Activation of cyclic AMP-generating systems in brain membranes and slices by the diterpene forskolin: augmentation of receptor-mediated responses

Optimized 3D Culture of Hepatic Cells for Liver Organoid Metabolic Assays

The liver is among the principal organs for glucose homeostasis and metabolism. Studies of liver metabolism are limited by the inability to expand primary hepatocytes in vitro while maintaining their metabolic functions. Human hepatic three-dimensional (3D) organoids have been established using defined factors, yet hepatic organoids from adult donors showed impaired expansion. We examined conditions to facilitate the expansion of adult donor-derived hepatic organoids (HepAOs) and HepG2 cells in organoid cultures (HepGOs) using combinations of growth factors and small molecules. The expansion dynamics, gluconeogenic and HNF4α expression, and albumin secretion are assessed. The conditions tested allow the generation of HepAOs and HepGOs in 3D cultures. Nevertheless, gluconeogenic gene expression varies greatly between conditions. The organoid expansion rates are limited when including the TGFβ inhibitor A8301, while are relatively higher with Forskolin (FSK) and Oncostatin M (OSM). Notably, expanded HepGOs grown in the optimized condition maintain detectable gluconeogenic expression in a spatiotemporal distribution at 8 weeks. We present optimized conditions by limiting A8301 and incorporating FSK and OSM to allow the expansion of HepAOs from adult donors and HepGOs with gluconeogenic competence. These models increase the repertoire of human hepatic cellular tools available for use in liver metabolic assays.

The selectivity of α-adrenoceptor agonists for the human α1A, α1B, and α1D-adrenoceptors

Highly selective drugs offer a way to minimize side-effects. For agonist ligands, this could be through highly selective affinity or highly selective efficacy, but this requires careful measurements of intrinsic efficacy. The α1-adrenoceptors are important clinical targets, and α1-agonists are used to manage hypotension, sedation, attention deficit hypersensitivity disorder (ADHD), and nasal decongestion. With 100 years of drug development, there are many structurally different compounds with which to study agonist selectivity. This study examined 62 α-agonists at the three human α1-adrenoceptor (α1A, α1B, and α1D) stably expressed in CHO cells. Affinity was measured using whole-cell 3 H-prazosin binding, while functional responses were measured for calcium mobilization, ERK1/2-phosphorylation, and cAMP accumulation. Efficacy ratios were used to rank compounds in order of intrinsic efficacy. Adrenaline, noradrenaline, and phenylephrine were highly efficacious α1-agonists at all three receptor subtypes. A61603 was the most selective agonist and its very high α1A-selectivity was due to selective α1A-affinity (>660-fold). There was no evidence of Gq-calcium versus ERK-phosphorylation biased signaling at the α1A, α1B, or α1D-adrenoceptors. There was little evidence for α1A calcium versus cAMP biased signaling, although there were suggestions of calcium versus cAMP bias the α1B-adrenoceptor. Comparisons of the rank order of ligand intrinsic efficacy suggest little evidence for selective intrinsic efficacy between the compounds, with perhaps the exception of dobutamine which may have some α1D-selective efficacy. There seems plenty of scope to develop affinity selective and intrinsic efficacy selective drugs for the α1-adrenoceptors in future.

3-Iodothyronamine Activates a Set of Membrane Proteins in Murine Hypothalamic Cell Lines

3-Iodothyronamine (3-T1AM) is an endogenous thyroid hormone metabolite. The profound pharmacological effects of 3-T1AM on energy metabolism and thermal homeostasis have raised interest to elucidate its signaling properties in tissues that pertain to metabolic regulation and thermogenesis. Previous studies identified G protein-coupled receptors (GPCRs) and transient receptor potential channels (TRPs) as targets of 3-T1AM in different cell types. These two superfamilies of membrane proteins are largely expressed in tissue which influences energy balance and metabolism. As the first indication that 3-T1AM virtually modulates the function of the neurons in hypothalamus, we observed that intraperitoneal administration of 50 mg/kg bodyweight of 3-T1AM significantly increased the c-FOS activation in the paraventricular nucleus (PVN) of C57BL/6 mice. To elucidate the underlying mechanism behind this 3-T1AM-induced signalosome, we used three different murine hypothalamic cell lines, which are all known to express PVN markers, GT1-7, mHypoE-N39 (N39) and mHypoE-N41 (N41). Various aminergic GPCRs, which are the known targets of 3-T1AM, as well as numerous members of TRP channel superfamily, are expressed in these cell lines. Effects of 3-T1AM on activation of GPCRs were tested for the two major signaling pathways, the action of Gαs/adenylyl cyclase and Gi/o. Here, we demonstrated that this thyroid hormone metabolite has no significant effect on Gi/o signaling and only a minor effect on the Gαs/adenylyl cyclase pathway, despite the expression of known GPCR targets of 3-T1AM. Next, to test for other potential mechanisms involved in 3-T1AM-induced c-FOS activation in PVN, we evaluated the effect of 3-T1AM on the intracellular Ca2+ concentration and whole-cell currents. The fluorescence-optic measurements showed a significant increase of intracellular Ca2+ concentration in the three cell lines in the presence of 10 μM 3-T1AM. Furthermore, this thyroid hormone metabolite led to an increase of whole-cell currents in N41 cells. Interestingly, the TRPM8 selective inhibitor (10 μM AMTB) reduced the 3-T1AM stimulatory effects on cytosolic Ca2+ and whole-cell currents. Our results suggest that the profound pharmacological effects of 3-T1AM on selected brain nuclei of murine hypothalamus, which are known to be involved in energy metabolism and thermoregulation, might be partially attributable to TRP channel activation in hypothalamic cells.

Modulation of cGMP accumulation by adenosine A1 receptors at the hippocampus: influence of cGMP levels and gender

Adenosine A1 receptor is highly expressed in hippocampus where it inhibits neurotransmitter release and has neuroprotective activity. Similar actions are obtained by increasing cGMP concentration, but a clear link between adenosine A1 receptor and cGMP levels remains to be established. The present work aims to investigate if cGMP formation is modulated by adenosine A1 receptors at the hippocampus and if this effect is gender dependent. cGMP accumulation, induced by phosphodiesterases inhibitors Zaprinast (100 μM) and Bay 60-7550 (10 μM), and cAMP accumulation, induced by Forskolin (20 μM) and Rolipram (50 μM), were quantified in rat hippocampal slices using specific enzymatic immunoassays. N6-cyclopentyladenosine (CPA, 100 nM) alone failed to modify basal cGMP accumulation. However, the presence of adenosine deaminase (ADA, 2 U/ml) unmasked a CPA (0.03-300 nM) stimulatory effect on basal cGMP accumulation (EC50: 4.2±1.4 nM; Emax: 17±0.9%). ADA influence on CPA activity was specific for cGMP, since inhibition of cAMP accumulation by CPA was not affected by the presence of ADA, though ADA inhibited cAMP accumulation in the absence of CPA. Increasing cGMP accumulation, by about four-fold, with sodium nitroprusside (SNP, 100 μM) abolished the CPA (100 nM) effect on cGMP accumulation in males but did not modify the effect of CPA in female rats. This effect was reversed by 8-Cyclopentyl-1,3-dipropylxanthine (DPCPX, 100 nM), indicating an adenosine A1 receptor mediated effect on cGMP accumulation. In conclusion, adenosine A1 receptors increase intracellular cGMP formation at hippocampus both in males and females under basal conditions, but only in females when cGMP levels are increased by SNP.

Non-raft adenylyl cyclase 2 defines a cAMP signaling compartment that selectively regulates IL-6 expression in airway smooth muscle cells: differential regulation of gene expression by AC isoforms

Adenylyl cyclase (AC) isoforms differ in their tissue distribution, cellular localization, regulation, and protein interactions. Most cell types express multiple AC isoforms. We hypothesized that cAMP produced by different AC isoforms regulates unique cellular responses in human bronchial smooth muscle cells (BSMC). Overexpression of AC2, AC3, or AC6 had distinct effects on forskolin (Fsk)-induced expression of a number of known cAMP-responsive genes. These data show that different AC isoforms can differentially regulate gene expression. Most notable, overexpression and activation of AC2 enhanced interleukin 6 (IL-6) expression, but overexpression of AC3 or AC6 had no effect. IL-6 production by BSMC was induced by Fsk and select G protein-coupled receptor (GPCR) agonists, though IL-6 levels did not directly correlate with global cAMP levels. Treatment with PKA selective 6-Bnz-cAMP or Epac selective 8-CPT-2Me-cAMP cAMP analogs revealed a predominant role for PKA in cAMP-mediated induction of IL-6. IL-6 promoter mutations demonstrated that AP-1 and CRE transcription sites were required for Fsk to stimulate IL-6 expression. Our present study defines an AC2 cAMP signaling compartment that specifically regulates IL-6 expression in BSMC via Epac and PKA and demonstrates that other AC isoforms are excluded from this pool.

Determination of endogenous GABA released from the cerebral cortex slices of the rat by high-performance liquid chromatography with a series-dual electrochemical detector

A new, simple, rapid and sensitive method for the determination of ?-aminobutylic acid (GABA) has been developed by high-performance liquid chromatography with electrochemical detection (LCEC). A new and unique technique for LCEC by using the reductive-oxidative mode of a dual electrochemical detector provided a simple and sensitive assay method for GABA. The standard curve was linear over a concentration range of 1-1000 ng. The detection limit for GABA was less than 0.5 ng. This new method was adapted to the assay of the transmitter released endogenously from the cerebral cortical slices of the rat. Endogenous GABA release evoked by high K(+) was reduced when superfusion was performed in the presence of 100 ?M forskolin.

cAMP and fibroblast growth factor 2 regulate bone sialoprotein gene expression in human prostate cancer cells

Bone sialoprotein (BSP) is a noncollagenous protein of the extracellular matrix in mineralized connective tissues that has been implicated in the nucleation of hydroxyapatite. Forskolin (FSK), an activator of adenylate cyclase, increased the intracellular cAMP level, which stimulates the proliferation and differentiation of osteoblasts. Fibroblast growth factor 2 (FGF2) is a potent mitogen in many cell types, including osteoblasts. In human prostate cancer DU145 cells, FSK (1 μM) and FGF2 (10 ng/ml) increased BSP and Runx2 mRNA and protein levels at 3 and 12h, respectively. Transient transfection analyses were performed using chimeric constructs of the human BSP gene promoter linked to a luciferase reporter gene. Treatment of DU145 cells with FSK (1 μM) and FGF2 (10 ng/ml) increased the luciferase activities of constructs between -60LUC to -927LUC and -108LUC to -927LUC, including the human BSP gene promoter. Effects of FSK and FGF2 abrogated in constructs included 2bp mutations in the two cAMP response elements (CRE1 and CRE2). Luciferase activities induced by FSK and FGF2 were blocked by protein kinase A and tyrosine kinase inhibitors. Gel mobility shift analyses showed that FSK and FGF2 increased the binding of CRE1 and CRE2. CRE1-protein complexes were supershifted by phospho-CREB1 and c-Fos antibodies, and disrupted by CREB1, c-Jun, JunD, Fra2, p300, Runx2, Dlx5 and Smad1 antibodies. CRE2-protein complexes were disrupted by CREB1, phospho-CREB1, c-Fos, c-Jun, JunD, Fra2, p300, Runx2, Dlx5 and Smad1 antibodies. These studies demonstrate that FSK and FGF2 stimulate BSP transcription in DU145 human prostate cancer cells by targeting the CRE1 and CRE2 elements in the human BSP gene promoter.

Sleep deprivation impairs cAMP signalling in the hippocampus

Millions of people regularly obtain insufficient sleep1. Given the impact of sleep deprivation on our lives, understanding the cellular and molecular pathways affected by sleep deprivation is clearly of social and clinical importance. One of the major effects of sleep deprivation on the brain is to produce memory deficits in learning paradigms that are dependent on the hippocampus2–5. In this study, we have identified a molecular mechanism by which brief sleep deprivation alters hippocampal function. Sleep deprivation selectively impaired cAMP/PKA-dependent forms of synaptic plasticity6 in the hippocampus, reduced cAMP signaling, and increased activity and protein levels of phosphodiesterase-4 (PDE4), an enzyme that degrades cAMP. Treatment with PDE inhibitors rescued the sleep deprivation-induced deficits in cAMP signaling, synaptic plasticity, and hippocampus-dependent memory. These findings demonstrate that brief sleep deprivation disrupts hippocampal function by interfering with cAMP signaling through increased PDE4 activity. Thus drugs that enhance cAMP signaling may provide a novel therapeutic approach to counteract the cognitive effects of sleep deprivation.

Estradiol increases guanosine 5'-triphosphate cyclohydrolase expression via the nitric oxide-mediated activation of cyclic adenosine 5'-monophosphate response element binding protein

A number of studies have demonstrated that estradiol can stimulate endothelial nitric oxide synthase expression and activity, resulting in enhanced nitric oxide (NO) generation. However, its effect on the NO synthase cofactor, tetrahydrobiopterin are less clear. Cellular tetrahydrobiopterin levels are regulated, at least in part, by GTP cyclohydrolase 1 (GCH1). Thus, the purpose of this study was to determine the effect of estradiol on GCH1 expression and the regulatory mechanisms in pulmonary arterial endothelial cells. Our data indicate that 17beta-estradiol (E2) increases GCH1 transcription in a dose- and time-dependent manner, whereas estrogen receptor antagonism or NO synthase inhibition attenuated E2-stimulated GCH1 expression. Analysis of the GCH1 promoter fragment responsive to E2 revealed the presence of a cAMP response element, and we found that E2 triggers a rapid but transient elevation of phospho-cAMP response element-binding protein (CREB; <1 h) followed by a second sustained rise after 6 h. EMSA analysis revealed an increase in the binding of CREB during E2 treatment and mutation of the cAMP response element in the GCH1 promoter attenuated the E2-mediated increase in transcription. Furthermore, inhibition of the cAMP-dependent kinase, protein kinase A (PKA) completely abolished the E2-stimulated GCH1 promoter activity, whereas the stimulation of cAMP levels with forskolin increased GCH1 promoter activity, indicating the key role of cAMP in regulating GCH1 promoter activity. In conclusion, our results demonstrate that estradiol can modulate GCH1 expression via NO-mediated activation of CREB in pulmonary arterial endothelial cells. These findings provide new insight into the vascular protective effect of estradiol.

GTP cyclohydrolase I expression is regulated by nitric oxide: role of cyclic AMP

Our previous studies have demonstrated that nitric oxide (NO) leads to nitric oxide synthase (NOS) uncoupling and an increase in NOS-derived superoxide. However, the cause of this uncoupling has not been adequately resolved. The pteridine cofactor tetrahydrobiopterin (BH(4)) is a critical determinant of endothelial NOS (eNOS) activity and coupling, and GTP cyclohydrolase I (GCH1) is the rate-limiting enzyme in its generation. Thus the initial purpose of this study was to determine whether decreases in BH(4) could underlie, at least in part, the NO-mediated uncoupling of eNOS we have observed both in vitro and in vivo. Initially we evaluated the effect of inhaled NO levels on GCH1 expression and BH(4) levels in the intact lamb. Contrary to our hypothesis, we found that there was a significant increase in both plasma BH4 levels and peripheral lung GCH1 protein levels. Furthermore, in vitro, we found that exposure to the NO donor spermine NONOate (SPNONO) led to an increase in GCH1 protein and BH(4) levels in both COS-7 and pulmonary arterial endothelial cells. However, SPNONO treatment also caused a significant increase in phospho-cAMP response element binding protein (CREB) levels, as detected by Western blot analysis, and significantly increased cAMP levels, as detected by enzyme immunoassay. Furthermore, utilizing GCH1 promoter fragments fused to a luciferase reporter gene, we found that GCH1 promoter activity was enhanced by SPNONO in a CREB-dependent manner, and electromobility shift assays revealed an NO-dependent increase in the nuclear binding of CREB. These data suggest that NO increases BH(4) levels through a cAMP/CREB-mediated increase in GCH1 transcription and that the eNOS uncoupling associated with exogenous NO does not involved reduced BH(4) levels.

Partial compensation for N-type Ca(2+) channel loss by P/Q-type Ca(2+) channels underlines the differential release properties supported by these channels at cerebrocortical nerve terminals

N-type and P/Q-type Ca(2+) channels support glutamate release at central synapses. To determine whether the glutamate release mediated by these channels exhibits distinct properties, we have isolated each release component in cerebrocortical nerve terminals from wild-type mice by specifically blocking N-type Ca(2+) channels with omega-conotoxin-GVIA and P/Q-type Ca(2+) channels with omega-agatoxin-IVA. In addition, we have determined the release properties at terminals from mice lacking the alpha(1B) subunit of N-type channels (Ca(v) 2.2) to test the possibility that P/Q-type channels can compensate for the loss of N-type Ca(2+) channels. We recently demonstrated that, while evoked glutamate release depends on P/Q- and N-type channels in wild-type nerve terminals, only P/Q-type channels participate in these knockout mice. Moreover, in nerve terminals expressing solely P/Q-type channels, metabotropic glutamate receptor 7 (mGluR7) fails to inhibit the evoked Ca(2+) influx and glutamate release. Here, we show that the failure of mGluR7 to modulate evoked glutamate release is not due to a lack of receptors, as nerve terminals from mice lacking N-type Ca(2+) channels express mGluR7. Indeed, we show that other receptor responses, such as the inhibition of forskolin-induced release, are preserved in these knockout mice. N-type channels are more loosely coupled to release than P/Q-type channels in nerve terminals from wild-type mice, as reflected by the tighter coupling of release in knockout nerve terminals. We conclude that the glutamate release supported by N- and P/Q-type channels exhibits distinct properties, and that P/Q-type channels cannot fully compensate for the loss of N-type channels.

In vitro skin diffusion study of pure forskolin versus a forskolin-containing Plectranthus barbatus root extract

An in vitro skin diffusion study of pure forskolin (1) versus a 1-containing Plectranthus barbatus root extract (P. barbatus extract) in hairless guinea pig skin and human skin in a flow-through diffusion cell system was conducted and is being reported for the first time. Both topical agents were formulated in a solution of 70% ethanol and 30% propylene glycol (v/v). The results showed that forskolin can be delivered through the stratum corneum and that the flux of this compound was enhanced when 1 was delivered as a constituent of the P. barbatus extract as compared to an equivalent amount in pure form. These results suggest that the P. barbatus extract used contains permeation enhancement activity from other compound(s) contained in the crude root extract. It is possible that P. barbatus root extract may be used as an economical source of 1 to perform topical chemical manipulation of pigmentation in high-risk populations.

Proteasome function is regulated by cyclic AMP-dependent protein kinase through phosphorylation of Rpt6

Dysregulation of the proteasome has been documented in a variety of human diseases such as Alzheimer, muscle atrophy, cataracts etc. Proteolytic activity of 26 S proteasome is ATP- and ubiquitin-dependent. O-GlcNAcylation of Rpt2, one of the AAA ATPases in the 19 S regulatory cap, shuts off the proteasome through the inhibition of ATPase activity. Thus, through control of the flux of glucose into O-GlcNAc, the function of the proteasome is coupled to glucose metabolism. In the present study we found another metabolic control of the proteasome via cAMP-dependent protein kinase (PKA). Contrary to O-Glc-NAcylation, PKA activated proteasomes both in vitro and in vivo in association with the phosphorylation at Ser(120) of another AAA ATPase subunit, Rpt6. Mutation of Ser(120) to Ala blocked proteasome function. The stimulatory effect of PKA and the phosphorylation of Rpt6 were reversible by protein phosphatase 1 gamma. Thus, hormones using the PKA system can also regulate proteasomes often in concert with glucose metabolism. This finding might lead to novel strategies for the treatment of proteasome-related diseases.

Characterization of [3H]-forskolin binding sites in young and adult rat brain cortex: identification of suramin as a competitive inhibitor of [3H]-forskolin binding

Little is know about forskolin binding in the rat brain during ontogenetic development. For this paper, we have characterized specific binding sites for [3H]-forskolin in cerebrocortical membranes from young (12-day-old) and adult (90-day-old) rats. High-affinity, as well as super-high-affinity, [3H]-forskolin binding sites were detected in samples from both age groups tested, and the binding parameters of these sites differed significantly. Whereas the number of high-affinity [3H]-forskolin binding sites was higher by about 50% in adult than in young rats, their affinity was markedly (about 4 times) lower. In the presence of AlF4-, the number high-affinity [3H]-forskolin binding sites in samples from young rats rose to the level determined in samples from adult animals, and the number of super-high-affinity sites considerably increased in both age groups. The different characteristics of [3H]-forskolin binding found in cerebrocortical membranes from young and adult rats may be closely related to markedly diminished adenyl cyclase activity in preparations from adult animals. Results of our experiments with suramin indicated that this drug may act as a competitive inhibitor of [3H]-forskolin binding.

Human thyrotropin (TSH) variants designed by site-directed mutagenesis block TSH activity in vitro and in vivo

TSH is a heterodimeric glycoprotein hormone synthesized in the pituitary and composed of a specific beta-subunit and a common alpha-subunit shared with FSH, LH, and human chorionic gonadotropin. The heterodimer was previously converted into a biologically active single chain protein by genetic fusion of the genes coding to both subunits in the presence of the carboxy-terminal sequence of human (h) chorionic gonadotropin-beta subunit as a linker [hTSHbeta-carboxyl-terminal peptide (CTP)-alpha]. N-linked carbohydrate-free single-chain TSH variants were constructed by site-directed mutagenesis and overlapping PCR: one devoid of both N-linked oligosaccharide chains on the alpha-subunit (hTSHbeta-CTP-alpha(deg)) and the other lacking also the oligosaccharides on the beta-subunit (hTSHbeta(deg)-CTP-alpha(deg)). These variants were expressed in Chinese hamster ovary cells and secreted into the culture media. We have previously reported that the variants block the activities of hTSH and thyroid-stimulating immunoglobulins in cultured human thyroid follicles. In the present study, binding affinity of hTSH variants to hTSH receptor and the localization of the antagonistic effect were examined. Moreover, the effect of these variants on TSH activity was tested in vivo. The results of the present study indicate that the hTSH variants bind to the hTSH receptor with high affinity. Experiments using forskolin also indicated that the N-linked carbohydrate-free TSH single-chain variants inhibit TSH activity at the receptor-binding site and not at a postreceptor level. Moreover, the variants significantly inhibited (about 50%) TSH activity with respect to thyroid hormone secretion in vivo in mice. These variants may offer a novel therapeutic strategy in treating hyperthyroidism.

Up-regulation of NMDAR1 subunit gene expression in cortical neurons via a PKA-dependent pathway

Transcription mediated by protein kinase A and the cAMP response element binding protein (CREB) has been linked to the establishment of long-term memory and cell survival. However, all of the major targets for activated CREB have yet to be identified. Given the fact that CREB-mediated transcription is intimately involved in cellular processes of learning and memory and that CREB activity can be regulated by synaptic N-methyl-d-aspartate receptors (NMDARs) and metabotropic GABA receptors, we have studied the role of the cAMP-dependent signaling pathway in the regulation of the NMDA receptor subunit 1 (NMDAR1), a subunit required for functional receptor formation. We now report that levels of NMDAR1 subunit protein in primary neocortical cultures are increased 66% in response to forskolin, an activator of adenylyl cyclase. Up-regulation of NMDAR1 is paralleled by a twofold increase in mRNA levels and an 83% increase in NMDAR1 promoter/luciferase reporter activity that is dependent on protein kinase A. Three cAMP regulatory elements (CREs) in the rat NMDAR1 promoter (- 228, - 67, and - 39) bind CREB in vitro and forskolin increases binding to two of the sites (- 228 and - 67). Chromatin immunoprecipitation of neuronal rat genomic DNA reveals that CREB is bound in vivo to the endogenous NMDAR1 gene. Increased presence of the activated Ser133 phosphorylated form is dependent on the length of exposure to forskolin. Taken together with the results of mutational analysis, the findings strongly suggest that transcription of NMDAR1 is regulated by the c-AMP signaling pathway, most likely through the binding of CREB and its activation by signal-dependent phosphorylation.

Forskolin as a tool for examining adenylyl cyclase expression, regulation, and G protein signaling

1. As initially shown by Seamon and Daly, the diterpene forskolin directly activates adenylyl cyclase (AC) and raises cyclic AMP levels in a wide variety of cell types. In this review, we discuss several aspects of forskolin action that are often unappreciated. These include the utility of labeled forskolin as a means to quantitate the number of AC molecules; results of those types of studies, coupled with efforts to increase AC expression, document that such expression stoichiometrically limits cyclic AMP formation by hormones and neurotransmitters. 2. Response to forskolin is also strongly influenced by the activation of AC by the heterotrimeric G-protein, Gs. Gs-promoted enhancement of AC activity in response to forskolin occurs not only when cells are incubated with exogenously administered agonists that activate G-protein-coupled receptors but also by agonists that can be endogenously released by cells. 3. Such agonists, which include ATP and prostaglandins, serve as autocrine/paracrine regulators of cellular levels of cyclic AMP under "basal" conditions and also in response to forskolin and to agonists that promote release of such regulators. 4. The ability of forskolin to prominently activate cyclic AMP generation has proved valuable for understanding stoichiometry of the multiple components involved in "basal" cyclic AMP formation, in enzymologic studies of AC as well as in defining responses to cyclic AMP in cells within and outside the nervous system.

PKA and PKC enhance excitatory synaptic transmission in human dentate gyrus

cAMP-dependent protein kinase (PKA) and protein kinase C (PKC) are two major modulators of synaptic transmission in the CNS but little is known about how they affect synaptic transmission in the human CNS. In this study, we used forskolin, a PKA activator, and phorbol ester, a PKC activator, to examine the effects of these kinases on synaptic transmission in granule cells of the dentate gyrus in human hippocampal slices using whole-cell recording methods. We found that both forskolin and phorbol ester increased the frequency of spontaneous and miniature excitatory postsynaptic currents (sEPSCs and mEPSCs) but left the amplitude unaffected. Inactive forskolin and phorbol ester had no effect on sEPSCs in human dentate granule cells. Prior application of forskolin occluded the effects of phorbol ester on mEPSC frequency. Tetanic stimulation applied to the perforant path induced short-term depression in dentate gyrus granule cells. Both forskolin and phorbol ester significantly enhanced this short-term depression. Taken together, these results demonstrate that PKA and PKC are involved in up-regulation of excitatory synaptic transmission in human dentate granule cells, primarily by presynaptic mechanisms. In addition, the occlusion experiments suggest that the two kinases may share a common signal pathway.

Histamine H(2) -like receptors in chick cerebral cortex: effects on cyclic AMP synthesis and characterization by [(3) H]tiotidine binding

In this study, histamine (HA) receptors in chick cerebral cortex were characterized using two approaches: (1) analysis of the effects of HA-ergic drugs on the cAMP-generating system, and (2) radioreceptor binding of [(3) H]tiotidine, a selective H(2) antagonist. HA was a weak activator of adenylyl cyclase in a crude membrane preparation of chick cerebrum. On the other hand, HA (0.1-1000 microm) potently and concentration dependently stimulated cAMP production in [(3) H]adenine pre-labelled slices of chick cerebral cortex, displaying an EC(50) value (concentration that produces 50% of maximum response) of 2.65 microm. The effect of HA was mimicked by agonists of HA receptors with the following rank order of potency: HA >or= 4-methylHA (H(2)) >or= N alpha,N alpha-dimethylHA (H(3) >> H(2) = H(1)) >> 2-methylHA (H(1)) >> 2-thiazolylethylamine (H(1)) >or= R alpha-methylHA (H(3)) >> amthamine, dimaprit (H(2)), immepip (H(3), H(4)). The HA-evoked increase in cAMP production in chick cerebral cortex was antagonized by selective H(2) receptor blockers (aminopotentidine >or= tiotidine > ranitidine >> zolantidine), and not significantly affected by mepyramine and thioperamide, selective H(1) and H(3) /H(4) receptor blockers, respectively. A detailed analysis of the antagonistic action of aminopotentidine (vs. HA) revealed a non-competitive mode of action. The binding of [(3) H]tiotidine to chick cortical membranes was rapid, stable and reversible. Saturation analysis resulted in a linear Scatchard plot, suggesting binding to a single class of receptor binding site with high affinity [equilibrium dissociation constant (K (d)) = 4.42 nm] and high capacity [maximum number of binding sites (B (max) ) = 362 fmol/mg protein]. The relative rank order of HA-ergic drugs to inhibit [(3) H]tiotidine binding to chick cerebrum was: antagonists - tiotidine >> aminopotentidine = ranitidine >or= zolantadine >> thioperamide - triprolidine; agonists - HA >or= 4-methylHA >> 2-methylHA >or=R alpha-methylHA - dimaprit. In conclusion, chick cerebral cortex contains H(2) -like HA receptors that are linked to the cAMP-generating system and are labelled with [(3) H]tiotidine. The pharmacological profile of these receptors is different from that described for their mammalian counterpart. It is suggested that the studied receptors represent either an avian-specific H(2) -like HA receptors or a novel subtype of HA receptors.

Presynaptic H3 autoreceptors modulate histamine synthesis through cAMP pathway

Histamine H3 receptors modulate histamine synthesis, although little is known about the transduction mechanisms involved. To investigate this issue, we have used a preparation of rat brain cortical miniprisms in which histamine synthesis can be modulated by depolarization and by H3 receptor ligands. When the miniprisms were incubated in presence of forskolin, dibutyryl-cAMP, or 3-isobutyl-1-methylxanthine (IBMX), histamine synthesis was stimulated in 34, 29, and 47%, respectively. These stimulations could be prevented by the selective cAMP protein kinase blocker Rp-adenosine 3',5'-cyclic monophosphothioate triethylamine (Rp-cAMPs). Preincubation with the H3 receptor agonist imetit prevented IBMX- (100% blockade) and forskolin- (70% blockade) induced stimulation of histamine synthesis. The H3 inverse agonist thioperamide enhanced histamine synthesis in the presence of 1 mM IBMX or 30 mM potassium (+47 and +45%, respectively). Similarly, the H3 antagonist clobenpropit enhanced histamine synthesis in the presence of 30 mM potassium (+ 59%). The cAMP-dependent protein kinase blockers Rp-cAMPs and PKI14-22 could impair the effects of thioperamide and clobenpropit, respectively. These results indicate that the adenylate cyclase-protein kinase A pathway is involved in the modulation of histamine synthesis by H3 autoreceptors present in histaminergic nerve terminals.

Environmental enrichment modifies the PKA-dependence of hippocampal LTP and improves hippocampus-dependent memory

cAMP-dependent protein kinase (PKA) is critical for the expression of some forms of long-term potentiation (LTP) in area CA1 of the mouse hippocampus and for hippocampus-dependent memory. Exposure to spatially enriched environments can modify LTP and improve behavioral memory in rodents, but the molecular bases for the enhanced memory performance seen in enriched animals are undefined. We tested the hypothesis that exposure to a spatially enriched environment may alter the PKA dependence of hippocampal LTP. Hippocampal slices from enriched mice showed enhanced LTP following a single burst of 100-Hz stimulation in the Schaffer collateral pathway of area CA1. In slices from nonenriched mice, this single-burst form of LTP was less robust and was unaffected by Rp-cAMPS, an inhibitor of PKA. In contrast, the enhanced LTP in enriched mice was attenuated by Rp-cAMPS. Enriched slices expressed greater forskolin-induced, cAMP-dependent synaptic facilitation than did slices from nonenriched mice. Enriched mice showed improved memory for contextual fear conditioning, whereas memory for cued fear conditioning was unaffected following enrichment. Our data indicate that exposure of mice to spatial enrichment alters the PKA dependence of LTP and enhances one type of hippocampus-dependent memory. Environmental enrichment can transform the pharmacological profile of hippocampal LTP, possibly by altering the threshold for activity-dependent recruitment of the cAMP-PKA signaling pathway following electrical and chemical stimulation. We suggest that experience-dependent plasticity of the PKA dependence of hippocampal LTP may be important for regulating the efficacy of hippocampus-based memory.

Evidence for a G protein-coupled gamma-hydroxybutyric acid receptor

gamma-Hydroxybutyric acid (GHB) is a naturally occurring metabolite of GABA that has been postulated to exert ubiquitous neuropharmacological effects through GABA(B) receptor (GABA(B)R)-mediated mechanisms. The alternative hypothesis that GHB acts via a GHB-specific, G protein-coupled presynaptic receptor that is different from the GABA(B)R was tested. The effect of GHB on regional and subcellular brain adenylyl cyclase in adult and developing rats was determined and compared with that of the GABA(B)R agonist (-)-baclofen. Also, using guanosine 5'-O:-(3-[(35)S]thiotriphosphate) ([(35)S]GTPgammaS) binding and low-K:(m) GTPase activity as markers the effects of GHB and (-)-baclofen on G protein activity in the brain were determined. Neither GHB nor baclofen had an effect on basal cyclic AMP (cAMP) levels. GHB significantly decreased forskolin-stimulated cAMP levels by 40-50% in cortex and hippocampus but not thalamus or cerebellum, whereas (-)-baclofen had an effect throughout the brain. The effect of GHB on adenylyl cyclase was observed in presynaptic and not postsynaptic subcellular tissue preparations, but the effect of baclofen was observed in both subcellular preparations. The GHB-induced alteration in forskolin-induced cAMP formation was blocked by a specific GHB antagonist but not a specific GABA(B)R antagonist. The (-)-baclofen-induced alteration in forskolin-induced cAMP formation was blocked by a specific GABA(B)R antagonist but not a specific GHB antagonist. The negative coupling of GHB to adenylyl cyclase appeared at postnatal day 21, a developmental time point that is concordant with the developmental appearance of [(3)H]GHB binding in cerebral cortex, but the effects of (-)-baclofen were present by postnatal day 14. GHB and baclofen both stimulated [(35)S]GTPgammaS binding and low-K:(m) GTPase activity by 40-50%. The GHB-induced effect was blocked by GHB antagonists but not by GABA(B)R antagonists and was seen only in cortex and hippocampus. The (-)-baclofen-induced effect was blocked by GABA(B)R antagonists but not by GHB antagonists and was observed throughout the brain. These data support the hypothesis that GHB induces a G protein-mediated decrease in adenylyl cyclase via a GHB-specific G protein-coupled presynaptic receptor that is different from the GABA(B)R.

Modulation of the ischemia-induced taurine release by adenosine receptors in the developing and adult mouse hippocampus

The release of the inhibitory amino acid taurine is markedly enhanced under ischemic conditions in both adult and developing hippocampus, together with a pronounced increase in the release of excitatory amino acids and the neuromodulator adenosine. We studied the effects of adenosine receptor agonists and antagonists as well as adenosine transport inhibitors on hippocampal [(3)H]taurine release in normoxia and ischemia, using a superfusion system. Under standard conditions the adenosine A(1) receptor agonists N(6)-cyclohexyladenosine and R(-)N(6)-(2-phenylisopropyl)adenosine potentiated basal taurine release in developing mice and depressed the release in adults in a receptor-mediated manner. Adenosine A(2) receptor compounds had only minor effects on the basal release and the K(+)-stimulated release was not affected by these drugs. The adenosine uptake inhibitor dipyridamole enhanced basal taurine release in the developing hippocampus and reduced it in the adult. In ischemia the adenosine compounds had no marked effects on taurine release in immature animals, whereas A(1) receptor activation was still able to evoke taurine release in adults by a receptor-mediated mechanism. The results show that the basal release of taurine is modulated by A(1) receptors in both mature and immature hippocampus, whereas in ischemia these receptors potentiate taurine release only in adults. The elevated taurine levels together with the depression of excitatory amino acid release by adenosine receptor activation could be beneficial under ischemic conditions, protecting neural cells against excitotoxicity and hyperexcitation.

Histamine H(3) receptors mediate inhibition of noradrenaline release from intestinal sympathetic nerves

1. The present study investigates whether presynaptic histamine receptors regulate noradrenaline release from intestinal sympathetic nerves. The experiments were performed on longitudinal muscle-myenteric plexus preparations of guinea-pig ileum, preincubated with [(3)H]-noradrenaline. 2. In the presence of rauwolscine, electrically-induced [(3)H]-noradrenaline release was inhibited by histamine or R-alpha-methylhistamine, whereas it was unaffected by pyridylethylamine, impromidine, pyrilamine, cimetidine, thioperamide or clobenpropit. The inhibitory effects of histamine or R-alpha-methylhistamine were antagonized by thioperamide or clobenpropit, but not by pyrilamine or cimetidine. In the absence of rauwolscine, none of these drugs modified the release of [(3)H]-noradrenaline. 3. The modulatory action of histamine was attenuated by pertussis toxin and abolished by N-ethylmaleimide. Tetraethylammonium or 4-aminopyridine enhanced the evoked tritium outflow and counteracted the inhibitory effect of histamine. However, the blocking effects of tetraethylammonium and 4-aminopyridine were no longer evident when their enhancing actions were compensated by reduction of Ca(2+) concentration in the superfusion medium. 4. Histamine-induced inhibition of tritium output was enhanced by omega-conotoxin or low Ca(2+) concentration, whereas it was not modified by nifedipine, forskolin, rolipram, phorbol myristate acetate, H7 or lavendustin A. 5. The present results indicate that presynaptic H(3) receptors, located on sympathetic nerve endings, mediate an inhibitory control on intestinal noradrenergic neurotransmission. It is suggested that these receptors are coupled to G(i)/G(o) proteins which modulate the activity of N-type Ca(2+) channels through a direct link, thus reducing the availability of extracellular Ca(2+) at the level of noradrenergic nerve terminals.

Potentiation of receptor-mediated cAMP production: role in the cross-talk between vasopressin V1a and V2 receptor transduction pathways

Cross-talk between the phospholipase C and adenylyl cyclase signalling pathways was investigated in Chinese hamster ovary (CHO) cells transfected with the V1a and V2 vasopressin receptors. Cell lines expressing V1a, V2, or both V1a and V2 receptors, were established and characterized. Stimulation of V2 receptors by vasopressin induced a dose-dependent increase in cAMP accumulation, whereas stimulation of V1a receptor resulted in an increase in intracellular calcium without any change in basal cAMP. The simultaneous stimulation of V2 and V1a receptors by vasopressin elicited an intracellular cAMP accumulation which was twice that induced by stimulation of V2 receptor alone with deamino-[d-Arg8]vasopressin. This potentiation between V1a and V2 receptors was mimicked by activation of protein kinase C (PKC) with PMA, and was suppressed when PKC activity was inhibited by bisindolylmaleimide. The potentiation was observed in the presence or absence of 1 mM 3-isobutyl-1-methylxanthine, a phosphodiesterase inhibitor, implying that an alteration in cAMP hydrolysis was not involved. Vasopressin, as well as PMA, had no effect on the forskolin-induced cAMP accumulation, suggesting that PKC did not directly stimulate the cyclase activity. On the other hand, vasopressin, like PMA, potentiated the cAMP accumulation induced by cholera toxin, an activator of Galphas protein. These results suggest that, in CHO cells, vasopressin V1a receptor potentiates the cAMP accumulation induced by the V2 receptor through a PKC-dependent increase in the coupling between Gs protein and adenylyl cyclase.

Effects of the Aconitum alkaloid mesaconitine in rat hippocampal slices and the involvement of alpha- and beta-adrenoceptors

1 The effects of mesaconitine, the main alkaloid contained in Aconiti tuber, were investigated by use of extracellular recordings of stimulus-evoked population spikes and field excitatory postsynaptic potentials (e.p.s.ps) in the CA1 region of rat hippocampal slices. 2 At a concentration of 10 nM, mesaconitine evoked excitations, which were manifested as an increase in the amplitude of the orthodromic spike and the appearance of multiple spikes following the first postsynaptic spike, without affecting the magnitude of paired-pulse facilitation. The increase in spike amplitude was persistent and was not reversed by up to 90 min of washout. At concentrations of 30 and 100 nM, the alkaloid produced a biphasic effect, that is an excitation followed by an inhibition without having any effect upon the field e.p.s.p. At concentrations above 100 nM, mesaconitine suppressed the orthodromic population spike and the field e.p.s.p. 3 The excitatory effect was also observed when electrical stimulation was stopped completely during the application of mesaconitine (10 nM) and during the first 15 min of washout. 4 The enhancement of the population spike and the appearance of multiple spikes induced by mesaconitine (10-100 nM) were blocked by pretreatment with the beta-adrenoceptor antagonists propranolol (1 microM) and timolol (1 microM), whereas the inhibitory effect was blocked by the alpha-adrenoceptor antagonists yohimbine (1 microM) and phentolamine (10 microM). However, when the beta-adrenoceptor antagonist timolol was added 10 min after the application of mesaconitine, it failed to block the long-lasting enhancement of the spike amplitude and the appearance of multiple population spikes. 5 Application of the selective beta-adrenoceptor agonist isoprenaline (500 nM) to the hippocampal slices induced an increase in the amplitude of the orthodromic population spike and elicited 2-3 additional spikes. Mesaconitine (10 nM) did not further potentiate this enhancement of the spike amplitude when added after a 15 min pretreatment with isoprenaline. 6 Perfusion of forskolin, which directly activates adenylate cyclase, enhanced the population spike. Mesaconitine had no additional effect when applied after pretreatment with forskolin. 7 It is concluded that the excitatory effects evoked by lower concentrations of the plant alkaloid mesaconitine are mediated by stimulation of beta-adrenoceptors and the consequent activation of intracellular processes which lead to the long-lasting changes in excitability.

Modulation of forskolin-mediated adenylyl cyclase activation by constitutively active G(S)-coupled receptors

In transfected CHO cells constitutively active histamine H2 receptors not only increase the basal cAMP level, but also enhance forskolin-induced cAMP production. The increased forskolin response was inhibited by inverse H2 agonists with potencies similar to those determined at basal levels. The modulation of the forskolin response was also observed after H2 receptor expression in HEK-293 and Sf9 cells or TSH receptor expression in COS-7 cells. The enhancement of forskolin-induced cAMP production seems to be a general characteristic of constitutively active G(S)-coupled receptors and can be very useful to study inverse agonism at wild-type receptors.

Forskolin potentiates isoprenaline-induced glycerol output and local blood flow in human adipose tissue in vivo

The synergistic action of forskolin on beta-adrenoceptor-mediated glycerol output and changes in local blood flow were investigated in situ, in human adipose tissue of healthy subjects, by the use of microdialysis. The addition of isoprenaline 0.1-1.0 microM or forskolin 10-100 microM to the perfusion solvent caused a concentration-dependent, marked and sustained increase in the levels of glycerol in the dialysate (lipolysis index) as compared to the solvent alone. On a molar basis, isoprenaline was almost one thousand times more potent than forskolin. Isoprenaline caused a rapid and concentration-dependent decrease in the ethanol clearance ratio (index of local blood flow, i.e. a decrease in ethanol ratio implies an increase in blood flow). Forskolin had no effect on the ethanol ratio at either 1.0 microM or 10 microM, while forskolin at 100 microM induced a significant decrease in the ethanol ratio. When adipose tissue was pre-treated with forskolin, the subsequent addition of isoprenaline to the microdialysate resulted in a significantly higher glycerol output and a significantly more prominent decrease in the ethanol ratio than with isoprenaline alone. In conclusion, the data demonstrate that forskolin and the (beta-adrenoceptor-agonist both stimulate lipolysis and local blood flow in human adipose tissue in vivo. Furthermore, forskolin, at concentrations that are ineffective alone, potentiates the actions of isoprenaline on lipolysis and blood flow.

Potentiation of the relaxing action of isoproterenol by forskolin in rabbit aortic rings: the involvement of beta 2-adrenoceptors

1. Forskolin, an activator of adenylate cyclase, potentiated the relaxing response to isoproterenol in rabbit aortic rings precontracted by phenylephrine (PE). 2. The potentiating effect of forskolin was inhibited by propranolol, a beta-adrenoceptor inhibitor, but not by methylene blue, a guanylate cyclase inhibitor. 3. The relaxing response to terbutaline, a beta 2-adrenoceptor agonist, but not lower concentrations of dobutamine, a beta 1-adrenoceptor agonist, also was potentiated by forskolin. Forskolin, however, potentiated the relaxing response to high concentrations of dobutamine, which activates both beta 1- and beta 2-adrenoceptors. 4. Yohimbine, an alpha 2-adrenoceptor inhibitor, glyburide, an ATP-sensitive K+ channel inhibitor, iberiotoxin, a Ca(2+)-activated K+ channel inhibitor, or endothelium-removal failed to affect the potentiating effect of forskolin. 5. Dibutyryl cyclic AMP (cAmp) also potentiated the relaxing response to terbutaline. 6. These results suggest that in rabbit aortic rings forskolin causes the apparent potentiation of isoproterenol-induced relaxation by mainly affecting the relaxing response due to the activation of beta 2-adrenoceptors by the forskolin-induced increase in the level of cAMP.

1,25-dihydroxyvitamin D3 and agents that increase intracellular adenosine 3',5'-monophosphate synergistically inhibit fibroblast proliferation

Agents that increase intracellular cAMP (cAMP elevating agents) and 1, 25(OH)2D3 inhibit the proliferation of many cell types. We investigated the combined effect of 1,25(OH)2D3 and cAMP elevating agents on exponentially growing mouse 3T3 fibroblasts. The following cAMP elevating agents were used: theophylline and pentoxyfilline, which inhibit cAMP-dependent phosphodiesterase; prostaglandin E2 which activates adenylate cyclase by a receptor-mediated mechanism; forskolin, which directly stimulates adenylate cyclase; and the cell permeable cAMP analogs 8-bromo cAMP and N6 benzoyl cAMP. 1,25(OH)2D3 and cAMP elevating agents were added to exponentially growing fibroblasts cultured in 96-well microtiter plates and cell number was monitored 3-7 d later. 1,25(OH)2D3 and the cAMP elevating agents as single agents inhibited the growth of the 3T3 cells. The combined treatment of the fibroblasts with 1,25(OH)2D3 and the cAMP elevating agents resulted in an antiproliferative effect that was more than additive. The synergistic interaction depended on the dose of 1,25(OH)2D3 and was apparent already at 10(-8) M of the hormone. The specificity of the effect of 1,25(OH)2D3 was demonstrated by the finding that 24,25-dihydroxyvitamin D3, a vitamin D metabolite with low affinity for the vitamin D receptor, did not affect the antiproliferative effect of cAMP elevating agents. From the synergistic interaction between 1,25(OH)2D3 and the cell permeable cAMP analogs, we infer that the site of interaction between the two signaling pathways is distal to the cAMP generating and degrading machinery.

Neurochemical changes in cerebellum of goldfish exposed to various temperatures

Acclimation of goldfish at 35 degrees C increased the cerebellar content of aspartate, glutamate, and taurine and [3H]glutamate uptake. Acclimation at 4 degrees C increased the levels of glutamine, serine, and alanine and glutamine synthetase (GS) activity. Adenosine content increased in cerebellum of fish acclimated to warm temperature. K+-evoked release of endogenous and exogenous glutamate from cerebellar slices increased in fish acclimated at 35 degrees C compared to 4 degrees C. The basal level of cyclic adenosine 3':5'-monophosphate (cAMP) in perfused cerebellar slices in fish acclimated at 35 degrees C was much higher than in fish acclimated at 5 degrees and 22 degrees C. It is concluded that variations of environmental temperature produces large neurochemical changes in goldfish cerebellum.

cAMP-dependent facilitation of glutamate release by beta-adrenergic receptors in cerebrocortical nerve terminals

We have investigated the presence of a cAMP-protein kinase A-dependent pathway in cerebrocortical nerve terminals and its role in the modulation of glutamate release. The activation of adenylyl cyclase with forskolin enhances intrasynaptosomal cAMP and induces Ca2+-dependent glutamate release. The membrane permeant analogue dibutyryl cAMP mimics this facilitatory effect, whereas the inactive compound 1,9-dideoxyforskolin is without effect. This cAMP-induced facilitation is consistent with the induction of spontaneous action potentials that are abolished by the Na+ channel blocker tetrodotoxin and by reducing nerve terminal excitability with arachidonic acid. We have also demonstrated that a beta-adrenergic receptor is linked to this pathway because isoproterenol increases cAMP levels and glutamate release, and both actions are antagonized by the receptor antagonist propanolol and the protein kinase A inhibitors H89 and 8-chloroadenosine 3',5'-monophosphorothioate ((Rp)-isomer). The finding that the increase in cytoplasmic free Ca2+ concentration induced by synaptic activity reduces the concentration of agonist required to maximally activate adenylyl cyclase suggests that this enzyme may act as a coincidence detector, integrating glutamatergic neurotransmission and noradrenaline release.

Brain pertussis toxin-sensitive G proteins are involved in the flavoxate hydrochloride-induced suppression of the micturition reflex in rats

The effect of flavoxate hydrochloride (flavoxate), an anti-pollakiurea agent, on cyclic AMP (cAMP) formation was investigated in the rat brain and a possible involvement of brain G proteins in the action of flavoxate on the bladder function was subsequently examined. Flavoxate (10(-8)-10(-5) M) inhibited cAMP formation in a concentration-dependent manner, an action which was completely abolished by pretreating the membranes with pertussis toxin (PTX). The inhibitory effect of flavoxate was also completely antagonized by combined treatment with any two antagonists for adenosine A1 (8-cyclopentyl-1,3-dipropylxanthine), dopamine D2 (sulpiride) or adrenergic alpha 2 (yohimbine) receptors, although each antagonist alone did not significantly block the flavoxate-induced inhibition of cAMP formation. Radioligand binding studies indicated that flavoxate at micro- or submicromolar concentrations has affinity for Gi-coupled receptors such as A1, D2 and alpha 2 receptors. Therefore, flavoxate may inhibit cAMP formation by the stimulation of A1, D2 and alpha 2 receptors. To clarify the involvement of brain Gi proteins in the flavoxate-induced inhibition of the micturition reflex, the effect of pretreatment with PTX (i.c.v.) on the flavoxate-induced inhibition of isovolumetric rhythmic bladder contractions was examined in rats. Flavoxate (3 mg/kg, i.v.) completely abolished rhythmic bladder contractions in vehicle-pretreated rats, but not in PTX-pretreated rats. These findings suggest that signal transduction via Gi-coupled receptors is involved, at least in part, in the inhibition of the micturition reflex by flavoxate in rats. These results also provide the first evidence suggesting a negative role of brain PTX-sensitive G proteins in the micturition reflex.

Possible participation of histamine H3 receptors in the modulation of noradrenaline release from rat spinal cord slices

Rat spinal cord slices prelabelled with [3H]noradrenaline were superfused with a medium containing 1 mu M desipramine plus 0.3 mu M phentolamine. Histamine (0.01-10 mu M) and the selective histamine H3 receptor agonist R-(-)-alpha-methylhistamine (0.001-10 mu M) caused a concentration-dependent decrease in the release of radioactivity evoked by electrical field stimulation (0.8 Hz, 20 mA, 2 min). The inhibitory effect of histamine was not modified by either pyrilamine (1 mu M) or ranitidine (10 mu M), but it was antagonized by burimamide (1 mu M). The inhibitory action of histamine (1 mu M) was attenuated by pertussis toxin (3 mu g/ml) and was abolished by N-ethylmaleimide (30 mu M). Neither forskolin (10 mu M) nor rolipram (100 mu M), nor the combination of both drugs, modified the inhibitory effect of histamine. Histamine (1 mu M) did not modify the overflow of tritium induced by electrical stimulation in the absence of phentolamine. The present results suggest that in the rat spinal cord the release of noradrenaline elicited by electrical stimulation is negatively modulated by histamine, probably through the activation of histamine H3 receptors. This modulatory mechanism is likely to involve the participation of regulatory Go/Gi proteins.

Osmotic dilution stimulates axonal outgrowth by making axons more sensitive to tension

Mechanical tension is a potent stimulator of axonal growth rate, which is also stimulated by osmotic dilution. We wished to determine the relationship, if any, between osmotic stimulation and tensile regulation of axonal growth. We used calibrated glass needles to apply constant force to elongate axons of cultured chick sensory neurons. We find that a neurite being pulled at a constant force will grow 50-300% faster following a 50% dilution of inorganic ions in the culture medium. That is, osmotic dilution appears to cause axons to increase their sensitivity to applied tensions. Experimental interventions suggest that this effect is not mediated by dilution of extracellular calcium, or to osmotic stimulation of adenylate cyclase, or to osmotic stimulation of mechanosensitive ion channels. Rather, experiments measuring the static tension normally borne by neurites suggest a direct mechanical effect on the cytoskeletal proteins of the neurite shaft. Our results are consistent with a formal thermodynamic model for axonal growth in which removing a compressive load on axonal microtubules promotes their assembly, thus promoting axonal elongation.

Subtypes of Guanine-Nucleotide-Binding Regulatory Proteins at the Locus coeruleus Involved in Fentanyl-Induced Muscular Rigidity in the Rat

Previous results from our laboratory have established that the G(o) subtype of guanine nucleotide (GTP)-binding regulatory protein at the locus coeruleus (LC) may participate in the elicitation of muscular rigidity by fentanyl. The present study further examined the involvement of other subtypes of GTP-binding regulatory proteins at the LC in this process, using Sprague-Dawley rats anesthetized with ketamine (120 mg/kg, i.p., with 30 mg/kg/h i.v. infusion supplements) and under mechanical ventilation. Intravenous administration of fentanyl (100 &mgr;g/kg) induced a significant increase in electromyographic signals recorded from the sacrococcygeus dorsi lateralis muscle. Power spectral analysis revealed that this was accomplished by a decrease in the mean power frequency and an increase in the root mean square values of the signals. The above responses were appreciably antagonized by pretreating animals with bilateral microinjection into the LC of pertussis toxin (80 or 160 fmol), N-ethylmaleimide (16 pmol) or 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (100 or 200 fmol); but not by cholera toxin (120 or 240 fmol), forskolin (240 or 480 pmol) or N-ethylmaleimide at a higher dose (32 pmol). These results suggest that, in addition to G(o) protein, fentanyl-induced muscular rigidity may also involve other pertussis toxin-sensitive GTP-binding regulatory proteins, possibly G(i) and G(p) subtypes, in the signal transduction processes following activation of &mgr;-opioid receptors at the LC. Copyright 1995 S. Karger AG, Basel

Analysis of effects of corticotropin, forskolin and fluoride on activity of adenylate cyclase of bovine adrenal cortex

A mathematical model relating the activity of adenylate cyclase (AC) with concentrations of stimulators, equilibrium dissociation constants, specific activity and efficacies of AC depending on the states of its binding sites has been developed and used for analysis of the data on activation of AC of bovine adrenal cortex plasma membranes presented in (De Foresta et al. (1987) FEBS Lett. 216, 107-112). Equilibrium dissociation constants. chi h and chi l, corresponding to high- and low-affinity forskolin-binding sites were estimated to be 0.37 and 17 microM: these constants characterize forskolin's potency more adequately than does ED50, the concentration eliciting half-asymptotic activity of AC. Corticotropin does not affect the affinity of AC for forskolin whereas fluoride increases this affinity, thus augmenting forskolin's potency. Hormone receptor of adenylate cyclase of bovine adrenal cortex has been suggested to have two or more binding sites for corticotropin. Some unidentified factor(s) may be responsible for the differences found in adenylate cyclase activity in different experiments carried out under similar conditions. The model applied for the analysis may be thought to be the best means for the moment to relate dose-response dependencies with what is known or can be hypothesized about the mechanisms underlying activation of adenylate cyclase.

Acetylcholine elicits a rebound stimulation of Ca2+ current mediated by pertussis toxin-sensitive G protein and cAMP-dependent protein kinase A in atrial myocytes

Cholinergic inhibition of atrial contraction is typically followed by a rebound positive inotropic response. In the present study, we used a nystatin-perforated patch whole-cell recording method to determine whether acetylcholine (ACh) elicits a rebound stimulation of L-type Ca2+ current (ICa,L) in cat atrial myocytes. ACh (1 mumol/L) decreased basal ICa,L (-19 +/- 2%). Within approximately 30 s of returning to ACh-free solution, basal ICa,L exhibited a rebound increase above the control level (+61 +/- 7%) that returned to the control level within 4 to 5 minutes. ACh elicited concomitant changes in cell shortening, ie, a decrease followed by a rebound increase. The EC50 and maximal response of ACh-induced inhibition and rebound stimulation of ICa,L were 1.9 x 10(-9) mol/L and -30%, respectively, and 2.9 x 10(-8) mol/L and +64%, respectively. All effects of ACh on ICa,L were blocked by prior exposure to 1 mumol/L atropine or 100 mumol/L AFDX116 and unaffected by 0.2 mumol/L pirenzepine or 1 mumol/L propranolol. In the presence of ACh, exposure to atropine elicited stimulation of ICa,L.ACh-induced inhibition and rebound stimulation of current were independent of external Ca2+. Rebound stimulation of ICa,L was associated with a negative shift in the voltage dependence of ICa,L activation. Inhibition of protein kinase A by 50 mumol/L Rp-cAMPs decreased basal ICa,L by 36 +/- 1% and abolished the rebound stimulation of ICa,L. Forskolin (0.01 mumol/L) or isoproterenol (0.01 mumol/L) had no effect on basal ICa,L, but each accentuated the rebound increase in ICa,L. When adenylate cyclase was maximally stimulated with 1 mumol/L isoproterenol plus 2 mumol/L forskolin, ACh decreased ICa,L but failed to elicit rebound stimulation of ICa,L. Milrinone (10 mumol/L) increased basal ICa,L by 70 +/- 7% and significantly attenuated the rebound stimulation of ICa,L. Exposure to 1 mmol/L 8-bromo-cGMP elicited a small decrease in basal ICa,L, attenuated ACh-induced inhibition, and enhanced the rebound stimulation of ICa,L. Incubation in pertussis toxin prevented all ACh-induced changes in ICa,L. Inhibition of nitric oxide synthase by 100 mumol/L NG-monomethyl-L-arginine (L-NMMA) decreased basal ICa,L by -20 +/- 5%, prevented ACh-induced inhibition, and markedly attenuated the rebound stimulation of ICa,L. We conclude that in cat atrial myocytes ACh acts via M2 muscarinic receptors and pertussis toxin-sensitive G protein to inhibit basal ICa,L and that on withdrawal ACh elicits a rebound stimulation of ICa,L. Rebound stimulation of ICa,L is mediated via cAMP-dependent protein kinase A enhanced by ACh-induced inhibition of phosphodiesterase.(ABSTRACT TRUNCATED AT 400 WORDS)

Remoxipride shows low propensity to block functional striatal dopamine D2 receptors in vitro

The effect of remoxipride ((S)(-)3-bromo-N-[(1-ethyl-2-pyrrolidinyl)methyl]-2,6-dimethoxybenzam ide) on dopamine D2 receptor-mediated inhibition of cAMP formation in rat striatal tissues pieces was established together with that of a number of other dopamine D2 receptor antagonists. The action of remoxipride, three other substituted benzamides, (-)-sulpiride, raclopride and NCQ 298 ((S)-3-iodo-N-[(1-ethyl-2-pyrrolidinyl)methyl]-5,6-dimethoxysalicylam ide mesylate) and haloperidol, a butyrophenone, was studied in the presence of (I) (+/-) SKF 38393 (7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine) hydrochloride (100 microM) plus pergolide (1 microM) or (II) forskolin (1 microM) plus dopamine (100 microM). In addition, four of the metabolites of remoxipride: FLA 797 ((S)-3-bromo-N[(1-ethyl-2-pyrrolidinyl)methyl]-2-hydroxy-6- methoxybenzamide), NCR 181 ((S)(-)-5-bromo-N-[(1-ethyl-2-pyrrolidinyl)methyl]-2-hydroxy-6- methoxybenzamide tartrate), NCQ 436 ((S)(-)-3-bromo-N-[(1-ethyl-2-pyrrolidinyl)methyl]-5,6-dihydroxy-2- methoxybenzamide semioxalate) and NCQ 469 ((S)(-)-3-bromo-N-[(1-ethyl-2-pyrrolidinyl)methyl]-5-hydroxy-2,6- dimethoxybenzamide hydrochloride), mainly found in rodents, were studied using test system I. The results demonstrate that remoxipride is significantly weaker in blocking functional striatal dopamine D2 receptors than either of the reference compounds studied and three of the four metabolites. The studies also demonstrate that dopamine D1 and D2 receptor interactions at the level of cAMP formation in the striatum are independent of action potentials or Ca2+.

Further Elucidation of a Pertussis Toxin-Sensitive Transmembrane Signaling Mechanism Involved in Central alpha(2)-Adrenoceptor Activation in the Rat

In adult male Sprague-Dawley rats anesthetized with pentobarbital sodium, we elucidated the molecular consequence of central alpha(2)-adrenoceptor activation. The hypotensive and negative chronotropic and inotropic actions of the alpha(2)-adrenoceptor agonist guanabenz were used as our experimental index. Intracerebroventricular administration of pertussis toxin (2.5 &mgr;g) significantly attenuated the cardiovascular suppressant effects of the aminoguanidine compound (100 &mgr;g/kg i.v.). However, application of N-ethylmaleimide (0.125 or 0.250 &mgr;g), phorbol 12-myristate 13-acetate (1.25 or 2.50 &mgr;g), cholera toxin (1.25 or 2.50 &mgr;g) or forskolin (12.5 or 25.0 &mgr;g) into the lateral cerebral ventricle elicited no appreciable blunting effect on the circulatory depression produced by guanabenz. These results were essentially duplicated when pertussis toxin (0.125 or 0.250 &mgr;g), N-ethylmaleimide (0.0125 or 0.05 &mgr;g), phorbol 12-myristate 13-acetate (0.125 or 0.25 &mgr;g), cholera toxin (0.125 or 0.25 &mgr;g) or forskolin (1.25 or 2.50 &mgr;g) was microinjected bilaterally to the nucleus reticularis gigantocellularis, a medullary site believed to be intimately related to the antihypertensive action of guanabenz. These findings suggest that stimulation of the alpha(2)-adrenoceptors in the medulla oblongata may result in the activation of a pertussis toxin-sensitive GTP-binding regulatory protein. They further suggest that the biologic signals subsequent to this action may not be linked to Gs, Gi or Gp but possibly Go. Copyright 1994 S. Karger AG, Basel

Evidence that the presynaptic A2a-adenosine receptor of the rat motor nerve endings is positively coupled to adenylate cyclase

The action of the A2a-adenosine analogue, CGS 21680C, on electrically evoked [3H]acetylcholine ([3H]-ACh) release, and its interaction with forskolin (an activator of adenylate cyclase), MDL 12,330A (an irreversible inhibitor of adenylate cyclase), rolipram (an inhibitor of cyclic AMP specific phosphodiesterase), dibutyryl- (db-cAMP) and 8-bromo- (8-Br-cAMP) cyclic AMP analogues (substances that mimic intracellular actions of cyclic AMP), were investigated using rat phrenic nerve-hemidiaphragm preparations. CGS 21680C facilitated [3H]-ACh release. Forskolin (but not 1,9-dideoxy forskolin), rolipram, db-cAMP and 8-Br-cAMP also increased evoked neurotransmitter release in a concentration-dependent manner. When the evoked [3H]-ACh release that is dependent on stimulation of the adenylate cyclase/cyclic AMP transduction system was supramaximally stimulated by these compounds, CGS 21680C (3 nmol/l) could not further increase [3H]-ACh release. Phosphodiesterase inhibition with low concentrations (< or = 30 mumol/l) of rolipram significantly potentiated the augmenting effect of CGS 21680C (1 nmol/l) on evoked [3H]-ACh release. MDL 12,330A (an irreversible inhibitor of adenylate cyclase) decreased evoked [3H]-ACh release. The irreversible blocking action of MDL 12,330A on [3H]-ACh release was overcome by by-passing cyclase activation with db-cAMP and 8-Br-cAMP, but could not be overcome with FSK or CGS 21680C. The inhibitory effect of MDL 12,330A on evoked [3H]-ACh release was not mimicked by nifedipine. It is concluded that the increase in [3H]-ACh release caused by CGS 21680C results from activation of an A2a-adenosine receptor positively linked to the adenylate cyclase/cyclic AMP system.

Anomalies in ion transport in CF mouse tracheal epithelium

The cystic fibrosis (CF) mouse trachea has become a model for gene transfer. To characterize ion transport properties of tracheal epithelium from normal and CF mice, tracheas were excised, mounted in Ussing chambers, and basal properties and responses to pharmacological agents and/or ion substitution protocols measured. No difference in basal short-circuit (Isc) was observed between normal (29.1 +/- 3.8 muA/cm2, n = 21) and CF (34.7 +/- 4.5 muA/cm2, n = 16) tracheas. The relative contribution of Na+ transport to basal Isc was small (30-40%). Ionomycin stimulated large increases in Isc in both normal and CF murine tracheas [change in Isc (delta Isc) with ionomycin: 30.5 +/- 8.8 muA/cm2, n = 11, normal; 27.3 +/- 6.7 muA/cm2, n = 6, CF]. Unexpectedly, forskolin increased Isc in both CF and normal amiloride-pretreated tracheas (delta Isc: 10.5 +/- 2.1 muA/cm2, n = 21, normal; 13 +/- 2.3 muA/cm2, n = 16, CF). Forskolin was observed to increase intracellular Ca2+ in both normal and CF tracheal cells, suggesting this as a mechanism to induce Cl- secretion. These similarities in ion transport, in part reflecting the dominance of Ca(2+)-regulated Cl- conductance, suggest that the murine trachea is not an ideal target for assessment of CF correction by gene transfer.

Stimulation of postsynaptic and inhibition of presynaptic adenylyl cyclase activity by metabotropic glutamate receptor activation

To determine the subcellular distribution of cyclic AMP-coupled metabotropic glutamate receptors (mGluRs), the effects of glutamate agonists on adenylyl cyclase activity were examined using two hippocampal membrane preparations. These were synaptosomes (SY), which are composed of presynaptic terminals, and synaptoneurosomes (SN), which are composed of both pre- and postsynaptic elements. In SY, a water-soluble analogue of forskolin (7 beta-forskolin) increased enzyme activity approximately 10-fold at the highest concentration tested. The selective metabotropic receptor agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) inhibited enzyme activity as did glutamate and quisqualate. L-Amino-4-phosphobutanoate (L-AP4) had no effect on enzyme activity at any concentration tested. The metabotropic receptor antagonist L-2-amino-3-phosphopropionic acid (L-AP3) was not effective in the SY in antagonizing the agonist-induced decreases in adenylyl cyclase activity by glutamate or 1S,3R-ACPD. It was, however, effective at antagonizing quisqualate-induced decreases in enzyme activity. In SN, at the highest concentration tested, 7 beta-forskolin produced a 60-fold increase in adenylyl cyclase activity. As was observed in SY, glutamate decreased adenylyl cyclase activity in SN. In contrast, 1S,3R-ACPD, quisqualate, and L-AP4 increased adenylyl cyclase activity. In the SN, L-AP3 was ineffective in antagonizing any agonist-induced increases (1S,3R-ACPD, L-AP4, and quisqualate) or decreases (glutamate) in adenylyl cyclase activity. The data suggest that postsynaptic metabotropic glutamate receptor activation results in stimulation of adenylyl cyclase activity, whereas inhibition of this enzyme appears to be mediated at least partly through presynaptic mechanisms.

Enhancement of NMDA receptor-mediated synaptic potential by isoproterenol is blocked by Rp-adenosine 3',5'-cyclic monophosphothioate

The intracellular mechanisms underlying the facilitatory action of isoproterenol (Iso) on the NMDA receptor-mediated synaptic potential (EPSPNMDA) was investigated in an in vitro slice preparation of rat amygdala. Intracellular recordings were made from basolateral amygdala neurons in the presence of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) and picrotoxin (50 microM) which block non-NMDA and GABAA receptors, respectively. Superfusion of Iso (15 microM) produced a sustained increase in EPSPNMDA. Rp-adenosine-3',5'-cyclic monophosphotioate (Rp-cAMPS), a potent inhibitor of protein kinase A (PKA) alone decreased the amplitude of EPSPNMDA below baseline values and prevented the subsequent potentiation by Iso. Forskolin, a direct activator of adenylate cyclase, mimics the effect of Iso, and Rp-cAMPS also reversed forskolin-induced enhancement of EPSNMDA. These results suggest that cAMP-dependent protein kinase mediates the enhancement of EPSPNMDA by Iso in the amygdala.

N-acetylaspartylglutamate inhibits forskolin-stimulated cyclic AMP levels via a metabotropic glutamate receptor in cultured cerebellar granule cells

The neuronal dipeptide N-acetylaspartylglutamate (NAAG) fulfills several of the criteria for classification as a neurotransmitter including localization in synaptic vesicles, calcium-dependent release after neuronal depolarization, and low potency activation of N-methyl-D-aspartate receptors. In the present study, the influence of NAAG on metabotropic receptor activation in cerebellar granule cells was examined in cell culture. Stimulation of granule cell adenylate cyclase with forskolin increased cyclic AMP (cAMP) several hundredfold above basal levels within 10 min in a concentration-dependent manner. Although glutamate, NAAG, and the metabotropic receptor agonist trans-1-amino-1,3-cyclopentanedicarboxylic acid did not alter the low basal cAMP levels, the application of 300 microM glutamate or NAAG or trans-1-amino-1,3-cyclopentanedicarboxylic acid reduced forskolin-stimulated cAMP in granule cells by 30-50% in the absence or presence of inhibitors of ionotropic acidic amino acid receptors, as well as 2-amino-4-phosphonobutyrate. No additivity in the inhibition of cAMP was found when 300 microM NAAG and trans-1-amino-1,3-cyclopentanedicarboxylic acid were coapplied. The beta-analogue of NAAG failed to reduce cAMP levels. Similar effects of NAAG and glutamate were obtained under conditions of inhibition of phosphodiesterase activity and were prevented by pretreatment of the cells with pertussis toxin. These data are consistent with the activation by NAAG of a metabotropic acidic amino acid receptor coupled to an inhibitory G protein. In contrast, the metabotropic acidic amino acid receptor coupled to phosphoinositol turnover in these cells was not activated by NAAG.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenocorticotropin/alpha-melanocyte-stimulating hormone (ACTH/MSH)-like peptides modulate adenylate cyclase activity in rat brain slices: evidence for an ACTH/MSH receptor-coupled mechanism

The regulation of adenylate cyclase activity by adrenocorticotropin/alpha-melanocyte-stimulating hormone (ACTH/MSH)-like peptides was investigated in rat brain slices using a superfusion method. Adenylate cyclase activity was concentration-dependently increased by ACTH-(1-24), alpha-MSH (EC50 values 16 and 6 nM, respectively), and [Nle4,D-Phe7]alpha-MSH (EC50 value 1.6 nM), in the presence of forskolin (1 microM, optimal concentration). 1-9-Dideoxyforskolin did not augment the response of adenylate cyclase to ACTH-(1-24). Various peptide fragments were tested for their ability to enhance [3H]cyclic AMP production. [Nle4,D-Phe7]alpha-MSH increased [3H]cyclic AMP formation with a maximal effect of 30% and was more potent than ACTH-(1-24), ACTH-(1-16)-NH2, alpha-MSH, ACTH-(1-13)-NH2, [MetO4]alpha-MSH, [MetO2(4),D-Lys8,Phe9]ACTH-(4-9), ACTH-(7-16)-NH2, ACTH-(1-10), and ACTH-(11-24), in order of potency. This structure-activity relationship resembles that found for the previously described peptide-induced display of excessive grooming. ACTH-(1-24) stimulated adenylate cyclase activity in both striatal (maximal effect, approximately 20%) and septal slices (maximal effect, approximately 40%), but not in hippocampal or cortical slices. Lesioning of the dopaminergic projections to the striatum did not result in a diminished effect of [Nle4,D-Phe7]alpha-MSH on [3H]cyclic AMP accumulation, which indicates that the ACTH/MSH receptor-stimulated adenylate cyclase is not located on striatal dopaminergic terminals. ACTH-(1-24) did not affect the dopamine D1 or D2 receptor-mediated modulation of adenylate cyclase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Vip-induced cross-talk between G-proteins in membranes from rat anterior pituitary cells

In order to study the activation mechanism of heterotrimeric G-proteins by agonist-liganded receptors, GTP gamma S binding to membranes was measured in rat adenohypophyseal cells after addition of dopamine (DA) or vasoactive intestinal peptide (VIP), which, respectively, inhibit and activate pituitary adenylyl cyclase. G-protein subunit present in anterior pituitary cells was characterized by either ADP-ribosylation catalysed by Bordetella pertussis and cholera toxins or by immunoblot using specific antisera. Binding of GTP gamma S was found to depend upon GTP gamma S and Mg2+ concentrations; it was sensitive to pretreatment of the cells with cholera and Bordetella pertussis toxins (IAP). DA increased binding of the nucleotide. Paradoxically, VIP decreased the rate of GTP gamma S binding; the effect was suppressed by prior treatment of the cells with either cholera toxin or IAP. VIP also increased [33P]ADPribose incorporation in Gi/Go-proteins catalysed by IAP. Forskolin was also able to decrease GTP gamma S binding, thus suggesting that the binding of forskolin with the adenylyl cyclase catalytic unit might activate Gs proteins through an increased interaction between Gs and adenylyl cyclase. Taken together, these results suggest that VIP, as well as forskolin, may both accelerate the activation of Gs and suppress the inhibitory effect of activated Gi/Go-proteins. Interactions between Gs and Gi/Go subunits mediated by beta gamma and/or adenylyl cyclase might thus result in a kinetic coupling of transduction pathways involving distinct G-proteins.