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

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.

Development of renovascular hypertension after central serotonin depletion

The participation of the central serotonergic system in the development of two-kidney, two clip (2K2C) Goldblatt renovascular hypertension in the rat has been examined. Half of the rats were treated with desmethylimipramine intraperitoneally and 5,7-dihydroxytryptamine intracisternally; the other half received only desmethylimipramine and the 5,7-dihydroxytryptamine vehicle. Two days later, a silver clip was placed in both renal arteries in half of the rats of each group. A sham operation was performed in the remaining rats. Blood pressure was recorded during the 5 weeks after treatment. At the end of the experiment, blood and cerebrospinal fluid samples were obtained. The brain was dissected into several areas and kept frozen. Norepinephrine, serotonin, angiotensinogen, and renin-like concentration were evaluated in the brain areas. Plasma renin activity and angiotensinogen concentration in the plasma and cerebrospinal fluid were estimated. In the sham-operated groups, blood pressure was lower in the treated than in the control rats. The curve of blood pressure increase, as well as the final blood pressure, was similar in the treated and control 2K2C rats. Serotonin was significantly depleted by the 5,7-dihydroxytryptamine treatment in all brain areas. Treatment did not induce any changes in central norepinephrine concentration. Plasma renin activity was diminished in the treated sham-operated rats. These data indicate that the central serotonin depletion does not prevent the development of hypertension and confirm the role of the amine in normal blood pressure regulation. On the other hand, the peripheral renin-angiotensin system might participate in the development of high blood pressure in serotonin-depleted animals.

Altered levels of neuropeptides in the medulla and spinal cord of spontaneously hypertensive rats

1. Spontaneously hypertensive rats (SHR) are useful for investigating the possible pathophysiological and neurochemical basis of human essential hypertension. 2. The accepted pathogenic mechanism of hypertension in SHR is an increased central sympathetic drive which results in an increased peripheral resistance. 3. The neurochemical basis of the increased sympathetic drive is unknown. The observation that there are reduced levels of neuropeptides (vasoactive intestinal peptide, neuropeptide Y, cholecystokinin octapeptide, neurotensin and calcitonin gene related peptide) in the spinal cord in SHR rats compared with age and gender matched Wistar-Kyoto normotensive rats could provide a basis for understanding the mechanism of hypertension in SHR.

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.

Interaction of propranolol with central serotonergic neurons

Central monoaminergic mechanisms are believed to be involved in cardiovascular regulation. The present study was designed to evaluate the involvement of central serotonergic pathways in the antihypertensive action of propranolol in pentobarbital anesthetized mongrel dogs. Ventriculocisternal perfusion of propranolol (25 ug/kg/min for 30 min) decreased serotonin turnover as indicated by a significant decrease in cerebrospinal fluid levels of 5-hydroxyindoleacetic acid (5-HIAA). This effect was accompanied by a significant reduction in mean arterial pressure and heart rate. These results indicate that propranolol decreases central serotonergic activity and suggests a possible role for central serotonergic pathways in the antihypertensive action of propranolol. Several studies have indicated that central serotonergic pathways participate in the regulation of blood pressure. Brainstem regions including the nucleus tractus solitarius, the raphe nucleus and the anterior hypothalamic preoptic region are involved in cardiovascular control and contain a dense population of serotonergic neurons. A centrally-mediated hypotensive effect of propranolol has been demonstrated. Centrally administered propranolol increases cerebrospinal fluid (CSF) levels of norepinephrine and reduces blood pressure possibly due to decreased peripheral sympathetic nerve activity. Central serotonergic pathways may also be involved in the antihypertensive action of some beta-adrenoceptor antagonists. Destruction of central serotonergic neurons with 5,7-dihydroxytryptamine and desipramine abolished the antihypertensive effect of intracisternal propranolol in sinoaortic denervated dogs. Acute administrations of (-)-propranolol and (-)-pindolol decreased the synthesis rate of serotonin, while acute administration of salbutamol, a beta 2-adrenoceptor agonist, increased 5-HIAA levels in rat brain structures. The present study was designed to evaluate the involvement of central serotonergic pathways in the antihypertensive action of propranolol.

Substance P mechanisms of the spinal cord related to vasomotor tone in the spontaneously hypertensive rat

This report deals with substance P (SP) mechanisms involved in regulation of vasomotor tone at the spinal cord levels in normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). Our results indicate the following. (1) Intrathecal injections of the SP antagonist, D-Pro,D-Trp-SP cause dose-dependent decreases in mean arterial pressure and heart rate in Sprague-Dawley rats, WKYs and SHRs; the maximal blood pressure decreases are equal to those seen after cervical spinal cord transection. (2) Intrathecal injections of this antagonist into the L1 spinal level in WKYs or SHRs that had previously had their C8 spinal cords transected caused a rise in blood pressure and heart rate, suggesting that intrinsic spinal SP mechanisms are probably not involved in vasomotor tone. (3) The intermediolateral cell column region (IML) of 16-week-old WKYs and SHRs has a single high affinity and saturable binding component with approximately the same dissociation constant (Kd = 1.21 nM for WKYs; Kd = 1.25 nM for SHRs); the SHRs showed a higher number of sites (Bmax = 24.5 fmol/mg protein) than WKY rats (Bmax = 19.9 fmol/mg protein). The Kd and Bmax obtained from IML sections from 4-week-old WKYs and SHRs exhibit no differences, although their binding values with 2 nM [3H]SP are higher than those obtained from the 16-week-old animals. (4) D-Pro4,D-Trp7,9-SP4-11 has the same relatively low (micromolar range) potency for displacing [3H]SP binding in the IML of WKYs and SHRs. (5) SHRs (16 weeks old) contain 20% more SP immunoreactivity in the IML than WKY rats (834 +/- 36 pg/mg protein vs 694 +/- 50 pg/mg protein); 4-week-old rats do not show such differences. The potential significance of these results is discussed in relation to the control of vasomotor tone.

Blood pressure control by neurotransmitters in the medulla oblongata and spinal cord

The roles of putative central neurotransmitters in the control of blood pressure have been reviewed with respect to the cardiovascular functions of individual nerve pathways in the medulla oblongata and spinal cord. Vasomotor activity of sympathetic preganglionic neurones originates from spinally-projecting neurones in the ventrolateral medulla which may include adrenaline neurones of the C1 group and serotonin neurones in the lateral B1 and B3 groups. Other bulbospinal monoamine nerves may modulate vasomotor activity at the spinal level, but the mechanism of this modulation is controversial. Evidence for two descending sympatho-inhibitory pathways has emerged: a noradrenergic projection from the A5 cell group and a serotonergic projection from the medullary raphe (medial B1 and B2 groups). The vasomotor influence of other bulbospinal pathways is unclear. Baroreflex control of blood pressure is mediated through the solitary tract nucleus (NTS). L-Glutamate and substance P are considered as candidates for transmitters in baroreceptor afferents to the NTS. Transmitters in efferent nerves relaying baroreflex activity from the NTS to cardiovagal motoneurones, medullary vasomotor neurones or sympathetic preganglionic neurones have not been identified but the monoamine transmitters present in the NTS appear to modulate baroreflexes. Noradrenaline and serotonin nerve endings may facilitate the vasodepressor component of the baroreflex while adrenaline nerves possibly inhibit the cardiovagal mechanism. Enkephalins and vasopressin act in the NTS to raise blood pressure and nerves containing these neuropeptides may constitute important links in reciprocal cardiovascular pathways between the lower brainstem and hypothalamus.

Effects of LM 5008, a selective inhibitor of 5-hydroxytryptamine uptake, on blood pressure and responses to sympathomimetic amines

LM 5008 (4-[2-(3-indolyl)ethyl]piperidine) (10, 20 and 50 mg kg-1) had no significant effect on pressor responses to noradrenaline or tyramine in rats anaesthetized with urethane. Desmethylimipramine (1 mg kg-1) blocked the response to tyramine but chlorimipramine (5 mg kg-1) had no significant effect on responses to noradrenaline or tyramine. In the rabbit, anaesthetized with chloralose, LM 5008 (5 mg kg-1) had no effect on pressor responses to noradrenaline, tyramine or angiotensin II, while desmethylimipramine (0.25 mg kg-1) inhibited responses to tyramine and potentiated those to noradrenaline. LM 5008 (10 mg kg-1) had no effect on resting blood pressure of conscious normotensive or DOCA-saline hypertensive rats. Tranylcypromine (5 mg kg-1) produced a fall in blood pressure in conscious normotensive and in DOCA hypertensive rats. Treatment with a combination of LM 5008 (10 mg kg-1) and tranylcypromine (5 mg kg-1) resulted in the appearance of a behavioural hyperactivity syndrome, but blood pressure was not different from that of animals treated with tranylcypromine alone. These results further demonstrate the selectivity of LM 5008 for 5-hydroxytryptamine as opposed to catecholamine uptake.