Pacemaking in Dopaminergic Ventral Tegmental Area Neurons: Depolarizing Drive from Background and Voltage-dependent Sodium Conductances

Dopaminergic neurons in the ventral tegmental area (VTA) fire spontaneously in a pacemaker-like manner. We analyzed the ionic currents that drive pacemaking in dopaminergic VTA neurons, studied in mouse brain slices. Pacemaking was not inhibited by blocking hyperpolarization-activated cation current (I(h)) or blocking all calcium current by Mg(2+) replacement of Ca(2+). Tetrodotoxin (TTX) stopped spontaneous activity and usually resulted in stable resting potentials near -60 mV to -55 mV, 10-15 mV below the action potential threshold. When external sodium was replaced by N-methyl-D-glucamine (NMDG) with TTX present, cells hyperpolarized by an average of -11 mV, suggesting a significant resting sodium conductance not sensitive to TTX. Voltage-clamp experiments using slow (10 mV/s) ramps showed a steady-state, steeply voltage-dependent current blocked by TTX that activates near -60 mV, as well as a sodium "background" current with little voltage sensitivity, revealed by NMDG replacement for sodium with TTX present. We quantified these two components of sodium current during the pacemaking trajectory using action potential clamp. The initial phase of depolarization, up to approximately -55 mV, is driven mainly by non-voltage-dependent sodium background current. Above -55 mV, TTX-sensitive voltage-dependent "persistent" Na current helps drive the final phase of depolarization to the spike threshold. Voltage-dependent calcium current is small at all subthreshold voltages. The pacemaking mechanism in VTA neurons differs from that in substantia nigra pars compacta (SNc) neurons, where subthreshold calcium current plays a dominant role. In addition, we found that non-voltage-dependent background sodium current is much smaller in SNc neurons than VTA neurons.