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 (C(1)) 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 C(1) cells (96%) and C(3) cells (100%) with hypothalamic projections but in only 9% of C(1) cells and 58% of C(3) cells projecting to thoracic segment 3 (T(3)) or T(6) of the spinal cord. To identify the electrophysiological properties of the C(1) cells that express NPY mRNA, we recorded from baroinhibited neurons within the C(1) region of the ventrolateral medulla (RVLM) and tested for projections to segment T(3), 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 T(3) 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 C(1) cells with projection to the hypothalamus. NPY-positive C(1) neurons are barosensitive, have unmyelinated axons, and have a very low rate of discharge. Most bulbospinal C(1) cells with a putative sympathoexcitatory role do not make NPY. Copyright 1999 Wiley-Liss, Inc.