Regulation of central corticosteroid receptors following short-term activation of serotonin transmission by 5-hydroxy-L-tryptophan or fluoxetine

Anxiolytic and antioxidant effects of Euterpe oleracea Mart. (açaí) seed extract in adult rat offspring submitted to periodic maternal separation

Many studies suggest a protective role of phenolic compounds in mood disorders. We aimed to assess the effect of Euterpe oleracea (açaí) seed extract (ASE) on anxiety induced by periodic maternal separation (PMS) in adult male rats. Animals were divided into 6 groups: control, ASE, fluoxetine (FLU), PMS, PMS+ASE, and PMS+FLU. For PMS, pups were separated daily from the dam for 3 h between postnatal day (PN) 2 and PN21. ASE (200 mg·kg^-1·day^-1) and FLU (10 mg·kg^-1·day^-1) were administered by gavage for 34 days after stress induction, starting at PN76. At PN106 and PN108, the rats were submitted to open field (OF) and forced swim tests, respectively. At PN110, the rats were sacrificed by decapitation. ASE increased time spent in the center area in the OF test, glucocorticoid receptors in the hypothalamus, tropomyosin receptor kinase B (TRKB) levels in the hippocampus, and nitrite levels and antioxidant activity in the brain stem (PMS+ASE group compared with PMS group). ASE also reduced plasma corticotropin-releasing hormone levels, adrenal norepinephrine levels, and oxidative damage in the brain stem in adult male offspring submitted to PMS. In conclusion, ASE treatment has an anti-anxiety effect in rats submitted to PMS by reducing hypothalamic-pituitary-adrenal axis reactivity and increasing the nitric oxide (NO)-brain-derived neurotrophic factor (BDNF)-TRKB pathway and antioxidant defense in the central nervous system. Novelty ASE has anti-anxiety and antioxidant effects in early-life stress. ASE reduces hypothalamic-pituitary-adrenal axis reactivity. The anxiolytic effect of ASE may involve activation of the NO-BDNF-TRKB pathway in the central nervous system.

Cellular and molecular mechanisms of sexual differentiation in the mammalian nervous system

Neuroscientists are likely to discover new sex differences in the coming years, spurred by the National Institutes of Health initiative to include both sexes in preclinical studies. This review summarizes the current state of knowledge of the cellular and molecular mechanisms underlying sex differences in the mammalian nervous system, based primarily on work in rodents. Cellular mechanisms examined include neurogenesis, migration, the differentiation of neurochemical and morphological cell phenotype, and cell death. At the molecular level we discuss evolving roles for epigenetics, sex chromosome complement, the immune system, and newly identified cell signaling pathways. We review recent findings on the role of the environment, as well as genome-wide studies with some surprising results, causing us to re-think often-used models of sexual differentiation. We end by pointing to future directions, including an increased awareness of the important contributions of tissues outside of the nervous system to sexual differentiation of the brain.

Early life environmental and pharmacological stressors result in persistent dysregulations of the serotonergic system

Dysregulations in the brain serotonergic system and exposure to environmental stressors have been implicated in the development of major depressive disorder. Here, we investigate the interactions between the stress and serotonergic systems by characterizing the behavioral and biochemical effects of chronic stress applied during early-life or adulthood in wild type (WT) mice and mice with deficient tryptophan hydroxylase 2 (TPH2) function. We showed that chronic mild stress applied in adulthood did not affect the behaviors and serotonin levels of WT and TPH2 knock-in (KI) mice. Whereas, maternal separation (MS) stress increased anxiety- and depressive-like behaviors of WT mice, with no detectable behavioral changes in TPH2 KI mice. Biochemically, we found that MS WT mice had reduced brain serotonin levels, which was attributed to increased expression of monoamine oxidase A (MAO A). The increased MAO A expression was detected in MS WT mice at 4 weeks old and adulthood. No change in TPH2 expression was detected. To determine whether a pharmacological stressor, dexamethasone (Dex), will result in similar biochemical results obtained from MS, we used an in vitro system, SH-SY5Y cells, and found that Dex treatment resulted in increased MAO A expression levels. We then treated WT mice with Dex for 5 days, either during postnatal days 7–11 or adulthood. Both groups of Dex treated WT mice had reduced basal corticosterone and glucocorticoid receptors expression levels. However, only Dex treatment during PND7–11 resulted in reduced serotonin levels and increased MAO A expression. Just as with MS WT mice, TPH2 expression in PND7–11 Dex-treated WT mice was unaffected. Taken together, our findings suggest that both environmental and pharmacological stressors affect the expression of MAO A, and not TPH2, when applied during the critical postnatal period. This leads to long-lasting perturbations in the serotonergic system, and results in anxiety- and depressive-like behaviors.

5-HT1A/1B receptors as targets for optimizing pigmentary responses in C57BL/6 mouse skin to stress

Stress has been reported to induce alterations of skin pigmentary response. Acute stress is associated with increased turnover of serotonin (5-hydroxytryptamine; 5-HT) whereas chronic stress causes a decrease. 5-HT receptors have been detected in pigment cells, indicating their role in skin pigmentation. To ascertain the precise role of 5-HT in stress-induced pigmentary responses, C57BL/6 mice were subjected to chronic restraint stress and chronic unpredictable mild stress (CRS and CUMS, two models of chronic stress) for 21 days, finally resulting in abnormal pigmentary responses. Subsequently, stressed mice were characterized by the absence of a black pigment in dorsal coat. The down-regulation of tyrosinase (TYR) and tyrosinase-related proteins (TRP1 and TRP2) expression in stressed skin was accompanied by reduced levels of 5-HT and decreased expression of 5-HT receptor (5-HTR) system. In both murine B16F10 melanoma cells and normal human melanocytes (NHMCs), 5-HT had a stimulatory effect on melanin production, dendricity and migration. When treated with 5-HT in cultured hair follicles (HFs), the increased expression of melanogenesis-related genes and the activation of 5-HT1A, 1B and 7 receptors also occurred. The serum obtained from stressed mice showed significantly decreased tyrosinase activity in NHMCs compared to that from nonstressed mice. The decrease in tyrosinase activity was further augmented in the presence of 5-HTR1A, 1B and 7 antagonists, WAY100635, SB216641 and SB269970. In vivo, stressed mice received 5-HT precursor 5-hydroxy-l-tryptophan (5-HTP), a member of the class of selective serotonin reuptake inhibitors (fluoxetine; FX) and 5-HTR1A/1B agonists (8-OH-DPAT/CP94253), finally contributing to the normalization of pigmentary responses. Taken together, these data strongly suggest that the serotoninergic system plays an important role in the regulation of stress-induced depigmentation, which can be mediated by 5-HT1A/1B receptors. 5-HT and 5-HTR1A/1B may constitute novel targets for therapy of skin hypopigmentation disorders, especially those worsened with stress.

The effects of the perinatal treatment with 5-hydroxytryptophan or tranylcypromine on the peripheral and central serotonin homeostasis in adult rats

Serotonin (5HT) is a biologically active amine present in mammals in the brain and the peripheral tissues. Autism is a neurodevelopmental disorder in which 5HT homeostasis is disturbed both centrally and peripherally, but the relationship between the 5HT disturbances in the two compartments is not understood. In an attempt to explore the relationship between the disturbed peripheral 5HT homeostasis and central 5HT functioning, we exposed the developing rat brain to increased 5HT concentrations, by treatment of rats with subcutaneous injections of the immediate 5HT precursor 5-hydroxy-L-tryptophan (5HTP, 25 mg/kg), or the non-selective MAO inhibitor tranylcypromine (TCP, 2 mg/kg), during the period of the most intensive development of 5HT neurons--from gestational day 13 to post-natal day 21. The effects of the mentioned treatments on peripheral and central 5HT levels were then studied in adult rats. Platelet and plasma 5HT concentrations (measured by ELISA), as well as cortical and midbrain 5HT, tryptophan and 5-hydroxyindoleacetic acid levels (measured by HPLC) were determined in twelve 5HTP treated and eight TCP treated rats, and compared with the values measured in 10 control, saline treated rats. Treatment with 5HTP significantly raised peripheral but not central 5HT concentrations. At adult age, peripheral 5HT homeostasis was re-established, while modest decrease in 5HT concentration was observed in frontal cortex, presumably due to hyperserotonemia-induced loss of 5HT terminals during brain development. Treatment with TCP induced significant 5HT elevations in both compartments. At adult age, permanent changes in 5HT homeostasis were observed, both peripherally (as hyperserotonemia) and centrally (as altered 5HT metabolism with decreased 5HT concentrations). Further studies are planned in order to explore the nature of the different disturbances of 5HT homeostasis induced by the two compounds, and their results are expected to shed some light on the role of hyperserotonemia in autism.

Antidepressant-like effects of mineralocorticoid but not glucocorticoid antagonists in the lateral septum: interactions with the serotonergic system

The lateral septum (LS) is a limbic brain region that receives serotonergic projections from raphe neurons and participates in the modulation of stress responses and affective states. The present study determined whether mineralocorticoid receptors (MRs) and/or glucocorticoid receptors (GRs) located in the LS interact with the serotonergic system in the regulation of depressive-like behavior of rats subjected to the forced swimming test (FST). We also studied the effect of corticosterone release induced by the FST on MR- and GR-mRNA expression in the LS. Specifically, we studied the antidepressant-like effects of spironolactone (a MR antagonist), mifepristone (a GR antagonist), and the antidepressant clomipramine (CMI) administered directly into the LS. In addition, spironolactone and CMI actions were studied in animals with serotonergic depletion induced by dl-p-chlorophenylalanine (pCPA). Finally, adrenalectomized and Sham-operated rats were subjected to the FST to determine MR- and GR-mRNA expression in the LS at different post-FST intervals. The results showed that intraseptal injection of spironolactone, but not mifepristone induced antidepressant-like actions in the FST; this effect was blocked by pCPA treatment. CMI and spironolactone increased 5-HT concentrations in the LS of rats subjected to the FST. Increases in corticosterone release, induced by the FST, correlated with a decrease in MR-mRNA expression in the LS; no correlation was found with GR-mRNA expression. In conclusion, MRs in the lateral septum, but not GRs, participate in the regulation of depressive-like behavior of animals subjected to the FST. Both serotonin and corticosterone play an important role in MR actions in the LS.

Sex differences in the serotonergic influence on the hypothalamic-pituitary-adrenal stress axis

Appropriate interactions between serotonin (5-HT) and stress pathways are critical for maintaining homeostasis. Dysregulation of hypothalamic-pituitary-adrenal (HPA) stress axis is a common feature in affective disorders in which an involvement of 5-HT neurocircuitry has been implicated in disease vulnerability and treatment responsiveness. Because there is a greater prevalence of affective disorders in women, sex differences in the 5-HTergic influence on stress pathways may contribute to disease disparity. Therefore, our studies compared stress or citalopram-induced corticosterone levels in male and female mice. To determine whether sex-dependent HPA axis responsiveness was mediated by the difference in testosterone levels, testosterone-treated females were also examined. Gene expression patterns in 5-HTergic and stress neurocircuitry were analyzed to determine sites of potential sex differences and mechanisms of testosterone action. As expected, restraint stress corticosterone levels were higher in intact females and were masculinized by testosterone. Interestingly, citalopram administration independent of stress resulted in a greater corticosterone response in females, which was also masculinized by testosterone. Analyses along the 5-HT-HPA axis revealed sex differences including greater pituitary 5-HT receptors and adrenal weights in females. Moreover, in stress-regulatory regions, we found sex differences in glucocorticoid receptor and glutamic acid decarboxylase expression supportive of greater inhibitory modulation and feedback potential in males. Taken together, these data suggest that multiple sites related to 5-HTergic stimulation, corticosterone production, and negative feedback of HPA neurocircuitry combine to produce higher female stress responsiveness. These studies support a potential for sex-specific involvement of 5-HT and stress pathways in the etiology of affective disorders.

Immunohistochemical localization of CB1 cannabinoid receptors in frontal cortex and related limbic areas in obese Zucker rats: effects of chronic fluoxetine treatment

In the present study, we report on the application of two specific polyclonal antibodies to different intracellular domains of the CB1 cannabinoid receptor to define the expression of the neural CB1 cannabinoid receptor at the histochemical level in frontal cortex and related limbic areas of the obese Zucker rats. Higher levels of CB1 receptor expression in frontal, cingulated and piriform cortex, without differences in temporal, parietal and occipital cortex, were observed in obese Zucker rats, with respect to their lean littermates. CB1 phosphorylated receptor (CB1-P) levels were also higher in frontal, temporal, parietal and occipital cortex in obese rats with respect to lean controls. Potential involvement of brain cortical CB1 cannabinoid receptors in the long-term effects of fluoxetine was studied. Experimental animals were administered with fluoxetine (10 mg/kg, i.p.) daily for 3 weeks, whereas the control group was given 0.9% NaCl solution. In obese Zucker rats, a significant decrease in CB1 receptor levels, measured by western blot, was observed in brain cortex after fluoxetine treatment. Immunostaining for CB1 receptor expression was also carried out, showing a significant decrease in the density of neural cells positive for CB1 receptor in frontal, cingulate and piriform cortex, without changes in parietal, temporal and occipital regions. Regional prosencephalic immunostaining for CB1-P receptor level showed a significant decrease in the density of stained neural cells in frontal, temporal and parietal cortex, without changes in cingulated, piriform and occipital cortex. These results suggest the involvement of endocannabinoid system in the chronic effects of fluoxetine, especially in the frontal cortex.

Daily male exposure attenuates estrous cycle disruption by fluoxetine

Fluoxetine (Prozac) produces sexual dysfunction in a substantial number of patients. In the few animal studies designed to address this sexual dysfunction in females, data have been inconsistent. Some investigators report that the drug disrupts sexual behavior without affecting the estrous cycle while we have reported robust effects of fluoxetine on the estrous cycle. The current studies were designed to initiate examination of procedural differences that may account for these contradictory outcomes. In the first experiment, intact, regularly cycling female rats were injected daily for 10 days with 10 mg/kg fluoxetine (intraperitoneally) or vehicle. Male-exposed, fluoxetine- or vehicle-treated rats were housed in a room with males and placed for 5 min/day into a male's cage. Other fluoxetine-treated females were housed in a room separate from males. In the second experiment, this protocol was repeated for 20 days and an additional group of females were exposed to male bedding for 5 min/day. Without male exposure, fluoxetine rapidly disrupted vaginal estrus and sexual receptivity so that approximately 50% of the rats failed to show vaginal estrus during the first 5 days; and the majority of the rats had a blocked cycle by 10 days of treatment. With male exposure, these reproductive effects were attenuated. The majority of rats cycled normally during the first 5 days of treatment and more than half cycled throughout the experiment. Loss of behavioral receptivity occurred even when normal estrous cyclicity was present. Although exposure to the male's bedding may have delayed the onset of estrous cycle disruption, five min daily exposure to a male's bedding did not prevent the disruptive effects of fluoxetine. These findings are consistent with evidence that fluoxetine's effect on female sexual dysfunction may result, in part, from the drugs' disruption of the hypothalamic-pituitary-gonadal axis. However, the data also evidence dissociation between the effects of fluoxetine on vaginal and behavioral estrus. These findings may also explain why different laboratories have reported the presence or absence of estrous cycle disturbances following daily treatment with fluoxetine.

Glucocorticoids and serotonin alter glucocorticoid receptor mRNA levels in fetal guinea-pig hippocampal neurons, in vitro

The hypothalamic-pituitary-adrenal (HPA) axis is susceptible to programming during fetal life. Such programming occurs, at least partially, at the level of the hippocampus. The hippocampus plays a central role in regulation of the HPA axis and release of endogenous glucocorticoids, via mediation of glucocorticoid negative feedback. Fetal exposure to synthetic glucocorticoids can permanently alter glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) levels within the hippocampus, and serotonin is thought to be involved in this process. In the present study, we hypothesised that dexamethasone, cortisol and serotonin exposure would modify GR mRNA expression within fetal guinea-pig hippocampal cultures. Cultures were derived from 40-day-old guinea-pig fetuses, and were exposed to 0, 1, 10 and 100 nM dexamethasone, cortisol or serotonin for 4 days. Expression of GR and MR mRNA was examined by in situ hybridisation followed by high-resolution silver emulsion autoradiography. Four-day exposure to dexamethasone (P < 0.05; 100 nM) or cortisol (P = 0.08; 100 nM) downregulated the expression of GR mRNA within neurons. There was no change in the expression of MR mRNA levels following cortisol treatment. Exposure to serotonin (100 nM) significantly increased GR mRNA levels in hippocampal neurons. We conclude that synthetic and endogenous glucocorticoids, as well as serotonin, can influence GR expression during hippocampal development and in this way may act to permanently programme HPA function.

Fluoxetine disrupts food intake and estrous cyclicity in Fischer female rats

Adult, regularly cycling female Fischer rats were injected daily with 10 mg/kg fluoxetine for 12-23 days. In the first experiment, body weight and vaginal smears were monitored daily. Fluoxetine treatment reduced body weight within the first 24 h of treatment. Fluoxetine treatment also elongated the estrous cycle, reduced blood levels of progesterone, and eliminated lordosis behavior. In the second experiment, body weight and food intake were examined and a pair-fed group was included to determine if fluoxetine-induced anorexia contributed to the disturbance of the estrous cycle. In pair-fed rats, effects similar to fluoxetine treatment were present. These results lead to the suggestion that fluoxetine's anorectic properties could disrupt the female's normal endocrine cyclicity and that this disruption could be relevant to the reduction in sexual behavior and motivation. However, when the duration of fluoxetine treatment was extended beyond 16 to 17 days, fluoxetine-treated female rats reinitiated vaginal cyclicity and showed evidence of normal sexual receptivity. In contrast, the estrous cycles of their pair-fed counterparts remained disrupted. Thus, restricted food intake appears to contribute to the disruption of the estrous cycle and elimination of sexual receptivity during fluoxetine treatment. However, compensatory changes in the serotonergic system that are associated with chronic fluoxetine administration may contribute to the gradual recovery of estrous cyclicity and sexual receptivity of the fluoxetine-treated animals.

Serotonin a la carte: supplementation with the serotonin precursor 5-hydroxytryptophan

This paper reviews the preclinical and clinical evidence regarding the use of the dietary supplement 5-hydroxytryptophan (5-HTP) for the treatment of depression. In the absence of supplementation with exogenous 5-HTP, the amount of endogenous 5-HTP available for serotonin synthesis depends on the availability of tryptophan and on the activity of various enzymes, especially tryptophan hydroxylase, indoleamine 2,3-dioxygenase, and tryptophan 2,3-dioxygenase (TDO). Factors affecting each of these are reviewed. The amount of 5-HTP reaching the central nervous system (CNS) is affected by the extent to which 5-HTP is converted to serotonin in the periphery. This conversion is controlled by the enzyme amino acid decarboxylase, which, in the periphery, can be blocked by peripheral decarboxylase inhibitors (PDIs) such as carbidopa. Preclinical and clinical evidence for the efficacy of 5-HTP for depression is reviewed, with emphasis on double-blind, placebo-controlled (DB-PC) trials. Safety issues with 5-HTP are also reviewed, with emphasis on eosinophilia myalgia syndrome (EMS) and serotonin syndrome.

Effects of exposure to a predator on behaviour and serotonergic neurotransmission in different brain regions of C57bl/6N mice

Clinical studies and animal models have provided evidence that stress and serotonin may play a role in the aetiology of psychiatric diseases such as depression and anxiety. In addition, reciprocal interactions between stress and serotonergic neurotransmission have been demonstrated. However, the relationships between stress, serotonin and behaviour are far from completely understood. In this integrative study, we aimed to elucidate the effect of the psychological stress model predator exposure on behaviour and serotonergic neurotransmission in mice. We used a high time-resolution microdialysis method to measure extracellular levels of serotonin (5-hydroxytryptamine, 5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in the hippocampus, prefrontal cortex, lateral septum and caudate putamen of C57BL/6N [corrected] mice, before (08:30-10:30 h), during (10:30-11:00 h) and after exposure (11:00-14:00 h) to a rat. Detailed behavioural observations were also made. Rat exposure resulted in behavioural activation, with predominant risk-assessment activities, and in increases in hippocampal, cortical, septal but not striatal 5-HT and 5-HIAA. When rat exposure was repeated on the consecutive day, small behavioural differences and reductions in 5-HIAA levels, but no differences in the 5-HT response, as compared with the first exposure were observed. As increases in 5-HT often coincide with behavioural activation, it was particularly interesting to find that 5-HT also increased in periods when mice only made minor movements such as sniffing, and that an effect of predator stress was absent in the caudate putamen. Our results indicate that the presence of the rat leads to differential activation of serotonergic neurotransmission in higher brain structures, probably involved in the coping response to this potentially life-threatening situation.

Chronic treatment with imipramine reverses immobility behaviour, hippocampal corticosteroid receptors and cortical 5-HT1A receptor mRNA in prenatally stressed rats

Prenatal stress in the rat induces enhanced reactivity of the hypothalamus-pituitary-adrenal (HPA) axis, disturbances in a variety of circadian rhythms and increased anxiety-like behaviour. Such abnormalities parallel those found in human depressed patients. Prenatally stressed (PS) rats could represent, therefore, an interesting animal model for the evaluation of the efficacy of pharmacotherapeutic intervention in psychiatric disorders that has often been addressed using control animals. In the present study, PS and non-stressed rats were chronically treated with the tricyclic antidepressant imipramine (10 mg/kg i.p. for 21 days) and assessed in the forced swim test. Glucocorticoid receptor binding sites in the hippocampus were measured and 5-HT(1A) receptor mRNA levels in the frontal cortex were also assessed. PS rats were characterised by increased immobility in the forced swim test, reduced hippocampal corticosteroid receptor binding and increased levels of cortical 5-HT(1A) mRNA. All these parameters were significantly reversed by chronic imipramine treatment. Conversely, no significant effects were observed for non-stressed rats. All these effects are consistent with the expected pharmacotherapy of depression-like abnormalities in PS rats. These results further indicate that PS rats are a relevant animal model of depression.

Do antidepressants regulate how cortisol affects the brain?

Although the effects of antidepressants on glucocorticoid hormones and their receptors are relevant for the therapeutic action of these drugs, the molecular mechanisms underlying these effects are unclear. Studies in depressed patients, animals and cellular models have demonstrated that antidepressants increase glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) expression and function; this, in turn, is associated with enhanced negative feedback by endogenous glucocorticoids, and thus with reduced resting and stimulated hypothalamic-pituitary-adrenal (HPA) axis activity. In a series of studies conducted over the last few years, we have shown that antidepressants modulate GR function in vitro by inhibiting membrane steroid transporters that regulate the intracellular concentration of glucocorticoids. In this paper, we will review the effects of membrane steroid transporters and antidepressants on corticosteroid receptors. We will then present our unpublished data on GR live microscopy in vitro, showing that ligand-induced translocation of the GR starts within 30 seconds and is completed within minutes. Furthermore, we will present our new data using an in situ brain perfusion model in anaesthetised guinea-pigs, showing that entry of cortisol to the brain of these animals is limited at the blood-brain barrier (BBB). Finally, we will present a comprehensive discussion of our published findings on the effects of chemically unrelated antidepressants on membrane steroid transporters, in mouse fibroblasts and rat cortical neurones. We propose that antidepressants in humans could inhibit steroid transporters localised on the BBB and in neurones, like the multidrug resistance p-glycoprotein, and thus increase the access of cortisol to the brain and the glucocorticoid-mediated negative feedback on the HPA axis. Enhanced cortisol action in the brain might prove to be a successful approach to maximise therapeutic antidepressant effects.

Serotonin-induced increases in adult cell proliferation and neurogenesis are mediated through different and common 5-HT receptor subtypes in the dentate gyrus and the subventricular zone

Increase in serotonin (5-HT) transmission has profound antidepressant effects and has been associated with an increase in adult neurogenesis. The present study was aimed at screening the 5-HT receptor subtypes involved in the regulation of cell proliferation in the subgranular layer (SGL) of the dentate gyrus (DG) and the subventricular zone (SVZ) and to determine the long-term changes in adult neurogenesis. The 5-HT1A, 5-HT1B, and 5-HT2 receptor subtypes were chosen for their implication in depression and their location in/or next to these regions. Using systemic administration of various agonists and antagonists, we show that the activation of 5-HT1A heteroreceptors produces similar increases in the number of bromodeoxyuridine-labeled cells in the SGL and the SVZ (about 50% over control), whereas 5-HT2A and 5-HT2C receptor subtypes are selectively involved in the regulation of cell proliferation in each of these regions. The activation of 5-HT2C receptors, largely expressed by the choroid plexus, produces a 56% increase in the SVZ, while blockade of 5-HT2A receptors produces a 63% decrease in the number of proliferating cells in the SGL. In addition to the influence of 5-HT1B autoreceptors on 5-HT terminals in the hippocampus and ventricles, 5-HT1B heteroreceptors also regulate cell proliferation in the SGL. These data indicate that multiple receptor subtypes mediate the potent, partly selective of each neurogenic zone, stimulatory action of 5-HT on adult brain cell proliferation. Furthermore, both acute and chronic administration of selective 5-HT1A and 5-HT2C receptor agonists produce consistent increases in the number of newly formed neurons in the DG and/or olfactory bulb, underscoring the beneficial effects of 5-HT on adult neurogenesis.

Differential regulation of corticosteroid receptors by monoamine neurotransmitters and antidepressant drugs in primary hippocampal culture

Hyperactivity of the hypothalamic-pituitary-adrenal axis is a characteristic feature of depressive illness. The centrally located corticosteroid receptors, the glucocorticoid and mineralocorticoid receptors, are thought to be important modulators of this axis and changes in the levels of these receptors, particularly in the hippocampus, may underlie the hyperactivity observed. Various antidepressant drugs increase hippocampal mineralocorticoid and glucocorticoid receptor levels in vivo. These effects are thought to be mediated via alterations in monoaminergic neurotransmission. We examined whether serotonin (5HT) and noradrenaline (NA) have direct effects on glucocorticoid receptor and mineralocorticoid receptor expression in primary hippocampal neurones, and whether antidepressants also exert direct effects on target neurones. Exposure of hippocampal cells to 5HT for 4 days increased both glucocorticoid and mineralocorticoid receptor mRNA and protein expression. The induction of mineralocorticoid receptor mRNA was completely blocked by the 5HT(7) receptor antagonist SB 269970. In contrast glucocorticoid receptor induction was insensitive to the 5HT(7) receptor, whilst studies with the 5HT(1A) receptor agonist 8-hydroxy-2-(di-n-proplamino) tetralin hydrochloride and the 5HT(1A) receptor antagonist N-[2-[4-2-[O-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexane carboxamide trihydrochloride (WAY 100635) suggest a partial role for 5HT(1A) receptors in hippocampal glucocorticoid receptor regulation. Treatment with NA for 4 days also increased glucocorticoid receptor expression but had no effect on mineralocorticoid receptor expression. This was blocked by propanolol suggesting action via beta-adrenergic receptors. Similarly to NA, fluoxetine and amitriptyline also selectively increased glucocorticoid receptor mRNA and protein levels over this time course. However, glucocorticoid receptor induction by fluoxetine or amitriptyline was not blocked by WAY 100635 or propanolol. These results show that 5HT, NA and antidepressants act directly but via distinct mechanisms on hippocampal neurones to regulate mineralocorticoid and glucocorticoid receptor expression. Thusly, manipulation of neurotransmitter or antidepressant levels in the brain may aid in reversing hypothalamic-pituitary-adrenal axis hyperactivity by restoring hippocampal corticosteroid receptor balance.

Obese Zucker rats have reduced mineralocorticoid receptor and 11beta-hydroxysteroid dehydrogenase type 1 expression in hippocampus-implications for dysregulation of the hypothalamic-pituitary-adrenal axis in obesity

Obese Zucker rats have elevated basal corticosterone levels and an increased stress response suggestive of an increased activity of the hypothalamic-pituitary-adrenal (HPA) axis. We hypothesized that altered central expression of glucocorticoid receptors (GR), mineralocorticoid receptors (MR), and/or 11beta-hydroxysteroid dehydrogenase type 1 (11betaHSD1) contribute to these changes. In brains from young adult male rats, in situ hybridization and Western blotting showed that obese rats had normal hippocampal GR mRNA and protein levels. In contrast, in obese rats, 11betaHSD1 mRNA levels were reduced in a subpopulation of hippocampal cells in the main neuronal layers (by 37-47%, P < 0.05), whereas 11betaHSD1 levels in sparse high-expressing cells did not differ. MR mRNA was decreased in all regions of the hippocampus (by 37-49%, P < 0.05 for CA1-2 and P < 0.01 for dentate gyrus) and in frontal cortex (by 16%, P < 0.05) in obese rats. In whole hippocampal homogenates, however, neither the protein concentration of MR by Western blot nor activity of 11betaHSD1 was measurably different between the phenotypes. To test the functional importance of lower central MR expression, groups of lean and obese rats were given spironolactone before restraint stress. In vehicle-treated animals, obese rats had higher plasma corticosterone levels than lean rats after stress (by ANOVA, P < 0.05). Spironolactone markedly increased the corticosterone response in both groups, but the incremental rise was smaller in the obese rats, so that spironolactone abolished the differences between groups. We conclude that lower levels of MR, but not GR, contribute to the increased HPA activity in the obese Zucker rats and that this seems more influential during stress than in the basal state. This may be exacerbated by impaired local regeneration of corticosterone by 11betaHSD1. These abnormalities could contribute to the subtle changes in the HPA axis in rodent and human obesity.

The effects of long-term adrenalectomy on 5-HT1B receptors mRNA expression in cerebellum, striatum, frontal cortex and hippocampus of rats

The brain serotonin (5-HT) system and circulating corticosteroids are in close interaction and both implicated in the pathogenesis of affective disorders. We evaluated the effects of adrenalectomy (ADX) on 5-HT(1B) receptors mRNA expression in cerebellum, frontal cortex, striatum and hippocampus in rats, using the RNase protection assay technique. Eight weeks after bilateral adrenalectomy, 5-HT(1B) receptor mRNA levels were decreased in the cerebellum and in the frontal cortex. The expression of 5-HT(1B) receptors mRNA was unchanged in the hippocampus and in the striatum. This data indicates regional differences in the effects of long term adrenalectomy on the expression of 5-HT(1B) receptors.

Mss4 gene is up-regulated in rat brain after chronic treatment with antidepressant and down-regulated when rats are anhedonic

Differential display reverse transcription-polymerase chain reaction was used to identify mRNAs that are differentially expressed in the brain of rats treated chronically with the reference tricyclic antidepressant, imipramine, in comparison with control rats. The gene encoding for a mutation suppressor for Sec4-8 yeast (Mss4) transcript is overexpressed in the amygdala of treated rats after 3 weeks of daily administration. This overexpression is also found in the hippocampus of rats treated chronically with either tianeptine or fluoxetine. Mss4 protein has the properties of a guanine nucleotide exchange factor, interacting with several members of the Rab family implicated in Ca(2+)-dependent exocytosis of neurotransmitters. Mss4 was also overexpressed in other brain structures as judged by in situ hybridization. The kinetics of the up-regulation of Mss4 gene expression measured by Northern blot during the imipramine, tianeptine, or fluoxetine treatments are consistent with an antidepressant effect that occurs after 3 weeks. In rats in which anhedonia was induced by chronic mild stress during 3 weeks, Mss4 transcripts were specifically down-regulated in hippocampus and amygdala compared with control rats. It is proposed that Mss4 protein, which stimulates exocytosis in vivo, participates in the potentiation of the activity of neurotransmitter pathways implicated in the action of several antidepressants and constitutes one of the common functional molecules induced after chronic antidepressant treatment.