Figure 2.

Sub-populations of VTA GABA neurons exhibit qualitatively distinct burst firing activity after membrane hyperpolarization. In (A) we show our current clamp protocol (above) and a corresponding neuronal membrane output (below) that allows for investigation of both tonic firing from increases in membrane voltage and burst firing after membrane hyperpolarization. A population study of 58 VTA GABA neurons found 3 phenotypically distinct firing patterns after injection of a hyperpolarizing current, as show in pie chart (B). We categorized these observed phenotypes as Type 1, Type 2, or Type 3 based on their response to a hyperpolarizing current of 500 ms. Type 1 neurons as shown in panel (C) displayed fairly typical T-channel activity, marked by a classical depolarizing potential lasting 100–200 ms which included the firing of 1 or more action potentials during the slow depolarization. This low threshold calcium spike was blocked by 5 µM TTA-P2 Type 2 neurons, as shown in (D) also exhibited spiking after hyperpolarization. However, instead of a classical T-channel depolarization they generally exhibited fast membrane hyperpolarization following action potentials and a longer duration of spontaneous action potentials after hyperpolarizing pulses. Type 3 neurons, shown in (E), were completely silent after hyperpolarization.