Histamine and related substances influence neurotransmission in the semicircular canal

Histaminergic System and Vestibular Function in Normal and Pathological Conditions

Most neurotransmitter systems are represented in the central and peripheral vestibular system and are thereby involved both in normal vestibular signal processing and the pathophysiology of vestibular disorders. However, there is a special relationship between the vestibular system and the histaminergic system. The purpose of this review is to document how the histaminergic system interferes with normal and pathological vestibular function. In particular, we will discuss neurobiological mechanisms such as neuroinflammation that involve histamine to modulate and allow restoration of balance function in the situation of a vestibular insult. These adaptive mechanisms represent targets of histaminergic pharmacological compounds capable of restoring vestibular function in pathological situations. The clinical use of drugs targeting the histaminergic system in various vestibular disorders is critically discussed.

Ménière's disease and allergy: Epidemiology, pathogenesis, and therapy

The etiology of Ménière's disease (MD) remains controversial. Allergies are potential extrinsic factors that, in conjunction with underlying intrinsic factors, may cause MD. The link between allergies and MD was first described in 1923. For nearly a century, studies have demonstrated a possible link between allergies and MD, even though a causal relationship has not been definitively determined. Previous reviews have mainly focused on clinical epidemiology studies of patients. In this review, we shed light on the association between allergies and MD not only in terms of its epidemiology, but also from an immunology, pathophysiology, and immunotherapy perspective in both patients and animal models. Patients with MD tend to have a high risk of comorbid allergies or an allergy history, showing positive allergy immunology characteristics. Other MD-related diseases, such as migraine, may also interact with allergies. Allergy mediators such as IgE may worsen the symptoms of MD. Deposits of IgE in the vestibular end organs indicate the ability of the inner ear to participate in immune reactions. Allergic challenges can induce vertigo in animals and humans. Anti-allergy therapy plays a positive role in patients with MD and animal models of endolymphatic hydrops.

Histamine and Its Receptors in the Mammalian Inner Ear: A Scoping Review

BACKGROUND

Histamine is a widely distributed biogenic amine with multiple biological functions mediated by specific receptors that determine the local effects of histamine. This review aims to summarize the published findings on the expression and functional roles of histamine receptors in the inner ear and to identify potential research hotspots and gaps.

METHODS

A search of the electronic databases PubMed, Web of Science, and OVID EMBASE was performed using the keywords histamine, cochlea*, and inner ear. Of the 181 studies identified, 18 eligible publications were included in the full-text analysis.

RESULTS

All four types of histamine receptors were identified in the mammalian inner ear. The functional studies of histamine in the inner ear were mainly in vitro. Clinical evidence suggests that histamine and its receptors may play a role in Ménière's disease, but the exact mechanism is not fully understood. The effects of histamine on hearing development remain unclear.

CONCLUSIONS

Existing studies have successfully determined the expression of all four histamine receptors in the mammalian inner ear. However, further functional studies are needed to explore the potential of histamine receptors as targets for the treatment of hearing and balance disorders.

Does Mucosal Chronic Otitis Media Leads to Sensorineural Hearing Loss

Sensori-neural hearing loss (SNHL) results from inner ear damage or injury to the neural pathways that relay signals from the inner ear to the brain. A serious sequelae of COM is damage to the inner ear. This study aimed at finding the occurrence of SNHL in unilateral mucosal COM. One Hundred patients who had unilateral mucosal COM were enrolled in the study and underwent pure tone audiometry using Audio 4002 system in acoustically treated room. Bone conduction (BC) threshold less than 20 dB was taken as normal. Significant SNHL of 23% was seen in mucosal COM. Unilateral mucosal COM leads to significant impact on BC thresholds. The risk of SNHL increases with larger perforation size and increasing duration and active stage of disease.

Effects of Several Therapeutic Agents on Mammalian Vestibular Function: Meclizine, Diazepam, and JNJ7777120

Management of vestibular dysfunction may include treatment with medications that are thought to act to suppress vestibular function and reduce or eliminate abnormal sensitivity to head motions. The extent to which vestibular medications act centrally or peripherally is still debated. In this study, two commonly prescribed medications, meclizine and diazepam, and a candidate for future clinical use, JNJ7777120, were evaluated for their effects on short latency compound action potentials generated by the peripheral vestibular system and corresponding central neural relays (i.e., vestibular sensory-evoked potentials, VsEPs). The effects of the selected drugs developed slowly over the course of two hours in the mouse. Findings indicate that meclizine (600 mg/kg) and diazepam (> 60 mg/kg) can act on peripheral elements of the vestibular maculae whereas diazepam also acts most effectively on central gravity receptor circuits to exert its suppressive effects. The novel pharmacological agent JNJ7777120 (160 mg/kg) acts in the vestibular periphery to enhance macular responses to transient stimuli (VsEPs) while, hypothetically, suppressing macular responses to sustained or slowly changing stimuli.

Mast Cells in the Auditory Periphery of Rodents

Mast cells (MCs) are densely granulated cells of myeloid origin and are a part of immune and neuroimmune systems. MCs have been detected in the endolymphatic sac of the inner ear and are suggested to regulate allergic hydrops. However, their existence in the cochlea has never been documented. In this work, we show that MCs are present in the cochleae of C57BL/6 mice and Wistar rats, where they localize in the modiolus, spiral ligament, and stria vascularis. The identity of MCs was confirmed in cochlear cryosections and flat preparations using avidin and antibodies against c-Kit/CD117, chymase, tryptase, and FcεRIα. The number of MCs decreased significantly during postnatal development, resulting in only a few MCs present in the flat preparation of the cochlea of a rat. In addition, exposure to 40 µM cisplatin for 24 h led to a significant reduction in cochlear MCs. The presence of MCs in the cochlea may shed new light on postnatal maturation of the auditory periphery and possible involvement in the ototoxicity of cisplatin. Presented data extend the current knowledge about the physiology and pathology of the auditory periphery. Future functional studies should expand and translate this new basic knowledge to clinics.

Effect of the novel histamine H4 receptor antagonist SENS-111 on spontaneous nystagmus in a rat model of acute unilateral vestibular loss

BACKGROUND AND PURPOSE

Histamine H₄ receptors are expressed in the peripheral vestibular system, and their selective inhibition improves vertigo symptoms in rats with unilateral vestibular lesions. The effects of SENS-111, a selective oral H₄ receptor antagonist with high affinity to both animal and human receptors, on vertigo symptoms was evaluated in a translational in vivo model of unilateral vestibular loss.

EXPERIMENTAL APPROACH

Pharmacokinetics of SENS-111 in rats was determined to aid dose selection for efficacy testing. Vestibular lesions were induced in rats by unilateral transtympanic injection of kainic acid. The effect of SENS-111 (10 or 20 mg·kg^-1 ) on spontaneous nystagmus was evaluated compared with placebo vehicle using video-nystagmography, and the effective dose was compared with those of similar drugs used clinically, as single agents or combined with SENS-111.

KEY RESULTS

Doses were selected for plasma exposure were consistent with published phase 1 results from healthy volunteers. SENS-111 of 10 mg·kg^-1 gave a 21-22% reduction in nystagmus at 1 hr post-administration, whereas a loss of efficacy was seen with 20 mg·kg^-1 . Compared with SENS-111, meclizine and methylprednisolone had minimal effects on nystagmus as single agents, and meclizine abolished the effect of SENS-111 when combined with SENS-111. All evaluated drugs were well tolerated.

CONCLUSIONS AND IMPLICATIONS

The exposure-efficacy relationship for improved spontaneous nystagmus seen with SENS-111 in this in vivo model is consistent with phase 1 clinical results and provides preclinical support for pharmacokinetic/pharmacodynamic modelling and selection of effective clinical drug concentrations.

LINKED ARTICLES

This article is part of a themed section on New Uses for 21st Century. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v177.3/issuetoc.

Management of peripheral vertigo with antihistamines: New options on the horizon

Vertigo is associated with a wide range of vestibular pathologies. It increasingly affects the elderly, with a high cost to society. Solutions include vestibular suppressants and vestibular rehabilitation, which form the mainstay of therapy. Antihistamines represent the largest class of agents used to combat vestibular vertigo symptoms. Agents targeting the H₁ and H₃ receptors have been in clinical use for several decades as single agents. Nonetheless, effective management of vertigo proves elusive as many treatments largely address only associated symptoms, and with questionable efficacy. Additionally, the primary and limiting side effect of sedation is counterproductive to normal functioning and the natural recovery process occurring via central compensation. To address these issues, the timing of administration of betahistine, the mainstay H₃ antihistamine, can be fine-tuned, while bioavailability is also being improved. Other approaches include antihistamine combination studies, devices, physical therapy and behavioural interventions. Recently demonstrated expression of H₄ receptors in the peripheral vestibular system represents a new potential drug target for treating vestibular disorders. A number of novel selective H₄ antagonists are active in vestibular models in vivo. The preclinical potential of SENS-111 (Seliforant), an oral first-in-class selective H₄ antagonist is the only such molecule to date to be translated into the clinical setting. With an excellent safety profile and notable absence of sedation, encouraging outcomes in an induced vertigo model in healthy volunteers have led to ongoing clinical studies in acute unilateral vestibulopathy, with the hope that H₄ antagonists will offer new effective therapeutic options to patients suffering from vertigo.

Safety, tolerability, pharmacokinetics and pharmacokinetic-pharmacodynamic modelling of the novel H4 receptor inhibitor SENS-111 using a modified caloric test in healthy subjects

AIM

A Phase 1 study was performed to evaluate safety, pharmacokinetics (PK) and pharmacodynamics (PD) of the selective histamine H₄ receptor antagonist SENS-111, an oral small molecule.

METHODS

One hundred healthy subjects were randomized in a placebo-controlled, double-blind study evaluating single-ascending doses (SAD; 100-500 mg) and multiple-ascending doses (MAD; 50-150 mg day^-1 , 4 days; 200-250 mg day^-1 , 7 days). Effects of SENS-111 on nystagmus and vertigo induced by modified caloric tests were measured in the MAD studies. Population PK and PK/PD models were developed using a nonlinear mixed-effects approach.

RESULTS

SENS-111 was well tolerated with mild to moderate events. No sedation was reported. A maximal tolerated dose was not reached. Dose-proportional increases in concentrations were seen up to 200 mg and more than dose-proportional thereafter, with mean half-life between 24 and 56 h. The caloric test induced mild but measurable vertigo and nystagmus with large intra/inter-individual variation for all parameters. SENS-111 did not significantly impact nystagmus but significantly improved latency of vertigo appearance/disappearance, duration and European Evaluation of Vertigo questionnaire parameters vs. baseline. A two-compartment model with first-order absorption, distribution and elimination best fit the data. PK/PD indirect modelling applied to vertigo duration and latency of appearance indicated maximum activity between 100 and 500 ng ml^-1 plasma concentrations, corresponding to 100 and 200 mg day^-1 , which are appropriate for clinical efficacy evaluations in vestibular diseases.

CONCLUSIONS

SENS-111 is a well-tolerated first-in-class H₄ receptor antagonist with acceptable PK for oral daily dosing. PK/PD modelling determined plasma concentrations and doses for efficacy studies in patients with vertigo symptoms.

Localization of histamine (H1, H2, H3 and H4) receptors in mouse inner ear

Conclusion The present findings show that all four types of histamine receptors (H1R, H2R, H3R, and H4R) are present in the inner ear, thus supporting the hypothesis that histamine plays a physiological role in the inner ear. Objective To analyse the presence of histamine receptors in the normal mouse inner ear. Methods CBA/J mice were used in this study. The localization of H1R, H2R, H3R, and H4R in the inner ear, i.e. cochlea, vestibular end organs, vestibular ganglion, and endolymphatic sac, was studied by real-time PCR and immunohistochemistry. Results The mRNA for each receptor sub-type was detected in the inner ear. In the immunohistochemical study, the organ of Corti, spiral ganglion, vestibular ganglion, vestibular sensory epithelium, and endolymphatic sac cells showed an immunofluorescent reaction to all histamine receptors.

Principles of vestibular pharmacotherapy

Ideally, vestibular pharmacotherapy is intended, through specific and targeted molecular actions, to significantly alleviate vertigo symptoms, to protect or repair the vestibular sensory network under pathologic conditions, and to promote vestibular compensation, with the eventual aim of improving the patient's quality of life. In fact, in order to achieve this aim, considerable progress still needs to be made. The lack of information on the etiology of vestibular disorders and the pharmacologic targets to modulate, as well as the technical challenge of targeting a drug to its effective site are some of the main issues yet to be overcome. In this review, my intention is to provide an account of the therapeutic principles that have shaped current vestibular pharmacotherapy and to further explore crucial questions that must be taken into consideration in order to develop targeted and specific pharmacologic therapies for each type and stage of vestibular disorders.

Current concepts in the pathogenesis and treatment of chronic suppurative otitis media

Otitis media (OM) is an inflammation of the middle ear associated with infection. Despite appropriate therapy, acute OM (AOM) can progress to chronic suppurative OM (CSOM) associated with ear drum perforation and purulent discharge. The effusion prevents the middle ear ossicles from properly relaying sound vibrations from the ear drum to the oval window of the inner ear, causing conductive hearing loss. In addition, the inflammatory mediators generated during CSOM can penetrate into the inner ear through the round window. This can cause the loss of hair cells in the cochlea, leading to sensorineural hearing loss. Pseudomonas aeruginosa and Staphylococcus aureus are the most predominant pathogens that cause CSOM. Although the pathogenesis of AOM is well studied, very limited research is available in relation to CSOM. With the emergence of antibiotic resistance as well as the ototoxicity of antibiotics and the potential risks of surgery, there is an urgent need to develop effective therapeutic strategies against CSOM. This warrants understanding the role of host immunity in CSOM and how the bacteria evade these potent immune responses. Understanding the molecular mechanisms leading to CSOM will help in designing novel treatment modalities against the disease and hence preventing the hearing loss.

Danhong enhances recovery from residual dizziness after successful repositioning treatment in patients with benign paroxysmal positional vertigo

OBJECTIVES

Although the repositioning maneuvers are usually very effective in patients with BPPV, some patients still complain residual dizziness. Danhong injection (DHI), a traditional Chinese medicine, can effectively dilate blood vessels and improve microcirculation, and has been proven to be effective in improving cervical vertigo and posterior circulation ischemic vertigo. The aim of this study was to evaluate the effects of DHI on residual dizziness after successful repositioning treatment in patients with BPPV.

METHODS

Eighty-six patients with BPPV were randomized into two treatment groups, DHI group and non DHI group. The DHI group received the same repositioning treatment as the non-DHI group, with the addition of DHI therapy. The durations of residual dizziness of DHI group and non-DHI group were compared. In addition, the scores of the dizziness handicap inventory of these two groups were calculated.

RESULTS

The durations of residual dizziness of DHI group were shorter than that of non-DHI group. There were no significant differences in the scores of dizziness handicap inventory in the first week between these two groups, and there were much significant differences in the second, the fourth, the sixth and eighth weeks.

CONCLUSIONS

The results demonstrate that DHI can significantly improve the residual dizziness after successful repositioning treatment in patients with BPPV.

Neuropharmacology of vestibular system disorders

This work reviews the neuropharmacology of the vestibular system, with an emphasis on the mechanism of action of drugs used in the treatment of vestibular disorders. Otolaryngologists are confronted with a rapidly changing field in which advances in the knowledge of ionic channel function and synaptic transmission mechanisms have led to the development of new scientific models for the understanding of vestibular dysfunction and its management. In particular, there have been recent advances in our knowledge of the fundamental mechanisms of vestibular system function and drug mechanisms of action. In this work, drugs acting on vestibular system have been grouped into two main categories according to their primary mechanisms of action: those with effects on neurotransmitters and neuromodulator receptors and those that act on voltage-gated ion channels. Particular attention is given in this review to drugs that may provide additional insight into the pathophysiology of vestibular diseases. A critical review of the pharmacology and highlights of the major advances are discussed in each case.

Histamine H1 receptors are expressed in mouse and frog semicircular canal sensory epithelia

Histamine-related drugs are commonly used in the treatment of vertigo and related vestibular disorders. Their site and mechanism of action, however, are still poorly understood. To increase our knowledge of the histaminergic system in the vestibular organs, we have investigated the expression of H1 and H3 histamine receptors in the frog and mouse semicircular canal sensory epithelia. Analysis was performed by mRNA reverse transcriptase-PCR, immunoblotting and immunocytochemistry experiments. Our data show that both frog and mouse vestibular epithelia express H1 receptors. Conversely no clear evidence for H3 receptors expression was found.

Histamine (H3) receptors modulate the excitatory amino acid receptor response of the vestibular afferents

Although the effectiveness of histamine-related drugs in the treatment of peripheral and central vestibular disorders may be explained by their action on the vestibular nuclei, it has also been shown that antivertigo effects can take place at the peripheral level. In this work, we examined the actions of H3 histaminergic agonists and antagonists on the afferent neuron electrical discharge in the isolated inner ear of the axolotl. Our results indicate that H3 antagonists such as thioperamide, clobenpropit, and betahistine (BH) decreased the electrical discharge of afferent neurons by interfering with the postsynaptic response to excitatory amino acid agonists. These results lend further support to the idea that the antivertigo action of histamine-related drugs may be caused, at least in part, by a decrease in the sensory input from the vestibular endorgans. The present data show that the inhibitory action of the afferent neurons discharge previously described for BH is not restricted to this molecule but is also shared by other H3 antagonists.

Sensorineural hearing loss in chronic otitis media

OBJECTIVE

To determine whether chronic suppurative otitis media may cause sensorineural hearing loss.

METHODS

The files of 121 patients with unilateral chronic suppurative otitis media were reviewed in a retrospective study. Air conduction and bone conduction threshold averages were calculated over the speech frequencies (500 Hz, 1,000 Hz, and 2,000 Hz). Thresholds at 4 kHz were examined separately but in a similar way. Multiple linear regression models were used to clarify the relationships between sensorineural hearing loss and chronic otitis media.

RESULTS

Chronic suppurative otitis media was seen to be associated with sensorineural hearing loss. When age and normal side were corrected for, pure-tone threshold and bone conduction threshold at either the speech frequencies or at 4 kHz increased gradually according to the duration of the chronic suppurative otitis media. The threshold shift was more accentuated as age increased. The sensorineural hearing loss at 4 kHz seemed to be higher than that at the speech frequencies.

CONCLUSIONS

The inner ear is vulnerable against chronic suppurative otitis media. Older age increases this vulnerability. The proximity of the sensory cells to the potential source of harm (inflamed middle ear) may mean higher exposure, as reflected by the fact that sensory cells processing higher frequencies are more seriously damaged.

Histamine and betahistine in the treatment of vertigo: elucidation of mechanisms of action

The aim of this review is to provide clinicians with a picture of the mechanisms by which: histamine and histaminergic agonists act on the vestibular system both peripherally and centrally; and histaminergic agonists and antagonists interfere with the recovery process after peripheral vestibular lesion. We have focused on betahistine, a structural analogue of histamine with weak histamine H(1) receptor agonist and more potent H(3) receptor antagonist properties, to review the currently available data on the role of the histaminergic system in the recovery process after peripheral vestibular deficits and the effects of histamine analogues in the clinical treatment of vertigo. This review provides new insights into the basic mechanisms by which betahistine improves vestibular compensation in animal models of unilateral vestibular dysfunction, and elucidates particularly the mechanisms of action of this substance at the level of the CNS. It is proposed that betahistine may reduce peripherally the asymmetric functioning of the sensory vestibular organs in addition to increasing vestibulocochlear blood flow by antagonising local H(3) heteroreceptors. Betahistine acts centrally by enhancing histamine synthesis within tuberomammillary nuclei of the posterior hypothalamus and histamine release within vestibular nuclei through antagonism of H(3) autoreceptors. This mechanism, together with less specific effects of betahistine on alertness regulation through cerebral H(1) receptors, should promote and facilitate central vestibular compensation. Elucidation of the mechanisms of action of betahistine is of particular interest for the treatment of vestibular and cochlear disorders and vertigo.

Effect of L-carnosine on the hyperglycemia caused by intracranial injection of 2-deoxy-D-glucose in rats

Effects of a chicken essence and one of its components, L-carnosine, on the hyperglycemia caused by intracranial injection of 2-deoxy-D-glucose (2DG-hyperglycemia) in unanesthetized rats were examined. The chicken essence inhibited the 2DG-hyperglycemia. Central or peripheral administration of specific doses of L-carnosine reduced the 2DG-hyperglycemia. L-carnosine inhibited neural activities of sympathetic efferent nerves innervating the adrenal gland and liver and facilitated the activity of vagal celiac nerve innervating the pancreas in urethane anesthetized rats. Specific doses of histamine also suppressed the 2DG-hyperglycemia, and thioperamide eliminated the inhibiting actions of both histamine and L-carnosine on the 2DG-hyperglycemia. Considering mammalian muscles contain L-carnosine, these facts suggest a possibility that L-carnosine might be an endogenous control factor of the blood glucose level through autonomic nerves via H3-receptor.

The effects of histamine and its antagonists on the cochlear microphonic and the compound action potential of the guinea pig

OBJECT

we studied the effects of histamine, the H1 receptor antagonist pyrilamine, and the H2 receptor antagonist cimetidine on the cochlear potential of guinea pigs (cochlear microphonic, CM; compound action potential, CAP).

METHODS

histamine was applied into the cochlear perilymph at three different dosages (10 microM, 50 microM or 10 mM). Pyrilamine and cimetidine were applied at 50 microM each.

RESULTS

histamine increased the CAP at 10 and 50 microM without any significant effects on the CM. The effects of histamine at 50 microM were suppressed by the 50-microM of pyrilamine and cimetidine. At 10 mM of histamine, CAP and CM amplitudes were significantly decreased.

CONCLUSION

in low concentrations, histamine may act as an extracellular signal on inner hair cells (IHCs) or it may stimulate the afferent nerve by binding to their H1 and H2 receptors. A possible explanation for the inhibitory effects of histamine at 10-mM dosage was apparently found in that the effects of the high concentration may be supraphysiological; and furthermore, there is a difference in the mechanism by which histamine exerts its effects mediated by the histamine receptors on the cochlea.

Glutamate and carnosine in the vestibular system of the frog

We demonstrate that both glutamate-like and carnosine-like immunoreactivities are present in hair cells and in fibers of the vestibular organ of the frog inner ear. Comparison of the two immunoreactivity patterns indicates that glutamate and carnosine might be colocalized in some hair cells. The presence of glutamate-like immunoreactivity in hair cells is consistent with biochemical and pharmacological data indicating glutamate as the excitatory neurotransmitter in these sensory receptors. There is also evidence that carnosine might have a neuromodulatory function.

Influence of ATP and ATP agonists on the physiology of the isolated semicircular canal of the frog (Rana pipiens)

In the present study, the influence of extracellular ATP and ATP agonists in the physiology of the vestibular organs was examined, using the in vitro model of the isolated semicircular canal of the frog (Rana pipiens). The firing activity of the afferent nerve, the d.c. nerve potential and the transepithelial potential were measured in the absence and presence of mechanical stimulation of the sensory epithelium. Administration of ATP into the perilymphatic compartment, from 10(-12) to 10(-3) M, increased the firing rate of the afferent fibers recorded in the absence of mechanical stimulation. Recordings of the d.c. nerve potential indicated that the afferent fibers were hyperpolarized. The presence of the purine also modified the transepithelial potential. During mechanical stimulation of the sensory epithelium, both the evoked afferent firing and the evoked variation of the d.c. nerve potential were reduced in the presence of ATP. However, ATP did not effect the evoked modulation of the transepithelial potential, evoked by the mechanical stimulation. Administration of the P2x purinoceptor agonists, alpha, beta-methylene-ATP and beta, gamma-methylene-ATP, at concentrations between 10(-12) and 10(-3) M, did not significantly modify the different bioelectrical activities investigated. In contrast, 2-methylthio-ATP, a P2y purinoceptor agonist, more potent and efficacious than ATP in its effect on the spontaneous firing. Concurrently, no modification of the d.c. nerve potential, the transepithelial potential and their variation during mechanical stimulation was observed. In opposition to the ATP effect, the total amplitude of the evoked firing was increased in the presence of 2-methylthio-ATP. These data suggest that extracellular ATP, present in the perilymphatic compartment, may act as a neuromodulator in the vestibular physiology. The effects of the purine appear to be mediated by the activation of a P2y subtype of purinoceptor. The absence of an effect of ATP and 2-methylthio-ATP on the evoked variation of the transepithelial potential suggest that the purine did not affect the processes responsible for the generation of the receptor potential but more likely modified the mechanisms involved in the release of the neurotransmitter from the hair cells and/or acted on the afferent endings.

Choline acetyltransferase immunoreactive neurons innervating labyrinthine and lateral line sense organs in amphibians

The goal of the present study was to investigate aspects of the central organization of the neurons belonging to the octavolateralis efferent system of amphibians. The perikarya of three genera, Pleurodeles, Xenopus, and Discoglossus, were located in the brainstem by applying retrograde tracers to the appropriate cranial nerves and choline acetyltransferase immunohistochemistry was used to identify cholinergic neurons. The efferent neurons supplying lateral line (Pleurodeles, Xenopus) and labyrinthine (Pleurodeles, Xenopus, and Discoglossus) end organs were found to intermingle in a single octavolateralis efferent nucleus. The neurons lie bilateral to the labelled nerves in Pleurodeles and ipsilateral in Xenopus and Discoglossus. Separate labelling of the anterior and posterior octavus rami provided no evidence for distinct groupings of efferent neurons that could be associated with auditory and vestibular end organs. In all three species many if not all octavolateral efferent neurons displayed immunoreactivity for choline acetyltransferase. They could be distinguished from the cholinergic facial motoneurons, with which they sometimes intermingle, on the basis of either their distinctive size and shape (Pleurodeles, Xenopus) or their location (Discoglossus). Double labelling in Xenopus confirmed the cholinergic nature of the efferent neurons.

The correlated blanching of synaptic bodies and reduction in afferent firing rates caused by transmitter-depleting agents in the frog semicircular canal

Synaptic bodies (SBs) associated with rings of synaptic vesicles and well-defined, pre- and post-synaptic membrane structures are indicators of maturity in most hair cell-afferent nerve junctions. The role of the SBs remains elusive despite several experiments showing that they may be involved in storage of neurotransmitter. Our results demonstrate that SBs of the adult posterior semicircular canal (SCC) cristae hair cells become less electron dense following incubation of the SCC with the transmitter-depleting drug tetrabenazine (TBZ). Objective quantification and comparison of the densities of the SBs in untreated and TBZ-treated frog SCC demonstrated that TBZ significantly decreased the electron density of SBs. This reduction in electron density was accompanied by a reduction in firing rates of afferent fibers innervating the posterior SCC. A second transmitter-depleting drug, guanethidine, previously shown to reduce the electron density of hair cell SBs, also reduced the firing rates of afferent fibers innervating the posterior SCC. In contrast, the electron density of dense granules (DG), similar in size and shape to synaptic bodies (SB) in hair cells, did not change after incubation in TBZ, thus indicating that granules and SBs are not similar in regard to their electron density. The role of SBs in synaptic transmission and the transmitter, if any, stored in the SBs remain unknown. Nonetheless, the association of the lessening of electron density with a reduction in afferent firing rate provides impetus for the further investigation of the SB's role in neurotransmission.

Expression of carnosine-like immunoreactivity during retinal development in the clawed frog (Xenopus laevis)

The development of neurons immunoreactive to carnosine (beta-alanyl-L-histidine) was studied in the retina of Xenopus laevis during the premetamorphic period. Carnosine-like immunoreactivity was detected in photoreceptors from stage 39/40 (according to Nieuwkoop and Faber [Normal Tables of Xenopus laevis (Daudin), Elsevier, Amsterdam, 1956]) and in bipolar cells and their processes in the inner plexiform layer from stage 44/45. At all the developmental stages studied, neuroepithelial cells at the ciliary margin were completely unstained, suggesting that carnosine is only present in postmitotic retinal neurons. This study demonstrates a correlation between the times of appearance of carnosine-like immunoreactivity during retinal development and the onset of visual function.

Carnosine-like immunoreactivity is associated with synaptic vesicles in photoreceptors of the frog retina

The distribution of carnosine-like immunoreactivity in photoreceptors of the frog retina was studied by post-embedding electron microscope immunocytochemistry. Different fixation and embedding procedures were applied and the best results were achieved on sections from tissue embedded in the hydrophilic resin LR White. All photoreceptor types present in the frog retina (red and green rods, single and double cones) were intensely reactive for the carnosine antiserum. The immunoreactivity was particularly prominent in the synaptic terminal, nucleus and myoid, but decreased in the ellipsoid and in the outer segment. Müller glial cells and horizontal and bipolar cell processes in the outer plexiform layer were completely unstained. At the level of the photoreceptor terminals, the presence of gold particles within the synaptic vesicles could be demonstrated. This study is the first to provide the anatomical evidence for the presence of carnosine in the synaptic vesicles of a nervous cell. The present data may be relevant to the understanding of the biological functions of carnosine (and related dipeptides) and offer an exploitable system in which to verify its proposed role as a neurotransmitter or neuromodulator in sensory systems.

Cyclic AMP modulates sensory-neural communication at the vestibular end organ

Adenosine 3':5'-cyclic phosphate (cAMP) is a second messenger that plays an important role in mediating neuronal interactions in many systems. A possible role for cAMP in sensorineural communication at the vestibular end organ was studied. The putative roles for cAMP action investigated here were: the ability of cAMP to act as the second messenger for the efferent transmitter, acetylcholine, and the possible involvement of cAMP in modulating spontaneous or mechanically-evoked afferent nerve firing. Levels of cAMP were increased pharmacologically with forskolin, 3-isobutyl-1-methyl xanthine (IBMX) and dibutyryl cAMP. Changes in multiunit afferent nerve firing measured from the ampullar nerve of the semicircular canal, and the transepithelial potential measured across the neuroepithelium of the semicircular canal were recorded. At selected doses, all drugs produced a similar increase in spontaneous multiunit afferent nerve firing with a concomitant decrease in the transepithelial potential. Mechanically-evoked hair cell activity and the response to exogenously applied acetylcholine were unaffected by these drugs. We are suggesting that the excitatory aspects of the acetylcholine response are not mediated via a cAMP-dependent mechanism. However, cAMP does play an important role in modulating spontaneous afferent nerve firing in the semicircular canal. The finding that spontaneous afferent nerve firing can be biochemically modulated without altering mechanically-induced afferent firing is novel and deserves further investigation.

N-acetylhistidine, glutamate, and beta-alanine are concentrated in a receptor cell layer of the trout inner ear

An epithelial sheet isolated from the trout saccular macula, highly enriched in acousticolateralis receptor cells (hair cells), has been analyzed for primary amine-containing compounds. The hair cell preparation, compared to the saccular nerve, was found to contain elevated levels of the presumptive receptoneural transmitter, glutamate, as well as beta-alanine, and components eluting in the positions of the standards phosphoserine and phosphoethanolamine on cation-exchange HPLC. Saccular nerve contained a different spectrum of primary amines and was elevated specifically in carnosine/homocarnosine. Acid hydrolysis of perchlorate extracts of both hair cell and nerve fractions yielded large amounts of histidine. For the saccular nerve fraction, production of histidine by acid hydrolysis was matched by production of beta-alanine and gamma-aminobutyric acid (GABA) and disappearance of carnosine/homocarnosine. The dipeptides carnosine and homocarnosine have been chromatographically resolved by expanded HPLC and found to be present in saccular nerve in a ratio of approximately 10:1, respectively. Production of histidine in the hair cell extract was not coupled with production of beta-alanine and GABA. The hair cell histidine-containing unknown, present in millimolar concentration, has been identified as N-acetylhistidine by the hydrolysis and rechromatography of fractions from cation-exchange HPLC. The large and specific presence of N-acetylhistidine in the hair cell preparation, together with electrophysiological evidence for its facilitatory action on afferent fibers in the frog semicircular canal, is suggestive of a role for this molecule as well as glutamate in acousticolateralis receptoneural transmission.

Cholinergically-induced changes in outward currents in hair cells isolated from the semicircular canal of the frog

Two cholinergically-induced modulations of membrane conductances have been identified in hair cells isolated from the crista ampullaris of the leopard frog (Rana pipiens), using the whole cell recording configuration of the patch clamp technique. Of 56 crista hair cells tested, 28 showed drug-induced changes in membrane current or membrane potential which were repeatable and could be reversed with washout of drug. The predominant effect (observed in 20 hair cells) of acetylcholine (Ach, 100 microM) to 1mM) or carbachol (1 microM to 50 microM) applied to these hair cells was the reduction of an outward current corresponding to a change in conductance of approximately -0.22 nS. This action by Ach on hair cells has been inferred from previous studies of afferent fiber discharge which reported an increase in firing rate with stimulation of efferent fibers or exogenous application of cholinomimetics (Bernard et al., 1985; Valli et al., 1986; Guth et al., 1986; Norris et al., 1988a). The Ach-induced reduction in outward current was associated with a depolarization of the zero-current membrane potential by approximately +2.5 mV. In a total of 8 hair cells, an Ach-induced reversible increase in outward current was recorded. Changes in conductance were approximately +0.13 nS and were associated with a hyperpolarization of the zero-current membrane potential by approximately -2.2 mV. This current increase is likely to be responsible for the inhibitory post-synaptic potentials (IPSPs) which have previously been recorded intracellularly from acoustico-lateralis hair cells during stimulation of the efferent innervation (Flock and Russell, 1976; Ashmore and Russell, 1982; Art et al., 1984, 1985). Of the remaining 28 hair cells, six cells failed to exhibit any change in membrane conductance or membrane potential in the presence of cholinomimetics while an additional 15 cells exhibited decreases, and 7 cells exhibited increases in outward conductance, during application of Ach or carbachol, which were neither reversible with washout nor repeatable. The Ach-induced decrease in outward current could be reversible blocked by removal of Ca2+ from the external solution. The antagonism of the Ach-induced decrease in outward current by atropine (10(-5) M) suggests that this current may correspond to a facilitatory, 'atropine-preferring' Ach receptor mediated response previously identified in the isolated semicircular canal (Norris et al., 1988a).(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiological properties and morphology of hair cells isolated from the semicircular canal of the frog

Hair cells isolated from the crista ampullaris of the frog (Rana pipiens) remained viable for up to 5 h and were studied using whole cell voltage- and current clamp recordings. Morphological characteristics of isolated crista hair cells were compared with hair cells studied in situ using light- and electron microscopy. While other labyrinthine hair cells such as mammalian inner and outer hair cells of the cochlea, saccular hair cells of the frog, and cochlear hair cells of the turtle typically have a cylindrical shape, the crista hair cells in the frog are predominantly bulbous, having a thin elongated trunk projecting from a spherical base just large enough to enclose the nucleus. This shape correlates well with the compressed packing configuration of hair cells of the crista ampullaris observed in situ in the histological material. The support cells often failed to separate adjacent hair cells, particularly the apical ends of the hair cells. Maximal cell density on the sensory epithelial ridge appears to be achieved by this arrangement. The mean resting membrane potential (Vz) of isolated crista hair cells was -44.8 mV. Cells with smooth surfaces and apparent opacity had the most negative Vz potentials. As the cells appeared to deteriorate, there was development of transparency and cell surface granulation. Such cells had more positive initial Vz values. Cells with Vz values more positive than -15 mV exhibited a distinct, contoured nucleus. Cells lacking these indicators of deterioration were characterized by input resistances of 1.9 +/- 0.31 G omega and membrane time constants of 13 +/- 2.5 ms. A large complex outwardly rectifying current was identified which was abolished by substituting Cs+ for K+ in the internal solution. The outward K+ current had two major components: a fast tetraethylammonium (TEA)-insensitive, voltage dependent I(A)-type current which showed voltage dependent inactivation; and a TEA sensitive current which had characteristics of a calcium dependent IK(Ca)-type current. Transient changes (20 ms duration) in membrane potential mimicking that which could be produced by the transduction current during cilial displacement potently modulated the I(A) current. Depolarizing current pulses of greater than 63800 pA were required to elicit membrane voltage oscillations. The resulting membrane potential offset of at least 40 mV is well beyond the magnitude of hair cell receptor potentials making it unlikely that these oscillations would play a role in enhancing frequency selectivity.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacological alterations of the activity of afferent fibers innervating hair cells

To determine whether some of the substances that may be present in hair-cell sensory organs could affect neural activity in afferent fibers, we examined 56 compounds for the ability to alter the discharge rate of afferent fibers innervating hair cells in the lateral line organ of Xenopus laevis, the African clawed frog. These compounds included amino acids, glutamyl dipeptides, standard neurotransmitter candidates, and other constituents of tissues and body fluids. Substances found to be excitatory included some neutral amino acids (alanine, serine, threonine, asparagine, glutamine, and proline), ATP, carnosine, histidine, and barium chloride. Compounds that suppressed discharge included the aromatic amino acids (phenylalanine, tryptophan, and tyrosine), serotonin, and gamma-glutamyl dipeptides. GABA and acidic amino acids (glutamate, aspartate, and cysteine sulfinate) produced a brief excitation followed by a suppression of discharge rate. Several of these substances were active at sufficiently low concentrations that their presence in body fluids may affect afferent fiber discharge rate under normal or pathological conditions.