Inhibition of efferent sympathetic nerve activity by 5-hydroxytryptophan and centrally administered 5-hydroxytryptamine

Effects of intracerebroventricular injections of 5-HT on systemic vascular resistances of conscious rats

The aims of this study were to determine (i) the effects of intracerebroventricular (i.c.v.) injections of 5-hydroxytryptamine (5-HT, 10μg) on mean arterial blood pressure (MAP), heart rate (HR) and mesenteric (MR), renal (RR) and hindquarter (HQR) vascular resistances of conscious rats, (ii) the central 5-HT receptor subtype which mediates these effects, and (iii) the role of nitric oxide (NO) in the expression of these responses. The i.c.v. injection of 5-HT had minor effects on MAP but produced a decrease in HR (-18±4%), which lasted for 20min. The i.c.v. injection of 5-HT elicited marked increases in MR (+50±7%) and reductions in HQR (-31±3%). These responses occurred promptly and lasted for 25-35min. 5-HT also produced a transient decrease in RR (-26±8% at 10min). All of these responses were prevented by the prior i.c.v. injection of the 5-HT1/5-HT2-receptor antagonist, methysergide (10μg). The intravenous injection of the NO synthesis inhibitor, L-NAME (25μmol/kg), produced a sustained pressor response, bradycardia and increases in MR, RR and HQR. Subsequent i.c.v. injection of 5-HT produced a minor pressor response (+7±2%), bradycardia (-18±3%), an increase in MR (+52±8%) but no decreases in RR or HQR. This study demonstrates that i.c.v. 5-HT differentially affects peripheral vascular resistances by activation of central 5-HT1/5-HT2-receptors. It appears that L-NAME did not interfere with the central actions of 5-HT as it did not prevent the 5-HT-induced bradycardia or mesenteric vasoconstriction. Since the 5-HT-induced falls in RR and HQR were abolished by L-NAME, it is possible that these responses are mediated by an active neurogenic process involving the release of NO within the vasculature.

Mechanism of the inhibitory effect of electroacupuncture on experimental arrhythmias

Clinical observations reported that acupuncture can alleviate several kinds of arrhythmias. To explore its efficacy and mechanism, we have studied the electroacupuncture (EA) inhibition on experimental arrhythmias in rabbits since 1980s and analyzed its mechanism. These studies were mostly conducted in the Department of Physiology, Shanghai Medical University; recently the mechanism of acupuncture's effect on arrhythmias was analyzed in the School of Medicine, University of California, Irvine, which involves the following: (1)the inhibitory effect of EA on ventricular extrasystoles can be induced by hypothalamic defense area stimulation: a low-current and low-frequency stimulation of the median nerve underneath acupoints P 5 or deep peroneal nerve underneath S 36 can activate arcuate nucleus-ventral periaqueductal gray -nuclei raphe pathway and release endorphin, enkephaline, gamma-aminobutyric acid (GABA), 5-hydroxytryptamine (5-HT), etc., thus inhibiting the rostral ventrolateral medulla (rVLM), decreasing sympathetic outflow, and alleviating ventricular extrasystoles; (2) the bradycardia induced by stimulation of aortic depressor nerve can be blocked by stimulation of superficial radial nerve (underneath LI 6-7) or superficial peroneal nerve (underneath G37-39): these stimulations activate the rVLM release opioids and GABA to inhibit nucleus tractus solitarius and vagal nuclei, and block vagal bradycardia. These experimental data explore the mechanism of acupuncture's effect on arrhythmias and are useful for clinical application.

Central serotoninergic response to orthostatic challenge in patients with neurocardiogenic syncope

AIMS

To determine whether central serotoninergic system activity is impaired by orthostatic challenge in patients with neurocardiogenic syncope (NCS).

METHODS AND RESULTS

Thirty-five [mean age: 24 (SD): 6 years] patients with a clinical history of NCS and positive head-up tilt test and 35 age-matched healthy volunteers (CON = 25+/-5 years) with negative response were studied. Overnight dexamethasone suppression test (DST) (1.5 mg given at 11 p.m.) was performed to assess the sensitivity of the hypothalamic-pituitary-adrenal axis by measuring next day cortisol (microg/dL) at 8 a.m. and 4 p.m. Cardiac autonomic function, cortisol, and prolactin (ng/dL) were also determined at baseline supine (BAS) and after 5, 10, and 15 min of orthostatic stress (OS) at 60 degrees . No significant differences were observed in cortisol plasma levels after the DST: CON = 0.6+/-0.6 microg/dL vs. NCS = 0.6+/-0.5; P = 0.7. Cardiac autonomic function, cortisol, and prolactin responses were similar in both study groups (CON vs. NCS; P > 0.05) during BAS: cortisol = 8.6+/-4 vs.8.7+/-4 microg/dL and prolactin = 16.8+/-9 vs. 16.8+/-9 ng/dL; OS-5: cortisol = 8.7+/-5 vs. 8.5+/-4 microg/dL and prolactin = 16.9+/-9 vs. 15.8+/-9 ng/dL; OS-10: cortisol = 8.5+/-5 vs. 8.1+/-3 microg/dL; prolactin = 16.2+/-9 vs. 15.8+/-9 ng/dL, and OS-15: cortisol = 9.0+/-5 vs. 8.4+/-4 microg/dL; prolactin = 17.1+/-9 vs. 15.5+/-9 ng/dL.

CONCLUSION

Central serotoninergic response during orthostatic challenge was not impaired in patients with recurrent NCS. These findings suggest that the activation of the hypothalamic-pituitary-adrenal axis is not altered in patients with recurrent NCS.

Psychotropic drugs, cardiac arrhythmia, and sudden death

A variety of drugs targeted towards the central nervous system are associated with cardiac side effects, some of which are linked with reports of arrhythmia and sudden death. Some psychotropic drugs, particularly tricyclic antidepressants (TCAs) and antipsychotic agents, are correlated with iatrogenic prolongation of the QT interval of the electrocardiogram (ECG). In turn, this is associated with the arrhythmia (TdP). This review discusses the association between psychotropic agents, arrhythmia and sudden death and, focusing on TCAs and antipsychotics, considers their range of cellular actions on the heart; potentially pro-arrhythmic interactions between psychotropic and other medications are also considered. At the cellular level TCAs, such as imipramine and amitriptyline, and antipsychotics, such as thioridazine, are associated with inhibition of potassium channels encoded by In many cases this cellular action correlates with ECG changes and a risk of TdP. However, not all psychotropic agents that inhibit HERG at the cellular level are associated equally with QT prolongation in patients, and the potential for QT prolongation is not always equally correlated with TdP. Differences in risk between classes of psychotropic drugs, and between individual drugs within a class, may result from additional cellular effects of particular agents, which may influence the consequent effects of inhibition of repolarizing potassium current.

Monoaminergic and Peptidergic Innervation of the Intermedio-Lateral Horn of the Spinal Cord

In the rat the monoaminergic and neuropeptidergic innervation of the sympathetic visceral nuclei of the entire thoracic spinal cord has been analysed in serial horizontal sections using immunocytochemistry. Tyrosine hydroxylase (TH), Phenyl-ethanolamine-N-methyl-transferase (PNMT), 5-hydroxytryptamine (5-HT), substance P (SP) and enkephalin (ENK) immunoreactive (IR) nerve terminals form tufts of plexa with strong IR in the principal part of the intermediolateral nucleus (ILp) with the terminals in an extraperikaryal location. High densities of these strongly IR terminals are also found in the principal part of the intercalated nucleus (ICp) and in the paraependymal part of the intercalated nucleus (ICpe). The various types of IR nerve terminals also form rostro-caudally oriented and latero-medially oriented strands of strongly IR nerve terminals at regular intervals within each segment. Outside these sympathetic nuclei the terminals are absent or only weakly to moderately IR. The similar pattern of monoamine and peptide innervation of the putative preganglionic sympathetic neurons along the entire thoracic spinal cord may be related to the general three dimensional architecture of the preganglionic multipolar neurons. Thus, these inputs tend to cover the entire surface area of the preganglionic neurons in a uniform way. Some heterogeneities have been observed for the TH, PNMT and neuropeptide Y (NPY) innervation which may contribute to a differential control of sympathetic preganglionic neurons. It is suggested that the unique features of the descending monoaminergic or peptidergic neurons to sympathetic spinal nuclei are related to a demand for maintained transmission upon prolonged activation in these cardiovascular systems, allowing the maintenance of cardiovascular homeostasis.

Management of vasovagal syncope

Vasovagal syncope is a common disorder of autonomic cardiovascular regulation that can be very disabling and result in a significant level of psychosocial and physical limitations. The optimal approach to treatment of patients with vasovagal syncope remains uncertain. Although many different types of treatment have been proposed and appear effective based largely on small nonrandomized studies and clinical series, there is a remarkable absence of data from large prospective clinical trials. However, based on currently available data, the pharmacologic agents most likely to be effective in the treatment of patients with vasovagal syncope include beta blockers, fludrocortisone, and alpha-adrenergic agonists. In this article, we provide a summary of the various therapeutic options that have been proposed for vasovagal syncope and review the clinical studies that form the basis of present therapy for this relatively common entity.

Malignant vasovagal syncope: a randomised trial of metoprolol and clonidine

OBJECTIVE

To evaluate the efficacy of head up tilt guided treatment with metoprolol and clonidine in preventing the recurrence of syncope in patients with malignant vasovagal syncope.

PATIENTS

20 patients (9 men and 11 women, mean age 33 (SD 17), range 14 to 62 years) with severe symptoms.

DESIGN

Randomised double blind crossover trial; efficacy was assessed by head up tilt testing.

RESULTS

Metoprolol was more effective than clonidine in abolishing syncope (19/20 v 1/20, P < 0.001) but clonidine showed some beneficial effects on time to syncope and severity of hypotension in 12 patients. During an average follow up of 15 (3) months there was a significant reduction in the recurrence of symptoms compared with the previous year in patients who had tilt up guided treatment (18 metoprolol, 1 clonidine).

CONCLUSIONS

Treatment guided by head up tilting is a reliable method of treating patients with malignant vasovagal syndrome. Metoprolol was an effective long term treatment for preventing syncope. High doses were more effective and a careful dose titration period helped to minimise withdrawal symptoms and side effects.

The cardiovascular and renal functional responses to the 5-HT1A receptor agonist flesinoxan in two rat models of hypertension

1. This study investigated the importance of renal sympathetic nerves in regulating sodium and water excretion from the kidneys of stroke prone spontaneously hypertensive and 2K1C Goldblatt hypertensive rats anaesthetized with chloralose/urethane (17.5/300 mg initially and supplemented at regular intervals), and prepared for measurement of renal function. 2. In stroke prone spontaneously hypertensive rats, flesinoxan, 30-1000 micrograms kg-1, i.v., caused graded reductions in blood pressure and heart rate of 74 +/- 5 mmHg and 63 +/- 9 beats min-1, respectively at the highest dose (P < 0.001). Renal blood flow did not change at any dose of drug while glomerular filtration rate fell by some 20% (P < 0.001) at the highest dose of drug, absolute and fractional sodium excretions, approximately doubled at 100 micrograms kg-1, and thereafter fell to below the baseline level at 1000 micrograms kg-1. 3. This pattern of excretory response was abolished following acute renal denervation when flesinoxan caused dose-related reductions in urine flow and sodium excretion, similar to that obtained by a mechanical reduction of renal perfusion pressure. 4. Flesinoxan administration (30-1000 micrograms kg-1, i.v.) into 2K1C Goldblatt hypertensive rats caused a maximum decrease in blood pressure and heart rate (both P < 0.001) of 34 +/- 3 mmHg and 20 +/- 6 beats min-1 and while renal blood flow and glomerular filtration rate were autoregulated, from 160 to 125 mmHg, there were dose-related decreases in urine volume and sodium excretion from the clipped and non-clipped kidneys of approximately 50-60% at the highest dose. 5. These findings suggest that in the stroke prone spontaneously hypertensive rat the renal nerves importantly control sodium and water reabsorption at the level of the tubules, whereas in 2K1C Goldblatt hypertensive rats, they play a minor role.

Neurally mediated syncope and serotonin reuptake inhibitors

Serotonin (5-hydroxytryptamine or 5HT) is a neurotransmitter which appears to play a prominent role in central regulation of heart rate and blood pressure. Recent evidence suggests that the activation of cerebral serotonin receptors results in a depressor effect principally through sympatho-inhibition. Several common clinical disorders resulting in hypotension leading to syncope are neurally mediated syncope, carotid sinus hypersensitivity and orthostatic hypotension, each of which may involve a serotonergic component. This brief review provides a summary of serotonergic blood pressure regulation, as well as the initial experience with the clinical effects of the serotonin reuptake inhibitors in the therapy of the aforementioned disorders.

The renal functional responses to 5-HT1A receptor agonist, flesinoxan, in anaesthetized, normotensive rat

1. The present study was designed to examine the effects of a centrally acting 5-HT1A receptor agonist, flesinoxan, on the cardiovascular system and renal haemodynamics and excretory function. 2. In chloralose-urethane anaesthetized Wistar rats, i.v. administration of bolus doses of flesinoxan, at 30, 100, 300 and 1000 micrograms kg-1, caused significant, dose-dependent decreases in mean arterial pressure, of 33 +/- 2 mmHg (P < 0.001) and heart rate of 57 +/- 9 beats min-1 (P < 0.001) at the highest dose used. Despite this substantial fall in perfusion pressure there were no meaningful changes in the renal excretion of water and sodium. In a second group of rats, reduction of renal perfusion pressure mechanically to the same values as observed in rats given flesinoxan (i.e. 100, 92, 84 and 76 mmHg) produced reductions in urine flow, absolute and fractional sodium excretions reaching a maximum of 74, 86 and 84% respectively (all P < 0.001) at the lowest pressure. These reductions were significantly larger than those seen in the previous group of animals. 3. In the group of rats subjected to renal denervation, flesinoxan produced changes in blood pressure and heart rate which were not different from those observed in intact animals. However, the reduction in pressure was accompanied by significant decreases in urine flow of 71%, absolute sodium excretion of 68% and fractional sodium excretion of 67% (all P < 0.001) at the highest dose, which were all significantly greater than the changes seen in the innervated animals but were not different from those observed when renal perfusion pressure was reduced mechanically. 4. The findings of this investigation showed that flesinoxan was effective in lowering blood pressure and heart rate in the anaesthetized rat, which was probably due to decreased sympathetic nerve activity.Renal excretion of water and sodium was well preserved in the face of the flesinoxan-induced hypotension.The maintenance of fluid excretion with flesinoxan appeared to be mediated via changes in renal nerve activity, since it did not occur when the kidney was denervated.

Effect of dietary T-2 toxin on biogenic monoamines in discrete areas of the rat brain

Acute T-2 toxin treatments alter biogenic monoamine concentrations in the brain; however, these perturbations have not been well documented or demonstrated in feeding trials. In this study, the effect of dietary T-2 toxin on regional brain concentrations of biogenic monoamines and their metabolites was investigated in male rats fed a semi-synthetic diet containing 0, 2.5 or 10 ppm T-2 toxin for either 7 or 14 days. Reduction in feed consumption, feed efficiency and weight gain was observed in rats fed either 2.5 or 10 ppm T-2 toxin. This effect was transient in animals fed the 10 ppm T-2 toxin diet, with feed consumption, feed efficiency and weight gain improving significantly during wk 2. T-2 toxin affected brain biogenic monoamine concentrations. In the nucleus raphe magnus, serotonin, 5-hydroxy-3-indoleacetic acid and norepinephrine increased in a dose-dependent manner, and dopamine increased transiently. In the substantia nigra of rats fed 10 ppm T-2, epinephrine increased after 7 days and norepinephrine decreased after 14 days, when compared with controls. Dihydroxyphenylacetic acid concentrations in the paraventricular nucleus and medial forebrain bundle were lower in T-2 toxin-treated rats than in control animals. The observed effects of T-2 toxin on brain monoamines and the resulting neurochemical imbalance may account for the physiological manifestation of trichothecene intoxication.

Central serotonergic uptake mechanisms in hypertensive rats: effects of clonidine and centhaquin

The binding of a highly specific ligand for serotonin (5-HT) uptake sites, [3H]paroxetine, was studied in brain regions of normotensive Wistar-Kyoto rats (WKY) and spontaneous hypertensive rats (SHR). [3H]Paroxetine bound to a single, high affinity binding site in the brain. In midbrain, the density (Bmax values) of [3H]paroxetine binding were significantly reduced (27.16%) in SHR as compared to WKY. The affinity (Kd values) were found to be similar in SHR and WKY. The Kd and Bmax values of [3H]paroxetine binding were found to be similar in spinal cord, pons and medulla and cerebral cortex of WKY and SHR. The effect of centrally acting hypotensive agents, clonidine and centhaquin, on [3H]paroxetine binding was also determined and compared with imipramine, a known 5-HT uptake inhibitor. Clonidine did not displace [3H]paroxetine binding at any concentration (10(-4) to 10(-7) M). On the other hand, centhaquin, which produces hypotension similar to clonidine, could displace [3H]paroxetine binding in a concentration dependent manner. In cerebral cortex and brainstem (midbrain, pons and medulla) membranes, the IC50 values of imipramine and centhaquin for [3H]paroxetine binding were found to be similar in WKY and SHR. The IC50 of centhaquin in displacing paroxetine from 5-HT uptake sites, was 10 times lower in the cerebral cortex and 4 times lower in the brainstem membranes when compared to imipramine. Clonidine had no effect on 5-HT uptake sites. The results indicate that (1) the density of 5-HT uptake sites is reduced in the midbrain of hypertensive rats, and (2) centhaquin, a centrally acting hypotensive agent, acts on 5-HT transporter sites.(ABSTRACT TRUNCATED AT 250 WORDS)

The action of (+/-)-MDMA on medial prefrontal cortical neurons is mediated through the serotonergic system

The mechanism of action of systemically administered (+/-)-MDMA (3,4-methylenedioxymethamphetamine) on spontaneously active neurons in the medial prefrontal cortex (mPFc) of chloral hydrate anesthetized rats was examined using standard single unit extracellular recording techniques. Intravenously administered MDMA dose-dependently decreased the firing rates of the majority of mPFc neurons in control rats. In contrast, in rats that were pretreated with p-chlorophenylalanine (PCPA), which depletes the brain serotonin (5-hydroxytryptamine, 5-HT) content by inhibiting tryptophan hydroxylase, the rate-limiting enzyme in the synthesis of 5-HT, MDMA was largely ineffective in inhibiting the firing of mPFc cells. In PCPA-treated animals, the administration of 5-hydroxytryptophan (5-HTP), which presumably restored the brain 5-HT content, but not L-DOPA, reinstated MDMA's inhibitory action in PCPA-treated rats. In rats that were pretreated with alpha-methyl-p-tyrosine (AMPT), which depletes the brain dopamine (DA) content by inhibiting tyrosine hydroxylase, the rate-limiting enzyme in the synthesis of DA, MDMA inhibited the firing of all of the mPFc cells. MDMA's effect on mPFc neurons was reversed by 5-HT receptor antagonists such as granisetron and metergoline. These results strongly suggest that MDMA exerts its action on mPFc cells indirectly by releasing endogenous 5-HT.

The pulmonary vascular responses to peripheral and central applications of acetylcholine

Acetylcholine (ACh) applied within the pulmonary artery elicited either a pressor or depressor response in the pulmonary vascular bed of rabbits, while ACh applied at the 4th ventricle induced a depressor response only. Both responses could be blocked by atropine and the pressor response could also be blocked partially by hexamethonium and phentolamine. Damage to the sympathoinhibition center in the median area of the medulla significantly reduced the centrogenic ACh-induced depressor response in both carotid and pulmonary arteries; the response could also be greatly diminished by the 5-HT blockers and naloxone applied centrally. It is thus suggested that ACh may exert a direct action on the M-receptors of the pulmonary vessels. It may also activate the N-receptors in the sympathetic ganglion which then induces a pulmonary vasoconstriction as a consequence. The centrogenic ACh-induced depressor responses involve the central M-receptors and are mediated by the medullary 5-HT and endorphin neuronal systems, resulting in suppression of the sympathetic tonus.

Toluene and benzene inhalation influences on ventricular arrhythmias in the rat

We have previously found that toluene did not share the capacity of benzene for increasing the arrhythmogenic action of epinephrine in the rat, but appeared to elicit the opposite effect. The present experiments were carried out to verify this observation in rats subjected to more severe ventricular arrhythmias. In animals previously inhaling either air, toluene or benzene and anesthetized with pentobarbital, arrhythmias were produced by coronary ligation or aconitine. In both models, toluene decreased and benzene increased the number of ectopic ventricular beats in the 30 min following induction of arrhythmia. Gas chromatographic measurement of toluene levels in the heart during and after inhalation revealed essentially constant concentrations at the time of arrhythmia evaluation, equivalent to approximately one-third the peak levels observed at the end of inhalation. Although the mechanism of the effect of toluene on arrhythmia could not be ascertained, nonspecific membrane stabilization or central serotonergic stimulation were considered as possible explanations. Since both mechanisms could be operant also in the case of benzene, the opposite effects of the solvents on arrhythmia could not be readily accounted for.

Failure of serotonin antagonist pizotifen to stimulate feeding or weight gain in free-feeding rats

The serotonin antagonist pizotifen (BC-105) is prescribed as an appetite and weight enhancer (Mosegor--Wander, also commercialized under brand names Sanmigran or Sandomigran--Sandoz, Switzerland) for anorectic and convalescent humans. There has been, however, difficulty in demonstrating any orexigenic effect of pizotifen in laboratory animals. In the present report, the influence of chronic administration of pizotifen (0.1-30.0 mg/kg b.wt. per day, SC) on food intake and body weight gains was studied in rats given a standard diet (SD-energy content 14.5 kJ/g, 9% fibre), and in rats either habituated to a low energy content, carbohydrate-free diet (DD-7.3 kJ/g, 45% fibre), or given the DD after habituation to the SD. Pizotifen failed to increase food intake or weight gain. Nor did it shorten a period of initial depression of intake of the unfamiliar DD. On the contrary, pizotifen seemed to diminish food intake and weight gain in rats fed the low energy content diet. Since it has been reported that other 5-HT antagonists, e.g., cyproheptadine, methysergide, and ritanserin can enhance feeding, it is of some interest that pizotifen failed to affect food intake or weight gain in rats. The results suggest that the effects of pizotifen (and, possibly, of serotonin) in rats may differ from those in man. The possibility that feeding in the rat is mediated by 5-HT1 rather than 5-HT2 receptors is discussed.

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.

Evidence that the putative 5-HT1A receptor agonists, 8-OH-DPAT and ipsapirone, have a central hypotensive action that differs from that of clonidine in anaesthetised cats

Thoracic preganglionic sympathetic nerve activity, blood pressure, heart rate and femoral arterial conductance were recorded in anaesthetised, paralysed cats. Cumulative dose-response curves were constructed for 8-OH-DPAT, ipsapirone and clonidine. All three drugs caused dose-related falls in blood pressure which were associated with minimal changes in femoral arterial conductance. However, 8-OH-DPAT and ipsapirone differed from clonidine in that their hypotensive action was associated with moderate sympathoinhibition and a profound bradycardia, whereas clonidine caused profound sympathoinhibition and, as it did not increase central vagal tone, only a moderate bradycardia. 8-OH-DPAT also caused sympathoinhibition in bi-vagotomised cats and decreased carotid sinus nerve activity along with blood pressure. As 8-OH-DPAT and ipsapirone bind selectively to central 5-HT1A receptors it is concluded that central stimulation of these receptors causes sympathoinhibition and an increase in vagal tone, whereas stimulation of central alpha 2-adrenoceptors causes only sympathoinhibition. In addition, the present data suggest a peripheral vasodilator mechanism may also contribute to the hypotensive effects of 8-OH-DPAT and ipsapirone in the cat. The nature and relative importance of this remains to be established.

The central site of the sympatho-inhibitory action of 5-hydroxytryptamine in the cat

The aim of this study was to determine the site in the CNS at which 5-hydroxytryptamine (5-HT) inhibits efferent sympathetic nerve activity in the cat. 5-Hydroxytryptamine (3 and 10 micrograms/kg), given into the lateral cerebral ventricle, produced immediate non dose related increases in mean blood pressure (MBP), heart rate (HR) and renal nerve activity (RNA). Larger doses (30 and 100 micrograms/kg i.c.v.) produced gradual decreases in blood pressure, heart rate and renal nerve activity, which did not occur when access of the drug to the fourth ventricle was prevented. Administration of 5-HT (10 and 30 micrograms/kg) into the fourth ventricle produced only decreases in blood pressure, heart rate and renal nerve activity after 15-40 min, which were accompanied by decreases in cardiac output and renal vascular resistance, but little or no change in total peripheral resistance. Application of 5-HT onto the ventral surface of the medulla, into the subarachnoid space at various levels along the spinal cord or into various parts of the nucleus tractus solitarius produced no effect on blood pressure heart rate or renal nerve activity. However, application of a cotton wool pledget soaked in a 5-HT solution (3 mg/ml) over the entire obex/NTS region produced immediate decreases in blood pressure, heart rate and renal nerve activity. These studies suggest that the sympatho-inhibitory effect of 5-HT is due to an action at a site near the caudal end of the dorsal surface of the medulla.

The cardiovascular effects of centrally administered 5-hydroxytryptamine in the conscious normotensive and hypertensive rat

The cardiovascular effects of centrally administered 5-hydroxytryptamine (5-HT) have been analysed in conscious normotensive and hypertensive rats. In conscious normotensive rats, 5-HT, (1-30 micrograms) administered intracerebroventricularly (i.c.v.) produced profound and immediate dose-related decreases in heart rate and small increases in blood pressure. The initial pressor responses were followed by secondary secondary depressor responses at high doses of 5-HT. Similar effects were produced by 5-HT i.c.v. in conscious DOCA-salt and spontaneously hypertensive rats, although the magnitude of the pressor responses was substantially greater in hypertensive than normotensive rats. Pretreatment with either N-methylatropine or atenolol intra-arterially reduced the 5-HT-induced bradycardia in normotensive rats; the reduction was enhanced when both antagonists were given in combination. The 5-HT2 antagonist, cyproheptadine (10 micrograms i.c.v.) increased basal blood pressure and heart rate in normotensive rats. Subsequent administration of 5-HT i.c.v. produced biphasic effects on heart rate consisting of an initial tachycardia followed by a marked bradycardia. Methysergide (10 micrograms i.c.v.) pretreatment did not alter resting heart rate, but attenuated the 5-HT induced bradycardia. A higher dose of methysergide, (30 micrograms i.c.v.), decreased resting blood pressure and heart rate. This study has demonstrated, therefore, that the 5-HT induced bradycardia is produced by not only a centrally mediated decrease in sympathetic tone, but also an increase in vagal drive to the heart. The bradycardia is antagonised by centrally administered methysergide, but not by cyproheptadine, which suggests that it is probably mediated through a '5-HT1-like' receptor mechanism.

Pizotifen (BC-105) attenuates orienting and Pavlovian heart rate conditioning in rabbits

The cardiac component of the orienting reflex (OR) was elicited in rabbits by 75 dB, 4-sec duration tones of either 304 or 1216 Hz. The conditioned cardiac response was also studied using the same tones and paraorbital electric shock as conditioned and unconditioned stimuli, respectively, using a differential Pavlovian conditioning paradigm. Subcutaneous injections of the central 5-HT antagonist pizotifen (BC-105), the peripheral 5-HT antagonist xylamidine, the central 5-HT agonist d-lysergic acid diethylamide (LSD), and LSD in conjunction with BC-105 were administered 15 min prior to behavioral assessment. Both the heart rate (HR) conditioned response (CR) and the OR consisted of bradycardia. BC-105 attenuated, but xylamidine had no effect on, OR habituation. LSD reduced the magnitude of the OR, an effect which was blocked by BC-105. BC-105 also produced a dose-related attenuation of the bradycardiac HR CR; however, xylamidine had no effect on HR conditioning, suggesting that the attenuation of the HR CR by BC-105 was central rather than peripheral in origin. LSD potentiated the bradycardiac HR CR, but BC-105 in conjunction with LSD attenuated this response. These results suggest that central 5-HT neurons may modulate the magnitude of bradycardiac responses during orienting and aversive Pavlovian conditioning.

Acute administration of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a selective 5-HT-receptor agonist, causes a biphasic blood pressure response and a bradycardia in the normotensive Sprague-Dawley rat and in the spontaneously hypertensive rat

8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) is a new serotonin (5-HT) receptor agonist that binds selectively to the 5-HT1A binding site. In the present paper we investigated the cardiovascular effects of 8-OH-DPAT in the normotensive Sprague-Dawley rat and in the spontaneously hypertensive rat. The acute i.v. administration of 8-OH-DPAT (5-150 micrograms/kg) was in both rat strains associated with a biphasic blood pressure response and a bradycardia. The initial pressor response was due to a direct vascular effect of 8-OH-DPAT involving activation of alpha-adrenoceptors since it was present in pithed rats and in reserpine pretreated rats and since it was attenuated by prazosin. The longer lasting hypotension was not due to a direct vascular relaxation or a presynaptic inhibition of transmitter release since the hypotension was not evident in pithed rats and since 8-OH-DPAT did not influence the pressor responses to electrical stimulation in pithed rats. Rather, the combination of hypotension and bradycardia would suggest a central site of action although the intracerebroventricular (lat. ventricles) route of administration was not more efficient (to induce hypotension) than i.v. administration. At least the bradycardia was mediated by changes in vagal as well as sympathetic discharge since it was prevented by pretreatment with atropine and propranolol in combination but not by pretreatment with either agent alone. The cardiovascular effects of 8-OH-DPAT were not prevented by pretreatment with methergoline, methiothepin, pirenperone or cianserine or by 5-HT depletion by means of p-chlorophenylalanine, which suggests that the putative 5-HT receptor that is responsible for the hypotension and bradycardia to 8-OH-DPAT is not of a presynaptic type and does not have the pharmacological characteristics of a general 5-HT1 receptor.

Cardiovascular regulation and lesions of the central nervous system

The central nervous system has an important role in the second-to-second regulation of cardiac activity and vasomotor tone. Central lesions that lead to a disturbance in autonomic activity tend to cause electrocardiographic and pathological evidence of myocardial damage, cardiac arrhythmias, and disturbances of arterial blood pressure regulation. To a great extent such cardiovascular disturbances result from alterations in sympathetic activity. Similar alterations in sympathetic activity can occur under conditions of emotional stress and precipitate cardiac arrhythmias that can themselves lead to the syndrome of sudden death. Experimental and clinical evidence suggests that central neural mechanisms may be involved in this important human syndrome, but no central lesion has yet been identified to account for it. Recent experimental evidence, derived from hypertension research, suggests that chemical disturbances in the central nervous system, without accompanying structural lesions, may be found to explain cardiovascular disturbances such as sudden death and hypertension.

Analysis of the cardiovascular responses to central injection of tryptamine in rats

Tryptamine (2-20 micrograms), administered into the lateral cerebral ventricle of the rat, evoked a pressor response which was sometimes followed by a prolonged depressor response. The intracisternal administration of tryptamine (7-20 micrograms) caused a slow progressive and long-lasting depressor effect without or with an initial pressor effect. The pressor response was accompanied by variable changes in heart rate, whilst the pure depressor response was accompanied by a decrease in heart rate. After transection of the spinal cord between C1 and C2 the pressor response was substantially reduced or abolished. Methysergide, injected centrally, antagonized in a dose-dependent manner the pressor effect, whilst p-chlorophenylalanine, atropine and hexamethonium, administered by the same route, did not diminish this effect. It is concluded that tryptamine, injected centrally, causes both increases and decreases in arterial blood pressure and heart rate. The pressor response to tryptamine results from the activation of central noncholinergic, methysergide-sensitive, receptor sites and the depressor response to tryptamine may be due to a centrally-induced reduction in sympathetic nervous activity. It is tentatively suggested that tryptamine, like 5-hydroxytryptamine, participates in the physiological regulation of the cardiovascular system of the rat, as both a central excitatory and inhibitory regulator.

Cardiovascular effects in the Sprague-Dawley rat of 8-hydroxy-2(di-N-propylamino) tetralin, a selective 5-hydroxytryptamine receptor agonist

The intravenous administration of 8-hydroxy-2(di-N-propylamino) tetralin, a selective 5-HT receptor agonist, caused a biphasic blood pressure response and bradycardia in Sprague-Dawley rats. The initial pressor response involved peripheral alpha 1-adrenoceptors since it was present in pithed rats and was antagonized by prazosin. Though the intracerebroventricular route of administration was not more effective the hypotension and bradycardia were probably of central origin. The bradycardia was prevented by pretreatment with atropine and propranolol suggesting an involvement of vagal as well as sympathetic activity. These results support the view that central 5-HT receptor activation reduces the blood pressure and heart rate.

The distribution of substance P, enkephalin, and serotonin immunoreactivities in the area postrema of the rat and cat

With the use of the peroxidase-antiperoxidase technique the distribution of substance P (SP), enkephalin (ENK), and serotonin (5HT) immunoreactivities were described in the area postrema of the rat and cat. In both species, immunoreactivity in the area postrema was differentially distributed as either fibers only, or cell bodies and fibers. In the rat and cat, ENK had the greatest accumulation of immunoreactive fibers, followed by 5HT and SP. In the area postrema of the rat the majority of SP-, ENK-, and 5HT-immunoreactive fibers were along the ventral and ventrolateral borders, with fewer immunostained fibers at the dorsal surface. The area postrema of the cat had the majority of SP-, ENK-, and 5HT-immunoreactive fibers at the ventral and lateral borders, with fewer immunostained fibers at the dorsal and medial borders. In both species, the area postrema's central region contained the fewest immunostained fibers. In general, for each putative neurotransmitter examined, immunostained fibers in both species progressively decreased in number rostrocaudally. Serotonin- and ENK-immunoreactive cell bodies were found in the rat area postrema; in the cat area postrema only ENK-immunoreactive cell bodies were present. The area postrema of both species lacked SP-immunoreactive cell bodies. The heterogeneous distribution of immunoreactive fibers and cell bodies within the area postrema of the rat and cat may reflect the different functions of the area postrema in both species.

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.

Serotonergic excitation of sympathetic preganglionic neurons: a microiontophoretic study

The effects of microiontophoretically applied serotonin on the extracellularly recorded discharges of sympathetic preganglionic neurons (SPNs) were studied in anesthetized cats. Thoracic SPNs were identified on the basis of constancy of antidromic activation and collision. Low ejecting currents of serotonin (5-30 nA) invariably excited spontaneously active SPNs. Serotonin also excited the vast majority of quiescent SPNs, as well as neurons brought to discharge threshold by the excitatory amino acid L-glutamate. A population of SPNs was identified which was insensitive to the excitatory effects of both serotonin and L-glutamate. Iontophoretic or intravenous administration of the putative serotonin antagonists methysergide and metergoline blocked the excitatory effects of serotonin on SPNs. The blockade of the serotonin-induced excitation was not associated with a local anesthetic action of methysergide or metergoline. Methysergide and metergoline also reduced the firing rate of SPNs in intact but not in spinal animals. These data provide strong evidence to support the contention that serotonergic neurons provide a tonic excitatory input to SPNs.

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.

The regulation of cardiovascular functions by monoamine neurotransmitters in the brain

The complexity of the central nervous system's influence on cardiovascular integration is underscored by the extensive neural interconnections between the various higher and lower brain structures shown by electrophysiological studies to have an influence on cardiovascular function. Furthermore, complex neural connections exist within each level of cardiovascular integration (for instance, the medullary reticular formation, or the limbic system). The electrophysiological analysis of the integrative aspects of central cardiovascular regulation has been the subject of several reviews (Reis, 1972; Smith, 1974; Calaresu et al., 1975; Oberg, 1976). In recent years, the methodology of analyzing the anatomical neural pathways of central cardiovascular regulation by lesion-degeneration, evoked potential, and fluorescent histochemical procedures, has been improved with the introduction of anterograde and retrograde tracer techniques. The result is the delineation of previously undetected neural interconnections among various brain areas, long known as cardiovascular centres, such as the NTS, the parabrachial nucleus, the paraventricular nucleus of the hypothalamus, the central nucleus of the amygdala, and the bed nucleus of the stria terminalis (Loewy & McKellar, 1980). Undoubtedly the next few years will see major advances in the understanding of the neuroanatomical pathways involved in central cardiovascular control.

Long-lasting cardiovascular depressor response following sciatic stimulation in spontaneously hypertensive rats. Evidence for the involvement of central endorphin and serotonin systems

A naloxone-reversible long-lasting depressor response induced by a prolonged low frequency stimulation of the sciatic nerve in conscious spontaneously hypertensive rats (SHRs) was reported in a previous paper. In the present study pharmacological tools were used to further investigate the neurotransmitters involved in this phenomenon. Naloxone infusion (20--25 mg/kg/h following a bolus dose of 10 mg/kg i.v.) attenuated significantly the depressor response, while dexamethasone pretreatment had no such effect, suggesting an important role of the brain endorphin system, but not of the pituitary beta-endorphin, in this depressor response. Since the concomitant increase in pain threshold produced by the sciatic stimulation exhibited a different time course of development and naloxone reversibility, it is suggested that the depressor response and the hypalgesic effect produced by the same stimulation are mediated via different types of opiate receptors in the brain. On the other hand, PCPA abolished the post-stimulatory depressor response whereas 5-HTP and zimelidine had additive effects on the sciatic stimulation-induced depressor response, suggesting the involvement of central serotonin systems in the mechanism of the response. The interaction between the central endorphin and the serotonin systems in the mediation of the post-stimulatory depressor response is discussed.

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.

Comparison of the cardiovascular responses to intracerebroventricular administration of tryptamine, 5-hydroxytryptamine, tryptophan and 5-hydroxytryptophan in rats

General characteristics of the cardiovascular responses to intracerebroventricular (i.c.v.) injection of tryptamine, 5-hydroxytryptamine (5-HT), tryptophan and 5-hydroxytryptophan (5-HTP) were compared. Relatively small doses of tryptamine and 5-HT (0.005-0.1 microM) produced considerable, long-lasting and dose-dependent pressor effects, which sometimes were followed by prolonged depressor effects. Tryptophan (0.02-0.5 microM) and 5-HTP (0.02-0.2 microM) caused variable and usually slight, but long-lasting, vascular responses or no vascular response A large dose of tryptamine (0.5 microM) evoked variable vascular effects, while the same dose of 5-HT and 5-HTP evoked marked and prolonged depressor effects. The vascular responses to the drugs were accompanied by variable changes in heart rate. Tryptamine, 5-HT and 5-HTP, in the majority of rats, produced a bradycardia. The present study provides evidence that the cardiovascular response to i.c.v. administration of tryptamine is similar to that of 5-HT, supporting the idea that tryptamine, in addition to 5-HT, participates in the central physiological regulation of the rat cardiovascular system. The role of tryptophan and 5-HTP by themselves in this regulation, if any is of secondary importance.

Neurophysiology and neuropharmacology of cardiovascular regulation and stress

Evidence has accumulated over the past several years indicating that environmental factors can have a substantial influence on cardiovascular dynamics. It has been hypothesized by many investigators that through these influence environmental stressors may be important to the etiology and maintenance of cardiovascular diseases. Since the nervous system is intimately involved in the regulation of cardiovascular function it may be assumed that environmental influences on cardiovascular dynamics are to a large extent mediated by the nervous system. This assumption is supported by the literature reviewed which indicates that there are many nervous system nuclei and neurotransmitter systems involved in the regulation of cardiovascular dynamics which are also involved in an organisms adjustment to environmental stressors. The conclusion is reached that further multidisciplinary research will reveal underlying neurophysiological and neuropharmacological mechanisms responsible for stress induced cardiovascular disease and lead to new methods of treatment.

Tyrosine administration decreases vulnerability to ventricular fibrillation in the normal canine heart

Intravenous infusion of tyrosine (1, 2, or 4 milligrams per kilogram) for 20 to 30 minutes caused dose-dependent increases in the ventricular fibrillation threshold in normal dogs. Administration of valine, a neutral amino acid that competes with tyrosine for uptake at the blood-brain barrier, in a dose equimolar to the most effective dose of tyrosine, slightly decreased the ventricular fibrillation threshold when given alone and significantly blocked elevation of the ventricular fibrillation threshold after tyrosine infusion. Hence, tyrosine, presumably acting in the central nervous system, can protect against certain ventricular arrhythmias.

Contrasting effects of clonidine and 5-hydroxytryptophan on spinal sympathetic pathways

The effects of clonidine HCI were compared with those of 5-HTP on transmission through two spinal sympathetic pathways, segmental spinal reflex pathways and descending intraspinal excitatory pathways, in unanesthetized spinal cats. Evoked sympathetic discharges were recorded from upper thoracic preganglionic rami. Clonidine (5-50 microgram/kg) produced a parallel, dose-dependent depression of transmission through each pathway. The intraspinal pathway was five time more sensitive than the spinal reflex pathway (ED50's, 6 and 30 microgram/kg), and the spinal reflex pathway could not be depressed by more than 60% even by higher doses. In contrast, 5-HTP was more effective in depressing the spinal reflex than the intraspinal pathway (ED50's 32 and 44 mg/kg), and both pathways could be depressed completely. Small doses of tolazoline or yohimbine rapidly antagonized the effects of clonidine but not 5-HTP. Clonidine and 5-HTP appear to depress the excitability of sympathetic preganglionic neurons by activating alpha2- and 5-HT receptors, respectively. Each mechanism may contribute independently to regulation of the sympathetic outflow.

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.