Adrenal steroids and extinction behavior: antagonism by progesterone, deoxycorticosterone and dexamethasone of a specific effect of corticosterone

Differential effects of glucocorticoids on human auditory perception

It is assumed that glucocorticoids and mineralocorticoids, the hormones produced by the adrenal cortex, act as modulators of central nervous functions in addition to their well-known role in endocrine stress responses. More specifically, it has been suggested that adrenal corticoids cause changes in sensory thresholds. In two double-blind crossover studies we evaluated auditory perception in healthy volunteers after treatment with hydrocortisone, dexamethasone, and the respective placebos. Stapedial reflexes, pure tone and speech audiograms, and brainstem electric response audiometry served as the dependent variables. As circadian changes in the density and occupancy of the brain corticosteroid receptors have been postulated, we performed Study I in the morning and Study II in the evening. Treatment effects clearly varied with time of day. A differential effect of the two corticoids occurred on contralateral stapedial reflexes. These results parallel results obtained with gustatory stimuli, and suggest that glucocorticoids modulate sensory perception across modalities.

Regulation of 5-HT receptors by corticosteroids: where do we stand?

Twenty five years ago, experimental procedures such as adrenalectomy and corticosteroid administration (to intact rats) allowed the recognition of direct and indirect controls of central 5-HT synthesis rate by corticosteroids. These effects indicated that the activity of the hypothalamo-pituitary-adrenal (HPA) axis, whether under basal conditions or during stress, is endowed with a modulatory action upon serotonergic neurons. Nowadays, in situ hybridisation, in vitro autoradiography, and radioligand binding on the one hand, and electrophysiological, behavioural, and neuroendocrinological responses on the other hand, are tools that allow the analysis of direct corticosteroid effects upon 5-HT receptors. Among the dozen of 5-HT receptors identified so far, four receptors (namely the 5-HT1A, 5-HT1B, 5-HT2A, and 5-HT2C receptors)--and the 5-HT uptake system--have been the focus of studies aimed at detecting corticosteroid modulatory effects. The results that are reviewed herein indicate that hippocampal 5-HT1A receptors are under the tonic inhibitory control of corticosterone. This control is directly exerted at the level of the 5-HT1A receptor gene, essentially through mineralocorticoid receptors; as well, electrophysiological findings bring support for an additional modulation of hippocampal 5-HT1A receptor-mediated functions by indirect (ie 5-HT1A receptor gene-independent) genomic actions of corticosteroids. In keeping with the respective effects of stressful stimuli and psychotropic drugs upon the HPA axis and central serotonergic systems, it is likely that these corticosteroid-5-HT1A receptor interactions in the hippocampus have consequences in the pathophysiology of mood disorders. However, because the data regarding a corticosteroid control of other 5-HT receptors are either scarce and contradictory (eg 5-HT1B, 5-HT2A, 5-HT2C receptors and 5-HT uptake systems) or lacking, it is at the present time unknown whether corticosteroids exert other effects on 5-HT receptor-mediated functions, including those related to homeostasis.

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)

Involvement of corticosterone in the modulation of ethanol consumption in the rat

Several studies report that rats exposed to stressful conditions may increase their ethanol consumption. Stress is accompanied by a rise in the secretion of adrenocortical hormones, and the possibility that these hormones exert an influence on ethanol consumption should be considered. The present investigation addressed this issue by studying the effect of adrenalectomy (ADX) and subsequent corticosterone (CORT) or aldosterone (ALDO) treatment on ethanol intake. The results showed that ADX rats decreased their ethanol intake compared to the sham-operated controls and that treatment with CORT restored the intake of ethanol to the preoperative level. In contrast, treatment with ALDO (0.25 or 0.75 mg/kg) had no effect on ethanol intake. Biochemical analyses showed increases in monoamine turnover in the brain stem and limbic forebrain after ADX. The reduction of ethanol consumption caused by ADX may thus be specifically attributed to the loss of one of the adrenal hormones, CORT. The results indicate that CORT may be a factor of importance in the modulation of alcohol consumption.

The effects of corticosteroids on lateral bias in female rats

In Experiment 1 female rats were given one trial per day for 8 days in a T-maze, and their initial direction of choice (left/right) was noted. Vaginal smears were also obtained daily. After this some animals were adrenalectomized and given Ringer's solution; others were adrenalectomized and given hydrocortisone replacement; a third group was sham adrenalectomized, and a fourth group was an intact control. A week after surgery the animals were again tested for 8 days in the T-maze and vaginal smears were obtained. In Experiment 2 rats were subjected to the same surgical treatments as described above and were then tested for 8 days in the T-maze. In Expt. 1 there was no direction bias among the four groups prior to surgery. However, after surgery the Adrenalectomy+Ringer's group showed a significant increase in their rightward choices in the T-maze. This was also found in Expt. 2. Both adrenalectomized groups in Expt. 1 had a significant reduction in the duration of the estrus phase of their cycle. We conclude that corticosterone can affect lateral preference in a T-maze through a mechanism independent of the hormonal changes involved in the estrus cycle.

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.

Adrenalectomy attenuates the improvement of memory in rats by peripheral application of Des-Tyr-D-Pro4-casomorphin

beta-Casomorphin derivatives without the N-terminal amino acid tyrosine possess memory-improving effects after central and peripheral application. We investigated the significance of adrenal glands for the memory improving effect of the systemically applied beta-casomorphin derivative des-Tyr-D-Pro4CM (Pro-Phe-D-Pro-Gly) in a learning experiment. Seven-week-old rats were adrenalectomized or sham operated. One week after surgery the rats were trained in an active avoidance task in a shuttle box. Five avoidance reactions were taken as learning criterion. After training 10 nmol/kg des-Tyr-D-Pro4CM or saline (10 ml/kg) was subcutaneously applied. There were no differences in acquisition between adrenalectomized and sham-operated rats. The memory retention of sham-operated animals was improved by des-Tyr-D-Pro4CM. In adrenalectomized rats this positive effect could not be observed. The involvement of adrenal glands in the peptide effect during learning and retention is discussed.

Inverted-U relationship between the level of peripheral corticosterone and the magnitude of hippocampal primed burst potentiation

Studies have shown that peripheral levels of corticosterone correlate with the magnitudes of two well-described physiological models of memory, long-term potentiation (LTP) and primed burst (PB) potentiation. In the present experiments, the authors investigated the effects of experimenter-controlled manipulations of the levels of corticosterone on the magnitude of hippocampal PB potentiation in urethane-anesthetized rats. Primed burst potentiation is a long-lasting (at least 30 minutes) increase in the amplitude of the CA1 population spike and EPSP slope in response to physiologically patterned stimulation of the hippocampal commissure. The levels of serum corticosterone were controlled by implanting corticosterone pellets in adrenalectomized rats (ADX/PELLET). In the first experiment, a significant negative linear correlation between elevated (stress) levels of serum corticosterone (greater than 20 micrograms/dL) and the magnitude of PB potentiation in ADX/PELLET subjects (r = 0.60, P < .05) was found. In the second experiment, the shape of the corticosterone-PB potentiation function was different at low and intermediate levels of corticosterone than it was at high levels of corticosterone: There was a positive correlation at low levels (0-10 micrograms/dL), a peak response at intermediate levels (11-20 micrograms/dL), and a negative correlation at high levels (21-93 micrograms/dL) of corticosterone. Thus, the overall relationship between corticosterone and PB potentiation is an inverted-U function. These findings provide strong support for the hypothesis that corticosterone exerts a concentration-dependent biphasic influence on the expression of hippocampal plasticity.

The effect of aging on stress responsiveness and central corticosteroid receptors in the brown Norway rat

The present study examined the stress responsiveness of the hypothalamic-pituitary-adrenal axis in relation to the properties of corticosteroid receptors in the brain and pituitary of old (30 months) and young (3 months) male Brown Norway rats. Adrenocorticotropin hormone (ACTH) and corticosterone (B) were measured following exposure to novelty and to a conditioned emotional stimulus in blood samples sequentially obtained from chronically cannulated animals. Mineralocorticoid (MR) and glucocorticoid (GR) receptors were quantified by radioligand binding assay and in situ hybridization. The receptor binding constants were determined in tissue of rats that were adrenalectomized 24 hours previously, whereas gene expression was measured in the brain of intact animals. Aged Brown Norway rats showed a small but significant elevation in basal circulating ACTH level. The conditioned emotional stimulus, rather than the exposure to novelty, triggered a more than two-times higher ACTH response in the aged compared to the young rat. The termination of the stress-induced ACTH response seemed to proceed more efficiently in the aged rat. Basal and stress-induced total plasma B level did not differ in the young and old rats. The latter showed a 65% lower binding capacity of corticosteroid-binding globulin (CBG). Interestingly, in the aged rat the stress-induced rise in free circulating plasma B level was not elevated, but only prolonged. The hippocampus of aged rats displayed a decrease of maximally 44% in the apparent Bmax of MR, but no change in GR number. The Bmax of GR showed a 40% reduction in the hypothalamus and a 50% reduction in the anterior pituitary. GR affinity was considerably increased in the anterior pituitary, but was unchanged in the hippocampus and hypothalamus. Old age affected MR and GR gene expression differentially. GR mRNA was significantly reduced in cell field CA3 (-42%), CA4 (-41%) and the dentate gyrus (-26%) of the dorsal hippocampus, but did not change either in hippocampal cell field CA1 or in the hypothalamic paraventricular nucleus (PVN) of the old rat. There was no significant difference in MR mRNA between young and aged rats in the different cell fields of the hippocampus. The aged rat, therefore, is characterized by site- and receptor-specific changes in binding constants as well as by changes in receptor transcription and translation. The data demonstrate that in the old Brown Norway rats, a conditioned emotional stimulus results in enhanced pituitary ACTH release.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of prenatal ethanol exposure on glucocorticoid receptors in rat hippocampus

Animals exposed to ethanol in utero are typically hyperresponsive to stressors in adulthood as indicated by increased adrenocortical activation and/or deficits in response inhibition or recovery following stress. In the present study we reasoned that a deficit in feedback control of pituitary-adrenal activity might underlie this hyperresponsiveness in fetal ethanol-exposed (FEE) animals. Further, we hypothesized that a long-term decrease in hippocampal glucocorticoid receptor concentration, induced by prenatal ethanol exposure, might mediate such a deficit in pituitary-adrenal feedback regulation. Male and female Sprague-Dawley rats from prenatal ethanol (E), pair-fed (PF), and control (C) treatment groups were tested in adulthood for determination of cytosolic hippocampal glucocorticoid receptor binding. No significant differences in specific binding (Bmax) or binding affinity (Kd) of either type I or type II glucocorticoid receptors were found among animals from E, PF, and C conditions. There were, however, significant sex differences in receptor concentration and binding affinity; females showed significantly greater maximal binding and significantly lower binding affinity than males. These data do not support the hypothesis that prenatal ethanol exposure induces a long-term decrease in hippocampal glucocorticoid receptors in animals tested under basal nonstressed conditions. However, these data do not preclude the possibility that receptor binding capacity may be differentially affected in E, PF, and C animals during stress.

Brain corticosteroid receptor gene expression and neuroendocrine dynamics during aging

The present study examined the stress responsiveness of the hypothalamic-pituitary-adrenal axis in relation to the properties of corticosteroid receptors in the brain and pituitary in old (30 months) and young (3 months) male Brown Norway rats. The data demonstrate that circulating ACTH rather than the corticosteroid plasma level was elevated under basal conditions and following stress. Furthermore, a reduction of mineralocorticoid receptor (MR) number in the hippocampus and of glucocorticoid receptor (GR) number in the hypothalamus and the pituitary correspond to increased neuroendocrine responsiveness and negative feedback following stress. The changes in receptor binding do not parallel the changes in the amount of MR and GR mRNA measured with in situ hybridization. This suggests that the processing rather than the receptor gene expression is affected in senescence.

Behavioural physiology of serotonergic and steroid-like anxiolytics as antistress drugs

Pharmacological studies are useful tools to understand the neurobiological basis of behavioural and physiological stress mechanisms. Ipsapirone, a 5-HT1A autoreceptor agonist is a representative of novel anxiolytics without the disadvantages of benzodiazepam-like drugs. Behavioural, physiological and neuroendocrine studies in the rat are reviewed which were aimed to investigate the antistress properties of ipsapirone during reexposure to various conditioned emotional stress situations. It is demonstrated that in certain situations, probably due to a stress-induced sensitisation of postsynaptic 5-HT1A receptors, anxiolytic doses of the drug may show prostress (anxiogenic) behavioural and neuroendocrine effects. Furthermore, brain corticosteroid receptors, probably interacting with the serotonergic transmission, are involved in anxiogenic/prostress processes. In this respect antagonists of the brain mineralocorticoid-like (type I) receptors may be important antistress drugs of the future.

Hippocampal glucocorticoid receptor and behavior: a correlative study in rats and mice

A correlation has been demonstrated between binding capacity for [3H]corticosterone in the hippocampus and the performance in passive and active avoidance in the rat, and impaired behavior in adrenalectomized rats by exogenous corticosterone is restored. On this basis we have studied the possible correlation between strain-dependent behavioral differences and the glucocorticoid binding capacity in the hippocampus in mice and rats. In Naples high- (NHE) and low-excitability (NLE) rat strains, genetically selected on the basis of divergent locomotor activity upon forced exposure to a spatial novelty situation, no differences were found in glucocorticoid maximal binding capacity while both strains had a lower capacity than Naples random-breed (NRB) control rats. However, the intra-strain correlative analysis of hippocampal glucocorticoid receptors number and behavioral scores demonstrated that motor and emotional indexes of arousal to novelty were positively correlated in NLE-and negatively in NHE- while no correlation was present in NRB rats. Using two inbred strains of mice, C57BL/6 and Balb/c, extensively investigated for learning abilities, the lower active avoidance score of C57BL/6 was associated with a lower binding capacity for [3H]corticosterone in the hippocampus. Altogether the above results support the involvement of the hippocampal glucocorticoid receptor in the modulation of some adaptive behavioral responses, while do not prove that genetic differences in behavior rest on parallel differences in binding capacity for glucocorticoid hormone.

Aldosterone receptors are involved in the mediation of the memory-enhancing effects of piracetam

The blockade of the memory-enhancing effects of piracetam resulting from adrenalectomy can be abolished by substitution with either corticosterone or aldosterone. However, corticosterone substitution does not reinstate these effects if the aldosterone receptors are blocked by the aldosterone antagonist epoxymexrenon.

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

Pyramidal neurons in the rat hippocampus contain mineralocorticoid receptors (MRs) and glucocorticoid receptors (GRs) to which the adrenal steroid corticosterone binds with differential affinity. We have used intracellular recording techniques to examine MR-mediated effects on membrane properties of CA1 pyramidal neurons in hippocampal slices from adrenalectomized rats. Low doses of corticosterone (1 nM) applied by perfusion for 20 min decreased the spike accommodation observed during a depolarizing current pulse (0.5 nA for 500 ms) and the amplitude of the subsequent afterhyperpolarization without affecting other membrane properties tested. The decrease became apparent ca. 15 min after steroid perfusion was started and reached its peak value 10-20 min after the steroid perfusion was terminated. The steroid effect was blocked by the MR antagonist spironolactone and mimicked by the natural MR ligand aldosterone (1 nM). Neurons recorded 30-90 min after termination of aldosterone application still displayed a decreased spike accommodation. However, 30-90 min after corticosterone application, the decrease in spike accommodation/afterhyperpolarization appeared to be reversed. Higher doses of corticosterone (greater than or equal to 30 nM) induced a significant increase in accommodation and amplitude of the afterhyperpolarization, as was previously observed for selective GR ligands. The data indicate that MR and GR activations induce opposite actions on the spike accommodation/afterhyperpolarization of CA1 pyramidal neurons, an important intrinsic mechanism of these neurons to regulate their response to excitatory input. We suggest that occupation of both MR and GR by the endogenous ligand corticosterone will result in an initial MR-mediated enhanced cellular excitability, which is gradually reversed and overridden by a GR-mediated suppression of cellular activity.

ICV infusion of corticosterone antagonizes ICV-aldosterone hypertension

There is evidence of crucial central nervous system involvement in the pathogenesis of mineralocorticoid-excess salt hypertension, as well as data indicating that corticosterone is the predominant ligand for the type I adrenocorticoid receptor in the brain. Miniosmotic pumps were used to deliver artificial cerebrospinal fluid (CSF), aldosterone (10 ng/h), corticosterone (10 or 20 ng/h), aldosterone (10 ng/h) plus corticosterone [10 ng/h intracerebroventricularly (icv)], or aldosterone (10 ng/h) plus corticosterone (20 ng/h icv). All animals were sensitized to mineralocorticoid hypertension by removing the right kidney and offering saline to drink. Indirect blood pressure by the unheated tail-cuff method and weights were measured twice weekly; 24-h urine volumes were measured once a week. The blood pressures of the four groups did not differ statistically before infusion. The blood pressures of those animals receiving CSF or corticosterone were not significantly elevated after 4-5 wk of intracerebroventricular infusion, whereas the aldosterone group had become significantly elevated within 2 wk. A similar study was done comparing the effects of intracerebroventricular infusion of aldosterone (10 ng/h), aldosterone (10 ng/h) and RU26988 (20 ng/h), and RU26988 (20 ng/h). RU26988, a selective type II receptor agonist, had no effect on the blood pressure, nor did it alter the pressor effect of intracerebroventricular aldosterone. The concomitant infusion of corticosterone antagonized the increase in blood pressure in a dose-dependent manner. Neither steroid nor their combinations produced significant differences in daily urine volume or body weight gain compared with the CSF group.

Neurochemical, endocrine and immunological responses to stress in young and old Fischer 344 male rats

Two experiments were performed. In the first, a 20 min conditioned emotional response (CER) paradigm was used to compare the neurochemical, endocrine and immunological responses to stress of 7- and 22-month-old Fischer 344 (F344) male rats. In the second, corticosterone levels 20 min following ether stress, and regional brain type I and II corticosterone receptor densities were examined using 7- and 17.5-month-old F344 male rats. Dopamine (DA) metabolism in old nonstressed rats was significantly reduced in the medial frontal cortex, neostriatum, nucleus accumbens and hypothalamus, but not in the amygdala. The CER procedure, nevertheless, increased medial frontal cortical, nucleus accumbens and amygdaloid DA turnover in both the young and old rats. The young and old nonstressed rats did not evidence differences in norepinephrine (NE) and serotonin (5-HT) concentrations. However, stress resulted in a decrease in medial frontal cortical 5-hydroxyindoleacetic acid (5-HIAA) and hypothalamic 5-HT levels in old but not in young animals. These observations suggest age-related differences in the response of central NE and 5-HT systems to stress. Ether and the CER procedure led to exaggerated corticosterone responses in the old rats (17.5 and 22 month, respectively). Hippocampal type I but not type II corticosterone receptors were decreased by 47% in the 17.5-month-old rats. Thus, age-related changes in hippocampal corticosterone receptor types do not occur in unison, and the exacerbated corticosterone response to stress precedes the reported down-regulation of hippocampal type II corticosterone receptors in aged rats. Age-related changes were not observed in the concentrations of corticosterone receptors in other brain regions, or in the prolactin response to stress. The old rats, however, evidenced a reduction in the availability of the renin substrate, angiotensinogen, and in stress-induced renin secretion. Immune function was impaired in the old nonstressed rats, and further compromised by exposure to the CER procedure. In comparison to the young control rats, the old nonstressed rats showed an increased percentage of splenic large granular lymphocytes, reduced splenic natural killer cytotoxicity, and impaired Con-A-stimulated splenic T lymphocyte proliferation. Reductions in T splenic cell proliferation and natural killer cytotoxicity were observed in the young rats subjected to the CER paradigm, but not to the same extent as in the old rats. These observations indicate that aging male F344 rats evidence major alterations in basal central monoamine, endocrine and immune functions, and an increased sensitivity of these systems to stress.

Charting of type II glucocorticoid receptor-like immunoreactivity in the rat central nervous system

The rat brain and spinal cord have been mapped for Type II glucocorticoid receptor-like immunoreactivity in neurons and glia, using a monoclonal antibody, BUGR2, which recognizes an epitope close to the DNA-binding domain of the rat Type II receptor. The study revealed a widespread distribution of Type II-like immunoreactive neurons and glia, and a heterogeneity of densities and intensities of immunoreactive elements. Our results corresponded to a large extent with previous immunocytochemical mapping using Ig2a, a monoclonal antibody against a different epitope in the variable domain, with some notable differences in the hippocampus, hypothalamus and cerebellum. There was also a good correlation between immunocytochemical mapping and binding studies, [3H]steroid autoradiography and mRNA localization of the Type II receptor.

Central action of adrenal steroids during stress and adaptation

Corticosteroids interact with receptors in the central nervous system. These receptors display heterogeneity and can be distinguished as corticosterone- and aldosterone-binding mineralocorticoid receptors and dexamethasone-binding glucocorticoid receptors. Ligand specificity of mineralocorticoid receptors for either corticosterone or aldosterone seems to be determined by co-localized transcortin and the enzyme, 11 beta-hydroxysteroid dehydrogenase. Aldosterone-selective mineralocorticoid receptors appear to be present in the circumventricular organs and the AV3V region of the hypothalamus and mediate behavior that is driven by salt appetite. Highest concentrations of mineralocorticoid receptors are found in neurons of the hippocampus. These limbic mineralocorticoid receptor sites mediate tonic influences of corticosterone on brain processes. Glucocorticoid receptors bind corticosterone with a tenfold lower affinity than do mineralocorticoid receptors, and are widely distributed in neuronal and glial cells of the brain. Glucocorticoid receptors are involved in the termination of the stress response (negative feedback). Studies involving measurement of glucocorticoid receptor mRNA and binding sites have revealed that glucocorticoid receptors are subject to autoregulation. After ADX, glucocorticoid receptor concentration increases, but is reduced after chronic stress, chronic administration of glucocorticoids, and at senescence. A diminished glucocorticoid receptor concentration may compromise the negative feedback action exerted by glucocorticoids after stress. After ADX, mineralocorticoid receptor binding is acutely up-regulated and reaches its maximum between 7 and 24 hours post-ADX. Mineralocorticoid receptor mRNA level shows a transient increase following ADX. Long-term ADX has no effect on the mineralocorticoid receptor concentration, but, interestingly, chronic dexamethasone treatment results in an up-regulation of mineralocorticoid receptors. Mineralocorticoid receptor level is decreased at senescence, but this age-related decrement can be reversed by chronic treatment with the ACTH4-9 analog, ORG 2766. Functionally, mineralocorticoid receptors and glucocorticoid receptors are involved in different aspects of the organization of the stress response, and in conjunction they control the stress responsiveness of the animal.

Acetyl-L-carnitine reduces the age-dependent loss of glucocorticoid receptors in the rat hippocampus: an autoradiographic study

Brain autoradiography in adrenalectomized rats injected with 3H-corticosterone 2 hr before sacrifice was used to study the effect of aging and long-term acetyl-l-carnitine treatment on the hippocampal glucocorticoid receptor. Densitometric analysis of silver grains in individual nerve cells of the hippocampus showed that pyramidal neurones of the CA1 field and granular cells of the dentate gyrus are richest in 3H-corticosterone binding sites, whereas pyramidal neurons of the CA3 field have the lowest number of binding sites. There was a significant decline in the number of glucocorticoid receptors within the various hippocampal areas, both as the total number of 3H-corticosterone binding sites and as the number per single pyramidal or granule neuron associated with aging and perhaps due to loss of adrenocorticoid-competent neurons. The dentate gyrus and the CA1 region were mostly affected by the age-dependent decrease in glucocorticoid receptors of the hippocampus. Twenty-eight-month-old rats, treated with acetyl-l-carnitine for 7 months, showed a significantly higher number of 3H-corticosterone binding sites within the various hippocampal regions examined than did age-matched controls. The CA1 and the dentate gyrus were the regions most susceptible to amelioration by acetyl-l-carnitine treatment. These findings suggest a positive effect of acetyl-l-carnitine treatment on age-related changes which occur in the hippocampus.

Adrenocorticoid receptor binding in the rat hippocampus: strain-dependent covariations with arousal and habituation to novelty

In order to investigate whether the genotype-dependent behaviour of the Naples high-(NHE) and low-excitability (NLE) rat strains was modulated by differences in the capacity of hippocampal adrenocorticoid receptors, a correlative analysis was made among behavioural scores from exposure to a Làt-maze and in vitro [3H]corticosterone hippocampal binding capacity in these rats and in their random-bred controls (NRB). As previously shown, NHE/NLE-rats differed markedly upon forced exposure to the maze, with the NRB group occupying an intermediate position. No differences were found in maximal binding capacity (Bmax) and dissociation constant (Kd), nor in the individual maximal binding capacity (IMBC) between the two strains, while both showed lower IMBC than NRB-rats. These results tend to exclude that the genetic differences in the behaviour of NHE/NLE-rats are due to distinct patterns in the adrenocorticoid binding capacity in the hippocampus (HPC). Moreover, the intrastrain correlative analysis among IMBC (in the whole HPC and in its dorsal and ventral portion) and the behavioural scores showed that (1) motor and emotional correlates of 'arousal' to novelty were positively correlated in NLE and negatively in NHE-, but not in NRB-rats; (2) a consistent correlation was found with the intertrial activity decrement (long-term habituation), which was negative in both strains, and it was positive in NRB-rats. These complex co-variations are envisioned as possibly due to the differential modulatory components of the activation and inhibition of novelty-induced arousal response. However, our findings support the hypothesized involvement of the HPC, where adrenocorticoid receptors are selectively concentrated, in the modulation of some adaptive behavioural responses.

Different regulation of serotonin receptors following adrenal hormone imbalance in the rat hippocampus and hypothalamus

Adrenal influence on serotonin (5-HT) transmission in the hippocampal and hypothalamic areas was studied in adrenalectomized rats receiving or not corticosterone replacement. After adrenalectomy, the 5-HT presynaptic receptors were desensitized both in hippocampus and hypothalamus: a significant increase in 5-HT 1 and 5-HT 2 receptor binding numbers took place in membranes from the hippocampus, but not in hypothalamus, while no changes in affinity of receptors to radioligands were observed in either brain area. Corticosterone treatment restored the adrenalectomy-impaired 5-HT autoreceptor sensitivity in hippocampus and hypothalamus and 5-HT density receptor sites in the hippocampus. Serotonin autoreceptor down-regulation following adrenalectomy may increase 5-HT release to maintain the constancy of serotonergic transmission in the brain and 5-HT modulated CRH-ACTH release to compensate the plasma corticosteroid drop. Corticosterone seems to display a distinct tonic control on serotonin transmission in both hippocampus and hypothalamus, the diversity being due to the different roles played by the hormone in these brain regions.

Chronic immobilization stress: evidence for decreases of 5-hydroxy-tryptamine immunoreactivity and for increases of glucocorticoid receptor immunoreactivity in various brain regions of the male rat

Male rats were exposed to severe 14 day immobilization stress. Body weight, body temperature, food and water intake, behavioral parameters, and serum corticosterone levels were measured during and after the stress period. On the 7th day after cessation of stress the experimental animals together with the control rats were taken to immunocytochemical analysis involving morphometry and microdensitometry of tyrosine hydroxylase (TH), 5-hydroxytryptamine (5-HT), various neuropeptide, and glucocorticoid receptor (GR) immunoreactivities (IRs) in a large number of regions of the central nervous system. In addition, adrenocorticotropic hormone (ACTH) IR was analyzed in the pituitary gland. Seven days following cessation of the chronic stress food intake, total locomotion and forward locomotion had been restored to normal. Serum corticosterone levels appeared to remain increased even 6 days following cessation of the chronic immobilization stress, probably caused by increased release of ACTH. Paraventricular corticotropin releasing hormone (CRF) IR was negatively correlated with the pituitary ACTH IR, indicating that the increase in ACTH release was produced by an increased release of CRF from the hypothalamus. The major immunocytochemical change observed 7 days after cessation of stress was a disappearance of 5-HT IR in the 5-HT cell groups B1, B2, B3, and B7. 5-HT IR in nerve terminals was only affected in the dorsal horn, where 5-HT IR was increased in the substantia gelatinosa. GR IR was found to be significantly increased in monoaminergic cell groups: serotoninergic B7, dopaminergic A12, and noradrenergic A1, A2, and A6. A trend for a reduction of TH IR was observed in nigral DA cells associated with significant reductions in TH IR in striatal DA nerve terminals. Finally, increases in 5-HT and substance P (SP) IR were found in the nerve terminals of the substantia gelatinosa of the cervical spinal cord in the stress group. In the present experimental model evidence has been obtained for a maintained activation of the hypothalamic-pituitary-adrenal axis as evaluated 7 days after cessation of severe chronic immobilization stress. The reduction of 5-HT IR in various 5-HT cell groups indicates a reduction of 5-HT synthesis, which may also be associated with reduced 5-HT release from the nerve terminals, since no depletion was observed in terminal regions and in one case an increase in 5-HT IR was noted (substantia gelatinosa).(ABSTRACT TRUNCATED AT 400 WORDS)

Immobilization stress and direct glucocorticoid effects on rat septohippocampus

The rat septohippocampal cholinergic system to a large extent regulates the adaptive physiological and behavioral response to stress. The mesoseptal dopaminergic (DA) system, one of the converging inputs to the lateral septum, exerts a tonic inhibitory action on the septohippocampal cholinergic neurons. High concentrations of pituitary-adrenocortical hormones in plasma may activate the septohippocampal cholinergic system. We have sought to determine whether this mode of activation may be directly initiated by hormonal action on the cholinergic terminals, or indirectly induced through an alteration in the DA septal inputs. The results indicate that stress initiates rapid and transient changes in DA uptake by septal DA terminals, changes which probably contribute to the initial transient activation of the hippocampal cholinergic system. While the effects of glucocorticoids, observed in vitro, may mimic the enhanced ACh release in stress, they do not mimic the increased choline uptake. Nevertheless, high glucocorticoid concentrations may act directly on septal dopaminergic terminals to reduce their DA uptake capacity. These results imply that the septohippocampal cholinergic activity represents an integrative pathway for neuronal and hormonal signals of stress.

Neurotrophic ACTH analogue promotes plasticity of type I corticosteroid receptor in brain of senescent male rats

Age-related changes were studied in the concentration of type-I and type-II corticosteroid receptors in the hippocampus of young adult (3 months) and aged (28.5 to 30.5 months) male rats. Using 3H-labelled ligands, in vitro binding of type-I and type-II corticosteroid receptors in the soluble cell fraction (cytosol) revealed an age-related decrease in concentration of both receptor types of 52% and 28%, respectively. Infusion of young and aged male rats for 2 weeks with the ACTH4-9 [adrenocorticotropin4-9] peptide analogue ORG 2766 (0.5 micrograms/0.5 microliter/hr) resulted in only a minor increase (+8%) in the number of type-I receptors in young rats. In the aged animals, however, the type-I receptor concentration was 68% higher than in the vehicle-treated aged animals. In contrast, no effect of the peptide treatment was noted on the concentration of type-II receptors in either young or aged rats. Furthermore, no effect was found for either age or treatment with peptide on the affinity of type-I and type-II receptors for their respective ligands. Binding of 3H-labelled ligands to brain sections of young and aged rats was performed using in vitro autoradiography. Quantitative image analysis of the film showed that in senescence there is a marked reduction in both type-I (62-75%) and type-II (29-56%) receptor concentrations in the hippocampal subregions (CA1, CA2, CA3 and dentate gyrus) as well as in the lateral septum. Treatment of aged rats with ORG 2766 selectively reversed the age-associated reduction in type-I receptors, while the peptide did not affect the type-II receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Activated type II receptors in brain cannot rebind glucocorticoids: relationship to progesterone's antiglucocorticoid actions

Exchange assays have often been used to quantitate steroid receptors when endogenous ligands are present; however, there are no reports of their successful application to activated glucocorticoid-Type II receptor complexes. In addition to investigating the reasons for this failure, the present study also examined the effects of progesterone on glucocorticoid dissociation from, and reassociation with unactivated and activated Type II receptors. Molybdate-stabilized brain cytosol from adrenal-ovariectomized mice was incubated with [3H]dexamethasone ( +/- [1H]DEX) for 40 h at 0 degree C. Afterwards free steroid was removed on Sephadex G-25 columns in the presence (unactivated receptors) or absence (activated receptors) of molybdate. Activation, as measured by DNA-cellulose binding, was achieved by incubating molybdate-free cytosol at 22 degrees C for 20 min followed by G-25 filtration in the presence of molybdate. The rates of dissociation and reassociation were then measured by incubating cytosol with [1H]triamcinolone acetonide (TA) or [3H]TA ( +/- [1H]TA) at 12 degrees C. An exchange assay was also employed in which cytosol was incubated first with [1H]DEX for 40 h at 0 degree C followed by bound-free steroid separations and 12 degrees C incubations with [3H]TA ( +/- [1H]TA). Both approaches revealed that even though activation reduced the rate of DEX dissociation from Type II receptors by 40%, it eliminated the ability of the newly unoccupied receptors to rebind glucocorticoid. Adding [1H]progesterone to occupied receptor preparations increased dissociation rate constants by nearly 3-fold, for both unactivated and activated Type II receptors. Since [1H]TA failed to prevent this effect, progesterone appears to act at an allosteric site(s) which cannot be occupied by glucocorticoids. Exchange assays revealed that progesterone-facilitated dissociation increased the rate of glucocorticoid rebinding to unactivated, but not activated Type II receptors. These results suggest that spontaneous and progesterone-facilitated termination of glucocorticoid genomic actions could be mediated by steroid dissociation since unoccupied activated Type II receptors do not rebind agonist steroid.

Corticosteroid receptor analyses in rat and hamster brains reveal species specificity in the type I and type II receptors

In vitro cytosol binding, receptor autoradiography with radiolabelled corticosteroid analogs, and immunocytochemistry with monoclonal antibodies have revealed the presence of two receptor systems for corticosteroids in rat and hamster brains. The type I receptor is found mainly in the hippocampal region, and in the hamster it binds cortisol (F) and corticosterone (B) with similar affinity while in the rat (a species which unlike the hamster secretes solely B) the type I receptor shows high affinity to B and not to F. The type II receptor is more widely distributed in the brain and it binds to F (hamster) or B (rat) with affinity 4-6-fold lower than to the type I. in vivo, the hamster type I and II retain F much more than B while those in the rat show the opposite. In conclusion, the present study clearly indicates species-specificity in type I and type II receptor systems in these animals. Furthermore, the type I receptor displays in vivo stringent preference for retention of the animal's predominantly circulating corticosteroid (F in hamster, in B in rat).

Adrenocortical hormone regulation of nicotine sensitivity in mice

The possibility that nicotine-induced corticosterone (CCS) release regulates nicotine sensitivity was investigated in female mice of the C3H strain. Adrenalectomy (ADX) resulted in an increase in nicotine sensitivity as measured in a number of physiological and behavioral tests. In ADX animals, chronic CCS (100 micrograms/ml) administered in the drinking solution normalized nicotine sensitivity. Dexamethasone (DEX), a potent synthetic glucocorticoid which interacts with a distinct population of CNS steroid receptors, did not reverse the effects of ADX. Unoperated animals administered CCS (200 micrograms/ml) were protected from the effects of nicotine for several test battery parameters. ADX had no effect on the number of brain nicotinic cholinergic receptors and also did not alter nicotine metabolism. These data support the hypothesis that CCS secretion modulates nicotine sensitivity in the mouse; however, the mechanisms by which this regulation occurs are unknown.

In vitro presynaptic modulation of cholinergic hippocampal activity by pituitary-adrenocortical hormones

In vivo studies have shown that high blood concentrations of pituitary-adrenocortical hormones can activate the hippocampal cholinergic terminals. Incubation of hippocampal synaptosomal preparations with methylprednisolone, or with ACTH at concentrations comparable to stress-induced high concentrations in plasma, did not have any significant effects on the cholinergic parameters measured under unactivated conditions. In the presence of either high K+ or of ACh, choline uptake was decreased. This decrease was not affected by methylprednisolone. However, methylprednisolone did enhance ACh release both after a previous increase (induced by K+) or a decrease (induced by ACh) of ACh release. In contrast, ACTH had no direct effects on either unactivated or K+-stimulated synaptosomes. Thus, a differential effect was exerted by methylprednisolone on the two presynaptic regulatory mechanisms: choline uptake (no change) and ACh release (increase). We suggest that the activation, observed in vivo, resulted mainly from indirect action of the hormones on the hippocampal cholinergic terminals, in view of the fact that the direct effect in vitro was partial.

Feedback action and tonic influence of corticosteroids on brain function: a concept arising from the heterogeneity of brain receptor systems

Two types of corticosteroid receptors can be distinguished in rat brain. The type 1 receptor resembles the kidney mineralocorticoid receptor and has two functional expressions in brain, i.e. type 1 corticosterone (CORT) preferring sites (CR) and mineralocorticoid receptors (MR). The type 2 receptor is similar to the liver glucocorticoid receptor (GR). CORT binds to both CR and GR. The localization, binding specificity, and capacity of the receptor systems have served as criteria to evaluate steroid dependent events in brain biochemistry and behaviour. The GR is widely distributed in neurons and glial cells, with the highest density in frontal brain regions. The GR becomes occupied concomitant with rising plasma CORT levels after stress and as part of the circadian rhythm. The GR mediates the feedback action of CORT on stress-activated brain processes. The CR has its predominant localization in neurons of the septo-hippocampal complex and has a ten-fold higher affinity for CORT than that of the GR. The CR is, at all times of intact adrenocortical secretion, 90% or more occupied by endogenous hormone. The CR mediates a tonic influence exerted with stringent specificity by CORT on hippocampus-associated functions, e.g. cognition, mood, and affect. CORT, via the CR, thus contributes to hippocampus function in interpretation of sensory information, leading to appropriate neuroendocrine and behavioural responses, which are themselves subsequently subject to feedback action via the GR. The MR mediates the mineralocorticoid effect on salt and water balance and its behavioural corollary of salt appetite. The anatomical localization of the MR system is as yet ill-defined, although functional studies suggest circumventricular organs as mineralocorticoid target sites. The CR and the MR have in common the high affinity for mineralocorticoids, but the CR is defined by its exclusive responsiveness to CORT as its agonist. The CR and MR probably represent the same chemical receptor modality (type 1), which is expressed differentially depending on the presence of extravascular corticosteroid binding globulin (CBG) in the vicinity of the receptor. GR capacity is subject to autoregulation. Chronic stress, senescence, and chronic CORT administration reduce GR number, with, as a consequence, a less efficient feedback signal. The CR number seems not to be under the control of corticosteroids, probably since the receptor sites are extensively occupied by endogenous hormones. The CR number displays a circadian rhythm and is reduced during senescence.(ABSTRACT TRUNCATED AT 400 WORDS)

Function and plasticity of brain corticosteroid receptor systems: action of neuropeptides

Two types of corticosteroid receptors may be distinguished in rat brain. Type 1 resembles the kidney mineralocorticoid receptor and Type 2 is similar to the liver glucocorticoid receptor (GR). Type 1 receptor system displays two functional expressions, i.e. Type 1 corticosterone (CORT)-preferring sites (CR) and Type 1 mineralocorticoid receptors (MR). MR occurs in circumventricular organs and mediates behaviors such as salt appetite. CR has its principal localization in neurons of the hippocampus, and mediates tonic influences of CORT on hippocampus-associated functions. CR responds with stringent specificity to CORT. Differentiation between CR and MR is due to a different accessibility of the receptor by CORT and ALDO, which seems dependent on the presence of extravascular corticosteroid binding globulin (CBG). GR has a wide distribution in brain, occurs in neurons and glial cells and has its principal localization in such regions as the paraventricular nucleus and the n. tractus solitarii (site of CRF synthesis and of blood pressure regulation, respectively). GR mediates the feedback action of CORT on stress-activated brain processes. GR is subject to autoregulation by CORT. Chronic stress, senescence, and chronic CORT administration reduce receptor number, while GR capacity is increased after adrenalectomy. Reduced GR receptor number results in a less-efficient feedback action. The CORT signal via CR probably can only be altered via changes in CR density evoked rather by neural factors than by autoregulation. CR density is reduced at senescence, but is increased to receptor number of young control animals after chronic treatment with a behaviorally potent ACTH-(4-9) analogue, Org 2766. CR plasticity is prominent for the hippocampus, which is a structure involved in cognition, emotional state and subtle regulation of pituitary-adrenal function.

Anatomical resolution of two types of corticosterone receptor sites in rat brain with in vitro autoradiography and computerized image analysis

The rat brain contains two receptor systems for corticosterone (CORT): the glucocorticoid (GR) and corticosterone or mineralocorticoid-like (CR) receptor sites. We have studied the localization of these receptors by in vitro autoradiography and by in vitro cytosol binding assays in microdissected brain areas. In vitro autoradiography revealed that CR receptor sites are almost entirely restricted to the septal-hippocampal complex, whereas the presence of GR extends throughout the brain. Highest levels of GR are present in the lateral septum, hippocampal, cortical and thalamic regions and the paraventricular nucleus. In vitro determination of binding of 3H-labelled steroids to CR and GR in cytosol of "punched out" brain tissue revealed a similar neuroanatomical distribution as observed with the autoradiographic analysis. In addition, it was found that CORT binds to CR (KD approximately 0.5 nM) with 5-10-fold higher affinity than to GR (KD approximately 2.5-5 nM).

Spatial learning and the hippocampal corticosterone receptor system of old rats: effect of the ACTH4-9 analogue ORG 2766

Old (26 months) and young (6 months) male Wistar rats were treated chronically for 2 weeks with ORG 2766 or with vehicle, delivered via subcutaneously implanted minipumps (0.5 microgram peptide/0.5 microliter/h). Learning of a spatial task was not impaired in the old animals, except for one measure, i.e. the latency to find the goal box. In neither age group did ORG 2766 influence behavioral performance. The number of corticosterone receptor sites was decreased in the hippocampus of senescent rats, but restored to the level observed in young rats following ORG 2766 treatment. It is concluded that the number of hippocampal corticosterone receptor sites is a sensitive index of brain aging and effectiveness of ORG 2766.

Hippocampal kindling: corticosterone modulation of induced seizures

The effect of adrenalectomy (ADX) and corticosterone replacement was studied on seizures induced by hippocampal kindling. A complex series of changes occurred in after-discharge (AD) and behavioural depression (BD) during the immediate hours after ADX, culminating at day 1 in markedly decreased AD and BD, which returned to normal over the next several days. These changes were normalized after replacement of the ADX group with low doses of corticosterone. It is concluded that the expression and maintenance of hippocampal kindled seizures is under short-term control of corticosterone.

Relative binding affinity of steroids for the corticosterone receptor system in rat hippocampus

In cytosol of the hippocampus corticosterone displays highest affinity for the sites that remain available for binding in the presence of excess RU 26988, which is shown to be a "pure" glucocorticoid. A rather high affinity (greater than or equal to 25%) was found for 11 beta-hydroxyprogesterone, 21-hydroxyprogesterone, 5 alpha-corticosterone, 19-nor-deoxycorticosterone, 11-deoxycorticosterone and cortisol. A moderate affinity (greater than 5% and less than 25%) was displayed by about 14 steroids among which progesterone, aldosterone, 9 alpha-fluorocortisol and dexamethasone. Corticosterone also shows highest affinity to plasma transcortin and thymus cytosol in the presence of RU 26988. However, the rank-order in affinity by the competing steroids was distinctly different from that observed in the hippocampus; cf. aldosterone and dexamethasone displaced [3H]corticosterone from sites unoccupied by RU 26988 in the hippocampus but not from transcortin or sites in thymus cytosol. In thymus cytosol some potent glucocorticoids have higher affinity for the [3H]dexamethasone labeled sites than dexamethasone. The binding of [3H]dexamethasone in thymus cytosol is completely abolished in the presence of a 100-fold excess of RU 26988. We conclude that our data support the evidence for RU 26988 as a selective ligand for glucocorticoid receptors. RU 26988 leaves binding sites available with highest affinity for corticosterone in hippocampus cytosol that are distinct from transcortin-like sites as found in thymus cytosol or from plasma transcortin.

Behavioral actions of vasoactive intestinal peptide (VIP)

The effect of vasoactive intestinal peptide (VIP) was studied on fear-motivated behaviours, exploration of a novel environment and on novelty and ACTH-induced grooming. VIP was administered via a plastic cannula into the lateral ventricle. Retention of a step-through passive avoidance task was inhibited by 10 and 30 ng VIP injected 1 hour before the retention test. Extinction of pole-jumping active avoidance behaviour was facilitated by 10 and 100 ng VIP. Mild effects were observed in an open field test on exploration and grooming activity. In conclusion, VIP produces inhibitory effects on fear-motivated behaviours.

Acute changes in dopamine metabolism in the medio-basal hypothalamus following adrenalectomy

During the first hour following adrenalectomy the alpha-MPT-induced disappearance of dopamine was increased in the arcuate nucleus compared to that in sham-operated rats. In a number of other brain regions of both adrenalectomized and sham-adrenalectomized rats only stress-induced changes were observed in catecholamine utilization. These data suggest that corticosterone selectively modulates dopamine utilization in the medio-basal hypothalamus.

Corticosterone decreases the efficacy of adrenaline to affect passive avoidance retention of adrenalectomized rats

Short-term (48h) adrenalectomy (ADX) resulted in a deficit in the retention of a passive avoidance response. An inverted U-shaped dose-response relationship was found following immediate post-learning administration of adrenaline (A). A in a dose range of 0.005 - 5 micrograms/kg s.c. facilitated later retention. While corticosterone (CS) replacement alone had no effect, pretreatment with CS (300 micrograms/kg) was followed by a shift in the dose-response curve of A in ADX rats. Ten thousand times higher doses of A were required to improve retention behavior. Administration of the potent synthetic glucocorticoid dexamethasone failed to affect the responsiveness to A. It is concluded that corticosterone decreases the efficacy by which adrenaline affects later retention behavior of ADX rats. The specificity of corticosterone in this interaction suggests the involvement of the corticosterone receptor system which has its predominant localization in hippocampal neurons.

Temperature-dependent kinetic correlates of the activation of the glucocorticoid-receptor complex

The effects of temperature on the kinetics of activation were studied in [3H]triamcinolone acetonide[( 3H]TA)-labeled cytosol preparations from mouse whole brain. After removal of unbound [3H]TA and molybdate (which prevents activation) from the unactivated steroid-receptor complex by gel exclusion chromatography, activation was initiated by incubation at 6-30 degrees C for 0.75-24 min and then rapidly quenched at -5 degrees C with Na2MoO4 (20 mM final concentration). The loss of the 9.2S (unactivated) form of the [3H]TA-receptor complex and the concomitant formation of the 3.8S (activated) form increased dramatically with increases in the activation temperature. These hydrodynamic changes were correlated directly with rapid time- and temperature-dependent increases in the binding of [3H]TA-labeled cytosol to DNA-cellulose (DNA-C). Further analyses of these data revealed a greater than 50-fold increase in the apparent first-order rate constant for the increased binding to DNA-C as the activation temperature was increased from 6 degrees C to 30 degrees C. An Arrhenius plot of these temperature-dependent kinetic constants revealed an energy of activation of 116 kJ. These data support a proposed model for activation of the glucocorticoid-receptor complex that includes the splitting of a 297 kDa, unactivated species into a 92 kDa, activated species.

Adrenal steroids as modulators of nerve cell function

Adrenal steroids modulate the function of nerve cells. Some, but not all actions of these steroids take place after binding to intracellular receptor systems and translocation of the steroid-receptor complex into the cell nucleus. Studies on the rat brain revealed heterogeneity of receptors. One population of receptor sites is present in abundance in extrahypothalamic limbic brain regions, e.g. neurons of the hippocampus, septum and amygdala. This neuronal receptor system displays a stringent binding specificity towards corticosterone, which is the naturally occurring glucocorticoid of the rat. Focussing the studies on the corticosterone receptor system in hippocampal neurons has provided further insight in the understanding of some of the actions of the steroid. Certain hippocampus-associated behaviors and indices of neurotransmission (serotonin) were disturbed after removal of the adrenals, but selectively restored after replacement with a low dose of corticosterone. The specificity, localization and dose-dependency of the corticosterone action on behavior and neurotransmission corresponds to the properties of its receptor system. The responsiveness to corticosterone is altered after changes in number of receptor sites. Chronic stress or high doses of exogenous corticosterone cause a long-term reduction. Other factors involved in regulation of receptor number are the neurotransmitter serotonin and neuropeptides related to ACTH and vasopressin. These substances restore changes in number of hippocampal corticosterone receptor sites due to aging, endocrine or neural deficiencies. Our results show that the number of corticosterone receptors is a sensitive index for brain functioning. Thus, the receptor system mediates some of the modulatory actions of corticosterone on nerve cell function and it may adjust its capacity under the influence of neural and endocrine factors.

Hippocampal corticosterone receptors and novelty-induced behavioral activity: effect of kainic acid lesion in the hippocampus

Rats were injected bilaterally in the dorsal and ventral hippocampus with kainic acid (KA) or with artificial CSF and their behavior and brain corticosterone (B) receptor systems were studied. The hippocampal KA injection destroyed part of the pyramidal neurons and of the dentate gyrus neurons. These neurons contain a receptor system for B. At 2 weeks after the KA lesion this B receptor system displays an increase in apparent maximal binding capacity (Bmax) of approximately 25%. The compensatory increase in B receptor concentration is reflected in an increased uptake of [3H]B in cell nuclei of hippocampal slices incubated in vitro with saturating concentrations of the steroid. Administration of a tracer dose of [3H]B shows that labelled steroid can enter in vivo the cell nuclear compartment of the KA-lesioned lobe. The role of B was investigated on novelty-induced behavioral activities of KA-lesioned and sham-lesioned animals in a large open and a small closed field at 10 days after bilateral adrenalectomy (ADX) or sham-ADX which is 14 days after the (sham) lesion. B (300 micrograms/kg rat) was administered s.c. 1 h prior to the test. KA lesion resulted in an increase in exploratory activity and a reduction in grooming and immobility. After ADX the effect of KA on exploration was reduced in the 5 min open field and abolished in the 30 min closed field. ADX animals displayed more grooming behavior (closed-field). B replacement of ADX rats reinstated the exploratory hyperactivity of KA-lesioned rats. On some components of the behavior such as ambulation in open-field and locomotion in closed field, there was even a larger responsiveness to B in the KA-lesioned rats than in the control animals. It is concluded that (1) after KA lesion of receptor containing neurons, the remaining tissue displays a compensatory increase in number of B receptor sites; (2) B is required for full expression of exploratory activity of rats with or without KA lesions; (3) the KA-lesioned rats display a larger responsiveness to B; and (4) the increased number of B receptor sites may underlie the larger responsiveness to B.

Inhibitory avoidance deficit following short-term adrenalectomy in the rat: the role of adrenal catecholamines

Impaired retention of an inhibitory avoidance response was observed in rats subjected to adrenalectomy (ADX) up to 120 hr before the single learning trial. Corticosterone substitution failed to normalize this behavioral deficit. Rats ADX 240 hr prior to the learning trial showed a normalized behavior. Adrenomedullectomy (ADXM) 48 or 240 hr before learning caused a similar impairment as in short-term ADX rats. The 240-hr ADX rats subjected to corticosterone substitution showed the same behavioral deficit as short-term ADX rats or ADXM ones. Immediate postlearning subcutaneous injection of adrenaline in a dose range of 0.005-5.0 micrograms/kg or of noradrenaline (0.005-0.5 microgram/kg) to 48-hr ADX rats resulted in a dose-related improvement of later retention behavior. Higher doses of catecholamines were less or ineffective. Postlearning treatment of 48-hr ADXM rats with adrenaline (0.5-500 micrograms/kg) caused a similar pattern of behavioral changes. It is concluded that adrenal catecholamines play an important role in the modulation of consolidation of memory. In addition, the high circulating ACTH levels that follow long-term ADX may correct for the behaviorial deficit induced by the absence of adrenomedullary catecholamines.

Renal mineralocorticoid receptors and hippocampal corticosterone-binding species have identical intrinsic steroid specificity

There is current evidence for two classes of hippocampal glucocorticoid receptors (GR)--one classical, [3H]dexamethasone [( 3H]Dex)-binding sites in glial cells, and the other [3H]corticosterone-preferring sites in neuronal cells. In the presence of 1 microM of the synthetic glucocorticoid RU26988 (11 beta, 17 beta-dihydroxy-17 alpha-propynylandrost-1,4,6,-trien-3-one) to exclude tracer from [3H]Dex sites, hippocampal cytosol from adrenalectomized/ovariectomized Sprague-Dawley rats binds [3H]Dex to sites (Kd at 4 degrees C, 0.77 X 10(-9) M; 65 fmol/mg of protein) with the following order of specificity: aldosterone (Aldo) = 9 alpha-fluorocortisol (9 alpha F-cortisol) = deoxycorticosterone (DOC) = corticosterone greater than cortisol much greater than Dex; [3H]Aldo, [3H]DOC, and [3H]corticosterone binding show identical specificity in the presence of RU26988. Addition of 1% adrenalectomized/ovariectomized rat plasma (but not plasma heated at 56 degrees C for 30 min) alters the specificity to: 9 alpha F-cortisol greater than or equal to Aldo greater than or equal to DOC much greater than Dex greater than or equal to corticosterone greater than or equal to cortisol, consistent with sequestration of DOC, corticosterone, and cortisol by transcortin and similar to classical mineralocorticoid receptor (MR) binding of [3H]Aldo in renal cytosol (9 alpha F-cortisol greater than or equal to Aldo greater than or equal to DOC much greater than corticosterone greater than or equal to cortisol greater than or equal to Dex). Separation of other renal binders from transcortin by hydroxylapatite adsorption established the intrinsic specificity of [3H]Aldo binding to MR as: DOC greater than or equal to Aldo greater than or equal to 9 alpha F-cortisol greater than or equal to corticosterone greater than cortisol much greater than Dex, parallel to that of the [3H]corticosterone-binding sites in hippocampus. These studies suggest (i) that hippocampal [3H]corticosterone-binding sites and renal MR may have identical intrinsic specificity for steroids, with apparent specificity differences the result of tissue-specific sequestration of naturally occurring steroids other than Aldo and (ii) that an identical steroid-binding species may thus be occupied under physiological conditions by a mineralocorticoid in one tissue (kidney) and a glucocorticoid in another (hippocampus).

Glucocorticoids modulate the response of ornithine decarboxylase to unilateral removal of the dorsal hippocampus

The effect of unilateral removal of the dorsal hippocampus and of glucocorticoid administration was measured on the activity of ornithine decarboxylase (ODC) in the remaining contralateral hippocampus lobe. Unilateral hippocampectomy (Hx) resulted in a rapid rise of ODC activity in the contralateral lobe. The effect on ODC was maximal at 6 h after surgery and lasted two days. In the absence of the adrenals the effect of Hx on the enzyme was more potent and more prolonged. Elevated ODC activity was still detectable at 5 days after surgery, but not at 10 days. Chronic replacement with dexamethasone (DEX) offered in drinking water decreased the Hx-induced ODC response of ADX rats at 3 days after surgery to the level of enzyme activity observed in the S-ADX Hx subject. The effect of the steroid seemed related to the extent of occupation of the pool of glucocorticoid receptor sites in cytosol of rat hippocampus. In contrast, a single injection of a high dose of DEX to Hx-ADX animals at 3 days after surgery increased ODC activity in addition to the lesion-induced ODC in the contralateral lobe. It is concluded that after unilateral removal of the dorsal hippocampus ODC is a biochemical marker for cellular responses taking place in the contralateral lobe. Glucocorticoids modulate the lesion-induced ODC response.

Opioid modulation of appetite

The discovery of opiate receptors and endogenous opioid peptides within the central nervous system has resulted in a number of speculations concerning the physiological significance of these peptides. In the present article, we review the evidence suggesting a primary role for some of the opioid peptides as regulators of ingestive behavior. In particular, we elaborate a hypothesis in which we suggest that in some species opioid peptides may play a role as a tonic inducer of ingestive behaviors, held in check by a variety of neuropeptides and monoamines. This review explores in detail the role of the opioid peptides as major mediators of the reward system and as a link between reward and feeding behaviors. Finally, a teleological role for opioid peptides in species preservation, which may explain the discrepancies in the role of the opioid peptides in feeding behavior in different species is proposed. It is suggested that the feeding profile of the animal provides important clues as to whether or not the animal has an opiate-sensitive feeding system. We stress that interactions with ingested nutrients and the milieu interieur provide an important means by which animals modulate the opiate-entrained feeding drives.

Antagonistic effects of aldosterone on corticosterone-mediated changes in exploratory behavior of adrenalectomized rats

The effect of aldosterone administration on exploratory activity of chronic adrenalectomized (10 days) male rats was investigated. Aldosterone (30 micrograms/100 g body wt sc) administered 1 hr or 30 min prior to the behavioral test failed to normalize disturbed exploratory activity of adrenalectomized rats, in contrast to the restoration observed after corticosterone, the naturally occurring glucocorticoid of the rat. Administration of the mineralocorticoid 30 min prior to corticosterone prevented the normalization of the behavioral response by the latter steroid. Administration of the same dose of aldosterone 30 min prior to a tracer amount of [3H]corticosterone effectively blocked cell nuclear uptake of radioactive-labeled hormone in the hippocampus. The specific action of corticosterone on exploratory behavior corresponds with the stringent specificity of the neuronal hippocampal corticosterone receptor system. Mineralocorticoid receptors do not seem to be involved in effects on this behavior. The antagonistic action of aldosterone is probably exerted by competitive binding to the corticosterone receptor.

Aldosterone blocks the response to corticosterone in the raphe-hippocampal serotonin system

The accumulation of serotonin induced by the monoamino oxidase inhibitor pargyline was used as an index for 5-HT turnover in the dorsal hippocampus and raphe area. A low dose of corticosterone administered s.c. immediately after adrenalectomy significantly increased serotonin turnover in both regions over the subsequent 1 h interval. The same dose of aldosterone was ineffective, but pretreatment with aldosterone blocked the serotonin response to corticosterone in the acutely adrenalectomized rat. [3H]Corticosterone administered to adrenalectomized rats was not retained by cell nuclei of the raphe area in a limited capacity manner as occurred in the hippocampus. Pretreatment with aldosterone blocked the uptake of [3H]corticosterone in hippocampal cell nuclei. It is concluded that corticosterone triggers a serotonin response and that the specificity of the corticosterone action suggests involvement of the steroid receptor system located postsynaptically to the raphe-hippocampal serotonin projection.