GIRK2 expression in dopamine neurons of the substantia nigra and ventral tegmental area

G-protein-regulated inward-rectifier potassium channel 2 (GIRK2) is reported to be expressed only within certain dopamine neurons of the substantia nigra (SN), although very limited data are available in humans. We examined the localization of GIRK2 in the SN and adjacent ventral tegmental area (VTA) of humans and mice by using either neuromelanin pigment or immunolabeling with tyrosine hydroxylase (TH) or calbindin. GIRK2 immunoreactivity was found in nearly every human pigmented neuron or mouse TH-immunoreactive neuron in both the SN and VTA, although considerable variability in the intensity of GIRK2 staining was observed. The relative intensity of GIRK2 immunoreactivity in TH-immunoreactive neurons was determined; in both species nearly all SN TH-immunoreactive neurons had strong GIRK2 immunoreactivity compared with only 50-60% of VTA neurons. Most paranigral VTA neurons also contained calbindin immunoreactivity, and approximately 25% of these and nearby VTA neurons also had strong GIRK2 immunoreactivity. These data show that high amounts of GIRK2 protein are found in most SN neurons as well as in a proportion of nearby VTA neurons. The single previous human study may have been compromised by the fixation method used and the postmortem delay of their controls, whereas other studies suggesting that GIRK2 is located only in limited neuronal groups within the SN have erroneously included VTA regions as part of the SN. In particular, the dorsal layer of dopamine neurons directly underneath the red nucleus is considered a VTA region in humans but is commonly considered the dorsal tier of the SN in laboratory species.