Vasomotor sympathetic neurons are more excitable than secretomotor sympathetic neurons in bullfrog paravertebral ganglia.

We compared the excitability of secretomotor B and vasomotor C neurons using virtual nicotinic synapses implemented with the dynamic clamp technique. In response to fast synaptic conductance (g(syn)) waveforms modeled after B cell synaptic currents, it took 17.1+/-1.2nS to elicit spikes in 104 B cells and 3.3+/-0.3nS in 35 C cells. After normalizing for whole-cell capacitance, C cells were still more excitable than B cells (76+/-5pS/pF vs. 169+/-8pS/pF). Stimulating C cells with slower g(syn) waveforms, identical to synaptic currents in C cells, further accentuated the difference between cell types. The phenotypic excitability difference did not correlate with time in culture (1-12days) and could not be explained by resting potential (B cells: -65.6+/-0.9mV, C cells: -63.1+/-1.6mV) or input conductance density, which was greater in C cells (24.4+/-4.3pS/pF) than B cells (14.5+/-1.5pS/pF). Action potentials elicited by virtual EPSPs had a threshold voltage for firing that was -28.4+/-0.7mV in C cells and -19.7+/-0.4mV B cells, and an upstroke velocity and peak spike potential that were greater in B cells. The repetitive firing properties of B and C cells were similar; 69-78% phasic, 11-16% adapting and 11-15% tonic. We propose that B and C neurons express different types of Na(+) channels that shape how they integrate nicotinic synaptic potentials.