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Both dopamine D3 receptor (D3R) partial agonists and antagonists have been implicated as potential medications for substance use disorders. In contrast to antagonists, partial agonists may cause fewer side effects since they maintain some dopaminergic tone and may be less disruptive to normal neuronal functions. Here, we report three sets of 4-phenylpiperazine stereoisomers that differ considerably in efficacy: the (R)-enantiomers are antagonists/weak partial agonists, whereas the (S)-enantiomers are much more efficacious. To investigate the structural basis of partial agonism, we performed comparative microsecond-scale molecular dynamics simulations starting from the inactive state of D3R in complex with these enantiomers. Analysis of the simulation results reveals common structural rearrangements near the ligand binding site induced by the bound (S)-enantiomers, but not by the (R)-enantiomers, that are features of partially activated receptor conformations. These receptor models bound with partial agoni...

Background and Purpose A ligand is believed to produce either positive or negative responses, or to block both of them. However, an indole compound was found to promote both positive and negative effects at the 5-HT3AB receptor, which displays a low level of spontaneous activity. The present study attempted to delineate the mechanisms underlying this phenomenon. Experimental Approach The spontaneously active V291S 5-HT3A receptor was used to explore the properties of 5-hydroxyindole (5-HoI) and 5-methoxyindole (5-MoI), structural analogues of 5-HT, either alone or in combination with orthosteric probes. Key Results Two types of efficacy switching were initiated by altering ligand structure and concentration. At lower concentrations, a subtle structural change at position 5 of the indole molecule resulted in opposite effects. 5-HoI apparently elicited partial allosteric inverse agonism, whereas 5-MoI induced allosteric agonism. Interestingly, at a higher concentration, these indoles produced distinct auto-inhibition, manifested as a switch from positive to negative effects. 5-HoI induced a transition from orthosteric agonism to allosteric inverse agonism, whereas 5-MoI produced a shift from allosteric agonism to orthosteric inverse agonism. The auto-inhibition appears to involve communication between orthosteric and allosteric sites of the active receptor conformation and/or between inactive and active conformations. An additive effect of orthosteric and allosteric inverse agonism and insensitivity of allosteric agonism to orthosteric antagonism were also demonstrated. Conclusions and Implications Together, the results suggest that the moiety at position 5 of the indole structure is a critical determinant of a ligand's properties at the 5-HT3A receptor, providing new insights into understanding ligand–receptor interactions.

Drug discovery strives for selective ligands to achieve targeted modulation of tissue function. Here we introduce engineered context-sensitive agonism as a postreceptor mechanism for tissue-selective drug action through a G protein-coupled receptor. Acetylcholine M2-receptor activation is known to mediate, among other actions, potentially dangerous slowing of the heart rate. This unwanted side effect is one of the main reasons that limit clinical application of muscarinic agonists. Herein we show that dualsteric (orthosteric/allosteric) agonists induce less cardiac depression ex vivo and in vivo than conventional full agonists. Exploration of the underlying mechanism in living cells employing cellular dynamic mass redistribution identified context-sensitive agonism of these dualsteric agonists. They translate elevation of intracellular cAMP into a switch from full to partial agonism. Designed context-sensitive agonism opens an avenue toward postreceptor pharmacologic selectivity, which even works in target tissues operated by the same subtype of pharmacologic receptor.

A great deal is known about the evolutionary significance of body size and development time. They are determined by the nonlinear interaction of three physiological traits: two hormonal events and growth rate (GR). In this study we investigate how the genetic architecture of the underlying three physiological traits affects the simultaneous response to selection on the two life-history traits in the hawkmoth Manduca sexta. The genetic architecture suggests that when the two life-history traits are both selected in the same direction (to increase or decrease) the response to selection is primarily determined by the hormonal mechanism. When the life-history traits are selected in opposite directions (one to increase and one to decrease) the response to selection is primarily determined by factors that affect the GR. To determine how the physiological traits affect the response to selection of the life-history traits, we simulated the predicted response to 10 generations of selection. A total of 83% of our predictions were supported by the simulation. The main components of this physiological framework also exist in unicellular organisms, vertebrates, and plants and can thus provide a robust framework for understanding how underlying physiology can determine the simultaneous evolution of life-history traits.

If one drug, B, antagonizes another, A, by producing the opposite physiological effect, the antagonist concentration-effect curves should be affected by the recording system, which limits the range of agonist responses. With pieces of isolated guinea-pig ileum taken from adjacent parts of the same animal, one recorded isotonically, the other isometrically with the same load, the isotonic IC50 values for (−)isoprenaline opposing carbachol or histamine were lower than the isometric values (P

The ability of four endogenous vasodilators, nitric oxide (NO; 0.01 - 30 microM), atrial (ANP), brain (BNP) and C-type (CNP) natriuretic peptide (0.1 - 300 nM), to reverse endothelin-1 (ET-1; 10 nM) constrictions in human resistance and conductance coronary arteries (CA) in vitro was investigated. ET-1 (0.1 - 300 nM) constricted resistance CA more potently than conductance CA (P

Bombesin along with several closely related neuropeptides elicit scratching behavior when administered centrally. The first part of the study was designed to determine the antagonistic effects of a novel phyllolitorin analogue [desTrp3,Leu8]phyllolitorin (DTP) on scratching induced by three peptides (bombesin, neuromedin-C, and [Leu8]phyllolitorin). In addition, the binding affinity of each peptide for the bombesin receptor site was determined. DTP (30 μg) inhibited scratching induced by these peptides, but unlike the peptides, DTP had no affinity for the bombesin site, thereby suggesting that DTP is displaying physiological antagonism through an unknown mechanism.

Abstract —Previous biochemical experiments have revealed two antisense peptide antagonists to human angiotensin II (Ang II), one encoded in the cDNA in the antiparallel reading, the other in the parallel reading. Neither peptide’s ability to produce physiological antagonism has been demonstrated previously. Both peptides were tested for their ability to antagonize Ang II–induced contractions on rabbit aorta smooth muscle. Neither peptide had any direct contractile activity. The antiparallel Ang II peptide had physiological antagonism to Ang II contractions at a lower sensitivity than reported in biochemical studies, and its antagonist activity was partially blocked by Ang II antiserum, suggesting that it is not an antipeptide but an Ang II homologue. The parallel Ang II antipeptide also required high concentrations for physiological inhibition. Its contractile inhibition was not affected by Ang II antiserum and diminished the Ang II contraction at high micromolar concentrations, findings consistent with physicochemical data showing that it is an Ang II complement. The concentration of either peptide required to produce an antagonistic physiological effect was too high to predict any pharmacological usefulness. The parallel antipeptide, however, significantly increased the force of muscle contractions at high nanomolar concentrations, thus displaying a unique dual augmentation/antagonist activity. This antipeptide seems to have highly sequence-specific activity because other similar parallel antipeptides had no activity. The parallel antipeptide augmentation mimics the shift in the Ang II dose-response curve produced in hypertension studies of the slow pressor effect of Ang II and may be useful in deducing the currently unknown cause of the slow pressor effect. It may also have some uses in migraine studies.

1. In the present study, we have pharmacologically characterized two novel mixed endothelin ETA/ETB receptor antagonists, namely BQ-928 and BQ-238, in ETA and ETB preparations, the rabbit carotid artery (RbCA) and the rabbit pulmonary artery (RbPA), respectively. These two antagonists were compared to established ETA (BQ-123 and BMS 182874), ETB (BQ-788) and mixed ETA/ETB (SB 209670) receptor antagonists. 2. In the RbCA, the ETA monoreceptor preparation, BQ-238 and BQ-928 had apparent affinities (pA2) of 7.42 +/- 0.22 and 7.22 +/- 0.18, respectively, BQ-788 being inactive in this preparation. In the ETB monoreceptor preparation, the RbPA (when IRL-1620 was used as an ETB receptor agonist), the pA2 for BQ-238 was 7.05 +/- 0.14 and for BQ-928 was 8.43 +/- 0.04. BQ-123 and BMS 182874 were inactive in this preparation. Similar to SB 209670, BQ-238 but not BQ-928 had a higher affinity for the ETA than the ETB receptor. 3. All of the antagonists were tested for their ability to block and reverse endothelin-l-induced vasoconstrictions in the rabbit perfused kidney. In this preparation endothelin-1-induced increases in vascular resistance have been shown to be mediated solely by ETA receptors. All compounds (except BQ-788) blocked the pressor effects of endothelin within the kidney; the calculated IC50 values for BQ-123, BMS 182874, SB 209670, BQ-928 and BQ-238 were 0.4 microM, 2 microM, 0.01 microM, 0.4 microM and 0.09 microM, respectively. 4. In all experiments in the rabbit perfused kidney, endothelin-1 was readministered for a third time, 60 min following cessation of infusion of the above-mentioned antagonists. The response to the third infusion of endothelin-1 following cessation of infusion of BQ-123, BMS 182874 and SB 209670 was not significantly different from that to the third infusion of endothelin in control conditions. However, the response to endothelin-1 was significantly higher than control in tissues pre-infused with BQ-788 or BQ-928 (56 +/- 9 and 41.6 +/- 15%, respectively, n = 8 each, P < 0.05). 5. Our results suggest that in a system where ETA receptor activation is responsible for vasoconstriction and ETB-receptor activation for vasodilatation. ETA receptor selective antagonists or mixed ETA/ETB receptor antagonists which possess high affinity for ETA receptors do not induce hyperresponsiveness to endothelin-1. In contrast, ETB selective antagonists or mixed antagonists possessing a high affinity for ETB receptors (such as BQ-928) interfere with the ETB-receptor-dependent physiological antagonism of endothelin-1-induced pressor responses in these same tissues.

See related article, pp 753–758 As noted by Romo et al,1 the past decade has witnessed an increased incidence, in the general population, of intrauterine growth retardation (IUGR), currently averaged at 5.13% of all newborns. An elegant survey by Hutter et al2 subsequently highlighted the major causes of intrauterine hypoxia often associated with IUGR. Those authors additionally argued in favor of an important role for intrauterine hypoxia-induced alteration of gene expression in the increased incidence of risk factors in premature cardiovascular diseases at later life stages.2 In the present study, Bourque et al3 provide experimental evidence toward a role for the precursor of endothelin-1 (ET-1), big-ET-1, as a new stressor in aged male rats that experienced prenatal hypoxia-induced IUGR. Interestingly, the authors demonstrated a vascular hyper-responsiveness to big-ET-1 and a 2-fold reduction in nitric oxide (NO)–dependent physiological antagonism of the precursor’s contractile properties in vessels derived from aged male but not female rats. Furthermore, an orally available mixed ETA/ETB antagonist, tezosentan,4 attenuated the hypertensive state of aged IUGR …

TRPV1 has emerged as a promising therapeutic target for pain as well as a broad range of other conditions such as asthma or urge incontinence. The identification of resiniferatoxin as an ultrapotent ligand partially able to dissect the acute activation of TRPV1 from subsequent desensitization and the subsequent intense efforts in medicinal chemistry have revealed that TRPV1 affords a dramatic landscape of opportunities for pharmacological manipulation. While agonism and antagonism have represented the primary directions for drug development, the pharmacological complexity of TRPV1 affords additional opportunities. Partial agonism/partial antagonism, its modulation by signaling pathways, variable desensitization, and slow kinetics of action can all be exploited through drug design.

+/- -Higenamine (demethylcoclaurine), a cardiotonic principle from aconite root, chronotropic and inotropic actions mediated through beta 1-adrenergic receptors. We have investigated the influence of cholera toxin (CTX), a Gs-protein activator, and pertussis toxin (PTX), a Gi-protein inhibitor on the chronotropic interaction between higenamine and a muscarinic agonist, acetylcholine (ACh) in the isolated right atria of mice. CTX (100nm, 1h) pretreatment accentuated the inhibitory responses to cumulative applications of ACh (30nM--30 microns for the positive chronotropic effects induced by higenamine (100nM), isoproterenol (3 and 10 nM) or dobutamine (100nM). In normal atria (CTX-untreated), ACh physiologically antagonized the positive chronotropic effects of these beta-adrenergic agonists. Pretreatment with PTX (150 microgram/kg, i.p., 3d) abolished the CTX (100nm, 1 h)-induced accentuation in the inhibitory effect of ACh against higenamine. PTX pretreatment also attenuated the physiological antagonism by ACh against higenamine in normal atria. The negative chronotropic effect of ACh was not affected by a submaximal concentration of forskolin (1 micron). The These results suggest an accentuated antagonism between higenamine and ACH in CTX-treated, but not in untreated, isolated right atria of mice, which may occur through a functional interaction between the beta1-adrenergic-Gs and muscarinic-Gi systems.

Nitroglycerin (NTG) has been demonstrated to provide uterine relaxation in the management of various obstetric complications. A 32-yr-old woman presented 40 min postpartum for manual removal of a retained placenta. Repeated, alternating doses of NTG 250 micrograms and syntocinon (SYN) 10U iv were used over 15 min to produce periods of uterine relaxation and contraction respectively for uterine exploration. Multiple attempts to extract the placenta failed and a diagnosis of placenta accreta was made. There were no major side effects from this combination of drugs apart from a transient 20% decrease in blood pressure after NTG, which responded to ephedrine 10-15 mg iv. The rapid change in uterine tone was believed to be due not only to the short duration of action of NTG and SYN, but also to the possible physiological antagonism between the two drugs. The mechanism of interaction may involve calcium mobilization and myosin light chain phosphorylation. We conclude that NTG and SYN can be used to produce alternating periods of uterine relaxation and contraction rapidly and consistently with little sustained effects from either agents.

1 The incorporation of [3H]-thymidine into guinea-pig lymphocytes stimulated by a plant lectin (concanavalin A), soluble antigen (tuberculin (P.P.D.)) and syngeneic hepatoma cells, was partially inhibited (50%) by histamine in vitro. 2 The effect of histamine on both mitogen and antigen dose-response curves suggests a non-competitive, probably physiological antagonism. 3 The inhibitory dose range of histamine lay between 10 nM and 30 microM with an ID50 of approximately 400 nM. 4 The potency order for histamine analogues for the inhibition of lymphocyte activation was histamine greater than or equal to 4-methylhistamine greater than 2-methylhistamine greater than 3-methylhistamine. This is in accord with the mediation of the response through an H2-receptor. 5 H2-receptor antagonists reversed the inhibitory effect of histamine in a dose-related manner, but both metiamide and burimamide, in high concentrations, augmented lymphocyte activation in their own right. This precluded the determination of affinity constants and made it impossible to state with certainty that the inhibition of lymphocyte activation by histamine was mediated by an H2-receptor.

The present investigation reports on the accumulation and damage caused by copper in the liver and kidney of albino rats. Simultaneous treatment with copper and molybdenum shows that no traces of copper are retained in these tissues. Thus these observations lead to the conclusion that copper is ineffective in the presence of molybdenum exhibiting physiological antagonism. Several hypothesises have been considered for their antagonistic behaviour. Present authors suggest that molybdenum may act on copper as a chelating agent binding with it and forming either cupric molybdate or copper thiomolybdate which is ultimately excreted. Intake of molybdenum with the diet has also been suggested for the occupational victims of copper.

Summary 1 The hyperactivity and hyperpyrexia produced by l-tryptophan in rats treated with a monoamine oxidase inhibitor was inhibited by chlorpromazine. 2 Chlorpromazine did not inhibit the increased rate of synthesis of brain 5-hydroxytryptamine (5-HT) produced by tryptophan loading. 3 Hyperactivity and hyperpyrexia were also produced by 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) in rats. Pretreatment with a monoamine oxidase inhibitor potentiated the hyperactivity response. Pretreatment of rats with p-chlorophenylalanine did not inhibit hyperactivity produced by 5-MeODMT. 4 Chlorpromazine inhibits hyperactivity caused by tryptophan or 5-MeODMT after monoamine oxidase inhibition either by competition with 5-HT or 5-MeODMT, respectively, at receptor sites or by physiological antagonism.

Summary 1 The dose of clonidine (given intravenously) required to elicit sleep in the young chick is 1/25th to 1/50th of an equiactive dose of noradrenaline. The approximate ed50 is 001 μmol/kg. Phentolamine (10–15 mg/kg, but not 5 mg/kg) antagonizes the action of both clonidine and noradrenaline. 2 Intensive treatment with p-chlorophenylalanine (700 mg/kg for 3–4 days) does not prevent the hypnotic effect of clonidine in the chick, although brain 5-HT is reduced to 15% of normal. Neither is natural sleep modified. 3 Sleep after clonidine is not affected by methysergide (0·1–1 μmol/kg, i.m.), but prevented by LSD (0·1–0·3 μmol/kg). The effect of LSD is interpreted as a physiological antagonism. 4 Clonidine (50 mg/kg) injected intravenously into adult rats causes sleep which is not abolished by phentolamine (5 mg/kg) or by p-chlorophenylalanine in doses which interfere with natural sleep. 5 When, per kg body weight, the same dose of clonidine is injected into the lateral cerebral ventricle of rats, sleep ensues in more than half the animals, and persistent eating in about a third; only one of seventeen rats showed no change in behaviour. Eating and sleeping remained unaltered after p-chlorophenylalanine. The actual dose of clonidine injected into the lateral ventricle was 0·037 μmol, amounting to about 0·15 μmol/kg or 15 times the dose required intravenously in the chick. Noradrenaline 0·15 μmol per (intraventricular) injection caused eating but no sleep, whereas higher doses produced ataxia and paresis. 6 The work suggests that clonidine does not elicit sleep by an action requiring the integrity of the 5-HT-containing neurones arising in the raphe nuclei, and that its action is not on tryptamine receptors. In the chick, sleep appears to be produced by a central sympathomimetic effect; it is possible, but not certain, that this also holds for the rat. 7 The intravenous hypnotic dose of clonidine for the cat is about the same as that for the rat, but injection is not accompanied by signs of peripheral sympathetic stimulation.

The pressor effects of porcine (ET-1) and rat (ET-3) endothelins were studied in the pithed rat along with the actions of cyclo-oxygenase inhibitors upon these responses. Indomethacin (15 mumol/kg i.v.) when given prior to endothelin had no effect on the pressor responses to ET-1 or ET-3. However, when indomethacin or piroxicam was administered between two doses of ET-1 or ET-3, the second response was significantly potentiated. This potentiation was abolished when the adrenal glands were excluded from the circulation but partially restored when neuropeptide Y (NPY) (1 nmol/kg i.v.) was administered. Radiolabeled microspheres were used to measure regional blood flow and from these measurements, regional vascular resistance was calculated. From these data, it is evident that ET-1 caused a generalized increase in vascular resistance, and only in the large intestine and epididymal fat pad was this attenuated by indomethacin. In the gastric vasculature, the effects of ET-1 were potentiated by indomethacin. In the bronchial vasculature, ET-1 caused a reduction in vascular resistance possibly due to the bronchoconstrictor actions of ET-1 and the concomitant release of vasodilators such as histamine. When the fraction of the cardiac output received by each vascular bed is taken into account, the gastrointestinal tract, kidneys, and skeletal muscle account for most of the increase in total peripheral resistance induced by ET-1. Prostanoids have a role in the pressor response to ET-1 and ET-3 that is more complex than one of simple physiological antagonism or potentiation at the level of the vascular smooth muscle and possibly act as modulators of other regulatory factors such as NPY.

An established cellular line (MDCK) derived from canine kidney showed specific binding for prostaglandin E2, both when the determination of specific binding was carried out in cells attached to a semipermeable support and when this binding was determined in suspended cells in a calcium-free medium. Specific binding for tritiated prostaglandin F2 alpha was also observed, and it was inhibited by increasing concentrations of non-radioactive PGF2 alpha in a dose-related pattern. Cross-reactivity between PGE2 binding and PGF2 alpha was present. Arginine-vasopressin induced an increase in the apical-to-basal water flux when applied to the basolateral surface. No change was observed when the hormone was in contact with the apical surface of the monolayer. Prostaglandin E2 also induced an increase in water flux, with a slower time course than that of arginine-vasopressin. The presence of PGE2 inhibited the increase in water flux induced by antidiuretic hormone. The results suggest that MDCK monolayers depict a physiological antagonism between ADH and prostaglandins which is similar to that observed in natural epithelia.

1. The relationship between the vasodilator and inhibitory effects of prostaglandin E(1) (PGE(1)), E(2) and A(2) on responses to nerve stimulation, noradrenaline and angiotensin was evaluated in the dog hindlimb preparation.2. PGE(1) and PGE(2) were equipotent as vasodilators in the hindlimb; however, PGE(1) was much more potent as an inhibitor of vasoconstrictor responses to nerve stimulation, noradrenaline and angiotensin.3. PGA(2) and PGE(2) were approximately equal as inhibitors of vasoconstrictor responses to noradrenaline, nerve stimulation and angiotensin; however, PGE(2) was far more potent as a vasodilator.4. Since there is no relationship between the vasodilator and inhibitory effects of PGE(1), E(2) and A(2), and since the inhibitory effect of PGA(2) was present at a time when hindlimb perfusion pressure had returned to control value, it is concluded that the inhibitory action is probably not the result of a physiological antagonism.5. Since each prostaglandin inhibited responses to nerve stimulation and noradrenaline to approximately the same extent and responses to angiotensin were also inhibited, it is suggested that these agents antagonize vasoconstrictor responses by a nonspecific depressant effect on smooth muscle cells.

CHEMICALLY, magnesium and calcium are related since both elements are members of Group II of the Periodic Table. Physiologically, however, in plants1 or in animals2 their actions are often antagonistic. It has been shown that magnesium-deficient rats absorb more calcium from the gastrointestinal tract than do control rats3 and the active transport of calcium ions across the intestinal wall of several species is depressed by magnesium ions4. These experiments indicate again a physiological antagonism between these ions and also suggest the possibility of a common absorptive pathway. However, the experimental conditions in the former study3 are abnormal since magnesium is an integral part of normal diets and magnesium deficiency may have caused changes in the intestinal mucosa which allowed more calcium to be absorbed. In vitro type experiments4 are, by their nature, unphysiological and extrapolations from such isolated systems to interpretations concerning overall calcium absorption in vivo may not be valid.