5-Hydroxytryptamine-stimulated inositol phospholipid hydrolysis in rat cerebral cortex slices: pharmacological characterization and effects of antidepressants

The ability of 5-hydroxytryptamine (5-HT) and related agonists to stimulate hydrolysis of inositol phospholipids was examined in rat cerebral cortex slices using a direct assay which involves prelabeling with [3H]inositol and assaying [3H]inositol phosphates in the presence of lithium. 5-HT agonists stimulated [3H]inositol phosphate accumulation in a dose-related but biphasic manner and only the high-affinity component of the dose-response curve was sensitive to antagonists. This response to 5-HT was blocked potently by ketanserin and other putative 5-HT2 antagonists but the overall pattern of apparent drug affinities was inconsistent with that seen at 5-HT2 sites labeled with [3H]ketanserin in cortical membranes. Pretreatment of slices with the alkylating antagonist phenoxybenzamine reduced the inositol phospholipid response to 5-HT to a greater extent than the suppression of [3H]ketanserin binding. Similarly, chronic but not acute treatment of rats with the antidepressants iprindole and imipramine resulted in a greater loss of 5-HT-induced inositol phospholipid hydrolysis than specific [3H]ketanserin binding. However, the effect of antidepressants was agonist-specific in that neither alpha-1 adrenoceptor nor muscarinic receptor stimulation were altered by acute or chronic treatment.     

Developmental changes in muscarinic receptor-stimulated phosphoinositide metabolism in rat brain

Muscarinic receptor-stimulated phosphoinositide hydrolysis was investigated in rat brain during ontogeny by measuring the accumulation of [3H]inositol phosphates ([3H]InsPs) in cerebral cortex slices at various ages. Experiments with carbachol and acetylcholine showed that [3H]InsPs accumulation was maximal in 7-day-old rats (1477 +/- 98% of basal) and lowest in adult (75 days) rats (428 +/- 24% of basal). No differences were found in the EC50 values for both cholinergic agonists. This effect appeared to be mediated by the M1-muscarinic receptor subtype as it was blocked by pirenzepine with Ki = 29.1 +/- 7.1 nM (adults) and 87.9 +/- 18.2 nM (7-day-old rats). Incorporation of [3H]inositol into phospholipid decreased from day 3 to adulthood; however, when data of [3H]InsPs release were corrected for the incorporation at a given age, the highest stimulation by cholinergic agonists was still observed in 7-day-old rats. Among the other neurotransmitters tested (norepinephrine, histamine and serotonin), all known to stimulate phosphoinositide metabolism, none had the same developmental profile of [3H]InsPs accumulation as cholinergic agonists. In contrast to carbachol- and acetylcholine-stimulated phosphoinositide hydrolysis, the density of muscarinic binding sites, measured by [3H]quinuclidinyl benzilate binding, increased from day 3 to day 75. Acetylcholinesterase activity also increased during development. The dissociation of receptor binding sites from receptor-stimulated phosphoinositide metabolism suggests the presence of a more effective receptor-effector coupling at specific times of neonatal development, particularly 1 week. Furthermore, the fact that maximal stimulation of phosphoinositide hydrolysis coincides with the period of brain growth spurt in the rats suggests that this system in the cerebral cortex might be involved in the processes of cell division and differentiation.     

Characterization of alpha-1 adrenergic receptors linked to [3H]inositol metabolism in rat cerebral cortex

The properties of alpha-1 adrenergic receptors in rat cerebral cortex were examined by measuring increases in [3H]inositol metabolism in brain slices. Slices of rat cerebral cortex were incubated in the presence of 0.23 microM [3H]inositol in Krebs-Ringer-bicarbonate buffer containing 10 mM lithium chloride, and the production of water-soluble [3H]inositol phosphates was monitored after extraction and anion-exchange chromatography. Norepinephrine caused a 4- to 6-fold increase in [3H]inositol metabolism in cerebral cortical slices, and this response was blocked much more potently by the alpha-1-selective antagonist prazosin than by the alpha-2-selective antagonist yohimbine. Epinephrine and norepinephrine were both full agonists and stimulated [3H]inositol metabolism to the same extent in this system. The synthetic drugs phenylephrine and methoxamine were both partial agonists at these receptors, with intrinsic activities only 56 to 58% of epinephrine and norepinephrine. A variety of imidazoline and other partial agonists caused no measurable stimulation of [3H]inositol metabolism in this preparation. The response to norepinephrine was completely blocked by alpha adrenergic receptor antagonists with the potency order prazosin greater than BE 2254 greater than indoramin = phentolamine greater than azapetine greater than piperoxan greater than yohimbine. In the absence of calcium, basal [3H]inositol metabolism was increased, but norepinephrine caused the same 5-fold stimulation as in the presence of 2.5 mM CaCl2. The potencies of both antagonists and agonists in inhibiting or activating [3H]inositol metabolism in slices of rat cerebral cortex were highly correlated with their ability to displace the alpha-1 adrenergic receptor selective radioligand [125I]BE 2254 from specific binding sites in membrane preparations of rat cerebral cortex.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mu and delta opioid receptors inhibit serotonin release in rat hippocampus

We studied the influence of opioid agonists on the release of serotonin (5-HT) elicited by K+ (20 mM) in superfused slices of rat hippocampus. K+-evoked outflow of serotonin was inhibited significantly up to 50% in the presence of the mu-selective agonist [D-Ala2,N-methyl-Phe4,Gly5-ol]enkephalin (DAGO) and of the delta-selective agonist [D-Pen2,D-Pen5]enkephalin (DPDPE). U50,488H a selective kappa agonist, at concentrations between 0.1 to 1 microM, produced an inhibition of 5-HT-release lower than that observed in the presence of mu and delta agonists. The delta antagonist ICI 174,864 (N,N-diallyl-Tyr1,Aib2,Aib3)Leu-enkephalin potently inhibited the effect of DPDPE but did not affect the inhibition produced by DAGO. In contrast, the mu-selective antagonist D-Phe-Cys-Tyr-D-Trp-Nle-Thr-Pen-Thr-NH2 at 1 microM significantly reversed the inhibitory effect produced by a maximal dose of DAGO (0.1 microM) but not the corresponding effect produced by a maximal dose of DPDPE (1 microM). Naloxone was a competitive antagonist of DAGO but noncompetitive antagonist of DPDPE. Treatment of hippocampal slices with pertussis toxin did not alter the K+-evoked release of 5-HT but abolished the inhibitory effect of both DAGO and DPDPE.     

Characterization of cholinergic muscarinic receptor-stimulated phosphoinositide metabolism in brain from immature rats

Hydrolysis of phosphoinositides elicited by stimulation of cholinergic muscarinic receptors has been studied in brain from neonatal (7-day-old) rats in order to determine: 1) whether the neonatal rat could provide a good model system to study this signal-transduction pathway; and 2) whether potential differences with adult nerve tissue would explain the differential, age-related effects of cholinergic agonists. Accumulation of [3H] inositol phosphates in [3H]inositol prelabeled slices from neonatal and adult rats was measured as an index of phosphoinositide metabolism. Full (acetylcholine, methacholine, carbachol) and partial (oxotremorine, bethanechol) agonists had qualitatively similar, albeit quantitatively different, effects in neonatal and adult rats. Atropine and pirenzepine effectively blocked the carbachol-induced response with inhibition constants of 1.2 and 20.7 nM, respectively. In all brain areas, response to all agonists was higher in neonatal than adult rats, and in hippocampus and cerebral cortex the response was higher than in cerebellum or brainstem. The relative intrinsic activity of partial agonists was higher in the latter two areas (0.6-0.7) than in the former two (0.3-0.4). Carbachol-stimulated phosphoinositide metabolism in brain areas correlated well with the binding of [3H]QNB (r2 = 0.627) and, particularly, with [3H]pirenzepine (r2 = 0.911). In cerebral cortex the effect of carbachol was additive to that of norepinephrine and glutamate. The presence of calcium (250-500 microM) was necessary for maximal response to carbachol to be elicited; the EC50 value for Ca2+ was 65.4 microM. Addition of EDTA completely abolished the response. Removal of sodium ions from the incubation medium reduced the response to carbachol by 50%.(ABSTRACT TRUNCATED AT 250 WORDS)     

Relative efficacies of piperazines at the phosphoinositide hydrolysis-linked serotonergic (5-HT-2 and 5-HT-1c) receptors

Serotonin (5-HT)-stimulated phosphoinositide hydrolysis is mediated by the 5-HT-2 receptor in rat cerebral cortex and by the 5-HT-1c receptor in rat choroid plexus. These systems were used to determine relative efficacies of piperazine derivatives at the 5-HT-2 and 5-HT-1c receptors. Both quipazine and 6-chloro-2-[1-piperazinyl]-pyrazine (MK-212) stimulated phosphoinositide hydrolysis in cerebral cortex, and these effects were blocked by ketanserin. The maximum responses to these agonists were 80% of the maximum response to 5-HT. m-Trifluoromethylphenylpiperazine (TFMPP), m-chlorophenylpiperazine (MCPP) and 1-(1-naphthyl)-piperazine (1-NP) did not stimulate phosphoinositide hydrolysis in cerebral cortex at concentrations that blocked the effect of 5-HT. In the choroid plexus, TFMPP and MCPP, as well as MK-212 and quipazine, increased phosphoinositide hydrolysis and mianserin blocked these effects. MK-212 had an efficacy which was equal to that of 5-HT, whereas quipazine, MCPP and TFMPP were partial agonists in the choroid plexus. 1-NP did not stimulate phosphoinositide hydrolysis in choroid plexus but completely blocked the effect 5-HT. On the basis of these data, we conclude that quipazine and MK-212 are partial agonists at 5-HT-2 receptors in cerebral cortex, whereas 1-NP, TFMPP and MCPP are pure antagonists of the cortical 5-HT-2 receptor. However, TFMPP and MCPP as well as quipazine and MK-212 are agonists at the 5-HT-1c receptor, while 1-NP is a pure antagonist of the 5-HT-1c receptor in choroid plexus.     

Pharmacological evidence for a functional serotonin-2B receptor in a human uterine smooth muscle cell line

The present study investigated the serotonin-induced increase in phosphoinositide hydrolysis and mobilization of intracellular Ca2+ ([Ca2+]i) in human uterine smooth muscle cells (HUSMCs) to identify the serotonergic receptor positively coupled to phospholipase C in these cells. In phosphoinositide (PI) assays, serotonin (5-HT) and alpha-methyl-5-HT were potent, full agonists (EC50 = 20 and 4.1 nM, respectively), whereas the phenylethylamine, R-(-)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane hydrochloride, was less active (EC50 = 63 nM). Proposed 5-HT2B-selective agonists, BW-723C86 [alpha-methyl-5-(2-thienylmethoxy)-1H-indole-3-ethanamine hydrochloride] and (+)-norfenfluramine, exhibited strong agonist potency and efficacy comparable with 5-HT (EC50 = 18 and 33 nM, respectively) and approximately 15-fold more potency than (-)-norfenfluramine (EC50 = 500 nM). 5-HT2C receptor agonists m-chlorophenylpiperazine and MK-212 [6-chloro-2-(1-piperaxinyl)pyrazine] were weak agonists in these cells, with potencies of 110 and 880 nM, respectively. A similar rank order of potency was observed in [Ca2+]i mobilization assays (r = 0.9, p < 0.005) in the HUSMC and with contraction of rat stomach fundus strips that contain a 5-HT2B receptor (r = 0.9, p < 0.001). Antagonist studies revealed that a 5-HT2B-selective antagonist, RS-127445 [2-amino-4-(4-fluoronaphth-1-yl)-6-isopropylpyrimidine] (Ki = 0.13 nM), was significantly more effective at inhibiting 5-HT-induced activity than a 5-HT2A antagonist, M-100907 (R-(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenyl)ethyl]-4-piperidinemethanol]) (Ki= 914 nM) and the 5-HT2C antagonists RS-102221 (8-[5-(2,4-dimethoxy-5-(4-trifluoromethylsulfo-amido)phenyl-5-oxopentyl]-1,3,8-triazaspiro[4.5]decane-2,4-dione hydrochloride) (Ki = 2.5 microM) and SB-242084 (6-chloro-5-methyl-1-[6-92-methylpyridin-3-yloxy) pyridine-3-ylcarbamoyl] indoline) (Ki = 42.4 nM) in the HUSMC PI turnover assays. Taken together, these studies strongly suggest the presence of a functionally active 5-HT2B receptor subtype in HUSMCs. The physiological role of this receptor in these cells remains to be defined.     

Characterization of serotonergic receptors mediating contraction of ovine umbilical artery

Responses to serotonergic agonists were studied in isolated umbilical arteries obtained from fetal lambs within 2 weeks of term. The order of potency of the agonists was determined to be 2,5-dimethoxy-4-methyl-amphetamine (DOM) greater than 5-hydroxytryptamine (5-HT) greater than alpha-methyl-5-HT greater than 1-(3-chlorophenyl) piperazine = m-trifluoromethyl-phenylpiperazine greater than 8-hydroxy-dipropylaminotetralin greater than 2-methyl-5-HT greater than 1-(2-methoxyphenyl) piperazine. Variations in the sensitivity and potency of the agonists results primarily from the variation in the affinity for the 5-HT2 receptor and less so in the efficacy, alpha-Methyl-5-HT was a full agonist compared to 5-HT. The others were partial agonists. The mean KA values for 5-HT and DOM were 4.71 +/- 0.62 x 10(-7) and 0.36 +/- 0.04 x 10(-7) M, respectively. Contractions to 5-HT and DOM were antagonized by ketanserin with pA2 values being 9.4 and 9.1, respectively, suggesting that they act on the same receptor and that their responses are mediated by 5-HT2 receptors. Contractile responses to 8-hydroxy-dipropylaminotetralin, 2-methyl-5-HT and the phenylpiperazines [m-trifluoromethyl-phenylpiperazine and 1-(3-chlorophenyl) piperazine] were also blocked by ketanserin (10(-8) M), indicating that contractions produced by these agonists were mediated by 5-HT2 receptors. No antagonism by MDL 72222 (3-tropanyl-3,5-dichlorobenzoate) of responses to 5-HT indicates that 5-HT3 receptors are not present in this tissue.     

Characterization of the contractile serotonergic receptor in guinea pig trachea with agonists and antagonists.

5-Hydroxytryptamine (5-HT)2 receptors can be partially characterized by their sensitivity to ketanserin blockade and increase in phosphoinositide turnover upon stimulation. Previously, the contraction of guinea pig trachea to 5-HT was shown to be antagonized by the 5-HT2 receptor antagonists ketanserin and LY53857. However, 5-HT did not dramatically increase phosphoinositide turnover in guinea pig trachea, suggesting that the contractile receptor may be different from the classically defined 5-HT2 receptor. The present in vitro studies better characterize this receptor, using diverse serotonergic agonists and antagonists to profile in more detail the contractile serotonergic receptor in guinea pig trachea. With regard to agonists, the 5-HT2 receptor agonists DOI and alpha-methyl-5-HT contracted guinea pig trachea with greater potency than quipazine, 5-methoxytryptamine, 5-carboxamidotryptamine, 8-hydroxy-2-(di-N-propylamino)tetralin and 2-methyl-5-HT. Sumatriptan and 1-(3-chlorophenyl)-piperazine (10 nM-100 microM) were inactive as agonists. A strong correlation between agonist potency (EC50) and reported 5-HT receptor binding affinities was found for both the 5-HT1C (r = 0.890) and 5-HT2 (r = 0.831) receptor. Ketanserin, spiperone, ritanserin, LY53857, 1-napthylpiperazine, 1-(3-chlorophenyl)-piperazine, rauwolscine, ICS 205-930, cyanopindolol and sumatriptan all blocked 5-HT-induced contractions in guinea pig trachea. As occurred with agonist potencies, strong correlations were found between reported 5-HT1C (r = 0.814) and 5-HT2 (r = 0.912) receptor binding affinities in brain membranes and apparent dissociation constants (KB) for the 10 antagonists of 5-HT induced contraction in guinea pig trachea.(ABSTRACT TRUNCATED AT 250 WORDS)     

Release of endogenous dopamine by stimulation of 5-hydroxytryptamine3 receptors in rat striatum

5-Hydroxytryptamine (5-HT) caused a persistent, concentration-dependent increase of spontaneous release of endogenous dopamine (DA) from superfused rat striatal slices. 2-Methyl-5-HT, a selective 5-HT3 agonist, mimicked the 5-HT response with a potency only slightly less than that of 5-HT. A highly selective 5-HT3 antagonist, ICS 205-930 [(3-alpha-tropanyl)1H-indole-3-carboxylic acid ester], inhibited the effect of both agonists with a pKB value characteristic of 5-HT3 receptors. 5-HT-evoked DA release was resistant to antagonism by methiothepin and methysergide, antagonists at 5-HT 1-like and 5-HT2 receptors. Neither (2,5-dimethoxy-4-iodophenyl)-2-aminopropane, the selective 5-HT2 receptor agonist, nor 5-carboxamidotryptamine, the selective 5-HT 1-like receptor agonist, altered DA release. The release of DA by 5-HT3 stimulation was Ca++-dependent and partially sensitive to tetrodotoxin. 5-HT and 2-methyl-5-HT also increased K+-evoked DA release. These observations constitute direct, unambiguous evidence that in rat striatum 5-HT3 receptors modulate release of DA.     

Interaction between serotonin uptake inhibitors and alpha-2 adrenergic heteroreceptors in the rat hypothalamus

The effectiveness of presynaptic receptor agonists to inhibit the electrically evoked release of [3H]monoamines from brain slices is attenuated in the presence of blockade of neuronal uptake for the serotonin (5-HT) and the norepinephrine (NE) systems. There is controversy, however, as to the existence of a functional link between the presynaptic receptors and the neuronal uptake carriers. An alternative hypothesis involves competition for the presynaptic receptor sites between the exogenous agonist and the released neurotransmitter. In order to examine the proposed functional interaction, we studied the alpha-2 adrenoceptor-mediated inhibition of the electrically evoked release of [3H]-5-HT from slices of the rat hypothalamus, a model in which endogenous NE does not activate the alpha-2 heteroreceptors located on 5-HT terminals. The inhibitors of 5-HT uptake, citalopram (0.01-1 microM) and paroxetine (1 microM), which by themselves did not modify [3H]-5-HT release, antagonized the inhibition of [3H]-5-HT overflow produced by UK 14.304, an alpha-2 adrenoceptor agonist. The inhibition of the electrically evoked release of [3H]-5-HT by exogenous NE (0.1-1 microM) was also attenuated in the presence of citalopram. In contrast, citalopram did not modify the electrically evoked release of [3H]-NE or the inhibition of [3H]-NE release mediated by UK 14.304. When the 5-HT autoreceptor was blocked by cyanopindolol, the inhibitory effect of UK 14.304 on [3H]-5-HT release was unaltered in the presence of citalopram.(ABSTRACT TRUNCATED AT 250 WORDS)     

Lorcaserin, a novel selective human 5-hydroxytryptamine2C agonist: in vitro and in vivo pharmacological characterization

5-Hydroxytryptamine (5-HT)(2C) receptor agonists hold promise for the treatment of obesity. In this study, we describe the in vitro and in vivo characteristics of lorcaserin [(1R)-8-chloro-2,3,4,5-tetrahydro-1-methyl-1H-3 benzazepine], a selective, high affinity 5-HT(2C) full agonist. Lorcaserin bound to human and rat 5-HT(2C) receptors with high affinity (K(i) = 15 +/- 1 nM, 29 +/- 7 nM, respectively), and it was a full agonist for the human 5-HT(2C) receptor in a functional inositol phosphate accumulation assay, with 18- and 104-fold selectivity over 5-HT(2A) and 5-HT(2B) receptors, respectively. Lorcaserin was also highly selective for human 5-HT(2C) over other human 5-HT receptors (5-HT(1A), 5-HT(3), 5-HT(4C), 5-HT5(5A), 5-HT(6), and 5-HT(7)), in addition to a panel of 67 other G protein-coupled receptors and ion channels. Lorcaserin did not compete for binding of ligands to serotonin, dopamine, and norepinephrine transporters, and it did not alter their function in vitro. Behavioral observations indicated that unlike the 5-HT(2A) agonist (+/-)-1-(2,5-dimethoxy-4-phenyl)-2-aminopropane, lorcaserin did not induce behavioral changes indicative of functional 5-HT(2A) agonist activity. Acutely, lorcaserin reduced food intake in rats, an effect that was reversed by pretreatment with the 5-HT(2C)-selective antagonist 6-chloro-5-methyl-1-[6-(2-methylpyridin-3-yloxy)pyridin-3-yl-carbamoyl]indoline (SB242,084) but not the 5-HT(2A) antagonist (R)-(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine-methanol (MDL 100,907), demonstrating mediation by the 5-HT(2C) receptor. Chronic daily treatment with lorcaserin to rats maintained on a high fat diet produced dose-dependent reductions in food intake and body weight gain that were maintained during the 4-week study. Upon discontinuation, body weight returned to control levels. These data demonstrate lorcaserin to be a potent, selective, and efficacious agonist of the 5-HT(2C) receptor, with potential for the treatment of obesity.     

Single and combined effects of desimipramine and lithium on serotonergic receptor number and second messenger function in rat brain

The effects of chronic administration of desimipramine (DMI, 10 mg/kg i.p. daily for 4 or 5 weeks), short-term administration of lithium (Li, 0.2% in food for 10 days) and a combination of these treatments on serotonergic receptors and second messengers were studied in the rat brain. DMI alone had no effect on [3H]5-HT binding but reduced [3H]ketanserin binding in cortical membranes, 5-HT-stimulated inositol phosphate (IP) formation in cortical slices and the degree of inhibition of forskolin-stimulated adenylate cyclase by 5-HT in hippocampal membranes. Li alone reduced [3H]5-HT binding and the degree of inhibition of forskolin-stimulated adenylate cyclase by 5-HT in hippocampal membranes, and also reduced [3H]ketanserin binding and 5-HT-stimulated IP formation in the cortex. The two treatments combined in general produced effects similar to those of Li alone, but the decrease in [3H]ketanserin binding in cortical membranes was significantly greater than that given by Li alone, whereas the reduction in the degree of inhibition of forskolin-stimulated adenylate cyclase by 5-HT in hippocampal membranes was significantly greater than that produced by DMI alone. It is concluded that the therapeutic action of Li when added to tricyclic antidepressants in the treatment of refractory depression may partly have its basis in potentiation of effects on the serotonergic system in the brain.     

Serotonergic modulation of the release of endogenous norepinephrine from rat hypothalamic slices

Ca+(+)-dependent release of endogenous norepinephrine (NE) and dopamine from superfused rat hypothalamic slices was stimulated by 40 mM K+. 20 mM K+ released only NE. Two consecutive exposures to 20 mM K+ (S1 and S2, respectively) produced NE release of similar magnitude (S2/S1 = 1.03 +/- 0.08). Serotonin (5-HT), 3 to 10 microM, in the presence of methylsergide or ritanserin (antagonists at 5-HT1-like and 5-HT2 receptors), caused a concentration-dependent decrease of K(+)-evoked NE release. 5-HT alone did not alter K(+)-evoked NE release. 2-Methyl-serotonin, 2-methyl-5-hydroxytryptamine, 3 to 10 microM (a selective 5-HT3 agonist), mimicked the 5-HT response in the presence and in the absence of ritanserin. A highly selective 5-HT3 antagonist, (3 alpha-tropanyl)1H-indole-3-carboxylic acid ester (ICS 205-930), 1 nM, inhibited the effect of both agonists. The isomers of another highly selective 5-HT3 antagonist, zacopride, inhibited the effect of 2-methyl-serotonin, 2-methyl-5-hydroxytryptamine, at a concentration range, 0.03 to 20 nM, characteristic of their interaction with 5-HT3 receptors. alpha-Methyl-serotonin, alpha-methyl-5-hydroxytryptamine, a selective 5-HT1-like/5-HT2 agonist, failed to affect the K(+)-evoked NE release, but antagonized the effect of 2-methyl-serotonin, 2-methyl-5-hydroxytryptamine. These observations provide direct evidence that, in rat hypothalamus, 5-HT modulates release of endogenous NE through activation of 5-HT3 and, possibly, 5-HT1C receptors.     

Ligand dependency of 5-hydroxytryptamine 2C receptor internalization

Agonist-induced internalization of G protein-coupled receptors (GPCRs) is a well characterized phenomenon believed to contribute to receptor desensitization. The 5-hydroxytryptamine (5-HT)2C subtype of serotonin receptor is a GPCR that we have shown to internalize upon agonist incubation. In this study, we have examined the effects of 5-HT2C receptor agonists serotonin, Ro 60-0175 [(S)-2-(6-chloro-5-fluoroindol-1-yl)-1-methylethylamine], and WAY-161503 [(4aR)-8,9-dichloro-2,3,4,4a-tetrahydro-1H-pyrazino[1,2-a]quinoxalin-5(6H)-one]; partial agonists mCPP [1-(m-chlorophenyl)piperazine] and DOI [(+)-1-(2,5-dimethoxy-4-iodophenyl)-2-amino-propane]; inverse agonists SB-206553 [N-3-pyridinyl-3,5-dihydro-5-methylbenzo(1,2-b:4,5-b')dipyrrole-1(2H)carboxamide] and mianserin; and neutral antagonists SB-242084 [6-chloro-5-methyl-1-[[2-[(2-methyl-3-pyridyl)oxy]-5-pyridyl]carbamoyl]-indoline] and 5-methoxygramine on the internalization of a C-terminal green fluorescent protein (GFP)-tagged 5-HT2C receptor (VSV isoform) expressed in transiently transfected human embryonic kidney cells. We detected internalization with an automated, cell-based fluorescence-imaging system (Arrayscan) and monitored function with intracellular Ca2+ measurements (flourometric imaging plate reader). The 5-HT2C-GFP construct exhibited appropriate pharmacology, and we observed that although all three agonists resulted in similar magnitudes of dose-dependent internalization, the partial agonists resulted in approximately 50% less internalization, and the inverse agonists and neutral antagonists failed to induce internalization. These results were confirmed by confocal microscopy. They demonstrate that the 5-HT2C receptor is internalized by incubation with agonists and partial agonists but not with inverse agonists or neutral antagonists.     

5-HT-induced transferrin production by choroid plexus epithelial cells in culture: role of 5-HT1c receptor

Previous studies in our laboratory have demonstrated that serotonin (5-HT) elevates transferrin production by choroid plexus epithelial cells in primary culture in a time- and concentration-dependent fashion. The present study shows that 5-HT stimulates phosphoinositide hydrolysis in these cells and further demonstrates that the phosphoinositide hydrolysis response is mediated by the 5-HT1c receptor. To determine if the effect on transferrin is also mediated by the 5-HT1c receptor, the effects of 5-HT receptor agonists and antagonists were examined in choroid plexus epithelial cells in primary culture. The stereoisomers of lysergic acid diethylamide (LSD) and the piperazine derivative 6-chloro-2-[1-piperazinyl]-piperazine (MK-212) were evaluated as potential agonists. MK-212 and (+)LSD mimicked 5-HT, increasing transferrin levels to the same extent. The levorotary isomer, (-)LSD, had no effect. This agonist profile agrees with that previously found for 5-HT1c receptor-mediated phosphoinositide hydrolysis. Three antagonists with varying potencies to block 5-HT1c receptor-mediated phosphoinositide hydrolysis were examined: ritanserin, mianserin and spiperone. The results of these studies were less clear-cut. Neither mianserin nor ritanserin significantly reduced the effects of 5-HT on transferrin, even though they markedly reduced 5-HT-induced phosphoinositide hydrolysis. Consistent with its low potency at the 5-HT1c receptor, spiperone, a 5-HT2 and 5-HT1a antagonist, was a less effective antagonist of the phosphoinositide hydrolysis response than were ritanserin and mianserin. Spiperone also failed to block the effect of 5-HT on transferrin.(ABSTRACT TRUNCATED AT 250 WORDS)     

5-hydroxytryptamine (5-HT)1A receptors and the tail-flick response. I. 8-hydroxy-2-(di-n-propylamino) tetralin HBr-induced spontaneous tail-flicks in the rat as an in vivo model of 5-HT1A receptor-mediated activity

This study pharmacologically characterizes a novel behavioral response as a potential in vivo model of serotonin (5-HT)1A receptor-mediated activity. In rats restrained in horizontal cylinders, the selective 5-HT1A agonist, 8-hydroxy-2-(di-n-propylamino) tetralin HBr (8-OH-DPAT), dose-dependently (0.04-10.0 mg/kg s.c.) elicited spontaneous tail-flicks (STFs). This action was mimicked by other ligands possessing high affinity and high efficacy at 5-HT1A sites: RU 24969 [(5-methoxy-3-(1,2,3,6-tetrahydropyridin-4-yl)-1H-indole], lisuride, (+)-lysergic acid diethylamide and 5-methoxy-N,N-dimethyltryptamine hydrogen oxalate. The response could not be elicited by CGS 12066B [7-trifluormethyl-4-(4-methyl-l-piperazonyl)-pyrrolol- [1-2-a] quinoxaline dimaleate], mCPP 1-(3-chlorophenyl)-piperazine-2-HCl, TFMPPm-trifluromethylphenylpiperazine HCl, MK 212 [6-chloro-2-(l-piperzinyl)pyrazine], quipazine and DOI (+-)-2,5-dimethoxy-4-iodophenyl-2-aminopropane HCl, which act in vivo as agonists at 5-HT1B, 5-HT1C and/or 5-HT2 receptors, or by the 5-HT3 agonist, 2-methyl-5-HT. p-chloroamphetamine, which releases endogenous 5-HT, also evoked STFs; in contrast, d-amphetamine, a preferential releaser of catecholamines, was inactive, as were agonists and antagonists at alpha-1, alpha-2, beta-1, beta-2, dopamine D1 and D2 sites. 8-OH-DPAT-elicited STFs were blocked by the 5-HT1/2 antagonist, methiothepin, but not by the 5-HT1C/5-HT2 antagonists, mianserin, ritanserin and ICI 169,369 [2-(2-dimethylaminoetheylthio)-3-phenylquinoline] nor by the 5-HT3 antagonists, GR 38032F [(1,2,3,9-tetrahydro-9-methyl-3-[(2-methyl-1H-imidazol-l-yl)methyl]-4H- carbazol-4-one HCl], ICS 205,930 [(3 alpha-tropanyl)-1H-indol-3-carboxylic acid ester] and MDL 72222 [(1 alpha H, 3 alpha, 5 alpha H)-tripan-3-yl-3,5- dichlorobenzoate]. beta-Blockers with 5-HT1A affinity i.e., (-)-alprenolol, (+/-)-isamoltane and, stereoselectivity, (-)-but not (+)-pindolol, blocked the action of 8-OH-DPAT. Spiperone and spiroxatrine, D2 antagonists with high 5-HT1A affinity, also inhibited 8-OH-DPAT-induced STFs. Selective beta-blockers and D2 antagonists with low 5-HT1A affinity were inactive. 5-HT1A partial agonists, the pyrimidinylpiperazines, buspirone, gepirone and ipsapirone, the halogenated phenylpiperazine, LY 165,163 [1-(2-(4-aminophenyl) ethyl-4-(3-trifluoromethylphenyl)-piperazine], and the benzodioxane, MDL 72832 [8-(4-(1,4-benzodioxan-2-yl-methylamino)-butyl-8-azaspiro-(4 ,5)-decane- 7,9-dione] did not elicit STFs and antagonized the effect of 8-OH-DPAT.(ABSTRACT TRUNCATED AT 400 WORDS)     

Interaction between tricyclic and nontricyclic 5-hydroxytryptamine uptake inhibitors and the presynaptic 5-hydroxytryptamine inhibitory autoreceptors in the rat hypothalamus

In slices of the rat hypothalamus prelabeled with [3H]-5-hydroxytryptamine [( 3H]-5-HT), exposure to lysergic acid diethylamide or 5-methoxytryptamine decreased, in a concentration-dependent manner, the release of 3H-transmitter elicited by electrical stimulation. These inhibitory effects were antagonized by the 5-HT receptor antagonist methiothepin (1 microM). Exposure to methiothepin on its own increased in a concentration-dependent manner the electrically evoked overflow of [3H]-5-HT. Exposure to tricyclic antidepressants, like imipramine and amitriptyline, and to nontricyclic 5-HT uptake inhibitors, like paroxetine and citalopram, did not modify by themselves the electrically evoked overflow of [3H]-5-HT. Yet, the four inhibitors of neuronal uptake of 5-HT, antagonized the inhibition by lysergic acid diethylamide or 5-methoxytryptamine of the electrically induced release of [3H]-5-HT. After depletion of endogenous stores of 5-HT by pretreatment with para-chlorophenylalanine (300 mg/kg i.p.), the inhibitors of 5-HT uptake increased the electrically evoked release of [3H]-5-HT in a concentration-dependent manner. Their order of potency to enhance 5-HT overflow after pretreatment with parachlorophenylalanine paralleled their potency at inhibiting neuronal uptake of 5-HT (paroxetine = citalopram greater than imipramine greater than amitriptyline). In para-chlorophenylalanine-treated rat hypothalamic slices, these inhibitors of 5-HT uptake antagonized the inhibition by 5-HT autoreceptor agonists of the electrically evoked release of [3H]-5-HT to a similar extent than was observed in control rats. It is concluded that inhibition of 5-HT uptake reduces the effectiveness of 5-HT autoreceptor agonists to inhibit the electrically evoked release of [3H]-5-HT, irrespective of the chemical structure of the uptake inhibitor or of the levels of endogenous 5-HT achieved in the synaptic gap.     

Pharmacological characterization of 5-hydroxytryptamine2 and 5-hydroxytryptamine3 receptors in rat dorsal root ganglion cells

5-Hydroxytryptamine (5-HT) depolarized 87% of the rat dorsal root ganglion cells recorded. 5-HT increased the input resistance (Rin) in 50%, decreased Rin in 41% and produced both responses in 9% of the responding cells. When 5-HT increased the Rin, the response was mimicked by the 5-HT2 agonists alpha-methyl-5-HT, (+/-)-1-(2,5-dimethoxy-4 iodophenyl)-2-aminopropane HCl, quipazine and MK 212 (6-chloro-1-[1-piperazinyl]-pyrazine), but not by 2-methyl-5-HT or carboxamidotryptamine. The response to 5-HT was antagonized by ketanserin, spiperone and methiothepin. The unsurmountable blockade induced by higher concentrations of ketanserin was not explained by pseudo-irreversible antagonism or multiple receptor subtypes, but could result from a two-state receptor model or multiple subtypes of the 5-HT2 receptor. This conclusion is supported by the partial agonist action of DOI. Cells responding to 5-HT with depolarization and decreased Rin responded similarly to 2-methyl-5-HT and phenylbiguanide, but not to alpha-methyl-5-HT or carboxyamidotryptamine. This response was surmountably blocked by ICS 205-930 (3-tropanyl-indole-3-carboxylate) (pA2 = 10.3) and MDL 72222 (3-tropanyl-3,5-dichlorobenzoate)(pA2 = 7.8). The arylpiperazines, quipazine and MK 212, antagonized the action of 2-methyl-5-HT with IC50 values of 8 and 4 nM, respectively. These data indicate that 5-HT2 receptors mediate the increased Rin and 5-HT3 receptors mediate the decreased Rin.     

Inhibition of carbachol-induced inositol phosphate accumulation by phencyclidine, phencyclidine-like ligands and sigma agonists involves blockade of the muscarinic cholinergic receptor: a novel dioxadrol-preferring interaction

The effect of phencyclidine (PCP) on carbachol-induced phosphoinositol hydrolysis was examined in rat brain slices taken from cortex, caudate-putamen and hippocampus. In all three regions studied, PCP significantly inhibited carbachol-induced [3H]inositol phosphate accumulation working as low as 10(-6) M in the cerebral cortex. Because PCP has been shown to act at two sites, a PCP-site and a sigma site, various PCP-like agonists [levoxadrol (Lev), dexoxadrol (Dex) and MK-801 [(+)-5-methyl-10,11-dihydro- 5H-dibenzo(a,b)cyclo-hepaten-5, 10-imine maleate]] as well as sigma agonists [(+)-SKF10047 and 1,3-di(2-toly)guanidine (DTG) were examined for their effects on carbachol-induced phosphoinositol hydrolysis. All but MK-801 significantly inhibited the carbachol action; however, their order of potencies, Lev greater than or equal to Dex much greater than PCP greater than or equal to DTG greater than or equal to (+)-SKF10047 differed from those of other known PCP interactions at PCP and sigma sites. Inasmuch as it is known that PCP competes for binding at muscarinic sites, we examined the effects of PCP, Lev, Dex, DTG and MK-801 on the binding of L-[3H]-3-quinuclidinyl benzilate to its muscarinic site. All blocked L-[3H]-3-quinuclidinyl benzilate binding and exhibited a rank order of potency almost identical to that obtained in the inositol studies with Lev greater than Dex much much greater than DTG much greater than PCP MK-801. In addition, the IC50 values obtained from both studies were very similar. It is concluded that PCP, PCP-like compounds and sigma agonists block carbachol-induced inositol-phosphate accumulation by blockade of muscarinic receptors.(ABSTRACT TRUNCATED AT 250 WORDS)