Charles H. Norris

The vestibular hair cells: post-transductional signal processing.

Hair cells in mechanosensory systems transduce mechanical stimuli into biological signals to be presented to and analyzed by the brain. Vestibular hair cells transduce stimuli primarily associated with the organisms orientation and motion in space. When examined superficially it may appear that the hair cells act as passive transducers whereby mechanical stimulation of their hair bundle results in transmitter release at their afferent synapses. In fact, hair cell functions are more complicated, and the mechanical signals are heavily processed even before being encoded in afferent nerve activity. Hair cells are different from one another in morphology, biophysics, transmitter and transmitter receptor complements, not only across different organs (as one might expect), but even in the same organ. This review focuses on hair cell morpho-physiological properties, ionic conductances, neurotransmitters/modulators and their receptors, second messengers and effectors. Special features of hair cell neurotransmission, as the synaptic body and the presence of autoreceptors and local circuits, are also discussed, as is the possibility of a differential modulation of hair cell transmitter release in the resting and mechanically-stimulated states.

Differential modulation of spontaneous and evoked neurotransmitter release from hair cells: Some novel hypotheses

It has been generally accepted that even in the absence of mechanical stimulation of the transductional elements, a resting depolarizing current exists which is ultimately responsible for the spontaneous release of neurotransmitter. Movement of the transductional elements modulates this resting current and thereby the evoked release of neurotransmitter occurs. Recent data from our laboratory and others have led us to question whether the relationship between spontaneous and evoked neurotransmitter release is as simple as stated. Indeed, a variety of experimental manipulations appear to influence the two modes of release differently. Examination of our results and the results of others has led us to four hypotheses: 1. the two modes of neurotransmitter release are processed differently by the hair cells; 2. cyclic AMP is involved in spontaneous but not evoked neurotransmitter release; 3. there is a positive feedback step involving an excitatory amino acid and its receptor on the hair cell in evoked neurotransmitter release and; 4. different pools of calcium are involved according to the mode of release. Accordingly, there may be several biochemical steps between the transductional movement of the stereocilia at the apex of the hair cells and the ultimate release of the neurotransmitter at the base of these cells. Some of these biochemical steps are different depending on whether the mode of release is spontaneous or evoked. These biochemical steps may amplify or at least interact with the biophysical processes previously described in the hair cells.

The hair cell acetylcholine receptors: a synthesis

In this article the evidence concerning the nature of the acetylcholine (ACh) receptors on hair cells is reviewed. A schematic organization of these receptors is offered, based on the evidence as follows. (1) There are two kinds of ACh receptors on hair cells: muscarinic-like and nicotinic-like. (2) The nicotinic-like receptor mediates a hyperpolarizing response to ACh and a consequent reduction in afferent firing. (3) The muscarinic-like receptors mediate both a depolarization and a hyperpolarization of hair cells. (4) The hyperpolarization results in a reduction in afferent firing and (5) the depolarization results in an increase in afferent firing.

Histamine and related substances influence neurotransmission in the semicircular canal

Histamine and other imidazole-containing substances were found to increase ampullar nerve afferent firing rate while both H1 and H2 histamine antagonists effectively inhibited ampullar nerve activity. A specific inhibitor of histidine decarboxylase, the enzyme which catalyses the synthesis of histamine, reduced ampullar nerve firing in a dose-dependent manner. These observations suggest a physiological role for histamine in the inner ear. Maintenance of a response to histamine after de-efferentation of the crista ampullaris supports the hypothesis that the site of action is the hair cell; antagonism of the histamine response by a cholinergic antagonist, atropine, and antagonism of a cholinergically mediated facilitation by the histaminergic antagonist pyrilamine, indicate that the site of action may involve the acetylcholine receptor complex on the crista ampullaris hair cells. The observation that imidazole-containing compounds cause significant effects on semicircular canal neurotransmission provides an important finding with regard to the site of action of antihistamines used for the treatment of vertigo and motion sickness.

Cholinomimetics Mimic Efferent Effects on Semicircular Canal Afferent Activity in the Frog

Acetylcholine (Ach) has received strong support as the neurotransmitter at vestibular efferent nerve endings. Ach, cholinomimetics and cholinergic antagonists were therefore applied to frog isolated whole labyrinths and isolated semicircular canals. Both spontaneous and evoked single unit and multiple unit activities were recorded from the decentralized posterior semicircular canal afferent nerve. In a manner analogous to efferent nerve stimulation, Ach produced both facilitatory and inhibitory changes in afferent firing rates. The facilitatory effect is likely mediated by muscarinic receptors (i.e. atropine antagonizes it at low concentrations). The facilitatory effect can also be elicited by muscarine and carbachol and it is likely produced presynaptically on the vestibular sensory cell. That is, the effects of Ach are not changed by removal of the efferent neurons but they are absent when afferent transmitter release is blocked. The inhibitory effect is not as well characterized as is the facilitatory effect but it can be blocked by strychnine. The results are consistent with the hypothesis that Ach is the transmitter responsible for both the facilitatory and the inhibitory effects of efferent vestibular nerve stimulation.

The Release of Acetylcholine (Ach) By the Crossed Olivo-Cochlear Bundle (Cocb)

A method has been developed to detect acetyl-choline-like activity in the perilymph of guinea pigs. The acetylcholine-like activity in the guinea pig perilymph is not significantly increased by moderate acoustical stimuli but it is significantly increased when the crossed olivocochlear bundle is tetanically stimulated. The criteria for identifying acetylcholine as the chemical mediator at the crossed olivo-cochlear nerve/hair cell junction are now largely fulfilled.

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.

The cholinergic pharmacology of the frog saccule.

Stimulation of the efferent nerves to the vestibular organs of the frogs inner ear produces either facilitation or inhibition of afferent firing. Similarly, application of acetylcholine (ACH), the major transmitter of the efferents, can produce both facilitation and/or inhibition as previously reported [Guth et al. (1986) Acta Otolaryngol. 102, 194-204; Norris et al. (1988) Hear. Res. 32, 197-206]. The firing rates of afferent neurons of the semicircular canal (SCC) using multiunit recordings are generally facilitated by ACH. Conversely, the firing rates of afferent units innervating the saccule are generally inhibited by ACH. This latter inhibition is antagonized by strychnine more potently than by curare, which is more potent than atropine. When inhibition is antagonized by strychnine or curare an underlying facilitation is revealed. The inhibition of saccular afferents by ACH shows desensitization requiring about 20 min to recover. The ACH-induced inhibition is mimicked by nicotine at very high concentrations but not by dimethyl phenylpiperazinium or cytisine. The fact that multiunit afferent firing from the SCC is generally facilitated while that from the saccule is generally inhibited by ACH suggests a different distribution of ACH receptors and receptor types (i.e. muscarinic or nicotinic and their subtypes) in the two organs and demonstrates the usefulness of recording from multiple units simultaneously. The difference in distribution of ACH receptors may be important for understanding the physiology of vestibular efferents.

Drugs affecting the inner ear: a review of their clinical efficacy mechanisms of action, toxicity, and place in therapy

SummaryMany drugs have a site of action within the inner ear. The list includes therapeutic, diagnostic and ototoxic agents.Therapeutic agents are most useful in cases of infections, endolymphatic hydrops, vascular insufficiency, vertigo of peripheral origin, autoimmune disease, otosclerosis (otospongiosis), sudden hearing loss and tinnitus. For infections, the most widely used antimicrobial agents are the penicillins and cephalosporins. There are no antiviral agents that have been proven useful for inner ear viral infections. However, steroids have been of some value for controlling some of the sequelae. Steroids have also been useful in conjunction with ampicillin in cases of syphilitic hearing loss. In cases of endolymphatic hydrops, the diuretics chlorthalidone, hydrochlorothiazide and acetazolamide have been useful. When diuretic and diet therapy cannot control endolymphatic hydrops, ototoxic drugs such as streptomycin have been used. In cases of vascular insufficiency within the inner ear, vasodilators such as carbon dioxide, papaverine, buphenine (nylidrin), naftidrofuryl (nafronyl) and thymoxamine have been recommended, but their true efficacy is questionable. Some success with betahistine has been achieved but the mechanism of this drug’s action may be other than vasodilatation. Vertigo is best controlled with antihistamines and anti-cholinergics and with certain calcium channel blockers. Autoimmune inner ear disease appears to respond to a combination of steroids and cyclophosphamide. Although controversial, current pharmacotherapy for otosclerosis includes sodium fluoride. Sudden hearing loss is treated with a ’shotgun’ combination of drugs and/or bed rest. There are as yet no drugs which can be used to routinely reduce tinnitus although some medications may help the patient tolerate the problem. Lignocaine (lidocaine) is useful in diagnosing, and very evanescently reducing, tinnitus. Glycerin (glycerol) is useful in diagnosing endolymphatic hydrops and may at times transiently reduce tinnitus.The drugs most noted for their ototoxicity are the aminoglycoside antibiotics, certain diuretics, non-steroidal anti-inflammatory agents, certain anticancer agents and some miscellaneous chemicals.Some new research drugs are in clinical trials for tinnitus, hearing loss and vertigo, and the rational search for new otopharmacotherapeutic agents is increasing.

Three tests of the hypothesis that glutamate is the sensory hair cell transmitter in the frog semicircular canal

Three series of experiments were devised to test the hypothesis that glutamate is the transmitter released by sensory hair cells of the frog semicircular canal. These three tests were: 1 - The Tolerance experiment (i.e. making the preparation tolerant to injected Glu yet still capable of responding to endogenous transmitter). 2 - The Glu Decarboxylase experiment (i.e. bathing the preparation in sufficient enzyme to prevent the effects of exogenous Glu by degrading it without affecting the response to endogenous transmitter) and; 3 - The Diltiazem experiment (i.e. using the calcium channel antagonist, diltiazem, to prevent the effect of exogenous Glu and yet not to interfere with endogenous transmitter release and action). The Tolerance and Diltiazem experiments produced results indicative of a clear dissociation between exogenous Glu and natural transmitter. The Glu decarboxylase experiment results were not so clear, producing both evidence for and against the hypothesis.