Effects of adrenocortical steroids on long-term potentiation in the limbic system: basic mechanisms and behavioral consequences

Dissociation of dorsal hippocampal regional activation under the influence of stress in freely behaving rats

Stress has deleterious effects on brain, body, and behavior in humans and animals alike. The present work investigated how 30-min acute photic stress exposure impacts on spatial information processing in the main sub-regions of the dorsal hippocampal formation [CA1, CA3, and dentate gyrus (DG)], a brain structure prominently implicated in memory and spatial representation. Recordings were performed from spatially tuned hippocampal and DG cells in rats while animals foraged in a square arena for food. The stress procedure induced a decrease in firing frequencies in CA1 and CA3 place cells while sparing locational characteristics. In contrast to the CA1-CA3 network, acute stress failed to induce major changes in the DG neuronal population. These data demonstrate a clear dissociation of the effects of stress on the main hippocampal sub-regions. Our findings further support the notion of decreased hippocampal excitability arising from behavioral stress in areas CA1 and CA3, but not in DG.

Stress, the hippocampus, and epilepsy

Stress is among the most frequently self-reported precipitants of seizures in patients with epilepsy. This review considers how important stress mediators like corticotropin-releasing hormone, corticosteroids, and neurosteroids could contribute to this phenomenon. Cellular effects of stress mediators in the rodent hippocampus are highlighted. Overall, corticosterone--with other stress hormones--rapidly enhances CA1/CA3 hippocampal activity shortly after stress. At the same time, corticosterone starts gene-mediated events, which enhance calcium influx several hours later. This later effect serves to normalize activity but also imposes a risk for neuronal injury if and when neurons are concurrently strongly depolarized, for example, during epileptic activity. In the dentate gyrus, stress-induced elevations in corticosteroid level are less effective in changing membrane properties such as calcium influx; here, enhanced inhibitory tone mediated through neurosteroid effects on gamma-aminobutyric acid (GABA) receptors might dominate. Under conditions of repetitive stress (e.g., caused from experiencing repetitive and unpredictable seizures) and/or early life stress, hormonal influences on the inhibitory tone, however, are diminished; instead, enhanced calcium influx and increased excitation become more important. In agreement, perinatal stress and elevated steroid levels accelerate epileptogenesis and lower seizure threshold in various animal models for epilepsy. It will be interesting to examine how curtailing the effects of stress in adults, for example, by brief treatment with antiglucocorticoids, may be beneficial to the treatment of epilepsy.

Attenuating corticosterone levels on the day of memory assessment prevents chronic stress-induced impairments in spatial memory

This study investigated whether chronic stress-induced spatial memory deficits were caused by changes in the hypothalamic-pituitary-adrenal axis, such as corticosterone (CORT) elevations on the day of memory assessment, rather than the consequence of structural changes in the hippocampus. Male Sprague-Dawley rats were restrained for 6 h/day/21 days, and spatial memory was assessed on the Y-maze on day 22. Ninety minutes before training, rats received a subcutaneous injection of vehicle or metyrapone, a CORT synthesis inhibitor, and then spatial memory was determined 4-h later. The highest dose of metyrapone (75 mg/kg, s.c.) was most effective at preventing stress-induced spatial memory deficits. Chronic stress increased total CORT levels following Y-maze exposure, while acute metyrapone treatment dose-dependently attenuated total and free (unbound) CORT levels in both stress and control conditions. Blood samples taken from a separate subset of chronically stressed rats showed that baseline CORT levels were similar across the restraint period. Finally, chronic stress down-regulated glucocorticoid, but not mineralocorticoid, receptor mRNA expression within the hippocampus (dentate gyrus, CA1, CA2, CA3). These findings suggest that chronic stress-induced spatial memory deficits may be mediated by hypothalamic-pituitary-adrenal axis dysregulation. Specifically, CORT elevations and reductions in hippocampal glucocorticoid receptor expression, at the time of behavioural assessment may be involved, as opposed to a direct effect that is solely dependent upon hippocampal structural changes. These results have significance for treating cognitive decline in conditions associated with elevated glucocorticoids that include subpopulations in ageing, depression, Cushing's disease and Alzheimer's disease.

Neurosteroids, neuroactive steroids, and symptoms of affective disorders

Neurosteroids (NS) are steroids synthesized by the brain. Neuroactive steroids (NAS) refers to steroids that, independent of their origin, are capable of modifying neural activities. NAS bind and modulate different types of membrane receptors. The gamma amino butyric acid (GABA) and sigma receptor complexes have been the most extensively studied. Oxidized ring A reduced pregnanes, tetrahydroprogesterone (THP), and tetrahydrodeoxycorticosterone (THDOC) bind to the progesterone intracellular receptor (PR), and in this way can also regulate gene expression. Animal experimentation showed that salient symptoms of depression, viz., anxiety, sleep disturbances, and memory and sexual dysfunctions, are modulated by NAS. In turn, psychotropic drugs modulate NS and NAS levels. NS levels as well as NAS plasma concentrations change in patients with depression syndromes, the levels return to normal baseline with recovery, but normalization is not necessary for successful therapy. Results from current studies on the evolution of nervous systems, including evolutionary developmental biology as well as anatomical and physiological findings, almost preclude a categorical classification of the psychiatric ailments the human brain succumbs to. The persistence in maintaining such essentialist classifications may help to explain why up to now the search for biological markers in psychiatry has been an unrewarding effort. It is proposed that it would be more fruitful to focus on relationships between NAS and symptoms of psychiatric disorders, rather than with typologically defined disorders.

Effects of stress and corticosterone on activity and plasticity in the amygdala

The basolateral amygdala (BLA) has been repeatedly shown to mediate the effects of stress on memory-related processes. However, the way in which stress influences BLA itself has not been fully explored. We studied the effects of stress and corticosterone (CORT) on activity and plasticity in the BLA in the rat, using the electrophysiological procedure of long-term potentiation (LTP) induction in vivo. Rats were exposed to an acute elevated-platform stress or administered vehicle or 5 mg/kg, 10 mg/kg, or 25 mg/kg of CORT systemically, after which they were anesthetized and prepared for field potential recording in the BLA, in response to stimulation of the entorhinal cortex. The elevated platform stress enhanced baseline responses in BLA and plasma CORT but inhibited amygdalar LTP. Systemic injections of CORT enhanced baseline responses in BLA in a dose-dependent manner but did not influence amygdalar LTP. Posttetanic potentiation (PTP) was similarly reduced in CORT- and vehicle-injected groups, possibly because of an additional stress from the injection, thus implying that PTP and LTP in the amygdala differentially react to stress. These results suggest that the increase in amygdalar baseline activity following the exposure to stress may be mediated by the concomitant increase in plasma CORT. However, the suppression of amygdalar LTP is not a result of elevated levels of CORT, suggesting that activity and plasticity in the amygdala might be mediated by different mechanisms.

Steroids, neuroactive steroids and neurosteroids in psychopathology

The term "neurosteroid" (NS) was introduced by Baulieu in 1981 to name a steroid hormone, dehydroepiandrosterone sulfate (DHEAS), that was found at high levels in the brain long after gonadectomy and adrenalectomy, and shown later to be synthetized by the brain. Later, androstenedione, pregnenolone and their sulfates and lipid derivatives as well as tetrahydrometabolites of progesterone (P) and deoxycorticosterone (DOC) were identified as neurosteroids. The term "neuroactive steroid" (NAS) refers to steroids which, independent of their origin, are capable of modifying neural activities. NASs bind and modulate different types of membrane receptors. The GABA and sigma receptor complexes have been the most extensively studied, while glycine-activated chloride channels, nicotinic acetylcholine receptors, voltage-activated calcium channels, although less explored, are also modulated by NASs. Within the glutamate receptor family, N-methyl-d-aspartate (NMDA) receptors, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and kainate receptors have also been demonstrated to be a target for steroid modulation. Besides their membrane effects, once inside the neuron oxidation of Ring A reduced pregnanes, THP and THDOC, bind to the progesterone intracellular receptor and regulate gene expression through this path. The involvement of NASs on depression syndromes, anxiety disorders, stress responses to different stress stimuli, memory processes and related phenomena such as long-term potentiation are reviewed and critically evaluated. The importance of context for the interpretation of behavioral effects of hormones as well as for hormonal levels in body fluids is emphasized. Some suggestions for further research are given.

Effects of the active neurosteroid allotetrahydrodeoxycorticosterone on long-term potentiation in the rat hippocampus: implications for depression

We studied the effects of the active neurosteroid (ANS) allotetrahydrodeoxy corticosterone (ATHDOC) on long-term potentiation (LTP) in the dentate gyrus (DG) of intact, urethane anesthetized rats. Intravenous injection of the hormone at two doses, 0.1 and 0.5 mg/kg, produced a significant decrease in both components of the response: excitatory postsynaptic potentials (EPSP) and population spikes (PS). The effects were similar for the two doses. The results are discussed in terms of the potential mechanism by which ATHDOC modulates neural processes associated with symptoms present in depression syndromes.

Corticosterone acutely prolonged N-methyl-d-aspartate receptor-mediated Ca2+ elevation in cultured rat hippocampal neurons

This work reports the first demonstration that corticosterone (CORT) has a rapid and transient effect on NMDA receptor-mediated Ca2+ signaling in cultured rat hippocampal neurons. Using single cell Ca2+ imaging, CORT and agonists of glucocorticoid receptors were observed to modulate the NMDA receptor-mediated Ca2+ signals in a completely different fashion from pregnenolone sulfate. In the absence of steroids, 100 micro m NMDA induced a transient Ca2+ signal that lasted for 30-70 s in 86.1% of the neurons prepared from postnatal rats (3-5 days old). After pre-treatment with 0.1-100 micro m CORT for 10-20 min, NMDA induced extremely prolonged Ca2+ elevation. This prolonged Ca2+ elevation was terminated by the application of MK-801 and followed by washing out of CORT. The proportion of CORT-modulated neurons within the NMDA-responsive cells increased from 25.1 to 95.5% when the concentration of CORT was raised from 0.1 to 50 micro m. Substitution of BSA-conjugated CORT produced essentially the same results. When hippocampal neurons were preincubated with 10 micro m cortisol and 1 micro m dexamethasone for 20 min, a very prolonged Ca2+ elevation was also observed upon NMDA stimulation. The CORT-prolonged Ca2+ elevation caused a long-lasting depolarization of the mitochondrial membrane, as observed with rhodamine 123. In contrast, incubation with 100 micro m pregnenolone sulfate did not considerably alter the time duration of NMDA-induced transient Ca2+ elevation, but caused a significant increase in the peak amplitude of Ca2+ elevation in hippocampal neurons. These results imply that high levels of CORT induce a rapid and non-genomic prolongation of NMDA receptor-mediated Ca2+ elevation, probably via putative membrane surface receptors for CORT in the hippocampal neurons.

Effects of androstenedione on long term potentiation in the rat dentate gyrus. Relevance for affective and degenerative diseases

We studied the effects of the androgenic hormone androstenedione, a 17-ketosteroid, on long term potentiation (LTP) in the dentate gyrus (DG) of intact, urethane anesthetized rats. Intravenous injection of 10mg of the hormone dissolved in Nutralipid produced a significant increase of the population spike (PS), but not of the excitatory post-synaptic potentials (EPSPs). The results are discussed in terms of the potential enhancement that androstenedione may have on some aspects of memory processes as reported for other androgenic steroids. Also noted are the plausible beneficial effects of the hormone on depression as well as in recovery following both central and peripheral neural injury.

Dynamics of neural networks: a proposed mechanism to account for changes in clinical symptomatology through time in patients with psychotic diseases

The classical Kraepelinean dichotomy between manic depressive insanity and the schizophrenias has been recently challenged from clinical and neurobiological quarters. It is not so infrequent to see patients shift from a manic to a schizophrenic symptomatology and vice versa. This paper proposes neurobiological mechanisms as to how these changes may occur, based on recent data on the functioning of neural networks at different modes.

Androsterone sulfate increases dentate gyrus population spike amplitude following tetanic stimulation

We studied the effects of the androgenic hormone, androsterone sulfate, a 17-ketosteroid, on long term potentiation in the dentate gyrus (DG) of urethane anesthesized rats. Intravenous injection of 10 mg of the hormone dissolved in Nutralipid produced a significant increase of the population spike (PS), but not of the excitatory post-synaptic potentials (EPSP). The results are discussed in terms of the potential enhancement that androsterone sulfate may have on memory as was described for one of its parent compounds, dehydroepiandrosterone (DHEA) and its potential use as an antidepressant.

Dehydroepiandrosterone sulfate (DHEAS) counteracts decremental effects of corticosterone on dentate gyrus LTP. Implications for depression

It is well-established that levels of corticosterone sufficient to occupy Type II glucocorticoid receptors produce a decrement in long-term potentiation (LTP) in the dentate gyrus of the hippocampus in rats. In the present series of experiments we investigate the interaction of corticosterone and the neurosteroid dehydroepiandrosterone sulfate (DHEAS) on LTP in the rat dentate gyrus. In confirmation of previous studies, we found that corticosterone (2 mg/kg) had decremental effects on LTP. However, simultaneous injection of corticosterone and DHEAS (30 mg/kg) elicited excitatory post-synaptic potentials and population spikes that were not significantly different from those observed in control animals. The results are discussed in terms of the interaction of the two hormones, the agonist effects of DHEAS on sigma receptors, and their relation with the antidepressant effects of DHEA.

The specificity of stress responses to different nocuous stimuli: neurosteroids and depression

The role that adrenal cortex and neurosteroid hormones may have in the etiology and/or maintenance of depressive diseases is discussed. Selye's concept of stress as the summation of unspecific body responses of the autonomic central nervous system (CNS) and hypothalamic pituitary adrenal axis (HPAA) as the main characteristic of it is contrasted with Mason's view of stress responses as being specific for different stimuli, i.e., the neuroendocrine system responds with the production of a hormonal profile individualized and characteristic for the various stimuli applied. The data reviewed provides support for Mason's interpretation of stress as fundamentally a behavioral response. In turn, the high relevance of emotional factors in the determination of stress responses led to a reconsideration of cognitive-affective interactions in nervous systems. Recent results revealed that improvement in depression treated with antidepressants (ADs) is associated with an increase in the neurosteroid 3alpha 5alpha tetrahydroprogesterone, both in the blood and cerebrospinal fluid of recovered patients. The increase occurs with both selective serotonin reuptake inhibitors and tricyclic ADs. An evaluation of the possible and putative roles for neurosteroids in the CNS is presented and suggestions for enhancing the type of supporting data from the laboratory diagnosis of depressions are advanced.

Regulation of gene expression by corticoid hormones in the brain and spinal cord

Glucocorticoids (GC) and mineralocorticoids (MC) have profound regulatory effects upon the central nervous system (CNS). Hormonal regulation affects several molecules essential to CNS function. First, evidences are presented that mRNA expression of the alpha3 and beta1-subunits of the Na,K-ATPase are increased by GC and physiological doses of MC in a region-dependent manner. Instead, high MC doses reduce the beta1 isoform and enzyme activity in amygdaloid and hypothalamic nuclei, an effect which may be related to MC control of salt appetite. The alpha3-subunit mRNA of the Na,K-ATPase is also stimulated by GC in motoneurons of the injured spinal cord, suggesting a role for the enzyme in GC neuroprotection. Second, we provide evidences for hormonal effects on the expression of mRNA for the neuropeptide arginine vasopressin (AVP). Our data show that GC inhibition of AVP mRNA levels in the paraventricular nucleus is sex-hormone dependent. This sexual dimorphism may explain sex differences in the hypothalamic-pituitary-adrenal axis function between female and male rats. Third, steroid effects on the astrocyte marker glial fibrillary acidic protein (GFAP) points to a complex regulatory mechanism. In an animal model of neurodegeneration (the Wobbler mouse) showing pronounced astrogliosis of the spinal cord, in vivo GC treatment down-regulated GFAP immunoreactivity, whereas the membrane-active steroid antioxidant U-74389F up-regulated this protein. It is likely that variations in GFAP protein expression affect spinal cord neurodegeneration in Wobbler mice. Fourth, an interaction between neurotrophins and GC is shown in the injured rat spinal cord. In this model, intensive GC treatment increases immunoreactive low affinity nerve growth factor (NGF) receptor in motoneuron processes. Because GC also increases immunoreactive NGF, this mechanism would support trophism and regeneration in damaged tissues. In conclusion, evidences show that some molecules regulated by adrenal steroids in neurons and glial cells are not only involved in physiological control, but additionally, may play important roles in neuropathology.

Sex differences in pain-induced effects on the septo-hippocampal system

In addition to its role in the modulation of functions such as arousal and attention, learning and memory, the limbic system has repeatedly been described to be involved in the regulation of several behavioral aspects concerning the adaptation to aversive situations, including pain. A key role in these processes seems to be played by the septo-hippocampal system. This paper, far from being a comprehensive review of all the data available about the limbic system, describes some of the circuits participating in the septo-hippocampal system, with the aim of contributing to an understanding of the sex differences in the behavioral, hormonal and neuronal responses to aversive stimuli. It will appear that the complex anatomical and functional interactions between the different neurotransmitters acting at this level prevent one from indicating a certain substance as more important than others in determining a difference between the two sexes. This leads to the conclusion that the septo-hippocampal formation in toto plays a key role in determining the sex differences in the 'pain experience'.

The corticosteroid synthesis inhibitors metyrapone and aminoglutethimide impair long-term memory for a passive avoidance task in day-old chicks

Long-term memory for a passive avoidance task in day-old chicks has proved to depend upon an action of the adrenal steroid corticosterone through specific receptors in a brain region, the intermediate medial hyperstriatum ventrale (IMHV), involved in learning the task. In this study, we questioned whether pretraining peripheral administration of drugs described to inhibit either basal levels of corticosterone - aminoglutethimide - or treatment-induced stimulated corticosterone secretion - metyrapone - might interfere with retention for the task at 24 h post-training. The results showed a dose-dependent effect of the inhibitors, with the highest doses tested for both drugs (10 and 50 mg/kg for metyrapone, and 50 mg/kg for aminoglutethimide) being amnestic for the task. Additional experiments, in which we studied possible effects of the inhibitors on concomitant aspects of learning (i.e., reactivity to novelty, and pecking pattern), show that the drugs did not affect general behavioural reactivity. The present results thus support the idea that training-induced corticosterone release plays a key role in the neurobiological processes that determine the establishment of a persistent memory for the aversively motivated avoidance response. In addition, they point to corticosteroid inhibitor drugs as potential tools for the study of the interactions between steroid hormones and cognition-enhancing compounds in this learning task.

Natural steroids counteracting some actions of putative depressogenic steroids on the central nervous system: potential therapeutic benefits

Psychological similarities in the symptomatology of Cushing's and depressive diseases led to repeated attempts of treatment of the affective disease by suppression of adrenocortical secretion. While successful in many patients, all drugs employed-metyrapone, ketoconazole and aminoglutethimide-carry the danger of inducing adrenal insufficiency. In addition, their undesirable side effects were also a main reason for treatment suspension. In our 1990 proposal for the treatment of depression through control of adrenal steroid levels, we set as one of the goals the identification of steroids which can antagonize each other on their effects on the central nervous system. Specifically, we looked first at steroids that could counter each other's effects on long-term potentiation, a putative memory mechanism in the central nervous system. One reason for this was the consensus that memory mechanisms are affected in both Cushing's and depressive patients. Another was the fact that cortisol-type hormones which underlie, at least in part, the depressogenic actions of adrenal steroids also have inhibitory effects on LTP. We conjectured, then, that a steroid with opposite effects, one that could enhance long-term potentiation and, further, that could counter the depressant effects of corticosterone on long-term potentiation, could be of use in the treatment of depression. Dehydroepiandrosterone sulfate increases long-term potentiation in a dose-related manner, and preliminary data suggest that it also counteracts the depressant effects of corticosterone on long-term potentiation when injected simultaneously on experimental animals. Potentially at least, rather than resort to total suppression of adrenocortical activity, it may be possible to treat depression just by counteracting some of the effects of cortisol-like hormone actions in the central nervous system. Further, both in clinical trials as well as in experimental animals, dehydroepiandro-sterone sulfate has been shown to enhance performance in memory-requiring tasks.

Reduced hippocampal in vitro CA1 long-term potentiation in rat offsprings with increased circulating corticosterone during neonatal life

A moderate increase in plasma level of corticosterone was induced in dams by adding the hormone (200 micrograms/ml) to the drinking water from the day after delivery to weaning. This procedure produces a parallel increase in plasma levels of the hormone in the pups (from 0.7 +/- 0.1 to 1.2 +/- 0.2 micrograms/100 ml) at 10 days of lactation. A significant (P < 0.01) reduction in the magnitude of the long-term potentiation (LTP) of the CA1 population spike occurred in hippocampal slices obtained from 30-45 day old male corticosterone-nursed rats with respect to controls, while no significant difference occurred in the magnitude of the basal CA1 evoked extracellular somatic field potentials with respect to controls. The results demonstrate that a moderate increase in plasma corticosterone during neonatal life, obtained through maternal milk, has long-lasting effects on the hippocampal CA1 synaptic plasticity. In addition, these results together with our previous findings [Catalani, A. et al., Brain Res., 624 (1993) 209-215], demonstrating that 30 day old corticosterone-nursed offsprings perform better than controls in the place learning version of the Morris water maze, show no relationships between in vitro CA1 LTP induction and spatial learning in agreement with literature data.

Formalin-induced changes in adrenocorticotropic hormone and corticosterone plasma levels and hippocampal choline acetyltransferase activity in male and female rats

Formalin (10%) induces higher levels of licking and flexing in female than in male rats, as shown in the present study. In order to ascertain the neural and hormonal modifications that accompany these behavioural differences, we determined the activity of choline acetyltransferase in the hippocampus and the levels of adrenocorticotropic hormone and corticosterone in the plasma. Two concentrations of formalin were used (50 microliters; 0.1% or 10%). Formalin was injected subcutaneously in the dorsal part of the hindpaw, and the animal's behaviour was then recorded for 60 min in a familiar open-field apparatus. Hippocampal choline acetyltransferase activity did not differ between the two genders in controls, while a significant gender difference was present in both formalin-injected groups, with higher levels in females than in males. This was the result of a decrease in males but not in females. In contrast, adrenocorticotropic hormone was increased by both formalin concentrations in females; corticosterone was not affected by treatment in either gender. Results are discussed in the light of the morphological and functional differences between the two genders in the hippocampus and in the hypothalamo-pituitary-adrenal axis.

A novel cytochrome P450 expressed primarily in brain

hct-1 (hippocampal transcript) was detected in a differential screen of a rat hippocampal cDNA library. Expression of hct-1 was enriched in the formation but was also detected in rat liver and kidney, though at much lower levels; expression was barely detectable in testis, ovary, and adrenal. In liver, unlike brain, expression was sexually dimorphic; hepatic expression was greatly reduced in female rats. In mouse, brain expression was widespread, with the highest levels being detected in corpus callosum; only low levels were detected in liver. Sequence analysis of rat and mouse hct-1 cDNAs revealed extensive homologies with cytochrome P450s (CYPs), a diverse family of heme-binding monooxygenases that metabolize a range of substrates including steroids, fatty acids, and xenobiotics. Among the CYPs, hct-1 is most similar (39% at the amino acid sequence) to cholesterol 7 alpha-hydroxylase (CYP7) and contains a postulated steroidogenic domain present in other steroid-metabolizing CYPs but clearly represents a type of CYP not previously reported. Genomic Southern analysis suggests that a single gene corresponding to hct-1 is present in mouse, rat, and human. hct-1 is unusual in that, unlike all other CYPs described, the primary site of expression is in the brain. Similarity to CYP7 and other steroid-metabolizing CYPs may argue that hct-1 (CYP7B) plays a role in steroid metabolism in brain, notable because of the documented ability of brain-derived steroids (neurosteroids) to modulate cognitive function in vivo.

Opposing roles of type I and type II adrenal steroid receptors in hippocampal long-term potentiation

Studies were performed in vivo on the dentate gyrus to investigate the possible involvement of Type I and Type II adrenal steroid receptors in the mediation of reported adrenal steroid effects on long-term potentiation, through the use of specific Type I and Type II receptor agonists and antagonists. In adrenalectomized rats, administration of aldosterone, a specific Type I agonist, produced a marked enhancement in long-term potentiation, in comparison to either the adrenalectomized or sham adrenalectomized controls. Administration of RU 28318, a Type I antagonist, which by itself had minimal effects, blocked the aldosterone enhancement. In contrast, administration of the specific Type II agonist, RU 28362, produced a marked decrement in the induction of long-term potentiation. The RU 28362 effect was blocked by a prior injection of the Type II antagonist, RU 38486. Neither adrenalectomy nor administration of any of the steroid agonists or antagonists had noticeable effects on neuronal excitability (as determined by the field potentials), nor on post-tetanic potentiation. These findings are consistent with other studies that have shown a biphasic effect of increasing levels of corticosterone on long-term or prime burst potentiation. Taken together, these studies suggest that Type I receptors, with a high affinity for corticosterone, and Type II receptors, having a lower affinity for corticosterone, form a two-level recognition system to modulate induced synaptic plasticity in opposite directions in the dentate gyrus and possibly also in Ammon's horn.