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

Cortisol Stress Response and in Vivo PET Imaging of Human Brain Serotonin 1A Receptor Binding

BACKGROUND

Abnormalities in the hypothalamic-pituitary-adrenal axis, serotonergic system, and stress response have been linked to the pathogenesis of major depressive disorder. State-dependent hyper-reactivity of the hypothalamic-pituitary-adrenal axis is seen in major depressive disorder, and higher binding to the serotonin 1A receptor is observed as a trait in both currently depressed and remitted untreated major depressive disorder. Here, we sought to examine whether a relationship exists between cortisol secretion in response to a stressor and serotonin 1A receptor binding throughout the brain, both in healthy controls and participants with major depressive disorder.

METHODS

Research participants included 42 medication-free, depressed subjects and 31 healthy volunteers. Participants were exposed to either an acute, physical stressor (radial artery catheter insertion) or a psychological stressor (Trier Social Stress Test). Levels of serotonin 1A receptor binding on positron emission tomography with [11C]WAY-100635 were also obtained from all participants. The relationship between [11C]WAY-100635 binding and cortisol was examined using mixed linear effects models with group (major depressive disorder vs control), cortisol, brain region, and their interactions as fixed effects and subject as a random effect.

RESULTS

We found a positive correlation between post-stress cortisol measures and serotonin 1A receptor ligand binding levels across multiple cortical and subcortical regions, independent of diagnosis and with both types of stress. The relationship between [11C]WAY-100635 binding and cortisol was homogenous across all a priori brain regions. In contrast, resting cortisol levels were negatively correlated with serotonin 1A receptor ligand binding levels independently of diagnosis, except in the RN. There was no significant difference in cortisol between major depressive disorder participants and healthy volunteers with either stressor. Similarly, there was no correlation between cortisol and depression severity in either stressor group.

CONCLUSIONS

This study suggests that there may be a common underlying mechanism that links abnormalities in the serotonin system and hypothalamic-pituitary-adrenal axis hyper-reactivity to stress. Future studies need to determine how hypothalamic-pituitary-adrenal axis dysfunction affects mood to increase the risk of suicide in major depression.

Effects of maternal corticosterone and stress on behavioral and hormonal indices of formalin pain in male and female offspring of different ages

In previous studies, we showed for the first time that prenatal stress in rats produces long-term alterations of formalin-induced pain behavior that are dependent on age and sex, and we demonstrated an important role of the serotonergic system in mechanisms of prenatal stress (Butkevich, I.P. and Vershinina, E.A., 2001; Butkevich, I.P. and Vershinina, E.A., 2003; Butkevich, I.P., Mikhailenko, V.A., Vershinina, E.A., Khozhai, L.I., Grigorev, I.P., Otellin, V.A., 2005; Butkevich, I.P., Mikhailenko, V.A., Khozhai, L.I., Otellin, V.A., 2006). In the present study, we focus on the influence of the maternal corticosterone milieu and its role in the effects of stress during pregnancy on formalin-induced pain and the corticosterone response to it in male and female offspring of different ages. For this purpose, we used adrenalectomy (AD) in female rats 3-4 weeks before mating (as distinct from AD typically performed at the beginning of pregnancy). Since AD is considered a reliable method to treat hypercortisolism, researches on the effects of long-term AD in dams on the systems responsible for adaptive behavior in offspring are important (such studies are not described in the literature). The results demonstrate that the differences in the corticosterone response to injection of formalin and saline are obvious in 90-day-old (adult) female offspring but masked in 25-day-old ones. AD promoted the corticosterone response to formalin-induced pain but not to injection of saline in prenatally non-stressed female offspring of both ages. Prenatal stress canceled the differences in corticosterone response to injection of formalin and saline in 25-day-old offspring of AD dams and in adult offspring of sham-operated (SH) dams but caused similar differences in adult offspring of AD dams. Sex differences were found in basal corticosterone levels in AD prenatally stressed rats of both age groups, with a higher level in females, and in the corticosterone response to formalin-induced pain in the adult rats of all groups investigated, with higher corticosterone levels in females. In regard to pain behavior, AD induced significant changes in flexing+shaking in prenatally non-stressed adult offspring and canceled the differences in this behavior between non-stressed and stressed 25-day-old offspring. There were sex differences in pain behavior of the adult rats: greater flexing+shaking in AD non-stressed males but in SH non-stressed females; greater licking in prenatally-stressed AD and SH females. These results indicate that the long-term influences of maternal corticosterone on formalin-induced pain and the corticosterone response to it are determined by the sex and age of the offspring and suggest that other mechanisms, including serotonergic ones revealed in our previous studies, are involved in the effects of prenatal stress on inflammatory pain behavior.

Steroids, neuroactive steroids and neurosteroids in psychopathology

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

Role of maternal adrenal glands on the developing serotoninergic and aminoacidergic systems of the postnatal rat brain

Serotonin, gamma-aminobutyric acid and glutamate, which are regulated by glucocorticoids in the central nervous system, are involved in neuroendocrine functions and the development of the brain. The present study investigates the effect of maternal adrenalectomy on the developing serotoninergic, GABAergic and glutamatergic systems. Neurotransmitter levels were measured in four brain areas of both male and female offspring on postnatal days 1, 8, 12 and 22. At postnatal day 1 and 8, the pups of adrenalectomized dams showed higher concentrations of serotonin than controls in all the brain areas studied. Serotonin levels decreased significantly in males at postnatal day 22 in the hippocampus and cortex. During the first 2 weeks of postnatal life, the lack of maternal corticosterone produced an increase in glutamate and a reduction in gamma-aminobutyric acid concentrations, mainly in males. Further, on postnatal day 1, increased serotonin and glutamate levels and lower levels of gamma-aminobutyric were observed in the hypothalamus of male pups born to adrenalectomized dams. The absence of maternal corticosterone affects the pattern of development of the serotoninergic system, especially in the hippocampus and cortex, and particularly in males. A delay in the maturation of the aminoacidergic systems, mainly of the GABAergic system and in males, was also seen. A sexually dimorphic response to the removal of maternal glucocorticoids was seen in terms of neurotransmitter levels, mainly in the hippocampus and hypothalamus.

Serotonin, stress and corticoids

There is evidence for stressor- and brain region-specific selectivity in serotonergic transmission responses to aversive stimuli. The aim of the present review is to provide an overview of the effects of different acute and repeated/chronic stressors on serotonin (5-HT) release and reuptake, extracellular 5-HT levels, and 5-HT pre- and postsynaptic receptors in areas tightly linked to the control of fear and anxiety, namely the dorsal and median raphe nuclei, the frontal cortex, the amygdala and the hippocampus. In addition, our knowledge of the impacts of corticoids on serotonergic systems in these brain areas is also briefly provided to examine whether the hypothalamo-pituitary-adrenal axis may play a role in stress-induced alterations in 5-HT neurotransmission. Taken together, the data presented reinforce the hypothesis that stress affects such a transmission, partly through the actions of corticoids. However, we are still left with unanswered, albeit crucial questions. First, the question of the specificity of the serotonergic responses to stress, with regard to the site of action and the nature of the stressor still remains open due to the heterogeneity of the results obtained so far. This could indicate that environmental factors, other than the stressor itself, may have enduring consequences on 5-HT sensitivity to stress. Second, the question regarding the role of stress-elicited changes in 5-HT transmission within coping processes finds in most cases no clearcut answer. In keeping with human symptomatology, the need to consider the environment (including the early one) and the genetic status when assessing the effects of stress on 5-HT neurotransmission is underlined. Such a consideration could help to answer the questions raised.

Central 5-HT(1) and 5-HT(2) binding sites in transgenic mice with reduced glucocorticoid receptor number

Transgenic mice bearing a transgene coding for a glucocorticoid receptor antisense mRNA, which partially blocks glucocorticoid receptor expression, were used in order to clarify the role of glucocorticoid receptors in the regulation of 5-HT(1A), 5-HT(1nonA) and 5-HT(2) binding sites labelled by quantitative autoradiography in the frontal and prefrontal cortex, striatum, hypothalamus, amygdala and raphe nuclei. We found that 1 nM [3H]8-hydroxy-2-[di-N-propylamino]tetralin ([3H]8-OH-DPAT) binding to 5-HT(1A) sites was decreased in strata oriens (-15.1+/-3.5%) and radiatum-lacunosum-moleculare (-13.3+/-4.3%) of the hippocampal CA(3) area, and 2 nM [3H]5-hydroxytryptamine binding to 5-HT(1nonA) sites in the presence of 100 nM 8-OH-DPAT and mesulergine was decreased in the dorsal subiculum (-17.8+/-6.9%). By contrast, 5-HT(2) sites labelled by 0.5 nM of (+/-)-1-(2, 5-dimethoxy-4-[125I]iodophenyl)-2-aminopropane was increased in the dorsal subiculum (+35.2+/-11.5%) and CA(2) area (+29.2+/-11.3%). The observed differences in binding to 5-HT(1) and 5-HT(2) sites were all located in areas of the hippocampus that contain both gluco- and mineralo-corticoid receptors, and no difference was observed in anatomical structures which contain only glucocorticoid receptors. Therefore, it seems that the important factor for the regulation of these 5-HT receptors is the interaction between gluco- and mineralo-corticoid receptors rather than the absolute density of glucocorticoid receptors. These results suggest that some of the alterations of the serotonergic neurotransmission observed in depressed patients might be secondary to an altered glucocorticoid receptor function.

The influence of corticosterone on serotonergic stereotypy and sexual behavior in the female rat

The effects of adrenalectomy and chronic corticosterone treatment on sexual behavior in the ovariectomized female rat were investigated. The serotonergic type 2A (5-HT2A) receptor-mediated behavior 'wet dog shakes' (WDS) was measured concurrently. In Experiment 1, adrenalectomy reduced the frequency of WDS following the administration of the 5-HT(2A/2C) receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) but had no effect on spontaneous WDS. In Experiment 2, chronic corticosterone treatment increased DOI-induced WDS in both adrenalectomized and sham-adrenalectomized rats. In Experiment 3, adrenalectomized and sham-adrenalectomized rats were compared on measures of spontaneous WDS and sexual behavior following the administration of estrogen alone, or estrogen in combination with progesterone. Chronic corticosterone and acute progesterone administration increased WDS and facilitated sexual receptivity and proceptivity, while adrenalectomy decreased WDS, facilitated sexual receptivity and inhibited proceptivity. These findings suggest that the behavioral effects seen following hypothalamic-pituitary-adrenal (HPA) axis disruption may, in part, be mediated by altered 5-HT2A receptor responsivity.

Inhibition of hypothalamic GnRH secretion in the ewe by antigonadotropic decapeptide during the estrous cycle and nonbreeding season

Previous experiments from our laboratory and others have shown that the peptide antigonadotropic decapeptide (AGD) has marked inhibitory effects on luteinizing hormone (LH) secretion in rats and ewes. The first objective of this study was to determine whether AGD inhibits LH secretion by regulating hypothalamic release of gonadotropin hormone (GnRH). AGD (200 microg in 200 microL of 0.3% bovine serum albumin [BSA] saline) or vehicle was infused into the lateral ventricle of ovariectomized (OVX) ewes with hypophyseal-portal cannulae, and GnRH secretion was monitored. The frequency of GnRH and LH pulses in AGD-treated ewes was significantly decreased (p < 0.05) but did not change in the control ewes. The second objective of this investigation was to evaluate changes in hypothalamic sensitivity to AGD in the ewe during the estrous cycle and nonbreeding season. During the estrous cycle, the effects of AGD on LH secretion were assessed following ovariectomy, during the metestrous, diestrous, and proestrous phases of the estrous cycle. The response to AGD during the estrous cycle was compared to its effect during the anestrous season. LH, cortisol, and prolactin (PRL) concentrations were assayed in peripheral blood samples obtained at 10-min intervals over a 6-h period prior to injection of either vehicle (200 microL of 0.3% BSA in 0.9% saline) or AGD (200 microg in 200 microL of vehicle), and for an additional 10 h following treatment. LH pulse frequency decreased after treatment with AGD (p < 0.05) at all times in OVX and intact ewes compared to vehicle-treated controls. During the anestrous season, AGD treatment was more effective in inhibiting LH pulse frequency than during the breeding season (p < 0.05). Furthermore, there was a significant increase (p < 0.05) in mean cortisol concentrations after AGD infusion in all AGD-treated groups compared to controls independent of season or reproductive status. PRL concentrations were also increased (p < 0.05) following treatment with AGD. These results suggest that inhibition of pulsatile LH release induced by AGD is modulated by alterations in frequency of hypothalamic discharges of GnRH. Furthermore, changes in the inhibitory actions of AGD may contribute to the seasonal regulation of hypothalamic GnRH secretion in the ewe.

Chronic stress effects on sexual behavior in male and female rats: mediation by 5-HT2A receptors

The effects of chronic psychosocial stress on sexual behavior and on the serotonergic type 2A (5-HT2A) receptor-mediated behavior "wet dog shakes" (WDS) were investigated in male and female rats. In Experiment 1, both bilaterally adrenalectomized and sham-adrenalectomized female rats were assigned to either a psychosocial stress condition or a control condition for 62 days. On the 63rd day, estrogen-primed females were compared on measures of sexual behavior and WDS. Immediately after the behavioral tests, the same rats were primed with a subthreshold level of progesterone. Three hours after the administration of progesterone, rats were again scored for sexual behavior and WDS. Psychosocial stress was found to facilitate sexual behavior and increase WDS in sham-adrenalectomized female rats providing they were primed with both estrogen and progesterone. In Experiment 2, intact male rats were assigned to either the psychosocial stress condition or the control condition for 30 days. On the 31st day, males were compared on measures of sexual behavior and WDS. No significant differences were revealed on the spontaneous expression of sexual behavior and WDS. Subsequently, males were retested following the administration of the 5-HT2A agonist, DOI. Psychosocial stress resulted in a significant decrease in male sexual behavior and a concurrent increase in WDS, following the administration of DOI. Taken together, these results suggest that chronic psychosocial stress facilitates female sexual behavior and inhibits male sexual behavior, and that the effects of stress on sexual behavior may be mediated by 5-HT2A receptor activity.

Sexual behavior and wet dog shakes in the male rat: regulation by corticosterone

The potential involvement of adrenal steroids in the regulation of 'wet dog shakes' (WDS) and sexual behavior was investigated in male rats treated or not with the serotonergic type 2A (5-HT2A) agonist DOI (5-HT2A receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane). In Experiment 1, the frequency of both spontaneous and DOI-induced WDS were compared in adrenalectomized and sham-adrenalectomized rats. Adrenalectomy significantly reduced the frequency of DOI-induced WDS. In Experiment 2, adrenalectomized and sham-adrenalectomized rats received either corticosterone or oil chronically and were again scored for WDS behavior. Corticosterone effectively blocked the adrenalectomy-induced reduction of WDS in the DOI treatment condition. In Experiment 3, intact male rats were chronically administered either corticosterone or oil treatment. Animals were then compared on measures of both spontaneous and DOI-induced WDS and sexual behavior. Corticosterone significantly increased WDS and inhibited sexual behavior in both the spontaneous and DOI treatment conditions. These results suggest that the adrenal steroid corticosterone is important in the regulation of WDS and sexual behavior in the male rat and that this regulation may be mediated by activity at 5-HT2A receptors.

Effects of housing conditions and 5-HT2A activation on male rat sexual behavior

Adult male rats were housed individually or in groups for a period of 39 days. In Experiment 1, the effects of housing conditions on sexual behavior and concurrent spontaneous "wet dog shaking" (WDS) were investigated. Individual housing significantly impaired male sexual behavior and resulted in a trend toward increased WDS. In Experiment 2, the effects of housing conditions were examined following administration of the serotonergic type 2A (5-HT2A) agonist DOI. Individual housing significantly increased DOI-induced WDS. The implications of these findings are discussed in the context of stress-induced corticosterone secretion and the possible regulatory effect on 5-HT2A receptors.

Multiple serotonin receptors: too many, not enough, or just the right number?

In this manuscript, current knowledge about central nervous system serotonin (5-HT) receptors is discussed with an emphasis toward describing the functional significance of the multiple 5-HT receptors. Five characteristics of 5-HT receptors, which are hypothesized to contribute to this functional significance, are discussed: (a) 5-HT has varying affinity and potency for the different receptor subtypes; (b) multiple transduction pathways are used by the different receptor subtypes; (c) receptor subtypes differ in their susceptibility to agonist-mediated desensitization/downregulation; (d) receptor subtypes interact in mediating cellular responses to the neurotransmitter; and (e) receptor subtypes respond differently to changes in the physiological environment. It is hypothesized that these characteristics of the multiple neurotransmitter receptors provide the nervous system with a capacity for coding and decoding of 5-HT-mediated neuronal transmission that could not take place with a single neurotransmitter receptor. Serotonergic regulation of female reproduction and regulation of glucocorticoid release are used to illustrate the integrative potential deriving from the existence of multiple 5-HT receptors.

Regulation of hippocampal 5-HT1A receptor mRNA and binding in transgenic mice with a targeted disruption of the glucocorticoid receptor

Corticosterone is known to suppress levels of 5-HTA(1A) receptor mRNA in rat hippocampus. We describe hippocampal 5-HT(1A) receptor mRNA regulation in mice that have a targeted disruption of the glucocorticoid receptor gene. 5-HT(1A) receptor mRNA levels as well as binding of [3H]8-OH-DPAT, were measured in the hippocampus of heterozygous and homozygous GR-deficient mice and in wild-type control mice. The effect of adrenalectomy in wild-type mice and heterozygous knockouts was also studied. We hypothesized that if the glucocorticoid receptor is important as a mediator of the suppressive effect of corticosterone, this would be revealed by changed (enhanced) expression of 5-HT(1A) receptor mRNA in mice with a genetically changed glucocorticoid receptor status. It was found that 5-HT(1A) receptor mRNA levels and 5-HT(1A) receptor binding were not different in GR-deficient mice. The 5-HT(1A) receptor mRNA levels were responsive to corticosterone, as adrenalectomy led to increased levels of hippocampal 5-HT(1A) receptor mRNA both in wild-type as in heterozygous knockout mice. These increases were paralleled by small but statistically significant changes in [3H]8-OH-DPAT binding. These results support a suppressive control of B over 5-HT(1A) receptor expression in the hippocampus of the mouse, which is predominantly mediated via the mineralocorticoid receptor. The data indicates that no interaction between the two corticosteroid receptors is required for this effect of corticosterone, and that mineralocorticoid receptor-mediated suppression of gene expression can take place in the complete absence of glucocorticoid receptor.

Acute pharmacological blockade of corticosterone secretion reverses food restriction-induced sensitization of the locomotor response to cocaine

Several data indicate that a blockade of stress-induced corticosterone secretion prevents the development of the stress-induced sensitization of the behavioral effects of drugs of abuse. In this report we investigated if an acute blockade of corticosterone secretion could reverse stress-induced sensitization once it is already established. Food restriction (90% of initial body weight) was used as stressor. Corticosterone secretion was blocked by the corticosterone synthesis inhibitor metyrapone (100 mg/kg). After 8 days of food restriction, animals received an injection of metyrapone and 3 h later they were tested either for the locomotor response to cocaine or for the corticosterone secretion in response to stress (restraint, 30 min). Neither metyrapone nor food restriction had any effect on the locomotor response to a saline injection. In contrast, food-restricted animals, compared to ad libitum-fed controls, showed a higher locomotor response to cocaine and higher corticosterone levels. Treatment with metyrapone totally abolished these effects. Food-restricted animals, receiving a single injection of metyrapone, did not differ from ad libitum-fed controls for both locomotor response to cocaine and corticosterone secretion. Metyrapone treatment also similarly reduced the response to cocaine and corticosterone secretion in ad libitum-fed controls. In conclusion, this study provides further evidence that the enhancement in drug effects produced by stress depends on an increase in corticosterone levels. Since stress-induced sensitization is considered one of the conditions predisposing to drug abuse, the present results might have implications for the treatment of addiction.

5-HT1A receptor expression is modulated by corticosteroid receptor agonists in primary rat hippocampal culture

The effects of corticosteroid receptor agonists on the expression of 5-HT1A receptor mRNA were measured in rat hippocampal cultures using in situ hybridization histochemistry. In our normal culture system, grown in serum and steroid-free media, moderate to heavy signal for 5-HT1A mRNA transcripts were detected in hippocampal neurons and glial cells. Aldosterone, a type I corticosteroid receptor agonist (10(-9) M), significantly reduced the expression of 5-HT1A mRNA both in neurons and glial fibrillary acidic protein (GFAP)-immunoreactive (IR) cells. The type II corticosteroid receptor agonist, Ru28362 (10(-8) M), also significantly decreased neuronal 5-HT1A mRNA expression. However, it was not as effective as aldosterone in reducing the label over GFAP-IR cells. These data indicate that corticosteroids may directly regulate the expression of hippocampal 5-HT1A receptors at the mRNA level in cultured hippocampal cells.

Effects of adrenodemedullation and adrenalectomy on the 5-HT2 receptor agonists DOI-and mCPP-induced hypophagia in rats

Effects of adrenodemedullation and adrenalectomy on the serotonin2 (5-HT2) receptor agonists-induced hypophagia were investigated. Hypophagia induced by both the 5-HT2A/2C, receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and the 5-HT2C-receptor agonist 1-(3-chlorophenyl)piperazine (mCPP) were not affected by adrenodemedullation. In adrenalectomized rats. DOI did not induce hypophagia, while mCPP elicited apparent effects. These results suggest that neither DOI- nor mCPP-induced hypophagia was not associated with adrenaline release from the adrenal medulla. Moreover, our results suggest that DOI-induced hypophagia is involved in corticosterone-sensitive feeding, although other mechanisms which are not related to corticosterone are involved in mCPP-induced anorexia.

Hippocampal 5-HT1A receptor binding site densities, 5-HT1A receptor messenger ribonucleic acid abundance and serotonin levels parallel the activity of the hypothalamo-pituitary-adrenal axis in rats

We have previously demonstrated that susceptibility of the Lewis rat to inflammatory disease, compared to the relatively resistant Fischer F344 rat, is related to a hyporesponsive hypothalamopituitary adrenal axis to inflammatory and other stress mediators. Since 5-HT and the 5HT1A receptor are important stimulators of this axis, we have investigated the levels of 5-HT1A receptor binding sites and encoding mRNA, 5-HT and 5-hydroxyindole acetic acid in various brain regions of Lewis, Harlan Sprague Dawley and Fischer F344 rats. Lewis rats expressed significantly less hippocampal and frontal cortical 5-HT1A receptor binding sites and mRNA than Harlan Sprague-Dawley and Fischer F344 rats. Adrenalectomy increased the number of 5HT1A receptor binding sites and mRNA expression in the hippocampus of all three strains. The levels of hippocampal 5-HT in Fischer F344 rats were significantly greater than levels detected in the same regions for the other two strains. Hypothalamic 5-HT and 5-hydroxyindole acetic acid levels in Harlan Sprague-Dawley rats were higher than the same area from the other two strains. Adrenalectomy increased the levels of 5-hydroxyindole acetic acid in the hypothalamus of all three strains. We conclude that hippocampal 5-HT1A receptor densities and 5-HT levels in the rat parallel the the activity and responsiveness of the hypthalamopituitary-adrenal axis. We have published these data in an earlier report.

Adrenalectomy increases serotonin turnover in brains of obese Zucker rats

Because adrenalectomy tends to normalize many metabolic abnormalities of obese Zucker rats, we hypothesized that it would also normalize the depressed serotonergic turnover in their ventromedial nucleus (VMN). Lean (Fa/Fa) and obese (fa/fa) male Zucker rats were adrenalectomized or sham operated when 5 wks old and sacrificed at 11 wks. Their brains were frozen, and 13 areas were dissected for HPLC-EC analysis of monoamines and metabolites. Consistent with previous studies, VMN serotonin turnover (indexed by 5-HIAA/5-HT) was lower in obese than lean sham-operated rats. Monoamine and metabolite concentrations were altered in several other brain areas as well. Adrenalectomy reduced percent body fat and elevated VMN serotonergic turnover more in obese than in lean rats. It also stimulated serotonergic turnover in almost every brain area examined. We conclude that in obese Zucker rats: monoaminergic activity is altered in several brain areas involved in regulating energy balance; adrenalectomy normalizes the reduced VMN serotonergic turnover seen in the obese rats; and adrenalectomy results in a generalized increase in central serotonergic turnover. These data are consistent with serotonin's role in inhibiting food intake and enhancing sympathetic stimulation of energy metabolism.

Long-term control of neuronal excitability by corticosteroid hormones

Hippocampal CA1 neurons express both mineralocorticoid and glucocorticoid receptors. Due to the difference in affinity of the two receptor types for corticosterone and variations in endogenous steroid levels, occupation of the receptors will range between a situation of predominant mineralocorticoid receptor activation and conditions where both receptor types are occupied. It was observed that local signal transduction is regulated by activation of the corticosteroid receptors. Particularly, transmission mediated by biogenic amines appears to be sensitive to steroid control. The data indicate that cholinergic and serotonergic responses are small with predominant mineralocorticoid receptor activation, while additional glucocorticoid receptor activation results in large responses; the reverse has been found for noradrenalin. The steroid-dependent control over transmission by biogenic amines will influence local excitability and therefore functional processes in which the hippocampal system is involved.

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.

Transcriptional regulation of hippocampal 5-HT1a receptors by corticosteroid hormones

5-HT1a receptors in the hippocampus play a critical role in modulating limbic system output. The activity and level of 5-HT1a receptors are modulated by glucocorticoid levels. The present study was undertaken to test the hypothesis that glucocorticoids attenuate the transcriptional activity of the 5-HT1a receptor gene. Using in situ hybridization and RNase protection assays, we observed a substantial increase in 5-HT1a mRNA expression after adrenalectomy in the same hippocampal regions in which 5-HT1a binding sites are increased. This increase in 5-HT1a mRNA expression occurs as early as 1 h after adrenalectomy and precedes the increase in receptor binding sites. Further in situ hybridization analysis showed that 5-HT1a mRNA is increased within individual hippocampal cells after adrenalectomy. Administration of dexamethasone completely prevents the adrenalectomy-induced elevation in hippocampal 5-HT1a receptor mRNA. Nuclear run-on assays showed that the rate of transcription of 5-HT1a mRNA after adrenalectomy increased 70% above the rate from control preparations and could be reduced to basal levels by the administration of dexamethasone. Adrenalectomy did not cause an increase in functional coupling of 5-HT1a receptors to adenylyl cyclase or phospholipase C. These results suggest that transcription of hippocampal 5-HT1a receptor mRNA is under negative regulation by corticosteroid hormones.

Inhibition of corticosterone synthesis by Metyrapone decreases cocaine-induced locomotion and relapse of cocaine self-administration

Several studies have recently shown that basal and stress-induced secretion of corticosterone may enhance vulnerability to drugs of abuse. In this report, we studied the effects of metyrapone, an inhibitor of the synthesis of corticosterone, on cocaine-induced locomotion and on the relapse of cocaine self-administration. Locomotor response to cocaine was studied because psychomotor effects of drugs have been shown to be related to their reinforcing properties. Self-administration was studied in the relapse phase since blockade of relapse is central to the therapy of addiction. Before these behavioral tests, rats in different experimental groups were injected subcutaneously with either metyrapone (100 mg/kg) or vehicle, twice a day for 8 days. Metyrapone treatment reduced cocaine-induced locomotor activity and relapse of cocaine self-administration, without inducing a nonspecific disruption of motor or food-directed behaviors. Under these experimental conditions, the metyrapone treatment totally blocked stress-induced corticosterone secretion but did not modify basal corticosterone levels. These results confirm the involvement of glucocorticoids in the pathophysiological mechanisms underlying vulnerability to drug abuse, and may have implications for the development of new therapeutic strategies of drug addiction.

Effects of adrenalectomy and type I or type II glucocorticoid receptor activation on 5-HT1A and 5-HT2 receptor binding and 5-HT transporter mRNA expression in rat brain

We evaluated the effects of adrenalectomy (ADX) and replacement with glucocorticoid receptor agonists on serotonin (5-HT) 5-HT1A and 5-HT2 receptor binding in rat brain. 5-HT1A receptor binding was increased in the CA2-CA4 and the dentate gyrus of the hippocampus 1 week after ADX. This effect was prevented by the systemic administration of aldosterone (10 micrograms/microliters/h) but not by RU28362 (10 micrograms/microliters/h). No significant effect was observed on 5-HT2 receptor binding in rat cortex. The expression of 5-HT transporter mRNA was unchanged in the raphe nucleus as measured by in situ hybridization.

Mineralocorticoid and glucocorticoid receptors in the brain. Implications for ion permeability and transmitter systems

In this review we have argued that corticosteroid hormones represent an endocrine signal that can influence neuronal communication. The steroids bind to intracellular receptors in the brain, resulting in slow effects that involve gene transcription, but they may also evoke rapid effects via membrane receptors. The signal carried by the corticosteroids is therefore divergent with respect to the dimension of space and time. Within the rat brain, at least two intracellular receptor subtypes, i.e. MRs and GRs, bind corticosterone. The affinity, density and localization of the MRs is different from the GRs, although the actual properties may vary somewhat depending on the condition of the animal. In general, due to the difference in affinity, low corticosteroid levels result in a predominant MR occupation, while higher steroid levels additionally occupy GRs. Recent studies indicate that predominant MR occupation is important for the maintenance of ongoing transmission in certain brain regions and for neuroprotection. By contrast, additional GR occupation (for a limited period of time) results in an attenuation of local excitability; yet, prolonged exposure to high steroid levels may become an endangering condition for neurons. Since predominant MR occupation on the one hand and additional GR occupation on the other hand induce different cellular actions, the ratio of MR/GR occupation is an important factor determining the net effect of corticosteroid hormones in the brain. How coordinated MR- and GR-mediated effects control neuronal communication under various physiological and pathological conditions will be a challenge for future research.

Behavioural and biochemical evidence that glucocorticoids are not involved in DOI-elicited 5-HT2 receptor down-regulation

Numerous studies have brought evidence for reciprocal relationships between glucocorticoids and 5-HT2 receptors; however, whether glucocorticoids affect 5-HT2 receptor regulation is still unknown. Herein, we have analyzed whether 5-HT2 receptor down-regulation following repeated administration of the 5-HT2/5-HT1C receptor agonist 1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane (DOI) is affected by glucocorticoid removal. Compared with sham surgery, adrenalectomy (11-15 days beforehand) did not affect either frontal cortex [3H]ketanserin binding nor the number of head shakes elicited by a single administration of DOI (2.5 mg/kg s.c.). Pretreatment with DOI (2.5 mg/kg s.c. x 4 in 48 h) decreased to similar extents the head shake response to DOI injection in sham (-88%) and adrenalectomised (-95%) rats. Confirmingly, this paradigm was found to diminish the Bmax for [3H]ketanserin binding in sham and adrenalectomised rats by 64% and 46%, respectively. From these data, it is concluded that glucocorticoid removal does not alter 5-HT2 receptor binding and function nor does it affect 5-HT2 receptor down-regulation.

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

Hippocampal structures are a major target for adrenal steroid hormones, and hence these neural regions are some of the most likely mediators of the effects of adrenocortical steroids on behavior. Memory disturbance, in particular biasing toward negative contents, are part of the symptomatology presented by depressive patients. In turn, a sizeable subset of depression also presents with hypercortisolemia. Adrenocortical hormones are also known to affect memory processes. Hippocampal formation is essential for declarative memory. We thought it appropriate then to study the effects of adrenal steroids on long-term potentiation, a putative memory mechanism in the hippocampus. Two clearly distinguished components of the evoked response to perforant path stimulation can be studied in the hippocampus: the excitatory postsynaptic potential (EPSP) which denotes the graded depolarization of the somatodendritic region of the neuron and the population spike (PS), a manifestation of the all-or-none-discharge of the cell action potential. Corticosterone had a significant depressant effect on the EPSP component of the evoked response immediately and 15 min after injection. Thereafter EPSP amplitudes were within normal values. Corticosterone significantly decreased the PS immediately after the train, the component remaining low 30 min after the train. 5 alpha-Dihydrocorticosterone (a ring A-reduced metabolite of corticosterone) significantly reduced the PS component of the response at all times after injection. 18-Hydroxydeoxycorticosterone and deoxycorticosterone significantly decreased both EPSP and PS components of the evoked response from the time of infusion. Contrary to expectation, tetrahydrodeoxycorticosterone was ineffective in decreasing and if anything, enhanced the development of long-term potentiation. 18-Hydroxydeoxycorticosterone 21-acetate behaved like vehicle, except for the first 30 min after injection when the EPSP was decreased. Allotetrahydroprogesterone decreased all EPSP's values and had no effect in the PS development in comparison with vehicle. The suggestion is made that the study of steroidal effects on hippocampal LTP can serve as a preclinical model of some aspects of depression in a specific subset of the disease.

Effects of chronic corticosterone ingestion on spatial memory performance and hippocampal serotonergic function

The effects of chronic ingestion of corticosterone (8 weeks via the drinking water, 400 micrograms/ml) on spatial memory performance and on monoamine levels in brain areas related to memory were investigated. Corticosterone treatment was associated with a long lasting (5 weeks post treatment) increase in 5-HT levels (44%) in the dentate gyrus of the hippocampus and decrease in 5-HT (50%) and NE (36%) levels in the frontal cortex. No effects were found in CA1, CA3 or in nucleus basalis. Performance of the rats on an 8-arm radial arm maze showed no overall effect of corticosterone treatment on trials to criterion or choice accuracy scores. However, three of the treated rats, who had consumed the most corticosterone during treatment, 12.5 +/- 0.3 mg/day, were impaired relative to all subjects. Thus, these results suggest that hippocampal serotonergic terminals show long lasting effects from corticosterone and may also be an early indicator of deleterious effects of glucocorticoids on hippocampal function. However, since only a small number of corticosterone-treated rats showed behavioral changes, future experiments are necessary to address the possibility that a higher level of corticosterone intake alters spatial memory as well as brain morphology and neurochemistry. Additional studies are also needed to determine whether such changes represent a threshold effect of the steroid or a dose-response function.

Hippocampal 8-[3H]hydroxy-2-(di-n-propylamino) tetralin binding site densities, serotonin receptor (5-HT1A) messenger ribonucleic acid abundance, and serotonin levels parallel the activity of the hypothalamopituitary-adrenal axis in rat

We have previously demonstrated that susceptibility of the Lewis rat to inflammatory disease, compared with the relatively resistant Fischer F344/N rat, is related to a hyporesponsive hypothalamopituitary-adrenal axis to inflammatory and other stress mediators. Because serotonin (5-HT) and the 5-HT1A receptor are important stimulators of this axis, we have investigated the levels of 8-[3H]-hydroxy-2,3-(di-n-propylamino)tetralin binding sites, 5-HT1A mRNA, 5-HT, and 5-hydroxyindoleacetic acid in various brain regions of Lewis, outbred Harlan Sprague Dawley, and Fischer F344/N rats. Lewis rats expressed significantly fewer hippocampal and frontal cortical 8-[3H]-hydroxy-2,3-(di-n-propylamino)tetralin binding sites and less 5-HT1A mRNA than Harlan Sprague Dawley and Fischer F344/N rats. Adrenalectomy increased the number of 8-[3H]hydroxy-2,3-(di-n-propylamino)tetralin binding sites and 5-HT1A mRNA expression in the hippocampus of all three strains. Levels of hippocampal 5-HT in Fischer F344/N rats were significantly greater than levels detected in the same regions from Lewis and Harlan Sprague Dawley rats. Hypothalamic 5-HT and 5-hydroxyindoleacetic acid levels in Harlan Sprague Dawley rats were higher than the same area from the other two strains. Adrenalectomy increased the levels of 5-hydroxyindoleacetic acid in the hypothalamus of all three strains. We conclude that hippocampal 5-HT1A receptor densities and 5-HT levels in the rat parallel the activity and responsiveness of the hypothalamopituitary-adrenal axis.

Adrenalectomy or metyrapone-pretreatment abolishes cerebral metabolic responses to the serotonin agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) in the hippocampus

1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), a serotonin type 2 (5-HT2) agonist, elevates plasma corticosterone levels and reduces the cerebral metabolic rate for glucose (rCMRglc) in the hippocampus, a structure which possesses few 5-HT2 receptors but a large number of steroid receptors. To explore the hypothetical interaction between 5-HT and steroid mechanisms in the hippocampus, we measured rCMRglc in intact, adrenalectomized and metyrapone-pretreated rats after saline or DOI administration. Metyrapone pretreatment alone had no significant effect on rCMRglc, but adrenalectomy produced widespread rCMRglc increases in the cortex, hippocampus and monoaminergic brainstem nuclei. In intact rats, DOI 10 mg/kg reduced rCMRglc in limbic areas and increased it in the interanteromedial and paracentral thalamic nuclei. Metyrapone pretreatment and adrenalectomy abolished rCMRglc responses to DOI in hippocampal areas and enhanced those in thalamic nuclei. These results indicate that brain responses to DOI are dependent upon the functional state of the hypothalamus-pituitary-adrenal cortex axis.

Repeated corticosterone administration sensitizes the locomotor response to amphetamine

Repeated exposures to stressful situations has been shown to increase individual reactivity to psychostimulants, although the biological factors involved in such stress-induced changes are still poorly understood. In this study, we investigated the role of corticosterone in the effects of stress on the response to psychostimulants. We found that repeated corticosterone administration (both 1.5 mg/kg, intraperitoneally and 50 micrograms/ml in drinking water, once per day for 15 days) increased the locomotor response to amphetamine (1.15 mg/kg, i.p.). At the doses used in these experiments, corticosterone administration induced similar increases in plasma levels of the hormone to those induced by stress. These results suggest that corticosterone secretion may be one of the mechanisms by which repeated stress increases the behavioral responses to amphetamine. Since an enhanced reactivity to psychostimulants has been found to be an index of a propensity for drug self-administration and a model of certain psychopathological conditions, these findings point to a role for glucocorticoids in such abnormal states.

Effect of ACTH, adrenalectomy and the combination treatment on the density of 5-HT2 receptor binding sites in neocortex of rat forebrain and 5-HT2 receptor-mediated wet-dog shake behaviors

The effect of ACTH and/or adrenalectomy on serotonin (5-HT)2 receptor binding sites was evaluated in the neocortex of rat forebrain. One day after the adrenalectomy or sham operation, ACTH (50 micrograms/day) was injected subcutaneously into adult male SD rats for 10 consecutive days. Saturation analysis showed that subchronic ACTH treatment significantly increased the Bmax values for 3H-ketanserin binding without any change in the Kd values. Moreover, this ACTH-induced increase in the Bmax values was prevented by adrenalectomy. The concentrations of 5-HT and 5-hydroxyindole acetic acid (5-HIAA) measured by HPLC-ECD were not altered by these manipulations. Ten-day administration of corticosterone (20 and 50 mg/kg) also increased 5-HT2 receptor density in the neocortex of rat forebrain. 5-HT2 (and 5-HT1C) receptor agonist, (+/-)DOI-induced wet-dog shakes in ACTH and/or adrenalectomy-treated rats were also examined. Ten-day administration of ACTH enhanced (+/-)DOI-induced wet-dog shakes and this increase was prevented by adrenalectomy. These results indicate that subchronic adrenocorticotropin-adrenal axis activation of rats increases both the number of 5-HT2 receptors in neocortex of forebrain and the wet-dog shake responses induced by (+/-)DOI.