Attenuation of aortic baroreflex responses by microinjections of endomorphin-2 into the rostral ventrolateral medullary pressor area of the rat

• Published in: American journal of physiology. Regulatory, integrative and comparative physiology Vol. 289; no. 1; pp. R59 - R67
• Main Authors: Kasamatsu, Ken, Sapru, Hreday N
• Format: Journal Article
• Language: English
• Published: United States 01.07.2005
• Subjects: Animals; Aorta - innervation; Aorta - physiology; Baroreflex - drug effects; Blood Pressure - physiology; Dose-Response Relationship, Drug; Electric Stimulation; Heart Rate - drug effects; Heart Rate - physiology; Male; Medulla Oblongata - drug effects; Medulla Oblongata - physiology; Microinjections; Naloxone - administration & dosage; Naloxone - analogs & derivatives; Naloxone - pharmacology; Nervous System Physiological Phenomena - drug effects; Oligopeptides - administration & dosage; Oligopeptides - pharmacology; Rats; Rats, Wistar; Receptors, Opioid, mu - agonists; Receptors, Opioid, mu - physiology; Vagus Nerve - physiology
• ISSN: 0363-6119; 1522-1490
• DOI: 10.1152/ajpregu.00007.2005

Department of Neurological Surgery, University of Medicine and Dentistry of New Jersey-New Jersey Medical School, Newark, New Jersey Submitted 6 January 2005 ; accepted in final form 12 February 2005 The presence of µ-opioid receptors and endomorphins has been demonstrated in the general area encompassing the rostral ventrolateral medullary pressor area (RVLM). This investigation was carried out to test the hypothesis that endomorphins in the RVLM may have a modulatory role in regulating cardiovascular function. Blood pressure and heart rate (HR) were recorded in urethane-anesthetized male Wistar rats. Unilateral microinjections of endomorphin-2 (0.0125–0.5 mmol/l) into the RVLM elicited decreases in mean arterial pressure (16–30 mmHg) and HR (12–36 beats/min), which lasted for 2–4 min. Bradycardia was not vagally mediated. The effects of endomorphin-2 were mediated via µ-opioid receptors because prior microinjections of naloxonazine (1 mmol/l) abolished these responses; the blocking effect of naloxonazine lasted for 15–20 min. Unilateral stimulations of aortic nerve for 30 s (at frequencies of 5, 10, and 25 pulses/s; each pulse 0.5 V and 1-ms duration) elicited depressor and bradycardic responses. These responses were significantly attenuated by microinjections of endomorphin-2 (0.2 and 0.4 mmol/l). The inhibitory effect of endomorphin-2 on baroreflex responses was prevented by prior microinjections of naloxonazine. Microinjections of naloxonazine alone did not affect either baseline blood pressure and HR or baroreflex responses. These results indicate that endomorphin-2 elicits depressor and bradycardic responses and inhibits baroreflex function when injected into the RVLM. These effects are consistent with the known hyperpolarizing effect of opioid peptides on RVLM neurons. bradycardia; depressor responses; naloxonazine; opioid peptides Address for reprint requests and other correspondence: H. N. Sapru, Dept. of Neurological Surgery, MSB H-586, UMDNJ-New Jersey Medical School, 185 South Orange Ave., Newark, NJ 07103 (e-mail: sapru{at}umdnj.edu )