NPY is Co‐localized with TH in Ventrolateral Medullary Neurons Projecting to the Hypothalamus in Mice

The catecholamine (CA) neurons of the ventrolateral medulla (VLM) are activated by many visceral stressors, including hypovolemia, hypoxia, hypoglycemia and infection and their activation results in equally diverse physiological output, including elevated BP, ventilation, arousal and neuroendocrine activation. In the rat, we previously described two subsets of VLM‐CA neurons: spinal‐ and hypothalamic‐projecting. In the current study we have identified the same subsets in mice. We distinguished between two VLM‐CA subtypes by rostral‐caudal location and neuropeptide Y (NPY) expression. In mice, Fluorogold (FG) was injected into either the spinal cord or paraventricular nucleus of the hypothalamus to identify spinal‐projecting and hypothalamic‐projecting VLM‐CA neurons, respectively. After allowing 5‐7 days for retrograde transport, the mice were perfused and brains processed for combined immunohistochemistry and in situ hybridization (ISH) to visualize VLM‐CA neuronal somata (TH‐ir) containing FG and NPY (ISH). We found that more than half (66.1 ± 2.5 %, n = 8) of the VLM‐CA neurons are positive for NPY mRNA. Spinal‐projecting VLM‐CA neurons generally resided more rostral, just behind and medial to the facial nucleus, and did not express NPY mRNA (0.5 ± 0.3 %, n = 4).Conversely, hypothalamic‐projecting VLM‐CA neurons were located slightly more caudal and most expressed NPY mRNA (63.8 ± 19.6 %, n = 3). In addition, over 90 % (n = 8) of NPY‐positive neurons co‐expressed TH, indicating that, within the mouse VLM, NPY is predominantly expressed by VLM‐CA neurons. NPY expression is a selective marker for hypothalamic‐projecting VLM‐CA neurons in mice as well as rats.