Review of the role of the central serotonergic neuronal system in blood pressure regulation

Physiopharmacological interactions between stress hormones and central serotonergic systems

The present review tries to delineate some mechanisms through which the sympathetic nervous system (SNS) and the hypothalamo-pituitary-adrenal (HPA) interact with central serotonergic systems. The recent progress in 5-hydroxytryptamine (5-HT) receptor pharmacology has helped to define the means by which central serotonergic activity may alter the respective activities of the SNS (sympathetic nerves and adrenomedulla) and of the HPA axis. These pharmacological findings have also helped to characterize the differential effects of central 5-HT upon different branches of the SNS and the numerous sites at which 5-HT exerts stimulatory influences upon the HPA axis. Although relevant to stress-related neuroendocrinology, the extent to which these interactions are involved in the antidepressant/anxiolytic properties of some serotonergic agents still remains to be clarified. Beside these findings, there is also abundant evidence for a tight control of central serotonergic systems by stress hormones. Activation of the SNS increases, by numerous means, central availability of tryptophan, whereas glucocorticoids exert differential actions upon the intra- and the extraneuronal regulation of 5-HT function. Actually, a significant number of these mechanisms is involved in the maintenance of homeostasis during stressful events, thereby conferring to these mechanisms a key role in adaptation processes.

Cardiovascular effects of the 5-HT1A receptor ligand, MDL 73005EF, in conscious spontaneously hypertensive rats

The effects of pretreatment with the potent and selective 5-HT1A receptor ligand, MDL 73005EF, on the cardiovascular responses to administration of the 5-HT1A receptor agonists, 8-hydroxy-2-(di-n- propylamino)tetralin (8-OH-DPAT), flesinoxan and 5-methylurapidil were studied in conscious spontaneously hypertensive rats (SHR) and compared with those of putative 5-HT1A receptor antagonists. MDL 7300EF (0.1-3 mg/kg) induced a dose-dependent but transient decrease in mean arterial pressure (MAP). Pretreatment with doses of 1 or 3 mg/kg MDL 73005EF significantly inhibited the hypotensive and bradycardiac effects of 8-OH-DPAT (0.03-1 mg/kg). Pretreatment with 1 mg/kg MDL 73005EF similarly reduced the hypotensive actions of flesinoxan (0.3-1 mg/kg) and 5-methylurapidil (0.1 mg/kg). In contrast, MDL 73005EF did not significantly affect the decrease in blood pressure induced by administration of 0.01 mg/kg clonidine, 0.3 mg/kg hydralazine or 0.2 mg/kg nifedipine. The effect of 8-OH-DPAT (0.1 mg/kg) on MAP was also reduced by pretreatment with 1 mg/kg BMY 7378, buspirone or pindolol, but not NAN 190 or spiperone. BMY 7378, NAN 190, pindolol and spiperone induced a significant decrease in blood pressure. To rule out the possibility that the reduced baseline may have influenced responses to 8-OH-DPAT, we showed that pretreatment with the vasodilator, hydralazine (0.3 mg/kg), had no effect on the MAP response to 8-OH-DPAT although it significantly reduced MAP. We conclude that MDL 73005EF acts as a mixed agonist/antagonist at 5-HT1A receptors since it caused a decrease in blood pressure, but also reduced the cardiovascular responses to the 5-HT1A receptor agonists, 8-OH-DPAT, flesinoxan and 5-methylurapidil.

Serotonergic receptors and drugs in hypertension

The possible role of 5-hydroxytryptamine (5HT) and 5HT-receptors in hypertension, already suggested by Page in 1954, has been subject to a renaissance of interest owing to the development of antihypertensive drugs which interact with 5HT-receptors. These drugs, like ketanserin, urapidil and flesinoxan are used as tools to study the role of 5HT and its receptors in hypertension. Some arguments would plead in favour of a certain role of 5HT and 5HT-receptors in the pathogenesis and maintenance of hypertension: hyperresponsiveness of blood vessels from hypertensive patients and animals to 5HT-induced constriction; the antihypertensive/vasodilator activity of the 5HT2-receptor antagonist ketanserin; enhanced sensitivity of platelets from hypertensives to 5HT. However, there are also several arguments which do not support a causal role of 5HT in hypertensive disease: 5HT is not a generally accepted pressor agent, whereas its concentration in the circulating blood is subthreshold; the 5HT2-receptor antagonist ketanserin is the only agent of this type which lowers blood pressure, other 5HT2-receptor blockers (ritanserin; LY 53587) being inactive. The various data and arguments available do not unequivocally support a relevant role of peripheral 5HT and its receptors in hypertensive disease. 5HT2-receptor blockade may, however, have a favourable effect on the microcirculation under pathological conditions. The stimulation of central 5HT1A-receptors by drugs like urapidil, 8-OH-DPAT or flesinoxan, has been demonstrated to induce peripheral sympathoinhibition and a fall in blood pressure. This mechanism appears to be a novel target for centrally acting antihypertensives, clearly different from that of clonidine and related drugs, which are centrally acting alpha 2-adrenoceptor agonists.

The hypothalamic paraventricular and lateral parabrachial nuclei receive collaterals from raphe nucleus neurons: a combined double retrograde and immunocytochemical study

Retrograde tracer injections of fluorescein- and rhodamine-labelled latex microspheres centered in the parvicellular zone of the hypothalamic paraventricular nucleus and pontine lateral parabrachial nucleus revealed that 36% of the labelled neurons in the dorsal raphe nucleus send collaterals to both structures. These cells were organized in a well-distinguishable cluster within the dorsal raphe nucleus. By combining retrograde tracing with immunocytochemistry, it was found that less than 8% of the double-labelled cells stained positively for serotonin. Of the remaining raphe nuclei that were examined, only the median raphe nucleus contributed a minor nonserotoninergic projection to the paraventricular or lateral parabrachial nuclei. Few of the retrogradely labelled cells in the median raphe nucleus contained both tracers. These results suggest that nonserotoninergic and serotoninergic neurons in the dorsal raphe nucleus, via collateral branching, may simultaneously influence the activity of two central nervous system nuclei involved in autonomic control.

Factors influencing platelet serotonin uptake in essential hypertension

In a study of the mechanism(s) of platelet serotonin uptake alteration in essential hypertension, a total of 90 blood samples were analysed for platelet count and platelet serotonin uptake. These included 20 blood samples each of hypertensives, controls before and after cross-incubation experiments and 10 samples of hypertensives after control of blood pressure. It was observed that serotonin uptake was markedly reduced in hypertensive platelets. Diminished serotonin uptake in essential hypertension correlated directly with diastolic and mean arterial blood pressure and inversely with plasma total cholesterol values. In cross-incubation experiments using control platelets and hypertensive plasma, there was a significant reduction in platelet serotonin uptake (303.06 +/- 86.28 cpm/10(8) vs. 204.26 +/- 66.45 cpm/10(8); P less than 0.001), whereas hypertensive platelets when incubated with control plasma, showed increased serotonin uptake (233.50 +/- 75.19 cpm/10(8) vs. 312.64 +/- 79.54 cpm/10(8); P less than 0.01). Upon control of blood pressure, the platelet serotonin uptake improved significantly (205.45 +/- 70.0 cpm/10(8) vs. 266.77 +/- 61.68 cpm/10(8); P less than 0.05-0.01). From these results, it appears that reduced platelet serotonin uptake in essential hypertension is a reversible phenomenon probably governed by the presence of plasma factor(s) and/or altered platelet-membrane function.

Aging reduces cardiovascular and sympathetic responses to NTS injections of serotonin in rats

Serotonergic mechanisms for baroreflex modulation could become altered with age. This possibility was explored by comparing cardiovascular and sympathetic effects elicited in 2-month- and 24-month-old rats by injecting serotonin (5-HT) directly into the nucleus tractus solitarius (NTS) which is the primary baroreflex relay station in the medulla. Ensuing decreases in mean pressure, heart rate, and renal nerve firing were significantly smaller in 24-month-old than in 2-month-old rats. By contrast, similar injections into the NTS of the vehicle alone were ineffective in both age groups. Postmortem examination of brain sections showed that NTS injection sites were equally distributed in both age groups, thereby indicating that the brain areas affected by 5-HT were identical regardless of age. Reduced sensitivity of peripheral myocardial beta-adrenergic and vascular alpha-adrenergic receptors was considered partly responsible because 5-HT injected into the NTS lowers blood pressure by decreasing sympathetic vasomotor tone and slows the heart by increasing vagal tone with reciprocal sympatho-inhibition. But since reduced adrenergic sensitivity would not account for the concurrent decrease in renal nerve firing, a more logical explanation is that the sensitivity of serotonergic mechanisms in the NTS for inhibiting blood pressure, heart rate, and renal nerve activity decreases with age.

Cardiovascular effects of the local injection of 5,7-dihydroxytryptamine into the nodose ganglia and nucleus tractus solitarius in awake freely moving rats

The role of the nucleus tractus solitarius (NTS) serotonergic afferents in cardiovascular (CV) regulation is yet to be established. However, several findings suggest that in this nucleus the serotonergic endings coming from the nodose ganglia (NG) are involved in the control of blood pressure (BP). The purpose of the present study was to identify the CV effects of the destruction of this NG-NTS serotonergic pathway. For that, the BP, BP variability (BPV) and heart rate (HR) effects of the local microinjection of 5,7-dihydroxytryptamine (5,7-DHT), into the NG and NTS were investigated in awake freely moving rats. The local degeneration of serotonergic elements was associated with a significant decrease in the 5-HT and 5-hydroxyindole acetic acid levels within the NG and NTS in 5,7-DHT treated rats. In addition, the microinjection of the neurotoxin in both structures produced a transient and significant increase in BP. This effect was of greater amplitude and associated with an increase in BPV in NG lesioned rats. These results may indicate that the NG-NTS serotonergic pathway participates in the transfer of the messages arising from the aortic baroreceptors. However, the vagal component of the baroreflex assessed with the phenylephrine test was not significantly modified in NG lesioned animals as compared to controls. Consequently, if the present data suggest that the NG-NTS serotonergic pathway plays a depressor role in BP regulation, its involvement in the reflex CV responses triggered by the stimulation of the aortic baroreceptors has yet to be established.

Hypothalamic-midbrain dysregulation syndrome: hypertension, hyperthermia, hyperventilation, and decerebration

Certain decerebrate lesions of brain stem or hypothalamus induce pharmacologically reversible hypertension and hyperthermia in animals. We observed three young patients with episodic decerebration, hyperthermia, hypertension, and hyperventilation during recovery from comas of different etiologies. The shared pathology on neurologic examinations and computed tomographic scans was hypothalamic-mesencephalic dysfunction, suggesting a diencephalic-brain-stem disconnection syndrome or brain-stem release mechanism. Propranolol was the most effective drug tested, but only two patients responded, one dramatically. This novel clinical syndrome may have localizing and therapeutic significance in pediatric coma that needs to be further defined in future studies.

Haemodynamic, metabolic and endocrine effects of short-term dexfenfluramine treatment in young, obese women

Seventeen normotensive, premenopausal women were treated with the 5-hydroxytryptamine-reuptake inhibitor dexfenfluramine 30 mg per day, for 4 days in a randomised double-blind, cross-over, placebo controlled trial. Energy intake was held constant during the study as the aim was to study the endocrine and metabolic effects of dexfenfluramine dissociated from its weight-lowering properties. Body weight, blood glucose, plasma insulin, cholesterol triglycerides and C-peptide after an overnight fast and during an oral load of 100 g glucose did not change after dexfenfluramine compared to placebo. Supine and standing systolic and diastolic blood pressures were significantly decreased, while heart rate remained unchanged. Plasma noradrenaline and plasma renin were markedly reduced by dexfenfluramine, and cortisol, beta-endorphin and thyroid hormones were not changed. Thus, dexfenfluramine has a significant hypotensive effect in normotensive, obese women after 4 days of treatment, independent of a negative energy balance. This was associated with decreased circulating plasma noradrenaline, indicating decreased sympathetic nerve activity. Dexfenfluramine may be a candidate drug for longer-term trials in the treatment of primary hypertension associated with obesity.

Drug-induced supraventricular tachycardia: a case report of fluoxetine

We report the occurrence of supraventricular tachycardia and hypotension in a 54-year-old woman after maintenance therapy with fluoxetine. Although cases of tachycardia and palpitations have been reported, supraventricular tachycardia and hypotension have not been directly attributed to fluoxetine.

Effects of tryptophane on SHRSP offspring growth

The effects of tryptophane on growth and hypertension of offspring obtained from mothers given L-tryptophane prior to mating: In the tryptophane group, body weights were heavier and blood pressures were higher than in the control group. Stroke lesion incidence rates were also much higher in the tryptophane group. At 20 weeks of age, brain enzyme activities were lower and serotonin content was higher in the tryptophane group in comparison with the control. These results suggest that dietary tryptophane may affect precocious maturation and as well as affect elevation in blood pressures due to brain serotonin turnover.

Evidence suggesting that the 5-HT2 antagonist ICI 169,369 activates vagal afferents and in addition has a central hypotensive action in anaesthetized rats

1. The cardiovascular effects of the 5-HT2 antagonist ICI 169,369 have been investigated in pentobarbitone anaesthetized rats and in pithed rats whose blood pressure was supported by a vasopressin infusion. 2. In anaesthetized rats, cumulative doses (0.1-3.0 mg kg-1) of ICI 169,369 caused dose-related large transient falls in heart rate and blood pressure followed by a slow dose-related decline in both parameters. 3. Pretreatment with atropine methonitrate (1 mg kg-1) alone or in combination with atenolol (1 mg kg-1) or bi-vagotomy blocked the transient changes in blood pressure and heart rate caused by ICI 169,369. The long-term fall in heart rate was also attenuated by either atropine or atenolol; however, the combination of atenolol and atropine was more effective. None of the above pretreatments affected the long-term fall in blood pressure caused by ICI 169,369. 4. The cardiovascular effects of ICI 169,369 were unaffected by pretreatment with MDL 72222 (1 mg kg-1) and failed to show cross-tachyphylaxis with phenylbiguanide. 5. In pithed rats whose blood pressure was maintained with vasopressin, ICI 169,369 failed to cause any of the above transient and long-term effects on blood pressure and heart rate. 6. The study indicates that ICI 169,369 is capable of stimulating cardiopulmonary afferents through a non 5-HT3 receptor mechanism and has a central hypotensive action.

Cardiovascular effects of irindalone, a new S2-serotonergic antagonist, in the rat

Irindalone is a new antihypertensive agent with affinity to serotonin (5-HT2) receptors and at higher concentrations also to alpha 1-adrenoceptors. The present study was designed to evaluate the relative importance of the antagonism of central and peripheral alpha 1- and 5-HT2-receptors in the blood pressure lowering properties or irindalone after acute administration. In conscious Sprague-Dawley rats intravenous irindalone (0.05-1.5 mg/kg) dose-dependently reduced the blood pressure. In the same dose-range irindalone antagonized pressor responses to phenylephrine and electrical stimulation of the spinal sympathetic outflow (SNS) in the pithed rats, indicating that the acute blood pressure lowering effect is primarily related to the blockade of alpha 1-adrenoceptors. However, the concomitant 5-HT2-receptor blockade may contribute since irindalone in a dose (0.15 mg/kg) where it had no alpha-adrenoceptor blocking properties enhanced the hypotensive response to selective alpha 1-adrenoceptor blockade by prazosin (1 micrograms/kg). We found no evidence that central mechanisms contributed to the blood pressure lowering effect of irindalone. In anaesthetized rats irindalone (1 mg/kg) did not reduce the directly recorded sympathetic nerve activity. Intracerebroventricular administration of irindalone in conscious rats (10-100 micrograms) had no consistent effects on the blood pressure and did not enhance the hypotensive response to intracerebroventricularly administered prazosin (10 micrograms). Finally, the hypotensive response to irindalone was not influenced by depletion of central serotonin stores (by PCPA). It is concluded that the blood pressure lowering effect of irindalone following acute administration is related primarily to blockade of peripheral alpha-adrenoceptors but that the concomitant blockade of 5-HT2-receptors may contribute.

Studies on the site and mechanism of the sympatholytic action of 8-OH DPAT

Studies in our laboratory indicate that the 5-HT1A agonist 8-OH DPAT acts in the central nervous system at postsynaptic receptor sites to inhibit sympathetic nerve activity and lower arterial blood pressure. The present study was designed to investigate possible postsynaptic sites on central sympathetic neurons where 8-OH DPAT might produce its sympatholytic action in anesthetized cats. The sympatholytic effect of 8-OH DPAT was compared in midcollicular transected and sham operated control animals. Administration of 8-OH DPAT (0.01-1.0 mg/kg, i.v.) inhibited sympathetic activity and decreased blood pressure in both the transected and sham animals to a similar degree. The effects of microiontophoretically applied 8-OH DPAT and 5-HT on antidromically identified sympathetic preganglionic neurons were determined. Microiontophoretically applied 5-HT consistently increased the firing rate of sympathetic preganglionic neurons. Iontophoretic 8-OH DPAT failed to affect the firing of sympathetic preganglionic neurons but blocked the excitatory effects of 5-HT. The effects of 8-OH DPAT and 5-HT on the firing of sympathoexcitatory neurons located in the rostral ventrolateral medulla were also determined. Sympathoexcitatory neurons were identified using spike triggered averaging techniques and by their response to baroreceptor activation. Intravenous administration of 8-OH DPAT inhibited the firing of sympathoexcitatory neurons in the rostral ventrolateral medulla. The inhibition of unit firing produced by 8-OH DPAT was exactly paralleled by the shutoff of inferior cardiac nerve activity. Microiontophoretic application of 8-OH DPAT and 5-HT onto sympathoexcitatory neurons in the rostral ventrolateral medulla failed to affect the firing rate of these neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of enflurane on 5-hydroxytryptamine transport in synaptosomes from rat brain

The administration of volatile anesthetics to laboratory animals has been reported to alter brain 5-hydroxytryptamine (serotonin, 5-HT) homeostasis. To examine a potential anesthetic action that could account for these observations, the effect of enflurane on 5-HT accumulation by rat brain synaptosomes was examined. Established techniques were used to prepare synaptosomes and perform uptake assays using [3H]5-hydroxytryptamine as substrate. Exposure of the synaptosomes to enflurane resulted in a concentration-dependent inhibition of serotonin uptake; the apparent I50 was 1.4 +/- 0.3 mM enflurane. Maximum inhibition was observed between enflurane concentrations of 2.6 and 4.3 mM, which inhibited uptake between 62 and 70%. The inhibition was rapid and reversible, and kinetic analysis of the inhibition was consistent with competitive inhibition by enflurane of 5-HT uptake with an apparent KI of 1.61 +/- 0.07 mM. In summary, exposure of synaptosomes to clinically relevant concentrations of enflurane resulted in a rapid, concentration-dependent, and reversible inhibition of 5-HT accumulation. These observations could represent a molecular interaction contributing to the anesthetic properties of enflurane and other volatile anesthetics.

Monoamine and amino acid content in brain regions of Brattleboro rats

Monoamine and amino acid content were measured in brain regions from 12 week old male, homozygous Brattleboro (DI, n = 12) and Long-Evans control (LE, n = 12) rats. Norepinephrine (NE) content was significantly elevated (16-25%) in the spinal cord, pons-medulla and anterior hypothalamus of DI rats when compared to LE controls. NE content of the neurointermediate lobe of pituitary in DI rats was almost twice that of LE controls. Serotonin content was also significantly elevated in the spinal cord, pons-medulla, anterior hypothalamus and forebrain of DI rats relative to the LE controls. Taurine content in DI rats was increased (31-42%) above that of LE rats in the anterior hypothalamus, striatum and forebrain. Glutamine content was also greater in DI rats than LE in the spinal cord, pons-medulla, anterior hypothalamus, striatum, hippocampus and forebrain. The changes in monoamine and amino acid content were discussed in relation to the cardiovascular and osmoregulatory deficits that are present in DI rats due to arginine vasopressin (AVP) deficiency. The possible role of AVP in modulating NE turnover was also discussed. The increase in brain TAU content in DI rats may be a physiological response to hypernatremia.

Comparison of the cardiovascular effects of the 5-HT1A receptor agonist flesinoxan with that of 8-OH-DPAT in the rat

The cardiovascular response to flesinoxan and 8-OH-DPAT (8-hydroxy-2-(di-N-propylamino)tetralin), 5-HT1A receptor agonists, has been investigated in anaesthetized Wistar rats and spontaneously hypertensive rats (SHR) and in conscious SHR. Flesinoxan and 8-OH-DPAT potently lowered blood pressure and heart rate in these models. In conscious SHR, atropine reversed the bradycardia induced by flesinoxan partially and that induced by 8-OH-DPAT completely. In pithed rats with vasopressin-raised blood pressure, neither flesinoxan nor 8-OH-DPAT lowered blood pressure or heart rate. Intracisternal administration of either flesinoxan or 8-OH-DPAT was less efficacious than intravenous administration. The cardiovascular responses to flesinoxan and 8-OH-DPAT in the anaesthetized Wistar were inhibited by the putative 5-HT1A antagonists methiothepin, buspirone, spiroxatrine and 8-MeO-C1EPAT (8-methoxy-2-(N-2-cholroethyl-N-n-propylamino)tetralin). 8-MeO-C1EPAT appeared to be the most suitable antagonist in this model. The 5-HT1C, antagonist ritanserin or the 5-HT3 antagonist GR 38032F had no effect on the responses to flesinoxan or 8-OH-DPAT. In conscious SHR however, 8-MeO-C1EPAT did not antagonize these cardiovascular responses. This study confirms the involvement of central 5-HT1A receptors in the cardiovascular effects of flesinoxan and 8-OH-DPAT.

Role of central serotonergic systems in the development of hypertension in spontaneously hypertensive rats

The spontaneously hypertensive rat (SHR) maintained a higher blood pressure level at and after 8 weeks old than the genetical control Wistar-Kyoto strain (WKY). At 10 weeks old, the turnover rate of 5-hydroxytryptamine (5-HT) was lower in the hypothalamus of SHR than of WKY. Following portacaval anastomosis (PCA) in SHR, the blood pressure was significantly decreased in comparison with that of sham-operated control SHR. In WKY, no significant change in the blood pressure response was observed. PCA treatment increased the 5-HT turnover including that in SHR. If the SHR with PCA was bred with food pellets containing higher concentrations of leucine and isoleucine, the blood pressure increased and the 5-HT turnover decreased. These findings suggest that the central serotonergic system is involved in the development of hypertension.

Studies on the site and mechanism of the sympathoexcitatory action of 5-HT2 agonists

Intravenous administration of the selective 5-HT2 agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), produced an increase in sympathetic activity recorded from the inferior cardiac nerve in chloralose-anesthetized cats. Microiontophoretically applied 5-HT increased the firing rate of antidromically identified sympathetic preganglionic neurons. Microiontophoretic DOI failed to affect the firing of sympathetic preganglionic neurons. The effect of DOI was also studied on medullospinal sympathoexcitatory neurons located in the rostral ventrolateral medulla. Intravenous DOI increased the firing of sympathoexcitatory neurons and sympathetic nerve discharge to a similar extent. Microiontophoretic application of DOI failed to affect the firing of sympathoexcitatory neurons. The data are discussed in relation to the site and mechanism of the sympathoexcitatory action of 5-HT2 agonists.

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.

Halothane inhibits 5-hydroxytryptamine uptake by synaptosomes from rat brain

The activity of 5-hydroxytryptamine (serotonin; 5-HT) in the central nervous system modulates sleep, the perception of pain and other functions of the body which might possibly relate to mechanisms of general anesthetic action. While administration of anesthetics has inconsistent effects on the content of 5-HT in brain, in vivo, accumulated data suggest that anesthetic drugs alter 5-HT homeostasis in the central nervous system. In an effort to identify one possible site of anesthetic action, the effect of halothane on the uptake of 5-HT was studied in synaptosomes isolated from the brain of rat. Established techniques were used to prepare the synaptosomal fractions and measure high affinity transport of radiolabelled 5-HT. Halothane inhibited synaptosomal accumulation of 5HT in a concentration-dependent manner, but had little effect on the passive or spontaneous release of the accumulated 5-HT. Rates of uptake of 5HT were inhibited by 43% in the presence of 1 mM halothane and by 75% of control in the presence of 5 mM halothane; the apparent I50 for halothane was 1.0 +/- 0.1 mM and Lineweaver-Burk analysis indicated the inhibition to be competitive at concentrations around the I50.

Role of neurotransmitters in the central regulation of the cardiovascular system

The last decade has seen tremendous progress in determining the nature of the neurotransmitters which regulate central nervous system pathways involved in the regulation of blood pressure. Investigations are now pursuing the identity and functional importance of neurotransmitters contained within pathways shown to be important in cardiovascular regulation. In addition, several key components of the brain stem networks involved in the control of sympathetic activity have been identified. For example, numerous studies indicate the importance of neurons located in the rostral ventrolateral medulla in the regulation of SPN. Indeed, this area contains medullospinal sympathoexcitatory neurons which represent the final site of integration of many brain stem and reflex pathways involved in the regulation of sympathetic nerve activity. The neurotransmitter which is utilized by this medullospinal pathway remains unknown. Epinephrine, substance P and glutamate have all been hypothesized as primary chemical mediators in the descending pathway from the brain stem to SPN. Interestingly, lesions of, or antagonists to, epinephrine, substance P, glutamate and 5-HT neurons all abolish sympathetic activity and reduce blood pressure to a level similar to that in a spinal animal. Clearly, not all these transmitters are primary mediators of sympathetic information carried from the brain stem to the spinal cord. It is likely that monoamines and neuropeptides act in the IML, as in other area of the central nervous system, as neuromodulators to set the level of excitability of SPN rather than relaying sympathetic information over a functionally specific medullospinal pathway. This conclusion is supported by the observation that midline medullary 5-HT neurons provide a tonic excitatory input to SPN, but receive no afferent inputs from other central sympathetic or baroreceptor pathways. However, the firing of 5-HT neurons appears to relate to the state of vigilance of the animal. This suggests that 5-HT neurons may lower the threshold of SPN to sympathetic inputs during states of wakefulness. In addition, the time course of the norepinephrine-mediated slow EPSPs and IPSPs in SPN is consistent with a gain-setting function. By analogy, epinephrine is likely to act as a neuromodulator in the IML rather than to serve as the primary mediator of sympathetic information descending from the rostral ventrolateral medulla.(ABSTRACT TRUNCATED AT 400 WORDS)

The role of 5-hydroxytryptamine and 5-hydroxytryptaminergic mechanisms in hypertension

1. The possible role of 5-hydroxytryptamine (5-HT) and 5-HT-receptors in hypertension, already suggested by Page in 1954, has been subject to a renaissance of interest owing to the development of antihypertensive drugs which interact with 5-HT-receptors. These drugs, like ketanserin, urapidil and flesinoxan are used as tools to study the role of 5-HT and its receptors in hypertension. 2. The following arguments would plead in favour of a certain role of 5-HT and 5-HT-receptors in the pathogenesis and maintenance of hypertension: hyperresponsiveness of blood vessels from hypertensive patients and animals to 5-HT-induced constriction; the antihypertensive/vasodilator activity of the 5-HT2-receptor antagonist ketanserin; enhanced sensitivity of platelets from hypertensives to 5-HT. 3. However, at least as many arguments would deny a role of peripheral 5-HT in hypertension, like for instance: the fact that 5-HT is not a generally accepted pressor agent, whereas its concentration in the circulating blood is subthreshold; the hyperresponsiveness of vessels from hypertensives is not specific for 5-HT; the elevated local concentration of 5-HT released by platelets is mainly limited to the microcirculation; the 5-HT2-receptor antagonist ketanserin is the only agent of this type which lowers blood pressure, other 5-HT2-receptor blockers like ritanserin and LY 53587 being inactive. 4. The various data and arguments available do not unequivocally support a relevant role of peripheral 5-HT and its receptors in hypertensive disease.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of dietary tryptophan on the development of hypertension in the Dahl salt-sensitive rat

Dahl salt-sensitive rats are inbred on the basis of their tendency to develop hypertension when placed on a high salt diet. The present study investigated the effects of chronic dietary tryptophan (trp) at 50 g/kg food on the development of hypertension in these animals under conditions of both normal and elevated dietary salt. Dietary trp attenuated the development of hypertension in inbred Dahl salt-sensitive (DS/JR) rats and had no effect upon the patterns of development of systolic blood pressures in the normotensive controls, the inbred Dahl salt-resistant (DR/JR) rat and the outbred parental Sprague Dawley (SD) rat. Cardiac hypertrophy, which is associated with Dahl salt-induced hypertension, was blocked by the high trp diet. Further work will be necessary to elucidate the mechanisms by which dietary trp protected against the development of hypertension and cardiac hypertrophy in DS/JR rats.

Central serotonergic mechanisms in cardiovascular regulation

This paper reviews the role of central serotonin-containing neurons in the control of blood pressure. Central serotonin nerves have their cell bodies in the brainstem in a number of discrete collections, from where they ascend to ramify throughout the brain, descend to terminate in the spinal cord, or send shorter projections terminating in medulla, pons, and midbrain. Activation of one important ascending serotonin pathway innervating the preoptic region of the hypothalamus causes an increase in blood pressure. Activation of a bulbospinal serotonin projection descending from the ventrolateral medulla (the B3 cell group) to terminate in the intermediolateral cell column (IML) also evokes a pressor response. This pressor response is independent of that elicited by stimulation of the ventrolateral medulla in the adjacent but separate area containing the C1 adrenaline cell group. The pressor action appears to depend on increased release of serotonin, as detected by microdialysis in the area of the IML, and to be mediated by serotonin receptors of the 5HT1 subclass, probably located on sympathetic preganglionic neurons. It is possible that neuroactive excitatory amino acids, such as glutamate or aspartate, and neuropeptides such as substance P, also play a part in the pressor response evoked by stimulation of the ventrolateral medulla in the area of the lateral B3 serotonin cells. This descending serotonin pathway also appears important in mediating the hypotensive action of the antihypertensive drugs methyldopa and clonidine.

Kinetics of serotonin in platelets in essential hypertension

In a study of the kinetics of 5-hydroxytryptamine (5-HT) in platelets in 26 hypertensive subjects with a mean systolic and diastolic blood pressure of 153.9 +/- 26.9 and 106.9 +/- 9.1 mm Hg respectively, it has been observed that in hypertensive platelets there was a marked decrease in 5-HT uptake and content, an increase in 5-HT efflux and an accompanying increase in Plasma 5-HT and 5-HIAA levels. Regression analysis of the data indicated a significant correlation between rise in diastolic blood pressure and these changes in 5-HT kinetics.

Serotonin and 5-hydroxytryptophan on blood pressure and renal blood flow in anesthetized rats

The effects of exogenous (injected) serotonin on mean blood pressure and renal blood flow in male Sprague-Dawley rats were compared with the effects of enhanced endogenous serotonin, accomplished by injections of 5-hydroxytryptophan (5-HTP). Responses were evaluated with and without carbidopa, which blocks peripheral conversion of 5-HTP to serotonin, and with and without indomethacin to assess the contribution of prostanoids. Both serotonin (0.25-1.0 micrograms) and 5-HTP (50-200 micrograms) decreased blood pressure and renal blood flow and increased renal vascular resistance in dose-dependent fashion. Hypotensive responses with both agents were greater after arterial injections into the aortic arch than after intravenous injections (into the jugular vein). Carbidopa had no significant effects on responses to serotonin but ablated the ability of 5-HTP to increase renal vascular resistance and decrease renal blood flow. Carbidopa did not alter the hypotensive action of 5-HTP. Indomethacin had no significant effect on responses to serotonin or 5-HTP. These results support a direct renal vasoconstrictor effect for exogenous and endogenously formed serotonin and a hypotensive effect probably arising in the central nervous system.

Changes in serotonin metabolism in the rat raphe magnus and cardiovascular modifications following systemic administration of clonidine and other central alpha 2-agonists: an in vivo voltammetry study

By using the in vivo voltammetry, it was demonstrated that an injection of clonidine induced both cardiovascular modifications (hypotension and bradycardia) and a decrease in the level of 5-hydroxyindolacetic acid (5-HIAA) in the ventromedial B3 serotonergic (5-HT) cell bodies of the medulla oblongata of the rat. The cardiovascular effects of clonidine and of two other imidazolic compounds (detomidine and medetomidine) are likely to be related to their alpha 2 adrenoceptor agonist properties since hypotension and bradycardia were completely antagonized by idazoxan. The decrease in levels of 5-HIAA, induced by these three imidazolic compounds is likely to represent the combination of two additional mechanisms: (i) the stimulation of the alpha 2 adrenoceptors which could contribute to 55% of the decrease observed for the extracellular 5-HIAA and (ii) the interaction with a non-alpha 2 site (through a putative imidazole recognition site), corresponding to the part of the decrease (about 45%) which was not prevented by idazoxan.

Ventrolateral medullary pressor area: site of hypotensive and sympatho-inhibitory effects of (+/-)8-OH-DPAT in anaesthetized dogs

Injections of 8-OH-DPAT (0.1-6 micrograms/kg) into the vertebral artery or into the cisterna magna (5 micrograms/kg) produced a dose-dependent decrease in blood pressure, heart rate and renal sympathetic nerve activity in intact anaesthetized dogs and baroreceptor-denervated dogs. 8-OH-DPAT reduced the renal sympathetic nerve activity without changing the blood pressure or heart rate in catecholamine-depleted animals. Methiothepin (0.2 mg/kg) injected into the vertebral artery reduced the blood pressure without changing the heart rate or renal sympathetic nerve activity in baroreceptor-denervated dogs. The pressor response to i.v. phenylephrine was largely attenuated. Subsequent administration of 8-OH-DPAT (3 micrograms/kg) into the vertebral artery failed to alter the sympathetic discharge. Methiothepin (0.2 mg/kg) injected into the vertebral artery reversed the sympatho-inhibitory effect of 8-OH-DPAT (3 micrograms/kg) injected by the same route without changing the blood pressure. (+/-)Pindolol (0.2 mg/kg) injected into the vertebral artery of baroreceptor-denervated dogs reduced the blood pressure and heart rate without changing renal sympathetic activity. Subsequent administration of 8-OH-DPAT (3 micrograms/kg) failed to alter the sympathetic discharge. Bilateral microinjection of 8-OH-DPAT (1 microgram) into the nucleus tractus solitarii or into the medullary raphe nuclei failed to alter the blood pressure, heart rate or renal sympathetic activity. In contrast, bilateral microinjections of 8-OH-DPAT into the ventrolateral pressor area (VLPA) (0.2 microgram) produced a marked decrease in blood pressure, heart rate and renal sympathetic nerve activity. These effects were prevented or reversed by microinjections of methiothepin (10 micrograms) at the same sites. These results indicate that the central sympatho-inhibitory effects of 8-OH-DPAT were due to the stimulation of 5-HT1A receptors located in the ventrolateral pressor area. 5-HT autoreceptors did not seem to be involved.

Evidence for the interaction of urapidil with 5-HT1A receptors in the brain leading to a decrease in blood pressure

Current knowledge about the role of serotonin (5-HT) in central cardiovascular regulation is reviewed. Results from experiments with the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) suggest that activation of somatodendritic 5-HT1A receptors in the medulla oblongata decreases the firing of serotoninergic neurons and thus reduces their excitatory input to the sympathetic neurons in the intermediolateral cell column. As a consequence, blood pressure is reduced by 5-HT1A receptor agonists. Urapidil is an antihypertensive drug that has a dual mode of action: peripheral alpha-adrenoceptor antagonism and interaction with 5-HT1A receptors in the brain. This profile can adequately explain the vasodilation and lack of significant sympathetic activation observed during urapidil treatment.

Monoamine depletion does not alter angiotensin II binding sites in the rat brain

6-Hydroxydopamine (6-OHDA) or 5,7-dihydroxytryptamine (5,7-DHT) was administered in the lateral cerebral ventricles of rats. These respective treatments caused reductions in norepinephrine (NE) and 5-hydroxytryptamine (5-HT) levels in the hypothalamus-thalamus-septum-midbrain (HTSM; 62% and 44%) and brainstem (57% and 26%), respectively. Catecholamine depletion in 6-OHDA-treated rats resulted in a 55% decrease in the pressor response to ICV Ang II which was not associated with any significant change in the binding of 125I-sarcosine1, isoleucine8 Ang II to Ang II binding sites in the HTSM or brainstem. The radioligand binding was not significantly affected by the 5,7-DHT treatment. These results indicate that brain Ang II binding sites are not predominantly localized on NE- or 5-HT-containing nerve terminals. They do not, however, preclude the existence of a small proportion of Ang II binding sites on these neuronal populations.

Reduced urinary serotonin excretion after intake of high doses of hypoxanthine

Two healthy volunteers were treated with hypoxanthine 3 x 1 g and allopurinol 3 x 100 mg daily for 1 week. During this treatment serum oxypurine concentration and urinary oxypurine excretion increased as expected. No side effects were observed except for some mild daytime drowsiness and lethargy. Measurements of urinary serotonin (5-HT) excretion showed decreases to as much as 60% below initial values. Decreased urinary 5-HT excretion was also found in a patient with incomplete Lesch-Nyhan syndrome during treatment with high doses of hypoxanthine. His neurological symptoms improved slightly. The results suggest that high doses of hypoxanthine exert a nonspecific sedative effect on both patients with Lesch-Nyhan syndrome and healthy controls. The cause is probably a reduced synthesis or release of 5-HT.

Comparison between the cardiovascular effects of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) and clonidine in the conscious sino-aortic denervated rat

1. The purpose of this study was to investigate the role of an intact baroreceptor reflex mechanism in the expression of the cardiovascular response to 8-OH-DPAT and to determine whether there are any differences between the activation of central alpha 2-adrenoreceptors and 5-HT1A receptors in this respect. To this end, the effects of 8-OH-DPAT and clonidine have been assessed on blood pressure, heart rate, ECG and cardiac contractility indices in conscious sino-aortic baroreceptor denervated (SAD) rats and their sham-operated controls. 2. In both sham-operated and SAD rats, intravenous (i.v.) administration of 8-OH-DPAT (32 micrograms kg-1) and clonidine (8 micrograms kg-1) produced falls in systemic blood pressure, left ventricular systolic pressure and dP/dtmax. 3. 8-OH-DPAT produced similar bradycardia in each group of rats; in contrast, clonidine had a greater effect in the SAD animals. Increases of the PQ interval mirrored the heart-rate changes with both compounds. 4. No significant changes in end diastolic blood pressure or in the myocardial contractility indices dP/dtmax/P and Vmax were evident. 5. This study provides support for the view that i.v. 8-OH-DPAT lowers blood pressure and heart rate through a central mechanism. The effects occur independently of an intact baroreceptor reflex and are not associated with effects on myocardial contractility. 8-OH-DPAT shows close qualitative similarities to clonidine in this model.

Interaction of thyrotropin-releasing hormone (TRH) and serotonin in cardiovascular control

Intravenous injection of the synthetic TRH analog, MK-771, to anesthetised cats raised the blood pressure by a central mechanism, i.e. by activating the outflow of sympathetic stimuli from the CNS to the periphery and raising the plasma concentration of adrenaline and noradrenaline. In contrast, noradrenaline in the CSF was lowered by intravenously injected MK-771. The concentration of 5-HT in the CSF was increased and the pressor action of the peptide was correspondingly enhanced by 5-HT antagonists. These findings suggest that the release of 5-HT into the CSF lowers blood pressure and thus reduces the hypertensive effect of MK-771. Methysergide acts synergistically with MK-771 to combat the sudden fall in pressure induced by acute hemorrhage in the anesthetised cat. Combination of the two drugs enhanced and accelerated recovery from this hypotensive state.

Changes in neurotransmitter turnover in locus coeruleus produced by changes in arterial blood pressure

The effect of drug-induced hypertension and hypotension on neurotransmitter metabolism in the locus coeruleus (LC) of urethane anesthetized rats was studied using in vivo electrochemical methods. Peaks were seen at +0.15 V and +0.28 V. Studies with alpha-methylparatyrosine, fusaric acid and pargyline showed the first peak was produced by extracellular fluid dihydroxyphenylacetic acid (DOPAC) while the second peak was 5-hydroxyindoleacetic acid (5-HIAA). Phenylephrine was infused intravenously to raise the blood pressure by 50 mmHg, nitroprusside IV was used to reduce the blood pressure by 20 mmHg. During phenylephrine hypertension, the electrochemical signal for DOPAC showed an initial small reduction followed by a later significant increase which persisted even after the infusion was stopped. The signal for 5-HIAA rose with the onset of hypertension and remained elevated. Nitroprusside hypotension did not change the DOPAC peak but did lead to an immediate and persistent increase in the electrochemical 5-HIAA peak. To confirm the electrochemical findings, other groups of rats were decapitated during and after hypertensive and hypotensive drug infusions and the LC was assayed for norepinephrine, dopamine, DOPAC, serotonin, and 5-HIAA using HPLC with electrochemical detection. Changes in tissue DOPAC and 5-HIAA concentrations supported the electrochemical electrode observations. The effect of clonidine on the electrochemical recordings from LC was also studied. Clonidine reduced the catechol peak. No change was observed in the 5-HIAA peak during the infusion, but the 5-HIAA peak went up after the infusion was stopped. These experiments show that hypertension, hypotension, and alpha-2 agonists lead to changes in catecholamine and indoleamine metabolism in LC.

Chromatography of urinary indole derivatives

Latest strategies are discussed for the routine chromatographic analysis of clinically important indole derivatives in urine. Analysis of 5-hydroxyindoleacetic acid and, perhaps more importantly, serotonin and 5-hydroxytryptophan remains attractive in the screening for carcinoid tumours and their differentiation. Analyses of two precursors of the skin pigment eumelanin seem to be promising in the monitoring of treatment of malignant melanoma and screening for pigmentation disorders and gallstone formation. Studies on the clinical relevance of the determination of tetrahydro-beta-carbolines and melatonin-related indoles await routine application of chromatographic methods designed to take into consideration the relative instability of these compounds. Application of GC-MS, although confined to larger and/or governmental laboratories remains attractive as a way of improving the specificity of analyses and in establishing reference methods. As for HPLC, the recent development of chromatographic and detection methods for the concurrent determination of different clinically important and metabolically related compounds from the same sample, preferably by direct injection techniques, seems to be fruitful and should be continued.

5-HT2 receptor agonists increase spontaneous sympathetic nerve discharge

The selective 5-HT2 agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) produced a marked increase in spontaneous sympathetic nerve discharge recorded from the inferior cardiac nerve in chloralose anesthetized cats. DOI (0.01-1.0 mg/kg i.v.) increased sympathetic nerve discharge to a maximum of 1750% of control values. The increase in sympathetic nerve discharge produced by DOI was reversed by the 5-HT2 antagonists ketanserin and LY 53857. In addition, pretreatment with ketanserin, but not prazosin, completely prevented the increase in sympathetic nerve discharge produced by DOI. These data are discussed in relationship to the role of serotonin in the regulation of activity in central sympathetic pathways.

Excitatory and inhibitory actions of intrathecally administered 5-hydroxytryptamine on sympathetic nerve activity in the rat

In chloralose/urethane anaesthetised rats the effect of intrathecal (i.t.) administration of 5-hydroxytryptamine (5-HT) on activity in sympathetic nerves to the kidney was studied. Intrathecal 5-HT (20-100 micrograms, 10 microliter) increased activity in renal nerves (mean change for 50 micrograms dose 157% +/- 39). At higher doses of 5-HT the initial excitatory response was followed by inhibition 64% +/- 15. Intrathecally, alpha methyl 5-HT (10-400 micrograms) mimicked only the inhibitory action of 5-HT in a dose-dependent manner. These inhibitions were completely antagonised by i.t. ketanserin (25-100 micrograms). Intrathecal 2 methyl 5-HT (10-200 micrograms) had little effect on renal nerve activity. Intrathecal 5 carboxyamidotryptamine (10-200 micrograms) had a more powerful excitatory action on renal nerve activity than did 5-HT. Intrathecally, 8 OH-DPAT (10-200 micrograms) was without effect. Neither the inhibitory action of 5-HT or the excitatory action was affected by i.t. ICS 205 930 (1-10 micrograms) or MDL 72222 (200 micrograms). It was concluded that the actions of 5-HT were within the spinal cord and that the inhibitory effect on sympathetic activity is mediated via a 5-HT2-like receptor whilst the receptors involved in the excitatory action of 5-HT are not M- or 5-HT3-like but appear to be more like the 5-HT1 subtype.

Antihypertensive effects of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) in conscious dogs

Low doses of 8-OH-DPAT (10 and 25 micrograms/kg) administered subcutaneously (s.c.) to renal hypertensive mongrel dogs caused decreases in systolic blood pressure which persisted for 3 h. Mild salivation and hyperventilation were observed with both doses. A short-lasting (less than 60 min) depressor response was seen with 100 micrograms/kg s.c. Prominent hyperventilation and salivation accompanied this response. A still higher dose (250 micrograms/kg s.c.) induced tremor, signs of anxiety and occasional vomiting in addition to the hyperventilation and salivation. Paradoxically, no cardiovascular activity was noted at this dose.

Fenfluramine-induced behavior changes in rats prefed serotonin-altering amounts of tryptophan and pyridoxine

It has been well established that elevated dietary tryptophan (TRP) levels can increase brain serotonin concentrations, thereby influencing serotonergic transmission. We previously examined interaction between dietary substrate (TRP: 0.15 and 0.6%) and the cofactor precursor (pyridoxine HCl: 3 and 3,000 mg/kg) on brain serotonin metabolism, observing significant increases in serotonin concentrations from such dietary interaction. The present experiments were designed to explore possible behavioral consequences of the substrate-cofactor interaction. After the IP injection of fenfluramine (FA: at 5, 10, 15, and 20 mg/kg), serotonin-mediated behavior traits and the appearance of flushing were observed in rats fed experimental diets as stated above. With a 5 mg/kg dose of FA, a differential dietary effect was most visible. However, at higher FA levels (15 and 20 mg/kg), such dietary effects were no longer discernible. The appearance of flushing was also dependent on dietary TRP intake and the dosage of FA. These results indicate a clear substrate-cofactor interaction on certain serotonin-mediated behavior traits in the rat.

Haemorrhage-induced release of noradrenaline, 5-hydroxytryptamine and uric acid in the supraoptic nucleus of the rat, measured by microdialysis

Concentrations of monoamines and metabolites and uric acid were measured in sequential 15-min microdialysis samples taken from the area of the supraoptic nucleus (SON) in 9 urethane-anaesthetized rats. A decrease in blood pressure of 40 mm Hg was induced by withdrawing 3-4 ml of blood from an indwelling jugular catheter. This decreased blood pressure was maintained for 30 min and produced a significant rise in the concentrations of noradrenaline (NA) and its metabolite 4-hydroxy-3-methoxyphenylethan-1,2-diol (MHPG) and also in 5-hydroxytryptamine (5-HT) and the purine metabolite uric acid (UA). When blood pressure was returned to normal by replacing the withdrawn blood, the concentrations of NA, MHPG, 5-HT and UA returned to pre-haemorrhage levels within 15 min. No significant changes in concentrations of dopamine and its metabolites were found, and in 7 control animals sampled 1.5-2 mm away from the SON, in the area of the olfactory tubercle, concentrations of NA, MHPG, 5-HT and UA were not significantly altered by haemorrhage-induced decreases in blood pressure.

Hypotensive spinal serotonergic effect. Are S1 or S2 receptors involved?

Since previous data obtained in anesthetized rats supported the idea that the activation of spinal serotonergic receptors induced a hypotensive effect, it was decided to characterize more closely the serotonergic spinal involvement and to elucidate the serotonergic receptor type involved in this effect. After female Wistar rats were anesthetized, the femoral artery (for blood pressure measurement) and vein (for parenteral injection of drugs) were cannulated. An intrathecal catheter was positioned with the tip at the T6-L3 intervertebral space. The results showed that the dose-dependent decrease in mean blood pressure induced by serotonin administered at the T6-L3 level was prevented by giving the serotonergic S2 antagonist ritanserin intravenously. The intravenous administration of 5-methoxy-N,N-dimethyltryptamine, a direct serotonergic agonist, induced a dose-dependent hypotension previously shown to originate at spinal cord level. This effect was prevented by intrathecal administration of ketanserin, an S2-receptor antagonist. The selective agonist of the S1-type receptors, 8-hydroxy-dipropylaminotetralin, given at the same level of the spinal cord, failed to induce any effect on mean blood pressure. The results suggest that the hypotensive effect obtained after the spinal serotonergic activation involves serotonin receptors of the S2 type.

Serotoninergic mechanisms in hypertension. Focus on the effects of ketanserin

Aggregating platelets release serotonin, which induces contraction of most vascular smooth muscle by activation of S2-serotoninergic receptors. Serotonin released in the circulation may contribute to the increase in peripheral resistance of hypertension as the responsiveness of blood vessels from hypertensive animals and humans to the vasoconstrictor action of the monoamine is augmented. The data obtained with the new antihypertensive agent ketanserin may favor that interpretation. Ketanserin is a selective S2-serotoninergic antagonist with additional alpha 1-adrenergic blocking properties. In humans, it has a terminal half-life of 12 to 25 hours and is eliminated predominantly by the liver. The hemodynamic profile of ketanserin is that of a vasodilator drug with actions on both resistance and capacitance vessels. On short-term intravenous administration, it lowers blood pressure in hypertensive patients with minimal reflex changes in cardiovascular function. When given orally long term to hypertensive patients, ketanserin causes a sustained reduction in arterial blood pressure, comparable to that obtained with either beta-adrenergic blockers or diuretics. Several studies have shown a greater efficacy in older (greater than 60 years of age) than in younger patients independent of starting pressure. Side effects mainly consist of dizziness, somnolence, and dry mouth, but they are usually not severe. The mechanism underlying the antihypertensive effect of ketanserin is unclear. It cannot be attributed to either S2-serotoninergic or alpha 1-adrenergic blockade alone, but an interaction between the two effects appears to be required.

Role of central serotonergic (5-HT2) receptor in blood pressure regulation in rats

To investigate the role of the serotonergic nervous system in blood pressure regulation, 5 micrograms of 5-hydroxytryptamine (5-HT) was given i.c.v. before and after 1 microgram of i.c.v. xylamidine or 200 micrograms of i.c.v. ketanserin or 200 micrograms of i.v. ketanserin in conscious Wistar Kyoto rats. Also i.v. (0.5, 1, 2 micrograms) or i.c.v. (1 microgram) phenylephrine (PHE) were given before and after 1 microgram of i.c.v. xylamidine. I.c.v. 5-HT elicited a consistent pressor response of approximately 27mmHg and slight decrease in heart rate. MAP and heart rate did not change after xylamidine or ketanserin. Whereas pressor response to i.c.v. 5-HT after i.c.v. ketanserin or i.c.v. xylamidine was suppressed, it did not change after i.v. ketanserin. Neither i.c.v. nor i.v. PHE-induced pressor response was influenced by i.c.v. xylamidine pretreatment. These data suggest that the central 5-HT2 receptor may subserve pressor function in rats.

The effects of the 5 HT2 antagonist ritanserin on blood pressure and serotonin-induced platelet aggregation in patients with untreated essential hypertension

We have given the selective 5 HT2 antagonist ritanserin in a dose of 10 mg twice daily for 4 weeks in a double-blind, randomized, placebo-controlled, parallel group study of 18 patients with untreated essential hypertension. The fall in single platelet count due to 5 HT-induced platelet aggregation was significantly reduced by ritanserin compared with placebo (p less than 0.05). There were no significant changes in supine or erect blood pressure or heart rate after ritanserin compared to placebo. Forearm blood flow, measured by mercury-in-strain gauge venous occlusion plethysmography, was not significantly altered by ritanserin. Ritanserin caused prolongation of the QTc interval by 41 (SEM 11) ms (p less than 0.05 compared to placebo) but had no detectable effect on QRS duration, features suggestive of Class III antiarrhythmic activity. These findings do not support an independent role of the 5 HT2 receptor in maintaining raised arterial pressure in essential hypertension.

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.