Prostaglandin uptake and catabolism by the choroid plexus during development in sheep

Restoration of Blood Pressure by Centrally Injected U-46619, a Thromboxane A2 Analog, in Hemorrhaged Hypotensive Rats: Investigation of Different Brain Areas

In the present study, we investigated the cardiovascular effects of centrally injected U-46619, a thromboxane A(2) (TXA(2)) analog, and the central and peripheral mechanisms of these effects in hemorrhagic shock conditions. Hemorrhage was performed by withdrawing a total volume of 2.1 ml of blood/100 g body weight over a period of 10 min. Injections were made into the lateral cerebral ventricle (LCV), nucleus tractus solitarius (NTS), rostral ventrolateral medulla (RVLM) and paraventricular nucleus of hypothalamus (PVN). U-46619 (0.1, 1 and 2 microg) increased blood pressure and reversed hypotension in hemorrhagic shock. The pressor effect was dose- and time-dependent in all investigated brain areas. Heart rate changes were not significantly different in all groups. Pretreatment of rats with an injection of SQ-29548 (4 or 8 microg), a TXA(2) receptor antagonist, into the LCV, NTS, RVLM and PVN completely blocked the pressor effect of U-46619 (1 microg) injected into respective brain areas. Hemorrhage itself increased plasma adrenaline, noradrenaline, vasopressIN levels and renin activity. U-46619 (1 microg) injected into the LCV, PVN, RVLM and NTS produced additional increases in these hormone levels and in renin activity. Intravenous pretreatments of rats with prazosin (0.5 mg/kg), an alpha(1)-adrenoceptor antagonist, [beta-mercapto-beta,beta-cyclopentamethylenepropionyl(1), O-Me-Tyr(2),Arg(8)]- vasopressin (10 microg/kg), a vasopressin V(1)-receptor antagonist, or saralasin (250 microg/kg), an angiotensin II receptor antagonist, in hemorrhaged rats partially blocked the pressor response to U-46619 (1 microg) injected into the LCV, PVN, RVLM and NTS. Results show that centrally administered U-46619, a TXA(2) analog, increases blood pressure and reverses hypotension in hemorrhagic shock. Activation of central TXA(2) receptors mediates the pressor effect of the drug. Furthermore, the increases in plasma adrenaline, noradrenaline, vasopressin levels and renin activity are involved in these effects.

The involvement of the blood–brain and the blood–cerebrospinal fluid barriers in the distribution of leptin into and out of the rat brain

Leptin is a 16 kDa hormone that is produced by adipose tissue and has a central effect on food intake and energy homeostasis. The ability of leptin to cross the blood-brain and blood-cerebrospinal fluid (CSF) barriers and reach or leave the CNS was studied by the bilateral in situ brain perfusion and isolated incubated choroid plexus techniques in the rat. Brain perfusion results indicated that [(125)I]leptin reached the CNS at higher concentrations than the vascular marker, confirming that [(125)I]leptin crossed the brain barriers. Leptin distribution varied between CNS regions and indicated that the blood-brain barrier, in contrast to the blood-CSF route, was the key pathway for [(125)I]leptin to reach the brain. Further perfusion studies revealed that [(125)I]leptin movement into the arcuate nucleus, thalamus, frontal cortex, choroid plexuses and CSF was unaffected by unlabelled human or murine leptin at a concentration that reflects the upper human and rat plasma leptin concentration (2.5 nM). In contrast, the cerebellum uptake of [(125)I]leptin was decreased by 73% with 2.5 nM human leptin. Thus, this site of dense leptin receptor expression would be sensitive to physiological changes in leptin plasma concentrations. The highest rate (K(in)) of [(125)I]leptin uptake was into the choroid plexuses (307.7+/-68.0 microl/min/g); however, this was not reflected in the CSF (8.9+/-4.1 microl/min/g) and indicates that this tissue tightly regulates leptin distribution. The multiple-time brain uptake of [(125)I]leptin was non-linear and suggested leptin could also be removed from the CNS. Studies using the incubated rat choroid plexus model found that [(125)I]leptin could cross the apical membrane of the choroid plexus to leave the CSF. However, this movement was not sensitive to unlabelled human leptin or specific transport inhibitors/modulators (including probenecid, digoxin, deltorphin II, progesterone and indomethacin).This study supports the concept of brain-barrier regulation of leptin distribution to the CNS, and highlights an important link between leptin and the cerebellum.

Maturation attenuates the effects of cGMP on contraction, [Ca2+]i and Ca2+ sensitivity in ovine basilar arteries

The present study explores the hypothesis that age-related variations in cerebrovascular responses to vasodilators reflect corresponding age-dependent differences in the mechanisms coupling changes in cytosolic cGMP to vasorelaxation. The experiments focused on cGMP's ability to decrease either [Ca2+]i or myofilament Ca2+ sensitivity, because both effects can contribute to cGMP-induced vasodilation. Use of the cGMP analog 8-pCPT-cGMP minimized problems associated with limited cell permeation or cGMP hydrolysis. In fetal basilars contracted with 10 microM serotonin, the EC30 for 8-pCPT-cGMP-induced relaxation was 6 microM. In fura-2 loaded fetal basilars, pretreatment with 6 microM 8-pCPT-cGMP significantly depressed the sensitivity of [Ca2+]i to 5HT, and also myofilament sensitivity to calcium, but only in fetal arteries. In fetal basilar arteries contracted with 120 mM potassium, the EC30 for 8-pCPT-cGMP-induced relaxation was 25 microM. In fura-2 loaded ovine arteries, pretreatment with 25 microM 8-pCPT-cGMP had no effect on the ability of graded concentrations of potassium to elevate [Ca2+]i but reduced potassium's ability to induce contraction and attenuated myofilament calcium sensitivity; these latter effects were significant only in fetal arteries. In alpha-toxin permeabilized preparations, 25 microM 8-pCPT-cGMP significantly depressed both basal- and agonist-stimulated myofilament calcium sensitivity, only in fetal but not in adult basilars. Together, these results demonstrate that: (1) sensitivity to cGMP is greater in fetal than adult sheep arteries independent of method of contraction; (2) cGMP can reduce [Ca2+]i but only in agonist-contracted and not in potassium-contracted arteries; (3) and cGMP attenuates myofilament calcium sensitivity regardless of method of contraction. Overall, the data demonstrate that variations in the ability of cGMP to produce vasodilatation reflect age-, artery-, and agonist-dependent differences in the combination of mechanisms mediating responses to cGMP.

Perinatal changes in choroidal 15-hydroxyprostaglandin dehydrogenase: implications for prostaglandin removal from brain

We have previously shown in the sheep fetus at 0.7 and 0.9 gestation that the choroid plexus, unlike brain parenchyma, catabolizes prostaglandins (PGs). Peculiarly, in the choroid plexus, PGE(2) catabolism persists throughout the neonatal period to abate in the adult, while PGF(2alpha) catabolism abates shortly after birth. To explain this differential behavior and elucidate the function of catabolic enzymes, we examined the cellular location and activity of the rate-limiting enzyme for PGE(2) and PGF(2alpha) catabolism, 15-hydroxyprostaglandin dehydrogenase (15-PGDH). Immunofluorescence histochemistry and immunogold electronmicroscopy revealed abundant 15-PGDH expression in the epithelial cytosol close to the brush-border membrane at 0.7 and 0.9 gestation. In contrast, at 5 and 15 days postnatal, 15-PGDH was found throughout the cytosol of stromal fibroblasts. No staining was observed at either location in pregnant adults. PGF(2alpha) catabolism was minimal in the total homogenate and 100000xg supernatant of the fetal choroid plexus at 0.7 and 0.9 gestation, while PGE(2) catabolism was evident at 0.7 gestation only. In contrast, both PGs were catabolized in minced specimens at either age. In conclusion, our study shows immunoreactive 15-PGDH in the choroid plexus from fetal and neonatal, but not pregnant adult, sheep. Results suggest that PGE(2) catabolism is not as critically dependent as that of PGF(2alpha) on tissue integrity and 15-PGDH location. Given the key role being assigned to the choroid plexus in PG removal from brain, we speculate that persistence of PGE(2) catabolism into the early postnatal period protects against central respiratory depression caused by the compound during this susceptible stage of development.

Differential uptake and catabolism of prostaglandin (PG)E(2) versus PGF(2alpha) in the sheep choroid plexus during development

The early postnatal decrease in prostaglandin (PG)E(2) levels in cerebrospinal fluid (CSF) likely contributes to the establishment of continuous breathing. To elucidate mechanisms underlying this event, choroid plexuses from lateral (L-CP) and third/fourth (III/IV-CP) ventricles were incubated with [3H]-PGE(2) and label uptake (tissue-to-medium ratio for radioactivity, T/M) and catabolism (%radioactivity associated with metabolites, PGM) were measured. [3H]-PGF(2alpha) was a reference. Uptake of [3H]-PGE(2) was lower than [3H]-PGF(2alpha) in the term fetus (L-CP: 5.9+/-0.5 vs. 9.6+/-0. 9, n=11; III/IV-CP: 2.7+/-0.4 vs. 7.7+/-1.0, n=5) and 17 d lamb (L-CP: 5.3+/-0.8 vs. 11.0+/-1.2, n=7; III/IV-CP: 3.1+/-0.2 vs. 11. 6+/-2.8, n=3 and 4, respectively). This difference was not significant in the pregnant adult. Release of the two compounds was similar and did not change with age. [3H]-PGE(2) uptake was reduced by probenecid (1 mM) and excess PG (60 microM PGE(2) or PGF(2alpha)). Excess PG also reduced catabolism in the fetus, which was extensive for [3H]-PGE(2) and [3H]-PGF(2alpha)60%). In the lamb, catabolism remained high for [3H]-PGE(2) (L-CP: 64+/-4%, n=7; III/IV-CP: 41+/-4%, n=3), but not [3H]-PGF(2alpha) (L-CP: 26+/-4%, n=7; III/IV-CP: 4+/-1%, n=4). In the pregnant adult, catabolism was above background only for [3H]-PGE(2) in the L-CP (26+/-5%, n=11). Unlike the perinatal animal, this catabolism was reduced by probenecid. In conclusion, PGE(2) uptake and catabolism operate independently in the choroid plexus from perinatal sheep. Differences between PGE(2) and PGF(2alpha) are developmentally-regulated for both mechanisms. While neither process explains the postnatal decrease in CSF PGE(2), both may help keep CSF levels low during early postnatal development.

Suppression of PGE(2) fever at near term: reduced thermogenesis but not enhanced vasopressin antipyresis

Fevers are known to be suppressed near term in the mother, but the mechanism responsible for this phenomenon is not understood. We tested the hypothesis that the suppression of fever at term is a result of enhanced vasopressin-induced antipyresis. Effects of intracerebroventricular prostaglandin E(2) (PGE(2)) were examined in rats at gestational days 16-17 and 19-20 (near term) and days 1-2 postpartum. PGE(2) (50 ng) elevated body and interscapular brown adipose tissue (iBAT) temperatures and increased sympathetic nerve activity to the iBAT. PGE(2)-induced changes in iBAT temperature and nerve activity, as well as in rectal temperature, were reduced or eliminated near term, and responses were recovered in the postpartum period. Blood pressure and heart rate changes induced by central PGE(2) were also decreased at near term. Coinfusion of Manning compound, a V(1) vasopressin receptor antagonist, with PGE(2) throughout the peripartum period did not reverse the suppressed iBAT temperature and nerve activity or body temperature responses to PGE(2). Microdialysis experiments revealed unchanged terminal release of vasopressin in the ventral septal area after PGE(2) infusion in either pregnant or parturient rats. These results suggest that fever reduction at near term is not associated with enhanced vasopressin antipyresis, but may be a result of reduced sympathetic tone and in particular a reduced sympathetic drive to the iBAT. This finding may reflect a generalized reduction in autonomic output around the time of parturition.

Effects of ischemia on prostaglandin H synthase-2 expression in piglet choroid plexus

We examined effects of ischemia on expression of prostaglandin H synthase-1 (PGHS-1) and prostaglandin H synthase-2 (PGHS-2) in piglet choroid plexus. Ten minutes of ischemia was induced by increasing intracranial pressure. Whole choroid plexus was removed and fixed and/or frozen after 1, 2, 4, and 8 h of recovery from anoxic stress. In addition, tissues were obtained from untreated animals or from time control animals. Tissues were analyzed for mRNA, using RNase protection assays, and for proteins, using immunohistochemical approaches. Limited, but detectable PGHS-2 immunoreactivity was present in choroid plexus under normal conditions, and there was no difference between time-control and non-treated animals. Further, PGHS-2 mRNA increased by 2-4 h after ischemia, and enhanced immunoreactivity for PGHS-2 was present at 8 h after ischemia. Enhanced immunoreactivity for PGHS-2 was present in vascular endothelial cells as well as cuboidal epithelial cells and macrophages. In contrast, PGHS-1 mRNA did not increase following ischemia. We conclude that PGHS-2 is present in piglet choroid plexus under normal conditions and that ischemia increases levels of PGHS-2 in choroid plexus.

Thromboxane A2 acts on the brain to mediate hemodynamic, adrenocorticotropin, and cortisol responses

Conditions that increase the formation of thromboxane A2 (TxA2) also result in activation of hemodynamic and adrenocortical responses. The purpose of this study was to test the hypothesis that TxA2 acts directly on the brain to mediate these responses. Adult sheep were chronically instrumented with vascular and intracerebroventricular catheters. The TxA2 analog U-46619 (0, 100, or 1,000 ng.kg-1.min-1) and artificial cerebrospinal fluid (CSF) were infused intracerebroventricularly for 30 min. Heart rate increased in response to 100 ng.kg-1.min-1 U-46619 infusions. Heart rate did not change over preinfusion values in response to the highest infusion rate, but values were elevated compared with the postinfusion period. Mean arterial pressure, ACTH, cortisol, hematocrit, and arterial pH (pHa) increased, and arterial partial CO2 pressure (PaCO2) fell in response to 1,000 ng.kg-1.min-1 infusions of U-46619. Plasma vasopressin concentrations and arterial partial O2 pressure did not change. In a second study, U-46619 or artificial CSF was infused intracerebroventricularly during prostaglandin synthase blockade. Blockade reduced but did not prevent blood pressure responses to U-46619 infusion, suggesting that the U-46619 infusions increased prostaglandin synthase metabolism to contribute de novo TxA2 or a second metabolite to augment the blood pressure response. Heart rate, pHa, PaCO2, ACTH, and cortisol responses to U-46619 were not different with blockade. We conclude that TxA2 acts on the brain to mediate blood pressure, heart rate, pHa, PaCO2, hematocrit, ACTH, and cortisol responses. These findings support the hypothesis that TxA2 acts directly on the brain to promote cardiovascular and hormonal responses that may serve a protective function during conditions when TxA2 formation is increased.