title: A mapping study of cardiorespiratory responses to chemical stimulation of the midline medulla oblongata in ventilated and freely breathing rats.
abstract: The aim of this study was to examine the cardiorespiratory effects of chemically stimulating neurons in the midline medulla oblongata (MM) of artificially ventilated and freely breathing anesthetized rats. Earlier studies reported that stimulation of the MM elicits increases or decreases in mean arterial pressure (MAP) and phrenic nerve activity, depending on the mode and site of stimulation, anesthetic, and species. In the first series of experiments, rats were anesthetized with urethane, artificially ventilated, paralyzed, and bilaterally vagotomized. The rostrocaudal extent of the MM was mapped by microinjections of DL-homocysteic acid or L-glutamate (both 100 mM, 100 nl), and, in line with previous studies, most injections produced only small responses in MAP, heart rate, and splanchnic sympathetic nerve activity. Increases in respiratory parameters were evoked in caudal regions. However, activation of a discrete region of the MM at the level of the caudal pole of the facial nucleus (CP7) consistently caused a dramatic reduction in phrenic nerve amplitude and/or frequency and, in six rats, produced a prolonged apnea. The second series of experiments was carried out on freely breathing pentobarbitone sodium-anesthetized rats, with a diaphragmatic electromyogram used to monitor respiratory activity. Respiratory activity could again be abolished at CP7 after microinjections of glutamate (100 mM, 50 nl); however, these responses were accompanied by large decreases in MAP and moderate reductions in heart rate. This depression of respiratory activity may be due to activation of propriobulbar inhibitory neurons that project to known respiratory centers in the brain stem.
authors.affiliation: Hypertension and Stroke Research Laboratories, Department of Physiology, University of Sydney, Royal North Shore Hospital, St. Leonards NSW 2065, Australia.
authors.forename: Todd A
authors.initials: TA
authors.lastname: Verner
authors.forename: Ann K
authors.initials: AK
authors.lastname: Goodchild
authors.forename: Paul M
authors.initials: PM
authors.lastname: Pilowsky
mesh_terms: D000759:Adjuvants, Anesthesia; D018686:Anesthetics, Intravenous; D000818:Animals; D001049:Apnea / Q000139:chemically induced / Q000503:physiopathology; D001919:Bradycardia / Q000139:chemically induced / Q000503:physiopathology; D001931:Brain Mapping; D018698:Glutamic Acid / Q000494:pharmacology*; D006710:Homocysteine / Q000494:pharmacology; D008297:Male; D008526:Medulla Oblongata / Q000187:drug effects* / Q000502:physiology*; D008845:Microinjections; D010424:Pentobarbital; D010791:Phrenic Nerve / Q000187:drug effects / Q000502:physiology; D011903:Raphe Nuclei / Q000187:drug effects / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012121:Respiration, Artificial; D015656:Respiratory Mechanics / Q000187:drug effects* / Q000502:physiology; D013268:Stimulation, Chemical; D014520:Urethane
chemical_list: D000759:Adjuvants, Anesthesia; D018686:Anesthetics, Intravenous; D006710:Homocysteine; D014520:Urethane; D018698:Glutamic Acid; D010424:Pentobarbital