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

Abstract

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.