Effects of dopamine and noradrenaline on Ca channels of cultured sensory and sympathetic neurons of chick

The effects of noradrenaline and dopamine on voltage-dependent Ca currents were investigated in cultured dorsal root and sympathetic ganglion neurons from chick embryos. At concentrations of 1 to 10 ?M, bath application of the neurotransmitters caused a general depression of inward Ca currents. Above -20 mV the decrease of the current amplitude was reversible and accompanied by a 2-10-fold prolongation of the activation time course. Below -20 mV, where a low voltage-activated Ca component is turned on, the size of the currents was reduced by 40% with little effect on the time course. Despite extensive wash-out, little sign of reversibility was observed in this case. Single-channel current recording in outside-out membrane patches revealed that a low membrane potentials dopamine and noradrenaline reversibly reduced single Ca-channel activity. This finding supports the view that in sensory and sympathetic neurons, both neurotransmitters affect the membrane conductance by modulating Ca permeability and not by activating catecholamine-specific channels able to carry transient outward currents. The probability of Ca channel opening is strongly reduced by addition of 10 ?M of either catecholamine to the bath. The possible involvement of a voltage-dependent block of Ca channels by the neurotransmitters is discussed.