Intraischemic Cerebral Blood Flow Research Articles

Abstract TP327: Plant Long-Chain Isoprenoid Alcohols (Polyprenols) Protect Against Cerebral Ischemia and Reperfusion Injury in a Mouse Model of Stroke

Introduction: Plant long-chain isoprenoid alcohols (polyprenols) are biologically active molecules affecting the isoprenoid pathway. Polyprenols have broad therapeutic actions including for infections and inflammatory, hepatic and neurological conditions. The aim of this study was to evaluate the protective effects of pharmaceutical-grade polyprenols on experimental stroke in a mouse middle cerebral artery occlusion (MCAO) followed by reperfusion model. Methods: Pharmaceutical-grade polyprenol substance, Ropren®, is isolated from green verdure of ( Picea abies (L) Karst) . Adult (12-14 weeks old) C57BL/6 male mice were anesthetized (1.5% of isoflurane in 30% of O 2 and 70% of N 2 O) and body temperature was maintained (36.5-37.0°C) during 30-min filament-mediated MCAO and 48 h of reperfusion. Cerebral laser-Doppler flowmetry was utilized during MCAO to test whether regional cerebral blood flow (CBF) differences correlated with stroke outcome. Before and 24 h after MCAO, the experimental group was treated with Ropren® intraperitoneal injections (12 mg/kg) diluted in 100 μl sunflower oil. The control group was treated with 100 μl oil only at the same time points. Mice were euthanized 48 h after MCAO and infarct volumes were calculated (indirect method) after TTC staining. Results: Experimental stroke revealed a decrease in the cerebral infarct volume in the Ropren-treated group (51±16 mm 3 , Mean±SD) in comparison with the control group (67±13) (n=10/group, p < 0.05, two-tailed t-test). Neurological deficit measured by a 4-point scale was improved in Ropren-treated mice (1.2 points) compared with control mice (1.8) (n=10/group, p=0.05). We did not find a significant difference both between intra-ischemic CBF and CBF during reperfusion between groups (15th minute of reperfusion: 62±24% in Ropren-treated mice; 77±35% in control mice (n=10/group, NS)). This suggests Ropren® renders its neuroprotective effects by mechanisms independent of CBF. Conclusion: This study shows the protective effects of pharmaceutical-grade polyprenols (Ropren®) during cerebral ischemia-reperfusion injury in the mouse MCAO model. Further research is required to understand the mechanisms of Ropren-mediated neuroprotection in this ischemic stroke model.

Excess salt increases infarct size produced by photothrombotic distal middle cerebral artery occlusion in spontaneously hypertensive rats.

Cerebral circulation is known to be vulnerable to high salt loading. However, no study has investigated the effects of excess salt on focal ischemic brain injury. After 14 days of salt loading (0.9% saline) or water, spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) were subjected to photothrombotic middle cerebral artery occlusion (MCAO), and infarct volume was determined at 48 h after MCAO: albumin and hemoglobin contents in discrete brain regions were also determined in SHR. Salt loading did not affect blood pressure levels in SHR and WKY. After MCAO, regional cerebral blood flow (CBF), determined with two ways of laser-Doppler flowmetry (one-point measurement or manual scanning), was more steeply decreased in the salt-loaded group than in the control group. In SHR/Izm, infarct volume in the salt-loaded group was 112±27 mm3, which was significantly larger than 77±12 mm3 in the control group (p = 0.002), while the extents of blood-brain barrier disruption (brain albumin and hemoglobin levels) were not affected by excess salt. In WKY, salt loading did not significantly increase infarct size. These results show the detrimental effects of salt loading on intra-ischemic CBF and subsequent brain infarction produced by phototrhombotic MCAO in hypertensive rats.

Dichotomous Effects of Chronic Intermittent Hypoxia on Focal Cerebral Ischemic Injury

Obstructive sleep apnea, a condition associated with chronic intermittent hypoxia (CIH), carries an increased risk of stroke. However, CIH has been reported to either increase or decrease brain injury in models of focal cerebral ischemia. The factors determining the differential effects of CIH on ischemic injury and their mechanisms remain unclear. Here, we tested the hypothesis that the intensity of the hypoxic challenge determines the protective or destructive nature of CIH by modulating mitochondrial resistance to injury. Male C57Bl/6J mice were exposed to CIH with 10% or 6% O2 for ≤35 days and subjected to transient middle cerebral artery occlusion. Motor deficits and infarct volume were assessed 3 days later. Intraischemic cerebral blood flow was measured by laser-Doppler flowmetry and resting cerebral blood flow by arterial spin labeling MRI. Ca2+-induced mitochondrial depolarization and reactive oxygen species production were evaluated in isolated brain mitochondria. We found that 10% CIH is neuroprotective, whereas 6% CIH exacerbates tissue damage. No differences in resting or intraischemic cerebral blood flow were observed between 6% and 10% CIH. However, 10% CIH reduced, whereas 6% CIH increased, mitochondrial reactive oxygen species production and susceptibility to Ca2+-induced depolarizations. The influence of CIH on the ischemic brain is dichotomous and can be attributed, in part, to changes in the mitochondrial susceptibility to injury. The findings highlight a previously unappreciated complexity in the effect of CIH on the brain, which needs to be considered in evaluating the neurological effect of conditions associated with cyclic hypoxia.

Lipoprotein Receptor–Related Protein-6 Protects the Brain From Ischemic Injury

Loss-of-function mutations of the lipoprotein receptor-related protein-6 (LRP6), a coreceptor in the Wingless-related integration site-β-catenin prosurvival pathway, have been implicated in myocardial ischemia and neurodegeneration. However, it remains to be established whether LRP6 is also involved in ischemic brain injury. We used LRP6+/- mice to examine the role of this receptor in the mechanisms of focal cerebral ischemia. Focal cerebral ischemia was induced by transient occlusion of the middle cerebral artery. Motor deficits and infarct volume were assessed 3 days later. Glycogen-synthase-kinase-3β (GSK-3β) phosphorylation was examined by Western blotting with phosphospecific antibodies, and the mitochondrial membrane potential changes induced by Ca2+ were also assessed. LRP6+/- mice have larger stroke and more severe motor deficits, effects that were independent of intraischemic cerebral blood flow, vascular factors, or cytosolic β-catenin levels. Rather, LRP6 haploinsufficiency increased the activating phosphorylation and decreased the inhibitory phosphorylation of GSK-3β, a kinase involved in proinflammatory signaling and mitochondrial dysfunction. Accordingly, postischemic inflammatory gene expression was enhanced in LRP6+/- mice. Furthermore, the association of mitochondria with activated GSK-3β was increased in LRP6+/- mice, resulting in a reduction in the Ca2+ handling ability of mitochondria. The mitochondrial dysfunction was reversed by pharmacological inhibition of GSK-3β. LRP6 activates an endogenous neuroprotective pathway that acts independently of β-catenin by controlling GSK-3β activity and preventing its deleterious mitochondrial and proinflammatory effects. The findings raise the possibility that emerging treatment strategies for diseases attributable to LRP6 loss-of-function mutations could also lead to new therapeutic avenues for ischemic stroke.

Obligatory Role of Inducible Nitric Oxide Synthase in Ischemic Preconditioning

Sublethal insults can induce a transient tolerance toward subsequent lethal ischemia, a phenomenon termed ischemic preconditioning (IPC). In the myocardium, nitric oxide derived from 'inducible' nitric oxide synthase (iNOS or NOS II) plays a critical role in the expression of IPC produced by sublethal ischemia. Here, we investigated whether iNOS is involved in IPC in brain. Ischemic preconditioning was produced in mice by three episodes of 1-min bilateral common carotid artery (BCCA) occlusion, each followed by 5 mins of reperfusion. After 24 h, mice underwent middle cerebral artery (MCA) occlusion for 20 mins. Intraischemic cerebral blood flow was monitored during both in BCCA and MCA occlusion (MCAO) by laser-Doppler flowmetry. Mice were killed 3 days after MCAO, and infarct volume was determined in thionine-stained sections. Infarct volume was significantly reduced 24 h after IPC (70%; P<0.05). Treatment with the iNOS inhibitor aminoguanidine (400 mg/kg), abolished the IPC-induced protection. Furthermore, IPC failed to induce ischemic tolerance in iNOS-null mice. In wild-type mice, IPC increased the resistance to Ca(2+)-mediated depolarization in isolated brain mitochondria. However, in iNOS-null mice IPC failed to induce such resistance. We conclude that iNOS is required for the full expression of IPC and that such effect is coupled to an increased resistance of mitochondria to injury. Thus, iNOS-derived nitric oxide, in addition to its deleterious effects on the late stages of ischemic brain damage, can also be beneficial by promoting ischemic tolerance through signaling, ultimately resulting in mitochondrial protection.

Ischemic Nitric Oxide and Poly (ADP-Ribose) Polymerase-1 in Cerebral Ischemia: Male Toxicity, Female Protection

It is well established that tissue damage and functional outcome after experimental or clinical stroke are shaped by biologic sex. We investigated the novel hypothesis that ischemic cell death from neuronally derived nitric oxide (NO) or poly-ADP ribose polymerase (PARP-1) activation is sexually dimorphic and that interruption of these molecular death pathways benefits only the male brain. Female neuronal nitric oxide synthase (nNOS) knockout (nNOS-/-) mice exhibited exacerbated histological injury after middle cerebral artery occlusion (MCAO) relative to wild-type (WT) females, unlike the protection observed in male nNOS-/- littermates. Similarly, treatment with the nNOS inhibitor (7-nitroindozole, 25 mg/kg) increased infarction in female C57Bl6 WT mice, but protected male mice. The mechanism for this sexually specific response is not mediated through changes in protein expression of endothelial NOS or inducible NOS, or differences in intraischemic cerebral blood flow. Unlike male PARP-1 knockouts (PARP1-/-), female PARP1-/- littermates sustained grossly increased ischemic damage relative to sex-matched WT mice. Treatment with a PARP inhibitor (PJ-34, 10 mg/kg) resulted in identical results. Loss of PARP-1 resulted in reversal of the neuroprotective activity by the female sex steroid, 17beta estradiol. These data suggest that the previously described cell death pathways involving NO and PARP ischemic neurotoxicity may be operant solely in male brain and that the integrity of nNO/PARP-1 signaling is paradoxically protective in the female.

Severe hypotension is not essential for isoflurane neuroprotection against forebrain ischemia in mice.

Volatile anesthetics provide protection in experimental models of global cerebral ischemia. To date, all models evaluated have included profound systemic arterial hypotension as a component of the ischemic insult. This study was designed to determine if isoflurane protection persists in a global insult devoid of hypotension. C57BL/6J mice having a high incidence of posterior communicating artery atresia were anesthetized with isoflurane (1.2%) or fentanyl/N2O and subjected to bilateral carotid artery occlusion for 15 min or 20 min with normotension (80-110 mmHg mean arterial pressure) or for 10 min with hypotension (35 mmHg mean arterial pressure). Three days later, neurologic function and histologic damage were assessed. Other mice underwent measurement of intraischemic cerebral blood flow (4-iodo-N-methyl-[14C]antipyrine autoradiography) or plasma norepinephrine. Isoflurane reduced the percentage of hippocampal CA1 dead neurons (e.g., 10 min bilateral carotid occlusion + hypotension: 43 +/- 18 (isoflurane) vs. 67 +/- 20 (fentanyl/N2O), P = 0.003; 20 min bilateral carotid occlusion + normotension: 49 +/- 27 (isoflurane) vs. 71 +/- 22 (fentanyl/N2O), P = 0.003). Isoflurane also reduced CA3 damage and improved neurologic function under all conditions. Intraischemic forebrain blood flow was similar during bilateral carotid occlusion plus normotension for the two anesthetic states. Plasma norepinephrine values were greater when hypotension was added to the ischemic insult. Isoflurane resulted in improved neurologic function and reduced histologic damage regardless of the presence or absence of systemic hypotension during the ischemic insult. This indicates that beneficial effects of isoflurane are most likely attributable to direct effects at the neuronal level as opposed to indirect effects resulting from interactions with profound hypotension.

High-dose ibuprofen for reduction of striatal infarcts during middle cerebral artery occlusion in rats.

Ibuprofen is an antiinflammatory drug that disrupts leukocyte-endothelial cell interactions by limiting expression of endothelial adhesion molecules such as intercellular adhesion molecule-1 (ICAM-1), also known as CD54. The authors hypothesized that ibuprofen could reduce the size of the infarct associated with transient focal ischemia by inhibition of ICAM-1 expression, and they evaluated its effects in rats treated with middle cerebral artery (MCA) occlusion. Ibuprofen treatment was compared with mild systemic hypothermia, which is known to be neuroprotective and is commonly used during neurosurgical procedures. The maximum ibuprofen dose (240 mg/kg/day) that could be tolerated with no systemic toxicity was established in the initial experiments. In the efficacy experiment, rats were pretreated with vehicle, ibuprofen, or hypothermia (33 degrees C) prior to 2 hours of MCA occlusion; then their brains were harvested at 24 hours of reperfusion for histological studies. End-ischemic cerebral blood flow (CBF) was evaluated using [14C]iodoantipyrine autoradiography in additional cohorts. Expression of ICAM-1 within ischemic compared with nonischemic caudate nucleus and putamen (striatum) or cortex was evaluated using immunohistochemical studies. Compared with vehicle treatment, ibuprofen produced a 46.2% reduction (p = 0.01) in striatal infarcts, which was comparable to hypothermia (48.7% reduction, p = 0.02). Ibuprofen did not alter end-ischemic CBF in any region studied, and the ibuprofen treatment group had the lowest proportion of animals with marked ICAM-1 staining. Ibuprofen given in maximum tolerated doses reduces the striatal infarct size after focal cerebral ischemia. The neuroprotective mechanism does not work through preservation of intraischemic CBF and is consistent with inhibition of ICAM-1 expression; however, at the doses used in this study, other effects of ibuprofen on platelet and endothelial function are possible.

Anesthetic choice of halothane versus propofol: impact on experimental perioperative stroke.

It is not known whether preischemic exposure to anesthetic agents affects the amount of damage from transient focal ischemia that occurs after cessation of the anesthetic. We compared the effect of prior exposure to halothane or propofol on infarction size after transient middle cerebral artery occlusion (MCAO) induced in the awakening animal to test the hypothesis that anesthetic type and exposure duration would independently affect the amount of brain injury. Male Wistar rats (weight, 200 to 300 g) were anesthetized briefly with halothane for placement of hemodynamic instrumentation. Twenty-four hours later, rats were treated with either a short (approximately 1 hour) or long (8 hours) duration of inhaled halothane (1% to 2%) or intravenous propofol (10 mg/kg bolus, 30 mg/kg per hour infusion). Each cohort (n=8 per group) was then subjected to 2-hour MCAO by the intraluminal suture technique. All anesthesia was discontinued once MCAO was achieved. Infarct volume was measured at 22 hours of reperfusion. In a second cohort, regional cerebral blood flow (CBF) was measured ([(14)C]iodoantipyrine autoradiography) at end-occlusion in short-duration halothane (n=5) or short-duration propofol (n=5) anesthesia groups and in corresponding surgical shams (n=3 each). Pericranial temperature, PaO(2), PaCO(2), and blood pressure were controlled and not different among groups before or during occlusion. MCAO resulted in a similar immediate reduction in laser-Doppler flow signal after discontinuation of anesthesia in the awakening animals. Infarct volume was smaller in rats exposed to short-duration halothane in cortex (87.5+/-16.6 mm(3)) (mean+/-SEM) and caudoputamen (38.3+/-13.7 mm(3)) compared with rats exposed to short-duration propofol (cortex, 177.5+/-16.9 mm(3); caudoputamen, 47.8+/-2.9 mm(3)). Infarct volume was not different in long-duration halothane versus long-duration propofol treatment. Absolute cortical or caudoputamen intraischemic CBF was not different between short-duration halothane or short-duration propofol treatment. These data demonstrate that short-duration halothane exposure before MCAO in the awakening animal attenuates infarction volume compared with propofol. This protection by halothane is not mediated through preservation of intraischemic CBF. Longer durations of halothane exposure may activate secondary injury pathways, which negate the protective effects of short-term halothane preischemic treatment.

Inhibition of N-glycan processing alters axonal transport of synaptic glycoproteins in vivo

The principal aim of this study was to examine the relative contributions from the neuronal and endothelial isoforms of nitric oxide synthase (nNOS and eNOS, respectively) in their capacity to modulate intra-ischemic cerebral blood flow (CBF) changes, in the ischemically vulnerable hippocampus and striatum. CBF changes were monitored, using laser-Doppler flowmetry, in rats subjected to 30 min of forebrain ischemia (right common carotid occlusion+hemorrhagic hypotension). Rats were pretreated with a selective nNOS inhibitor (ARR 17477), a NOS inhibitor that blocks both eNOS and nNOS (N(G)-nitro-L-arginine; L-NNA), or saline (control). In initial experiments, where ischemic MABP was targeted to exactly 30 mmHg, NOS inhibition reduced intra-ischemic cortical CBF from the control level of approximately 20% of baseline to 3% (L-NNA) or 6% (ARR 17477) of baseline. The statistically similar effects of the two NOS inhibitors confirmed that nNOS is the predominant NO source supporting intra-ischemic vasodilation in the cortex. In subsequent experiments, CBF was measured in the right hippocampus, and striatum, as well as the cortex, and, to reduce data variability, blood withdrawal was adjusted to achieve an intra-ischemic cortical CBF of 20% (controls) or 5% (NOS inhibited rats) of baseline. In those groups, mean ischemic MABP levels ranged from 28 to 32 mmHg. In controls, intra-ischemic CBF fell to 20%, 45%, and 47% of baseline in the cortex, hippocampus, and striatum, respectively. With nNOS inhibition, intra-ischemic CBF was further reduced to 5%, 15%, and 18% of baseline, respectively. However, with combined eNOS/nNOS inhibition, the CBF values were 5%, 37%, and 21%, respectively. These results suggest that the nNOS contribution to intra-ischemic vasodilation in vulnerable regions is substantially greater than eNOS. The significantly higher intra-ischemic CBF level in the hippocampus in combined eNOS/nNOS vs nNOS-inhibited rats may relate, in contrast to other regions, to a low eNOS influence on vascular function in that structure and CBF redistribution to the hippocampus when eNOS activity is blocked globally.

Relative contributions from neuronal and endothelial nitric oxide synthases to regional cerebral blood flow changes during forebrain ischemia in rats

The principal aim of this study was to examine the relative contributions from the neuronal and endothelial isoforms of nitric oxide synthase (nNOS and eNOS, respectively) in their capacity to modulate intra-ischemic cerebral blood flow (CBF) changes, in the ischemically vulnerable hippocampus and striatum. CBF changes were monitored, using laser-Doppler flowmetry, in rats subjected to 30 min of forebrain ischemia (right common carotid occlusion+hemorrhagic hypotension). Rats were pretreated with a selective nNOS inhibitor (ARR 17477), a NOS inhibitor that blocks both eNOS and nNOS (N(G)-nitro-L-arginine; L-NNA), or saline (control). In initial experiments, where ischemic MABP was targeted to exactly 30 mmHg, NOS inhibition reduced intra-ischemic cortical CBF from the control level of approximately 20% of baseline to 3% (L-NNA) or 6% (ARR 17477) of baseline. The statistically similar effects of the two NOS inhibitors confirmed that nNOS is the predominant NO source supporting intra-ischemic vasodilation in the cortex. In subsequent experiments, CBF was measured in the right hippocampus, and striatum, as well as the cortex, and, to reduce data variability, blood withdrawal was adjusted to achieve an intra-ischemic cortical CBF of 20% (controls) or 5% (NOS inhibited rats) of baseline. In those groups, mean ischemic MABP levels ranged from 28 to 32 mmHg. In controls, intra-ischemic CBF fell to 20%, 45%, and 47% of baseline in the cortex, hippocampus, and striatum, respectively. With nNOS inhibition, intra-ischemic CBF was further reduced to 5%, 15%, and 18% of baseline, respectively. However, with combined eNOS/nNOS inhibition, the CBF values were 5%, 37%, and 21%, respectively. These results suggest that the nNOS contribution to intra-ischemic vasodilation in vulnerable regions is substantially greater than eNOS. The significantly higher intra-ischemic CBF level in the hippocampus in combined eNOS/nNOS vs nNOS-inhibited rats may relate, in contrast to other regions, to a low eNOS influence on vascular function in that structure and CBF redistribution to the hippocampus when eNOS activity is blocked globally.

Estrogen receptor antagonist ICI182,780 exacerbates ischemic injury in female mouse.

Recent findings in animals emphasize that experimental ischemic brain damage can be strikingly reduced by estrogen: however, the neuroprotective mechanisms are not well understood. It was hypothesized that estrogen signaling via cognate estrogen receptors (ERs) within the vasculature is an important aspect of cerebral ischemic protection in the female brain, in part by amplifying intraischemic cerebral blood flow (CBF). In the present study, the hypothesis that chronic treatment with the pure ER antagonist ICI182,780 (ICI) would increase ischemic brain damage by a blood flow-mediated mechanism was investigated. Adult C57B1/6J mice were pretreated with either subcutaneous ICI (100 microg/day) or oil/ethanol vehicle for 1 week before 2 hours of middle cerebral artery occlusion (MCAO) and 22 hours of reperfusion. End-ischemic regional CBF was evaluated in additional cohorts using [14C]iodoantipyrine autoradiography. Infarction volume as measured by cresyl violet histology was greater in the striatum of ICI-treated females (70 +/- 3% of contralateral striatum vs. 40 +/- 12% in vehicle-treated females). Cortical injury was not enhanced relative to control animals (39 +/- 6% of contralateral cortex in ICI group vs. 27 +/- 8% in vehicle-treated group). Physiologic variables and ischemic reduction of the ipsilateral cortical laser-Doppler flow signal were similar between groups. Further, ICI treatment did not alter end-ischemic cortical or striatal CBF. The deleterious effect of ICI was limited to females, as there were no differences in stroke damage or CBF between male treatment groups. These data suggest that estrogen inhibits ischemic brain injury in striatum of the female by receptor-mediated mechanisms that are not linked to preservation of intraischemic CBF.

Abstracts of literature

Abstracts of literature

Mice overexpressing extracellular superoxide dismutase have increased resistance to focal cerebral ischemia

Transgenic mice, which had been transfected with the human extracellular superoxide dismutase gene, causing an approximate five-fold increase in brain parenchymal extracellular superoxide dismutase activity, were used to investigate the role of extracellular superoxide dismutase in ischemic brain injury. Transgenic ( n=21) and wild-type ( n=19) mice underwent 90 min of intraluminal middle cerebral artery occlusion and 24 h of reperfusion. Severity of resultant hemiparesis and cerebral infarct size were measured. Wild-type mice had larger infarcts (cortex: wild type=37±14 mm 3, transgenic=27±13 mm 3, P=0.03; subcortex: wild type=33±14 mm 3, transgenic=23±10 mm 3, P=0.02). Neurological scores, however, were similar ( P=0.29). Other mice underwent autoradiographic determination of intra-ischemic cerebral blood flow. The volume of tissue at risk of infarction (defined as volume of tissue where blood flow was <25 ml/100 g/min) was similar between groups (cortex: wild type=51±15 mm 3, transgenic=47±9 mm 3, P=0.65; subcortex: wild type=39±16 mm 3, transgenic=37±17 mm 3, P=0.81). These results indicate that antioxidant scavenging of free radicals by extracellular superoxide dismutase plays an important role in the histological response to a focal ischemic brain insult.

Brain Temperature Reduction During Ischemia Increases with Immaturity† 1051

Brain temperature (Tb) is a critical determinant of the response and outcome to ischemia. Different Tb may exist in immature vs. mature animals during ischemia due to differences in the balance between heat loss and production, but this has not been explored. To examine this issue, Tb was measured during control and at 2 min intervals during 16 min of partial brain ischemia in 8 newborn miniswine (Nb, age 5±3d, weight 1.4±.2 Kg, x±SD) and 10 older miniswine (Old, age 34±5d, weight 6.1±1.2 Kg). Tb was measured directly with thermocouples implanted to a depth of 2 cm beneath the cortical surface, and cerebral blood flow (CBF) was measured with fluorescent microspheres during control and at 2 intervals during ischemia (6-9 and 12-15 min of the 16 min interval) to verify a constant intraischemic CBF reduction in each animal. Ischemia was induced by inflation of a neck cuff plus hypotension. At control rectal temperature (Tr) did not differ between groups (38.4±0.2 vs 38.6±0.2°C for Nb and Old, respectively), but Tb was lower in Nb compared to Old(38.4±0.2 vs. 38.8±0.3°C, p<.01). By design a broad range of CBF reductions were studied in each group during ischemia. In Nb and Old mean intraischemic CBF (% of control, 100%) was 31±27 and 33± 33% respectively, and in each group CBF at 6-9 and 12-15 min did not differ. During ischemia Tr was stable and never decreased by more than 0.2°C in either group. Changes in Tb were expressed as rectal-brain temperature difference (ΔTrb). In Nb ΔTrb was 0.0±0.1°C at control and increased to 0.9±0.7°C at 16 min of ischemia. In Old ΔTrb was -0.2±0.3 at control (p=.02 vs Nb) and increased to 0.1±0.4°C at 16 min of ischemia (p<.01 vs Nb). CBF was inversely correlated with ΔTrb for Nb when measured from 6-9 min of ischemia(r2=.69, p=.01) and 12-15 min of ischemia (r2=.64, p=.02). In contrast there was no correlation between CBF and ΔTrb for Old during ischemia. The thickness of the overlying scalp plus calvarium differed between groups; 2.69±0.55 vs 6.55±1.26 mm in Nb and Old respectively(p<.01). Ischemia results in lower Tb in Nb compared to Old, and the lower Tb could contribute to the resistance of brain to ischemia in younger compared to older mammals. The presence of less insulating tissue (scalp, calvarium) in Nb compared to Old may be responsible for greater intraischemic heat loss in Nb relative to Old.