In vitro identification and electrophysiological characterization of dopamine neurons in the ventral tegmental area

Dopamine (DA) neurons in the ventral tegmental area (VTA) have been implicated in brain mechanisms related to motivation, reward, and drug addiction. Successful identification of these neurons in vitro has historically depended upon the expression of a hyperpolarization-activated current (I(h)) and immunohistochemical demonstration of the presence of tyrosine hydroxylase (TH), the rate-limiting enzyme for DA synthesis. Recent findings suggest that electrophysiological criteria may be insufficient for distinguishing DA neurons from non-DA neurons in the VTA. In this study, we sought to determine factors that could potentially account for the apparent discrepancies in the literature regarding DA neuron identification in the rodent brain slice preparation. We found that confirmed DA neurons from the lateral VTA generally displayed a larger amplitude I(h) relative to DA neurons located in the medial VTA. Measurement of a large amplitude I(h) (>100 pA) consistently indicated a dopaminergic phenotype, but non-dopamine neurons also can have I(h) current. The data also showed that immunohistochemical TH labeling of DA neurons can render false negative results after relatively long duration (>15 min) whole-cell patch clamp recordings. We conclude that whole-cell patch clamp recording in combination with immunohistochemical detection of TH expression can guarantee positive but not negative DA identification in the VTA.