Autoradiography of serotonin 5-HT1A receptor-activated G proteins in guinea pig brain sections by agonist-stimulated [35S]GTPgammaS binding

G protein activation mediated by serotonin 5-HT1A and 5-HT(1B/D) receptors in guinea pig brain was investigated by using quantitative autoradiography of agonist-stimulated [35S]GTPgammaS binding to brain sections. [35S]GTPgammaS binding was stimulated by the mixed 5-HT1A/5-HT(1B/D) agonist L694247 in brain structures enriched in 5-HT1A binding sites, i.e., hippocampus (+140 +/- 14%), dorsal raphe (+70 +/- 8%), lateral septum (+52 +/- 12%), cingulate (+36 +/- 8%), and entorhinal cortex (+34 +/- 5%). L694247 caused little or no stimulation of [35S]GTPgammaS binding in brain regions with high densities of 5-HT(1B/D) binding sites (e.g., substantia nigra, striatum, central gray, and dorsal subiculum). The [35S]GTPgammaS binding response was antagonized by WAY100635 (10 microM) and methiothepin (10 microM). In contrast, the 5-HT1B inverse agonist SB224289 (10 microM) did not affect the L694247-mediated [35S]GTPgammaS binding response, and the mixed 5-HT(1B/D) antagonist GR127935 (10 microM) yielded a partial blockade. The distribution pattern of the [35S]GTPgammaS binding response and the antagonist profile suggest the L694247-mediated response in guinea pig brain to be mediated by 5-HT1A receptors. In addition to L694247, 8-hydroxy-2-(di-n-propylamino)tetralin, and flesinoxan also stimulated [35S]GTPgammaS binding; their maximal responses varied between 46 and 52% compared with L694247, irrespective of the brain structure being considered. Sumatriptan, rizatriptan, and zolmitriptan (10 microM) stimulated [35S]GTPgammaS binding in the hippocampus by 20-50%. Naratriptan, CP122638, and dihydroergotamine stimulated [35S]GTPgammaS binding to a similar level as L694247 in hippocampus, lateral septum, and dorsal raphe. It appears that under the present experimental conditions, G protein activation through 5-HT1A but not 5-HT(1B/D) receptors can be measured in guinea pig brain sections.

Pharmacological analysis of the haemodynamic effects of 5-HT1B/D receptor agonists in the normotensive rat

1 The receptors involved in mediating the haemodynamic effects of three 5-HT1B/D receptor agonists were investigated in pentobarbitone anaesthetized rats (n = 6-17 per group). 2 Cumulative intravenous (i.v.) infusions of rizatriptan and sumatriptan (from 0.63 to 2500 microg kg(-1); each dose over 5 min) induced dose-dependent and marked hypotension (-42+/-6 and -34+/-4 mmHg at the highest dose, respectively; both P<0.05 vs vehicle: +5+/-3 mmHg) and bradycardia (-85+/-16 and -44+/-12 beats min(-1) at the highest dose, respectively; both P<0.05 vs vehicle: +16+/-6 beats min(-1)). Zolmitriptan evoked only moderate hypotension at the highest dose (-19+/-9 mmHg; P<0.05 vs vehicle). 3 A high dose of the 5-HT1B/D receptor antagonist, GR 127935 (0.63 mg kg(-1), i.v.), failed to antagonize the hypotension and bradycardia evoked by sumatriptan (-35+/-6 mmHg and -52+/-19 beats min(-1), respectively; both not significant vs sumatriptan in untreated rats), but moderately reduced the hypotension and bradycardia evoked by rizatriptan (-20+/-5 mmHg and -30+/-17 beats min(-1), respectively; both P<0.05 vs vehicle and vs rizatriptan in untreated rats). 4 The selective 5-HT1A receptor antagonist, WAY 100635 (0.16 and 0.63 mg kg(-1), i.v.), dose-dependently attenuated the haemodynamic responses evoked by rizatriptan and sumatriptan, which were almost abolished by the higher dose of WAY 100635 (-4+/-3 mmHg and -15+/-8 beats min(-1); both not significant vs vehicle and P<0.05 vs rizatriptan in untreated rats). A slight but statistically significant reduction in mean arterial pressure (MAP) persisted at the highest dose of sumatriptan (-13+/-4 mmHg following the higher dose of WAY 100635; P<0.05 vs vehicle). 5 In pithed rats with MAP normalized by angiotensin II, rizatriptan failed to induce hypotension or bradycardia (+5+/-4 mmHg and -6+/-16 beats min(-1), respectively; both NS vs vehicle and P<0.05 vs rizatriptan in untreated rats). Similarly, sumatriptan failed to induce bradycardia in pithed rats (+5+/-6 beats min(-1); not significant vs vehicle and P<0.05 vs sumatriptan in untreated rats), whereas a slight but statistically significant reduction in MAP, compared to controls, occurred at the highest dose (-9+/-9 mmHg; P<0.05 vs both vehicle and sumatriptan in untreated rats). 6 In bilaterally vagotomized and atropine-treated (1 mg kg(-1), i.v.) rats, the reductions in MAP and heart rate evoked by rizatriptan (-31+/-4 mmHg and -64+/-9 beats min(-1), respectively; both P<0.05 vs vehicle and not significant vs rizatriptan in controls) and sumatriptan (-47+/-8 mmHg and -56+/-10 beats min(-1), respectively; both P<0.05 vs vehicle and not significant vs sumatriptan in controls) were not statistically significantly different from those observed in controls. 7 In conclusion, the 5-HT1B/D receptor agonists, rizatriptan and sumatriptan, elicit hypotension and bradycardia in the normotensive anaesthetized rat predominantly via activation of central 5-HT1A receptors, and a consequent reduction in sympathetic outflow.

Pharmacological analysis of G-protein activation mediated by guinea-pig recombinant 5-HT1B receptors in C6-glial cells: similarities with the human 5-HT1B receptor

1. The guinea-pig recombinant 5-hydroxytryptamine1B (gp 5-HT1B) receptor stably transfected in rat C6-glial cells was characterized by monitoring G-protein activation in a membrane preparation with agonist-stimulated [35S]-GTPgammaS binding. The intrinsic activity of 5-HT receptor ligands was compared with that determined previously at the human recombinant 5-HT1B (h 5-HT1B) receptor under similar experimental conditions. 2. Membrane preparations of C6-glial/gp 5-HT1B cells exhibited [3H]-5-carboxamidotryptamine (5-CT) and [3H]-N-[4-methoxy-3,4-methylpiperazin-1-yl) phenyl]-3-methyl-4-(4-pyridinyl)benzamide (GR 125743) binding sites with a pKd of 9.62 to 9.85 and a Bmax between 2.1 to 6.4 fmol mg(-1) protein. The binding affinities of a series of 5-HT receptor ligands determined with [3H]-5-CT and [3H]-GR 125743 were similar. Ligand affinities were comparable to and correlated (r2: 0.74, P<0.001) with those determined at the recombinant h 5-HT1B receptor. 3. [35S]-GTPgammaS binding to membrane preparations of C6-glial/gp 5-HT1B cells was stimulated by the 5-HT receptor agonists that were being investigated. The maximal responses of naratriptan, zolmitriptan, sumatriptan, N-methyl-3-[pyrrolidin-2(R)-ylmethyl]-1H-indol-5-ylmethyl sulphonamide (CP 122638), rizatriptan and dihydroergotamine were between 0.76 and 0.85 compared to 5-HT. The potency of these agonists showed a positive correlation (r2: 0.72, P=0.015) with their potency at the recombinant h 5-HT1B receptor. 1-naphthylpiperazine, (+/-)-cyanopindolol and (2'-methyl-4'-(5-methyl[1,2,4] oxadiazole-3-yl)biphenyl-4-carboxylic acid [4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]amide (GR 127935) elicited an even smaller response (Emax: 0.32 to 0.63). 4. The ligands 1'-methyl-5-(2'-methyl-4'-(5-methyl-1,2,4-oxadiazole-3-yl) biphenyl-4-carbonyl)-2,3,6,7tetrahydrospiro [furo[2,3-f]indole-3-spiro-4'-piperidine] (SB224289), methiothepin and ritanserin displayed inhibition of basal [35S]-GTPgammaS binding at concentrations relevant to their binding affinity for the gp 5-HT1B receptor. Methiothepin and SB224289 behaved as competitive antagonists at gp 5-HT1B receptors; pA2 values were 9.74 and 8.73, respectively when 5-HT was used as an agonist. These estimates accorded with the potencies measured in antagonism of zolmitriptan-mediated inhibition of forskolin-stimulated cyclic AMP formation. Ketanserin acted as a weak antagonist (pK(B): 5.87) at gp 5-HT1B receptors. 5. In conclusion, the recombinant gp 5-HT1B receptor shares important pharmacological similarities with the recombinant h 5-HT1B receptor. The finding that negative activity occurs at these receptors further suggests that SB224289, methiothepin and ritanserin are likely to be inverse agonists.

Stimulated [35S]GTP gamma S binding by 5-HT1A receptor agonists in recombinant cell lines. Modulation of apparent efficacy by G-protein activation state

G-protein activation by different 5-HT receptor ligands was investigated in h5-HT1A receptor-transfected C6-glial and HeLa cells using agonist-stimulated [35S]-GTP gamma S binding to membranes in the presence of excess GDP. 5-HT (10 microM) stimulated [35S]GTP gamma S binding in the C6-glial membrane preparation to a larger extent than in the HeLa preparation; maximal responses with 30 microM GDP were 490 +/- 99 and 68 +/- 12%, respectively. With the 5-HT receptor agonists that were being investigated, the two preparations displayed the same rank order of potency for stimulation of [35S]GTP gamma S binding. In the C6-glial preparation at 0.3 microM GDP, the rank order of maximal effects was: 5-HT (1.00) > 8-OH-DPAT (0.90) = R(+)-8-OH-DPAT (0.87) = 5-CT (0.86) = L694247 (0.84) > S(-)8-OH-DPAT (0.68) = buspirone (0.67) = spiroxatrine (0.67) = flesinoxan (0.64) > ipsapirone (0.53) = (-)-pindolol (0.50) > SDZ216525 (0.25). However, differences in maximal response in the C6-glial preparation were magnified by increasing the GDP concentrations, indicating that the activity state of G-proteins can affect the maximal response. With the exception of 5-CT and L694247, increasing the amount of GDP to 30 microM and higher concentrations resulted in an attenuation of both the ligand's maximal effect (24 to 56%) and apparent potency (6 to 24-fold). Each of the [35S]GTP gamma S binding responses was mediated by a 5-HT1A receptor as indicated by the competitive blockade by WAY100635 and spiperone. Only 5-CT and L694247 in some conditions displayed an efficacy similar to that of 5-HT at the h5-HT1A receptor; the other agents with intrinsic activity are partial agonists at this receptor. The data also suggest that the activity state of the G-proteins is involved in the maximal effects that can be produced by activating the h5-HT1A receptor.

Evidence for partial agonist properties of daltroban (BM 13,505) at TP receptors in the anaesthetized open-chest rat

We sought to determine whether the intrinsic pulmonary hypertensive activity of the purported thromboxane A2/prostanoid (TP) receptor antagonist, daltroban, was mediated by TP receptors, using the high efficacy TP receptor agonist, U-46619, and the silent TP receptor antagonist, SQ 29,548. In pentobarbitone-anesthetized, open-chest rats (n = 4-10 per group), non-cumulative injections of U-46619, dose-dependently increased mean pulmonary arterial pressure (MPAP) with an ED50 (geometric mean with 95% confidence limits in parentheses) of 1.4 (1.1-2.3) microg/kg i.v.. Daltroban increased MPAP in a bell-shaped manner, with an apparent ED50 [29 (21-35) microg/kg i.v.] being 21 fold less potent than that of U-46619. The maximal pulmonary hypertensive responses evoked by daltroban represented about half those induced by U-46619 (25.4+/-1.0 vs. 12.7+/-2 mmHg; P < 0.05 between groups). The TP receptor antagonist SQ 29,548 fully antagonized increases in MPAP evoked by equihypertensive doses of U-46619 (1.25 microg/kg) or daltroban (80 microg/kg). Further experiments were carried out to determine whether daltroban antagonized the pulmonary hypertensive responses evoked by the high efficacy agonist, U-46619, or by itself as receptor theory would predict for a partial agonist. Daltroban (10-2500 microg/kg) antagonized, although not fully, U-46619 (20 microg/kg)-evoked pulmonary hypertensive responses, since prominent intrinsic pulmonary hypertensive effects of daltroban were observed in the same range of doses. Furthermore, in contrast to U-46619 (1.25 microg/kg), daltroban (80 microg/kg) failed to evoke a second pulmonary hypertensive response following a previous injection, as would be expected for a partial agonist. Collectively, the results strongly suggest that daltroban behaves as a partial agonist at TP receptors in the pulmonary vascular bed of the rat in vivo.

Regulation of metallothionein-III (GIF) mRNA in the brain of patients with Alzheimer disease is not impaired

Contradictory results have been reported on the downregulation and role of the brain-specific protein metallothionein-III (MT-III, GIF) in Alzheimer disease (AD). In this article, the importance of MT-III downregulation in AD brain was re-evaluated in temporal and frontal cortex, hippocampus, and cerebellum of 11 AD patients and two groups of five and six control subjects, respectively. Reverse transcription-polymerase chain reaction (RT-PCR) was used to quantify the levels of MT-III mRNA relative to the levels of three constitutive RNAs: beta-actin, glyceraldehyde-3-phosphate dehydrogenase (G3PDH), and ribosomal RNA 18S (rRNA 18S). The distribution of MT-III was similar to that of each of the three constitutive RNAs. The relative levels of each of these RNAs was high in brain regions examined in both AD patients and control subjects. Our findings do not support a downregulation of MT-III mRNA in the frontal cortex as well as the temporal cortex and hippocampus of AD patients. However, the level of MT-III mRNA was not constant in the investigated samples, suggesting that MT-III mRNA regulation could be controlled by factors other than AD pathology. Brain-derived neurotrophic factor (BDNF) mRNA levels were hardly detectable by RT-PCR in human brain tissue; a trend for a decrease was apparent in the temporal cortex of AD patients. In conclusion, the content of MT-III mRNA in the brain of AD patients was not detectably impaired, whereas BDNF mRNA may be affected.

How efficacious are 5-HT1B/D receptor ligands: an answer from GTP gamma S binding studies with stably transfected C6-glial cell lines

The intrinsic activity of a series of 5-hydroxytryptamine (serotonin, 5-HT) receptor ligands was analysed at recombinant h5-HT1B and h5-HT1D receptor sites using a [35S]GTP gamma S binding assay and membrane preparations of stably transfected C6-glial cell lines. Compounds either stimulated or inhibited [35S]GTP gamma S binding to a membrane preparation containing either h5-HT1B or h5-HT1D receptors. The potencies observed for most of the compounds at the h5-HT1B receptor subtype correlated with their potencies measured by inhibition of stimulated cAMP formation on intact cells. Apparent agonist potencies in the [35S]GTP gamma S binding assay to C6-glial/h5-HT1D membranes were, with the exception of 2-[5-[3-(4-methylsulphonylamino)benzyl-1 2,4-oxadiazol-5-yl]-1H-indol-3-yl] ethanamine (L694247), 5- to 13-times lower than in the cAMP assay on intact cells. This suggests that receptor coupling in the h5-HT1D membrane preparation is less efficient than that in the intact cell. It further appeared that 6-times more h5-HT1D than h5-HT1B binding sites were required to attain a similar, maximal (73%), 5-HT-stimulated [35S]GTP gamma S binding response: Hence, the h5-HT1B receptor in C6-glial cell membranes could be more efficiently coupled, even though some compounds more readily displayed intrinsic activity at h5-HT1D receptor sites [e.g. dihydroergotamine and (2'-methyl-4'-(5-methyl[1,2,4]oxadiazol-3-yl)biphenyl-4-carboxylic acid [4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]amide (GR127935)]. Efficacy differences were apparent for most of the compounds (sumatriptan, zolmitriptan, rizatriptan, N-methyl-3-[pyrrolidin-2(R)-ylmethyl]-1H-indol-5-ylmethyl sulfonamide (CP122638), dihydroergotamine, naratriptan and GR127935) that stimulated [35S]GTP gamma S binding compared to the native agonist 5-HT. The observed maximal responses were different for the h5-HT1B and h5-HT1D receptor subtypes. Few compounds behaved as full agonists: L694247, zolmitriptan and sumatriptan did so at the h5-HT1B receptor and only L694247 at the h5-HT1D receptor. GR127935 (10 microM) exerted little effect on [35S]GTP gamma S binding via h5-HT1B receptors (10% stimulation), but potently (pA2: 9.11) antagonized h5-HT1B receptor-stimulated [35S]GTP gamma S binding. Ketanserin and methiothepin inhibited [35S]GTP gamma S binding (by 13-28%) in the absence of an agonist, but were potent and competitive antagonists in the presence of an agonist via h5-HT1B (methiothepin) and h5-HT1D (methiothepin and ketanserin) receptors. The results document the utility of using [35S]GTP gamma S binding studies to assess agonist efficacy, and to characterize 5-HT1B/D receptor ligands as apparently neutral antagonists and inverse agonists at the G-protein level.

Sequence and functional analysis of cloned guinea pig and rat serotonin 5-HT1D receptors: common pharmacological features within the 5-HT1D receptor subfamily

This study was undertaken to investigate the pharmacology of cloned guinea pig and rat 5-hydroxytryptamine (serotonin; 5-HT)1D receptor sites. Guinea pig, rat, and mouse 5-HT1D receptor genes were cloned, and their amino acid sequences were compared with those of the human, dog, and rabbit. The overall amino acid sequence identity between these 5-HT1D receptors is high and varies between 86 and 99%. The sequence homology is slightly more divergent (13-27%) in the N-terminal extracellular region of these 5-HT1D receptors. Guinea pig and rat 5-HT1D receptors, stably and separately expressed in rat C6 glial cells, are negatively coupled to cyclic AMP formation upon stimulation with agonists, as previously found for cloned human 5-HT1D receptor sites. The cyclic AMP data show some common pharmacological features for the 5-HT1D receptors of guinea pig, rat, and human: an almost similar rank order of potency for the investigated 5-HT1D receptor agonists, stereoselectivity for the binding affinity and agonist potency of R(+)-8-hydroxy-2-(di-n-propylamino)tetralin, and equal 5-HT1D receptor-mediated antagonist potency for methiothepin and the 5-HT2 receptor antagonists ritanserin and ketanserin. In conclusion, the pharmacology of the cloned 5-HT1D receptor subtype seems, unlike the 5-HT1B receptor subtype, conserved among various mammal species such as the human, guinea pig, and rat.

Recombinant saphenous vein 5-HT1B receptors of the rabbit: comparative pharmacology with human 5-HT1B receptors

1. The rabbit recombinant saphenous vein 5-hydroxytryptamine1B (r 5-HT1B) receptor stably transfected in rat C6-glial cells was characterized by measuring adenosine 3':5'-cyclic monophosphate (cycle AMP) formation upon exposure to various 5-HT receptor ligands. The effects of agonists and antagonists were compared with their effects determined previously at the human cloned 5-HT1B (h 5-HT1B) receptor under similar experimental conditions. 2. Intact C6-glial cells expressing rb HT1B receptors exhibited [3H]-5-carboxamidotryptamine (5-CT) binding sites with a Kd of 0.80 +/- 0.13 nM and a Bmax between 225 to 570 fmol mg-1 protein. The binding affinities of a series of 5-HT receptor ligands determined in a membrane preparation with [3H]-5-CT or [3H]-N-[4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]-3-methyl-4-(-4 -pyridyl) benzamide (GR 125,743) were similar. With the exception of ketanserin, ligand affinities were comparable to those determined at the clones h 5-HT1B receptor site. 3. rb 5-HT1B receptors were negatively coupled to cyclic AMP formation upon stimulation with 5-HT agonists. Of the several 5-HT agonists tested, 5-CT was the most potent, the potency rank order being: 5-CT > 5-HT > zolmitriptan > naratriptan > rizatriptan > sumatriptan > R (+)-8-(hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). The maximal responses of these agonists were similar to those induced by 5-HT. The potency of these agonists showed a positive correlation (r2 = 0.87; P < 0.002) with their potency at the cloned h 5-HT1B receptor subtype. 4. 2'-Methyl-4-(5-methyl-[1,2,4]oxadiazol-3-yl)-biphenyl-4-carboxylic acid [4-methoxy-e-(4-methyl-piperazin-1-yl)-phenyl]-amide (GR 127,935), methiothepin and ketanserin each behaved as silent, competitive antagonists at rb 5HT1B receptors; pKB values were 8.41, 8.32 and 7.05, respectively when naratriptan was used as an agonist. These estimates accorded with their binding affinities and the potencies found on 5-HT and/or sumatriptan-mediated contraction of isolated rabbit saphenous vein segments. 5. In conclusion, the recombinant saphenous vein 5-HT1B receptor of the rabbit shares important pharmacological similarities with the cloned h 5-HT1B receptor. However, ketanserin is a more potent antagonist of rb 5-HT1B receptors.

Promotion of cell growth by stimulation of cloned human 5-HT1D receptor sites in transfected C6-glial cells is highly sensitive to intrinsic activity at 5-HT1D receptors

5-Hydroxytryptamine (serotonin, 5-HT), essentially known as a neurotransmitter and vasoactive agent, also functions as a mitogen in various cell types through several different second messenger systems. Stimulation of cloned human 5-HT1D receptor sites by sumatriptan in stably transfected rat C6-glial/5-HT1D cells promotes cell growth (Pauwels et al. (1996) Naunyn-Schmiedeberg's Arch Pharmacol 353:144-156). In the present study, the pharmacology of this growth response was investigated using a broad series of 5-HT receptor ligands. The data were compared with the responses obtained by measuring inhibition of forskolin-stimulated cAMP formation. 5-HT (EC50: 25 nM) promoted cell growth of C6-glial/5HT1D cells, and this in contrast to the absence of any measurable effect in pcDNA3-plasmid transfected and non-transfected C6-glial cells. The 5-HT effect could be mimicked by the following compounds (EC50 in nM): zolmitriptan (0.41), 2'-methyl-4'-(-methyl[1,2,4] oxadiazol-3-yl)biphenyl-4-carboxylic acid [4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]amid (GR 127,935;0.86), naratriptan (0.92), metergoline (1.9), sumatriptan (2.9), (N,N-dimethyl-2-[5-(1,2,4-triazol-1-ylmethyl)-1H-indol-3-yl] ethylamine (MK-462; 3.0), and R(+)-8-(hydroxy-2-(di-n-propylamino)tetralin (R(+)-8-OH-DPAT; 30.7). These EC50 -values correspond to the compounds binding affinities at the human 5-HT1D receptor site and, with the exception of GR 127,935 and metergoline also to the EC50-values found by measuring over 5 min inhibition of forskolin (100 microM)-stimulated cAMP formation. Prolonged exposure of GR 127,935(3 h) and metergoline (30 min) to cells yielded EC50 values in the cAMP assay more close to those measured in the mitogenic response. The growth response to sumatriptan, 5-HT, GR 127,935 and metergoline was blocked by the apparently silent antagonist methiothepin, ritanserin and ketanserin with potencies similar to blockade of inhibition of stimulated cAMP formation. The 8-OH-DPAT effect also is likely mediated by 5-HT1D receptors; stereoselectivity was found with its enantiomers at this receptor site and the effect was blocked by ketanserin (1 microM) but not by spiperone (1 microM). Micromolar concentrations of the 5-HT1B receptor agonist 3-(1,2,5,6-tetrahydro)-4-pyridil-5-pyrrolo[3,2-b]pyril-5-one (CP 93,129) and of the 5-HT2 receptor agonist 1-(2,5-dimethoxy-4-iodophenyl_-2-aminopropane (DOI) induced cell growth with a potency that accorded with the affinity of these compounds for the human 5-HT1D receptor site. These effects were sensitive to ketanserin (1 microM) antagonism, but not to blockade by beta-adrenergic blockers and the 5-HT2 receptor antagonist 2-anilino-N-[2-(3-chlorophenoxy)-propyl] acetamidine hydroiodide (BW 501-C-67). The findings suggest that 5-HT1A, 5-HT1B and 5-HT2 receptors are not implicated in 5-HT-stimulated C6-glial/5-HT1D cell growth. In conclusion, human 5-HT1D receptors are involved in the growth of C6-glial/5-HT1D cells. This cellular response is highly sensitive to the intrinsic activity of compounds at 5-HT1D receptors.

Stereoselectivity of 8-OH-DPAT enantiomers at cloned human 5-HT1D receptor sites

The cAMP responses of (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and its enantiomers were measured at cloned human 5-HT1D alpha and 5-HT1D beta receptors in transfected C6-glial cells. R(+)-8-OH-DPAT demonstrated potent intrinsic activity (EC50 value: 30 nM) at 5-HT1D alpha receptor sites, its maximal effect being comparable to that of sumatriptan. Racemic 8-OH-DPAT and S(-)-8-OH-DPAT showed similar agonist efficacy but were respectively 2 and 75 times less potent than R(+)-8-)OH-DPAT. This differs from the lack of stereoselectivity of the 8-OH-DPAT enantiomers for 5-HT1A receptors.

Selective antagonism of human 5-HT1D and 5-HT1B receptor-mediated responses in stably transfected C6-glial cells by ketanserin and GR 127,935

The antagonist effects of ketanserin and 2'-methyl-4'-(5-methyl-1,2,4)oxadiazol-3-yl)-biphenyl-[4-carboxyli c acid 4-methoxy-3-(4-methyl-piperazin-1-yl)-phenyl]-amide (GR 127,935) were compared to naratriptan-induced inhibition of cAMP formation in C6-glial cell lines stably expressing human 5-HT1D or 5-HT1B receptor sites. Ketanserin demonstrated potent (pA2: 7.76), competitive antagonism of naratriptan-induced inhibition of forskolin (100 microM)-stimulated cAMP formation in C6-glial/5-HT1D cells. Whereas GR 127,935 was ineffective as an antagonist in these cells, it produced and intrinsic activity (pEC50: 6.98) that was sensitive to ketanserin (10 microM) blockade. Unlike ketanserin, GR 127,935 potently antagonised the naratriptan response in C6-glial/5-HT1B cells while also depressing the maximum response. The differential antagonist effects of ketanserin and GR 127,935 on naratriptan responses elicited in C6-glial/5-HT1D and C6-glial/5-HT1B cells demonstrate these compounds do selectively block human 5-HT1D and 5-HT1B receptors, respectively.

Pharmacology of cloned human 5-HT1D receptor-mediated functional responses in stably transfected rat C6-glial cell lines: further evidence differentiating human 5-HT1D and 5-HT1B receptors

This study was undertaken to investigate the pharmacology of human serotonin (5-HT)1D receptor sites by measuring two functional cellular responses, inhibition of forskolin-stimulated cAMP formation and promotion of cell growth, using transfected rat C6-glial cell lines and a broad series of 5-HT receptor agonists. Stable and separate transfection of a pcDNA3 or pRcRSV plasmid, each containing a cloned human 5-HT1D receptor gene, in rat C6-glial cells was confirmed with RT-PCR of 5-HT1D receptor mRNA and radioligand binding with [3H] 5-carboxamidotryptamine (5-CT) and [3H] sumatriptan. The 5-HT1D receptor density was 350 and 1050 fmol/mg protein for the C6-glial/pcDNA3/5-HT1D and C6-glial/pRcRSV/5-HT1D cell line, and forskolin (100 microM)-induced cAMP formation was inhibited by 45 and 78% in the presence of 1 microM 5-HT, respectively. A comparison of the intrinsic agonist activities for sixteen 5-HT receptor ligands with their corresponding binding affinities for the human 5-HT1D receptor site showed similar results for both cell lines with the exception of the partial agonist m-trifluoro-phenyl-piperazine (TFMPP). Three classes of compounds were observed: 1) efficacious agonists, such as 5-CT, 5-methoxytryptamine, 5-HT, sumatriptan, bufotenine, 5-methoxy-3(1,2,3,6-tetrahydro-4-pyridinyl)1H-indole (RU 24,969), tryptamine and 8-hydroxy-2(di-n-propilamino)tetralin (8-OH-DPAT), with agonist potency close to their binding affinity; 2) the partial agonists metergoline, 7-trifluoromethyl-4(4-methyl-1-piperazinyl)-pyrolo-(1,2-a) quinoxaline (CGS 12066B), 1-naphthylpiperazine and 2'-methyl-4-(5-methyl-[1,2,4]oxadiazol-3-yl)-biphenyl-4-carboxylic acid [4-methoxy-3-(4-methylpiperazin-1-yl)-phenyl]-amide (GR 127,935) with marked intrinsic agonist activity but at concentrations higher than their binding affinity; and 3) the silent antagonists ritanserin, ketanserin and methiothepin, apparently free of intrinsic agonist activity, with antagonist potency close to their binding affinity. The cAMP data were further supported by the observed promotion of cell growth by stimulation of both transfected cell lines with sumatriptan under serum-free conditions; half-maximal stimulation was obtained at 4.4 nM (C6-glial/pcDNA3/5-HT1D) fully in agreement with its EC50-value (5.7 nM) for inhibition of cAMP formation. This growth promoting effect was antagonised by 1 microM methiothepin and not observed in pcDNA3-plasmid-transfected and non-transfected C6-glial cells. A comparative study with a C6-glial/pcDNA3/5-HT1B cell line expressing a similar amount of cloned human 5-HT1B receptors (Bmax: 360 fmol/mg protein) showed almost no intrinsic agonist activity for metergoline, 1-naphtylpiperazine and GR 127,935. Together with the 5-HT1D receptor binding selectivity and antagonist activity of ketanserin and ritanserin, the findings define important pharmacological differences between cloned human 5-HT1D and 5-HT1B receptor sites.

Stimulation of cloned human serotonin 5-HT1D beta receptor sites in stably transfected C6 glial cells promotes cell growth

The involvement of serotonin 5-HT1D beta receptor sites was investigated in the growth of rat C6 glial cells permanently transfected with a gene encoding a human 5-HT1D beta receptor. The 5-HT receptor identity of control and transfected C6 glial/5-HT1D beta cells was determined by reverse transcription-polymerase chain reaction using primers specific for rat 5-HT1A, rat 5-HT1B, rat 5-HT1D alpha, human 5-HT1D beta, and rat 5-HT2A receptor genes. Constitutive mRNA for 5-HT2A receptors was present in control and transfected C6 glial/5-HT1D beta cells, whereas mRNA for 5-HT1D beta receptor sites was only present in the transfected C6 glial/5-HT1D beta cell line. 5-HT inhibited forskolin-stimulated cyclic AMP formation and promoted cell growth, in contrast to the absence of any measurable effect in pcDNA3 plasmid-transfected and nontransfected C6 glial cells. The 5-HT effects could be mimicked by sumatriptan (EC50 = 44-76 nM) and were totally and partially blocked by methiothepin (IC50 = 9 nM) and GR 127,935 (2'-methyl-4'-(5-methyl[1,2,4]oxadiazol-3-yl)-biphenyl-4-carbox yli c acid [4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]amide; IC50 = 97 pM), respectively. No effect on cell growth was measured with the 5-HT2 receptor agonist DOI [1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane; 10 microM], suggesting that 5-HT2A receptors are not involved in the 5-HT-stimulated C6 glial/5-HT1D beta cell growth. Dibutyryl-cyclic AMP (0.3 mM)-treated cultures did not show sumatriptan-promoted cell growth, indicating an inhibitory role for cyclic AMP in the cell growth mediated by 5-HT1D beta receptor sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Differentiation between partial and silent 5-HT1D beta receptor antagonists using rat C6-glial and Chinese hamster ovary cell lines permanently transfected with a cloned human 5-HT1D beta receptor gene

Intrinsic activities of serotonin (5-HT) receptor ligands at cloned human 5-HT1D beta receptor sites were determined by measuring cAMP responses in two permanently transfected cell types: rat C6-glial and Chinese hamster ovary (CHO)-K1 cells. Both transfected cell lines expressed a similar 5-HT1D beta receptor density (361 to 448 fmol/mg protein) and displayed a number of similar cAMP responses: marked inhibition of forskolin-stimulated cAMP formation by 5-HT; a similar agonist potency and efficacy with 5-carboxamidotryptamine (5-CT), 5-methoxytryptamine, bufotenine, sumatriptan, 7-trifluoromethyl-4(4-methyl-1-piperazinyl)-pyrolo-(1,2-a)quinoxal ine (CGS 12066B), 5-methoxy-3(1,2,3,6-tetrahydro-4-pyridinyl)1H-indole (RU 24,969), and tryptamine, their maximal effect being comparable to that of 5-HT; less agonist efficacy with m-trifluoro-phenyl-piperazine (TFMPP) (it inhibited at most 63% of stimulated cAMP formation); and antagonist activity against the 5-CT-mediated agonist response with methiothepin, 2'-methyl-4-(5-methyl-[1,2,4]oxadiazol-3-yl)-biphenyl-4-carboxylic acid [4-methoxy-3-(4-methyl-piperazin-1-yl)-phenyl]-amide (GR 127,935), and ritanserin. Metergoline and 1-naphtylpiperazine showed different intrinsic activities. In contrast to their pronounced antagonist activity in the transfected CHO-K1 cell line, the antagonist effect was only partial and absent for metergoline and 1-naphtylpiperazine in the transfected C6-glial cell line, respectively. In conclusion, these cell lines are useful as a tool to measure with high sensitivity differences in intrinsic activities of 5-HT receptor ligands and, therefore, discriminate between silent antagonists (no intrinsic activity) and antagonists with intrinsic activity (i.e. partial agonists), even though this intrinsic activity may be relatively weak.

Investigation of the mechanism by which ketanserin prolongs the duration of the cardiac action potential

Action potential duration (APD) lengthening is believed to underlie the cardiac arrhythmogenicity of ketanserin, a serotonin (5-HT)2A/2C receptor antagonist. We wished to determine (a) whether this activity involves blockade of 5-HT2A/2C receptors and (b) the precise mechanism of ketanserin-induced APD prolongation. APs were recorded in guinea pig isolated papillary muscles by conventional "floating" microelectrodes, and potassium currents in guinea pig isolated myocytes were recorded in the whole-cell configuration. Ketanserin (1-10 microM) increased APD (EC50 value for enhancing APD at 90% repolarization (APD90) 3.1 +/- 2.7 microM, n = 24), without affecting resting potential, maximum upstroke velocity (Vmax) or AP amplitude (APA). Pirenperone (10 microM), a ketanserin congener, similarly increased APD90 from 204 +/- 3 to 241 +/- 7 ms (p < 0.001, n = 6). No increase in APD was observed, however, with ritanserin or ICI 170809, even at high concentrations (10 microM, n = 6, respectively), two 5-HT2A/2C receptor antagonists chemically distinct from ketanserin, thereby excluding the involvement of 5-HT2A/2C receptors in mediating APD lengthening. That APD prolongation was mediated specifically by the benzolyl-piperidine moiety of ketanserin and pirenperone was confirmed by 1-propyl-4(4-fluorobenzoyl)piperidine (PFBP), which evoked APD lengthening effects remarkably similar to those produced by ketanserin and pirenperone (EC50 3.73 +/- 2.6 microM, n = 12). In isolated cardiomyocytes, ketanserin (1-32 microM) selectively and concentration-dependently reduced the IKr component of the delayed outward current (IK) without affecting the inward rectifier current, IK1. Thus, ketanserin (32 microM) significantly reduced IK at a potential value of -20 mV from 813 +/- 65 to 569 +/- 55 pA (p < 0.001, n = 6), whereas at a potential value of -110 mV, IK1 was not significantly affected (730 +/- 103 vs. 603 +/- 143 pA, respectively; n=6). The results demonstrate that APD is prolonged by ketanserin and congeners but not be chemically different 5-HT2A/2C receptor antagonists. The benzoyl-piperidine moiety appears to mediate the APD-prolonging effects of ketanserin and pirenperone specifically. Furthermore, ketanserin-induced APD lengthening does not appear to involve 5-HT2A/2C receptors but is consecutive to direct blockade of myocardial potassium channels.

Empirical evidence that the state dependence and drug discrimination paradigms can generate different outcomes

The study compared the outcomes generated by the State Dependence and Drug Discrimination paradigms with ethanol in the rat. Food-deprived rats learned to complete a fixed-ratio 10 schedule of bar presses for food within 120 s while treated with 320- to 1250-mg/kg doses of ethanol. Subsequent tests of recall of this response with saline failed to generate any evidence that transfer was hampered following the drug-to-saline state change. In contrast, each of 14 rats learned to discriminate 1250 mg/kg ethanol from saline in a Drug Discrimination procedure that also required the animals to press one of two levers for food according to a fixed- ratio 10 schedule. The results offer the first empirical evidence to demonstrate directly that the State Dependence and Drug Discrimination paradigms can generate different outcomes in otherwise identical experimental conditions.

Use of the pithed rat model to determine the relative potencies of 5-HT2A/2C receptor antagonists following acute intravenous and oral administration

We investigated the usefulness of the pithed rat model to determine the relative potencies of 5-HT2A/2C receptor antagonists following acute intravenous and oral administration in inhibiting 5-HT-induced pressor responses. The 5-HT2A/2C receptor antagonists, pirenperone, ketanserin, and ritanserin, all dose-dependently inhibited 5-HT-induced pressor responses when administered either intravenously 5 min prior to commencing 5-HT injections or acutely via the oral route 30 min before pithing. All three compounds were found to be of comparable potency in inhibiting 5-HT-induced pressor responses when given i.v. (range of ID50s 3.7-27.5 micrograms/kg), whereas substantial differences became apparent after oral administration (range of ID50s 13.0-214.0 micrograms/kg), as reflected also by the p.o./i.v. ID50 ratios for the drugs. In conclusion, inhibition of 5-HT-induced pressor responses in the pithed rat preparation provides a reproducible and robust procedure in which comparisons can be made of relative potencies of 5-HT2A/2C receptor antagonists, not only by the intravenous route, but also following acute oral administration.

Intrinsic activity of the non-prostanoid thromboxane A2 receptor antagonist, daltroban (BM 13,505), in human platelets in vitro and in the rat vasculature in vivo

1. We evaluated the effects of daltroban on (i) human platelet shape change and aggregation in vitro, and (ii) mean systemic and pulmonary arterial pressures (MAP and MPAP, respectively) as well as haematocrit, in anaesthetized, open-chest Sprague-Dawley rats, compared with those of a chemically distinct prostanoid thromboxane A2 (TxA2) receptor antagonist, SQ 29,548, and agonist, U-46619. 2. In human platelets in vitro, daltroban (10 nM-100 microM; n = 6 per group) concentration-dependently induced shape change, attaining at 50 microM, a maximum amplitude of 0.83 +/- 0.09 mV representing 46.4 +/- 4.8% of that evoked by U-46619 (1.78 +/- 0.20 mV at 0.2 microM; n = 9); and inhibited U-46619-induced platelet aggregation with an IC50 of 77 (41-161)nM. SQ 29,548 (10 nM-100 microM; n = 6 per group) failed to evoke any platelet shape change, but potently inhibited U-46619-induced platelet aggregation with an IC50 < 10 nM. 3. In anaesthetized rats in vivo, daltroban (10-2500 micrograms kg-1, i.v. infused over 2 min; n = 4-8 per group) produced a bell-shaped dose-response curve for MPAP and haematocrit, and evoked maximal increases of 12.7 +/- 2.1 mmHg and 5.8 +/- 1.5% at 80 micrograms kg-1 (n = 6) and 630 micrograms kg-1 (n = 8), respectively (both P < 0.05) with ED50s of 20 (16-29) and 217 (129-331) micrograms kg-1, respectively. By comparison, U-46619(0.16-20 microg kg-1, i.v.), induced dose-dependent increases in MPAP and haematocrit (25.4 +/- 1.0 mmHg and 16.1 +/- 2.9% at the highest dose; n = 12, both P<0.01), with ED50s of 1.8 (1.3-2.5) and 3.9(3.5- 5.4) microg kg- 1, respectively. Daltroban dose-dependently increased MAP with a maximum amplitude of 42.2 +/- 4.4 mmHg at a dose of 80 microg kg-1 [ED50 = 94 (64-125) microg kg-1], similar to that induced by U-46619 (41.3 +/- 9.6 mmHg) at a dose of 0.63 microg kg-1 [ED50= 0.22 (0.13-0.24) microg kg-1]. SQ 29,548(10-2500 microg kg-1, i.v.; n =4 per group) failed to modify significantly any of these parameters.4. Our results clearly demonstrate that daltroban, in a similar manner to the TxA2 analogue, U-46619,but unlike the TxA2 receptor antagonist, SQ 29,548, exhibits significant intrinsic activity in human platelets in vitro and in the rat vasculature in vivo, possibly through TxA2 receptor activation.

The 5-HT1D receptor antagonist GR 127,935 is an agonist at cloned human 5-HT1D alpha receptor sites

The cAMP response of the 5-HT1D receptor antagonist GR 127,935 was compared with 5-CT and ketanserin at cloned human 5-HT1D alpha receptor sites in transfected C6-glial cells. GR 127,935 showed marked agonist activity (EC50-value: 141 nM), its maximal effect being comparable to that of the agonist 5-CT (EC50-value: 0.91 nM), unlike the apparently silent antagonist ketanserin (KB-value: 34 nM).

Ketanserin inhibits the transient outward current in rabbit ventricular myocytes

The effects of ketanserin on the calcium-independent component of the transient outward potassium current (Ito) was investigated in isolated rabbit ventricular myocytes using the whole-cell patch-clamp technique. Ketanserin concentration-dependently (0.1-100 microM) reduced Ito (IC50 2.25 +/- 2.2 microM; Hill coefficient, nH 0.91), and decreased both the fast and slow Ito inactivation time constants so that the decay of the peak outward current was significantly accelerated. Our results indicate that, in addition to its antagonist activity on 5HT2A/C receptors, ketanserin reduces the calcium-independent component of Ito. Blockade of Ito could explain the proarrhythmic properties of ketanserin in vivo.

Can the DRL 72s schedule selectively reveal antidepressant drug activity?

The effects of three antidepressants, desipramine (2.5-20 mg/kg) tranylcypromine (0.63-2.5 mg/kg) mianserin (1.25-10 mg/kg) and three non-antidepressants, chlordiazepoxide (CDP; 1.25-10 mg/kg) haloperidol (0.02-0.16 mg/kg) d-amphetamine (0.31-1.25 mg/kg) were evaluated in rats responding for water reinforcement under a DRL 72s schedule. The antidepressants all produced dose-related decreases in overall response rates, but no significant changes in reinforcement frequency. In contrast, the anxiolytic CDP did increase the number of reinforcers obtained. Haloperidol decreased both reinforcers and responses whilst d-amphetamine stimulated responding, thereby decreasing reinforcement frequency. An analysis of the modes of inter-response times (IRTs) revealed no significant shifts in the peaks of the IRT distributions for most of the drugs tested. Amphetamine, however, (0.31 and 0.63 mg/kg) decreased the modal values in correspondence with the shift to the left of the peak of responding caused by this compound. These results are discussed in the context of the use of the DRL 72s procedure as a screening test for antidepressant drugs.

Inhibition of methylphenidate-induced behaviors in rats: differences among neuroleptics

Methylphenidate (MPD) increased in rats the incidence of sniffing, rearing and locomotion, and this along dose-response curves that had an inverted U-shape; at 40 mg/kg, MPD exclusively induced stereotyped gnawing, which was inhibited by neuroleptics. However, as gnawing induced by 40 mg/kg MPD was inhibited in a dose-dependent manner, other responses (i.e., sniffing, rearing and locomotion) appeared. Higher doses of neuroleptics also inhibited these latter responses, so that the behavior of the MPD-treated animals became similar to that of normal controls. Only some nonsedative neuroleptics appeared able to normalize the behavior of MPD-treated rats at doses that induced neither complete behavioral suppression nor adverse effects. The neuroleptics differed markedly, however, in terms of the relative doses at which they 1) inhibited gnawing, 2) inhibited the other effects of MPD and 3) induced complete behavioral suppression and/or adverse effects. This variation among neuroleptics, which appears to represent a novel aspect of their ability to antagonize behavioral effects of central nervous system stimulants, may be based on differences in the extent to which they exert agonist activity at dopamine receptors. Assuming that MPD-induced behaviors model the positive symptoms of schizophrenia, it may be hypothesized that the differences among the neuroleptics observed here provide an indication of their efficacy to reduce positive symptoms in schizophrenics.