Prolonged effects of p-chlorophenylalanine on the blood pressure of conscious normotensive and DOCA/saline hypertensive rats

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.

Vascular reactivity, 5-HT uptake, and blood pressure in the serotonin transporter knockout rat

The handling of serotonin [5-hydroxytryptamine (5-HT)] depends on the serotonin transporter (SERT). A SERT knockout (KO) 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 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; and KO, 1.0 +/- 0.5; P < 0.05). Immunohistochemistry and Western blot analyses validated the presence of the SERT protein in the WT rats and a lesser expression in the KO rat. The aorta isolated from KO rats had a normal contraction to phenylephrine and norepinephrine and a normal relaxation to the endothelium-dependent agonist acetylcholine compared with the aorta from WT. In contrast, the potency of 5-HT was increased in the aorta from KO rats compared with WT rats [-log EC(50) (M); WT, 5.71 +/- 0.08; and KO, 6.7 +/- 0.18] and maximum contraction was reduced [%phenylephrine (10 muM) contraction, WT, 113 +/- 6%; and KO, 52 +/- 12%]. 5-HT uptake was reduced but not abolished in arteries of the KO compared with the WT rats. Diurnal mean arterial blood pressure, heart rate, and locomotor activity level of the KO rats were similar to the WT rats. These data suggest that there are other mechanisms of 5-HT uptake in the arteries of the rat and that although the absence of circulating 5-HT and/or SERT function sensitizes arteries to 5-HT, SERT dysfunction does not impair normal blood pressure.

5-HT in systemic hypertension: foe, friend or fantasy?

Since its discovery by Erspamer in the 1930s and identification by Page in the 1950s, 5-HT (5-hydroxytryptamine; serotonin) has been an elusive candidate as a substance that plays a role in the disease of high blood pressure, also known as hypertension. In both animal and human hypertension, arterial contraction to 5-HT is profoundly enhanced. Additionally, 5-HT is a vascular smooth muscle cell mitogen. Because both increased arterial contractility and smooth muscle growth contribute to the disease of hypertension, it is logical to believe that 5-HT is a potential cause of disease, and thus a foe. However, decades of research have produced conflicting results as to the potential role of 5-HT in hypertension. This review will discuss historical findings which both support and refute the involvement of 5-HT in hypertension, and pose some new questions that may reveal novel ways for 5-HT to modify vascular control of blood pressure.

Effect of central serotonin depletion on blood pressure and the renin system in rats

In the present study we examined the effect of depletion of central nervous system serotonin by 5,7-dihydroxytryptamine on blood pressure in male Wistar rats. We also analyzed the relationship between the serotonergic and renin-angiotensin systems. Blood pressure was determined before and after intracisternal administration of 5,7-dihydroxytryptamine, 200 micrograms in saline with 1 mg/ml ascorbic acid (n = 9). Control rats (n = 8) received intracisternal vehicle. Before sacrifice, blood and cerebrospinal fluid samples were obtained. The brain was dissected in several areas. Serotonin, norepinephrine, angiotensinogen, and reninlike concentrations were determined in the brain parenchyma; angiotensinogen concentration was evaluated in cerebrospinal fluid and plasma samples; plasma renin activity was also measured. Treatment produced a significant decrease in blood pressure (-10 mm Hg; p less than 0.025) and, simultaneously, a high depletion of serotonin stores in the studied central areas (p less than 0.001), except in the cerebral cortex. Reninlike concentration was increased in the medulla oblongata (p less than 0.005) and the brainstem (p less than 0.02) after 5,7-dihydroxytryptamine treatment. Angiotensinogen concentration was decreased in the hypothalamus and elevated in the spinal cord. Angiotensinogen concentration in cerebrospinal fluid, plasma angiotensinogen concentration, and plasma renin activity did not change with treatment. Serotonin concentration in the cerebrospinal fluid remained unchanged, while the 5-hydroxyindoleacetic acid level was diminished (-47%; p less than 0.001). Intracisternal administration of 5,7-dihydroxytryptamine produced a hypotensive effect in normal rats and several modifications of the renin-angiotensin complex, suggesting a relationship between the monoaminergic and peptidergic systems.

Failure to produce blood pressure changes following pharmacological or surgical depletion of brain serotonin in the spontaneously hypertensive rat

Spontaneously hypertensive (SH) and normotensive (Wistar-Kyoto, WKY) rats were examined for blood pressure changes following depletion of CNS serotonin (5-HT) by 3 separate techniques: (1) p-chlorophenylalanine, (2) 5,7-dihydroxytryptamine, and (3) a lesion of the dorsal and median raphe nuclei. All of these procedures failed to alter blood pressure in either hypertensive or normotensive rats, despite marked reductions (75-85%) in forebrain 5-HT. Moreover, treatment of 10 day-old hypertensive rat pups with intracisternal injections of 5,7-DHT (10 microgram) failed to alter the development of hypertension despite a 75-80% decrease in spinal cord 5-HT. These findings, which show that 5-HT depletion does not alter blood pressure in the SH or the WKY rat, do not lend support to the idea that 5-HT is involved in the regulation of blood pressure or in the development and maintenance of hypertension in the SH rat.

d-Tyrosine prevents hypertension in DOCA-saline treated uninephrectomised rats

High blood pressure in DOCA-saline treated uninephrectomised rats is prevented or even reversed by tyrosine, tyramine or by treatments which - based on circumstantial evidence - might increase local brain tyramine concentration.

Bulbospinal tryptaminergic neurones. A search for the role of bulbospinal tryptaminergic neurones in the control of sympathetic activity

1. The possible role of bulbospinal tryptaminergic neurones in the control of sympathetic activity has been investigated in anaesthetised cats. 2. Depletion of spinal cord stores of 5-hydroxytryptamine was achieved by systemic administration of p-chlorophenylalanine or by intraspinal microinjections of 5,6-dihydroxytryptamine. 3. Blood pressure was little changed by these treatments, neither was the pattern of ongoing activity in sympathetic nerves (arterial pulse rhythmicity and respiratory modulation), the influence of pulmonary afferent nerves on this activity (determined by an airway occlusion technique), the sympatho-inhibitory influence of the carotid sinus baroreceptors, nor the sympatho-inhibitory or -excitatory influences of somatic afferent nerves. 4. Since both p-chlorophenylalanine and 5,6-dihydroxytryptamine treatment caused extensive disruption of the bulbospinal tryptaminergic neurones, it was concluded that these play no significant role in the mediation of the responses tested in anaesthetised cats in the present study.

Central tyramine prevents hypertension in uninephrectomized DOCA-saline treated rats

Prevention of high blood pressure in uninephrectomized, DOCA-saline treated rats was observed after treatment with central tyramine precursors. We suggest that the high blood pressure is either due to relative lack of tyrosine, which might be caused by the hyperactivity of tyrosine hydroxylase, or to hypoactivity of the decarboxylase: in both cases the result is diminished tyramine synthesis.