Antihypertensive effects of fluoxetine and L-5-hydroxytryptophan in rats

Hypertension associated with serotonin reuptake inhibitors: A new analysis in the WHO pharmacovigilance database and examination of dose-dependency

INTRODUCTION

Recent literature has reported instances of drug associated with hypertension with serotonin reuptake inhibitors (SRIs). Nonetheless, the association between SRIs and hypertension development is the subject of ongoing debate. It remains uncertain whether this is indicative of a class effect, and if dose-effect exist. To investigate the potential class effect associating SRIs with hypertension reporting, we utilized real-world data from VigiBase®, the World Health Organization (WHO) pharmacovigilance database.

METHODS

We conducted an updated disproportionality analysis within VigiBase® to identify a signal of hypertension reporting with individual SRIs by calculating adjusted reporting odds ratios (aRORs) within a multivariate case/non-case study design. Additionally, we explored the presence of a dose-effect relationship.

RESULTS

The database contained 13,682 reports of SRI associated with hypertension (2.2%), predominantly in women (70.0%). Hypertension was most reported in the 45-64 years old age group (44.8%). A total of 3,879 cases were associated with sertraline, 2,862 with fluoxetine, 2,516 with citalopram, 2,586 with escitalopram, 2,441 with paroxetine, 201 with fluvoxamine and 8 with zimeldine. A significant ROR was observed for all SRIs in both univariate (RORs ranging from 1.39 to 1.54) and multivariable analyses (aRORs ranging from 1.16 to 1.40) after adjustments for age group, sex, concurrent antihypertensive medication and drugs knowns to induce hypertension, except for fluvoxamine and zimeldine. No dose-response relationship was identified.

CONCLUSION

This investigation, conducted under real life conditions, unveils a notable pharmacovigilance safety signal associating SRI usage with hypertension reporting. No dose-response effect was detectable. Further longitudinal studies are warranted.

Drug-Related Orthostatic Hypotension: Beyond Anti-Hypertensive Medications

Orthostatic hypotension (OH) is an abnormal blood pressure response to standing, which is associated with an increased risk of adverse outcomes such as syncope, falls, cognitive impairment, and mortality. Medical therapy is one the most common causes of OH, since numerous cardiovascular and psychoactive medications may interfere with the blood pressure response to standing, leading to drug-related OH. Additionally, hypotensive medications frequently overlap with other OH risk factors (e.g., advanced age, neurogenic autonomic dysfunction, and comorbidities), thus increasing the risk of symptoms and complications. Consequently, a medication review is recommended as a first-line approach in the diagnostic and therapeutic work-up of OH, with a view to minimizing the risk of drug-related orthostatic blood pressure impairment. If symptoms persist after the review of hypotensive medications, despite adherence to non-pharmacological interventions, specific drug treatment for OH can be considered. In this narrative review we present an overview of drugs acting on the cardiovascular and central nervous system that may potentially impair the orthostatic blood pressure response and we provide practical suggestions that may be helpful to guide medical therapy optimization in patients with OH. In addition, we summarize the available strategies for drug treatment of OH in patients with persistent symptoms despite non-pharmacological interventions.

Treatment with escitalopram modulates cardiovascular function in rats

Considering depression is three times more common in cardiac patients compared to the normal population and selective serotonin reuptake inhibitors (SSRI) as drug of choice for treating patients with cardiovascular disease and depression, our work aims to evaluate the cardiovascular effects of treatment for 21 days with escitalopram (5 mg/kg/day, ip) in rats. The treatment caused an increase in mean arterial pressure concomitant with a decrease in heart rate. Concerning heart rate variability, there was a significant reduction in the sympathetic component and an elevation of the parasympathetic component, indicating that escitalopram caused an autonomic imbalance with parasympathetic predominance. In addition, we observed a decrease in both low and very low frequency power in blood pressure variability. The cardiac autonomic blockade indicated an increase in parasympathetic modulation to the heart with escitalopram chronic treatment. However, no change was observed on baroreflex activity. On the other hand, there was a decrease in pressure response during acute restraint stress with no changes in the tachycardia response. These findings showed that despite the escitalopram be a relatively safe drug it can cause tonic effects on cardiovascular function as well as during aversive situations.

HBK-14 and HBK-15 Do Not Influence Blood Pressure, Lipid Profile, Glucose Level, or Liver Enzymes Activity after Chronic Treatment in Rats

Older and even new antidepressants cause adverse effects, such as orthostatic hypotension, hyper- or hypoglycemia, liver injury or lipid disorders. In our previous experiments we showed significant antidepressant- and anxiolytic-like activities of dual 5-HT_1A and 5-HT₇ antagonists with α₁-adrenolitic properties i.e. 1-[(2,6-dimethylphenoxy)ethoxyethyl]-4-(2-methoxyphenyl)piperazine hydrochloride (HBK-14) and 1-[(2-chloro-6-methylphenoxy)ethoxyethyl]-4-(2-methoxyphenyl)piperazine hydrochloride (HBK-15). Here, we evaluated the influence of chronic administration of HBK-14 and HBK-15 on blood pressure (non-invasive blood pressure measurement system for rodents), lipid profile (total cholesterol, low density lipoproteins—LDL, high density lipoproteins—HDL, triglycerides), glucose level, and liver enzymes activity (aspartate aminotransferase, alanine aminotransferase, γ-glutamyl transferase). We determined potential antihistaminic (isolated guinea pig ileum) and antioxidant properties (ferric reducing ability of plasma–FRAP, non-protein thiols–NPSH, stable free radical diphenylpicrylhydrazyl—DPPH) cytotoxicity. Our experiments revealed that HBK-14 and HBK-15 did not influence blood pressure, lipid profile, glucose level or liver enzymes activity in rats after 2-week treatment. We also showed that none of the compounds possessed antioxidant or cytotoxic properties at antidepressant- and anxiolytic-like doses. HBK-14 and HBK-15 very weakly blocked H₁ receptors in guinea pig ileum. Positive results of our preliminary experiments on the safety of HBK-14 and HBK-15 encourage further studies concerning their effectiveness in the treatment of depression and/or anxiety disorders.

An evaluation of the cardiovascular safety profile of duloxetine: findings from 42 placebo-controlled studies

BACKGROUND AND OBJECTIVE

In recent years, new classes of medication, such as the serotonin-noradrenaline reuptake inhibitors (SNRIs), have been developed for use in the treatment of major depressive disorder (MDD). For many years, treatment options were largely limited to the use of monoamine oxidase inhibitors (MAOIs), tricyclic antidepressants (TCAs) and selective serotonin reuptake inhibitors (SSRIs). However, there have been published reports of orthostatic hypotension, arrhythmias and corrected QT (QTc) interval changes in patients treated with TCAs. As new medications become available, it is important to understand how their cardiovascular safety profile compares with that of more established agents to aid clinicians and patients in choosing the best treatment options. This study was designed to evaluate the cardiovascular safety profile of the SNRI duloxetine through evaluation of cardiovascular-related parameters and adverse events (AEs).

METHODS

The cardiovascular safety of duloxetine was assessed using all placebo-controlled duloxetine clinical trial data as of December 2005. This consisted of data from 42 placebo-controlled clinical trials of 8504 patients who were treated with duloxetine. Additional information from a high-dose clinical pharmacology study and postmarketing safety surveillance are also presented. Of the placebo-controlled trials included in this analysis, clinical indications under investigation included MDD (15 studies), diabetic peripheral neuropathic pain (3 studies), fibromyalgia (2 studies), generalised anxiety disorder (3 studies) and lower urinary tract disorders (19 studies, all related to incontinence). Cardiovascular safety was evaluated based on vital signs, ECGs and the incidence of treatment-emergent AEs potentially related to cardiovascular safety. These safety parameters were analysed across all indications. To identify both serious and non-serious cardiovascular-related AEs, as well as AEs reported as the reason for discontinuation, a comprehensive list of terms derived from the Medical Dictionary for Regulatory Activities (version 8.0) was generated and used to search the duloxetine databases for cardiovascular-related events.

RESULTS

Calculation of change from baseline to maximum in ECG parameters showed significant differences between treatment groups for all parameters, with decreases from baseline in RR, QRS and QT intervals for patients receiving duloxetine and increases from baseline for patients treated with placebo. These shifts were related to small heart rate changes, but the mean differences were not considered clinically relevant. Categorical analyses of shifts from normal to abnormal (or abnormal to normal) for heart rate and QT corrected for heart rate using Fridericia's formula (QTcF) values showed that most patients did not shift from their baseline category. Patients with MDD who were treated for up to 1 year with duloxetine had blood pressure changes early in treatment that then stabilised. Even in patients with elevated blood pressure at baseline in these clinical trials, no increased risk of sustained blood pressure elevation with duloxetine treatment was found.

CONCLUSION

Overall, the findings presented here support our conclusions that use of duloxetine does not appear to be associated with significant cardiovascular risks in patients with conditions for which the drug has been approved or studied.

Serotonin transporter gene functional polymorphism: a plausible candidate gene for increased vascular risk in depression

The evidence of increased vascular morbidity and mortality associated with depression has generated research interest in studying the mechanisms or causal pathways underlying this association. Recent advances in molecular genetics have demonstrated that serotonin transporter gene functional polymorphism may confer susceptibility for affective disorder as well as for some cardiovascular risk factors. Taking into account these genetic findings, this article proposes a hypothesis that serotonin transporter gene functional polymorphism may be a plausible candidate gene to study the genetic mechanisms of depression-related increased vascular morbidity and mortality. Future research projects to test this hypothesis is warranted.

Selective serotonin-reuptake inhibitor antidepressants increase the risk of falls and hip fractures in elderly people by inhibiting cardiovascular ion channels

Surprising results from recently published retrospective studies show that the use of new selective serotonin-reuptake inhibitor antidepressants (SSRIs), similarly to the older tricyclic antidepressants (TCAs), increases the risk of falls and hip fractures among elderly people.The mechanism whereby antidepressants increase this risk is complex and may include orthostatic hypotension, arrhythmias, sedation and confusion. The increased risk of falls and hip fractures with the use of TCAs is not surprising considering their well-known cardiovascular, anticholinergic and antihistaminergic side-effects. But the increased risk of falls with SSRIs is highly unexpected since these drugs are believed to be free from the disadvantages of TCAs. We hypothesized that the new SSRI antidepressants may also have cardiovascular effects similarly to the older TCA compounds, which may be an explanation for the increased rate of falls and hip fractures. The experimental and clinical evidence in support of this hypothesis are discussed.

Effects of clomipramine hydrochloride on heart rate and rhythm in healthy dogs

OBJECTIVE

To determine the effects of clomipramine hydrochloride on heart rate and rhythm in dogs.

ANIMALS

17 healthy Beagles.

PROCEDURES

In experiment 1, 8 dogs received placebo or clomipramine (20 mg/kg of body weight, q 24 h, PO) for 7 days in a 2-way crossover design. In experiment 2, 9 dogs were evaluated for 48 hours before and 24 hours after oral administration of clomipramine (4 or 12 mg/kg) in a 2-way crossover design. Electrocardiogram and heart rate were monitored continuously by use of telemetry.

RESULTS

A significant diurnal rhythm in heart rate was detected; minimum values were recorded at night. Administration of 20 mg of clomipramine/kg induced a significant reduction in heart rate, with peak effect achieved approximately 12 hours after dosing. Administration of 4 or 12 mg of clomipramine/kg did not result in significant changes in heart rate. Sinoatrial and second-degree atrioventricular block and ventricular escape beats were observed during periods of slow heart rate in more dogs that received clomipramine (3 to 4 of 8 dogs), compared with dogs that received placebo (1 to 2 of 8 dogs), but this difference was not significant.

CONCLUSIONS AND CLINICAL RELEVANCE

Short-term administration of clomipramine induced benign cardiovascular effects in dogs rather than the potentially dangerous arrhythmias or tachycardia reported following administration of tricyclic antidepressants to humans. Precautions regarding cardiovascular effects may not be needed for the use of clomipramine in healthy dogs.

Serotonin reuptake inhibitor, fluoxetine, dilates isolated skeletal muscle arterioles. Possible role of altered Ca2+ sensitivity

1. Inhibitors of serotonin reuptake in the central nervous system, such as fluoxetine, may also affect the function of vascular tissues. Thus, we investigated the effect of fluoxetine on the vasomotor responses of isolated, pressurized arterioles of rat gracilis muscle (98 +/- 4 microns in diameter at 80 mmHg perfusion pressure). 2. We have found that increasing concentrations of fluoxetine dilated arterioles up to 155 +/- 5 microns with an EC50 of 2.5 +/- 0.5 x 10(-6) M. 3. Removal of the endothelium, application of 4-aminopyridine (4-AP, an inhibitor of aminopyridine sensitive K+ channels), or use of glibenclamide (an inhibitor of ATP-sensitive K+ channels) did not affect the vasodilator response to fluoxetine. 4. In the presence of 10(-6), 2 x 10(-6) or 10(-5) M fluoxetine noradrenaline (NA, 10(-9)-10(-5) M) and 5-hydroxytryptamine (5-HT, 10(-9)-10(-5)M)-induced constrictions were significantly attenuated resulting in concentration-dependent parallel rightward shifts of their dose-response curves (pA2 = 6.1 +/- 0.1 and 6.9 +/- 0.1, respectively). 5. Increasing concentrations of Ca2+ (10(-4) 3 x 10(-2) M) elicited arteriolar constrictions (up to approximately 30%), which were markedly reduced by 2 x 10(-6)M fluoxetine, whereas 10(-5)M fluoxetine practically abolished these responses. 6. In conclusion, fluoxetine, elicits substantial dilations of isolated skeletal muscle arterioles, a response which is not mediated by 4-AP- and ATP-sensitive K+ channels or endothelium-derived dilator factors. The findings that fluoxetine had a greater inhibitory effect on Ca2+ elicited constrictions than on responses to NA and 5-HT suggest that fluoxetine may inhibit Ca2+ channel(s) or interfere with the signal transduction by Ca2+ in the vascular smooth muscle cells.

Serotonin and heart rate in hypertensive and normotensive subjects

Plasma and platelet levels of 5-hydroxytryptamine (5 HT) may be altered in essential hypertension. To establish the determinants and correlates of 5 HT in plasma and platelets, we studied 53 untreated subjects with essential hypertension (26 men; 30 whites; mean supine blood pressure 172/101 mm Hg; mean age 49.3 +/- 1.5 years) and 61 normotensive subjects (37 men; 47 whites; mean supine blood pressure 128/78 mm Hg; mean age 42.8 +/- 1.6 years). Plasma and platelet 5 HT were assayed by reverse-phase high performance liquid chromatography with electrochemical detection. No significant difference was found in platelet-poor plasma or platelet 5 HT levels in hypertensive or normotensive subjects (plasma: 43.0 +/- 4.2 and 39.6 +/- 4.4 nmol/L; platelet: 1.65 +/- 1.22 and 1.70 +/- 1.39 nmol/10(9) cells in hypertensive and normotensive subjects, respectively). No significant correlation was found between plasma or platelet 5 HT and systolic or diastolic blood pressure (plasma: r = 0.01 and 0.01 in normotensive subjects and r = 0.01 and -0.14 in hypertensive subjects; platelet: r = 0.12 and 0.13 in normotensive subjects and r = 0.02 and -0.09 in hypertensive subjects). However, plasma 5 HT was associated with supine and standing pulse rates (supine: r = 0.27, p = 0.05 in normotensive subjects and r = 0.54, p < 0.001 in hypertensive subjects; standing: r = 0.19 and r = 0.46, p < 0.001, respectively). Significant relations were also found between platelet 5 HT levels and supine and standing heart rate in the subjects mentioned above (supine: r = 0.28, p = 0.05 in normotensive subjects and r = 0.64, p < 0.001 in hypertensive subjects; standing: r = 0.24 and r = 0.51, p < 0.001, respectively). These associations were stronger in the hypertensive group as a whole, and they held when adjustment was made for differences in age and total blood cholesterol. The present study showed that plasma and platelet 5 HT are not significantly altered in hypertensive subjects. However, plasma and platelet 5 HT levels showed a significant association with supine and standing pulse rate predominantly in hypertensive subjects. This is consistent with experimental evidence of a positive chronotropic effect of 5 HT on perfused hearts and it suggests a possible role of plasma serotonin in the regulation of heart rate.

Serum corticosterone increases reflect enhanced uptake inhibitor-induced elevation of extracellular 5-hydroxytryptamine in rat hypothalamus

The increase in extracellular 5-hydroxytryptamine (5-HT) in rat hypothalamus following administration of fluoxetine, a 5-HT-uptake inhibitor, was enhanced by the injection of LY206130(1-[1-H-indol-4-yloxy]-3-[cyclohexylamino]-2-prop ano l maleate), a 5HT1A receptor antagonist, or by L-5-hydroxytryptophan (L-5-HTP), the 5-HT precursor. Elevation of serum corticosterone, measured as a functional output of hypothalamic 5-HT pathways, was greater in rats treated with fluoxetine plus LY206130 or with fluoxetine plus L-5-HTP than in rats treated with the agents alone. Synergism between effects of fluoxetine and L-5HTP has often been reported, but this is the first report of an increased functional effect when a 5-HT1A receptor antagonist is combined with a 5-HT uptake inhibitor to augment the increase in extracellular 5-HT.

Serotonin uptake inhibitors: uses in clinical therapy and in laboratory research

Fluoxetine, zimelidine, sertraline, paroxetine, fluvoxamine, indalpine and citalopram are the selective inhibitors of serotonin uptake that have been most widely studied. Some of these compounds are or have been used clinically in the treatment of mental depression, obsessive-compulsive disorder and bulimia, and therapeutic benefit has been claimed in additional diseases as well. By blocking the membrane uptake carrier which transports serotonin from the extracellular space to inside the serotonin nerve terminals, these compounds increase extracellular concentrations of serotonin and amplify signals sent by serotonin neurons. Because serotonin neurons are widespread in the central nervous system, the functional consequences of blocking serotonin uptake are diverse, but are generally subtle. Animals treated with serotonin uptake inhibitors look normal in gross appearance, but effects such as reduced aggressive behavior, decreased food intake and altered food selection, analgesia, anticonvulsant activity, endocrine changes and neurochemical changes have been demonstrated and characterized. Serotonin uptake inhibitors have helped in revealing some dynamics of serotonin neurons; for example, when uptake is inhibited and extracellular serotonin concentration increases, presynaptic as well as postsynaptic receptors for serotonin are activated to a greater degree. A consequence of increased activation of autoreceptors on serotonin cell bodies and nerve terminals is a reduction in firing of serotonin neurons and a decrease in serotonin synthesis and release. The result is a limit on the degree to which extracellular serotonin and serotonergic neurotransmission are increased.(ABSTRACT TRUNCATED AT 250 WORDS)

Extracellular 5-hydroxytryptamine concentration in rat hypothalamus after administration of fluoxetine plus L-5-hydroxytryptophan

Fluoxetine (10 mg kg-1, i.p.) caused a three- to fourfold increase in extracellular 5-hydroxytryptamine (5-HT) concentration measured by microdialysis in hypothalamus of freely moving rats. The addition of L-5-hydroxytryptophan at 20 or 40 mg kg-1, i.p. doses, magnified the increase in extracellular 5-HT to as much as 16 times basal levels, although these doses of L-5-hydroxytryptophan alone had only small effects on extracellular 5-HT. The increased formation of 5-HT following L-5-hydroxytryptophan administration appears to overcome homeostatic mechanisms that limit the increases in extracellular 5-HT caused by uptake inhibition.

Drugs affecting serotonin neurons

Advances in serotonin pharmacology, the development of drugs that intervene at specific sites to modify serotonergic function, have accompanied advances in the understanding of physiologic roles of serotonin present in neurons and elsewhere and of serotonin receptors that are widely distributed in brain and many peripheral tissues. The pharmacologic advances have sometimes been stimulated by developments in serotonin physiology, such as the recognition of multiple serotonin receptor subtypes, and in other cases have been a major factor in providing new insights into physiologic roles of serotonin. Drugs that modify serotonin function have a variety of therapeutic applications currently and many more potential therapeutic uses to be explored in the future. Having drugs that act with high specificity or selectivity on particular enzymes in serotonin biosynthesis, on particular serotonin receptors, or at other sites such as uptake carriers for serotonin not only offers the hope of improved clinical therapy in diseases caused by abnormal serotonergic function or in which alteration of serotonergic function can alleviate symptoms, but also provides valuable pharmacologic tools for learning more about serotonin physiology and probing the functional status of serotonergic systems. The next few years promise to yield important new serotonergic drugs.

Fluoxetine. A review of its pharmacodynamic and pharmacokinetic properties, and therapeutic efficacy in depressive illness

Fluoxetine is a new antidepressant which enhances serotoninergic neurotransmission through potent and selective inhibition of neuronal reuptake of serotonin. Metabolism by N-desmethylation occurs in man yielding desmethylfluoxetine, which also inhibits serotonin reuptake. Both the parent compound and metabolite possess elimination half-lives of several days facilitating the maintenance of steady-state plasma concentrations during long term treatment. Fluoxetine has overall therapeutic efficacy comparable with imipramine, amitriptyline and doxepin in patients with unipolar depression treated for 5 to 6 weeks, although it may be less effective than tricyclic antidepressants in relieving sleep disorders in depressed patients. Geriatric patients also responded as well to fluoxetine as to doxepin. The symptomatic improvement in patients with unipolar depression during short term fluoxetine treatment has been satisfactorily maintained when therapy was extended for at least 6 months: the relapse rate was low and similar to that of imipramine. Preliminary data have shown that patients with bipolar depression gained similar therapeutic benefit from fluoxetine or imipramine. Other preliminary trials have indicated that fluoxetine may be useful in obsessive-compulsive disorders. Usual doses of fluoxetine cause significantly fewer anticholinergic-type side effects than tricyclic antidepressants. Nausea, nervousness and insomnia are the most frequently reported fluoxetine-related adverse effects, but these have usually not been severe. Therapeutic doses of fluoxetine do not affect cardiac conduction intervals in patients without pre-existing cardiovascular disease and fluoxetine has been relatively safe in the small number of patients who have taken overdoses. It has not been clearly established whether some types of depression may respond more readily to fluoxetine than other antidepressants, and its overall therapeutic efficacy has not been compared with other second generation antidepressants. Thus, with its different and perhaps improved side effect profile compared with older tricyclic antidepressants, fluoxetine offers properties that could be used to advantage in many patients with depression.

Prevention of DOCA-induced hypertension in rats by chronic treatment with tryptophan

Hypertension developed within 5 weeks in uninephrectomized rats administered deoxycorticosterone acetate (DOCA, 30 mg/kg, s.c., weekly) and given isotonic saline to drink. Chronic dietary administration of tryptophan (50 g/kg food) reduced intake of saline solution and prevented the elevation of systolic blood pressure induced by treatment with DOCA alone. Treatment with tryptophan also protected against the reduction in urinary concentrating ability during a 24 h dehydration that is characteristic of DOCA-treated rats. Other tests were carried out to assess the responsiveness to the beta-adrenergic agonist, isoproterenol. The tests included measurement of drinking and heart rate following acute administration of isoproterenol. The characteristically depressed drinking response of DOCA-treated rats to acute administration of isoproterenol was returned to that of untreated controls by chronic treatment with tryptophan. However, the reduced chronotropic response of the heart of DOCA-treated rats to administration of isoproterenol was unaffected. The cardiac hypertrophy characteristic of DOCA-treatment was attenuated significantly by chronic treatment with tryptophan. These results suggest that tryptophan provides significant protection against the development of DOCA-induced hypertension, polydipsia, and cardiac hypertrophy in rats. The mechanism by which tryptophan protects is unknown and requires additional study.

Hemodynamic and electrocardiographic effects of fluoxetine and its major metabolite, norfluoxetine, in anesthetized dogs

The cardiovascular effects of the selective serotonin uptake inhibitor, fluoxetine, and its N-desmethyl metabolite, norfluoxetine, were studied in anesthetized dogs during constant iv infusion of supratherapeutic doses (0.1 mg/kg/min for 50 min). Fluoxetine and norfluoxetine did not significantly affect mean blood pressure, pulmonary artery wedge pressure, or heart rate compared to a corresponding vehicle group. Cardiac output fell 15 to 20% during fluoxetine infusion due to nonsignificant decreases in both heart rate (10%) and stroke volume (5 to 10%). In contrast, the tricyclic antidepressant agent, amitriptyline, infused at the same dose, decreased both mean pressure and systemic vascular resistance (20%) and increased heart rate (20%). Pulmonary wedge pressure rose by 35%, and stroke volume fell by 20% suggesting impaired ventricular contractility. Both intramyocardial and infranodal conduction was slowed during amitriptyline infusion as indicated by increases in the QRS duration, and the PQ and HV interval. Fluoxetine and norfluoxetine had no influence on cardiac impulse conduction velocity as assessed by either surface or intracardiac conduction indices. Plasma concentrations of fluoxetine, norfluoxetine, and amitriptyline reached during infusion ranged from 1.0 to 2.5 micrograms/ml. Platelet [3H]serotonin uptake was inhibited by 95% during infusion of fluoxetine and about 75% during infusion of norfluoxetine or amitriptyline. These observations indicate that large iv doses of fluoxetine or norfluoxetine lack prominent cardiodepressant effects in dogs, suggesting a greater margin of safety for fluoxetine compared to tricyclic antidepressant drugs.

Cardiovascular and sympathetic responses to intracarotid and intravenous injections of serotonin in rats

The possible central regulation of cardiovascular system by serotoninergic neurones was investigated in rats by using a cross-circulated head preparation. Intracarotid injections of serotonin (5-HT, 0.1-10 micrograms) consistently produced neurally mediated vasodepression in the recipient body. Although intravenous injections of 5-HT (0.1-1 micrograms) caused similar depressor responses, larger doses (5-10 micrograms) caused biphasic responses, consisting of vasodepression followed by a sustained pressor response. Despite the biphasic blood pressure responses, sympathetic nerve activity was consistently reduced by the intravenous 5-HT. Bilateral vagotomy abolished the vasodepressor responses to intravenous injections of 5-HT (10 micrograms) and made the responses purely pressor, but the vasodepressor responses to intracarotid injections of 5-HT (10 micrograms) were largely unaffected. Following destruction of central noradrenergic neurones by intracerebroventricular injections of 6-hydroxydopamine, the vasodepressor responses to intracarotid 5-HT (1-10 micrograms), but not to the intravenous 5-HT, were significantly reduced. These results suggest that intravenous injections of 5-HT reduce nerve activity by affecting the reflex mechanism via the vagus nerves. On the other hand, the vasodepression and reduction of the nerve activity by the intracarotid injections appears to be of central origin, and the vasodepression could be mediated via noradrenergic mechanisms in the brain.

A serotonin-containing pathway from the area postrema to the parabrachial nucleus in the rat

A combined retrograde tracing-immunofluorescent technique was used to identify the relationships between the cellular population projecting to the parabrachial nucleus and the serotonin-immunoreactive cell population of the area postrema in rats. The retrograde fluorescent tracer True Blue was injected in the parabrachial region and 3 days later the animals were perfused. Serial cryostat sections were processed for serotonin immunofluorescence. Three different groups of labeled cells were identified in the area postrema. First, True Blue-positive cells (up to 250/section) that project to the parabrachial nucleus were observed distributed throughout the area postrema. Second, in the pargyline (a monoamine oxidase inhibitor)-treated animals a large number of serotonergic cells (up to 125/section) was observed distributed throughout the area postrema. There was a tendency to a heavier distribution of serotonin-immunoreactive cells in the dorsal two-thirds of the area postrema. Third, double-labelled cells were also seen. Twenty percent of the True Blue-labelled cells projecting to the parabrachial nucleus were serotonin-immunoreactive. Thirty nine percent of the serotonin-immunoreactive population was retrogradely labelled with True Blue. Thus a new serotonergic pathway from the area postrema to the parabrachial nucleus is described; this pathway may be important in the ascending transmission and modulation of chemical and visceral sensory input.

The hypotensive action of 4-(5,6-dimethyl-2-benzofuranyl) piperidine HCl (CGP 6085 A) in spontaneously hypertensive rats

CGP 6085 A, [4-(5,6-dimethyl-2-benzofuranyl)piperidine HCl], has been found to be a mild to moderately potent hypotensive agent. One hour following CGP 6085 A administration (10 mg/kg, i.p.), a maximal reduction in blood pressure of approximately 20-30 mm Hg is observed in spontaneously hypertensive rats. The maximal reduction in blood pressure was observed at a dose of 3 mg/kg. CGP 6085 A blocks 5-HT uptake in the brainstem when assessed in vivo by use of the serotonin depletor, H 75/12 (3-hydroxy-4-methyl-alpha-ethyl-phenylethylamine). The maximal inhibitory effect on 5-HT uptake occurred at 10 mg/kg CGP 6085 A. The reduction in blood pressure correlates well with the ability of the drug to inhibit 5-HT uptake as assayed by H 75/12, with a correlation coefficient of 0.71 for SH rats. However, since the drug has not been widely characterized, alternate explanations for the cardiovascular pharmacological properties of CGP 6085 A are also proposed.

Decreased uptake of 3H-serotonin and endogenous content of serotonin in blood platelets in hypertensive patients

The uptake and content of serotonin in blood platelets were studied in patients with essential hypertension and in five families in which at least one member was hypertensive. Blood was obtained from male and female normotensive volunteers and hypertensive patients who were free of medication. Lineweaver-Burk plots of 3H-serotonin uptake from both control subjects and hypertensive patients were linear, which suggested simple Michaelis-Menten uptake kinetics. The maximal uptake velocity (Vmax) in hypertensive patients was significantly lower than in control subjects (control = 41.7 +/- 3.3 pmol/min/10(8) platelets, n = 17; hypertensive = 26.6 +/- 3.0 pmol/min/10(8) platelets, n = 16; p less than 0.005). The affinity constant (Km) was slightly but significantly lower in hypertensive patients (control = 0.70 +/- 0.08 microM; hypertensive = 0.46 +/- 0.08 microM; p less than 0.05). The serotonin content in blood platelets determined by high pressure liquid chromatography with electrochemical detection was significantly lower in hypertensive patients (control = 165.0 +/- 12.9 nmol/10(11) platelets, n = 29; hypertensive = 105.9 +/- 10.4 nmol/10(11) platelets, n = 27; p less than 0.001). In the five families investigated, the lowered serotonin content was observed in some normotensive members. The reduced number of carriers of serotonin uptake and the slight decrease in the affinity constant observed in platelets of patients with essential hypertension suggest that serotonin metabolism is altered in essential hypertension and that blood platelets may be a useful model in studying the serotonergic modifications at the molecular level.

Cardiovascular responses to centrally administered serotonin in conscious normotensive and spontaneously hypertensive rats

Serotonin (5-HT, 1-10 micrograms), injected into the lateral ventricle of the urethanized, and conscious normotensive and spontaneously hypertensive rats produced a dose-dependent increase in blood pressure. In conscious rats, there was mainly a decrease in heart rate while variable changes in heart rate were elicited by intraventricular (i.c.v.) administration of 5-HT in anesthetized animals. These pressor responses and bradycardia caused by 5-HT in conscious rats were reduced by pretreatment with i.c.v. methysergide (25 micrograms). Microinjection of 5-HT (2.5-5 micrograms) directly into the medial hypothalamus and the anterior hypothalamus/preoptic area of conscious normotensive rats caused a pressor response accompanied by variable changes in heart rate. The present results indicate that urethane can affect the HR response to 5-HT injected i.c.v. without having a marked influence on the pressor response. These findings, which show that 5-HT produced a rise in BP independent of the anesthetized or conscious state and of normotension or hypertension, further confirm the idea that 5-HT plays a pressor role in the central regulation of the cardiovascular system.

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.

Fluoxetine and two other serotonin uptake inhibitors without affinity for neuronal receptors

Fluoxetine and nine other antidepressant drugs which interact with brain receptors for neurotransmitters were studied in vitro using radioligand-binding techniques and transmitter-coupled adenylate cyclase assays. Tricyclic antidepressant drugs (desipramine, imipramine, clomipramine, amitriptyline and doxepin) had marked affinity for alpha-adrenergic, muscarinic cholinergic and histaminergic H1 receptors, and lesser affinity for serotonin and dopamine receptors. Mianserin was relatively similar to some of the tricyclic compounds, whereas trazodone had less affinity for most receptors except serotonin and alpha-adrenergic receptors. Fluoxetine had little affinity for any of these receptors, and the same was true for zimelidine and fluvoxamine, two other selective inhibitors of serotonin uptake. None of the compounds showed much affinity for beta-adrenergic receptors, opiate receptors, gamma-aminobutyric acid receptors, or benzodiazepine receptors. The present findings with fluoxetine are consistent with the virtual absence of anticholinergic or other side effects often observed with tricyclic antidepressant drugs in animal models or during the treatment of depressed patients.

Alpha-Adrenergic and 5-hydroxytryptaminergic receptor stimulants as new antihypertensive drugs, with observations on involvement of opiate receptors

Cardiovascular actions of central alpha-adrenergic and 5-hydroxytryptaminergic stimulants are discussed in terms of overall effects, correlation of various activities, receptor activation and side-effects. The involvement of opiate receptors in their actions is also examined.

Pressor effects of dorsal raphe stimulation and intrahypothalamic application of serotonin in the spontaneously hypertensive rat

Electrical stimulation of the dorsal raphe nucleus or direct microinjection of serotonin into the preoptic region of the anterior hypothalamus produces a transient rise in arterial blood pressure in both spontaneously hypertensive rats (SRH) and Wistar--Kyoto (WKY) controls. SRHs are more responsive to raphe stimulation but are somewhat less responsive to serotonin injections when compared to WKYs. The serotonin antagonist metergoline blocks the pressor response to serotonin in both strains. These results suggest that the central serotonergic neuronal system play a similar, but not identical, role in blood pressure modulation in hypertensive and normotensive rats.

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.

Lowering of blood pressure by direct- and indirect-acting serotonin agonists in spontaneously hypertensive rats

1-(m-Trifluoromethylphenyl)piperazine, a serotonin agonist, lowered blood pressure in spontaneously hypertensive rats (SHR) at doses of 2 to 10 mg/kg s.c. A structurally related compound lacking serotonin agonist activity, 4-(m-trifluoromethylphenyl)piperidine, was ineffective. Quipazine, another serotonin agonist, lowered blood pressure in SHR at doses of 0.1 to 2 mg/kg s.c. Fenfluramine, a serotonin-releasing drug, lowered blood pressure in SHR at doses of 2 and 5 mg/kg s.c. Metergoline (3 mg/kg s.c.), a serotonin antagonist, elevated blood pressure and prevented the decrease by all of the above agents. These findings are consistent with the view that enhancement of central serotonergic function lowers blood pressure in SHR.