The membrane current activated by fast nicotinic excitation of intact and mature rat sympathetic neurons was studied at 37°C, by using the two- microelectrode voltage-clamp technique. The excitatory postsynaptic current (EPSC) was modeled as the difference between two exponentials. A fast time constant (r2; mean value 0.57 ms), which proves to be virtually voltage- independent, governs the current rise phase and a longer time constant (r1; range 5.2-6.8 ms in 2 mM Ca2+) describes the current decay and shows a small negative voltage dependence. A mean peak synaptic conductance of 0.58 μS per neuron is measured after activation of the whole presynaptic input in 5 mM Ca2+ external solution (0.40 μS in 2 mM Ca2+). The miniature EPSCs also rise and decay with exponential time constants very similar to those of the compound EPSC recorded at the same voltage. A mean peak conductance of 4.04 nS is estimated for the unitary event. Deconvolution procedures were employed to decompose evoked macrocurrents. It is shown that under appropriate conditions the duration of the driving function describing quantal secretion can be reduced to < 1 ms. The shape of the EPSC is accurately mimicked by a complete mathematical model of the sympathetic neuron incorporating the kinetic properties of five different voltage- dependent current types, which were characterized in a previous work. We show that I(A) channels are opened by depolarizing voltage steps or by synaptic potentials in the subthreshold voltage range, provided that the starting holding voltage is sufficiently negative to remove I(A) steady-state inactivation (less than -50 mV) and the voltage trajectories are sufficiently large to enter the I(A) activation range (greater than -65 mV). Under current-clamp conditions, this gives rise to an additional fast component in the early phase of membrane repolarization-in response to voltage pulses-and to a consistent distortion of the excitatory postsynaptic potential (EPSP) time course around its peak-in response to the synaptic signal. When the stimulation initiates an action potential, I(A) is shown to significantly increase the synaptic threshold conductance (up to a factor of 2 when I(A) is fully deinactivated), compared with that required when I(A) is omitted. The voltage dependence of this effect is consistent with the I(A) steady-state inactivation curve. It is concluded that I(A), in addition to speeding up the spike repolarization process, also shunts the excitatory drive and delays or prevents the firing of the neuron action potential.
Synaptic current at the rat ganglionic synapse and its interactions with the neuronal voltage-dependent currents
SACCHI, Oscar;ROSSI, Marialisa;CANELLA, Rita;
1998
Abstract
The membrane current activated by fast nicotinic excitation of intact and mature rat sympathetic neurons was studied at 37°C, by using the two- microelectrode voltage-clamp technique. The excitatory postsynaptic current (EPSC) was modeled as the difference between two exponentials. A fast time constant (r2; mean value 0.57 ms), which proves to be virtually voltage- independent, governs the current rise phase and a longer time constant (r1; range 5.2-6.8 ms in 2 mM Ca2+) describes the current decay and shows a small negative voltage dependence. A mean peak synaptic conductance of 0.58 μS per neuron is measured after activation of the whole presynaptic input in 5 mM Ca2+ external solution (0.40 μS in 2 mM Ca2+). The miniature EPSCs also rise and decay with exponential time constants very similar to those of the compound EPSC recorded at the same voltage. A mean peak conductance of 4.04 nS is estimated for the unitary event. Deconvolution procedures were employed to decompose evoked macrocurrents. It is shown that under appropriate conditions the duration of the driving function describing quantal secretion can be reduced to < 1 ms. The shape of the EPSC is accurately mimicked by a complete mathematical model of the sympathetic neuron incorporating the kinetic properties of five different voltage- dependent current types, which were characterized in a previous work. We show that I(A) channels are opened by depolarizing voltage steps or by synaptic potentials in the subthreshold voltage range, provided that the starting holding voltage is sufficiently negative to remove I(A) steady-state inactivation (less than -50 mV) and the voltage trajectories are sufficiently large to enter the I(A) activation range (greater than -65 mV). Under current-clamp conditions, this gives rise to an additional fast component in the early phase of membrane repolarization-in response to voltage pulses-and to a consistent distortion of the excitatory postsynaptic potential (EPSP) time course around its peak-in response to the synaptic signal. When the stimulation initiates an action potential, I(A) is shown to significantly increase the synaptic threshold conductance (up to a factor of 2 when I(A) is fully deinactivated), compared with that required when I(A) is omitted. The voltage dependence of this effect is consistent with the I(A) steady-state inactivation curve. It is concluded that I(A), in addition to speeding up the spike repolarization process, also shunts the excitatory drive and delays or prevents the firing of the neuron action potential.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
