Chronic intermittent hypoxia impairs heart rate responses to AMPA and NMDA and induces loss of glutamate receptor neurons in nucleus ambiguus of F344 rats

1 Biomolecular Science Center, Burnett School of Biomedical Sciences, College of Medicine, University of Central Florida, Orlando, Florida; 2 Department of Physiology, Harbin Medical University, Harbin, China; 3 Department of Pediatrics, Kosair Children's Hospital Research Institute, and 4 Department of Physiology and Biophysics, University of Louisville, Kentucky; and 5 Department of Physiology, Loyola University Stritch School of Medicine, Maywood, Illinois Submitted 7 May 2008 ; accepted in final form 28 October 2008 Chronic intermittent hypoxia (CIH), as occurs in sleep apnea, impairs baroreflex-mediated reductions in heart rate (HR) and enhances HR responses to electrical stimulation of vagal efferent. We tested the hypotheses that HR responses to activation of -amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and N -methyl- D -aspartate (NMDA) receptors in the nucleus ambiguus (NA) are reduced in CIH-exposed rats and that this impairment is associated with degeneration of glutamate receptor (GluR)-immunoreactive NA neurons. Fischer 344 rats (3–4 mo) were exposed to room air (RA) or CIH for 35–50 days ( n = 18/group). At the end of the exposures, AMPA (4 pmol, 20 nl) and NMDA (80 pmol, 20 nl) were microinjected into the same location of the left NA (–200 µm to +200 µm relative to caudal end of area postrema; n = 6/group), and HR and arterial blood pressure responses were measured. In addition, brain stem sections at the level of –800, –400, 0, +400, and +800 µm relative to obex were processed for AMPA and NMDA receptor immunohistochemistry. The number of NA neurons expressing AMPA receptors and NMDA receptors (NMDARs) was quantified. Compared with RA, we found that after CIH 1 ) HR responses to microinjection of AMPA into the left NA were reduced (RA –290 ± 30 vs. CIH –227 ± 15 beats/min, P < 0.05); 2 ) HR responses to microinjection of NMDA into the left NA were reduced (RA –302 ± 16 vs. CIH –238 ± 27 beats/min, P < 0.05); and 3 ) the number of NMDAR1, AMPA GluR1, and AMPA GluR2/3-immunoreactive cells in the NA was reduced ( P < 0.05). These results suggest that degeneration of NA neurons expressing GluRs contributes to impaired baroreflex control of HR in rats exposed to CIH. brain stem; parasympathetic efferent; heart; baroreflex; sleep apnea Address for reprint requests and other correspondence: Z. J. Cheng, BMS Bldg. 20, Rm. 230, Biomolecular Science Center, Burnett School of Biomedical Sciences, College of Medicine, Univ. of Central Florid