Neurochemically and Hodologically Distinct Ascending VGLUT3 versus Serotonin Subsystems Comprise the r2-Pet1 Median Raphe.

Brainstem median raphe (MR) neurons expressing the serotonergic regulator gene Pet1 send collateralized projections to forebrain regions to modulate affective, memory-related, and circadian behaviors. Some Pet1 neurons express a surprisingly incomplete battery of serotonin pathway genes, with somata lacking transcripts for tryptophan hydroxylase 2 (Tph2) encoding the rate-limiting enzyme for serotonin [5-hydroxytryptamine (5-HT)] synthesis, but abundant for vesicular glutamate transporter type 3 (Vglut3) encoding a synaptic vesicle-associated glutamate transporter. Genetic fate maps show these nonclassical, putatively glutamatergic Pet1 neurons in the MR arise embryonically from the same progenitor cell compartment--hindbrain rhombomere 2 (r2)--as serotonergic TPH2+ MR Pet1 neurons. Well established is the distribution of efferents en masse from r2-derived, Pet1-neurons; unknown is the relationship between these efferent targets and the specific constituent source-neuron subgroups identified as r2-Pet1Tph2-high versus r2-Pet1Vglut3-high. Using male and female mice, we found r2-Pet1 axonal boutons segregated anatomically largely by serotonin1 versus VGLUT31 identity. The former present in the suprachiasmatic nucleus, paraventricular nucleus of the thalamus, and olfactory bulb; the latter are found in the hippocampus, cortex, and septum. Thus r2-Pet1Tph2-high and r2-Pet1Vglut3-high neurons likely regulate distinct brain regions and behaviors. Some r2-Pet1 boutons encased interneuron somata, forming specialized presynaptic "baskets" of VGLUT31 or VGLUT3+/5-HT+ identity; this suggests that some r2-Pet1Vglut3-high neurons may regulate local networks, perhaps with differential kinetics via glutamate versus serotonin signaling. Fibers from other Pet1 neurons (non-r2-derived) were observed in many of these same baskets, suggesting multifaceted regulation. Collectively, these findings inform brain organization and new circuit nodes for therapeutic considerations. [ABSTRACT FROM AUTHOR]