The Ventral Tegmental Area has calbindin neurons with the capability to co‐release glutamate and dopamine into the nucleus accumbens

The ventral tegmental area (VTA) has three major classes of neurons: dopaminergic (expressing tyrosine hydroxylase; TH), GABAergic (expressing vesicular GABA transporter; VGaT) and glutamatergic (expressing vesicular glutamate transporter 2; VGluT2). While VTA dopaminergic and GABAergic neurons have been further characterized by expression of calcium‐binding proteins (calbindin, CB; calretinin, CR or parvalbumin, PV), it is unclear whether these proteins are expressed in rat VTA glutamatergic neurons. Here, by a combination of in situ hybridization (for VGluT2 mRNA detection) and immunohistochemistry (for CB‐, CR‐ or PV‐detection), we found that among the total population of VGluT2 neurons, 30% coexpressed CB, 3% coexpressed PV and <1% coexpressed CR. Given that some VGluT2 neurons coexpress TH or VGaT, we examined whether these neurons coexpress CB, and found that about 20% of VGluT2‐CB neurons coexpressed TH and about 13% coexpressed VGaT. Because VTA TH‐CB neurons are known to target the nucleus accumbens (nAcc), we determined whether VGluT2‐CB‐TH neurons innervate nAcc, and found that about 80% of VGluT2‐CB neurons innervating the nAcc shell coexpressed TH. In summary, (a) CB, PV and CR are detected in subpopulations of VTA‐VGluT2 neurons; (b) CB is the main calcium‐binding protein present in VTA‐VGluT2 neurons; (c) one‐third of VTA‐VGluT2 neurons coexpress CB; (d) some VTA‐VGluT2‐CB neurons have the capability to co‐release dopamine or GABA, and (e) a subpopulation of VTA glutamatergic‐dopaminergic neurons innervates nAcc shell. These findings further provide evidence for molecular diversity among VTA‐VGluT2 neurons, neurons that may play a role in specific circuitry and behaviours.