Orexins/hypocretins are involved in mechanisms of emotional arousal and short-term regulation of feeding. The dense projection of orexin neurons from the lateral hypothalamus to mesocorticolimbic dopaminergic neurons in the ventral tegmental area (VTA) is likely to be important in both of these processes. We used single-unit extracellular and whole-cell patch-clamp recordings to examine the effects of orexins (A and B) and melanin-concentrating hormone (MCH) on neurons in this region. Orexins caused an increase in firing frequency (EC(50) 78 nm), burst firing, or no change in firing in different groups of A10 dopamine neurons. Neurons showing oscillatory firing in response to orexins had smaller afterhyperpolarizations than the other groups of dopamine neurons. Orexins (100 nm) also increased the firing frequency of nondopaminergic neurons in the VTA. In the presence of tetrodotoxin (0.5 microm), orexins depolarized both dopaminergic and nondopaminergic neurons, indicating a direct postsynaptic effect. Unlike the orexins, MCH did not affect the firing of either group of neurons. Single-cell PCR experiments showed that orexin receptors were expressed in both dopaminergic and nondopaminergic neurons and that the calcium binding protein calbindin was only expressed in neurons, which also expressed orexin receptors. In narcolepsy, in which the orexin system is disrupted, dysfunction of the orexin modulation of VTA neurons may be important in triggering attacks of cataplexy.