preproenkephalin and Hypotension

Previous studies have demonstrated that a range of stimuli activate neurons, including catecholaminergic neurons, in the ventrolateral medulla. Not all catecholaminergic neurons are activated and other neurochemical content is largely unknown hence whether stimulus specific populations exist is unclear. Here we determine the neurochemistry (using in situ hybridization) of catecholaminergic and noncatecholaminergic neurons which express c-Fos immunoreactivity throughout the rostrocaudal extent of the ventrolateral medulla, in Sprague Dawley rats treated with hydralazine or saline. Distinct neuronal populations containing PPCART, PPPACAP, and PPNPY mRNAs, which were largely catecholaminergic, were activated by hydralazine but not saline. Both catecholaminergic and noncatecholaminergic neurons containing preprotachykinin and prepro-enkephalin (PPE) mRNAs were also activated, with the noncatecholaminergic population located in the rostral C1 region. Few GlyT2 neurons were activated. A subset of these data was then used to compare the neuronal populations activated by 2-deoxyglucose evoked glucoprivation (Brain Structure and Function (2015) 220:117). Hydralazine activated more neurons than 2-deoxyglucose but similar numbers of catecholaminergic neurons. Commonly activated populations expressing PPNPY and PPE mRNAs were defined. These likely include PPNPY expressing catecholaminergic neurons projecting to vasopressinergic and corticotrophin releasing factor neurons in the paraventricular nucleus, which when activated result in elevated plasma vasopressin and corticosterone. Stimulus specific neurons included noncatecholaminergic neurons and a few PPE positive catecholaminergic neuron but neurochemical codes were largely unidentified. Reasons for the lack of identification of stimulus specific neurons, readily detectable using electrophysiology in anaesthetized preparations and for which neural circuits can be defined, are discussed.

Topics: Animals; Antihypertensive Agents; Catecholamines; Deoxyglucose; Enkephalins; Gene Expression Regulation; Glycine Plasma Membrane Transport Proteins; Hydralazine; Hypotension; Male; Medulla Oblongata; Nerve Tissue Proteins; Neurochemistry; Neurons; Neuropeptide Y; Pituitary Adenylate Cyclase-Activating Polypeptide; Protein Precursors; Proto-Oncogene Proteins c-fos; Rats; Rats, Sprague-Dawley; Tachykinins

According to prior evidence opioid and serotonin release by lower brain stem neurons may contribute to hemorrhage-induced sympathoinhibition (HISI). Here we seek direct evidence for the activation of opioidergic, GABAergic, or serotonergic neurons by severe hemorrhage in the medulla oblongata. Blood was withdrawn from awake rats (40-50% total volume) causing hypotension and profound initial bradycardia. Other rats received the vasodilator hydralazine, causing tachycardia and hypotension. Neuronal activation was gauged by the presence of Fos-immunoreactive (ir) nuclei after 2 h. Serotonergic, enkephalinergic, and GABAergic neurons were identified by the presence of a diagnostic enzyme or mRNA. Hemorrhaged rats had 30% fewer non-GABAergic Fos-ir neurons in the rostral ventrolateral medulla (RVLM) than hydralazine-treated rats, but they had six times more Fos-ir neurons within the subependymal parapyramidal nucleus (SEPPN). Fos-labeled SEPPN neurons were serotonergic (40-60%), GABAergic (31%), enkephalinergic (15%), or had mixed phenotypes. The data suggest that a reduced sympathoexcitatory drive from RVLM may contribute to HISI. SEPPN neuronal activation may also contribute to HISI or could mediate defensive thermoregulatory mechanisms triggered by hemorrhage-induced hypothermia.

Topics: Animals; Blood Pressure; Enkephalins; gamma-Aminobutyric Acid; Heart Rate; Hydralazine; Hypotension; Male; Medulla Oblongata; Neurons; Phenotype; Protein Precursors; Proto-Oncogene Proteins c-fos; Rats; Rats, Sprague-Dawley; RNA, Messenger; Serotonin; Shock, Hemorrhagic; Vasodilator Agents

The autonomic regions of the thoracolumbar spinal cord receive a dense enkephalinergic (ENK) innervation from supraspinal sources, including the rostral ventrolateral medulla (RVLM). In the present study, we sought to determine 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 retrogradely labeled throughout the ventrolateral medulla. At the most rostral RVLM level, 29% of bulbospinal PPE+ cells were tyrosine hydroxylase-immunoreactive (TH-ir) and the latter constituted 19.4% of the bulbospinal TH-ir cells. We determined whether the bulbospinal PPE(+) RVLM neurons are barosensitive in two ways. First, we examined Fos production by FG-labeled RVLM neurons after 2 hours of hydralazine-induced hypotension (to 73 +/- 2 mm Hg) in conscious rats. Hydralazine (10 mg/kg i.v.) increased the number of Fos-ir neurons by two- to eightfold at all levels of the ventrolateral medulla examined. 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 anesthetized rats and filled them juxtacellularly with biotinamide. Most biotinamide-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 is predominantly expressed by RVLM BSBS neurons with lightly myelinated spinal 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.

Topics: Anesthesia; Animals; Blood Pressure; Consciousness; Enkephalins; Gene Expression; Hypotension; In Situ Hybridization; Male; Medulla Oblongata; Neural Pathways; Neurons; Pressoreceptors; Protein Precursors; Proto-Oncogene Proteins c-fos; Rats; Rats, Sprague-Dawley; RNA, Messenger; Spinal Cord; Sympathetic Nervous System; Tyrosine 3-Monooxygenase