Expression of serotonin receptor mRNAs in blood vessels.

Using RT-PCR we distinguished mRNAs for all known G-protein coupled serotonin receptors expressed in various rat and porcine blood vessels. Nearly all vessels expressed 5HT1D beta, 5-HT2A, 5-HT2B, 5-HT4, and 5-HT7 receptor mRNA to different extents. New splice variants of the porcine 5-HT4 receptor were observed. Similar PCR assays were performed with endothelial and smooth muscle cells from human pulmonary artery, aorta, and with endothelial cells from human coronary artery and umbilical vein. All endothelial cells expressed 5-HT1D beta, 5-HT2B, and 5-HT4 receptor mRNA, whereas in smooth muscle cells 5-HT1D beta, 5-HT2A, 5-HT7, and in some experiments 5-HT2B receptor mRNA were found. A model for the regulation of vascular tone by different 5-HT receptors is proposed.

Extended radioligand binding profile of iloperidone: a broad spectrum dopamine/serotonin/norepinephrine receptor antagonist for the management of psychotic disorders.

Iloperidone is a novel psychotropic compound currently undergoing Phase III trials. Its affinity for human dopamine and 5-HT(2A) and 5-HT(2C) receptors has been reported previously. This report presents the affinity of iloperidone for a largely extended number of human neurotransmitter receptors. In a few instances human receptors were not available and receptor studies were performed on tissues from laboratory animals. The present data, supplemented with those of, indicate that iloperidone displays high affinity (K(I) < 10 nM) for norepinephrine alpha(1)-adrenoceptors, dopamine D(3) and serotonin 5-HT(2A) receptors. Intermediate affinity (10-100 nM) was found for norepinephrine alpha(2C)-adrenoceptors, dopamine D(2A) and D(4) receptors and serotonin 5-HT(1A), 5-HT(1B), 5-HT(2C) and 5-HT(6) receptors. The affinity for all other receptors was below 100 nM, including norepinephrine alpha(2A), alpha(2B), beta(1), and beta(2), muscarine M(1)-M(5), histamine H(1), dopamine D(1) and D(5), CCK(A) and CCK(B), 5-HT(7), dopamine and norepinephrine transporters. Thus, iloperidone targets a selective set of dopamine, norepinephrine and serotonin receptor subtypes. The affinity for this particular set of receptors indicates that iloperidone has the potential to be a broad spectrum antipsychotic, with efficacy against positive, negative, depressive and cognitive symptoms of schizophrenia, and a low propensity to induce side effects.

(+)-cis-4,5,7a,8,9,10,11,11a-octahydro-7H-10-methylindolo[1,7- bc][2,6]-naphthyridine: a 5-HT2C/2B receptor antagonist with low 5-HT2A receptor affinity.

The indolonaphthyridine 8 is described as a selective 5-HT2C/2B vs 5-HT2A receptor antagonist. The compound was synthesized in seven steps starting from indoline and isonicotinic acid chloride. The key step is a photocyclization of the indolinyl tetrahydropyridinocarbamic acid ethyl ester 4 to the cis-octahydroindolo[1, 7-bc][2,6]naphthyridinecarbamic acid ethyl ester 5. The synthesis was accomplished by reduction with aluminum hydride and racemic resolution. The indolonaphthyridine 8 exerted the binding profile of a selective 5-HT2C receptor ligand (pKD 7.8) and behaved as an antagonist on the 5-HT-induced accumulation of inositol phosphates in pig choroid plexus cells (pKB 7.13). Compound 8 dose-dependently inhibited the ACTH response to MK-212 in rats and the MK-212-induced hypophagic effect with an ID50 value of 0.3 mg/kg sc. Compound 8 acted as a 5-HT2B receptor antagonist at the rat stomach fundus with a pKB value of 7.34.

Centrally acting alpha 1-adrenoceptor agonists based on hexahydronaphth[2,3-b]-1,4-oxazines and octahydrobenzo[g]quinolines.

Centrally acting alpha 1-agonists may be of therapeutic value in dementias and other CNS disorders characterized by symptoms of noradrenergic insufficiency. Therefore, on the basis of known peripherally acting alpha 1-agonists two new groups of centrally acting alpha 1-agonists with improved lipophilicity, the hexahydronaphth[2,3-b]-1,4-oxazines type A and the octahydrobenzo[g]quinolines type B were designed. The N-methylated derivatives 14 and 33 demonstrate potent, direct agonistic activity at postjunctional alpha 1-receptors. Ring substituent alterations in compounds of type A and B change the potency of compounds on the rabbit ear artery by over 3 orders of magnitude (pD2 = 5.35-8.40). The efficacy of these compounds varies from 42 to 110%. Those alpha 1-agonists which were selective in the pithed rat increase vigilance in rats. Compound 14 was found to be a centrally acting alpha 1-agonist with good tolerability in different animal species and in healthy volunteers. Furthermore, 14 selectively stimulates the breakdown of phosphatidylinositol in rat cerebral cortex slices. In vivo, the compound reverses behavioral deficits in animals which received noradrenergic lesions following DDC or DPS4 treatment. Oxazine 14 and its close derivatives are by far more lipophilic than commonly known alpha 1-agonists. This is demonstrated in a ClogP-PROBIS plot.

Zacopride and BRL 24924 induce an increase in EEG-energy in rats.

1. The substituted benzamides, zacopride and BRL 24924 induced dose-dependent increases of the total EEG-energy of rats when applied intracerebroventricularly (i.c.v.) with ED50 values of 8.0 +/- 0.6 and 3.6 +/- 0.9 micrograms, respectively. Not only the energy of the low frequency hippocampal theta rhythm but also that of the other frequency bands was increased. 2. In contrast to i.c.v. application intraperitoneal administration of zacopride or BRL 24924 (1 and 10 mg kg-1) did not lead to an increase in EEG-energy. 3. The increase in EEG-energy induced by zacopride (10 micrograms, i.c.v.) was blocked by ICS 205-930 (1 microgram, i.c.v.). Neither the 5-HT3 receptor agonist 2-methyl-5-hydroxytryptamine (30 micrograms, i.c.v.) nor the selective 5-HT3 receptor antagonist MDL 72222 (30 micrograms, i.c.v.) had any effect upon rat EEG. 4. Scopolamine (0.01 micrograms and 0.1 micrograms, i.c.v.) dose-dependently antagonized the effect of zacopride (10 micrograms, i.c.v.). 5. An agonist action of zacopride and BRL 24924 and inhibition of these effects by ICS 205-930 but not by MDL 72222 was recently described in isolated colliculi neurones from neonatal mice. The receptor involved was described as '5-HT4'. The present results indicate that the central effects of zacopride and BRL 24924 may be due to activation of such a 5-HT receptor.

The role of alpha2-adrenoceptor antagonism in the anti-cataleptic properties of the atypical neuroleptic agent, clozapine, in the rat.

1. The mechanism underlying the anticataleptic properties of the atypical neuroleptic agent, clozapine, has been investigated in the rat. 2.The close structural analogues of clozapine, loxapine (0.1 mg kg(-1) s.c.) and iso-clozapine (1 and 3 mg kg(-1) s.c.) induced catalepsy in rats. In contrast, clozapine and the regio-isomer of loxapine, iso-loxapine (up to 10 mg kg(-1) s.c.) did not produce catalepsy, but at a dose of 1 mg kg(-1) significantly inhibited catalepsy induced by loxapine (0.3 mg kg(-1) s.c.). 3. Radioligand binding assays showed that cataleptogenic potential was most clearly predicted by the D2/5-HT1A, D2/5-HT1B/1D and D2/alpha2-receptor affinity (KD) ratios: i.e. 30-100-fold higher ratios were calculated for loxapine and iso-clozapine, whereas the ratios were less than 1 for clozapine and iso-loxapine. The ratios of affinities for D2 to 5-HT2A, 5-HT2C or D1 did not reflect the grouping of cataleptic and non-cataleptic compounds. 4. Co-treatment with the alpha2-adrenoceptor antagonists, yohimbine (1-10 mg kg(-1) s.c.), RX 821002 (1-10 mg kg(-1) s.c.) and MK-912 (0.3 and 1 mg kg(-1) s.c.) dose-dependently inhibited the cataleptic response to loxapine (0.3 mg kg(-1)). Yohimbine (1-10 mg kg(-1) s.c.) also dose-dependently inhibited the cateleptic response to haloperidol (0.3 mg kg(-1) s.c.). The alpha2-adrenoceptor antagonists had no effect per se. 5. Neither yohimbine (10 mg kg(-1)) nor RX821002 (3 mg kg(-1)) altered the cataleptic response to the D1 receptor antagonist, SCH 23390 (1 mg kg(-1) s.c.), while, like clozapine, both compounds abolished the response to the 5-HT2A receptor antagonist, MDL 100,151 (3 mg kg(-1) s.c.). 6. The present data strongly implicate alpha2-adrenoceptor blockade in the anticataleptic properties of clozapine and suggest that its lack of extrapyramidal side effects in the clinic may also be a consequence of this property.

Discriminative stimulus properties of 8-OH-DPAT in rats are not altered by pretreatment with para-chlorophenylalanine.

The role of 5-HT1A autoreceptors in the discriminative stimulus properties of 8-OH-DPAT (0.1 mg/kg, SC) in rats, was investigated. Drug lever appropriate responding to 8-OH-DPAT (0.1 mg/kg, SC) and ipsapirone (3 mg/kg, SC) was measured before and after treatment with para-chlorophenylalanine (pCPA) at a dose (150 mg/kg IP, -3 and -2 days) which causes severe depletion of brain 5-HT stores. The recognition of the drug stimulus was not significantly altered by pCPA. These results indicate that activation of 5-HT1A autoreceptors is of minimal importance to the 8-OH-DPAT cue.

Beta-adrenoceptor blockade in rats enhances the ambulation induced by 5-HT1A receptor agonists.

The mixed beta-adrenoceptor and 5-HT1A receptor antagonists, (-)-pindolol and propranolol, enhance rather than inhibit the hyperlocomotion induced in rats by the 5-HT1A receptor agonist, 8-OH-DPAT. The mechanism of this effect was now investigated. The rats were pretreated with the beta-adrenoceptor antagonist or saline and with the agonist 45 min later. Ambulation was quantified as the number of quadrants entered during a 15 min observation period. (-)-Pindolol, alprenolol, betaxolol, ICI 118,551 and a combination of betaxolol and ICI 118,551 (all at 1 mg/kg) significantly enhanced the locomotion induced by 8-OH-DPAT (0.24 mg/kg). Timolol (1 and 10 mg/kg) given 45 min before 8-OH-DPAT was inactive; however, given at 10 mg/kg 15 min prior to 8-OH-DPAT, the compound enhanced locomotion. (-)-Pindolol (1 mg/kg) also enhanced the locomotion induced by the putative selective 5-HT1A receptor partial agonists, flesinoxan and ipsapirone, but not that induced by 5-OH-DPAT, a DA2 receptor agonist. These results suggest that beta 1- or beta 2-adrenoceptor antagonism can enhance the locomotion induced by 5-HT1A receptor agonists. In the case of mixed 5-HT1A and beta-adrenoceptor antagonists, the beta-adrenoceptor-mediated effect may mask the inhibition of locomotion expected from 5-HT1A receptor antagonism.

Determination of the 5-HT receptor subtype involved in 8-OH-DPAT-induced hyperlocomotion: potential difficulties arising from inadequate pharmacological tools.

Selective 5-HT1A receptor agonists such as 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin), flesinoxan, 5-methylurapidil and others, increased locomotor behaviour in rats following s.c. injection. Unfortunately, all available 5-HT1A receptor antagonists are non-selective, a fact which severely hampers their use. The inhibitory effects of (+)- and (-)-pindolol (1-10 mg/kg, -45 min s.c.) on locomotion induced by 8-OH-DPAT were investigated in rats pretreated with the beta-adrenoceptor antagonists, ICI 118,551 and betaxolol (both 1 mg/kg, -45 min s.c.). ICI 118,551 and betaxolol significantly enhanced the response to 8-OH-DPAT. Co-administration of (-)-pindolol (2-10 mg/kg) dose dependently antagonised the hyperlocomotion whereas (+)-pindolol was weakly effective at 10 mg/kg. The partial agonists ipsapirone (1-10 mg/kg, -45 min s.c.) and buspirone (0.1-1 mg/kg, -45 min) antagonised the effects of 8-OH-DPAT. This inhibition may, however, be due to alpha 1-adrenoceptor blockade and DA receptor antagonism, respectively. Finally, although spiroxatrine (0.1 and 1 mg/kg, -45 min s.c.) inhibited the response to 8-OH-DPAT, this inhibition may also be non-specific since spiroxatrine strongly reduced spontaneous locomotor activity. In conclusion, whilst much of our data is consistent with 8-OH-DPAT-induced locomotion being mediated by 5-HT1A receptors, only the data obtained with the pindolol enantiomers provide direct evidence for this. The results also suggest that, under normal circumstances, the 5-HT1A receptor antagonist effect of pindolol may be masked by the facilitating effect of beta-adrenoceptor blockade.

Evidence for a 5-HT1D receptor-mediated hypothermic effect of the alpha 1-adrenoceptor agonist, SDZ NVI-085, in guinea-pigs.

The alpha 1-adrenoceptor agonist, SDZ NVI-085 ((-)-(4aR,10aR)-3,4,4a,5,10,10a-hexahydro-6-methoxy-4- methyl-9-(methylthio)-2H-naphth[2,3-b]-1,4-oxazine.HCl; 1 mg/kg i.p.), decreased body temperature of guinea-pigs. Two 5-HT1D receptor antagonists, GR127935 (N-[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]-2'-methyl-4'-(5-methyl- 1,2,4-oxadiazol-3yl)[1,1-biphenyl]-4-carboxamide) and PAPP (p-aminophenylethyl-m-trifluoromethylphenyl piperazine; both compounds at 1 mg/kg i.p., -30 min) blocked this response, whilst the alpha 1-adrenoceptor blocker prazosin (1 mg/kg i.p.) and the 5-HT1A receptor antagonist, SDZ 216-525 (methyl 4-(-[4-(1,1m3-trioxo-2H-1,2-benzoisothiazol-2-yl)butyl ]-1-piperazinyl)1H- indole-2-carboxylate; 1 mg/kg i.p.) were inactive. Another alpha 1-adrenoceptor agonist, St 587 (2-(2-chloro-5-trifluoromethylphenylimino)-imidazoline; 1 mg/kg i.p.) did not alter body temperature. SDZ NVI-085-induced hypothermia in guinea-pigs is probably mediated by 5-HT1D receptors.

Inhibition of 5-carboxamidotryptamine-induced relaxation of guinea-pig ileum correlates with [125I]LSD binding.

5-Hydroxytryptamine (5-HT), 5-carboxamidotryptamine (5-CT), 8-OH-DPAT and RU 24969 relax the guinea-pig ileum precontracted with histamine. This relaxation was characterized using 5-CT as agonist with a series of 8 competitive antagonists. [125I]LSD binding was measured in a membrane preparation of the longitudinal muscle of the guinea-pig ileum in the presence of 3 X 10(-7) M cinanserin in order to suppress binding to 5-HT2 receptors. There was a significant correlation (P less than 0.01) between the antagonism of 5-CT-induced relaxation (pA2 values) and the affinity values of the antagonists for [125I]LSD binding (pKD values). It was also shown that 5-CT relaxed histamine-precontracted longitudinal muscle strips of the guinea-pig ileum. The results suggest that the 5-HT receptor-mediating relaxation in the guinea-pig ileum can be labelled with [125I]LSD and that this receptor does not belong to the 5-HT2, 5-HT1A, 5-HT1B or 5-HT1C receptor subtypes.

Characterization of the 5-HT1B recognition site in rat brain: binding studies with (-)[125I]iodocyanopindolol.

(-)[125I]Iodocyanopindolol ([125I]CYP) labels rat brain membrane sites which display high affinity for several serotonergic and beta-adrenergic compounds. The binding of [125I]CYP to these serotonergic recognition sites was evaluated in the presence of 30 microM (-)isoprenaline in order to suppress binding to beta-adrenoceptors. [125I]CYP binds in rat cortex membranes rapidly, reversibly and stereoselectively to a finite number of recognition sites: Bmax = 180 fmol/mg, KD = 230 pM. Similar affinity values of [125I]CYP were obtained in membranes from rat hippocampus and striatum. Kinetic, saturation and competition experiments suggest that under these conditions [125I]CYP binds to a single serotonergic recognition site named 5-HT1B. The pharmacological profile of 5-HT1B sites is characteristic of a 5-HT1 binding site and shows the following rank order of affinity for agonists: RU 24969, (5-methoxy-3-[1,2,3,6-tetrahydropyridin-4-yl]1H-indole) greater than 5-CT, (5-carboxamidotryptamine) greater than 5-HT, (5-hydroxytryptamine, serotonin) greater than 5-OCH3-T, (5-methoxytryptamine) much greater than 2-CH3-5-HT, (2-methylserotonin) greater than 8-OH-DPAT, (8-hydroxy-2-(di-n-pro-pylamino)-tetralin). The rank order of affinity for antagonists is: (+/-)ICYP, ((+/- )-3-I-cyano-pindolol) greater than (-)21-009, (4-[3-ter-butyl-amino-2-hydroxy-propoxy]-indol-2-carbonic acid isopropyl ester) greater than (+)21-009 greater than (-)propranolol greater than metitepin greater than (-)pindolol much greater than ketanserin greater than spiroperidol greater than mesulergine. 5-HT1B recognition sites display low affinity for selective beta 1- and beta 2-adrenoceptor antagonists, e.g. atenolol, betaxolol, ICI 89-406 and ICI 118-551. The low affinity of 5-HT1B recognition sites for some 5-HT1A, 5-HT1C and 5-HT2 selective compounds (e.g. 8-OH-DPAT, mesulergine, ketanserin) suggests that 5-HT1B recognition sites are pharmacologically different from 5-HT1A, 5-HT1C and 5-HT2 recognition sites.

Molecular pharmacology of 5-HT1 and 5-HT2 recognition sites in rat and pig brain membranes: radioligand binding studies with [3H]5-HT, [3H]8-OH-DPAT, (-)[125I]iodocyanopindolol, [3H]mesulergine and [3H]ketanserin.

The pharmacological characteristics of the binding of [3H]8-OH-DPAT ([3H]8-hydroxy-2(di-n-propylamino)tetralin, [125I]CYP ((-)[125I]iodocyanopindolol) (in the presence of 30 microM (-)isoprenaline) and [3H]mesulergine to 5-HT1 recognition sites were studied in rat and pig brain membranes. [3H]8-OH-DPAT bound in rat and pig cortex to the 5-HT1A recognition site characterized by high affinity for 5-CT (5-carboxamido-tryptamine), 8-OH-DPAT, 5-HT (5-hydroxytryptamine, serotonin) and low affinity for pirenperone, ketanserin and mesulergine. [125I]CYP bound in rat but not in pig cortex to the 5-HT1B site which shows high affinity for (-)21-009 (4[3-ter-butyl-amino-2-hydroxy-propoxy]indol-2-carbonic acid isopropyl ester), (+/-)ICYP (3-I-cyanopindolol), 5-HT, RU 24969 (5-methoxy-3-[1,2,3,6-tetrahydropyridon-4-yl]1H-indole) and low affinity for 8-OH-DPAT, mesulergine and pirenperone. [3H]Mesulergine bound in pig choroid plexus and in rat cortex (besides binding to 5-HT2 sites in rat cortex) to the 5-HT1C recognition site characterized by high affinity for metergoline, mesulergine, 5-HT and methergine and low affinity for (-)21-009, ICYP, 8-OH-DPAT and spiroperidol. The pharmacological profile of 5-HT1A sites in rat and pig cortex appears to be identical; 5-HT1C sites in pig choroid plexus and rat cortex show no differences. In contrast, it was not possible to label 5-HT1B sites with [125I]CYP in pig brain membranes indicating that like 5-HT2 receptors, 5-HT1 recognition sites show species differences. The pharmacological profiles of the three 5-HT1 recognition sites are clearly different from one another. Furthermore, the pharmacological profile of each individual 5-HT1 recognition site is also different from that of the 5-HT2 receptors labelled with [3H]ketanserin in rat cortex membranes although some similarities exist between 5-HT2 and 5-HT1C sites. Finally, the beta-adrenoceptor antagonist (-)21-009 which has different affinities for 5-HT1A, 5-HT1B and 5-HT1C recognition sites, yielded triphasic competition curves for [3H]5-HT binding in rat cortex membranes providing evidence that [3H]5-HT labels three distinct 5-HT1 sites in these membranes.

Cataleptogenic effect of subtype selective 5-HT receptor antagonists in the rat.

5-HT receptor antagonists with selectivity for 5-HT1A WAY-100635 (N-[2-[-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohe xanecarboxamide), 5-HT1B GR 127935 (N-[methoxy-3-(4-methyl-1-piperazinyl)phenyl]-2'-methyl-4'(5-methyl-1,2, 4-oxadiazol-3-yl)[1,1-biphenyl]-4-carboxamide x HCl), 5-HT2C SB 200646A (N-(1-methyl-5-indolyl)-N'-(3-pyridyl)urea x HCl) and 5-HT2A (ketanserin, fananserin and MDL 100,151 ((+/-)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenyl)ethyl]-4-pipe ridinemethanol) receptors were tested for cataleptogenic responses in rats. WAY-100635 (0.1-3 mg/kg, s.c.), ketanserin (0.1-3 mg/kg, s.c.), MDL 100,151 (0.3-3 mg/kg, s.c.) and fananserin (RP 62203; 3 mg/kg, s.c.) induced a significant catalepsy. GR 127935 (1 mg/kg, s.c.), SB 200646A (without effect per se at 10 mg/kg, s.c.) and MDL 100,151 (0.3 mg/kg, s.c.) did not inhibit the cataleptic response to the dopamine D2 receptor antagonist, loxapine (0.3 mg/kg, s.c.). Catalepsy induced by MDL 100,151 (3 mg/kg) was blocked by co-treatment with clozapine, but not by SB 200646A (both at 10 mg/kg, s.c.). Although clozapine displays significant affinity to 5-HT1A, 5-HT1B, 5-HT2A and 5-HT2C receptors, the present results suggest that blockade of these receptors is not responsible for clozapine's anticataleptic activity.

Effects of (+)- and (-)-mianserin on alpha-adrenoceptors and tyramine-induced tachycardia in rats.

The individual stereoisomers of mianserin were tested in pithed normotensive rats for their antagonistic activity at vascular postjunctional alpha 1- and alpha 2-adrenoceptors as well as their interaction with the 'amine pump receptor'. (+)-Mianserin was found approximately five times more potent than (-)-mianserin in antagonizing the increase in diastolic pressure brought about by either selective alpha 1 (agonist: methoxamine)- or alpha 2 (agonist: B-HT 920)-adrenoceptor stimulation. (+)-Mianserin also behaved as the more effective displacer of [3H]prazosin and [3H]clonidine binding to rat brain membranes. Tyramine, an indirectly acting sympathomimetic agent, increased the resting heart rate of pithed rats. This effect was competitively antagonised by the noradrenaline uptake inhibitor desipramine. (-)-Mianserin was inactive in inhibiting the tyramine-induced increase in heart rate. (+)-Mianserin showed some activity, but was much less potent than desipramine. The present results clearly indicate that (+)-mianserin is the pharmacologically more active isomer.

Involvement of alpha 1- and alpha 2-adrenoceptors in the vasoconstriction caused by ergometrine.

In pithed normotensive rats, the pressor effects evoked by i.v. serotonin (5-HT) and ergometrine were analysed using the relatively selective alpha 1-, alpha 2- and serotonin antagonists prazosin, yohimbine methysergide, respectively. The pressor response to ergometrine was reduced by both prazosin and yohimbine but only moderately affected by methysergide. On the other hand, the rise in diastolic pressure brought about by serotonin was strongly depressed by methysergide but not influenced by the blockade of either alpha 1- or alpha 2-adrenoceptors. The calcium antagonist nifedipine shifted the dose-pressor response curve of ergometrine to the right in a dose-related, non-parallel manner. The maximum pressor response was diminished by nifedipine. In contrast, the rise in pressure provoked by serotonin was not affected by nifedipine. The results thus show that in the pithed rat the vasoconstrictor response to ergometrine is mediated by alpha-adrenoceptors (both alpha 1 and alpha 2) rather than by serotonin receptors. These findings question the role of serotonin receptors in the vasoconstriction induced by ergometrine.

Differential selectivities of RU 24969 and 8-OH-DPAT for the purported 5-HT1A and 5-HT1B binding sites. Correlation between 5-HT1A affinity and hypotensive activity.

RU 24969 and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) inhibited the specific binding of [3H]5-HT (2 nM) to rat brain membranes with shallow displacement curves. The displacement data were best fitted with a model of two independent, high and low affinity binding sites. Following addition of spiperone (1 microM) as a selective ligand for the putative 5-HT1A recognition site of [3H]5-HT, the displacement curve of RU 24969 underwent a leftward shift, whereas spiperone induced a shift to the right for the displacement curve of 8-OH-DPAT. In contrast to spiperone, pindolol (1 microM) shifted the displacement curve of RU 24969 to the right. These results suggest that RU 24969 possesses preference for the purported 5-HT1B subtype of central 5-HT1 recognition site. The reported significant linear correlation between hypotensive activity following intravenous (i.v.) administration to anesthetized rats and affinity for the central 5-HT1 binding site could only be maintained by incorporation of the affinity of RU 24969 for its low and 8-OH-DPAT for its high affinity binding site. Based on the proposal that the 5-HT1A site corresponds to the high affinity site of 8-OH-DPAT and the low affinity site of RU 24969, it is hypothesized that the late depressor phase of 5-HT agonists in rats is mediated by activation of peripheral (vascular) 5-HT receptors which have similarities with the 5-HT1A subtype of central 5-HT1 recognition site.

Vascular smooth muscle contraction initiated by postsynaptic alpha 2-adrenoceptor activation is induced by an influx of extracellular calcium.

Vasoconstriction in pithed, normotensive rats elicited via stimulation of postsynaptic alpha 2-adrenoceptors by B-HT 920 was antagonized by EDTA and the calcium antagonists nifedipine, D 600 and verapamil, whereas pressor responses to the alpha 1-agonist methoxamine were unaffected. This indicates that vasoconstriction in vivo initiated via postsynaptic alpha 2-adrenoceptors requires an influx of extracellular calcium. Thus, the antihypertensive effect of calcium antagonists may be based upon a diminution of vascular tone maintained by postsynaptic alpha 2-adrenoceptors.

Alpha 1/alpha 2-adrenoceptor agonist selectivity of mono- and dihydroxy-2-N,N-DI-n-propylaminotetralins.

The pressor activities and the identity of the postjunctional alpha-adrenoceptors involved were determined for a series of congeneric mono- and dihydroxy-substituted 2-N,N-di-n-propylaminotetralins and N,N-di-n-propyldopamine (DPDA) following i.v. administration to pithed normotensive rats. The affinity for alpha 1- and alpha 2-adrenoceptor-like binding sites was obtained from radioligand displacement studies. The 5- and 7-OH substituted tetralins as well as DPDA were reasonably potent and about equieffective pressor agents. The 6-OH congener had almost no vasoconstrictor effects whereas the 8-OH positional isomer occupied an intermediate position. The 5,6- and 6,7-di-OH analogs very effectively raised the diastolic pressure of pithed rats. On account of the inhibition exerted by prazosin (0.1 mg/kg) and yohimbine (1 mg/kg) the 5- and 7-OH isomers as well as DPDA can be classified as mixed alpha 1/alpha 2-adrenoceptor agonists, the alpha 1-adrenoceptor-stimulating potency being more pronounced especially for the 5-OH congener. In addition, a significant contribution of serotonin receptors to the pressor responses to the 8-OH compound was detected. Similarly, alpha 2-adrenoceptors were mainly responsible for the vasoconstriction caused by the 6,7-di-OH isomer, whereas the 5,6-di-OH congener very selectively stimulated this alpha 2-type receptor in the lower dose range and alpha 1-adrenoceptor stimulation predominated at higher doses of this agonist. The 6,7-di-OH compound failed to activate vascular postjunctional beta 2-adrenoceptors. The results indicate that the alpha 1/alpha 2-adrenoceptor agonist selectivity depends on the position(s) and the number of hydroxy groups present as well as on the alkyl substitution at the amino function. 2-N,N-Di-n-propylamino-6,7-dihydroxytetralin may be a more suitable alpha 2-adrenoceptor selective agonist than M-7.

Effect of captopril on sympathetic neurotransmission in pithed normotensive rats.

Captopril significantly diminished the basal diastolic blood pressure and the vasopressor response to electrical stimulation of the thoracic-lumbar spinal cord in pithed normotensive rats. The reduction of the hypertensive response to electrical stimulation was more pronounced after bilateral adrenalectomy. Captopril also diminished the vasopressor response to intravenously administered (-)-noradrenaline. Pretreatment of the animals with indomethacin had no effect on the vasopressor response to electrical stimulation and did not affect the sympathoinhibition of captopril. After bilateral nephrectomy, 18-24 h previously, the basal diastolic blood pressure and the vasopressor response to electrical stimulation were reduced and captopril had no additional inhibitory effect on these parameters. In indomethacin-pretreated animals with intact kidneys, restoration of the basal diastolic blood pressure with angiotensin II (AII) completely abolished the sympathodepressive effect of captopril. When the reduction in basal diastolic blood pressure with captopril was prevented by vasopressin, converting enzyme inhibition had no depressive effect on the hypertensive response to intravenously administered (-)-noradrenaline and did not influence sympathetic neurotransmission in animals with intact adrenals. However, a small, but significant reduction of the hypertensive response to electrical stimulation by captopril was still detectable in bilaterally adrenalectomized rats. The results suggest that endogenous AII facilitates sympathetic neurotransmission in vascular smooth muscle of pithed normotensive rats. However, the modulatory action of endogenous AII largely results from an effect on basal arteriolar smooth muscle tone and should, therefore, be considered as non-specific facilitation. Genuine prejunctional facilitation of the sympathetic neurotransmission in vascular smooth muscle can only be observed after bilateral adrenalectomy but this effect of endogenous AII appears of minor significance, at least in pithed normotensive rats.