Steroid-sensitive single neurons in rat hypothalamus and midbrain: identification by microelectrophoresis

Membrane-initiated actions of sex steroids and reproductive behavior: A historical account

It was assumed for a long time that sex steroids are activating reproductive behaviors by the same mechanisms that produce their morphological and physiological effects in the periphery. However during the last few decades an increasing number of examples were identified where behavioral effects of steroids were just too fast to be mediated via changes in DNA transcription. This progressively forced behavioral neuroendocrinologists to recognize that part of the effects of steroids on behavior are mediated by membrane-initiated events. In this review we present a selection of these early data that changed the conceptual landscape and we provide a summary the different types of membrane-associated receptors (estrogens, androgens and progestagens receptors) that are playing the most important role in the control of reproductive behaviors. Then we finally describe in more detail three separate behavioral systems in which membrane-initiated events have clearly been established to contribute to behavior control.

Glucocorticoids, metabolism and brain activity

The review integrates different experimental approaches including biochemistry, c-Fos expression, microdialysis (glutamate, GABA, noradrenaline and serotonin), electrophysiology and fMRI to better understand the effect of elevated level of glucocorticoids on the brain activity and metabolism. The available data indicate that glucocorticoids alter the dynamics of neuronal activity leading to context-specific changes including both excitation and inhibition and these effects are expected to support the task-related responses. Glucocorticoids also lead to diversification of available sources of energy due to elevated levels of glucose, lactate, pyruvate, mannose and hydroxybutyrate (ketone bodies), which can be used to fuel brain, and facilitate storage and utilization of brain carbohydrate reserves formed by glycogen. However, the mismatch between carbohydrate supply and utilization that is most likely to occur in situations not requiring energy-consuming activities lead to metabolic stress due to elevated brain levels of glucose. Excessive doses of glucocorticoids also impair the production of energy (ATP) and mitochondrial oxidation. Therefore, glucocorticoids have both adaptive and maladaptive effects consistently with the concept of allostatic load and overload.

Hypothalamic-Pituitary--Adrenal Axis-Feedback Control

The hypothalamo-pituitary-adrenal axis (HPA) is responsible for stimulation of adrenal corticosteroids in response to stress. Negative feedback control by corticosteroids limits pituitary secretion of corticotropin, ACTH, and hypothalamic secretion of corticotropin-releasing hormone, CRH, and vasopressin, AVP, resulting in regulation of both basal and stress-induced ACTH secretion. The negative feedback effect of corticosteroids occurs by action of corticosteroids at mineralocorticoid receptors (MR) and/or glucocorticoid receptors (GRs) located in multiple sites in the brain and in the pituitary. The mechanisms of negative feedback vary according to the receptor type and location within the brain-hypothalmo-pituitary axis. A very rapid nongenomic action has been demonstrated for GR action on CRH neurons in the hypothalamus, and somewhat slower nongenomic effects are observed in the pituitary or other brain sites mediated by GR and/or MR. Corticosteroids also have genomic actions, including repression of the pro-opiomelanocortin (POMC) gene in the pituitary and CRH and AVP genes in the hypothalamus. The rapid effect inhibits stimulated secretion, but requires a rapidly rising corticosteroid concentration. The more delayed inhibitory effect on stimulated secretion is dependent on the intensity of the stimulus and the magnitude of the corticosteroid feedback signal, but also the neuroanatomical pathways responsible for activating the HPA. The pathways for activation of some stressors may partially bypass hypothalamic feedback sites at the CRH neuron, whereas others may not involve forebrain sites; therefore, some physiological stressors may override or bypass negative feedback, and other psychological stressors may facilitate responses to subsequent stress.

Brain estrogen signaling effects acute modulation of acoustic communication behaviors: A working hypothesis

Although estrogens are widely considered circulating "sex steroid hormones" typically associated with female reproduction, recent evidence suggests that estrogens can act as local modulators of brain circuits in both males and females. The functional implications of this newly characterized estrogen signaling system have begun to emerge. This essay summarizes evidence in support of the hypothesis that the rapid production of estrogens in brain circuits can drive acute changes in both the production and perception of acoustic communication behaviors. These studies have revealed two fundamental neurobiological concepts: (1) estrogens can be locally produced in brain circuits, independent of levels in nearby circuits and in the circulation and (2) estrogens can have very rapid effects within these brain circuits to modulate social vocalizations, acoustic processing, and sensorimotor integration. This vertebrate-wide span of research, including vocalizing fishes, amphibians, and birds, emphasizes the importance of comparative model systems in understanding principles of neurobiology.

Glucocorticoid acts on a putative G protein-coupled receptor to rapidly regulate the activity of NMDA receptors in hippocampal neurons

Glucocorticoids (GCs) have been demonstrated to act through both genomic and nongenomic mechanisms. The present study demonstrated that corticosterone rapidly suppressed the activity of N-methyl-D-aspartate (NMDA) receptors in cultured hippocampal neurons. The effect was maintained with corticosterone conjugated to bovine serum albumin and blocked by inhibition of G protein activity with intracellular GDP-β-S application. Corticosterone increased GTP-bound G(s) protein and cyclic AMP (cAMP) production, activated phospholipase Cβ(3) (PLC-β(3)), and induced inositol-1,4,5-triphosphate (IP(3)) production. Blocking PLC and the downstream cascades with PLC inhibitor, IP(3) receptor antagonist, Ca(2+) chelator, and protein kinase C (PKC) inhibitors prevented the actions of corticosterone. Blocking adenylate cyclase (AC) and protein kinase A (PKA) caused a decrease in NMDA-evoked currents. Application of corticosterone partly reversed the inhibition of NMDA currents caused by blockage of AC and PKA. Intracerebroventricular administration of corticosterone significantly suppressed long-term potentiation (LTP) in the CA1 region of the hippocampus within 30 min in vivo, implicating the possibly physiological significance of rapid effects of GC on NMDA receptors. Taken together, our results indicate that GCs act on a putative G protein-coupled receptor to activate multiple signaling pathways in hippocampal neurons, and the rapid suppression of NMDA activity by GCs is dependent on PLC and downstream signaling.

Effects of hydrocortisone sodium succinate on voltage-gated sodium current in trigeminal ganglion neurons of rat

OBJECTIVE

The effects of hydrocortisone sodium succinate on voltage-gated sodium current (I(Na)) were investigated by using the patch-clamp technique in trigeminal ganglion (TG) neurons of rat.

METHODS

I(Na) was recorded by whole-cell patch-clamp techniques after different concentrations of hydrocortisone sodium succinate were perfused in TG neurons of rat.

RESULTS

The results showed that hydrocortisone sodium succinate could inhibit I(Na) in concentration-dependent manner. Hydrocortisone sodium succinate 0.1, 0.3, 1, 3 μmol/l reduced I(Na) by 19.4±4.3, 26.7±3.9, 38.1±6.1, 69.6±5.4% respectively. The IC(50) was 1.58 μmol/l. This inhibitory effect occurred quickly (within 1 minute). However, hydrocortisone sodium succinate had no significant effect on the activation and inactivation courses of I(Na).

CONCLUSION

It is suggested that the rapid inhibition of I(Na) in TG neurons by hydrocortisone sodium succinate is probably related to non-genomic effect. This inhibition might participate in the relaxation of pain in some emergency states.

Nongenomic actions of adrenal steroids in the central nervous system

Mineralocorticoids and glucocorticoids are steroid hormones that are released by the adrenal cortex in response to stress and hydromineral imbalance. Historically, adrenocorticosteroid actions are attributed to effects on gene transcription. More recently, however, it has become clear that genome-independent pathways represent an important facet of adrenal steroid actions. These hormones exert nongenomic effects throughout the body, although a significant portion of their actions are specific to the central nervous system. These actions are mediated by a variety of signalling pathways, and lead to physiologically meaningful events in vitro and in vivo. We review the nongenomic effects of adrenal steroids in the central nervous system at the levels of behaviour, neural system activity, individual neurone activity and subcellular signalling activity. A clearer understanding of adrenal steroid activity in the central nervous system will lead to a better ability to treat human disease as well as reduce the side-effects of the steroid treatments already in use.

Fast effects of glucocorticoids on memory-related network oscillations in the mouse hippocampus

Transient or lasting increases in glucocorticoids accompany deficits in hippocampus-dependent memory formation. Recent data indicate that the formation and consolidation of declarative and spatial memory are mechanistically related to different patterns of hippocampal network oscillations. These include gamma oscillations during memory acquisition and the faster ripple oscillations (approximately 200 Hz) during subsequent memory consolidation. We therefore analysed the effects of acutely applied glucocorticoids on network activity in mouse hippocampal slices. Evoked field population spikes and paired-pulse responses were largely unaltered by corticosterone or cortisol, respectively, despite a slight increase in maximal population spike amplitude by 10 microm corticosterone. Several characteristics of sharp waves and superimposed ripple oscillations were affected by glucocorticoids, most prominently the frequency of spontaneously occurring sharp waves. At 0.1 microm, corticosterone increased this frequency, whereas maximal (10 microm) concentrations led to a reduction. In addition, gamma oscillations became slightly faster and less regular in the presence of high doses of corticosteroids. The present study describes acute effects of glucocorticoids on sharp wave-ripple complexes and gamma oscillations in mouse hippocampal slices, revealing a potential background for memory deficits in the presence of elevated levels of these hormones.

Non-genomic effects of glucocorticoids in the neural system. Evidence, mechanisms and implications

Complementing the classical concept of genomic steroid actions, here we (i) review evidence showing that important neural effects of glucocorticoids are exerted by non-genomic mechanisms; (ii) describe known mechanisms that may underlie such effects; (iii) summarize the functions and implications of non-genomic mechanisms and (iv) outline future directions of research. The role of non-genomic mechanisms is to shape the response of the organism to challenges that require a substantial reorganization of neural and somatic functions and involve massive behavioral shifts. Non-genomic effects may (i) prepare the cell for subsequent glucocorticoid-induced genomic changes, (ii) bridge the gap between the early need of change and the delay in the expression of genomic effects and (iii) may induce specific changes that in some instances are opposite to those induced by genomic mechanisms. The latter can be explained by the fact that challenging situations require different responses in early (acute) and later (chronic) phases. Data show that non-genomic mechanisms of glucocorticoid action play a role in both pathological phenomena and the expression of ameliorative pharmacological effects. Non-genomic mechanisms that underlie many glucocorticoid-induced neural changes constitute a for long overlooked controlling factor. Despite the multitude and the variety of accumulated data, important questions remain to be answered.

Pleiotropic signaling pathways in rapid, nongenomic action of glucocorticoid

The traditional genomic theory of steroid action does not fully explain the rapid effects of hormonal steroids, and it is thought that the nongenomic actions mediated by a putative membrane receptor may provide a plausible explanation. Although there is a rich body of evidence to substantiate the rapid, nongenomic effects of steroid hormones, the signal transduction pathways involved have proved to be complex and pleiotropic. Based on previous studies on the rapid, nongenomic actions of glucocorticoid (GC) and the G-protein-protein kinase pathways involved, including our own studies on PC12, SK-N-SH, BT-325 cells, and synaptosomes, in this review we will discuss the issue of multiple signal transduction pathways involved in the rapid, nongenomic effects of GC.

The effects of steroid hormones on electrical activity of excitable cells

Steroid hormones influence the electrical activity of many neurons and effectors by regulating the transcription of their ion channels and neurotransmitter receptors, or by modulating the activity of their channels and receptors through second messenger-coupled membrane receptors, or both. In this article, four cell types with known functions and distinct electrical activities are focused on to illustrate how different steroids act synergistically with, or in opposition to, each other to modulate specific electrical phenomena such as spontaneous regular firing (GH3 cells, a pituitary cell line), action potential duration (electric organ cells), and intrinsic excitability and sensitivity to neurotransmitters (GnRH and opioidergic neurons).These examples illustrate how steroids might influence electrical activity in neurons involved in more complex central circuits.

Rapid and opposite effects of dexamethasone on in vivo and in vitro hypothalamic somatostatin release

We have previously reported the rapid response of hypothalamic somatostatin (SS) neurons to acute stress. Since it is well known that glucocorticoids (GC) are involved in neuroendocrinal stress regulation, we investigate in this study the effects of acute administration of dexamethasone (Dex) on both in vivo and in vitro SS release. Freely moving animals received stereotaxic implant of a push-pull cannula into the median eminence for 10 days, and then they were perfused with artificial cerebrospinal fluid for 120-150 min. An i.p. injection of Dex (200 or 300 micrograms/100 g) induced, 15-30 min later, a mean increase in SS hypothalamic output of 62.6 +/- 6.2% of basal secretion. By contrast, after 15 min incubation of hypothalamic fragments with either 10(-7) or 10(-6) M Dex, SS release decreased abruptly to 57.3 +/- 3.3% (n = 16; P < 0.001 compared with basal release) and 78.0 +/- 9.5% (n = 13; P < 0.05 compared with basal release) of basal release, respectively. Other Dex concentrations induced no variations, giving the dose-effect curve an abrupt "on-off" effect. The inhibitory effect was blocked by picrotoxin (10(-4) M) and was immediately reversed when Dex was removed from the medium. Specificity was tested by using another steroid, estradiol, and another tissue, cortex. The rapid action of GC whatever the model used and in particular the blocking in vitro effect of picrotoxin could suggest that GCs act at the level of the membrane and could operate physiologically in response to stress. In addition, the opposite in vivo and in vitro effects on SS release would indicate that GCs exert two different controls on SS neurons.

Prednisolone excitation of medial vestibular nucleus neurons in cats

An electrophysiological study was performed to determine whether prednisolone hydrochloride directly influenced neuronal activities of the medial vestibular nucleus (MVN) in alpha-chloralose-anesthetized cats. Single neuronal activities of MVN were recorded extracellularly with a glass-insulated silver wire microelectrode attached along a seven-barreled micropipette. Each barrel was filled with prednisolone, glutamate, glutamic acid diethylester (GDEE) or CoCl2. Except for prednisolone, which was administered both intravenously and microiontophoretically, other chemicals were applied microiontophoretically to the immediate vicinity of the target neurons. These MVN neurons were classified as type I and II neurons according to their responses to horizontal and sinusoidal rotations. Intravenous prednisolone (up to 5 mg/kg) enhanced spontaneous and rotation-induced neuronal firings of both type I and II neurons in a dose-dependent manner. In a similar tendency, microiontophoretically applied prednisolone (50-200 nA) dose-dependently increased spontaneous and rotation-induced firings of both type I and II neurons. Microiontophoretic GDEE, a non-selective glutamate receptor antagonist, inhibited glutamate- and rotation-induced neuronal discharges without affecting prednisolone-induced increases in neuronal responses of MVN. In addition, iontophoretically applied CoCl2, a Ca2+ channel blocker, did not affect prednisolone-, glutamate- and rotation-induced neuronal findings of MVN. These results suggest that prednisolone induces excitation of type I and II neurons, probably by acting directly on the membrane of MVN neurons. Thus, glucocorticoids such as prednisolone may be effective for the treatment of vertigo resulting from hypofunction of vestibular nucleus neurons.

Corticosteroids enhance convulsion susceptibility via central mineralocorticoid receptors

Recently, interest in the roles of central nervous system mineralocorticoid receptors (MR) and glucocorticoid receptors (GR) has increased. In vitro results have implicated MR in the enhancing effects of corticosteroids and GR in the suppressing effects of corticosteroids on hippocampal excitability. Although indirect evidence exists suggesting that opposing actions of central MR and GR occur in vivo, direct evidence from studies employing receptor agonists and antagonists is only beginning to emerge. Work in our laboratory suggests that increased corticosterone levels are associated with increased severity of ethanol, pentobarbital, and diazepam withdrawal. Further work with chemical convulsants suggests that MR mediate excitatory effects of corticosteroids on convulsion susceptibility. The circadian rhythm in convulsion susceptibility varies with the circadian rhythm of plasma corticosterone levels and MR binding. The types of convulsions affected by manipulations of MR activity are believed to be of limbic origin, suggesting that limbic convulsions may be alleviated by the use of specific MR antagonists. In addition, because MR are substantially bound at rest and maximally occupied during the circadian peak in corticosteroid levels and during stressor exposure, these receptors are implicated in the maintenance of and in changes in the arousal state of animals.

Corticosterone increases severity of acute withdrawal from ethanol, pentobarbital, and diazepam in mice

It has been suggested that withdrawal from several subclasses of central nervous system (CNS) depressants involves common underlying mechanisms. For example, mice genetically selected for severe ethanol withdrawal convulsions (Withdrawal Seizure Prone or WSP) have also been found to express severe withdrawal following treatment with barbiturates and benzodiazepines. Corticosteroids appear to modulate severity of withdrawal from CNS depressants. Therefore, it was hypothesized that corticosterone would enhance withdrawal convulsions following acute ethanol, pentobarbital, and diazepam in WSP mice. Corticosterone (20 mg/kg) administered following each of these drugs significantly increased severity of handling-induced convulsions during withdrawal. Corticosterone did not affect pre-withdrawal convulsion scores or handling-induced convulsions of drug-naive mice. These results suggest that withdrawal convulsions following acute ethanol, pentobarbital, and diazepam are sensitive to modulation by corticosterone and they support the hypothesis that stress may increase drug withdrawal severity.

Type I corticosteroid receptors modulate PTZ-induced convulsions of withdrawal seizure prone mice

Corticosteroids have been shown to modulate convulsion expression in humans and animals. It is hypothesized that type I corticosteroid receptors mediate the excitatory effects of corticosteroids in vivo based on low-dose efficacy of corticosterone, and differential effects of mineralocorticoids vs. glucocorticoids on convulsions. In the present experiments, the effects of altering corticosterone levels, and the role of the type I receptor in mediating these effects, were examined using pentylenetetrazol (PTZ)-induced convulsions in ethanol withdrawal seizure prone (WSP) mice. It was hypothesized that stimulation of type I receptors partially mediates the expression of tonic hindlimb extensor (THE) convulsions produced by PTZ. Aminoglutethimide, a steroid synthesis inhibitor, increased latencies to PTZ-induced THE. This anticonvulsant effect was reversed by corticosterone and the type I agonist, deoxycorticosterone (DOC), but not by the type II agonist, dexamethasone. Furthermore, two type I receptor antagonists, spironolactone and RU26752, increased latencies to PTZ-induced THE, suggesting that they have anticonvulsant action. In summary, the results of these experiments suggest that type I corticosteroid receptors are important for expression of PTZ-induced convulsions.

Potentiation of the depression by adenosine of rat cerebral cortical neurones by progestational agents

The effects of four progestational agents pregnenolone sulphate, cyproterone acetate, norethindrone acetate and progesterone, on adenosine-evoked depression of the firing of rat cerebral cortical neurones have been studied. When applied iontophoretically, pregnenolone sulphate, cyproterone, and norethindrone enhanced the actions of iontophoretically applied adenosine and failed to potentiate the depressant effects of adenosine 5'-N-ethylcarboxamide and gamma-aminobutyric acid. Cyproterone acetate (50 micrograms kg-1) and progesterone (200 micrograms kg-1) administered intravenously enhanced the depressant actions of iontophoretically applied adenosine. When applied by large currents, cyproterone, and less frequently norethindrone, depressed the firing of cerebral cortical neurones. The depressant effects of cyproterone were antagonized by caffeine. Pregnenolone sulphate tended to excite cortical neurones but neither this action, nor its potentiation of adenosine were reproduced by application of sulphate ions. It is hypothesized that some of the psychotropic actions of progestational agents may involve an enhancement of 'purinergic' tone in the central nervous system.

Role of central nervous system monoamines in cardiopulmonary effects of Althesin in rabbit and man

The steroid induction agent, Althesin, infused intravenously in light anesthetic doses in otherwise unsedated man (84 micrograms kg-1 min-1) and rabbit (140 micrograms kg-1 min-1) causes similar autonomic and somatic effects. In the rabbit, the rise in heart rate (mainly due to central vagal blockade) and the selective depressant effects on respiratory rate are independent of CNS 5-hydroxytryptamine and noradrenaline. The rise in arterial pressure and the fall in hindlimb conductance is dependent on CNS 5-hydroxytryptamine and noradrenaline synthesizing neurons, which are probably arranged in series. These findings provide a working hypothesis for the mechanisms of action of Althesin on central cardiopulmonary controls in man.

Estradiol and progesterone potentiate adenosine's depressant action on rat cerebral cortical neurons

The effects of 17 beta-estradiol and progesterone on the uptake of adenosine by rat cerebral cortical synaptosomes have been correlated with the ability of these gonadal steroids to potentiate the depressant actions of adenosine on the spontaneous firing of rat cerebral cortical neurons. 17 beta-estradiol hemisuccinate competitively inhibited adenosine uptake with a Ki of 0.5 microM (Lineweaver-Burk plot) or 0.78 microM (Dixon plot). The Ki for progesterone was 0.34 microM (Lineweaver-Burk plot) or 0.36 microM (Dixon plot). When applied by iontophoresis, both steroids potentiated the depressant effects of adenosine on the firing of rat cerebral cortical neurons. Potentiation of the effect of endogenously released adenosine would account for the central depressant actions of these steroids.

Effects of corticosterone on the electrophysiology of hippocampal CA1 pyramidal cells in vitro

Modulation of CA1 field potential amplitudes by normal and stress concentrations of corticosterone (CT) was observed in hippocampal slice preparations from adrenalectomized rats. Slices exposed to CT levels characteristic of a morning (4 nM) or evening (7 nM) resting state showed increased population spike amplitudes in the CA1 pyramidal cell field within 10 min. A stress concentration (15 nM) also increased spike amplitudes, but only at the higher stimulus intensities. The effects of these doses were essentially the same 10 and 60 min after administration. The hormone facilitated responding more in morning resting concentrations than in concentrations characteristic of the evening resting state. This occurred, however, only for relatively low intensity stimuli. The data provide some support for the suggestion that circadian fluctuations in magnitude of long-term potentiation result from corresponding changes in CT level. The rapid onset of the observed changes is difficult to account for in terms of generally accepted mechanisms of receptor binding.

Increased uptake of [3H]choline by rat superior cervical ganglion: an effect of dexamethasone

The high affinity uptake of [3H]choline by the superior cervical ganglion, isolated from the rat, was found to be increased by dexamethasone. Maximal increase (60-65% above control values) occurred at the steroid concentration of 5 X 10(-5) M. Other glucocorticoids (triamcinolone, corticosterone and hydrocortisone) were without an effect on the [3H]choline uptake. Following administration of dexamethasone (25 mg/kg, i.p.), there was a marked increase in the level of choline in the ganglion. The increase was 3-fold at 1 hr and 10-fold at 6 hr, and by 24 hr the choline levels still remained higher in the steroid-treated animals than in the controls. Levels of acetylcholine in the ganglion were also increased, beginning at 1 hr after the injection of steroid. The increase was 85% by 3 hr and 60% by 6 hr. Triamcinolone, a glucocorticoid that was without an effect on [3H]choline uptake in vitro, was also ineffective in altering the levels of choline and acetylcholine in vivo. It seems probable that the increase of choline uptake in the ganglion induced by dexamethasone may, at least in part, occur in the preganglionic cholinergic terminals, leading to increased synthesis of acetylcholine. Such an effect of dexamethasone provides another case of a selective steroid acting directly on nerve terminals by altering a transport mechanism.

Steroid binding to synaptic plasma membrane: differential binding of glucocorticoids and gonadal steroids

The specific binding of [3H]-corticosterone, [3H]-17 beta-estradiol, [3H]-testosterone, and [3H]-progesterone to synaptic plasma membrane (SPM) prepared from rat brain has been characterized. The dissociation constant is estimated as on the order of 1 x 10(-7) M for corticosterone and 1 x 10(-8) M for the other three steroids. In a competition experiment, none of the 3H-steroids was displaced by the other steroids at 500-fold excess, indicating the presence of specific binding sites on the membrane for each type of steroid. Moreover, pre-incubation of the SPM with phospholipase A2 or phospholipase C totally destroys the membrane binding of corticosterone and testosterone, but the binding of estradiol and progesterone remains intact. Since the SPM prepared from brain tissue is derived from many different neuronal cell types, it is possible that the membrane binding sites for glucocorticoids and for gonadal steroids are present in different neurons.

Influences of adrenocortical hormones on pituitary and brain function

Adrenocortical secretions influence neuroendocrine function and behavior, and it is possible to recognize separate physiologic actions of gluco- and mineralocorticoids. The search for neuroanatomical sites and cellular modes of adrenocorticoid action has revealed a system of putative glucocorticoid receptors in neurons of the hippocampus, septum, amygdala, and entorhinal cortex, and in the pituitary. No part of the brain is totally devoid of receptor activity, however, and glial cells may also contain glucocorticoid receptors. Mineralocorticoid receptors are less well characterized neuroanatomically or biochemically. One reason for this is the considerable degree to which both gluco- and mineralocorticoids bind to both classes of receptors in vitro. Another reason may be the overwhelming quantitative predominance of glucocorticoid over mineralocorticoid receptors in neural tissue. Glucocorticoid receptors of the pituitary, which have a high avidity for dexamethasone, appear to participate in the delayed negative feedback effects of glucocoticoids. Functional correlates of neural glucocorticoid receptors remain to be clearly established. Among the possibilities are several reported effects on hippocampal neural activity that have an onset latency of 20--30 min and a duration of several hours. The relative rapidity of such effects does not preclude genomic mediation, as genomic effects of glucocorticoids on thymus lymphocytes have been detected within as little as 15 min of steroid application [117]. What are not so far explained by the intracellular receptor mechanism are the extremely rapid effects of glucocorticoids such as the rate-sensitive negative feedback on CRF and ACTH secretion. These may involve a direct action of the steroid on cell membranes in the pituitary and hypothalamus.

The localization over time of exogenous aldosterone and angiotensin II in various organs

Central nervous system effects have been demonstrated for angiotensin II and suggested for aldosterone. In order to determine whether either of these chemicals naturally crosses the blood-brain barrier, radioactive aldosterone and angiotensin II were introduced via intracardiac injections in rats. Samples of blood, liver, kidney, adrenals, cerebral cortex, and hypothalamus were collected at three, 15, and 60 minutes, frozen, dissolved, and counted. Blood levels for aldosterone and angiotensin II remained constant over 60 minutes. Aldosterone accumulated in the liver, kidney, adrenals and hypothalamus three minutes after injection, and levels diminished over time. Angiotensin II levels peaked in the adrenal, kidney, and liver after three minutes, and in the hypothalamus after 15 minutes. Cerebral cortex levels were lower than hypothalamic levels by 30% for aldosterone and 50% for angiotensin II. This suggests that both drugs may enter the central nervous system and selectively accumulate in the hypothalamus.

Cerebrospinal fluid and plasma free cortisol concentrations in depression

Cerebrospinal fluid (CSF) cortisol levels were examined in a total group of 65 patients. Those who were not depressed (ND), and those suffering from depressive neuroses (DN) had marginally elevated values. Patients with unipolar depression (UD) and bipolar depression (BD) had levels twice as high as the ND and DN patients. Psychotic UD and BD patients had the highest values, three to four times as high as the ND and DN subjects. A significant reduction of CSF cortisol levels was observed following treatment and recovery. Manic patients had moderately elevated CSF cortisol values. The CSF results were in good agreement with plasma total cortisol levels and with urinary free cortisol excretion. Age and sex effects were not responsible for the observed differences; similar results were found in patient subgroups studied in Australia and in the United States. Preliminary equilibrium dialysis data are presented for plasma and CSF cortisol binding. CSF cortisol was 20% bound and 80% free. Plasma free cortisol levels were in good agreement with CSF free cortisol values. Depressed patients have increased tissue and central nervous system (CNS) exposure to free, physiologically active glucocorticoids. The appearance of severe depressive symptoms which manifest a diurnal rhythm may be determined in part by excesssve CNS exposure to glucocorticoids.

Single unit recording in hypothalamus and preoptic area of estrogen-treated and untreated ovariectomized female rats

Single unit activity was recorded with micropipettes in the medial hypothalamus and preoptic area of urethane-anesthetized ovariectomized female rats. Some females had received long-term estradiol treatment, while others had been left untreated. In the medial preoptic region and bed nucleus of the stria terminalis, estrogen-treated rats had fewer cells (compared to untreated rats) with recordable spontaneous activity, due primarily to a loss of cells with very slow firing rates. In the basomedial hypothalamus, estrogen-treated rats had more cells (than untreated rats) with recordable spontaneous activity, due primarily to an increase in the number of cells with slow firing rates. Responsiveness of neurons to somatosensory stimulation was generally low. If present it was depressed by estrogen treatment in medial preoptic area and bed nucleus of stria terminalis, while it tended to be elevated by estrogen treatment in medial anterior hypothalamus and basomedial hypothalamus. Differences in the effects of long-term systemic estrogen treatment on medial preoptic neurons compared to basomedial hypothalamus are paralledled by differences in the control of lordosis by these neurons in female rats.

Neuronal correlates of behavior in freely moving rats

Firing patterns of single neurons in the hypothalamus, preoptic area, midbrain reticular system, and hippocampus of awake, freely moving female rats were temporally correlated with exploratory sniffing and vibrissa twitching, feeding, lordosis, locomotion, and (or) arousal. These relationships were remarkably stable during continuous observations lasting many hours. During extended periods when certain of these movements were not performed, the correlated neurons showed no action potentials for minutes at a time. Electrical stimulation at certain recording sites elicited behavior patterns whose spontaneous occurrence was accompanied by neuronal activation. Self-stimulation was elicited from sites spontaneously activated during exploratory behavior.