ELECTRICAL AND INTEGRATIVE PROPERTIES OF RABBIT SYMPATHETIC NEURONS REEVALUATED BY PATCH CLAMPING NONDISSOCIATED CELLS

1. Voltage recordings were performed on non-dissociated sympathetic neurones from rabbit coeliac ganglia using the whole-cell configuration of the patch clamp technique. 2. Cells were classified depending on their firing pattern as silent cells (63%) producing either phasic (24%) or tonic (76%) spike discharge in response to depolarizing currents, and pacemaker cells (37 %). 3. All the cells produced large overshooting spikes and prolonged postspike after-hyperpolarization. The peak-to-peak spike amplitude was 113.8 +/- 1 mV. Spikes were shortened and the after-hyperpolarization was suppressed when calcium channel blockers (Cd2+ and La3+) were added. 4. Silent cells have a resting potential of - 58- 8 +/- 1-5 mV. At potentials ranging from - 50 to - 90 mV, the input impedance was 490 +/- 27 MOMEGA at 22-24-degrees-C and 426 +/- 47 MOMEGA at 35-36-degrees-C. The time constant at voltages corresponding to the high input impedance region was 126 +/- 7 ms at 22-24-degrees-C and 86 +/- 7 ms at 35-36-degrees-C. 5. The firing frequency of the pacemaker cells was 3.2 +/- 0.5 Hz at 35.36-degrees-C in the presence of nicotinic blockers. Evidence is given that the firing did not result from cell injury but was induced by an intrinsic pacemaker mechanism. Input impedance of pacemaker neurones was 580 +/- 47 MOMEGA at 22-24-degrees-C and 473 +/- 56 MOMEGA at 35-36-degrees-C. 6. Most of the pacemaker cells (63 %) were motoneurones, since they were antidromically fired by stimulating post-ganglionic nerves. ln addition, they received synaptic inputs from both preganglionic fibres (splanchnic nerves) and the periphery (postganglionic nerves). Long-lasting depolarizations were induced in either silent or pacemaker cells by single shocks applied to pre- and postganglionic nerves. 7. Slowly rising voltage ramps revealed the presence of an N-shaped current-voltage relationship in voltage clamped pacemaker cells. The negative slope was located in a subthreshold voltage range, between -83.4 +/- 1.4 and -59.0 +/- 1.8 mV. It was induced by the activation of a low threshold persistent inward current. Although it was tiny (22 +/- 3 pA at its peak level) this current brought the null-current voltage up to -41.0 +/- 1.4 mV, which resulted in continuous firing. 8. Due to the instability introduced by the N-shaped I-V relationship, pacemaker cells can display bistable behaviour characterized by hyperpolarizing responses. 9. Our data provide new information about the electrical and integrative properties of the mammalian sympathetic neurones. These neurones are electrotonically compact and have pacemaker properties.