Structural and Functional Insights into CRF Peptides and Their Receptors.

Corticotropin-releasing factor or hormone (CRF or CRH) and the urocortins regulate a plethora of physiological functions and are involved in many pathophysiological processes. CRF and urocortins belong to the family of CRF peptides (CRF family), which includes sauvagine, urotensin, and many synthetic peptide and non-peptide CRF analogs. Several of the CRF analogs have shown considerable therapeutic potential in the treatment of various diseases. The CRF peptide family act by interacting with two types of plasma membrane proteins, type 1 (CRF1R) and type 2 (CRF2R), which belong to subfamily B1 of the family B G-protein-coupled receptors (GPCRs). This work describes the structure of CRF peptides and their receptors and the activation mechanism of the latter, which is compared with that of other GPCRs. It also discusses recent structural information that rationalizes the selective binding of various ligands to the two CRF receptor types and the activation of receptors by different agonists.

Rational Design, Synthesis and Binding Affinity Studies of Anthraquinone Derivatives Conjugated to Gonadotropin-Releasing Hormone (GnRH) Analogues towards Selective Immunosuppression of Hormone-Dependent Cancer.

Gonadotropin-releasing hormone (GnRH) is pivotal in regulating human reproduction and fertility through its specific receptors. Among these, gonadotropin-releasing hormone receptor type I (GnRHR I), which is a member of the G-protein-coupled receptor family, is expressed on the surface of both healthy and malignant cells. Its presence in cancer cells has positioned this receptor as a primary target for the development of novel anti-cancer agents. Moreover, the extensive regulatory functions of GnRH have underscored decapeptide as a prominent vehicle for targeted drug delivery, which is accomplished through the design of appropriate conjugates. On this basis, a rationally designed series of anthraquinone/mitoxantrone-GnRH conjugates (con1-con8) has been synthesized herein. Their in vitro binding affinities range from 0.06 to 3.42 nM, with six of them (con2-con7) demonstrating higher affinities for GnRH than the established drug leuprolide (0.64 nM). Among the mitoxantrone based GnRH conjugates, con3 and con7 show the highest affinities at 0.07 and 0.06 nM, respectively, while the disulfide bond present in the conjugates is found to be readily reduced by the thioredoxin (Trx) system. These findings are promising for further pharmacological evaluation of the synthesized conjugates with the prospect of performing future clinical studies.

Design, synthesis, structural optimization, SAR, in silico prediction of physicochemical properties and pharmacological evaluation of novel & potent thiazolo[4,5-d]pyrimidine corticotropin releasing factor (CRF) receptor antagonists.

Corticotropin-releasing factor (CRF) is a 41-amino-acid neuropeptide secreted from the hypothalamus and is the main regulator of the hypothalamus-pituitary-adrenocortical (HPA) axis. CRF is the master hormone which modulates physiological and behavioral responses to stress. Many disorders including anxiety, depression, addictive disorders and others are related to over activation of the CRF system. This suggests that new molecules which can interfere with CRF binding to its receptors may be potential candidates for neuropsychiatric drugs to treat stress-related disorders. Previously, three series of pyrimidine and fused pyrimidine CRF1 receptor antagonists were synthesized by our group and specific binding assays, competitive binding studies and determination of the ability to antagonize the agonist-stimulated accumulation of cAMP (the second messenger for CRF receptors) were reported. In continuation of our efforts in this direction, in the current manuscript, we report the synthesis & biological evaluation of a new series of CRF1 receptor antagonists. Seven compounds showed promising binding affinity with the best two compounds (compounds 6 & 43) displaying a superior binding affinity to all of our previous compounds. Compounds 6 & 43 have only 4 times and 2 times less binding affinity than the standard CRF antagonist antalarmin, respectively. Thus, our two best lead compounds (compound 6 & 43) can be considered potent CRF receptor antagonists with binding affinity of 41.0 & 19.2 nM versus 9.7 nM for antalarmin.

Analgesic and Anti-Inflammatory Effects of the Synthetic Neurosteroid Analogue BNN27 during CFA-Induced Hyperalgesia.

Dehydroepiandrosterone (DHEA), an adrenal and neurosteroid hormone with strong neuroprotective and immunomodulatory properties, and ligand for all high-affinity neurotrophin tyrosine kinase receptors (Trk), also exerts important effects on hyperalgesia. Its synthetic, 17-spiro-epoxy analogue, BNN27, cannot be converted to estrogen or androgen as DHEA; it is a specific agonist of TrkA, the receptor of pain regulator Nerve Growth Factor (NGF), and it conserves the immunomodulatory properties of DHEA. Our study aimed to evaluate the anti-nociceptive and anti-inflammatory properties of BNN27 during Complete Freund's Adjuvant (CFA)-induced inflammatory hyperalgesia in mice. Hyperalgesia was evaluated using the Hargreaves test. Inflammatory markers such as cytokines, NGF and opioids were measured, additionally to corticosterone and the protein kinase B (AKT) signaling pathway. We showed for the first time that treatment with BNN27 reversed hyperalgesia produced by CFA. The effect of BNN27 involved the inhibition of NGF in the dorsal root ganglia (DRG) and the increased synthesis of opioid peptides and their receptors in the inflamed paw. We also found alterations in the cytokine levels as well as in the phosphorylation of AKT2. Our findings strongly support that BNN27 represents a lead molecule for the development of analgesic and anti-inflammatory compounds with potential therapeutic applications in inflammatory hyperalgesia.

Synthesis, in vitro biological investigation, and molecular dynamics simulations of thiazolopyrimidine based compounds as corticotrophin releasing factor receptor-1 antagonists.

Corticotrophin releasing factor receptor-1 (CRFR1) is a potential target for treatment of depression and anxiety through modifying stress response. A series of new thiazolo[4,5-d]pyrimidine derivatives were designed, prepared and biologically evaluated as potential CRFR1 antagonists. Four compounds produced more than fifty percent inhibition in the [125I]-Tyr0-sauvagine specific binding assay. Assessment of binding affinities revealed that compound (3-(2,4-dimethoxyphenyl)-7-(dipropylamino)-5-methylthiazolo[4,5-d]pyrimidin-2(3H)-one) 8c was the best candidate with highest binding affinity (Ki = 32.1 nM). Further evaluation showed the ability of compound 8c to inhibit CRF induced cAMP accumulation in a dose response manner. Docking and molecular dynamics simulations were used to investigate potential binding modes of synthesized compounds as well as the stability of 8c-CRFR1 complex. These studies suggest similar allosteric binding of 8c compared to that of the co-crystalized ligand CP-376395 in 4K5Y pdb file.

Discovery of a stable tripeptide targeting the N-domain of CRF1 receptor.

The corticotropin-releasing factor (CRF) and its CRF1 receptor (CRF1R) play a central role in the maintenance of homeostasis. Malfunctioning of the CRF/CRF1R unit is associated with several disorders, such as anxiety and depression. Non-peptide CRF1R-selective antagonists have been shown to exert anxiolytic and antidepressant effects on experimental animals. However, none of them is in clinical use today because of several side effects, thus demonstrating the need for the development of other more suitable CRF1R antagonists. In an effort to develop novel CRF1R antagonists we designed, synthesized and chemically characterized two tripeptide analogues of CRF, namely (R)-LMI and (S)-LMI, having their Leu either in R (or D) or in S (or L) configuration, respectively. Their design was based on the crystal structure of the N-extracellular domain (N-domain) of CRF1R/CRF complex, using a relevant array of computational methods. Experimental evaluation of the stability of synthetic peptides in human plasma has revealed that (R)-LMI is proteolytically more stable than (S)-LMI. Based on this finding, (R)-LMI was selected for pharmacological characterization. We have found that (R)-LMI is a CRF antagonist, inhibiting (1) the CRF-stimulated accumulation of cAMP in HEK 293 cells expressing the CRF1R, (2) the production of interleukins by adipocytes and (3) the proliferation rate of RAW 264.7 cells. (R)-LMI likely blocked agonist actions by interacting with the N-domain of CRF1R as suggested by data using a constitutively active chimera of CRF1R. We propose that (R)-LMI can be used as an optimal lead compound in the rational design of novel CRF antagonists.

Novel stable analogues of the neurotensin C-terminal hexapeptide containing unnatural amino acids.

Neurotensin (NT) (pGlu-Leu-Tyr-Glu-Asn-Lys-Pro-Arg-Arg-Pro-Tyr-Ile-Leu) exerts a dual function as a neurotransmitter/neuromodulator in the central nervous system and as a hormone/cellular mediator in periphery. This dual function of NT establishes a connection between brain and peripheral tissues that renders this peptide a central player in energy homeostasis. Many biological actions of NT are mediated through its interaction with three types of NT receptors (NTS receptors). Despite its role in energy homeostasis, NT has a short half-life that hampers further determination of the biological actions of this peptide and its receptors in brain and periphery. The short half-life of NT is due to the proteolytic degradation of its C-terminal side by several endopeptidases. Therefore, it is important to synthesize NT analogues with resistant bonds against metabolic deactivation. Based on these findings, we herein report the synthesis of ten linear, two cyclic and two dimeric analogues of NT with modifications in its structure that improve their metabolic stability, while retaining the ability to bind to NTS receptors. Modifications at position 11 (introduction of D-Tyrosine (OEthyl) [D-Tyr(Et)] or D-1-naphtylalanine [D-1-Nal] were combined with introduction of a L-Lysine or a D-Arginine at positions 8 or 9, and 1-[2-(aminophenyl)-2-oxoethyl]-1H-pyrrole-2-carboxylic acid (AOPC) at positions 7 or 8, resulting in compounds NT4-NT21. AOPC is an unnatural amino acid with promise in applications as a building block for the synthesis of peptidomimetic compounds. To biologically evaluate these analogues, we determined their plasma stability and their binding affinities to type 1 NT receptor (NTS1), endogenously expressed in HT-29 cells, Among the fourteen NT analogues, compounds, NT5, NT6, and NT8, which have D-Tyr(Et) at position 11, bound to NTS1 in a dose-response manner and with relatively high affinity but still lower than that of the natural peptide. Despite their lower binding affinities compared to NT, the NT5, NT6, and NT8 exhibited a remarkably higher stability, as a result of their chemistry, which provides protection from enzymatic activity. These results will set the basis for the rational design of novel NT molecules with improved pharmacological properties and enhanced enzymatic stability.

Effects of Cholesterol on GPCR Function: Insights from Computational and Experimental Studies.

The extensive experimental and computational evidences revealed that cholesterol is involved in the drug binding to G protein-coupled receptor (GPCR) targets that is influenced by the membrane environment and external functions. These multifunctional factors make the understanding of the molecular mechanism of action in greater detail an entirely difficult task. Significant efforts have been made for better understanding the role of multi-directional specific, receptor-dependent interactions of cholesterol, and its effects on drug design and development. Additional efforts must be made in this complex system in order to shed more light on cholesterol molecular basis of action. The results of molecular simulations that complemented experimental data may reveal new aspects of GPCR-cholesterol interactions and may provide a comprehensive understanding of receptor function.

Selective antagonism of CRF1 receptor by a substituted pyrimidine.

The corticotrophin-releasing factor (CRF) and its type 1 receptor (CRF1R) regulate the hypothalamic-pituitary-adrenal axis, as well as other systems, thus playing a crucial role in the maintenance of homeostasis. Non-peptide CRF1R-selective antagonists exert therapeutic effects on experimental animals with abnormal regulation of their homeostatic mechanisms. However, none of them is as yet in clinical use. In an effort to develop novel small non-peptide CRF1R-selective antagonists, we have synthesized a series of substituted pyrimidines described in a previous study. These small molecules bind to CRF1R, with analog 3 having the highest affinity. Characteristic structural features of analog 3 are a N,N-bis(methoxyethyl)amino group at position 6 and a methyl in the alkythiol group at position 5. Based on the binding profile of analog 3, we selected it in the present study for further pharmacological characterization. The results of this study suggest that analog 3 is a potent CRF1R-selective antagonist, blocking the ability of sauvagine, a CRF-related peptide, to stimulate cAMP accumulation in HEK 293 cells via activation of CRF1R, but not via CRF2R. Moreover, analog 3 blocked sauvagine to stimulate the proliferation of macrophages, further supporting its antagonistic properties. We have also constructed molecular models of CRF1R to examine the interactions of this receptor with analog 3 and antalarmin, a prototype CRF1R-selective non-peptide antagonist, which lacks the characteristic structural features of analog 3. Our data facilitate the design of novel non-peptide CRF1R antagonists for clinical use.

Development of bioactive gemcitabine-D-Lys6-GnRH prodrugs with linker-controllable drug release rate and enhanced biopharmaceutical profile.

Peptide-drug conjugates have emerged as a potent approach to enhance the targeting and pharmacokinetic profiles of drugs. However, the impact of the linker unit has not been explored/exploited in depth. Gemcitabine (dFdC) is an anticancer agent used against a variety of solid tumours. Despite its potency, gemcitabine suffers mostly due to its unspecific toxicity, lack of targeting and rapid metabolic inactivation. To minimize these limitations and enable its targeting to tumours overexpressing the GnRH receptor, we examined the peptide-drug conjugation approach. Our design hypothesis was driven by the impact that the linker unit could have on the peptide-drug conjugate efficacy. Along these lines, in order to exploit the potential to manipulate the potency of gemcitabine through altering the linker unit we constructed three different novel peptide-drug conjugates assembled of gemcitabine, the tumour-homing peptide D-Lys6-GnRH and modified linker building blocks. Specifically, the linker was sculpted to either allow slow drug release (utilizing carbamate bond) or rapid disassociation (using amide and ester bonds). Notably, the new analogues possessed up to 95.5-fold enhanced binding affinity for the GnRH receptor (GnRH-R) compared to the natural peptide ligand D-Lys6-GnRH. Additionally, their in vitro cytotoxicity was evaluated in four different cancer cell lines. Their cellular uptake, release of gemcitabine and inactivation of gemcitabine to its inactive metabolite (dFdU) was explored in a representative cell line. In vitro stability and the consequent drug release were evaluated in cell culture medium and human plasma. In vivo pharmacokinetic studies were performed in mice, summarizing the relative stability of the three conjugates and the released levels of gemcitabine in comparison with dFdU. These studies suggest that the fine tuning of the linkage within a peptide-drug conjugate affects the drug release rate and its overall pharmaceutical profile. This could eventually emerge as an intriguing medicinal chemistry approach to optimize bio-profiles of prodrugs.

Nonconventional glucagon and GLP-1 receptor agonist and antagonist interplay at the GLP-1 receptor revealed in high-throughput FRET assays for cAMP.

G protein-coupled receptors (GPCRs) for glucagon (GluR) and glucagon-like peptide-1 (GLP-1R) are normally considered to be highly selective for glucagon and GLP-1, respectively. However, glucagon secreted from pancreatic α-cells may accumulate at high concentrations to exert promiscuous effects at the ß-cell GLP-1R, as may occur in the volume-restricted microenvironment of the islets of Langerhans. Furthermore, systemic administration of GluR or GLP-1R agonists and antagonists at high doses may lead to off-target effects at other receptors. Here, we used molecular modeling to evaluate data derived from FRET assays that detect cAMP as a read-out for GluR and GLP-1R activation. This analysis established that glucagon is a nonconventional GLP-1R agonist, an effect inhibited by the GLP-1R orthosteric antagonist exendin(9-39) (Ex(9-39)). The GluR allosteric inhibitors LY2409021 and MK 0893 antagonized glucagon and GLP-1 action at the GLP-1R, whereas des-His1-[Glu9]glucagon antagonized glucagon action at the GluR, while having minimal inhibitory action versus glucagon or GLP-1 at the GLP-1R. When testing Ex(9-39) in combination with des-His1-[Glu9]glucagon in INS-1 832/13 cells, we validated a dual agonist action of glucagon at the GluR and GLP-1R. Hybrid peptide GGP817 containing glucagon fused to a fragment of peptide YY (PYY) acted as a triagonist at the GluR, GLP-1R, and neuropeptide Y2 receptor (NPY2R). Collectively, these findings provide a new triagonist strategy with which to target the GluR, GLP-1R, and NPY2R. They also provide an impetus to reevaluate prior studies in which GluR and GLP-1R agonists and antagonists were assumed not to exert promiscuous actions at other GPCRs.

Host-Guest Interactions between Candesartan and Its Prodrug Candesartan Cilexetil in Complex with 2-Hydroxypropyl-ß-cyclodextrin: On the Biological Potency for Angiotensin II Antagonism.

Renin-angiotensin aldosterone system inhibitors are for a long time extensively used for the treatment of cardiovascular and renal diseases. AT1 receptor blockers (ARBs or sartans) act as antihypertensive drugs by blocking the octapeptide hormone Angiotensin II to stimulate AT1 receptors. The antihypertensive drug candesartan (CAN) is the active metabolite of candesartan cilexetil (Atacand, CC). Complexes of candesartan and candesartan cilexetil with 2-hydroxylpropyl-ß-cyclodextrin (2-HP-ß-CD) were characterized using high-resolution electrospray ionization mass spectrometry and solid state 13C cross-polarization/magic angle spinning nuclear magnetic resonance (CP/MAS NMR) spectroscopy. The 13C CP/MAS results showed broad peaks especially in the aromatic region, thus confirming the strong interactions between cyclodextrin and drugs. This experimental evidence was in accordance with molecular dynamics simulations and quantum mechanical calculations. The synthesized and characterized complexes were evaluated biologically in vitro. It was shown that as a result of CAN's complexation, CAN exerts higher antagonistic activity than CC. Therefore, a formulation of CC with 2-HP-ß-CD is not indicated, while the formulation with CAN is promising and needs further investigation. This intriguing result is justified by the binding free energy calculations, which predicted efficient CC binding to 2-HP-ß-CD, and thus, the molecule's availability for release and action on the target is diminished. In contrast, CAN binding was not favored, and this may allow easy release for the drug to exert its bioactivity.

Current understanding of the structure and function of family B GPCRs to design novel drugs.

Family B of G-protein-coupled receptors (GPCRs) and their ligands play a central role in a number of homeostatic mechanisms in the endocrine, gastrointestinal, skeletal, immune, cardiovascular and central nervous systems. Alterations in family B GPCR-regulated homeostatic mechanisms may cause a variety of potentially life-threatening conditions, signifying the necessity to develop novel ligands targeting these receptors. Obtaining structural and functional information on family B GPCRs will accelerate the development of novel drugs to target these receptors. Family B GPCRs are proteins that span the plasma membrane seven times, thus forming seven transmembrane domains (TM1-TM7) which are connected to each other by three extracellular (EL) and three intracellular (IL) loops. In addition, these receptors have a long extracellular N-domain and an intracellular C-tail. The upper parts of the TMs and ELs form the J-domain of receptors. The C-terminal region of peptides first binds to the N-domain of receptors. This 'first-step' interaction orients the N-terminal region of peptides towards the J-domain of receptors, thus resulting in a 'second-step' of ligand-receptor interaction that activates the receptor. Activation-associated structural changes of receptors are transmitted through TMs to their intracellular regions and are responsible for their interaction with the G proteins and activation of the latter, thus resulting in a biological effect. This review summarizes the current information regarding the structure and function of family B GPCRs and their physiological and pathophysiological roles.

Integration of multi-scale molecular modeling approaches with experiments for the in silico guided design and discovery of novel hERG-Neutral antihypertensive oxazalone and imidazolone derivatives and analysis of their potential restrictive effects on cell proliferation.

AT1 antagonists is the most recent drug class of molecules against hypertension and they mediate their actions through blocking detrimental effects of angiotensin II (A-II) when acts on type I (AT1) A-II receptor. The effects of AT1 antagonists are not limited to cardiovascular diseases. AT1 receptor blockers may be used as potential anti-cancer agents - due to the inhibition of cell proliferation stimulated by A-II. Therefore, AT1 receptors and the A-II biosynthesis mechanisms are targets for the development of new synthetic drugs and therapeutic treatment of various cardiovascular and other diseases. In this work, multi-scale molecular modeling approaches were performed and it is found that oxazolone and imidazolone derivatives reveal similar/better interaction energy profiles compared to the FDA approved sartan molecules at the binding site of the AT1 receptor. In silico-guided designed hit molecules were then synthesized and tested for their binding affinities to human AT1 receptor in radioligand binding studies, using [125I-Sar1-Ile8] AngII. Among the compounds tested, 19d and 9j molecules bound to receptor in a dose response manner and with relatively high affinities. Next, cytotoxicity and wound healing assays were performed for these hit molecules. Since hit molecule 19d led to deceleration of cell motility in all three cell lines (NIH3T3, A549, and H358) tested in this study, this molecule is investigated in further tests. In two cell lines (HUVEC and MCF-7) tested, 19d induced G2/M cell cycle arrest in a concentration dependent manner. Adherent cells detached from the plates and underwent cell death possibly due to apoptosis at 19d concentrations that induced cell cycle arrest.

Role of Central and Peripheral CRH in Skin.

BACKGROUND: Hypothalamic corticotrophin-releasing hormone (CRH) has a key role in coordinating and controlling complex responses to stress, both systemically, by stimulating the expression of the pituitary POMC gene, and thus, resulting in increased production of ACTH and adrenal glucocorticoid release, and locally since CRH has been identified in several peripheral tissues. CRH seems to exert its effects through interaction with two known so far receptors, CRF1R and CRF2R. The mRNA and protein of CRH family of peptides and their receptors are expressed at several peripheral tissues including rodent and human skin. In addition to CRH, skin expresses POMC and its products, including ACTH while recent studies have shown the presence of glucocorticoids also in skin. OBJECTIVE: This review aims to summarize the role of CRH in the physiology and pathophysiology of human and rodent skin. RESULTS: It is clear that a) locally produced CRH is involved in the inflammatory process, b) CRH has been shown to stimulate angiogenesis in vivo and chemotaxis of endothelial cells in vitro, and c) CRH mRNA and peptide have been identified in skin. CONCLUSION: Based on the above we hypothesize that CRH plays a crucial role in several inflammatory pathologies of the skin as well as in cutaneous wound healing, which are all discussed in the present review.

The Multi-faceted Profile of Corticotropin-releasing Factor (CRF) Family of Neuropeptides and of their Receptors on the Paracrine/Local Regulation of the Inflammatory Response.

BACKGROUND: The corticotropin releasing factor (CRF) family of neuropeptides, CRF and the Urocortins, and their receptors are present not only within the central nervous system but also in the periphery at various locations and at the sites of inflammation where they influence its progress in a complex local / paracrine manner. OBJECTIVE AND METHODS: This review summarizes current knowledge regarding the regulation of inflammatory process by CRF family of neuropeptides and receptors with a special sight into their role in inflammatory pain and in chronic low grade inflammation that occurs in obesity. For this purpose, we searched for relevant peer-reviewed research articles using bibliographic databases. RESULTS: The CRF neuropeptides are either produced locally, by components of the inflammatory response or they may reach the inflammation sites via postganglionic sympathetic and sensory afferent nerve transport. It now appears that most immune cells taking part in the inflammatory process express CRF receptor type 1 (CRF1R) and type 2 (CRF2R) and thus represent targets of CRF neuropeptides. Indeed, mast cells, monocytes / macrophages, neutrophils and other types of immune cells express both types of the CRF receptors. In addition to their role in the pathophysiology of inflammation, CRF and its receptors also exert modulatory effects on inflammatory pain. Finally, it now appears that the CRF system is also present in adipose tissue and may play a crucial role in the development of the chronic low grade inflammation, which is characteristic of obesity. CONCLUSION: The local effects of the CRF family of neuropeptides can be either pro- or antiinflammatory depending on concentration of each type of neuropeptide present and the ratio of the local expression of their receptors CRF1R and CRF2R.

Synthesis of 2-imino and 2-hydrazono thiazolo[4,5-d]pyrimidines as corticotropin releasing factor (CRF) antagonists.

Corticotropin-releasing factor (CRF) is an important neuropeptide hormone which controls the body's overall response to stress. It plays a crucial role in regulating the behavioral, cardiovascular, immune and gastrointestinal systems. Over-activation of the CRF system has been implicated in many disorders including anxiety, depression, drug addiction, hypertension, Irritable Bowel Syndrome (IBS), peptic ulcers, inflammation and others. Thus, binding of CRF to its receptors is an attractive target to develop new medications which aim at treating ailments associated with chronic stress. Numerous small-molecule non-peptide CRF receptor antagonists were developed and many are in various stages in clinical trials. Many showed great promise in treatment of anxiety, depression, peptic ulcers, inflammation, IBS and drug addiction. In our recent previous work, the development of two series of pyrimidine and fused pyrimidine CRF antagonists were described. In continuation of our efforts in this direction, in the current manuscript, the synthesis of a third series of CRF receptor antagonists is described. The binding affinities of select compounds for the type 1 receptor of CRF (CRF1R) were determined and compared to a standard CRF antagonist drug antalarmin. A lead compound was identified and further evaluated by measuring its effect on the inhibition of the agonist-stimulated accumulation of second messengers.