Sleep improvement in dogs after oral administration of mioflazine, a nucleoside transport inhibitor

The Mechanism Enabling Hibernation in Mammals

Some rodents including squirrels and hamsters undergo hibernation. During hibernation, body temperature drops to only a few degrees above ambient temperature. The suppression of whole-body energy expenditure is associated with regulated, but not passive, reduction of cellular metabolism. The heart retains the ability to beat constantly, although body temperature drops to less than 10 °C during hibernation. Cardiac myocytes of hibernating mammals are characterized by reduced Ca²⁺ entry into the cell membrane and a concomitant enhancement of Ca²⁺ release from and reuptake by the sarcoplasmic reticulum. These adaptive changes would help in preventing excessive Ca²⁺ entry and its overload and in maintaining the resting levels of intracellular Ca²⁺. Adaptive changes in gene expression in the heart prior to hibernation may be indispensable for acquiring cold resistance. In addition, protective effects of cold-shock proteins are thought to have an important role. We recently reported the unique expression pattern of cold-inducible RNA-binding protein (CIRP) in the hearts of hibernating hamsters. The CIRP mRNA is constitutively expressed in the heart of a nonhibernating euthermic hamster with several different forms probably due to alternative splicing. The short product contained the complete open reading frame for full-length CIRP, while the long product had inserted sequences containing a stop codon, suggesting production of a C-terminal deletion isoform of CIRP. In contrast to nonhibernating hamsters, only the short product was found in hibernating animals. Thus, these results indicate that CIRP expression in the hamster heart is regulated at the level of alternative splicing, which would permit a rapid increment of functional CIRP when entering hibernation. We will summarize the current understanding of the cold-resistant property of the heart in hibernating animals.

Nucleoside transporters: from scavengers to novel therapeutic targets

Hydrophilic purine and pyrimidine nucleosides rely on specialized carrier proteins for their membrane translocation. The recent identification of two gene families encoding equilibrative and concentrative nucleoside transporters in mammals and other organisms has provided the essential breakthrough to a more complete understanding of the biological significance of nucleoside transport. Although nucleoside salvage is a primary function of these proteins, recent data indicate functions beyond metabolic recycling. In brain and spinal cord, for example, nucleoside transporters have the potential to regulate synaptic levels of neuroactive purines such as adenosine and, thereby, indirectly modulate physiological processes through G-protein-coupled purine P1 receptors. As described in this review, recent research indicates novel putative functions for CNS nucleoside transporters in sleep, arousal, drug and alcohol addiction, nociception and analgesia. The therapeutic use of nucleoside analogue drugs and nucleoside transporter inhibitors in viral, neoplastic, cardiovascular and infectious disease is also described.

Phase-specific central regulatory systems of hibernation in Syrian hamsters

The central body temperature (T(b)) regulation system during hibernation was investigated in Syrian hamsters of either sex. Hibernation induced in Syrian hamsters by housing them in a cold room under short day-light/dark cycle was confirmed by marked reductions in the heart rate, T(b) and respiratory rate. The hibernation of hamsters was classified into (i) entrance, (ii) maintenance and (iii) arousal phases according to T(b) changes. In hibernating hamsters, T(b) elevations were phase-selectively elicited by intracerebroventricular (ICV) injection of 8-cyclopenthyltheophylline (CPT; a selective A1-adenosine receptor antagonist) and naloxone (a non-selective opioid receptor antagonist) during the entrance and maintenance phases, respectively. Moreover, a similar T(b) elevation tendency during the maintenance phase was also induced by ICV naloxonazine, (a selective mu1-opioid receptor antagonist), although such was not the case for naltrindole (a selective delta-opioid receptor antagonist) or nor-binaltorphimine (nor-BNI, a selective kappa-opioid receptor antagonist). Furthermore, T(b) elevations in hibernating hamsters were similarly induced with ICV thyrotropin-releasing hormone (TRH) during the entrance and maintenance phases. Furthermore, ICV injection of the anti-TRH antibody ameliorated the T(b) elevations induced by tactile stimulation. These results suggest that activation of the A1-receptor by adenosine is important for the generation of hypothermia in the entrance phase, and that activation of the mu1-opioid receptor by opioid peptides is required for perpetuation of hypothermia in the maintenance phase. In addition, TRH is a key endogenous substance involved in T(b) elevations during the arousal phase of hibernating hamsters.

Adenosine uptake inhibitors

Adenosine is a purine nucleoside and modulates a variety of physiological functions by interacting with cell-surface adenosine receptors. Under several adverse conditions, including ischemia, trauma, stress, seizures and inflammation, extracellular levels of adenosine are increased due to increased energy demands and ATP metabolism. Increased adenosine could protect against excessive cellular damage and organ dysfunction. Indeed, several protective effects of adenosine have been widely reported (e.g., amelioration of ischemic heart and brain injury, seizures and inflammation). However, the effects of adenosine itself are insufficient because extracellular adenosine is rapidly taken up into adjacent cells and subsequently metabolized. Adenosine uptake inhibitors (nucleoside transport inhibitors) could retard the disappearance of adenosine from the extracellular space by blocking adenosine uptake into cells. Therefore, it is expected that adenosine uptake inhibitors will have protective effects in various diseases, by elevating extracellular adenosine levels. Protective or ameliorating effects of adenosine uptake inhibitors in ischemic cardiac and cerebral injury, organ transplantation, seizures, thrombosis, insomnia, pain, and inflammatory diseases have been reported. Preclinical and clinical results indicate the possibility of therapeutic application of adenosine uptake inhibitors.

Sleep as neuronal detoxification and restitution

The classical 'hypnotoxin theory' was followed by extensive search for an endogenous sleep substance. Brain tissues and body fluids of sleeping and sleep-deprived animals contained active sleep-inducing factors like the sleep-promoting substance (SPS). Uridine and oxidized glutathione (GSSG), two components of SPS, seem to regulate physiological sleep differentially. Uridine may facilitate the inhibitory neurotransmission at the synaptic level of the GABAA-uridine receptor complex. In contrast, GSSG may inhibit the excitatory neurotransmission at the synaptic level of the glutamate receptor. Thus, the two SPS components promote sleep by exerting a complementary action on the two major neurotransmitter systems in the brain that have mutually reciprocal functions. Further, among multidimensional functions of sleep, uridine may contribute to recover the activity of neurons, while glutathione may counteract excitotoxic events. Hence sleep at the behavioral level is a process of neuronal restitution and detoxification at the cellular level. Such a concept can be regarded as a modern version of the Ishimori-Piéron's hypnotoxin theory proposed early in this century.

Nucleoside transport sites in a cultured human retinal cell line established by SV-40 T antigen gene

Adenosine, an important neuromodulatory compound in the brain and retina, is a potent vasodilator in most vascular beds throughout the body. Its actions are potentiated by inhibitors of nucleoside transport into cells. Knowledge of the existence of specific adenosine uptake systems in mammalian retina and the inhibition of the uptake by nitrobenzylthioinosine (NBMPR), a potent inhibitor of nucleoside transport, raises the possibility that the associated nucleoside transport system may be of pharmacological importance in retinal function. We have characterized the binding of the nucleoside transporter probe, [3H]NBMPR, to a cultured human retinal cell line established by transfection of SV-40 T antigen plasmid-DNA. The binding was specific, saturable and reversible. Scatchard analysis of the saturation data revealed that NBMPR binds to a homogeneous population of high affinity binding sites (KD = 0.65 +/- 0.22 nM; Bmax = 466 +/- 157 fmol/mg protein) characteristically similar to the binding sites in human retinal tissue (KD = 0.32 +/- 0.01 nM; Bmax = 292 +/- 41 fmol/mg protein). Selected compounds inhibited the binding in the cell line and retinal tissue with the same rank order of potency, suggesting that the transporters in the cell line and retinal tissue are similar. The data showed that the cell line is a useful model for the study of nucleoside transporter function in human retina.

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.

[3H]adenosine transport in synaptoneurosomes of postmortem human brain

[3H]Adenosine transport was characterized in cerebral cortical synaptoneurosomes prepared from postmortem human brain using an inhibitor-stop/centrifugation method. The adenosine transport inhibitors dipyridamole and dilazep completely and rapidly blocked transmembrane fluxes of [3H]adenosine. For 5-s incubations, two kinetically distinguishable processes were identified, i.e., a high-affinity adenosine transport system with Kt and Vmax values of 89 microM and 0.98 nmol/min/mg of protein, respectively, and a low-affinity adenosine transport system that did not appear to be saturable. For incubations with 1 microM [3H]adenosine as substrate, intrasynaptoneurosomal concentrations of [3H]adenosine were 0.26 microM at 5 s and 1 microM at 600 s. Metabolism of accumulated [3H]adenosine to adenine nucleotides was 15% for 5-s, 23% for 15-s, 34% for 30-s, 43% for 60-s, and 80% for 600-s incubations. The concentrations (microM) of total accumulated 3H-purines ([3H]adenosine plus metabolites) at these times were 0.3, 0.5, 1.0, 1.3 and 5.6, respectively. These results indicate that in the presence of extensive metabolism, the intrasynaptoneurosomal accumulation of 3H-purines was higher than the initial concentration of 1 microM [3H]adenosine in the reaction medium. For 5-, 15-, 30-, 60-, and 600-s incubations in the presence of the adenosine deaminase inhibitor EHNA and the adenosine kinase inhibitor 5'-iodotubercidin, metabolism of the transported [3H]adenosine was 14, 14, 16, 14, and 38%, respectively. During these times, total 3H-purine accumulation was 0.3, 0.5, 0.5, 0.7, and 1.8 microM, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Novel therapeutics acting via purine receptors

A recent conference entitled Purines in Cell Signalling: Targets for New Drugs, held in Rockville, Maryland, in September, 1989, was one indication of the increasing interest in developing agonists and antagonists of P₁-(adenosine) and P₂-(ATP) purinoceptors [1] as potential therapeutic agents. Extracellular adenosine, acting at its membrane bound A₁ and A₂ receptors, is a ubiquitous modulator of cellular activity. The purine can arise from several sources including ATP hydrolysis by ectokinase activity in the region of the nerve terminal [2] and from S-adenosylhomocysteine [3] and ATP within the cell. Together with its more stable analogs, adenosine is a potent inhibitor of neurotransmitter release in both the central and peripheral nervous systems, and in cardiac, adipose and other tissues. Adenosine can also affect blood pressure and heart rate as well as modulate the function of the immune, inflammatory, gastrointestinal, renal and pulmonary systems, either via its effects on transmitter release or directly via receptor mechanisms altering intracellular transduction processes.

In vivo effectiveness of several nucleoside transport inhibitors in mice and hamsters

The in vivo nucleoside transport inhibitory effects of 6-[(4-nitrobenzyl)-mercapto]purine ribonucleoside (NBMPR), used as its 5'-monophosphate derivative (NBMPR-P), dilazep, mioflazine and its derivatives soluflazine, R57974 and R75231, were investigated in BALB/c mice. The extent and duration of action were followed by assaying adenosine transport in blood cells sampled at time intervals following i.p. administration (ca. 20 mg/kg). Dilazep and R57974 were found to be short-acting inhibitors, while NBMPR-P and R75231 were similar in their action and caused essentially full inhibition of adenosine transport over a 4- to 5-h period. Mioflazine and soluflazine were rather ineffective, causing only partial inhibition. R75231 was also active after oral administration which, when repeated three times in 4-h intervals, resulted in essentially full transport inhibition up to 20 h following the initial dose. Effects of NBMPR-P, R57974 and dilazep on adenosine transport in blood cells were also measured in blood cells of hamsters after i.p. administration of the same doses. All three drugs caused full transport inhibition, but the action of dilazep and R75231 showed reversal within about 30 min and 2 h, respectively, while NBMPR-P caused full inhibition for at least 3-4 h. These results demonstrate the potential of the mioflazine derivative R75231 to be useful in vivo, possibly even after p.o. administration, for host protection against the actions of cytotoxic nucleosides used in experimental antiparasitic therapy or other studies requiring suppression of nucleoside transport.

Effect of propentofylline (HWA 285) on extracellular purines and excitatory amino acids in CA1 of rat hippocampus during transient ischaemia

1. The adenosine uptake blocker propentofylline (HWA 285) has previously been shown to protect hippocampal CA1 pyramidal cells from ischaemia-induced delayed neuronal death. The influence of propentofylline, on the extracellular concentrations of purines, aspartate and glutamate in the CA1 of the rat hippocampus during transient forebrain ischaemia was investigated. 2. Twenty min of ischaemia was induced by four-vessel occlusion in Wistar rats, extracellular compounds were sampled by use of microdialysis and EEG was recorded by a tungsten electrode attached to the dialysis probe. 3. Propentofylline (10 mg kg-1 i.p.) did not influence the basal levels of any of the compounds in the hippocampal dialysates. 4. The EEG became isoelectric within 20 s after induction of ischaemia. 5. Extracellular adenosine, inosine, hypoxanthine, aspartate and glutamate increased several fold during ischaemia and remained elevated during early reflow. Within 2 h of reperfusion the concentration of all compounds was normalized. Xanthine increased upon reperfusion and remained elevated after 2 h. 6. Propentofylline (10 mg kg-1 i.p.) administered 15 min before ischaemia significantly enhanced the ischaemia-evoked increase of adenosine but attenuated the increases of the other purine catabolites and of glutamate. 7. In separate in vitro experiments, propentofylline did not inhibit adenosine deaminase activity. 8. The present data show that propentofylline enhances extracellular adenosine and lowers extracellular glutamate in vivo during ischaemia. These findings may be important in relation to the neuroprotective properties of propentofylline.

Adenosine transport by rat and guinea pig synaptosomes: basis for differential sensitivity to transport inhibitors

Adenosine transport by rat and guinea pig synaptosomes was studied to establish the basis for the marked differences in the potency of some transport inhibitors in these species. An analysis of transport kinetics in the presence and absence of nitrobenzylthioinosine (NBTI) using synaptosomes derived from several areas of rat and guinea pig brain indicated that at least three systems contributed to adenosine uptake, the Km values of which were approximately 0.4, 3, and 15 microM in both species. In both species, the system with the Km of 3 microM was potently (IC50 of approximately 0.3 nM) and selectively inhibited by NBTI. This NBTI-sensitive system accounted for a greater proportion of the total uptake in the guinea pig than in the rat and was inhibited by dipyridamole, mioflazine, and related compounds more potently in the guinea pig. Preliminary experiments with other species indicate that adenosine transport in the mouse is similar to that in the rat, whereas in the dog and rabbit, it is more like that in the guinea pig. In the rat, none of the systems appeared to require Na+, but the two systems possessing the higher affinities for adenosine were inhibited by veratridine- and K(+)-induced depolarization. The transport systems were active over a broad pH range, with maximal activity between pH 6.5 and 7.0. Our results are consistent with the possibility that adenosine transport systems may be differentiated into uptake and release systems.

Adenosinergic modulation of homocysteine-induced seizures in mice

Homocysteine thiolactone (HTL) elicits seizures in mice at a dose of 850 mg/kg (95-100% of animals) with an average latency time of 19.5 min. These seizures are reversed by both 5' N-ethylcarboximide adenosine (NECA) and flunitrazepam, with respective ED50 doses of 0.025 and 0.20 mg/kg. NECA was approximately four-fold more potent as an inhibitor of HTL-induced seizures than of seizures induced by pentylenetetrazol (PTZ, 75 mg/kg). Flunitrazepam was equipotent in both seizure paradigms. The purine precursor 5-amino-4-imidazole carboxamide riboside, (AICAr), although virtually ineffective against PTZ-induced seizures at doses greater than 1 g/kg, was able to inhibit HTL-induced seizures with an ED50 of approximately 350 mg/kg. The anticonvulsant effect of AICAr was dose and time dependent. The anticonvulsant potency of AICAr was increased by simultaneous administration of the adenosine uptake blocker Mioflazine, whereas the central nervous system (CNS)-impermeable adenosine uptake blocker dipyridamole had no effect. The ability of AICAr to permeate the blood-brain barrier (BBB) is limited (less than 1%) and may explain its low potency as an anticonvulsant. AICAr also has very low potency at brain adenosine A1 and A2 receptors as well as adenosine uptake sites (IC50 greater than 10(-3) M), suggesting that its anticonvulsant properties are not mediated by direct action at these sites. The results indicate that AICAr does have frank anticonvulsant effects and further suggest that HTL-induced seizures may represent a useful paradigm for evaluation of adenosinergic agents. AICAr or more potent derivatives thereof may represent a new class of anticonvulsants with the ability to target seizure foci selectively.

Adenosine analogs elevate N-acetylserotonin and melatonin in rat pineal gland

Adenosine, S-adenosyl-L-homocysteine, adenosine analogs such as 5'-N-ethylcarboxamidoadenosine and mioflazine, a nucleoside transport inhibitor, injected intraperitoneally at 09.00 h during the light period increased melatonin levels in the pineal gland of the rat. The largest increase occurred with 1 mg/kg 5'-N-ethylcarboxamidoadenosine. A representative time-response curve with 5'-N-ethylcarboxamidoadenosine (1 mg/kg) showed a maximal peak of N-acetylserotonin and melatonin 2 and 4 h after injection, respectively. These results are discussed in relation with the possible modulation through A2 receptors of melatonin synthesis in the pineal gland.

Effects of two nucleoside transport inhibitors, dipyridamole and soluflazine, on purine release from the rat cerebral cortex

The effects of two nucleoside transport inhibitors, dipyridamole and soluflazine, on adenosine, inosine and oxypurine release from the normoxic and hypoxic/ischemic rat cerebral cortex have been studied. Dipyridamole (500 micrograms/kg) enhanced adenosine release during hypoxic/ischemic challenges in comparison with saline-injected controls. It decreased the hypoxia/ischemia-elicited releases of inosine, hypoxanthine and xanthine. Both basal and hypoxia/ischemia-elicited releases of uric acid were elevated. Soluflazine, administered topically or systemically, failed to enhance adenosine release and did not consistently alter the hypoxia/ischemia-evoked releases of inosine, hypoxanthine and xanthine. Basal release of uric acid was elevated. The failure of either drug to elevate the basal or hypoxia/ischemia-evoked releases of adenosine above predrug levels illustrates one of the problems which may be inherent in the use of bidirectional nucleoside transport inhibitors for the manipulation of adenosine levels in the cerebral interstitial fluid.

Chronic administration of diazepam downregulates adenosine receptors in the rat brain

Following chronic administration (10 or 20 days) of diazepam (5 mg/kg/day, subcutaneous pellets) or RO 15-1788 (5 mg/kg/day, intraperitoneally), adenosine and benzodiazepine receptors in different rat brain areas were assessed by radioligand binding studies using [3H]R-PIA for A1 receptors, [3H]NECA and [3H]R-PIA for A2 receptors and [3H]FNZ for benzodiazepine receptors. Chronic administration of diazepam for 10, but not for 20 days, decreased A2 receptors in the striatum by 46% (p less than 0.05) and A1 receptors in the hippocampus by 13% (p less than 0.05). Administration of diazepam for 10 days and 20 days failed to alter [3H]FNZ binding in all brain areas studied. However, 20 days of diazepam administration decreased the magnitude of GABA enhancement of [3H]FNZ binding in the cortex by 25% (p less than 0.05). In contrast, chronic administration of RO 15-1788 failed to alter [3H]R-PIA, [3H]NECA and [3H]FNZ binding in all brain areas. These results suggest that adenosine receptors may play a role in the CNS actions of benzodiazepines.

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.

The nucleoside-transport inhibitor soluflazine (R 64 719) increases the effects of adenosine in the guinea-pig hippocampal slice and is antagonized by adenosine deaminase

Field EPSP slope and population spike (PS) amplitude were measured in the CA1 pyramidal cell region after double-pulse stimulation of the striatum radiatum in hippocampal slices of guinea-pig. Iontophoresis of adenosine reduced the EPSP slope to 77.9 +/- 5.0% (mean +/- S.E.M.) and PS amplitude to 32.9 +/- 9.7% of the control values. Recovery was 98.7 +/- 3% for the EPSP and 82.9 +/- 7.0% for the PS 1.5 min after iontophoresis was stopped. In the presence of soluflazine 10(-6) M the effects of adenosine iontophoresis on the PS amplitude were significantly increased and the recovery of the EPSP and PS was significantly delayed. Soluflazine perfusion alone gradually decreased EPSP slope and PS amplitude as with adenosine. The reductions in EPSP slope and PS amplitude produced by soluflazine were antagonized by adenosine deaminase. An increase in EPSP slope and PS amplitude was seen when adenosine deaminase was given first. This increase was not reduced by exposure to soluflazine. These results are compatible with the hypothesis that soluflazine acts as a nucleoside transport inhibitor in the CNS, where it may increase the extracellular concentration of adenosine.