Effects of 5-hydroxytryptamine and related compounds on the sympathetic nerves of the rabbit heart

Serotonin and beyond-a tribute to Manfred Göthert (1939-2019)

Manfred Göthert, who had served Naunyn-Schmiedeberg's Arch Pharmacol as Managing Editor from 1998 to 2005, deceased in June 2019. His scientific oeuvre encompasses more than 20 types of presynaptic receptors, mostly on serotoninergic and noradrenergic neurones. He was the first to identify presynaptic receptors for somatostatin and ACTH and described many presynaptic receptors, known from animal preparations, also in human tissue. In particular, he elucidated the pharmacology of presynaptic 5-HT receptors. A second field of interest included ligand-gated and voltage-dependent channels. The negative allosteric effect of anesthetics at peripheral nACh receptors is relevant for the peripheral clinical effects of these drugs and modified the Meyer-Overton hypothesis. The negative allosteric effect of ethanol at NMDA receptors in human brain tissue occurred at concentrations found in the range of clinical ethanol intoxication. Moreover, the inhibitory effect of gabapentinoids on P/Q Ca2+ channels and the subsequent decrease in AMPA-induced noradrenaline release may contribute to their clinical effect. Another ligand-gated ion channel, the 5-HT3 receptor, attracted the interest of Manfred Göthert from the whole animal via isolated preparations down to the cellular level. He contributed to that molecular study in which 5-HT3 receptor subtypes were disclosed. Finally, he found altered pharmacological properties of 5-HT receptor variants like the Arg219Leu 5-HT1A receptor (which was also shown to be associated with major depression) and the Phe124Cys 5-HT1B receptor (which may be related to sumatriptan-induced vasospasm). Manfred Göthert was a brilliant scientist and his papers have a major impact on today's pharmacology.

Serotonin discovery and stepwise disclosure of 5-HT receptor complexity over four decades. Part I. General background and discovery of serotonin as a basis for 5-HT receptor identification

This review contains background information on the serotonin system, furthermore the suggestion to introduce the term Contemporary Witness Report (CWR) for a novel type of review and, as the main part, an overview over the history of serotonin discovery as a basis for the identification of its receptor heterogeneity and the increase in complexity by genetic and allosteric variation. The present article conforms to CWRs in historical and autobiographical elements, in more emphasis on the author's work than in conventional reviews and in aspects neglected in previous reviews, but not in the main feature namely the work of a scientist with comprehensive expertise in a field in which, over long time, he/she continuously performed research and published. A scientist complying with these requirements is a contemporary witness in that field. His report on the scientific achievements in that period, a CWR, comprises confirmation and putative re-interpretation of data from a superior viewpoint. Identification of serotonin's vascular properties (publication year: 1912) as an "adrenaline mimicking substance" (without attempt to isolate it) by O'Connor preceded the discovery of serotonin in the gastrointestinal tract by Erspamer [1937] and in blood by Rapport [1948, 1949], who identified its structure as 5-hydroxytryptamine [1949]. Detection as a neurotransmitter in invertebrates suggested its occurrence in vertebrate CNS as well. This was verified by finding it in dog, rat and rabbit brain [1953]. The Falck-Hillarp technique [1962] visualized serotonin neurones as fluorescent structures. The neurotoxin 5,7-dihydroxytryptamine [1972] indirectly proved the involvement of 5-HT in multiple CNS functions.

Presynaptic lonotropic receptors

The release of transmitters through vesicle exocytosis from nerve terminals is not constant but is subject to modulation by various mechanisms, including prior activity at the synapse and the presence of neurotransmitters or neuromodulators in the synapse. Instantaneous responses of postsynaptic cells to released transmitters are mediated by ionotropic receptors. In contrast to metabotropic receptors, ionotropic receptors mediate the actions of agonists in a transient manner within milliseconds to seconds. Nevertheless, transmitters can control vesicle exocytosis not only via slowly acting metabotropic, but also via fast acting ionotropic receptors located at the presynaptic nerve terminals. In fact, members of the following subfamilies of ionotropic receptors have been found to control transmitter release: ATP P2X, nicotinic acetylcholine, GABA(A), ionotropic glutamate, glycine, 5-HT(3), andvanilloid receptors. As these receptors display greatly diverging structural and functional features, a variety of different mechanisms are involved in the regulation of transmitter release via presynaptic ionotropic receptors. This text gives an overview of presynaptic ionotropic receptors and briefly summarizes the events involved in transmitter release to finally delineate the most important signaling mechanisms that mediate the effects of presynaptic ionotropic receptor activation. Finally, a few examples are presented to exemplify the physiological and pharmacological relevance of presynaptic ionotropic receptors.

Molecular mechanisms underlying the modulation of exocytotic noradrenaline release via presynaptic receptors

The release of noradrenaline from nerve terminals is modulated by a variety of presynaptic receptors. These receptors belong to one of the following three receptor superfamilies: transmitter-gated ion channels, G protein-coupled receptors (GPCR), and membrane receptors with intracellular enzymatic activities. For representatives of each of these three superfamilies, receptor activation has been reported to cause either an enhancement or a reduction of noradrenaline release. As these receptor classes display greatly diverging structures and functions, a multitude of different molecular mechanisms are involved in the regulation of noradrenaline release via presynaptic receptors. This review gives a short overview of the presynaptic receptors on noradrenergic nerve terminals and summarizes the events involved in vesicle exocytosis in order to finally delineate the most important signaling cascades that mediate the modulation via presynaptic receptors. In addition, the interactions between the various presynaptic receptors are described and the underlying molecular mechanisms are elucidated. Together, these presynaptic signaling mechanisms form a sophisticated network that precisely adapts the amount of noradrenaline being released to a given situation.

Noradrenergic mediation of spinal antinociception by 5-hydroxytryptamine: characterization of receptor subtypes

The present study examined the involvement of spinal noradrenergic mechanisms in spinal antinociception by the 5-hydroxy-tryptamine (5-HT) receptor-selective agonists CGS 12066B (5-HT1B; 7-trifluoromethyl-4(4-methyl-1-piperazinyl)-pyrrolo[1,2-a]quinoxaline), TFMPP (5-HT1C; M-trifluoromethylphenyl-piperazine) and DOI (5-HT2; 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane) using the rat hot plate test. Effects of alpha-adrenoreceptor antagonists (phentolamine, yohimbine), the adrenergic neurotoxin 6-hydroxydopamine, and the selective noradrenergic uptake blocker desipramine were determined. CGS 12066B, TFMPP and DOI produced dose-related antinociception. The antinociceptive effect of each agent was reduced by pretreatment with both phentolamine and yohimbine (15-60 micrograms). Pretreatment with 6-hydroxydopamine (100 micrograms, intrathecal) for 7-10 days, which reduced spinal cord levels of noradrenaline by 87%, inhibited the action of TFMPP (and 5-HT), but not CGS 12066B or DOI. Pretreatment with desipramine (25 mg/kg, systemic) potentiated the action of TFMPP but not CGS 12066B or DOI (or 2-methyl-5-HT). These results suggest that antinociception by TFMPP is dependent on release of endogenous noradrenaline from the spinal cord, while that produced by CGS 12066B and DOI is not. As TFMPP exhibits a close similarity to 5-HT in these experiments, the 5-HT receptor subtype being activated to induce noradrenaline release may either be a 5-HT1C or a 5-HT1S subtype. Other mechanisms account for the observed blockade of the action of CGS 12066B and DOI by alpha-adrenoreceptor antagonists.

5-HT3 receptors are not involved in the modulation of the K(+)-evoked release of [3H]5-HT from spinal cord synaptosomes of rat

The ability of 5-HT3 receptor agonists to modulate the resting efflux or K(+)-evoked release of [3H]5-HT from superfused synaptosomes from the spinal cord of the rat was investigated. Phenylbiguanide did not alter the resting efflux of [3H]5-HIAA or [3H]5-HT or modify the K(+)-evoked release of [3H]5-HT. 2-Methyl-5-HT (10 microM) caused an increase in resting efflux of [3H]5-HIAA, an effect that was blocked by the inhibitor of the uptake of 5-HT fluoxetine. No effect on K(+)-evoked release of tritium was observed. Bufotenine (100-1000 nM) increased the resting efflux of [3H]5-HT and [3H]5-HIAA. These effects were not antagonized by the 5-HT3 antagonist ICS 205-930 but were antagonized by fluoxetine. The drug ICS 205-930 (1 microM) did not alter resting efflux or block the ability of serotonin (30 and 100 nM) to decrease K(+)-evoked release of tritium. Quipazine, a potent antagonist of peripheral 5-HT3 receptors (subnanomolar concentrations), was also unable to alter resting or K(+)-evoked release of [3H]5-HT. It did, however, attenuate the inhibitory effect 5-HT on K(+)-evoked release. The concentrations required were in the micromolar range, consistent with the ability of the drug to antagonize the 5-HT1B autoreceptor. These results support the idea that 5-HT3 receptors do not act as nerve terminal autoreceptors in the spinal cord of the rat.

Noradrenergic and purinergic involvement in spinal antinociception by 5-hydroxytryptamine and 2-methyl-5-hydroxytryptamine

The adrenergic involvement in spinal antinociception by 5-hydroxytryptamine (5-HT) and 2-methyl-5-hydroxytryptamine (2-Me-5-HT) was examined using the alpha-adrenoceptor antagonists phentolamine, yohimbine and prazosin, and the neurotoxin 6-hydroxydopamine. Intrathecal pretreatment with phentolamine and yohimbine (7.5-30 micrograms), but not prazosin (30 micrograms), reduced the action of 5-HT and 2-Me-5-HT in both the tail flick and hot plate tests. Pretreatment with 6-hydroxydopamine (100 micrograms) reduced (5-HT) or increased (2-Me-5-HT) antinociception in the hot plate test, while tail flick responses were largely unaffected. In other experiments, 8-phenyltheophylline, an adenosine receptor antagonist, reduced the action of 5-HT, but not 2-Me-5-HT, in both tests. These results indicate that (a) antinociception by both 5-HT and 2-Me-5-HT involves some form of interaction with spinal alpha 2-adrenoceptors, but the nature of the interaction for these two agents is different because only 5-HT is dependent on endogenous noradrenaline, (b) release of adenosine from the spinal cord contributes to spinal antinociception by 5-HT but not by 2-Me-5-HT.

Hemodynamic and renin responses to (+-)-DOI, a selective 5-HT2 receptor agonist, in conscious rats

The effect of the selective 5-HT2 agonist (+-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI) on arterial pressure (AP), heart rate (HR), renal blood flow (RBF) and plasma renin activity (PRA) was determined in conscious rats. DOI increased AP and PRA, but decreased HR and RBF. All responses to DOI were abolished by central (LY 53857) or peripheral (xylamidine) 5-HT2 antagonists. Prazosin did not alter the AP or HR response to DOI. Chlorisondamine abolished the bradycardia but slightly increased the hypertension produced by DOI, while enalapril attenuated the pressor response. No further reduction was produced by the combination of enalapril and prazosin. Propranolol attenuated but did not eliminate the renin response, and blocked the bradycardia elicited by DOI. The data suggest that DOI activates 5-HT2 receptors located on vascular smooth muscle and/or the circumventricular organs of the brain to: (1) increase AP and reflexly decrease HR, (2) decrease RBF and (3) increase PRA. The hypertension is mediated by angiotensin II and direct vascular effects whereas the increase in PRA is mediated by an interaction of increased sympathetic nerve activity and decreased renal perfusion pressure.

Inhibition of noradrenaline release via presynaptic 5-HT1B receptors of the rat vena cava

In the rat inferior vena cava preincubated with 3H-noradrenaline, the effects of nine serotonin (5-HT) receptor agonists and of eight antagonists (including two beta-adrenoceptor blocking agents) on the electrically evoked 3H overflow were determined. 1. 5-HT, 5-carboxamido-tryptamine, 5-methoxy-3(1,2,3,6-tetrahydropyridine-4-yl)-1H-indole (RU 24969), 5-methoxytryptamine, N,N-dimethyl-5HT, tryptamine and 5-aminotryptamine inhibited the evoked 3H overflow. The potencies of these agonists in inhibiting overflow were significantly correlated with their affinities for 5-HT1B binding sites, but not with their affinities for 5-HT1A, 5-HT1C or 5-HT2 binding sites. 8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A receptor agonist, and ipsapirone, a partial agonist at these receptors, did not inhibit overflow. 2. Cyanopindolol facilitated the evoked 3H overflow, an effect which was abolished by propranolol. The maximum inhibition of overflow obtainable with 5-HT was diminished by cyanopindolol. 3. The concentration-response curve for 5-HT was shifted to the right by metitepine, metergoline, quipazine, 6-chloro-2-(1-piperazinyl)pyrazine (MK 212) and propranolol which, given alone, did not affect 3H overflow. The apparent pA2 values of these antagonists tended to be correlated with their affinities for 5-HT1B (but not 5-HT1A, 5-HT1C or 5-HT2) binding sites. Ketanserin, a 5-HT2 receptor antagonist, and spiperone, which blocks 5-HT2 and 5-HT1A but not 5-HT1B or 5-HT1C receptors, failed to antagonize the effect of 5-HT.(ABSTRACT TRUNCATED AT 250 WORDS)

Noradrenergic denervation prevents sensitization of rat forebrain neurons to serotonin by tricyclic antidepressant treatment

The effect of prior lesioning of noradrenergic neurons with 6-hydroxydopamine (6-OHDA) on the ability of amitriptyline to enhance the responsiveness of hippocampal pyramidal neurons to serotonin (5-HT) was examined in Sprague-Dawley rats. In control rats, amitriptyline (5 mg/kg/day for 14 days), administered either by daily injection or by an osmotic minipump, enhanced the responsiveness of these neurons to microiontophoretically applied 5-HT, leaving their responsiveness to norepinephrine (NE) and to acetylcholine unaltered. However, in 6-OHDA-pretreated rats, amitriptyline failed to enhance the responsiveness of hippocampal pyramidal neurons to 5-HT. The effect of NE and, to a lesser extent, that of 5-HT were prolonged in 6-OHDA-pretreated rats, presumably because NE and 5-HT released by microiontophoresis are taken up by NE terminals in the intact rat. The present results provide additional evidence that the integrity of the NE system is required for antidepressant treatments to modify 5-HT neurotransmission.

Serotonin (5-HT) enhances hippocampal noradrenaline (NA) release: evidence for facilitatory 5-HT receptors within the CNS

Slices of rabbit hippocampus were preincubated with 3H-noradrenaline (3H-NA), then superfused continuously in the presence of the noradrenaline (NA) uptake inhibitor (+)oxaprotilin and twice stimulated electrically. The stimulation induced tritium overflow was increased by the 5-HT receptor agonists, 5-HT, 2-methyl-5-HT and 5-carboxamidotryptamine in a concentration dependent manner; a tyramine-like displacement of NA by the 5-HT agonists was prevented by (+)oxaprotilin. The 5-HT M-receptor antagonists, MDL 72222 and ICS 205-930, inhibited the facilitatory effects of 5-HT agonists as well as the enhanced tritium overflow due to the selective 5-HT uptake inhibitor, 6-nitroquipazine: in each case, concentrations much higher than those required to block M-receptors of the periphery were necessary. At high concentrations MDL 72222, in contrast to ICS 205-930, seems to have alpha-adrenoceptor antagonistic activity. The 5-HT2 receptor antagonist, ketanserin, had no effect on 5-HT-induced facilitation of transmitter release; metitepin facilitated stimulation-evoked transmitter release per se both in the absence and presence of phentolamine. From our results we conclude that, as on peripheral nerve endings, also on central noradrenergic terminals, facilitatory 5-HT receptors are present that modulate NA release. The enhanced tritium overflow following 6-nitroquipazine may be due to an increased release of endogenous 5-HT, a suggestion which supports the hypothesis of a physiological innervation of these facilitatory 5-HT receptors on NA terminals.

The depolarizing action of 5-hydroxytryptamine on rabbit vagal afferent and sympathetic neurones in vitro and its selective blockade by ICS 205-930

Depolarizing responses to 5-hydroxytryptamine (5-HT) were recorded from rabbit nodose (NG) and superior cervical (SCG) ganglia using the sucrose-gap technique. The antagonist potency and selectivity of ICS 205-930 ([3 alpha-tropanyl]-1H-indole-3-carboxylic acid ester) were investigated. In NG, 5-HT (5 to 80 nmol) evoked depolarizations of graded amplitude. The ED50 was 18.2 (10.9-30.5) nmol (geometric mean, 95% confidence limits). Responses were blocked surmountably by ICS 205-930, 10(-11) and 10(-10) M, the threshold for blockade being below 10(-11) M. Parallel, rightward shifts in dose-response curves were seen with these concentrations of antagonist, but at higher concentrations (10(-9) and 10(-8) M) there was a further rightward shift with reduction in slope and maximum of the curves. In SCG, where 5-HT (20 to 320 nmol) evoked depolarizations of graded amplitude and the ED50 was 55.8 (22.3-139.6) nmol (geometric mean, 95% confidence limits), ICS 205-930 had a similar inhibitory effect to that observed in NG. The apparent pA2 values for the surmountable blockade produced by ICS 205-930 at concentrations of 10(-11) and 10(-10) M were 10.2 +/- 0.2 for NG and 10.4 +/- 0.1 for SCG (means +/- s.e. mean). ICS 205-930 was selective in its action since it had no effect on dimethylphenylpiperazinium (DMPP) responses in either ganglion or on GABA responses in NG. This study provides quantitative evidence on the blocking action of ICS 205-930 at neuronal 5-HT receptors using a technique that allows the depolarizing responses evoked by the amine to be directly recorded.

An active pathway for serotonin synthesis by renal proximal tubules

Serotonin (5HT) has significant effects on renal metabolism and glomerular function and is a potent renal vasoconstrictor. In this study we describe and localize a highly active biosynthetic pathway for serotonin in the kidney. Rat kidneys were dissected into cortical and medullary fractions; in some experiments the cortex was also separated into subfractions enriched with glomeruli or proximal tubules. Serotonin and tryptophan hydroxylase (TyOH) were measured by radioenzymatic techniques. (table; see text) Renal denervation did not alter tryptophan hydroxylase activity. In kidneys from human cadaveric donors, cortical tryptophan hydroxylase (4.13 +/- 0.68 nM/30 min/g) exceeded that in the medulla (1.96 +/- 0.86 nM/30 min/g). Aromatic L-amino acid decarboxylase, the remaining enzyme for serotonin synthesis, is present in both rat renal cortex and medulla; however, we found 15-fold greater decarboxylase activity in proximal tubular (2070 nM/30 min/g) as compared to glomerular (131 nM/30 min/g) subfractions. We were able to demonstrate that under physiological conditions, free urine serotonin reflects actual biosynthesis by the kidney. Thus, although serotonin stores retained by the kidney appear small and relatively localized to the medulla, the enzymatic activity for the synthesis of serotonin in the kidney is comparable to that in the brain, with the complete pathway localized to renal cortical proximal tubules. These data suggest that further studies of renal serotonin metabolism may contribute to our understanding of renal function in health and disease.

The depolarizing action of 5-hydroxytryptamine on rabbit vagal primary afferent and sympathetic neurones and its selective blockade by MDL 72222

MDL 72222 (1 alpha H,3 alpha,5 alpha H-tropan-3-yl-3,5-dichlorobenzoate) is a novel compound with potent and selective blocking actions at certain excitatory 5-hydroxytryptamine (5-HT) receptors on mammalian peripheral neurones. In the present study, the sucrose-gap technique has been used to record depolarizing responses to 5-HT from the cells of the rabbit nodose and superior cervical ganglia and to investigate the potency and selectivity of MDL 72222 as an antagonist of these responses. On nodose ganglia, responses to 5-HT were inhibited surmountably by MDL 72222 at concentrations up to 100 nmol/l. The threshold for antagonism was 2-10 nmol/l and the apparent pA2 value (Schild 1947) was 7.7 +/- 0.2, n = 10. Blockade was selective since responses to GABA and noradrenaline were unaffected by MDL 72222, 100 nmol/l. With concentrations of MDL 72222 higher than 100 nmol/l, antagonism was concentration-related but not in a manner consistent with simple competitive antagonism and even a concentration of 1 mumol/l failed to abolish the response to 5-HT. The results from the superior cervical ganglion were essentially similar to those obtained from the nodose ganglion. The threshold concentration of MDL 72222 for inhibition of 5-HT was 1-10 nmol/l and blockade was selective in that depolarizing responses to dimethylphenyl-piperazinium (DMPP) was unaffected by a concentration of MDL 72222 of 1 mumol/l. The data provide direct evidence that MDL 72222 is a potent and selective antagonist of the receptors for 5-HT which mediate depolarizing responses in vagal primary afferent cell bodies and in sympathetic ganglion cells.

MDL 72222: a potent and highly selective antagonist at neuronal 5-hydroxytryptamine receptors

The properties of MDL 72222 (1 alpha H, 3 alpha, 5 alpha H-tropan-3-yl-3,5-dichlorobenzoate), a novel compound with potent and selective blocking actions at certain excitatory 5-hydroxytryptamine (5-HT) receptors on mammalian peripheral neurones, are described. On the rabbit isolated heart, MDL 72222 was a potent antagonist of responses mediated through the receptors for 5-HT present on the terminal sympathetic fibres. The threshold for antagonism was approximately 0.1 nM and the negative logarithm of the molar concentration of MDL 72222 which reduced the chronotropic response of the isolated rabbit heart to twice an ED50 of 5-HT to that of the ED50 was 9.27. MDL 72222 was also highly selective since responses to the nicotine receptor agonist, dimethylphenylpiperazinum iodine (DMPP), were inhibited only at concentrations more than 1000 times those necessary to inhibit 5-HT. In the anaesthetised rat, MDL 72222 produced marked blockade of the Bezold-Jarisch effect of 5-HT. Again, inhibition was selective since much higher doses of MDL 72222 failed to alter the response to electrical stimulation of the efferent vagus nerves. In contrast, MDL 72222 proved only a weak and essentially non-selective antagonist of responses mediated by the 5-HT M-receptor present on the cholinergic nerves of the guinea-pig ileum. MDL 72222 does not block smooth muscle contractile responses elicited by oxytocin or mediated through 5-HT D-receptors, muscarinic or nicotinic cholinoceptors or histamine H1-receptors except at relatively high concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Dual presynaptic effects of 5-hydroxytryptamine on peripheral noradrenergic synapses

The aim of the present experiments was to study the effects of 5-hydroxytryptamine (5-HT) on the responses to postganglionic stimulation of two models of the peripheral sympathetic nervous system: the isolated nictitating membrane of the cat and the guinea-pig isolated atria. In the nictitating membrane of the cat, 5-HT (0.1 microM) shifted to the left the frequency-response curve to nerve stimulation. This potentiating effect was prevented by 5-HT receptor antagonists (0.1 microM methysergide, 0.1 microM pizotifen and 0.1 microM morphine) and also by the beta-adrenoreceptor blocker propranolol (0.1 microM). The alpha 2-adrenoreceptor antagonist yohimbine (0.1 microM) had no effect on the 5-HT-induced potentiation. In the guinea-pig isolated atria the responses to cardioaccelerans nerve stimulation were diminished by 5-HT (0.1 to 1.0 microM). The shift to the right in the frequency-response curve induced by 5-HT (1.0 microM) was additive to the antagonism caused in the atria by propranolol (0.1 microM). The inhibitory effect of 5-HT on the pacemaker responses to nerve stimulation was prevented by the 5-HT receptor antagonists methysergide (1.0 microM) and pizotifen (1.0 microM) and also by the alpha-adrenoreceptor antagonist phentolamine (0.1 microM). The selective alpha 2-adrenoreceptor agonist clonidine (0.01 microM) reduced to the same extent as 5-HT (1.0 microM) the responses to the guinea-pig atria to nerve stimulation. The inhibitory effect of clonidine was prevented by the alpha-adrenoreceptor blocker phentolamine (0.1 microM) but not by the 5-HT receptor blocker pizotifen (1.0 microM). With the exception of propranolol, which in the atria shifted to the right the concentration-response curve to exogenous noradrenaline (NA), neither 5-HT nor the different antagonists employed modified the sensitivity to NA in the tissues studied. The present observations show that 5-HT can produce a dual effect on the sympathetic neurotransmission. It is proposed that a modification in the overflow of NA in response to nerve stimulation is caused by 5-HT and results from the interaction of 5-HT with specific receptors located on the sympathetic fibres. These presynaptic 5-HT receptors behave as excitatory (cat nictitating membrane) or inhibitory (guinea-pig atria) depending on the tissue studied.

Evidence for common pharmacological properties of [3H]5-hydroxytryptamine binding sites, presynaptic 5-hydroxytryptamine autoreceptors in CNS and inhibitory presynaptic 5-hydroxytryptamine receptors on sympathetic nerves

The affinities of 16 5-hydroxytryptamine (5-HT) receptor agonists (indole derivatives) and 7 5-HT receptor antagonists for [3H]5-hydroxytryptamine [( 3H]5-HT) binding sites in rat cerebral cortex membranes were determined. In addition, the potencies of the agonists for inhibiting the electrically induced tritium overflow from rat brain cortex slices preincubated with [3H]5-HT and from canine saphenous veins preincubated with [3H]noradrenaline were measured. Furthermore, the potencies of the indole derivatives for inducing contractile responses of canine saphenous veins were recorded. In addition, the interaction of the antagonists with unlabelled 5-HT at the 5-HT autoreceptor was studied in rat brain cortex slices. There was a good correlation between the binding affinities of the indole derivatives for the [3H]5-HT sites of rat brain cortex membranes and their potencies for inhibiting the evoked tritium overflow from both rat brain cortex slices and strips of canine saphenous vein. Comparison of the inhibition constants derived from the overflow experiments in both tissues again revealed a high correlation coefficient while there was only weak correlation between the binding affinities in rat brain cortex and the contractile potencies of the drugs in canine saphenous vein strips. When 5-HT receptor antagonists were investigated, metitepin and metergoline showed moderate affinities for the 5-HT autoreceptors in rat brain cortex slices, whereas quipazine had only weak affinity, and ketanserin, metoclopramide, cinanserin and cyproheptadine exhibited no antagonistic property. In binding experiments, the competition curves of most 5-HT receptor antagonists were biphasic, suggesting that the [3H]5-HT binding sites are heterogeneous.(ABSTRACT TRUNCATED AT 250 WORDS)

Autoreceptor-mediated inhibition of 3H-5-hydroxytryptamine release from rat brain cortex slices by analogues of 5-hydroxytryptamine

Rat brain cortex slices preincubated with 3H-5-hydroxytryptamine (3H-5-HT) were superfused with physiological salt solution containing paroxetine, an inhibitor of 5-hydroxytryptamine (5-HT) uptake. The effects of various indolethylamines on the electrically evoked tritium overflow (containing 66.3% unmetabolized 3H-5-HT) were investigated (the percentage of unmetabolized 3H-5-HT was not altered by the indolethylamines or metitepin). 6,7-Dihydroxytryptamine (6,7-DHT) did not affect the stimulation-evoked tritium overflow, whereas the latter was inhibited by the other tryptamine derivatives investigated; when the compounds were compared to each other on the basis of their inhibitory potencies the following rank order was obtained: unlabelled 5-HT greater than 5-methoxytryptamine greater than 4-HT greater than 6-HT greater than 5,6-DHT greater than tryptamine greater than 7-HT greater than 5,7-DHT. The inhibitory effects of these compounds were antagonized by metitepin. It is concluded that the indolethylamines inhibit the stimulation-evoked 3H-5-HT release by activating the presynaptic 5-HT autoreceptors on the 5-HT neurones of the rat brain cortex. Similarities may exist between these receptors and the postsynaptic 5-HT1 binding sites of this brain area.

Differences between receptors for 5-hydroxytryptamine on autonomic neurones revealed by nor-(-)-cocaine

1 The interaction between nor-(-)-cocaine and 5-hydroxytryptamine (5-HT) at autonomic neuronal 5-hydroxytryptamine (5-HT) receptors has been investigated on the rabbit heart and guinea-pig ileum. 2 The chronotropic response to 5-HT mediated through activation of receptors on the terminal sympathetic fibres of the rabbit heart was antagonized selectively and surmountably by nor-(-)-cocaine. Schild analysis yielded a pA2 value for the interaction of 7.79 +/- 0.11, n = 13. 3 On the guinea-pig ileum induced to contract by activation of 5-HT receptors on the intramural cholinergic nerves, nor-(-)-cocaine was also a selective and surmountable antagonist of 5-HT, although the concentrations needed were higher than those shown to be effective on the heart. The pA2 value for the interaction was 6.49 +/- 0.07, n = 16. 4 The differential blocking effect of nor-(-)-cocaine in the two tissues suggests that the receptors for 5-HT on the cholinergic nerves of the ileum and the sympathetic fibres of the heart are different. The data thus lend support to the earlier similar conclusion based on the observation that 5-methoxytryptamine is a potent agonist on the ileum yet inactive on the heart.

Inhibition of 5-hydroxytryptamine-evoked autonomic transmitter release by apomorphine

Apomorphine inhibited chronotropic responses of the isolated rabbit heart to 5-HT by 40% at 1.17 micrometers and by 90% at 4.68 micrometers and strongly inhibited the outflow of 3H following preloading of hearts with [3H]- (-)-noradrenaline. Apomorphine, 4.68 micrometers, had no significant effects on transmitter release evoked by DMPP or tyramine but inhibited the responses to SNS at frequencies up to 3.2 Hz. The inhibitory effects of apomorphine on 5-HT were resistant to blockade by chlorpromazine, 1.4 micrometers, haloperidol, 1.6 micrometers, spiroperidol, 2.5 micrometers, or yohimbine, 2.8 micrometers. In contrast, the inhibitory effects of apomorphine on low frequency SNS were abolished by yohimbine. On the guinea-pig ileum treated with methysergide, apomorphine, 1.17-4.68 micrometers, blocked the indirect cholinergic responses to 5-HT less markedly than it blocked the indirect sympathomimetic responses to 5-HT on the rabbit heart. Moreover, the effects were non-selective since responses to DMPP and transmural stimulation of the intramural cholinergic nerves were similarly reduced. Modification of 5-HT receptor function is the most likely explanation for the action of apomorphine with the differential effect on 5-HT in the heart and ileum reflecting differences in the receptors and/or post receptorial events at the two sites.

Trichlorethylene and halothane inhibit uptake of 5-hydroxytryptamine in the isolated perfused rat lung

Lung uptake of 5-hydroxytryptamine (5-HT) was determined in isolated perfused and ventilated rat lung, and was found to decrease with time according to a two-compartmental model. When the lungs were exposed to either trichlorethylene (TRI) or halothane, the uptake of 5-HT was drastically reduced. Both TRI and halothane gave log dose inhibition curves, which were superimposed, i.e. they were equally potent to inhibit lung uptake of 5-HT. At a concentration TRI of 18,000 ppm, the extraction of 5-HT was inhibited by 80 +/- 2 (X +/- S.E.M.) per cent, at 8500 ppm the inhibition was 65 +/- 6 per cent and 25 +/- 1 per cent at 3000 ppm. When the lungs were exposed to halothane, the inhibition was 85 +/- 6 per cent at 40,000 ppm, 48 +/- 1 per cent at 6000 ppm, and 15 +/- 0.3 per cent at 2000 ppm. When exposure to the solvent was discontinued, extraction of 5-HT was rapidly normalized. There was no detectable displacement of [3H]-5-HT from lungs saturated with the amine when they subsequently were exposed to solvent-containing atmosphere. This inhibition of lung uptake of 5-HT from the circulation is therefore postulated as to be an effect dependent on concentration solvent in the tissue, and is probably due to a reversible membrane stabilization of the endothelium.

Lack of modulation by presynaptic alpha 2-adrenoceptors of adrenergic transmitters release evoked by activation of 5-hydroxytryptamine and nicotine receptors

Clonidine (4.3 X 10(-6) M) and yohimbine (2.8 X 10(-6) M) have been used to stimulate and to block alpha 2-adrenoceptors in the isolated perfused rabbit heart. Transmitter release from the terminal sympathetic fibres as a result of stimulation of the nerves leaving the stellate ganglion (SNS; 0.32-10 Hz) and bolus injections of 5-hydroxytryptamine (5-HT; 2.8-182 nmol) or dimethylphenylpiperazinium (DMPP; 26-418 nmol) was estimated from changes in the chronotropic response of the heart under conditions of constant end organ sensitivity to injected noradrenaline (0.06-15.1 nmol). In accordance with the literature, clinidine inhibited responses to SNS and was more effective against low than against high frequencies of stimulation; similarly, yohimbine enhanced responses to SNS. In contrast, neither clonidine no yohimbine had any effect on the indirect sympathomimetic response to 5-HT. Similarly yohimbine did not alter responses to DMPP. Clonidine produced inconsistent effects on the response to DMPP; the response to 105 nmol was unchanged whereas the response to 209 nmol was reduced. The results suggest that transmitter release from the cardiac sympathetic nerves of the rabbit heart evoked by 5-HT or DMPP is not subject to control through activation of alpha 2-adrenoceptors by the released transmitter. They lend support to the suggestion of Stjärne that alpha 2-adrenoceptor inhibition of transmitter release arises primarily from the suppression of impulse transmission from varicosity to varicosity within the adrenergic ground plexus rather than interference with a Ca2+-dependent step in excitation-secretion coupling.