Robert Patrick Davis

Serotonin and Blood Pressure Regulation

5-Hydroxytryptamine (5-HT; serotonin) was discovered more than 60 years ago as a substance isolated from blood. The neural effects of 5-HT have been well investigated and understood, thanks in part to the pharmacological tools available to dissect the serotonergic system and the development of the frequently prescribed selective serotonin-reuptake inhibitors. By contrast, our understanding of the role of 5-HT in the control and modification of blood pressure pales in comparison. Here we focus on the role of 5-HT in systemic blood pressure control. This review provides an in-depth study of the function and pharmacology of 5-HT in those tissues that can modify blood pressure (blood, vasculature, heart, adrenal gland, kidney, brain), with a focus on the autonomic nervous system that includes mechanisms of action and pharmacology of 5-HT within each system. We compare the change in blood pressure produced in different species by short- and long-term administration of 5-HT or selective serotonin receptor agonists. To further our understanding of the mechanisms through which 5-HT modifies blood pressure, we also describe the blood pressure effects of commonly used drugs that modify the actions of 5-HT. The pharmacology and physiological actions of 5-HT in modifying blood pressure are important, given its involvement in circulatory shock, orthostatic hypotension, serotonin syndrome and hypertension.

5-hydroxtryptamine receptors in systemic hypertension: an arterial focus.

BACKGROUND Serotonin (5-hydroxytryptamine [5-HT]) was named for its isolation from blood serum (sero-) and ability to contract smooth muscle (-tonin). Thus, its relationship with the cardiovascular system began with its discovery. AIMS This review will focus on the effects of 5-HT and its receptors in the vasculature, with a focus on their involvement in high blood pressure (hypertension). Two seemingly contradictory bodies of evidence exist that make it difficult to assign any one function to 5-HT in vascular control of blood pressure. RESULTS  In vitro, 5-HT is an established vasoconstrictor, the effects of which are amplified in hypertension. By contrast, 5-HT (or its precursor 5-hydroxytryptophan) lowers blood pressure when given chronically in vivo. We will discuss ideas that might help us understand these differences, discuss relatively new pharmacology parameters (e.g. biased, inverse agonism) as they pertain to 5-HT receptors, and pose questions that are vital to answer so as to understand the role played by 5-HT in control of blood pressure, especially as it pertains to vascular function. CONCLUSIONS Our goal is to understand if the actions of 5-HT in hypertension are physiologically and clinically relevant. The community understands 5-HT has complex cardiovascular effects, and clinical studies have proven equivocal in terms of the involvement of 5-HT. This article provides a balanced view of evidence/literature that illustrates involvement of 5-HT in hypertension as controversial. It contributes new pharmacological knowledge of 5-HT compounds, and poses timely questions as to how this field can move forward. The take home message is that the cardiovascular effects of 5-HT are markedly complex such that we have not yet answered the question of whether 5-HT is beneficial or detrimental to hypertension.

Drug Delivery: Enabling Technology for Drug Discovery and Development. iPRECIO Micro Infusion Pump: Programmable, Refillable, and Implantable.

Successful drug delivery using implantable pumps may be found in over 12,500 published articles. Their versatility in delivering continuous infusion, intermittent or complex infusion protocols acutely or chronically has made them ubiquitous in drug discovery and basic research. The recent availability of iPRECIO®, a programmable, refillable, and implantable infusion pump has made it possible to carry out quantitative pharmacology (PKPD) in single animals. When combined with specialized catheters, specific administration sites have been selected. When combined with radiotelemetry, the physiologic gold standard, more sensitive and powerful means of detecting drug induced therapeutic, and/or adverse effects has been possible. Numerous application examples are cited from iPRECIO® use in Japan, United States, and Europe with iPRECIO® as an enabling drug delivery device where the refillable and programmability functionality were key benefits. The ability to start/stop drug delivery and to have control periods prior dosing made it possible to have equivalent effects at a much lower dose than previously studied. Five different iPRECIO® applications are described in detail with references to the original work where the implantable, refillable, and programmable benefits are demonstrated with their different end-points.

5-hydroxytryptamine (5-HT) reduces total peripheral resistance during chronic infusion: direct arterial mesenteric relaxation is not involved

Serotonin (5-hydroxytryptamine; 5-HT) delivered over 1 week results in a sustained fall in blood pressure in the sham and deoxycorticosterone acetate (DOCA)-salt rat. We hypothesized 5-HT lowers blood pressure through direct receptor-mediated vascular relaxation. In vivo, 5-HT reduced mean arterial pressure (MAP), increased heart rate, stroke volume, cardiac index, and reduced total peripheral resistance during a 1 week infusion of 5-HT (25 µg/kg/min) in the normotensive Sprague Dawley rat. The mesenteric vasculature was chosen as an ideal candidate for the site of 5-HT receptor mediated vascular relaxation given the high percentage of cardiac output the site receives. Real-time RT-PCR demonstrated that mRNA transcripts for the 5-HT2B, 5-HT1B, and 5-HT7 receptors are present in sham and DOCA-salt superior mesenteric arteries. Immunohistochemistry and Western blot validated the presence of the 5-HT2B, 5- HT1B and 5-HT7 receptor protein in sham and DOCA-salt superior mesenteric artery. Isometric contractile force was measured in endothelium-intact superior mesenteric artery and mesenteric resistance arteries in which the contractile 5- HT2A receptor was antagonized. Maximum concentrations of BW-723C86 (5- HT2B agonist), CP 93129 (5-HT1B agonist) or LP-44 (5-HT7 agonist) did not relax the superior mesenteric artery from DOCA-salt rats vs. vehicle. Additionally, 5-HT (10–9 M to 10–5 M) did not cause relaxation in either contracted mesenteric resistance arteries or superior mesenteric arteries from normotensive Sprague- Dawley rats. Thus, although 5-HT receptors known to mediate vascular relaxation are present in the superior mesenteric artery, they are not functional, and are therefore not likely involved in a 5-HT-induced fall in total peripheral resistance and MAP.

Long-Term Inhibition of Xanthine Oxidase by Febuxostat Does Not Decrease Blood Pressure in Deoxycorticosterone Acetate (DOCA)-Salt Hypertensive Rats

Xanthine oxidase and its products, uric acid and ROS, have been implicated in the pathogenesis of cardiovascular disease, such as hypertension. We have previously reported that allopurinol inhibition of XO does not alter the progression of deoxycorticosterone acetate (DOCA)-salt hypertension in rats. However other researchers have observed a reduction in blood pressure after allopurinol treatment in the same model. To resolve this controversy, in this study we used the newer and more effective XO inhibitor febuxostat, and hypothesized that a more complete XO blockade might impair hypertension development and its end-organ consequences. We used DOCA-salt hypertensive rats and administered vehicle (salt water) or febuxostat (orally, 5 mg/kg/day in salt water) in a short-term “reversal” experiment (2 weeks of treatment 3 weeks after DOCA-salt beginning) and a long-term “prevention” experiment (treatment throughout 4 weeks of DOCA-salt). We confirmed XO inhibition by febuxostat by measuring circulating and tissue levels of XO metabolites. We found an overall increase in hypoxanthine (XO substrate) and decrease in uric acid (XO product) levels following febuxostat treatment. However, despite a trend for reduced blood pressure in the last week of long-term febuxostat treatment, no statistically significant difference in hemodynamic parameters was observed in either study. Additionally, no change was observed in relative heart and kidney weight. Aortic media/lumen ratio was minimally improved by long-term febuxostat treatment. Additionally, febuxostat incubation in vitro did not modify contraction of aorta or vena cava to norepinephrine, angiotensin II or endothelin-1. We conclude that XO inhibition is insufficient to attenuate hypertension in the rat DOCA-salt model, although beneficial vascular effects are possible.

Allopurinol does not decrease blood pressure or prevent the development of hypertension in the deoxycorticosterone acetate-salt rat model.

Reactive oxygen species play an important role in the pathogenesis of hypertension, disease in which reactive oxygen species levels and markers of oxidative stress are increased. Xanthine oxidase (XO) is a reactive oxygen species-producing enzyme the activity of which may increase during hypertension. Studies on XO inhibition effects on blood pressure have yielded controversial results. We hypothesized that XO inhibition would decrease blood pressure or attenuate the development of deoxycorticosterone acetate (DOCA)-salt hypertension. We administered the XO inhibitor, allopurinol (50 mg/kg per day, orally) or its vehicle to rats during the established or development stages of DOCA-salt hypertension. We validated XO inhibition by high-performance liquid chromatography measurements of XO metabolites in urine, serum, and tissues demonstrating a decrease in products, increase in substrates, and detection of the active metabolite of allopurinol, oxypurinol. We monitored blood pressure continuously through radiotelemetry and performed gross evaluations of target organs of hypertension. Allopurinol treatment did not impact the course of DOCA-salt hypertension regardless of the timing of administration. Aside from a significant decrease in pulse pressure in allopurinol-treated rats, no positive differences were observed between the allopurinol and the vehicle-treated rats. We conclude that XO does not play an important role in the development or maintenance of hypertension in the rat DOCA-salt hypertension model.

One-month serotonin infusion results in a prolonged fall in blood pressure in the deoxycorticosterone acetate (DOCA) salt hypertensive rat.

A 7-day infusion of serotonin (5-hydroxytryptamine, 5-HT) causes a sustained fall in elevated blood pressure in the male deoxycorticosterone acetate (DOCA)-salt rat. As hypertension is a long-term disease, we presently test the hypothesis that a longer (30 day) 5-HT infusion could cause a sustained fall in blood pressure in the established hypertensive DOCA-salt rat. This time period (∼4 weeks) was also sufficient to test whether 5-HT could attenuate the development of DOCA-salt hypertension. 5-HT (25 μg/kg/min; sc) or vehicle (Veh) was delivered via osmotic pump to (1) established DOCA-salt rats for one month, (2) Sprague-Dawley rats prior to DOCA-salt administration for one month, and blood pressure and heart rate measured telemetrically. On the final day of 5-HT infusion, free platelet poor plasma 5-HT concentrations were significantly higher in 5-HT versus Veh-infused rats, and mean arterial pressure was significantly lower in 5-HT-infused (135 ± 4 mmHg vs Veh-infused 151 ± 7 mmHg) established DOCA-salt rats. By contrast, 5-HT-infusion did not prevent the development of DOCA-salt hypertension (144 ± 7 mmHg vs Veh = 156 ± 6 mmHg). Isometric contraction of aortic strips was measured, and neither the potency nor maximum contraction to the alpha adrenergic receptor agonist phenylephrine (PE) or 5-HT were modified by infusion of 5-HT (established or preventative infusion), and maximum aortic relaxation to acetylcholine (ACh) was modestly but not significantly enhanced (∼15% improvement). This study demonstrates 5-HT is capable of lowering blood pressure in established DOCA-salt hypertensive rats over the course of one month in a mechanism that does not significantly modify or is dependent on modified vascular responsiveness. This finding opens the possibility that elevation of 5-HT levels could be useful in the treatment of hypertension.

Comparison of the function of the serotonin transporter in the vasculature of male and female rats.

1. The serotonin transporter (SERT) handles serotonin (5‐hydroxytryptamine (5‐HT)) and is blocked by the antidepressant SERT inhibitors fluoxetine and fluvoxamine. Although the importance of SERT in the central nervous system is clear, SERT also functions in the peripheral vasculature. In the present study, we tested the hypothesis that the vasculature from female rats has increased SERT function compared with male rats because females are more responsive to SERT inhibitors.

Ehrhart Series, Unimodality, and Integrally Closed Reflexive Polytopes

An interesting open problem in Ehrhart theory is to classify those lattice polytopes having a unimodal h*-vector. Although various sufficient conditions have been found, necessary conditions remain a challenge. In this paper, we consider integrally closed reflexive simplices and discuss an operation that preserves reflexivity, integral closure, and unimodality of the h*-vector, providing one explanation for why unimodality occurs in this setting. We also discuss the failure of proving unimodality in this setting using weak Lefschetz elements.

5-Hydroxytryptamine does not reduce sympathetic nerve activity or neuroeffector function in the splanchnic circulation

Infusion of 5-hydroxytryptamine (5-HT) in conscious rats results in a sustained (up to 30 days) fall in blood pressure. This is accompanied by an increase in splanchnic blood flow. Because the splanchnic circulation is regulated by the sympathetic nervous system, we hypothesized that 5-HT would: 1) directly reduce sympathetic nerve activity in the splanchnic region; and/or 2) inhibit sympathetic neuroeffector function in splanchnic blood vessels. Moreover, removal of the sympathetic innervation of the splanchnic circulation (celiac ganglionectomy) would reduce 5-HT-induced hypotension. In anaesthetized Sprague-Dawley rats, mean blood pressure was reduced from 101±4 to 63±3mm Hg during slow infusion of 5-HT (25μg/kg/min, i.v.). Pre- and postganglionic splanchnic sympathetic nerve activity were unaffected during 5-HT infusion. In superior mesenteric arterial rings prepared for electrical field stimulation, neither 5-HT (3, 10, 30nM), the 5-HT1B receptor agonist CP 93129 nor 5-HT1/7 receptor agonist 5-carboxamidotryptamine inhibited neurogenic contraction compared to vehicle. 5-HT did not inhibit neurogenic contraction in superior mesenteric venous rings. Finally, celiac ganglionectomy did not modify the magnitude of fall or time course of 5-HT-induced hypotension when compared to animals receiving sham ganglionectomy. We conclude it is unlikely 5-HT interacts with the sympathetic nervous system at the level of the splanchnic preganglionic or postganglionic nerve, as well as at the neuroeffector junction, to reduce blood pressure. These important studies allow us to rule out a direct interaction of 5-HT with the splanchnic sympathetic nervous system as a cause of the 5-HT-induced fall in blood pressure.