Receptor-mediated stimulation of aortic prostacyclin release by 5-hydroxytryptamine

5-Hydroxytryptamine (5-HT) stimulated prostacyclin release, measured by radioimmunoassay as 6-keto-prostaglandin F1 alpha, from rat aorta in a dose-dependent manner (3 X 10(-7)-10(-4) M). A statistically significant stimulation was observed at concentrations higher than 3 X 10(-6) M. Norepinephrine also increased prostacyclin release but only at a high concentration (10(-4) M) and histamine (3 X 10(-5) and 10(-4) M) had no significant effect. Among some structurally 5-HT-related analogues, only tryptamine exhibited a dose-dependent stimulatory effect on prostacyclin release but it was slightly less potent than 5-HT. Tryptophan and 5-hydroxytryptophan had no effects whereas 5-hydroxyindoleacetic acid at 10(-4) M stimulated slightly. Prostacyclin release stimulated by 5-HT was depressed by non-specific 5-HT antagonists, methysergide, mianserin and cyproheptadine. In contrast, a specific 5-HT2 antagonist ketanserin (3 X 10(-7)-10(-5) M) had no antagonistic effect. These results suggest that 5-HT stimulates the release of prostacyclin from rat aorta by interaction with receptors distinct from its 5-HT2 subtype.     

Evidence for a central 5-hydroxytryptamine receptor stimulation by lysergic acid diethylamide

1. Lysergic acid diethylamide (LSD) and the 5-hydroxytryptamine (5-HT) precursor, 5-hydroxytryptophan produced similar functional effects in rat spinal cord and brain to the 5-hydroxytryptamine precursor 5-hydroxytryptophan, which indicates that LSD stimulates central 5-HT receptors.2. By means of combined histochemical and biochemical techniques it was found that LSD reduced the turnover rate of brain and spinal cord 5-HT, studied after inhibition of the tryptophan hydroxylase by alpha-propyldopacetamide. The turnover of brain noradrenaline but not dopamine was somewhat accelerated.3. The functional and chemical effects by LSD were related to dose and to time. They were not observed after the LSD analogues 2-bromo-LSD and methylsergide.4. The retardation of the 5-HT turnover by LSD may be due to negative feed-back mechanisms evoked by direct stimulation of the central 5-HT receptors.     

Mechanisms of stimulation of rat cardiac muscle by 5-hydroxytryptamine

Administration of 5-hydroxytryptamine to isolated perfused working rat heart preparations caused an increase in contractile activity. The increase in activity due to low concentrations of 5-hydroxytryptamine (10^?5 M) could be blocked by either methysergide or propanolol but not by atenolol. At high concentrations of 5-hydroxytryptamine (10^?3 M) no increase in tissue cyclic AMP was found but a significant rise in the value of the cyclic AMP-dependent protein kinase activation ratio was observed, suggesting a slight increase in intracellular cyclic AMP had occurred. It was also observed that while low concentrations of 5-hydroxytryptamine (up to 10^?5 M) did not provoke catecholamine release from the intra-cardiac stores higher concentrations (10^?4 M) did. It is suggested that low concentrations of 5-hydroxytryptamine have a direct action on cardiac muscle but that at high concentrations a direct action cannot be separated from an action by which catecholamines are released from intra-cardiac stores.     

The putative 5-HT1 receptor agonist, RU 24969, inhibits the efflux of 5-hydroxytryptamine from rat frontal cortex slices by stimulation of the 5-HT autoreceptor

The putative central 5-HT receptor agonist, 5-methoxy-3(1,2,3,6-tetrahydropyridin-4-yl)-1H-indole succinate (RU 24969), was found to be a potent inhibitor of the continuous K+ evoked efflux of [3H]5-HT from superfused rat frontal cortex slices (pD2 7.45). The effects of RU 24969 were attenuated by the putative 5-HT autoreceptor antagonists, methiothepin, quipazine and (-)-propranolol but not by the alpha 2-adrenoceptor antagonist, idazoxan. It is concluded that RU 24969 inhibits K+ evoked efflux of [3H]5-HT from rat frontal cortex slices by stimulation of the 5-HT autoreceptor. Moreover, since RU 24969 potently displaced ligand binding to the 5-HT1 and 5-HT1B recognition sites but was only weakly active at the 5-HT2 receptor, the results lend support to the claim for a pharmacological resemblance between the 5-HT autoreceptor and the 5-HT1 recognition site and in particular the low affinity 5-HT1B subtype.     

Effect of ergot drugs on central 5-hydroxytryptamine neurons: Evidence for 5-hydroxytryptamine release or 5-hydroxytryptamine receptor stimulation

In combined biochemical and functional studies it has been possible to show that ergocornine (0.5-5 mg/kg) and the ergolene derivative (5R,8R)-8-(4-p-methoxyphenyl-1-piperazinylmethyl)-6-methylergolene (PTR 17402; MPME) (0.25-5 mg/kg) reduce in a dose-dependent way brain 5-hydroxytryptamine (5-HT) turnover in rat as evaluated with the tryptophan hydroxylase inhibitor, alpha-propyl-dopacetamide (H 22/54), whereas 2-Br-alpha-ergocryptine (CB 154; Br-EC) had no effect on brain 5-HT turnover. Effects on 5-HT receptor activity were evaluated using the extensor hindlimb reflex of acutely spinalized rats. It was found that ergocornine increased the 5-HT receptor activity independent of presynaptic 5-HT stores and that it didnot have any effects on uptake, retention and spontaneous overflow of 3-H-5-HT in vitro but reduced the fiedl stimulation-induced release of 3-H-5-HT in vitro. Therefore, it is suggested that ergocornine is a 5-HT recpetor-stimulating agent, an effect which may lead to reduction of nervous impulse flow in the 5-HT neurons and subsequently of 5-HT release and turnover. MPME, on the other hand, seems to increase 5-HT receptor release of 5-HT stores, mainly from extragranular sites. Thus, the increase in extensor reflex activity found after MPME was reduced by reserpine and H 22/54 and enhanced by nialamide and in vitro MPME markedly increased 3-H-5-HT overflow in cortical slices of nialamide-pretreated rats and inhibited uptake and retention of 3-H-5-HT (EC50 equals 1.6 times 10-minus 6 M) in cortical slices of normal rats. Inhibition of the 5-HT membrane pump does not seem to be of any major importance, since chlorimipramine was only weakly active on the extensor reflex in the pharmacological models used and since MPME did not block but rather enhanced the 5-HT depletion caused by 4-methyl-alpha-ethyl-m-tyramine. It is suggested that MPME is a releaser of extragranular 5-HT stores leading to increased 5-HT receptor activity and reduction of 5-HT turnover in the same way as indicated for ergocornine. This new ergolene derivative may represent a new class of antidepressant drugs acting via release of extragranular 5-HT stores.     

The interaction of some kynurenine pathway metabolites with 5-hydroxytryptophan and 5-hydroxytryptamine

The kynrenine pathway metabolites kynurenine, 3-hydroxykynurenine and xanthurenic acid have been tested against 5-hydroxytryptamine (5-HT) and 5-hydroxytryptophan (5-HTP)-induced head twitches in the mouse in a dose-range of 0.5–5.0 mg/kg. Kynurenine and 3-hydroxykynurenine were highly active. Low doses caused marked potentiation of the twitch response to both 5-HT and 5-HTP with increased toxicity of 5-HT. High doses caused antagonism of both responses. Xanthurenic acid was inactive over the same dose range. The effects of kynurenine could not be duplicated in the guinea-pig ileum. The relevance of these results to the involvement of kynurenine pathway metabolites in depressive illness is discussed.     

Fenfluramine antagonizes the stimulation of food intake induced by the putative 5-hydroxytryptamine1A agonist, isapirone, in non-fasted rats

Isapirone, a 5-hydroxytryptamine1A receptor agonist, stimulated food intake in non-fasted rats in a dose-dependent manner (1, 3 and 10 mg kg-1 s.c.). Fenfluramine, an antiobesity agent and a 5-HT releaser, at 3 and 10 mg kg-1 s.c. antagonized the isapirone-induced (3 mg kg-1 s.c.) feeding. These results are consistent with the known inhibitory role of 5-HT in the control of food intake in rats.     

Inhibition of reflex responses of neonate rat lumbar spinal cord by 5-hydroxytryptamine.

1. Monosynaptic (MSR) and polysynaptic (PSR) segmental reflex responses were recorded from a ventral root of the neonate rat hemisected spinal cord. Amplitudes of the two components were monitored with a peak height detector. 2. 5-Hydroxytryptamine (5-HT) depressed the MSR and PSR in a concentration-dependent manner. The IC50 for MSR depression was 9.5 +/- 3.2 microM and for PSR depression was 9.0 +/- 4.8 microM. 3. Blockade of neuronal uptake of 5-HT by citalopram (0.1 microM) greatly increased sensitivity to 5-HT. In the presence of citalopram, the IC50 for MSR depression was 30 +/- 18 nM and for PSR depression was 89 +/- 23 nM. 4. 5-HT did not depress the MSR or the PSR by releasing glycine since strychnine (1 microM) did not prevent these actions of 5-HT. 5. 5-Carboxamidotryptamine (5-CT), 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), RU 24969, 1-[3-(trifluoromethyl)phenyl]-piperazine (TFMPP) and methysergide were full agonists for depression of the MSR. The IC50 for 5-CT was 3.6 +/- 0.5 nM, for 8-OH-DPAT was 0.4 +/- 0.04 microM, for TFMPP was 0.93 +/- 0.3 microM and for methysergide was 21.8 +/- 3.0 nM. The order of potency was 5-CT greater than methysergide greater than 5-HT greater than 8-OH-DPAT greater than TFMPP. 6. 8-OH-DPAT, RU 24969, TFMPP and methysergide had either no or only a minor action in reducing the PSR. 5-CT caused a 50% depression at the highest concentration tested (30 nM).(ABSTRACT TRUNCATED AT 250 WORDS)     

Barium enhances the excitability of a motoneurone from the cockroach (Periplaneta americana)

• 1.1. An isolated ganglion preparation was used to investigate the effects of barium ions (Ba²⁺) on excitability of the soma of the ‘fast’ coxal depressor motoneurone (D_f) from the cockroach Periplaneta americana.
• 2.2. Under current-clamp, short (50 ms) depolarising pulses applied to the soma of D_f elicited damped membrane oscillations in normal external solution. In the presence of 10 mM Ba²⁺, similar pulses produced all-or-none action potentials (n = 12).
• 3.3. Under voltage-clamp, addition of Ba²⁺ to the external solution suppressed the Ca-dependent K⁺ conductance (I_c) in D_f(n = 4).
• 4.4. Modulation of I_c may offer a means of altering the excitability, and therefore output, of motoneurone D_f.

     

The inactivation of 5-hydroxytryptamine by blood platelets in mental deficiency with elevated serum 5-hydroxytryptamine

Summary The mentally defective patients were divided into two groups according to the 5-hydroxytryptamine (5HT) content in serum. In the first (normal) group were those with values <200 ng/ml (6 mongolisms, 4 cerebral palsies) and in the second those with values >200 ng/ml (7 cerebral palsies, 5 encephalopathies).The content of 5HT per platelet was twice as high in the second than in the first group. When the platelet-rich plasma was incubated with tetrabenazine in vitro equal amounts of 5HT were liberated from platelets in both groups. During the liberation slightly more of the 5 HT (in percent from released) was inactivated in the first group than in the second. The ability of platelets to inactivate 5 HT under these experimental conditions does not explain the higher serum and platelet values found in some patients.     

Efflux of 3H-5-hydroxytryptamine from rat hypothalamic slices by continuous electrical stimulation: Frequency-dependent responses to serotonergic antagonists and 5-hydroxytryptamine

Rat hypothalamic slices were incubated with 3H-5-hydroxytryptamine and superfused in the presence of paroxetine to inhibit 5-hydroxytryptamine (5-HT) reuptake. The slices were continuously stimulated electrically with rectangular pulses at varying frequencies. Continuous stimulation for up to 42 min at 1 Hz or at 3 Hz evoked a steady efflux of tritium that slowly decayed with time. The efflux produced by continuous stimulation at 5 Hz declined more rapidly with time. Continuous stimulation at 1 Hz in the presence of increasing concentrations of unlabelled 5-HT produced a concentration-dependent decrease in tritium efflux. The presence of methiothepin (0.5 mumol/l), quipazine (10 mumol/l) and (-)- but not (+)-propranolol (1 mumol/l) attenuated this response to 5-HT. From these data, the apparent pA2 values were calculated and found to be in agreement with published values. Frequency-dependent responses were determined using a "cumulative stimulation" protocol whereby the slices were subjected to three consecutive 14 min periods of stimulation at increasing frequencies (1, 3 and then 5 Hz). Unlabelled 5-HT (1 mumol/l) inhibited electrically-evoked tritium efflux more at 1 than at 5 Hz. Methiothepin (0.5 mumol/l) and quipazine (10 mumol/l) enhanced the stimulated efflux in a manner inversely related to the frequency of stimulation. Neither (+)- nor (-)-propranolol enhanced stimulated tritium efflux at any of the three frequencies tested. It is concluded that continuous electrical stimulation of rat hypothalamic slices at a low frequency provides a rapid means of obtaining apparent affinities and intrinsic activities of drugs that modify the serotonergic autoreceptor.(ABSTRACT TRUNCATED AT 250 WORDS)     

Behavioural profiles of putative 5-hydroxytryptamine receptor agonists and antagonists in developing rats

The effects of a variety of 5-hydroxytryptamine (5-HT) receptor agonists and antagonists on behaviour in 5- and 20-day old rat pups have been investigated. Increased locomotion and head-weaving responses were induced in both age groups by 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin; 5-HT1A agonist); 5-MeODMT (5-methoxy-N,N-dimethyltryptamine; 5-HT1) and RU 24969 (5-methoxy-3(1,2,3,6-tetrahydropyrindin-4-yl)-1H-indole; 5-HT1B/5-5HT1A). The putative 5-HT1A-agonist LY165163 (1-2-(4-aminophenyl)ethyl 4-(3-trifluoromethylphenyl)piperazine) also produced hyperactivity in the developing pups. In contrast, locomotion was not affected by buspirone (5-HT1A); mCPP (1-(3-chlorophenyl)piperazine; 5-HT1B/5-HT1C) and DOI (1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane; 5-HT2) though buspirone produced a small increase in head-weaving at 5- and 20-days. The full 5-HT syndrome was induced in older animals (but not neonates) by both 8-OH-DPAT and 5-MeODMT. Large doses of buspirone, mCPP and DOI also produced signs of reciprocal forepaw treading and flattened body posture at 20-days. In addition, mCPP induced grooming and stereotyped mouthing, while DOI increased sniffing behaviour in the young rats. Catecholaminergic mechanisms were implicated in the head-weaving and locomotor responses to 8-OH-DPAT and RU 24969, following experiments with a number of monoamine receptor antagonists. Preliminary findings with (-)-pindolol, which was high affinity for 5-HT1-receptors, suggested that this subtype of receptor may play a role in hyperlocomotion induced by RU 24969.(ABSTRACT TRUNCATED AT 250 WORDS)     

Uptake of 5-hydroxytryptamine by platelets

Human blood platelets were incubated with 5-hydroxytryptamine and with tryptamine and the uptake of each amine measured. The uptake of tryptamine, unlike that of 5-hydroxytryptamine, was linearly related to the concentration of the amine in the surrounding fluid, was similar in amount at 0° and 37° C and varied directly with the pH of the solution. When both amines were present together the uptake of 5-hydroxytryptamine was depressed. The antagonism of tryptamine was found to be competitive, and the possible site of its action is discussed. The effect of a number of other substances on the uptake of 5-hydroxytryptamine by platelets was examined; of these imipramine, cocaine and chlorpromazine were more potent and dihydroergotamine and lysergic acid diethylamide somewhat less potent than tryptamine in inhibiting uptake.     

Repeated chlorpromazine administration increases a behavioural response of rats to 5-hydroxytryptamine receptor stimulation.

1 The hyperactivity syndrome produced in rats by administration of tranylcypromine (20 mg/kg i.p.) followed 30 min later by L-tryptophan (50 mg/kg i.p.) is generally considered to be due to increased 5-hydroxytryptamine (5-HT) functional activity. It is inhibited by chlorpromazine (30 mg/kg i.p.) injected 60 min before the tranylcypromine. However, chlorpromazine injection for 4 days either at a dose of 30 mg/kg once daily or 5 mg/kg twice daily results in an enhanced hyperactivity response to tranylcypromine and L-tryptophan administration 24 h after the final dose of chlorpromazine. 2 One injection of chlorpromazine (30 mg/kg) did not produce enhancement 24 h later and the inhibition of the tranylcypromine/L-tryptophan hyperactivity observed after acute chlorpromazine injection was seen if the rats were given tranylcypromine and L-tryptophan 1 h after the fourth chlorpromazine (30 mg/kg) dose. 3 Chlorpromazine (30 mg/kg) once daily or 5 mg/kg twice daily for 4 days resulted in rats displaying enhanced behavioral responses to the suggested 5-HT agonist 5-methoxy N,N-dimethyltryptamine (2 mg/kg) on day 5. 4 Chlorpromazine (30 mg/kg) once daily for 4 days produces a slight increase in brain 5-hydroxytryptamine (5-HT) concentration on day 5, but no difference in the rate of brain 5-HT synthesis or the rate of 5-HT accumulation after tranylcypromine and L-tryptophan administration. 5. There is some evidence that chlorpromazine blocks 5-HT receptors. It has also been observed that several other neuroleptic drugs do not produce enhanced 5-HT responses after repeated administration. It is suggested therefore that the enhanced behavioural response to 5-HT receptor stimulation following repeated chlorpromazine administration may be because this drug blocks 5-HT receptors.     

Involvement of 5-hydroxytryptamine7 receptors in inhibition of porcine myometrial contractility by 5-hydroxytryptamine

1. 5-Hydroxytryptamine (5-HT; 1 nM–100 μM) concentration-dependently inhibited the amplitude and frequency of spontaneous contractions in longitudinal and circular muscles of the porcine myometrium. The circular muscle (EC₅₀; 68–84 nM) was more sensitive than the longitudinal muscle (EC₅₀; 1.3–1.44 μM) to 5-HT. To characterize the 5-HT receptor subtype responsible for inhibition of myometrial contractility, the effects of 5-HT receptor agonists on spontaneous contractions and of 5-HT receptor antagonists on inhibition by 5-HT were examined in circular muscle preparations.
2. Pretreatment with tetrodotoxin (1 μM), propranolol (1 μM), atropine (1 μM), guanethidine (10 μM) or L-NAME (100 μM) failed to change the inhibition by 5-HT, indicating that the inhibition was due to a direct action of 5-HT on the smooth muscle cells.
3. 5-CT, 5-MeOT and 8-OH-DPAT mimicked the inhibitory response of 5-HT, and the rank order of the potency was 5-CT>5-HT>5-MeOT>8-OH-DPAT. On the other hand, oxymethazoline, α-methyl-5-HT, 2-methyl-5-HT, cisapride, BIMU-1, BIMU-8, ergotamine and dihydroergotamine had almost no effect on spontaneous contractions, even at 10–100 μM.
4. Inhibition by 5-HT was not decreased by either pindolol (1 μM), ketanserin (1 μM), tropisetron (10 μM), MDL72222 (1 μM) or {"type":"entrez-nucleotide","attrs":{"text":"GR113808","term_id":"238362519","term_text":"GR113808"}}GR113808 (10 μM), but was antagonized by the following compounds in a competitive manner (with pA₂ values in parentheses): methiothepin (8.05), methysergide (7.92), metergoline (7.4), mianserin (7.08), clozapine (7.06) and spiperone (6.86).
5. Ro 20-1724 (20 μM) and rolipram (10 μM) significantly enhanced the inhibitory response of 5-HT, but neither zaprinast (10 μM) nor dipyridamole (10 μM) altered the response of 5-HT.
6. 5-HT (1 nM–1 μM) caused a concentration-dependent accumulation of intracellular cyclic AMP in the circular muscle.
7. From the present results, the 5-HT receptor, which is functionally correlated with the 5-HT₇ receptor, mediates the inhibitory effect of 5-HT on porcine myometrial contractility. This inhibitory response is probably due to an increase in intracellular cyclic AMP through the activation of adenylate cyclase that is positively coupled to 5-HT₇ receptors.

     

Analgesia induced by brief footshock is inhibited by 5-hydroxytryptamine but unaffected by antagonists of 5-hydroxytryptamine or by naloxone

Exposure to footshock (1 mA) for 30 sec induced a marked analgesia that was enhanced by pretreatment with the 5HT synthesis inhibitor, p-chlorophenylalanine, and attenuated by the 5HT releasing drugs p-chloroamphetamine and fenfluramine, by the 5HT re-uptake inhibitor, fluoxetine and by the 5HT agonists, 5-methoxy-N,N-dimethyltryptamine and MK212. However, agonists, quipazine and trifluoromethylphenylpiperazine, with greated reported affinities for 5HT binding sites on rat brain membranes than MK212 were without effect as were the antagonists metergoline, methysergide, cyproheptadine, mianserine and methiothepin. The specific opioid antagonist naloxone was also without effect. The results in general indicate that analgesia induced by brief footshock (1 mA, 30 sec) is inversely related to 5HT availability but thereis little evidence of involvement of known 5HT receptors.