Effects of adenosine receptor agonists and antagonists on acquisition of passive avoidance learning

Exploration of chalcones and related heterocycle compounds as ligands of adenosine receptors: therapeutics development

Adenosine receptors (ARs) are ubiquitously distributed throughout the mammalian body where they are involved in an extensive list of physiological and pathological processes that scientists have only begun to decipher. Resultantly, AR agonists and antagonists have been the focus of multiple drug design and development programmes within the past few decades. Considered to be a privileged scaffold in medicinal chemistry, the chalcone framework has attracted a substantial amount of interest in this regard. Due to the potential liabilities associated with its structure, however, it has become necessary to explore other potentially promising compounds, such as heterocycles, which have successfully been obtained from chalcone precursors in the past. This review aims to summarise the emerging therapeutic importance of adenosine receptors and their ligands, especially in the central nervous system (CNS), while highlighting chalcone and heterocyclic derivatives as promising AR ligand lead compounds.

CA2 beyond social memory: Evidence for a fundamental role in hippocampal information processing

Hippocampal region CA2 has received increased attention due to its importance in social recognition memory. While its specific function remains to be identified, there are indications that CA2 plays a major role in a variety of situations, widely extending beyond social memory. In this targeted review, we highlight lines of research which have begun to converge on a more fundamental role for CA2 in hippocampus-dependent memory processing. We discuss recent proposals that speak to the computations CA2 may perform within the hippocampal circuit.

Caffeine impairs the acquisition and retention, but not the consolidation of Pavlovian conditioned freezing in mice

RATIONALE

The psychoactive substance, caffeine, may improve cognitive performance, but its direct impact on learning and memory remains ill defined. Conflicting reports suggest that caffeine may impair as well as enhance Pavlovian fear conditioning in animals and its effect may vary across different phases of learning.

OBJECTIVES

The purpose of this study is to dissect the effect of a motor-stimulant dose of caffeine (30 mg/kg intraperitoneal (i.p.)) on acquisition, retrieval or consolidation of conditioned fear in C57BL/6 mice.

METHODS

Fear conditioning was evaluated in a conditioned freezing paradigm comprising 3 tone-shock pairings and a two-way active avoidance paradigm lasting two consecutive days with 80 conditioning trials per test session.

RESULTS

Conditioning to both the discrete tone-conditioned stimulus (CS) and the context was markedly impaired by caffeine. The deficits were similarly evident when caffeine was administered prior to acquisition or retrieval (48 and 72 h after conditioning); and the most severe impairment was seen in animals given caffeine before acquisition and before retrieval. A comparable deficit was observed in the conditioned active avoidance test. By contrast, caffeine administered immediately following acquisition neither affected the expression of tone freezing nor context freezing.

CONCLUSIONS

The present study challenges the previous report that caffeine primarily disrupts hippocampus-dependent conditioning to the context. At the relevant dose range, acute caffeine likely exerts more widespread impacts beyond the hippocampus, including the amygdala and striatum that are anatomically connected to the hippocampus; together, they support the acquisition and retention of fear memories to discrete stimuli as well as diffused contextual cues.

Purinergic signalling: from normal behaviour to pathological brain function

Purinergic neurotransmission, involving release of ATP as an efferent neurotransmitter was first proposed in 1972. Later, ATP was recognised as a cotransmitter in peripheral nerves and more recently as a cotransmitter with glutamate, noradrenaline, GABA, acetylcholine and dopamine in the CNS. Both ATP, together with some of its enzymatic breakdown products (ADP and adenosine) and uracil nucleotides are now recognised to act via P2X ion channels and P1 and P2Y G protein-coupled receptors, which are widely expressed in the brain. They mediate both fast signalling in neurotransmission and neuromodulation and long-term (trophic) signalling in cell proliferation, differentiation and death. Purinergic signalling is prominent in neurone-glial cell interactions. In this review we discuss first the evidence implicating purinergic signalling in normal behaviour, including learning and memory, sleep and arousal, locomotor activity and exploration, feeding behaviour and mood and motivation. Then we turn to the involvement of P1 and P2 receptors in pathological brain function; firstly in trauma, ischemia and stroke, then in neurodegenerative diseases, including Alzheimer's, Parkinson's and Huntington's, as well as multiple sclerosis and amyotrophic lateral sclerosis. Finally, the role of purinergic signalling in neuropsychiatric diseases (including schizophrenia), epilepsy, migraine, cognitive impairment and neuropathic pain will be considered.

Adenosine hypothesis of schizophrenia--opportunities for pharmacotherapy

Pharmacotherapy of schizophrenia based on the dopamine hypothesis remains unsatisfactory for the negative and cognitive symptoms of the disease. Enhancing N-methyl-D-aspartate receptors (NMDAR) function is expected to alleviate such persistent symptoms, but successful development of novel clinically effective compounds remains challenging. Adenosine is a homeostatic bioenergetic network modulator that is able to affect complex networks synergistically at different levels (receptor-dependent pathways, biochemistry, bioenergetics, and epigenetics). By affecting brain dopamine and glutamate activities, it represents a promising candidate for reversing the functional imbalance in these neurotransmitter systems believed to underlie the genesis of schizophrenia symptoms, as well as restoring homeostasis of bioenergetics. Suggestion of an adenosine hypothesis of schizophrenia further posits that adenosinergic dysfunction might contribute to the emergence of multiple neurotransmitter dysfunctions characteristic of schizophrenia via diverse mechanisms. Given the importance of adenosine in early brain development and regulation of brain immune response, it also bears direct relevance to the aetiology of schizophrenia. Here, we provide an overview of the rationale and evidence in support of the therapeutic potential of multiple adenosinergic targets, including the high-affinity adenosine receptors (A(1)R and A(2A)R), and the regulatory enzyme adenosine kinase (ADK). Key preliminary clinical data and preclinical findings are reviewed.

Adenosine receptors and neurological disease: neuroprotection and neurodegeneration

Adenosine receptors modulate neuronal and synaptic function in a range of ways that may make them relevant to the occurrence, development and treatment of brain ischemic damage and degenerative disorders. A(1) adenosine receptors tend to suppress neural activity by a predominantly presynaptic action, while A(2A) adenosine receptors are more likely to promote transmitter release and postsynaptic depolarization. A variety of interactions have also been described in which adenosine A(1) or A(2) adenosine receptors can modify cellular responses to conventional neurotransmitters or receptor agonists such as glutamate, NMDA, nitric oxide and P2 purine receptors. Part of the role of adenosine receptors seems to be in the regulation of inflammatory processes that often occur in the aftermath of a major insult or disease process. All of the adenosine receptors can modulate the release of cytokines such as interleukins and tumor necrosis factor-alpha from immune-competent leukocytes and glia. When examined directly as modifiers of brain damage, A(1) adenosine receptor (AR) agonists, A(2A)AR agonists and antagonists, as well as A(3)AR antagonists, can protect against a range of insults, both in vitro and in vivo. Intriguingly, acute and chronic treatments with these ligands can often produce diametrically opposite effects on damage outcome, probably resulting from adaptational changes in receptor number or properties. In some cases molecular approaches have identified the involvement of ERK and GSK-3beta pathways in the protection from damage. Much evidence argues for a role of adenosine receptors in neurological disease. Receptor densities are altered in patients with Alzheimer's disease, while many studies have demonstrated effects of adenosine and its antagonists on synaptic plasticity in vitro, or on learning adequacy in vivo. The combined effects of adenosine on neuronal viability and inflammatory processes have also led to considerations of their roles in Lesch-Nyhan syndrome, Creutzfeldt-Jakob disease, Huntington's disease and multiple sclerosis, as well as the brain damage associated with stroke. In addition to the potential pathological relevance of adenosine receptors, there are earnest attempts in progress to generate ligands that will target adenosine receptors as therapeutic agents to treat some of these disorders.

Improved spatial recognition memory in mice lacking adenosine A2A receptors

Adenosine receptors play an important role in learning and memory as their antagonists have been found to facilitate learning and memory in various tasks in rodents. However, few studies have examined the effect of adenosine A2A receptor deficiency on cognition. In the present study, we therefore used the Y-maze, a simple two-trial recognition test to measure spatial recognition memory in mice lacking adenosine A2A receptors. The results showed that adenosine A2A receptor knockout mice had a higher percentage of novel arm visits as first choice than wild-type CD1 mice. Moreover, these mice showed longer duration of visits in the novel arm when compared with controls, suggesting that the lack of adenosine A2A receptors improved spatial recognition memory. On the other hand, mice lacking the adenosine A2A receptors had low scores in the number of arm visits, suggesting that they were hypoactive. In conclusion, these data suggest the involvement of adenosine receptors in modulating spatial recognition memory in mice, consistent with earlier findings using adenosine receptor antagonists.

Antagonistic interaction between adenosine A2A and dopamine D2 receptors modulates the social recognition memory in reserpine-treated rats

Increasing evidence suggests that antagonistic interactions between specific subtypes of adenosine and dopamine receptors in the basal ganglia are involved in the control of motor activity. However, there are few studies investigating this interaction in other brain regions and its role in additional functions. In the present study, we evaluated whether reserpine-treated rats (1.0 mg/kg, i.p.) exhibit altered social recognition memory abilities. The effects of acute administration of the dopamine receptor agonists 7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3 benzazepine (SKF 38393, dopamine D(1) receptor agonist) and quinpirole (dopamine D(2) receptor agonist), together with the adenosine receptor antagonists caffeine (non-selective), 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, adenosine A(1) receptor antagonist) and 4-(2-[7-amino-2-{2-furyl}{1,2,4}triazolo-{2,3-a}{1,3,5}triazin-5-yl-amino]ethyl)phenol (ZM241385, adenosine A(2A) receptor antagonist), were also investigated. Twenty-four hours after treatment, reserpine-treated rats exhibited a significant disruption in the ability to recognize a juvenile rat after a short period of time. These animals did not show any motor deficit. The social recognition disruption induced by reserpine was reversed by acute treatment with quinpirole (0.05-0.1 mg/kg, i.p.), caffeine (10.0-30.0 mg/kg, i.p.) or ZM241385 (0.5-1.0 mg/kg, i.p.), but not with SKF 38393 (0.5-3.0 mg/kg, i.p.) or DPCPX (0.5-3.0 mg/kg, i.p.). Moreover, a synergistic response was observed following the co-administration of 'non-effective' doses of ZM241385 (0.1 mg/kg, i.p.) and quinpirole (0.01 mg/kg, i.p.). These results reinforce and extend the notion of antagonistic interactions between adenosine and dopamine receptors, and demonstrate, for the first time, that the blockade of adenosine A(2A) receptors and the activation of dopamine D(2) receptors can reverse the social recognition deficits induced by reserpine in rats.

Mice lacking the adenosine A1 receptor have normal spatial learning and plasticity in the CA1 region of the hippocampus, but they habituate more slowly

Using mice with a targeted disruption of the adenosine A1 receptor (A1R), we examined the role of A1Rs in hippocampal long-term potentiation (LTP), long-term depression (LTD), and memory formation. Recordings from the Shaffer collateral-CA1 pathway of hippocampal slices from adult mice showed no differences between theta burst and tetanic stimulation-induced LTP in adenosine A1 receptor knockout (A1R-/-), heterozygote (A1R+/-), and wildtype (A1R+/+) mice. However, paired pulse facilitation was impaired significantly in A1R-/- slices as compared to A1R+/+ slices. LTD in the CA1 region was unaffected by the genetic manipulation. The three genotypes showed similar memory acquisition patterns when assessed for spatial reference and working memory in the Morris water maze tasks at 9 months of age. However, 10 months later A1R-/- mice showed some deficits in the 6-arm radial tunnel maze test. The latter appeared, however, not due to memory deficits but to decreased habituation to the test environment. Taken together, we observe normal spatial learning and memory and hippocampal CA1 synaptic plasticity in adult adenosine A1R knockout mice, but find modifications in arousal-related processes, including habituation, in this knockout model.

Blockade of adenosine A2A receptors reverses short-term social memory impairments in spontaneously hypertensive rats

Spontaneously hypertensive rats (SHR) exhibit impairment across several cognitive domains such as attention, short-term memory and spatial reference memory. These cognitive deficits have been variously attributed to disrupted dopaminergic, cholinergic and adenosinergic neurotransmitter function. However, social memory in SHR has not been investigated. In the present study, we therefore evaluated whether SHR exhibit altered short-term social memory abilities compared to normotensive Wistar rats (WIS) through two experimental paradigms (social recognition and habituation-dishabituation tests). We also compared the performance of SHR and WIS rats in the object recognition test. SHR exhibited significantly impaired performance in both models of social memory, but not in the object recognition test, demonstrating a selective deficit in the ability to recognize a juvenile rat after a short period of time. The administration of acute doses of the non-selective adenosine receptor antagonist caffeine (3.0 or 10.0 mg/kg, i.p.) and the adenosine A2A receptor antagonist 4-(2-[7-amino-2-[2-furyl][1,2,4]triazolo-[2,3-a][1,3,5]triazin-5-yl-amino]ethyl) phenol (ZM241385, 0.5 or 1.0 mg/kg, i.p.) but not the adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 1.0 or 3.0 mg/kg, i.p.) reversed this social memory impairment in SHR, but these treatments did not alter the hypertension state. These results demonstrate an impairment of short-term social memory in SHR and the involvement of the adenosine A2A receptors in this alteration.

Modulation of short-term social memory in rats by adenosine A1 and A(2A) receptors

The recognition of an unfamiliar juvenile rat by an adult rat has been shown to imply short-term memory processes. The present study was designed to examine the role of adenosine receptors in the short-term social memory of rats using the social recognition paradigm. Adenosine (5.0-10.0 mg/kg), the selective adenosine A1 receptor agonist 2-chloro-N6-cyclopentyladenosine (CCPA, 0.025-0.05 mg/kg) and the selective adenosine A(2A) receptor agonist N6-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)ethyl]adenosine (DPMA, 1.0-5.0 mg/kg), given by i.p. route 30 min before the test, disrupted the juvenile recognition ability of adult rats. This negative effect of adenosine (5.0 mg/kg, i.p.) on social memory was prevented by pretreatment with the non-selective adenosine receptor antagonist caffeine (10.0 mg/kg, i.p.), the adenosine A1 antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 1.0 mg/kg, i.p.) and the adenosine A(2A) antagonist 4-(2-[7-amino-2-{2-furyl}{1,2,4}triazolo-{2,3-a}{1,3,5}triazin-5-yl-amino]ethyl)phenol (ZM241385, 1.0 mg/kg, i.p.). Furthermore, acute administration of caffeine (10.0-30.0 mg/kg, i.p.), DPCPX (1.0-3.0 mg/kg, i.p.) or ZM241385 (0.5-1.0 mg/kg, i.p.) improved the short-term social memory in a specific manner. These results indicate that adenosine modulates the short-term social memory in rats by acting on both A1 and A(2A) receptors, with adenosine receptor agonists and antagonists, respectively, disrupting and enhancing the social memory.

Mice lacking the adenosine A1 receptor are anxious and aggressive, but are normal learners with reduced muscle strength and survival rate

Behavioural assessment of mice lacking adenosine A1 receptors (A1Rs) showed reduced activity in some phases of the light-dark cycle, reduced exploratory behaviour in the open-field and in the hole-board, increased anxiety in the plus maze and dark-light box and increased aggressiveness in the resident-intruder test. No differences were found in spatial reference and working memory in several Morris water maze tasks. Both mutant mice had reduced muscle strength and survival rate. These results confirm the involvement of adenosine in motor activity, exploratory behaviour, anxiety and aggressiveness. A1Rs also appear to play a critical role in ageing-related deterioration.

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.

Interaction of angiotensin II and III with adenosine A(1) receptor-related drugs in passive avoidance conditioning in rats

The present study examines the functional interaction between angiotensins (ANG II and III) and adenosine A(1) receptor-related drugs on passive avoidance (step-through) conditioning in rats. ANG II and III were administered intracerebroventricularly (i.c.v.) while N(6)-cyclohexyladenosine (CHA) and theophylline-intraperitoneally (i.p.), immediately after the training trial. ANG II (0.1,0.5,1 microg) induced dose-dependent (inverted-U) increase of the retention while ANG III in the same doses decreased it upon re-testing of rats 24 h and 7 days later. The selective adenosine A(1) receptor agonist CHA (0.1 mg/kg) attenuated memory-enhancing effect of ANG II (0.1 microg) 24 h but not 7 days after the training session. Conversely, CHA had opposite i.e. facilitating effect on ANG III (0.1 microg) response upon re-testing 24 h and 7 days later. The pretreatment with ANG III attenuated the retention-improving effect exerted by the non-specific adenosine A(1)/A(2) receptor antagonist theophylline (75 mg/kg) 24 h and 7 days after the training trial. Taken together, the results show a mutual interaction of the drugs belonging to the adenosine and angiotensin modulatory systems in memory consolidation of rats.

Adenosine A1 receptor activation selectively impairs the acquisition of contextual fear conditioning in rats

Three experiments were conducted to examine the importance of adenosine A1 receptors for the acquisition and expression of hippocampal-dependent and hippocampal-independent forms of conditioned fear. In Experiment 1, the selective adenosine A1 receptor agonist, N6-cyclopentyladenosine (CPA), or saline was administered intraperitoneally to male rats 30 min prior to Pavlovian fear conditioning, which consisted of 7 tone-shock pairings. Adenosine A1 receptor activation dose-dependently and selectively disrupted the acquisition of contextual fear conditioning while sparing tone-shock associations. Experiments 2 and 3 demonstrated that CPA's selective disruption of contextual learning could not be attributed to context being weaker than tone conditioning or to state-dependent learning. Adenosine A1 receptor activation also impaired the expression of both context- and tone-elicited fear. These results suggest that endogenous adenosine modulates the acquisition and expression of emotional (fear) memories by acting on A1 receptors in brain regions underlying fear conditioning.

Effects of adenosine receptor agonists and antagonists on pentylenetetrazole-induced amnesia

The effect of adenosine agents on amnesia induced by pentylenetetrazole was examined in mice. Post-training administration of pentylenetetrazole (50 and 60 mg/kg) disrupted 24-h retention of a single-trial passive avoidance task. The adenosine receptor antagonists, theophylline (2.5-25 mg/kg) and 8-phenyltheophylline (0.5-2 mg/kg), administered 30 min before and just after training at doses which did not affect retention, reduced the amnestic effect of pentylenetetrazole in a dose-dependent manner. Post-training administration of the adenosine A(1) receptor agonists, N(6)-cyclohexyladenosine (CHA, 0.1 and 0.5 mg/kg) and N(6)-phenylisopropyladenosine (R-PIA, 0.03 and 0.1 mg/kg), but not the adenosine A(2) receptor agonist, 5'-N-ethylcarboxamidoadenosine (NECA, 0.01 and 0.001 mg/kg), impaired retention. Nonamnestic doses of CHA and R-PIA potentiated the disruption induced by a lower dose of pentylenetetrazole (40 mg/kg). NECA did not induce any response in this respect. It is suggested that an adenosine A(1) receptor mechanism is involved in amnesia induced by pentylenetetrazole.

Design, synthesis and biological evaluation of a novel series of potent, orally active adenosine A1 receptor antagonists with high blood-brain barrier permeability

A novel series of 3-(2-substituted-3-oxo-2,3-dihydropyridazin-6-yl)-2-phenylpyrazolo[1,5-a]pyridines (5-38) were synthesized and evaluated for their in vitro adenosine A1 and A(2A) receptor binding activities, and in vitro metabolism by rat liver in order to search for orally active compounds. Most of the test compounds were potent adenosine A1 receptor antagonists with high A1 selectivity and the A1 affinity and A1 selectivity of carbonyl derivatives (5-11) was particularly high. In particular, compound 7 was an extremely potent and selective adenosine A1 antagonist with high A1 selectivity (Ki=0.026 nM, A(2A)/A1=5400). In terms of metabolic stability, 2-oxopropyl (5), 2-hydroxypropyl (12), N-methylacetamide (16), 2-(piperidin-1-yl)ethyl (28) and 1-methylpiperidin-4-yl (32, FR194921) were the most stable compounds in this series of analogues. Further in vivo evaluation indicated that compounds 5, 13, 17, 28 and 32 were detected in both plasma and brain after oral administration in rats. In particular, 32 displayed good plasma and brain concentrations (dose: 32 mg/kg (n=3); after 30 min, plasma conc.=3390+/-651nM, brain conc.=3670+/-496nM; after 60min, plasma conc.=1580+/-348nM, brain conc.=2143+/-434nM), and a good brain/plasma ratio (1.11+/-0.060 (30min), 1.39+/-0.172 (60min)). As a result, we could show that 32 is a good candidate for an orally active adenosine A1 receptor antagonist with high blood-brain barrier permeability and good bioavailability (Ki=6.6nM, A(2A)/A1=820, BA=60.6+/-4.9% (32 mg/kg)).

Interaction between angiotensin IV and adenosine A1 receptor related drugs in passive avoidance conditioning in rats

The functional interaction between ANG (3--8) (ANG IV) and adenosine A(1) receptor related drugs in passive avoidance (step-through) task in rats was studied in Wistar rats. ANG IV exerted dose-dependent (inverted-U) improvement of the retention while sarilesin (an angiotensin II analog) impaired this effect. Co-administration of theophylline and ANG IV, both in ineffective doses, enhanced the retention. The selective adenosine A(1) receptor agonist cyclopentyladenosine (CPA) attenuated ANG IV-induced memory enhancement.

Facilitative effects of an adenosine A1/A2 receptor blockade on spatial memory performance of rats: selective enhancement of reference memory retention during the light period

The present experiment was designed to examine the role of adenosine in spatial working and reference memory in rats using an 8-arm radial maze task which requires the integrity of the hippocampal formation. We investigated the effects of the unselective adenosine A1/A2 receptor antagonist theophylline on acquisition and retention of spatial working and reference memory. As there is evidence that brain extracellular adenosine levels vary significantly during the light-dark cycle, we tested the effects of theophylline both during the light and the dark period. Acquisition of the task was investigated for 10 consecutive days after rats received daily injections of vehicle or theophylline (15 mg/kg, intraperitoneally). Retention was tested in two nondrug sessions 7 and 14 days after completion of acquisition. The results demonstrate that in saline-treated control rats acquisition and retention of reference memory and, to a lesser extent, working memory was superior in the dark period. The results further revealed that daily administration of theophylline interacted with days to selectively enhance reference memory acquisition in the light, but not in the dark, period. In addition, reference memory retention was significantly enhanced in those rats who learned the task under theophylline treatment during the light period. Overall, the results show that in saline-treated control rats the effectiveness of acquisition and retention of spatial information in a radial maze strongly depends on the time of day. The higher levels of maze performance in the dark period might be related to a better functioning of involved brain systems in the active period of the rat. Furthermore, theophylline-induced blockade of adenosine A1/A2 receptors in the light, but not in the dark, period selectively enhanced reference memory acquisition and retention. Variations of brain extracellular adenosine levels during the light-dark cycle might account for the restriction of reference memory enhancing effects of theophylline to the light period.

N(6)-2-(4-aminophenyl)ethyl-adenosine enhances the anticonvulsive action of conventional antiepileptic drugs in the kindling model of epilepsy in rats

APNEA [(N(6)-2-(4-aminophenyl)ethyl-adenosine; a non-selective adenosine A(3) receptor agonist; 2-4 mgkg(-1)] had no significant effect on seizure parameters (seizure severity, seizure duration and afterdischarge duration) in amygdala-kindled rats. Subsequently, APNEA was combined with antiepileptic drugs administered at doses ineffective in fully kindled rats. Co-administration of APNEA (0.5-2 mg kg(-1)) with carbamazepine (2.5-20 mg kg(-1)) resulted in the significant reduction of all studied seizure parameters. Moreover, 8-cyclopentyl-1,3-dimethylxanthine 8-CPX (a selective adenosine A(1) receptor antagonist; 5 mg kg(-1)) partially reduced the anticonvulsive activity of a combination of APNEA (2 mg kg(-1)) with carbamazepine (20 mg kg(-1)), but not that of carbamazepine (20 mgkg(-1))+APNEA (0.5 mg kg(-1)). When APNEA (2 mg kg(-1)) was combined with phenobarbital (20 mg kg(-1)), valproate (75 mg kg(-1)) or clonazepam (0.003 mg kg(-1)), seizure and afterdischarge durations were significantly shortened. 8-CPX (5 mg kg(-1)) totally reversed the APNEA (2 mg kg(-1))-induced enhancement of the anticonvulsive action of valproate. However, when the non-selective adenosine A(3) receptor agonist was administered together with diphenylhydantoin, no protection was observed in the kindling model of epilepsy. The interaction at the pharmacokinetic level can be excluded because APNEA did not interfere with the free plasma level of antiepileptics used in this study. It may be concluded that the interaction of APNEA with carbamazepine involves A(3) adenosine receptor-dependent events.

Adenosine-amino acid interactions in the chick brain: a role in passive avoidance learning

The present work describes interactions between adenosine and the amino acids glutamate and GABA in slices of intermediate medial hyperstriatum ventrale (IMHV), an area of the chick brain known to be involved in learning and memory events associated with a one-trial passive avoidance task. In slices derived from the IMHV of untrained chicks, the A(1) receptor agonist N(6)-cyclohexyladenosine (CHA; 10 microM) specifically inhibited glutamate release. Conversely, cyclopentyltheophylline (CPT; 100 microM an A(1) antagonist) increased glutamate release from the slices and blocked the CHA-induced inhibition of glutamate. The A(2) receptor agonist 2-p-(2-carboxylethyl)-phenylamino-5'-N-ethylcarboxamido adenosine hydrochloride (CGS 21680) selectively increased glutamate release when applied at 5 microM while it selectively inhibited GABA release at a lower concentration (10 nM). The addition of NMDA to the medium, resulted in increased adenosine release equivalent to that found following stimulation with 50 mM KCl. Both the NMDA and the KCl-induced increases were eliminated by addition of D-2-amino-5 phosphopentanoic acid (D-AP5), an NMDA-receptor antagonist. Slices prepared from the IMHV of chicks following successful training on the task showed enhanced adenosine release 30 min, 1, 3 and 6.5 h after training compared to chicks trained to peck a water-coated bead. The results show that changes in adenosine release from the IMHV accompany memory formation in the chick. We suggest that adenosine-amino acid transmitter interactions potentially via the activation of NMDA receptors, a necessary step in long-term memory formation for the task, may modulate the formation of memory for the one-trial passive avoidance task.

Effects of xanthine derivatives on phosphatidylcholine secretion in rat type II pneumocytes in the presence of activated eosinophils

We have previously reported that activated eosinophils enhanced the phosphatidylcholine (PC) secretion in type II pneumocytes. In this study, we have determined whether xanthine derivatives affect the PC secretion increased by activated eosinophils. Theophylline enhanced the increased PC secretion at 10(-5) M. 8-Phenyltheophylline dose-dependently enhanced the PC secretion. The enhanced secretion by either theophylline at 10(-5) M or 8-phenyltheophylline was suppressed by superoxide dismutase in combination with catalase. Pentoxifylline did not enhance the PC secretion increased by activated eosinophils, although it increased the PC secretion by itself. The PC secretion increased by theophylline at 10(-3) M or pentoxifylline was not suppressed by superoxide dismutase in combination with catalase. The present results suggest that xanthine derivatives increased the PC secretion in the co-culture of type II pneumocytes and activated eosinophils possibly through the inhibition of phosphodiesterases or the antagonism of adenosine receptors of the eosinophils.

The adenosine A1 receptor antagonist BIIP 20 counteracts scopolamine-induced behavioral deficits in the passive avoidance task in the rat

The present study investigated the effects of the putative adenosine A1 receptor antagonist BIIP 20 ((S)-(-)-8-(3-oxocyclopentyl)-1,3-dipropyl-7H-purine-2,6-dione) on counteracting scopolamine-induced behavioral deficits in the rat in the passive avoidance paradigm. A single oral application of BIIP 20 (1 and 3 mg/kg) 90 min before the rats received the noxious stimulus significantly attenuated the scopolamine-induced deficits observed during the retention trial of this task.

N6-2-(4-aminophenyl)ethyl-adenosine enhances the anticonvulsive activity of antiepileptic drugs

N6-2-(4-Aminophenyl)ethyl-adenosine (APNEA, a non-selective agonist of the adenosine A3 receptors), at the subprotective dose of 1 mg/kg against electroconvulsions, significantly potentiated the anticonvulsive action of phenobarbital, diphenylhydantoin and valproate against maximal electroshock, being ineffective at lower doses. APNEA (0.0039-1 mg/kg) also enhanced the protective activity of carbamazepine. Aminophylline (5 mg/kg) and 8-cyclopentyl-1,3-dimethylxanthine (8-CPX, 5 mg/kg), reversed the APNEA (1 mg/kg)-induced enhancement of the anticonvulsive action of phenobarbital, diphenylhydantoin and valproate, but not that of carbamazepine produced by APNEA at 0.0039 mg/kg. The adenosine agonist did not alter the plasma levels of antiepileptic drugs studied, so a pharmacokinetic interaction is not probable. Finally, APNEA (0.0156 and 1 mg/kg) administered alone or in combination with carbamazepine significantly decreased the body temperature and impaired long-term memory. Our results suggest that APNEA at low doses potentiates the protective activity of carbamazepine most likely through the A subtype of adenosine receptors. At higher doses, APNEA seems to enhance the anticonvulsive effect of other antiepileptics via adenosine A1 receptors.

Adenosine receptor ligands: differences with acute versus chronic treatment

Adenosine receptors have been the target of intense research with respect to potential use of selective ligands in a variety of therapeutic areas. Caffeine and theophylline are adenosine receptor antagonists, and over the past three decades a wide range of selective agonists and antagonists for adenosine receptor subtypes have been developed. A complication to the therapeutic use of adenosine receptor ligands is the observation that the effects of acute administration of a particular ligand can be diametrically opposite to the chronic effects of the same ligand. This 'effect inversion' is discussed here by Ken Jacobson and colleagues, and has been observed for effects on cognitive processes, seizures and ischaemic damage.

Effects of purine analogues on spontaneous alternation in mice

The Y-maze was used to examine the effects of purines acting at A1 and A2 adenosine receptors upon spontaneous alternation, a model of working memory, in mice. In support of previous work, scopolamine produced a loss of spontaneous alternation behaviour to the 0.5 chance level. The A1 receptor selective agonist N6- cyclopentyladenosine (CPA) did not change spontaneous alternation behaviour alone, but it prevented the decrease of spontaneous alternation scores produced by scopolamine. The A1 receptor selective antagonist 1,3-dipropyl-8-cyclopentylxanthine (CPX) blocked the effect of CPA in combination with scopolamine but had no effect alone. The A2 receptor selective agonist (N6-[2-(3,5-dimethoxyphenyl)-2-(2- methylphenyl)ethyl] adenosine (DPMA), and the A2 receptor selective antagonist 3,7-dimethyl-1-propargylxanthine (DMPX) had no effect of alternation behaviour alone and did not modify the effect of scopolamine. The results indicate the ability of A1 but not A2 receptor activation to modify working memory deficits induced by scopolamine, but suggest that endogenous adenosine does not normally participate in working memory processes.

Effects of nicotine on memory retrieval in mice

The effect of nicotine was tested on retrieval 24 h after training on a passive avoidance task. Intraperitoneal (i.p.) injection of nicotine (0.25-1.5 mg/kg) increased the step-down latency in mice dose dependently. Pretreatment with the nicotinic receptor antagonist mecamylamine (0.5-1 mg/kg) decreased, whereas pretreatment with the dopamine D1 receptor antagonist SCH 23390 (R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepine-7-ol maleate) (0.01, 0.05 and 0.1 mg/kg) and the beta-adrenoreceptor antagonist propranolol (10 mg/kg) increased the nicotine response. The dopamine receptor D2 receptor antagonist sulpiride (5-10 mg/kg), the anti-muscarinic agent atropine (2.5-10 mg/kg), the peripheral nicotinic receptor antagonist hexamethonium (0.01-0.5 mg/kg), the alpha-adrenoceptor antagonist phenoxybenzamine (1 and 10 mg/kg) and the peripheral dopamine D2 receptor antagonist domperidone (5 and 10 mg/kg) did not change the response induced by nicotine. Single administration of the antagonists did not cause response; however, a high dose of domperidone (10 mg/kg) and propranolol alone increased the step-down latencies. It may be concluded that a nicotinic receptor mechanism is involved in the nicotine-induced improvement of memory retrieval.

Effects of high doses of theophylline on memory acquisition

Intraperitoneal (IP) injection of high doses of theophylline decreased memory acquisition dose dependently. Low but not high doses of the dopamine receptor D2 antagonist sulpiride and the dopamine D1 receptor antagonist SCH 23390 decreased the theophylline response. Pimozide and the beta-adrenoreceptor propranolol also decreased the drug response. The anti-muscarinic agent atropine, the alpha-adrenoceptor phenoxybenzamine and the 5-HT antagonist metergoline did not affect the response induced by theophylline. It is concluded that dopamine and beta-adrenergic receptor mechanisms may be involved in the theophylline-induced attenuation of memory acquisition.