An inward Ca2+ -current and two outward K+-currents have recently been identified in frog vestibular hair cells. The Ca2+- current is more effectively carried by Ba2+ and it is suppressed by Ca2+ channel-blockers (Cd2+- Co2+). One of the two K+-currents, is Ca2+-dependent and is abolished by Ba2+, TEA and the divalent cations that block the Ca2+ -current; the other is a transient K+-current of the A-type which is specifically cancelled by 4-AP. In the frog labyrinth the resting receptor discharge depends on the external Ca2+ level and its frequency increases on increasing Ca2+ concentration in the bathing medium. Since the pre-synaptic currents most likely control the transmitter release at the cytoneural junction, their modifications will have a counterpart at the post-synaptic level. Therefore, it seems to be of interest to investigate the effect of Ba2+, TEA, 4-AP and CoCl2 on the EPSP and spike discharges recorded at rest from single fibres of the posterior nerve in the isolated frog labyrinth. In Ca-free Ringer solution Ba2+ preserved, at low concentration (0.3 mM), the resting activity and at higher levels (up to 6 mM) it resulted in a pronounced facilitation of the EPSP and spike discharges. The amplitude and shape of the sensory spikes were not modified by Ba2+ which therefore proved not to interfere with the K+-channel of the myelinated fibres. Facilitation increased on increasing Ba2+ concentration up to 4-5 mM and it was more prominent in those units exhibiting a low resting spike firing, The effect of Ba2+ (1 mM) was completely antagonized by 10 mM Ca2+. CoCl2 (3 mM) suppressed the resting rate at the normal external Ca2+ concentration (1.8 mM); the synaptic block operated by Co2+ was partially releaved by 1.8 mM Ba2+ . TEA (20 mM) evoked a clear-cut increase in the EPSP and spike discharges which, however, was less consistent than that produced by Ba2+. By comparing the effect of TEA on the spike frequency with that obtained at different Ba2+ levels, the Ba2+ capacity to carry the Ca2+-current was dissected. Such an effect is dose- dependent and it is more evident in low-frequency units. Conversely, 4-AP (4 mM) did not affect the resting discharge frequency. A more close evaluation of the synaptic effect of such ions was obtained when Ba2+ (1 mM) or TEA (20 mM) were added to a TTX-treated preparation, since their effect on the EPSP frequency was uncompiicated by the spike discharge. These experiments reveal that the Ba2+- and the Ca2+ -activated K+-current produced an increase in the EPSP frequency of 30 and 15%, respectively. These results indicate that either the Ca2+ - or the Ba2+-current sustain the transmitter release at the cytoneural junction. The effect of TEA suggests that the Ca2+ - dependent K+-current may play an important role in supporting the neurosecretory process by controlling the membrane potential of the hair cells.
Effect of some ions on the sensory activity of the frog labyrinth
ROSSI, Marialisa;MARTINI, Marta
1986
Abstract
An inward Ca2+ -current and two outward K+-currents have recently been identified in frog vestibular hair cells. The Ca2+- current is more effectively carried by Ba2+ and it is suppressed by Ca2+ channel-blockers (Cd2+- Co2+). One of the two K+-currents, is Ca2+-dependent and is abolished by Ba2+, TEA and the divalent cations that block the Ca2+ -current; the other is a transient K+-current of the A-type which is specifically cancelled by 4-AP. In the frog labyrinth the resting receptor discharge depends on the external Ca2+ level and its frequency increases on increasing Ca2+ concentration in the bathing medium. Since the pre-synaptic currents most likely control the transmitter release at the cytoneural junction, their modifications will have a counterpart at the post-synaptic level. Therefore, it seems to be of interest to investigate the effect of Ba2+, TEA, 4-AP and CoCl2 on the EPSP and spike discharges recorded at rest from single fibres of the posterior nerve in the isolated frog labyrinth. In Ca-free Ringer solution Ba2+ preserved, at low concentration (0.3 mM), the resting activity and at higher levels (up to 6 mM) it resulted in a pronounced facilitation of the EPSP and spike discharges. The amplitude and shape of the sensory spikes were not modified by Ba2+ which therefore proved not to interfere with the K+-channel of the myelinated fibres. Facilitation increased on increasing Ba2+ concentration up to 4-5 mM and it was more prominent in those units exhibiting a low resting spike firing, The effect of Ba2+ (1 mM) was completely antagonized by 10 mM Ca2+. CoCl2 (3 mM) suppressed the resting rate at the normal external Ca2+ concentration (1.8 mM); the synaptic block operated by Co2+ was partially releaved by 1.8 mM Ba2+ . TEA (20 mM) evoked a clear-cut increase in the EPSP and spike discharges which, however, was less consistent than that produced by Ba2+. By comparing the effect of TEA on the spike frequency with that obtained at different Ba2+ levels, the Ba2+ capacity to carry the Ca2+-current was dissected. Such an effect is dose- dependent and it is more evident in low-frequency units. Conversely, 4-AP (4 mM) did not affect the resting discharge frequency. A more close evaluation of the synaptic effect of such ions was obtained when Ba2+ (1 mM) or TEA (20 mM) were added to a TTX-treated preparation, since their effect on the EPSP frequency was uncompiicated by the spike discharge. These experiments reveal that the Ba2+- and the Ca2+ -activated K+-current produced an increase in the EPSP frequency of 30 and 15%, respectively. These results indicate that either the Ca2+ - or the Ba2+-current sustain the transmitter release at the cytoneural junction. The effect of TEA suggests that the Ca2+ - dependent K+-current may play an important role in supporting the neurosecretory process by controlling the membrane potential of the hair cells.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
