multiple Sources of Cholinergic Input to the Superior Olivary Complex

The superior olivary complex (SOC) is a major computation center in the brainstem auditory system. Despite previous reports of high expression levels of cholinergic receptors in the SOC, few studies have addressed the functional role of acetylcholine in the region. The source of the cholinergic innervation is unknown for all but one of the nuclei of the SOC, limiting our understanding of cholinergic modulation. The medial nucleus of the trapezoid body, a key inhibitory link in monaural and binaural circuits, receives cholinergic input from other SOC nuclei and also from the pontomesencephalic tegmentum. Here, we investigate whether these same regions are sources of cholinergic input to other SOC nuclei. We also investigate whether individual cholinergic cells can send collateral projections bilaterally (i.e., into both SOCs), as has been shown at other levels of the subcortical auditory system. We injected retrograde tract tracers into the SOC in gerbils, then identified retrogradely-labeled cells that were also immunolabeled for choline acetyltransferase, a marker for cholinergic cells. We found that both the SOC and the pontomesencephalic tegmentum (PMT) send cholinergic projections into the SOC, and these projections appear to innervate all major SOC nuclei. We also observed a small cholinergic projection into the SOC from the lateral paragigantocellular nucleus of the reticular formation. These various sources likely serve different functions; e.g., the PMT has been associated with things such as arousal and sensory gating whereas the SOC may provide feedback more closely tuned to specific auditory stimuli. Further, individual cholinergic neurons in each of these regions can send branching projections into both SOCs. Such projections present an opportunity for cholinergic modulation to be coordinated across the auditory brainstem.

Silence, Solitude, and Serotonin: Neural Mechanisms Linking Hearing Loss and Social Isolation

For social animals that communicate acoustically, hearing loss and social isolation are factors that independently influence social behavior. In human subjects, hearing loss may also contribute to objective and subjective measures of social isolation. Although the behavioral relationship between hearing loss and social isolation is evident, there is little understanding of their interdependence at the level of neural systems. Separate lines of research have shown that social isolation and hearing loss independently target the serotonergic system in the rodent brain. These two factors affect both presynaptic and postsynaptic measures of serotonergic anatomy and function, highlighting the sensitivity of serotonergic pathways to both types of insult. The effects of deficits in both acoustic and social inputs are seen not only within the auditory system, but also in other brain regions, suggesting relatively extensive effects of these deficits on serotonergic regulatory systems. Serotonin plays a much-studied role in depression and anxiety, and may also influence several aspects of auditory cognition, including auditory attention and understanding speech in challenging listening conditions. These commonalities suggest that serotonergic pathways are worthy of further exploration as potential intervening mechanisms between the related conditions of hearing loss and social isolation, and the affective and cognitive dysfunctions that follow.

Characterization of serotonin-induced inhibition of excitatory synaptic transmission in the anterior cingulate cortex

Excitatory synaptic transmission in central synapses is modulated by serotonin (5-HT). The anterior cingulate cortex (ACC) is an important cortical region for pain perception and emotion. ACC neurons receive innervation of projecting serotonergic nerve terminals from raphe nuclei, but the possible effect of 5-HT on excitatory transmission in the ACC has not been investigated. In the present study, we investigated the role of 5-HT on glutamate neurotransmission in the ACC slices of adult mice. Bath application of 5-HT produced dose-dependent inhibition of evoked excitatory postsynaptic currents (eEPSCs). Paired pulse ratio (PPR) was significantly increased, indicating possible presynaptic effects of 5-HT. Consistently, bath application of 5-HT significantly decreased the frequency of spontaneous and miniature excitatory postsynaptic currents (sEPSCs and mEPSCs). By contrast, amplitudes of sEPSCs and mEPSCs were not significantly affected. After postsynaptic application of G protein inhibitor GDP-β-S, 5-HT produced inhibition of eEPSCs was significantly reduced. Finally, NAN-190, an antagonist of 5-HT_1A receptor, significantly reduced postsynaptic inhibition of 5-HT and abolished presynaptic inhibition. Our results strongly suggest that presynaptic as well as postsynaptic 5-HT receptor including 5-HT1A subtype receptor may contribute to inhibitory modulation of glutamate release as well as postsynaptic responses in the ACC.

Tonotopic action potential tuning of maturing auditory neurons through endogenous ATP

KEY POINTS

Following the genetically controlled formation of neuronal circuits, early firing activity guides the development of sensory maps in the auditory, visual and somatosensory system. However, it is not clear whether the activity of central auditory neurons is specifically regulated depending on the position within the sensory map. In the ventral cochlear nucleus, the first central station along the auditory pathway, we describe a mechanism through which paracrine ATP signalling enhances firing in a cell-specific and tonotopically-determined manner. Developmental down-regulation of P2X2/3R currents along the tonotopic axis occurs simultaneously with an increase in AMPA receptor currents, suggesting a high-to-low frequency maturation pattern. Facilitated action potential (AP) generation, measured as higher firing rate, shorter EPSP-AP delay in vivo and shorter AP latency in slice experiments, is consistent with increased synaptic efficacy caused by ATP. The long lasting change in intrinsic neuronal excitability is mediated by the heteromeric P2X2/3 receptors.

ABSTRACT

Synaptic refinement and strengthening are activity-dependent processes that establish orderly arranged cochleotopic maps throughout the central auditory system. The maturation of auditory brainstem circuits is guided by action potentials (APs) arising from the inner hair cells in the developing cochlea. The AP firing of developing central auditory neurons can be modulated by paracrine ATP signalling, as shown for the cochlear nucleus bushy cells and principal neurons in the medial nucleus of the trapezoid body. However, it is not clear whether neuronal activity may be specifically regulated with respect to the nuclear tonotopic position (i.e. sound frequency selectivity). Using slice recordings before hearing onset and in vivo recordings with iontophoretic drug applications after hearing onset, we show that cell-specific purinergic modulation follows a precise tonotopic pattern in the ventral cochlear nucleus of developing gerbils. In high-frequency regions, ATP responsiveness diminished before hearing onset. In low-to-mid frequency regions, ATP modulation persisted after hearing onset in a subset of low-frequency bushy cells (characteristic frequency< 10 kHz). Down-regulation of P2X2/3R currents along the tonotopic axis occurs simultaneously with an increase in AMPA receptor currents, thus suggesting a high-to-low frequency maturation pattern. Facilitated AP generation, measured as higher firing frequency, shorter EPSP-AP delay in vivo, and shorter AP latency in slice experiments, is consistent with increased synaptic efficacy caused by ATP. Finally, by combining recordings and pharmacology in vivo, in slices, and in human embryonic kidney 293 cells, it was shown that the long lasting change in intrinsic neuronal excitability is mediated by the P2X2/3R.

Pb exposure prolongs the time period for postnatal transient uptake of 5-HT by murine LSO neurons

Pb exposure is associated with cognitive deficits including Attention Deficit Hyperactivity Disorder (ADHD) in children and alters auditory temporal processing in humans and animals. Serotonin has been implicated in auditory temporal processing and previous studies from our laboratory have demonstrated that developmental Pb decreases expression of serotonin (5-HT) in the adult murine lateral superior olive (LSO). During development, certain non-serotonergic sensory neurons, including auditory LSO neurons, transiently take up 5-HT through the serotonin reuptake transporter (SERT). The uptake of 5-HT is important for development of sensory systems. This study examines the effect of Pb on the serotonergic system in the LSO of the early postnatal mouse. Mice were exposed to moderate Pb (0.01mM) or high Pb (0.1mM) throughout gestation and postnatal day 4 (P4) and P8. We found that Pb exposure prolongs the normal developmental expression of 5-HT by LSO neurons and this is correlated with expression of SERT on LSO cell bodies. The prolonged expression of 5-HT by postnatal LSO neurons is correlated with decreased synaptic immunolabeling within the LSO. This Pb-associated decrease in synaptic density within the LSO could contribute to the auditory temporal processing deficits and cognitive deficits associated with developmental Pb exposure.

Modulation of auditory brainstem responses by serotonin and specific serotonin receptors

The neuromodulator serotonin is found throughout the auditory system from the cochlea to the cortex. Although effects of serotonin have been reported at the level of single neurons in many brainstem nuclei, how these effects correspond to more integrated measures of auditory processing has not been well-explored. In the present study, we aimed to characterize the effects of serotonin on far-field auditory brainstem responses (ABR) across a wide range of stimulus frequencies and intensities. Using a mouse model, we investigated the consequences of systemic serotonin depletion, as well as the selective stimulation and suppression of the 5-HT1 and 5-HT2 receptors, on ABR latency and amplitude. Stimuli included tone pips spanning four octaves presented over a forty dB range. Depletion of serotonin reduced the ABR latencies in Wave II and later waves, suggesting that serotonergic effects occur as early as the cochlear nucleus. Further, agonists and antagonists of specific serotonergic receptors had different profiles of effects on ABR latencies and amplitudes across waves and frequencies, suggestive of distinct effects of these agents on auditory processing. Finally, most serotonergic effects were more pronounced at lower ABR frequencies, suggesting larger or more directional modulation of low-frequency processing. This is the first study to describe the effects of serotonin on ABR responses across a wide range of stimulus frequencies and amplitudes, and it presents an important step in understanding how serotonergic modulation of auditory brainstem processing may contribute to modulation of auditory perception.

The dorsal raphe modulates sensory responsiveness during arousal in zebrafish

During waking behavior, animals adapt their state of arousal in response to environmental pressures. Sensory processing is regulated in aroused states, and several lines of evidence imply that this is mediated at least partly by the serotonergic system. However, there is little information directly showing that serotonergic function is required for state-dependent modulation of sensory processing. Here we find that zebrafish larvae can maintain a short-term state of arousal during which neurons in the dorsal raphe modulate sensory responsiveness to behaviorally relevant visual cues. After a brief exposure to water flow, larvae show elevated activity and heightened sensitivity to perceived motion. Calcium imaging of neuronal activity after flow revealed increased activity in serotonergic neurons of the dorsal raphe. Genetic ablation of these neurons abolished the increase in visual sensitivity during arousal without affecting baseline visual function or locomotor activity. We traced projections from the dorsal raphe to a major visual area, the optic tectum. Laser ablation of the tectum demonstrated that this structure, like the dorsal raphe, is required for improved visual sensitivity during arousal. These findings reveal that serotonergic neurons of the dorsal raphe have a state-dependent role in matching sensory responsiveness to behavioral context.

From behavioral context to receptors: serotonergic modulatory pathways in the IC

In addition to ascending, descending, and lateral auditory projections, inputs extrinsic to the auditory system also influence neural processing in the inferior colliculus (IC). These types of inputs often have an important role in signaling salient factors such as behavioral context or internal state. One route for such extrinsic information is through centralized neuromodulatory networks like the serotonergic system. Serotonergic inputs to the IC originate from centralized raphe nuclei, release serotonin in the IC, and activate serotonin receptors expressed by auditory neurons. Different types of serotonin receptors act as parallel pathways regulating specific features of circuitry within the IC. This results from variation in subcellular localizations and effector pathways of different receptors, which consequently influence auditory responses in distinct ways. Serotonin receptors may regulate GABAergic inhibition, influence response gain, alter spike timing, or have effects that are dependent on the level of activity. Serotonin receptor types additionally interact in nonadditive ways to produce distinct combinatorial effects. This array of effects of serotonin is likely to depend on behavioral context, since the levels of serotonin in the IC transiently increase during behavioral events including stressful situations and social interaction. These studies support a broad model of serotonin receptors as a link between behavioral context and reconfiguration of circuitry in the IC, and the resulting possibility that plasticity at the level of specific receptor types could alter the relationship between context and circuit function.

Expression of glutamate and inhibitory amino acid vesicular transporters in the rodent auditory brainstem

Glutamate is the main excitatory neurotransmitter in the auditory system, but associations between glutamatergic neuronal populations and the distribution of their synaptic terminations have been difficult. Different subsets of glutamatergic terminals employ one of three vesicular glutamate transporters (VGLUT) to load synaptic vesicles. Recently, VGLUT1 and VGLUT2 terminals were found to have different patterns of organization in the inferior colliculus, suggesting that there are different types of glutamatergic neurons in the brainstem auditory system with projections to the colliculus. To positively identify VGLUT-expressing neurons as well as inhibitory neurons in the auditory brainstem, we used in situ hybridization to identify the mRNA for VGLUT1, VGLUT2, and VIAAT (the vesicular inhibitory amino acid transporter used by GABAergic and glycinergic terminals). Similar expression patterns were found in subsets of glutamatergic and inhibitory neurons in the auditory brainstem and thalamus of adult rats and mice. Four patterns of gene expression were seen in individual neurons. 1) VGLUT2 expressed alone was the prevalent pattern. 2) VGLUT1 coexpressed with VGLUT2 was seen in scattered neurons in most nuclei but was common in the medial geniculate body and ventral cochlear nucleus. 3) VGLUT1 expressed alone was found only in granule cells. 4) VIAAT expression was common in most nuclei but dominated in some. These data show that the expression of the VGLUT1/2 and VIAAT genes can identify different subsets of auditory neurons. This may facilitate the identification of different components in auditory circuits.

Context-dependent modulation of auditory processing by serotonin

Context-dependent plasticity in auditory processing is achieved in part by physiological mechanisms that link behavioral state to neural responses to sound. The neuromodulator serotonin has many characteristics suitable for such a role. Serotonergic neurons are extrinsic to the auditory system but send projections to most auditory regions. These projections release serotonin during particular behavioral contexts. Heightened levels of behavioral arousal and specific extrinsic events, including stressful or social events, increase serotonin availability in the auditory system. Although the release of serotonin is likely to be relatively diffuse, highly specific effects of serotonin on auditory neural circuitry are achieved through the localization of serotonergic projections, and through a large array of receptor types that are expressed by specific subsets of auditory neurons. Through this array, serotonin enacts plasticity in auditory processing in multiple ways. Serotonin changes the responses of auditory neurons to input through the alteration of intrinsic and synaptic properties, and alters both short- and long-term forms of plasticity. The infrastructure of the serotonergic system itself is also plastic, responding to age and cochlear trauma. These diverse findings support a view of serotonin as a widespread mechanism for behaviorally relevant plasticity in the regulation of auditory processing. This view also accommodates models of how the same regulatory mechanism can have pathological consequences for auditory processing.

Serotonin in the inferior colliculus fluctuates with behavioral state and environmental stimuli

Neuromodulation by serotonin (5-HT) could link behavioral state and environmental events with sensory processing. Within the auditory system, the presence of 5-HT alters the activity of neurons in the inferior colliculus (IC), but the conditions that influence 5-HT neurotransmission in this region of the brain are unknown. We used in vivo voltammetry to measure extracellular 5-HT in the IC of behaving mice to address this issue. Extracellular 5-HT increased with the recovery from anesthesia, suggesting that the neuromodulation of auditory processing is correlated with the level of behavioral arousal. Awake mice were further exposed to auditory (broadband noise), visual (light) or olfactory (2,5-dihydro-2,4,5-trimethylthiazoline, TMT) stimuli, presented with food or confined in a small arena. Only the auditory stimulus or restricted movement increased the concentration of extracellular 5-HT in the IC. Changes occurred within minutes of stimulus onset, with the auditory stimulus increasing extracellular 5-HT by an average of 5% and restricted movement increasing it by an average of 14%. These findings suggest that the neuromodulation of auditory processing by 5-HT is a dynamic process that is dependent on internal state and behavioral conditions.

Normal hearing is required for the emergence of long-lasting inhibitory potentiation in cortex

Long-term synaptic plasticity is a putative mechanism for learning in adults. However, there is little understanding of how synaptic plasticity mechanisms develop or whether their maturation depends on experience. Since inhibitory synapses are particularly malleable to sensory stimulation, long-lasting potentiation of inhibitory synapses was characterized in auditory thalamocortical slices. Intracortical high-frequency electrical stimulation led to a 67% increase in inhibitory synaptic currents. In the absence of stimulation, inhibitory potentiation was induced by a brief exposure to exogenous brain-derived neurotrophic factor (BDNF). BDNF exposure occluded any additional potentiation by high-frequency afferent stimulation, suggesting that BDNF signaling is sufficient to account for inhibitory potentiation. Moreover, inhibitory potentiation was reduced significantly by extracellular application of a BDNF scavenger or by intracellular blockade of BDNF receptor [tropomyosin-related kinase B (TrkB)] signaling. In contrast, glutamatergic or GABAergic antagonists did not prevent the induction of inhibitory potentiation. Since BDNF and TrkB expression are influenced strongly by activity, we predicted that inhibitory potentiation would be diminished by manipulations that decrease central auditory activity, such as hearing loss. Two forms of hearing loss were examined: conductive hearing loss in which the cochleae are not damaged or sensorineural hearing loss in which both cochleae are removed. Both forms of hearing loss were found to reduce significantly the magnitude of inhibitory potentiation. These data indicate that early experience is necessary for the normal development of BDNF-mediated long-lasting inhibitory potentiation, which may be associated with perceptual deficits at later ages.

Presynaptic 5-HT(1B) receptor-mediated serotonergic inhibition of glutamate transmission in the bed nucleus of the stria terminalis

Activation of neurons in the bed nucleus of the stria terminalis (BNST) plays a critical role in stress and anxiety-related behaviors. Previously, we have shown that serotonin (5-HT) can directly modulate BNST neuronal excitability by an action at postsynaptic receptors. In this study we built upon that work to examine the effects of 5-HT on excitatory neurotransmission in an in vitro rat BNST slice preparation. Bath application of 5-HT reversibly reduced the amplitude of evoked excitatory postsynaptic currents (eEPSCs). These effects were mimicked by the 5-HT(1B/D) receptor agonist, sumatriptan, and by the 5-HT(1B) receptor selective agonist, CP93129. Conversely, the effects of 5-HT and sumatriptan could be blocked by the 5-HT(1B) receptor-selective antagonist, GR55562. In contrast, the 5-HT(1A) receptor agonist 8-OH DPAT or antagonist WAY 100635 could not mimic or block the effect of 5-HT on eEPSCs. Together, these data suggest that the 5-HT-induced attenuation of eEPSCs was mediated by 5-HT(1B) receptor activation. Moreover, sumatriptan had no effect on the amplitude of the postsynaptic current elicited by pressure applied AMPA, suggesting a possible presynaptic locus for the 5-HT(1B) receptor. Furthermore, 5-HT, sumatriptan and CP93129 all increased the paired pulse ratio of eEPSCs while they concomitantly decreased the amplitude of eEPSCs, suggesting that these agonists act to reduce glutamate release probability at presynaptic locus. Consistent with this observation, sumatriptan decreased the frequency of miniature EPSCs, but had no effect on their amplitude. Taken together, these results suggest that 5-HT suppresses glutamatergic neurotransmission in the BNST by activating presynaptic 5-HT(1B) receptors to decrease glutamate release from presynaptic terminals. This study illustrates a new pathway by which the activity of BNST neurons can be indirectly modulated by 5-HT, and suggests a potential new target for the development of novel treatments for depression and anxiety disorders.

Transient gain adjustment in the inferior colliculus is serotonin- and calcium-dependent

In the inferior colliculus (IC), a brief period of acoustic conditioning can transiently enhance evoked discharge rate. The cellular basis of this phenomenon was assessed with whole cell current-clamp recordings in a gerbil IC brain slice preparation. The current needed to elicit a single action potential was first established for each neuron. A 5s synaptic stimulus train was delivered to the lateral lemniscus (LL), and followed immediately by the initial current pulse to assess a change in postsynaptic gain. The majority of IC neurons (66%) displayed an increase in current-evoked action potentials (Positive Gain). Despite the blockade of ionotropic glutamate receptors, this effect was correlated with membrane depolarization that occurred during the synaptic train. The postsynaptic mechanism for positive gain was examined by selective blockade of specific neurotransmitter receptors. Gain in action potentials was enhanced by antagonists of metabotropic glutamate, acetylcholine, GABA(A) and glycine receptors. In contrast, the gain was blocked or reduced by an antagonist to ionotropic serotonin receptors (5-HT(3)R). Blocking voltage-activated calcium channels with verapamil also reduced the effect. These results suggest that 5-HT(3)R activation, coupled with increased intracellular calcium, can transiently alter postsynaptic excitability in IC neurons.

Ontogeny of serotonin and serotonin2A receptors in rat auditory cortex

Maturation of the mammalian cerebral cortex is, in part, dependent upon multiple coordinated afferent neurotransmitter systems and receptor-mediated cellular linkages during early postnatal development. Given that serotonin (5-HT) is one such system, the present study was designed to specifically evaluate 5-HT tissue content as well as 5-HT(2A) receptor protein levels within the developing auditory cortex (AC). Using high performance liquid chromatography (HPLC), 5-HT and the metabolite, 5-hydroxyindoleacetic acid (5-HIAA), was measured in isolated AC, which demonstrated a developmental dynamic, reaching young adult levels early during the second week of postnatal development. Radioligand binding of 5-HT(2A) receptors with the 5-HT(2A/2C) receptor agonist, (125)I-DOI ((+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl; in the presence of SB206553, a selective 5-HT(2C) receptor antagonist, also demonstrated a developmental trend, whereby receptor protein levels reached young adult levels at the end of the first postnatal week (P8), significantly increased at P10 and at P17, and decreased back to levels not significantly different from P8 thereafter. Immunocytochemical labeling of 5-HT(2A) receptors and confocal microscopy revealed that 5-HT(2A) receptors are largely localized on layer II/III pyramidal cell bodies and apical dendrites within AC. When considered together, the results of the present study suggest that 5-HT, likely through 5-HT(2A) receptors, may play an important role in early postnatal AC development.

"Non-serotonergic" lateral superior olivary neurons of the neonatal mouse contain serotonin

In mutant mice which have high serotonin (5-HT) levels perinatally, neurons in the lateral superior olive (LSO, which are not serotonergic), contain 5-HT transiently in the neonatal period of development. We determined that the same phenomenon occurs in non-genetically altered mice: LSO neurons were 5-HT-immunoreactive at postnatal days 1 and 8. These findings suggest accumulation of 5-HT in by LSO neurons during postnatal development.

Different serotonin receptor agonists have distinct effects on sound-evoked responses in inferior colliculus

The neuromodulator serotonin has a complex set of effects on the auditory responses of neurons within the inferior colliculus (IC), a midbrain auditory nucleus that integrates a wide range of inputs from auditory and nonauditory sources. To determine whether activation of different types of serotonin receptors is a source of the variability in serotonergic effects, four selective agonists of serotonin receptors in the serotonin (5-HT) 1 and 5-HT2 families were iontophoretically applied to IC neurons, which were monitored for changes in their responses to auditory stimuli. Different agonists had different effects on neural responses. The 5-HT1A agonist had mixed facilitatory and depressive effects, whereas 5-HT1B and 5-HT2C agonists were both largely facilitatory. Different agonists changed threshold and frequency tuning in ways that reflected their effects on spike count. When pairs of agonists were applied sequentially to the same neurons, selective agonists sometimes affected neurons in ways that were similar to serotonin, but not to other selective agonists tested. Different agonists also differentially affected groups of neurons classified by the shapes of their frequency-tuning curves, with serotonin and the 5-HT1 receptors affecting proportionally more non-V-type neurons relative to the other agonists tested. In all, evidence suggests that the diversity of serotonin receptor subtypes in the IC is likely to account for at least some of the variability of the effects of serotonin and that receptor subtypes fulfill specialized roles in auditory processing.

Postnatal Expression of the Serotonin Transporter in Auditory Brainstem Neurons

To investigate the putative role of serotonin (5-HT) in auditory brainstem development, the expression of the 5-HT transporter (5-HTT) was evaluated in the normal mouse brainstem at 6 different postnatal ages. The brains of C3H/HeJ mice at birth (P0) and P1, P8-P9, P13, P21-P22, P35-P36 and P48-P50 were collected and processed immunohistochemically with an antibody raised against the 5-HTT. 5-HTT immunoreactivity (5-HTT-IR) was first observed in P8 mice and was localized to cell bodies in the ventral cochlear nucleus (VCN) and principal nuclei of the superior olivary complex, including the medial nucleus of the trapezoid body. Labeled neurons were found in similar regions in older mice except at P48-50, where labeled neurons were observed in the VCN only. 5-HTT-IR was especially prominent in VCN neurons at P21 and was observed in all of the brains examined at this age. These results indicate that auditory brainstem neurons of the normal inbred mouse express the 5-HTT postnatally. The presence of 5-HTT-IR in neurons located in the VCN indicates a regional expression of the 5-HTT that is related to the ascending auditory pathway. The timing of 5-HTT expression indicates that 5-HT may modulate developmental processes that rely on cochlear input.

Long-term depression of synaptic inhibition is expressed postsynaptically in the developing auditory system

Inhibitory transmission is critically involved in the functional maturation of neural circuits within the brain. However, the mechanisms involved in its plasticity and development remain poorly understood. At an inhibitory synapse of the developing auditory brain stem, we used whole cell recordings to determine the site of induction and expression of long-term depression (LTD), a robust activity-dependent phenomenon that decreases inhibitory synaptic gain and is postulated to underlie synapse elimination. Recordings were obtained from lateral superior olivary (LSO) neurons, and hyperpolarizing inhibitory potentials were evoked by stimulation of the medial nucleus of the trapezoid body (MNTB). Both postsynaptic glycine and GABAA receptors could independently display LTD when isolated pharmacologically. Focal application of GABA, but not glycine, on the postsynaptic LSO neuron was sufficient to induce depression of the amino acid-evoked response, or MNTB-evoked inhibitory postsynaptic potentials. This GABA-mediated depression, in the absence of MNTB stimulation, was blocked by a GABAB receptor antagonist. To assess whether a change in neurotransmitter release is associated with the LTD, the polyvalent cation, ruthenium red, was used to increase the frequency of miniature inhibitory synaptic events. Consistent with a postsynaptic locus of expression, we found that the mean amplitude of miniature events decreased after LTD with no change in their frequency of occurrence. Furthermore, there was no change in the paired-pulse ratio or release kinetics of evoked inhibitory responses. Together, these results provide direct evidence that activity-dependent LTD of inhibition has a postsynaptic locus of induction and alteration, and that GABA but not glycine plays a pivotal role.

Allelic variants of the functional promoter polymorphism of the human serotonin transporter gene is associated with auditory cortical stimulus processing

The loudness dependence (LD) of the auditory-evoked N1/P2 component has been shown to be related to the central serotonergic neurotransmission. Allelic variants in the promoter region of the 5-hydroxytryptamine transporter (5-HTT) gene were shown to modulate serotonergic activity. It was hypothesized that the three genotypes (l/l, s/l, s/s) differ with respect to LD. Allelic variants of the 5-HTT promoter region and LD at the Cz electrode were determined in 185 healthy subjects prospectively. A significant association was found between LD and genotype (ANOVA: F=4.172, p=0.017). Individuals homozygous for the l allele exhibited a weaker LD compared to heterozygous subjects. The results are consistent with the reported association between 5-HTT genotype and serotonin transport capacity in lymphoblasts, and indicate that auditory stimulus processing is associated with genetic variants of the brain serotonergic system. The LD may serve as endophenotype in human serotonin research.

Serotonin in the inferior colliculus

It has been recognized for some time that serotonin fibers originating in raphe nuclei are present in the inferior colliculi of all mammalian species studied. More recently, serotonin has been found to modulate the responses of single inferior colliculus neurons to many types of auditory stimuli, ranging from simple tone bursts to complex species-specific vocalizations. The effects of serotonin are often quite strong, and for some neurons are also highly specific. A dramatic illustration of this is that serotonin can change the selectivity of some neurons for sounds, including species-specific vocalizations. These results are discussed in light of several theories on the function of serotonin in the IC, and of outstanding issues that remain to be addressed.

Serotonin effects on frequency tuning of inferior colliculus neurons

We investigated the modulatory effects of serotonin on the tuning of 114 neurons in the central nucleus of the inferior colliculus (ICc) of Mexican free-tailed bats and how serotonin-induced changes in tuning influenced responses to complex signals. We obtained a "response area" for each neuron, defined as the frequency range that evoked discharges and the spike counts evoked by those frequencies at a constant intensity. We then iontophoretically applied serotonin and compared response areas obtained before and during the application of serotonin. In 58 cells, we also assessed how serotonin-induced changes in response areas correlated with changes in the responses to brief frequency-modulated (FM) sweeps whose structure simulated natural echolocation calls. Serotonin profoundly changed tone-evoked spike counts in 60% of the neurons (68/114). In most neurons, serotonin exerted a gain control, facilitating or depressing the responses to all frequencies in their response areas. In many cells, serotonergic effects on tones were reflected in the responses to FM signals. The most interesting effects were in those cells in which serotonin selectively changed the responsiveness to only some frequencies in the neuron's response area and had little or no effect on other frequencies. This caused predictable changes in responses to the more complex FM sweeps whose spectral components passed through the neurons' response areas. Our results suggest that serotonin, whose release varies with behavioral state, functionally reconfigures the circuitry of the IC and may modulate the perception of acoustic signals under different behavioral states.

Serotonergic innervation of the auditory brainstem of the Mexican free-tailed bat,Tadarida brasiliensis

Anatomical and electrophysiological evidence suggests that serotonin alters the processing of sound in the auditory brainstem of many mammalian species. The Mexican free-tailed bat is a hearing specialist, like other microchiropteran bats. At the same time, many aspects of its auditory brainstem are similar to those in other mammals. This dichotomy raises an interesting question regarding the serotonergic innervation of the bat auditory brainstem: Is the serotonergic input to the auditory brainstem similar in bats and other mammals, or are there specializations in the serotonergic innervation of bats that may be related to their exceptional hearing capabilities? To address this question, we immunocytochemically labeled serotonergic fibers in the brainstem of the Mexican free-tailed bat, Tadarida brasiliensis. We found many similarities in the pattern of serotonergic innervation of the auditory brainstem in Tadarida compared with other mammals, but we also found two striking differences. Similarities to staining patterns in other mammals included a higher density of serotonergic fibers in the dorsal cochlear nucleus and in granule cell regions than in the ventral cochlear nucleus, a high density of fibers in some periolivary nuclei of the superior olive, and a higher density of fibers in peripheral regions of the inferior colliculus compared with its core. The two novel features of serotonergic innervation in Tadarida were a high density of fibers in the fusiform layer of the dorsal cochlear nucleus relative to surrounding layers and a relatively high density of serotonergic fibers in the low-frequency regions of the lateral and medial superior olive.

Plasticity of the superior olivary complex

The superior olivary complex (SOC) is part of the auditory brainstem of the vertebrate brain. Residing ventrally in the rhombencephalon, it receives sensory signals from both cochleae through multisynaptic pathways. Neurons of the SOC are also a target of bilateral descending projections. Ascending and descending efferents of the SOC affect the processing of auditory signals on both sides of the brainstem and in both organs of Corti. The pattern of connectivity indicates that the SOC fulfills functions of binaural signal integration serving sound localization. But whereas many of these connectional features are shared with the inferior colliculus (with the important exception of a projection to the inner ear), cellular and molecular investigations have shown that cells residing in SOC are unique in several respects. Unlike those of other auditory brainstem nuclei, they specifically express molecules known to be involved in development, plasticity, and learning (e.g., GAP-43 mRNA, specific subunits of integrin). Moreover, neurons of the SOC in adult mammals respond to various kinds of hearing impairment with the expression of plasticity-related substances (e.g., GAP-43, c-Jun, c-Fos, cytoskeletal elements), indicative of a restructuring of auditory connectivity. These observations suggest that the SOC is pivotal in the developmental and adaptive tuning of binaural processing in young and adult vertebrates.

Afferent projections of the superior olivary complex

Based on current literature, the afferents of the superior olivary complex (SOC) are described including those from the cochlear nucleus, inferior colliculus, thalamus, and auditory cortex. Intrinsic SOC afferents and non-auditory afferents from the serotoninergic and noradrenergic systems are also described. New data are provided that show a differential distribution of serotoninergic afferents within the SOC: serotoninergic fibers were relatively sparse in the lateral and medial superior olives and the medial nucleus of the trapezoid body and were most numerous in periolivary regions. There are variations in the density of serotoninergic fibers within periolivary regions themselves. New data is also provided on auditory and non-auditory afferents to SOC neurons, which have known targets. These include: cochlear nucleus afferents to periolivary (lateral nucleus of the trapezoid body, LNTB) cells that project to the inferior colliculus; cortical afferents to periolivary (ventral nucleus of the trapezoid body, VNTB) cells that project to the cochlear nucleus; and serotoninergic and noradrenergic afferents to periolivary (LNTB and VNTB) cells that project to the cochlear nucleus. The relationships between other types of afferents and SOC neurons with known projections are also described as functional circuits. The circuits include those that are part of the ascending auditory system (to the inferior and superior colliculi, lateral lemniscus, and medial geniculate nucleus), the descending auditory system (to the cochlea and cochlear nucleus), and the middle ear reflex circuits.

Chemoarchitecture of the anuran auditory midbrain

The anuran torus semicircularis consists of several subnuclei that are part of the ascending auditory pathway as well as audiomotor interface structures. Additionally, recent anatomical studies suggest that the midbrain tegmentum is an integral part of the audiomotor network. To describe the chemoarchitecture of these nuclei, taking into account the toral subdivisions, we investigated the distribution of serotonin, leucine-enkephalin, substance P, tyrosine-hydroxylase, dopamine D2-receptor, parvalbumin, aspartate, GABA, and estrogen-binding protein-immunoreactivity in the midbrain of Bombina orientalis, Discoglossus pictus and Xenopus laevis. In the torus semicircularis, the highest density of immunoreactive fibers and terminals for all transmitters was found in the laminar nucleus. Parvalbumin-like immunoreactivity was highest in the principal nucleus, and D2-receptor-like immunoreactivity was uniformly distributed throughout the torus. In the tegmentum, axons and/or dendrites were stained with all antibodies except estrogen-binding protein. Additionally, heavily stained enkephalin and substance P-immunopositive fiber plexus were found in the lateral and dorsal tegmentum. The immunostainings revealed no qualitative differences between the three species. Immunopositive cell bodies were labeled in several brain areas, the connectivity of which with torus and tegmentum is discussed on the background of functional questions. The putative neuromodulatory innervation of both the laminar nucleus of the torus semicircularis and the tegmentum may be the anatomical basis for the influence of the animal's endogenous state on the behavioral reaction to sensory stimuli. These data corroborate earlier anatomical and physiological findings that the neurons of these nuclei are key elements in the audio-motor interface.

Long-lasting inhibitory synaptic depression is age- and calcium-dependent

The developmental refinement of excitatory synapses is often influenced by neuronal activity, and underlying synaptic mechanisms have been suggested. In contrast, few studies have asked whether inhibitory synapses are reorganized during development and whether this is accompanied by use-dependent changes of inhibitory synaptic strength. The topographic inhibitory projection from the medial nucleus of the trapezoid body (MNTB) to the lateral superior olive (LSO) undergoes synapse elimination during development (Sanes and Takács, 1993). To determine whether there is an associated period of synaptic plasticity, whole-cell recordings were obtained from developing LSO neurons of gerbils in a brain slice preparation. In current-clamp recordings, low-frequency stimulation of the MNTB led to a decline in IPSP amplitude by 43%. In voltage-clamp recordings, hyperpolarized LSO neurons also exhibited a long-lasting depression of MNTB-evoked inhibitory synaptic currents (34%) after low-frequency stimulation. When LSO neurons were depolarized, low-frequency stimulation of the MNTB produced a significantly larger inhibitory synaptic depression (59%). This synaptic plasticity declined dramatically by postnatal days 17-19. Similar to well studied forms of excitatory synaptic plasticity, inhibitory depression depended on postsynaptic calcium. We propose that such activity-dependent synaptic depression may support the developmental rearrangement of inhibitory terminals as they compete with neighboring excitatory and/or inhibitory inputs.