Haemorrhage-evoked compensation and decompensation are mediated by distinct caudal midline medullary regions in the urethane-anaesthetised rat

• Published in: Neuroscience Vol. 113; no. 3; pp. 555 - 567
• Main Authors: Heslop, D.J, Keay, K.A, Bandler, R
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.01.2002; Elsevier
• Subjects: anaesthesia; Anesthetics, Inhalation - pharmacology; Anesthetics, Intravenous - pharmacology; Anesthetics, Local - pharmacology; Animals; arterial pressure; Biological and medical sciences; blood loss; Blood Pressure - drug effects; bradycardia; Excitatory Amino Acids - pharmacology; Fundamental and applied biological sciences. Psychology; Halothane - pharmacology; Heart Rate - drug effects; Hemorrhage - physiopathology; hypertension; hypotension; Hypotension - physiopathology; hypovolaemia; Lidocaine - pharmacology; Male; Medulla Oblongata - drug effects; Medulla Oblongata - physiopathology; Microinjections; Motor control and motor pathways. Reflexes. Control centers of vegetative functions. Vestibular system and equilibration; Pressoreceptors - drug effects; raphe nuclei; Raphe Nuclei - physiopathology; Rats; Rats, Sprague-Dawley; shock; Sympathetic Nervous System - drug effects; Urethane - pharmacology; Vertebrates: nervous system and sense organs; hypotension; EAA, excitatory amino acid; SPN, sympathetic preganglionic neurone; blood loss; DLH, D,L-homocysteic acid; shock; HR, heart rate; CMM, caudal midline medulla; arterial pressure; raphe nuclei; PB, phosphate buffer; WGA-HRP, wheatgerm agglutinin conjugated to horseradish peroxidase; NTS, nucleus of the solitary tract; CVLM, caudal ventrolateral medulla; MAP, mean arterial pressure; bradycardia; AP, arterial pressure; 5-HT, 5-hydroxytryptamine; IML, intermediolateral cell column; RVLM, rostral ventrolateral medulla; hypertension; hypovolaemia; anaesthesia; Rat; Rodentia; Central nervous system; Sympathetic nervous system; Hemorrhage; Vertebrata; Mammalia; Anesthetic; Autonomic nervous system; Arterial pressure; Raphe nucleus; Hemodynamics; Brain (vertebrata)
• ISSN: 0306-4522; 1873-7544
• DOI: 10.1016/S0306-4522(02)00161-6

Previous research using microinjections of excitatory amino acids suggested that the caudal midline medulla (including nucleus raphe obscurus and nucleus raphe pallidus) contained a mixed population of sympathoexcitatory and sympathoinhibitory neurones. The results of this study indicate that different anaesthetic regimes (urethane versus halothane) determine whether sympathoexcitatory (urethane only) or sympathoinhibitory (halothane only) responses are evoked by stimulation within distinct caudal midline medullary regions. In addition, anaesthetic regimes also affect the caudal midline medullary-mediated response to haemorrhage. Specifically, under conditions of urethane anaesthesia, inactivation (lignocaine) of the midline medullary region immediately caudal to the obex, prematurely triggered and dramatically potentiated the hypotension and bradycardia evoked by 15% haemorrhage; whereas under halothane anaesthesia, inactivation of the same region had no effect. In contrast, under urethane anaesthesia, inactivation of the midline medullary region immediately rostral to the obex, delayed the onset of the hypotension and bradycardia to 15% haemorrhage; inactivation of the same region under halothane anaesthesia blocked haemorrhage-evoked hypotension and bradycardia. Our findings indicate that topographically distinct parts of the caudal midline medulla contain neurones (i) that differentially regulate the timing and magnitude of the compensatory (normotensive) versus decompensatory (hypotensive) phases of the response to haemorrhage; and (ii) whose activity is altered by urethane versus halothane anaesthesia.