The selective neurotoxin DSP-4 impairs the noradrenergic projections from the locus coeruleus to the inferior colliculus in rats

Auditory disturbances in patients with complex regional pain syndrome

ABSTRACT

Complex regional pain syndrome (CRPS) is often associated with reduced sound tolerance (hyperacusis) on the affected side, but the mechanism of this symptom is unclear. As compensatory increases in central auditory activity after cochlear injury may trigger hyperacusis, hearing and discomfort thresholds to pure tones (250, 500, 1000, 2000, 3000, 4000, 6000, and 8000 Hz) were assessed in 34 patients with CRPS and 26 pain-free controls. In addition, in 31 patients and 17 controls, auditory-evoked potentials to click stimuli (0.08 ms duration, 6 Hz, 60 dB above the hearing threshold) were averaged across 2000 trials for each ear. Auditory discomfort thresholds were lower at several pitches on the CRPS-affected than contralateral side and lower at all pitches on the affected side than in controls. However, ipsilateral hyperacusis was not associated with psychophysical or physiological signs of cochlear damage. Instead, neural activity in the ipsilateral brainstem and midbrain was greater when repetitive click stimuli were presented on the affected than contralateral side and greater bilaterally than in controls. In addition, click-evoked potentials, reflecting thalamo-cortical signal transfer and early cortical processing, were greater contralaterally in patients than controls. Together, these findings suggest that hyperacusis originates in the ipsilateral brainstem and midbrain rather than the peripheral auditory apparatus of patients with CRPS. Failure of processes that jointly modulate afferent auditory signalling and pain (eg, inhibitory influences stemming from the locus coeruleus) could contribute to ipsilateral hyperacusis in CRPS.

Pupillary Reflexes in Complex Regional Pain Syndrome: Asymmetry to Arousal Stimuli Suggests an Ipsilateral Locus Coeruleus Deficit

Converging lines of evidence suggest that autonomic and nociceptive pathways linked with the locus coeruleus are disrupted in complex regional pain syndrome (CRPS). To investigate this, pupillary dilatation to arousal stimuli (which reflects neural activity in the locus coeruleus) and pupillary reflexes to light were assessed in a cross-sectional study of 33 patients with CRPS. Moderately painful electrical shocks were delivered to the affected or contralateral limb and unilateral 110 dB SPL acoustic startle stimuli were delivered via headphones. To determine whether the acoustic startle stimuli inhibited shock-induced pain, startle stimuli were also administered bilaterally 200 ms before or after the electric shock. The pupils constricted briskly and symmetrically to bright light (500 lux) and dilated symmetrically in dim light (5 lux). However, the pupil on the CRPS-affected side was smaller than the contralateral pupil before and after the delivery of painless and painful arousal stimuli. Auditory sensitivity was greater on the affected than unaffected side but acoustic startle stimuli failed to inhibit shock-induced pain. Together, these findings suggest that neural activity in pathways linked with the locus coeruleus is compromised on the affected side in patients with CRPS. This may contribute to autonomic disturbances, auditory discomfort and pain. Perspective: The locus coeruleus is involved not only in modulation of pain but also regulates sensory traffic more broadly. Hence, fatigue of neural activity in the ipsilateral locus coeruleus might not only exacerbate pain and hyperalgesia in CRPS but could also contribute more generally to hemilateral disturbances in sensory processing.

Phasic activation of the locus coeruleus attenuates the acoustic startle response by increasing cortical arousal

An alerting sound elicits the Acoustic Startle Response (ASR) that is dependent on the sound volume and organisms’ state, which is regulated by neuromodulatory centers. The locus coeruleus (LC) neurons respond to salient stimuli and noradrenaline release affects sensory processing, including auditory. The LC hyperactivity is detrimental for sensorimotor gating. We report here that priming microstimulation of the LC (100-ms at 20, 50, and 100 Hz) attenuated the ASR in rats. The ASR reduction scaled with frequency and 100 Hz-stimulation mimicked pre-exposure to a non-startling tone (prepulse). A rapid (~ 40 ms) EEG desynchronization following the LC stimulation suggested that the ASR reduction was due to elevated cortical arousal. The effects of LC stimulation on the ASR and EEG were consistent with systematic relationships between the ASR, awake/sleep state, and the cortical arousal level; for that matter, a lower ASR amplitude corresponded to a higher arousal level. Thus, the LC appears to modulate the ASR circuit via its diffuse ascending projections to the forebrain saliency network. The LC modulation directly in the brainstem and/or spinal cord may also play a role. Our findings suggest the LC as a part of the brain circuitry regulating the ASR, while underlying neurophysiological mechanisms require further investigation.

Acoustic startle stimuli inhibit pain but do not alter nociceptive flexion reflexes to sural nerve stimulation

Acoustic startle stimuli inhibit pain, but whether this is due to a cross-modal inhibitory process or some other mechanism is uncertain. To investigate this, electrical stimulation of the sural nerve either preceded or followed an acoustic startle stimulus (by 200 ms) or was presented alone in 30 healthy participants. Five electrical stimuli, five acoustic startle stimuli, 10 startle + electrical stimuli, and 10 electrical + startle stimuli were presented in mixed order at intervals of 30-60 s. Effects of the startle stimulus on pain ratings, pupillary dilatation and nociceptive flexion reflexes to the electric shock were assessed. The acoustic startle stimulus inhibited electrically evoked pain to the ensuing electric shock (p < .001), and the electrical stimulus inhibited the perceived loudness of a subsequent acoustic startle stimulus (p < .05). However, the startle stimulus did not affect electrically evoked pain when presented 200 ms after the electric shock, and electrically evoked pain did not influence the perceived loudness of a prior startle stimulus. Furthermore, stimulus order did not influence the pupillary responses or nociceptive flexion reflexes. These findings suggest that acoustic startle stimuli transiently inhibit nociceptive processing and, conversely, that electrical stimuli inhibit subsequent auditory processing. These inhibitory effects do not seem to involve spinal gating as nociceptive flexion reflexes to the electric shock were unaffected by stimulus order. Thus, cross-modal interactions at convergence points in the brainstem or higher centers may inhibit responses to the second stimulus in a two-stimulus train.

The Inferior Colliculus in Alcoholism and Beyond

Post-mortem neuropathological and in vivo neuroimaging methods have demonstrated the vulnerability of the inferior colliculus to the sequelae of thiamine deficiency as occurs in Wernicke-Korsakoff Syndrome (WKS). A rich literature in animal models ranging from mice to monkeys-including our neuroimaging studies in rats-has shown involvement of the inferior colliculi in the neural response to thiamine depletion, frequently accomplished with pyrithiamine, an inhibitor of thiamine metabolism. In uncomplicated alcoholism (i.e., absent diagnosable neurological concomitants), the literature citing involvement of the inferior colliculus is scarce, has nearly all been accomplished in preclinical models, and is predominately discussed in the context of ethanol withdrawal. Our recent work using novel, voxel-based analysis of structural Magnetic Resonance Imaging (MRI) has demonstrated significant, persistent shrinkage of the inferior colliculus using acute and chronic ethanol exposure paradigms in two strains of rats. We speculate that these consistent findings should be considered from the perspective of the inferior colliculi having a relatively high CNS metabolic rate. As such, they are especially vulnerable to hypoxic injury and may be provide a common anatomical link among a variety of disparate insults. An argument will be made that the inferior colliculi have functions, possibly related to auditory gating, necessary for awareness of the external environment. Multimodal imaging including diffusion methods to provide more accurate in vivo visualization and quantification of the inferior colliculi may clarify the roles of brain stem nuclei such as the inferior colliculi in alcoholism and other neuropathologies marked by altered metabolism.

Foreground stimuli and task engagement enhance neuronal adaptation to background noise in the inferior colliculus of macaques

Auditory neuronal responses are modified by background noise. Inferior colliculus (IC) neuronal responses adapt to the most frequent sound level within an acoustic scene (adaptation to stimulus statistics), a mechanism that may preserve neuronal and behavioral thresholds for signal detection. However, it is still unclear whether the presence of foreground stimuli and/or task involvement can modify neuronal adaptation. To investigate how task engagement interacts with this mechanism, we compared the response of IC neurons to background noise, which caused adaptation to stimulus statistics, while macaque monkeys performed a masked tone detection task (task-driven condition) with responses recorded when the same background noise was presented alone (passive listening condition). In the task-dependent condition, monkeys performed a Go/No-Go task while 50-ms tones were embedded within an adaptation-inducing continuous background noise whose levels changed every 50 ms and were drawn from a probability distribution. The adaptation to noise stimulus statistics in IC neuronal responses was significantly enhanced in the task-driven condition compared with the passive listening condition, showing that foreground stimuli and/or task-engagement can modify IC neuronal responses. Additionally, the response of IC neurons to noise was significantly affected by the preceding sensory information (history effect) regardless of task involvement. These studies show that dynamic range adaptation in IC preserves behavioral and neurometric thresholds irrespective of noise type and a dependence of neuronal activity on task-related factors at subcortical levels of processing.NEW & NOTEWORTHY Auditory neuronal responses are influenced by maskers and distractors. However, it is still unclear whether the neuronal sensitivity to the masker stimulus is influenced by task-dependent factors. Our study represents one of the first attempts to investigate how task involvement influences the neural representation of background sounds in the subcortical, midbrain auditory neurons of behaving animals.

Bimodal neuromodulation combining sound and tongue stimulation reduces tinnitus symptoms in a large randomized clinical study

Tinnitus is a phantom auditory perception coded in the brain that can be bothersome or debilitating, affecting 10 to 15% of the population. Currently, there is no clinically recommended drug or device treatment for this major health condition. Animal research has revealed that sound paired with electrical somatosensory stimulation can drive extensive plasticity within the brain for tinnitus treatment. To investigate this bimodal neuromodulation approach in humans, we evaluated a noninvasive device that delivers sound to the ears and electrical stimulation to the tongue in a randomized, double-blinded, exploratory study that enrolled 326 adults with chronic subjective tinnitus. Participants were randomized into three parallel arms with different stimulation settings. Clinical outcomes were evaluated over a 12-week treatment period and a 12-month posttreatment phase. For the primary endpoints, participants achieved a statistically significant reduction in tinnitus symptom severity at the end of treatment based on two commonly used outcome measures, Tinnitus Handicap Inventory (Cohen’s d effect size: −0.87 to −0.92 across arms; P < 0.001) and Tinnitus Functional Index (−0.77 to −0.87; P < 0.001). Therapeutic improvements continued for 12 months after treatment for specific bimodal stimulation settings, which had not previously been demonstrated in a large cohort for a tinnitus intervention. The treatment also achieved high compliance and satisfaction rates with no treatment-related serious adverse events. These positive therapeutic and long-term results motivate further clinical trials toward establishing bimodal neuromodulation as a clinically recommended device treatment for tinnitus.

Differential encoding of signals and preferences by noradrenaline in the anuran brain

Social preferences enable animals to selectively interact with some individuals over others. One influential idea for the evolution of social preferences is that preferred signals evolve because they elicit greater neural responses from sensory systems. However, in juvenile plains spadefoot toad (Spea bombifrons), a species with condition-dependent mating preferences, responses of the preoptic area, but not of the auditory midbrain, mirror adult social preferences. To examine whether this separation of signal representation from signal valuation generalizes to other anurans, we compared the relative contributions of noradrenergic signalling in the preoptic area and auditory midbrain of S. bombifrons and its close relative Spea multiplicata We manipulated body condition in juvenile toads by controlling diet and used high pressure liquid chromatography to compare call-induced levels of noradrenaline and its metabolite MHPG in the auditory midbrain and preoptic area of the two species. We found that calls from the two species induced different levels of noradrenaline and MHPG in the auditory system, with higher levels measured in both species for the more energetic S. bombifrons call. In contrast, noradrenaline levels in the preoptic area mirrored patterns of social preferences in both S. bombifrons and S. multiplicata That is, noradrenaline levels were higher in response to the preferred calls within each species and were modified by diet in S. bombifrons (with condition-dependent preferences) but not S. multiplicata (with condition-independent preferences). Our results are consistent with a potentially important role for preoptic noradrenaline in the development of social preferences and indicate that it could be a target of selection in the evolution of condition-dependent social preferences.

Integration of locomotion and auditory signals in the mouse inferior colliculus

The inferior colliculus (IC) is the major midbrain auditory integration center, where virtually all ascending auditory inputs converge. Although the IC has been extensively studied for sound processing, little is known about the neural activity of the IC in moving subjects, as frequently happens in natural hearing conditions. Here, by recording neural activity in walking mice, we show that the activity of IC neurons is strongly modulated by locomotion, even in the absence of sound stimuli. Similar modulation was also found in hearing-impaired mice, demonstrating that IC neurons receive non-auditory, locomotion-related neural signals. Sound-evoked activity was attenuated during locomotion, and this attenuation increased frequency selectivity across the neuronal population, while maintaining preferred frequencies. Our results suggest that during behavior, integrating movement-related and auditory information is an essential aspect of sound processing in the IC.

Noninvasive Bimodal Neuromodulation for the Treatment of Tinnitus: Protocol for a Second Large-Scale Double-Blind Randomized Clinical Trial to Optimize Stimulation Parameters

Background

There is increasing evidence from animal and human studies that bimodal neuromodulation combining sound and electrical somatosensory stimulation of the tongue can induce extensive brain changes and treat tinnitus.

Objective

The main objectives of the proposed clinical study are to confirm the efficacy, safety, and tolerability of treatment demonstrated in a previous large-scale study of bimodal auditory and trigeminal nerve (tongue) stimulation (Treatment Evaluation of Neuromodulation for Tinnitus - Stage A1); evaluate the therapeutic effects of adjusting stimulation parameters over time; and determine the contribution of different features of bimodal stimulation in improving tinnitus outcomes.

Methods

This study will be a prospective, randomized, double-blind, parallel-arm, comparative clinical trial of a 12-week treatment for tinnitus using a Conformité Européenne (CE)–marked device with a pre-post and 12-month follow-up design. Four treatment arms will be investigated, in which each arm consists of two different stimulation settings, with the first setting presented during the first 6 weeks and the second setting presented during the next 6 weeks of treatment. The study will enroll 192 participants, split in a ratio of 80:80:16:16 across the four arms. Participants will be randomized to one of four arms and stratified to minimize baseline variability in four categories: two separate strata for sound level tolerance (using loudness discomfort level as indicators for hyperacusis severity), high tinnitus symptom severity based on the Tinnitus Handicap Inventory (THI), and tinnitus laterality. The primary efficacy endpoints are within-arm changes in THI and Tinnitus Functional Index as well as between-arm changes in THI after 6 weeks of treatment for the full cohort and two subgroups of tinnitus participants (ie, one hyperacusis subgroup and a high tinnitus symptom severity subgroup). Additional efficacy endpoints include within-arm or between-arm changes in THI after 6 or 12 weeks of treatment and in different subgroups of tinnitus participants as well as at posttreatment assessments at 6 weeks, 6 months, and 12 months. Treatment safety, attrition rates, and compliance rates will also be assessed and reported.

Results

This study protocol was approved by the Tallaght University Hospital/St. James’s Hospital Joint Research Ethics Committee in Dublin, Ireland. The first participant was enrolled on March 20, 2018. The data collection and database lock are expected to be completed by February 2020, and the data analysis and manuscript submission are expected to be conducted in autumn of 2020.

Conclusions

The findings of this study will be disseminated to relevant research, clinical, and health services and patient communities through publications in peer-reviewed journals and presentations at scientific and clinical conferences.

Trial Registration

ClinicalTrials.gov NCT03530306; https://clinicaltrials.gov/ct2/show/NCT03530306

International Registered Report Identifier (IRRID)

DERR1-10.2196/13176

Effect of monoamine reuptake inhibition and α1 blockade on respiratory arrest and death following electroshock-induced seizures in mice

OBJECTIVE

Sudden unexpected death in epilepsy (SUDEP) is the leading cause of death in patients with refractory epilepsy. Although the mechanisms for SUDEP are incompletely understood, seizure-induced respiratory arrest (S-IRA) has been strongly and consistently implicated. A body of evidence indicates that serotonin (5-HT), a modulator of breathing, plays a critical role in SUDEP. Because the 5-HT and norepinephrine (NE) systems interact in many biologic processes and NE is known to modulate breathing and seizures, we hypothesized that NE may play a role in S-IRA and SUDEP.

METHODS

We examined the effects of pharmacologic manipulation of 5-HT and NE on S-IRA and death following maximal electroshock (MES)-induced seizures in adult wild-type (WT) mice, genetically 5-HT neuron-deficient (Lmx1bf/f/p ) mice, and chemically NE neuron-deficient mice. Mice were treated with pharmacologic agents targeting the serotonergic and noradrenergic systems and subjected to seizure induction via MES while breathing was measured via whole-body plethysmography.

RESULTS

S-IRA and death was reduced in WT mice with NE reuptake inhibitors (NRIs), reboxetine and atomoxetine, selective serotonin reuptake inhibitors (SSRIs), fluoxetine and citalopram, and the dual 5-HT/NE reuptake inhibitor (SNRI), duloxetine. S-IRA and death was also reduced in Lmx1bf/f/p mice with reboxetine and fluoxetine. The protective effects of the reuptake inhibitors were prevented by the α1 antagonist, prazosin. Citalopram did not reduce S-IRA and death in NE neuron-deficient mice.

SIGNIFICANCE

These data suggest that 5-HT and NE critically interact in the modulation of breathing following a seizure and potentially inform preventive strategies for SUDEP.

The Neurotoxin DSP-4 Induces Hyperalgesia in Rats that is Accompanied by Spinal Oxidative Stress and Cytokine Production

Central neuropathic pain (CNP) a significant problem for many people, is not well-understood and difficult to manage. Dysfunction of the central noradrenergic system originating in the locus coeruleus (LC) may be a causative factor in the development of CNP. The LC is the major noradrenergic nucleus of the brain and plays a significant role in central modulation of nociceptive neurotransmission. Here, we examined CNS pathophysiological changes induced by intraperitoneal administration of the neurotoxin DSP-4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride). Administration of DSP-4 decreased levels of norepinephrine in spinal tissue and cerebrospinal fluid (CSF) and led to the development of thermal and mechanical hyperalgesia over 21 days, that was reversible with morphine. Hyperalgesia was accompanied by significant increases in noradrenochrome (oxidized norepinephrine) and expression of 4-hydroxynonenal in CSF and spinal cord tissue respectively at day 21, indicative of oxidative stress. In addition, spinal levels of pro-inflammatory cytokines (interleukins 6 and 17A, tumor necrosis factor-α), as well as the anti-inflammatory cytokine interleukin10 were also significantly elevated at day 21, indicating that an inflammatory response occurred. The inflammatory effect of DSP-4 presented in this study that includes oxidative stress may be particularly useful in elucidating mechanisms of CNP in inflammatory disease states.

Morphological correlates of sex differences in acoustic startle response and prepulse inhibition through projections from locus coeruleus to cochlear root neurons

The noradrenergic locus coeruleus (LC) plays an important role in the promotion and maintenance of arousal and alertness. Our group recently described coerulean projections to cochlear root neurons (CRNs), the first relay of the primary acoustic startle reflex (ASR) circuit. However, the role of the LC in the ASR and its modulation, prepulse inhibition (PPI), is not clear. In this study, we damaged LC neurons and fibers using a highly selective neurotoxin, DSP-4, and then assessed ASR and PPI in male and female rats. Our results showed that ASR amplitude was higher in males at 14 days after DSP-4 injection when compared to pre-administration values and those in the male control group. Such modifications in ASR amplitude did not occur in DSP-4-injected females, which exhibited ASR amplitude within the range of control values. PPI differences between males and females seen in controls were not observed in DSP-4-injected rats for any interstimulus interval tested. DSP-4 injection did not affect ASR and PPI latencies in either the male or the female groups, showing values that were consistent with the sex-related variability observed in control rats. Furthermore, we studied the noradrenergic receptor system in the cochlear nerve root using gene expression analysis. When compared to controls, DSP-4-injected males showed higher levels of expression in all adrenoceptor subtypes; however, DSP-4-injected females showed varied effects depending on the receptor type, with either up-, downregulations, or maintenance of expression levels. Lastly, we determined noradrenaline levels in CRNs and other LC-targeted areas using HPLC assays, and these results correlated with behavioral and adrenoceptor expression changes post DSP-4 injection. Our study supports the participation of LC in ASR and PPI, and contributes toward a better understanding of sex-related differences observed in somatosensory gating paradigms.

Dopaminergic Input to the Inferior Colliculus in Mice

The response of sensory neurons to stimuli can be modulated by a variety of factors including attention, emotion, behavioral context, and disorders involving neuromodulatory systems. For example, patients with Parkinson’s disease (PD) have disordered speech processing, suggesting that dopamine alters normal representation of these salient sounds. Understanding the mechanisms by which dopamine modulates auditory processing is thus an important goal. The principal auditory midbrain nucleus, the inferior colliculus (IC), is a likely location for dopaminergic modulation of auditory processing because it contains dopamine receptors and nerve terminals immunoreactive for tyrosine hydroxylase (TH), the rate-limiting enzyme in dopamine synthesis. However, the sources of dopaminergic input to the IC are unknown. In this study, we iontophoretically injected a retrograde tracer into the IC of mice and then stained the tissue for TH. We also immunostained for dopamine beta-hydroxylase (DBH), an enzyme critical for the conversion of dopamine to norepinephrine, to differentiate between dopaminergic and noradrenergic inputs. Retrogradely labeled neurons that were positive for TH were seen bilaterally, with strong ipsilateral dominance, in the subparafascicular thalamic nucleus (SPF). All retrogradely labeled neurons that we observed in other brain regions were TH-negative. Projections from the SPF were confirmed using an anterograde tracer, revealing TH-positive and DBH-negative anterogradely labeled fibers and terminals in the IC. While the functional role of this dopaminergic input to the IC is not yet known, it provides a potential mechanism for context dependent modulation of auditory processing.

Relationships between Pupil Diameter and Neuronal Activity in the Locus Coeruleus, Colliculi, and Cingulate Cortex

Changes in pupil diameter that reflect effort and other cognitive factors are often interpreted in terms of the activity of norepinephrine-containing neurons in the brainstem nucleus locus coeruleus (LC), but there is little direct evidence for such a relationship. Here, we show that LC activation reliably anticipates changes in pupil diameter that either fluctuate naturally or are driven by external events during near fixation, as in many psychophysical tasks. This relationship occurs on as fine a temporal and spatial scale as single spikes from single units. However, this relationship is not specific to the LC. Similar relationships, albeit with delayed timing and different reliabilities across sites, are evident in the inferior and superior colliculus and anterior and posterior cingulate cortex. Because these regions are interconnected with the LC, the results suggest that non-luminance-mediated changes in pupil diameter might reflect LC-mediated coordination of neuronal activity throughout some parts of the brain.

Dopamine in the auditory brainstem and midbrain: co-localization with amino acid neurotransmitters and gene expression following cochlear trauma

Dopamine (DA) modulates the effects of amino acid neurotransmitters (AANs), including GABA and glutamate, in motor, visual, olfactory, and reward systems (Hnasko et al., 2010; Stuber et al., 2010; Hnasko and Edwards, 2012). The results suggest that DA may play a similar modulatory role in the auditory pathways. Previous studies have shown that deafness results in decreased GABA release, changes in excitatory neurotransmitter levels, and increased spontaneous neuronal activity within brainstem regions related to auditory function. Modulation of the expression and localization of tyrosine hydroxylase (TH; the rate limiting enzyme in the production of DA) in the IC following cochlear trauma has been previously reported (Tong et al., 2005). In the current study the possibility of co-localization of TH with AANs was examined. Changes in the gene expression of TH were compared with changes in the gene expression of markers for AANs in the cochlear nucleus (CN) and inferior colliculus (IC) to determine whether those deafness related changes occur concurrently. The results indicate that bilateral cochlear ablation significantly reduced TH gene expression in the CN after 2 months while in the IC the reduction in TH was observed at both 3 days and 2 months following ablation. Furthermore, in the CN, glycine transporter 2 (GLYT2) and the GABA transporter (GABAtp) were also significantly reduced only after 2 months. However, in the IC, DA receptor 1 (DRDA1), vesicular glutamate transporters 2 and 3 (VGLUT2, VGLUT3), GABAtp and GAD67 were reduced in expression both at the 3 days and 2 months time points. A close relationship between the distribution of TH and several of the AANs was determined in both the CN and the IC. In addition, GLYT2 and VGLUT3 each co-localized with TH within IC somata and dendrites. Therefore, the results of the current study suggest that DA is spatially well positioned to influence the effects of AANs on auditory neurons.

Catecholaminergic connectivity to the inner ear, central auditory, and vocal motor circuitry in the plainfin midshipman fish porichthys notatus

Although the neuroanatomical distribution of catecholaminergic (CA) neurons has been well documented across all vertebrate classes, few studies have examined CA connectivity to physiologically and anatomically identified neural circuitry that controls behavior. The goal of this study was to characterize CA distribution in the brain and inner ear of the plainfin midshipman fish (Porichthys notatus) with particular emphasis on their relationship with anatomically labeled circuitry that both produces and encodes social acoustic signals in this species. Neurobiotin labeling of the main auditory end organ, the saccule, combined with tyrosine hydroxylase immunofluorescence (TH-ir) revealed a strong CA innervation of both the peripheral and central auditory system. Diencephalic TH-ir neurons in the periventricular posterior tuberculum, known to be dopaminergic, send ascending projections to the ventral telencephalon and prominent descending projections to vocal-acoustic integration sites, notably the hindbrain octavolateralis efferent nucleus, as well as onto the base of hair cells in the saccule via nerve VIII. Neurobiotin backfills of the vocal nerve in combination with TH-ir revealed CA terminals on all components of the vocal pattern generator, which appears to largely originate from local TH-ir neurons but may include input from diencephalic projections as well. This study provides strong neuroanatomical evidence that catecholamines are important modulators of both auditory and vocal circuitry and acoustic-driven social behavior in midshipman fish. This demonstration of TH-ir terminals in the main end organ of hearing in a nonmammalian vertebrate suggests a conserved and important anatomical and functional role for dopamine in normal audition.

The noradrenergic projection from the locus coeruleus to the cochlear root neurons in rats

The cochlear root neurons (CRNs) are key components of the primary acoustic startle circuit; mediating auditory alert and escape behaviors in rats. They receive a great variety of inputs which serve to elicit and modulate the acoustic startle reflex (ASR). Recently, our group has suggested that CRNs receive inputs from the locus coeruleus (LC), a noradrenergic nucleus which participates in attention and alertness. Here, we map the efferent projection patterns of LC neurons and confirm the existence of the LC-CRN projection using both anterograde and retrograde tract tracers. Our results show that each LC projects to the CRNs of both sides with a clear ipsilateral predominance. The LC axons terminate as small endings distributed preferentially on the cell body and primary dendrites of CRNs. Using light and confocal microscopy, we show a strong immunoreactivity for tyrosine hydroxylase and dopamine β-hydroxylase in these terminals, indicating noradrenaline release. We further studied the noradrenergic system using gene expression analysis (RT-qPCR) and immunohistochemistry to detect specific noradrenergic receptor subunits in the cochlear nerve root. Our results indicate that CRNs contain a noradrenergic receptor profile sufficient to modulate the ASR, and also show important gender-specific differences in their gene expression. 3D reconstruction analysis confirms the presence of sexual dimorphism in the density and distribution of LC neurons. Our study describes a coerulean noradrenergic projection to the CRNs that might contribute to neural processes underlying sensory gating of the ASR, and also provides an explanation for the gender differences observed in the behavioral paradigm.

Sexually-dimorphic expression of tyrosine hydroxylase immunoreactivity in the brain of a vocal teleost fish (Porichthys notatus)

Vocal communication has emerged as a powerful model for the study of neural mechanisms of social behavior. Modulatory neurochemicals postulated to play a central role in social behavior, related to motivation, arousal, incentive and reward, include the catecholamines, particularly dopamine and noradrenaline. Many questions remain regarding the functional mechanisms by which these modulators interact with sensory and motor systems. Here, we begin to address these questions in a model system for vocal and social behavior, the plainfin midshipman fish (Porichthys notatus). We mapped the distribution of immunoreactivity for the catecholamine-synthesizing enzyme tyrosine hydroxylase (TH) in the midshipman brain. The general pattern of TH(+) cell groups in midshipman appears to be highly conserved with other teleost fish, with a few exceptions, including the apparent absence of pretectal catecholamine cells. Many components of the midshipman vocal and auditory systems were innervated by TH(+) fibers and terminals, including portions of the subpallial area ventralis, the preoptic complex, and the anterior hypothalamus, the midbrain periaqueductal gray and torus semicircularis, several hindbrain auditory nuclei, and parts of the hindbrain vocal pattern generator. These areas thus represent potential sites for catecholamine modulation of vocal and/or auditory behavior. To begin to test functionally whether catecholamines modulate vocal social behaviors, we hypothesized that male and female midshipman, which are sexually dimorphic in both their vocal-motor repertoires and in their responses to hearing conspecific vocalizations, should exhibit sexually dimorphic expression of TH immunoreactivity in their vocal and/or auditory systems. We used quantitative immunohistochemical techniques to test this hypothesis across a number of brain areas. We found significantly higher levels of TH expression in male midshipman relative to females in the TH cell population in the paraventricular organ of the diencephalon and in the TH-innervated torus semicircularis, the main teleost midbrain auditory structure. The torus semicircularis has been implicated in sexually dimorphic behavioral responses to conspecific vocalizations. Our data thus support the general idea that catecholamines modulate vocal and auditory processing in midshipman, and the specific hypothesis that they shape sexually dimorphic auditory responses in the auditory midbrain.

Dopamine modulates auditory responses in the inferior colliculus in a heterogeneous manner

Perception of complex sounds such as speech is affected by a variety of factors, including attention, expectation of reward, physiological state, and/or disorders, yet the mechanisms underlying this modulation are not well understood. Although dopamine is commonly studied for its role in reward-based learning and in disorders, multiple lines of evidence suggest that dopamine is also involved in modulating auditory processing. In this study, we examined the effects of dopamine application on neuronal response properties in the inferior colliculus (IC) of awake mice. Because the IC contains dopamine receptors and nerve terminals immunoreactive for tyrosine hydroxylase, we predicted that dopamine would modulate auditory responses in the IC. We recorded single-unit responses before, during, and after the iontophoretic application of dopamine using piggyback electrodes. We examined the effects of dopamine on firing rate, timing, and probability of bursting. We found that application of dopamine affected neural responses in a heterogeneous manner. In more than 80 % of the neurons, dopamine either increased (32 %) or decreased (50 %) firing rate, and the effects were similar on spontaneous and sound-evoked activity. Dopamine also either increased or decreased first spike latency and jitter in almost half of the neurons. In 3/28 neurons (11 %), dopamine significantly altered the probability of bursting. The heterogeneous effects of dopamine observed in the IC of awake mice were similar to effects observed in other brain areas. Our findings indicate that dopamine differentially modulates neural activity in the IC and thus may play an important role in auditory processing.

Tonotopic and localized pathways from primary auditory cortex to the central nucleus of the inferior colliculus

Descending projections from the cortex to subcortical structures are critical for auditory plasticity, including the ability for central neurons to adjust their frequency tuning to relevant and meaningful stimuli. We show that focal electrical stimulation of primary auditory cortex in guinea pigs produces excitatory responses in the central nucleus of the inferior colliculus (CNIC) with two tonotopic patterns: a narrow tuned pattern that is consistent with previous findings showing direct frequency-aligned projections; and a broad tuned pattern in which the auditory cortex can influence multiple frequency regions. Moreover, excitatory responses could be elicited in the caudomedial portion along the isofrequency laminae of the CNIC but not in the rostrolateral portion. This descending organization may underlie or contribute to the ability of the auditory cortex to induce changes in frequency tuning of subcortical neurons as shown extensively in previous studies.