ergoline and 2-5-dimethoxy-4-bromoamphetamine

5-HT2C receptor antagonists, such as mianserin and mesulergine, exhibit negative intrinsic activity, defined as a decrease in agonist-independent, receptor-mediated, phosphoinositide hydrolysis in cells transfected with the 5-HT2C receptor cDNA. These drugs are classified as inverse agonists. Guanine nucleotides reciprocally modulate the binding of an agonist and inverse agonist, suggesting that an inverse agonist binds preferentially to the G protein-uncoupled form of the 5-HT2C receptor. Another 5-HT2C receptor antagonist, 2-bromolysergic acid diethylamide, functions as a neutral antagonist with no intrinsic activity, but is able to block both agonist and inverse agonist. Chronic treatment of choroid plexus cells with an inverse agonist, but not with the neutral antagonist, causes 5-HT2C receptor down-regulation, suggesting that the biological effects of 5-HT2C receptor antagonists are not solely due to antagonism of endogenous agonist. These results provide evidence that constitutively active 5-HT2C receptors are biologically significant. The functionally distinct properties of inverse agonists and neutral antagonists may elucidate the mechanisms controlling basal receptor activity states and lead to novel approaches in the development of therapeutic agents.

Topics: 3T3 Cells; Animals; Antiparkinson Agents; Cell Membrane; Cells, Cultured; Choroid Plexus; Cloning, Molecular; DNA, Complementary; DOM 2,5-Dimethoxy-4-Methylamphetamine; Epithelium; Ergolines; Guanosine 5'-O-(3-Thiotriphosphate); Kinetics; Lysergic Acid Diethylamide; Mianserin; Mice; Radioligand Assay; Rats; Receptors, Serotonin; Serotonin Antagonists; Transfection

The serotonin2A and serotonin2C receptors are unique among receptors coupled to guanine nucleotide binding proteins in that chronic treatment in vivo with agonists as well as antagonists decreases receptor density. In an attempt to uncover molecular events involved in down-regulation of the serotonin2A receptor, the ability of agonists and antagonists to alter receptor density was examined in three heterologous expression systems, i.e., transfected NIH 3T3, transfected Madin-Darby canine kidney, and transfected AtT-20 cells. All three transfected cell lines exhibited pharmacological properties consistent with that predicted for cells expressing the serotonin2A receptor. However, the three cell lines displayed different receptor regulation properties in response to drugs acting at the serotonin2A receptor. In transfected NIH 3T3 cells, neither agonist nor antagonist treatment altered receptor density. Treatment with agonist as well as antagonist led to up-regulation of the serotonin2A receptor in transfected Madin-Darby canine kidney cells. In transfected AtT-20 cells, treatment with agonist led to receptor down-regulation, whereas antagonist treatment increased receptor density. Thus, the cellular background in which the serotonin2A receptor is expressed appears to determine the regulation properties of the receptor.

Topics: 3T3 Cells; Animals; Antiparkinson Agents; Binding, Competitive; Cell Line; Dogs; DOM 2,5-Dimethoxy-4-Methylamphetamine; Ergolines; Gene Expression Regulation; Ketanserin; Kidney; Kinetics; Mice; Pituitary Neoplasms; Receptor, Serotonin, 5-HT2A; Receptors, Serotonin; Recombinant Proteins; Transfection; Tumor Cells, Cultured

Expression of the 5-hydroxytryptamine type 2C (5-HT 2C) receptor in NIH/3T3 fibroblasts results in agonist-independent 5-HT2C receptor activation. Some 5-HT2c receptor antagonists decrease this activation and are termed inverse agonists. The present study uses this system to evaluate functional and receptor binding properties of other 5-HT2C receptor antagonists. A number of inverse agonists, including clozapine, and a neutral antagonist (methysergide) were identified in a functional assay. Guanine nucleotides increased the affinity of a radiolabeled inverse agonist ([3H]mesulergine), suggesting that inverse agonists bind the G protein-uncoupled form of the 5-HT2C receptor with high affinity. Competition binding was performed using conditions that separately labeled the G protein-coupled and -uncoupled forms of the receptor. These studies demonstrated that inverse agonists bound the uncoupled form of the 5-HT2C receptor with higher affinity, compared with the G protein-coupled form. Agonists, on the other hand, had higher affinity for the coupled form whereas neutral antagonists had equal affinity for both forms of the receptor. Thus, 5-HT2C receptor neutral antagonists exhibited functional and receptor binding properties consistent with those of classical receptor antagonists. However, 5-HT2C receptor inverse agonists displayed functional and receptor binding properties that were opposite those of agonists.

Topics: 3T3 Cells; Animals; Binding, Competitive; Cell Line; DOM 2,5-Dimethoxy-4-Methylamphetamine; Ergolines; Guanine Nucleotides; Hydrolysis; Mice; Phosphatidylinositols; Radioligand Assay; Receptor, Serotonin, 5-HT2C; Receptors, Serotonin; Recombinant Proteins; Serotonin Antagonists; Serotonin Receptor Agonists

Behavioral, electrophysiological and biochemical evidence suggest that the 5HT2 receptor plays a role in the action of hallucinogenic agents. Considering the structural and functional similarities between the 5HT2 and 5HT1C receptors, we hypothesized that the 5HT1C receptor may also be an important site of action of hallucinogens. The present manuscript evaluates this hypothesis by examining the properties of hallucinogens in the phenalkylamine and indolealkylamine classes at 5HT1C receptors. Epithelial cells isolated from the rat choroid plexus have a high density of 5HT1C receptors linked to phosphoinositide hydrolysis. Comparison of the actions of drugs in cultured cells and whole choroid plexus confirmed that the cell culture system can serve as an in vitro model of 5HT1C receptor activation. 2,5-Dimethoxy-4-bromoamphetamine (DOB), 2,5-dimethoxy-4-methylamphetamine (DOM), 2,5-dimethoxy-4-iodoamphetamine (DOI) and 3,4-methylenedioxyamphetamine (MDA) were evaluated. The rank order of potency to activate 5HT1C receptors [(-)DOB greater than (+/-) DOI greater than (+)DOB greater than (-)DOM much greater than (-)MDA greater than (+) MDA] was consistent with the rank order of effective behavioral doses in rats and humans. The indolealkylamine hallucinogen, 5-methoxy-N,N-dimethyltryptamine was also a 5HT1C receptor agonist, as is the primary amine, 5-methoxytryptamine. These data, combined with previous studies showing that (+)LSD potently activates 5HT1C receptors, suggest that future investigations of the mechanism of action of hallucinogens should consider the role of 5HT1C receptors in addition to the more commonly investigated 5HT2 receptors.

Topics: 3,4-Methylenedioxyamphetamine; Amphetamines; Animals; Antiparkinson Agents; Cells, Cultured; Choroid Plexus; DOM 2,5-Dimethoxy-4-Methylamphetamine; Epithelial Cells; Ergolines; Hallucinogens; Male; Phosphatidylinositols; Quipazine; Radioligand Assay; Rats; Rats, Inbred Strains; Receptors, Serotonin; Serotonin; Serotonin Antagonists; Stereoisomerism

The aim of the present study was to characterize 5-hydroxytryptamine2 (5-HT2) receptors in the rat medial prefrontal cortex (mPFc) by single cell recording and microiontophoretic techniques. This was accomplished using 5-HT2 receptor agonists 1-[2,5-dimethoxy-4-iodophenyl]-2-aminopropane [(+/-)-DOI] and 1-[2,5-dimethoxy-4-bromophenyl]-2-aminopropane [(+/-)-DOB]. DOI ejected at a low current (0.5 nA) potentiates glutamate (GLU)-induced activation of mPFc neurons and this effect is blocked by spiperone. At higher currents. DOI invariably inhibits GLU-induced neuronal activity. The microiontophoretic ejection of both DOI and DOB predominantly inhibits spontaneously active mPFc cells. The inhibitory action of DOI on spontaneously active cells is dose-dependent and is blocked by putative 5-HT2 receptor antagonists, with a rank order of potency as follows: ritanserin greater than metergoline approximately LY-53857 greater than spiperone greater than mesulergine greater than mianserin approximately ketanserin. Interestingly, ketanserin and mianserin only weakly block the effect of DOI. The suppressant action of DOI is probably not related to its interaction with 5-HT10 sites as spiperone, which has low affinity for these sites, potently blocks the effect of DOI. The suppressant effect of DOI is not blocked by other receptor antagonists such as BRL-43694 (5-HT3), (+/-)-pindolol (5HT 1a,1b, beta adrenergic, beta), prazosin (adrenergic1, alpha-1), pyrilamine (histamine1, H1), l-sulpiride (dopamine2, D2) or SR 95103 (gamma-aminobutyric acid, GABAA). Overall our results indicate that DOI predominantly inhibits mPFc cells in a direct manner and this effect is mediated by 5-HT2 receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: 8-Hydroxy-2-(di-n-propylamino)tetralin; Amphetamines; Animals; DOM 2,5-Dimethoxy-4-Methylamphetamine; Ergolines; Frontal Lobe; Ketanserin; Magnesium; Male; Pyridazines; Rats; Rats, Inbred Strains; Receptors, Serotonin; Serotonin Antagonists; Sodium Glutamate; Tetrahydronaphthalenes