• hypocretin;
  • hypothalamus;
  • mouse;
  • orexin


Orexin/hypocretin (orx/hcrt) neurons are thought to ensure that reward-seeking is accompanied by alertness, but the underlying circuit organization is unclear. Reports of differential regulation of lateral versus medial orx/hcrt cells produced a hypothesis of ‘efferent dichotomy’, in which lateral orx/hcrt cells innervate the ventral tegmental area (VTA) and control reward, while medial orx/hcrt cells innervate locus coeruleus (LC) and control arousal. Two distinct types of orx/hcrt cells also emerged from analysis of intrinsic and input-driven single-cell electrical activity. To examine the projections of these emerging orx/hcrt subtypes to LC and VTA, we injected retrograde tracer into these regions in the mouse brain in vivo, and then examined the properties of tracer-containing orx/hcrt cells in hypothalamic slices. VTA- and LC-projecting orx/hcrt cells were found across the entire orx/hcrt field, including the zona incerta, perifornical area, dorsomedial/anterior and lateral hypothalamus. Within these areas, orx/hcrt cells had similar probabilities of projecting to VTA or LC. Examination of lateral versus medial sections revealed that VTA and LC received inputs from both lateral and medial orx/hcrt cells, but, unexpectedly, lateral orx/hcrt cells were more likely to project to LC than medial orx/hcrt cells. Finally, patch-clamp recordings revealed that VTA and LC received projections from both electrical classes of orx/hcrt cells, which had similar likelihoods of projecting to VTA or LC. Contrary to previous predictions, our data suggest that medial and lateral orx/hcrt cells, and the different electrical and morphological subclasses of orx/hcrt cells identified to date, send projections to both LC and VTA.