Preproenkephalin mRNA is expressed by C1 and non-C1 barosensitive bulbospinal neurons in the rostral ventrolateral medulla of the rat

The autonomic regions of the thoracolumbar spinal cord receive a dense enke phalinergic (ENK) innervation from supraspinal sources, including the rostr al ventrolateral medulla (RVLM). In the present study, we sought to determi ne whether the barosensitive bulbospinal (BSBS) neurons of the RVLM express preproenkephalin (PPE) mRNA. After injection of Fluoro-Gold (FG) into the upper thoracic spinal cord, neurons with PPE mRNA (PPE+ neurons) were retro gradely labeled throughout the ventrolateral medulla. At the most rostral R VLM level, 29% of bulbospinal PPE + cells were tyrosine hydroxylase-immunor e active (TH-ir) and the latter constituted 19.4% of the bulbospinal TH-ir cells. We determined whether the bulbospinal PPE+ RVLM neurons are barosens itive in two ways. First, we examined Fos production by FG-labeled RVLM neu rons after 2 hours of hydralazine-induced hypotension (to 73 +/- 2 mm Hg) i n conscious rats. Hydralazine (10 mg/kg i.v.) increased the number of Fos-i r neurons by two- to eightfold at all levels of the ventrolateral medulla e xamined. In the RVLM, 54% of bulbospinal PPE+ neurons were Fos-ir, whereas such cells were more rarely found at caudal ventrolateral medullary levels. Second, we recorded individual BSBS RVLM units extracellularly in anesthet ized rats and filled them juxtacellularly with biotinamide. Most biotinamid e-filled neurons were PPE+ (10 of 17), and the PPE+ BSBS cells had a faster axonal conduction velocity than those without PPE mRNA (4.2 vs. 0.67 m/sec ). Four of the 10 PPE+ BSBS RVLM neurons were TH-ir. In summary, PPE mRNA i s predominantly expressed by RVLM BSBS neurons with lightly myelinated spin al axons. PPE mRNA is present in most noncatecholaminergic BSBS neurons and also in approximately 20% of the bulbospinal C1 neurons. BSBS RVLM neurons most likely provide a major ENK input to sympathetic preganglionic neurons and PPE mRNA is the first identified positive phenotype of the non-C1 BSBS RVLM neurons. (C) 2001 Wiley-Liss, Inc.