The dual dopamine‐glutamate phenotype of growing mesencephalic neurons regresses in mature rat brain

Coexpression of tyrosine hydroxylase (TH) and vesicular glutamate transporter 2 (VGLUT2) mRNAs in the ventral tegmental area (VTA) and colocalization of these proteins in axon terminals of the nucleus accumbens (nAcb) have recently been demonstrated in immature (15‐day‐old) rat. After neonatal 6‐hydroxydopamine (6‐OHDA) lesion, the proportion of VTA neurons expressing both mRNAs and of nAcb terminals displaying the two proteins was enhanced. To determine the fate of this dual phenotype in adults, double in situ hybridization and dual immunolabeling for TH and VGLUT2 were performed in 90‐day‐old rats subjected or not to the neonatal 6‐OHDA lesion. Very few neurons expressed both mRNAs in the VTA and substantia nigra (SN) of P90 rats, even after neonatal 6‐OHDA. Dually immunolabeled terminals were no longer found in the nAcb of normal P90 rats and were exceedingly rare in the nAcb of 6‐OHDA‐lesioned rats, although they had represented 28% and 37% of all TH terminals at P15. Similarly, 17% of all TH terminals in normal neostriatum and 46% in the dopamine neoinnervation of SN in 6‐OHDA‐lesioned rats were also immunoreactive for VGLUT2 at P15, but none at P90. In these three regions, all dually labeled terminals made synapse, in contradistinction to those immunolabeled for only TH or VGLUT2 at P15. These results suggest a regression of the VGLUT2 phenotype of dopamine neurons with age, following normal development, lesion, or sprouting after injury, and a role for glutamate in the establishment of synapses by these neurons. J. Comp. Neurol. 517:873–891, 2009. © 2009 Wiley‐Liss, Inc.