Structure-activity relationships and mechanism of action of hallucinogenic agents based on drug discrimination and radioligand binding studies.
Explore the source record for details and available documents.
Animals↗
Biomedical subjects
M Titeler
Publications and source records attributed to M Titeler.
[3H]DOB: a specific agonist radioligand for 5-HT2 serotonin receptors.
Explore the source record for details and available documents.
DOM 2,5-Dimethoxy-4-Methylamphetamine↗
[3H]Nitrendipine binding to calcium channels in bovine and rat pituitary.
[3H]Nitrendipine was used to label sites in homogenates of bovine anterior and neurointermediate lobes of the pituitary gland. The amount of specific binding in the anterior lobe was 1.82 +/- 0.30 pmol/g wet weight of tissue and the KD was 1.44 +/- 0.02 X 10(-10) M. Preliminary experiments indicated a similar amount of binding in bovine neurointermediate lobe. In competition studies nimodipine and nisoldipine (two potent voltage-sensitive calcium channel blockers) displayed IC50 values of 1.6 and 6.8 X 10(-10) M, respectively. Verapamil and the verapamil-like calcium channel blockers D-600 and tiapamil competed in a complex manner for the [3H]nitrendipine specific binding to bovine anterior pituitary homogenates. Autoradiographical studies demonstrated specific [3H]nitrendipine binding sites distributed approximately equally in the anterior and posterior lobes, but not in the intermediate lobe of the rat pituitary. In general the properties of [3H]nitrendipine binding in the pituitary tissue resemble strongly the properties of [3H]nitrendipine binding in the brain which is believed to be to voltage-sensitive calcium channels. These results provide support for the hypothesis that calcium channels are involved in pituitary hormone secretion and that drugs that interact with calcium channels may modulate the secretory process directly at the level of the pituitary.
Animals↗
Evidence for 5-HT2 involvement in the mechanism of action of hallucinogenic agents.
The affinities (Ki values) of twenty two psycho-active agents, including LSD, 5-OMe DMT and a series of phenalkylamine derivatives, for cortical 5-HT1 and 5-HT2 binding sites were compared with two measures of behavioral activity. It was found that a significant correlation (r = 0.938) exists between the 5-HT2 binding affinities of these agents and their ED50 values as determined in tests of stimulus generalization using 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM) as the training drug. Furthermore, for fifteen of these agents where human data were available, a significant correlation (r = 0.924) also exists between 5-HT2 binding affinities and their human hallucinogenic potencies. The results of this study suggest that the mechanism of action of these agents involves 5-HT2-related events.
DOM 2,5-Dimethoxy-4-Methylamphetamine↗
5-HT1 and 5-HT2 binding properties of derivatives of the hallucinogen 1-(2,5-dimethoxyphenyl)-2-aminopropane (2,5-DMA).
The affinities of a series of 1-(2,5-dimethoxyphenyl)-2-aminopropane (2,5-DMA) derivatives, most of which are hallucinogenic in man, and several related agents were determined for rat cortical serotonin (5-HT) binding sites. Competition assays were performed in which these agents were competed for the 5-HT2 binding of [3H]ketanserin, or the 5-HT1 binding of [3H]LSD (in the presence of ketanserin). The R(-)-isomers of DOI, DOM and DON (i.e. the 4-iodo, -methyl and -nitro derivatives of 2,5-DMA) were found to be more potent than their racemates and demonstrated selectivity for 5-HT2 sites. These same agents in competing for [3H]ketanserin binding resulted in Hill coefficients significantly less than unity; computer-assisted analysis indicated a two-state model better fit the data. In the presence of 10(-4) M Gpp(NH)p the competition curve for R(-)-DOI produced a Hill coefficient close to unity. These results are consistent with the hypothesis that certain derivatives of 2,5-DMA, in particular R(-)-DOI, may be potent and selective agonists at 5-HT2 binding sites, sites that may constitute a serotonin receptor that is regulated by a guanine nucleotide regulatory protein. Conversely, the interactions of these agents at 5-HT1 sites was with a lower affinity and a lack of stereoselectivity. Although DOI and DOM are amongst the most potent of these agents as hallucinogens, it is still too premature to draw any conclusions regarding a possible relationship between 5-HT binding and hallucinogenic potency.
Amphetamines↗
Guanyl nucleotide and divalent cation regulation of cortical S2 serotonin receptors.
Computer-assisted quantitative analysis of radioligand binding to rat cortical S2 serotonin receptors indicates the existence of two affinity states of the same receptor population. Monophasic antagonist competition curves for [3H]ketanserin-labelled sites suggest a uniform population of receptors with one affinity state for antagonists. Biphasic competition curves of agonists suggest that agonists discriminate high- and low-agonist-affinity forms of the S2 receptors. The affinities of agonists for the high- and low-affinity states, and the apparent percentages of high agonist-affinity forms varies with different agonists. The guanine nucleotides GTP and guanyl-5'-imido-diphosphate [Gpp(NH)p], as well as divalent cations, modulate the proportion of the sites with high affinity for agonists as evidenced by their ability to shift the agonist competition curves for [3H]ketanserin-labelled S2 receptors. GTP and Gpp(NH)p effects appear to be agonist-specific, as they do not affect antagonist competition for [3H]ketanserin-labelled S2 receptors, or [3H]ketanserin binding to S2 receptors. ATP and ADP have little or no effect on the binding properties of S2 serotonin receptors, whereas GDP is less potent than GTP. The presence of these specific nucleotide effects are the first evidence suggesting involvement of a guanine nucleotide-binding protein in the mechanism of agonist interaction with the S2 serotonin receptor. In general, the binding properties of [3H]ketanserin-labelled S2 serotonin receptors strongly resemble those of adenylate-cyclase coupled receptors such as the beta-adrenergic, the alpha 2-receptor, and the D-2 dopamine receptor. This may indicate the S2 serotonin receptor is coupled to adenylate cyclase activity, through a GTP binding protein.
Animals↗
Guanine nucleotides modulate cortical S2 serotonin receptors.
Many radiolabelled receptors coupled to intracellular adenylate cyclase activity have been found to be modulated by physiological modulators such as GTP (guanosine triphosphate) and Gpp(NH)p (guanosine-imido-diphosphate). In particular, the apparent affinity of agonists competing for the binding of 3H-antagonist-labelled receptors is reduced in the presence of GTP and Gpp(NH)p. We report herein the agonist-specific effects of GTP and Gpp(NH)p on rat brain cortical S2 serotonin receptors. The agonists serotonin, 5-methoxytryptamine, bufotenine, and tryptamine display threefold lower affinities for S2 serotonin receptors in the presence of 10(-4)M GTP or Gpp(NH)p than in the absence of the nucleotides. The antagonists spiperone, cinanserin, cyproheptadine and methysergide are unaffected by the guanine nucleotides. The Hill coefficients of the agonists increase from between 0.70-0.80 to 0.90-1.00 due to guanine nucleotides. ATP, ADP, and GDP have little or no effect. This pattern of guanine nucleotide effects has been found with receptors which are modulated by a guanine nucleotide regulatory protein and may indicate that the S2 serotonin receptor may be coupled to intracellular adenylate cyclase activity.
Animals↗
Alpha 1 adrenergic receptors in the porcine pituitary neurointermediate lobe: detection with [3H]ketanserin.
[3H]Ketanserin, a serotonin receptor antagonist, labelled high affinity, saturable sites in homogenates of porcine neurointermediate lobe tissue. Cinanserin, a potent and selective serotonin receptor antagonist, inhibited the specific binding of 5 X 10(-10)M [3H]ketanserin with a high affinity component representing 20% of the total binding. Prazosin, a potent and selective alpha 1 adrenergic antagonist, inhibited [3H]ketanserin binding with a high affinity component representing 60% of total binding. The prazosin-specific component was demonstrated to be distinct from the cinanserin-specific component. 10(-7)M cinanserin was co-incubated with [3H]ketanserin to eliminate the serotonergic component of the binding and allow pharmacological characterization of the remaining prazosin-specific component. The prazosin-specific binding of [3H]ketanserin binding closely resembled the results of experiments using [3H]prazosin to label alpha 1 receptors in neurointermediate lobe tissue homogenates. Ketanserin was found to be seven-fold more potent in inhibiting [3H]prazosin binding to alpha 1 adrenergic receptors in the neurointermediate lobe tissue than in brain tissue. This observation explains why low concentrations of [3H]ketanserin can selectively label serotonin receptors in the brain but will label both adrenergic and serotonin receptors in the neurointermediate lobe.
Animals↗
pH-dependent modulation of agonist interactions with [3H]-ketanserin-labelled S2 serotonin receptors.
The effects of varying the pH on the properties of S2 serotonin receptors labelled by [3H]-ketanserin were examined. Between pH 7.0 and 8.2 the agonist affinities, as determined by competition experiments, increased dramatically. Serotonin, 5-methoxytryptamine, tryptamine, bufotenine and quipazine, demonstrated 15,16,8,6 and 5-fold increases in apparent affinity between pH 7.0 and 8.2. On the other hand the antagonists, ketanserin, cinanserin, and spiperone demonstrated little or no affinity changes between pH 7.0 and 8.2. The largest shift in affinity for an antagonist occurred with spiperone, which displayed a two-fold shift. Although changing pH is a rather non-specific manipulation, the selective affect on agonist interaction with S2 receptors indicates further investigation of this pH effect may aid in discovering the difference in receptor interactions between serotonin agonists and antagonists.
Animals↗
Modulation of brain S2 serotonin receptors by lithium, sodium and potassium chloride.
The effect of LiCl, NaCl, and KCl on serotonin competition for 3H-ketanserin binding to S2 serotonin receptors in homogenates of rat prefrontal cortex were investigated. LiCl was the most potent of the ionic modulators in lowering the apparent affinity of serotonin for the S2 serotonin receptor. A threshold effect was noted at 12 mM LiCl (a 60% change in IC50); at 120 mM LiCl a nine-fold shift in the serotonin IC50 was noted. 120 mM NaCl or KCl demonstrated similar effects as 12 mM LiCl in reducing serotonin's apparent affinity. These results indicated that monovalent cations modulate S2 serotonin receptor affinity for serotonin and that lithium ion is more potent than sodium or potassium.
Animals↗
Properties of [3H]prazosin-labeled alpha 1-adrenergic receptors in rat brain and porcine neurointermediate lobe tissue.
[3H]Prazosin binding to alpha 1 receptors in homogenates of rat prefrontal cortical tissue and porcine pituitary neurointermediate lobe tissue was investigated. Competition curves produced by coincubating adrenergic agonists and antagonists with 0.5 nM [3H]prazosin and tissue revealed some anomalous binding properties. In the brain and pituitary tissue, agonist competition curves produced "shallow" slopes, with Hill coefficients significantly lower than unity. The IC50 of the agonists epinephrine, norepinephrine, and clonidine for inhibition of 0.5 nM [3H]prazosin binding were significantly lower in the porcine pituitary than in the rat brain. Most antagonists, such as prazosin, chlorpromazine, and piperoxan, produced "steep" competition curves with Hill coefficients close to unity, with two notable exceptions. WB-4101 and phentolamine produced competition curves with Hill coefficients significantly less than unity in the rat brain preparation. Ketanserin, an antagonist, displayed a sevenfold higher affinity for the alpha 1 sites in the pituitary tissue than in the brain tissue. These anomalies in the binding results may indicate the presence of an endogenous modulatory factor affecting agonist and antagonist affinities for the alpha 1 receptor.
Animals↗
Initial detection of [3H]prazosin-labeled alpha 1-receptors in the porcine pituitary neurointermediate lobe.
[3H]Prazosin binding to alpha 1-adrenergic receptors was investigated in homogenates of tissue from the porcine pituitary neurointermediate lobe. Potent alpha 1-adrenergic antagonists such as prazosin, WB-4101, and phentolamine displayed high potency in competing for [3H]prazosin specific binding in pituitary tissue. Epinephrine and norepinephrine were the only neurotransmitters which displayed high potency in competing for [3H]prazosin binding sites. There was a strong correlation in the pharmacological properties of the alpha 1 sites in pituitary tissue and the alpha 1 sites detected by [3H]prazosin in rat brain tissue. Alpha-adrenergic agonists demonstrated significantly higher potency for the alpha 1-receptor sites in pituitary tissue than in brain tissue preparations. The antagonist ketanserin demonstrated significantly higher potency at alpha 1 sites in pituitary tissue than in brain tissue. This latter observation has important implications for investigators attempting to use [3H]ketanserin as a selective serotonin receptor ligand in pituitary tissue. The results of this study represent the first demonstration of alpha 1-receptors in mammalian neurointermediate lobe tissue.
Adrenergic alpha-Agonists↗
[3H]N-propylapomorphine and [3H]spiperone binding in brain indicate two states of the D2-dopamine receptor.
[3H]N-propylapomorphine ([3H]NPA) a dopaminergic catecholamine derivative, labels a sub-set of D2-dopamine receptors in bovine caudate particulate preparation. [3H]Spiperone, a dopamine receptor antagonist, labels twice as many sites as [3H]NPA. Dopaminergic ergots and potent neuroleptics compete for both radioactive ligands with similar high affinities. Catecholamines and catecholamine derivatives compete more potently for [3H]NPA binding than for [3H]spiperone binding. Guanyl nucleotides reduce both [3H]NPA binding and the high affinity phase of catecholamine and catecholamine derivative competition for [3H]spiperone binding. These results are similar to binding results reported in studies of two-state receptors linked to adenylate cyclase such as the beta-adrenergic receptors. These observations indicate that the D2-dopamine receptor in the brain may exist in two states and may be inversely coupled to brain adenylate cyclase activity.
Animals↗
Effects of antihypertensive clonidine congeners on alpha-adrenergic receptors.
Evidence in the literature suggests that the antihypertensive effects of clonidine stem from its action on alpha-2 adrenergic receptors. In order to examine this possibility we tested the effects of 13 congeners of clonidine on the binding of [3H]WB-4101 and [3H]clonidine to calf frontal cortex homogenates; [3H]WB-4101 served as a label for alpha-1 receptors while [3H]clonidine served to label alpha-2 adrenergic receptors. All the substituted imidazolines were two to three orders more potent in inhibiting the binding of [3H]clonidine than they were against [3H]WB-4101. There was a strong correlation between the antihypertensive doses of these congeners and their concentrations required to inhibit the binding of [3H]clonidine. The results are compatible with the concept that the antihypertensive action of clonidine is more likely due to interactions with alpha-2 adrenergic receptors than with alpha-1 receptors.
Animals↗
Irreversible blockade of [3H]spiperone- but not [3H]dopamine-labeled dopamine receptors with phenoxybenzamine.
Explore the source record for details and available documents.
Animals↗
Direct binding of 3H-lisuride to adrenergic and serotonergic receptors.
Explore the source record for details and available documents.
Animals↗
D2- but not D3-dopamine receptors detected in the anterior pituitary.
Using conditions which revealed high affinity binding sites for [3H]ADTN (KD of 1.6 nM), [3H]apomorphine (DK of 3.5 nM) and [3H]dopamine (KD of 1,5 nM) in the calf caudate nucleus, no such high affinity sites could be detected in the calf anterior pituitary gland. In contrast, high affinity binding sites for [3H]-spiperone were found in both the caudate and the anterior pituitary. The apparent absence of high affinity sites for these [3H]-catecholamine ligands (in the 1-10 nM region) and the presence of the [3H]spiperone binding site in the anterior pituitary is compatible with the view that the pituitary [3H]neuroleptic receptor with its low affinity for dopamine, may be the physiological site of dopamine action in the pituitary. THe data, therefore, support the presence of D2- but not D3-dopamine receptor sites in the calf anterior pituitary.
Animals↗
Presynaptically acting catecholamines bind to alpha 2-adrenoceptors labelled by 3H-clonidine.
It is known that certain catecholamine congeners can decelerate the heart rate by inhibiting the prejunctional sympathetic neurones to the atrium. The present study was done to determine whether this presynaptic action might be associated with either dopamine receptors or alpha-adrenergic receptors. The effects of 10 catecholamine congeners wee tested on the specific binding of 0.2 nM 3H-WB-4101 and 0.2 nM 3H-clonidine to calf frontal cortex homogenates, and that of 2 nM 3H-apomorphine to calf caudate nucleus homogenates. The drugs tested were apomorphine, the aminotetralin M-7, 3 dialkylated dopamine congeners, and 5 congeners of octahydrobenzo(f)-quinoline. The IC50 values (concentrations for 50% inhibition of binding) ranged from 400 to 28,000 nM for 3H-WB-4101, from 3 to 270 nM for 3H-apomorphine, and from 9 to 1000 nM for 3H-clonidine. Only the IC50 values for 3H-clonidine binding correlated with the in vitro IC50 values for inhibiting atrial acceleration (data from Long et al., 1975, 1979). These findings suggest that 3H-clonidine appears to bind to the same site in brain (alpha 2-adrenoceptor) on which catecholaminergic drugs act to produce cardiodeceleration.
Animals↗