Characteristics of the release of adenosine from slices of rat cerebral cortex

Circadian clock organization in the retina: From clock components to rod and cone pathways and visual function

Circadian (24-h) clocks are cell-autonomous biological oscillators that orchestrate many aspects of our physiology on a daily basis. Numerous circadian rhythms in mammalian and non-mammalian retinas have been observed and the presence of an endogenous circadian clock has been demonstrated. However, how the clock and associated rhythms assemble into pathways that support and control retina function remains largely unknown. Our goal here is to review the current status of our knowledge and evaluate recent advances. We describe many previously-observed retinal rhythms, including circadian rhythms of morphology, biochemistry, physiology, and gene expression. We evaluate evidence concerning the location and molecular machinery of the retinal circadian clock, as well as consider findings that suggest the presence of multiple clocks. Our primary focus though is to describe in depth circadian rhythms in the light responses of retinal neurons with an emphasis on clock control of rod and cone pathways. We examine evidence that specific biochemical mechanisms produce these daily light response changes. We also discuss evidence for the presence of multiple circadian retinal pathways involving rhythms in neurotransmitter activity, transmitter receptors, metabolism, and pH. We focus on distinct actions of two dopamine receptor systems in the outer retina, a dopamine D4 receptor system that mediates circadian control of rod/cone gap junction coupling and a dopamine D1 receptor system that mediates non-circadian, light/dark adaptive regulation of gap junction coupling between horizontal cells. Finally, we evaluate the role of circadian rhythmicity in retinal degeneration and suggest future directions for the field of retinal circadian biology.

Cortical spreading depression-induced preconditioning in mouse neocortex is lamina specific

Cortical spreading depression (CSD) is able to confer neuroprotection when delivered at least 1 day in advance of an ischemic event. However, its ability to confer neuroprotection in a more immediate time frame has not previously been investigated. Here we have used mouse neocortical brain slices to study the effects of repeated episodes of CSD in layer V and layer II/III pyramidal neurons. In layer V, CSD evoked at 15-min intervals caused successively smaller membrane depolarizations and increases in intracellular calcium compared with the response to the first CSD. With an inter-CSD interval of 30 min this preconditioning effect was much less marked, indicating that preconditioning lasts between 15 and 30 min. A single episode of CSD also provided a degree of protection in oxygen-glucose deprivation (OGD) by significantly lengthening the time a cell could withstand OGD before anoxic depolarization occurred. In layer II/III pyramidal neurons no preconditioning by CSD on subsequent episodes of CSD was observed, demonstrating that the response of pyramidal neurons to repeated CSD is lamina specific. The A1 receptor antagonist 8-cyclopentyl theophylline (8-CPT) reduced the layer V preconditioning in a concentration-related manner. Inhibition of extracellular formation of adenosine by blocking ecto-5'-nucleotidase with α,β-methyleneadenosine 5'-diphosphate prevented preconditioning in most but not all cells. Block of equilibrative nucleoside transporters 1 and 2 with dipyramidole alone or in combination with 6-[(4-nitrobenzyl)thio]-9-β-d-ribofuranosylpurine also prevented preconditioning in some but not all cells. These data provide evidence that rapid preconditioning of one CSD by another is primarily mediated by adenosine.

Effect of Climbing Fibre Deprivation on the K+-evoked Release of Endogenous Adenosine from Rat Cerebellar Slices

We report the identification of a compound whose K+-induced Ca2+-dependent release in rat cerebellar slices was reduced following climbing fibre deprivation by 3-acetylpyridine (3-AP) treatment. Based on HPLC retention time, UV absorption spectrum, and mass spectrometry, this compound was identified as adenosine. The K+-induced, Ca2+-dependent release of adenosine was subsequently quantified in control and 3-AP-treated rats. It decreased by 60 - 70% in both the cerebellar vermis and hemispheres following climbing fibre deprivation, while 3-AP treatment had no effect on adenosine release in the cerebral cortex. Inhibition of ecto-5'-nucleotidase by alpha,beta-methylene ADP and GMP decreased basal and stimulated efflux of adenosine in the cerebellum by 50 - 60%, indicating that a significant proportion of adenosine was derived from the extracellular metabolism of released nucleotides. Taken with the reports of other groups on adenosine in cerebellum, these results suggest that climbing fibre activity increases the extracellular level of adenosine, probably through the metabolism of released nucleotides. This adenosine could then cause presynaptic inhibition of the release of the parallel fibre transmitter, which is presumably glutamate. This may account for the climbing fibre-evoked depression of Purkinje cell sensitivity to parallel fibre input.

Adenosine in the central nervous system: release mechanisms and extracellular concentrations

Adenosine has several functions within the CNS that involve an inhibitory tone of neurotransmission and neuroprotective actions in pathological conditions. The understanding of adenosine production and release in the brain is therefore of fundamental importance and has been extensively studied. Conflicting results are often obtained regarding the cellular source of adenosine, the stimulus that induces release and the mechanism for release, in relation to different experimental approaches used to study adenosine production and release. A neuronal origin of adenosine has been demonstrated through electrophysiological approaches showing that neurones can release significant quantities of adenosine, sufficient to activate adenosine receptors and to modulate synaptic functions. Specific actions of adenosine are mediated by different receptor subtypes (A(1), A(2A), A(2B) and A(3)), which are activated by various ranges of adenosine concentrations. Another important issue is the measurement of adenosine concentrations in the extracellular fluid under different conditions in order to know the degree of receptor stimulation and understand adenosine central actions. For this purpose, several experimental approaches have been used both in vivo and in vitro, which provide an estimation of basal adenosine levels in the range of 50-200 nM. The purpose of this review is to describe pathways of adenosine production and metabolism, and to summarize characteristics of adenosine release in the brain in response to different stimuli. Finally, studies performed to evaluate adenosine concentrations under physiological and hypoxic/ischemic conditions will be described to evaluate the degree of adenosine receptor activation.

Early changes in adenosine A1 receptors in cerebral cortex slices submitted to in vitro ischemia

The effects of brain ischemia on the maximum binding capacity (Bmax) and affinity (Kd) of A1 receptors were studied in the rat cerebral cortex, with an in vitro approach. The results were correlated with changes in 3H-adenosine release, studied under identical experimental conditions. Fifteen minutes of in vitro 'ischemia' (hypoxic, glucose-free medium) induced a significant increase in both Bmax (2398+/-132 fmol/mg protein, 151% of the control, P < 0.05) and in Kd (2.43+/-0.12 nM, 161% of the control, P < 0.01). At the same time, an increase in tritium efflux from [3H]-adenosine labeled cerebral cortex slices to 324% of the control was observed. A trend toward normalization was evident 5-15 min after 'reoxygenation' (restoring normal medium), but the binding parameters were still altered after 60 min (Bmax 2110+/-82 fmol/mg protein, Kd 2.26+/-0.14 nM, P < 0.01 vs the corresponding control) as was adenosine release (196% of the control). These findings suggest that the increased availability of adenosine and its receptors may be a defense mechanism against ischemic injury, while the reduced affinity of A1 receptors, possibly due to desensitization, may be a sign of ischemia-induced cellular damage.

Adenosinic modulation of 7-14 Hz spindle rhythms in interconnected thalamic relay and nucleus reticularis neurons

Intrathalamic spindle rhythms have been shown in vitro and in vivo to recur at frequencies of 0.1-0.3 Hz and last for periods of 1-3 s depending on the species observed. Although it is now generally agreed that both the intrinsic properties of relay and reticularis neurons, as well as their circuit dependency, contribute to the 7-14 Hz interburst spindle frequency, it is not presently known what mechanisms generate these slower oscillations. A model of interconnected thalamocortical relay and nucleus reticularis thalami cells was developed to investigate these properties. The model suggests that modulation of the hyperpolarization-activated cation current, Ib, by the nucleoside adenosine can serve to regulate both the duration of spindling and frequency of recurrence. These results suggest that the waxing and waning characteristics of thalamic spindle rhythms are, in part, dependent on the changing levels of extracellular adenosine and its influence on Ib in thalamocortical relay cells within this neuronal network.

Release and actions of adenosine in the central nervous system

Adenosine is released from active neurons into the extracellular fluid at a concentration of about 1 mumol/l. Neither the precise cellular origin nor the biochemical form of release has been firmly established, though the nucleotide is probably released partly directly, as a result of raised intracellular levels, and partly as nucleotides, which are subsequently hydrolysed. Once in the extracellular medium, adenosine markedly inhibits the release of excitatory neurotransmitters and modulatory peptides and has direct inhibitory effects on postsynaptic excitability via A1 receptors. A population of A2 receptors may mediate depolarization and enhanced transmitter release. Adenosine also modulates neuronal sensitivity to acetylcholine and catecholamines, all these effects probably contributing to the behavioural changes observed in conscious animals. As a result of their many actions, adenosine analogues are being intensively investigated for use as anticonvulsant, anxiolytic, and neuroprotective agents.

The chemical nature of the main central excitatory transmitter: a critical appraisal based upon release studies and synaptic vesicle localization

The chemical nature of the central transmitter responsible for fast excitatory events and other related phenomena is analysed against the historical background that has progressively clarified the structure and function of central synapses. One of the problems posed by research in this field has been whether one or more of the numerous excitatory substances endogenous to the brain is responsible for fast excitatory synaptic transmission, or if such a substance is, or was, a previously unknown one. The second question is related to the presence in the CNS of three main receptor types related to fast excitatory transmission, the so-called alpha-amino-3-hydroxy-5-methylisoxazole propionic acid, kainate and N-methyl-D-aspartate receptors. This implies the possibility that each receptor type might have its own endogenous agonist, as has sometimes been suggested. To answer such questions, an analysis was done of how different endogenous substances, including L-glutamate, L-aspartate, L-cysteate, L-homocysteate, L-cysteine sulfinate, L-homocysteine sulfinate, N-acetyl-L-aspartyl glutamate, quinolinate, L-sulfoserine, S-sulfo-L-cysteine, as well as possible unknown compounds, were able to fulfil the more important criteria for transmitter identification, namely identity of action, induced release, and presence in synaptic vesicles. The conclusion of this analysis is that glutamate is clearly the main central excitatory transmitter, because it acts on all three of the excitatory receptors, it is released by exocytosis and, above all, it is present in synaptic vesicles in a very high concentration, comparable to the estimated number of acetylcholine molecules in a quantum, i.e. 6000 molecules. Regarding a possible transmitter role for aspartate, for which a large body of evidence has been presented, it seems, when this evidence is carefully scrutinized, that it is either inconclusive, or else negative. This suggests that aspartate is not a classical central excitatory transmitter. From this analysis, it is suggested that the terms alpha-amino-3-hydroxy-5-methylisoxazole propionic acid, kainate and N-methyl-D-aspartate receptors, should be changed to that of glutamate receptors, and, more specifically, to GLUA, GLUK and GLUN receptors, respectively. When subtypes are described, a Roman numeral may be added, as in GLUNI, GLUNII, and so on.

Neuronal injury evoked by depolarizing agents in rat cortical cultures

Chemical depolarization is often used to study neurotransmitter release. Three commonly used depolarizing agents, veratridine, potassium, and glutamate, were evaluated for neurotoxicity. Neuronal survival and lactate dehydrogenase efflux were measured to assay irreversible injury. In addition, video-enhanced differential interference contrast microscopy was used to measure acute neuronal swelling. We found that lactate dehydrogenase efflux and cell death associated with exposure to potassium and glutamate could be blocked by the competitive N-methyl-D-aspartate antagonist amino-phosphonovaleric acid. Neuronal swelling was observed with all three agents, and could not be blocked by amino-phosphonovaleric acid. These results suggest multiple mechanisms of neuronal injury accompanying chemical depolarization. A 60-min exposure to 100 microM veratridine increased lactate dehydrogenase appearing in the medium at the end of this exposure to 615% of control and produced a 62% loss of neurons after 20-24 h. These effects could not be blocked by amino-phosphonovaleric acid at 500 microM. Differential interference contrast imaging revealed acute neuronal swelling in response to veratridine within 5 min of exposure, and this swelling could not be blocked by amino-phosphonovaleric acid. A 60-min exposure to medium supplemented with 50 mM KCl caused a lactate dehydrogenase efflux of 204% of control and produced a 48% loss of neurons. Amino-phosphonovaleric acid blocked both the neuronal loss and the excess lactate dehydrogenase efflux. In addition, differential interference contrast monitoring showed no KCl-evoked swelling. In contrast, isotonic substitution of 50 mM KCl for NaCl resulted in acute swelling which could not be blocked by amino-phosphonovaleric acid, in addition to neuronal death and lactate dehydrogenase release. Glutamate was, as expected, neurotoxic, and as has been shown before, this toxicity could be blocked by amino-phosphonovaleric acid. Observation of neurons exposed to 300 microM glutamate revealed that this treatment was invariably associated with neuronal swelling. In the presence of amino-phosphonovaleric acid, 81% of neurons swelled to greater than 110% by 30 min exposure to glutamate. These results suggest that experimental paradigms which investigate the effects of chemical depolarization upon central neurons are likely to be associated with reversible and irreversible forms of injury. This is of special importance to any study of the mechanisms of release of substances from central neurons.

Developmental regulation of adenosine A1 receptors, uptake sites and endogenous adenosine in the chick retina

Although adenosine A1 receptors mediate the inhibition of dopamine-dependent stimulation of adenylate cyclase activity in the developing chick retina, their localization and function are unknown. We have examined the localization of these receptors, and of endogenous adenosine and adenosine uptake sites at several stages of chick retinal development. A1 receptors were already localized predominantly to plexiform regions by embryonic day 12 (E12) with no gross changes at subsequent stages. Adenosine immunoreactivity was absent from retina at E8 but was detected at E12 in the ganglion cell layer, as well as cells in the inner nuclear cell layer and photoreceptors. At more advanced developmental stages the immunoreactivity was greater, but displayed similar localizations. Uptake sites labeled with [3H]nitrobenzylthioinosine (NBI) were detected even earlier using binding and autoradiographic methods. [3H]NBI binding was saturable, and Scatchard analysis demonstrated a single class of sites with a Kd of 0.91 nM and Bmax of 298 fmol/mg protein in E15 retinal membranes. The binding was displaced by unlabeled NBI and dipyridamole. NBI binding sites differentiated earlier than adenosine A1 receptors or endogenous adenosine immunoreactivity, showing a diffuse distribution at E8, but predominating in the plexiform layers of more developed retinas. The results indicate that elements of a putative purinergic system differentiate at specific localizations early in retinal development.

Release of endogenous adenosine and its metabolites by the activation of NMDA receptors in the rat hippocampus in vivo

1. The effects of N-methyl-D-aspartate (NMDA), KCl, and veratridine on the release of endogenous adenosine and its metabolites, inosine and hypoxanthine, from the rat hippocampus have been studied by in vivo microdialysis. 2. In the hippocampus of rats anaesthetized with urethane the adenosine level reached a stable state estimated at 0.93 microM during the first 2 h after the implantation of the dialysis probe. NMDA (50 microM to 25 mM) in the perfusate evoked a concentration-dependent release of adenosine, inosine and hypoxanthine with an EC50 of 180 microM. The release was reduced by 93% by the specific NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid (2-AP5) at 200 microM, indicating an NMDA receptor-mediated process. In addition, the 100 mM KCl-evoked release of adenosine was also substantially reduced by 77% by 2-AP5, suggesting that a large component of the K(+)-evoked release is NMDA-receptor-mediated. 3. Perfusion with zero-Ca2+ artificial cerebrospinal fluid attenuated the NMDA-evoked release of adenosine only by 16% (not significant) but depressed the K(+)-evoked release by 62%, indicating that most of the NMDA-evoked release is directly receptor-mediated, whereas a large component of the K(+)-evoked release could be via the release of an excitatory amino acid acting at the NMDA receptors.

Adenosine inhibition of the regeneration in vitro of adult frog sciatic sensory axons

The sensory axons of the adult frog sciatic nerve have earlier been shown to regenerate in vitro. If a local test crush is made at the initiation of culturing, regeneration starts after 3.4 days and proceeds at a rate of about 0.8-0.9 mm/day for several days. In the present experiments regeneration was inhibited by adenosine in a reversible and dose-dependent fashion. Similarly, both an adenosine analogue, 2-chloroadenosine (2-CA), and a non-hydrolyzable ATP analogue, AMP-PNP, reduced the outgrowth of sensory axons. The effect of adenosine was partially antagonized by theophylline at a critical concentration. Using a compartmental system, it could clearly be shown that adenosine exerted its effects at the outgrowth region. Adenosine, 2-CA, and AMP-PNP were also found to inhibit the proliferation of Schwann cells in the regenerating nerve. Various experiments showed that the latter can not explain the outgrowth inhibitory effects, which could be mediated by adenosine receptors associated with the elongating axons.

Adenosine in vertebrate retina: localization, receptor characterization, and function

1. The uptake of [3H] adenosine into specific populations of cells in the inner retina has been demonstrated. In mammalian retina, the exogenous adenosine that is transported into cells is phosphorylated, thereby maintaining a gradient for transport of the purine into the cell. 2. Endogenous stores of adenosine have been demonstrated by localization of cells that are labeled for adenosine-like immunoreactivity. In the rabbit retina, certain of these cells, the displaced cholinergic, GABAergic amacrine cells, are also labeled for adenosine. 3. Purines are tonically released from dark-adapted rabbit retinas and cultured embryonic chick retinal neurons. Release is significantly increased with K+ and neurotransmitters. The evoked release consists of adenosine, ATP, and purine metabolites, and while a portion of this release is Ca2+ dependent, one other component may occur via the bidirectional purine nucleoside transporter. 4. Differential distributions of certain enzymes involved in purine metabolism have also been localized to the inner retina. 5. Heterogeneous distributions of the two subtypes of adenosine receptors, A1 and A2, have been demonstrated in the mammalian retina. Coupling of receptors to adenylate cyclase has also been demonstrated. 6. Adenosine A1 receptor agonists significantly inhibit the K(+)-stimulated release of [3H]-acetylcholine from the rabbit retina, suggesting that endogenous adenosine may modulate the light-evoked or tonic release of ACh.

Regional differences in the electrically stimulated release of endogenous and radioactive adenosine and purine derivatives from rat brain slices

The release of both radioactive and endogenous purines was investigated in rat brain cortical, hippocampal and striatal slices at rest and following stimulation with electrical fields. Purines were labelled by incubating the slices with 3H-adenine. The purine efflux at rest and that evoked by electrical stimulation (10 Hz. 5 min) was analyzed by HPLC with ultraviolet absorbance detection. Both radioactive and endogenous purines in the effluent consisted mainly of hypoxanthine, xanthine, inosine and adenosine. No qualitative differences in the composition of the released purines were found in the three areas investigated. Electrical stimulation evoked a net increase in both radioactive and endogenous purine release. However the increase in 3H-adenosine following electrical stimulation was twice as large as that of endogenous adenosine. The electrically evoked release of both radioactive and endogenous purines was greatest in hippocampal slices and progressively smaller in cortical and striatal slices. In the three areas the addition of 0.5 microM tetrodotoxin to the superfusing Krebs solution brought about a similar (83-100%) reduction in evoked 3H-purine and endogenous purine release. Superfusion of the slices with calcium-free Krebs solution containing 0.5 mM EGTA reduced evoked release of 3H-purines by 58-60% and that of endogenous purine components by 54-89%. The results demonstrate similar characteristics for both radioactive and endogenous purine release but indicate that the most recently synthetized adenosine is the most readily available for release. The features of the electrically evoked purine release support a neuronal origin of adenosine and derivatives and are consistent with the hypothesis of discrete regional differences in adenosine neuromodulation.

Role of excitatory amino acid receptors in K+- and glutamate-evoked release of endogenous adenosine from rat cortical slices

K+ and glutamate released endogenous adenosine from superfused slices of rat parietal cortex. The absence of Ca2+ markedly diminished K+- but not glutamate-evoked adenosine release. Tetrodotoxin decreased K+- and glutamate-evoked adenosine release by 40 and 20%, respectively, indicating that release was mediated in part by propagated action potentials in the slices. Inhibition of ecto-5'-nucleotidase by alpha,beta-methylene ADP and GMP decreased basal release of adenosine by 40%, indicating that part of the adenosine was derived from the extracellular metabolism of released nucleotide. In contrast, inhibition of ecto-5'-nucleotidase did not affect release evoked by K+ or glutamate, suggesting that adenosine was released as such. Inhibition of glutamate uptake by dihydrokainate potentiated glutamate-evoked release of adenosine. Glutamate-evoked adenosine release was diminished 50 and 55% by the N-methyl-D-aspartate (NMDA) receptor antagonists, DL-2-amino-5-phosphonovaleric acid and (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801), respectively. The remaining release in the presence of MK-801 was diminished a further 66% by the non-NMDA receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione, suggesting that both NMDA and non-NMDA receptors were involved in glutamate-evoked adenosine release. Surprisingly, K+-evoked adenosine release was also diminished about 30% by NMDA antagonists, suggesting that K+-evoked adenosine release may be partly mediated indirectly through the release of an excitatory amino acid acting at NMDA receptors.

Adenosine antagonists delay hypoxia-induced depression of neuronal activity in hippocampal brain slice

Submerged rat hippocampal slices were exposed to hypoxic medium prepared with 95% N2/5% CO2. The population spikes recorded from CA1 cell layer were completely blocked within a range of 5-10 min. The adenosine antagonist theophylline (100 microM) delayed and partially prevented the hypoxia-induced depression. Increasing concentrations of the more potent adenosine antagonist 8-phenyltheophylline (8-PT; 0.1, 1, 10 microM) resulted in progressively less hypoxia-induced depression. The antidromically elicited afterpotentials recorded in the absence of synaptic transmission in low calcium, high magnesium medium were blocked within 8 min of hypoxia. Theophylline (100 microM) and 8-PT (10 microM) delayed to a similar extent the hypoxia-induced depression of the first afterpotential but did not prevent its complete depression.

Effect of acute ethanol on release of endogenous adenosine from rat cerebellar synaptosomes

The effects of pharmacologically relevant concentrations of ethanol on the release of endogenous adenosine from rat cerebellar synaptosomes were investigated. Release was conducted for 5, 10, 30, or 60 s after which time the incubation medium (containing the released adenosine) was rapidly separated from the synaptosomal membranes by vacuum filtration. The adenosine content of the filtrate was measured by HPLC-fluorescence detection. Both basal and KCl-stimulated adenosine release consisted of an initial rapid phase, for the first 10 s, that was followed by a relatively slower phase. Basal endogenous adenosine release was estimated as 199 +/- 14 pmol/mg protein/5 s. Potassium (chloride) increased adenosine release from the basal level to 433 +/- 83 pmol/mg protein/5 s. Ethanol caused a dose-dependent increase of adenosine release. The interaction between dilazep and ethanol indicates that ethanol-stimulated release does not involve the dilazep-sensitive transport system. The results support previous findings that indicate that cerebellar adenosine is involved in the mediation of ethanol-induced motor disturbances in the rat.

Cholinergic and noradrenergic denervations decrease labelled purine release from electrically stimulated rat cortical slices

The origin of cortical purine release was investigated by measuring [3H]purine release from electrically stimulated cortical slices of rats after neurotoxic lesions of cholinergic, noradrenergic and serotoninergic pathways innervating the cortex. Purines were labelled by incubating the cortical slices with [3H]adenine. The 3H efflux at rest and during stimulation, analysed by high performance liquid chromatography, consisted of adenosine, inosine, hypoxanthine and a small amount of nucleotides. Twenty days after unilateral or bilateral lesion of the nucleus basalis a marked decrease in choline acetyltransferase activity was associated with a decrease in [3H]purine release. A linear relationship was found between the decrease in choline acetyltransferase activity and [3H]purine release. A partial recovery in both choline acetyltransferase activity and [3H]purine release was observed eight months after the lesion. Twenty days after intra-cerebroventricular injection of 6-hydroxydopamine a 59% decrease in cortical noradrenaline content was associated with a 44% decrease in [3H]purine release. Conversely, no change in [3H]purine release was found in rats in which a 89% decrease in cortical serotonin content was induced by intra-cerebroventricular injection of 5,7-dihydroxytryptamine. The decrease in [3H]purine release after the lesion of the cholinergic and noradrenergic pathways may depend on metabolic changes, a loss of a stimulating influence of acetylcholine and noradrenaline or may indicate a release of [3H]purine from cholinergic and noradrenergic fibres.

Origin of ATP release in the rat vas deferens: concomitant measurement of [3H]noradrenaline and [14C]ATP

High pressure liquid chromatography (HPLC) combined with radiochemical detection was used to show that [3H]noradrenaline (NA) and [14C]ATP were released concomitantly during field stimulation of the rat vas deferens. The release of radioactivity (3H and 14C) in response to three consecutive field stimulations was constant (S2/S1 and S3/S2: 0.89 +/- 0.10 and 0.91 +/- 0.04 for 3H and 1.03 +/- 0.12 and 1.08 +/- 0.07 for 14C, respectively) and Ca2+-dependent. Tetrodotoxin completely inhibited the release of both NA and ATP. However, the alpha 1-antagonist, prazosin, enhanced the release of [3H]NA, and significantly reduced that of [14C]ATP. Exogenously applied NA produced an increase in ATP release which was antagonized by prazosin. 4-Aminopyridine substantially enhanced the release of ATP in the presence of prazosin, clearly indicating that some ATP originates from nerves. It is concluded that part of the ATP collected during stimulation of the rat vas deferens was released from nerves together with NA. In addition, a significant amount of ATP was released secondarily as a result of NA action via alpha 1-adrenoceptors. The experiments provide no information about whether the prazosin-sensitive component of release of ATP originates from smooth muscle or from axon terminals.

Release of purines from postsynaptic structures of amphibian ganglia

Isolated sympathetic paravertebral ganglia of the frog were incubated for 1 h with [3H]adenosine. Then, after washout of excess label, the contribution of pre- and post-synaptic activation on the release of 3H-labeled purines was studied. The ganglion was superfused with Ringer's solution at room temperature, and extracellular electrodes were used for stimulation and recording. Preganglionic stimulation enhanced overall release of 3H-labeled purines. At rest, the release of 3H-labeled purines per minute represented 0.62 +/- 0.02% of the total 3H-label in the ganglion, and this fraction increased depending on the frequency of orthodromic stimulation. Analyses of the effluent from resting and stimulated ganglia showed that in both cases the nonnucleotide fractions constituted greater than 97% of the total counts in the medium: adenosine (58.4 +/- 10.1%); inosine (31.7 +/- 12.9%); hypoxanthine (7.1 +/- 2.4%); and AMP, ADP, and ATP together (1.6 +/- 0.9%) (n = 11). Nucleotides were released, but their levels were not increased significantly during stimulation. Inclusion of ectophosphatase inhibitors slightly enhanced nucleotide release (from 1.1 +/- 0.5 to 1.8 +/- 0.7%; n = 5) but did not alter the amount of nucleosides. Hence, nucleosides are the main products released by the ganglion and do not arise from hydrolysis of extracellular ATP. Preganglionic stimulation enhanced release of labeled purines, which was frequency dependent from 1 to 20 Hz. Atropine (2 microM) and tubocurarine (150 microM) totally blocked the release of 3H-labeled purines associated with preganglionic stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of neuronal excitability by endogenous adenosine in the absence of synaptic transmission

Rat hippocampal slices were superfused with low calcium, high magnesium medium. Reductions in flow rate were associated with a marked depression of antidromically elicited afterpotentials with little change in the initial antidromic population spike recorded from CA1 pyramidal neurons. The depression of the afterpotential at the lower flow rates was largely reversed by the adenosine antagonist, theophylline (100 microM), by adenosine deaminase (10 micrograms/ml) and was mimicked by the application of the adenosine reuptake blocker, dipyridamole (100 microM). Since synaptic transmission was blocked, it is concluded that sufficient endogenous adenosine exists in the absence of synaptic function to alter neuronal excitability.

Sodium and calcium dependence of purine release from rat cerebral cortical slices

Electrically evoked purine release from rat cerebral cortical slices was evaluated, using a HPLC analysis combined with radioactivity measurement of the identified fractions. Two different pools of released purines have been identified: one probably related to cell metabolism and the other strictly linked to the nervous transmission. Since a linear increase, due to the stimulation frequencies, was found for the purines released from this second pool, a possible dependence on sodium and calcium transmembrane fluxes was evaluated. Pretreatment of the slices with TTX (5 x 10(-7) M) caused only a partial inhibitory effect on purine release (50%). This effect was probably related to the drug activity on the neuronal component of slices, since TTX induces an almost complete inhibition of purine release from isolated neurons "in cultures" and does not affect it from glial cells. Verapamil (1 x 10(-4) M), a calcium-channel blocker at glial and neuronal level, and TEA (3 x 10(-2) M), a specific inhibitor of calcium-mediated potassium efflux from glial cells, administered to the slices alone or in combination, showed a partial calcium-dependence of purine release. These results suggest a glial role in modulation of electrically-evoked purine release. These cells could exert a "buffering action" that regulates the calcium-mediated potassium availability, by which neuronal activity might be influenced.

Endogenous adenosine inhibits hippocampal CA1 neurones: further evidence from extra- and intracellular recording

Extracellular and intracellular recordings from CA1 pyramidal neurones of rats in vitro were used to study the effects of endogenous and exogenously applied adenosine. The adenosine receptor antagonist, caffeine, enhanced the intracellular recorded e.p.s.p.-i.p.s.p. sequence evoked by stimulation of the stratum radiatum which is antagonized by exogenous adenosine. The late, potassium dependent i.p.s.p. was not antagonized. The adenosine uptake inhibitor, nitrobenzylthioinosine (NBTI), mimicked the effects of exogenously applied adenosine. The effects of NBTI and of exogenously applied adenosine were antagonized by caffeine in the same manner. Exposure to adenosine deaminase enhanced the evoked field e.p.s.p. During this enhancement caffeines effects were significantly reduced. In low calcium high magnesium medium which abolishes synaptic activity, adenosine deaminase increased, NBTI decreased cell firing. We conclude that endogenous adenosine, release by a calcium independent mechanism, can exert an inhibitory tone on CA1 neurones in vitro. This is consistent with a role for adenosine as a mediator of negative feedback between the metabolic state and electrophysiological activity of nervous tissue.

Muscarinic modulation of purine release from electrically stimulated rat cortical slices

The release of 3H-labeled purines at rest and during electrical stimulation was investigated in slices of rat cortex prelabeled with [3H]adenine and perfused with Krebs solution. A linear relationship was found between radioactivity efflux and stimulation frequency from 2.5 to 20 Hz. At frequencies of less than 2.5 Hz, no increase in radioactivity efflux was detected. The amount of tritium released per pulse increased with stimulation frequency up to 10 Hz and declined at 20 Hz. The tritium efflux from the slices at rest and at a stimulation frequency of 10 Hz, analyzed by HPLC with ultraviolet absorbance detection at 254 nm, consisted mostly of adenosine, inosine, and hypoxanthine. The 3H-labeled purine release evoked by 10-Hz stimulation increased with current intensity from 15 to 100 mA/cm2. At 20 mA/cm2, addition of 0.5 microM tetrodotoxin to the superfusing Krebs solution brought about a 98% decrease of 3H-labeled purine release. At higher current strength, the percentage of tetrodotoxin-sensitive-evoked tritium efflux was smaller. At 30 mA/cm2, 86% of the evoked release was tetrodotoxin sensitive. Under these stimulation conditions, tritium efflux showed a 69% decrease when the slices were superfused with calcium-free Krebs solution containing 0.5 mM EGTA. The muscarinic agonist oxotremorine (30 microM) significantly enhanced the 10-Hz-stimulated 3H-labeled purine release. The effect of oxotremorine was partially prevented by tetrodotoxin, was antagonized by atropine (1.5 microM), and was mimicked by addition of physostigmine (3.8 microM) to the superfusion fluid. Atropine alone did not affect the evoked release, and none of the drugs modified the basal tritium efflux.(ABSTRACT TRUNCATED AT 250 WORDS)

Purinergic modulation of cortical acetylcholine release is decreased in aging rats

The effect of adenosine, N-ethylcarboxamide adenosine (NECA), and caffeine on acetylcholine (ACh) release was investigated in cortical slices prepared from 3 and 22-24-month-old rats. The slices were perfused with Krebs solution and electrically stimulated at 0.2, 1, and 5 Hz stimulation frequency. In old rats, ACh released by stimulation at 1 and 5 Hz was about half as large as in adult rats. In 22-24-month-old rats, the potency of adenosine was strongly reduced, and a similar significant inhibition of ACh release was obtained with concentrations of 1 microM adenosine in adult and 300 microM in old rats. Conversely, NECA, which has no effect on ACh release in adult rats, brought about a 40% decrease in old rats. Caffeine at 50 microM concentration enhanced, and at 500 microM inhibited, the evoked ACh release in adult rats, but was inactive in old rats. The possibility is envisaged that aging may modify purinergic modulation of ACh release by inducing conformational changes in purinergic receptors or changing adenosine metabolism.

The distribution of adenosine A1 receptors and 5'-nucleotidase in the hippocampal formation of several mammalian species

The distributions of adenosine A1 receptors, as demonstrated by 3H-cyclohexyladenosine (3H-CHA) binding, and the adenosine-producing enzyme 5'-nucleotidase were examined in the hippocampal formation of the rat, mouse, gerbil, cat, hamster, rabbit, and guinea pig. The enzyme and binding sites were restricted to subregions and often individual layers of this structure. The distribution of 3H-CHA binding was consistent among the species with the strata radiatum and oriens of fields CA1 and CA3 exhibiting the highest levels of binding. A distinct band of 3H-CHA binding was observed in the stratum moleculare of the dentate gyrus; and in most species, this band was restricted to the inner one-third of the stratum moleculare (i.e., proximal to the stratum granulosum). The strata pyramidale, granulosum, and lucidum were in general only weakly positive for 3H-CHA binding. The binding to the stratum lacunosum/moleculare (or the distinct strata lacunosum and moleculare in the rabbit and cat) was moderate. In contrast to the relative consistency of the patterns of 3H-CHA binding in these species, 5'-nucleotidase exhibited wide variations in both the absolute amount of activity and its localization. In all species, the strata granulosum and pyramidale appeared devoid of 5'-nucleotidase activity. The only clear exception to this rule was the CA3 region of the cat where activity was seen between the cell bodies of stratum pyramidale. The strata radiatum and oriens of CA1 were strongly positive in the rat and hamster but only low to moderately stained in the other species examined.(ABSTRACT TRUNCATED AT 250 WORDS)

Nifedipine: more than a calcium channel blocker

Nifedipine exhibits a greater incidence of side effects than the other currently marketed calcium channel antagonists. In addition to those effects attributable to calcium channel blockade, nifedipine produces side effects similar to the effects of adenosine. It is probable that nifedipine exerts part of its physiological actions through potentiation of adenosine. Adenosine, an endogenous calcium channel blocker, modifies synaptic events throughout the nervous system and causes sedation, smooth and skeletal muscle relaxation, anticonvulsion, hypotension and hypothermia, all reversible by caffeine or theophylline administration. Nifedipine inhibits adenosine uptake from, and release into, the extracellular space and binds at an adenosine receptor. Both nifedipine and adenosine interact with benzodiazepine binding sites. Interaction between nifedipine and adenosine should be kept in mind when treating patients with nifedipine.

Release of purines, noradrenaline, and GABA from rat hippocampal slices by field stimulation

Labelled adenine, noradrenaline (NA), and gamma-aminobutyric acid (GABA) were taken up by the transversely cut hippocampal slice. [3H]NA and [14C]GABA were retained as such, [3H]- (or [14C]-) adenine mainly as adenine nucleotides. There was a spontaneous overflow of all three types of compounds ranging from 0.1 (GABA) to 0.21 (NA) %/min. The rate of [3H]NA overflow increased rapidly during electrical field stimulation. The release rate was well maintained over a 15-min period. The rate of [14C]GABA release also increased rapidly but it was not maintained over a 15-min period even if uptake and/or metabolism was inhibited by nipecotic acid (1 mM) and aminooxyacetic acid (AOAA, 0.1 mM). The bulk of the purines was released after the stimulation period. For all compounds the amounts released were frequency- and calcium-dependent. At a frequency of 3 Hz a 10 V stimulation was sufficient to cause a maximal [3H]NA release and 20 V to cause maximal [14C]GABA release, but 14C-purine release was increased further by increasing the voltage to 40 V. The evoked purine release was inhibited by a nucleoside uptake inhibitor (dipyridamole). On stimulation of [3H]NA-labelled slices the released radioactivity was composed of greater than 95% unchanged NA. The specific activities of NA in the slice and in the superfusate were practically identical. In [3H]adenine-labelled slices the released radioactivity was composed of adenosine, inosine, and hypoxanthine, but the activity in the slice of ATP, ADP, and AMP.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Effects of ouabain on carotid body chemoreceptor activity in the cat

The effects of infusions of ouabain on chemoreceptor activity recorded from the peripheral end of a sectioned carotid sinus nerve were studied in cats anaesthetized with pentobarbitone. Ouabain caused a marked increase in chemoreceptor discharge followed by a decline in discharge to frequencies near or below the pre-ouabain level; during the latter period further administration of ouabain had no effect. Infusion of ouabain during hypoxia further increased the chemoreceptor discharge, but this effect was short-lasting. On intracarotid administration ouabain was less effective in cats with the ganglioglomerular (sympathetic) nerves cut, whereas on intravenous administration no significant difference was observed. Following intravenous administration of ouabain the chemoreceptor peak discharge occurred with dose levels similar to those needed to cause cardiac arrhythmias, but following intracarotid administration the chemoreceptor discharge peaked at doses about 40% of those causing arrhythmias. During ouabain-induced excitation the stimulatory action of NaCN, CO2-equilibrated Locke solution and acetylcholine was potentiated, as was the chemo-inhibition induced by dopamine. During the post-excitatory period the responses evoked by these substances were reduced or abolished. Neither mecamylamine, a nicotinic antagonist, nor physostigmine, an anti-cholinesterase, affected the response of the carotid chemoreceptors to ouabain. The major finding of this study was that ouabain initially 'sensitizes' the carotid body chemoreceptors and then 'desensitizes' them. The most likely mechanism responsible for these effects is the well established Na+--K+-ATPase-inhibiting property of ouabain.

Release of purines and noradrenaline by ouabain and potassium chloride from vascular adrenergic nerves

1 Release of [3H]-noradrenaline and 3H-purine by ouabain (10(-4)M) or high KCl (50 mM) was investigated in the superfused rabbit pulmonary arterial segment preincubated with [3H]-noradrenaline or [3H]-adenosine. 2 Ouabain elicited a delayed large contraction and a parallel [3H]-noradrenaline efflux. These were substantially inhibited by Ca2+-free medium or preincubation with 6-hydroxydopamine (30 micrograms/ml, 30 min). 3 Ouabain caused an 3H-purine efflux which was slower than the [3H]-noradrenaline efflux. This was inhibited by 6-hydroxydopamine and in part by phentolamine (3 X 10(-6)M), indicating both neuronal and extraneuronal origins of purines. 4 In contrast to ouabain, high KCl appeared to induce predominantly purine efflux, which was phentolamine-insensitive and 6-hydroxydopamine-sensitive, indicative of neuronal origin. 5 It is suggested that the purine efflux evoked by ouabain and high KCl may originate from different neuronal vesicles.

Potassium-stimulated release of radiolabelled taurine and glycine from the isolated rat retina

The release of preloaded [3H]glycine and [3H]taurine in response to a depolarising stimulus (12.5-50 mM KCl) has been studied in the superfused rat retina. High external potassium concentration immediately increased the spontaneous efflux of [3H]glycine, the effect of 50 mM K+ apparently being abolished by omitting calcium from the superfusing medium. In contrast, although high potassium concentrations increased the spontaneous efflux of [3H]taurine from the superfused rat retina, this release was not evident until the depolarising stimulus was removed from the superfusing medium. The magnitude of this "late" release of [3H]taurine was dependent on external K+ concentrations, and appeared immediately after cessation of the stimulus irrespective of whether it was applied for 4, 8, or 12 min. Potassium (50 mM)-induced release of taurine appeared partially calcium-dependent, being significantly reduced (p less than 0.01) but not abolished by replacing calcium with 1 mM EDTA in the superfusate. High-affinity uptake systems for both [3H]glycine and [3H]taurine were demonstrated in the rat retina in vitro (Km values, 1.67 microM and 2.97 microM; Vmax values, 19.3 and 23.1 nmol/g wet weight tissue/h, respectively). The results are discussed with respect to the possible neurotransmitter roles of both amino acids in the rat retina.

Depolarizing agent-induced cyclic AMP accumulation in isolated rat spinal cord

The stimulation of cyclic adenosine 3',5'-monophosphate (cyclic AMP) accumulation by the depolarizing agents K+, ouabain and veratridine, was studied in rat and guinea pig spinal cord tissue slices. Significantly increased accumulation of cyclic AMP was produced by each of the agents in a concentration-dependent manner. Veratridine and ouabain were equipotent (EC50 = 5 x 10(-5)M) and approximately 500 fold more potent than K+ (EC50 = 10(-2)M). Depolarizing agent-induced cyclic AMP accumulation in slices from guinea pig spinal cord was approximately double the response in rat spinal cord. Maximum stimulation occurred within 2.5 min of incubation with these agents and lasted for at least 30 min. Regional studies demonstrated that the maximal accumulation of cyclic AMP occurred to a greater degree in tissue slices from the dorsal section of spinal cord from both rat and guinea pig. Whereas the ouabain and veratridine stimulatory responses are completely dependent on extracellular Ca++, the K+ response is only partially dependent. Stimulation due to ouabain and veratridine is dependent, and K+ is independent, of release of neurohumoral substances such as norepinephrine or adenosine from spinal neurons. These experiments indicate the possible modulatory role of depolarization-linked events in regulating the spinal cord cyclic AMP system.

Possible selective inhibition of [3H]adenosine uptake by papaverine in vascular adrenergic nerves

The effect of the adenosine uptake inhibitors dipyridamole, papaverine and diazepam on the [3H]adenosine uptake was assessed using the rabbit pulmonary arterial segment. [3H]Adenosine uptake into the vascular segment was significantly diminished by dipyridamole (10(-6) -10(-5) M), papaverine (10(-5) -10(-4) M) or diazepam (5 x 10(-5) M) with potencies: dipyridamole greater than papaverine greater than diazepam. Dipyridamole (10(-6) M) or diazepam (5 x 10(-5) M) added 30 min before the incubation with [3H]adenosine reduced the high KCl-induced and epinephrine-induced [3H]purine effluxes equally, whereas papaverine (10(-5) M) selectively diminished the former efflux. KCl and epinephrine have been shown to act preferentially on the neuronal and extraneuronal sites, respectively. These results suggest that papaverine inhibits adenosine uptake into the vascular neuronal compartment, in preference to that into the extraneuronal compartment.

A combined in vivo/in vitro study of the presynaptic release of adenosine derivatives in the hippocampus

To investigate the release of adenine compounds from defined neuronal pathways, we employed a hippocampal slice preparation in which a selective-loading of the releasable pools was achieved in vivo with the aid of axonal transport. By injecting radioactive adenosine stereotaxically into the entorhinal cortex, the major afferent system to the dentate gyrus (the perforant path) was loaded within 20-36 h, at which time the rats were killed and hippocampal slices were prepared. The efflux of radioactive material, as recovered from the perfusate and measured in a scintillation counter, was found to be significantly increased in response to electrophysiologically controlled stimulation of the perforant path but not to stimulation of an alternative fiber tract, the fimbria. These findings provide supportive and more direct evidence for an activation-coupled release of adenosine derivatives from presynaptic sites in the central nervous system.

The effects of morphine and methionine-enkephalin on the release of purines from cerebral cortex slices of rats and mice

1 Slices of cerebral cortex from Wistar rats, TO mice or C57 mice were preincubated with [3H]-adenosine, and labelled purines were subsequently releases by electrical stimulation or by perfusing with ouabain, 100 micro M. 2 Electrically-evoked purine release was substantially reduced when the Ca2+ concentration in the medium was lowered from 2.4 to 0.1 mM. In both rats and mice, the electrically-evoked release was increased by morphine and methionine-enkephalin (Met-enkephalin), 10 micro M, and in rats and TO mice by morphine 1 micro M, both drug effects being prevented by naloxone. 3 Purine release evoked by ouabain was also increased by morphine 1 and 10 micro M, though not by Met-enkephalin, from slices of rat cortex. Ouabain-induced release from TO mice was reduced by morphine, and from C57 mice was unchanged. 4 The enhancement by morphine of electrically-evoked purine release may indicate that purines mediate some effects of morphine in the CNS.

Methylxanthines modulate adenosine release from slices of cerebral cortex

Using slices of mouse or rat cerebral cortex incubated with [3H]adenosine or [3H]adenine and/or [14C]GABA we have examined factors affecting the release of these compounds, and especially the influence of methylxanthines. Although release of purines and GABA could be induced by ouabain (10(-4) M), or p-hydroxymercuribenzoate (5 x 10(-4) M) no release was produced by ethacrynic acid (10(-3) or 10(-4) M) phenytoin (10(-3) M), noradrenaline or SC 13504. Release is probably not therefore related to (Na+, K+) ATPase or Mg2+-ATPase inhibition. At concentrations of 10(-3) and 10(-4) M, caffeine, theophylline, aminophylline and isobutyl-methylxanthine (IBMX) markedly depressed the release of purines evoked by ouabain. Non-xanthine inhibition of phosphodiesterase had much weaker though statistically significant effects. The methylxanthines had no significant effect on GABA release. It is suggested that the results can be explained on the basis of a positive feedback system in which released adenosine activates membranal adenylate cyclase, and the increased concentration of cyclic AMP which results form or origin of much of the adenosine released subsequently. However, we cannot exclude the existence of an intracellular receptor for methylxanthines which causes directly the inhibition of purine release.

Neuronal (Na+,K+)-ATPase and the release of purines from mouse and rat cerebral cortex

Slices of mouse or rat cerebral cortex were incubated with [3H]adenine or [3H]adenosine, and [14C]GABA. Purines and GABA could subsequently be released by ouabain. The release of purines previously shown to occur on restoring elevated K+ levels to normal was not mimicked by noradrenaline at concentrations which activate (Na+,K+)-ATPase. Potassium-free solutions evoked no release of purine during the test period, but resulted in a large release when K+ was restored to normal. K+-free solutions evoked an immediate release of GABA. It is concluded that (Na+,K+)-ATPase is not involved in purine release.