Uptake, exchange, and release of GABA by cerebellar glomeruli

Glomerular particles were isolated from the bovine cerebellar vermis and studied in vitro to further assess the possibility that gamma-aminobutyric acid (GABA) is utilized as a neurotransmitter in this synaptic complex. Cerebellar glomeruli accumulated [3H]GABA at two different high affinity sites, with affinities (KT) of 2.2 X 10(-6) M and 3.0 X 10(-5) M. These uptake sites could not be distinguished on the basis of their temperature sensitivities, sodium dependence, substrate specificities or responses to metabolic inhibitors. Although an exchange process contributed to the uptake measured in these experiments, a considerable amount of the [3H]GABA accumulated by glomerular particles was stored in an osmotically-sensitive, nonexchangeable pool. Glomerular particles preloaded with [3H]GABA exhibited a Ca2+-independent release of this amino acid in response to membrane depolarization. However, when preloaded glomerular particles were exposed to unlabeled GABA, which presumably displaced [3H]GABA from the exchangeable pool, a K+-evoked and Ca2+-dependent release of the remaining [3H]GABA occurred. The observed net uptake, together with the depolarization-induced and Ca2+-dependent release, of [3H]GABA from glomerular particles supports the suggestion that functionally active GABAergic synapses are present in these structures.

Chronic ethanol inhibits rat hippocampal "stimulus-secretion" coupling mechanism for 5-hydroxytryptamine in vitro

Effects of ethanol on serotonergic neurotransmission were investigated in crude mitochondrial fraction (P2 fraction) from rat brain hippocampus and hypothalamus. The [14C]5-HT preloaded P2 fraction was exposed to 45 mM KCl to induce 5-hydroxytryptamine release in vitro. Ethanol in vitro did not produce any significant inhibition of [14C]5-HT release until its concentration was greater than 100 mM. The K+-evoked 45Ca uptake of hippocampal P2 fraction was unaffected by 100 mM. However, 200 mM ethanol inhibited approximately 63% of K+-evoked 45Ca uptake. Chronic ethanol (10 g/kg/day) for 6 days inhibited [14C]5-HT release from hippocampus whereas it did not affect [14C]5-HT release from hypothalamus. Results indicate that chronic ethanol treatment may decrease serotonergic neurotransmission in selective brain regions. The reduction in 5-hydroxytryptamine release was the result of inhibition in "stimulus-secretion" coupling mechanism.

Up-regulation of brain [3H]diazepam binding sites in chronic caffeine treated rats

Brain [3H]diazepam and [3H]L-phenylisopropyladenosine binding sites in caffeine treated (75 mg/kg/day, i.p. 12 days) and caffeine withdrawn (30 days) rats were examined. Treatment with caffeine (75 mg/kg/day) for 12 days increases the Bmax (maximum binding capacity) for [3H]diazepam binding by 30.9% whereas the same treatment increases the Bmax for [3H]L-PIA binding by 165%. The Bmax for [3H]diazepam binding sites returns to slightly below control levels but [3H]L-PIA binding sites remain elevated after 30 days of caffeine withdrawal. These findings suggest that the up-regulation of [3H]diazepam binding sites seen in caffeine treated rats may not be adequately explained by a direct antagonism of caffeine on benzodiazepine receptors. Other modes of interaction therefore must be considered.

Potent depressant effects of adenosine analogs on hippocampal slow-wave activity in the unanesthetized rat

Adenosine and its analogs have previously been shown to exert a depressant effect on several measures of hippocampal excitability in the hippocampal slice and intact anesthetized preparation. In the present report, we examined the effects of intraventricular injections of adenosine analogs on hippocampal slow-wave activity in the freely moving rat. Each of three adenosine analogs-5'-N-ethylcarboxamidoadenosine (NECA) and N6-(phenylisopropyl) adenosine (L- and D-PIA)--were found to strongly suppress hippocampal electroencephalographic (EEG) activity. For instance, low doses of NECA (0.5 micrograms) produced an 80-90% decrease in the amplitude of the hippocampal EEG. NECA was approximately 20-fold more potent than L-PIA, and L-PIA was twice the potency of D-PIA. In separate experiments in the anesthetized rat, NECA and L-PIA were found to block completely the activation of the hippocampal theta rhythm elicited with brain-stem stimulation. The effects of adenosine analogs on both the hippocampal EEG and theta rhythm were very effectively reversed with methylxanthine, 8-para-sulphophenyl-theophylline (8-PSPT). The present findings demonstrate that adenosine analogs exert a powerful depressant effect on the hippocampal EEG in the natural unanesthetized state, and suggest that changes in the levels of endogenous adenosine may play a significant role in modulating the normal activity and function of the hippocampus.

Isolation of glomeruli from areas of bovine cerebellum and comparison of [3H]serotonin uptake

A method for bulk preparation of glomerular particles from subdivisions of the bovine cerebellum is presented. This method represents a modification of that previously reported by Hajos et al. [8], which increases the yield of glomerular protein by five-fold (5.5 mg/g wet wt) without compromising structural integrity or homogeneity. In addition, it offers the advantage of allowing one to study intraregional variations in the metabolic properties of cerebellar glomeruli. [3H]Serotonin (5-HT) uptake was measured in this preparation and it was demonstrated that glomeruli possess an active high affinity mechanism for this substrate. Comparison of [3H]5-HT uptake by glomeruli isolated from the cerebellar cortices of the lateral hemispheres and vermis revealed no differences in their kinetic properties.

2-Chloro-[3H]-adenosine binding in isolated rat kidney membranes

Binding sites for 2-chloro-[3H]-adenosine were found to be present in rat kidney membranes. Binding was inhibited by adenosine receptor agonists (5'-N-ethylcarboxamide adenosine, NECA; L-phenylisopropyl adenosine, L-PIA) and antagonists (theophylline; 3-isobutyl-1-methylxanthine, IBMX) but unaffected by an adenosine uptake inhibitor (papaverine). The binding sites exhibited the properties of an adenosine A2 receptor: micromolar affinity for 2-chloroadenosine; nearly identical potencies of theophylline and IBMX for inhibition of binding; NECA more potent than L-PIA for inhibition of binding.

Organic calcium channel blockers enhance [3H]purine release from rat brain cortical synaptosomes

The release of [3H]purines was investigated in a crude mitochondrial fraction (P2 fraction) from rat brain cortex pre-loaded with [3H]adenosine for 30 sec at 37 degrees C in vitro. Potassium, veratridine and glutamate were used as depolarizing agents to evoke the release of [3H]purines. Ca2+ removal, the addition of EGTA, and treatment with organic or inorganic Ca2+ antagonists did not inhibit [3H]purine release in this preparation. On the other hand, Ca2+ removal and the addition of EGTA greatly enhanced 3H-purine release induced by glutamate. D-600 and diltiazem enhanced K+-evoked [3H]purine release, and nifedipine increased veratridine evoked [3H]purine release indicating that either these Ca2+ antagonists have different sites of action, or that K+ and veratridine may release [3H]purine from different metabolic pools. Organic Ca2+ antagonists failed to enhance the [3H]purine release evoked by glutamate, further supporting the notion that various depolarizing agents may release [3H]purines from different cellular compartments.

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

Rats implanted with chronic indwelling cannulae were injected in the lateral cerebral ventricle with two adenosine analogues and the effects on spontaneous locomotor activity and blood pressure recorded. Both analogues produced dose-related decreases in locomotor activity, with 5'-N-ethylcarboxamidoadenosine (NECA) exhibiting slightly more potent depressant activity than (-)-N-(1-methyl-2-phenylethyl)adenosine (L-phenylisopropyladenosine) (L-PIA). NECA and L-PIA also produced dose-related reductions in blood pressure but the threshold dose for hypotensive activity was 10-100-fold higher than the dose required for depression of spontaneous locomotor activity. The depression of locomotor activity and the hypotensive effect of both analogues were antagonized by parenteral injections of caffeine. These results show that the hypoactive and hypotensive effects of adenosine analogues can be dissociated and that methylxanthines probably exert an antagonism of central adenosine receptors in the rat.

Up-regulation of brain [3H]diazepam binding sites in chronic caffeine-treated rats

Brain [3H]diazepam and L-[3H]phenylisopropyladenosine binding sites in caffeine-treated (75 mg/kg/day, i.p. 12 days) and caffeine-withdrawn (30 days) rats were examined. Treatment with caffeine (75 mg/kg/day) for 12 days increases the Bmax (maximum binding capacity) for [3H]diazepam binding by 30.9% whereas the same treatment increased the Bmax for L-[3H]PIA binding by 120%. The Bmax for [3H]diazepam binding sites returns to slightly below control levels but L-[3H]PIA binding sites still remain elevated after 30 days of caffeine withdrawal. The up-regulation of [3H]diazepam binding sites seen in caffeine-treated rats may indicate an interaction between caffeine and benzodiazepines at the receptor level and it may account for the supersensitivity to benzodiazepines seen in behavioral testing.

Further studies on the inhibition of adenosine uptake into rat brain synaptosomes by adenosine derivatives and methylxanthines

Various adenosine derivatives, methylxanthines and other compounds were tested for their abilities to inhibit the rapid uptake of adenosine by rat cerebral cortical synaptosomes. Several pharmacologically potent derivatives of adenosine were weak inhibitors of uptake with IC20 values in excess of 10(-5) M. Derivatives in this category were adenosine-5'-N-ethylcarboxamide, adenosine-5'-cyclopropylcarboxamide, N6-cyclohexyladenosine, L-N6-phenylisopropyladenosine, 1-methylisoguanosine, 2-phenylaminoadenosine and 5-iodotubercidin. Several methylxanthines were very weak inhibitors of adenosine uptake. These included pentoxifylline, n-hexyltheophylline, n-butyltheobromine, and isoamyltheobromine. HL 725, a pyrimido-isoquinoline with potent phosphodiesterase inhibitory activity, inhibited adenosine uptake with an IC20 of 2.0 X 10(-6) M. PK 11195, a putative ligand for the peripheral benzodiazepine binding site did not alter uptake at a concentration of 10(-4) M.

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.

Morphine enhances the release of 3H-purines from rat brain cerebral cortical prisms

In vitro experiments have shown that 3H-purines can be released from 3H-adenosine preloaded rat brain cortical prisms by a KCl-evoked depolarization. The KCl-evoked release of 3H-purines is dependent on the concentration of KCl present in the superfusate. At concentrations of 10(-7) approximately 10(-5)M morphine did not influence the basal release of 3H-purines from the prisms, although it enhanced the KCl-evoked release of 3H-purines. The enhancement of KCl-evoked 3H-purine release by morphine was concentration-dependent and was antagonized by naloxone, suggesting the involvement of opiate receptors. Uptake studies with rat brain cerebral cortical synaptosomes show that morphine is a very weak inhibitor of adenosine uptake. Comparisons with dipyridamole, a potent inhibitor of adenosine uptake, suggest that this low level of inhibition of the uptake did not contribute significantly to the release of 3H-purine by morphine seen in our experiments. It is therefore suggested that morphine enhances KCl-evoked 3H-purine release by an interaction with opiate receptors and that the resultant increase in extracellular purine (adenosine) levels may account for some of the actions of morphine.

Adenosine mediates sedative action of various centrally active drugs

Adenosine and its analogs depress the firing of neurons in various brain regions. The primary mode of action of adenosine in exerting this action appears to be the depression of calcium entry, thus decreasing presynaptic neurotransmitter release. Adenosine uptake inhibitors and adenosine deaminase inhibitors potentiate the depressant actions of adenosine. Caffeine and theophylline, methylxanthines, antagonize these actions. Adenosine is therefore likely to be released and to exert an ongoing modulation of the neuron excitability in the intact brain. Adenosine uptake by nerve terminals appears to be important in regulating the extracellular concentration of adenosine and thus of adenosine's action. A number of groups of centrally active sedative, anxiolytic and anticonvulsant drugs inhibit adenosine uptake by brain synaptosomal preparations. It is proposed that these agents exert their sedative effects by inhibiting adenosine uptake and thus potentiating depressant actions by locally released adenosine on neuronal activity.

Adenosine receptor agonists inhibit K+-evoked Ca2+ uptake by rat brain cortical synaptosomes

The uptake of Ca2+ by a K+-depolarized rat brain cerebral cortical crude synaptosomal preparation (P2 fraction) was investigated. The characteristics of the Ca2+ uptake system are similar to those observed by other investigators. The preparation is also a suitable model with which to study the effects of adenosine on Ca2+ uptake and neurotransmitter release, as it is generally accepted that K+-evoked Ca2+ uptake is intimately related to depolarization-induced release of neurotransmitters. We have demonstrated that an extracellular receptor is involved in mediating the adenosine-evoked inhibition of K+-evoked Ca2+ uptake. The pharmacological properties of the receptor suggest that it may be similar in some respects to the A2-receptor associated with adenylate cyclase. The adenosine uptake inhibitor, dipyridamole, potentiated the action of adenosine, suggesting that re-uptake is important in controlling the extracellular adenosine concentration and thus in the regulation of the adenosine receptor. The adenosine receptor antagonist theophylline inhibited the effects of adenosine. Calmodulin inhibited K+-evoked uptake of Ca2+ by the synaptosomal fraction.

The effect of alpha-, beta adrenergic receptor agonists and antagonists on the efflux of 22Na and uptake of 42K by rat brain cortical slices

The effects of norepinephrine on ion fluxes in rat brain cortical slices have now been ascertained. 22Na efflux and 42K influx are enhanced by norepinephrine. The increase in ion fluxes can be blocked by ouabain, phentolamine and propranolol, suggesting that the catecholamine activates a membrane sodium pump by a receptor-mediated step. The facilitation of 22Na efflux is stereospecific as demonstrated by the very weak action of D-norepinephrine at 10(-5) M concentration. Various alpha-adrenergic and beta-adrenergic receptor agonists, including oxymetazoline, naphazoline, clonidine, tramazoline, methoxamine, phenylephrine, L-isoproterenol and methoxyphenamine are potent stimulants of the sodium pump as demonstrated by their enhancement of ion fluxes in rat brain cortical slices. Our results are consistent with the hypothesis that norepinephrine hyperpolarizes central neurons by activating an ouabain-sensitive, receptor-mediated sodium pump.

Uptake of adenosine by isolated rat brain capillaries

Adenosine uptake by isolated rat brain capillaries is a carrier-mediated, temperature- and pH-sensitive process. The Km value for adenosine uptake is 4.74 microM and the Vmax is 21.7 picomol/mg protein/10 min. This is a high-affinity uptake system that can be cross-inhibited by several nucleosides and by the adenosine analogs tubercidin and 5'-deoxyadenosine. The uptake is very sensitive to inhibition by papaverine, hexobendine, and dipyridamole. These results confirm the existence of a nucleoside transport system associated with the blood-brain barrier observed during in vivo studies.

Adenosine receptors in rat brain membranes: characterization of high affinity binding of [3H]-2-chloroadenosine

1. [3H]-2-chloroadenosine has been found to be a suitable ligand for the study of adenosine receptors in rat brain synaptic membranes. 2. Binding sites labelled by [3H]-2-chloroadenosine had a high affinity with a KD value of 23.5 nM. 3. Binding is heat sensitive, pH dependent and probably involves protein molecules. 4. The IC50 values for 2-chloroadenosine, adenosine, L-N6-phenylisopropyladenosine and D-N6-phenylisopropyladenosine, N6-cyclohexyladenosine and adenosine-5'-N-ethyl-carboxamide inhibition of [3H]2-chloroadenosine binding are in good agreement with the values obtained in studies of the ability of these compounds to inhibit adenylate cyclase, suggesting that [3H]-2-chloroadenosine binding sites reported here are comparable to the adenosine A1 receptor site. 5. There are regional differences in [3H]-2-chloroadenosine binding to brain membranes. 6. This difference is probably due to the discrepancies in the number of binding sites, and is probably not caused by changing affinities of receptors to the ligand.

The effect of various centrally active drugs on adenosine uptake by the central nervous system

1. Adenosine and its analogs depress the firing of neurons in various brain regions. The primary mode of action of adenosine in exerting this effect appears to be the depression of transmitter release from presynaptic nerve terminals. This is a result of reduced calcium mobilization. 2. Adenosine uptake inhibitors and deaminase inhibitors depress the firing of central neurons. Adenosine antagonists, caffeine and theophylline, excite central neurons. Adenosine is therefore likely to be released in sufficient quantities to exert an ongoing modulation of synaptic transmission in the intact brain. 3. A number of groups of centrally active drugs inhibit adenosine uptake by brain synaptosomal preparations. These include the benzodiazepines, phenothiazines, various other sedatives and hypnotics, tricyclic antidepressants, non-steroidal anti-inflammatory analgesics, some steroids, diphenylhydantoin, puromycin and toyocamycin. 4. It is proposed that many agents with anxiolytic, sedative, analgesic or anti-convulsant actions may achieve their effects by inhibiting adenosine uptake and thus potentiating extracellular adenosine levels. 5. Morphine also elevates extracellular adenosine levels but achieves this by enhancing adenosine release.

Nucleoside transport in rat cerebral cortical synaptosomal membrane: a high affinity probe study

1. The nucleoside transport system in rat cerebral cortical synaptosomes was investigated using [H3]p-nitrobenzylthioinosine (NBMPR) as a high affinity probe. 2. There are high affinity and low affinity binding sites for NBMPR on rat synaptosomal membranes. The high affinity sites showed a KD value of 0.05 nM and a Bmax value of 113 fmol/mg protein. 3. Biochemical characterization of the high affinity [H3]NBMPR binding sites indicated that they probably correspond to nucleoside transport sites. 4. Several known adenosine uptake inhibitors including clonazepam were tested for their interaction with this high affinity binding site. 5. The results suggest that hexobendine and papaverine inhibit adenosine uptake by occupying the [H3]NBMPR high affinity binding sites. 6. Clonazepam and dipyridamole appear to inhibit adenosine uptake in rat cerebral cortical synaptosomes via an interaction at a different site.

Some biochemical properties of the rapid adenosine uptake system in rat brain synaptosomes

The rapid uptake of adenosine into rat brain cortical synaptosomes is mediated by a facilitated diffusion process. The carrier mediated uptake is sensitive to pH and temperature. The average Q10 value for the system is approximately 1.77 and the necessary activation energy (Ea) is estimated to be 8870 cal/mol. These values are essentially in agreement with values reported for adenosine uptake carriers of other tissues. Substrate specificity of the uptake system in the CNS demonstrates that nucleotides do not interact with the carrier until they have been hydrolyzed to nucleosides. Structural modification of the purine moiety at the "2" position did not have a profound effect on the ability of the molecule to serve as a substrate for the uptake system. Competitive inhibition by sulfhydryl reagents, p-chloromercuribenzoate, and N-ethylmaleimide on adenosine uptake suggests a direct involvement of sulfhydryl group(s) in the uptake mechanism. Other purines and pyrimidines also inhibit adenosine uptake, suggesting that a variety of nucleosides can interact with a common carrier system.

Phenothiazines inhibit adenosine uptake by rat brain synaptosomes

Phenothiazine (chlolpromazine, trifluoperazine, thioridazine, and fluphenazine) are potent inhibitors of the rapid uptake of adenosine by rat cerebral cortical synaptosomes. Other antipsychotics with fewer sedative side effects (clozapine, pimozide, haloperidol) were weaker inhibitors of adenosine uptake. It is hypothesized that inhibition of adenosine uptake is a significant factor in the sedative, antianxiety, and analgesic actions of the phenothiazines.

The rapid uptake and release of [3H]adenosine by rat cerebral cortical synaptosomes

Adenosine, a putative inhibitory transmitter or modulator in the brain, is rapidly transported by rat cerebral cortical synaptosomes. The uptake may represent a facilitated diffusion process, which is saturable and temperature-dependent. In this study, the uptake process was very rapid, reaching completion within 60 s of incubation at 37 degrees C, and had an apparent Km value of 0.9 microM and a Vmax value of 5.26 pmol/mg protein/30 s. Over 70% of the adenosine taken up remained unchanged, whereas 14% was metabolized to inosine. Twelve percent of the adenosine was converted to nucleotides. Rapid uptake of adenosine into rat cerebral cortical synaptosomes was partially inhibited by replacing Na+ with choline chloride in the medium. Ca2+ ion is important for the uptake process, as inhibition of adenosine uptake occurs in the presence of either Co2+ or EGTA. Rapid uptake of adenosine is apparently mediated by a nucleoside carrier, a conclusion based on its inhibition by a variety of purine and pyrimidine nucleosides. Uptake was inhibited by dipyridamole, hexobendine, papaverine, flurazepam, and morphine. Over 60% of the adenosine taken up by the rapid uptake system (30 s) was released by depolarizing agents. In contrast, only 30% of the adenosine taken up during a 15-min incubation period was released under the same conditions. [3H]Adenosine was the predominant purine released in the presence or absence of depolarizing agents. The basal and KCl-evoked release mechanisms were found to be at least partially Ca2+-dependent, however, the release of adenosine by veratridine was increased in the presence of EGTA. This finding is in agreement with the reported Ca2+-independent release of ATP from brain synaptosomes. The present findings suggest that there are at least two functional pools of adenosine in synaptosomes. Adenosine taken up by different uptake systems may be destined for different uses (metabolism or release) in the neuron.

Effects of diazepam on adenosine and acetylcholine release from rat cerebral cortex: further evidence for a purinergic mechanism in action of diazepam

1 Diazepam administered intraperitoneally (0.25 mg/kg) enhanced the rate of efflux of [3H]-adenosine and its metabolites from rat cerebral cortex. At a lower dose (0.05 mg/kg), this effect could be detected in only one of four rats. 2 Diazepam (0.05 and 0.25 mg/kg i.p.) depressed acetylcholine release from the rat cerebral cortex. Its effect was reversed by theophylline. 3 Theophylline (15 and 30 mg/kg) enhanced acetylcholine release from the rat cerebral cortex. Diazepam (0.25 mg/kg) administered after theophylline failed to cause a reduction in the rate of release, rather there appeared to be a further enhancement of release. 4 Pentobarbitone sodium (5, 10 and 15 mg/kg i.p.) did not elicit any increase in adenosine release. 5 These results support the proposal that benzodiazepines may exert their pharmacological actions by preventing adenosine uptake, thus enhancing the levels of extracellular adenosine.

The characterization of [3H] adenosine uptake into rat cerebral cortical synaptosomes

Uptake of adenosine, a putative inhibitory transmitter or modulator, was investigated in rat cerebral cortical synaptosomes. The accumulation of [3H] adenosine into synaptosomes, using an adenosine concentration of 10 microM, was linear for 30 min at 30 degrees C. The uptake appeared to be mediated by kinetically saturable processes with apparent Km's of 1 microM ("high-affinity A") and 5 microM ("high-affinity B"), both of which were partially sensitive to the presence of external sodium and calcium ions. Both uptake processes were partially inhibited by 2,4-dinitrophenol, implying the presence of active uptake and diffusional components. A study of the metabolites of adenosine taken up by the two uptake systems indicates that the major metabolites were adenosine and nucleotides. However, adenosine incorporated by the high-affinity A uptake system is more likely to form deaminated metabolites, such as hypoxanthine and inosine, indicating a possible functional difference between the two uptake processes. A detailed comparison of the inhibitory properties of certain adenosine analogues and other pharmacological agents has revealed differences between the two adenosine uptake systems. Since the glial contamination in synaptosomal preparations is well established, one of the uptake systems we observed in the present study might be of glial origin. This notion is supported by the findings that the Km values and kinetic properties of papaverine action in he synaptosomal high-affinity A uptake system are similar to those of astrocytes reported in the literature. In conclusion, the uptake processes of synaptosomal preparations show that accumulation of adenosine into neuronal (and possibly glial) elements may play a major role in regulating the extracellular adenosine concentration. Uptake inhibitors, such as diazepam, may exert, at least in part, their pharmacological actions by interfering with the regulation of extracellular adenosine concentrations.

The effect of morphine on purine and acetylcholine release from rat cerebral cortex: evidence for a purinergic component in morphine's action

Morphine enhances the release of adenosine and its metabolites from the rat cerebral cortex and inhibits the release of acetylcholine. Naloxone antagoinizes the effects of morphine on both purine and acetylcholine release. The adenosine antagonists, caffeine and theophylline, reduce morphine's effects on acetylcholine release, and at the same time increase the spontaneous release of acetylcholine. It is suggested that morphine, acting at a naloxone-sensitive site, enhances the level of extracellular adenosine, which in turn inhibits the release of acetylcholine, and that some of morphine's actions are mediated by a purinergic step.

Interactions between the benzodiazepines, methylxanthines, and adenosine

Experimental evidence is cited in support of the proposal that benzodiazepines exert their anxiolytic effects by inhibiting the uptake of adenosine by central neurons and glia.

Morphine enhances adenosine release from the in vivo rat cerebral cortex

Morphine (1 and 5 mg/kg), administered intravenously, increased the rate of efflux of purines from intact rat cerebral cortices prelabelled with 3H-adenosine. Naloxone antagonized morphine's action. It is suggested that the depressant actions of morphine on transmission in the central nervous system may be related to this enhancement of extracellular levels of adenosine and the adenine nucleotides.

Specific binding of 2-[3H]chloroadenosine to rat brain cortical membranes

2-[3H]Chloroadenosine was used as a ligand to investigate the presence of adenosine receptors on rat brain cortical membranes. The binding studies reveal the presence of a single binding site which has an apparent dissociation constant (KD) of 23.5 nM and a maximal binding capacity (Bmax) of 476 fmol/mg protein. This binding can be inhibited by adenosine, adenine, nicotinamide adenine dinucleotide phosphate, inosine, theophylline, and isobutylmethylxanthine. These findings strongly support the suggestion that there is an adenosine receptor on brain cell membranes.

Topographical distribution of ATP in rat brain

Studies on the distribution of ATP in microdissected segments of the rat brain indicate that the nucleotide is concentrated in gray matter, and especially in the thalamus, hippocampus, entorhinal cortex and sensorimotor cortex. These distribution studies in conjunction with previous neuropharmacological studies, support the concept that adenine nucleotides may function as intercellular mediators in various regions of the brain.