Actions of some analogues of 5-hydroxytryptamine on the isolated rat uterus and the rat fundus strip preparations

alpha-Ethyltryptamine (alpha-ET) as a discriminative stimulus in rats

alpha-Ethyltryptamine (etryptamine, alpha-ET) is a drug of abuse that first appeared on the clandestine market in the mid-1980s. Its pharmacological actions are poorly understood. In this investigation, it is reported for the first time that alpha-ET serves as a training drug in drug discrimination studies. Male Sprague-Dawley rats were trained to discriminate (30-min pretreatment time) 2.5 mg/kg of alpha-ET (ED(50)=1.3 mg/kg) from saline vehicle using a standard two-lever operant paradigm and a VI-15s schedule of reinforcement for appetitive reward. Once established, the alpha-ET stimulus was shown to have an onset to action of 30 min and a duration of effect of at least 4 h. In tests of stimulus generalization (substitution), the alpha-ET stimulus generalized to S(-)alpha-ET (ED(50)=1.6 mg/kg) and R(+)alpha-ET (ED(50)=1.3 mg/kg). Tests of stimulus generalization were also conducted with prototypical phenylisopropylamines: (+)amphetamine, 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM), and N-methyl-1-(4-methoxyphenyl)-2-aminopropane (PMMA). The alpha-ET stimulus generalized to DOM (ED(50)=0.4 mg/kg) and PMMA (ED(50)=0.7 mg/kg), but only partially generalized (ca. 40% maximal drug-appropriate responding) to (+)amphetamine. The results suggest that alpha-ET produces a complex stimulus.

Effects on amine oxidase of substances which antagonize 5-hydroxytryptamine more than tryptamine on the rat fundus strip

Certain substances, 2-bromolysergic acid diethylamide, dimethyltryptamine (3-(2-dimethylaminoethyl)indole), 2-methyldimethyltryptamine (3-(2-dimethylaminoethyl)-2-methylindole), and 5-benzyloxydimethyltryptamine (5-benzyloxy-3-(2-dimethylaminoethyl)indole), antagonize the effects of 5-hydroxytryptamine on the rat fundus strip more than those of tryptamine. These substances have been tested for their ability to inhibit the oxidation of tryptamine and 5-hydroxytryptamine by suspensions of guinea-pig liver and rat fundus. 2-Bromolysergic acid diethylamide has virtually no inhibitory activity and it is doubtful if the others produce any significant inhibition of amine oxidase in the concentrations which antagonize the effects of 5-hydroxytryptamine more than those of tryptamine. It seems that the differential character of the blocking action of these compounds should be ascribed either to interference with the transport of tryptamine (but not 5-hydroxytryptamine) through the cell wall, coupled with the block of a receptor common to both tryptamine and 5-hydroxytryptamine, or to the existence of separate tryptamine and 5-hydroxytryptamine receptors.The amine oxidases of the guinea-pig liver and rat fundus appear to be a mixture of at least two types of enzyme, one of which has a higher affinity for 5-hydroxytryptamine than the other and is more susceptible to inhibition by 2-methyldimethyltryptamine.

Pharmacological studies upon neurones of the lateral geniculate nucleus of the cat

Indoles related to 5-hydroxytryptamine, lysergic acid derivatives, phenethylamine derivatives and some other compounds have been applied electrophoretically to the neurones of the lateral geniculate nucleus of the cat anaesthetized with pentobarbitone sodium. Many of these compounds, particularly 4-, 5- and 7-hydroxytryptamine and ergometrine, depress the orthodromic excitation of the neurones by volleys in optic nerve fibres, but do not affect antidromic excitation by volleys in the optic radiation or chemical excitation by L-glutamic acid. It is concluded that the active depressants either block the access of the excitatory transmitter to subsynaptic receptors or prevent the release of the transmitter from optic nerve terminals. The structure-activity relationships of the depressant substances are discussed.

Further characterization of 5-hydroxytryptamine receptors (putative 5-HT2B) in rat stomach fundus longitudinal muscle

1. The present study was undertaken to isolate and characterize pharmacologically homogeneous populations of 5-hydroxytryptamine (5-HT) receptors from a possible mixed receptor population mediating concentration of the longitudinal muscle of rat stomach fundus. Our aim was to extend the pharmacological characterization of the 5-HT2B receptor which is reported to be expressed in this preparation. 2. To minimize spontaneous activity and any influence of circular muscle on the contractile response, narrow (1-1.5 x 20 mm) segments of mucosa-denuded longitudinal muscle were used. Under these conditions, blockade of monoamine oxidase with pargyline (100 microM for 15 min) caused a leftward displacement of concentration-effect curves for both 5-methoxytryptamine (5-MeO-T) and tryptamine. Neither pargyline nor a number of uptake inhibitors affected responses to 5-HT. 3. In pargyline pretreated preparations, the order of potency of a number of tryptamine analogues was as follows: 5-MeO-T > or = alpha-Me-5-HT > or = 5-HT > 5-carboxamidotryptamine (5-CT) > tryptamine > 2-Me-5-HT. In addition several ligands known to act as agonists at either 5-HT2A or 5-HT2C receptors including 1-m-chlorophenylpiperazine (m-CPP), Ru 24969, MK 212 and SCH 23390 were also agonists in rat fundus whilst sumatriptan, renzapride and 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) were very weak or inactive. With the exception of 2-Me-5-HT and m-CPP, most agonists produced monophasic concentration-effect curves consistent with an interaction at a single site. High concentrations of 2-Me-5-HT evoked relaxations which were blocked by phentolamine (1 MicroM) suggesting an interaction with alpha-adrenoceptors. m-CPP often evoked biphasic concentration-effect curves with a second contractile phase which was insensitive to yohimbine at concentrations higher than required for antagonism of responses to 5-HT.4. LY 53857, methiothepin, methysergide, ritanserin and ICI 170809 were potent but non-surmountable antagonists of 5-HT in rat fundus. In contrast, several ligands behaved as surmountable antagonists with the following order of potency: rauwolscine >yohimbine = mesulergine > mianserin = SB 204070 >WY 26703 > SB 200646> pirenpirone> renzapride. DAU 6285, granisetron, spiperone, ketanserin,phentolamine and GR 127935 did not affect responses to 5-HT at concentrations up to 1 pM. The agonist and concentration independent profile of antagonism supported a single site interaction for both agonists and antagonists.5. We conclude that despite small differences concerning the enantiomeric selectivity and affinity of rauwolscine and yohimbine, the close pharmacological identity of 5-HT receptors in rat stomach fundus and the recently cloned 5-HT2B receptor is maintained. SB 200646, which demonstrates some selectivity for 5-HT receptors in rat stomach fundus, should provide a useful ligand for confirmation of this view and allow discrimination of 5-HT2B function both in vitro and in vivo.

Pharmacological characterization of the novel nonpeptide angiotensin II receptor antagonist, BIBR 277

1. The pharmacological profile of BIBR 277, 4'-[(1,4'-dimethyl-2'-propyl[2,6'-bi-1H-benzimidazol]-1'-yl)methyl ]- [1,1'-biphenyl]-2-carboxylic acid, a novel, nonpeptide angiotensin II receptor antagonist has been investigated by use of receptor binding studies, enzymatic assays, functional in vitro assays in rabbit aorta as well as in vivo experiments in pithed, anaesthetized and conscious rats. 2. BIBR 277 potently interacted with rat AT1 receptors (Ki 3.7 nM). Competitive receptor interaction was shown by radioligand saturation experiments performed in the presence of BIBR 277. The failure to inhibit radioligand binding to AT2 sites demonstrates the selectivity of BIBR 277 for AT1 receptors. This is further substantiated by the findings that BIBR 277 neither interacted with other receptor systems investigated nor affected the activity of components of the human renin-angiotensin system, such as plasma renin or serum converting enzyme. 3. In rabbit aorta, BIBR 277 had no agonistic properties and was shown to be an insurmountable antagonist of angiotensin II-induced contractions (KB 0.33 nM). The antagonistic effect persisted even after several wash-out procedures. However, this interaction was not irreversible since the insurmountable antagonism was concentration-dependently reversed when BIBR 277 (0.1 microM) and the surmountable antagonist, losartan (0.1 and 1.0 microM) were incubated simultaneously. The specificity of BIBR 277 for the AT1 receptor was further substantiated in this preparation since micromolar concentrations of BIBR 277 neither affected potassium chloride and noradrenaline-induced contractions nor acetylcholine-mediated tissue relaxation. 4. In pithed rats, i.v. administration of BIBR 277 (0.1, 0.3 and 1.0 mg kg-1) shifted the dose-pressor response curve to angiotensin II dose-dependently to the right with ED50 values of 0.23 microg kg-1 (control)and 1.4 microg kg-1, 4.7 microg kg-1 and 20 microg kg-1, respectively. As observed in the in vitro experiments no agonistic effect was detected and the maximum of the blood pressure response to angiotensin II at the highest dose of BIBR 277 was decreased by 29%.5. In anaesthetized rats, bolus i.v. administration of 0.1, 0.3 and 1.0 mg kg-1 BIBR 277 attenuated the blood pressure response to bolus i.v. injections of angiotensin 11 (0.1 microg kg-1). At the highest dose an almost complete blockade was observed even after 2 h.6. Single oral administration of BIBR 277 (0.3 and 1.0 mg kg-1) to conscious, chronically instrumented renovascular hypertensive rats dose-dependently decreased the mean arterial blood pressure by 15 and 30 mmHg, respectively. At the higher dose a significant antihypertensive effect was maintained for more than 24 h. Moreover, consecutive daily dosing of 1 mg kg-1 orally resulted in a sustained reduction in blood pressure over the 4 day observation period.7. It is concluded that BIBR 277 is an effective and selective angiotensin II antagonist with antihypertensive activity after oral administration.

Dual mechanism of the stimulant action of N,N-dimethyl-5-hydroxy-tryptamine (bufotenine) on cardiac sympathetic nerves

The indirect sympathomimetic effects of N,N-dimethyl-5-hydroxytryptamine (bufotenine) have been analysed on the rabbit heart perfused in vitro by the Langendorff technique. Comparisons have been made with the effects of 5-hydroxytryptamine (5-HT), which activates tryptamine receptors, and dimethylphenylpiperazinium (DMPP), which stimulates nicotine receptors. Bufotenine, 5-HT and DMPP stimulated the rate and force of cardiac contraction but whereas all were powerful stimulants of cardiac rate, bufotenine and DMPP were much stronger stimulants of atrial and particularly ventricular tension than 5-HT. Responses to 5-HT were markedly reduced by perfusion of hearts with an excess of 5-HT, and those to DMPP, during perfusion with hexamethonium. A combination of 5-HT with hexamethonium was necessary to abolish the effects of bufotenine. The data suggest a dual mechanism of stimulant action of bufotenine on the cardiac sympathetic nerves of the rabbit heart involving activation of receptors sensitive to 5-HT and nicotine receptors.

The action of 5-hydroxthryptamine and related compounds on the activity of Retzius cells of the leech Hirudo medicinalis

1 The equipotent molar ratios of a range of tryptamine analogues, as compared with 5-hydroxytryptamine (5-HT), have been determined on the basis of their ability to hyperpolarize the membrane potential of the Retzius cell of the leech, Hirudo medicinalis.2 The substitution of methyl, fluoro, chloro, methoxy or acetyl groups onto the 5-HT molecule progressively reduced the potency.3 5-Methoxylation or terminal N-methylation of tryptamine considerably increased the potency of tryptamine but these compounds tended to depolarize cells rather than cause hyperpolarization. In some experiments they were ineffective on preparations pretreated with 5-HT.4 It is suggested that these compounds may act by a different mechanism from the 5-hydroxylated indoles, perhaps involving a different receptor.