Noradrenaline-induced afterdepolarization in cat sympathetic preganglionic neurons in vitro

Sympathetic preganglionic neurons of the intermediolateral nucleus were identified by antidromic stimulation in the slice of the T2 or T3 segment of the cat spinal cord. In normal Krebs solution, the action potential of these neurons had a shoulder on the repolarization phase and was followed by a long-lasting afterhyperpolarization (AHP). The AHP had a fast and a slow component. Superfusion of the slice with noradrenaline (NA), 10-50 microM, resulted in depression of the shoulder on the repolarization phase of the action potential, in the appearance of an afterdepolarization (ADP), which was absent in control conditions, and in depression of the slow component of the AHP. These effects were present whether the membrane potential of the sympathetic preganglionic neurons was decreased, increased, or not changed by NA. A typical ADP had time to peak of 50 ms and decay time of 200-500 ms; the amplitude was variable and large ADPs could be suprathreshold, causing repetitive firing. The amplitude and duration of the ADP increased with NA concentration. The appearance of the ADP seemed to be independent of the depressant effect of NA on the slow AHP. The ADP was associated with a decrease in neuron input resistance and was voltage dependent, being depressed in nonlinear fashion by membrane hyperpolarization. The ADP decreased in amplitude or disappeared within a range of membrane potentials from -70 to -90 mV. The ADP was reversibly suppressed by the Ca-channel blocker cobalt (2 mM), by low Ca Krebs (0.25 mM), and by iontophoretic injection of ethyleneglycol-bis(B-aminoethyl-ether)-N,N'-tetraacetic acid into the cell. Increasing Ca concentration from 2.5 to 10.0 mM had no effect. The ADP was unaffected by tetrodotoxin, at a concentration blocking the Na spike, but was suppressed in Na-free medium, even when the Ca spike was prolonged by tetraethylammonium 20 mM. Changes in external K concentration from 3.6 to 2.5 or 10.0 mM did not change the ADP. Increasing intracellular Cl concentration or decreasing extracellular Cl concentration had no effect on the ADP. It is concluded that the ADP, evoked by NA, is due to an increase in membrane conductance involving Na and Ca ions, possibly a Ca-activated Na conductance. The ADP provides a mechanism with which NA may modulate sympathetic preganglionic neuron responsiveness to excitatory synaptic inputs.