Introduction: Frog canal hair cells synapse with the afferent nerve fibres and release glutamate that sustains the afferent discharge. In the present work the role played by intracellular Ca2+ stores (CICR mechanism) in afferent transmission was investigated. Material and Methods: In the intact frog labyrinth, mEPSPs and spikes were intracellularly recorded at rest and during sinusoidal canal rotation (0.1 Hz, peak acceleration 12 deg/s2) by inserting glass microelectrodes into the posterior nerve close to the synapse (at about 500 microm). In the different units mEPSP frequency is high (>100/s) and a correct evaluation of their rate was performed after Wiener filtering the traces to eliminate event overlap by shortening their duration. Spikes were previously counted and then digitally subtracted. Changes in mEPSP size and waveform were also evaluated: the events were selected and averaged and the averaged mEPSP waveform was fit using a modified gamma-distribution function. In the same unit, the afferent discharge was evaluated in control conditions as well as in the presence of caffeine (20 mM) or ryanodine (1 mM). Results and Conclusions: Caffeine increased spontaneous spike activity as well as spike discharge evoked by rotation. In the presence of caffeine the frequency of spontaneous and mechanically evoked spikes increased to 372±120% (n=5) and 237±151% (n=3) of the control, respectively. Opposite effects were produced by ryanodine: the frequency of spontaneous and mechanically evoked spikes decreased to 65±6% (n=3) and 40±10% (n=2) of the control, respectively. Caffeine increased the discharge of both spontaneous (238±38% of control, n=7) and evoked mEPSPs (230±47% of control, n=3). Conversely, ryanodine decreased resting and evoked mEPSP frequency to 58±12% and 59±32% of the control (n=3), respectively. Neither drug altered mEPSP amplitude or time course, and transmitter release remained uncorrelated and asynchronous. Positive modulation by caffeine and negative modulation by ryanodine would suggest that CICR plays an important role in this system, amplifying Ca2+ signals due to Ca2+ influx across the plasma membrane and producing a cycle by cycle effect on synaptic transmission.
Caffeine and ryanodine affect transmitter release at the cytoneural junction in the frog labyrinth
ROSSI, Marialisa;MARTINI, Marta;RISPOLI, Giorgio;
2003
Abstract
Introduction: Frog canal hair cells synapse with the afferent nerve fibres and release glutamate that sustains the afferent discharge. In the present work the role played by intracellular Ca2+ stores (CICR mechanism) in afferent transmission was investigated. Material and Methods: In the intact frog labyrinth, mEPSPs and spikes were intracellularly recorded at rest and during sinusoidal canal rotation (0.1 Hz, peak acceleration 12 deg/s2) by inserting glass microelectrodes into the posterior nerve close to the synapse (at about 500 microm). In the different units mEPSP frequency is high (>100/s) and a correct evaluation of their rate was performed after Wiener filtering the traces to eliminate event overlap by shortening their duration. Spikes were previously counted and then digitally subtracted. Changes in mEPSP size and waveform were also evaluated: the events were selected and averaged and the averaged mEPSP waveform was fit using a modified gamma-distribution function. In the same unit, the afferent discharge was evaluated in control conditions as well as in the presence of caffeine (20 mM) or ryanodine (1 mM). Results and Conclusions: Caffeine increased spontaneous spike activity as well as spike discharge evoked by rotation. In the presence of caffeine the frequency of spontaneous and mechanically evoked spikes increased to 372±120% (n=5) and 237±151% (n=3) of the control, respectively. Opposite effects were produced by ryanodine: the frequency of spontaneous and mechanically evoked spikes decreased to 65±6% (n=3) and 40±10% (n=2) of the control, respectively. Caffeine increased the discharge of both spontaneous (238±38% of control, n=7) and evoked mEPSPs (230±47% of control, n=3). Conversely, ryanodine decreased resting and evoked mEPSP frequency to 58±12% and 59±32% of the control (n=3), respectively. Neither drug altered mEPSP amplitude or time course, and transmitter release remained uncorrelated and asynchronous. Positive modulation by caffeine and negative modulation by ryanodine would suggest that CICR plays an important role in this system, amplifying Ca2+ signals due to Ca2+ influx across the plasma membrane and producing a cycle by cycle effect on synaptic transmission.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
