Crypt neurons are a third type of olfactory receptor neurons with a highly unusual “one cell type - one receptor” mode of expression, the same receptor being expressed by the entire population of crypt neurons. Attempts to identify the target region(s) of crypt neurons have been inconclusive so far. We report that TrkA-like immunoreactivity specifically labeled somata, axons, and terminals of zebrafish crypt neurons and reveal a single glomerulus, mdg2 of the dorsomedial group, as target glomerulus of crypt neurons. Injection of a fluorescent tracing dye into the mdg2 glomerulus retrogradely labeled mostly crypt neurons, as assessed by quantitative morphometry, whereas no crypt neurons were found after injections in neighboring glomeruli. Our data provide strong evidence that crypt neurons converge onto a single glomerulus, and thus form a labeled line consisting of a single sensory cell type, a single olfactory receptor and a single target glomerulus.

Graphical Abstract Highlights d Most dopaminergic lamprey SNc/VTA neurons express glutamate d Individual co-expressing SNc/VTA neurons project to both striatum and optic tectum d SNc/VTA fibers in the striatum release both dopamine and glutamate d SNc/VTA fibers in the optic tectum release dopamine, but not glutamate erez-Ferná ndez Correspondence sten.grillner@ki.se In Brief von Twickel et al. show that most dopaminergic neurons in the lamprey SNc/VTA co-express glutamate and individual co-expressing neurons project to both the striatum and optic tectum.Remarkably, glutamate co-release is restricted to striatum, indicating differential neurotransmitter release depending on target area.

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