Low doses of dexamethasone can produce a hypocorticosteroid state in the brain.

The synthetic glucocorticoid dexamethasone (dex) blocks stress-induced hypothalamic-pituitary-adrenal (HPA) activation primarily at the level of the anterior pituitary because multidrug resistance P-glycoprotein hampers its penetration in the brain. Here, we tested the hypothesis that central components of the HPA axis would escape dex suppression under conditions of potent peripheral glucocorticoid action. We subchronically treated rats with low or high doses of dex. The animals were subjected on the last day of treatment for 30 min to a restraint stressor after which central and peripheral markers of HPA axis activity were measured. Basal and stress-induced corticosterone secretion, body weight gain, adrenal and thymus weight, as well as proopiomelanocortin mRNA in the anterior pituitary were reduced in a dose-dependent manner by dex administered either 5 d sc or 3 wk orally. In the brain, the highest dose dex suppressed CRH mRNA and CRH heteronuclear RNA in the paraventricular nucleus (PVN). However, in the peripherally active low-dose range of dex CRH mRNA and heteronuclear RNA showed resistance to suppression, and CRH mRNA expression in the PVN was in fact enhanced under the long-term treatment condition. In the PVN, c-fos mRNA was suppressed by the highest dose of dex, but this effect showed a degree of resistance after long-term oral treatment. c-fos mRNA responses in the anterior pituitary followed those in PVN and reflect central drive of the HPA axis even if corticosterone responses are strongly reduced. The results support the concept that low doses of dex can create a hypocorticoid state in the brain.

Multidrug resistance P-glycoprotein hampers the access of cortisol but not of corticosterone to mouse and human brain.

In the present study, we investigated the role of the multidrug resistance (mdr) P-glycoprotein (Pgp) at the blood-brain barrier in the control of access of cortisol and corticosterone to the mouse and human brain. [(3)H]Cortisol poorly penetrated the brain of adrenalectomized wild-type mice, but the uptake was 3.5-fold enhanced after disruption of Pgp expression in mdr 1a(-/-) mice. In sharp contrast, treatment with [(3)H]corticosterone revealed high labeling of brain tissue without difference between both genotypes. Interestingly, human MDR1 Pgp also differentially transported cortisol and corticosterone. LLC-PK1 monolayers stably transfected with MDR1 complementary DNA showed polar transport of [(3)H]cortisol that could be blocked by a specific Pgp blocker, whereas [(3)H]corticosterone transport did not differ between transfected and host cells. Determination of the concentration of both steroids in extracts of human postmortem brain tissue using liquid chromatography mass spectrometry revealed that the ratio of corticosterone over cortisol in the brain was significantly increased relative to plasma. In conclusion, the data demonstrate that in both mouse and human brain the penetration of cortisol is less than that of corticosterone. This finding suggests a more prominent role for corticosterone in control of human brain function than hitherto recognized.

Cell- and tIssue-specific effects of corticosteroids in relation to glucocorticoid resistance: examples from the brain.

The biological mechanisms that determine cell-specific responses to glucocorticoid hormones may overlap with those that are associated with acquired glucocorticoid resistance. Cell and tIssue specificity can be brought about in many different ways. Studies on the brain, an important glucocorticoid target tIssue, may provide examples of regulatory mechanisms underlying response specificity at multiple levels. In this commentary a number of such mechanisms are discussed, with emphasis on regulation of glucocorticoid bio-availability by the efflux transporter P-glycoprotein and on the variable presence of nuclear proteins which modulate or interfere with gluco- and mineralocorticoid receptor-mediated transcription.

The role of the efflux transporter P-glycoprotein in brain penetration of prednisolone.

In the present study, we have investigated the role of the multidrug resistance (mdr) P-glycoprotein (Pgp) at the blood-brain barrier in hampering the access of the synthetic glucocorticoid, prednisolone. In vivo, a tracer dose of [(3)H]prednisolone poorly penetrated the brain of adrenalectomised wild-type mice, but the uptake was more than threefold enhanced in the absence of Pgp expression in mdr1a (-/-) mice. In vitro, in stably transfected LLC-PK1 monolayers the human MDR1 P-glycoprotein was able to transport prednisolone present at a micromolar concentration. A specific Pgp blocker, LY 335979, could block this polar transport of [(3)H]prednisolone. Human Pgp does not transport all steroids, as cortexolone was not transported at all and aldosterone was only weakly transported. The ability of Pgp to export the synthetic glucocorticoid, prednisolone, suggests that uptake of prednisolone in the human brain is impaired, leading to a discrepancy between central and peripheral actions. Furthermore, the ensuing imbalance in activation of the two types of brain corticosteroid receptors may have consequences for cognitive performance and mood.

Localization of mRNA expression of P-glycoprotein at the blood-brain barrier and in the hippocampus.

The multidrug resistance (mdr) P-glycoprotein is an energy-dependent efflux transporter that protects the brain against a wide variety of neurotoxic compounds. This transmembrane protein is a well-known functional component of the blood-brain barrier and might be present in other brain cells as well. We have developed a riboprobe against the murine mdr1 mRNA recognizing both isoforms of the rodent mdr1 gene to determine the exact localization of P-glycoprotein expression. We have also studied the effects of treatment with a known inducer of P-glycoprotein expression. In situ mRNA hybridization demonstrates that mdr1 mRNA is present in the endothelial cells of brain capillaries throughout the rat brain, indicating that P-glycoprotein is expressed at the endothelial cells forming the blood-brain barrier. Surprisingly, specific mdr1 mRNA expression was also found in neuronal layers of hippocampal fields, particularly in the granule cells of the dentate gyrus. Kainic acid treatment decreased the expression levels of mdr1 mRNA in the dentate gyrus 6 and 24 h after treatment. Our data indicate that P-glycoprotein is expressed by endothelial cells and possibly dentate gyrus neurons The functional role of P-glycoprotein at dentate gyrus neurons is presently unknown.

A crossed projection from the optic tectum to craniocervical premotor areas in the brainstem reticular formation. An anterograde and retrograde tracing study in the mallard (Anas platyrhynchos L.).

The optic tectum in birds receives visual information from the contralateral retina. This information is passed through to other brain areas via the deep layers of the optic tectum. In the present study the crossed tectobulbar pathway is described in detail. This pathway forms the connection between the optic tectum and the premotor area of craniocervical muscles in the contralateral paramedian reticular formation. It originates predominantly from neurons in the ventromedial part of stratum griseum centrale and to a lesser extent from stratum album centrale. The fibers leave the tectum as a horizontal fiber bundle, and cross the midline through the caudal radix oculomotorius and rostral nucleus oculomotorius. On the contralateral side fibers turn to ventral and descend caudally in the contralateral paramedian reticular formation to the level of the obex. Labeled terminals are found in the ipsilateral medial mesencephalic reticular formation lateral to the radix and motor nucleus of the oculomotor nerve, and in the contralateral paramedian reticular formation, along the descending tract. Neurons in the medial mesencephalic reticular formation in turn project to the paramedian reticular formation. Through the crossed tectobulbar pathway visual information can influence the activity of craniocervical muscles via reticular premotor neurons.

Stress-induced changes in primate prefrontal profiles of gene expression.

Stressful experiences that consistently increase cortisol levels appear to alter the expression of hundreds of genes in prefrontal limbic brain regions. Here, we investigate this hypothesis in monkeys exposed to intermittent social stress-induced episodes of hypercortisolism or a no-stress control condition. Prefrontal profiles of gene expression compiled from Affymetrix microarray data for monkeys randomized to the no-stress condition were consistent with microarray results published for healthy humans. In monkeys exposed to intermittent social stress, more genes than expected by chance appeared to be differentially expressed in ventromedial prefrontal cortex compared to monkeys not exposed to adult social stress. Most of these stress responsive candidate genes were modestly downregulated, including ubiquitin conjugation enzymes and ligases involved in synaptic plasticity, cell cycle progression and nuclear receptor signaling. Social stress did not affect gene expression beyond that expected by chance in dorsolateral prefrontal cortex or prefrontal white matter. Thirty four of 48 comparisons chosen for verification by quantitative real-time polymerase chain reaction (qPCR) were consistent with the microarray-predicted result. Furthermore, qPCR and microarray data were highly correlated. These results provide new insights on the regulation of gene expression in a prefrontal corticolimbic region involved in the pathophysiology of stress and major depression. Comparisons between these data from monkeys and those for ventromedial prefrontal cortex in humans with a history of major depression may help to distinguish the molecular signature of stress from other confounding factors in human postmortem brain research.