11ß-Hydroxysteroid dehydrogenase 1 deficiency prevents PTSD-like memory in young adult mice.

Post-traumatic stress disorder (PTSD) is characterized by the co-existence of a persistent strong memory of the traumatic experience and amnesia for the peritraumatic context. Most animal models, however, fail to account for the contextual amnesia which is considered to play a critical role in the etiology of PTSD intrusive memories. It is also unclear how aging affects PTSD-like memory. Glucocorticoids alter the formation and retention of fear-associated memory. Here, we investigated whether a deficiency of 11ß-hydroxysteroid dehydrogenase type 1 (11ß-HSD1) (an intracellular glucocorticoid generating enzyme) and aging modulates fear conditioning and PTSD-like memory in mice. We first measured memory in 6 months and 24 months old 11ß-HSD1 deficient (HSD1 KO) and wildtype (WT) mice following paired tone-shock fear conditioning. Then, separate groups of mice were exposed to restraint stress immediately after unpaired tone-shock contextual fear conditioning. Compared with young controls, aged WT mice exhibited enhanced auditory cued fear memory, but contextual fear memory was not different. Contextual fear memory retention was attenuated in both young and aged HSD1 KO mice. In contrast, auditory cued fear memory was reduced 24 h after training only in aged HSD1 KO mice. When fear conditioned with stress, WT mice displayed PTSD-like memory (i.e., increased fear to tone not predictive of shock and reduced fear to 'aversive' conditioning context); this was unchanged with aging. In contrast, young HSD1 KO mice fear conditioned with stress showed normal fear memory (i.e., increased fear response to conditioning context), as observed in WT mice fear conditioned alone. While aged HSD1 KO mice fear conditioned with stress also displayed normal contextual fear memory, the fear response to the 'safe' tone remained. Thus, a deficiency of 11ß-HSD1 protects against both amnesia for the conditioning context and hypermnesia for a salient tone in young adult mice but only contextual amnesia is prevented in aged mice. These results suggest that brain 11ß-HSD1 generated glucocorticoids make a significant contribution to fear conditioning and PTSD-like memory. 11ß-HSD1 inhibition may be useful in prevention and/or treatment of PTSD.

Mining for Oxysterols in Cyp7b1-/- Mouse Brain and Plasma: Relevance to Spastic Paraplegia Type 5.

Deficiency in cytochrome P450 (CYP) 7B1, also known as oxysterol 7α-hydroxylase, in humans leads to hereditary spastic paraplegia type 5 (SPG5) and in some cases in infants to liver disease. SPG5 is medically characterized by loss of motor neurons in the corticospinal tract. In an effort to gain a better understanding of the fundamental biochemistry of this disorder, we have extended our previous profiling of the oxysterol content of brain and plasma of Cyp7b1 knockout (-/-) mice to include, amongst other sterols, 25-hydroxylated cholesterol metabolites. Although brain cholesterol levels do not differ between wild-type (wt) and knockout mice, we find, using a charge-tagging methodology in combination with liquid chromatography-mass spectrometry (LC-MS) and multistage fragmentation (MSn), that there is a build-up of the CYP7B1 substrate 25-hydroxycholesterol (25-HC) in Cyp7b1-/- mouse brain and plasma. As reported earlier, levels of (25R)26-hydroxycholesterol (26-HC), 3ß-hydroxycholest-5-en-(25R)26-oic acid and 24S,25-epoxycholesterol (24S,25-EC) are similarly elevated in brain and plasma. Side-chain oxysterols including 25-HC, 26-HC and 24S,25-EC are known to bind to INSIG (insulin-induced gene) and inhibit the processing of SREBP-2 (sterol regulatory element-binding protein-2) to its active form as a master regulator of cholesterol biosynthesis. We suggest the concentration of cholesterol in brain of the Cyp7b1-/- mouse is maintained by balancing reduced metabolism, as a consequence of a loss in CYP7B1, with reduced biosynthesis. The Cyp7b1-/- mouse does not show a motor defect; whether the defect in humans is a consequence of less efficient homeostasis of cholesterol in brain has yet to be uncovered.

Midlife stress alters memory and mood-related behaviors in old age: Role of locally activated glucocorticoids.

Chronic exposure to stress during midlife associates with subsequent age-related cognitive decline and may increase the vulnerability to develop psychiatric conditions. Increased hypothalamic-pituitary-adrenal (HPA) axis activity has been implicated in pathogenesis though any causative role for glucocorticoids is unestablished. This study investigated the contribution of local glucocorticoid regeneration by the intracellular enzyme 11ß-hydroxysteroid dehydrogenase type 1 (11ß-HSD1), in persisting midlife stress-induced behavioral effects in mice. Middle-aged (10 months old) 11ß-HSD1-deficient mice and wild-type congenic controls were randomly assigned to 28 days of chronic unpredictable stress or left undisturbed (non-stressed). All mice underwent behavioral testing at the end of the stress/non-stress period and again 6-7 months later. Chronic stress impaired spatial memory in middle-aged wild-type mice. The effects, involving a wide spectrum of behavioral modalities, persisted for 6-7 months after cessation of stress into early senescence. Enduring effects after midlife stress included impaired spatial memory, enhanced contextual fear memory, impaired fear extinction, heightened anxiety, depressive-like behavior, as well as reduced hippocampal glucocorticoid receptor mRNA expression. In contrast, 11ß-HSD1 deficient mice resisted both immediate and enduring effects of chronic stress, despite similar stress-induced increases in systemic glucocorticoid activity during midlife stress. In conclusion, chronic stress in midlife exerts persisting effects leading to cognitive and affective dysfunction in old age via mechanisms that depend, at least in part, on brain glucocorticoids generated locally by 11ß-HSD1. This finding supports selective 11ß-HSD1 inhibition as a novel therapeutic target to ameliorate the long-term consequences of stress-related psychiatric disorders in midlife.

Cognitive and Disease-Modifying Effects of 11ß-Hydroxysteroid Dehydrogenase Type 1 Inhibition in Male Tg2576 Mice, a Model of Alzheimer's Disease.

Chronic exposure to elevated levels of glucocorticoids has been linked to age-related cognitive decline and may play a role in Alzheimer's disease. In the brain, 11ß-hydroxysteroid dehydrogenase type 1 (11ß-HSD1) amplifies intracellular glucocorticoid levels. We show that short-term treatment of aged, cognitively impaired C57BL/6 mice with the potent and selective 11ß-HSD1 inhibitor UE2316 improves memory, including after intracerebroventricular drug administration to the central nervous system alone. In the Tg2576 mouse model of Alzheimer's disease, UE2316 treatment of mice aged 14 months for 4 weeks also decreased the number of ß-amyloid (Aß) plaques in the cerebral cortex, associated with a selective increase in local insulin-degrading enzyme (involved in Aß breakdown and known to be glucocorticoid regulated). Chronic treatment of young Tg2576 mice with UE2316 for up to 13 months prevented cognitive decline but did not prevent Aß plaque formation. We conclude that reducing glucocorticoid regeneration in the brain improves cognition independently of reduced Aß plaque pathology and that 11ß-HSD1 inhibitors have potential as cognitive enhancers in age-associated memory impairment and Alzheimer's dementia.

Diurnal and stress-induced intra-hippocampal corticosterone rise attenuated in 11ß-HSD1-deficient mice: a microdialysis study in young and aged mice.

11ß-Hydroxysteroid dehydrogenase type 1 (11ß-HSD1) locally regenerates active glucocorticoids from their inert forms thereby amplifying intracellular levels within target tissues including the brain. We previously showed greater increases in intra-hippocampal corticosterone (CORT) levels upon Y-maze testing in aged wild-type than in 11ß-HSD1(-/-) mice coinciding with impaired and intact spatial memory, respectively. Here we examined whether ageing influences 11ß-HSD1 regulation of CORT in the dorsal hippocampus under basal conditions during the diurnal cycle and following stress. Intra-hippocampal CORT levels measured by in vivo microdialysis in freely behaving wild-type mice displayed a diurnal variation with peak levels in the evening that were significantly elevated with ageing. In contrast, the diurnal rise in intra-hippocampal CORT levels was greatly diminished in 11ß-HSD1(-/-) mice and there was no rise with ageing; basal intra-hippocampal CORT levels were similar to wild-type controls. Furthermore, a short (3 min) swim stress induced a longer lasting increase in intra-hippocampal CORT levels in wild-type mice than in 11ß-HSD1(-/-) mice despite no genotypic differences in elevation of plasma CORT. These data indicate that 11ß-HSD1 activity contributes substantially to diurnal and stress-induced increases in hippocampal CORT levels. This contribution is even greater with ageing. Thus, 11ß-HSD1 inhibition may be an attractive target for treating cognitive impairments associated with stress or ageing.

Intrahippocampal glucocorticoids generated by 11ß-HSD1 affect memory in aged mice.

11Beta-hydroxysteroid dehydrogenase type 1 (11ß-HSD1) locally amplifies active glucocorticoids within specific tissues including in brain. In the hippocampus, 11ß-HSD1 messenger RNA increases with aging. Here, we report significantly greater increases in intrahippocampal corticosterone (CORT) levels in aged wild-type (WT) mice during the acquisition and retrieval trials in a Y-maze than age-matched 11ß-HSD1(-/-) mice, corresponding to impaired and intact spatial memory, respectively. Acute stress applied to young WT mice led to increases in intrahippocampal CORT levels similar to the effects of aging and impaired retrieval of spatial memory. 11ß-HSD1(-/-) mice resisted the stress-induced memory impairment. Pharmacologic inhibition of 11ß-HSD1 abolished increases in intrahippocampal CORT levels during the Y-maze trials and prevented spatial memory impairments in aged WT mice. These data provide the first in vivo evidence that dynamic increases in hippocampal 11ß-HSD1 regenerated CORT levels during learning and retrieval play a key role in age- and stress-associated impairments of spatial memory.

Short-term inhibition of 11ß-hydroxysteroid dehydrogenase type 1 reversibly improves spatial memory but persistently impairs contextual fear memory in aged mice.

High glucocorticoid levels induced by stress enhance the memory of fearful events and may contribute to the development of anxiety and posttraumatic stress disorder. In contrast, elevated glucocorticoids associated with ageing impair spatial memory. We have previously shown that pharmacological inhibition of the intracellular glucocorticoid-amplifying enzyme 11ß-hydroxysteroid dehydrogenase type 1 (11ß-HSD1) improves spatial memory in aged mice. However, it is not known whether inhibition of 11ß-HSD1 will have any beneficial effects on contextual fear memories in aged mice. Here, we examined the effects of UE2316, a selective 11ß-HSD1 inhibitor which accesses the brain, on both spatial and contextual fear memories in aged mice using a vehicle-controlled crossover study design. Short-term UE2316 treatment improved spatial memory in aged mice, an effect which was reversed when UE2316 was substituted with vehicle. In contrast, contextual fear memory induced by foot-shock conditioning was significantly reduced by UE2316 in a non-reversible manner. When the order of treatment was reversed following extinction of the original fear memory, and a second foot-shock conditioning was given in a novel context, UE2316 treated aged mice (previously on vehicle) now showed increased fear memory compared to vehicle-treated aged mice (previously on UE2316). Renewal of the original extinguished fear memory triggered by exposure to a new environmental context may explain these effects. Thus 11ß-HSD1 inhibition reverses spatial memory impairments with ageing while reducing the strength and persistence of new contextual fear memories. Potentially this could help prevent anxiety-related disorders in vulnerable elderly individuals.

Cholestenoic acids regulate motor neuron survival via liver X receptors.

Cholestenoic acids are formed as intermediates in metabolism of cholesterol to bile acids, and the biosynthetic enzymes that generate cholestenoic acids are expressed in the mammalian CNS. Here, we evaluated the cholestenoic acid profile of mammalian cerebrospinal fluid (CSF) and determined that specific cholestenoic acids activate the liver X receptors (LXRs), enhance islet-1 expression in zebrafish, and increase the number of oculomotor neurons in the developing mouse in vitro and in vivo. While 3ß,7α-dihydroxycholest-5-en-26-oic acid (3ß,7α-diHCA) promoted motor neuron survival in an LXR-dependent manner, 3ß-hydroxy-7-oxocholest-5-en-26-oic acid (3ßH,7O-CA) promoted maturation of precursors into islet-1+ cells. Unlike 3ß,7α-diHCA and 3ßH,7O-CA, 3ß-hydroxycholest-5-en-26-oic acid (3ß-HCA) caused motor neuron cell loss in mice. Mutations in CYP7B1 or CYP27A1, which encode enzymes involved in cholestenoic acid metabolism, result in different neurological diseases, hereditary spastic paresis type 5 (SPG5) and cerebrotendinous xanthomatosis (CTX), respectively. SPG5 is characterized by spastic paresis, and similar symptoms may occur in CTX. Analysis of CSF and plasma from patients with SPG5 revealed an excess of the toxic LXR ligand, 3ß-HCA, while patients with CTX and SPG5 exhibited low levels of the survival-promoting LXR ligand 3ß,7α-diHCA. Moreover, 3ß,7α-diHCA prevented the loss of motor neurons induced by 3ß-HCA in the developing mouse midbrain in vivo.Our results indicate that specific cholestenoic acids selectively work on motor neurons, via LXR, to regulate the balance between survival and death.

24S,25-Epoxycholesterol in mouse and rat brain.

24S,25-Epoxycholesterol is formed in a shunt of the mevalonate pathway that produces cholesterol. It is one of the most potent known activators of the liver X receptors and can inhibit sterol regulatory element-binding protein processing. Until recently analysis of 24S,25-epoxycholesterol at high sensitivity has been precluded by its thermal lability and lack of a strong chromophore. Here we report on the analysis of 24S,25-epoxycholesterol in rodent brain where its level was determined to be of the order of 0.4-1.4µg/g wet weight in both adult mouse and rat. For comparison the level of 24S-hydroxycholesterol in brain of both rodents was of the order of 20µg/g, while that of cholesterol in mouse was 10-20mg/g. By exploiting knockout mice for the enzyme oxysterol 7α-hydroxylase (Cyp7b1) we show that this enzymes is important for the subsequent metabolism of the 24S,25-epoxide.

Inhibiting 11ß-hydroxysteroid dehydrogenase type 1 prevents stress effects on hippocampal synaptic plasticity and impairs contextual fear conditioning.

11ß-Hydroxysteroid dehydrogenase type 1 (11ß-HSD1) catalyzes intracellular regeneration of corticosterone and cortisol, thereby enhancing glucocorticoid action. Inhibition of 11ß-HSD1 reverses the deficits in cognition with aging, a state of elevated glucocorticoid levels. However, any impact of 11ß-HSD1 inhibition during high glucocorticoid states in younger animals is unknown. Here we examined whether a single injection of the selective 11ß-HSD1 inhibitor UE2316 modifies the effect of stress on hippocampal long-term potentiation and fear conditioning, a learning paradigm that is strongly modulated by glucocorticoids. We found that novelty stress suppresses hippocampal synaptic potentiation. This effect was completely prevented by administration of UE2316 one hour before stress exposure. A single injection of UE2316 also impaired contextual, but not tone-cue-fear conditioning. These observations suggest that local metabolism of glucocorticoids is relevant for the outcome of stress effects on hippocampal synaptic plasticity and contextual fear conditioning. Selective 11ß-HSD1 inhibitors may be an interesting new approach to the prevention of trauma-associated psychopathology.

Local amplification of glucocorticoids in the aging brain and impaired spatial memory.

The hippocampus is a prime target for glucocorticoids (GCs) and a brain structure particularly vulnerable to aging. Prolonged exposure to excess GCs compromises hippocampal electrophysiology, structure, and function. Blood GC levels tend to increase with aging and correlate with impaired spatial memory in aging rodents and humans. The magnitude of GC action within tissues depends not only on levels of steroid hormone that enter the cells from the periphery and the density of intracellular receptors but also on the local metabolism of GCs by 11ß-hydroxysteroid dehydrogenases (11ß-HSD). The predominant isozyme in the adult brain, 11ß-HSD1, locally regenerates active GCs from inert 11-keto forms thus amplifying GC levels within specific target cells including in the hippocampus and cortex. Aging associates with elevated hippocampal and neocortical 11ß-HSD1 and impaired spatial learning while deficiency of 11ß-HSD1 in knockout (KO) mice prevents the emergence of cognitive decline with age. Furthermore, short-term pharmacological inhibition of 11ß-HSD1 in already aged mice reverses spatial memory impairments. Here, we review research findings that support a key role for GCs with special emphasis on their intracellular regulation by 11ß-HSD1 in the emergence of spatial memory deficits with aging, and discuss the use of 11ß-HSD1 inhibitors as a promising novel treatment in ameliorating/improving age-related memory impairments.

11beta-hydroxysteroid dehydrogenase type 1 deficiency prevents memory deficits with aging by switching from glucocorticoid receptor to mineralocorticoid receptor-mediated cognitive control.

Local brain amplification of glucocorticoids (GCs) by 11ß-hydroxysteroid dehydrogenase type 1 (11ß-HSD1) plays a pivotal role in age-related memory deficits. 11ß-HSD1 deficient mice are protected from spatial memory impairments with aging, but the underlying mechanisms are unknown. To determine which brain receptors [high-affinity mineralocorticoid receptors (MRs) or low-affinity glucocorticoid receptors (GRs)] are involved, spatial memory was measured in aged 11ß-HSD1(-/-) mice before and during intracerebroventricular infusion (10 d) of spironolactone (MR antagonist) or RU486 (GR antagonist). Aged C57BL/6J control mice showed impaired spatial memory in the Y-maze; this improved with GR blockade, while MR blockade had no effect. In contrast, aged 11ß-HSD1(-/-) mice showed intact spatial memory that became impaired with MR blockade, but not GR blockade. Hippocampal MR and GR mRNA expression and plasma corticosterone levels were not significantly altered with spironolactone or RU486 in either genotype. These data support the notion that 11ß-HSD1 deficiency in aging mice leads to lower intracellular GC concentrations in brain, particularly in the hippocampus, which activate predominantly MRs to enhance memory, while in aging C57BL/6J controls, the increased intracellular GCs saturate MRs and activate predominantly GRs, thus impairing memory, an effect reversed by GR blockade.

Manganese-enhanced magnetic resonance imaging (MEMRI) of rat brain after systemic administration of MnCl2: hippocampal signal enhancement without disruption of hippocampus-dependent behavior.

Manganese (Mn(2+))-enhanced magnetic resonance (MR) imaging (MEMRI) in rodents offers unique opportunities for the longitudinal study of hippocampal structure and function in parallel with cognitive testing. However, Mn(2+) is a potent toxin and there is evidence that it can interfere with neuronal function. Thus, apart from causing adverse peripheral side effects, Mn(2+) may disrupt the function of brain areas where it accumulates to produce signal enhancement and, thereby, Mn(2+) administration may confound cognitive testing. Here, we examined in male adult Lister hooded rats if a moderate systemic dose of MnCl2 (200 µmol/kg; two intraperitoneal injections of 100 µmol/kg separated by 1 h) that produces hippocampal MR signal enhancement would disrupt hippocampal function. To this end, we used a delayed-matching-to-place (DMP) watermaze task, which requires rapid allocentric place learning and is highly sensitive to interference with hippocampal function. Tested on the DMP task 1 h and 24 h after MnCl2 injection, rats did not show any impairment in indices of memory performance (latencies, search preference) or any sensorimotor effects. However, MnCl2 injection caused acute peripheral effects (severe ataxia and erythema, i.e. redness of paws, ears, and nose) which subsided over 30 min. Additionally, rats injected with MnCl2 showed reduced weight 1 day after injection and failed to reach the normal weight-growth curve of control rats within the 16 days monitored. Our results indicate that 200 µmol/kg MnCl2 produces hippocampal MR signal enhancement without disrupting hippocampus-dependent behavior on a rapid place learning task, even though attention must be paid to peripheral side effects.

Partial deficiency or short-term inhibition of 11beta-hydroxysteroid dehydrogenase type 1 improves cognitive function in aging mice.

11ß-Hydroxysteroid dehydrogenase type 1 (11ß-HSD1) regenerates active glucocorticoids (GCs) from intrinsically inert 11-keto substrates inside cells, including neurons, thus amplifying steroid action. Excess GC action exerts deleterious effects on the hippocampus and causes impaired spatial memory, a key feature of age-related cognitive dysfunction. Mice with complete deficiency of 11ß-HSD1 are protected from spatial memory impairments with aging. Here, we tested whether lifelong or short-term decreases in 11ß-HSD1 activity are sufficient to alter cognitive function in aged mice. Aged (24 months old) heterozygous male 11ß-HSD1 knock-out mice, with ∼60% reduction in hippocampal 11ß-reductase activity throughout life, were protected against spatial memory impairments in the Y-maze compared to age-matched congenic C57BL/6J controls. Pharmacological treatment of aged C57BL/6J mice with a selective 11ß-HSD1 inhibitor (UE1961) for 10 d improved spatial memory performance in the Y-maze (59% greater time in novel arm than vehicle control). These data support the use of selective 11ß-HSD1 inhibitors in the treatment of age-related cognitive impairments.

11beta-hydroxysteroid dehydrogenase type 1 expression is increased in the aged mouse hippocampus and parietal cortex and causes memory impairments.

Increased neuronal glucocorticoid exposure may underlie interindividual variation in cognitive function with aging in rodents and humans. 11beta-Hydroxysteroid dehydrogenase type 1 (11beta-HSD1) catalyzes the regeneration of active glucocorticoids within cells (in brain and other tissues), thus amplifying steroid action. We examined whether 11beta-HSD1 plays a role in the pathogenesis of cognitive deficits associated with aging in male C57BL/6J mice. We show that 11beta-HSD1 levels increase with age in CA3 hippocampus and parietal cortex, correlating with impaired cognitive performance in the water maze. In contrast, neither circulating corticosterone levels nor tissue corticosteroid receptor expression correlates with cognition. 11beta-HSD1 elevation appears causal, since aging (18 months) male transgenic mice with forebrain-specific 11beta-HSD1 overexpression ( approximately 50% in hippocampus) exhibit premature age-associated cognitive decline in the absence of altered circulating glucocorticoid levels or other behavioral (affective) deficits. Thus, excess 11beta-HSD1 in forebrain is a cause of as well as a therapeutic target in memory impairments with aging.

Enhanced hippocampal long-term potentiation and spatial learning in aged 11beta-hydroxysteroid dehydrogenase type 1 knock-out mice.

Glucocorticoids are pivotal in the maintenance of memory and cognitive functions as well as other essential physiological processes including energy metabolism, stress responses, and cell proliferation. Normal aging in both rodents and humans is often characterized by elevated glucocorticoid levels that correlate with hippocampus-dependent memory impairments. 11Beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) amplifies local intracellular ("intracrine") glucocorticoid action; in the brain it is highly expressed in the hippocampus. We investigated whether the impact of 11beta-HSD1 deficiency in knock-out mice (congenic on C57BL/6J strain) on cognitive function with aging reflects direct CNS or indirect effects of altered peripheral insulin-glucose metabolism. Spatial learning and memory was enhanced in 12 month "middle-aged" and 24 month "aged" 11beta-HSD1(-/-) mice compared with age-matched congenic controls. These effects were not caused by alterations in other cognitive (working memory in a spontaneous alternation task) or affective domains (anxiety-related behaviors), to changes in plasma corticosterone or glucose levels, or to altered age-related pathologies in 11beta-HSD1(-/-) mice. Young 11beta-HSD1(-/-) mice showed significantly increased newborn cell proliferation in the dentate gyrus, but this was not maintained into aging. Long-term potentiation was significantly enhanced in subfield CA1 of hippocampal slices from aged 11beta-HSD1(-/-) mice. These data suggest that 11beta-HSD1 deficiency enhances synaptic potentiation in the aged hippocampus and this may underlie the better maintenance of learning and memory with aging, which occurs in the absence of increased neurogenesis.

Forebrain mineralocorticoid receptor overexpression enhances memory, reduces anxiety and attenuates neuronal loss in cerebral ischaemia.

The nuclear mineralocorticoid receptor (MR), a high-affinity receptor for glucocorticoids, is highly expressed in the hippocampus where it underpins cognitive, behavioural and neuroendocrine regulation. Increased neuronal MR expression occurs early in the response to cellular injury in vivo and in vitro and is associated with enhanced neuronal survival. To determine whether increased neuronal MR might be causal in protecting against ischaemic damage in vivo we generated a forebrain-specific MR-overexpressing transgenic mouse (MR-Tg) under the control of the CamKII alpha promoter, and subjected mice to transient cerebral global ischaemia induced by bilateral common carotid artery occlusion for 20 min. We also separately assessed the effects of MR overexpression on hypothalamic-pituitary-adrenal (HPA) axis activity and cognitive and affective functions in noninjured animals. Our results showed that MR-Tg mice had significantly reduced neuronal death following transient cerebral global ischaemia compared to wild-type littermates. This effect was not associated with alterations in basal or poststress HPA axis function or in arterial blood pressure. MR-Tg mice also demonstrated improved spatial memory retention, reduced anxiety and altered behavioural response to novelty. The induction of neuronal MR appears to offer a protective response which has potential therapeutic implications in cerebral ischaemia and cognitive and affective disorders.

The antidepressant desipramine requires the ABCB1 (Mdr1)-type p-glycoprotein to upregulate the glucocorticoid receptor in mice.

The mechanisms by which antidepressants regulate the hypothalamic-pituitary-adrenal (HPA) axis are still unknown. The ABCB1-type multiple drug resistance (MDR) p-glycoprotein (PGP) regulates the HPA axis by limiting the access of glucocorticoids to the brain in mice and humans. Previous work in cell cultures has found that antidepressants enhance glucocorticoid receptor (GR) function in vitro by inhibiting MDR PGP, and therefore by increasing the intracellular concentration of glucocorticoids-but this model has never been tested directly in animals. Here, the tricyclic antidepressant, desipramine (20 mg/kg/day, i.p., for seven days), was administered to abcb1ab MDR PGP knockout mice (congenic on the FVB/N background strain) and to FVB/N controls. The hippocampal mRNA expression of GR, mineralocorticoid receptor (MR), MDR (Mdr1a) PGP, and 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) were measured, together with plasma corticosterone levels. In FVB/N controls, desipramine induced a significant upregulation of GR mRNA in the CA1 region (+31%; p=0.045); in contrast, in abcb1ab (-/-) mice, desipramine induced a significant downregulation of GR mRNA in the CA1 region (-45%; p=0.004). MR mRNA expression was unaltered. Desipramine decreased corticosterone levels in both FVB/N controls and in abcb1ab (-/-) mice, but in abcb1ab (-/-) mice the effects were smaller. Specifically, in FVB/N controls (but not in abcb1ab (-/-) mice), desipramine reduced corticosterone levels not only compared with saline-treated mice but also compared with the 'physiological' levels of untreated mice (-39%; p=0.05). Finally, desipramine reduced Mdr1a mRNA expression across all hippocampus areas (-9 to -23%), but had no effect on 11beta-HSD1 mRNA expression. These data support the notion that the MDR PGP is one of the molecular targets through which antidepressants regulate the HPA axis.

Central administration of a cytochrome P450-7B product 7 alpha-hydroxypregnenolone improves spatial memory retention in cognitively impaired aged rats.

Pregnenolone (PREG) and dehydroepiandrosterone (DHEA) have been reported to improve memory in aged rodents. In brain, these neurosteroids are transformed predominantly into 7alpha-hydroxylated metabolites by the cytochrome P450-7B1 (CYP7B). The biological role of steroid B-ring hydroxylation is unclear. It has been proposed to generate bioactive derivatives that enhance cognition, immune, and other physiological processes. In support, 7alpha-hydroxylated DHEA increases the immune response in mice with greater potency than the parent steroid. Whether the memory-enhancing effects of PREG in rats is mediated via its 7alpha-hydroxylated metabolite 7alpha-hydroxyPREG is not known. We investigated this by treating memory-impaired aged rats (identified by their spatial memory performances in the Morris water maze task compared with young controls) with 7alpha-hydroxyPREG or PREG administered intracerebroventricularly using osmotic minipumps and then tested the rats during week 2 of steroid treatment in the eight-arm radial-arm version of the water maze (RAWM) that allows repeated assessment of learning. CYP7B bioactivity in hippocampal tissue (percentage conversion of [14C]DHEA to [14C]7alpha-hydroxyDHEA) was decreased selectively in memory-impaired aged rats compared with both young and memory-intact aged rats. 7alpha-hydroxyPREG (100 ng/h) but not PREG (100 ng/h) administration to memory-impaired aged rats for 11 d enhanced spatial memory retention (after a 30 min delay between an exposure trial 1 and test trial 2) in the RAWM. These data provide evidence for a biologically active enzyme product 7alpha-hydroxyPREG and suggests that reduced CYP7B function in the hippocampus of memory-impaired aged rats may, in part, be overcome by administration of 7alpha-hydroxyPREG.

Differential regulation of variant glucocorticoid receptor mRNAs in the rat hippocampus by the antidepressant fluoxetine.

Adult rats were given antidepressant drugs orally. Fluoxetine, but not moclobemide, venlafaxine, tianeptine or desipramine, increased total glucocorticoid receptor (GR) mRNA in the hippocampus after 4 weeks. Further examination revealed that GR mRNA containing the brain-specific exon 1(7) was increased across all hippocampal subregions. In contrast, expression of the major exon 1(10) and another brain-specific exon 1(5)-containing GR mRNAs were unchanged. Tissue-specific first exon usage may contribute to the differential regulation of GR by fluoxetine in brain subregions.