The origin of slow potentials in semicircular canals of the frog

Effects of modifications of extracellular and intracellular calcium concentrations on the bioelectrical activity of the isolated frog semicircular canal

In the inner ear, calcium appears to play a major role in different processes including mechanoelectrical transduction, adaptation during prolonged stimulation and electrical resonance. The aim of the present study was to investigate the effect of an increase of the cytosolic calcium content and a reduction of the perilymphatic concentration of calcium, on the bioelectrical activity of the isolated frog semicircular canal. Under resting conditions, the spontaneous activity of the afferent fibers and the difference of potential between the endolymphatic and perilymphatic compartments, called endolymphatic potential, were recorded. When the sensory epithelium was mechanically stimulated three additional parameters were investigated: the variations of the endolymphatic potential (ampullar direct current), the variations of the ampullar nerve potential (nerve direct current) and the frequency of the evoked afferent spikes. Increase of the intracellular calcium concentration by administration of the calcium ionophore A23187 (3 x 10(-6) M, 20 min) into the perilymphatic compartment, caused a biphasic effect on the spontaneous activity of the ampullar nerve which increased rapidly, reaching a maximum within 15 min, and then gradually declined to stabilize at 74% of the control 1 h after withdrawal of A23187. A23187 did not induce any modifications of the endolymphatic potential, the ampullar direct current or the frequency of the evoked afferent spikes. In contrast, A23187 induced a significant reduction of the nerve direct current which decreased by 31% of the control 1 h after withdrawal of the ionophore. Gradual reduction of the perilymphatic concentration of calcium (from 2 to 1 mM) induced a dose-dependent increase of the spontaneous activity of the ampullar nerve and the frequency of the evoked afferent spikes. Reduction of the perilymphatic calcium concentration from 1.6 to 1.2 mM caused a transient increase of the endolymphatic potential, while 1 mM Ca2+ induced a decrease to 88% of the control. The nerve direct current slightly increased for calcium concentrations ranging from 1.8 to 1.4 mM and decreased in the presence of 1.2 mM CaCl2. These data suggest that an increase of calcium into the cytosol induces an alteration of the mechanisms responsible for the spontaneous release of the afferent neurotransmitter and the electrogenic spreading of the postsynaptic potentials. In contrast, an excess of calcium does not impair the mechanisms involved in the generation of the action potentials. Our results also suggest that reduction of the perilymphatic calcium concentration may lead to modifications of the physical and electrical properties of the cell membranes of the labyrinthine epithelium and/or the ampullar afferent fibers.

Perilymphatic potassium changes and potassium homeostasis in isolated semicircular canals of the frog

1. Endolymphatic and perilymphatic potassium concentrations were measured with K(+)-sensitive microelectrodes in isolated semicircular canals of the frog. K+ levels were evaluated both at rest and during sinusoidal stimulation (0.05 Hz) of the sensory organ. 2. Mechanical stimulation of hair cells was associated with sinusoidal changes (about 0.2 mM) in the perilymphatic K+ concentration. 3. Perilymphatic K(+)-fluctuations were modified neither by impairment of the synaptic transmission at cyto-neural junctions nor by chronic denervation of the crista ampullaris, thus indicating that K+ ions were actually released by hair cells. 4. Voltage-clamp experiments of the whole sensory organ showed that K+ flows across the crista ampullaris can vary from 3 X 10(11) molecules of K+ s-1 at rest up to about 15 X 10(11) molecules of K+ s-1 during mechanical stimuli. 5. Measurement of intra-ampullar K+ concentration demonstrated that the amount of K+ transported from the perilymph towards the endolymph can be rapidly altered by modifying its perilymphatic levels. This suggests that vestibular organs are endowed with K+ homeostatic mechanisms able to buffer in a very efficient way the concentration of K+ in both the fluids bathing the crista ampullaris. 6. The possible role of K+ homeostatic mechanisms in hair cell adaptation is discussed.

Differences between cats in response properties of horizontal semicircular canal primary afferents

The response properties of cat horizontal canal afferents (N = 81) were characterized by three parameters: their long time constants (tau), low frequency gain constants (G1), and middle frequency gain constants (Gm). An average value of each of these parameters was calculated for each of eight animals and comparisons were made across animals. There were significant differences between individual animals in their average values of tau and Gm. There was also a significant negative correlation between tau's and Gm's. An animal with a larger average tau tended to have a smaller average Gm. We also used anatomic data on membranous canal duct diameter from the literature to independently estimate the potential effect of interanimal anatomic variability on the predicted range of tau and Gm values in a population. We then compared the data from our 81 afferents with the predictions from the anatomic data.

Cupular movement and nerve impulse response in the isolated semicircular canal

The aim of the study was to record nerve impulse responses to controlled displacements of the cupula and relate these to observations of cupular motion. The preparation was the skate semicircular canal, isolated and maintained in a chamber. The canal was cut and tied tightly to an oil-filled tube driven by a 1 microliter syringe. In some experiments dye was used to make the face of the cupula visible. Small volume displacements (0.02 microliter) caused a billowing of the central portion of the cupula, whereas for repeated large displacements (0.1 microliter) the top of the cupula moved along the roof of the ampulla. Displacements of the latter amount were close to saturation in the characteristic functions (spike rate versus volume displacement) of nerve firing. It seems that the discharge rate of tonically responding units in the skate crista ampullaris rather faithfully record the force acting on the cupula. The motion of the base of the cupula rather than its tip appears to be the relevant parameter in setting the level of excitation in the sensory cells within the physiological range of stimulation. Such a mode of operation would ensure the highest sensitivity of the sense organ.

Frequency response of the lateral-line organ of Xenopus laevis

The stimulus response relation of the epidermal lateral-line organ of Xenopus laevis was studied by recording activity of single afferent nerve fibres in isolated preparations. Linear frequency response analysis over a frequency range of 0.1--100Hz was performed under steady-state conditions, using small amplitude, sinusoidal water displacements produced by a glass sphere at a short distance from the skin. Period histograms of afferent nerve activity were computed, and amplitude, phase and mean activity of the response were determined by means of Fourier analysis. A standardization procedure at the start of each experiment made scaling of the frequency responses of different preparations unnecessary. The results show that for small stimulus amplitudes the response of the lateral-line organ over the whole range of frequencies studied can adequately be described as a modulation of the spontaneous activity. The amplitude of the response is proportional to the stimulus amplitude, and the phase of the response is independent of stimulus amplitude. The lateral-line organ of Xenopus laevis can thus be regarded as a linear system for stimuli which produce modulation of the spontaneous activity. The frequency response demonstrates unequivocally that the lateral-line organ of Xenopus laevis functions as a water velocity detector. For frequencies of stimulation from 0.1--20Hz the gain increases with a slope of 7.5 dB/oct, and up to 5Hz the response is almost in phase with the water velocity. The extent to which the different transmission steps between stimulus and response will contribute to the frequency response is discussed.

Post-synaptic potentials recorded from afferent nerve fibres of the posterior semicircular canal in the frog

Glass microelectrode recordings were made from single fibres of the posterior ampullary nerve in the isolated labyrinth of the frog (Rana esculenta). Potentials were recorded both at rest and during rotatory stimulation of the canal. At rest, the tracings revealed an intense background of small, largely summated potentials (0.5-10 mV amplitude, 3-6 msec duration), which underlay the discharge of spikes in all the impaled units. The frequency of the subthreshold events was related to the frequency of the propagated spikes, the latter ranging from 0 to 40/sec. Stimulation modulated the frequency of both spikes and subthreshold potentials, whose summation during excitation led to a positive shift of the fibre membrane potential. The small potentials proved to be dependent on Ca2+ and Mg2+ levels in the bath. Antidromic-stimulation of the posterior ampullary nerve indicated that the observed events do not represent an artifact due to extracellular field interference related to spike activity in the neighbouring fibres. Tetrodotoxin (10(-7)-10(-6) g/ml) applied externally to the preparation or previously perfused through the frog vessels abolishes the propagated spikes but left unaffected the small potentials which, even under drug treatment, were normally modulated by the stimulus. The subthreshold potentials thus appear to be EPSPs generated at the cyto-neural junction between the hair cells and the endings of the ampullary nerve fibres.