Neuronal activity in the ventral tegmental area (VTA) of brain is thought to be involved in alcohol preference and reward mechanisms by regulating mesolimbic dopamine (DA) release. We investigated the spontaneous activity and properties of freshly isolated ventral tegmental area (VTA) principal neurons by whole-cell recording and single-cell RT-PCR. The VTA principal neurons, which were tyrosine hydroxylase-positive and glutamic acid decarboxylase (GAD67)-negative, exhibited low firing frequency and a long action potential (AP) duration. The VTA principal neurons exhibited a calretinin-positive and parvalbumin-negative Ca2+-binding protein mRNA expression pattern. The VTA principal neurons were classified into two subpopulations based on their firing frequency coefficient of variation (CV) at room temperature (21-23?C): irregular-type neurons with a large CV and tonic-type neurons with a small CV. These two firing patterns were also recorded at the temperature of 34?C and in nystatin-perforated patch recording. In VTA principal neurons, the AP afterhyperpolarization (AHP) amplitude contributed to the firing regularity and AHP decay slope contributed to the firing frequency. The AHP amplitude in the irregular-type VTA principal neurons was smaller than that in the tonic-type VTA principal neurons. There was no significant difference in the AHP decay slope between the two-types of VTA principal neurons. Apamin-sensitive small conductance Ca2+-activated K+ (SK) channels contributed to the AHP and the regular firing of the tonic-type neurons, but contributed little to the AHP and firing of the irregular-type neurons. In voltage-clamp tail-current analysis, in both conventional and nystatin-perforated whole-cell recording, the apamin-sensitive AHP current density of the tonic-type neurons was significantly larger than that of the irregular-type neurons. We suggest that apamin-sensitive SK current contributes to intrinsic firing differences between the two subpopulations of VTA principal neurons. Experiments are also in progress to elucidate the cellular mechanisms of alcohol and neuroactive substance action on excitability mechanisms in VTA and other types of neurons. These studies hold the promise that such investigations on excitability mechanisms will advance our knowledge of alcohol and neuroactive substance action in the nervous system and provide a foundation for understanding the role of excitability mechanisms in alcohol abuse and alcoholism.