Drugs and receptors. An overview of the current state of knowledge

Aging-related modifications to G protein-coupled receptor signaling diversity

Aging is a highly complex molecular process, affecting nearly all tissue systems in humans and is the highest risk factor in developing neurodegenerative disorders such as Alzheimer's and Parkinson's disease, cardiovascular disease and Type 2 diabetes mellitus. The intense complexity of the aging process creates an incentive to develop more specific drugs that attenuate or even reverse some of the features of premature aging. As our current pharmacopeia is dominated by therapeutics that target members of the G protein-coupled receptor (GPCR) superfamily it may be prudent to search for effective anti-aging therapeutics in this fertile domain. Since the first demonstration of GPCR-based β-arrestin signaling, it has become clear that an enhanced appreciation of GPCR signaling diversity may facilitate the creation of therapeutics with selective signaling activities. Such 'biased' ligand signaling profiles can be effectively investigated using both standard molecular biological techniques as well as high-dimensionality data analyses. Through a more nuanced appreciation of the quantitative nature across the multiple dimensions of signaling bias that drugs possess, researchers may be able to further refine the efficacy of GPCR modulators to impact the complex aberrations that constitute the aging process. Identifying novel effector profiles could expand the effective pharmacopeia and assist in the design of precision medicines. This review discusses potential non-G protein effectors, and specifically their potential therapeutic suitability in aging and age-related disorders.

A Miniaturized Screen of a Schistosoma mansoni Serotonergic G Protein-Coupled Receptor Identifies Novel Classes of Parasite-Selective Inhibitors

Schistosomiasis is a tropical parasitic disease afflicting ~200 million people worldwide and current therapy depends on a single drug (praziquantel) which exhibits several non-optimal features. These shortcomings underpin the need for next generation anthelmintics, but the process of validating physiologically relevant targets (‘target selection’) and pharmacologically profiling them is challenging. Remarkably, even though over a quarter of current human therapeutics target rhodopsin-like G protein coupled receptors (GPCRs), no library screen of a flatworm GPCR has yet been reported. Here, we have pharmacologically profiled a schistosome serotonergic GPCR (Sm.5HTR) implicated as a downstream modulator of PZQ efficacy, in a miniaturized screening assay compatible with high content screening. This approach employs a split luciferase based biosensor sensitive to cellular cAMP levels that resolves the proximal kinetics of GPCR modulation in intact cells. Data evidence a divergent pharmacological signature between the parasitic serotonergic receptor and the closest human GPCR homolog (Hs.5HTR7), supporting the feasibility of optimizing parasitic selective pharmacophores. New ligands, and chemical series, with potency and selectivity for Sm.5HTR over Hs.5HTR7 are identified in vitro and validated for in vivo efficacy against schistosomules and adult worms. Sm.5HTR also displayed a property resembling irreversible inactivation, a phenomenon discovered at Hs.5HTR7, which enhances the appeal of this abundantly expressed parasite GPCR as a target for anthelmintic ligand design. Overall, these data underscore the feasibility of profiling flatworm GPCRs in a high throughput screening format competent to resolve different classes of GPCR modulators. Further, these data underscore the promise of Sm.5HTR as a chemotherapeutically vulnerable node for development of next generation anthelmintics.

Parasitic flatworms express a diverse array of G protein coupled receptors, but our knowledge of their pharmacological profile is limited. No high throughput screen of a flatworm GPCR has been reported, even though these targets have precedent for high druggability and functionality in the chemotherapeutically vulnerable excitable cell niche. The goal of this study was to establish a method for profiling flatworm G protein coupled receptors that can be scaled to high content screening. Using a cAMP biosensor, we have performed a proof of principle miniaturized screen on a schistosome serotonergic GPCR that resolves new ligands that potently and selectivity block 5-HT receptor activity in vitro, and 5-HT evoked responses in schistosomules and adult worms. This approach evidences the pharmacological divergence of a parasitic GPCR from the closest human homolog and a capacity for high content interrogation of flatworm GPCR properties and ligand specificities.

In Vitro and In Vivo Characterization of the Alkaloid Nuciferine

Rationale

The sacred lotus (Nelumbo nucifera) contains many phytochemicals and has a history of human use. To determine which compounds may be responsible for reported psychotropic effects, we used in silico predictions of the identified phytochemicals. Nuciferine, an alkaloid component of Nelumbo nucifera and Nymphaea caerulea, had a predicted molecular profile similar to antipsychotic compounds. Our study characterizes nuciferine using in vitro and in vivo pharmacological assays.

Methods

Nuciferine was first characterized in silico using the similarity ensemble approach, and was followed by further characterization and validation using the Psychoactive Drug Screening Program of the National Institute of Mental Health. Nuciferine was then tested in vivo in the head-twitch response, pre-pulse inhibition, hyperlocomotor activity, and drug discrimination paradigms.

Results

Nuciferine shares a receptor profile similar to aripiprazole-like antipsychotic drugs. Nuciferine was an antagonist at 5-HT_2A, 5-HT_2C, and 5-HT_2B, an inverse agonist at 5-HT₇, a partial agonist at D_2, D₅ and 5-HT₆, an agonist at 5-HT_1A and D₄ receptors, and inhibited the dopamine transporter. In rodent models relevant to antipsychotic drug action, nuciferine blocked head-twitch responses and discriminative stimulus effects of a 5-HT_2A agonist, substituted for clozapine discriminative stimulus, enhanced amphetamine induced locomotor activity, inhibited phencyclidine (PCP)-induced locomotor activity, and rescued PCP-induced disruption of prepulse inhibition without induction of catalepsy.

Conclusions

The molecular profile of nuciferine was similar but not identical to that shared with several approved antipsychotic drugs suggesting that nuciferine has atypical antipsychotic-like actions.

Beta3-adrenoceptor function and long-term changes in body weight

BACKGROUND

The endogenous factors contributing to long-term changes in body weight are not known but the regulation of energy metabolism by different beta-adrenoceptors (beta(1)-AR, beta(2)-AR, beta(3)-AR) or alpha-adrenoceptors (alpha(2)-AR) may play a role.

METHODS

In a prospective study, we investigated beta-AR and alpha(2)-AR subtype function in subcutaneous fat cells of 85 healthy, non-obese women by using a standardized bioassay of lipolysis. Of these 73 were re-investigated on an average 10 years later to compare baseline function of beta(1)-AR, beta(2)-AR, beta(3)-AR and alpha(2)-AR with longitudinal weight changes.

RESULTS

Weight change over time was normally distributed ranging from-4 kg/m(2) to +6 kg/m(2) in body mass index. Long-term changes in body weight correlated inversely with beta(3)-AR function at base line (r=0.5, P=0.001). Those with low beta(3)-AR function gained weight, whereas the opposite was observed with those who had a high beta(3)-AR function. Nineteen percent of weight changes could be explained by beta(3)-AR status. No relationship with weight changes was observed as regards the function of alpha(2)-AR, beta(1)-AR or beta(2)-AR function.

CONCLUSIONS

Beta(3)-ARs are important for long-term changes in body weight putting energy metabolism in adipose tissue in frontline among endogenous factors that regulate body weight in adulthood.

Fetal alcohol exposure alters cerebrovascular reactivity to vasoactive intestinal peptide in adult sheep

Chronic fetal alcohol exposure impairs neural and vascular development. We have previously shown that fetal alcohol exposure is associated with attenuated hypoxic cerebral vasodilation and reduced neuronal vasoactive intestinal peptide (VIP) expression in fetal sheep. In the present study, we tested the hypothesis that fetal alcohol exposure alters vascular development, leading to altered cerebral vascular reactivity to VIP in adulthood. Penetrating intracerebral arterioles were harvested from the brains of adult (10-13 months old) offspring of ewes that had received intravenous infusions of alcohol (1.5 g/kg) or same-volume saline (90 min/day, 5 days/week) during days 30-82 of gestation (full term = 145 days). The isolated arterioles were cannulated with a micropipette system that allowed luminal perfusion and control of luminal pressure and developed spontaneous tone at 40 degrees C and 60 mm Hg luminal pressure. There was no difference in myogenic tone between arterioles exposed prenatally to alcohol (n = 18) and saline controls (n = 17). However, fetal alcohol exposure significantly (p = 0.03) enhanced the dilator responses of adult intracerebral arterioles to VIP [0.1 nM to 1 microM, logEC(50): -8.6 +/- 0.2 (alcohol) vs. -7.4 +/- 0.8 (saline)]. In contrast, there was no difference in dilator responses to H(+) (pH 6.8 buffer), to adenosine (10 nM to 0.1 mM), or to CGS21680 (an adenosine A(2A) receptor agonist, 0.01 nM to 10 microM). Thus, fetal alcohol exposure alters vasomotor sensitivity to VIP in adult intracerebral arterioles - perhaps a compensatory response to alcohol-induced underdevelopment of neurotransmitter pathways involved in cerebral vascular regulation.

GPCR functional selectivity has therapeutic impact

Many in vitro data show that some ligands can cause the differential activation of signaling pathways mediated by a single receptor (termed 'functional selectivity'). It remains unclear, however, whether functionally selective properties are meaningful in vivo. Data obtained with experimental compounds that are functionally selective at the dopamine D2L receptor in vitro suggest that these properties might predict atypical behavioral actions. Moreover, the antipsychotic drug aripiprazole is commonly thought to be a D2 partial agonist, but data clearly show that aripiprazole is functionally selective in vitro. It is proposed that the effects of aripiprazole in animal models and humans can be reconciled only with its functionally selective D2 properties, not its partial D2 agonism. Together, these data provide support for the hypothesis that compounds with functionally selective properties in vitro are likely to have novel actions in vivo, opening doors to new avenues of drug discovery.

Characterization of a selective and potent antagonist of human P2X(7) receptors, AZ11645373

BACKGROUND AND PURPOSE

The ATP-gated P2X(7) receptor has been shown to play a role in several inflammatory processes, making it an attractive target for anti-inflammatory drug discovery. We have recently identified a novel set of cyclic imide compounds that inhibited P2X(7) receptor-mediated dye uptake in human macrophage THP-1 cells. In this study the actions and selectivity of one of these compounds, AZ11645373, were characterized.

EXPERIMENTAL APPROACH

We measured membrane currents, calcium influx, and YOPRO-1 uptake from HEK cells expressing individual P2X receptors, and YOPRO1 uptake and interleukin-1beta release from THP-1 cells in response to ATP and the ATP analogue benzoylbenzoyl ATP (BzATP).

KEY RESULTS

AZ11645373 up to 10 microM, had no agonist or antagonist actions on membrane currents due to P2X receptor activation at human P2X(1), rat P2X(2), human P2X(3), rat P2X(2/3), human P2X(4), or human P2X(5) receptors expressed in HEK cells. AZ11645373 inhibited human P2X(7) receptor responses in HEK cells in a non-surmountable manner with K (B) values ranging from 5 - 20 nM, with mean values not significantly different between assays. K (B) values were not altered by removing extracellular calcium and magnesium. ATP-evoked IL-1beta release from lipopolysaccharide-activated THP-1 cells was inhibited by AZ11645373, IC(50) = 90 nM. AZ11645373 was > 500-fold less effective at inhibiting rat P2X(7) receptor-mediated currents with less than 50% inhibition occurring at 10 microM.

CONCLUSIONS AND IMPLICATIONS

AZ11645373 is a highly selective and potent antagonist at human but not rat P2X(7) receptors and will have much practical value in studies of human cells.

Perillyl Alcohol Inhibits a Calcium-Dependent Constitutive Nuclear Factor-κB Pathway

The cell death induced by the monoterpene anticancer agent perillyl alcohol correlates to the increased expression of certain proapoptotic genes known to influence cell survival. Whereas sequence-specific DNA-binding factors dictate the expression patterns of genes through transcriptional regulation, those transcriptional factors influencing constitutive cell survival with perillyl alcohol treatment are not well studied. Here, we investigated whether the monoterpenes can regulate the activity of nuclear factor-kappaB (NF-kappaB), a calcium-dependent transcription factor necessary for survival in the WEHI-231 B-lymphoma cells. Unique among the monoterpenes, perillyl alcohol short-term treatment induced a persistent decrease of calcium levels, whereas other various monoterpenes caused transient reductions in calcium levels. Perillyl alcohol treatment also rapidly elicited reductions of NF-kappaB DNA-binding activity and target gene induction, which was associated with an increase in apoptosis in these B-lymphoma cells. This apoptosis was directly due to NF-kappaB because its prior activation abolished the cell killing effects of perillyl alcohol treatment. Our findings suggest that perillyl alcohol can inhibit NF-kappaB function to modulate gene expression patterns and cell survival of certain B-lymphoma cells. The effects of perillyl alcohol were not limited to these B-lymphoma cells but were also observed in MDA-MB 468 cells, an estrogen receptor-negative breast cancer cell line. These results identify a calcium-dependent NF-kappaB pathway as a molecular target of perillyl alcohol activity in different cancer cell types.

Evidence for an important role of perilipin in the regulation of human adipocyte lipolysis

AIMS/HYPOTHESIS

We investigated the role of the adipocyte-specific protein perilipin for lipolysis in humans.

METHODS

Perilipin protein content and lipolysis rates were measured in human subcutaneous fat cells of non-obese (n=10) and obese (n=117) women. Single nucleotide polymorphisms in the perilipin gene were examined in obese subjects.

RESULTS

Basal and noradrenaline-induced rates of lipolysis were two to fourfold increased (p<0.01) and perilipin protein content decreased 50% (p=0.005) in adipocytes of the obese women. In subjects matched for body mass index and fat-cell volume, a high rate of lipolysis was associated with a low adipocyte content of perilipin (p=0.01). Adipocyte content of perilipin was inversely correlated with the circulating concentrations of glycerol (r=0.62) and non-esterified fatty acids (n=0.49). A gene polymorphism (rs891460 A/G) in intron 6 was common. In AA subjects basal and noradrenaline induced lipolysis were 50 to 100% times more rapid (p</=0.01) and the adipocyte perilipin content was about 80% reduced (p=0.005) as compared to GG carriers. Intermediate values were found in AG carriers.

CONCLUSIONS/INTERPRETATION

Perilipin seems important for the regulation of lipolysis in human fat cells. Obesity and a polymorphism in the perilipin gene associate with decreased protein content of perilipin and increased basal (unstrained) and noradrenaline-induced lipolysis. Low perilipin content also associate with high in vivo lipolytic activity. Perilipin could be a factor behind impaired lipolysis in insulin-resistant conditions.

Calpain-10 gene polymorphism is associated with reduced beta(3)-adrenoceptor function in human fat cells

Polymorphism in the calpain-10 gene is linked to type 2 diabetes, insulin resistance, and decreased thermogenesis. In view of the role of beta-adrenoceptors in thermogenesis we investigated the relationship between beta(1)-, beta(2)-, and beta(3)-adrenoceptor-stimulated lipolysis in abdominal sc fat cells and 3 different previously described single nucleotide polymorphisms (SNPs) in the calpain-10 gene (SNP-19, SNP-43, and SNP-63). The study sample comprised 240 healthy subjects. A strong association between lipolytic beta(3)-receptor function in adipocytes and the SNP-19, which is a deletion/insertion (1/2) was observed in overweight subjects (body mass index, >25 kg/m(2)), but not in lean ones. No association was found between any of the polymorphisms and lipolytic function of either beta(1)- or beta(2)-receptors. Carriers of 1/1 in SNP-19 had 30-fold decreased lipolytic sensitivity of beta(3)-adrenoceptors in comparison to 1/2 or 2/2 carriers (P = 0.0019, by ANOVA). This was found in both genders and was not influenced by SNP-43 or SNP-63 in the calpain-10 gene or by the Trp(64)Arg polymorphism in the beta(3)-adrenoceptor gene. In conclusion, a deletion/insertion polymorphism in the calpain-10 gene (SNP-19) is associated with reduced beta(3)-adrenoceptor function in obesity. This could be of importance for regulating thermogenesis in overweight subjects.

Effect of the (C825T) Gbeta(3) polymorphism on adrenoceptor-mediated lipolysis in human fat cells

A common Gbeta(3) gene polymorphism (C825T) influences G protein receptor-mediated signal transduction. We investigated whether this polymorphism influences lipolysis in isolated subcutaneous fat cells from 114 healthy obese subjects. The Gbeta(3) protein content was markedly decreased in adipocytes of TT carriers, but the alternatively spliced short form of Gbeta(3) previously shown in platelets of 825T carriers was not detected. Fat cells of TT carriers showed a significant 10-fold decrease in the half-maximum effective concentration of agonists selective for lipolytic beta(1)- and beta(2)-adrenoceptors as well as for the antilipolytic alpha(2)A-adrenoceptor. In TT carriers, maximum beta-adrenoceptor agonist-stimulated lipolysis was decreased, but the maximum antilipolytic effect of alpha(2)-adrenoceptors was less marked. Norepinephrine induced adipocyte lipolysis and circulating fasting levels of free fatty acids and glycerol were reduced by half in TT carriers. The polymorphism did not influence the adipocyte content of alpha(2)A-adrenoceptors, beta(2)-adrenoceptors, Galpha(i), or Galpha(s). In conclusion, the C825T variant of Gbeta(3) influences lipolysis. Adipocytes of TT carriers have a lower Gbeta(3) protein content and a decreased function of native G(s)- as well as G(i)-coupled adrenoceptors, which reduces the lipolytic effect of catecholamines. These data differ from those obtained in other cell systems that have shown increased expression of an alternative spliced Gbeta(3) variant and enhanced G protein signaling in 825T carriers, indicating that the polymorphism has cell type-specific effects that may be of importance for type 2 diabetes and other insulin-resistant conditions.

The Arg 389 Gly beta1-adrenergic receptor gene polymorphism and human fat cell lipolysis

BACKGROUND

The beta1-adrenoceptor is a candidate gene for obesity because of its role in catecholamine-induced energy homeostasis. A common Arg 389 Gly variant polymorphism has been shown in recombinant cells to influence its-coupling properties.

OBJECTIVE

To investigate the effect of the Arg 389 Gly beta1-adrenoceptor polymorphism on catecholamine-induced lipolysis in native human fat cells obtained by subcutaneous biopsy.

SUBJECTS

Two-hundred and ninety-eight apparently healthy male and female subjects with a wide variation in body mass index (BMI, 18-60 kg/m2).

MEASURES

The lipolytic sensitivities and maximum lipolytic action of noradrenaline and the selective adrenoceptor agonists dobutamine (beta1), terbutaline (beta2) and CGP 12177 (beta3) were determined in isolated subcutaneous adipocytes and related to beta-adrenoceptor radioligand binding parameters.

RESULTS

No differences in the sensitivity or maximum lipolytic capacity of the agonists were found between the genotypes. This was true both when all subjects were analyzed together and when subgroups (lean, obese, men, women) were analyzed separately. Radioligand binding to beta1- or beta2-adrenoceptors was also similar between genotypes. The polymorphism had no important influence on either BMI or the distribution of obese and non-obese subjects between the genotypes.

CONCLUSION

The distribution of the Arg 389 Gly polymorphism is similar in lean and obese subjects and has no apparent effect on the lipolytic response to beta-adrenergic stimulation in native human adipocytes. This suggests, despite the altered coupling properties reported in recombinant cells, that the Arg 386 Gly polymorphism has no important influence on human obesity.

A common hormone-sensitive lipase i6 gene polymorphism is associated with decreased human adipocyte lipolytic function

Hereditary factors may be involved in the pathogenesis of type 2 diabetes. A polymorphism in the hormone-sensitive lipase (HSL) gene (HSLi6) is associated with obesity and diabetes, although it is unknown whether the polymorphism is functional and thereby influences lipolysis. We genotyped 355 apparently healthy nonobese male and female subjects for the HSLi6 polymorphism. Allele 5 was found to be the most common allele (allele frequency 0.57). In 117 of the subjects, we measured abdominal subcutaneous fat cell lipolysis induced by drugs acting at various steps in the lipolytic cascade. The lipolysis rate induced by norepinephrine isoprenaline (acting on beta-adrenoceptors), forskolin (acting on adenylyl cyclase), and dibutyryl cyclic AMP (acting on HSL) were all decreased by approximately 50% in allele 5 homozygotes, as compared with noncarriers. Heterozygotes showed an intermediate lipolytic rate. The difference in lipolysis rate between genotypes was more pronounced in men than in women. We conclude that allele 5 of the HSLi6 polymorphism is associated with a marked decrease in the lipolytic rate of abdominal fat cells. This may in turn contribute to the development of obesity.

The effect of the beta(2) adrenoceptor gene Thr164Ile polymorphism on human adipose tissue lipolytic function

A rare beta(2)-adrenoceptor gene polymorphism, Thr164Ile, has been described that impairs receptor function when transfected into cell lines. We investigated whether the polymorphism influences native receptor function by studying lipolysis in freshly isolated subcutaneous fat cells from 236 apparently healthy subjects. Twelve subjects were heterozygous for the 164Ile variant. The fat cells of Ile carriers displayed a 6 fold increase (P=0.02) in the lipolytic EC(50) of terbutaline (a selective beta(2)-adrenoceptor agonist), but no change in the lipolytic action of dobutamine (a selective beta(1)-adrenoceptor agonist), compared with the Thr carriers. Maximum adrenoceptor agonist stimulated lipolysis did not differ between Thr and Ile carriers. The influence of two other polymorphisms (Arg16Gly and Gln27Glu) in the beta(2)-adrenoceptor gene was considered. Six 164Ile carriers also carried the 16Gly and 27Glu alleles. The latter combination occurred among 105 of the 164Thr carriers. For the 16Gly27Glu subgroup, the EC(50) of terbutaline was about 10 fold higher in 164Ile as than in 164Thr carriers (P=0.02) but there was no difference between genotypes in maximum terbutaline action. There was no difference between groups in dobutamine action. In conclusion, the 164Ile variant of the beta(2)-adrenoceptor is associated with a decreased native adipocyte receptor function, as evidenced by a marked increase in the half maximal effective concentration of the lipolytic action of a selective beta(2)-adrenoceptor agonist. This suggests that genetic variance in the beta(2)-adrenoceptor gene might be important for catecholamine function in humans, at least as far as adipocyte lipolysis is concerned.

Plasmon resonance studies of agonist/antagonist binding to the human delta-opioid receptor: new structural insights into receptor-ligand interactions

Structural changes accompanying the binding of ligands to the cloned human delta-opioid receptor immobilized in a solid-supported lipid bilayer have been investigated using coupled plasmon-waveguide resonance spectroscopy. This highly sensitive technique directly monitors mass density, conformation, and molecular orientation changes occurring in anisotropic thin films and allows direct determination of binding constants. Although both agonist binding and antagonist binding to the receptor cause increases in molecular ordering within the proteolipid membrane, only agonist binding induces an increase in thickness and molecular packing density of the membrane. This is a consequence of mass movements perpendicular to the plane of the bilayer occurring within the lipid and receptor components. These results are consistent with models of receptor function that involve changes in the orientation of transmembrane helices.

Long-term nitric oxide inhibition and chronotropic responses in rat isolated right atria

The long-term administration of nitric oxide synthesis inhibitors induces arterial hypertension accompanied by left ventricular hypertrophy and myocardial ischemic lesions. Because the enhancement of sympathetic drive has been implicated in these phenomena, the current study was performed to determine the potency of beta-adrenoceptor agonists and muscarinic agonists on the spontaneous rate of isolated right atria from rats given long-term treatment with the nitric oxide inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME). Atrial lesions induced by long-term treatment with L-NAME were also evaluated. Long-term L-NAME treatment caused a time-dependent, significant (P<0.05) increase in tail-cuff pressure compared with control animals. Our results showed that the potency of isoproterenol, norepinephrine, carbachol, and pilocarpine in isolated right atria from rats given long-term treatment with L-NAME for 7, 15, 30, and 60 days was not affected as compared with control animals. Addition of L-NAME in vitro (100 microl/L) affected neither basal rate nor chronotropic response for isoproterenol and norepinephrine in rat heart. Stereological analysis of the right atria at 15 and 30 days revealed a significant increase on amount of fibrous tissues in L-NAME-treated groups (27+/-2.3% and 28+/-1.3% for 15 and 30 days, respectively; P<0.05) as compared with the control group (22+/-1.1%). Our results indicate that nitric oxide does not to interfere with beta-adrenoceptor-mediated and muscarinic receptor-mediated chronotropic responses.

The influence of chronic hypokalemia on myocardial adrenergic receptor densities: enhanced sensitivity to epinephrine-induced arrhythmias

We studied the effects of a 30-day potassium (K+)-deficient diet on blood [K+] myocardial adrenergic receptor densities, serum catecholamines, and epinephrine arrhythmogenicity in adult laboratory rats (250 +/- 25 g). Within 3 days of beginning the K+-deficient diet, blood [K+] decreased by 50%. After 5 days, the myocardial alpha-1 density increased (62 +/- 2 vs 148 +/- 16 fmols/mg protein), and the total beta receptor increased (95 +/- 5 vs 273 +/- 49) without significant change in receptor affinity. However, 18-21 days of this diet was necessary to produce an increase in the duration of epinephrine arrhythmias (from 56 +/- 8 to 224 +/- 21 s). While prazosin block of the alpha-1 receptor in hypokalemic rats caused a significant, 42% reduction in arrhythmic duration and propranolol block caused a 62% reduction, both prazosin and propranolol were necessary to return arrhythmia times to normal (44 +/- 0.3 mmols/dL). Total serum catecholamines were reduced after 3 days of the diet (from 482 +/- 37 to 299 +/- 31 pg/ml) and remained depressed throughout the 30 days of the K+ diet. The results of this study indicate that prolonged restriction causes a reduction in serum catecholamines, an increase in myocardial alpha-1 and beta receptors densities, and an increase in epinephrine arrhythmogenicity. All of these changes were reversed within 5 days of initiating a normal dietary K+ intake.

Efficacy. I: A new method for estimating relative efficacy of full agonists via a newly defined efficacy related parameter

A new method for estimating relative efficacies and relative intrinsic efficacies of agonists is described. Relative efficacy is estimated by employing a newly defined efficacy related parameter (eES) and it may be estimated without prior knowledge of efficacy values or the value of the equilibrium dissociation constants, KA, of agonist-receptor complexes. The parameter eES is directly related to efficacy (e) and is defined as the ratio of maximal stimulus to maximal effect of an agonist. The value of eES indicates whether or not spare receptors are present for a particular agonist-effector system. The eES values of agonists are estimated by utilizing submaximal concentration-effect curves determined with fixed agonist-competitive antagonist concentration combinations and choosing a suitable reference (height of an agonistic concentration-effect curve) to which the height of the stimulus concentration-effect curves of the agonist may be compared. In addition to eES, other new agonist-effector parameters, namely SEm/Sm and phi min, were also defined.

Antagonism of the effects of (+)-PD 128907 on midbrain dopamine neurones in rat brain slices by a selective D2 receptor antagonist L-741,626

1. The ability of PD 128907 to activate dopamine receptors in the ventral tegmental area, substantia nigra pars compacta, and striatum was investigated by use of in vitro electrophysiological recording and fast cyclic voltammetry. The affinity of a novel D2 selective antagonist L-741,626 for receptors activated by this agonist was measured to determine if its effects were mediated by D2 or D3 receptors. 2. The active (+) enantiomer of PD 128907 bound with high affinity and selectivity to rat D3 dopamine receptors. The Ki values for (+)-PD 128907 were 620 nM at D2, 1 nM at D3 and 720 nM at D4 receptors. 3. (+)-PD 128907 inhibited cell firing in both the ventral tegmental area and substantia nigra pars compacta with EC50 values of 33 nM (pEC50 = 7.48 +/- 0.10, n = 10) and 38 nM (pEC50 = 7.42 +/- 0.15, n = 5), respectively. No effects of (+)-PD 128907 (100 nM) were observed on glutamate or GABA-mediated synaptic potentials elicited by focal bipolar stimulation. 4. L-741,626 antagonized these effects of (+)-PD 128907 in a concentration-dependent and surmountable manner with an affinity, determined from Schild analysis, of 20 nM (pKB = 7.71 +/- 0.14) in the ventral tegmental area and 11 nM (pKB = 7.95 +/- 0.18) in the substantia nigra pars compacta. 5. (+)-PD 128907 also inhibited dopamine release in the caudate-putamen with an EC50 of 66 nM (n = 5). The affinity of L-741,626 for these nerve terminal autoreceptors (pKB = 7.71 +/- 0.06; = 20 nM) was identical to that observed on midbrain dopamine neurones. 6. These data demonstrate that the D3 receptor ligand (+)-PD 128907 is a potent agonist on rat midbrain dopamine neurones. However, its lack of regional selectivity, and the high affinity of the selective D2 receptor antagonist L-741,626 for receptors activated by (+)-PD 128907, was more consistent with an action on D2 autoreceptors rather than upon a D3 dopamine receptor subtype.

Selective inactivation of muscarinic receptor subtypes

Muscarinic receptors exist in multiple subtypes, denoted as M1, M2, M3 and M4, encoded by four distinct but related genes. A fifth gene product, m5, has also been predicted although this sequence awaits a pharmacological equivalent. Many tissues express more than one muscarinic receptor subtype, which may couple to different intracellular effectors and thus have different physiological roles. One way to characterize the role of each receptor is to selectively inactivate one receptor population, thus pharmacologically 'isolating' the muscarinic receptor subtype of interest. Selective receptor inactivation can be achieved using either a selective, irreversible antagonist, or protection using a selective, reversible antagonist against a non-selective irreversible antagonist. Therefore, combination of these two approaches may provide optimal selective inactivation. Several muscarinic alkylating agents have been identified, including phenoxybenzamine, EEDQ (N-Ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline) and propylbenzilylcholine mustard. These irreversible antagonists do not, in general, discriminate between muscarinic receptor subtypes and are frequently used to estimate the affinity and relative efficacy of muscarinic agonists. Consequently, use of these irreversible antagonists provides estimations of the 'receptor reserve' associated with a response mediated by muscarinic receptor activation. In contrast, 4-DAMP mustard (4-diphenylacetoxy-N-(2-chloroethyl)piperidine) selectively inactivates M3 receptors, but will not discriminate between M1, M2 or M4 receptors. In the absence of highly selective alkylating agents, receptor protection by reversible antagonists may be used. Thus, reversible antagonists, such as pirenzepine, methoctramine or para-fluorohexahydrosiladifenidol, at appropriate fractional receptor occupancies, may protect M1, M2 or M3 receptors against alkylation by phenoxybenzamine. Selective alkylation of M3 receptors by 4-DAMP mustard is enhanced with concurrent M2 protection. This approach has been applied to defining the role of these muscarinic receptor subtypes in the control of ileal smooth muscle tone. These data suggest that, in ileum, M2 receptors may act to inhibit beta-adrenoceptor activation, thereby offsetting relaxation, while M3 receptors directly mediate contraction.

Effects of two truncated forms of human calcitonin-gene related peptide: implications for receptor classification

We investigated the possibility that human alpha-calcitonin-gene related peptide (CGRP)-(8-37) and human beta CGRP-(8-37) show some selectivity as antagonists of CGRP1 and CGRP2 receptor-mediated responses. Bindings assays showed that human alpha CGRP, human alpha CGRP-(8-37) and human beta CGRP-(8-37) showed high affinity (in the nanomolar concentration range) for CGRP receptors expressed in SK-N-MC cells and also in rat brain membrane preparations. Both human alpha CGRP-(8-37) and human beta CGRP-(8-37) were potent antagonists of human alpha CGRP-stimulated cAMP accumulation in SK-N-MC cells. However, both human alpha CGRP-(8-37) and human beta CGRP-(8-37) were weakly effective in antagonizing human alpha CGRP-stimulated responses in guinea-pig atria and rat vas deferens. In rat vas deferens, but not guinea-pig atria, the effects of human alpha CGRP and human alpha CGRP-(8-37) (but not human beta CGRP-(8-37)) were potentiated by thiorphan. Neither human alpha- nor human beta CGRP-(8-37) showed selectivity for supposedly CGRP1 and CGRP2 receptor-mediated responses. Furthermore, differences in the effects of the truncated CGRP analogues may reflect differences in enzyme distribution rather than the existence of CGRP receptor subtypes.

Comparison between the pharmacology of dopamine receptors mediating the inhibition of cell firing in rat brain slices through the substantia nigra pars compacta and ventral tegmental area

1. Electrophysiological recordings were made from presumed dopaminergic neurons in the substantia nigra pars compacta and ventral tegmental area of rat brain slices. The ability of selective dopamine receptor agonists to hyperpolarize neurones and inhibit cell firing, as well as the ability of dopamine receptor antagonists to block responses to quinpirole were compared. 2. Six dopamine receptor agonists were examined for their ability to hyperpolarize neurones within the substantia nigra pars compacta. Of these, the most potent ligand tested was naxagolide with an EC50 value of 20 nM and estimated maximum of 10 mV. The rank order of agonist potency was naxagolide > quinpirole > apomorphine > dopamine. 3. Quinpirole was more potent at inhibiting cell firing in the substantia nigra pars compacta (pIC50 = 7.65 +/ 0.06, n = 35) than in the ventral tegmental area (pIC50 = 7.24 +/- 0.06, n = 32; P < 0.01, Student's t test). 7-Hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT), a putative D3 selective agonist, had a comparable potency to quinpirole in both the ventral tegmental area (pIC50 = 7.39 +/- 0.26, n = 4), and substantia nigra pars compacta (pIC50 = 7.71 +/- 0.20; n = 4). 4. The inhibition of cell firing by quinpirole was antagonized by haloperidol, S(-)-sulpiride, clozapine, and ritanserin. S(-)-sulpiride and haloperidol had the highest estimated affinities in the substantia nigra, with pA2 values of 8.97 (slope = 0.85) and 8.20 (slope = 2.09) respectively. The pA2 values for S(-)-sulpiride and haloperidol in the ventral tegmental area were 8.07 (slope = 0.87) and 8.11 (slope = 1.48)respectively. Clozapine had a lower functional affinity than S(-)-sulpiride and haloperidol in both the substantia nigra (pA2 = 6.47, slope = 1.19) and ventral tegmental area (pA2 = 6.53, slope 0.87). Ritanserin,a 5-HT2 receptor antagonist that also binds to D2.u. dopamine receptors, caused a slight but significant shift in the concentration-effect curve to quinpirole with an estimated pKA of 6.97 +/- 0.13(n =4) in the substantia nigra and pKA of 7.12 +/- 0.22 (n =4) in the ventral tegmental area.5. Comparison of these data with the binding affinity for cloned dopamine receptors demonstrates that the responses to quinpirole on dopaminergic neurones in both the A9 (substantia nigra) and A10(ventral tegmental area) brain areas are consistent with the activation of predominantly D2, and not D3 or D4 dopamine receptors. Furthermore, the similarity in functional affinity of antagonists for these receptors suggest that the mesolimbic selectivity of atypical neuroleptics, like clozapine, may be a consequence of their actions on other receptors or their effects elsewhere in the brain.

Differences in the effects of NK1-receptor antagonists, (+/-)-CP 96,345 and CP 99,994, on agonist-induced responses in guinea-pig trachea

1. The effects of the NK1-receptor antagonists, (+/-)-CP 96,345 and CP 99,994, on NK1-agonist evoked contractions were compared in isolated rings of guinea-pig tracheal smooth muscle. 2. (+/-)-CP 96,345 and CP 99,994 were similarly effective in antagonizing responses evoked by septide, whereas CP 99,994 was more effective than (+/-)-CP 96,345 in inhibiting responses evoked by [Sar9Met11(O2)] substance P. 3. These results suggest that responses to septide and [Sar9Met11(O2)] substance P may be operated via different populations of NK1-receptors.

Differences in the effects of tachykinin NK1 receptor antagonists: neuronal versus smooth muscle tissues

The effects of three tachykinin NK1 receptor antagonists (L-668,169, (+/-)-RP 67580, and (+/-)-CP 96.345) were examined for their ability to antagonise responses evoked by substance P O-methyl ester (a selective NK1 receptor agonist) in isolated neuronal tissue (rat superior cervical ganglia and guinea-pig locus coeruleus) and smooth muscle tissues (rat urinary bladder and guinea-pig ileum longitudinal muscle/myenteric plexus). (+/-)-RP 67580 was similarly effective in antagonising responses in both rat superior cervical ganglia and urinary bladder (estimated pKa value = 7.4 for both tissues); however, (+/-)-CP 96,345 was 50-fold less effective in antagonising responses in guinea-pig locus coeruleus than in ileum longitudinal muscle/myenteric plexus (estimated pKa values = 7.6 and 9.3 respectively). It is suggested that the differential effects of (+/-)-CP 96,345 may reflect the existence of a population of NK1 receptors within guinea-pig locus coeruleus that are less sensitive to the effects of this NK1 receptor antagonist.

Muscarinic M3 receptors mediate total inositol phosphates accumulation in murine HSDM1C1 fibrosarcoma cells

Muscarinic receptors in murine fibrosarcoma HSDM1C1 cells were characterized using both radioligand binding and total inositol phosphates accumulation (IPs). Muscarinic agonists elicited a concentration-dependent enhancement of IPs accumulation with a maximum of 14-fold stimulation above basal level. The following potencies (-log EC50) were observed for the full agonists: (+)-cis-dioxolane 5.4, oxotremorine-M 5.3, (+)-muscarine 5.2 and carbachol 5.0. Bethanechol (4.1) and arecoline (5.0) were partial agonists, evoking 43 and 55%, respectively of the maximum level of stimulation to (+)-cis-dioxolane, whereas pilocarpine and McN-A-343 were inactive as agonists (1 mumol/l-1 mmol/1). The apparent affinities for muscarinic antagonists (-log KB) estimated by Schild regression were: 4-DAMP (4-diphenylacetoxy-N-methylpiperidine methiodide) 9.2, dicyclomine 7.0, pirenzepine 6.9, (+/-)-p-F-HHSiD (para-fluoro-hexahydro-siladifenidol) 7.0, AF-DX 116 6.2, methoctramine 5.7. In saturation binding studies using [3H]N-methylscopolamine a homogeneous population of sites was identified, with a density of 145 pmol/mg protein. In competition radioligand binding studies, the following apparent affinities (-log Ki) were observed: 4-DAMP 9.7, dicyclomine 8.3, (+/-)-p-F-HHSiD 7.6, AF-DX 116 6.8, methoctramine 6.6 and gallamine 6.8. In binding studies all antagonists studied recognized a single population of sites, as judged by the Hill coefficients from the displacement isotherms. These data are consistent with HSDM1C1 cells expressing an apparent homogeneous muscarinic M3 population that mediates a large level of total IPs accumulation. This clonal line may provide a useful model to further elucidate relationship between endogenous muscarinic M3 receptor stimulation and IPs accumulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Mediation by the same muscarinic receptor subtype of phasic and tonic contractile activities in the rat isolated portal vein

1. The effects of several agonists on the phasic and tonic contractile responses to muscarinic receptor stimulation have been investigated in the rat portal vein in vitro. 2. Neither chemical denervation with 6-hydroxydopamine nor the presence of the alpha 1-adrenoceptor antagonist, prazosin, influenced the spontaneous or the stimulated myogenic activity of the portal vein. 3. Indomethacin and NG-nitro-L-arginine were used to investigate the influence of vasoactive factors in this preparation. They slightly increased the frequency and the amplitude of the spontaneous myogenic activity of the portal vein, respectively. NG-nitro-L-arginine but not indomethacin enhanced the maximal phasic response to carbachol. Both indomethacin and NG-nitro-L-arginine failed to influence the tonic response to carbachol. 4. Muscarinic agonists increased phasic activity according to the rank order of potency: acetylcholine > muscarine > methacholine > carbachol > aceclidine > bethanechol. These effects were superimposed on a sustained contracture at higher concentrations. Oxotremorine was more potent than arecoline in increasing the mechanical phasic activity, without inducing a sustained contracture. Pilocarpine and McN A343 were weak agonists, producing submaximal effects only on phasic activity. 5. The muscarinic antagonists AF-DX116, 4-diphenylacetoxy-N-methylpiperidine (4-DAMP), P-fluorohexahydrosiladiphenidol (pFHHSiD) and pirenzepine antagonized the phasic and tonic mechanical responses to carbachol. Although the tonic contracture was slightly more sensitive to all antagonists studied, the rank order of potency: 4-DAMP > pFHHSiD > pirenzepine > AF-DX 116 was the same for both types of responses, which is indicative of a M3-receptor subtype. 6. The tonic contractile response of the rat portal vein to carbachol was more susceptible to partial receptor inactivation with propylbenzilylcholine mustard than the phasic contractile response. The dissociation constants (KA) obtained from an analysis according to Furchgott & Bursztyn (1967) were found to be 4.32 +/- 0.31 1AM for the phasic and 3.56 +/- 0.21 1AM for the tonic type of carbachol-induced response, respectively. Since the EC50-values for both carbachol-induced effects were different (phasic0.232 +/- 0.02 1AM; tonic 2.75 +/- 0.1 1AM) the phasic type of response appears to involve a large receptor reserve.

Angiotensin converting enzyme binding sites in human heart and lung: comparison with rat tissues

1. Angiotensin converting enzyme (ACE), a dipeptidyl carboxypeptidase which catalyzes the final activation step in the formation of angiotensin II, was identified by radioligand studies in rat heart and lung. In this work we identified ACE binding sites in human left ventricle and lung by radioligand binding using the ACE inhibitor [3H]-ramiprilat in all tissues tested was saturable, temperature and zinc-dependent, and inhibited by EDTA. In human left ventricle homogenate we found a density of binding sites of 121 +/- 15 fmol mg-1 protein (n = 4) with an affinity (Kd) of 850 +/- 55 pM, whereas in rat left ventricle the same values were 23 +/- 4 fmol mg-1 protein and 315 +/- 30 pM, (n = 4), respectively. 3. [3H]-ramiprilat binding to rat (n = 4) and human lung (n = 4) showed a binding site density of 2132 +/- 155 and 1085 +/- 51 fmol mg-1 protein respectively with an affinity of 639 +/- 54 and 325 +/- 22 pM. The lung:heart ratio of ACE binding site density was about 9:1 in man and 100:1 in rat. 4. The binding affinities of 13 ACE inhibitors were evaluated on human heart and lung: the drugs tested showed a wide range of affinities for the ACE binding sites in both tissues, and the affinity for lung was significantly greater than for heart for most of the drugs. 5. The greater potency of some ACE inhibitors in displacing [3H]-ramiprilat in human lung compared with the heart indicates differences between ACE binding sites in these tissues and suggests the possibility of a selective organ-targeted therapeutic approach.

L-689,660, a novel cholinomimetic with functional selectivity for M1 and M3 muscarinic receptors

1. L-689,660, 1-azabicyclo[2.2.2]octane, 3-(6-chloropyrazinyl)maleate, a novel cholinomimetic, demonstrated high affinity binding (pKD (apparent) 7.42) at rat cerebral cortex muscarinic receptors. L-689,660 had a low ratio (34) of pKD (apparent) values for the displacement of binding of the antagonist ([3H]-N-methylscopolamine ([3H]-NMS) compared with the displacement of the agonist [3H]-oxotremorine-M ([3H]-Oxo-M), in rat cerebral cortex. Low NMS/Oxo-M ratios have been shown previously to be a characteristic of compounds that are low efficacy partial agonists with respect to stimulation of phosphatidyl inositol turnover in the cerebral cortex. 2. L-689,660 showed no muscarinic receptor subtype selectivity in radioligand binding assays but showed functional selectivity in pharmacological assays. At M1 muscarinic receptors in the rat superior cervical ganglion, L-689,660 was a potent (pEC50 7.3 +/- 0.2) full agonist in comparison with (+/-)-muscarine. At M3 receptors in the guinea-pig ileum myenteric plexus-longitudinal muscle or in trachea, L-689,660 was again a potent agonist (pEC50 7.5 +/- 0.2 and 7.7 +/- 0.3 respectively) but had a lower maximum response than carbachol. In contrast L-689,660 was an antagonist at M2 receptors in guinea-pig atria (pA2 7.2 (95% confidence limits 7, 7.4)) and at muscarinic autoreceptors in rat hippocampal slices. 3. The putative M1-selective muscarinic agonist, AF102B (cis-2-methylspiro-(1,3-oxathiolane 5,3')-quinuclidine hydrochloride) was found to have a profile similar to L-689,660 but had up to 100 times less affinity in binding and functional assays.RS-86 (2-ethyl-8-methyl-2,8-diazospiro[4,5]decan 1,3-dionehydrochloride) also had lower affinity than L-689,660, and had no binding selectivity for muscarinic receptor subtypes. RS-86 had a higher NMS/Oxo-M ratio than L-689,660 and was a full agonist at MI,M2 and M3 receptors in the functional pharmacological assays.4. The functional selectivity of L-689,660 in muscarinic pharmacological assays is consistent with the effects of a low efficacy partial agonist in tissues with different effective receptor reserves.

Influence of lithium pretreatment and of cooling on the responsiveness of the rat isolated jejunum and urinary bladder to muscarinic agonists

1. The aim of the present study was to see whether contractile responses induced by muscarinic agonists in the rat jejunum and urinary bladder were differently affected by procedures that mainly influence the steps following agonist-receptor interaction. Thus, the effects of ex vivo lithium pretreatment (6.8 mmol kg-1 i.p. for 3 days) and in vitro cooling from 37 degrees C to 20 degrees C) on the contractile responses to full and partial agonists, carbachol, oxotremorine, muscarine and pilocarpine were studied. 2. Lithium pretreatment did not affect muscarinic responses on the urinary bladder. It significantly reduced responses to carbachol and oxotremorine but not to muscarine and pilocarpine on the rat jejunum. 3. A decrease of the bath temperature from 37 degrees C to 20 degrees C potentiated responses to carbachol, muscarine and oxotremorine and abolished those to pilocarpine in the urinary bladder. The same lowering of the bath temperature potentiated responses to carbachol, did not affect those to muscarine and to oxotremorine and reduced those to pilocarpine in the jejunum. 4. Together the findings indicate that differences exist in the stimulus-response coupling induced by muscarinic agonists between the two tissues and that, in a given tissue, differences exist among agonists in their ability to activate the agonist-receptor-transducer complex.

Biphasic dose-response curves to arecoline in rat atria-mediation by a single promiscuous receptor or two receptor subtypes?

Arecoline produces a biphasic response in rat left atria, i.e., a depression of basal inotropy at low doses and a positive inotropic effect at higher doses. These present studies were designed to determine whether it can be shown that the two separate responses to arecoline are mediated by two distinct cell surface muscarinic receptors. The antagonists scopolamine, 4-DAMP and AF-DX 116 produced apparent simple competitive antagonism of the negative responses to arecoline. Schild analysis was used to measure the equilibrium dissociation constant of the antagonist-receptor complex for antagonism of this response to arecoline by these antagonists. In atria from rats treated with pertussis toxin, the negative inotropy to arecoline was abolished and only the positive inotropic effects were observed. The antagonism of the positive inotropic response to arecoline by these antagonists was studied separately in atria from rats treated with pertussis toxin by the Schild technique. The pKB estimates made from the Schild regressions indicated no evidence to suggest that the two responses to arecoline (negative and positive inotropy) were mediated by two separate receptors in rat left atria. These data are discussed in terms of a single muscarinic receptor in this tissue mediating these two responses by interaction with two G-proteins in the same cell membrane. These data also are discussed in terms of the use of agonist potency ratios for the classification of receptors.

Promiscuous or heterogeneous muscarinic receptors in rat atria? I. Schild analysis with simple competitive antagonists

Carbachol has been shown to produce a biphasic response in rat left atria. At low concentrations, carbachol depresses basal inotropy, while at high doses a positive inotropic effect is observed. The negative inotropic response can be selectively eliminated by pretreatment of rats with pertussis toxin. The aim of these studies was to determine whether or not evidence could be obtained to show that different muscarinic receptors produced these different biochemical responses to the agonist carbachol. Schild analysis was used to measure the equilibrium dissociation constant of the antagonist-receptor complex for antagonism of the negative inotropy to carbachol by atropine, scopolamine 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) and AF-DX 116. The antagonism of the positive inotropic response to carbachol by these antagonists was studied in atria from rats pretreated with pertussis toxin where the negative inotropy was nearly completely abolished. In general, it was found that the antagonists did not produce simple competitive blockade of the positive inotropy but rather a nominal shift to the right of the dose-response curves followed by a depression of maximal responses. However, it was found that when pA2 or pKb values could be calculated, they coincided with those determined for the antagonism of the negative inotropy to carbachol. The conclusion drawn from these experiments was that no evidence was obtained to disprove the null hypothesis that a common receptor, interacting with two G-proteins, mediates these two effects of carbachol in rat left atria. The implications of these data for the classification of drug receptors with agonists is discussed.