Forms of adenylate cyclase, activation and/or potentiation by forskolin

Oleamide-induced modulation of 5-hydroxytryptamine receptor-mediated signaling

We investigated the effects of oleamide, a fatty acid amide isolated from the cerebrospinal fluid of sleep-deprived cats, on serotonin receptor-mediated signaling in cultured mammalian cells. Oleamide demonstrated opposing effects on 5-HT2A and 5-HT7 receptors, in rat pituitary cells and transfected HeLa cells, respectively. Oleamide caused a potentiation of 5-HT-elicited inositol phosphate formation mediated by the 5-HT2A receptor, but inhibited the effects of 5-HT on cAMP production mediated by the 5-HT7 receptor. In addition, oleamide alone caused a significant increase in cAMP accumulation that was dependent on the presence of the 5-HT7 receptor, but was not blocked by clozapine. These results demonstrate that oleamide can have diverse effects on 5-HT-mediated signal transduction at different subtypes of mammalian 5-HT receptors. Additionally, our data suggest that oleamide may act at an allosteric site on the 5-HT7 receptor and can elicit functional responses via activation of this site.

Rat striatal adenosinergic modulation of ethanol-induced motor impairment: possible role of striatal cyclic AMP

We have previously reported the involvement of the striatum in acute ethanol-induced motor incoordination and the striatal adenosinergic modulation of ethanol-induced motor incoordination through A1 receptor-mediated mechanism(s). The present study, a continuation of our previous work, was carried out to investigate the possible functional correlation between striatal cyclic AMP and ethanol-induced motor incoordination, and its modulation by striatal adenosine in Sprague-Dawley rats. Forskolin (0.1, 0.5 and 1.0 pmol), a known activator of adenylate cyclase, significantly attenuated ethanol-induced motor incoordination in a dose-dependent manner following its direct intrastriatal microinfusion. Forskolin also antagonized the accentuating effect of intrastriatal N6-cyclohexyladenosine on ethanol-induced motor incoordination. These results suggested that ethanol-induced motor incoordination might be functionally correlated to a decrease in the striatal cyclic AMP levels and that the striatal adenosine A1 receptors might modulate ethanol-induced motor incoordination through cyclic AMP signaling mechanism(s). Further support to this hypothesis was obtained by the actual measurement of the striatal cyclic AMP levels in the same experimental conditions as in motor coordination studies using high-performance liquid chromatography with fluoroscence detection. Regardless of the method (focused microwave irradiation, cervical dislocation or decapitation into a dry ice-ethanol mixture) used to kill the animals, a significant decrease in the striatal cyclic AMP levels was observed due to ethanol. Intrastriatal adenosine A1-selective agonist, N6-cyclohexyladenosine (24 ng), caused a further significant decrease in the striatal cyclic AMP levels in the ethanol- but not in the vehicle-treated animals. The further enhancement in the ethanol-induced decrease in the striatal cyclic AMP levels by intrastriatal N6-cyclohexyladenosine, therefore, functionally correlated with the observed potentiating effect of intrastriatal N6-cyclohexyladenosine on ethanol-induced motor incoordination. The effects of intrastriatal N6-cyclohexyladenosine+ethanol and of ethanol alone on the striatal cyclic AMP levels were blocked by intrastriatal pertussis toxin (500 ng) pretreatment, indicating the involvement of pertussis toxin-sensitive G-proteins (Gi, Go) and possibly of the adenosine A1 receptor coupled to the G-proteins in the striatum. Furthermore, ethanol alone significantly decreased the basal as well as the cyclic AMP-stimulated catalytic activities of the striatal cyclic AMP protein kinase, which were further reduced by intrastriatal N6-cyclohexyladenosine. The results of the present study therefore support an involvement of a cyclic AMP signaling pathway in the striatal adenosinergic modulation of ethanol-induced motor incoordination at the post-adenosine A1 receptor level.

Unique allosteric regulation of 5-hydroxytryptamine receptor-mediated signal transduction by oleamide

The effects of oleamide, an amidated lipid isolated from the cerebrospinal fluid of sleep-deprived cats, on serotonin receptor-mediated responses were investigated in cultured mammalian cells. In rat P11 cells, which endogenously express the 5-hydroxytryptamine2A (5HT2A) receptor, oleamide significantly potentiated 5HT-induced phosphoinositide hydrolysis. In HeLa cells expressing the 5HT7 receptor subtype, oleamide caused a concentration-dependent increase in cAMP accumulation but with lower efficacy than that observed by 5HT. This effect was not observed in untransfected HeLa cells. Clozapine did not prevent the increase in cAMP elicited by oleamide, and ketanserin caused an approximately 65% decrease. In the presence of 5HT, oleamide had the opposite effect on cAMP, causing insurmountable antagonism of the concentration-effect curve to 5HT, but had no effect on cAMP levels elicited by isoproterenol or forskolin. These results indicate that oleamide can modulate 5HT-mediated signal transduction at different subtypes of mammalian 5HT receptors. Additionally, our data indicate that oleamide acts at an apparent allosteric site on the 5HT7 receptor and elicits functional responses via activation of this site. This represents a unique mechanism of activation for 5HT G protein-coupled receptors and suggests that G protein-coupled neurotransmitter receptors may act like their iontropic counterparts (i.e., gamma-aminobutyric acid type A receptors) in that there may be several binding sites on the receptor that regulate functional activity with varying efficacies.

Age-related alteration in signal transduction: involvement of the cAMP cascade

In the present study, we have investigated the involvement of the cAMP signal transduction pathways in young and aged rats. A significantly higher endogenous adenosine 3':5'-cyclic monophosphate (cAMP) level and a significant decline of the adenylate cyclase [AC, ATP pyrophosphate-lyase (cyclizing), EC.4.6.1.1.] activity were observed in striatal tissue from young rats (3 months) in comparison to aged rats (approximately 40 months). In the nucleus accumbens (NA), no age-dependent changes in the cAMP concentration and in the AC basal activity were found. To address the question, whether the interactions of guanine nucleotide-binding protein (G-protein) subunits (G alpha s and Gi) with AC have changed in the aging process, various pharmacological agents that modulate the AC activity (e.g., beta, tau-imidoguanine 5'-triphosphate (GppNHp), sodium fluoride (NaF), forskolin (FSK), and the combinations of GppNHp plus FSK, NaF plus FSK, and NaF plus ethanol (ETOH)) were applied. In addition, a [3H]FSK binding test was carried out. In striatal and NA tissue, the stimulation of the AC activity by FSK was inhibited by GppNHp (via Gi-protein) and was superadditive by the combination of FSK and NaF (via Gs-protein). The absolute AC activity upon stimulation by all agents used was significantly lower in the aged striatum compared to young striatum. In the NA, however, the AC activity showed an age-dependent reduction only upon FSK and upon FSK plus GppNHp stimulation. There was no difference in the specific [3H]FSK binding to the G alpha s protein-coupled catalytic subunit of the AC between young and aged animals both in the striatum and NA.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization and pharmacological studies of an octopamine-sensitive adenylate cyclase from nerve cord of Locusta migratoria

An octopamine-sensitive adenylate cyclase that is insensitive to stimulation by dopamine (DA) and 5-hydroxytryptamine (5-HT), was studied in a homogenized nerve cord preparation of the migratory locust, Locusta migratoria. The enzyme complex is similar to catecholamine-sensitive cyclases from mammals with respect to pH-optimum, requirement for Mg++ and GTP, and sensitivity to forskolin. The octopamine-mediated elevation of adenylate cyclase activity is antagonized by a variety of drugs with the following order of potency: mianserin greater than phentolamine greater than promethazine greater than gramine greater than cyproheptadine greater than cis-flupenthixol greater than chlorpromazine greater than metoclopramide.

Forskolin activation of adenylate cyclase in rat myocardium with age: effects of guanine nucleotide analogs

Beta-adrenergic and post-receptor activation of adenylate cyclase decreases with age in the rat myocardium. Forskolin-stimulated adenylate cyclase activity decreases with age to the same extent as NaF or isoproterenol stimulation, suggesting that a loss of catalytic unit activity accounts for the loss of activity with age. However, recent evidence suggests that there are both a guanine nucleotide-dependent and independent component of forskolin activation. We assessed the possible role of each of these components with age by measuring forskolin dose-response stimulation of adenylate cyclase activity in the presence and absence of guanosine 5'-O-(2-thiodiphosphate) (GDP-beta S) and beta,gamma-imidoguanine 5'-triphosphate (Gpp(NH)p) in myocardial membranes from F-344 rats of 3, 12, and 24 months of age. Maximal forskolin stimulation of adenylate cyclase was least in the 24- (121 +/- 21 pmol cAMP/min/mg) as compared with either the 12- (212 +/- 29) or 3- (190 +/- 19) month-old rats. The presence of GDP-beta S had no effect on either the EC50 or maximum activity, and the age-related decline persisted. Gpp(NH)p plus isoproterenol enhanced forskolin activation but the effect was additive and not synergistic. There was no effect on the EC50, and the maximum activity was least in the 24-month-old rats. These data reconfirm the loss of catalytic unit activity with age and indicate that the reduced activity with age is independent of the presence or absence of activated G-protein.

Differential forskolin activation of rat heart and lung adenylate cyclase. Dependence on membrane-protein interactions

We have investigated whether the greater ability of forskolin to activate adenylate cyclase (EC 4.6.1.1) from rat heart compared with rat lung is due to interactions between G-proteins and catalytic units, isoforms of catalytic units or membrane-protein interactions. Interactions between Gs and catalytic units were found to be similar in both tissues with 10 microM Gpp(NH)p increasing activity up to 5-fold. While MnCl2 increased the response of the lung enzyme to forskolin, it reduced the response of the cardiac enzyme and uncoupled Gs from the cardiac catalytic units indicating that Gs interactions potentiate the response to forskolin. After enzyme solubilisation with n-octyl-beta-D-glucopyranoside, the response to forskolin was identical in heart and lung whether assayed with magnesium or manganese chloride, and not significantly different from the heart membrane enzyme. Overall, the results show that the relatively poor response of lung adenylate cyclase to forskolin is due to specific inhibitory interactions between the enzyme and lung membrane constituents.

Inhibition by forskolin of excitatory amino acid-induced accumulation of cyclic AMP in guinea pig hippocampal slices

The effect of forskolin on the excitatory amino acid-induced accumulation of cyclic AMP was examined in hippocampal preparations of the guinea pig. Forskolin at concentrations of 0.1-10 microM remarkably enhanced the stimulatory effects of histamine and adenosine, whereas it markedly attenuated the stimulation induced by cysteine sulfinate, an excitatory amino acid. Forskolin reduced the maximal response to cysteine sulfinate without affecting the apparent ED50. At a concentration of 1 microM, forskolin also inhibited the stimulatory effects of glutamate, N-methyl-DL-aspartate, and veratridine without affecting those of kainate and quisqualate. Pretreatment of the slices with 0.1 mM N-ethylmaleimide partially prevented the attenuation by forskolin of cysteine sulfinate-induced accumulation of cyclic AMP without affecting the stimulation induced by cysteine sulfinate. Forskolin at concentrations of less than 1 microM did not affect GTP-stimulated activity and Cl- -dependent activity of adenylate cyclase of the hippocampal membranes.

A dose-response study of forskolin, stimulatory hormone, and guanosine triphosphate analog on adenylate cyclase from several sources

We have described relationships involving forskolin stimulation of adenylate cyclase (AC) from a variety of sources and the potentiation of forskolin effects by stimulatory hormones (glucagon, ACTH, and epinephrine) and beta, gamma-imidoguanosine 5'-triphosphate (Gpp(NH)p). The effects on AC were examined using membrane preparations of rabbit adipocytes, rat adipocytes, rat erythrocytes, and rat liver. Also examined was the AC of liver membranes of rat pretreated with pertussis toxin as well as that solubilized from rat liver membranes. Maximal forskolin stimulation of AC in all preparations studied revealed a consistent 10-fold increase in AC activity. The EC50 for forskolin was 10 microM for rat liver, 15 microM for rabbit and rat adipocytes and 17 microM for rat erythrocyte AC stimulation. In all cases the AC activity attained by forskolin stimulation was further enhanced by stimulatory hormones in a dose-dependent manner. Furthermore, a combination of all three activators (forskolin, stimulatory hormone, and Gpp(NH)p) resulted in an even greater overall stimulation to levels ranging from 25- to 30-fold over unstimulated activity levels. In the presence of saturating levels of each stimulatory hormone and Gpp(NH)p, the EC50 for forskolin diminished markedly to the range of 0.5 to 4.0 microM. In the absence of any apparent tissue specificity for forskolin stimulation, the general pattern of these results further implicates the catalytic site of the AC complex as the site of forskolin activation. Furthermore, activation of additional components of the complex by Gpp(NH)p and tissue specific hormones may further influence the AC activity and thereby potentiate the stimulation by forskolin.

Conditional inhibition of forskolin-activated adenylate cyclase by guanosine diphosphate and its analog

Forskolin-activated adenylate cyclases (AC) in intact membranes, solubilized with Lubrol or eluted following adsorption on a forskolin-Sepharose column, were examined for inhibition by GDP and GDP beta S. AC in intact membranes of rat or rabbit adipocytes was activated by 100 microM forskolin and further potentiated by 10 microM Gpp(NH)p in combination with either 230 microM epinephrine or 50 mU X ml-1 ACTH. GDP (0-1 mM) or GDP beta S (0-500 microM) inhibited activation in a dose-dependent manner to a level similar to or slightly below that produced by 100 microM forskolin alone. Forskolin at 100 microM stimulated solubilized AC of rabbit adipocytes and rat liver membranes for 10 +/- 4 to 160 +/- 10 and from 26 +/- 2 to 274 +/- 21 pmol(mg X min)-1, respectively, in the absence of GDP beta S; forskolin-activated activity decreased from 160 +/- 10 to 157 +/- 6 and from 274 +/- 21 to 238 +/- 14 pmol(mg X min)-1 in the presence of 500 microM GDP beta S. Forskolin-activated solubilized enzyme was further potentiated by 10 microM Gpp(NH)p from 160 +/- 10 to 289 +/- 52 and from 274 +/- 21 to 702 +/- 50 pmol(mg X min)-1. GDP beta S at 500 microM inhibited 93 and 103% of the Gpp(NH)p-potentiated activity. AC of rat adipocytes eluted from forskolin-Sepharose affinity column with 500 mM NaCl and 100 microM forskolin was not significantly activated by Gpp(NH)p nor inhibited by GDP beta S. However, it was activated by forskolin. The lack of inhibition of unmodified forskolin-activated activity by GDP or GDP beta S in contrast to the inhibition of Gpp(NH)p-activated enzyme or Gpp(NH)p-potentiated forskolin-activated enzyme may be a general phenomenon descriptive of the action of forskolin on AC. Furthermore, inhibition of forskolin-activated AC by GDP and its analog may be a useful index in analyzing the degree of guanine nucleotide potentiation of this enzyme.