Glutamatergic Heterogeneity in the Neuropeptide Projections From the Lateral Hypothalamus to the Mouse Olfactory Bulb

The direct pathway from the lateral hypothalamus to the mouse olfactory bulb (OB) includes neurons that express the neuropeptide orexin-A and others that do not. The OB-projecting neurons that do not express orexin-A are located in an area of the lateral hypothalamus known to contain neurons that express the neuropeptide melanin-concentrating hormone (MCH). We used virally mediated anterograde tract tracing and immunohistochemistry for orexin-A and MCH to demonstrate that the OB is broadly innervated by axon projections from both populations of neurons with expression in each OB layer across the anterior-to-posterior axis and which overlapped with synaptophysin. Both orexin-A and MCH neurons are genetically heterogeneous, with subsets that co-express an isoform of vesicular glutamate transporter (VGLUT). We used confocal imaging to test whether the projections from orexin-A and MCH neurons to the OB reflect this glutamatergic heterogeneity. The majority of putative orexin-A axon terminals overlapped with VGLUT2, with smaller proportions that co-expressed VGLUT1 or that did not overlap with either VGLUT1 or VGLUT2. In contrast, a smaller proportion of MCH axon terminals overlapped with VGLUT2, with the majority being non-glutamatergic. Therefore, the projections from the lateral hypothalamus to the OB are genetically heterogeneous and include neurons that can release two different neuropeptides. The projections from orexin-A and MCH neuron populations are each genetically heterogeneous, with differing proportions of glutamatergic and non-glutamatergic axon terminals.