Forebrain-Specific Transgene Rescue of 11ß-HSD1 Associates with Impaired Spatial Memory and Reduced Hippocampal Brain-Derived Neurotrophic Factor mRNA Levels in Aged 11ß-HSD1 Deficient Mice.

Mice lacking the intracellular glucocorticoid-regenerating enzyme 11ß-hydroxysteroid dehydrogenase type 1 (11ß-HSD1) are protected from age-related spatial memory deficits. 11ß-HSD1 is expressed predominantly in the brain, liver and adipose tissue. Reduced glucocorticoid levels in the brain in the absence of 11ß-HSD1 may underlie the improved memory in aged 11ß-HSD1 deficient mice. However, the improved glucose tolerance, insulin sensitisation and cardioprotective lipid profile associated with reduced peripheral glucocorticoid regeneration may potentially contribute to the cognitive phenotype of aged 11ß-HSD1 deficient mice. In the present study, transgenic mice with forebrain-specific overexpression of 11ß-HSD1 (Tg) were intercrossed with global 11ß-HSD1 knockout mice (HSD1KO) to examine the influence of forebrain and peripheral 11ß-HSD1 activity on spatial memory in aged mice. Transgene-mediated delivery of 11ß-HSD1 to the hippocampus and cortex of aged HSD1KO mice reversed the improved spatial memory retention in the Y-maze but not spatial learning in the watermaze. Brain-derived neurotrophic factor (BDNF) mRNA levels in the hippocampus of aged HSD1KO mice were increased compared to aged wild-type mice. Rescue of forebrain 11ß-HSD1 reduced BDNF mRNA in aged HSD1KO mice to levels comparable to aged wild-type mice. These findings indicate that 11ß-HSD1 regenerated glucocorticoids in the forebrain and decreased levels of BDNF mRNA in the hippocampus play a role in spatial memory deficits in aged wild-type mice, although 11ß-HSD1 activity in peripheral tissues may also contribute to spatial learning impairments in aged mice.

Decreased Npas4 and Arc mRNA Levels in the Hippocampus of Aged Memory-Impaired Wild-Type But Not Memory Preserved 11ß-HSD1 Deficient Mice.

Mice deficient in the glucocorticoid-regenerating enzyme 11ß-HSD1 resist age-related spatial memory impairment. To investigate the mechanisms and pathways involved, we used microarrays to identify differentially expressed hippocampal genes that associate with cognitive ageing and 11ß-HSD1. Aged wild-type mice were separated into memory-impaired and unimpaired relative to young controls according to their performance in the Y-maze. All individual aged 11ß-HSD1-deficient mice showed intact spatial memory. The majority of differentially expressed hippocampal genes were increased with ageing (e.g. immune/inflammatory response genes) with no genotype differences. However, the neuronal-specific transcription factor, Npas4, and immediate early gene, Arc, were reduced (relative to young) in the hippocampus of memory-impaired but not unimpaired aged wild-type or aged 11ß-HSD1-deficient mice. A quantitative reverse transcriptase-polymerase chain reaction and in situ hybridisation confirmed reduced Npas4 and Arc mRNA expression in memory-impaired aged wild-type mice. These findings suggest that 11ß-HSD1 may contribute to the decline in Npas4 and Arc mRNA levels associated with memory impairment during ageing, and that decreased activity of synaptic plasticity pathways involving Npas4 and Arc may, in part, underlie the memory deficits seen in cognitively-impaired aged wild-type mice.

Dehydroepiandrosterone 7-hydroxylase CYP7B: predominant expression in primate hippocampus and reduced expression in Alzheimer's disease.

Neurosteroids such as dehydroepiandrosterone (DHEA), pregnenolone and 17beta-estradiol are synthesized by cytochrome P450s from endogenous cholesterol. We previously reported a new cytochrome P450 enzyme, CYP7B, highly expressed in rat and mouse brain that metabolizes DHEA and related steroids by hydroxylation at the 7alpha position. Such 7-hydroxylation can enhance DHEA bioactivity in vivo. Here we show that the reaction is conserved across mammalian species: in addition to mouse and rat, DHEA hydroxylation activity was present in brain extracts from sheep, marmoset and human. Northern blotting using a human CYP7B complementary deoxyribonucleic acid (cDNA) probe confirmed the presence of CYP7B mRNA in marmoset and human hippocampus; CYP7B mRNA was present in marmoset cerebellum and brainstem, with lower levels in hypothalamus and cortex. In situ hybridization to human brain revealed higher levels of CYP7B mRNA in the hippocampus than in cerebellum, cortex, or other brain regions. We also measured CYP7B expression in Alzheimer's disease (AD). CYP7B mRNA was significantly decreased (approximately 50% decline; P<0.05) in dentate neurons from AD subjects compared with controls. A decline in CYP7B activity may contribute the loss of effects of DHEA with ageing and perhaps to the pathophysiology of AD.

Differential regulation of corticosteroid receptors by monoamine neurotransmitters and antidepressant drugs in primary hippocampal culture.

Hyperactivity of the hypothalamic-pituitary-adrenal axis is a characteristic feature of depressive illness. The centrally located corticosteroid receptors, the glucocorticoid and mineralocorticoid receptors, are thought to be important modulators of this axis and changes in the levels of these receptors, particularly in the hippocampus, may underlie the hyperactivity observed. Various antidepressant drugs increase hippocampal mineralocorticoid and glucocorticoid receptor levels in vivo. These effects are thought to be mediated via alterations in monoaminergic neurotransmission. We examined whether serotonin (5HT) and noradrenaline (NA) have direct effects on glucocorticoid receptor and mineralocorticoid receptor expression in primary hippocampal neurones, and whether antidepressants also exert direct effects on target neurones. Exposure of hippocampal cells to 5HT for 4 days increased both glucocorticoid and mineralocorticoid receptor mRNA and protein expression. The induction of mineralocorticoid receptor mRNA was completely blocked by the 5HT(7) receptor antagonist SB 269970. In contrast glucocorticoid receptor induction was insensitive to the 5HT(7) receptor, whilst studies with the 5HT(1A) receptor agonist 8-hydroxy-2-(di-n-proplamino) tetralin hydrochloride and the 5HT(1A) receptor antagonist N-[2-[4-2-[O-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexane carboxamide trihydrochloride (WAY 100635) suggest a partial role for 5HT(1A) receptors in hippocampal glucocorticoid receptor regulation. Treatment with NA for 4 days also increased glucocorticoid receptor expression but had no effect on mineralocorticoid receptor expression. This was blocked by propanolol suggesting action via beta-adrenergic receptors. Similarly to NA, fluoxetine and amitriptyline also selectively increased glucocorticoid receptor mRNA and protein levels over this time course. However, glucocorticoid receptor induction by fluoxetine or amitriptyline was not blocked by WAY 100635 or propanolol. These results show that 5HT, NA and antidepressants act directly but via distinct mechanisms on hippocampal neurones to regulate mineralocorticoid and glucocorticoid receptor expression. Thusly, manipulation of neurotransmitter or antidepressant levels in the brain may aid in reversing hypothalamic-pituitary-adrenal axis hyperactivity by restoring hippocampal corticosteroid receptor balance.

Tonic activity and GABA responsiveness of medial vestibular nucleus neurons in aged rats.

The tonic discharge of rat medial vestibular nucleus (MVN) neurons, and their responsiveness to GABA receptor agonists were investigated in slices prepared from aged rats (24 months old). Aged MVN neurons showed regular spontaneous activity similar to that seen in slices from young adults. However the inhibitory effects of the GABA(A) agonist muscimol on the spontaneous activity of aged MVN neurons were significantly greater than in young MVN neurons. Inhibitory responses to the GABA(B) agonist baclofen also tended to be greater in slices from aged animals, but this difference was not statistically significant. The regular discharge of aged MVN neurons at firing rates similar to those in young animals suggests that the intrinsic excitability of MVN cells is maintained with age. The up-regulation of GABA(A) receptor efficacy in aged MVN neurons may compensate for changes in inhibitory inputs from vestibular commissures and cerebellum that may occur with neuronal loss in the aged brain.

Acute restraint stress increases 5-HT7 receptor mRNA expression in the rat hippocampus.

The brain serotonin (5-HT) system interacts closely with the hypothalamic-pituitary-adrenal axis. We examined the effects of stress on hippocampal 5-HT7 receptor and corticosteroid receptor (mineralocorticoid receptor (MR) and glucocorticoid receptor (GR)) mRNA expression measured by in situ hybridisation histochemistry. Acute restraint stress increased 5-HT7 receptor mRNA in CA2 and CA3 hippocampal subregions (32% increase) and had a small effect on GR but not MR mRNA (19% fall in GR in CA1). In contrast, chronic stress (1 week of variable stressors) had little effect on hippocampal 5-HT7 receptor mRNA (9% rise in CA3) but decreased MR mRNA (e.g. 34% decrease in CA2) and GR mRNA expression selectively in the dentate gyrus (26% decrease). The rise in 5-HT7 receptor mRNA expression following restraint stress parallels our previously reported increase in expression after inhibiting the synthesis of adrenal steroids. These data suggest that acute but not chronic stress regulates 5-HT7 receptor mRNA expression in a manner that is likely to be independent of its glucocorticoid actions.

Short-term administration of fluoxetine and venlafaxine decreases corticosteroid receptor mRNA expression in the rat hippocampus.

Chronic treatment with antidepressant drugs (2 weeks or longer) increases corticosteroid receptor mRNA expression in the hippocampus and reduces hypothalamic-pituitary-adrenal axis activity in parallel with improving mood and neuroendocrine function. Earlier effects are less well documented. We examined the effects of short term (9 days) treatment with fluoxetine (10 mg/kg) and venlafaxine (10 mg/kg) on hippocampal mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) mRNA expression and spatial memory in adult rats. In situ hybridization histochemistry showed that the antidepressants decreased MR mRNA expression in all hippocampal subregions (e.g. 45% decrease in CA1 with venlafaxine, P<0.001), while GR mRNA expression was selectively reduced in the CA3 subregion. There was a trend for decreased plasma corticosterone levels following fluoxetine (50% fall, P=0.07) and venlafaxine (30% fall, P=0.18) but neither antidepressants affected spatial memory in the watermaze. Thus antidepressants can have complex and opposing actions on hippocampal corticosteroid receptor expression depending on the duration of treatment.

Lack of tissue glucocorticoid reactivation in 11beta -hydroxysteroid dehydrogenase type 1 knockout mice ameliorates age-related learning impairments.

11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD-1) intracellularly regenerates active corticosterone from circulating inert 11-dehydrocorticosterone (11-DHC) in specific tissues. The hippocampus is a brain structure particularly vulnerable to glucocorticoid neurotoxicity with aging. In intact hippocampal cells in culture, 11beta-HSD-1 acts as a functional 11beta-reductase reactivating inert 11-DHC to corticosterone, thereby potentiating kainate neurotoxicity. We examined the functional significance of 11beta-HSD-1 in the central nervous system by using knockout mice. Aged wild-type mice developed elevated plasma corticosterone levels that correlated with learning deficits in the watermaze. In contrast, despite elevated plasma corticosterone levels throughout life, this glucocorticoid-associated learning deficit was ameliorated in aged 11beta-HSD-1 knockout mice, implicating lower intraneuronal corticosterone levels through lack of 11-DHC reactivation. Indeed, aged knockout mice showed significantly lower hippocampal tissue corticosterone levels than wild-type controls. These findings demonstrate that tissue corticosterone levels do not merely reflect plasma levels and appear to play a more important role in hippocampal functions than circulating blood levels. The data emphasize the crucial importance of local enzymes in determining intracellular glucocorticoid activity. Selective 11beta-HSD-1 inhibitors may protect against hippocampal function decline with age.

5'-heterogeneity of glucocorticoid receptor messenger RNA is tissue specific: differential regulation of variant transcripts by early-life events.

Glucocorticoid receptor (GR) gene expression is regulated in a complex tissue-specific manner, notably by early-life environmental events that program tissue GR levels. We have identified and characterized several new rat GR mRNAs. All encode a common protein, but differ in their 5'-leader sequences as a consequence of alternate splicing of, potentially, 11 different exon 1 sequences. Most are located in a 3-kb CpG island, upstream of exon 2, that exhibits substantial promoter activity in transfected cells. Ribonuclease (RNase) protection analysis demonstrated significant levels of six alternate exons 1 in vivo in rat, with differences between liver, hippocampus, and thymus reflecting tissue-specific differences in promoter activity. Two of the alternate exons 1 (exons 1(6) and 1(10)) were expressed in all tissues examined, together present in 77-87% of total GR mRNA. The remaining GR transcripts contained tissue-specific alternate first exons. Importantly, tissue-specific first exon usage was altered by perinatal environmental manipulations. Postnatal handling, which permanently increases GR in the hippocampus, causing attenuation of stress responses, selectively elevated GR mRNA containing the hippocampus-specific exon 1(7). Prenatal glucocorticoid exposure, which increases hepatic GR expression and produces adult hyperglycemia, decreased the proportion of hepatic GR mRNA containing the predominant exon 1(10), suggesting an increase in a minor exon 1 variant. Such tissue specificity of promoter usage allows differential GR regulation and programming.

Glucocorticoids and the ageing hippocampus.

Approximately 30 % of human and mammalian populations develop cognitive impairments with ageing. Many of these impairments have been linked to dysfunction of the hippocampus, a well studied area of the medial-temporal lobe, which is involved in episodic memory and control of the hypothalamo-pituitary-adrenal stress axis and, thus, of glucocorticoid secretion. This paper reviews the growing body of studies which explore a possible relationship between lifetime exposure to glucocorticoids and hippocampal impairment. There is now strong evidence which associates hypercortisolemia in aged men with later cognitive dysfunction and this complements a wealth of rodent and other human data. We conclude with a discussion of possible pharmacological and behavioural interventions.

Serotonin receptor subtype gene expression in the hippocampus of aged rats following chronic amitriptyline treatment.

The raphe-hippocampal 5-HT system plays a key role in the modulation of mood, memory and neuroendocrine responses. In the elderly, there is an increased incidence of disturbances of these functions. We examined the effects of ageing and of chronic antidepressant treatment upon 5-HT receptor subtype mRNA expression in the hippocampus and raphe of cognitively tested rats. Amitriptyline treatment decreased 5-HT1A receptor mRNA expression in the dorsal raphe nucleus of the aged rats (24% fall compared to saline treated controls, p<0.01) but not in the young rats. Neither age nor amitriptyline (10 mg/kg, i.p.) administration for 10 weeks altered 5-HT1A, 5-HT2A, 5-HT2C or 5-HT7 receptor mRNA expression in any hippocampal subregion. This suggests a difference in responsiveness to amitriptyline with ageing originating at the level of the raphe 5-HT1A autoreceptor gene expression.

Chronic amitriptyline administration increases serotonin transporter binding sites in the hippocampus of aged rats.

The effects of ageing and of chronic antidepressant treatment upon 5-HT transporter sites ([3H]paroxetine binding) in the rat hippocampus was examined. [3H]paroxetine binding to transporter sites was decreased with ageing in the hippocampus of control rats (38% decrease in dentate gyrus and CA4). Amitriptyline (10 mg/kg, i.p.) had no significant effect on [3H]paroxetine binding in 10 months old rats, but increased binding sites in 24 months rats in all hippocampal subregions (greatest increase of 109% in CA1 compared to saline controls). These data indicate an age-related decrease in hippocampal serotonin transporter sites and upregulation of these sites following 10 weeks of amitriptyline. The observed increase in transporter sites following amitriptyline may contribute to the general lower effectiveness of tricyclic antidepressants with ageing.

Continuous blockade of brain mineralocorticoid receptors impairs spatial learning in rats.

Hippocampal mineralocorticoid receptors (MR) are proposed to mediate facilitation of cognition in the short-term. The acute central blockade of MR increases plasma corticosterone levels which itself can affect cognition thus complicating the interpretation of such studies. We therefore investigated the effects of chronic continuous central MR antagonism by spironolactone infused intracerebroventricularly (i.c.v.) (100 ng/h) via Alzet minipumps for 12 days. Rats given chronic i.c.v. infusion of spironolactone spent significantly less time (20% decrease, P < 0.05) within the platform quadrant during the watermaze probe test. Neither hippocampal MR nor glucocorticoid receptor mRNA expression were affected by spironolactone. Morning plasma corticosterone levels and adrenal and thymic weights were unchanged. Chronic central MR antagonism impairs spatial memory in the watermaze independent of changes in receptors or circulating glucocorticoid levels.

Site-specific regulation of corticosteroid and serotonin receptor subtype gene expression in the rat hippocampus following 3,4-methylenedioxymethamphetamine: role of corticosterone and serotonin.

Abnormal interactions between serotonin (5-hydroxytryptamine) and glucocorticoids, notably in the hippocampus, may underpin neuroendocrine, affective and cognitive dysfunction in depression and ageing. Glucocorticoids act via intracellular glucocorticoid and mineralocorticoid receptors, whereas 5-hydroxytryptamine binds to a family of transmembrane sites; both cross- and auto-regulation have been proposed. To determine the roles of 5-hydroxytryptamine and corticosterone in the short-term control of hippocampal receptor gene expression, we used 3,4-methylenedioxymethamphetamine (20 mg/kg), which causes acute release of both 5-hydroxytryptamine and corticosterone. 3,4-methylenedioxymethamphetamine increased mineralocorticoid receptor messenger RNA expression throughout the hippocampus after 16 h. In rats with fixed glucocorticoid levels (adrenalectomy plus corticosterone pellets) this effect was lost in CA1-4, suggesting corticosterone-mediation, but maintained in the dentate gyrus, indicating 5-hydroxytryptamine involvement. In contrast, 3,4-methylenedioxymethamphetamine decreased glucocorticoid receptor messenger RNA expression in the dentate gyrus and CA1 within 4 h, but only in adrenal-intact rats, suggesting corticosterone control. 5-Hydroxytryptamine1A receptor messenger RNA expression was decreased in CA1 in both groups of rats, but increased in the dentate gyrus only in corticosterone-fixed rats, suggesting 5-hydroxytryptamine differentially regulates expression of this gene within hippocampal subfields. 5-hydroxytryptamine2C receptor messenger RNA was decreased in ventral CA1 only in adrenal-intact rats, suggesting a corticosterone effect, and decreased in the subiculum in both groups, indicating 5-hydroxytryptamine mediation. These results show the complexity and intricate subregional-specificity of 5-hydroxytryptamine and corticosterone interactions upon hippocampal corticosteroid and 5-hydroxytryptamine receptor gene expression. 3,4-Methylenedioxymethamphetamine-induced alterations in hippocampal receptor gene expression may play a role in the mood and behavioural changes associated with this drug of abuse.

Impact of adrenalectomy on 5-HT6 and 5-HT7 receptor gene expression in the rat hippocampus.

Both glucocorticoid excess and decreased serotonergic (5-HT) transmission may cause depression. The recently cloned 5-HT6 and 5-HT7 receptors have high affinity for antidepressants. Here, we show that pharmacological adrenalectomy increases 5-HT6 and 5-HT7 receptor mRNA expression in specific hippocampal subfields, effects partly reversed by corticosterone replacement. Increased 5-HT6 and 5-HT7 receptor expression may provide a basis, in part, for the therapeutic actions of adrenal steroid synthesis inhibitors in resistant depression.

Decreased NGFI-A gene expression in the hippocampus of cognitively impaired aged rats.

Hippocampal NGFI-A gene expression is increased following the induction of long-term potentiation, a form of activity-dependent synaptic plasticity that has been implicated in learning. In this study, we show a positive correlation between spatial learning and the constitutive expression of NGFI-A mRNA, selectively in CA1 pyramidal neurons. NGFI-A mRNA expression decreased with age in CA1, CA2 and neocortex. Long-term amitriptyline treatment, which improved spatial learning in young rats, had no significant effects on NGFI-A mRNA levels. Whether hippocampal NGFI-A plays a direct role in the mechanism of learning and memory remains to be determined.

Cloning and production of antisera to human placental 11 beta-hydroxysteroid dehydrogenase type 2.

By inactivating potent glucocorticoid hormones (cortisol and corticosterone), 11 beta-hydroxysteroid dehydrogenase type 2 (11 beta-HSD2) plays an important role in the placenta by controlling fetal exposure to maternal glucocorticoids, and in aldosterone target tissues by controlling ligand access to co-localized glucocorticoid and mineralocorticoid receptors. Amino acid sequence from homogeneous human placental 11 beta-HSD2 was used to isolate a 1897 bp cDNA encoding this enzyme (predicted M(r) 44126; predicted pI 9.9). Transfection into mammalian (CHO) cells produces 11 beta-HSD2 activity which is NAD(+)-dependent, is without reductase activity, avidly metabolizes glucocorticoids (Km values for corticosterone, cortisol and dexamethasone of 12.4 +/- 1.5, 43.9 +/- 8.5 and 119 +/- 15 nM respectively) and is inhibited by glycyrrhetinic acid and carbenoxolone (IC50 values 10-20 nM). Rabbit antisera recognizing 11 beta-HSD2 have been raised to an 11 beta-HSD2-(370--383)-peptide-carrier conjugate. Recombinant 11 beta-HSD2, like native human placental 11 beta-HSD2, is detectable with affinity labelling and anti-11 beta-HSD2 antisera, and appears to require little post-translational processing for activity. 11 beta-HSD2 mRNA (approximately 1.9 kb transcript) is expressed in placenta, aldosterone target tissues (kidney, parotid, colon and skin) and pancreas. In situ hybridization and immunohistochemistry localize abundant 11 beta-HSD2 expression to the distal nephron in human adult kidney and to the trophoblast in the placenta. 11 beta-HSD2 transcripts are expressed in fetal kidney (but not lung, liver or brain) at 21-26 weeks, suggesting that an 11 beta-HSD2 distribution resembling that in the adult is established by this stage in human development.

Glucocorticoids, hippocampal corticosteroid receptor gene expression and antidepressant treatment: relationship with spatial learning in young and aged rats.

The emergence of cognitive deficits in a subgroup of aged rats is associated with increased hypothalamic-pituitary-adrenal axis activity, decreased hippocampal mineralocorticoid and/or glucocorticoid receptor gene expression and neuronal loss. Short-term treatment with antidepressant drugs in young rats increases hippocampal corticosteroid receptor gene expression. In this study, the effects of chronic antidepressant administration on hippocampal mineralocorticoid and glucocorticoid receptor gene expression and spatial memory in young and aged rats were investigated. Young (eight months) and old (22 +/- 1 months) Lister-hooded rats were ranked according to watermaze performance. Matched pairs of rats were treated with amitriptyline (10 mg/kg) or saline daily for nine weeks, then reassessed in the watermaze. Amitriptyline significantly improved spatial memory in the young rats (33% increase in transfer test time) and increased hippocampal mineralocorticoid, but not glucocorticoid receptor messenger RNA expression. By contrast, in aged rats, amitriptyline had no effect on spatial memory or hippocampal corticosteroid receptor gene expression, either in cognitively unimpaired or cognitively-impaired animals. In aged rats, basal plasma corticosterone levels, which were significantly higher than in young animals, correlated negatively with spatial memory, while hippocampal glucocorticoid receptor mRNA expression correlated negatively with plasma corticosterone levels and positively with spatial memory. Amitriptyline had no significant effect on basal morning plasma corticosterone levels in either young or aged rats, but significantly decreased evening corticosterone levels in aged rats. Our data support the notion that corticosterone exerts a concentration-dependent biphasic influence, via selective activation of hippocampal mineralocorticoid and glucocorticoid receptor, on spatial memory. Amitriptyline improves spatial memory in young rats and increases hippocampal mineralocorticoid receptor gene expression. The lack of amitriptyline effect on spatial memory in aged rats may reflect decreased plasticity of both the synaptic processes underlying spatial memory and the regulation of hippocampal mineralocorticoid/glucocorticoid receptor expression, with mineralocorticoid receptors fully occupied due to elevated basal plasma corticosterone levels (in part a consequence of inadequate glucocorticoid receptor function).

The effect of adrenalectomy on 5-hydroxytryptamine and corticosteroid receptor subtype messenger RNA expression in rat hippocampus.

Both central serotonergic dysfunction and glucocorticoid hypersecretion have been separately implicated in the aetiology of affective disorders. The hippocampus highly expresses receptors for 5-hydroxytryptamine and glucocorticoids, and adrenalectomy alters the responsivity of hippocampal neurons to 5-hydroxytryptamine. The hippocampus thus represents a prime locus for interactions between the two systems. In this study we examined the effects of glucocorticoid manipulations on neuronal expression of messenger RNA encoding corticosteroid receptor and 5-hydroxytryptamine receptor subtypes in the hippocampus and 5-hydroxytryptamine1A messenger RNA expression in the dorsal raphe, in the rat. Interestingly, there was no effect of adrenalectomy on 5-hydroxytryptamine1A or 5-hydroxytryptamine2A receptor messenger RNA expression in the dorsal or ventral hippocampus at any time point measured. Furthermore, no changes in 5-hydroxytryptamine1A receptor gene expression were seen in the dorsal raphe (encoding autoreceptors) after adrenalectomy. However, 5-hydroxytryptamine2C (5-hydroxytryptamine1C) receptor messenger RNA expression was increased specifically in posterior CA1 and CA3 neurons following adrenalectomy, an effect that was reversed by glucocorticoid replacement. Following adrenalectomy, glucocorticoid and mineralocorticoid receptor messenger RNA expression increased in the dentate gyrus, CA1 and CA3 subfields of the hippocampus. These increases were apparent 6 h after adrenalectomy, were maintained at two days, but 14 days after adrenalectomy hippocampal glucocorticoid receptor and mineralocorticoid receptor gene expression had returned to control levels. These effects of adrenalectomy were abolished by dexamethasone, but not aldosterone administration, suggesting mediation by autoregulatory glucocorticoid receptors. Our results show that adrenalectomy only transiently increases corticosteroid receptor gene expression in the hippocampus, and selectively increases hippocampal 5-hydroxytryptamine2C receptor messenger RNA expression. The resulting change in 5-hydroxytryptamine2C receptor-mediated responses may produce the alterations in hippocampal neuronal activity in response to 5-hydroxytryptamine observed after adrenalectomy.