Mineralocorticoid receptor-mediated changes in membrane properties of rat CA1 pyramidal neurons in vitro

Corticosteroid actions in the hippocampus

Corticosteroid hormones can enter the brain and bind to two intracellular receptor types that regulate transcription of responsive genes: (i) the high affinity mineralocorticoid receptors and (ii) the glucocorticoid receptors with approximately 10-fold lower affinity. Although most cells in the brain predominantly express glucocorticoid receptors, principal cells in limbic structures such as the hippocampus often contain glucocorticoid as well as mineralocorticoid receptors. Recent electrophysiological studies have examined the consequences of transcriptional regulation via the two receptor types for information transfer in the hippocampus. It was found that, under resting conditions, corticosteroids do not markedly alter electrical activity. However, if neurones are shifted towards more depolarized or hyperpolarized potentials due to the action of neurotransmitters, slow and adaptive effects of the corticosteroid hormones become apparent. In general, mineralocorticoid receptor occupation maintains steady electrical activity in hippocampal neurones. Brief activation of glucocorticoid receptors leads to increased influx of calcium, which normally helps to slowly reverse temporarily raised electrical activity. These slow and persistent corticosteroid actions will alter network function within the hippocampus, thus contributing to behavioural adaptation in response to stress. Modulation of hippocampal activity by corticosteroids also affects hippocampal output (e.g. to inhibitory interneurones which control hypothalamic-pituitary-adrenal axis activity). The enhanced calcium influx after glucocorticoid receptor activation can become a risk factor when cells are simultaneously exposed to strong depolarizing inputs, such as those occurring during ischaemia. Similarly, chronically elevated corticosteroid levels (or lack of corticosteroids) could endanger hippocampal cell function. The latter may contribute to the precipitation of clinical symptoms in diseases associated with chronically aberrant corticosteroid levels.

Psychological stress increases hippocampal mineralocorticoid receptor levels: involvement of corticotropin-releasing hormone

We investigated whether acute stressors regulate functional properties of the hippocampal mineralocorticoid receptor (MR), which acts inhibitory on hypothalamic-pituitary-adrenocortical activity. Exposure of rats to forced swimming or novelty evoked a significant rise in density of MR immunoreactivity in all hippocampal subfields after 24 hr, whereas exposure to a cold environment was ineffective. Time course analysis revealed that the effect of forced swimming on MR peaked at 24 hr and returned to control levels between 24 and 48 hr. In pyramidal neurons of CA2 and CA3, marked rises were already observed after 8 hr. Radioligand binding assays showed that corticotropin-releasing hormone (CRH) injected intracerebroventricularly into adrenalectomized rats also produced a rise in hippocampal MR levels; an effect for which the presence of corticosterone, but not dexamethasone, at the time of injection was a prerequisite. Moreover, pretreatment with the CRH receptor antagonist (d-Phe(12),Nle(21,38),alpha-Me-Leu(37))-CRH(12-41) blocked the effect of forced swimming on hippocampal MR levels. To investigate whether the rise in MR levels had any functional consequences for HPA regulation, 24 hr after forced swimming, a challenge test with the MR antagonist RU 28318 was conducted. The forced swimming exposed rats showed an enhanced MR-mediated inhibition of HPA activity. This study identifies CRH as an important regulator of MR, a pathway with marked consequence for HPA axis regulation. We conclude that the interaction between CRH and MR presents a novel mechanism involved in the adaptation of the brain to psychologically stressful events.

Psychological stress increases hippocampal mineralocorticoid receptor levels: involvement of corticotropin-releasing hormone

We investigated whether acute stressors regulate functional properties of the hippocampal mineralocorticoid receptor (MR), which acts inhibitory on hypothalamic-pituitary-adrenocortical activity. Exposure of rats to forced swimming or novelty evoked a significant rise in density of MR immunoreactivity in all hippocampal subfields after 24 hr, whereas exposure to a cold environment was ineffective. Time course analysis revealed that the effect of forced swimming on MR peaked at 24 hr and returned to control levels between 24 and 48 hr. In pyramidal neurons of CA2 and CA3, marked rises were already observed after 8 hr. Radioligand binding assays showed that corticotropin-releasing hormone (CRH) injected intracerebroventricularly into adrenalectomized rats also produced a rise in hippocampal MR levels; an effect for which the presence of corticosterone, but not dexamethasone, at the time of injection was a prerequisite. Moreover, pretreatment with the CRH receptor antagonist (d-Phe(12),Nle(21,38),alpha-Me-Leu(37))-CRH(12-41) blocked the effect of forced swimming on hippocampal MR levels. To investigate whether the rise in MR levels had any functional consequences for HPA regulation, 24 hr after forced swimming, a challenge test with the MR antagonist RU 28318 was conducted. The forced swimming exposed rats showed an enhanced MR-mediated inhibition of HPA activity. This study identifies CRH as an important regulator of MR, a pathway with marked consequence for HPA axis regulation. We conclude that the interaction between CRH and MR presents a novel mechanism involved in the adaptation of the brain to psychologically stressful events.

The mineralocorticoid receptor expression in the mouse CNS is conserved during development

Using a specific polyclonal antiserum raised in rabbit against amino acids 1-23 of the mouse mineralocorticoid receptor (MR) we investigated the developmental profile of MR expression in the murine CNS by immunocytochemistry. MR protein appeared first at embryonic day E16.5 in the limbic system, i.e. in the hippocampus and induseum griseum. During development and in adulthood, high levels of MR expression were observed in the limbic system, whereas expression levels detectable in layers II, III, V of the neocortex and in motoneurons of cranial nerves and spinal cord were lower. No MR staining was found in the hypothalamus. Developmental MR expression was restricted to neuronal populations that also express MR protein in the adult CNS, indicating that the MR may fulfill the same functions in neurons during development and in adulthood.

Neuroactive steroids and seizure susceptibility

There is increasing clinical and experimental evidence that hormones, in particular sex steroid hormones, influence neuronal excitability and other brain functions. The term 'neuroactive steroids' has been coined for steroids that interact with neurotransmitter receptors. One of the best characterized actions of neuroactive steroids is the allosteric modulation of GABA(A)-receptor function via binding to a putative steroid-binding site. Since neuroactive steroids may interact with a variety of other membrane receptors, excitatory as well as inhibitory, they may have an impact on the excitability of specific brain regions. Neuronal excitability is enhanced by estrogen, whereas progesterone and its metabolites exert anticonvulsant effects. Testosterone and corticosteroids have less consistent effects on seizure susceptibility. Apart from these particular properties, neuroactive steroids may regulate gene expression via progesterone receptors. Based on their molecular properties, these compounds appear to have a promising therapeutical profile for the treatment of different neuropsychiatric diseases including epilepsy. This review focuses on the effects of neuroactive steroids on neuronal excitability and their putative impact on the physiology of epileptic disorders.

Adrenocorticosteroid receptor blockade and excitotoxic challenge regulate adrenocorticosteroid receptor mRNA levels in hippocampus

The mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) are glucocorticoid-activated transcription factors essential for maintenance of cellular homeostasis. Differential activation of these adrenocorticosteroid receptors (ACR) is thought to influence neuronal viability, particularly under challenging cellular conditions. The present study is designed to determine the effects of receptor blockade and excitotoxic insult on MR and GR mRNA expression and neuronal viability in hippocampus. Male Sprague--Dawley rats were pretreated for 48 hr with vehicle, MR antagonist spironolactone (SPIRO) (50 mg/kg, twice daily, s.c.), or GR antagonist RU486 (25 mg/kg, twice daily, s.c.) and subsequently injected with saline or the glutamate analog kainic acid (KA) (12 mg/kg i.p.). Twenty-four hr post-insult, MR and GR mRNA levels were assessed by in situ hybridization analysis, and hippocampal neurons were counted to assess KA-induced cell loss. MR blockade with SPIRO increased basal MR mRNA levels in hippocampal subregions CA1, CA3, and dentate gyrus (DG) and increased basal GR mRNA levels in CA3. GR blockade with RU486 increased basal GR mRNA levels in CA3. The excitotoxin KA decreased MR mRNA levels in CA1 and CA3, decreased GR mRNA levels in DG, and negated all antagonist-induced increases of ACR mRNAs. Cell counts quantifying KA damage indicated increased CA3 vulnerability to KA insult after treatment with MR antagonist spironolactone but demonstrated no significant cell loss in any other group or region. These results demonstrate dynamic regulation of hippocampal MR and GR mRNAs after ACR antagonist treatment and kainate toxicity, underscoring the potential importance of MR and GR availability to neuronal viability after insult.

Carbenoxolone depresses spontaneous epileptiform activity in the CA1 region of rat hippocampal slices

An important contributor to the generation of epileptiform activity is the synchronization of burst firing in a group of neurons. The aim of this study was to investigate whether gap junctions are involved in this synchrony using an in vitro model of epileptiform activity. Hippocampal slices (400 microm) were prepared from female Sprague-Dawley rats (120-170 g). The perfusion of slices with a medium containing no added magnesium and 4-aminopyridine (50 microM) resulted in the generation of spontaneous bursts of population spikes of a fast frequency along with less frequent negative-going bursts. The frequency of the bursts produced was consistent over a 3h period. Carbenoxolone (100 microM), a gap junction blocker and mineralocorticoid agonist, perfused for 75 min, reduced the frequency of both types of spontaneous burst activity. Perfusion of spironolactone (1 microM), a mineralocorticosteroid antagonist, for 15 min prior to and during carbenoxolone perfusion did not alter the ability of carbenoxolone to depress the frequency of spontaneous activity. The incubation of hippocampal slices in carbenoxolone prior to recording increased the time taken for the spontaneous activity to start on change to the zero magnesium/4-aminopyridine medium and decreased the total number of spontaneous bursts over the first 60 min period. The ability of carbenoxolone to delay induction of epileptiform activity and reduce established epileptiform activity suggests that gap junctions contribute to the synchronization of neuronal firing in this model.

The possibility of neurotoxicity in the hippocampus in major depression: a primer on neuron death

A number of studies indicate that prolonged, major depression is associated with a selective loss of hippocampal volume that persists long after the depression has resolved. This review is prompted by two ideas. The first is that overt neuron loss may be a contributing factor to the decrease in hippocampal volume. As such, the first half of this article reviews current knowledge about how hippocampal neurons die during insults, focusing on issues related to the trafficking of glutamate and calcium, glutamate receptor subtypes, oxygen radical generation, programmed cell death, and neuronal defenses. This is meant to orient the reader toward the biology that is likely to underlie any such instances of neuron loss in major depression. The second idea is that glucocorticoids, the adrenal steroids secreted during stress, may play a contributing role to any such neuron loss. The subtypes of depression associated with the hippocampal atrophy typically involve significant hypersecretion of glucocorticoids, and the steroid has a variety of adverse effects in the hippocampus, including causing overt neuron loss. The second half of this article reviews the steps in this cascade of hippocampal neuron death that are regulated by glucocorticoids.

Stress in the brain

Part I (first section) reports about research in the period 1964-1976, when the seminal observations were made on which today's concept of corticosteroid action on the brain is based. These key observations concern the discovery of nuclear corticosterone receptors in the limbic brain that mediate control over neuronal circuits underlying hypothalamic-pituitary-adrenal activity and behavioural adaptation. Part II (second section) covers the period of 1977-1989. It is about some aspects of the neuropeptide concept, the implementation of micro-neurochemistry using the "Palkovits punch", and the application of in vitro autoradiography. Vasopressin and oxytocin receptors were identified and their implication in behaviour was examined using the song control of the canary bird as a model system. Two distinct nuclear receptor types for corticosteroids were identified: mineralocorticoid receptors (MR) and glucocorticoid receptors (GR) which mediate in a coordinate manner the steroid control of hypothalamus-pituitary-adrenal activity and behaviour. Part III (third section) is from 1990 up to 2000. Focus is on the balance of MR- and GR-mediated actions in control of homeostasis as a determinant of health and disease. MR operates in pro-active mode to prevent homeostatic disturbance, while additional GR activation promotes in reactive fashion recovery after stress. An imbalance in MR and GR underlies behavioural deficits and neuroendocrine disturbances increasing vulnerability for stress-related brain disorders. The complete hippocampal genome is screened for corticosteroid responsive genes, which are potential targets for drugs promoting restorative capacity still present in the diseased brain.

Modulatory actions of steroid hormones and neuropeptides on electrical activity in brain

Electrophysiological studies over the past decades have shown that many compounds in addition to 'classical' neurotransmitters affect electrical activity in the brain. These compounds include neuropeptides synthesized in brain as well as compounds which are released from peripheral sources and subsequently enter the brain compartment, such as corticosteroid hormones from the adrenal gland. In the present review, this principle is illustrated by describing the effects of two substances, i.e. vasopressin and corticosterone. Neuropeptides and corticosteroid hormones add at least two essential aspects to information processing in the brain. First, they both act conditional, i.e. they modulate the actions of 'classical' neurotransmitters, rather than changing basal neuronal activity by themselves. Second, the time-frame in which modulation of electrical properties takes place differs from that generally seen with 'classical' neurotransmitters. Neuropeptides modulate electrical activity over a period of minutes, while effects of corticosteroid hormones usually become apparent after at least an hour but then last for hours. In this way, neuropeptides and steroid hormones expand the repertoire of responses through which the brain reacts to environmental challenges.

The brain mineralocorticoid receptor: greedy for ligand, mysterious in function

Glucocorticoids exert their regulatory effects on the hypothalamic-pituitary-adrenocortical axis via two types of corticosteroid receptors: the glucocorticoid receptor and the mineralocorticoid receptor. Whereas the glucocorticoid receptor has a broad distribution in the brain, highest levels of mineralocorticoid receptor are found in the hippocampus. Based on the differential occupancy profile by endogenous glucocorticoids, glucocorticoid receptors are thought to mediate negative feedback signals of elevated glucocorticoid levels, whereas mineralocorticoid receptors control the inhibitory tone of the hippocampus on hypothalamic-pituitary-adrenocortical axis activity. Dysfunction of mineralocorticoid receptors and glucocorticoid receptors are thought to be implicated in stress-related psychiatric diseases such as major depression. Because of its intriguing features, we focus in this review on the mineralocorticoid receptor and provide data which reveal novel aspects of the pharmacology and physiology of mineralocorticoid receptors. Newly obtained results are presented, which help to solve the paradox of why dexamethasone binds with high affinity to mineralocorticoid receptors in vitro, yet binds poorly in vivo. Until recently, mineralocorticoid receptor protein and mRNA levels could only be routinely studied with in vitro cytosol binding assays, in vitro and in vivo receptor autoradiography, Northern blot analysis, and in situ hybridization. These methods are unfortunately hampered by several flaws, such as the necessity of adrenalectomy, no or poor neuroanatomical resolution, the fact that mRNA does not provide the same information as protein, or combinations of these factors. We present immunohistochemical data on mineralocorticoid receptors in the brain obtained by using commercially available antibodies, which alleviate many of these shortcomings. Furthermore, an in vivo microdialysis method is presented which allows the assessment of free corticosterone levels in the brain, which is critical for the study of the pharmacological basis of mineralocorticoid receptor (and glucocorticoid receptor) function. Finally, a novel aspect of the regulation of mineralocorticoid receptors is described which provides evidence that this receptor system is dynamically regulated. In conjunction with previously reported effects of antidepressants, these results have initiated a new concept on the cause of the hypothalamic-pituitary-adrenocortical axis disturbances often seen in stress-related psychiatric disorders such as major depression.

Effects of mineralocorticoid and glucocorticoid receptors on long-term potentiation in the CA3 hippocampal field

We have previously shown that the two types of adrenal steroid receptors, mineralocorticoid MR. and glucocorticoid GR. produce opposite effects on long-term potentiation LTP. in the dentate gyrus in vivo. and CA1 hippocampal field in vitro. More specifically, MR activation enhanced and prolonged LTP, whereas GR activation suppressed LTP in these areas and also produced a long-term depression LTD. of the synaptic response. In the present experiment we investigated acute effects of MR and GR activation on LTP induction in the mossy fiber and commissural associational input to the CA3 hippocampal field, since the mechanisms underlying LTP induction in these two pathways differ, the former being N-methyl-D-aspartate receptor NMDAR. independent while the latter being NMDAR-dependent. Rats were either adrenalectomized ADX or adrenally intact. ADX animals were acutely injected with either the specific MR agonist, aldosterone, the specific GR agonist RU 28362 or vehicle. One hour following the injection, the animals were prepared for electrophysiological recording stimulation. Field potential recordings were performed in the radiatum or laconosum moleculare layers of the CA3 field, with stimulation of either the mossy fibers or the commissural associational input from the contralateral hemisphere. We also replicated our previous findings by recording in the dentate gyrus with stimulation of the medial perforant pathway, in the same animals. As observed in our previous study in the dentate gyrus, we found an enhancement and a suppression of LTP with MR and GR activation, respectively. Similarly, for the commissural associational input to CA3, MR activation enhanced LTP, while GR activation reduced it. In contrast, for the mossy fiber input to CA3, neither MR nor GR activation significantly affected LTP induction. These results indicate that adrenal steroids may modulate LTP induction in the hippocampus via an interaction with glutamatergic NMDAR.

Inhibition by aldosterone of insulin receptor mRNA levels and insulin binding in U-937 human promonocytic cells

The effect of aldosterone on insulin receptor (IR) expression was investigated in U-937 human promonocytic cells. The putative involvement of the mineralocorticoid receptor (MR) was also analysed. Aldosterone binding assays indicated the presence of MRs with high affinity and limited capacity in these cells. RNA blot assays showed that aldosterone treatment decreased the levels of the two major IR mRNAs (11 and 8.5 kb) present in these cells in a dose- and time-dependent manner. The partial reversal of such a decrease by the mineralocorticoid antagonist spironolactone suggested that MR was involved in the process. Experiments with the RNA synthesis inhibitor actinomycin D indicated that the decrease in IR mRNA content in aldosterone-treated cells was not the result of transcript destabilisation. The inhibitory action of aldosterone was not prevented by the simultaneous presence of the protein synthesis inhibitor cycloheximide, suggesting that the reduction of IR gene expression occurs as a direct response to the action of aldosterone. Furthermore, insulin binding assays showed that aldosterone decreased IR capacity but did not alter receptor affinity. In addition, the IR turnover resulted unaltered. These results provide the first evidence for an in vitro modulation of human IR expression by aldosterone.

Rapid glucocorticoid effects on excitatory amino acid levels in the hippocampus: a microdialysis study in freely moving rats

Glucocorticoids can rapidly affect neuronal function and behaviour in mammals. Several studies have suggested the possible existence of rapid, non-genomic effects of glucocorticoids in the hippocampus. To investigate whether glucocorticoids could affect neurotransmission in the hippocampus through rapid, non-genomic mechanisms, we studied the effects of acute glucocorticoid administration on extracellular amino acid levels in the CA1 area of the hippocampus. By means of microdialysis on freely moving rats, we observed that an intraperitoneal injection of corticosterone (2.5 mg/kg) induced a rapid (within 15 min) and transient (returning to basal levels by 35-45 min) increase in extracellular aspartate and glutamate levels ( approximately 155-160%), both in sham-operated and adrenalectomized rats. These effects occurred in parallel with a rise in corticosterone concentration, also detected by microdialysis, in this hippocampal area. Intrahippocampal perfusion of corticosterone by retrodialysis also produced the same fast and reversible effects on excitatory amino acid (EAA) levels. Extracellular concentrations of taurine and gamma-aminobutyric acid (GABA) were unchanged after intrahippocampal glucocorticoid administration. This corticosterone-mediated rise in EAA levels was not inhibited by the presence of specific antagonists for the two types of intracellular corticosteroid receptors, nor by a protein synthesis inhibitor, anisomycin. Perfusion of dexamethasone, a synthetic glucocorticoid, elicited a similar effect to that observed with corticosterone treatment in all studied cases. However, non-glucocorticoid steroids did not affect amino acid transmission in this hippocampal area. These results indicate that glucocorticoids induce a rapid and transient increase in hippocampal EAA levels in vivo that might be exerted through a novel non-genomic mechanism of action.

Dexamethasone blocks sleep induced improvement of declarative memory

To investigate the role of glucocorticoids for effects of early and late nocturnal sleep on declarative and procedural memory, 2 mg dexamethasone (versus placebo) were administered to healthy men 7 h prior to retention sleep. The retention sleep interval covered either the early or late half of nocturnal sleep. Following placebo, recall of a paired associate list (declarative memory) benefitted more from early than late sleep and recall of mirror tracing skills (procedural memory) benefitted more from late than early sleep. Dexamethasone did not affect slow wave sleep dominating early sleep, but blocked the beneficial effect of early sleep on recall of paired associates. Conversely, dexamethasone reduced rapid eye movement sleep dominating late sleep, but did not affect late sleeps beneficial effect on mirror tracing skills. The natural inhibition of endogenous glucocorticoid secretion during early sleep seems to be essential for a sleep-related facilitation of declarative memory.

Mineralocorticoid receptors regulate bcl-2 and p53 mRNA expression in hippocampus

The present study addressed the hypothesis that the neuronal mineralocorticoid receptor (MR) regulates genes associated with cell death, such as bax and p53, and cell viability, including bcl-2, BDNF, and NT-3. Rats were pretreated with either oil vehicle or the MR antagonist spironolactone (SPIRO) and subsequently injected with saline or kainic acid (KA). MR blockade significantly decreased basal mRNA expression of bcl-2 in CA1 of saline-treated animals and attenuated KA-induced increases in p53 mRNA levels in CA3. SPIRO pretreatment had no significant effect on expression of bax, NT-3, or BDNF mRNAs. The data suggest that the neuronal MR contributes to regulation of select cell survival and cell death-related genes in hippocampal pyramidal neurons.

Brain corticosteroid receptor balance in health and disease

In this review, we have described the function of MR and GR in hippocampal neurons. The balance in actions mediated by the two corticosteroid receptor types in these neurons appears critical for neuronal excitability, stress responsiveness, and behavioral adaptation. Dysregulation of this MR/GR balance brings neurons in a vulnerable state with consequences for regulation of the stress response and enhanced vulnerability to disease in genetically predisposed individuals. The following specific inferences can be made on the basis of the currently available facts. 1. Corticosterone binds with high affinity to MRs predominantly localized in limbic brain (hippocampus) and with a 10-fold lower affinity to GRs that are widely distributed in brain. MRs are close to saturated with low basal concentrations of corticosterone, while high corticosterone concentrations during stress occupy both MRs and GRs. 2. The neuronal effects of corticosterone, mediated by MRs and GRs, are long-lasting, site-specific, and conditional. The action depends on cellular context, which is in part determined by other signals that can activate their own transcription factors interacting with MR and GR. These interactions provide an impressive diversity and complexity to corticosteroid modulation of gene expression. 3. Conditions of predominant MR activation, i.e., at the circadian trough at rest, are associated with the maintenance of excitability so that steady excitatory inputs to the hippocampal CA1 area result in considerable excitatory hippocampal output. By contrast, additional GR activation, e.g., after acute stress, generally depresses the CA1 hippocampal output. A similar effect is seen after adrenalectomy, indicating a U-shaped dose-response dependency of these cellular responses after the exposure to corticosterone. 4. Corticosterone through GR blocks the stress-induced HPA activation in hypothalamic CRH neurons and modulates the activity of the excitatory and inhibitory neural inputs to these neurons. Limbic (e.g., hippocampal) MRs mediate the effect of corticosterone on the maintenance of basal HPA activity and are of relevance for the sensitivity or threshold of the central stress response system. How this control occurs is not known, but it probably involves a steady excitatory hippocampal output, which regulates a GABA-ergic inhibitory tone on PVN neurons. Colocalized hippocampal GRs mediate a counteracting (i.e., disinhibitory) influence. Through GRs in ascending aminergic pathways, corticosterone potentiates the effect of stressors and arousal on HPA activation. The functional interaction between these corticosteroid-responsive inputs at the level of the PVN is probably the key to understanding HPA dysregulation associated with stress-related brain disorders. 5. Fine-tuning of HPA regulation occurs through MR- and GR-mediated effects on the processing of information in higher brain structures. Under healthy conditions, hippocampal MRs are involved in processes underlying integration of sensory information, interpretation of environmental information, and execution of appropriate behavioral reactions. Activation of hippocampal GRs facilitates storage of information and promotes elimination of inadequate behavioral responses. These behavioral effects mediated by MR and GR are linked, but how they influence endocrine regulation is not well understood. 6. Dexamethasone preferentially targets the pituitary in the blockade of stress-induced HPA activation. The brain penetration of this synthetic glucocorticoid is hampered by the mdr1a P-glycoprotein in the blood-brain barrier. Administration of moderate amounts of dexamethasone partially depletes the brain of corticosterone, and this has destabilizing consequences for excitability and information processing. 7. The set points of HPA regulation and MR/GR balance are genetically programmed, but can be reset by early life experiences involving mother-infant interaction. 8. (ABSTRACT TRUNCATED)

Alterations in excitatory amino acid-mediated regulation of midbrain dopaminergic neurones induced by chronic psychostimulant administration and stress: relevance to behavioural sensitization and drug addiction

Repeated, intermittent administration of the psychostimulants d-amphetamine and cocaine, as well as other drugs of abuse, leads to an enduring augmentation of certain behavioural responses (e.g. locomotor activity) produced by these drugs. This behavioural sensitization has been the subject of considerable interest due to its potential relevance to drug addiction. Repeated administration of d-amphetamine also leads to an enhancement in the ability of electrical stimulation of the prefrontal cortex to induce burst firing in midbrain dopaminergic (DA) neurones. This hyper-responsiveness probably reflects a potentiation of transmission at excitatory amino acid (EAA)ergic synapses on DA neurones. In addition, we have previously reported that selective activation of mineralocorticoid receptors (MRs) by corticosterone leads to a potentiation of EAA-induced burst firing in midbrain DA neurones, an effect antagonized by glucocorticoid receptor (GR) activation. In this review article, we propose a model describing how drugs of abuse and stress alter EAA function at the level of DA cells in the ventral tegmental area (VTA), which can result in a long-lasting impact on behaviour. D-amphetamine produces a transitory increase in EAA-mediated transmission at the level of DA cells in the VTA, which triggers a more long-lasting change in EAAergic function resembling hippocampal long-term potentiation. Dopaminergic burst events are likely to be a critical link between enhanced EAAergic activity in afferents synapsing on DA neurones and plasticity at these synapses, by increasing calcium transport into the cell, which is known to be an important factor in synaptic plasticity. Selective MR occupation by corticosterone in the VTA facilitates the development of this plasticity. However, we hypothesize that during stress, GR-occupation also activates EAAergic afferents to DA neurones in a manner similar to that following psychostimulants. Under these circumstances, GR-occupation acts via circuitry external to the VTA, which may include the hippocampus. Thus, potentiation of EAAergic synapses on DA neurones in the VTA may represent a final common pathway by which two divserse means (psychostimulants and stress) achieve the same end (sensitization). Alterations in EAA-mediated transmission at the level of DA cells not only plays a critical role in the induction of behavioural sensitization, but probably continues to produce abnormal DA cell responses in the drug-free situation.

Aldosterone responsiveness of A6 cells is restored by cloned rat mineralocorticoid receptor

A6 cells, derived from Xenopus laevis renal tubule, form a high-resistance ion-transporting monolayer when grown on permeable supports and can generate a short-circuit current (SCC) that is stimulated by high levels of the mineralocorticoid aldosterone. Surprisingly, A6 SCC is more responsive to glucocorticoids than to mineralocorticoids, suggesting the possibility that these cells do not contain transcriptionally active mineralocorticoid receptor (MR) and that glucocorticoid receptor (GR) mediates MR-like responses in these collecting duct-like cells. We have examined the response of both SCC and a transfected reporter gene to mineralocorticoids and glucocorticoids in the presence and absence of transfected rat MR (rMR). We found that, in the absence of transfected MR, a reporter gene that can be activated by MR or GR was more responsive to glucocorticoids such as dexamethasone and RU-28362 than to mineralocorticoids such as aldosterone. Transfected rMR underwent mineralocorticoid-dependent nuclear localization and restored both transcriptional sensitivity of a reporter gene and SCC response to mineralocorticoids. These data demonstrate that A6 cells contain transcriptionally active GR but not MR and thus suggest a molecular basis for the defect in A6 cell SCC response to aldosterone. Our results also demonstrate that GR is capable of mediating hormone stimulation of SCC, a classic mineralocorticoid response. Finally, the observation that heterologous expression of rMR can localize normally to the A6 nucleus in a hormone-dependent fashion and restore both the transcriptional and SCC response to mineralocorticoids suggests that MR function is conserved in species as distantly related as toads and mammals.

Corticosteroids influence the action potential firing pattern of hippocampal subfield CA3 pyramidal cells

Corticosteroids regulate gene expression through the activation of mineralocorticoid and glucocorticoid receptors. The hippocampus contains the highest density of mineralocorticoid and glucocorticoid receptors in the central nervous system. The modulation of neuron excitability by corticosteroids in hippocampal subfield CA1 is well documented. However, it is not known whether corticosteroids produce different effects across the various hippocampal subfields. Therefore, we used intracellular recording techniques to examine the actions of chronic corticosteroid treatment (2 weeks) on the electrophysiological properties of rat hippocampal subfield CA3 pyramidal cells. The treatment groups used in this investigation were: adrenalectomy (ADX), selective mineralocorticoid receptor activation with aldosterone (ALD), mineralocorticoid and glucocorticoid receptor activation with high levels of corticosterone (HCT), and SHAM. Corticosteroid treatment altered the percentage of nonburst and burst firing neurons. The percentages of nonbursting cells were 74 and 62% in tissue from ADX and HCT animals compared to 42 and 41% in ALD and SHAM animals, respectively. The corticosteroid-induced effect on the ratio of nonbursting to bursting cells does not appear to be secondary to changes in the cell's membrane input resistance, resting potential, time constant, action potential, slow-or fast-afterhyperpolarizing potential properties. Based on these results we conclude that corticosteroids are important for maintaining the ratio of nonburst and burst firing pyramidal neurons in subfield CA3. These novel results are distinct from those previously reported for subfield CA1, suggesting that corticosteroids have different effects across hippocampal subfields.

Corticosteroids in the brain. Cellular and molecular actions

The rat adrenal hormone corticosterone reaches the brain and binds to intracellular receptors. These receptors comprise high-affinity mineralocorticoid and lower-affinity glucocorticoid receptors that, upon activation, affect the transcription rate of specific genes. The two receptor types are discretely localized in the brain, with particularly high expression levels in the hippocampus. Here we review recent studies showing that electrical properties and structural aspects of hippocampal principal neurons are specifically regulated by mineralocorticoid- or glucocorticoid-receptor activation. The molecular mechanisms by which these cellular effects could be accomplished are discussed.

Facilitation of feedback inhibition through blockade of glucocorticoid receptors in the hippocampus

In the present study the effects of intracerebroventricular (i.c.v.) and intrahippocampal administration of corticosteroid antagonists on basal hypothalamic-pituitary-adrenal (HPA) activity around the diurnal peak were compared in male Wistar rats. In two separate experiments the glucocorticoid receptor (GR) antagonist RU 38486 and the mineralocorticoid receptor (MR) antagonist RU 28318 were tested. One hour after GR antagonist injection, significant increases in plasma ACTH and corticosterone levels were observed in the i.c.v. treated rats, when compared to vehicle. In contrast, a significant decrease in ACTH levels, and a slight, but non-significant decrease in corticosterone concentrations were attained one hour after intrahippocampal injection of the GR antagonist. Injection of the MR antagonist, on the other hand, resulted in enhanced ACTH and corticosterone levels irrespective of the site of injection. These findings suggest that negative feedback inhibition at the circadian peak involves hippocampal MRs and extrahippocampal (hypothalamic) GRs. The latter feedback inhibition overrides a positive feedback influence exerted by endogenous corticosteroids through hippocampal GRs.

Dexamethasone in the treatment of the alcohol withdrawal syndrome

Pre-clinical studies and clinical case reports suggest that glucocorticoids may be efficacious in ameliorating the signs and symptoms of the alcohol withdrawal syndrome. In order to evaluate the efficacy of the glucocorticoid dexamethasone upon alcohol withdrawal, we administered 4 mg of dexamethasone intravenously to eight alcohol dependent men during withdrawal. Withdrawal severity, as determined by the amount of lorazepam required to ameliorate withdrawal symptoms, was compared to eight other withdrawing patients not administered dexamethasone. There was no significant difference between the two groups in the amount of lorazepam required to treat to withdrawal symptoms. This preliminary study suggests that dexamethasone, in doses expected to suppress hypothalmic-pituitary-adrenal axis activation, is not efficacious in the treatment of alcohol withdrawal.

Glucocorticoid receptor activation lowers the threshold for NMDA-receptor-dependent homosynaptic long-term depression in the hippocampus through activation of voltage-dependent calcium channels

The effects of the glucocorticoid receptor agonist RU-28362 on homosynaptic long-term depression (LTD) were examined in hippocampal slices obtained from adrenal-intact adult male rats. Field excitatory postsynaptic potentials were evoked by stimulation of the Schaffer collateral/commissural pathway and recorded in stratum radiatum of area CA1. Low-frequency stimulation (LFS) was delivered at LTD threshold (2 bouts of 600 pulses, 1 Hz, at baseline stimulation intensity). LFS of the Schaffer collaterals did not produce significant homosynaptic LTD in control slices. However, identical conditioning in the presence of the glucocorticoid receptor agonist RU-28362 (10 microM) produced a robust LTD, which was blocked by the selective glucocorticoid antagonist RU-38486. The LTD induced by glucocorticoid receptor activation was dependent on N-methyl-D-aspartate (NMDA) receptor activity, because the specific NMDA receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (D-AP5) blocked the facilitation. However, the facilitation of LTD was not due to a potentiation of the isolated NMDA receptor potential by RU-28362. The facilitation of LTD by RU-28362 was also blocked by coincubation of the L-type voltage-dependent calcium channel (VDCC) antagonist nimodipine. Selective activation of the L-type VDCCs by the agonist Bay K 8644 also facilitated LTD induction. Both nimodipine and D-AP5 were effective in blocking the facilitation of LTD by Bay K 8644. These results indicate that L-type VDCCs can contribute to NMDA-receptor-dependent LTD induction.

Determinants of specificity of transactivation by the mineralocorticoid or glucocorticoid receptor

Glucocorticoids and mineralocorticoids have distinct in vivo roles despite close structural homology and similarities in vitro. Known mechanisms of specificity focus on factors extrinsic to the receptor; interactions that directly regulate the receptor to confer specificity are less well understood, particularly for the mineralocorticoid receptor (MR). To examine relative MR vs. glucocorticoid receptor (GR) function in a more physiological context, we compared transactivation by GR and MR in the standard experimental fibroblast CV-1 cell line, the renal epithelial LLC-PK1 line, and neuronal medullary raphe RN33B cells. Maximal transactivational activity mediated by MR, relative to that mediated by GR, is enhanced in both of these cell lines and is primarily conferred by an N-terminal-mediated enhancement of the MR response. In addition, the ligand concentration required for maximal transcriptional activity of the GR varies significantly between cell lines. This is independent of binding affinity or 11beta-hydroxysteroid dehydrogenase-mediated inactivation and may contribute to in vivo tissue-specific differences in responses to the GR. Although ligand binding affinity is clearly conferred by the LBD, receptor-specific variations between cell lines in transcriptional sensitivity to ligand appear, rather, to be associated with the N-terminus. These studies demonstrate that the specificity of the MR vs. the GR response may be mediated via unique cellular factors, as well as suggesting a novel means of expanding the cellular response to cortisol.

Preferential occupation of mineralocorticoid receptors by corticosterone enhances glutamate-induced burst firing in rat midbrain dopaminergic neurons

Sensitisation to the behavioural effects of amphetamine, a phenomenon which appears to involve the potentiation of excitatory amino acid (EAA)-mediated transmission at the level of dopaminergic (DA) neurons in the ventral tegmental area (the A10 cell group), is known to be affected by corticosteroid manipulations. Since there is evidence that corticosteroid manipulations can also influence unpotentiated EAA-mediated transmission elsewhere in the brain, the possibility was examined that the same may be true for midbrain DA neurons. The effect of iontophoretically administered glutamate on the activity of A10 DA neurons was investigated in adrenalectomised animals given a low dose of corticosterone intravenously (equivalent to 13.4 micrograms/100 ml plasma - likely to preferentially occupy the mineralocorticoid subtype of corticosteroid receptor) at least 45 min (median 132.5) prior to recording. Cells from these animals were compared to those from adrenalectomised and sham operated animals administered saline. Adrenalectomy significantly reduced the firing rate of A10 cells, and this effect was reversed by corticosterone replacement. Adrenalectomy did not affect basal burst firing. However, in those cells which could be classified as "bursting' under basal conditions, cells from animals administered corticosterone showed enhanced glutamate-induced bursting relative to the other two groups. The degree of enhancement was strictly determined by the basal bursting level of the cell. Since the distinction between "bursting' and "non-bursting' DA neurons is probably not related to differences at the level of the EAA receptor/effector mediating bursting, it is argued that corticosterone's facilitation of glutamate-induced bursting is not produced at this level, but rather at the level of an intrinsic membrane property which modulates bursting.

CR16, a novel proline-rich protein expressed in rat brain neurons, binds to SH3 domains and is a MAP kinase substrate

CR16 is a glucocorticoid-regulated gene expressed in subpopulations of neurons in the brain, including the hippocampus. The CR16 open reading frame encodes a 45 kDa protein containing 32% proline. To begin characterizing the CR16 protein, a rabbit polyclonal antibody was raised against an Escherchia coli-produced fusion protein containing amino acids 370-438 of CR16. The antibody identifies a protein doublet of 68 and 72 kDa by sodium-dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) from hippocampal extracts and from insect cells expressing the CR16 open reading frame from a baculovirus construct. However, when hippocampal extracts are electrophoresed on nondenaturing polyacrylamide gels, the CR16 protein migrates as a 48 kDa protein that better correlates with the size of the open reading frame. Examination of the primary amino acid sequence reveals at least 12 sequence homologies to the abl-SH3 binding domain consensus sequence XPXXPPP psi XP. In addition, CR16 has at least 36 copies of the PXXP motif, which is contained in all known SH3 binding domains. Solution and filter binding assays confirm that CR16 selectively binds SH3 domains. The CR16 primary amino acid sequence also contains at least eight consensus MAP kinase phosphorylation sites, five of which are in the potential SH3 binding domains. The CR16 protein, immunoprecipitated from rat brain, is an in vitro substrate for the purified enzyme. However, phosphorylation of CR16 does not greatly affect the binding of the various SH3 domains in our assay system. These data strongly suggest that the function of CR16 is to mediate one or more signal transduction pathways in CNS neurons, in addition to being a glucocorticoid-regulated gene.

Stress, Glucocorticoids, and Damage to the Nervous System: The Current State of Confusion

An extensive literature demonstrates that glucocorticoids (GCs), the adrenal steroids secreted during stress, can have a broad range of deleterious effects in the brain. The actions occur predominately, but not exclusively, in the hippocampus, a structure rich in corticosteroid receptors and particularly sensitive to GCs. The first half of this review considers three types of GC effects: a) GC-induced atrophy, in which a few weeks' exposure to high GC concentrations or to stress causes reversible atrophy of dendritic processes in the hippocampus; b) GC neurotoxicity where, over the course of months, GC exposure kills hippocampal neurons; c) GC neuroendangerment, in which elevated GC concentrations at the time of a neurological insult such as a stroke or seizure impairs the ability of neurons to survive the insult. The second half considers the rather confusing literature as to the possible mechanisms underlying these deleterious GC actions. Five broad themes are discerned: a) that GCs induce a metabolic vulnerability in neurons due to inhibition of glucose uptake; b) that GCs exacerbate various steps in a damaging cascade of glutamate excess, calcium mobilization and oxygen radical generation. In a review a number of years ago, I concluded that these two components accounted for the deleterious GC effects. Specifically, the energetic vulnerability induced by GCs left neurons metabolically compromised, and less able to carry out the costly task of containing glutamate, calcium and oxygen radicals. More recent work has shown this conclusion to be simplistic, and GC actions are shown to probably involve at least three additional components: c) that GCs impair a variety of neuronal defenses against neurologic insults; d) that GCs disrupt the mobilization of neurotrophins; e) that GCs have a variety of electrophysiological effects which can damage neurons. The relevance of each of those mechanisms to GC-induced atrophy, neurotoxicity and neuroendangerment is considered, as are the likely interactions among them.

Brain-corticosteroid hormone dialogue: slow and persistent

1. The stress response system is shaped by genetic factors and life experiences, of which the effect of a neonatal life event is among the most persistent. Here we report studies focused on the "nature-nurture" question using rat lines genetically selected for extreme differences in dopamine phenotype as well as rats exposed as infants to the traumatic experience of maternal deprivation. 2. As key to the endocrine and behavioural adaptations occurring in these two animal models the hormone corticosterone (CORT) is considered. The stress hormone exerts slow and persistent genomic control over neuronal activity underlying the stress response system via high affinity mineralocorticoid (MR) and glucocorticoid receptors (GR). This action is exerted in a coordinate manner and involves after stress due to the rising CORT levels progressive activation of both receptor types. 3. Short periods of maternal separation (neonatal handling) trigger subsequently enhanced maternal care and sensory stimulation. However, a prolonged period (24 h) of depriving the infant of maternal care disrupts the stress hyporesponsive period (SHRP) and causes an inappropriate rise in CORT. During development exposure to CORT and to sensory stimulation has longlasting consequences for organization of the stress response system. 4. We find that these factors embodied by mother-pup interaction are critical for dopamine phenotype, CORT receptor dynamics and neuroendocrine regulation in adult life. The findings provide a conceptual framework to study dopamine-related psychopathology against a background of genetic predisposition, early life events, stress hormones and brain development.

Androgen modulates N-methyl-D-aspartate-mediated depolarization in CA1 hippocampal pyramidal cells

Previously, research elucidating steroid hormone actions in the central nervous system has focused on their role in sexual reproduction and maintaining homeostasis. The hippocampus is a target of steroid modulation and is involved in the development of emotional behavior and memory storage. Area CA1 of the hippocampus contains a high density of androgen receptor (AR) and N-methyl-D-aspartate (NMDA) receptors. NMDA receptors underlie excitatory synaptic transmission and excitotoxicity in CA1 neurons. The effects of AR activation on the neurophysiology of hippocampal pyramidal neurons is unknown. Standard intracellular recording techniques in hippocampal slices were used to investigate the effects of the non-aromatizable androgen, 5-alpha-dihydrotestos-terone-proprionate (DHTP), on CA1 pyramidal cell characteristics and NMDA receptor-mediated responses. Male Sprague-Dawley rats were unoperated, sham-operated (SHAM), gonadectomized (GDX), or gonadectomized with DHTP replacement therapy (GDX + DHTP). Neuronal AR was saturated by DHTP treatment as determined by binding studies and immunocytochemistry. Chronic DHTP treatment increased the action potential duration and decreased the fast afterhyperpolarization (fAHP) amplitude. To test the effect of DHTP on glutamate receptor-mediated responses, hippocampal slices were exposed to increasing concentrations of NMDA. In pyramidal cells from SHAM and GDX-treated animals, 30 microM NMDA induced an irreversible depolarization; the membrane potential of pyramidal cells from GDX + DHTP-treated animals recovered to baseline. The effect of DHTP was time dependent, implicating protein synthetic mechanisms. Our findings demonstrate that androgens can influence pyramidal cell characteristics and neurotransmitter-evoked actions in hippocampal CA1 pyramidal cells.

Sensory afferent processing in multi-responsive DRG neurons

The recent advance in molecular and neurobiological techniques disclosed the multi-responsive nature of DRG neurons. The survival, phenotype expression and electrical properties of these neurons are under the control of a variety of substances through their specific receptors. In pathological conditions, such as tissue inflammation or nerve injury, DRG neurons change their responsiveness through the dynamic reconstruction of their receptor system. This reconstruction is initiated by environmental stimuli. Thus the properties of polymodal nociceptors can be altered according to the environmental conditions. The whole story of this mechanism is not disclosed yet. In order to understand this mechanism, it is basically important to identify various receptor mRNAs in DRG neurons, precise localization of receptor proteins, site of synthesis and route of supply of ligands for these receptors.

Steroid receptor heterodimerization demonstrated in vitro and in vivo

The mineralocorticoid and glucocorticoid receptors (MR and GR, respectively) are members of the intracellular receptor superfamily that bind as homodimers to the same hormone response elements (HREs). Physiological evidence suggests that MR and GR interact with each other in cells that express both receptors, implying that they might directly interact in the regulation of transcription initiation. Indeed, we have found that coexpressed MR and GR interact functionally at the transcriptional level and furthermore that they interact physically through heterodimer formation at a shared HRE in vitro and in vivo. We suggest from these findings that heterodimerization may play an important role in steroid receptor transcriptional regulation.

In vitro studies of glucocorticoid effects on neurons and astrocytes

Studies using immunocytochemistry and RNase protection assay demonstrate that glucocorticoid and mineralocorticoid receptors (GR, MR) and their corresponding mRNAs are co-expressed in hippocampal neurons cultured in serum-free, defined medium and at lower levels in cultured astrocytes. Addition of serum or medium conditioned by astrocytes increases the levels of MR mRNA, but has little effect on the levels of GR mRNA. Cellular levels of both GR mRNA and MR mRNA are upregulated by growth of embryonic hippocampal neurons in corticosterone. This is in distinct contrast to regulation of receptor expression in vivo where mRNAs for these receptors are downregulated in the rat hippocampus by corticosterone treatment of the adult adrenalectomized rat. However, in cultured astrocytes, GR and MR mRNAs are also downregulated by corticosterone. To begin to define the role of glucocorticoids in gene expression in astrocytes, we have used giant two-dimensional (2D) gel electrophoresis to separate astrocyte cellular proteins and translation products synthesized in vitro from astrocyte poly A+ RNA. Analysis of approximately 1,500 in vitro translation products by giant 2D gel electrophoresis reveals 11 protein inductions and 1 repression that occur at the level of mRNA in the absence of protein synthesis following treatment of astrocytes with corticosterone. Interestingly, these changes appear to be mediated by GR, but not by MR. The in vitro studies described here are relevant to identifying the role of GR and MR in gene expression in specific cell types in the hippocampus.

Glucocorticoids and stress: permissive and suppressive actions

Protection against stress by glucocorticoids is discussed in relation to their permissive and suppressive actions. Evidence from the last decade is summarized regarding the physiological nature of the suppressive actions, and the hypothesis that they prevent stress-activated defense mechanisms from overshooting and damaging the organism. Support for this hypothesis has come from observations on how endogenous or administered glucocorticoids control inflammatory and immune responses, protect in endotoxic and hemorrhagic shock, regulate central nervous system responses to stimuli, and moderate many defense reactions through suppression of cytokines and other mediators. Studies showing that glucocorticoids permissively induce receptors for several mediators that they suppress have led to a model in which stimulated activity of a mediator system is increased permissively through induction of mediator receptors and decreased through suppression of mediator production.

The effect of corticosterone on reactivity to spatial novelty is mediated by central mineralocorticosteroid receptors

Corticosterone, secreted by the adrenal glands, binds to central mineralocorticoid receptors with high affinity and to glucocorticoid receptors with a tenfold lower affinity. In previous studies we have shown that the selective activation of either mineralocorticoid receptors or glucocorticoid receptors exerts distinctly different behavioural effects. In this study we examined in particular the mineralocorticoid receptor-mediated effect of corticosterone on the control of the behavioural response of male Wistar rats to spatial novelty. This analysis was based on our observation that in adrenal-intact rats the presence of an object in the centre of an open field alters the time spent and distance walked in the centre compared to the peripheral area, i.e. the pattern of reactive locomotor activity is changed. Using this paradigm we found that 1 day after removal of the adrenals the rats increased their behavioural reactivity towards the object. Treatment of adrenalectomized rats with a low dose of corticosterone (50 micrograms/kg s.c.) 1 h prior to testing restored the behavioural reactivity to the level of sham-operated, intact rats. Surprisingly, a high dose of corticosterone (1000 micrograms/kg s.c.) also increased the rat's reactivity towards the object. The same high dose of corticosterone given to adrenal-intact rats also increased behavioural reactivity. Pretreatment of these rats with an intracerebroventricular injection of the selective mineralocorticoid receptor antagonist RU28318 (100 ng/microliters) prevented the corticosterone-induced increase in behavioural reactivity, while the blockade of glucocorticoid receptors with the antagonist RU38486 (100 ng/microliters) was not effective. Administration of the mineralocorticoid receptor antagonist without corticosterone to adrenal-intact rats also increased behavioural reactivity, but this increase did not reach statistical significance.(ABSTRACT TRUNCATED AT 250 WORDS)

Aldosterone stimulates K secretion prior to onset of Na absorption in guinea pig distal colon

Distal colon from guinea pig was stimulated in vitro by aldosterone in Ussing chambers that allowed measurement of short-circuit current (Isc) and tissue conductance (Gt). The response to aldosterone was delayed by approximately 20 min and resulted in a negative Isc, consistent with K secretion. Approximately 1 h later the Isc began to increase and eventually became positive, consistent with subsequent stimulation of Na absorption. The Na-absorptive response could be inhibited by mucosal amiloride without altering the rate of K secretion. Similarly, K secretion could be inhibited by serosal bumetanide without altering Na absorption. In the presence of spironolactone, actinomycin D, or cycloheximide, aldosterone failed to stimulate both K secretion and Na absorption. A dose response to aldosterone provided an apparent Kd of 2.6 +/- 0.5 nM, consistent with a high-affinity receptor coupled to this secretory response. Stimulation by the K secretagogue epinephrine did not produce an additive increase in K secretion, suggesting that the same cell type responds to both aldosterone and epinephrine and that the protein induced by aldosterone was not one of the membrane proteins responsible for K secretion.

Regulation of adrenocorticosteroid receptor mRNA expression in the central nervous system

1. The adrenocorticosteroid receptors are hormone-activated transcription factors that have the potential to influence gene expression in a wide variety of CNS neurons. This review summarizes the present state of knowledge regarding the localization and regulation of glucocorticoid (or type II corticosteroid) receptor and mineralocorticoid (or type I corticosteroid) receptor mRNAs in brain, from the perspective of their potential influence on a wide variety of hormone-responsive genes. 2. Corticosteroid receptors are widely but not uniformly localized in the CNS and exhibit very complex regulation by glucocorticoids, gonadal steroids, neurotransmitter systems, and endogenous circadian drive. Both receptor species are present during development, implying an ability for these transcription factors to interact with neuronal differentiation, growth, and viability, and both receptors appear to regulate with age, suggesting relationships between adrenocorticosteroid receptor populations and brain aging. Regulation of adrenocorticosteroid receptor mRNA expression at the level of polyadenylation and splicing indicates that GR and MR biosynthesis is a dynamic process susceptible to numerous classes of information. 3. Further study of GR and MR biosynthesis at the gene, mRNA, and protein level is required to determine the true meaning of the regulatory complexities seen in defined neuronal circuits.

Functional implications of brain corticosteroid receptor diversity

1. Corticosteroids readily enter the brain and control gene expression in nerve cells via binding to intracellular receptors, which act as gene transcription factors. In the rat brain corticosterone binds to mineralocorticoid receptors (MRs) with a 10-fold higher affinity than to glucocorticoid receptors (GRs). As a consequence, these MRs are extensively occupied under basal resting conditions, while substantial GR occupation occurs at the circadian peak and following stress. Both receptors are colocalized in most, but not all, hippocampal neurons. In addition, some neurons contain aldosterone-selective MRs, if corticosterone is enzymatically inactivated. These aldosterone target neurons are presumably localized in the anterior hypothalamus, where they underlie central control of salt appetite and cardiovascular regulation. 2. The data show that MR- and GR-mediated effects proceed in a coordinate and often antagonistic mode of action: (i) in hippocampus MR activation maintains excitability, while GR occupancy suppresses excitability, which is transiently raised by excitatory stimuli; (ii) central MRs participate in control of the sensitivity of the neuroendocrine stress response system, while GRs are involved in termination of the stress response; (iii) MRs in the hippocampus have a role in regulation of behavioral reactivity and response selection. GR-mediated effects facilitate storage of information. 3. On the basis of these data, we propose that a relative deficiency or excess of MR- over GR-mediated neuronal effects may lead to a condition of enhanced or reduced responsiveness to environmental influences, alter behavioral adaptation, and promote susceptibility to stress. The findings may serve development of novel therapeutic strategies for treatment of stress-related brain diseases.

Association of glucocorticoid receptor immunoreactivity with cell membrane and transport vesicles in hippocampal and hypothalamic neurons of the rat

The aim of the present study was to reveal at the ultrastructural level cytoplasmic loci that display glucocorticoid receptor (GR) immunoreactivity in pyramidal neurons of the CA1 sector of the hippocampus and in cells of the medial parvicellular subnucleus of the hypothalamic paraventricular nucleus (PVN). Adrenalectomized male rats were injected intraperitoneally with corticosterone (CS) (1 mg/100 g bw) and sacrificed within 4 hr. Vibratome sections of the perfusion-fixed forebrains were processed for immunocytochemical detection of type 2 GR by means of the BuGr, anti-rat liver GR monoclonal antibody and silver-gold-intensified diaminobenzidine chromogen. The corticosterone administration gradually shifted the GR immunoreactivity (IR) from the cytoplasm to the nucleus. Samples taken 20-40 min after the steroid treatment demonstrated pyramidal cells expressing GR IR in both the cytoplasmic and nuclear pools. Although the chromatin-associated appearance of GR in the nucleus was identifiable at the light microscopic level, the nature of immunoreactive intracytoplasmic loci was not. Ultrastructural analysis of the cytoplasm indicated that fine silver-gold grains marking GR-immunoreactive sites associated with the plasma membrane and coated and regular vesicles. Noted occasionally beneath the plasma membrane of the cell bodies and dendrites, the vesicles also appeared at deeper locations in dendritic processes and around the cell nuclei. These results suggest that glucocorticoid receptors participate in signal transduction at the level of the cell membrane, as well as at the level of the genome in the cell nucleus.

Effects of glucocorticoids on hippocampal long-term potentiation

The effects of chronic and acute corticosterone (CORT) administration were investigated on hippocampal long-term potentiation (LTP) in the dentate gyrus granule cell layer of the rat. Electrophysiological experiments were performed in vivo under urethane anesthesia. Chronic CORT treatment (40 mg/kg/day) over 21 days decreased LTP compared to vehicle controls, even when LTP was measured 48 hours after cessation of CORT treatment, when serum CORT levels had returned to baseline. A single injection of CORT also decreased LTP compared to vehicle controls, but only when CORT levels were high, since at 48 hours after a single acute CORT injection LTP was not depressed. The decrements in LTP were seen both for the slope of the excitatory postsynaptic potential and for the population spike. Yet CORT had no effects on posttetanic potentiation or neuronal excitability. These findings are consistent with previous reports showing a reduction in LTP in the CA1 field of animals exposed to stress or acute CORT administration.

Glucocorticoid and mineralocorticoid receptors in rat neocortical and hippocampal brain cells in culture: characterization and regulatory studies

Glucocorticoid and mineralocorticoid binding sites were characterized in cell cultures derived from neocortical and hippocampal brain tissue from fetal (E18) rats. Specific and saturable binding was detected in living cells with a sensitive whole cell binding method using [3H]dexamethasone ([3H]DEX) and [3H]aldosterone ([3H]ALDO) (in the presence of RU 28362, a selective glucocorticoid receptor (GR) agonist) as ligands for the measurement of glucocorticoid and mineralocorticoid receptors (MRs), respectively. Specific corticosteroid binding was demonstrated as early as day 4 in culture in neocortical cells, with a time-dependent increase in binding sites during further culturing time. At 7-9 days in vitro, Scatchard analysis of [3H]DEX binding revealed a maximum binding capacity (Bmax) of 83.4 +/- 5.0 fmol/mg protein and a binding affinity (Kd) of 3.6 +/- 0.4 nM in neocortical brain cells. Competition binding studies with [3H]DEX demonstrated a glucocorticoid specificity of receptor sites (relative binding affinity: RU 28362 = DEX > PROG > ALDO). Similar binding characteristics were demonstrated for GRs in cultures derived from fetal hippocampal tissue (Bmax 49.1 +/- 5.8 fmol/mg protein, Kd 3.5 +/- 0.2 nM). Analysis of MRs with [3H]ALDO (+RU 28362) revealed specific and saturable binding in hippocampal cultures, with a Bmax of 8.0 +/- 0.5 fmol/mg protein and a Kd of 0.2 +/- 0.1 nM. Competition studies with [3H]ALDO showed a mineralocorticoid-like pattern of receptor binding (relative binding affinity: CORT = ALDO > PROG > DEX). In addition, small numbers of MRs were detectable in cortex-derived cultures (Bmax: 3.7 +/- 0.8 fmol/mg protein, Kd: 0.3 +/- 0.2 nM).(ABSTRACT TRUNCATED AT 250 WORDS)

Mineralocorticoid and glucocorticoid receptor activities distinguished by nonreceptor factors at a composite response element

Mineralocorticoid and glucocorticoid hormones elicit distinct physiologic responses, yet the mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) bind to and activate transcription similarly from a consensus simple hormone response element (HRE). The activities of GR and MR at plfG, a 25-base pair composite response element to which both the steroid receptors and transcription factor AP1 can bind, are analyzed here. Under conditions in which GR represses AP1-stimulated transcription from plfG, MR was inactive. With the use of MR-GR chimeras, a segment of the NH2-terminal region of GR (amino acids 105 to 440) was shown to be required for this repression. Thus, the distinct physiologic effects mediated by MR and GR may be determined by differential interactions of nonreceptor factors with specific receptor domains at composite response elements.