Microscopic heterogeneity in unitary N-type calcium currents in rat sympathetic neurons

1. Single N-type calcium (Ca2+) channels in rat superior cervical ganglion neurons display complex patterns of activity in both inactivating and non-inactivating gating modes. Unitary currents were elicited by holding the patch at -90 mV and stepping to +30 mV for 740 ms. Barium (110 mM) was used as the charge carrier. The dihydropyridine agonist (+)-202-791 was included in the bath to ensure that single channel recordings showed no L-type Ca2+ channel mode 2 activity. Using this protocol, we characterized three additional patterns of N-type Ca2+ channel activity named: (1) LLP for large unitary current amplitude (i = -0.92 pA) and low open probability (Po = 0.26); (2) SLP for small unitary current amplitude (i = -0.77 pA) and low open probability (Po = 0.25); and (3) SHP for its small unitary current (i = -0.77 pA) and higher open probability (Po = 0.39). 2. Transitions among these patterns of activity occur more slowly than transitions between closed and open states, resulting in significant clustering of like sweeps. Thus, the complicated gating of single N-type Ca2+ channels can be dissected into multiple, independent modes, each with the same reproducible pattern of activity. 3. This heterogeneous activity is not unique to sympathetic neurons, for inactivating (4), non-inactivating (4), SLP (4) and SHP (3 patches) gating modes were also observed in cell-attached patch recordings (n = 4) of single N-type Ca2+ channels in differentiated phaeochromocytoma (PC12) cells. 4. The 1568 sweeps from four single N-type Ca2+ channel recordings that used the same voltage protocol were categorized by mode to determine the frequency of occurrence of each. Of the 54% of sweeps that showed activity, 42% were inactivating and 58% were non-inactivating. The contribution by each mode to the sustained current was estimated using the equation: I = NPoi, where N is the frequency of occurrence of each mode and Po and i are the mean values of open probability and unitary current amplitude respectively. The LLP mode contributed 18%, the SLP mode 16%, and the SHP mode 66% of the sustained whole cell N-type Ba2+ current. 5. The variability in the incidence among these modes in other cell types may resolve some of the controversy surrounding the characterization of N- and L-type whole cell Ca2+ current components in peripheral neurons. In addition, the number of different modes provides a source of plasticity that may be a target of modulation by neurotransmitters and cellular signals.