Neurokinin B-expressing neurons of the central extended amygdala mediate inhibitory synaptic input onto melanin-concentrating hormone neuron subpopulations

Abstract

The lateral hypothalamic area (LHA) is a highly conserved brain region critical for maintaining physiological homeostasis and goal-directed behavior. LHA neurons that express melanin-concentrating hormone (MCH) are key regulators of arousal, energy balance, and motivated behavior. However, cellular and functional diversity among LHAMCH neurons is not well understood. Previous anatomic and molecular data suggest that LHAMCH neurons may be parsed into at least two distinct subpopulations, one of which is enriched in neurokinin-3 receptor (NK3R), the receptor for neurokinin B (NKB), encoded by the Tac2 gene. This tachykininergic ligand-receptor system has been implicated in reproduction, fear memory, and stress in other brain regions, but NKB interactions with LHAMCH neurons are poorly understood. We first identified how LHAMCH subpopulations may be distinguished anatomically and electrophysiologically. To dissect functional connectivity between NKB-expressing neurons and LHAMCH neurons, we used Cre-dependent retrograde and anterograde viral tracing in male Tac2-Cre mice and identified Tac2/EYFP1 neurons in the bed nucleus of the stria terminalis and central nucleus of the amygdala, the central extended amygdala, as major sources of NKB input onto LHAMCH neurons. In addition to innervating the LHA, these limbic forebrain NKB neurons also project to midbrain and brainstem targets. Finally, using a dual-virus approach, we found that optogenetic activation of these inputs in slices evokes GABA release onto a subset of LHAMCH neurons but lacked specificity for the NK3R1 subpopulation. Overall, these data define parallel tachykininergic/GABAergic limbic forebrain projections that are positioned to modulate multiple nodes of homeostatic and behavioral control.