5-HT1D Receptor Research Articles

Aspectos neuroquímicos: o papel da serotonina no TOC

The therapeutic action of selective serotonin (5-HT) reuptake inhibitors (SSRIs) in obsessive-compulsive disorder (OCD) points to the role of 5-HT in this condition. Functional neuroimaging studies showed hyperactivity of the nucleus caudatum in OCD patients, which disappears following treatment with either SSRIs or cognitive behavior therapy. Pharmacological results indicate hypersensitivity of the 5-HT1D receptor subtype in OCD, similarly reversed by chronic administration of SSRIs. Neurochemical data show that these receptors are selectively concentrated in the striatum, and are localized in 5-HT terminals, where they inhibit 5-HT release by the nerve impulse. Since 5-HT reduces the activity of post-synaptic neurons in the striatum, it has been suggested that the hypersensitivity of 5-HT1D receptors contribute to the genesis of OCD symptoms.

Safety and efficacy of PNU-142633, a selective 5-HT1D agonist, in patients with acute migraine.

In this randomized, double-blind, placebo-controlled, parallel-group study, patients received a single 50-mg oral dose of a 5-HT1D agonist, PNU-142633 (n = 34), or matching placebo (n = 35) during an acute migraine attack. No statistically significant treatment effects were observed at 1 and 2 h after dosing, even after stratifying by baseline headache intensity. At 1 and 2 h post-dose, 8.8% and 29.4% of the PNU-142633 group, respectively, and 8.6% and 40.0% of the placebo group, respectively, experienced headache relief; 2.9% and 8.8% of the PNU-142633 group and 0% and 5.7% of the placebo group were free of headache pain. Adverse events associated with PNU-142633 treatment included chest pain (two patients) and QTc prolongation (three patients). Results from this study suggest that anti-migraine efficacy is not mediated solely through the 5-HT1D receptor subtype, although this receptor may contribute, at least in part, to the adverse cardiovascular effects observed with 5-HT agonist medications.

Pharmacological profile of the mechanisms involved in the external carotid vascular effects of the antimigraine agent isometheptene in anaesthetised dogs.

The present study set out to investigate the external carotid vascular effects of isometheptene in vagosympathectomised dogs, anaesthetised with pentobarbital. One-minute intracarotid (intra-arterial; i.a.) infusions of isometheptene (10, 30, 100 and 300 microg/min) produced dose-dependent decreases in external carotid blood flow, without affecting blood pressure or heart rate. The vasoconstrictor responses to 100 microg/min and 300 microg/min of isometheptene were clearly attenuated in animals pretreated with reserpine (5,000 microg/kg). Moreover, after prazosin (an alpha1-adrenoceptor antagonist; 100 microg/kg), the responses to isometheptene remained unaltered in untreated as well as reserpine-pretreated dogs. In contrast, the responses to isometheptene were attenuated by rauwolscine (an alpha2-adrenoceptor antagonist; 300 microg/kg) in untreated animals, and were practically abolished in reserpine-pretreated dogs. Further investigation into the specific alpha2-adrenoceptor subtypes, using selective antagonists, showed that BRL44408 (alpha2A) and MK912 (alpha2C) markedly attenuated this response, while imiloxan (alpha2B) was ineffective. The involvement of 5-HT1B and 5-HT1D receptors seems highly unlikely since antagonists at 5-HT1B (SB224289) and 5-HT1D (BRL15572) receptors (both at 300 microg/kg) were ineffective. On this basis, it is concluded that isometheptene-induced canine external carotid vasoconstriction is mediated by both indirect (a tyramine-like action) and direct (acting at receptors) mechanisms, which mainly involve alphaA- and alpha2C-adrenoceptors, while the involvement of alpha1-adrenoceptors seems rather limited.

Co-localization of 5-HT(1B/1D/1F) receptors and glutamate in trigeminal ganglia in rats.

Anti-migraine triptan drugs are 5-HT(1B/1D) receptor agonists which are thought to block the neurotransmitter/neuropeptide release from sensory nerve terminals and directly constrict blood vessel smooth muscles. In the present study, we have investigated the anatomical basis for a possible modulation of glutamate release from trigeminal ganglion neurons by 5-HT(1B/1D) receptor agonists and by 5-HT1F receptor agonists, using double immunohistochemical staining technique in the rat. The majority of 5-HT1B, 5-HT1D or 5-HT1F receptor positive neurons were also glutamate positive, but both 5-HT1B, 5-HT1D or 5-HT1F receptor single-labeled and glutamate single-labeled neurons were observed. These results suggest that 5-HT(1B/1D/1F) receptor agonists may modulate glutamate release, and that one mechanism of their anti-migraine action could be the blockade of glutamate release.

Colocalization of CGRP with 5-HT1B/1D receptors and substance P in trigeminal ganglion neurons in rats.

Vasodilatation in the dura mater has been implicated in migraine pathogenesis. Anti-migraine triptan drugs block vasodilatation by binding to 5-HT1B/1D receptors localized on the peripheral sensory terminals and dural blood vessel smooth muscles. Previous studies suggest that calcitonin gene-related peptide (CGRP) released from Adelta-fibres plays a more important role than substance P (SP) released from C-fibres in inducing dural vasodilatation and that one of the antimigraine mechanisms of triptan drugs is inhibiting CGRP release. In the present study, the relationship between CGRP and 5-HT1B/1D receptors, and between CGRP and SP in the trigeminal ganglion neurons in rats was examined by double immunohistochemical staining. CGRP, 5-HT1B, 5-HT1D and SP-positive trigeminal ganglion neurons were all predominantly small and medium-sized. In the trigeminal ganglia, approximately 50% of CGRP-positive neurons were 5-HT1B positive. Similarly, approximately 55% of CGRP-positive neurons were 5-HT1D immunoreactive. Approximately 50% of CGRP-positive neurons were SP-positive, while 93% of SP-positive neurons were CGRP-positive, suggesting that nearly all SP-positive neurons also contain CGRP. The fibre types of the 5-HT1B- and 5-HT1D-positive neurons were further investigated with an antibody against the A-fibre marker 200-kDa neurofilaments (NF200). Approximately 46% of the 5-HT1B-positive and 43% of the 5-HT1D-positive trigeminal ganglion neurons were also NF200 positive, indicating that many A-fibre trigeminal neurons express 5-HT1B or 5-HT1D receptors. These results support the hypothesis that one important action of antimigraine drugs is the inhibition of CGRP release and that Adelta-fibres may play an important role in migraine pathogenesis.

Pharmacology and efficacy of eletriptan for the treatment of migraine attacks

We would like, to make a number of points on this Drug Focus article (IJCP December 2000; 670–674) and would be grateful for a response from Professor Diener on the following. Given that eletriptan has higher bioavailability (50%) than sumatriptan (140%),1 is two orders of magnitude more lipophilic than sumatriptan, and has four- and eightfold higher affinity than sumatriptan for the 5HT1B and 5HT1D receptor, it is odd that such a high dose of eletriptan is required to produce a therapeutic effect. One possible explanation may be that triptans act centrally as well as peripherally. If this theory is true, eletriptan would have to overcome the P-glycoprotein efflux pump to produce adequate cerebral concentrations to mediate a response.2 The authors state that eletriptan has a higher craniocoronary selectivity than sumatriptan. It would of course be quite inappropriate to extrapolate this finding to a benefit in clinical practice. The authors cite the study of Muir3 to contend that eletriptan produced no significant change in coronary artery diameters. In that same study one patient experienced marked segmental coronary artery spasm in association with the eletriptan infusion, which was unresponsive to sublingual glyceryl trinitrate. Furthermore, in a direct comparative clinical study, the incidence of chest symptoms was similar between the eletriptan and sumatriptan treatment groups.4 While we accept that such symptoms are usually not due to coronary ischaemia, this illustrates that preclinical findings should not be taken out of their proper context. Eletriptan may be absorbed rapidly in normal volunteers, with a Tmax of one hour, but this is not the case during a migraine attack, where gastric paresis may delay absorption. The Tmax for eletriptan during a migraine attack is prolonged to 2.8 hours.5 While eletriptan is not subject to drug interactions due to MAO metabolism, it will interact with potent inhibitors of cytochrome P450 3A4. The New Zealand product label for eletriptan recommends a reduction in dosage to 20 mg when being coadministered with erythromycin, clarithromycin, ketoconazole and fluconazole.4 The authors claim that eletriptan 80 mg has higher efficacy than eletriptan 40 mg. In none of the studies presented in the review was eletriptan 80 mg statistically superior to 40 mg.

Decreased sensitivity of 5-HT(1D) receptors in melancholic depression.

Brain serotonin (5-HT) function is abnormal in major depression, but the involvement of different 5-HT receptor subtypes has been little studied. The availability of selective ligands now makes it possible to test the sensitivity of 5-HT(1D) receptors in patients with depression. The aim of the study was to use the 5-HT(1D) receptor agonist, zolmitriptan, to test the sensitivity of 5-HT(1D) receptors in patients with depression before and after treatment with selective serotonin reuptake inhibitors (SSRIs). We measured the growth hormone response to zolmitriptan (5 mg orally) in patients with major depression before and after SSRI treatment. A matched sample of healthy subjects acted as a control group. The growth hormone response to zolmitriptan was blunted in patients with a melancholic depressive syndrome. SSRI treatment produced a marked reduction in zolmitriptan-induced growth hormone release. Patients with melancholic depression have impaired sensitivity of the post-synaptic 5-HT(1D) receptors that mediate growth hormone release. The reduction in 5-HT(1D) receptor sensitivity following SSRI treatment is probably an adaptive response to increased levels of synaptic 5-HT.

Neuroendocrine effects of subcutaneous sumatriptan in patients with migraine

We evaluated the sensitivity of 5-HT1D receptors in patients with migraine using sumatriptan as a pharmacological probe. The drug inhibits the release of ACTH, cortisol and prolactin and this effect may be used to explore the function of serotoninergic systems in vivo. We administered sumatriptan (6 mg sc) and placebo to 15 migraineurs, during the headache-free period, and to 10 healthy controls. Blood samples were collected -15, 0, 15, 30, 45, 60 and 90 min after injections. Sumatriptan induced a significant (p<0.01) decrease of ACTH, cortisol and prolactin concentrations both in patients with migraine and in controls. The neuroendocrine response was not significantly different in the two groups. Our results suggest that 5-HT1D receptor sensitivity is not altered in migraine.

Success and failure of triptans

Sumatriptan and the newer triptans (zolmitriptan, naratriptan, rizatriptan, eletriptan, almotriptan, frovatriptan and donitriptan) display high agonist activity at 5-HT1B and 5-HT1D receptors. Most triptans, but not all (donitriptan, frovatriptan and rizatriptan), also have a high affinity at the 5-ht1F receptor. In anaesthesisted animals, triptans decrease the arteriovenous anastomotic fraction of carotid blood flow. In isolated blood vessels, triptans cause contraction and this effect is more marked on cranial arteries. The 5-HT1B receptors and not 5-HT1D or 5-ht1F receptors mediate the vascoconstrictor effect of triptans. In animal studies, the triptans exert an inhibitory effect within the trigeminovascular system. The therapeutic effect of triptans is mediated mainly by their cranial vasoconstrictor property. Whether the inhibitory effects of the triptans on the trigeminovascular system contribute to their efficacy in migraine is still a moot point. The biggest success of triptans is that they provide an excellent therapeutic option for migraine therapy. This success has generated awareness for migraine in patients, clinicians and researchers alike. This, in turn, has increased our knowledge of the disease pathophysiology, which will ultimately lead to even better drugs in future. Among the failures of triptans, one may mention that a minority of patients respond poorly and others may have headache recurrence and chest syndromes. Overall, however, the advantages of triptans far outweigh their disadvantages and they represent a significant advance in medical therapy.

SLATE: a method for the superposition of flexible ligands.

A novel program for the superposition of flexible molecules, SLATE, is presented. It uses simulated annealing to minimise the difference between the distance matrices calculated from the hydrogen-bonding and aromaticring properties of two ligands. A method for generating a molecular stack using multiple pairwise matches is illustrated. These stacks are used by the program DOH to predict the relative positions of receptor atoms that could form hydrogen bonds to two or more ligands in the dataset. The methodology has been applied to ligands binding to dihydrofolate reductase, thermolysin. H3 histamine receptors, alpha2 adrenoceptors and 5-HT1D receptors. When there are sufficient numbers and diversity of molecules in the dataset, the prediction of receptor-atom positions is applicable to compound design.

Sumatriptan elicits both constriction and dilation in human and bovine brain intracortical arterioles.

1. Little is known about serotonin (5-HT) receptors present on brain microvessels that are innervated by brainstem serotonergic neurons. Using 5-HT, sumatriptan and subtype selective 5-HT(1) receptor agonists and/or the 5-HT(1) receptor antagonist GR127935, we characterized the 5-HT receptors involved in regulating microvascular tone of pressurized intracortical arterioles (approximately 40--50 microm) isolated from human and bovine cerebral cortex. The role of nitric oxide (NO) on these responses was assessed with the N(omega)-nitro-L-arginine (L-NNA, 10(-5) M), an inhibitor of NO synthesis. Bovine pial arteries were studied for comparative purposes. 2. At spontaneous tone, 5-HT induced a dose-dependent constriction of human and bovine microarteries (respective pD(2) values of 7.3+/-0.2 and 6.9+/-0.1); a response potently inhibited by GR127935 (pIC(50) value of 8.5+/-0.1) in bovine microvessels. 3. In both species, the 5-HT(1) receptor agonist sumatriptan induced a biphasic response consisting of a small but significant dilation at low concentrations (1 and/or 10 nM) followed by a constriction at higher doses (pD(2) for contraction of 6.9+/-0.1 and 6.6+/-0.2 in human and bovine vessels, respectively). Pre-incubation with L-NNA abolished the sumatriptan-induced dilation and significantly shifted the dose-response of the constriction curve to the left. In contrast, the selective 5-HT(1D) (PNU-109291) and 5-HT(1F) (LY344864) receptor agonists were devoid of any vasomotor effect. 4. In bovine pial vessels, 5-HT and sumatriptan elicited potent constrictions (respective pD(2) of 7.2+/-0.1 and 6.6+/-0.1), a weak dilation being occasionally observed at low sumatriptan concentrations. 5. A significant negative correlation was observed between pial and intracortical vessels diameter and the extent of the dilatory response to 10(-9) M sumatriptan. Together, these results indicate that sumatriptan, most likely via activation of distinctly localized microvascular 5-HT(1B) receptors, can induce a constriction and/or a dilation which is sensitive to inhibition of NO synthesis and dependent on the size and, possibly, the existing tone of the vessels.

Naratriptan

SUMMARYNaratriptan is a selective 5-HT1B/1D receptor agonist, with a high affinity at the 5-HT1B, 5-HT1D and 5-HT1F receptor subtypes. Naratriptan contracts a number of large isolated cerebral arteries from several species, and has little contractile effect on peripheral blood vessels. It has an inhibitory effect on the cranial neurogenic inflammation model. The clinically recommended dose is 2.5 mg. It was found significantly superior to placebo on headache relief and pain free at 2 and 4 hours, and in relieving nausea, photophobia and phonophobia. It also has a good within-patient consistency, and a low recurrence rate. The side-effect profile is that of the triptan class, with an incidence no different from placebo at the 25 mg dose. The contraindications are similar to any triptan, including coronary disease. Naratriptan is unlikely to affect metabolism of other drugs. In comparison with sumatriptan 100 mg, naratriptan 2.5 mg has a slower onset of action and a lower response rate at 4 h, but it has a lower recurrence rate, and is better tolerated.

Frovatriptan.

black triangle Frovatriptan, a new serotonin receptor agonist developed for the acute treatment of migraine, has high affinity for serotonin 5-HT1B and 5-HT1D receptor subtypes and is a potent stimulator of contraction in human basilar arteries. black triangle A long terminal elimination half-life (approximately 26 hours) is a distinctive pharmacokinetic feature of frovatriptan which appears to be independent of dose, age, gender and renal function. black triangle A single oral dose of frovatriptan 2.5mg was effective in the acute treatment of migraine providing meaningful relief within 2 hours to approximately twice as many recipients as placebo in clinical trials. black triangle Consistent relief of migraine symptoms was achieved in patients who treated a number of consecutive attacks with frovatriptan and the incidence of 24-hour migraine recurrence was reduced. black triangle Frovatriptan was well tolerated in clinical trials, with the overall incidence of adverse events occurring with frovatriptan 2.5mg only slightly higher than that reported with placebo. Mild to moderate fatigue, nausea and paraesthesia were the most commonly reported drug-related adverse events.

Unravelling the pharmacological profile of the canine external carotid vasodilator '5-HT1-like' receptors: coexistence of sympatho-inhibitory 5-HT1B and postjunctional 5-HT7 receptors.

It has been suggested that the external carotid vasodilatation produced by serotonin (5-hydroxytryptamine; 5-HT) in anaesthetised dogs with intact vagosympathetic trunks is mediated by sympatho-inhibitory '5-HT1D' receptors and musculotropic '5-HT1-like' receptors. The present study has re-analysed this suggestion with regard to the classification schemes recently proposed by the NC-IUPHAR subcommittee on 5-HT receptors. In pentobarbital-anaesthetised dogs with intact vagosympathetic trunks, 1-min intracarotid (i.c.) infusions of 5-carboxamidotryptamine (5-CT; 0.01-0.3 microg/min), 5-HT (0.3-30 microg/ min), 5-methoxytryptamine (5-MeO-T; 1-100 microg/min) or sumatriptan (1-100 microg/min) dose-dependently increased the external carotid blood flow without affecting blood pressure or heart rate. The selective 5-HT1D receptor agonist, PNU-142633 (1-1000 microg/min), was essentially inactive. After mesulergine (300 microg/kg, i.v.), an antagonist at cardiovascular 5-HT7 receptors, the above responses to 5-HT, 5-CT and 5-MeO-T were blocked, whilst those to sumatriptan remained unaffected. In contrast, after the 5-HT1B/1D receptor antagonist, GR127935 (10 microg/kg, i.v.), the responses to 5-HT, 5-CT and 5-MeO-T were not affected, but those to sumatriptan were abolished. Furthermore, after the selective 5-HT1B receptor antagonist, SB224289 (300 microg/kg, i.v.), the responses to 5-HT, 5-CT and 5-MeO-T were significantly enhanced, whereas those to sumatriptan were abolished. Interestingly, the responses to all these agonists remained unmodified after the selective 5-HT1D receptor antagonist, BRL15572 (300 microg/kg, i.v.). The above results suggest that the '5-HT1-like' receptors, which mediate canine external carotid vasodilatation, display the pharmacological profile of sympatho-inhibitory 5-HT1B receptors and musculotropic 5-HT7 receptors, and confirm the existence of vasoconstrictor 5-HT1B receptors.

Signalling Pathways Activated by 5-HT1B/5-HT1D Receptors in Native Smooth Muscle and Primary Cultures of Rabbit Renal Artery Smooth Muscle Cells

The potential of primary cultures of rabbit renal artery vascular smooth muscle cells (VSMCs) was assessed as a means to investigate the signalling pathways linked to 5-hydroxytryptamine (5-HT) 5-HT_1B/5-HT_1D receptors in native arteries. In renal artery segments denuded of endothelium, incubated with ketanserin and prazosin (each 1 µM), and prestimulated with 20 mM K⁺ Krebs buffer, 5-HT and CP 93,129, a 5-HT_1B receptor agonist, evoked concentration-dependent contractions. GR 127935, a 5-HT_1B/5-HT_1D receptor antagonist, significantly antagonised 5-HT-evoked contractions at nanomolar concentrations. Reverse transcription polymerase chain reaction (RT-PCR) of mRNA from smooth muscle cells from the isolated renal artery and from primary cultures of VSMCs from the same artery expressed mRNA transcripts for the 5-HT_1B receptor and the 5-HT_1D receptor in both preparations. The sequence of the PCR fragments corresponded to the known sequence for these receptors. Application of 5-HT evoked a concentration-dependent, pertussis toxin (PTx)-sensitive reduction in cyclic AMP in both cultured cells and intact artery (cyclic AMP concentration reduced by 65.53 ± 3.33 and 52.65 ± 5.34% from basal with 10 µM 5-HT, respectively). The effect of 10 µM 5-HT on cAMP was increased in the presence of 20 mM K⁺ (reduced by 82.50 ± 2.50 and 87.54 ± 3.97%, respectively). In intact arteries, contraction through 5-HT_1B/5-HT_1D receptors was significantly attenuated by inhibitors of phosphatidylinositol 3-kinase (wortmannin) and activated mitogen-activated protein kinase (MAPK), MEK (U01296). In the cultured VSMCs, activated MAPK was identified by immunocytochemistry and immunoblotting after stimulation with 5-HT, but only if 20 mM K⁺ was present at the onset of stimulation. These data provide the first direct evidence that 5-HT_1B/5-HT_1B receptors are linked to the activation of MAPK and indicate that primary cultures of renal VSMCs could provide a model system to study further the signalling pathways linked to these receptors.

Receptor-mediated regulation of serotonin output in the rat dorsal raphe nucleus: effects of risperidone.

The present study was undertaken to characterize the regulation of serotonin (5-HT) efflux and neuronal activity in the dorsal raphe nucleus (DRN) as well as to examine the potential ability of the antipsychotic drug risperidone to interfere with these mechanisms. By using microdialysis in freely moving rats, it was found that administration of the alpha2 adrenoceptor antagonist idazoxan (0.25 mg/kg, SC), the 5-HT1B/D receptor antagonist GR 127,935 (1.0 mg/kg, SC) and risperidone (0.6 or 2.0 mg/kg, SC) increased 5-HT output in the DRN. Local DRN perfusion with GR 127,935 or risperidone via reversed dialysis (100 or 10-100 microM, respectively) enhanced 5-HT efflux in this area, whereas idazoxan (10-100 microM) failed to affect this parameter. Both systemic administration and reversed DRN dialysis of the D2/3 and 5-HT2A receptor antagonists raclopride (2.0 mg/kg, SC or 10-100 microM) and MDL 100,907 (1.0 mg/kg, SC or 10-100 microM), respectively, were without effect. Intraraphe dialysis of the 5-HT1B/D receptor agonist CP 135,807 (0.2 microM) decreased the efflux of 5-HT in the DRN, an effect which was antagonized by co-administration of either GR 127,935 or risperidone (10 and 3.3 microM, respectively). By using single-cell recording, it was found that administration of GR 127,935 (50-400 microg/kg, IV) decreased, whereas CP 135,807 (2.5-20 microg/kg, IV) increased firing of 5-HT cells in the DRN. Our findings suggest a regulatory role of local 5-HT1B/D receptors on 5-HT efflux as well as cell firing in the DRN and indicate that risperidone may interfere with the regulation of 5-HT availability in this area primarily via blockade of 5-HT1D receptors.

ChemInform Abstract: 5‐Alkyltryptamine Derivatives as Highly Selective and Potent 5‐HT1D Receptor Agonists.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Characterization of mechanisms involved in presynaptic inhibition of sympathetic pressor effects induced by some 5-HT1 receptor antagonists.

1. In a previous study, we showed that the presynaptic inhibitory action of 5-hydroxytryptamine receptor agonists on sympathetic pressor effects obtained in the pithed rats were mainly mediated by activation of 5-HT1A and 5-HT1D receptor subtypes. At the time, we observed that some 5-HT1 receptors antagonists - WAY 100,635 and NAN-190 (both 5-HT1A receptor antagonists), methiothepin (a 5-HT1,2,5,6,7 receptor antagonist) and spiperone (a 5-HT1,2 receptor antagonist) - reduced per se the pressor effects obtained by electrical stimulation. The aim of the present work was to investigate the mechanism participating in this inhibitory effect. 2. The inhibition induced by WAY 100,635 (1000 microg kg-1, i.v.) was blocked after i.v. treatment with idazoxan, an alpha2-adrenoceptor antagonist (300 and 1000 microg kg-1) and was not modified after i.v. treatment with propranolol, a beta-adrenoceptor antagonist (1000 microg kg-1) and sulpiride, a D2 receptor antagonist (1000 microg kg-1). The inhibition induced by spiperone (500 microg kg-1 i.v.) was significantly blocked by sulpiride (1000 microg kg-1) and was not modified by idazoxan or propranolol. 3. Sulpiride (1000 microg kg-1) partially blocked the inhibition induced by methiothepin (50 microg kg-1 i.v.). Only pretreatment with idazoxan (300 microg kg-1) modified the inhibition induced by NAN-190 (100 microg kg-1 i.v.), such inhibition increasing after intravenous administration of idazoxan. 4. All the antagonists used in our experiments failed to inhibit the pressor responses elicited by i.v. noradrenaline administration. 5. The above results suggest that the inhibitory effects of these 5-HT1 receptor antagonists are presynaptic in nature, but not related to the blockade of 5-HT1 receptors subtypes. The simultaneous activation or inhibition of other receptor systems could explain the inhibition produced by each 5-HT1 receptor antagonist studied.

Coupling of canine serotonin 5-HT(1B) and 5-HT(1D) receptor subtypes to the formation of inositol phosphates by dual interactions with endogenous G(i/o) and recombinant G(alpha15) proteins.

Molecular cloning and expression of canine (ca) serotonin 5-HT(1B) and ca 5-HT(1D) receptor subtypes showed that besides the lower binding affinity of ketanserin for the ca 5-HT(1D) receptor, the ligand binding profiles were similar to their human homologues. Site-directed mutagenesis studies suggest that a Gln(189) residue in the second extracellular loop of the ca 5-HT(1D) receptor may partially account for the lower binding affinity of ketanserin. The coupling of ca 5-HT(1B) and ca 5-HT(1D) receptor subtypes to the phospholipase C pathway was analyzed by measuring stimulation of inositol phosphate formation in COS-7 cells. Zolmitriptan potently stimulated (EC(50) = 4.9 nM) the inositol phosphate formation at ca 5-HT(1D) receptors in a fully pertussis toxin (PTX)-dependent manner, whereas only a weak PTX-resistant inositol phosphate response (26-29% at 10 microM zolmitriptan) could be detected for the ca 5-HT(1B) receptor at a similar expression level. In contrast, both ca 5-HT(1B) and ca 5-HT(1D) receptor subtypes yielded a similar maximal magnitude of inositol phosphate formation (300-340% at 10 microM zolmitriptan) upon co-expression with a mouse (m) G(alpha15) protein. PTX treatment and co-expression with a beta-adrenergic receptor kinase C-terminal polypeptide partially (20-46%) abolished the m G(alpha15) protein-dependent ca 5-HT(1B) and ca 5-HT(1D) receptor-mediated stimulation of inositol phosphate formation. This study suggests both 5-HT receptor subtypes can activate betagamma subunits of endogenous G(i/o) proteins besides their coupling to recombinant m G(alpha15) protein.

Increased growth hormone response to sumatriptan challenge in adult autistic disorders

Serotonergic (5-HT) abnormalities have been documented in autism. To assess sensitivity of the 5-HT1d receptor, growth hormone response to the 5-HT1d receptor agonist sumatriptan was studied in adult autistic patients and matched normal controls. In this study, 11 adult patients with autism or Asperger’s disorder were compared with nine matched controls. All subjects were randomized to single dose sumatriptan (6 mg SQ) and placebo challenges, separated by a 1-week interval, and growth hormone was measured before and during the challenges. The results showed a highly significant diagnosis×drug×time interaction on repeated measure analysis covaried for baseline. This suggests that autistic patients had significantly greater growth hormone response to sumatriptan than normal controls, independent of placebo effects. Therefore, abnormalities in 5-HT regulation in autism may be related to increased sensitivity of the 5-HT1d inhibitory receptor in autism.