Phenothiazines inhibit adenosine uptake by rat brain synaptosomes

Neuroprotective potential of adenosine A1 receptor partial agonists in experimental models of cerebral ischemia

Cerebral ischemia is the second most common cause of death and a major cause of disability worldwide. Available therapies are based only on anticoagulants or recombinant tissue plasminogen activator. Extracellular adenosine increases during ischemia and acts as a neuroprotective endogenous agent mainly by activating adenosine A₁ receptors (A₁ Rs) which control calcium influx, glutamate release, membrane potential, and metabolism. Accordingly, in many experimental paradigms it has been already demonstrated that the stimulation of A₁ R with full agonists is able to reduce ischemia-related structural and functional brain damage; unfortunately, cardiovascular side effects and desensitization of A₁ R induced by these compounds have strongly limited their exploitation in stroke therapy so far. Among the newly emerging compounds, A₁ R partial agonists could be almost free of side effects and equally effective. Therefore, we decided to evaluate the neuroprotective potential of two A₁ R partial agonists, namely 2'-dCCPA and 3'-dCCPA, in in vitro and ex vivo experimental models of cerebral ischemia. Within the experimental paradigm of oxygen-glucose deprivation in vitro in human neuroblastoma (SH-SY5Y) cells both A₁ R partial agonists increased cell viability. Considering the high level of expression of A₁ Rs in the hippocampus and the susceptibility of CA1 region to hypoxia, we performed electrophysiological experiments in this subfield. The application of 7 min of oxygen-glucose deprivation constantly produces an irreversible synaptic failure in all the C57Bl/6 mice hippocampal slices evaluated; both tested compounds allowed a significant recovery of synaptic transmission. These findings demonstrate that A₁ R and its partial agonists are still of interest for cerebral ischemia therapy. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/.

Endogenous adenosine modulation of 22Na uptake by rat brain synaptosomes

To evaluate if endogenous extracellular adenosine influences sodium channel activity in nerve terminals, we investigated how manipulations of extracellular adenosine levels influence 22Na uptake by rat brain synaptosomes stimulated with veratridine (VT). To decrease extracellular adenosine levels, adenosine deaminase (ADA) that converts adenosine into an inactive metabolite was used. To increase extracellular adenosine levels, we used the adenosine deaminase inhibitor erythro-9(2-hydroxy-3-nonyl) adenine (EHNA), as well as the inhibitor of adenosine transport, nitrobenzylthioinosine (NBTI). ADA (0.1-5 U/ml) caused an excitatory effect on 22Na uptake stimulated by veratridine, which was abolished in the presence of the adenosine deaminase inhibitor erythro-9(2-hydroxy-3-nonyl) adenine (EHNA, 25 microM). Both the adenosine uptake inhibitor nitrobenzylthioinosine (NBTI, 1-10 microM) and the adenosine deaminase inhibitor EHNA (10-25 microM) inhibited 22Na uptake by rat brain synaptosomes. It is suggested that adenosine is tonically inhibiting sodium uptake by rat brain synaptosomes.

Monoclonal antibodies against a 60 kDa phenothiazine-binding protein from Trypanosoma brucei can discriminate between different trypanosome species

The dominant structure of the cytoskeleton of the Trypanosomatidae consists of a tight array of singlet pellicular microtubules, which surround the entire cell body. These microtubules are in close and stable contact with the cellular membrane. These contacts can be selectively disrupted by the action of phenothiazine drugs, which are potent trypanocides in vitro. Phenothiazine-affinity chromatography of detergent solubilized proteins from Trypanosoma brucei has resulted in the isolation of a protein of an apparent molecular weight of 60 000. Polyclonal antibodies raised against this protein (p60) have been used to investigate the presence of similar proteins in other protozoa. No such crossreacting proteins have been observed outside the family Trypanosomatidae. Within this family, a strong crossreactivity was observed with Crithidia fasciculata, while only a marginal reaction was seen with two species of Leishmania and, quite unexpectedly, also with the stercorarian trypanosomes T. cruzi and T. rangeli. Different monoclonal antibodies against p60 are able to clearly distinguish different subgenera of salivarian trypanosomes, and most notably to differentiate between various isolates of T. congolense. Therefore, these antibodies may prove valuable for diagnostic and epidemiological applications.

[3H]adenosine uptake and release from synaptosomes. Alterations by barbiturates

The effects of barbiturates on adenosine movements across the synaptic plasma membrane have been investigated using rodent whole brain synaptosomes. The hypothesis tested was that some of the depressant actions of these drugs may be mediated through interference with an endogenous adenosine system. Adenosine uptake was studied using synaptosomes prepared from Swiss-Webster mice. After preincubation at 37 degrees, [3H]adenosine was added to the synaptosomes in the presence or absence of pentobarbital, methohexital, phenobarbital, or 5-(2-cyclohexylideneethyl)-5-ethyl barbituric acid (CHEB) at various concentrations and times. All four compounds significantly inhibited [3H]adenosine uptake at concentrations of 100-300 microM. Pentobarbital did not affect the distribution of synaptosomal adenosine metabolites. Release of [3H]adenosine was studied using the P2 pellet from male CD-1 mice. Addition of 50 mM KCl caused an enhancement of 3H-efflux mainly due to increased release of adenosine and inosine. This effect was abolished in the presence of 250 microM ethylene glycol bis(beta-aminoethyl-ether)-N,N2-tetraacetic acid (EGTA). Pentobarbital, 0.3 mM, caused a significant increase in the net potassium-induced release of [3H]adenosine. These results suggest that some of the depressant effects of barbiturates may be due to inhibition of adenosine reuptake and enhancement of release resulting in elevated synaptic adenosine levels.

Behavioral interaction of adenosine and trifluoperazine in mice

Mice were implanted with chronic indwelling cannulae in the lateral cerebral ventricle. A dose-response curve was established for the effect of i.p. injections of trifluoperazine (TFP) on spontaneous locomotor activity. In addition, the behavioral interaction of i.p. injections of TFP with intracerebroventricular (i.c.v.) injections of adenosine (ADO) was examined. TFP depressed locomotor activity in a dose-dependent manner. A dose of ADO, which had no effect on locomotor activity when given alone, enhanced the depressant effects of TFP at all doses. As a control for the specificity of this behavioral interaction, mice also were given i.p. injections of TFP in combination with i.c.v. injections of 5'-N-ethylcarboxamidoadenosine (NECA), an uptake-resistant adenosine analogue. TFP and NECA did not interact to produce a significantly more pronounced locomotor depression. These results substantiate the notion that the sedative actions of TFP involve the inhibition of adenosine uptake and thus potentiation of extracellular adenosine levels.

[3H]nitrobenzylthioinosine binding to the guinea pig CNS nucleoside transport system: a pharmacological characterization

The binding of [3H]nitrobenzylthioinosine (NBMPR) to specific membrane sites in guinea pig brain was rapid, reversible, and saturable, and was dependent upon protein concentration, pH, and temperature. Mass law analysis of the binding data for cortical membranes indicated that NBMPR bound with high affinity to a single class of sites at which the equilibrium dissociation constant (KD) for NBMPR was 0.10-0.25 nM and which possessed a maximum binding capacity (Bmax) per mg of protein of 300 fmol of NBMPR. Kinetic analysis of the site-specific binding of NBMPR yielded an independent estimate of the KD of 0.16 nM. A relatively homogeneous subcellular distribution of the sites for NBMPR was found in cortical tissue. Recognized inhibitors of nucleoside transport were potent, competitive inhibitors of the binding of NBMPR in guinea pig CNS membranes whereas benzodiazepines and phenothiazines have low affinity for the sites. NBMPR sites in guinea pig cortical membranes have characteristics similar to those for NBMPR in human erythrocytes, the occupation of which is associated with inhibition of nucleoside transport. The comparable affinities for a range of agents for sites in human erythrocytes and guinea pig CNS membranes suggest that NBMPR also binds to transport inhibitory elements of the guinea pig CNS nucleoside transport system. It is proposed that the study of the binding of NBMPR provides an effective method by which to examine drug interactions with the membrane-located nucleoside transport system in CNS membranes.

Restoration of doxorubicin responsiveness in doxorubicin-resistant P388 murine leukaemia cells

The effects of certain compounds on the in vitro growth rate and the sensitivity to doxorubicin of P388 murine leukaemia cell line and of a doxorubicin-resistant subline (P388/ADR) were studied. The calcium channel blocking activity of these compounds was evaluated by measuring their effects on the sodium-dependent and membrane potential-dependent calcium uptake in synaptic plasma membrane vesicles. At non-inhibitory concentrations, verapamil, dipyridamole, meclizine and nicardipine were highly active in restoring the sensitivity to doxorubicin of P388/ADR cells. Moderately active were propranolol, N-(beta-diethylaminoethyl)-N-(beta-hydroxy-beta-phenylethyl)-2,5-dich loranaline (MDL-6792), thioridazine and chlorocyclizine, while nifedipine, guanethidine, phentolamine, chloroquine and papaverine had zero or only minimal synergistic activity to doxorubicin in this cell line. Doxorubicin synergistic activity could not be demonstrated in the parent drug-sensitive cell line. No sodium-dependent or membrane potential-dependent calcium uptake could be demonstrated in vesicles prepared from plasma membranes of either cell line. There is no correlation between the ability of these compounds to inhibit calcium uptake in synaptic vesicles and their potency in restoring the sensitivity of P388/ADR cells to doxorubicin.

Calmodulin antagonists inhibit adenosine uptake by rat brain cortical synaptosomes

The purpose of these experiments was to determine if the adenosine uptake process in brain synaptosomes is regulated by calmodulin. Several calmodulin antagonists including trifluoperazine, W-7 and R24571 were tested for their ability to inhibit adenosine uptake by rat brain cortical synaptosomes. The results indicate that these agents inhibit adenosine uptake in a competitive manner. Their potencies as inhibitors of uptake were in good agreement with those reported for their inhibition of identified calmodulin regulated reactions. It is therefore concluded that the adenosine uptake process in rat brain synaptosomes is regulated by calmodulin or a calmodulin-like protein.

Inhibition of adenosine uptake into rat brain synaptosomes by prostaglandins, benzodiazepines and other centrally active compounds

A number of compounds have been tested for their abilities to inhibit the rapid uptake of adenosine by rat cerebral cortical synaptosomes. Prostaglandins PGI2, PGA2, and PGE1 and PGE2 were potent inhibitors of adenosine uptake with IC20 values in the 10(-7) M-10(-6) M range. PGA1, PGD2 and PGF2 alpha also inhibited uptake but were less active. The benzodiazepine antagonist Ro 15-1788 inhibited adenosine uptake and failed to antagonize the effects of diazepam. Another antagonist, ethyl-beta-carboline-3-carboxylate, was a weak inhibitor of adenosine uptake. Ro 5-4864, the so-called peripheral benzodiazepine ligand, inhibited adenosine uptake. Hydroxyzine and tracazolate, two anxiolytic agents, inhibited uptake as did flunarizine, a coronary vasodilator. Two calmodulin antagonists, W7 and R 24571, were effective inhibitors of adenosine uptake. Their IC50 values were comparable to those at which they have been demonstrated to inhibit calmodulin-mediated reactions in other systems. These observations suggest that adenosine uptake may be a calmodulin-regulated process.