Dissociation of the locomotor and hypotensive effects of adenosine analogues in the rat

Adenosine receptors in the nervous system: pathophysiological implications

Adenosine is a ubiquitous homeostatic substance released from most cells, including neurones and glia. Once in the extracellular space, adenosine modifies cell functioning by operating G-protein-coupled receptors (GPCR; A(1), A(2A), A(2B), A(3)) that can inhibit (A(1)) or enhance (A(2)) neuronal communication. Interactions between adenosine receptors and other G-protein-coupled receptors, ionotropic receptors and receptors for neurotrophins also occur, and this might contribute to a fine-tuning of neuronal function. Manipulations of adenosine receptors influence sleep and arousal, cognition and memory, neuronal damage and degeneration, as well as neuronal maturation. These actions might have therapeutic implications for neurodegenerative diseases such as Parkinson's disease, Alzheimer's disease, as well as for other neurological situations such as epilepsy, idiopathic pain or even drug addition. Peripheral side effects associated with adenosine receptor agonists limit their usefulness in therapeutics; in contrast, adenosine receptor antagonists appear to have less side effects as it is the case of the well-known non-selective antagonists theophylline (present in tea) or caffeine (abundant in coffee and tea), and their emerging beneficial actions in Parkinson's disease and Alzheimer's disease are encouraging. A(1) receptor antagonism may also be useful to enhance cognition and facilitate arousal, as well as in the periphery when deficits of neurotransmitter release occur (e.g. myasthenic syndromes). Enhancement of extracellular adenosine levels through drugs that influence its metabolism might prove useful approaches in situations such as neuropathic pain, where enhanced activation of inhibitory adenosine A(1) receptors is beneficial. One might then consider adenosine as a fine-tuning modulator of neuronal activity, which via subtle effects causes harmonic actions on neuronal activity. Whenever this homeostasis is disrupted, pathology may be installed and selective receptor antagonism or agonism required.

Escape deficits induced by inescapable shock and metabolic stress are reversed by adenosine receptor antagonists

We examined the relationship between metabolic stress, brain adenosine regulation, and the learned helplessness effect in four experiments in rats. Glucoprivation and metabolic inhibition were induced by treating previously restrained (nonshocked) rats with 2-deoxy-D-glucose (2DG) shortly before escape testing. Experiment 1 demonstrated that 2-deoxy-D-glucose impairs escape performance in a dose-dependent manner. Experiment 2 showed that 2-deoxy-D-glucose and shock induced escape deficits are completely reversed by peripheral administration of the adenosine receptor antagonist caffeine. This result indicates that both inescapable shock and 2-deoxy-D-glucose result in compensatory adenosine regulation which, in turn, mediates the behavioral impairment. Experiment 3 determined that 8-[p-sulfophenyl]-theophylline, a peripheral adenosine receptor antagonist, fails to reverse the escape deficit resulting from metabolic stress, whereas centrally acting theophylline does. Experiment 4 showed that the behavioral impairments from both 2-deoxy-D-glucose and inescapable shock are reversed by intracranial ventricular (icv) caffeine treatment. The results of Experiments 3 and 4 indicate that the enhanced adenosine regulation and the ensuing performance deficit resulting from 2-deoxy-D-glucose treatment occurred in the central nervous system. These data are discussed in terms of the metabolic demands of neuronal over-activation during escape testing in inescapably shocked rats and the loss of normal behavioral function due to compensatory adenosine regulation in the brain.

Adenosine: does it have a neuroprotective role after all?

A neuroprotective role for adenosine is commonly assumed. Recent studies revealed that adenosine may unexpectedly, under certain circumstances, have the opposite effects contributing to neuronal damage and death. The basis for this duality may be the activation of distinct subtypes of adenosine receptors, interactions between these receptors, differential actions on neuronal and glial cells, and various time frames of adenosinergic compounds administration. If these aspects are understood, adenosine should remain an interesting target for therapeutical neuroprotective approaches after all.

GABAergic involvement in motor effects of an adenosine A(2A) receptor agonist in mice

Adenosine A(2A) agonists are known to induce catalepsy and inhibit dopamine mediated motor hyperactivity. An antagonistic interaction between adenosine A(2A) and dopamine D(2) receptors is known to regulate GABA-mediated neurotransmission in striatopallidal neurons. Stimulation of adenosine A(2A) and dopamine D(2) receptors has been shown to increase and inhibit GABA release respectively in pallidal GABAergic neurons. However, the role of GABAergic neurotransmission in the motor effects of adenosine A(2A) receptors is not yet known. Therefore in the present study the effect of GABAergic agents on adenosine A(2A) receptor agonist (NECA- or CGS 21680) induced catalepsy and inhibition of amphetamine elicited motor hyperactivity was examined. Pretreatment with GABA, the GABA(A) agonist muscimol or the GABA(B) agonist baclofen potentiated whereas the GABA(A) antagonist bicuculline attenuated NECA- or CGS 21680-induced catalepsy. However, the GABA(B) antagonists phaclophen and delta-aminovaleric acid had no effect. Administration of NECA or CGS 21680 not only reduced spontaneous locomotor activity but also antagonized amphetamine elicited motor hyperactivity. These effects of NECA and CGS 21680 were potentiated by GABA or muscimol and antagonized by bicuculline. These findings provide behavioral evidence for the role of GABA in the motor effects of adenosine A(2A) receptor agonists. Activation of adenosine A(2A) receptors increases GABA release which could reduce dopaminergic tone and induce catalepsy or inhibit amphetamine mediated motor hyperactivity.

N-substituted adenosines as novel neuroprotective A(1) agonists with diminished hypotensive effects

The synthesis and pharmacological profile of a series of neuroprotective adenosine agonists are described. Novel A(1) agonists with potent central nervous system effects and diminished influence on the cardiovascular system are reported and compared to selected reference adenosine agonists. The novel compounds featured are derived structurally from two key lead structures: 2-chloro-N-(1-phenoxy-2-propyl)adenosine (NNC 21-0041, 9) and 2-chloro-N-(1-piperidinyl)adenosine (NNC 90-1515, 4). The agonists are characterized in terms of their in vitro profiles, both binding and functional, and in vivo activity in relevant animal models. Neuroprotective properties assessed after postischemic dosing in a Mongolian gerbil severe temporary forebrain ischemia paradigm, using hippocampal CA1 damage endpoints, and the efficacy of these agonists in an A(1) functional assay show similarities to some reference adenosine agonists. However, the new compounds we describe exhibit diminished cardiovascular effects in both anesthetized and awake rats when compared to reference A(1) agonists such as (R)-phenylisopropyladenosine (R-PIA, 5), N-cyclopentyladenosine (CPA, 2), 4, N-[(1S,trans)-2-hydroxycyclopentyl]adenosine (GR 79236, 26), N-cyclohexyl-2'-O-methyladenosine (SDZ WAG 994, 27), and N-[(2-methylphenyl)methyl]adenosine (Metrifudil, 28). In mouse permanent middle cerebral artery occlusion focal ischemia, 2-chloro-N-[(R)-[(2-benzothiazolyl)thio]-2-propyl]adenosine (NNC 21-0136, 12) exhibited significant neuroprotection at the remarkably low total intraperitoneal dose of 0.1 mg/kg, a dose at which no cardiovascular effects are observed in conscious rats. The novel agonists described inhibit 6, 7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate-induced seizures, and in mouse locomotor activity higher doses are required to reach ED(50) values than for reference A(1) agonists. We conclude that two of the novel adenosine derivatives revealed herein, 12 and 5'-deoxy-5'-chloro-N-[4-(phenylthio)-1-piperidinyl]adenosine (NNC 21-0147, 13), representatives of a new series of P(1) ligands, reinforce the fact that novel selective adenosine A(1) agonists have potential in the treatment of cerebral ischemia in humans.

Antiparkinsonian effect of a new selective adenosine A2A receptor antagonist in MPTP-treated monkeys

BACKGROUND

Chronic treatment with L-3,4-dihydroxyphenylalanine (L-dopa) is often associated with motor side effects in PD patients. The search for new therapeutic approaches has led to study the role of other neuromodulators including adenosine. Among the four adenosine receptors characterized so far, the A2A subtype is distinctively present on striatopallidal output neurons containing enkephalin and mainly bearing dopamine (DA) D2 receptors (indirect pathway). Studies in DA-denervated rats suggest that blockade of adenosine A2A receptors might be used in PD.

OBJECTIVE

To evaluate the antiparkinsonian effect of a new selective adenosine A2A receptor antagonist, KW-6002, in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys.

METHODS

In the present study, we used six MPTP-exposed cynomolgus monkeys already primed and exhibiting L-dopa-induced dyskinesias to evaluate both the antiparkinsonian and dyskinetic effect upon challenge with two oral doses (60 and 90 mg/kg) of KW-6002 administered alone or in combination with L-dopa/benserazide (50/12.5 mg).

RESULTS

KW-6002 administered alone produced a dose-dependent antiparkinsonian response that reached the level of efficacy of L-dopa/benserazide but was less likely to reproduce dyskinesias in these animals. When co-administered, KW-6002 potentiated the effects of L-dopa/benserazide on motor activity (up to 30%) without affecting the dyskinetic response.

CONCLUSION

Adenosine A2A receptor antagonists have antiparkinsonian effects of their own with a reduced propensity to elicit dyskinesias. They might therefore be useful agents in the treatment of PD.

Adenosinergic modulation of ethanol-induced motor incoordination in the rat motor cortex

1. On going work in our laboratory has shown that adenosine modulates ethanol-induced motor incoordination (EIMI) when given systemically as well as directly into the cerebral ventricles, cerebellum and corpus striatum of the rat and/or mouse. 2. The objective of this study was to determine what effect adenosine agonists and antagonists would have within the rat motor cortex on EIMI. 3. The participation of the motor cortex in EIMI was suggested when microinfusion of the anti-ethanol compound, Ro15-4513, an inverse agonist of the benzodiazepine binding site, directly into the motor cortex significantly attenuated EIMI. Further, the adenosine agonists N6-cyclohexyladenosine (CHA) and 2-p-(2-carboxyethyl)-phenethylamino-5'-N-carboxaminoadenosine++ + hydrochloride (CGS-21680) significantly accentuated EIMI in a dose-related manner. The adenosine A1 receptor-selective agonist, CHA, appeared most potent in this modulatory effect when compared to the A2-selective agonist, CGS-21680. 4. The extent of diffusion of the adenosine drugs within the cortical tissue after their microinfusion was also checked by measuring the dispersion of microinfused [3H]CHA. The [3H]CHA dispersion study indirectly confirmed that the results of the present investigation were based on the effect of adenosine drugs within the motor cortex only. 5. Accentuation by the A1- and A2-selective adenosine agonists was significantly attenuated by the A1-selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) but not by the A2 receptor-selective antagonist 8-(3-chlorostyryl)caffeine (CSC) further suggesting modulation mainly by the A1-subtype. 6. Pretreatment of the motor cortex with pertussis toxin (PT) significantly reduced the capacity of both A1- and A2-selective adenosine agonists to accentuate EIMI suggesting the involvement of a PT-sensitive Gi/Go protein. 7. These data support earlier work which showed that adenosine modulates EIMI within the central nervous system (CNS), most likely via the A1 receptor, and moreover, extend that work by including the motor cortex as a brain area participating in the adenosinergic modulation of ethanol-induced motor impairment.

Promotion of sleep mediated by the A2a-adenosine receptor and possible involvement of this receptor in the sleep induced by prostaglandin D2 in rats

A 6-hr continuous infusion of 2-[p-(2-carboxyethyl)phenylethylamino]-5'-N-ethylcarboxamidoadenos ine (CGS21680), a selective A2a-adenosine agonist, into the subarachnoid space underlying the ventral surface region of the rostral basal forebrain, which has been defined as the prostaglandin (PG) D2-sensitive sleep-promoting zone, at rates of 0.02, 0.2, 2.0, and 12 pmol/min increased slow-wave sleep (SWS) and paradoxical sleep (PS) in a dose-dependent manner up to 183% and 202% of their respective baseline levels. The increments produced by the infusion of CGS21680 at 0.2 and 2.0 pmol/min were totally diminished when the rats had been pretreated with an i.p. injection of (E)-1,3-dipropyl-7-methyl-8-(3,4-dimethoxystyryl)xanthine (KF17837; 30 mg/kg of body weight), a selective A2-adenosine antagonist. In contrast, the infusion of N6-cyclohexyladenosine (CHA), a selective A1-adenosine agonist, at 2 pmol/min significantly suppressed SWS before causing an increase in SWS, and a decrease in PS was also markedly visible. Essentially the same effects of CGS21680 and CHA were observed when these compounds were administered to the parenchymal region of the rostral basal forebrain through chronically implanted microdialysis probes. Thus, we clearly showed that stimulation of A2a-adenosine receptors in the rostral basal forebrain promotes SWS and PS. Furthermore, i.p. injections of KF17837 at 30 and 100 mg/kg of body weight dose-dependently attenuated the magnitude of the SWS increase produced by the infusion of PGD2 into the subarachnoid space of the sleep-promoting zone, thus indicating that the A2a-adenosine receptors are crucial in the sleep-promoting process triggered by PGD2.

A role for central A3-adenosine receptors. Mediation of behavioral depressant effects

The behavioral effects of a selective A3 adenosine receptor agonist 3-IB-MECA (N6-(3-iodobenzyl)-5'-N-methylcarboxamidoadenosine) in mice and the localization of radioligand binding sites in mouse brain were examined. Low levels of A3 adenosine receptors were detected in various regions of the mouse brain (hippocampus, cortex, cerebellum, striatum), using a radioiodinated, high-affinity A3-agonist radioligand [125I]AB-MECA (N6-(3-iodo-4-aminobenzyl)-5'-N-methylcarboxamidoadenosine). Scatchard analysis in the cerebellum showed that the Kd value for binding to A3 receptors was 1.39 +/- 0.04 nM with a Bmax of 14.8 +/- 2.1 fmol/mg protein. 3-IB-MECA at 0.1 mg/kg i.p. was a locomotor depressant with > 50% reduction in activity. Although selective A1 or A2a antagonists reversed locomotor depression elicited by selective A1 or A2a agonists, respectively, the behavioral depressant effects of 3-IB-MECA were unaffected. 3-IB-MECA also caused scratching in mice, which was prevented by coadministration of the histamine antagonist cyproheptadine. The demonstration of a marked behavioral effect of A3 receptor activation suggests that the A3 receptor represents a potential new therapeutic target.

Effects of chronic administration of adenosine A1 receptor agonist and antagonist on spatial learning and memory

Spatial memory acquisition in Morris water maze was tested in C57BL/6 mice. Animals were injected once daily with different doses of either N6-cyclopentyladenosine (CPA) or 8-cyclopentyl-1,3-dipropylxanthine (CPX). Drugs were administered for 9 days either concurrently with water maze testing (drugs injected 1 h after each trial), or prior to the entire block of trials. In the latter case, 1 day without injections preceded water maze experiments. Chronic administration of CPA resulted in a significant, dose-dependent reduction of target latencies, rapid development of spatial preference, and the absence of animals unable to perform the task. CPX treated animals did not show significant performance changes, and failed to develop spatial preference. Locomotor disturbances were not the cause of the observed effects. Our results indicate that chronic treatment with agents acting at adenosine A1 receptors results in behavioral effects that are significantly different from those observed following their acute administration. Therefore, particular caution is required in development of adenosine-based strategies targeted at neurodegenerative or cognitive disorders in which chronic treatment is advocated.

Receptor-receptor interactions as an integrative mechanism in nerve cells

Several lines of evidence indicate that interactions among transmission lines can take place at the level of the cell membrane via interactions among macromolecules, integral or associated to the cell membrane, involved in signal recognition and transduction. The present view will focus on this last subject, i.e., on the interactions between receptors for chemical signals at the level of the neuronal membrane (receptor-receptor interaction). By receptor-receptor interaction we mean that a neurotransmitter or modulator, by binding to its receptor, modifies the characteristics of the receptor for another transmitter or modulator. Four types of interactions among transmission lines may be considered, but mainly intramembrane receptor-receptor interactions have been dealt with in this article, exemplified by the heteroregulation of D2 receptors via neuropeptide receptors and A2 receptors. The role of receptor-receptor interactions in the integration of signals is discussed, especially in terms of filtration of incoming signals, of integration of coincident signals, and of neuronal plasticity.

8-(3-Chlorostyryl)caffeine (CSC) is a selective A2-adenosine antagonist in vitro and in vivo

An adenosine antagonist, 8-(3-chlorostyryl)caffeine (CSC), was shown previously to be 520-fold selective for A2a-adenosine receptors in radioligand binding assays in the rat brain. In reversing agonist effects on adenylate cyclase, CSC was 22-fold selective for A2a receptors in rat phenochromocytoma cells (Kb 60 nM) vs. A1 receptors in rat adipocytes (Kb 1.3 microM). Administered i.p. in NIH mice at a dose of 1 mg/kg, CSC shifted the curve for locomotor depression elicited by the A2a-selective agonist APEC to the right (ED50 value for APEC shifted from 20 micrograms/kg i.p. to 190 micrograms/kg). CSC had no effect on locomotor depression elicited by an ED50 dose of the A1-selective agonist CHA. CSC alone at a dose of 5 mg/kg stimulated locomotor activity by 22% over control values. Coadministration of CSC and the A1-selective antagonist CPX, both at non-stimulatory doses, increased activity by 37% (P < 0.001) over CSC alone, suggesting a behavioral synergism of A1- and A2-antagonist effects in the CNS.

Stimulation of adenosine A2 receptors induces catalepsy

The central administration of the adenosine A2 agonist CGS 21680 induced catalepsy in the rat. This effect was counteracted by the previous systemic administration of the adenosine antagonist theophylline or the D2 agonist BHT-920. These results are in agreement with the view that adenosine A2 receptors regulate central dopamine D2 transmission and underline the potential antipsychotic activity of A2 agonists.

Do adenosinergic substrates mediate methylxanthine effects upon reinforcement thresholds for electrical brain stimulation in the rat?

Caffeine and other methylxanthines elevate reinforcement threshold for electrical brain stimulation with an order of potency suggesting that the effect is mediated by antagonism of adenosine A2 receptors. The purpose of this study was to evaluate further the possible mechanism by which caffeine and other methylxanthines elevate reinforcement thresholds for ICSS. Drugs known to affect adenosinergic transmission in predictable ways, adenosine receptor agonists and antagonists and benzodiazepine agonists and inverse agonists, were tested to determine their effect upon reinforcement threshold. Both the selective A1 adenosine agonist, R(-)-PIA, and the non-selective A1/A2 adenosine agonist NECA failed to alter reinforcement thresholds, as did CGS 15943, a potent non-xanthine non-selective adenosine receptor antagonist. Chlordiazepoxide, a benzodiazepine agonist, lowered reinforcement thresholds and FG 7142, a benzodiazepine inverse agonist, elevated reinforcement thresholds, perhaps corresponding to their anxiolytic and anxiogenic subjective effects in humans. However, another benzodiazepine agonist, midazolam and another inverse agonist, beta-CCE, did not alter reinforcement thresholds. These results fail to support a general role for adenosinergic systems in the threshold-elevating effect of methylxanthines.

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.

Antinociceptive interactions between intrathecally administered alpha noradrenergic agonists and 5'-N-ethylcarboxamide adenosine

Recently, it has been shown that intrathecal injection of norepinephrine and the mixed A1/A2 adenosine agonist 5'-N-ethylcarboxamide adenosine (NECA) interact in a supra-additive manner to produce antinociception. The present studies were designed to determine whether alpha 1 or alpha 2 noradrenergic receptors are involved in producing the antinociception induced by NECA and norepinephrine. The results indicated that intrathecal injection of NECA (0.97-4.9 nmol), the alpha 2 noradrenergic agonist clonidine (3.8-375 nmol), or the alpha 1 agonist phenylephrine (4.9-73.4 nmol) produced dose-dependent antinociception in rats. Furthermore, intrathecal injection of subeffective doses of NECA and clonidine interacted supra-additively to produce potent antinociception. In contrast, no supra-additive interaction was observed between NECA and phenylephrine. The supra-additive interaction of NECA and clonidine did not appear to result from alterations in cardiovascular tone because changes in blood pressure and nociceptive thresholds were not correlated in time. These results suggest that the noradrenergic component of the supra-additive interaction between adenosine A2 receptor agonists and noradrenergic agonists is mediated by alpha 2 noradrenergic receptors.

Antinociception produced by interactions between intrathecally administered adenosine agonists and norepinephrine

It is well-established that intrathecal injection of noradrenergic agonists produces dose-dependent antinociception in rats. Recently, the antinociceptive actions of norepinephrine in the central nervous system have been shown to be modulated by adenosine and adenosine analogs. This study examined whether there is an interaction between norepinephrine and adenosine analogs in the regulation of nociceptive transmission in the rat spinal cord using the tail flick and hot plate tests. The results indicate that dose-dependent antinociception was produced by intrathecal injection of norepinephrine (4.8-195 nmol), the A1/A2 adenosine agonist 5'-N-ethylcarboxamide adenosine (NECA; 0.97-4.9 nmol), and the A1 adenosine agonist R-phenylisopropyladenosine (R-PIA; 0.78-26 nmol). Furthermore, subeffective doses of NECA and norepinephrine interacted synergistically to produce potent antinociception. In contrast, no synergistic interaction was observed between norepinephrine and doses of R-PIA as high as 26 nmol. The antinociception produced by coadministration of norepinephrine and NECA appears to be mediated by adenosine receptors, since it was attenuated by pretreatment with theophylline, a non-selective adenosine antagonist. The synergistic interaction between NECA and norepinephrine did not appear to result from alterations in cardiovascular tone because blood pressure values were not significantly altered by drug administration. These results suggest that purinergic and noradrenergic systems interact synergistically to modify nociceptive transmission in the spinal cord. The purinergic component of this interaction may be mediated, in part, by adenosine A2 receptors.

Behavioral and electroencephalographic effects of the adenosine1 agonist, L-PIA

The effects of N6-(L-2-phenylisopropyl)-adenosine (L-PIA), an A1 agonist, were measured on both spontaneous locomotor activity and electroencephalographic (EEG) measures of sleep in rats. L-PIA strongly inhibited motor activity at 100 micrograms/kg intraperitoneally (IP), a dose which had no statistically significant effects on EEG-defined sleep. A higher dose of L-PIA (200 micrograms/kg) increased the latency to sleep initiation and inhibited later REM sleep. These results demonstrate that L-PIA can produce a state of apparent behavioral quiescence in the presence of EEG-defined arousal.

Endogenous adenosine and hemorrhagic shock: effects of caffeine administration or caffeine withdrawal

Plasma adenosine concentrations doubled when rats were subjected to 90 min of profound hemorrhagic shock. Administration of caffeine (20 mg per kg of body weight), an adenosine-receptor antagonist, attenuated the hemorrhage-induced decrease in blood pressure. In contrast, chronic caffeine consumption (0.1% in drinking water), followed by a brief period of caffeine withdrawal, amplified the hypotensive response to hemorrhage. These data suggest that endogenous adenosine participates in the hypotensive response to hemorrhage and that caffeine may protect against, and caffeine withdrawal may exacerbate, this response.

Central gastric antisecretory action of adenosine in the rat

The effects of intracerebroventricularly (i.c.v.) administered adenosine and some of its analogues on gastric secretion were studied in rats. The compounds inhibited the gastric output of acid, pepsin and fluid in pylorus-ligated rats in a dose-dependent manner with an order of potency: 5'-N-ethylcarboxamidoadenosine (NECA) greater than (-)N6-phenylisopropyladenosine (R-PIA) greater than (+)N6-phenylisopropyladenosine (S-PIA) greater than adenosine. Pretreatment with 10 and 30 mg/kg of theophylline i.v. or 5 mg/kg of 8-phenyltheophylline s.c. did not modify the antisecretory effect of 0.1 microgram of NECA i.c.v. NECA injected i.c.v. did not affect the secretion induced by carbachol in awake rats subjected to vagotomy or in anaesthetized rats with intact vagi. NECA i.c.v. had no effect on the serum concentration of gastrin. The depletion of brain monoamines (noradrenaline, dopamine and serotonin) with 6-OHDA i.c.v. significantly attenuated the inhibitory action of NECA. Pretreatment with 10 mg/kg of naloxone i.v. or indomethacin s.c. did not modify the antisecretory effect of NECA. The results indicate that adenosine inhibits gastric secretion in rats by a decrease in the stimulatory vagal impulses to the stomach, and that it acts in the brain via receptors insensitive to xanthines. Brain biogenic monoamines, but not opioid peptides or prostaglandins seem to be involved in the central gastric antisecretory action of adenosine.

Adenosine uptake site heterogeneity in the mammalian CNS? Uptake inhibitors as probes and potential neuropharmaceuticals

Inhibitors of adenosine uptake or transport have been used clinically for some time in certain cardiovascular diseases. More recently, some of them have also been investigated for possible clinical use in combination with antimetabolites based on the observed heterogeneity of nucleoside transport in mammalian tumor cells. Such a heterogeneity of adenosine uptake and uptake sites has now also been suggested in the mammalian CNS. The aim of this article is, therefore, to review the present status of our knowledge of adenosine uptake in the mammalian CNS, compare it with our far more advanced knowledge of nucleoside transport in other mammalian cells and suggest direction of future research. The possible implications for the development of adenosine uptake inhibitors as adenosinergic neuropharmaceuticals will be discussed based on our knowledge of the physiological function of adenosine in the CNS.

3,7-Dimethyl-1-propargylxanthine: a potent and selective in vivo antagonist of adenosine analogs

3,7-Dimethyl-1-propargylxanthine (DMPX), a caffeine analog that exhibits in vitro selectivity for A2-adenosine receptors, compared to A1-adenosine receptors, has now been investigated with respect to in vivo potency and selectivity. DMPX potently and selectively blocked the actions of the potent A2 adenosine agonist, 5'-N-ethylcarboxamidoadenosine (NECA), in DBA/2 mice, compared to blockade of the same responses elicited by the selective A1-adenosine agonist, N6-cyclohexyladenosine (CHA). DMPX was 57-fold more potent versus NECA-induced hypothermia than versus CHA-induced hypothermia and 11-fold more potent versus NECA-induced behavioral depression than versus CHA-induced behavioral depression. The hypothermia is mediated by peripheral receptors, based on blockade by 8-(p-sulfophenyl)theophylline (PSPT), while the behavioral depression is centrally mediated, based on lack of blockade by PSPT. DMPX was 28- and 15-fold more potent than caffeine in blocking peripheral and central NECA-responses, respectively. DMPX was equipotent with caffeine versus CHA-induced hypothermia and 2.5-fold more potent than caffeine versus CHA-induced behavioral depression. The motor stimulating potency of DMPX (ED50 10 mumol/kg) was slightly greater than caffeine.

Effects of caffeine and L-phenylisopropyladenosine on locomotor activity of mice

C57BL/6J and DBA/2J mice were used to determine if possible differences in the behavioral response to caffeine might be related to differences in A1 adenosine receptors. Caffeine stimulated locomotor activity of both strains, but the dose-response relationship and time course of drug action differed according to strain. Although their response to caffeine differed, the strains did not differ in response to the A1 adenosine agonist L-phenylisopropyladenosine (PIA) nor in the binding of the A1 agonist [3H]N6-cyclohexyladenosine (CHA) in various brain regions. Thus, the behavioral differences in response to caffeine could not be accounted for by differences in adenosine binding. Of alternative mechanisms, strain differences in A2 receptors appear to be the most promising.

Behavioral characteristics of centrally administered adenosine analogs

Mice were implanted with chronic indwelling cannulae in the lateral cerebral ventricle. A series of adenosine analogs and related compounds were injected into the lateral ventricle (ICVT) and their effects on spontaneous locomotor activity recorded. All analogs produced dose-related decreases in locomotor activity. 5'-N6-ethyl-carboxamidoadenosine (NECA) was the most potent compound tested, with a number of N6-substituted analogs also being effective depressants of activity. Caffeine, administered either intracerebroventricularly or intraperitoneally, antagonized the depressant effects of the adenosine analogs. 3-Isobutyl-1-methylxanthine, administered ICVT, depressed locomotor activity. However, after caffeine, IBMX elicited behavioral stimulation. Agents which inhibit the transport of adenosine (dipyridamole, dilazep, papaverine) depressed locomotor activity, as did erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA, an inhibitor of adenosine deaminase. The effects of dilazep, papaverine and EHNA, but not of dipyridamole, were antagonized by caffeine. These results further substantiate the notion that endogenous adenosine is involved in the regulation of central nervous system excitability.

Inhibition of the shake response in rats by adenosine and 2-chloroadenosine

Shaking movements, similar to those made by a dog when wet, were elicited in rats by immersion in ice-water, injections of icilin, a chemical that produces sensations of cold, and naloxone-precipitated morphine withdrawal. Adenosine and 2-chloroadenosine produced dose-dependent inhibition of shaking to ice-water and icilin. The 2-chloroadenosine effect was mediated centrally because the ICV dose required to produce inhibition was not effective when given IP. Caffeine antagonized the inhibitory effects of adenosine and 2-chloroadenosine. 2-Chloroadenosine suppressed morphine-abstinence shaking as well as the body weight loss that normally accompanies withdrawal.

Increase of rat serum prolactin by adenosine analogs and their blockade by the methylxanthine aminophylline

The administration of the stable adenosine analogs N6-[(R)-methyl-2-phenylethyl]adenosine (R-PIA; 0.01-1.0 mg/kg i.p.) and 1-(6-amino-9H-purin-9-yl)-1-deoxy-N-ethyl-beta-D-ribofuronamide (NECA; 0.01-1.0 mg/kg i.p.) caused a dose-related (NECA) or biphasic (R-PIA) increases in rat serum prolactin. The S-isomer of PIA was inactive up to 4 mg/kg i.p. The methylxanthine aminophylline (10 and 30 mg/kg i.p.) antagonized the R-PIA- and NECA-induced elevation of prolactin suggesting an adenosine receptor-mediated effect. The dopaminergic agents L-dopa and bromocriptine antagonized the R-PIA and NECA-induced increase in serum prolactin. Haloperidol (a dopamine antagonist) and alpha-methyl-p-tyrosine (a catecholamine synthesis inhibitor) potentiated the R-PIA-induced effects. R-PIA and NECA did not displace 3H-haloperidol from rat striatal membranes nor effect in vitro prolactin release from rat anterior pituitary cells grown in culture. Based upon these findings it is postulated that R-PIA and NECA may be increasing prolactin secretion in part by inhibiting central dopamine release, although other mechanisms may also be operating.

Adenosine receptor activation in brain reduces stress-induced ulcer formation

Rats restrained in a cold environment for 3 h developed a high incidence of gastric ulcers. Administration of adenosine receptor agonists prior to a restraint period significantly reduced ulcer formation and severity, and lowered plasma corticosterone levels. This protective effect was blocked by 8-phenyltheophylline, a methylxanthine type adenosine receptor antagonist able to permeate the blood-brain barrier. This finding together with the absolute and relative order of potencies with which adenosine receptor agonists produced their effects suggests that CNS adenosine A1 receptors are involved in blocking and methylxanthines in exacerbating stress-induced gastric pathology.