Inositol phospholipid metabolism during and following synaptic activation: role of adenosine

Possible involvement of A1 receptors in the inhibition of gonadotropin secretion induced by adenosine in rat hemipituitaries in vitro

We investigated the participation of A1 or A2 receptors in the gonadotrope and their role in the regulation of LH and FSH secretion in adult rat hemipituitary preparations, using adenosine analogues. A dose-dependent inhibition of LH and FSH secretion was observed after the administration of graded doses of the R-isomer of phenylisopropyladenosine (R-PIA; 1 nM, 10 nM, 100 nM, 1 microM and 10 microM). The effect of R-PIA (10 nM) was blocked by the addition of 8-cyclopentyltheophylline (CPT), a selective A1 adenosine receptor antagonist, at the dose of 1 microM. The addition of an A2 receptor-specific agonist, 5-N-methylcarboxamidoadenosine (MECA), at the doses of 1 nM to 1 microM had no significant effect on LH or FSH secretion, suggesting the absence of this receptor subtype in the gonadotrope. However, a sharp inhibition of the basal secretion of these gonadotropins was observed after the administration of 10 microM MECA. This effect mimicked the inhibition induced by R-PIA, supporting the hypothesis of the presence of A1 receptors in the gonadotrope. R-PIA (1 nM to 1 microM) also inhibited the secretion of LH and FSH induced by phospholipase C (0.5 IU/ml) in a dose-dependent manner. These results suggest the presence of A1 receptors and the absence of A2 receptors in the gonadotrope. It is possible that the inhibition of LH and FSH secretion resulting from the activation of A1 receptors may have occurred independently of the increase in membrane phosphoinositide synthesis.

2-Chloroadenosine reverses hyperglycemia-induced inhibition of phosphoinositide synthesis in cultured human retinal pigment epithelial cells and prevents reduced nerve conduction velocity in diabetic rats

The effect of the adenosine (AD) analog 2-chloroadenosine (C-AD) on glucose-induced inhibition of phosphoinositide synthesis was studied in human retinal pigment epithelial (RPE) cells by monitoring the level of the phosphatidylinositol (PI) synthase substrate, cytidine diphosphate diglyceride (CDP-DG). In high-aldose reductase (AR)-expressing RPE 91 cells, C-AD decreased CDP-DG at 5 mmol/L glucose and reversed the increase by 20 mmol/L glucose. AD deaminase (ADA), which inactivates endogenously released AD, potentiated the hyperglycemia-induced increase in CDP-DG. Theophylline, an AD-A1 and AD-A2 receptor antagonist, caused an increase in CDP-DG at 20 mmol/L glucose. C-AD did not alter CDP-DG in low-AR-expressing RPE 45 cells, but did decrease CDP-DG after cells were conditioned in 300 mmol/L glucose for 1 week (which induces AR). The mechanism by which AD regulates PI synthase in cells with high AR activity is unknown, but it is independent of Gi or Gs proteins, adenylate cyclase and phospholipase C (PLC) activation, myo-inositol (MI) uptake, or MI efflux. Administration of C-AD to streptozotocin-induced diabetic rats prevented the slowing of motor nerve conduction velocity (MNCV). Thus, AD derivatives, which reverse a glucose-induced deficit in phosphoinositide metabolism, might serve as a useful pharmacological tool to intervene in hyperglycemia-induced diabetic complications.

Adenosine acts by A1 receptors to stimulate release of prolactin from anterior-pituitaries in vitro

Adenosine has been identified in the anterior pituitary gland and is secreted from cultured folliculostellate (FS) cells. To determine whether adenosine controls the secretion of anterior pituitary hormones in vitro, adenosine was incubated with anterior pituitaries. It stimulated prolactin (PRL) release at the lowest concentration used (10(-10) M); the stimulation peaked at 10(-8) M with a threefold increase in release and declined to minimal stimulation at 10(-4) and 10(-3) M. Follicle-stimulating hormone release was maximally inhibited at 10(-8) M, whereas luteinizing hormone release was not significantly inhibited. Two selective A1 adenosine receptor antagonists (10(-7) or 10(-5) M) had no effect on basal PRL release, but either antagonist completely blocked the response to the most effective concentration of adenosine (10(-8) M). In contrast, a highly specific A2 receptor antagonist (10(-7) or 10(-5) M) had no effect on basal PRL release or the stimulation of PRL release induced by adenosine (10(-8) M). We conclude that adenosine acts to stimulate PRL release in vitro by activating A1 receptors. Since the A1 receptors decrease intracellular-free calcium, this would decrease the activation of nitric oxide synthase in the FS cells, resulting in decreased release of nitric oxide (NO). NO inhibits PRL release by activating guanylate cyclase that synthesizes cGMP from GTP; cGMP concentrations increase in the lactotrophs leading to inhibition of PRL release. In the case of adenosine, NO release from the FS cells decreases, resulting in decreased concentrations of NO in the lactotrophs, consequent decreased cGMP formation, and resultant increased PRL release.

Purinergic inhibition of neurotransmitter release in the central nervous system

Neurotransmitter release and the role of adenosine in its regulation has been investigated for more than twenty years, and it is now widely accepted that adenosine tonically inhibits the release of excitatory neurotransmitters. This effect of adenosine is operated by an A1 adenosine receptor. Since activation of this receptor could inhibit Ca2+ conductance, increase K+ conductance, inhibit adenylate cyclase or phospholipase C, it is not clear if there is only one mechanism or several mechanisms operated by adenosine to inhibit neurotransmitter release, and in that case, what is the relative importance of each mechanism. The mechanism by which adenosine inhibits evoked synchronous transmitter release might be different from that used by the nucleoside to inhibit spontaneous asynchronous release. In some systems adenosine triphosphate per se acts like adenosine and inhibits neurotransmitter release. However, in most cases the inhibitory effect of this adenine nucleotide depends upon its hydrolysis into adenosine by a cascade of ectoenzymes, the last step being mediated by ecto-5'-nucleotidase.

The role of adenosine in mediating cellular and molecular responses to ethanol

We have found that ethanol-induced increases in extracellular adenosine activate adenosine receptors which, in turn, mediate many of the acute and chronic effects of ethanol in the nervous system. Several laboratories have demonstrated the importance of adenosine in mediating the acute and chronic effects of ethanol at multiple levels of investigation in the nervous system. These include genetic selection for ethanol sensitivity in mice, behavioral responses to ethanol in naive and tolerant animals, neurophysiologic responses in hippocampal slices, and at the level of cAMP signal transduction and gene expression in cultured neural cells. In this review we present results from our laboratory which document the role of adenosine in mediating ethanol-induced changes in neural function at a cellular and molecular level. A schematic summary of our findings is: Etoh-->decreases Ado uptake-->increases Extracellular Ado-->Activation of Adenosine A2 receptor-->increases cAMP-->increases PKA-->-->-->Heterologous Desensitization (decreases cAMP)-->-->-->insensitivity of adenosine uptake to ETOH

Tolerance to adenosine's accentuation of ethanol-induced motor incoordination in ethanol-tolerant mice

Our previously published reports have provided data that have supported a functional correlation between ethanol-induced changes in the characteristics of adenosine receptor, adenosine uptake and release in the brain, and ethanol-induced motor incoordination. The present data demonstrated a cross-tolerance between ethanol and adenosine further supporting the hypothesis that brain adenosine modulates the motor impairing effects of ethanol. Mice that received (-)-N6-cyclohexyladenosine (CHA) [0.25 mg/kg/day, intraperitoneally (ip)] for 10 days exhibited marked attenuation (cross-tolerance) to acute ethanol-induced motor incoordination compared with chronic saline (ip) controls. The attenuation of acute ethanol-induced motor incoordination was essentially same in animals that received CHA (25 ng/5 microliters/day for 10 days) by the intracerebroventricular (icv) route as opposed to the controls that chronically received artificial cerebral spinal fluid by the same route. Similarly, tolerance was exhibited to acute CHA (0.125 mg/kg ip and 12.5 ng/5 microliters icv) by animals fed liquid ethanol (19.5 g/kg/24 hr) for 10 days compared with none in the pair-fed sucrose controls. Scatchard plots using cerebellar tissue homogenates from animals given chronic CHA or chronic ethanol indicated no change in Bmax and/or Kd values for CHA binding when compared with CHA binding in tissues from their respective controls. However, a lack of any change in the binding characteristics cannot rule out the involvement of adenosine receptors in the observed cross-tolerance between ethanol and CHA. The results may suggest desensitization of adenosine A1 receptors due to chronic CHA and ethanol as an alternate possible explanation in the development of cross-tolerance between adenosine (CHA) and ethanol.

Functional role of intravascular coronary endothelial adenosine receptors

The endothelium is relatively 'impermeable' to adenosine. In addition, infusion of adenosine deaminase and transient infusion of large size adenosine agonists (molecular weight 100 kD) which are confined to the intravascular space depress effects of endogenous adenosine and retain physiologic activity respectively. Accordingly, the concept that intravascular adenosine may exert some of its action on the capillary lumen was tested by coupling the agonists: N6-([aminoethylamino]carbonyl)methylphenyladenosine (ADAC) and N6-octylamine adenosine (NOA) to carboxylated latex microspheres (0.07 microns diameter); thus, insuring their intravascular confinement. Our results demonstrated that sustained infusion of these particles into isolated saline perfused guinea pigs hearts caused a decrease in coronary vascular resistance, ventricular contraction, spontaneous ventricular rhythm, inhibition of auricular ventricular transmission and glycolytic flux. These effects were reversible and specific since microspheres without purines had no effect and the adenosine antagonist sulphophenyltheophylline blocked these responses. Furthermore, the effects were not the result that during the passage of the sphere-agonist complex through the heart the covalent bond hydrolyzed, releasing free agonist. Our data indicate that selective activation of intravascular coronary purine receptors may cause the release of endothelial bioactive messengers that regulate the function and metabolism of vascular and cardiac cells.

Autoradiographic changes in brain adenosine A1 receptors and their coupling to G proteins following seizures in the developing rat

In the central nervous system, adenosine has been shown to be a major regulator of neuronal activity in convulsive disorders, mainly via the A1 receptor subtype. In a previous work, we have shown that seizures lead to an age-dependent upregulation of cerebral adenosine A1 sites measured in isolated rat cerebral membranes. However, information concerning regional changes in the receptor density was so far lacking. In the present study, the effects of bicuculline-induced seizures were investigated by quantitative autoradiography of central adenosine A1 receptors in developing rats and in adults. Animals were sacrificed 30 min after an intraperitoneal injection of either saline or a convulsive dose of bicuculline. Adenosine A1 receptors in brain sections were labeled by [3H]N6-cyclohexyladenosine (CHA), a potent receptor agonist. Generalized seizures induced a widespread increase in CHA-specific binding, with a marked enhancement in structures that mediate seizure activity, such as substantia nigra, amygdala, septum and hippocampus. Moreover, the addition of guanylyl-5'-imidodiphosphate, a GTP analogue, to the incubation medium reduced CHA binding by the same order of magnitude whether rats were given saline or bicuculline, suggesting that additional adenosine A1 receptors are also functionally linked to G proteins. The age-related postictal increase in adenosine receptors might contribute to facilitate adenosine anticonvulsant effect, especially in newborns.

Physiological and pharmacological properties of adenosine: therapeutic implications

Adenosine is a nucleoside which has been shown to participate in the regulation of physiological activity in a variety of mammalian tissues, and has been recognized as a homeostatic neuromodulator. It exerts its actions via membrane-bound receptors which have been characterized using biochemical, electrophysiological and radioligand binding techniques. Adenosine has been implicated in the pharmacological actions of several classes of drugs. A number of studies strongly suggest that the nucleoside may regulate cellular activity in many pathological disorders and, in that respect, adenosine derivatives appear as promising candidates for the development of new therapeutic compounds, such as anticonvulsant, anti-ischemic, analgesic and neuroprotective agents.

Quantitative autoradiographic study of the postnatal development of adenosine A1 receptors and their coupling to G proteins in the rat brain

Adenosine is now considered as a major regulatory agent in the mammalian central nervous system. Its actions are mediated by specific receptors which are coupled with an adenylate cyclase system via a G protein. The postnatal development of adenosine A1 receptors was studied by quantitative autoradiography using [3H]N6-cyclohexyladenosine, a potent receptor agonist in 42 rat brain structures. The coupling of these sites to G proteins was examined by measuring the effects of in vitro addition of guanylyl-5'-imidodiphosphate, a stable analogue of guanosine triphosphate, on N6-cyclohexyladenosine binding. [3H]N6-Cyclohexyladenosine-specific binding was quite low at birth, around 10% of adult levels, and exhibited a rather homogeneous distribution pattern, except in thalamic nuclei. Data showed a sequential development of adenosine A1 receptors in relation to the time course of maturation of cerebral structures with a proliferation peak which paralleled rapid brain growth. The time period by which adult levels are reached differed according to the cerebral region studied. N6-Cyclohexyladenosine-specific binding sites appeared to be functionally linked to G proteins in all structures and at all postnatal stages. However, the potency of guanylyl-5'-imidodiphosphate to displace N6-cyclohexyladenosine binding was significantly lower before 5 days of age, suggesting functional changes during postnatal maturation in cerebral pathways modulated by adenosine.

Analysis of adenosine immunoreactivity, uptake, and release in purified cultures of developing chick embryo retinal neurons and photoreceptors

We have investigated the presence of endogenous adenosine and of mechanisms for adenosine uptake and release in chick embryo retinal neurons and photoreceptors grown in purified cultures in the absence of glial cells. Simultaneous autoradiographic and immunocytochemical analysis showed that endogenous adenosine and the uptake mechanism for this nucleoside colocalize in practically all the photoreceptors, but only in approximately 20% of the neurons. Approximately 25% of the neurons showed either immunocytochemical labeling or autoradiographic labeling, while greater than 50% of the neurons were unlabeled with both techniques. [3H]Adenosine uptake was saturable and could be inhibited by nitrobenzylthioinosine and dipyridamole and by pretreatment of the [3H]adenosine with adenosine deaminase. Although these observations indicate that the uptake is specific for adenosine, only 35% of accumulated radioactivity was associated with adenosine, with the remaining 65% representing inosine, hypoxanthine, and nucleotides plus uric acid. Adenosine as well as several of its metabolites were released by the cells under basal as well as K(+)-stimulated conditions. Potassium-enhanced release was blocked by 10 mM CoCl2 or in Ca2(+)-free, Mg2(+)-rich solutions. The results indicate that retinal cells that synthesize, store, and release adenosine differentiate early during embryogenesis and are therefore consistent with a hypothetical role for adenosine in retinal development.

Effect of 5'-(N-ethylcarboxamido)adenosine on adenosine transport in cultured chromaffin cells

Extracellular adenosine is transported into chromaffin cells by a high-affinity transport system. The action of adenosine receptor ligands was studied in this cellular model. 5'-(N-Ethylcarboxamido)adenosine (NECA), an agonist of A2 receptors, activated adenosine transport. Km values for adenosine were 4.6 +/- 1.0 (n = 5) and 10.2 +/- 3.0 microM (n = 5) for controls and 100 nM NECA, respectively. The Vmax values were 66.7 +/- 23.5 and 170.2 +/- 30 pmol/10(6) cells/min for controls and 100 nM NECA, respectively. The A1 agonist N6-cyclohexyladenosine, the A1 antagonist 8-cyclopentyl-1, 3-dipropylxanthine, and the A1-A2 antagonist 1,3-dipropyl-8-(4-[(2-aminoethyl)amino]-carbonylmethyloxyphenyl)- xanthine did not significantly modify the adenosine transport in this system. Binding studies done with [3H]dipyridamole, a nucleoside transporter ligand, did not show changes in either the number or affinity of transporter sites after NECA treatment. This ligand can enter cells and quantifies the total number of transporters. The binding studies with [3H]-nitrobenzylthioinosine, which quantifies the plasma membrane transporters, showed a Bmax of 19,200 +/- 800 and 23,200 +/- 700 transporters/cell for controls and 100 nM NECA, respectively. No changes in the KD were obtained. The effects of NECA were not mediated through adenylate cyclase activation, because its action was not imitated by forskolin.

Interactions between adenosine and phorbol esters or lithium at the frog neuromuscular junction

1. Interactions between the effects of adenosine or 2-chloro-adenosine (CADO) and the effects of substances that interfere with the phosphoinositides/protein kinase C transducing system or with the adenylate cyclase transducing system, on endplate potentials (e.p.ps), were investigated. The preparation used was the innervated sartorius muscle of the frog in which twitches had been prevented with high magnesium concentrations. 2. The activator of protein kinase C, 4 beta-phorbol-12,13-diacetate (PDAc), reversibly increased the amplitude and the quantal content of e.p.ps and attenuated the inhibitory effects of adenosine and CADO on e.p.p. amplitude. The affinity of the adenosine receptor antagonist, 8-phenyltheophylline, was not modified by PDAc. 3. The phorbol ester 4 alpha-phorbol-12,13-didecanoate, which does not activate protein kinase C, did not modify either e.p.p amplitude or the inhibitory effect of adenosine on e.p.ps. 4. The inhibitor of protein kinase C, polymyxin B, reversibly decreased the amplitude and the quantal content of e.p.ps, prevented the enhancement caused by PDAc on e.p.p. amplitude, but did not modify the inhibitory effect of adenosine on e.p.ps. H-7, another inhibitor of protein kinases, also decreased e.p.p. amplitude but did not modify the effect of PDAc on the amplitude of e.p.ps. 5. Lithium chloride, which alters phosphoinositide signal transduction by inhibiting the breakdown of inositol phosphates, reversibly increased the amplitude and the quantal content of the e.p.ps. In the presence of adenosine or CADO the effect of lithium on e.p.p. amplitude was markedly attenuated. 6. The activator of adenylate cyclase, forskolin, reversibly increased the amplitude and the quantal content of the e.p.ps. 7. The results suggest that the phosphoinositides/protein kinase C transducing system, but not the adenylate cyclase transducing system, might be involved in the inhibitory effect of adenosine on neuromuscular transmission.