Functional hyperemia drives fluid exchange in the paravascular space.

The brain lacks a conventional lymphatic system to remove metabolic waste. It has been proposed that directional fluid movement through the arteriolar paravascular space (PVS) promotes metabolite clearance. We performed simulations to examine if arteriolar pulsations and dilations can drive directional CSF flow in the PVS and found that arteriolar wall movements do not drive directional CSF flow. We propose an alternative method of metabolite clearance from the PVS, namely fluid exchange between the PVS and the subarachnoid space (SAS). In simulations with compliant brain tissue, arteriolar pulsations did not drive appreciable fluid exchange between the PVS and the SAS. However, when the arteriole dilated, as seen during functional hyperemia, there was a marked exchange of fluid. Simulations suggest that functional hyperemia may serve to increase metabolite clearance from the PVS. We measured blood vessels and brain tissue displacement simultaneously in awake, head-fixed mice using two-photon microscopy. These measurements showed that brain deforms in response to pressure changes in PVS, consistent with our simulations. Our results show that the deformability of the brain tissue needs to be accounted for when studying fluid flow and metabolite transport.

Relationship between opioids and prostaglandins in hypoxia-induced vasodilation of pial arteries in the newborn pig.

Previously, it has been observed that methionine enkephalin and leucine enkephalin contribute to hypoxia-induced pial artery dilation in the newborn pig. It has also been observed that the cyclooxygenase inhibitor indomethacin attenuates hypoxic hyperemia in piglets. The present study was designed to determine the relationship between opioids and prostaglandins in hypoxia-induced pial artery dilation. Newborn pigs equipped with closed cranial windows were used to measure pial artery diameter and collect cortical periarachnoid cerebrospinal fluid (CSF) for assay of opioids and prostaglandins. Hypoxia-induced artery vasodilation was mildly attenuated during moderate hypoxia (PaCO2 approximately 35 mm Hg), while this response was blunted during severe hypoxia (PaO2 approximately 25 mm Hg) by indomethacin, 5 mg/kg iv (23% +/- 1 % vs 18% +/- 1% and 33% +/- 2% vs 21% +/- 2% for moderate and severe hypoxia in the absence and presence of indomethacin, respectively). Hypoxic dilation was accompanied by increased CSF prostaglandin E2 (PGE2) concentration (1260 +/- 37 vs 1734 +/- 67 and 1256 +/- 33 vs 2859 +/- 189 pg/ml for moderate and severe hypoxia, respectively). Similar changes in CSF 6 keto PGF1alpha concentration during hypoxia were also observed. Topical PGE2 (10,100 ng/ml) increased CSF methionine enkephalin (874 +/- 35, 1290 +/- 44, and 1791 +/- 143 pg/ml for control, 10 and 100 ng/ml PGE2 respectively). Similar increases in CSF methionine enkephalin concentration were observed for topical PGI2. Additionally, these two prostaglandins also increased CSF leucine enkephalin concentration. Furthermore, while indomethacin had no effect on the release of CSF methionine enkephalin during moderate hypoxia, it attenuated the release of this opioid during severe hypoxia (786 +/- 27 and 2633 +/- 74 vs 781 +/- 51 and 2467 +/- 52; 926 +/- 15 and 3489 +/- 156 vs 898 +/- 11 and 2314 +/- 124 pg/ml for control and moderate/severe hypoxia before and after indomethacin, respectively). Similar effects of indomethacin on hypoxic release of leucine enkephalin were also observed. These data indicate that prostaglandins contribute to hypoxic pial dilation. Additionally, these data show that prostaglandins release the opioids methionine enkephalin and leucine enkephalin. Finally, these data suggest that elevated prostaglandin concentrations during severe hypoxia release opioids which in turn contribute to hypoxic pial dilation.

Adenosine and cerebrovascular hyperemia during insulin-induced hypoglycemia in newborn piglet.

The present study tested the hypothesis that adenosine is involved in mediating the hyperemic response of the newborn brain to hypoglycemia. By use of the cranial window and microdialysis-H2 clearance methodologies, changes in the diameter of pial arterioles (25-50 microns), extracellular adenosine concentrations ([ADO]), and local cerebral blood flow (CBF) were examined in isoflurane-anesthetized piglets subjected to insulin-induced hypoglycemia. Blood glucose concentrations ranged from 10 to 18 mg/dl after insulin administration (25 IU/kg iv). Local CBF in the frontal cortex increased 36 +/- 12% (P = 0.014) at 30 min of hypoglycemia (group 1, n = 12; control = 43 +/- 3 ml.min-1.100 g-1). The mean increase in dialysate [ADO] sampled concurrently from the same cortical area was 59 +/- 29% (P = 0.011; control = 0.11 +/- 0.02 microM). At 30 min of hypoglycemia, pial diameters increased 55 +/- 10% (P = 0.001; group 2, n = 9). The [ADO] in cranial window cerebrospinal fluid (CSF) increased 217 +/- 71% (P = 0.04) in response to hypoglycemia (group 3, n = 8; control = 0.016 +/- 0.006 microM). Local administration of an adenosine antagonist, 10 microM 8-sulfophenyltheophylline, to the cerebral cortex before hypoglycemia caused a 38% reduction (P = 0.011) in the pial arteriolar response at 30 min of hypoglycemia (group 4, n = 9). Similarly, local superfusion of CSF with 3.7 mM glucose attenuated the hypoglycemia-induced pial dilation 33% (P = 0.039; group 5, n = 9). Perfusion of microdialysis probes with 3.7 mM glucose in the CSF abolished the hypoglycemia-induced increase in dialysate [ADO] (group 1).(ABSTRACT TRUNCATED AT 250 WORDS)