In vivo gene transfer demonstrates that endogenous eNOS activity in the nucleus tractus solitarii depresses the baroreceptor reflex in conscious rats

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  • In vivo gene transfer demonstrates that endogenous eNOS activity in the nucleus tractus solitarii depresses the baroreceptor reflex in conscious rats

University of Central Lancashire / University of Liverpool (2002) J Physiol 543P, S235

Communications: In vivo gene transfer demonstrates that endogenous eNOS activity in the nucleus tractus solitarii depresses the baroreceptor reflex in conscious rats

Hidefumi Waki*†, Sergey Kasparov*, David Murphy‡, Tsuyoshi Shimizu† and Julian F.R. Paton*

*Department of Physiology, School of Medical Sciences, University of Bristol, Bristol BS8 1TD, †Department of Physiology, Fukushima Medical University School of Medicine, Fukushima 960-1295, Japan and ‡University Research Centre for Neuroendocrinology, University of Bristol, Bristol Royal Infirmary, Bristol BS2 8HW, UK

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In a decerebrate rat model, the working heartÐbrainstem preparation, we demonstrated that angiotensin II activation of endothelial nitric oxide synthase (eNOS) within the nucleus tractus solitarii (NTS) depressed the baroreceptor reflex (Paton et al. 2001). In the present study, we have assessed whether endogenous eNOS activity in the NTS plays a role in determining the gain of the baroreceptor reflex in conscious rats.

A recombinant adenoviral vector directing expression of a dominant negative mutant form of eNOS (so-called TeNOS; Lee et al. 1995) was microinjected bilaterally into the NTS of rats to disable endogenous eNOS activity. We used radio-telemetry to acquire arterial pressure data and from this determined the spontaneous baroreceptor reflex gain (sBRG; see Oosting et al. 1997) before, and 4 weeks following, viral transfection. Both transmitter implantation and transfections were performed under anaesthesia using ketamine (60 mg kg-1) and medetomidine (250 mg kg-1) given I.M. and reversed with a subcutaneous injection of atipamezole (1 mg kg-1). Changes in the sBRG induced by both exercise (wheel running, 6 m min-1 X 10 min) and exposure to stress, caused by placing the rats in smaller cages, were assessed. Control groups received NTS microinjection of either an adenovirus that expressed enhanced green fluorescent protein (eGFP) or saline. TeNOS was detected in the NTS immunocytochemically.

There was no alteration in the basal sBRG in either control group during the 4 week observation period (eGFP, n = 6; saline, n = 4). However, over the same time course the resting sBRG increased gradually up to the third week from 1.04 ± 0.1 to 1.52 ± 0.3 ms mmHg-1 (mean ± S.E.M., n = 6, Student’s paired t test, P < 0.05) in NTS transfected rats. Despite this increase in baseline sBRG, dynamic exercise decreased this to sBRG to 0.51 ± 0.07 ms mmHg-1 in transfected rats, which was not different from the level measured prior to transfection (i.e. 0.50 ± 0.05 ms mmHg-1; n = 6). Similarly, stress reduced the sBRG to the same level before and after NTS transfection (i.e. 0.76 ± 0.08 vs. 0.87 ± 0.20 ms mmHg-1, n.s.). In all cases we found greater TeNOS immunoreactivity in the NTS of rats expressing the eNOS mutant protein versus those either expressing eGFP or microinjected with saline.

These results indicate the presence of endogenous eNOS activity in the NTS that controls the set-point level of the baroreceptor reflex gain. However, disabling eNOS activity does not alter the set-point gain reached during acute changes in baroreceptor reflex function that accompany exercise and stress.

The British Heart Foundation, Wellcome Trust and Japan Space Forum funded research.

All procedures accord with current UK legislation.

Where applicable, experiments conform with Society ethical requirements.

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