Repeated cocaine administration reduces bradykinin-induced dilation of pial arterioles.

Using the acute cranial window technique in rabbits under surgical anesthesia, we tested the vasoactivity of acetylcholine (ACh, 10(-8)-10(-5) M), bradykinin (BK, 10(-8)-10(-5) M), and asphyxia (10% O2, 9% CO2, balance N2) after subchronic pretreatment with cocaine. After repeated administration of cocaine (20 mg.kg-1.day-1 sc x 7 days), the BK-induced dilation of pial arterioles was reduced by 51%. Previous work showed that BK produces dilation of pial arterioles by a cyclooxygenase-dependent oxygen radical-mediated mechanism and that in rabbits the BK-induced dilation is dependent on both vascular and nonvascular cyclooxygenase. Selective blockade of vascular cyclooxygenase, in addition to cocaine treatment, did not produce any greater inhibition of the BK-induced dilation. The dilation in response to ACh and asphyxia was unaltered by cocaine. Levels of cerebrospinal fluid prostaglandins suggest cocaine pretreatment may inhibit cerebral vascular prostaglandin production. Together, cerebrospinal fluid prostaglandin and vasoreactivity data indicate cocaine pretreatment selectively inhibits the vascular cyclooxygenase-dependent mechanism mediating the BK-induced dilation. This decreased response to BK in cocaine-treated rabbits may result from decreased oxygen radical production concomitant with decreased vascular prostaglandin production. Alternatively, oxygen radical scavenging may be increased after cocaine treatment. We speculate that cocaine-induced alterations in cerebrovascular function and metabolism may be related to the increased incidence of stroke reported to occur in human cocaine users.

Effect of dietary n-3 fatty acids on cerebral microcirculation.

The purpose of these studies was to determine the effects of dietary n-3 fish oil on cerebrovascular reactivity and cerebrospinal fluid prostaglandin levels. Adult rabbits (n = 30) received fish oil (200 mg/kg eicosapentaenoic + 143 mg/kg docosahexaenoic acid), corn oil, or water by daily gavage for 6 wk and were then tested for their pial arteriolar diameter response to topical acetylcholine, bradykinin, or systemic asphyxia using the cranial window technique. Plasma and platelet fatty acids were measured by gas chromatography. Cerebrospinal fluid prostaglandin E and serum thromboxane B2 were measured by radioimmunoassay. n-3 Fatty acids were enriched in the plasma and platelets of the fish oil group (P less than 0.05). Serum thromboxane B2 was decreased by 31% in the fish oil group (P less than 0.05). The diameter response to acetylcholine and asphyxia was the same in all groups; however, the dilator response to bradykinin, which is known to be mediated by oxygen radicals, was significantly diminished in the fish oil group (P less than 0.05). Cerebrospinal fluid prostaglandin E concentration increased in response to acetylcholine, bradykinin, and asphyxia; however, the percent increase was less in the fish oil group. In summary, dietary n-3 fatty acids, which are purported to decrease heart disease, appear to selectively affect cerebral arteriolar reactivity, which is normally dependent on cyclooxygenase metabolism of arachidonic acid and formation of vasoactive oxygen radicals.

Restoration of cerebrovascular responsiveness to hyperventilation by the oxygen radical scavenger n-acetylcysteine following experimental traumatic brain injury.

Previous experiments have shown that, following experimental fluid-percussion brain injury, cyclo-oxygenase-dependent formation of oxygen radicals prevents arteriolar vasoconstriction in response to hyperventilation. The oxygen radical scavengers superoxide dismutase and catalase restore normal reactivity; however, they are not routinely available for clinical use. The present study tested whether n-acetylcysteine (Mucomyst), an agent currently available for acetaminophen toxicity, could be used as a radical scavenger to restore reactivity after brain injury. N-acetylcysteine (163 mg/kg) was given intraperitoneally prior to or 30 minutes after fluid-percussion brain injury (2.6 atm) in cats, and reactivity to hyperventilation was tested 1 hour after injury. The authors found either that pre- or postinjury administration led to normal reactivity. Additional experiments supported the hypothesis that n-acetylcysteine is an oxygen radical scavenger, since it reduced or prevented the free radical-dependent cerebral arteriolar dilation normally induced by the topical application of arachidonic acid or bradykinin. The mechanism by which n-acetylcysteine is effective in trauma may involve direct scavenging of radicals or stimulation of glutathione peroxidase activity. The results suggest that n-acetylcysteine may be useful for treatment of oxygen free radical-mediated brain injury.

Dilation of cerebral arterioles by cytochrome P-450 metabolites of arachidonic acid.

We have recently shown that brain tissue can synthesize cytochrome P-450 monooxygenase metabolites of arachidonic acid (AA), including 5,6-epoxyeicosatrienoic acid (5,6-EET), and 14,15-EET. The purpose of this investigation was to determine the vasoactivity of EETs and AA on the cerebral microcirculation. Pial arteriolar diameter was measured in rabbits and cats using in vivo microscopy and the closed cranial window technique. Prostaglandin (PG) E2 and 6-keto-PGF1 alpha formed by the brain cortex during application of these fatty acids was measured in cerebrospinal fluid by use of radioimmunoassay. A transient dose-dependent dilation was produced by 5,6-EET (1-15 micrograms/ml), with the maximum being 23% of control in both species. Other EETs had little or no activity, and AA-induced dilation was greater in rabbits than in cats. Indomethacin or superoxide dismutase plus catalase prevented dilation by 5,6-EET and AA, indicating that both produce dilation via cyclooxygenase-dependent oxygen radicals. PGE2 and 6-keto-PGF1 alpha levels were increased by AA but not by EETs, implying that EETs do not directly activate AA metabolism. Since 5,6-EET, but not other EETs, is known to be a substrate for cyclooxygenase, our data are consistent with brain cyclooxygenase metabolism of 5,6-EET with concomitant generation of dilator oxygen radicals. An implication of these results is that many previous studies of the cerebral circulation which based conclusions on results with cyclooxygenase inhibitors may need to be additionally interpreted.

Effect of fish oil n-3 fatty acids on cerebral microcirculation.

Dietary fish oil containing the n-3 polyunsaturated fatty acids eicosapentaenoic acid (EPA, 20:5) and docosahexaenoic acid (DHA, 22:6) is being consumed by many individuals in an effort to reduce thrombosis and heart disease. However, little is known about how these fatty acids can affect cerebrovascular function. The purpose of the present study was to begin to examine the effects of these fatty acids on cerebral arteriolar diameter and to compare their effects with that of arachidonic acid (AA). Pial arteriolar diameter responses to the topically applied fatty acids [0.2-200 micrograms/ml cerebrospinal fluid (CSF)] were measured in rabbits using in vivo microscopy and the acute cranial window technique. Prostaglandin E2 (PGE2) formed by the brain in response to AA, EPA, and DHA was measured in CSF using radioimmunoassay. EPA induced a dose-dependent dilation response of which the maximum was 29%, whereas the maximal dilation produced by AA was 100%. The arteriolar effect of EPA was reduced by indomethacin or superoxide dismutase plus catalase, indicating vasoactivity due to oxygen radicals formed by cyclooxygenase metabolism of EPA. DHA itself had no effect on diameter or adenosine-induced dilation but reduced dilation by AA when coapplied with AA. AA induced a 65-fold maximal increase in PGE2, whereas EPA and DHA had comparatively little effect. These results imply that substitution of n-3 fatty acids for AA in brain phospholipids may result in less cyclooxygenase-dependent cerebrovascular reactivity. This alteration in reactivity may produce important effects with respect to the brain's blood flow response to a number of physiological and pathological challenges.

Increased plasma PGE2, 6-keto-PGF1 alpha, and 12-HETE levels following experimental concussive brain injury.

Previous investigations have shown that brain prostaglandin levels are transiently elevated following experimental fluid percussion brain injury. Associated with these increased prostaglandin levels there is free radical production and abnormalities in cerebral arteriolar function. The purpose of this study was to determine whether experimental fluid percussion brain injury in cats is associated with increased systemic levels of prostaglandins and the lipoxygenase product, 12-HETE. Blood samples were collected before and at various periods of time after 2.7 atm of fluid percussion brain injury was produced in adult cats. Prostaglandin and 12-HETE analysis was performed by radioimmunoassay after extraction of the plasma samples. The control levels for 6-keto-PGF1 alpha, PGE2, and 12-HETE were 477 +/- 42, 2,372 +/- 431, and 13,328 +/- 1,769 pg/ml, respectively. Following injury all three eicosanoids reached peak plasma levels by 1-5 min after injury. The percentile increases for all eicosanoids were similar and increased from 70 to 110%. The increases were sustained at up to 30 min postinjury and by 1 h after injury were at control levels. As in previous studies, hypertension following injury was maximal by 1 min postinjury and blood pressure had returned to near normal levels by 5 min postinjury. These studies demonstrate prolonged systemic increases in eicosanoids following injury. Since free radical production and vascular damage occur concomitantly with eicosanoid production, the prolonged increases in these products suggest that there is an attainable therapeutic window following injury during which administration of free radical scavengers may decrease radical damage and reduce the consequences of injury.