The stressed brain of humans and rodents.

After stress, the brain is exposed to waves of stress mediators, including corticosterone (in rodents) and cortisol (in humans). Corticosteroid hormones affect neuronal physiology in two time-domains: rapid, non-genomic actions primarily via mineralocorticoid receptors; and delayed genomic effects via glucocorticoid receptors. In parallel, cognitive processing is affected by stress hormones. Directly after stress, emotional behaviour involving the amygdala is strongly facilitated with cognitively a strong emphasis on the "now" and "self," at the cost of higher cognitive processing. This enables the organism to quickly and adequately respond to the situation at hand. Several hours later, emotional circuits are dampened while functions related to the prefrontal cortex and hippocampus are promoted. This allows the individual to rationalize the stressful event and place it in the right context, which is beneficial in the long run. The brain's response to stress depends on an individual's genetic background in interaction with life events. Studies in rodents point to the possibility to prevent or reverse long-term consequences of early life adversity on cognitive processing, by normalizing the balance between the two receptor types for corticosteroid hormones at a critical moment just before the onset of puberty.

Forebrain glutamatergic, but not GABAergic, neurons mediate anxiogenic effects of the glucocorticoid receptor.

Anxiety disorders constitute a major disease and social burden worldwide; however, many questions concerning the underlying molecular mechanisms still remain open. Besides the involvement of the major excitatory (glutamate) and inhibitory (gamma aminobutyric acid (GABA)) neurotransmitter circuits in anxiety disorders, the stress system has been directly implicated in the pathophysiology of these complex mental illnesses. The glucocorticoid receptor (GR) is the major receptor for the stress hormone cortisol (corticosterone in rodents) and is widely expressed in excitatory and inhibitory neurons, as well as in glial cells. However, currently it is unknown which of these cell populations mediate GR actions that eventually regulate fear- and anxiety-related behaviors. In order to address this question, we generated mice lacking the receptor specifically in forebrain glutamatergic or GABAergic neurons by breeding GRflox/flox mice to Nex-Cre or Dlx5/6-Cre mice, respectively. GR deletion specifically in glutamatergic, but not in GABAergic, neurons induced hypothalamic-pituitary-adrenal axis hyperactivity and reduced fear- and anxiety-related behavior. This was paralleled by reduced GR-dependent electrophysiological responses in the basolateral amygdala (BLA). Importantly, viral-mediated GR deletion additionally showed that fear expression, but not anxiety, is regulated by GRs in glutamatergic neurons of the BLA. This suggests that pathological anxiety likely results from altered GR signaling in glutamatergic circuits of several forebrain regions, while modulation of fear-related behavior can largely be ascribed to GR signaling in glutamatergic neurons of the BLA. Collectively, our results reveal a major contribution of GRs in the brain's key excitatory, but not inhibitory, neurotransmitter system in the regulation of fear and anxiety behaviors, which is crucial to our understanding of the molecular mechanisms underlying anxiety disorders.

Spatial heterogeneity in the Mediterranean Biodiversity Hotspot affects barcoding accuracy of its freshwater fishes.

Incomplete knowledge of biodiversity remains a stumbling block for conservation planning and even occurs within globally important Biodiversity Hotspots (BH). Although technical advances have boosted the power of molecular biodiversity assessments, the link between DNA sequences and species and the analytics to discriminate entities remain crucial. Here, we present an analysis of the first DNA barcode library for the freshwater fish fauna of the Mediterranean BH (526 spp.), with virtually complete species coverage (498 spp., 98% extant species). In order to build an identification system supporting conservation, we compared species determination by taxonomists to multiple clustering analyses of DNA barcodes for 3165 specimens. The congruence of barcode clusters with morphological determination was strongly dependent on the method of cluster delineation, but was highest with the general mixed Yule-coalescent (GMYC) model-based approach (83% of all species recovered as GMYC entity). Overall, genetic morphological discontinuities suggest the existence of up to 64 previously unrecognized candidate species. We found reduced identification accuracy when using the entire DNA-barcode database, compared with analyses on databases for individual river catchments. This scale effect has important implications for barcoding assessments and suggests that fairly simple identification pipelines provide sufficient resolution in local applications. We calculated Evolutionarily Distinct and Globally Endangered scores in order to identify candidate species for conservation priority and argue that the evolutionary content of barcode data can be used to detect priority species for future IUCN assessments. We show that large-scale barcoding inventories of complex biotas are feasible and contribute directly to the evaluation of conservation priorities.

Knockdown of the glucocorticoid receptor alters functional integration of newborn neurons in the adult hippocampus and impairs fear-motivated behavior.

Glucocorticoids (GCs) secreted after stress reduce adult hippocampal neurogenesis, a process that has been implicated in cognitive aspects of psychopathology, amongst others. Yet, the exact role of the GC receptor (GR), a key mediator of GC action, in regulating adult neurogenesis is largely unknown. Here, we show that GR knockdown, selectively in newborn cells of the hippocampal neurogenic niche, accelerates their neuronal differentiation and migration. Strikingly, GR knockdown induced ectopic positioning of a subset of the new granule cells, altered their dendritic complexity and increased their number of mature dendritic spines and mossy fiber boutons. Consistent with the increase in synaptic contacts, cells with GR knockdown exhibit increased basal excitability parallel to impaired contextual freezing during fear conditioning. Together, our data demonstrate a key role for the GR in newborn hippocampal cells in mediating their synaptic connectivity and structural as well as functional integration into mature hippocampal circuits involved in fear memory consolidation.

Rapid effects of corticosterone in the mouse dentate gyrus via a nongenomic pathway.

Corticosterone activates two types of intracellular receptors in the rodent brain: the high affinity mineralocorticoid receptor (MR) and lower affinity glucocorticoid receptor (GR). These receptors act as transcriptional regulators and mediate slow changes in neuronal activity in a region-dependent manner. For example, in CA1 pyramidal cells, corticosterone slowly changes Ca(2+) currents and glutamate transmission but dentate granule cells appear to be resistant. Recent studies have shown that corticosteroids also exert rapid MR-dependent, nongenomic effects on hippocampal CA1 cells [e.g. increasing the frequency of miniature excitatory postsynaptic currents (mEPSCs)]. In the present study, we investigated whether dentate granule cells are also resistant to the rapid effects of corticosterone. We found that, comparable to the CA1 area, corticosterone quickly and reversibly increases mEPSC frequency but not amplitude of dentate cells. This effect did not require protein synthesis and displayed the pharmacological profile of an MR- rather than GR-dependent event. These data support the hypothesis that, unlike the slow gene-mediated effects of corticosterone, rapid hormonal actions are quite similar for CA1 and dentate cells.

Rapid changes in hippocampal CA1 pyramidal cell function via pre- as well as postsynaptic membrane mineralocorticoid receptors.

Corticosterone (100 nm) rapidly increases the frequency of miniature excitatory postsynaptic currents in mouse CA1 pyramidal neurons via membrane-located mineralocorticoid receptors (MRs). We now show that a presynaptic ERK1/2 signalling pathway mediates the nongenomic effect, as it was blocked by the MEK inhibitors U0126 (10 microm) and PD098059 (40 microm) and occluded in H-Ras(G12V)-mutant mice with constitutive activation of the ERK1/2 presynaptic pathway. Notably, the increase in mEPSC frequency was not mediated by retrograde signalling through endocannabinoids or nitric oxide, supporting presynaptic localization of the signalling pathway. Unexpectedly, corticosterone was also found to have a direct postsynaptic effect, rapidly decreasing the peak amplitude of I(A) currents. This effect takes place via postsynaptic membrane MRs coupled to a G protein-mediated pathway, as the effect of corticosterone on I(A) was effectively blocked by 0.5 mm GDP-beta-S administered via the recording pipette into the postsynaptic cell. Taken together, these results indicate that membrane MRs mediate rapid, nongenomic effects via pre- as well as postsynaptic pathways. Through these dual pathways, high corticosterone concentrations such as occur after stress could contribute to enhanced CA1 pyramidal excitability.

Opposite effects of glucocorticoid receptor activation on hippocampal CA1 dendritic complexity in chronically stressed and handled animals.

Remodeling of synaptic networks is believed to contribute to synaptic plasticity and long-term memory performance, both of which are modulated by chronic stress. We here examined whether chronic stress modulates dendritic complexity of hippocampal CA1 pyramidal cells, under conditions of basal as well as elevated corticosteroid hormone levels. Slices were prepared from naïve, handled or chronically stressed animals and briefly treated with vehicle or corticosterone (100 nM); neurons were visualized with a fluorescent dye injected into individual CA1 pyramidal cells. We observed that 21 days of unpredictable stress did not affect hippocampal CA1 apical or basal dendritic morphology compared with naïve animals when corticosteroid levels were low. Only when slices from stressed animals were also exposed to elevated corticosteroid levels, a significant reduction in apical (but not basal) dendritic length became apparent. Unexpectedly, animals that were handled or 3 weeks showed a reduction in both apical dendritic length and number of apical branch points when compared with naïve animals. Apical dendritic length and number of branch points were restored to levels found in naïve animals several hours after in vitro treatment with 100 nM corticosterone. All effects of acute corticosterone administration could be prevented by the glucocorticoid receptor antagonist RU38486 given during the last 4 days of the stress or handling protocol. We conclude that brief exposure to high concentrations of corticosterone can differently affect apical dendritic structure, depending on the earlier history of the animal, a process that critically depends on involvement of the glucocorticoid receptor.

Monitoring extracellular glutamate in hippocampal slices with a microsensor.

The direct local assessment of glutamate in brain slices may improve our understanding of glutamatergic neurotransmission significantly. However, an analytical technique that monitors glutamate directly in brain slices is currently not available. Most recording techniques either monitor derivatives of glutamate or detect glutamate that diffuses out of the slice. Microsensors provide a promising solution to fulfill this analytical requirement. In the present study we have implanted a 10 microm diameter hydrogel-coated microsensor in the CA1 area of hippocampal slices to monitor extracellular glutamate levels. The influence of several pharmacological agents, which facilitate glutamate release from neurons or astrocytes, was investigated to explore the applicability of the microsensor. It was observed that KCl, veratradine, alpha-latrotoxine (LTX), DL-threo-beta-benzyloxyaspartate (dl-TBOA) and L-cystine rapidly increased the extracellular glutamate levels. As far as we know this is the first study in which a microsensor is applied to monitor dynamic changes of glutamate in brain slices and in our opinion this type of research may contribute greatly to improve our understanding of the physiology of glutamatergic neurotransmission.

Acute activation of hippocampal glucocorticoid receptors results in different waves of gene expression throughout time.

Several aspects of hippocampal cell function are influenced by adrenal-secreted glucocorticoids in a delayed, genomic fashion. Previously, we used Serial Analysis of Gene Expression to identify glucocorticoid receptor (GR)-induced transcriptional changes in the hippocampus at a fixed time point. However, because changes in mRNA levels are transient and most likely precede the effects on hippocampal cell function, the aim of the current study was to assess the transcriptional changes in a broader time window by generating a time curve of GR-mediated gene expression changes. Therefore, we used rat hippocampal slices obtained from adrenalectomised rats, substituted in vivo with low corticosterone pellets, predominantly occupying the hippocampal mineralocorticoid receptors. To activate GR, slices were treated in vitro with a high (100 nM) dose of corticosterone and gene expression was profiled 1, 3 and 5 h after GR-activation. Using Affymetrix GeneChips, a striking pattern with different waves of gene expression was observed, shifting from exclusively down-regulated genes 1 h after GR-activation to both up and down regulated genes 3 h after GR-activation. After 5 h, the response was almost back to baseline. Additionally, real-time quantitative polymerase chain reaction was used for validation of a selection of responsive genes including genes involved in neurotransmission and synaptic plasticity such as the corticotropin releasing hormone receptor 1, monoamine oxidase A, LIMK1 and calmodulin 2. This permitted confirmation of GR-responsiveness of 15 out of 18 selected genes. In conclusion, direct activation of GR in hippocampal slices results in transient changes in gene expression. The pattern in which gene expression was modulated suggests that the fast genomic effects of glucocorticoids may be realised via transrepression, preceding a later wave of transactivation. Furthermore, we identified a number of interesting candidate genes which may underlie the glucocorticoid-mediated effects on hippocampal cell function.

Gene expression profiles associated with survival of individual rat dentate cells after endogenous corticosteroid deprivation.

Removal of circulating corticosterone by adrenalectomy (ADX) leads to apoptosis after 3 days in a small population of rat dentate granule neurons, whereas most surrounding cells remain viable. Interestingly, a specific expression profile is triggered in surviving granule cells that may enhance their survival. Hippocampal slices prepared 1, 2 or 3 days after ADX or sham operation were stained ex vivo with Hoechst 33258, which serves to identify apoptotic neurons. After electrophysiological analysis, multiple gene expression in surviving individual granule cells was assessed by linear antisense RNA amplification and hybridization to slot blots containing various neuronal cDNAs. Hierarchical clustering and principal component analysis was performed on two physiological variables and 14 mRNA ratios from ADX cells from every time point. Our results indicate that surviving 3-day ADX granule cells display lower membrane capacitance, lower relative N-methyl-d-aspartate (NMDA) R1 mRNA expression and higher relative mineralocorticoid receptor (MR), alpha1A voltage-gated Ca-channel, Bcl-2 and NMDA R2C mRNA expression. Some 1- and 2-day ADX cells cluster with these 3-day survivors; therefore, one or more components of their mRNA expression profile may represent predictive markers for apoptosis resistance. The functional relevance of two candidate genes was tested by in vivo local over-expression in the same model system; of these, Bcl-2 conferred partial protection when induced shortly before ADX. Therefore, removal of corticosteroids triggers a specific gene expression profile in surviving dentate granule cells; key components of this profile may be associated with their survival.

Acute stress increases calcium current amplitude in rat hippocampus: temporal changes in physiology and gene expression.

Activation of hippocampal glucocorticoid receptors in vitro increases calcium current amplitude through a process requiring DNA binding of receptor homodimers. We here investigated (i). whether similar increased calcium currents also occur following in vivo glucocorticoid receptor activation due to stress and (ii). if so, whether this can be explained by increased expression of calcium channel subunits. Rats were exposed to a novelty stress; some of the animals were pretreated with a glucocorticoid receptor antagonist. In subsequently prepared hippocampal slices, calcium currents were recorded from identified CA1 pyramidal neurons, after which RNA was collected, linearly amplified and hybridized with cDNA clones. Glucocorticoid receptor activation due to novelty exposure was associated with large total peak calcium currents and high-threshold noninactivating currents. Low-threshold calcium currents were not affected. Large total peak and noninactivating current amplitudes were also seen when animals received a more severe stressor, i.e. additional ether exposure. In the stressed groups, the total peak and high-threshold calcium current gradually increased with time resulting in a significant enhancement at >or=3 h after stress exposure. In the same cells, the summated (relative) RNA expression of various alpha1 calcium channel subunits was only transiently enhanced, prior to the functional changes. These data indicate that in vivo activation of glucocorticoid receptors due to stress gradually increases specific calcium current components. Prior to the functional change, increased expression of calcium channel subunits was observed, suggesting that the enhanced function could be explained by transcriptional regulation of the channels.

Morphological and functional properties of rat dentate granule cells after adrenalectomy.

After complete adrenalectomy, part of the granule cells in the dentate gyrus undergo apoptosis. Findings on morphological changes in non-apoptotic granule cells, though, have been equivocal. In the present study we examined the dendritic trees of dentate granule cells 7 days after adrenalectomy or sham operation, and tested the hypothesis that changes in dendritic trees have considerable consequences for ionic currents, as measured in the soma with whole cell recording. For the latter, we focussed on voltage-gated calcium currents, which are partly generated in distal dendrites. All cells were passively filled with a fluorescent dye via the patch pipette while recording calcium currents; subsequently the cells were three-dimensionally reconstructed with the use of a confocal microscope. In sham-operated rats, dendritic trees of cells with a soma located in the inner part of the granule cell layer (facing the hilus) were significantly smaller than trees of cells located in the outer part of the layer. Neurons from rats that had extremely low (undetectable-0.3 microg/dl) circulating levels of corticosterone displayed very small and simple dendritic trees compared to cells from adrenalectomized rats that still had residual levels of corticosterone (0.6-1.0 microg/dl), regardless of the location of their soma. Despite the observation that simple dendritic trees were seen in rats where corticosterone was extremely low, the whole cell calcium current amplitude recorded from the soma of these cells was not reduced compared to the remaining cells from adrenalectomized or sham-operated rats. Our data indicate that in the absence of corticosterone dendritic trees of dentate granule cells display atrophy but that this does not necessarily reduce ionic currents measured in the soma.

Calcium currents in rat dentate granule cells are altered after adrenalectomy.

Adrenal corticosteroid hormones modulate voltage-gated calcium currents in rat CA1 hippocampal neurons. In the present whole-cell recording study we examined whether calcium currents in dentate granule cells are also under control of corticosteroids. In a first series of experiments, in which the calcium chelator BAPTA was added to the recording solution, the amplitude of calcium currents induced by a voltage step to -10 mV was found to be enhanced shortly (1 or 2 days) after adrenalectomy compared to sham operation. No enhancement was seen when adrenalectomized animals received a low dose of corticosterone in the drinking water. By contrast, 3 or 7 days after adrenalectomy calcium current amplitude was decreased. Starting 3 days after adrenalectomy, some of the granule cells underwent apoptosis. This caused a bias in the recorded cell population towards relatively apoptosis-resistant cells, suggesting that restricted calcium influx may be a key feature of cells withstanding the apoptotic route. In accordance, cells from a small percentage ( approximately 20%) of animals that resisted apoptosis after adrenalectomy also displayed small calcium currents. In a second series without BAPTA, thus focusing on the endogenous calcium-buffering capacity, we found that the time constant for the decay of the calcium current was decreased after adrenalectomy, probably due to enhanced calcium-dependent inactivation of the current. The data indicate that cellular calcium current characteristics of dentate granule cells are altered after adrenalectomy and that the alterations may in part determine the vulnerability to undergo apoptosis.

Corticosteroid actions in hippocampus require DNA binding of glucocorticoid receptor homodimers.

Glucocorticoids are secreted from the adrenal gland in very high amounts after stress. In the brain, these stress hormones potently modulate ionic currents, monoaminergic transmission, synaptic plasticity and cellular viability, most notably in the hippocampus where corticosteroid receptors are highly enriched. Here we show that at least some of these actions require DNA binding of glucocorticoid receptor (GR) homodimers.

Episodic corticosterone treatment accelerates kindling epileptogenesis and triggers long-term changes in hippocampal CA1 cells, in the fully kindled state.

We tested the effect of episodic (approximately 10 days) corticosterone treatment on: (i) behavioural symptoms during kindling epileptogenesis; and (ii) electrical activity in the CA1 hippocampal area during epileptogenesis, and later on, in the fully kindled state. Male rats received a corticosterone-releasing pellet (100 mg/day) shortly before kindling was started, resulting in elevated hormone levels during the early and middle stages of epileptogenesis. The appearance of moderate behavioural signs of epilepsy and severe tonic-clonic seizures was significantly accelerated in corticosterone-treated animals compared to placebo controls. During epileptogenesis, corticosterone treatment did not affect the amplitude and paired-pulse characteristics of in vivo-recorded CA1 field responses, or the duration of the afterdischarge following tetanic stimulation of the Schaffer collaterals. However, other properties of CA1 cells studied in vitro, in the fully kindled state, were altered by the earlier episodic corticosterone treatment. Thus, in kindled rats, the amplitude of the population spike in the CA1 area was significantly enhanced after prior exposure to high corticosterone levels. Prior episodic steroid treatment resulted furthermore in a significantly increased amplitude of voltage-gated Ca currents, in kindled rats. At that time, corticosterone levels of animals which had received a corticosterone-releasing pellet earlier were no longer elevated compared to the placebo controls; the corticosteroid-treated rats did also not differ from the controls with respect to the mRNA expression levels for the two corticosteroid receptor subtypes in the hippocampus. The data suggest that exposure of animals to a period of stressful experiences during a critical phase in epileptogenesis could impose lasting deleterious effects on the course of epilepsy, even when CORT levels have been normalized again.

Effect of adrenalectomy in kindled rats.

It has previously been observed that adrenalectomy (ADX) increases the amplitude of transient Ca currents in CA1 hippocampal neurons. In the present study, the effect of ADX on cellular properties of CA1 neurons was investigated under conditions that challenge the CA1 network, i.e. in fully kindled rats, 5-6 weeks after the last kindling session. At this time, adrenally intact kindled rats showed, in comparison to non-kindled controls, a reduced population spike amplitude evoked in the CA1 area by Schaffer/commissural fiber stimulation, an increased amplitude of low-threshold, transient Ca currents and a decrease of the high-threshold, sustained Ca currents. As observed before, ADX in non-kindled rats increased the amplitude of transient Ca currents while sustained Ca currents were not affected. In kindled rats, ADX did not increase the transient Ca current amplitude beyond the level reached by kindling alone. The kindling-induced reduction of the sustained Ca current was partly normalized after ADX. Field responses of kindled rats were not affected by ADX. These data support the view that lack of adrenal hormones does not exacerbate but, to some extent, normalizes the kindling-induced changes in cellular properties and network function of the CA1 area. The hormonal effects on kindling may be reciprocal in nature, since kindling, in turn, was found to affect the stress response and the corticosteroid receptor mRNA expression in the hippocampus.

Effects of adrenalectomy on Ca2+ currents and Ca2+ channel subunit mRNA expression in hippocampal CA1 neurons of young rats.

Previously it was found that the amplitude of Ca currents in CA1 hippocampal neurons increases after adrenalectomy (ADX) of young adult rats. Preliminary data suggested that this effect of ADX is age-dependent. In the present study we therefore investigated the effect of ADX on Ca currents in three age groups: rats that were 1, 3, or 6 months old. It appeared that ADX of the youngest age group resulted in a selective enhancement of sustained, high threshold Ca currents. By contrast, ADX of the oldest rats enhanced only the low threshold, transient Ca current amplitude. The age dependency of the ADX-induced effects can be explained by developmental changes in Ca current properties in the adrenally intact rats with which ADX animals were compared. Data from acutely dissociated cells, which lack most of their dendrites, suggest that the ADX-induced changes of Ca current properties are at least partly targeted at currents generated in the distal dendrites. No significant changes were observed with age or after ADX for the mRNA expression of the alpha 1D Ca channel subunit, which forms the ionpore of the sustained high threshold L-type Ca channel. We can therefore presently not exclude that the altered amplitude of Ca current with age and ADX is due to changes in ion channel function rather than in the total number of these channels.

Effect of ORG 34116, a corticosteroid receptor antagonist, on hippocampal Ca2+ currents.

ORG 34116, a substituted 11,21-bisarylsteroid compound, binds selectively and with high affinity to human and rat glucocorticoid receptors. At the level of the hypothalamus it attenuates the negative feedback action of corticosterone, suggesting that it acts as an antagonist. In the present study we examined the effect of in vitro and in vivo administered ORG 34116 on cell properties of higher brain areas, i.e. on Ca2+ current characteristics of CAI hippocampal neurons recorded with whole cell techniques in hippocampal slices. We observed that in vitro applied ORG 34116 antagonized corticosterone induced effects on Ca2+ currents. Data observed after in vivo application of ORG 34116 corroborate these findings. The results furthermore suggest that pretreatment with the glucocorticoid receptor antagonist ORG 34116 also prevents the development of mineralocorticoid receptor mediated effects on Ca2+ currents. If ORG 34116 should indeed prove to be a corticosterone rather than glucocorticoid receptor selective antagonist, this drug may turn out to be an important tool in the treatment of stress-related disorders.

Hippocampal cell responses in mice with a targeted glucocorticoid receptor gene disruption.

Previous studies in rats have shown that cellular properties of hippocampal CA1 neurons are under coordinative control of mineralocorticoid and glucocorticoid receptors (MRs and GRs, respectively). In the present study, we examined electrical properties under conditions of exclusive MR occupation, by using mice with a genetic defect in GRs obtained by homologous recombination techniques. It appeared that in the animals homozygous for the genetic defect, the properties studied, i.e., the voltage-gated Ca currents and responses to serotonin and the cholinergic analog carbachol, resembled the effects observed in adrenalectomized mice, i.e., when no steroid receptors are activated. This may point to the necessity of functional GRs for the development of MR-induced actions. Ca current amplitude and transmitter responses in the heterozygous animals, which combine a reduced amount of GRs in the hippocampus with relatively high circulating levels of corticosterone, were large compared with those in the wild-type controls; this resembles the responses that were observed previously in rats subjected to a very high dose of corticosterone. These findings exemplify the use of GR knockout mice for the study of cellular properties in the brain. Further substantiation of the observations, however, awaits the development of site-specific, inducible GR knockouts.

Effects of estradiol and progesterone on voltage-gated calcium and potassium conductances in rat CA1 hippocampal neurons.

In this study we recorded voltage gated Ca and K conductances with patch electrodes in hippocampal slices from female rats, which were ovariectomized (OVX) 1 week before the experiment. One experimental group was primed with estradiol at day 3 and 4 after OVX and received progesterone 4 hr before the start of the experiment. The second group was treated with estradiol at day 3 and 4, but received vehicle at the day of the experiment. The third group was not treated with hormones (only vehicle injections). We observed that the amplitude of both sustained and transient Ca currents were significantly enhanced in CA1 pyramidal neurons recorded in tissue from estradiol primed rats receiving progesterone when compared to the nontreated OVX controls; without estradiol priming Ca current amplitudes were small. Current amplitudes in tissue from rats treated with estradiol only, were at intermediate values. The voltage for half-maximal steady state inactivation of the transient Ca current was at a less negative value for the estradiol/progesterone treated group in comparison to the OVX controls. Input resistances of the cells, and voltage or kinetic properties of the currents as recorded under these conditions were comparable for the three groups. Characteristics of two voltage gated K conductances, that is, the transient IA and the delayed rectifier, displayed no consistent differences for the three experimental groups. We conclude that long-term modulation of estradiol and progesterone levels alters the Ca but not the K currents that were tested in CA1 pyramidal neurons. These alterations may be linked to steroid-induced morphological changes in dendrites of CA1 pyramidal cells.