Effects of opioid be drugs on auditory evoked potentials suggest a role of lateral olivocochlear dynorphins in auditory function

Electrical signaling in cochlear efferents is driven by an intrinsic neuronal oscillator

Efferent neurons are believed to play essential roles in maintaining auditory function. The lateral olivocochlear (LOC) neurons-which project from the brainstem to the inner ear, where they release multiple transmitters including peptides, catecholamines, and acetylcholine-are the most numerous yet least understood elements of efferent control of the cochlea. Using in vitro calcium imaging and patch-clamp recordings, we found that LOC neurons in juvenile and young adult mice exhibited extremely slow waves of activity (∼0.1 Hz). These seconds-long bursts of Na⁺ spikes were driven by an intrinsic oscillator dependent on L-type Ca²⁺ channels and were not observed in prehearing mice, suggesting an age-dependent mechanism underlying the intrinsic oscillator. Using optogenetic approaches, we identified both ascending (T-stellate cells of the cochlear nucleus) and descending (auditory cortex) sources of synaptic excitation, as well as the synaptic receptors used for such excitation. Additionally, we identified potent inhibition originating in the glycinergic medial nucleus of trapezoid body (MNTB). Conductance-clamp experiments revealed an unusual mechanism of electrical signaling in LOC neurons, in which synaptic excitation and inhibition served to switch on and off the intrinsically generated spike burst mechanism, allowing for prolonged periods of activity or silence controlled by brief synaptic events. Protracted bursts of action potentials may be essential for effective exocytosis of the diverse transmitters released by LOC fibers in the cochlea.

Opioid receptor activation modulates the calcium current in the cochlear outer hair cells of the rat

Previous works showed that opioid peptides are produced by olivocochlear efferent neurons, while cochlear hair cells express opioid receptors. It has been proposed that opioids protect the auditory system from damage by intense stimulation, although their use for therapeutic or illicit purposes links to hearing impairment. Therefore, it is relevant to study the effect of opioids in the auditory system to define their functional expression and mechanism of action. This study investigated the modulation of the Ca²⁺ currents by opioid peptides in the rat outer hair cells (OHC) using the whole-cell patch-clamp technique. The influence of agonists of the three opioid receptor subtypes (μ, δ, and κ) was studied. The κ opioid receptor agonist U-50488 inhibits the Ca²⁺ currents in a partially reversible form. Coincidently, norbinaltorphimine (a κ receptor antagonist) blocked the U-50488 inhibitory effect on the Ca²⁺ current. The δ- and the μ opioid receptor agonists did not significantly affect the Ca²⁺ currents. These results indicate that the κ opioid receptor activation inhibits the Ca²⁺ current in OHC, modulating the intracellular Ca²⁺ concentration when OHCs depolarize. The modulation of the auditory function by opioids constitutes a relevant mechanism with a potential role in the physiopathology of auditory disturbances.

Effects of Calcitonin-Gene-Related-Peptide on Auditory Nerve Activity

Calcitonin-gene-related peptide (CGRP) is a lateral olivocochlear (LOC) efferent neurotransmitter. Depression of sound-driven auditory brainstem response amplitude in CGRP-null mice suggests the potential for endogenous CGRP release to upregulate spontaneous and/or sound-driven auditory nerve (AN) activity. We chronically infused CGRP into the guinea pig cochlea and evaluated changes in AN activity as well as outer hair cell (OHC) function. The amplitude of both round window noise (a measure of ensemble spontaneous activity) and the synchronous whole-nerve response to sound (compound action potential, CAP) were enhanced. Lack of change in both onset adaptation and steady state amplitude of sound-evoked distortion product otoacoustic emission (DPOAE) responses indicated CGRP had no effect on OHCs, suggesting the origin of the observed changes was neural. Combined with results from the CGRP-null mice, these results appear to confirm that endogenous CGRP enhances auditory nerve activity when released by the LOC neurons. However, infusion of the CGRP receptor antagonist CGRP (8–37) did not reliably influence spontaneous or sound-driven AN activity, or OHC function, results that contrast with the decreased ABR amplitude measured in CGRP-null mice.

Evidence for a dynorphin-mediated inner ear immune/inflammatory response and glutamate-induced neural excitotoxicity: an updated analysis

Acoustic overstimulation (AOS) is defined as the stressful overexposure to high-intensity sounds. AOS is a precipitating factor that leads to a glutamate (GLU)-induced Type I auditory neural excitotoxicity and an activation of an immune/inflammatory/oxidative stress response within the inner ear, often resulting in cochlear hearing loss. The dendrites of the Type I auditory neural neurons that innervate the inner hair cells (IHCs), and respond to the IHC release of the excitatory neurotransmitter GLU, are themselves directly innervated by the dynorphin (DYN)-bearing axon terminals of the descending brain stem lateral olivocochlear (LOC) system. DYNs are known to increase GLU availability, potentiate GLU excitotoxicity, and induce superoxide production. DYNs also increase the production of proinflammatory cytokines by modulating immune/inflammatory signal transduction pathways. Evidence is provided supporting the possibility that the GLU-mediated Type I auditory neural dendritic swelling, inflammation, excitotoxicity, and cochlear hearing loss that follow AOS may be part of a brain stem-activated, DYN-mediated cascade of inflammatory events subsequent to a LOC release of DYNs into the cochlea. In support of a DYN-mediated cascade of events are established investigations linking DYNs to the immune/inflammatory/excitotoxic response in other neural systems.

Mu-opioid receptor (MOR) expression in the human spiral ganglia

Opioid peptides and their receptors have been localized to the inner ear of the rat and guinea pig mammalian models. The expression of mu opioid receptor (MOR) in the human and mouse cochlea is not yet known. We present MOR protein localization by immunohistochemistry and mRNA expression by in situ hybridization in the human and mouse spiral ganglia (SG) and organ of Corti. In the human most of the (SG) neurons were immunoreactive; a subset was non-immunoreactive. In situ hybridization revealed a similar labeling pattern across the neurons of the SG. A similar distribution MOR pattern was demonstrated in the mouse SG. In the mouse organ of Corti MOR was expressed in inner and outer hair cells. Fibers underneath the inner hair cells were also MOR immunoreactive. These results are consistent with a role of MOR in neuromodulation of the auditory periphery. The present results show that the expression of MORs is well-conserved across multiple mammalian species, indicative of an important role in auditory processing.

Dynorphin release by the lateral olivocochlear efferents may inhibit auditory nerve activity: a cochlear drug delivery study

Dynorphin (dyn) is suggested to excite the auditory nerve (AN) when released by the lateral olivocochlear (LOC) efferents. However, previous studies evaluated either intravenously delivered dyn-like agents, raising the potential for systemic (central) effects, or agent concentrations unlikely to be achieved via endogenous cochlear release. This study tested the hypothesis that biologically relevant increases in dyn levels in the cochlea achieved via diffusion of the drug of (-)pentazocine across the round window membrane enhances AN firing. In general, amplitude of the cochlear whole-nerve action potential (CAP) was depressed following drug application. These results suggest that dyn released by the LOC neurons would likely act as an inhibitory transmitter substance in the LOC system; neurotransmission is one of the LOC system's vast unknowns.

Effects of isoflurane on auditory evoked potentials in the cochlea and brainstem of guinea pigs

Electrophysiological recordings of the auditory system are commonly performed in deeply anesthetized animals. This study evaluated the effects of various concentrations of the volatile anesthetic isoflurane (1-3%) on the compound action potential (CAP), cochlear microphonic (CM) and auditory brainstem response (ABR). Recordings were initiated in the awake, lightly restrained animal. Anesthesia was induced with a single dose of Hypnorm (fentanyl and fluanisone). After tracheostomy increasing isoflurane concentrations were applied in N(2)O/O(2) via controlled ventilation. Data were compared to recordings in the awake animal using repeated measures ANOVA and Dunnett's post hoc test. On average, isoflurane dose-dependently suppressed the amplitude and increased the latency of the CAP. CM amplitude was suppressed. These effects were most profound at high frequencies and were typically significant at isoflurane concentrations of 2.5% and 3%. Amplitude and latency of the second negative peak of the CAP (N(2)) were affected to a greater extent compared to the first peak (N(1)). On average, isoflurane dose-dependently reduced the amplitude and increased the latency of the ABR. These effects were typically significant at an isoflurane concentration of 2%. Effects on peak IV and V were more pronounced compared to the early peaks I and III.

Bi-phasic intensity-dependent opioid-mediated neural amplitude changes in the chinchilla cochlea: partial blockade by an N-Methyl-D-Aspartate (NMDA)-receptor antagonist

Dynorphins, glutamate, and glutamate-sensitive N-Methyl-D-Aspartate (NMDA) receptors exist in the mammalian cochlea. Dynorphins produce neural excitation and excitotoxic effects in the spinal cord through a kappa-opioid facilitation of NMDA receptor-sensitivity to glutamate. The kappa-opioid receptor drug agonists N-dimethylallyl-normetazocine [(-)-pentazocine (50 mmol)] and trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamide [U-50488H (100 mmol)] were administered across the cochlear round window membrane in the chinchilla. Each drug produced significant post-baseline amplitude changes in the click-evoked auditory nerve compound action potential. Amplitude changes at threshold amounted to increases in sensitivity that ranged from 4-8 decibels, measured in sound pressure level (dB SPL). The large neural amplitude increases at threshold were accompanied by progressively smaller amplitude changes at 5 and 10 dB above threshold (dB SL). However, at stimulus intensities > or =20 dB SL, post-baseline neural amplitudes were suppressed to levels below baseline and control values. These bi-phasic intensity-dependent neural amplitude changes have never before been observed following i.v. administered (-)-pentazocine in this species. Finally, the bi-phasic neural amplitude changes in U-50488H-treated (100 mmol) animals were partially blocked (except at 20 dB SL), following a round window pre-treatment with the NMDA receptor drug antagonist, dizocilpine hydrogen maleate [(+)-MK-801 (8 mmol)]. Our data suggests that endogenous dynorphins within lateral efferent olivocochlear neurons differentially modulate auditory neural excitation, possibly through cochlear NMDA receptors and glutamate. The role played by lateral efferent opioid neuromodulation at cochlear NMDA receptors, is discussed.

Selective removal of lateral olivocochlear efferents increases vulnerability to acute acoustic injury

Cochlear sensory cells and neurons receive efferent feedback from the olivocochlear (OC) system. The myelinated medial component of the OC system and its effects on outer hair cells (OHCs) have been implicated in protection from acoustic injury. The unmyelinated lateral (L)OC fibers target ipsilateral cochlear nerve dendrites and pharmacological studies suggest the LOC's dopaminergic component may protect these dendrites from excitotoxic effects of acoustic overexposure. Here, we explore LOC function in vivo by selective stereotaxic destruction of LOC cell bodies in mouse. Lesion success in removing the LOC, and sparing the medial (M)OC, was assessed by histological analysis of brain stem sections and cochlear whole mounts. Auditory brain stem responses (ABRs), a neural-based metric, and distortion product otoacoustic emissions (DPOAEs), an OHC-based metric, were measured in control and surgical mice. In cases where the LOC was at least partially destroyed, there were increases in suprathreshold neural responses that were frequency- and level-independent and not attributable to OHC-based effects. These interaural response asymmetries were not found in controls or in cases where the lesion missed the LOC. In LOC-lesion cases, after exposure to a traumatic stimulus, temporary threshold shifts were greater in the ipsilateral ear, but only when measured in the neural response; OHC-based measurements were always bilaterally symmetric, suggesting OHC vulnerability was unaffected. Interaural asymmetries in threshold shift were not found in either unlesioned controls or in cases that missed the LOC. These findings suggest that the LOC modulates cochlear nerve excitability and protects the cochlea from neural damage in acute acoustic injury.

Disruption of lateral olivocochlear neurons via a dopaminergic neurotoxin depresses sound-evoked auditory nerve activity

We applied the dopaminergic (DA) neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to the guinea pig cochlear perilymph. Immunolabeling of lateral olivocochlear (LOC) neurons using antibodies against synaptophysin was reduced after the MPTP treatment. In contrast, labeling of the medial olivocochlear innervation remained intact. As after brainstem lesions of the lateral superior olive (LSO), the site of origin of the LOC neurons, the main effect of disrupting LOC innervation of the cochlea via MPTP was a depression of the amplitude of the compound action potential (CAP). CAP amplitude depression was similar to that produced by LSO lesions. Latency of the N1 component of the CAP, and distortion product otoacoustic emission amplitude and adaptation were unchanged by the MPTP treatment. This technique for selectively lesioning descending LOC efferents provides a new opportunity for examining LOC modulation of afferent activity and behavioral measures of perception.

Modulation of cochlear afferent response by the lateral olivocochlear system: activation via electrical stimulation of the inferior colliculus

The olivocochlear (OC) efferent innervation of the mammalian inner ear consists of two subdivisions, medial (MOC) and lateral (LOC), with different peripheral terminations on outer hair cells and cochlear afferent terminals, respectively. The cochlear effects of electrically activating MOC efferents are well known, i.e., response suppression effected by reducing outer hair cells' contribution to cochlear amplification. LOC peripheral effects are unknown, because their unmyelinated axons are difficult to electrically stimulate. Here, stimulating electrodes are placed in the inferior colliculus (IC) to indirectly activate the LOC system, while recording cochlear responses bilaterally from anesthetized guinea pigs. Shocks at some IC sites produced novel cochlear effects attributable to activation of the LOC system: long-lasting (5-20 min) enhancement or suppression of cochlear neural responses (compound action potentials and round window noise), without changes in cochlear responses dominated by outer hair cells (otoacoustic emissions and cochlear microphonics). These novel effects also differed from classic MOC effects in their lack of dependence on the level and frequency of the acoustic stimulus. These effects disappeared on sectioning the entire OC bundle, but not after selective lesioning of the MOC tracts or the cochlea's autonomic innervation. We conclude that the LOC pathway comprises two functional subdivisions, capable of inducing slow increases or decreases in response magnitudes in the auditory nerve. Such a system may be useful in maintaining accurate binaural comparisons necessary for sound localization in the face of slow changes in interaural sensitivity.

Disruption of lateral efferent pathways: functional changes in auditory evoked responses

The functional consequences of selectively lesioning the lateral olivocochlear efferent system in guinea pigs were studied. The lateral superior olive (LSO) contains the cell bodies of lateral olivocochlear neurons. Melittin, a cytotoxic chemical, was injected into the brain stem using stereotaxic coordinates and near-field evoked potentials to target the LSO. Brain stem histology revealed discrete damage to the LSO following the injections. Functional consequences of this damage were reflected in depressed amplitude of the compound action potential of the eighth nerve (CAP) following the lesion. Threshold sensitivity and N1 latencies were relatively unchanged. Onset adaptation of the cubic distortion product otoacoustic emission (DPOAE) was evident, suggesting a reasonably intact medial efferent system. The present results provide the first report of functional changes induced by isolated manipulation of the lateral efferent pathway. They also confirm the suggestion that changes in single-unit auditory nerve activity after cutting the olivocochlear bundle are probably a consequence of disrupting the more lateral of the two olivocochlear efferent pathways.

Deciphering auditory processing disorders in children

APD is not a label for a unitary disease entity but rather a description of functional deficits [3]. It is a complex and heterogeneous group of auditory-specific disorders usually associated with a range of listening and learning deficits [3,4]. Underlying APD is a deficit observed in one or more of the auditory processes responsible for generating the auditory evoked potentials and the following behaviors: around localization and lateralization; auditory discrimination; auditory pattern recognition; temporal aspects of audition, including temporal resolution, masking, integration, and ordering; auditory performance with competing acoustic signals; and auditory performance with degraded acoustic signals [2]. Comprehensive assessment is necessary for the accurate differential diagnosis of APD from other "look-alike" disorders, most notably ADHD and language processing disorders. Speech-language pathologists, psychologists, educators, and physicians contribute to this more comprehensive assessment. The primary role of otolaryngologists is to evaluate and treat peripheral hearing disorders, such as otitis media. Children with APDs may present to an otolaryngologist, thus requiring the physician to make appropriate referral for assessment and intervention. Currently, diagnosis of APD is based on the outcomes of behavioral tests, supplemented by electroacoustic measures and, to a lesser extent, by electrophysiologic measures [1]. Intervention for APD focuses on improving the quality of the acoustic signal and the listening environment, improving auditory skills, and enhancing utilization of metacognitive and language resources [2]. Additional controlled case studies and single-subject and group research designs are needed to ascertain systematically the relative efficacy of various treatment and management approaches.

A biochemical model of peripheral tinnitus

Subjective tinnitus may be defined as the perceptual correlate of altered spontaneous neural activity occurring in the absence of an externally evoking auditory stimulus. Tinnitus can be caused or exacerbated by one or more of five forms of stress. We propose and provide evidence supporting a model that explains, but is not limited to, peripheral (cochlear) tinnitus. In this model, naturally occurring opioid dynorphins are released from lateral efferent axons into the synaptic region beneath the cochlear inner hair cells during stressful episodes. In the presence of dynorphins, the excitatory neurotransmitter glutamate, released by inner hair cells in response to stimuli or (spontaneously) in silence, is enhanced at cochlear N-methyl-D-aspartate (NMDA) receptors. This results in altered neural excitability and/or an altered discharge spectrum in (modiolar-oriented) type I neurons normally characterized by low rates of spontaneous discharge and relatively poor thresholds. It is also possible that chronic exposure to dynorphins leads to auditory neural excitotoxicity via the same receptor mechanism. Finally, the proposed excitatory interactions of dynorphins and glutamate at NMDA receptors need not be restricted to the auditory periphery.

Distribution and possible functional roles of some neuroactive peptides in the mammalian superior olivary complex

The mammalian superior olivary complex (SOC) is innervated by neuronal systems that contain a variety of neuroactive peptides. Conversely, neurones of the SOC form peptidergic projections to other targets. In this review, the peptides substance P, calcitonin-gene-related peptide, enkephalins and dynorphins, cholecystokinin and somatostatin are considered. Their distribution in fibres and cell bodies of the SOC are considered, with particular attention to differences between the SOC subdivisions. Evidence for the functional effects of these peptides is also reviewed and some brief speculations are offered about their possible functional role in hearing.

Olivocochlear neurons in the chinchilla: a retrograde fluorescent labelling study

Although the chinchilla is widely used as a model for auditory research, little is known about the distribution and morphology of its olivocochlear neurons. Here, we report on the olivocochlear neurons projecting to one cochlea, as determined by single and double retrograde fluorescent tracer techniques. 10 adult chinchillas were anesthetized and given either unilateral or bilateral injections of a fluorescent tracer (either Fluoro-Gold or Fast Blue) into scala tympani or as a control, a unilateral injection into the middle ear cavity. The results indicate that there are similarities as well as significant differences between the chinchilla and other species of rodents in the distributions of their olivocochlear neurons. Based on three well-labelled cases, there was a mean total of 1168 olivocochlear neurons in the chinchilla. Of these, the majority (mean 787) were small, lateral olivocochlear neurons found almost exclusively within the ipsilateral lateral superior olivary nucleus. The next largest group consisted of a mean of 280 medial olivocochlear neurons virtually all of which were located in the dorsomedial peri-olivary nucleus. Chinchilla medial olivocochlear neurons were more predominantly crossed in their projections (4:1) than in any known species. The smallest group of olivocochlear neurons (mean 101) consisted of larger lateral olivocochlear neurons (shell neurons) which were located on the margins of the superior olivary nucleus and which projected mainly (2.2:1) ipsilaterally. Double retrograde labelling was observed only in medial olivocochlear neurons and occurred in only 1-2% of these cells. The results confirm previous findings which indicated a relative paucity of fibers belonging to the uncrossed as compared to the crossed olivocochlear bundle. This, together with the strong apical bias of the uncrossed projection reported previously, offers possible explanations for the apparent absence of efferent-mediated suppressive effects of contralateral acoustic stimulation in this species. Regarding the lateral olivocochlear system, the chinchilla is shown to possess both intrinsic and shell neurons, as in the rat.

Effects of cholinergic blockers on auditory brain-stem evoked potentials in rats

The pharmacology of auditory brain-stem evoked potentials (ABEP) pathways is poorly understood. There are anecdotal reports on the involvement of various neurotransmitters but they were not investigated systematically. The aim of this study was to investigate the effects on ABEP of muscarinic and nicotinic blockers, administered into the cerebral ventricles. Atropine sulfate, d-Tubocurarine and saline were injected stereotactically into the lateral cerebral ventricle of anesthetized male rats. Auditory clicks were given at a rate of 20 s(-1). ABEP recording was performed before and 30 min after injection. Pre- and post-injection peak latencies and peak-to-peak amplitudes of positive waves were compared for each animal. Atropine reduced the amplitudes of waves P1, P3 and P4 and increased mildly the brain stem transmission time. d-Tubocurarine reduced the amplitudes of P1 and P4 with no significant effect on the peak latencies. Saline injection had no effect on any of the parameters. These results show that both cholinergic systems are involved in ABEP generation or transmission. Mechanism of action could be either direct inhibition of afferent pathways or indirect effect, via modulating efferent pathways.

Naloxone blockade of (-)pentazocine-induced changes in auditory function

OBJECTIVE

In a previous report, we found that intravenous (i.v.) (-)pentazocine improved auditory sensitivity and significantly altered compound action potential (CAP) amplitudes. Its sigma (sigma)-receptor-selective optical isomer (+)pentazocine administered at the same dose was without effect, suggesting that the observed auditory neural effects might be mediated by an opioid receptor. To directly test this hypothesis, in the present investigation we attempted to antagonize the auditory neural effects of (-)pentazocine using the pure, nonspecific drug antagonist naloxone.

DESIGN

In 25 normal-hearing, male, pigmented chinchillas, amplitude and latency changes in the click-evoked auditory nerve CAP (N1) and cochlear microphonic (CM) were tracked at six stimulus intensities during a baseline period and after the postbaseline administration of the opioid drug agonist (-)pentazocine (16 mg/kg; i.v.). In separate groups of chinchillas, (-)pentazocine was given alone or administered in combination with the standard opioid receptor antagonist naloxone administered at two doses.

RESULTS

Robust changes in CAP amplitudes after (-)pentazocine occurred in the absence of measurable alterations in CAP response latencies, CM amplitudes, or blood chemistries and were significantly antagonized when naloxone (5 mg/kg) was added to the i.v. infusion.

CONCLUSIONS

The observed blockade clearly indicates that the agonist effects of (-)pentazocine are opioid receptor-mediated and suggests a connection between opioid receptors and auditory neural function. Mechanisms of action and the connection between an opioid modulation of auditory function and stress, hyperacusis, and tinnitus are discussed.

Chemical synaptic transmission in the cochlea

The last two decades have witnessed major progress in the understanding of cochlear mechanical functioning, and in the emergence of cochlear neurochemistry and neuropharmacology. Recent models describe active processes within the cochlea that amplify and sharpen the mechanical response to sound. Although it is widely accepted that outer hair cells (OHCs) contribute to these processes, the nature of the medial efferent influence on cochlear mechanics needs further clarification. Acetylcholine (ACh) is the major transmitter released onto OHCs during the stimulation of these efferents. The inhibitory influence of this system is mediated by post- and presynaptic nicontinic and muscarinic receptors and the role of other neuroactive substances [gamma-aminobutyric acid (GABA), calcitonin gene-related peptide (CGRP), adenosine 5'-triphosphate (ATP) or nitric oxide (NO)] remains to be determined. The inner hair cells (IHCs) that transduce the mechanical displacements into neural activity, release glutamate on receptor-activated channels of AMPA, kainate, and NMDA types. This synapse is in turn controlled and/or regulated by the lateral efferents containing a cocktail of neuroactive substances (ACh, GABA, dopamine, enkephalins, dynorphin, CGRP). This glutamatergic nature of the IHCs is responsible for the acute destruction of the nerve endings and subsequently for neuronal death, damage usually described in various cochlear diseases (noise-induced hearing losses, neural presbycusis and certain forms of sudden deafness or peripheral tinnitus). These pathologies also include a regrowth of new dendritic processes by surviving neurons up to IHCs. Understanding the subtle molecular mechanisms which underly the control of neuronal excitability, synaptic plasticity and neuronal death in cochlear function and disease is a very important issue for the development of future therapies.

Enkephalin-like immunoreactivity in the chick brainstem: possible relation to the cochlear efferent system

Mammalian lateral olivocochlear (LOC) neurons that are immunoreactive for choline acetyltransferase (ChAT) are also immunoreactive for enkephalin (Enk). To determine whether cochlear efferent neurons in birds might also contain Enk-like immunoreactivity (Enk-LI), we studied the auditory brainstem of the domestic chicken using antisera to ChAT, leucine-enkephalin (L-Enk) and methionine-enkephalin (M-Enk). Enk-LI terminals are found around, but not within, the superior olivary nucleus (SO) and the nucleus of the lateral lemniscus, pars intermedia (LLi). A moderate concentration of Enk-LI terminals is found ventromedial to the ventral facial nucleus (VIIv) where the ventrolateral group of ChAT-I cochlear efferent neurons is located. After colchicine injections into the lateral ventricle, a population of intensely stained Enk-LI perikarya was found in the nucleus of the lateral lemniscus, pars ventralis (LLv) with scattered cells in the LLi and the nucleus subceruleus ventralis (SCv). The distribution of Enk-LI and ChAT-I somata, however, never overlapped, even in adjacent sections. Thus, in the chick, Enk-LI perikarya are not distributed in areas where cochlear efferent neurons are found. Instead, a dense concentration of Enk-I terminals can be found in areas containing ChAT-I cochlear efferent neurons. The source of these enkephalinergic terminals may be a population of Enk-LI cells in the LLv.

Acute effects of cocaine on cochlear function

The effects of a single administration of cocaine on the cochlea was evaluated by measuring amplitude-intensity functions of the N1 response of the auditory nerve. Amplitude-intensity functions of the N1 response to tone-pips of 500 Hz, 1, 2, 4 and 8 kHz were obtained before and after intraperitoneal injection of either saline, 3 mg/kg or 25 mg/kg of cocaine. N1 amplitudes were decreased after the administration of cocaine and this reduction was found to be dose dependent. The influence of cocaine on cochlear blood flow (CBF) was examined in order to test the possibility that cocaine induced reductions in CBF underlie these electro-physiological changes. Corresponding decreases in cochlear blood flow after cocaine exposure were observed.

Localization of preproenkephalin mRNA-expressing cells in rat auditory brainstem with in situ hybridization

Hair cells and auditory nerve dendrites in the inner ear are innervated by pontine neurons that have been demonstrated by immunochemical techniques to contain several neurotransmitters, including acetylcholine and the opioid peptide enkephalins and dynorphins. The functions of these nerve fibers are not known, but may involve modifying auditory sensitivity to low intensity stimuli. In the guinea pig the opioid pathways originate in the lateral superior olivary region. A recent study in the gerbil has reported cells expressing preproenkephalin mRNA present only in the ventral nucleus of the trapezoid body, and not in the superior olivary region. In the present study, a non-radioisotopically labeled in situ hybridization method was used to identify cells expressing mRNA coding for preproenkephalin in rat pontine neurons, specifically in the ventral nucleus of the trapezoid body. These cells may represent an enkephalin-containing medial olivocochlear system in the rat, the origin of the lateral system in the rat that differs markedly from the better-studied guinea pig and cat, or a non-olivocochlear enkephalin-containing system.

Endogenous opiates: 1991

This paper is the fourteenth installment of our annual review of research concerning the opiate system. It includes papers published during 1991 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.