Arcuate dopaminergic/GABAergic neurons project within the hypothalamus and to the median eminence.

Co-transmission, meaning the release of multiple neurotransmitters from one synapse, allows for increased diversity of signaling in the brain. Dopamine (DA) and γ-Aminobutyric acid (GABA) are known to co-express in many regions such as the olfactory bulb and the ventral tegmental area. Tuberoinfundibular dopaminergic neurons (TIDA) in the arcuate nucleus of the hypothalamus (Arc) project to the median eminence (ME) and regulate prolactin release from the pituitary, and prior work suggests dopaminergic Arc neurons also co-transmit GABA. However, the extent of co-transmission, and the projection patterns of these neurons have not been fully revealed. Here we used a genetic intersectional reporter expression approach to selectively label cells that express both tyrosine hydroxylase (TH), and vesicular GABA transporter (Vgat). Through this approach, we identified cells capable of both dopamine (DA) and GABA co-transmission in the Arc, periventricular (Pe), paraventricular (Pa), ventromedial, and the dorsolateral hypothalamic nuclei, in addition to a novel population in the caudate putamen. The highest density of labeled cells was in the Arc, 6.68% of DAPI-labeled cells at -2.06mm Bregma, and in the Pe, 2.83% of DAPI-labeled cells at -1.94mm Bregma. Next, we evaluated the projections of these DA/GABA cells by injecting an mCherry virus that fluoresces in DA/GABA cells. We observed a co-transmitting DA/GABA population, with projections within the Arc, and to the Pa and ME. These data suggest DA/GABA Arc neurons are involved in prolactin release as a subset of TIDA neurons. Further investigation will elucidate the interactions of dopamine and GABA in the hypothalamus.