Colocalization of tyrosine hydroxylase and GAD65 mRNA in mesostriatal neurons

Although dopamine has been considered as the only neurotransmitter in the n igrostriatal pathway, studies carried out in the last two decades have sugg ested the existence of a nondopaminergic nigrostriatal projection, and more recently, gamma -aminobutyric acid (GABA) has been identified as its neuro transmitter. In this study, we used the combination of immunocytochemistry for tyrosine hydroxylase (TH; a marker of dopaminergic neurons), in situ hy bridization (ISH) for two different isoforms of glutamic acid decarboxylase (GAD65 and GAD67, the rate-limiting enzyme in GABA synthesis) and retrogra de tracing techniques to investigate the possible existence of nigrostriata l neurons containing both neurotransmitters (dopamine and GABA) in the rat. Our results revealed that approximately 10% of mesostriatal dopaminergic n eurons, most of them lying in the medial region of the substantia nigra par s compacta (SNC) and neighbouring A10 region, contain GAD65 mRNA. These fin dings reveal a third nigrostriatal pathway formed by dopaminergic/GABAergic neurons. Contrasting with the idea that in the basal ganglia, dopamine and GABA are released from different cell populations, the results suggest a m ore complex dopamine/GABA interaction than previously assumed, probably inc luding cotransmission.