Hemispheric specializations are well studied at the functional level but less is known about the underlying neural mechanisms. We identified a small cluster of cholinergic neurons in the right dorsal habenula (dHb) of zebrafish, defined by their expression of the
lecithin retinol acyltransferase domain containing 2a
) gene and their efferent connections with a subregion of the ventral interpeduncular nucleus (vIPN). The unilateral
-expressing neurons are innervated by a subset of mitral cells from both the left and right olfactory bulb and are activated upon exposure of adult zebrafish to the aversive odorant cadaverine that provokes avoidance behavior. Using an intersectional strategy to drive expression of the botulinum neurotoxin specifically in these neurons, we find that adults no longer show protracted avoidance to cadaverine. Mutants with left-isomerized dHb that lack these neurons are less repelled by cadaverine and their behavioral response to alarm substance, a potent aversive cue, is diminished. However mutants in which both dHb have right identity appear more reactive to alarm substance. The results implicate an asymmetric dHb-vIPN neural circuit in processing of aversive olfactory cues and modulating resultant behavioral responses.