Effects of ethanol on Arrhenius parameters and activity of mouse striatal adenylate cyclase

International Union of Basic and Clinical Pharmacology. CI. Structures and Small Molecule Modulators of Mammalian Adenylyl Cyclases

Adenylyl cyclases (ACs) generate the second messenger cAMP from ATP. Mammalian cells express nine transmembrane AC (mAC) isoforms (AC1-9) and a soluble AC (sAC, also referred to as AC10). This review will largely focus on mACs. mACs are activated by the G-protein Gαs and regulated by multiple mechanisms. mACs are differentially expressed in tissues and regulate numerous and diverse cell functions. mACs localize in distinct membrane compartments and form signaling complexes. sAC is activated by bicarbonate with physiologic roles first described in testis. Crystal structures of the catalytic core of a hybrid mAC and sAC are available. These structures provide detailed insights into the catalytic mechanism and constitute the basis for the development of isoform-selective activators and inhibitors. Although potent competitive and noncompetitive mAC inhibitors are available, it is challenging to obtain compounds with high isoform selectivity due to the conservation of the catalytic core. Accordingly, caution must be exerted with the interpretation of intact-cell studies. The development of isoform-selective activators, the plant diterpene forskolin being the starting compound, has been equally challenging. There is no known endogenous ligand for the forskolin binding site. Recently, development of selective sAC inhibitors was reported. An emerging field is the association of AC gene polymorphisms with human diseases. For example, mutations in the AC5 gene (ADCY5) cause hyperkinetic extrapyramidal motor disorders. Overall, in contrast to the guanylyl cyclase field, our understanding of the (patho)physiology of AC isoforms and the development of clinically useful drugs targeting ACs is still in its infancy.

Effects of ethanol on basal and adenosine-induced increases in beta-endorphin release and intracellular cAMP levels in hypothalamic cells

Recently we have shown that the cAMP system is involved in ethanol-regulated beta-endorphin (beta-EP) release from rat hypothalamic neurons in primary cultures. The cascade of events that leads to activation of cAMP following ethanol treatment in hypothalamic beta-EP neurons is not apparent. In this study the role of adenosine, a cAMP regulator, in ethanol-regulated beta-EP release was determined by measuring the cellular incorporation of [3H]adenosine, intracellular cAMP levels and media immunoreactive (IR) beta-EP levels in cultures of rat hypothalamic cells following ethanol treatments in the presence and absence of an adenosine agonist and antagonist. Acute exposure to a 50 mM dose of ethanol for a period of 1 h increased media levels of IR-beta-EP and cellular contents of cAMP, but the ethanol treatment decreased [3H]adenosine uptake. Constant exposure to a 50 mM dose of ethanol for a period of 48 h, failed to alter media levels of IR-beta-EP, cell content of cAMP and [3H]adenosine uptake. The media level of IR-beta-EP was elevated following treatment with adenosine receptor agonist phenyl-isopropyl adenosine (PIA) and was reduced following treatment with adenosine receptor antagonist isobutylmethylxanthine (IBMX) or with adenosine uptake inhibitor adenosine deaminase. The level of cellular cAMP was also increased by PIA but was decreased by IBMX and adenosine deaminase. The stimulatory actions of the adenosine agonist PIA on IR-beta-EP release and on cAMP production were potentiated by simultaneous incubation with ethanol for 1 h. However, chronic ethanol exposure reduced PIA-induced IR-beta-EP release and cAMP production. Additionally, both IBMX and adenosine deaminase reduced ethanol-induced IR-beta-EP release and cAMP levels. These results suggest that ethanol inhibits adenosine uptake in IR-beta-EP neurons in the hypothalamus, thereby increasing extracellular levels of adenosine and leading to activation of membrane adenosine receptors, cAMP production and IR-beta-EP secretion from these neurons. Chronic ethanol desensitizes the adenosine-regulated cAMP production and IR-beta-EP release from hypothalamic neurons.

Ethanol modulates calmodulin-dependent Ca(2+)-activated ATPase in synaptic plasma membranes

The effects of ethanol in vitro on calmodulin-dependent Ca(2+)-activated ATPase (CaM-Ca(2+)-ATPase) activity were studied in synaptic plasma membranes (SPM) prepared from the brain of normal and chronically ethanol-treated rats. In SPM from normal animals, ethanol at 50-200 mM inhibited the Ca(2+)-ATPase activity. Lineweaver-Burk analysis indicates that the inhibition was the result of a decreased affinity of the enzyme for calmodulin, whereas the maximum activity of the enzyme was not changed. Arrhenius analysis indicates that the enzyme activity was influenced by lipid transition of the membranes, and ethanol in vitro resulted in a shift of the transition temperature toward a lower value. From animals receiving chronic ethanol treatment (3 weeks), the SPM were resistant to the inhibitory effect of ethanol on the enzyme activity. The resistance to ethanol inhibition was correlated with a higher enzyme affinity for calmodulin and a higher transition temperature, as compared with normal SPM. Since the calmodulin-dependent Ca(2+)-ATPase in synaptic plasma membranes is believed to be the Ca2+ pump controlling free Ca2+ levels in synaptic terminals, its inhibition by ethanol could therefore lead to altered synaptic activity.

Glucocorticoid actions on synaptic plasma membranes: modulation of [125I]calmodulin binding

The effects of corticosterone on Ca(2+)-dependent binding of [125I]calmodulin to purified synaptic plasma membranes (SPM) from rats brain were characterized. The enhancement of [125I]calmodulin binding was a sigmoidal function of steroid concentration, with the maximal increase (> 55% above control) occurring at a steroid concentration of 1 x 10(-6) M and EC50 estimated at 1-2 x 10(-7) M. Other glucocorticoids including hydrocortisone, dexamethasone and triamcinolone produced similar effects, whereas steroids without glucocorticoid activity such as 11-deoxycortisol, 11-deoxycorticosterone and cholesterol were ineffective. The steroid-induced increase of binding was correlated with an increase of membrane affinity for [125I]calmodulin as shown by Scatchard analysis, and a decrease of the rate of dissociation of [125I]calmodulin from the membranes as shown by kinetic analysis. Arrhenius analysis indicates that [125I]calmodulin binding was influenced by lipid transition of the membranes and that corticosterone resulted in a shift of membrane transition toward a higher temperature. Since a variety of biochemical processes associated with synaptic membranes are dependent upon calmodulin for their regulation, we hypothesize that the effects of glucocorticoids in promoting membrane binding of calmodulin may lead to a cascade of alterations in synaptic function.

Ethanol exposure results in a transient decrease in human platelet cAMP levels: evidence for a protein kinase C mediated process

At concentrations between 2 and 32 mM, ethanol is shown to depress human platelet cAMP levels. The effect is biphasic, maximal at 30 sec, with platelet concentrations of cAMP returning to baseline values at higher ethanol concentrations and at longer incubation times. The cAMP lowering effect of ethanol can be blocked by a phosphodiesterase (PPDE) inhibitor, 3-isobutyl-1-methyl-xanthine (IBMX), at a concentration of 2 mM, suggesting that an increase in PPDE activity may be responsible for this effect. Exposure of platelets to 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H7), a protein kinase C (PKC) inhibitor, blocks the ethanol-induced decrease in platelet cAMP, suggesting ethanol may be acting through activation of PKC.

Recent Advances in Pharmacological Research on Alcohol

Alcohol dependence is a major public health problem. Studies have shown that a person dependent on alcohol often coabuses other substances, such as cocaine. Cocaine is a powerful stimulant whereas ethanol is generally considered to be a depressant, with some stimulating properties. The subjective effects of these two substances in a dependent individual may often appear to be more similar than they are different. Animals also self-administer both substances. Basically, although both substances have anesthetic properties and both act to functionally increase catecholaminergic function, especially that of dopamine, there are some differences in their actions. Both alcohol and cocaine have various effects on several neurotransmitters and systems, which ultimately interact to produce the feeling of well-being avidly sought by many individuals today. This drive often eventually produces a dependence which has associated social and medical consequences. It seems likely that the neurochemical changes that ensue following abuse of these substances underlie the phenomena of dependence, tolerance, and subsequent withdrawal. The apparent similarities and differences between these two substances will be reviewed in this chapter.

Chronic exposure to alcohol during development alters the membrane properties of cerebellar Purkinje neurons in culture

The active and passive membrane properties of developing Purkinje neurons in control cultures and cultures chronically treated with 20 or 40 mM ethanol for 1 or 2 weeks were examined using whole-cell current-clamp techniques. The membrane properties were characterized by the features of the voltage responses evoked by intracellular current injection of a series of depolarizing and hyperpolarizing current pulses. Analysis of these responses and background spontaneous activity showed several differences between the control and ethanol-treated Purkinje neurons: (1) membrane input resistance was significantly larger in the ethanol-treated neurons; (2) the percentage of neurons exhibiting immature firing patterns was significantly higher in the ethanol-treated neurons; (3) the afterhyperpolarization following a current-evoked train of action potentials was significantly larger in the ethanol-treated neurons; (4) spontaneous activity (synaptic potentials and synaptically evoked spike events) was significantly reduced in neurons treated with 40 mM ethanol for 1 week; spontaneous activity in neurons treated with 20 mM ethanol for 1 or 2 weeks was similar to that observed in the control group. These differences indicate that ethanol exposure during development directly alters the physiological properties of this CNS neuronal type. These neuronal actions of ethanol may contribute to the behavioral deficits observed in animals models of fetal alcohol syndrome. Similar target sites of ethanol action are likely to be present in the human CNS neurons and may be involved in human fetal alcohol syndrome.

Regulation of adenylate cyclase by cannabinoid drugs. Insights based on thermodynamic studies

The abilities of lipophilic cannabinoid drugs to regulate adenylate cyclase activity in neuroblastoma cell membranes were analyzed by thermodynamic studies. Arrhenius plots of hormone-stimulated adenylate cyclase activity exhibited a break point at 20 degrees. The break point was reduced to 14 degrees by benzyl alcohol, consistent with results from other laboratories that have correlated this response with the increase in membrane fluidity induced by benzyl alcohol. Because cannabinoid drugs partition into membrane lipids and alter membrane fluidity parameters in a number of model systems, it was of interest to examine the influence of delta 9-tetrahydrocannabinol and cannabidiol on enzyme activity analyzed by the Arrhenius plot. delta 9-Tetrahydrocannabinol, known to inhibit adenylate cyclase, failed to decrease the transition temperature either at 1 microM or at concentrations exceeding its aqueous solubility (30 microM), suggesting that delta 9-tetrahydrocannabinol could not mimic the effects observed with benzyl alcohol. In contrast, 30 microM cannabidiol, which stimulated enzyme activity slightly, decreased the Arrhenius plot break point to 17.5 degrees. The decrease in the transition temperature in response to benzyl alcohol or cannabidiol was not accompanied by a change in activation energies above or below the transition temperature. delta 9-Tetrahydrocannabinol inhibits adenylate cyclase activity via Gi as does the muscarinic agonist carbachol (Howlett et al., Mol Pharmacol 29: 307-313, 1986). Both carbachol and delta 9-tetrahydrocannabinol decreased the enthalpy and entropy of activation. The net free energy of activation at 37 degrees was increased in the presence of both of these inhibitory agonists. These data suggest that, for the entropy-driven hormone-stimulated adenylate cyclase enzyme, less disorder of the system occurs in the presence of regulators that inhibit the enzyme via Gi. In summary, thermodynamic data suggest that cannabidiol can influence adenylate cyclase by increasing membrane fluidity, but that the inhibition of adenylate cyclase by delta 9-tetrahydrocannabinol is not related to membrane fluidization.

Effects of ethanol on VIP-and/or noradrenaline-stimulated cAMP formation in mouse brain

Among several effects, ethanol (EtOH) interferes with membrane fluidity and lipid-protein interactions. As proteins are influenced by surrounding lipids, the activity of membrane-bound enzymes such as adenylate cyclase (AC) could be modulated by EtOH, as shown in potentiating, at toxic concentrations, the stimulating effect of hormones or neurotransmitters. We have also found that EtOH potentiates in a dose-dependent manner (EC50 = 100 mM) the cAMP production elicited by vasoactive intestinal peptide (VIP), already noticeably at 70 mM, without affecting basal cAMP levels (up to 400 mM). Propanol produces a similar potentiation, whereas methanol was inactive. Butanol (200 mM) displays toxic effects. The potentiation induced by EtOH is similar for peptide- (VIP) or monoamine- (noradrenaline) stimulated cAMP formation, suggesting a primary action at a interaction between VIP and NA in stimulating cAMP formation.

Differential response of adenylate cyclase and ATPase activities after chronic ethanol exposure of PC12 cells

The direct effects of chronic ethanol administration on adenylate cyclase, Na,K-ATPase, and Mg-ATPase activities in a cell containing neuronal characteristics were investigated using PC12 pheochromocytoma cells. Exposure of PC12 cells to 0, 75, and 150 mM ethanol for 4 days caused a dose-dependent increase in the stimulation of adenylate cyclase by in vitro ethanol without altering activation of the enzyme by GTP, NaF, MnCl2, or 2-chloroadenosine. Conversely, a 4-day treatment with 150 mM ethanol increased Na,K-ATPase and Mg-ATPase activities without altering the inhibitory effects of in vitro ethanol. The increase in Na,K-ATPase activity was associated with an increase in Vmax without any change in the Km for KCl. Chronic ethanol exposure also increased the amount of [3H]ouabain specifically bound to PC12 cell membranes. Except for the increase in Mg-ATPase activity, the above results were also observed when chronic ethanol treatment was carried out in the presence of pyrazole. Although ethanol slowed PC12 cell growth, observed changes were not due to an ethanol-induced reduction in cellular density. A 4-day exposure of a nonneuronal cell line (Madin Darby canine kidney cell) to 150 mM ethanol did not alter adenylate cyclase or ATPase activities. The present study indicates that the direct effects of chronic ethanol exposure of a neuronal-like cell involve an increase in the density of sodium pumps per cell and an enhanced sensitivity of adenylate cyclase to activation by ethanol.

Reduced sensitivity of red blood cell (Na+,K+)-ATPase to ethanol in vitro in male alcoholic patients: relationship to clinical characteristics

We examined red blood cell (Na+,K+)-ATPase, its sensitivity to inhibition by ethanol in vitro, and its relationship to clinical characteristics and history in 41 newly admitted alcoholic patients and 14 age-matched healthy controls. Sensitivity to ethanol was significantly lower in the alcoholic patients and correlated negatively with ethanol intake. In addition, sensitivity of enzyme to ethanol was lower in patients with high agitation-anxiety ratings and correlated negatively with agitation and anxiety scores on the Brief Psychiatric Rating Scale. There were no relationships between (Na+,K+)-ATPase measures and depressive symptoms, history of treatment for depression, or family history of depression. These data suggest that tolerance to the effects of ethanol on (Na+,K+)-ATPase occurs in man and may be related to the severity of ethanol dependence or withdrawal.

Neuronal membrane enzymes in rat lines selected for differential motor impairment by ethanol

Neuronal membrane enzyme activities were determined in naive and ethanol-treated (30 min after 2 g/kg) male and female rats of lines developed for more (ANT) and less (AT) ethanol-induced motor impairment. Ethanol did not affect acetylcholinesterase, (Na+K)-ATPase or 5'-nucleotidase activities, but adenylate cyclase activities were lowered in both cerebellum and cerebrum. Cerebral acetylcholinesterase activities were higher in ANT than AT rats. No consistent line difference was observed regarding (Na+K)-ATPase activities. Slightly higher cerebellar 5'-nucleotidase activities were found in the ANT line. Cerebellar adenylate cyclase levels were substantially higher in the AT line. No line differences were displayed in the activation of adenylate cyclase activity by dopamine or norepinephrine. It is concluded that ethanol in vivo may inhibit neuronal adenylate cyclase activity and that cerebellar phosphorylation may be a regulator of motor impairment. Cholinergic mechanisms may also be connected to the ethanol-induced motor impairment.

Modulation by n-alkanols of rat cardiac adenylate cyclase activity

n-Alkanols (from methanol to decanol) have a biphasic effect on rat cardiac adenylate cyclase either basal or stimulated by GTP, GppNHp, NaF or hormones (isoproterenol, glucagon, secretin) in the presence of GTP. At high concentration, all the enzyme activities are inhibited. At low concentration, adenylate cyclase activity is either unchanged or potentiated depending on both the stimulus and the alkanols involved. Potentiation is due to an increase of maximum velocity with no change in the activation constant of the enzyme. Basal activity is unchanged as well as the isoproterenol- and glucagon-stimulated enzyme. The secretin-stimulated enzyme is potentiated. It is the guanyl nucleotide regulatory protein-mediated stimulation of adenylate cyclase which is mainly affected. An attempt was made to relate these effects on adenylate cyclase with physical parameters of the alkanols (partition coefficient). From the data obtained as a function of the alkanol chain-length and of temperature on the adenylate cyclase stimulated by GTP, GppNHp, NaF and permanently activated, it is concluded that the increase in efficacy observed in the presence of alkanol is due to an interaction with the protein moeity particularly with the guanyl nucleotide regulatory protein.

Inhibition by ethanol of forskolin-stimulated adenylate cyclase in a murine neuroblastoma clone (N1E-115)

Forskolin, a diterpene activator of adenylate cyclase, stimulated the formation of cyclic AMP in intact murine neuroblastoma clone N1E-115 cells and stimulated adenylate cyclase activity in a membranal preparation from these cells. Ethanol caused a concentration-dependent inhibition of the forskolin-stimulated responses in both preparations. In intact cells, the inhibition appeared to be noncompetitive. However, in the membranal preparation the inhibition was more of a competitive nature. In addition, there was also a large difference in the amount of inhibition in the two systems. Thus, the inhibition by ethanol was nearly twice as much with intact cells as with membranes. Sucrose appeared to mimic these effects of ethanol, suggesting that with intact cells the effect of this alcohol may be due, in part, to changes in cellular osmotic pressure.

Effects of ethanol, temperature, and endogenous regulatory factors on the characteristics of striatal opiate receptors

Ethanol can alter the affinity of mouse striatal opiate receptors for their ligands, and the present studies were aimed at determining the importance of the receptor microenvironment for this effect of ethanol. Changing the temperature of the binding assay, and thus altering the properties of neuronal membrane lipids, resulted in changes in the observed affinity of striatal binding sites for dihydromorphine (DHM), but not for D-Ala2, D-Leu5-enkephalin (ENK). The changes in temperature also differentially altered the response of the two binding sites to ethanol. Two other factors that regulate opiate receptor affinity, Na+ and GTP, also affected the response to ethanol. High concentrations of ethanol were more effective at decreasing receptor affinity for both DHM and ENK when the binding assays were performed in the presence of GTP or Na+. In addition, at 37 degrees C and in the presence of GTP or Na+, DHM binding, but not ENK binding, was significantly inhibited by a low, physiologically attainable concentration of ethanol. Our results suggest that the response of opiate receptors to ethanol is influenced by the microenvironment of the receptors, including the physical state of the membrane lipids and/or by the nature of the interactions of receptors with "coupling proteins" within the membrane. The differential responses of mu and delta receptors to temperature and to ethanol suggest that these receptors reside in specific membrane environments. Under physiological conditions, several different factors may contribute to a selective action of ethanol on particular subtypes of opiate receptors.

Ethanol and temperature effects on five membrane bound enzymes

The effects of ethanol on the activities of five membrane bound enzymes were determined using a crude membrane preparation obtained from cortex of long-sleep (LS) and short-sleep (SS) mice. These two mouse lines were selectively bred for differences in duration of ethanol-induced sleep time. The enzymes studied were two forms of NaK-ATPase, Mg-ATPase, 5'nucleotidase, and acetylcholinesterase. Arrhenius plots of the ethanol-temperature-enzyme activity studies indicate specificity in ethanol's actions. NaK-ATPase activity consists of two enzymes which were distinguished by sensitivity to ouabain. The Arrhenius plot of the high ouabain sensitivity enzyme (low Ki) exhibited a transition temperature which was reduced twice as much by ethanol in LS membranes as in SS membranes. Ethanol did not affect the transition temperature of the high Ki NaK-ATPase but the control (no ethanol) transition temperature was 2.7 degrees higher in SS membranes. Arrhenius plots of Mg-ATPase activity did not exhibit a transition temperature and ethanol did not alter enzyme activity. Ethanol did not alter the transition temperatures of 5'nucleotidase or acetylcholinesterase but the control transition temperature for acetylcholinesterase was 2.3 degrees higher in SS membranes. These results indicate specificity in ethanol's actions on membranes and that inhibition of the lipid-enzyme interactions for the low Ki NaK-ATPase is correlated with the difference in sensitivity to ethanol seen between the LS and SS mice.

Brain membrane sensitivity to ethanol during development of functional tolerance to ethanol in rats

Adult male rats were rendered progressively tolerant to ethanol by daily intragastric administration of doses of 3-6 g/kg body weight. Functional tolerance assessed by the hypothermic effect after injection of a challenge dose of ethanol developed slowly and was demonstrable after 2 weeks of treatment. Intrinsic crude synaptosomal membrane fluidity, as assessed by fluorescence polarization of DPH, as well as (Na+ + K+)ATPase activity, did not change significantly during the whole treatment. However, the extra addition of ethanol (0.175-0.700 M) in vitro to the membranes of rats, having received ethanol over a period of at least 2 weeks, fluidized less and inhibited the (Na+ + K+)ATPase activity less than in starch-fed controls. The time-course for the appearance of this membrane hyposensitivity was found to be the same as the time course for the development of functional tolerance to ethanol. The correlation between the degree of functional tolerance and the (Na+ + K+)ATPase sensitivity to ethanol appeared very significant, highlighting the sensitivity of membrane-bound enzymes to detecting adaptive changes in complex biological membranes tolerance.

Effect of chronic ethanol treatment on adenylate cyclase activity in rat striatum

Chronic ethanol consumption induces an increase in striatal adenylate cyclase enzymatic activity but is unable to further potentiate the dopamine stimulated production of cyclic-AMP. In striatal membranes obtained from chronic ethanol-treated rats, apomorphine exerts a more potent inhibition of [3H]spiperone binding when compared with controls, demonstrating that ethanol increases the affinity of the dopaminergic receptors associated with adenylate cyclase activity. In addition, GTP is unable to modify the agonist component of dopamine receptor in membrane exposed 'in vivo' to ethanol. Data are discussed in terms of a derangement of receptor-adenylate cyclase coupling system produced by chronic ethanol treatment.