Vglut2 and gad expression reveal distinct patterns of dual GABAergic versus glutamatergic cotransmitter phenotypes of dopaminergic and noradrenergic neurons in the zebrafish brain
Throughout the vertebrate lineage, dopaminergic neurons form important neuromodulatory systems that influence motor behavior, mood, cognition and physiology. Studies in mammals have established that dopaminergic neurons often use GABA- or glutamatergic cotransmission during development and physiological function. Here, we analyze vglut2, gad1b and gad2 expression in combination with TH immunoreactivity in four days old larval and 30 days old juvenile zebrafish brains to determine which dopaminergic and noradrenergic groups may use GABA or glutamate as second transmitter. Our results show that most dopaminergic neurons also express GABAergic markers, including the dopaminergic groups of the olfactory bulb (homologous to mammalian A16) and the subpallium, the hypothalamic groups (A12, A14), the prethalamic zona incerta group (A13), the preoptic groups (A15), and the pretectal group. Thus, the majority of catecholaminergic neurons is gad1b/2-positive and coexpresses GABA. Very few gad1/2-negative dopaminergic groups, however, express vglut2 instead and use glutamate as a second transmitter. These glutamatergic dual transmitter phenotypes are the Orthopedia transcription factor dependent, A11-type dopaminergic neurons of the posterior tuberculum. Altogether, our results demonstrate that all catecholaminergic groups in zebrafish are either GABA- or glutamatergic. Thus, cotransmission of dopamine and noradrenaline with either GABA or glutamate appears to be a regular feature of zebrafish catecholaminergic systems. We compare our results with those that have been described for mammalian systems, discuss the phenomenon of transmitter dualism in the context of developmental specification of GABAergic and glutamatergic regions in the brain, and put this phenomenon in an evolutionary perspective.