Location and electrophysiological characterization of rostral medullary adrenergic neurons that contain neuropeptide Y mRNA in rat medulla

The objective of this study was to characterize the projection pattern and electrophysiological properties of the rostral medullary adrenergic neurons (C1) that express neuropeptide Y (NPY) mRNA in rat. NPY mRNA was found in a variable fraction of tyrosine hydroxylase immunoreactive (TH‐IR) neurons depending on the medullary level. By retrograde labeling (Fast Blue, FluoroGold), NPY mRNA was detected in virtually all C1 cells (96%) and C3 cells (100%) with hypothalamic projections but in only 9% of C1 cells and 58% of C3 cells projecting to thoracic segment 3 (T3) or T6 of the spinal cord. To identify the electrophysiological properties of the C1 cells that express NPY mRNA, we recorded from baroinhibited neurons within the C1 region of the ventrolateral medulla (RVLM) and tested for projections to segment T3, the hypothalamus, or both. By using the juxtacellular method, we labeled these cells with biotinamide and determined whether the recorded neurons were TH‐IR and contained NPY mRNA. At rostral levels (Bregma −11.8 mm), barosensitive neurons had a wide range of conduction velocities (0.4–6.0 m/second) and discharge rates (2–28 spikes/second). Most projected to T3 only (27 of 31 cells), and 4 projected to both the hypothalamus and the spinal cord. Most of the baroinhibited cells with spinal projections but with no hypothalamic projections had TH‐IR but no NPY mRNA (11 of 17 cells). Only 1 cell had both (1 of 17 cells), and 5 cells had neither (5 of 17 cells). Both TH‐IR and NPY mRNA were found in neurons with dual projections (2 of 2 cells). At level Bregma −12.5 mm, baroinhibited neurons had projections to the hypothalamus only (13 of 13 cells) and had unmyelinated axons and a low discharge rate. Four of five neurons contained both TH‐IR and NPY mRNA, and 1 neuron contained neither. In short, NPY is expressed mostly by C1 cells with projection to the hypothalamus. NPY‐positive C1 neurons are barosensitive, have unmyelinated axons, and have a very low rate of discharge. Most bulbospinal C1 cells with a putative sympathoexcitatory role do not make NPY. J. Comp. Neurol. 415:482–500, 1999. © 1999 Wiley‐Liss, Inc.