Neurochemical characterization of melanin-concentrating hormone neurons in the mouse lateral hypothalamus

Abstract

Melanin concentrating hormone (MCH) is an orexigenic neuropeptide located in the lateral hypothalamus. Cells expressing MCH are heterogenous and can be marked by their coexpression of cocaine- and amphetamine-regulated transcript (CART) or the neurokinin 3 receptor (NK3R). Transcriptomic studies show that one-third of MCH neurons coexpress both CART and NK3R. However, the distribution of this MCH subpopulation has not yet been mapped in the mouse brain. We determined the percentage of MCH neurons that coexpress CART and/or NK3R and mapped their distribution to Allen Reference Atlas mouse brain templates. We identified MCH neurons by native EGFP fluorescence (EGFP-f) in Mch-cre;L10-Egfp mice and then labeled CART and NK3R immunoreactivity. We found that 96% of EGFP-f cells were MCH-positive. Within the lateral hypothalamus, EGFP-f cells may be clustered medial or lateral to the fornix. Nearly half (49%) of the EGFP-f cells counted expressed EGFP-f only and were most commonly found lateral to the fornix. In contrast, 47% of EGFP-f neurons coexpressed CART, and these cells were more common medial to the fornix. Of the CART-positive EGFP-f cells, half of them also coexpressed NK3R, which appeared evenly throughout the lateral hypothalamus. Lastly, 4% of EGFP-f neurons coexpressed NK3R only. Our results indicate a robust heterogeneity within MCH neurons, and we will expand our analysis to determine if these MCH subpopulations also express distinct electrical fingerprints based on their neurochemical properties.