Abstract 635: Knockdown Of Tyrosine Hydroxylase In The Nucleus Of The Solitary Tract (NTS) Reduces Persistent Increase In Blood Pressure During Chronic Intermittent Hypoxia (CIH)

Abstract

A2 neurons in caudal NTS express tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine synthesis, and respond to stressors such as hypoxia. We tested the hypothesis that TH knockdown in NTS reduces cardiovascular responses to CIH, an animal model that mimics the arterial hypoxemia observed during sleep apnea in humans. Adult male Sprague-Dawley rats were implanted with radiotelemetry transmitters and adeno-associated viral constructs (GeneDetect, Inc) with a GFP reporter having either short hair-pin RNA for TH (shRNA) or scrambled virus (scRNA) were injected into caudal NTS. A group of no injection, no CIH (control) rats were also used. Baseline mean arterial pressure (MAP) and heart rate (HR) were collected from all 3 groups for at least 4 days after which the virus injected rats were exposed to 7 days CIH (alternating 3 min periods of 10% O2 and 21% O2 from 8am to 4pm; from 4pm to 8am the rats were exposed to 21% O2). In shRNA injected rats (n=9) 7 day exposure to CIH increased MAP and HR during the day by 6.6±1.8 mmHg and 49.9±7.7 bpm and during the night by 0.3±1.8 mmHg and -4.5±1.3 bpm compared to baseline. In scRNA injected rats (n=10) 7 day exposure to CIH increased MAP and HR during the day by 7.7±1.2 mmHg and 65.3±7.9 bpm and during the night by 4.8±1.5 mmHg and 0.0±4.9 bpm compared to baseline. In control rats (n=8) MAP and HR were 1.5±1.0 mmHg and -8.4±3.8 bpm during the day and 1.8±0.9 mmHg and -10.8±2.9 bpm during the night compared to baseline. CIH induced significant changes in MAP and HR during the day in both the scRNA (p=.006 MAP; p<.001 HR) and shRNA (p=.007 MAP; p<.001 HR) groups. Compared to control rats, during the night CIH induced significant changes in MAP in the scRNA rats (p=0.02) but not in the shRNA group and no significant changes in HR in either group. The number of TH-immunoreactive neurons was reduced by 20% in sections with GFP fluorescence; shRNA 28±1 cells/section compared to scRNA 35±1 cells/section (p=.005). CIH induces an increase in MAP that persists beyond the period of exposure to CIH. Knockdown of TH in the NTS reduces this persistent increase in MAP induced by exposure to CIH. This indicates that NTS A2 neurons play a role in the cardiovascular responses to CIH and suggests they may play a similar role in the pathology of sleep apnea in humans.