Joyce L.W. Yau

Postnatal Handling Increases the Expression of cAMP-Inducible Transcription Factors in the Rat Hippocampus: The Effects of Thyroid Hormones and Serotonin

Postnatal handling increases glucocorticoid receptor expression in the rat hippocampus, thus altering the regulation of hypothalamic synthesis of corticotropin-releasing hormone and the hypothalamic–pituitary–adrenal response to stress. The effect on glucocorticoid receptor gene expression represents one mechanism by which the early environment can exert a long-term effect on neural development. The handling effect on hippocampal glucocorticoid receptor expression is dependent on peripheral thyroid hormone release and the activation of ascending serotonergic pathways. In primary hippocampal cell cultures, serotonin (5-HT) increases glucocorticoid receptor expression, and this effect appears to be mediated by increased cAMP levels. In the current studies we examined the in vivoeffects of handling on hippocampal cAMP–protein kinase A (PKA) activity. In 7-d-old rat pups, we found that (1) postnatal handling increased adenylyl cyclase activity and hippocampal cAMP levels, (2) the effect of handling on cAMP levels was completely blocked by treatment with either propylthiouracil (PTU), a thyroid hormone synthesis inhibitor, or the 5-HT receptor antagonist, ketanserin, and (3) handling also increased hippocampal PKA activity. We then examined the effects of handling on cAMP-inducible transcription factors. Handling rapidly increased levels of the mRNAs for nerve growth factor-inducible factor A (NGFI-A) (zif268,krox24) and activator protein-2 (AP-2) as well as for NGFI-A and AP-2 immunoreactivity throughout the hippocampus. Finally, we found that the effects of handling on NGFI-A and AP-2 expression were significantly reduced by concurrent treatment with either PTU or ketanserin, effects that paralleled those on cAMP formation. NGFI-A and AP-2 have been implicated in the regulation of glucocorticoid receptor expression during development. Thus, these findings suggest that postnatal handling might alter glucocorticoid receptor gene expression via cAMP–PKA pathways involving the activation of NGFI-A and AP-2.

11β-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. 11β-Hydroxysteroid dehydrogenase type 1 (11β-HSD1) catalyzes the regeneration of active glucocorticoids within cells (in brain and other tissues), thus amplifying steroid action. We examined whether 11β-HSD1 plays a role in the pathogenesis of cognitive deficits associated with aging in male C57BL/6J mice. We show that 11β-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. 11β-HSD1 elevation appears causal, since aging (18 months) male transgenic mice with forebrain-specific 11β-HSD1 overexpression (∼50% in hippocampus) exhibit premature age-associated cognitive decline in the absence of altered circulating glucocorticoid levels or other behavioral (affective) deficits. Thus, excess 11β-HSD1 in forebrain is a cause of as well as a therapeutic target in memory impairments with aging.

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

11β-hydroxysteroid dehydrogenase type 1 (11β-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, 11β-HSD-1 acts as a functional 11β-reductase reactivating inert 11-DHC to corticosterone, thereby potentiating kainate neurotoxicity. We examined the functional significance of 11β-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 11β-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 11β-HSD-1 inhibitors may protect against hippocampal function decline with age.

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).

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 20u2003min. 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.

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 Alzheimers 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.

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.

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.

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.

Enhanced Hippocampal Long-Term Potentiation and Spatial Learning in Aged 11β-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. 11β-Hydroxysteroid dehydrogenase type 1 (11β-HSD1) amplifies local intracellular (“intracrine”) glucocorticoid action; in the brain it is highly expressed in the hippocampus. We investigated whether the impact of 11β-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” 11β-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 11β-HSD1−/− mice. Young 11β-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 11β-HSD1−/− mice. These data suggest that 11β-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.