The effect of local infusion of adenosine and adenosine analogues on local cerebral blood flow

Characterization of the effects of adenosine receptor agonists on cerebral blood flow in uninjured and traumatically injured rat brain using continuous arterial spin-labeled magnetic resonance imaging

Hypoperfusion after traumatic brain injury may exacerbate damage. Adenosine, a vasodilator, regulates cerebral blood flow (CBF). Treatment with adenosine receptor agonists has shown benefit in experimental CNS trauma; however, their effects on CBF after injury remain undefined. We used magnetic resonance imaging to assess CBF in uninjured rats both early and at 24 h after intrahippocampal administration of either the nonselective adenosine receptor agonist 2-chloroadenosine (2-CA, 12 nmol) or the A(2A)-receptor agonist 2-p-(2-carboxyethyl)-phenethylamino-5'-N-ethylcarbox-amidoadenosine (CGS 21680, 6 nmol). We also assessed the effects of these agents on cerebral metabolic rate for glucose (CMRglu). We then assessed the effect of 2-CA on CBF at 3.5 to 5 h after controlled cortical impact (CCI). Injection of 2-CA into uninjured rat brain produced marked increases in CBF in ipsilateral hippocampus and cortex versus vehicle (P<0.05); CBF increases persisted even at 24 h. Measurement of hippocampal levels of 2-CA showed persistent increases to 24 h. CGS 21680 produced even more marked global increases in CBF than seen with 2-CA (2-6-fold versus vehicle, P<0.05 in 10/12 regions of interest (ROIs)). Neither agonist altered CMRglu versus vehicle. After CCI, 2-CA increased CBF in ipsilateral hippocampal and hemispheric ROIs (P<0.05 versus vehicle), but the response was attenuated at severe injury levels. We report marked increases in CBF after injection of adenosine receptor agonists into uninjured rat brain despite unaltered CMRglu. 2-Chloroadenosine produced enduring increases in CBF in uninjured brain and attenuated posttraumatic hypoperfusion. Future studies of adenosine-related therapies in CNS injury should address the role of CBF.

Vascular endothelial growth factor is increased in cerebrospinal fluid after traumatic brain injury in infants and children

OBJECTIVE

Vascular endothelial growth factor (VEGF) is an important regulator of angiogenesis, the formation of which is triggered by hypoxia, cytokines, and growth factors and is also induced by activation of the adenosine 2B receptor. VEGF is neuroprotective in several models of experimental brain injury and is increased in brain after traumatic brain injury (TBI) in humans and experimental animals. Adenosine is a neuroprotective purine metabolite that increases in cerebrospinal fluid (CSF) after clinical TBI in children. We hypothesized that VEGF levels would 1). be increased in CSF after TBI in infants and children, and 2). be preceded by increases in CSF adenosine. To test this hypothesis, we designed a case-control study to compare the CSF of infants and children after severe TBI with that of uninjured children.

METHODS

Using an Institutional Review Board-approved protocol, we compared CSF concentrations of VEGF (by enzyme-linked immunosorbent assay) and adenosine (by high-performance liquid chromatography) in 73 samples from 14 infants and children with severe TBI (Glasgow Coma Scale score <or=8) with those in CSF from 5 noninjured control subjects. Patients received standard neurointensive care.

RESULTS

Mean VEGF levels were increased after TBI versus control (39.8 +/- 6.2 versus 14.9 +/- 1.5 ng/dl, mean +/- standard error of the mean, P = 0.0002) and peaked at 91.6 +/- 26.4 ng/dl, approximately 6 times control (P = 0.001). Peak VEGF occurred at 22.4 hours after injury. There was a trend toward increased adenosine concentration after TBI versus control (18.3 +/- 3.5 versus 11.5 +/- 2.3 nmol/L), but this did not reach statistical significance. A multivariate regression model showed an independent, significant association between the concentrations of VEGF and adenosine.

CONCLUSION

VEGF is increased in CSF after pediatric TBI, and this increase is associated with an increase in CSF adenosine. These results may imply that a component of the vascular regenerative response of the brain is initiated rapidly after TBI and continues for several days after injury. Further investigation is warranted to determine 1). whether this association is causative, 2). the role of adenosine in triggering the increase in CSF VEGF concentration, and 3). the exact role VEGF that plays after injury.

Suppression of cortical functional hyperemia to vibrissal stimulation in the rat by epoxygenase inhibitors

Application of glutamate to glial cell cultures stimulates the formation and release of epoxyeicosatrienoic acids (EETs) from arachidonic acid by cytochome P-450 epoxygenases. Epoxygenase inhibitors reduce the cerebral vasodilator response to glutamate and N-methyl-D-aspartate. We tested the hypothesis that epoxygenase inhibitors reduce the somatosensory cortical blood flow response to whisker activation. In chloralose-anesthetized rats, percent changes in cortical perfusion over whisker barrel cortex were measured by laser-Doppler flowmetry during whisker stimulation. Two pharmacologically distinct inhibitors were superfused subdurally: 1) N-methylsulfonyl-6-(2-propargyloxyphenyl)hexanamide (MS-PPOH), an epoxygenase substrate inhibitor; and 2) miconazole, a reversible cytochrome P-450 inhibitor acting on the heme moiety. Superfusion with 5 micromol/l MS-PPOH decreased the hyperemic response to whisker stimulation by 28% (from 25 +/- 9 to 18 +/- 7%, means +/- SD, n = 8). With 20 micromol/l MS-PPOH superfusion, the response was decreased by 69% (from 28 +/- 9% to 9 +/- 4%, n = 8). Superfusion with 20 micromol/l miconazole decreased the flow response by 67% (from 31 +/- 6% to 10 +/- 3%, n = 8). Subsequent superfusion with vehicle restored the response to 26 +/- 11%. Indomethacin did not prevent MS-PPOH inhibition of the flow response, suggesting that EET-related vasodilation was not dependent solely on cyclooxygenase metabolism of 5,6-EET. Neither MS-PPOH nor miconazole changed baseline flow, reduced the blood flow response to an adenosine A(2) agonist, or decreased somatosensory evoked potentials. The marked reduction of the cortical flow response to whisker stimulation with two different types of epoxygenase inhibitors indicates that EETs play an important role in the physiological coupling of blood flow to neural activation.

Intracerebral adenosine infusion improves neurological outcome after transient focal ischemia in rats

Second Institute of New Drug Research, Otsuka Pharmaceutical Co., Ltd., Tokushima, Japan In order to elucidate the role of adenosine in brain ischemia, the possible protective effects of adenosine on ischemic brain injury were investigated in a rat model of brain ischemia both in vitro and in vivo. Exogenous adenosine dose-dependently rescued cortical neuronal cells from injury after glucose deprivation in vitro. Adenosine (1 mM) also significantly reduced hypoglycemia/hypoxia-induced glutamate release from the hippocampal slice. In a rat model of transient middle cerebral artery occlusion (MCAO), extracellular adenosine concentration was increased immediately after occlusion, and then returned to the baseline by 30 min after reperfusion. Adenosine infusion through a microdialysis probe into the ipsilateral striatum (1 mM adenosine, 2 microl min(-1), total 4.5 h from the occlusion to 3 h after reperfusion) showed a significant improvement in the neurological outcome, and about 25% reduction of infarct volume, although the effect did not reach statistical significance, compared with the vehicle-treated group at 20 h after 90 min of MCAO. These results demonstrated the neuroprotective effect of adenosine against ischemic brain injury both in vitro and in vivo, suggesting the possible therapeutic application of adenosine regulating agents, which inhibit adenosine uptake or metabolism to enhance or maintain extracellular endogenous adenosine levels, for stroke treatment.

Assessment of the effect of 2-chloroadenosine in normal rat brain using spin-labeled MRI measurement of perfusion

Adenosine analogs such as 2-chloroadenosine are potent cerebrovasodilators. Spin-labeled MRI was used to investigate the spatial distribution, dose-response, and timing of the effect of 2-chloroadenosine on cerebral blood flow (CBF) after intraparenchymal injection into rat brain. Sprague-Dawley rats (N = 10) were injected with 2-chloroadenosine at doses of 0.3, 6.0, or 12 nmoles, or saline vehicle (2-4 microL). CBF was serially quantified in a slice through the injection site in a circular (3.6 mm diameter) region of interest (ROI) around the injection and in ipsilateral hemispheric ROIs at approximately 90 min and approximately 180 min. Marked 3.77- and 3.93-fold increases in CBF (vs. vehicle) were seen in the circular ROI at approximately 90 min and approximately 180 min after 12-nmol injection, respectively. Similarly, 2.92- and 2.78-fold increases in hemispheric CBF were observed at approximately 90 min and approximately 180 min, respectively, after injection of 12 nmoles. Linear dose-response relationships were observed at both times after injection in both ROIs (all P < 0.01). Spin-labeling MRI assessment revealed that parenchymal injection of 2-chloroadenosine produces potent, dose-dependent, and sustained vasodilation over large areas of brain. This treatment and imaging paradigm should facilitate investigation of the effect of CBF promotion in models of traumatic and ischemic brain injury.

P-450 epoxygenase and NO synthase inhibitors reduce cerebral blood flow response to N-methyl-D-aspartate

Epoxyeicosatrienoic acids are cerebral vasodilators produced in astrocytes by cytochrome P-450 epoxygenase activity. The P-450 inhibitor miconazole attenuates the increase in cerebral blood flow (CBF) elicited by glutamate. We evaluated whether epoxygenase activity is involved in the CBF response to activation of the N-methyl-D-aspartate (NMDA) receptor subtype by using two structurally distinct inhibitors, miconazole and N-methylsulfonyl-6-(2-propargyloxyphenyl) hexanamide (MS-PPOH), a selective epoxygenase substrate inhibitor. Drugs were delivered locally through microdialysis probes in striata of anesthetized rats. Local CBF was measured by hydrogen clearance and compared with CBF in contralateral striatum receiving vehicle. Microdialysis perfusion of NMDA doubled CBF and increased nitric oxide (NO) production estimated by recovery of labeled citrulline in the dialysate during labeled arginine infusion. Perfusion of miconazole or MS-PPOH blocked the increase in CBF without decreasing citrulline recovery. Perfusion of N(omega)-nitro-L-arginine decreased baseline CBF and inhibited the CBF response to NMDA. Perfusion of MS-PPOH did not inhibit the CBF response to sodium nitroprusside. We conclude that both the P-450 epoxygenase and NO synthase pathways are involved in the local CBF response to NMDA receptor activation, and that the signaling pathway may be more complex than simply NO diffusion from neurons to vascular smooth muscle.

Neuroprotective FK506 does not alter in vivo nitric oxide production during ischemia and early reperfusion in rats

BACKGROUND AND PURPOSE

Previous studies have demonstrated that the immunosuppressant FK506 provides neuroprotection in experimental brain injury and suggest that this action may be mediated by suppression of neuronal nitric oxide synthase activation that occurs after ischemic depolarization. We sought to determine whether FK506 reduces histological injury after middle cerebral artery occlusion (MCAO) in the rat and whether the neuroprotective effect is mediated via suppression of in vivo nitric oxide (NO) production during ischemia or early reperfusion.

METHODS

Under controlled conditions of normoxia, normocarbia, and normothermia, halothane-anesthetized male Wistar rats were subjected to 2 hours of MCAO by the intraluminal occlusion technique in a blinded, randomized experimental trial. Ipsilateral parietal cortical laser-Doppler flowmetry was monitored throughout ischemia. Animals were randomly assigned to 4 pretreatment groups: intravenous FK506 0.3 mg/kg or 1. 0 mg/kg, vehicle (cremaphor), or an equivalent volume of saline administered 30 minutes before MCAO. Infarction volume was assessed by a triphenyltetrazolium chloride staining at 22 hours of reperfusion. In separate experiments, microdialysis probes were placed bilaterally into the striatum. Rats were perfused with artificial cerebrospinal fluid containing 3 micromol/L [14C]- L-arginine for 3 hours and then subjected to 2 hours of right MCAO. Intravenous 0.3 mg/kg FK506 or cremaphor was given 30 minutes before right MCAO. Right-left differences between [14C]-L-citrulline in the effluent were assumed to reflect differences in NO production.

RESULTS

All values are mean+/-SE. FK506 at 0.3 mg/kg reduced infarction volume in cortex: 40+/-12 mm3 compared with saline (109+/-15 mm3) and cremaphor vehicle (148+/-23) (P<0.05). Striatal infarction was also reduced by low-dose FK506: 16+/-4 mm3 versus 36+/-4 mm3 and 34+/-4 mm3 in saline and vehicle groups, respectively (P<0.05). High-dose treatment reduced infarction volume in cortex (61+/-14 mm3, P<0.05 from saline and vehicle groups) and in striatum (22+/-5 mm3, P<0.05 from saline and vehicle groups). [14C]-L-citrulline recovery via microdialysis was markedly enhanced in ischemic compared with nonischemic striatum. However, ischemia-evoked [14C]-L-citrulline recovery was not different in FK506-treated rats compared with vehicle-treated animals.

CONCLUSIONS

These data demonstrate that FK506 provides robust neuroprotection against transient focal cerebral ischemia in the rat. The mechanism of protection in vivo is not through attenuation of ischemia-evoked NO production during MCAO and early reperfusion.

Potent ς 1 -Receptor Ligand 4-Phenyl-1-(4-Phenylbutyl) Piperidine Modulates Basal and N -Methyl- d -Aspartate–Evoked Nitric Oxide Production In Vivo

Background and Purpose —ς-Receptor ligands ameliorate ischemic neuronal injury and modulate neuronal responses to N -methyl- d -aspartate (NMDA) receptor stimulation. Because NMDA-evoked synthesis of nitric oxide (NO) may play an important role in excitotoxic-mediated injury, we tested the hypothesis that ς-receptor ligands attenuate basal and NMDA-evoked NO production in the striatum in vivo.

Methods —Microdialysis probes were placed bilaterally into the striatum of halothane-anesthetized adult Wistar rats. Rats were divided into 7 treatment groups and perfused with artificial cerebrospinal fluid (aCSF) containing 3 μmol/L [ 14 C] l -arginine for 2 to 3 hours followed by NMDA in various combinations with the following drugs: l -nitroarginine (L-NNA); the ς 1 -receptor ligand 4-phenyl-1-(4-phenylbutyl) piperidine (PPBP); the selective ς 1 -receptor antagonist 1-(cyclopropylmethyl)-4-(2′-oxoethyl) piperidine hydrobromide (DuP 734); and the noncompetitive NMDA receptor blocker MK-801 in aCSF. Right-left differences between [ 14 C] l -citrulline in the effluent from rats treated with different drug combinations were assumed to reflect differences in NO production.

Results —After a 3-hour loading period with [ 14 C] l -arginine, addition of 1 mmol/L NMDA increased [ 14 C] l -citrulline recovery compared with aCSF alone. This NMDA-evoked increase was inhibited by 1 mmol/L of L-NNA and PPBP. Perfusion of 1 mmol/L of the ς 1 -receptor antagonist DuP 734 with 1 mmol/L PPBP augmented NMDA-evoked [ 14 C] l -citrulline recovery compared with perfusion with PPBP and NMDA. MK-801 attenuated the basal as well as NMDA-evoked [ 14 C] l -citrulline recovery. PPBP did not cause any further attenuation in the basal and NMDA-evoked [ 14 C] l -citrulline recovery in the presence of MK-801.

Conclusions —These data indicate that a ς 1 -receptor ligand attenuates basal as well as NMDA-evoked NO production. Because the attenuated NO production was reversed by DuP 734, PPBP appears to act as an agonist at the ς 1 -receptor. Attenuated NO production by ς 1 -receptor agonists provides one possible mechanism for focal ischemic neuroprotection.

Neurochemical and morphological responses to acutely and chronically implanted brain microdialysis probes

The purpose of this study was to compare, in rats, brain microdialysis results obtained using microdialysis probes implanted acutely for 2 h versus probes implanted chronically for 24 h in the caudate. Specific comparisons included: (1) dialysate purine and amino acid profiles during cerebral ischemia; (2) diffusional characteristics of the microdialysis probe; and (3) tissue morphology surrounding the probe. During ischemia, the increase in dialysate levels of adenosine, inosine, and hypoxanthine was less pronounced from probes implanted chronically, while dialysate xanthine levels increased to a greater extent. An increase in dialysate amino acid neurotransmitters during cerebral ischemia was observed in the acutely implanted probes within 10 min of the onset of cerebral ischemia; in the chronically implanted probes this increase did not occur until after 50 min of severe ischemia. Both in vitro and in vivo tests revealed a diffusional barrier in chronically implanted probes. Moreover, the tissue surrounding chronically implanted probes exhibited a high degree of inflammation, and fibrin deposits were substantial. In addition, uric acid levels (an indicator of tissue injury) sampled from chronically implanted probes were 7-fold greater than levels sampled from acutely implanted probes. These data raise concerns about the use of chronically implanted microdialysis probes for the measurement of purine and amino acid profiles during cerebral ischemia.

Role of adenosine and its receptors in the vasodilatation induced in the cerebral cortex of the rat by systemic hypoxia

1. In anaesthetized rats, we have examined the role of adenosine in vasodilatation evoked in the cerebral cortex by systemic hypoxia (breathing 8 % O2). Red cell flux was recorded from the surface of the exposed parietal cortex (CoRCF) by a laser Doppler probe, cortical vascular conductance (CoVC) being computed as CoRCF divided by mean arterial blood pressure. All agonists and antagonists were applied topically to the cortex. 2. Systemic hypoxia or adenosine application for 5 or 10 min, respectively, induced an increase in CoRCF and CoVC. These responses were substantially reduced by 8-phenyltheophylline (8-PT), an adenosine receptor antagonist which is non-selective between the adenosine A1 and A2A receptor subtypes. By contrast, the adenosine receptor antagonist 8-sulphophenyltheophylline (8-SPT) which is similarly non-selective, but unlike 8-PT, does not cross the blood-brain barrier, reduced the increases in CoRCF and CoVC induced by adenosine, but had no effect on those induced by hypoxia. 3. The A2A receptor agonist CGS21680 produced a substantial increase in CoRCF and CoVC, but the A1 receptor agonist 2-chloro-N6-cyclopentyladenosine had minimal effects. 4. The A2A receptor antagonist ZM241385 reduced the increase in CoRCF and CoVC induced by adenosine and reduced the increase in CoRCF induced by hypoxia. 5. We propose that exogenous adenosine that is topically applied to the cerebral cortex produces vasodilatation by acting on A2A receptors on the vascular smooth muscle. However, during systemic hypoxia, we propose that adenosine is released from endothelial cells and acts on endothelial A2A receptors to produce the major part of the hypoxia-induced dilatation in the cerebral cortex.

Characterization of ionotropic glutamate receptor-mediated nitric oxide production in vivo in rats

BACKGROUND AND PURPOSE

Glutamate receptor activation can stimulate nitric oxide (NO) production and possibly play a role in long-term potentiation and excitotoxic-mediated injury. We studied the differential effect of agonist-induced activation of ion channel-linked N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subtypes on NO production in vivo in rat hippocampus. We also studied whether dantrolene, a ryanodine calcium channel inhibitor previously shown to attenuate metabotropic glutamate receptor stimulation of NO production, also attenuated ionotropic glutamate receptor-mediated stimulation of NO production.

METHODS

Microdialysis probes were placed bilaterally into the CA3 region of the hippocampus of pentobarbital-anesthetized adult Sprague-Dawley rats and were perfused for 5 hours with artificial cerebrospinal fluid (CSF) containing 3 mumol/L [14C]L-arginine. Recovery of [14C]L-citrulline in the effluent was used as a marker of NO production. In 13 groups of rats, increases in [14C]L-citrulline recovery were compared between right- and left-sided probes perfused with no additional drugs versus combinations of NMDA, AMPA, the NO synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME), the non-competitive glutamate receptor blocker MK-801, the AMPA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and dantrolene.

RESULTS

Recovery of [14C]L-citrulline during perfusion with artificial CSF progressively increased to 272 +/- 73 fmol/min (+/-SEM) over 5 hours. Contralateral perfusion with 1 mmol/L L-NAME inhibited [14C]L-citrulline recovery. Perfusion with 1 mmol/L MK-801 or 1 mmol/L CNQX reduced [14C]L-citrulline recovery compared with contralateral perfusion with CSF alone. Perfusion with 1 mmol/L NMDA enhanced [14C]L-citrulline recovery, and this enhancement was attenuated by L-NAME, MK-801, and CNQX but not by dantrolene. Perfusion with 1 mmol/L AMPA enhanced [14C]L-citrulline recovery, and this enhancement was also attenuated by L-NAME, MK-801, and CNQX but not by dantrolene.

CONCLUSIONS

Through an indirect method of assessing NO production in vivo, results with MK-801 and CNQX indicate that NMDA and AMPA receptor activation contribute to basal NO production in the rat hippocampus. Enhanced NO production with NMDA and AMPA agonists appears to involve a complex neuronal interaction because the effect of NMDA was attenuated by both MK-801 and CNQX and because the effect of AMPA was attenuated by both CNQX and MK-801. In contrast to metabotropic glutamate receptor activation, release of calcium from intracellular ryanodine calcium channels does not appear to be a prominent mediator of ionotropic glutamate receptor stimulation of NO production.

Examination of potential mechanisms in the enhancement of cerebral blood flow by hypoglycemia and pharmacological doses of deoxyglucose

Cerebral blood flow (CBF) rises when the glucose supply to the brain is limited by hypoglycemia or glucose metabolism is inhibited by pharmacological doses of 2-deoxyglucose (DG). The present studies in unanesthetized rats with insulin-induced hypoglycemia show that the increases in CBF, measured with the [14C]iodoantipyrine method, are relatively small until arterial plasma glucose levels fall to 2.5 to 3.0 mM, at which point CBF rises sharply. A direct effect of insulin on CBF was excluded; insulin administered under euglycemic conditions maintained by glucose injections had no effects on CBF. Insulin administration raised plasma lactate levels and decreased plasma K+ and HCO3- concentrations and arterial pH. These could not, however, be related to the increased CBF because insulin under euglycemic conditions had similar effects without affecting CBF; furthermore, the inhibition of brain glucose metabolism with pharmacological doses (200 mg/kg intravenously) of DG increased CBF, just like insulin hypoglycemia, without altering plasma lactate and K+ levels and arterial blood gas tensions and pH. Nitric oxide also does not appear to mediate the increases in CBF. Chronic blockade of nitric oxide synthase activity by twice daily i.p. injections of NG-nitro-L-arginine methyl ester for 4 days or acutely by a single i.v. injection raised arterial blood pressure and lowered CBF in normoglycemic, hypoglycemic, and DG-treated rats but did not significantly reduce the increases in CBF due to insulin-induced hypoglycemia (arterial plasma glucose levels, 2.5-3 mM) or pharmacological doses of deoxyglucose.

Adenosine physiology and pharmacology: how about A2 receptors?

Adenosine participates in the physiology of central and peripheral tissues through several subtypes of G-protein-coupled receptors. Positively linked to adenylate cyclase, A2 receptors have been subdivided into A2a and A2b sites on the basis of their molecular, biochemical and pharmacological properties. They exhibit selective distribution, and are implicated in the modulation of psychomotor activity, circulation, respiration, and metabolism. Recent data support the evidence that adenosine A2 receptor properties may prove useful in future drug development, and selective manipulation of receptor-associated biologic effects might be relevant in the treatment of various disorders, including psychiatric diseases, hypoxia/ischemia, inflammation or erythrocytosis.

Effects of repeated administration of selective adenosine A1 and A2A receptor agonists on pentylenetetrazole-induced convulsions in the rat

The protective effects of the selective adenosine A1 receptor agonist 2-chloro-N(6)-cyclopentyladenosine (CCPA), the selective adenosine A2A receptor agonist, 2-hexynyl-5'-N-ethylcarboxamidoadenosine (2HE-NECA), and the non-selective agonist, 5'-N-ethylcarboxamidoadenosine (NECA) were studied against lethal seizures induced by intraperitoneal (i.p.) injection of pentylenetetrazole (80 mg/kg). In acute studies there was a dose-dependent reduction of lethal seizures, as shown by the low dose's protecting 50% of animals (PD50): 0.11, 0.05 and 0.05 mg/kg i.p. for CCPA, 2HE-NECA and NECA, respectively. In the repeated administration studies the animals received either vehicle or drug i.p. twice daily for 12 days. The drug doses were twice the PD50 value: 0.3 mg/kg for CCPA or 0.1 mg/kg for both 2HE-NECA and NECA. 2HE-NECA and NECA maintained their protective activity against pentylenetetrazole-induced seizures (63% or 60% vs. 60% or 58% in acute studies, respectively). Conversely, repeated treatment with CCPA resulted in a marked decrease of its effects (67% vs. 30% in acute studies; P < 0.05). The data indicate that in addition to adenosine A1 the A2A receptors also appear to be involved in the protection from seizures. The anticonvulsant effects induced by repeated stimulation of adenosine A1 receptors are subject to tolerance, whereas effects depending on adenosine A2A receptor activation are maintained.

Effects of adenosine and 2-chloroadenosine on cerebral collateral vessels

Adenosine is a potent cerebral vasodilator. We tested the hypothesis that dilatation of collateral vessels in cerebrum, in response to topical adenosine and 2-chloroadenosine (2-CAD), would increase blood flow to collateral-dependent cerebrum. In dogs anesthetized with halothane, a branch of the middle cerebral artery (MCA) was occluded proximally and cannulated distally. The collateral-dependent area at risk for infarction was perfused from a reservoir with microsphere-free blood, and blood flow to normal cerebrum and to cerebrum dependent on collateral flow was measured with radioactive microspheres injected into the left ventricle through a femoral artery catheter. Perfusion through the cannulated MCA branch was stopped, and flow to normal and collateral-dependent cerebrum was measured after adenosine (10(-2) M) or 2-CAD (10(-4) M) was added to the superfusate over the cerebrum. In normal cerebrum, topical application of adenosine increased flow to outer but not inner layers. Topical application of adenosine had little effect on flow to collateral-dependent tissue. In normal cerebrum, 2-CAD increased flow to outer layers, whereas flow to inner layers tended to increase. During 2-CAD, flow to outer cortical layers of collateral-dependent cerebrum increased from 140 +/- 20 ml/100 g/min (mean +/- SD) to 231 +/- 68, whereas flow to the inner collateral-dependent tissue did not change. The findings indicate that, after occlusion of a cerebral artery, topical 2-CAD increases blood flow to outer layers of collateral-dependent and normal cerebrum.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine modulates N-methyl-D-aspartate-stimulated hippocampal nitric oxide production in vivo

BACKGROUND AND PURPOSE

Adenosine acts presynaptically to inhibit release of excitatory amino acids (EAAs) and is thus considered to be neuroprotective. Because EAA-stimulated synthesis of nitric oxide (NO) may play an important role in long-term potentiation and excitotoxic-mediated injury, we tested the hypotheses that adenosine agonists attenuate basal and EAA-induced NO production in the hippocampus in vivo and that adenosine A1 receptors mediate this response.

METHODS

Microdialysis probes were placed bilaterally into the CA3 region of the hippocampus of adult Sprague-Dawley rats under pentobarbital anesthesia. Probes were perfused for 5 hours with artificial cerebrospinal fluid containing 3 mumol/L [14C]L-arginine. Recovery of [14C]L-citrulline in the effluent was used as a marker of NO production. In 10 groups of rats, time-dependent increases in [14C]L-citrulline recovery were compared between right- and left-sided probes perfused with various combinations of N-methyl-D-aspartate (NMDA), adenosine agonists, adenosine antagonists, and the NO synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME).

RESULTS

Recovery of [14C]L-citrulline during perfusion with artificial cerebrospinal fluid progressively increased to 141 +/- 27 fmol/min (+/- SEM) over 5 hours. Contralateral perfusion with 1 mmol/L NMDA augmented [14C]L-citrulline recovery to 317 +/- 62 fmol/min. Perfusion of 1 mmol/L L-NAME with NMDA inhibited [14C]L-citrulline recovery compared with NMDA alone. Perfusion with 0.1 mmol/L 2-chloroadenosine attenuated basal as well as NMDA-enhanced [14C]L-citrulline recovery. This action of 2-chloroadenosine was reversed by infusion of 0.1 mmol/L 8-cyclopentyl-1,3-dipropylxanthine, a specific A1 receptor antagonist. Infusion of 0.1 mmol/L (2S)-N6-[2-endo-norboryl]adenosine, a specific A1 receptor agonist, also attenuated the 0.1 mmol/L and 1 mmol/L NMDA-enhanced [14C]L-citrulline recovery.

CONCLUSIONS

Using an indirect method of assessing NO production in vivo, these data are consistent with in vitro results showing that NMDA receptor stimulation enhances NO production. Furthermore, we conclude that stimulation of A1 receptors can attenuate the basal as well as NMDA-induced production of NO. Because NMDA receptor stimulation amplifies glutamate release, our data are consistent with presynaptic A1 receptor-mediated inhibition of EAA release and consequent downregulation of NO production.

Salutary effects of exogenous adenosine administration on in vivo myocardial stunning

Augmentation of endogenous adenosine levels is associated with decreased myocardial ischemic-reperfusion injury. The purpose of this study was to determine whether exogenous adenosine administered before ischemia could attenuate postischemic myocardial dysfunction. Regional myocardial stunning was induced by 15 minutes of coronary artery occlusion and 90 minutes of reperfusion in an open-chest canine preparation. Regional ventricular function was assessed by measurement of systolic wall thickening. Control untreated hearts were compared with two groups of hearts treated immediately before ischemia with intracoronary adenosine (5 micrograms/kg per minute and 50 micrograms/kg per minute). A fourth group of hearts was treated for the first 30 minutes of reperfusion with adenosine (50 micrograms/kg per minute). Preischemic adenosine administration increased coronary flow sixfold to sevenfold without altering regional function, mean arterial pressure, or left ventricular end-diastolic pressure. Both adenosine pre-treatments attenuated stunning compared with results in control animals (14.7% +/- 5.1% and 21.6% +/- 7.3% of preischemic systolic wall thickness versus -14.0% +/- 10%). Adenosine treatment during reperfusion transiently increased function in parallel with increased coronary blood flow, but after termination of the infusion regional function was not different from that in control stunned hearts (-5.0% +/- 13.1% of preischemic systemic wall thickness). These results indicate that adenosine pretreatment is associated with attenuation of stunning, an effect that can be produced at doses that do not alter systemic hemodynamics.

Effects of selective adenosine A1 and A2 receptor agonists and antagonists on local rates of energy metabolism in the rat brain

The quantitative [14C]2-deoxyglucose autoradiographic technique was applied to the measurement of the cerebral metabolic effects of adenosine A1 and A2 receptor agonists and antagonists in adult rats. The adenosine A1 receptor agonist and antagonist, 2-chloro-N6-cyclopentyladenosine (CCPA) and 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) as well as the adenosine A2 receptor agonist, 2-[p-(2-carboxyethyl)phenylethylamino]-5'-ethylcarboxamidoadenosin e (CGS 21680), were injected at the dose of 0.01 mg/kg. The adenosine A2 receptor antagonist, 3,7-dimethyl-1-proparglyxanthine (DMPX) was injected at the dose of 0.3 mg/kg. These doses were chosen in accordance with the known affinity of the drugs for their respective receptor and to avoid peripheral effects. The adenosine A1 receptor agonist, CCPA, induced decreases in glucose utilization in three brain areas, the globus pallidus and two hypothalamic nuclei. The adenosine A2 receptor agonist, CGS 21680, induced more general depressant effects on energy metabolism which were significant in 17 brain areas, such as cerebral cortex, hippocampal and white matter regions plus motor and limbic structures. The adenosine A2 receptor antagonist, DMPX, decreased glucose utilization in the globus pallidus while increasing energy metabolism in the cochlear nucleus. The adenosine A1 receptor antagonist, DPCPX, depressed glucose utilization in the globus pallidus and dentate gyrus, and increased rates of energy metabolism in six regions, mainly hypothalamic, thalamic areas and in the cochlear nucleus. There was a mismatch between cerebral metabolic consequences of adenosine A1 and A2 receptor agonists and the localization of corresponding adenosine receptors. The metabolic effects of the adenosine A2 receptor agonist and antagonist were consistent with the known involvement of that type of receptor in the control of locomotion and its effects on neuronal firing in the hippocampus and cerebral cortex. The effects of the adenosine A1 receptor agonist were very discrete and mostly related to the transient decrease in blood pressure induced by the drug. The increases in glucose utilization induced in limbic regions by the adenosine A1 receptor antagonist are probably linked to the regulation by adenosine of arousal and cardiorespiratory function. These results are in good agreement with the neuroregulatory function of the adenosine system as previously shown by other methods.

Changes in extracellular adenosine during chemical or electrical brain stimulation

The purpose of this study was to determine the changes in adenosine and adenosine metabolites during graded electrical stimulation or kainic acid-induced activation and to assess the role of adenosine in the cerebral blood flow (CBF) response to increased brain activity. A modified brain microdialysis technique was used to sample cerebral interstitial fluid (ISF), deliver drugs locally to the brain, electrically stimulate the brain, and measure local CBF (H2 clearance). Microdialysis probes were implanted bilaterally in the caudate nuclei of ketamine-anesthetized rats. Graded electrical stimulation at 5, 15, and 30 Hz increased dialysate adenosine 1.5-fold, 2.3-fold, and 4.7-fold, respectively. Local infusion of kainic acid, an agonist of the excitatory amino acid neurotransmitter glutamate, produced a transient increase (2-fold) in dialysate adenosine and sustained increases in dialysate inosine (2-fold), hypoxanthine (4-fold) and CBF (2.4-fold). When the adenosine receptor antagonist 8(p-sulphophenyl)-theophylline (SPT, 10(-3) M) was co-administered with kainic acid, CBF increased only 1.6-fold, while the increase in dialysate adenosine was augmented by 40%. These data demonstrate that ISF adenosine increases during brain activation and suggest that adenosine contributes to active hyperemia in the brain.

Increases in interstitial adenosine and cerebral blood flow with inhibition of adenosine kinase and adenosine deaminase

The purpose of this study was to determine the changes in interstitial fluid (ISF) adenosine and cerebral blood flow (CBF) during inhibition of adenosine kinase or adenosine deaminase. Brain microdialysis was used to (a) measure CBF (H2 clearance), (b) sample cerebral ISF, and (c) deliver drugs locally to the brain. Microdialysis probes were implanted bilaterally in the caudate nucleus of halothane-anesthetized rats (n = 11). One probe was perfused with artificial cerebrospinal fluid (CSF) containing iodotubercidin (IODO), an adenosine kinase inhibitor, while the other probe was perfused with erythro-2-(2-hydroxy-3-nonyl)adenine (EHNA), an adenosine deaminase inhibitor. Both probes were subsequently perfused with EHNA+IODO. Finally, 8-(p-sulfophenyl)theophylline (SPT), an adenosine receptor antagonist, was added to EHNA + IODO in one probe, while the other probe continued to receive only EHNA + IODO. CBF and dialysate adenosine levels increased with either EHNA or IODO; however, the increases were greater with IODO. EHNA + IODO further increased CBF and dialysate adenosine. The hyperemia observed with EHNA + IODO was abolished by adenosine receptor blockade. These data suggest that basal adenosine levels are influenced to a greater extent by adenosine kinase than by adenosine deaminase. In addition, the increased CBF observed with inhibition of adenosine metabolism and the attenuation of this vasodilatory response with adenosine receptor blockade support a role for adenosine in CBF regulation.

Attenuation of ischemia-induced extracellular adenosine accumulation by homocysteine

The purpose of this study was to determine the effects of homocysteine, which consumes intracellular adenosine via formation of S-adenosylhomocysteine, on interstitial fluid (ISF) adenosine and cerebral blood flow (CBF) before, during, and after cerebral ischemia. Microdialysis probes, used to measure local CBF (H2 clearance) and to sample ISF, were implanted bilaterally into the caudate nucleus of halothane-anesthetized rats (n = 8). L-Homocysteine thiolactone was administered locally via one of the probes. Animals were exposed to 20 min of ischemia, induced by bilateral carotid occlusion plus hemorrhage to an arterial blood pressure of 50 mm Hg, followed by 60 min of reperfusion. Before ischemia, CBF and dialysate adenosine were decreased with homocysteine. During ischemia and early reperfusion, dialysate purine metabolites increased on both sides of the brain; however, the ischemia-induced increase in adenosine was attenuated on the side of local homocysteine. CBF was lower on the side of homocysteine throughout reperfusion. These data demonstrate that homocysteine (a) decreases basal ISF adenosine and CBF, (b) attenuates the increase in dialysate adenosine during ischemia, and (c) reduces hyperemia during early reperfusion.

Sciatic nerve stimulation does not increase endogenous adenosine production in sensory-motor cortex

Adenosine participates in the coupling of cerebral blood flow to oxygen consumption in the brain during such stimuli as hypoxia, ischemia, and seizures. It has been suggested that it also participates in the regulation of cerebral blood flow during somatosensory stimulation, a condition during which cerebral blood flow and oxygen consumption appear to be uncoupled. Interstitial adenosine was estimated by the microdialysis technique and cerebral blood flow was measured by hydrogen clearance in the hindlimb sensory-motor cortex during sciatic nerve stimulation. Cerebral blood flow increased from 102 to 188 ml min-1 100 g-1 (p less than 0.001) in the cortex contralateral to the stimulated leg without an associated increase in interstitial adenosine (baseline 0.624 microM, stimulation 0.583 microM). Infusion of the adenosine antagonist 8-sulfophenyltheophylline failed to block an increase in cerebral blood flow during central sciatic nerve stimulation, but decreased basal cerebral blood flow (69 ml min-1 100 g-1). These results suggest that adenosine does not mediate changes in cerebral blood flow during somatosensory stimulation, but may participate in the regulation of cerebral blood flow in the basal state.

Competitive inhibition of nitric oxide synthase prevents the cortical hyperemia associated with peripheral nerve stimulation

With combined microdialysis and hydrogen clearance techniques for simultaneous local delivery of drugs and blood-flow measurement in the rat hindlimb sensory-motor cortex, we examined the role of nitric oxide in cerebral blood-flow regulation during sciatic nerve stimulation. Infusion of 1 mM nitric oxide synthase antagonist, N eta-nitro-L-arginine methyl ester (L-NAME), blocked the cortical blood-flow response to sciatic nerve stimulation (152 +/- 43 ml.min-1.100 g-1 of tissue in controls and 73 +/- 11 ml.min-1.100 g-1 in the presence of L-NAME; P less than 0.05). Addition of 10 mM L-arginine to the dialysate containing L-NAME partially restored the hyperemic response to nerve stimulation (125 ml.min-1.100 g-1). L-NAME also produced a decrease in baseline cerebral blood flow when compared with the control (66 +/- 14 ml.min-1.100 g-1 vs. 93 +/- 25 ml.min-1.100 g-1). We conclude that nitric oxide from activated neurons participates in the local regulation of cortical blood flow in response to sciatic nerve stimulation and also in the maintenance of basal cortical blood flow.

Effect of 2-chloroadenosine on cerebrovascular reactivity to hypercapnia in newborn pig

The effect of local administration of vasodilative concentrations of the adenosine receptor agonist 2-chloroadenosine (2-CADO) on the hyperemic responses of the pial and parenchymal microcirculations to graded hypercapnia was determined. The cranial window and brain microdialysis-hydrogen clearance techniques were utilized in two groups of isoflurane-anesthetized newborn pigs to measure changes in pial diameters and local CBF, respectively, in response to graded hypercapnia in the absence and presence of 2-CADO. Progressive size-dependent dilations of pial arterioles [small = 41 +/- 7 microns (mean +/- SD), intermediate = 78 +/- 13 microns, and large = 176 +/- 57 microns in diameter] occurred in response to graded hypercapnia alone (PaCO2 = 58 and 98 mm Hg) and to superfusions of 2-CADO (10(-5) M) during normocapnia; the magnitude of the dilative response to each of these stimuli was inversely proportional to vessel size. When hypercapnia was induced concomitantly with 2-CADO superfusion, the dilative effects of each stimulus were directly additive. Similarly, local microdialysis infusion of 10(-5) M 2-CADO, which doubled CBF during normocapnia, did not affect the hyperemic response of the parenchymal circulation to graded hypercapnia (PaCO2 = 69 and 101 mm Hg). Our findings are consistent with the participation of adenosine in the mediation of cerebral hypercapnic hyperemia. If, however, adenosine is not involved in this dilative response, our results indicate that concomitant vascular and neuromodulatory actions induced by adenosine receptor stimulation do not affect the mechanism responsible for the hypercapnic hyperemic response.

Adenosine receptor blockade augments interstitial fluid levels of excitatory amino acids during cerebral ischemia

The excitotoxic hypothesis suggests that cerebral ischemic damage results in part from the accumulation of the excitatory and potentially toxic neurotransmitters glutamate and aspartate. Adenosine, which also increases during cerebral ischemia, is proposed to inhibit neurotransmitter release. The purpose of this study was to determine if adenosine receptor blockade exacerbates the accumulation of glutamate and aspartate during cerebral ischemia. Microdialysis probes, implanted bilaterally in the caudate nucleus of halothane-anesthetized rats, were used to (1) assess changes in interstitial fluid (ISF) glutamate, aspartate, adenosine, and adenosine metabolites; (2) measure local cerebral blood flow (H2 clearance); and (3) deliver 8-(p-sulfophenyl)theophylline (SPT), an adenosine receptor antagonist, locally to the brain. The probe on one side of the brain was perfused with artificial cerebrospinal fluid (CSF) containing 10(-3) M SPT, while the probe on the opposite side received only artificial CSF. Animals were exposed to 20 min of ischemia (carotid occlusion+arterial blood pressure = 50 mm Hg) followed by 60 min of reperfusion. Dialysate glutamate and aspartate increased during and after cerebral ischemia, but were increased to a greater extent in the presence of adenosine receptor blockade. Likewise, the increase in dialysate adenosine and adenosine metabolites was enhanced on the side of locally administered SPT. These data suggest that endogenous adenosine attenuates the accumulation of glutamate and aspartate during cerebral ischemia.

Evoked neuronal activity accompanied by transmitter release increases oxygen concentration in rat striatum in vivo but not in vitro

Dopamine and oxygen (O2) were measured in the caudate nucleus of anesthetized rats and in striatal slices during electrical stimulation. Simultaneous electrochemical detection of dopamine and O2 was accomplished with fast-scan cyclic voltammetry at a Nafion-coated carbon-fiber microelectrode. Stimulation of the medial forebrain bundle resulted in synaptic overflow of dopamine in the caudate nucleus. At the same time, O2 concentration increased in the extracellular fluid with two separate phases. The amplitude of the initial increase directly correlated with the frequency of the stimulus, with the time of maximum concentration reproducible across a range of frequencies. The second increase occurred at later times with a more random amplitude and with a broad, variable shape. Agents which blocked vasodilation affected both phases: atropine attenuated the initial increase, while the second feature was nearly absent after theophylline. Yohimbine and alpha-methyl-p-tyrosine did not affect the O2 responses. Local electrical stimulation of the slice preparation also resulted in dopamine overflow, but a prolonged decrease in O2 concentration accompanied this event. Striatal field stimulation in vivo produced changes in O2 concentration dependent on the relative position of the stimulating and working electrodes, but none of the responses resembled that seen in the caudate slice. Thus, while measurements in brain slices show O2 consumption as a result of stimulated neuronal activity, an apparent elevation of local cerebral blood flow during and after stimulation dominate the in vivo response.

Local cerebral blood flow response to locally infused 2-chloroadenosine during hypotension in piglets

Brain interstitial adenosine increases during hypotension in piglets. If adenosine is to participate in the regulation of neonatal cerebral blood flow (CBF) during hypotension, it must retain its vasodilatory action under that condition. To examine this issue, we studied the effects of locally infused 2-chloroadenosine (2-CADO), a stable adenosine analog, on local CBF in the piglet frontal cortex during normotension and graded hemorrhagic hypotension. We used the modified brain microdialysis/hydrogen clearance technique to simultaneously infuse 2-CADO into the frontal cortex and measure local CBF from the same area. When 2-CADO from 10(-8) M to 10(-3) M was infused under control conditions (n = 7), CBF increased 61% at 10(-5) M, 167% at 10(-4) M, and 210% at 10(-3) M. In hypotension experiments, local infusion of 10(-5) M 2-CADO (n = 8) caused significant increases in CBF (P less than 0.05) under control conditions (MABP = 65 mmHg) and at hypotensive blood pressures of 55 mmHg and 44 mmHg, respectively. At a blood pressure of 33 mmHg, however, infusion of the analog failed to increase CBF. Local infusion of 10(-3) M 2-CADO also produced a similar change in CBF during graded hypotension. These results indicate that 2-CADO dilates intracerebral vessels during normotension, and mild and moderate hypotension, and support the hypothesis that endogenous adenosine mediates autoregulatory adjustments of CBF during hypotension in newborn piglets.

Physiological and pharmacological properties of adenosine: therapeutic implications

Adenosine is a nucleoside which has been shown to participate in the regulation of physiological activity in a variety of mammalian tissues, and has been recognized as a homeostatic neuromodulator. It exerts its actions via membrane-bound receptors which have been characterized using biochemical, electrophysiological and radioligand binding techniques. Adenosine has been implicated in the pharmacological actions of several classes of drugs. A number of studies strongly suggest that the nucleoside may regulate cellular activity in many pathological disorders and, in that respect, adenosine derivatives appear as promising candidates for the development of new therapeutic compounds, such as anticonvulsant, anti-ischemic, analgesic and neuroprotective agents.

Effect of adenosine on total and regional cerebral blood flow of the newborn piglet

The effect of adenosine on total and regional CBF, measured by radiolabeled microspheres, was assessed in 16 anesthetized and ventilated newborn (1-3 days old) piglets. They received a ventriculocisternal perfusion containing either CSF alone (controls, n = 5) or CSF mixed with two different concentrations of adenosine (15 min each) randomly assigned using the following doses: 0.1 microM, 10 microM, 100 microM, 1 mM (n = 4), or 10 mM (n = 6). Mean CSF adenosine concentration (by HPLC) before perfusion was 0.6 +/- 0.4 microM. Total and regional CBF were not altered by the perfusion of CSF alone. All adenosine concentrations, except at low doses, increased total and regional CBF, without altering the cerebral metabolic rate for oxygen. Brainstem blood flow was increased by a mean of 110, 145, 306, and 378% with 10 microM, 100 microM, 1 mM, and 10 mM concentrations, respectively. Except for the highest concentration, CBF response was dose dependent in each region of the brain with the following order of potency: brainstem greater than periventricular area greater than telencephalon, midbrain, total brain, and cerebellum. These data indicate that, in the newborn, adenosine is a potent vasodilator of cerebral vessels. If the newborn brain can synthesize appropriate concentrations of adenosine, this nucleoside may play a major role in regional CBF regulation during the neonatal period.

Effects of caffeine, L-phenylisopropyladenosine and their combination on local cerebral blood flow in the rat

The quantitative [14C]iodoantipyrine autoradiographic method was applied to study the effects of acute administration of caffeine and L-phenylisopropyladenosine (LPIA) separately or in combination on local cerebral blood flow in the rat. After the injection of caffeine, cerebral blood flow rates were decreased in 13 out of the 61 structures studied, mainly in motor and auditory areas. The administration of LPIA induced a general decrease in local cerebral blood flow, which was significant in only 4 regions. The combined administration of caffeine and LPIA induced decreases in blood flow rates in 17 brain areas, motor, limbic and hypothalamic structures and increases in 3 limbic regions. The results confirm previous data on the effect of caffeine on cerebral circulation. The consequences of LPIA administration on blood flow may originate partly from peripheral effects and may also be the reflection of the reduction in the energy demand of the brain. Finally, LPIA also seems to be able to modulate caffeine effects on local cerebral blood flow when injected simultaneously with caffeine.