Submitted on December 7, 2007
Revised on January 22, 2008
Accepted on February 6, 2008
Vascular Reactivity, 5-HT Uptake and Blood Pressure in the Serotonin Transporter Knockout Rat
Aurea Elizabeth Linder1, Jessica Diaz1, Wei Ni1, Theodora Szasz, Robert Burnett1, and Stephanie W. Watts1*
1 Michigan State University
* To whom correspondence should be addressed. E-mail: wattss{at}msu.edu.
The handling of serotonin (5-hydroxytryptamine, 5-HT) depends on the serotonin transporter (SERT). A SERT knockout rat is a useful model to test the hypothesis that SERT is the primary mechanism for arterial 5-HT uptake, and to investigate the impact of SERT removal on blood pressure. Wild type (WT) and knockout (KO) rats were used to measure 5-HT content (plasma, raphe, aorta, carotid and mesenteric artery), aortic isometric contraction and blood pressure. HPLC supported the lack of circulating 5-HT in plasma (ng/ml plasma WT = 310±96; KO = 1.0±0.5; p< 0.05). Immunohistochemistry and Western analyses validated the presence of SERT protein in the WT and lesser expression in the KO. Aorta isolated from KO had a normal contraction to phenylephrine and norepinephrine, and a normal relaxation to the endothelium-dependent agonist acetylcholine when compared to aorta from WT. By contrast, the potency of 5-HT was increased in aorta from KO compared to WT (-log EC<sub>50</sub> [M]: WT = 5.71±0.08, KO = 6.7+0.18) and maximum contraction reduced (% phenylephrine [10 µM] contraction: WT = 113+6, KO = 52+12%). 5-HT uptake was reduced but not abolished in arteries of the KO compared to WT. Diurnal mean arterial blood pressure, heart rate and locomotor activity level of the KO rats were similar to the WT. These data suggest that there are other mechanisms of 5-HT uptake in arteries of the rat, and that while the absence of circulating 5-HT and/or SERT function sensitizes arteries to 5-HT, SERT dysfunction does not impair normal blood pressure.
