Adenosine as a neuromodulator and as a homeostatic regulator in the nervous system: different roles, different sources and different receptors

Ophidian envenomation strategies and the role of purines

Snake envenomation employs three well integrated strategies: prey immobilization via hypotension, prey immobilization via paralysis, and prey digestion. Purines (adenosine, guanosine and inosine) evidently play a central role in the envenomation strategies of most advanced snakes. Purines constitute the perfect multifunctional toxins, participating simultaneously in all three envenomation strategies. Because they are endogenous regulatory compounds in all vertebrates, it is impossible for any prey organism to develop resistance to them. Purine generation from endogenous precursors in the prey explains the presence of many hitherto unexplained enzyme activities in snake venoms: 5'-nucleotidase, endonucleases (including ribonuclease), phosphodiesterase, ATPase, ADPase, phosphomonoesterase, and NADase. Phospholipases A(2), cytotoxins, myotoxins, and heparinase also participate in purine liberation, in addition to their better known functions. Adenosine contributes to prey immobilization by activation of neuronal adenosine A(1) receptors, suppressing acetylcholine release from motor neurons and excitatory neurotransmitters from central sites. It also exacerbates venom-induced hypotension by activating A(2) receptors in the vasculature. Adenosine and inosine both activate mast cell A(3) receptors, liberating vasoactive substances and increasing vascular permeability. Guanosine probably contributes to hypotension, by augmenting vascular endothelial cGMP levels via an unknown mechanism. Novel functions are suggested for toxins that act upon blood coagulation factors, including nitric oxide production, using the prey's carboxypeptidases. Leucine aminopeptidase may link venom hemorrhagic metalloproteases and endogenous chymotrypsin-like proteases with venom L-amino acid oxidase (LAO), accelerating the latter. The primary function of LAO is probably to promote prey hypotension by activating soluble guanylate cyclase in the presence of superoxide dismutase. LAO's apoptotic activity, too slow to be relevant to prey capture, is undoubtedly secondary and probably serves principally a digestive function. It is concluded that the principal function of L-type Ca(2+) channel antagonists and muscarinic toxins, in Dendroaspis venoms, and acetylcholinesterase in other elapid venoms, is to promote hypotension. Venom dipeptidyl peptidase IV-like enzymes probably also contribute to hypotension by destroying vasoconstrictive peptides such as Peptide YY, neuropeptide Y and substance P. Purines apparently bind to other toxins which then serve as molecular chaperones to deposit the bound purines at specific subsets of purine receptors. The assignment of pharmacological activities such as transient neurotransmitter suppression, histamine release and antinociception, to a variety of proteinaceous toxins, is probably erroneous. Such effects are probably due instead to purines bound to these toxins, and/or to free venom purines.

Adenosine A(2A) receptors in portal hypertension: their role in the abnormal response to adenosine of the cranial mesenteric artery in rabbits

1. Adenosine is a regulator of mesenteric vasodilation involved in auto-regulation and post-prandial hyperemia, but the adenosine receptor subtype involved in this relaxant effect is poorly characterized. We have now pharmacologically characterized this receptor in rabbit mesenteric arteries and investigated how this adenosine receptor response changes in portal hypertensive animals since the adenosine response is decreased. 2. The closest non-metabolisable adenosine analogue, 2-chloroadenosine (CADO), the mixed A(1)/A(2) receptor agonist, 5'-ethylcarboxamidoadenosine (NECA), and the selective A(2A) receptor agonist, 2-[4-(2-p-carbonyethyl)phenylamino]-5'-N-ethylcarboxamidoadenosine (CGS 21680) (1 pM -- 1 mM) relaxed noradrenaline pre-contracted arteries with a rank order of potency of CGS 21680 (EC(50)=20 nM) > or = NECA (60 nM)>>CADO (640 nM). 3. The selective A(2A) receptor antagonist, 4-(2-[7-amino-2-(2-furyl)-[1,2,4]-triazolo[2,3-a][1,3,5]-triazin-5-ylamino]ethyl)phenol (ZM 241385, 100 nM), shifted to the right the CADO concentration-response curve. 4. In portal hypertensive animals, there was mainly a decreased potency but also a decreased efficacy of all tested adenosine agonists compared to normal animals. Concomitantly, there was a decreased adenosine plasma level and a decreased binding density of [(3)H]-CGS 21680 and [(3)H]-ZM 241385 to mesenteric artery membranes from portal hypertensive compared to normal rabbits. 5. These results indicate that A(2A) receptor activation is required for the adenosine-induced mesenteric relaxation and that the decreased density of A(2A) receptors may contribute to the decreased relaxation induced by adenosine of mesenteric arteries in portal hypertensive animals.

Blockade of adenosine A1 receptors in the posterior cingulate cortex facilitates memory in rats

Male Wistar rats were bilaterally implanted with indwelling cannulae in the caudal region of the posterior cingulate cortex. After recovery, animals were trained in a step-down inhibitory avoidance task (3.0-s, 0.4-mA foot shock) and received, immediately after training, a 0.5-microl infusion of the adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA; 1, 50 or 100 nM) or of the adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 1, 25 or 50 nM). Animals were tested twice, 1.5 h and, again, 24 h after training, in order to examine the effects of these agents on short- and long-term memory, respectively. Only 50-nM DPCPX was effective in altering memory, promoting a facilitation. These results suggest that adenosine A1 receptors in the posterior cingulate cortex inhibit memory consolidation in a way that their blockade facilitates memory for inhibitory avoidance in rats.

Effect of chronic and acute stress on ectonucleotidase activities in spinal cord

We have previously observed that, while acute stress induces analgesia, chronic stress causes a hyperalgesic response in male rats. No effect was observed in females. There is increasing evidence that both ATP and adenosine can modulate pain. Extracellular ATP and ADP are hydrolyzed by an apyrase in synaptosomes from the peripheral and central nervous systems. In the present study, we investigated the effect of chronic and acute stress on ATPase-ADPase and 5'-nucleotidase activities in spinal cord of male and female rats. Adult male and female Wistar rats were submitted to 1 h restraint stress/day for 1 day (acute) or 40 days (chronic) and were sacrificed 24 h later. ATPase-ADPase activities were assayed in the synaptosomal fraction obtained from the spinal cord of control and stressed animals. ADP hydrolysis was decreased 25% in chronically stressed males, while no change was observed on ATPase activity. There was an increase in the 5'-nucleotidase activity in the same group. No effect on ADPase, ATPase or on 5'-nucleotidase activity was observed in females with chronic stress, or after acute stress neither in males or females. Chronic stress reduced ADP hydrolysis and increased 5'-nucleotidase activity in the spinal cord in male rats.

Effects of angiotensin III and angiotensin IV on pentylenetetrazol seizure susceptibility (threshold and kindling): interaction with adenosine A(1) receptors

The effects of angiotensin (ANG) III and ANG IV on pentylenetetrazol (PTZ) seizure susceptibility--threshold and kindling in mice--as well as the influence of adenosine A(1) receptor agents (agonist and antagonist) on these effects were studied. It was found that ANG III and ANG IV increased dose-dependently the PTZ seizure threshold and decreased the seizure intensity in PTZ kindled mice. Cyclohexyladenosine (CHA), an adenosine A(1) receptor agonist, potentiated the effects of ANG III and ANG IV on the seizure threshold and kindling, whereas DPCPX (an A(1) receptor antagonist) reversed peptide-induced effects on the PTZ kindling. Taken together, ANG III and ANG IV decrease the PTZ seizure susceptibility. We could suggest that these effects are realized in part through interaction with adenosine A(1) receptors.

Pharmacological characterization of adenosine receptors in PGT-beta mouse pineal gland tumour cells

1. The adenosine receptor in mouse pinealocytes was identified and characterized using pharmacological and physiological approaches. 2. Expression of the two adenosine receptor subtypes A2B and A3 was detected in mouse pineal glands and PGT-beta cells by polymerase chain reaction and nucleotide sequencing. 3. Adenosine and 5'-N-ethylcarboxamidoadenosine (NECA) evoked cyclic AMP generation but the A2)-selective agonist 2-(4-(2-carboxyethyl)phenylethylamino)adenosine-5'-N-ethylcarboxamideadenosine (CGS 21680) and the A1-specific agonists R-N(6)-(2-phenylisopropyl)adenosine (R-PIA) and N(6)-cyclopentyladenosine (CPA) had little effect on intracellular cyclic AMP levels. The A2B receptor selective antagonists alloxazine and enprofylline completely blocked NECA-mediated cyclic AMP accumulation. 4. Treatment of cells with the A3-selective agonist N(6)-(3-iodobenzyl)-5'-(N-methylcarbamoyl)adenosine (IB-MECA) inhibited the elevation of the cyclic AMP level induced by NECA or isoproterenol in a concentration-dependent manner with maximal inhibition of 40 - 50%. These responses were blocked by the specific A3 adenosine receptor antagonist MRS 1191. Pretreatment of the cells with pertussis toxin attenuated the IB-MECA-induced responses, suggesting that this effect occurred via the pertussis toxin-sensitive inhibitory G proteins. 5. IB-MECA also caused a concentration-dependent elevation in [Ca(2+)]i and IP3 content. Both the responses induced by IB-MECA were attenuated by treatment with U73122 or phorbol 12-myristate 13-acetate. 6. These data suggest the presence of both A2B and A3 adenosine receptors in mouse pineal tumour cells and that the A2B receptor is positively coupled to adenylyl cyclase whereas the A3 receptor is negatively coupled to adenylyl cyclase and also coupled to phospholipase C.

Regulation of the ecto-nucleotidase pathway in rat hippocampal nerve terminals

Ecto-nucleotidases play a pivotal role in terminating the signalling via ATP and in producing adenosine, a neuromodulator in the nervous system. We have now investigated the pattern of adenosine formation with different concentrations of extracellular ATP in rat hippocampal nerve terminals. It was found that adenosine formation is delayed with increasing concentrations of ATP. Also, the rate of adenosine formation increased sharply when the extracellular concentrations of ATP + ADP decrease below 5 microM, indicating that ATP/ADP feed-forwardly inhibit ecto-5'-nucleotidase allowing a burst-like formation of adenosine possibly designed to activate facilitatory A2A receptors. Initial rate measurements of ecto-5'-nucleotidase in hippocampal nerve terminals, using IMP as substrate, showed that ATP and ADP are competitive inhibitors (apparent Ki of 14 and 4 microM). In contrast, in hippocampal immunopurified cholinergic nerve terminals, a burst-like formation of adenosine is not apparent, suggesting that channelling processes may overcome the feed-forward inhibition of ecto-5'-nucleotidase, thus favouring A1 receptor activation.

Adenosine A1 receptors reduce release from excitatory but not inhibitory synaptic inputs onto lateral horn neurons

Although adenosine is an important neuromodulator in the CNS, its role in modulating sympathetic outflow at the level of the spinal cord has not been studied. Because very little is known about adenosine A1 receptors (A1Rs) in the spinal cord, we determined their location and role with particular reference to the control of sympathetic preganglionic activity and interneuronal activity in the rat. High levels of immunoreactivity for A1Rs were observed throughout the spinal cord. Immunostaining was dense in the intermediolateral cell column (IML) and intercalated nucleus, regions containing retrogradely labeled sympathetic preganglionic neurons (SPNs). Electron microscopy revealed A1R immunoreactivity (A1R-IR) within presynaptic terminals and (to a lesser extent) postsynaptic structures in the IML, as well as the luminal membrane of endothelial cells lining capillaries. Using double-labeling techniques, some presynaptic terminals were observed to synapse onto SPNs. To investigate the effects of activating these A1Rs, visualized whole-cell patch-clamp recordings were made from electrophysiologically and morphologically identified SPNs and interneurons. Applications of the A1R agonist cyclopentyladenosine (CPA) reduced the amplitude of EPSPs elicited by stimulation of the lateral funiculus, an effect blocked by the A1R antagonist 8-cyclopentyl-1,3-dipropylxanthine. These effects were attributable to adenosine acting at a presynaptic site because CPA application increased the paired-pulse ratio. CPA did not affect evoked IPSPs. These data show that activating A1Rs reduces fast excitatory, but not inhibitory, transmission onto SPNs and interneurons in the IML and that A1Rs may play a protective role on neurons involved in the control of sympathetic outflow.

Age-dependent decrease in adenosine A1 receptor binding sites in the rat brain. Effect of cis unsaturated free fatty acids

Unsaturated free fatty acids and adenosine operate two neuromodulatory systems with opposite effects on neuronal function. Here, we tested if fatty acids controlled inhibitory adenosine A1 receptors. Arachidonate (AA, 10 microM) decreased the Bmax of an A1 receptor agonist, (R)-[3H]phenylisopropyladenosine (PIA; from 812 to 267 fmol x mg(-1) protein), and antagonist, [3H]1,3-dipropyl-8-cyclopentylxanthine (DPCPX; from 994 to 311 fmol x mg(-1) protein) and decreased the Kd of [3H]PIA (from 1.20 to 0.57 nM) binding to brain membranes of young adult rats (2 months old), these effects being mimicked by other cis but not trans unsaturated or saturated fatty acids. AA (10 microM) increased the potency of the A1 receptor agonist, 2-chloroadenosine to inhibit hippocampal synaptic transmission in young adult rats (EC50 decreased from 337 to 237 nM), which may constitute a safety feedback mechanism to control AA-induced neurotoxicity. Upon aging, there were increased free fatty acid levels and a concomitant decreased density of A1 receptors. This was more marked in hippocampal nerve terminals of aged rats (24 months old) and may be the determinant factor contributing to the lower potency of 2-choloroadenosine in aged rats (EC50 = 955 nM), in spite of the decreased Kd of PIA binding upon aging. The effects of AA on A1 receptor binding were attenuated upon aging, AA being devoid of effects in aged rats. Accordingly, AA (10 microM) failed to modify the potency of 2-choloroadenosine in aged rats (EC50 = 997 nM). However, albumin, which quenches free fatty acids, increased A1 receptor density by 65% and 2-chloroadenosine potency (EC50 = 703 nM) in aged rats, suggesting that the increased fatty acids levels in aged rats may contribute to the decreased potency of A1 receptor agonists in aged rats. Also, the observed saturation of the control by AA of A1 receptors may contribute to the decreased adaptability of neuromodulation to different firing conditions in aged rats.

Modulation of ethanol-induced motor incoordination by mouse striatal A(1) adenosinergic receptor

We have demonstrated that ethanol-induced motor incoordination is modulated by cerebellar adenosine A(1) receptor. This study represents an extension into another important brain motor area, the striatum that, unlike cerebellum, has high density of both A(1) and A(2A) receptors. Direct intra-striatal micro-infusion of Ro15-4513 (0.05, 0.5, 1 ng), a partial inverse-agonist of benzodiazepine, significantly and nearly dose-dependently attenuated ethanol-induced motor incoordination indicating mediation of ethanol's motor incoordination by striatum. Intra-striatal A(1)-selective agonist N(6)-cyclohexyladenosine (CHA; 1, 2, 4 ng), A(1) = A(2A) non-selective agonist, 5'-N-ethylcarboxamidoadenosine (NECA; 1.5, 3, 6 ng), and A(1)-selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 25, 50, 100 ng) dose-dependently accentuated and attenuated, respectively, ethanol-induced motor incoordination, strongly suggesting modulation by striatal adenosine A(1) receptor. Intra-striatal DPCPX significantly antagonized not only ethanol-induced motor incoordination but also its potentiation by intra-striatal CHA, R-(+)-N(6)-(2-phenylisopropyladenosine) (R-PIA), or NECA. No change in motor coordination occurred after the highest dose of CHA, R-PIA, or NECA followed by saline. Similarly, the highest intra-striatal dose of Ro15-4513 or DPCPX neither altered motor coordination or locomotor activity indicating relative selectivity of interaction with ethanol. Nearly 25-fold higher dose of A(2A)-selective agonist, CGS-21680, compared to CHA was necessary to produce a comparable potentiation of ethanol's motor incoordination perhaps suggesting a lack of or less significant striatal A(2A) involvement. Intra-striatal pertussis toxin (0.5 microg) pre-treatment markedly attenuated ethanol-induced motor incoordination as well as its potentiation by intra-striatal CHA. These results support that striatum is one of the brain motor areas mediating the motor impairing effects of acute ethanol and that the latter's modulation occurs via A(1)-selective receptors coupled to pertussis toxin-sensitive G proteins.

Modification by arachidonic acid of extracellular adenosine metabolism and neuromodulatory action in the rat hippocampus

Adenosine and arachidonate (AA) fulfil opposite modulatory roles, arachidonate facilitating and adenosine inhibiting cellular responses. To understand if there is an inter-play between these two neuromodulatory systems, we investigated the effect of AA on extracellular adenosine metabolism in hippocampal nerve terminals. AA (30 microm) facilitated by 67% adenosine evoked release and by 45% ATP evoked release. These effects were not significantly modified upon blockade of lipooxygenase or cyclooxygenase and were attenuated (52-61%) by the protein kinase C inhibitor, chelerythrine (6 microm). The ecto-5'-nucleotidase inhibitor, alpha,beta-methylene ADP (100 microm), caused a larger inhibition (54%) of adenosine release in the presence of AA (30 microm) compared with control (37% inhibition) indicating that the AA-induced extracellular adenosine accumulation is mostly originated from an increased release and extracellular catabolism of ATP. This AA-induced extracellular adenosine accumulation is further potentiated by an AA-induced decrease (48%) of adenosine transporters capacity. AA (30 microm) increased by 36-42% the tonic inhibition by endogenous extracellular adenosine of adenosine A(1) receptors in the modulation of acetylcholine release and of CA1 hippocampal synaptic transmission in hippocampal slices. These results indicate that AA increases tonic adenosine modulation as a possible feedback loop to limit AA facilitation of neuronal excitability.

Diadenosine polyphosphates facilitate the evoked release of acetylcholine from rat hippocampal nerve terminals

Diadenosine polyphosphates are present in synaptic vesicles, are released upon nerve stimulation and possess membrane receptors, namely in presynaptic terminals. However, the role of diadenosine polyphosphates to control neurotransmitter release in the CNS is not known. We now show that diadenosine pentaphosphate (Ap(5)A, 3-100 microM) facilitated in a concentration dependent manner the evoked release of acetylcholine from hippocampal nerve terminals, with a maximal facilitatory effect of 116% obtained with 30 microM Ap(5)A. The selective diadenosine polyphosphate receptor antagonist, diinosine pentaphosphate (Ip(5)I, 1 microM), inhibited by 75% the facilitatory effect of Ap(5)A (30 microM), whereas the P(2) receptor antagonists, suramin (100 microM) and pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS, 10 microM) only caused a 18-24% inhibition, the adenosine A(1) receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (20 nM), caused a 36% inhibition and the adenosine A(2A) receptor antagonist, 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo [2,3-a][1,3, 5]triazin-5-ylamino]ethyl)phenol (ZM 241385, 20 nM), was devoid of effect. These results show that diadenosine polyphosphates act as neuromodulators in the CNS, facilitating the evoked release of acetylcholine mainly through activation of diadenosine polyphosphate receptors.