Effect of adenosine receptor blockade on pial arteriolar dilation during sciatic nerve stimulation.

In the present study, we report the effects of adenosine receptor antagonists on pial vasodilatation during contralateral sciatic nerve stimulation (SNS). The pial circulation was observed through a closed cranial window in alpha-chloralose-anesthetized rats. In artificial cerebrospinal fluid (CSF), SNS resulted in a 30.5 +/- 13.2% increase in pial arteriolar diameter in the hindlimb somatosensory cortex. Systemic administration of the selective adenosine A2A receptor antagonist, 4-(2-[7-amino-2-[2-furyl][3,2,4]triazolol[2,3-a][1,3,5]triazin-5-yl-amino] ethyl)phenol (ZM-241385), significantly (P < 0.05, n = 6) attenuated the SNS-induced vasodilatation. Systemic administration of 8-(p-sulfophenyl)theophylline (8SPT), a nonselective antagonist that is blood-brain barrier (BBB) impermeable, had no effect on vasodilatation to SNS. In contrast, systemic theophylline, which readily penetrates the BBB, nearly abolished the SNS-induced vasodilatation (P < 0.01; n = 7). Topical superfusion of 8SPT significantly (P < 0.01; n = 6) attenuated vasodilatation during SNS. Topical superfusion of 8- cyclopentyl-1,3-dipropylxanthine (DPCPX), a selective adenosine A1 receptor antagonist, significantly potentiated SNS-induced vasodilatation (P < 0.01; n > or = 5). Hypercarbic vasodilatation and somatosensory-evoked potentials were not affected by any of the compounds tested. Our findings suggest that luminal endothelial adenosine receptors are not involved in the arteriolar response to SNS, as demonstrated by a lack of effect with systemic 8SPT. Furthermore, the adenosine A2A receptor subtype appears to be involved in the dilator response to SNS. Finally, the neuromodulatory action of adenosine, via the A1 receptor subtype, significantly influences SNS-induced vasodilatation. Thus the present study provides further evidence for a role of adenosine in the regulation of CBF during somatosensory stimulation.

Receptor subtypes mediating adenosine-induced dilation of cerebral arterioles.

The purpose of this study was to investigate the receptor subtypes that mediate the dilation of rat intracerebral arterioles elicited by adenosine. Penetrating arterioles were isolated from the rat brain, cannulated with the use of a micropipette system, and luminally pressurized to 60 mmHg. Both adenosine and the A2A receptor-selective agonist CGS-21680 induced dose-dependent vasodilation (-logEC(50): 6.5 +/- 0.2 and 8.6 +/- 0.3, respectively). However, adenosine, which is capable of activating both A2A and A2B receptors, caused a greater maximal dilation than CGS-21680. The A2A receptor-selective antagonist ZM-241385 (0.1 microM) only partially inhibited the dilation induced by adenosine but almost completely blocked CGS-21680-induced dilation. Neither 8-cyclopentyl-1,3-dipropylxanthine (0.1 microM), an A1 receptor-selective antagonist, nor MRS-1191 (0.1 microM), an A3 receptor-selective antagonist, attenuated adenosine dose responses. Moreover, ZM-241385 had no effect on the dilation induced by ATP (10 microM) or acidic (pH 6.8) buffer. We concluded that the A2A receptor subtype mediates adenosine-induced dilation of intracerebral arterioles in the rat brain. Furthermore, our results suggest that A2B receptors may also participate in the dilation response to adenosine.

Clinically relevant rat model for testing BOLD functional MR imaging techniques by using single-shot echo-planar imaging at 1.5 T.

By using a 1.5-T whole-body magnetic resonance (MR) imager, a high-spatial-resolution single-shot echo-planar technique was developed to perform blood oxygen level dependent functional MR imaging of rat sensory cortex during forepaw stimulation. This technique produced cubic 1-mm(3) voxels. Signal-to-noise ratio was 140-160 (43-44 dB). Optimal effective echo time was 50 msec. This system should prove useful for developing new functional MR imaging techniques with rapid adaptation to human use.

Frequency-dependent changes in cerebral blood flow and evoked potentials during somatosensory stimulation in the rat.

Contrary to the concept of neuronal-vascular coupling, cortical evoked potentials do not always correlate with blood flow responses during somatosensory stimulation at changing stimulus rates. The goal of this study is to clarify the effects of stimulus frequency on the relationship between somatosensory evoked potentials (SEPs) and cerebral blood flow. In rats anesthetized with alpha-chloralose, we measured SEPs by signal-averaging field potentials recorded with an electrode placed on dura overlying the hindlimb somatosensory cortex. Regional blood flow was simultaneously assessed in the same region with a laser-Doppler flow (LDF) probe. The contralateral sciatic nerve was stimulated with 0.1 A pulses at the frequencies of 1, 2, 5, 10 and 20 Hz. SEPs (both P1 and N1 components) declined with increasing frequency regardless whether stimulus duration (20 s) or number (100) were kept constant, suggesting that frequency is an important determinant of neuronal activity. In contrast, LDF responses increased to a maximum at 5 Hz, and do not correlate with SEPs. Because CBF should reflect integrated neuronal activity, we computed the sum of SEPS (summation operatorSEP = SEP x stimulus frequency) as an index of total neuronal activity at each frequency. Summation operatorSEP indeed correlates positively (P<0.001) with LDF responses. Thus, during somatosensory stimulation at various frequencies, cerebral blood flow is coupled to integrated neuronal activity but not to averaged evoked potentials.

Suppression of somatosensory evoked potentials by nitric oxide synthase inhibition in rats: methodological differences.

We have previously shown that topically applied N(G)-nitro-L-arginine (L-NNA), a nitric oxide synthase (NOS) inhibitor, suppressed both somatosensory evoked potentials (SEPs) and vascular responses during sciatic nerve stimulation in rats. Due to the normal tight coupling between cerebral blood flow and neuronal activity, we surmise that the vascular response attenuation may be secondary to the SEP decrease. However, a recent study, in which SEPs were recorded with a 'non-contact' electrode placed longitudinally across the cranial window without touching the cortex, did not find a SEP decrease following NOS inhibition. In the present study, we compared SEPs recorded with 'contact' and 'non-contact' electrodes. Regardless of stimulation methods (sciatic nerve or hindpaw), an electrode in contact with the pial surface overlying the hindlimb somatosensory cortex recorded a steady SEP decline during I-NNA application. In contrast, a 'non-contact' electrode did not detect a significant SEP change in the presence of I-NNA. The present results thus confirm the attenuation of SEPs by NOS inhibition.

Neurotoxicity in organotypic hippocampal slices mediated by adenosine analogues and nitric oxide.

Adenosine (ADO) and nitric oxide (NO) have been implicated in a variety of neurophysiological actions, including induction of long-term potentiation, regulation of cerebral blood flow, and neurotoxicity/neuroprotection. ADO has been shown to promote NO release from astrocytes by a direct effect on A1 and A2 receptors, thus providing a link between actions of NO and adenosine in the brain. However, while adenosine acts as an endogenous neuroprotectant, NO is believed to be the effector of glutamate neurotoxicity. To resolve this apparent paradox, we have further investigated the effects of adenosine and NO on neuronal viability in cultured organotypic hippocampal slices exposed to sub-lethal (20') in vitro ischemia. Up to a concentration of 500 microM ADO did not cause toxicity while exposures to 100 microM of the stable ADO analogue chloroadenosine (CADO) caused widespread neuronal damage when paired to anoxia/hypoglycemia. CADO effects were significantly prevented by the ADO receptor antagonist theophylline and blockade of NO production by L-NA (100 microM). Moreover, CADO effects were mimicked by the NO donor SIN-1 (100 microM). Application of 100 microM ADO following blockade of adenosine deaminase (with 10 microM EHNA) replicated the effects of CADO. CADO, ADO + EHNA but not ADO alone caused a prolonged and sustained release of nitric oxide as measured by direct amperometric detection. We conclude that at high concentrations and/or following blockade of its enzymatic catabolism, ADO may cause neurotoxicity by triggering NO release from astrocytes. These results demonstrate for the first time that activation of pathways other than those involving neuronal glutamate receptors can trigger NO-mediated neuronal cell death in the hippocampus.

L-NNA suppresses cerebrovascular response and evoked potentials during somatosensory stimulation in rats.

We studied the local cerebrovascular response and somatosensory-evoked potentials (SEPs) to stimulation of the sciatic nerve during both short- (< 30 min) and long-term (90-150 min) exposure to topically applied NG-nitro-L-arginine (L-NNA). The pial circulation was visualized through a cranial window in alpha-chloralose-anesthetized rats. The diameter of pial arterioles (25-45 microns) and laser-Doppler flow (LDF) in the hindlimb sensory cortex were simultaneously measured during sciatic nerve stimulation. Short-term (< 30 min) treatment with L-NNA (1 mM) abolished the dilation of pial arterioles induced by acetylcholine, whereas the response to sciatic nerve stimulation was not affected. When applied for > 30 min, L-NNA induced severe vasomotion and attenuated the vascular responses to sciatic nerve stimulation. Long-term exposure to topically (1 mM) or systemically (10 mg/kg i.v.) applied L-NNA also attenuated cortical SEPs to sciatic nerve stimulation. Thus L-NNA-induced inhibition of vascular responses may be secondary to suppression of neuronal activity and an L-arginine metabolite, such as nitric oxide, may be involved in neurotransmission in the cerebral cortex during somatosensory activity.

Anesthetic-dependent pial arteriolar response to ethanol.

Anesthetic agents are often administered in the presence of ethyl alcohol, both in research and in the clinical setting. The authors tested the hypothesis that anesthetic agents may affect cerebrovascular responses to ethanol. A closed cranial window preparation in the rat was used to compare the response of pial arterioles to topically applied ethanol (0.01% to 1% vol/vol) in the presence of alpha-chloralose/urethane (50 and 600 mg/kg, respectively) or halothane (0.5% to 1%) anesthesia. Heart rate, mean arterial blood pressure, and blood gas levels were maintained stable and within the physiological range throughout each experiment. Ethanol induced significant vasoconstriction in alpha-chloralose/urethane-anesthetized animals (multivariate analysis of variance (MANOVA), p = 0.039); conversely, ethanol induced significant vasodilation of the pial arterioles in halothane-anesthetized animals (MANOVA, p = 0.017). These responses were significantly different from one another (MANOVA, p = 0.001). Thus, the choice of anesthetic agent alters the cerebrovascular response to ethanol, and care should be taken to ascertain the influence of anesthesia in both research and clinical settings.

Simultaneous measurements of pial arteriolar diameter and laser-Doppler flow during somatosensory stimulation.

We simultaneously measured pial arteriolar diameter and changes in cortical blood flow during activation of the somatosensory cortex by sciatic nerve stimulation. The pial vasculature was visualized with a closed-cranial window technique in chloralose-anesthetized rats (n = 13). Local blood flow was monitored with laser-Doppler flowmetry. During stimulation of the sciatic nerve (0.2 V, 5 Hz, 20 s), vascular diameter and laser-Doppler flow consistently displayed similar response profiles. With 0.5-ms stimulation pulses, the responses showed an initial peak followed by a smaller but sustained plateau dilation. In contrast, 5-ms pulses evoked a monotonically rising response. Our results support the concept that pial arteriolar diameter changes reflect cortical blood flow responses during somatosensory stimulation.

Effects of theophylline and cyclohexyladenosine on brain injury following normo- and hyperglycemic ischemia: a histopathologic study in the rat.

The present study was designed to determine the effects of theophylline, an adenosine receptor antagonist, and cyclohexyladenosine (CHA), an adenosine receptor agonist, on ischemic brain injury following normo- and hyperglycemic ischemia and reperfusion in fasted male Wistar rats. Moderate hyperglycemia was achieved by administering 17% D-glucose (3 g/kg i.p.), whereas normoglycemic animals received an equal volume of saline. The animals were further divided into two groups: One group was pretreated with either theophylline (0.20 mumol/g i.p.) or an equal volume of saline; the second group received either intraventricular CHA (6.25 nmol) or mock CSF prior to the onset of ischemia. During ischemia, pericranial temperature was maintained at 36 degrees C and EEG was monitored. Cerebral ischemia was induced for 15 min, after which flow was restored and the animals were allowed to recover completely. There were no significant differences in physiologic parameters among the groups studied. Five days following the ischemic episode, the rats were perfused with formalin and the brains subserially sectioned (8 microns) in the coronal plane and stained with celestine blue/acid fuchsin. Histopathologic analysis was performed in a blinded fashion to determine percentage of dead neurons. Hyperglycemic animals had significantly greater ischemic injury in CA1, cortex, and caudate than the normoglycemic group (p < 0.01). Moreover, rats pretreated with theophylline had a significantly (p < 0.01) higher percentage of dead neurons in CA1, cortex, and caudate than corresponding controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Moderate hyperglycemia affects ischemic brain ATP levels but not intracellular pH.

We used 31P nuclear magnetic resonance (NMR) spectroscopy to study the effect of moderate hyperglycemia on brain ATP and intracellular pH in a model of severe incomplete forebrain ischemia. Plasma glucose in the hyperglycemic rats was 277 +/- 9 mg/100 ml compared with 115 +/- 10 mg/100 ml in the normoglycemic rats at the onset of ischemia. After 15 min of ischemia, brain ATP levels decreased to 31 +/- 8% in normoglycemic rats vs. 63 +/- 11% in hyperglycemic rats (P < 0.05). Phosphocreatine levels were 31 +/- 9 and 55 +/- 8% for normoglycemic and hyperglycemic rats, respectively. Intracellular pH decreased to the same level (approximately 6.5) in both normoglycemic and hyperglycemic animals after 15 min of ischemia. In summary, we found that moderate hyperglycemia during severe incomplete forebrain ischemia significantly increases ischemic brain ATP levels but does not have a significant effect on intracellular pH. These results support the hypothesis that alterations in brain ATP and adenosine concentrations may be important in the pathogenesis of ischemic tissue injury under moderate hyperglycemic conditions, whereas alterations in tissue pH may be less important.

Changes in pial arteriolar diameter and CSF adenosine concentrations during hypoxia.

We measured the changes in pial arteriolar diameter and CSF concentrations of adenosine, inosine, and hypoxanthine during hypoxia in the absence and presence of topically applied dipyridamole (10(-6) M) and erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA; 10(-5) M). Closed cranial windows were implanted in halothane-anesthetized adult male Sprague-Dawley rats for the observation of the pial circulation and collection of CSF. The mean resting arteriolar diameter in mock CSF was 31.2 +/- 5.9 microns. Topically applied dipyridamole and EHNA, in combination, caused a slight but significant (p < 0.05) increase in resting arteriolar diameter (33.8 +/- 4.3 microns). With mock CSF, moderate hypoxia caused a 22.1 +/- 9.7% increase in pial vessel diameter. Topically applied dipyridamole and EHNA significantly (p < 0.01) potentiated pial arteriolar vasodilation in response to hypoxia. Moreover, the potentiating effects of dipyridamole and EHNA during hypoxia were completely abolished by theophylline (0.20 mumol/g, i.p.; p < 0.05), an adenosine receptor antagonist. Resting concentrations of adenosine, inosine, and hypoxanthine in the subwindow CSF were 0.18 +/- 0.09, 0.35 +/- 0.21, and 0.62 +/- 0.12 microM, respectively. In the absence of dipyridamole and EHNA, these levels were not affected by sustained moderate hypoxia (PaO2 = 36 +/- 6 mm Hg). However, in the presence of dipyridamole and EHNA, the concentration of adenosine in the CSF during hypoxia was significantly (p < 0.05) increased. Our data indicate that dipyridamole and EHNA potentiate hypoxic vasodilation of pial arterioles while simultaneously increasing extracellular adenosine levels, thus supporting the hypothesis that adenosine is involved in the regulation of cerebral blood flow.

Adenosine release and changes in pial arteriolar diameter during transient cerebral ischemia and reperfusion.

We utilized the closed cranial window technique in the anesthetized rat to determine changes in CSF concentrations of adenosine, inosine, and hypoxanthine and pial arteriolar diameter during transient (20 min) forebrain ischemia and reperfusion. After mock CSF under the cranial window was allowed to equilibrate with cerebral interstitial fluid, endogenous adenosine concentration was found to be 0.16 +/- 0.05 microM, while inosine and hypoxanthine were 0.35 +/- 0.17 and 1.23 +/- 0.47 microM, respectively. The concentration of adenosine in CSF increased 4.2-fold during ischemia and 13.8-fold during the first 5 min of reperfusion. Inosine and hypoxanthine concentrations were also significantly increased during ischemia and reperfusion. After 1 h of reperfusion, CSF adenosine and inosine levels had decreased from peak value but remained significantly above preischemic values. In contrast, hypoxanthine remained at peak concentrations even after 60 min of reperfusion. Preischemic arteriolar diameter was 42.6 +/- 11.3 microns and was not significantly changed after 20 min of ischemia. However, during the first 5 min of reperfusion, arteriolar diameter increased significantly (p less than 0.05), coincident with peak adenosine concentrations. By 60 min of reperfusion, arteriolar diameter had returned to baseline. These results indicate that during the postischemic period, adenine nucleosides and hypoxanthine in CSF are elevated and could affect reperfusion.

Influence of hyperglycemia on cerebral adenosine production during ischemia and reperfusion.

We hypothesized that systemic hyperglycemia would alter cerebral adenosine concentrations during ischemia and reperfusion. In the present study, we analyzed brain tissue and cerebrospinal fluid (CSF) from hyperglycemic and normoglycemic rats before ischemia, after 15 min of incomplete forebrain ischemia, and during 60 min of reperfusion. Hyperglycemic rats received 3 g/kg of 17% D-glucose intraperitoneally, which increased blood glucose to 357 +/- 23 mg/100 ml compared with 128 +/- 12 mg/100 ml in normoglycemic rats. Brain tissue was sampled by the freeze-blow technique, and CSF was obtained by collecting cortical perfusate from the closed cranial window. Tissue and CSF were analyzed for adenosine and its metabolites inosine and hypoxanthine, and tissue was also analyzed for adenine nucleotides. Hyperglycemia significantly attenuated the increase in brain tissue and CSF adenosine and its metabolites during ischemia while preserving adenine nucleotide concentrates. This attenuation of ischemic adenosine production persisted after 5 min of reperfusion in tissue and throughout 60 min of reperfusion in CSF. Because adenosine, a cerebral vasodilator, can inhibit the release of neuronal excitotoxins as well as affect neutrophil-endothelial interactions, adenosine has been proposed as an endogenous neuroprotector. Thus the attenuation of adenosine and its metabolites may be a factor in the pathogenesis of increased ischemic brain injury associated with systemic hyperglycemia.

Lack of sympathetic and cholinergic influences on cerebral vasodilation caused by sciatic nerve stimulation in the rat.

We studied the influences of sympathetic and cholinergic mechanisms on pial arteriolar responses during cortical activation in the rat. Adult male Sprague-Dawley rats were anesthetized with alpha-chloralose and urethane and mechanically ventilated. Pial arterioles on the somatosensory cortex were visualized on a video monitor through a closed cranial window. Changes in arteriolar diameter induced by sciatic nerve stimulation (0.2 V, 5 Hz, 5 ms, for 20 s) were measured before and after (a) ipsilateral superior cervical ganglionectomy (n = 5), (b) intravenous (0.5 mg/kg) administration and topical (10(-5) M) application of atropine (n = 5), and (c) lesion of the nucleus basalis magnocellularis (the major source of intracerebral acetylcholine neurons, n = 7). Unilateral nucleus basalis magnocellularis lesions were performed stereotactically by injection of ibotenic acid (25 nmol/microliter). Sensory cortex cholinergic denervation was confirmed histologically. These treatments had no significant effect on arteriolar responses to sciatic nerve stimulation. Thus, the present results suggest that neither sympathetic nor cholinergic mechanisms play a significant role in somatosensory evoked cerebral vasodilation.

Effects of topical adenosine analogs and forskolin on rat pial arterioles in vivo.

We utilized the closed window technique to study the in vivo responses of rat pial arterioles to superfused adenosine agonists. Adenosine and its analogs dilated pial arterioles and exhibited the following order of potency: 5'N-ethylcarboxamide adenosine (NECA) greater than 2-chloroadenosine (2-CADO) greater than adenosine = R-N6-phenylisopropyladenosine (R-PIA) = S-PIA greater than N6-cyclohexyladenosine (CHA). This potency profile suggests that cerebral vasodilation is mediated through the A2 receptor. Forskolin (10(-9) M) potentiated the vasodilation caused by 10(-6) M NECA, thus implicating adenylate cyclase activation during NECA-induced vasodilation and providing further support for involvement of the A2 receptor.

The effects of dipyridamole and theophylline on rat pial vessels during hypocarbia.

Hypocarbia results in an increase in brain adenosine concentrations, presumably because of brain hypoxia associated with hypocarbic vasoconstriction. It was hypothesized that adenosine limits the degree of hypocarbic vasoconstriction. To test this hypothesis, the effects of dipyridamole and theophylline on CO2 reactivity during hypocarbia were investigated in anesthetized rats. Dipyridamole should reduce the vasoconstriction by potentiating adenosine action, whereas theophylline should increase the vasoconstriction by blocking adenosine receptors. Cortical pial arterioles of mechanically ventilated and anesthetized rats were displayed on a video monitor system through a closed cranial window. Arterial blood pressure and oxygen tension were stable. CO2 reactivity, formulated as 100 X [delta diameter (micron)/resting diameter (micron)]/delta PaCO2 (mmHg), in the hypocarbic phase was calculated before and after topical superfusion of dipyridamole (10(-6) M; n = 7) and theophylline (5 X 10(-5) M; n = 6). CO2 reactivity was significantly decreased after superfusion of dipyridamole (0.57 +/- 0.08; mean +/- SEM) as compared with mock cerebrospinal fluid (CSF) (0.97 +/- 0.17, p less than 0.05, n = 7). On the other hand, CO2 reactivity after superfusion of theophylline was increased (1.63 +/- 0.28) as compared with mock CSF (1.00 +/- 0.20, p less than 0.05, n = 6), indicating that adenosine is involved in hypocarbic vasoconstriction.