Ouabain and streptomycin: their different loci of action on saccular hair cells in goldfish

The contribution of the lateral line to 'hearing' in fish

In the underwater environment, sound propagates both as a pressure wave and as particle displacement, with particle displacement dominating close to the source (the nearfield). At the receptor level, both the fish ear and the neuromast hair cells act as displacement detectors and both are potentially stimulated by the particle motion component of sound sources, especially in the nearfield. A now common way to test 'hearing' in fish involves auditory evoked potentials (AEPs), with recordings made from electrodes implanted near the auditory brainstem. These AEP recordings are typically conducted in enclosed acoustic environments with the fish well within the nearfield, especially for lower frequencies. We tested the contribution of neuromast hair cells to AEP by first testing intact goldfish (Carassius auratus), then ablating their neuromasts with streptomycin sulphate--disabling superficial and canal neuromasts--and retesting the same goldfish. We performed a similar experiment where only the superficial neuromasts were physically ablated. At 100 and 200 Hz, there was a 10-15 dB increase in threshold after streptomycin treatment but no significant difference at higher frequencies. There was no difference in threshold in control fish or in fish that only had superficial neuromasts removed, indicating that the differential responses were driven by canal neuromasts. Taken together, these results indicate that AEP results at lower frequencies should be interpreted as multimodal responses, rather than as 'hearing'. The results also suggest that in natural situations both the ear and lateral line likely play an integrative role in detecting and localising many types of 'acoustic' stimuli.

Rapid resting ion fluxes in goldfish hair cells are balanced by (Na+,K+)-ATPase

Inhibition of sodium/potassium pumping by isolated goldfish hair cells led to a rapid gain of sodium and loss of potassium. Half-times for turnover were about 10 min, among the fastest of any cell type examined by electron-probe analysis. Pumping was inhibited by removal of extracellular potassium or by treatment with 1 mM ouabain, as expected of a classical (Na+,K+)-ATPase. The initial rate of entry of sodium after inhibition, about 4 mM/min, provided an estimate of resting sodium-entry and sodium-pumping rates. After return to control medium, cells loaded with sodium by removal of extracellular potassium could recover their normal high-potassium/low-sodium status. The initial rate of recovery (an estimate of the cells' maximum sodium-pumping rate) was sufficient to lower cell sodium by 10 mM/min. This functional estimate of hair-cell (Na+,K+)-ATPase activity was of the same order of magnitude as the biochemical activity of (Na+,K+)-ATPase previously reported for sensory epithelia of other species. The balance between sodium entry and sodium pumping determines hair-cell ionic composition, and thus the resting potential and the driving forces for sodium-coupled transport processes. Imbalance due to excess sodium entry or loss of pump capacity could have significant consequences for hair-cell function and integrity.

Streptomycin blocks the afferent synapse of the isolated semicircular canals of the frog

This study aimed to define the acute electrophysiological effects of the perilymphatic perfusion of streptomycin in the sensory apparatus of the semicircular canals of the frog. The ampullary DC potential, the vestibular nerve multiunit discharge, the nerve DC potential and the unitary EPSP activity were recorded in isolated semicircular canals of the frog (Rana esculenta L). The results demonstrated that perilymphatic microperfusion of streptomycin (0.1, 0.3, 1 and 3 mM) reduced both resting and mechanically evoked afferent discharge, while the response of the hair cells remains unchanged. Intracellular recordings from single afferent axons showed that the reduction of the afferent discharge was mainly due to a reduction of the amplitude, but not the frequency, of the EPSPs. These results indicate that streptomycin, when applied in the fluid bathing the synaptic pole of the sensory cells, can act as an antagonist of the vestibular afferent transmitter at the postsynaptic level.

Sensory hair cells of a fish ear: evidence of multiple types based on ototoxicity sensitivity

Sensory hair cells from the striolar region (striolar hair cells) of the utricle and the lagena of the ear of a teleost fish Astronotus ocellatus (Cuvier) ear are sensitive to gentamicin sulphate, an ototoxic drug. In contrast, sensory hair cells from outside the striolar region (extra-striolar hair cells) are not sensitive to gentamicin. These data, combined with results from studies showing different ultrastructural features and different immunoreactivity to a calcium binding protein, S-100, lead to the suggestion that there are distinguishable types of hair cells in these endorgans. These results add to the increasing evidence that classifying the sensory hair cells of fish ears only as the traditional 'vestibular type II' may be inadequate for properly understanding structure and function of the fish ear.

Blockage of the transduction channels of hair cells in the bullfrog's sacculus by aminoglycoside antibiotics

The action of aminoglycoside antibiotics on transduction by hair cells was investigated in isolated preparations of the bullfrog's sacculus. Bath application of aminoglycosides produced a reversible blockage of extracellularly recorded responses to displacements of the otolithic membrane. The half-blocking concentrations for various drugs were in the range 2-95 microM. The effect of dihydrostreptomycin on the receptor currents of individual hair cells was studied under two-electrode, voltage-clamp conditions. Iontophoretic application of drug to the apical cellular surface caused a reduction of the receptor current within 20 ms; the reduction was reversible within 1 s. The effect was most striking at holding potentials more negative than -60 mV and was relieved by depolarization. The effect of intracellular aminoglycosides was investigated in cells voltage-clamped with the tight-seal, whole-cell technique. Gentamicin and dihydrostreptomycin, at concentrations near 100 microM, did not block transduction under these conditions. The acute, reversible blocking effect of aminoglycosides therefore occurs from the extracellular membrane surface. The results are consistent with aminoglycosides' plugging the poorly ion-selective transduction channels of hair cells.

The Ca++ permeability of the apical membrane in neuromast hair cells

The mechanosensitivity of eel (Anguilla anguilla) neuromasts was measured by the impulse responses of single afferent nerve fibers to mechanical stimuli. It is dependent on the potential across the skin and on the ions in the water outside the apical membrane of the sensory cells. The mechanosensitivity decreases to zero when the skin is polarized by 10-100 mV cathodal DC (skin surface negative); it increases with increasing (10-60 mV) anodal DC and remains remarkably constant with higher polarization (Fig. 1). The mechanosensitivity increases with increasing concentrations of Ca++ outside the apical membrane of the sensory cells. Na+ and K+ have no influence. Addition of La , Co++, Mg++, D 600 and A-QA 39 inhibits the mechanosensitivity; the degree of inhibition varies with the inhibitor and the ratio [Ca++]/[inhibitor], indicating that the inhibition is competitive (Figs. 2, 3). We conclude that the apical membrane is specifically permeable to Ca++ ('late Ca channel') and that the inward receptor current through the apical membrane is carried by Ca++. Streptomycin also inhibits mechanosensitivity by competing with Ca++. With streptomycin, however, anodal polarization reduces, rather than increases, the mechanosensitivity (Fig. 4).(ABSTRACT TRUNCATED AT 250 WORDS)

Action of streptomycin and calcium on the apical membrane of hair cells of the frog isolated semicircular canal

An isolated frog posterior semicircular canal was placed in a two-compartment perspex chamber allowing separate perfusions of the peri- and endolymphatic space with adequate saline solutions. The ampullar receptors were mechanically stimulated with a hydraulic device which imposed sinusoidal movements to the cupulo-endolymphatic system at low frequency (0.10 Hz). Slow potentials recorded with non-polarizable Ag/AgCl electrodes near the crista ampullaris (Adc) and on the ampullar nerve (Ndc) were displayed together with an analog signal of the mechanical stimulation and with the nerve spike frequency, on a strip chart. The streptomycin action or the high calcium concentration action (these two substances being administered together or separately into the ampullar space) indicated that the streptomycin seemed to block mechano-dependent channels (non-specific) assumed to be included into the hair cell apical membrane. The synaptic transmission process was altered by streptomycin but was improved by the high calcium concentration. Also, the mechanisms which would control the spontaneous release of a neurotransmitter, seemed, at least partly, different from those controlling the neurotransmitter release in response to a mechanical stimulation; these last ones were less sensitive to the calcium action.

Electrical activity of isolated semicircular canal in the frog. Action of streptomycin

1. An isolated frog semicircular canal was placed in a two-compartment perspex chamber which allowed the fluids bathing the inside (endolymph) and the outside (perilymph) of the canal to be replaced separately. Mechanical stimulation (MS) of the ampullar receptors was performed by sinusoidal cupular movement. 2. Streptomycin sulfate in the perilymph reduced promptly, strongly and reversibly nerve spontaneous activity (SA) recorded on the ampullar nerve. Streptomycin sulfate also reduced reversible and weakly the slow potentials recorded either near the crista ampullaris (Adc) or on the ampullar nerve (Ndc) and the modulation of firing rate (delta Fms) all these activities being evoked by a mechanical stimulation. 3. Streptomycin sulfate injected into ampullar endolymph reduced promptly and strongly the Adc, Ndc, and delta Fms. When its concentration was higher 0.2 mg/ml, the SA was also reduced about 40 min later. 4. It is concluded that streptomycin sulfate alters weakly and reversibly spike generation process and synaptic transmission; it inhibits strongly and reversibly the resting discharge and it inhibits strongly and may be partly reversibly the transduction mechanisms.

Vestibular effects of antibiotics introduced in the inner ear. Behavioral and electrophysiological studies in the frog

The aim of this study was to find a rapid and suitable method for testing the toxicity of drugs upon the vestibular end organs. The experiments were performed in the frog and the action of two antibiotics, streptomycin sulphate and penicillin G, was studied by carefully observing the equilibrium behaviour of the animals and recording the spontaneous activity of the nerve of the horizontal semicircular canal. The results obtained show that: 1) injection of streptomycin sulphate (20 microgram) directly into the labyrinthic cavity elicited disorders of the equilibrium and reduced the spontaneous activity of the horizontal canal nerve; 2) these disorders were reversible or not depending on the animal; 3) there is a good correlation between behavioral observations and electrophysiological data (i.e. the activity of the ampullary nerve was low in frogs whose behaviour was impaired at the time of sacrifice; the activity did not differ from controls when the normal behaviour was completely restored); 4) injection of penicillin G or 0.7% NaCl into the labyrinthic cavity had no effect on the equilibrium of the frogs or on the activity of the ampullary nerve; 5) intramuscular or intralymphatic injections of streptomycin sulphate produced a neuromuscular block but did not alter the activity of the ampullary nerve. In conclusion, the question of ototoxicity is discussed.

Morphological basis for a mechanical linkage in otolithic receptor transduction in the frog

Observations made with a scanning electron microscope confirm the binding of the stereocilia to a matchhead-like bulbous terminal at the apex of the kinocilium in frog saccular receptor cells. Since the kinocilium is shown to rest on a portion of the receptor cell that lacks the rigid cuticular base of the stereocilia, movenment of the ciliary ensemble results in a "plunging-like" effect of the kinocilium which produces a distension of the membranc at its base. This membrane distension is envisaged as bringing about the ionic conductance changes necessary for the production of a generator potential and, thus, for the transduction of movement into vestibular nerve activity.