Increase of 5-HT7 (serotonin-7) and 5-HT1A (serotonin-1A) receptor mRNA expression in rat hippocampus after adrenalectomy

Neurochemical Anatomy of Cushing's Syndrome

The neurochemical anatomy underlying Cushing's syndrome is examined for regional brain metabolism as well as neurotransmitter levels and receptor binding of biogenic amines and amino acids. Preliminary studies generally indicate that glucose uptake, blood flow, and activation on fMRI scans decreased in neocortical areas and increased in subcortical areas of patients with Cushing's syndrome or disease. Glucocorticoid-mediated increases in hippocampal metabolism occurred despite in vitro evidence of glucocorticoid-induced decreases in glucose uptake or consumption, indicating that in vivo increases are the result of indirect, compensatory, or preliminary responses. In animal studies, glucocorticoid administration decreased 5HT levels and 5HT1A receptor binding in several brain regions while adrenalectomy increased such binding. Region-specific effects were also obtained in regard to the dopaminergic system, with predominant actions of glucocorticoid-induced potentiation of reuptake blockers and releasing agents. More in-depth neuroanatomical analyses are warranted of these and amino acid-related neurotransmission.

Attenuation of stress-induced behavioral changes by activation of serotonin type 7 receptors in the median raphe nucleus of rats

BACKGROUND

Exposure to stressful aversive situations induces physiological and behavioral changes. Serotonin has been suggested to mediate such changes, as well as adaptation to stressful events. Serotoninergic projections arising from the median raphe nucleus to the dorsal hippocampus have been suggested to promote adaptation to chronic aversive stimuli. Such pathway may involve serotonin type 1a receptor-mediated neurotransmission. However, the serotonin 7 receptor can also be found in the median raphe nucleus and may be involved in mechanisms underlying response to stress.

AIMS

In this work we sought to investigate if activation of serotonin type 7 receptors would attenuate stress-induced deficits in different animal models of depression.

METHODS

Male Wistar rats with a guide-cannula aimed to the median raphe nucleus were submitted to restraint or forced swim stress and were tested in an elevated plus maze or forced swim test, respectively, 24 h later. SB 258741 (serotonin type 7 receptor antagonist) and/or LP 44 (serotonin type 7 receptor agonist) were administered intra-median raphe nucleus immediately before or after exposure to stress or before test. Control groups received intra-median raphe nucleus treatment 24 h or immediately before test in the elevated plus maze or forced swim test.

RESULTS

LP 44 attenuated restraint-induced exploratory deficits independently of the moment it was administered. Similar results were observed in the forced swim test, with the exception on post-stress condition. These effects on adaptation to stress induced by serotonin type 7 receptor activation were prevented by previous treatment with SB 258741.

CONCLUSIONS

Our data support the idea that activation of median raphe nucleus serotonin 7 receptor is important to the development of adaptation to stress.

Ozone increases plasma kynurenine-tryptophan ratio and impacts hippocampal serotonin receptor and neurotrophic factor expression: Role of stress hormones

Air pollution is associated with adverse impacts on the brain, including cognitive decline and increased incidence of dementia, depression and anxiety; however, underlying mechanisms remain unclear. We have shown that both ozone and particulate matter activate the hypothalamic-pituitary-adrenal (HPA) axis, increasing plasma glucocorticoids and altering mRNA profiles in multiple tissues including the brain. HPA axis dysregulation has been associated with central nervous system impacts, including key effects in the hippocampus; accordingly, we hypothesized that pollutant-dependent increases in glucocorticoid levels impact biological pathways relevant to brain health. Fischer-344 rats were treated with metyrapone (0 or 50 mg/kg), a glucocorticoid synthesis inhibitor, and exposed to ozone (0 or 0.8 ppm) for 4 h (n = 5/group) to investigate the role of glucocorticoids in ozone-dependent effects on tryptophan metabolism and expression of serotonin receptors and neurotrophic factors. Ozone increased plasma levels of the tryptophan metabolite kynurenine (~2-fold) and decreased tryptophan levels (~1.2 fold). Hippocampal expression of serotonin receptors exhibited differential regulation following exposure, and expression of key neurotrophic factors (brain-derived neurotrophic factor, vascular endothelial growth factor A, insulin-like growth factor-1, tyrosine kinase receptor B, b-cell lymphoma 2) was decreased. Some, but not all effects were abrogated by metyrapone treatment, suggesting both glucocorticoid-dependent and -independent regulation. Exposure to exogenous corticosterone (10 mg/kg) followed by clean air reproduced the ozone effects that were blocked with metyrapone, confirming the specificity of effects to glucocorticoids. These results indicate that ozone can modify pathways relevant to brain health and establish a role for the HPA axis in mediating these effects.

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.

The 5-HT7 receptor antagonist SB 269970 ameliorates corticosterone-induced alterations in 5-HT7 receptor-mediated modulation of GABAergic transmission in the rat dorsal raphe nucleus

RATIONALE

Chronic stress and corticosterone have been shown to affect serotonin (5-HT) neurotransmission; however, the influence of stress on the activity of the dorsal raphe nucleus (DRN), the main source of 5-HT in the forebrain, is not well understood. In particular, it is unknown if and how stress modifies DRN 5-HT₇ receptors, which are involved in the modulation of the firing of local inhibitory interneurons responsible for regulating the activity of DRN projection cells.

OBJECTIVES

Our study aimed to investigate the effect of repeated corticosterone injections on the modulation of the inhibitory transmission within the DRN by 5-HT₇ receptors and whether it could be reversed by treatment with a 5-HT₇ receptor antagonist.

METHODS

Male Wistar rats received corticosterone injections repeated twice daily for 14 days. Spontaneous inhibitory postsynaptic currents (sIPSCs) were then recorded from DRN projection cells in ex vivo slice preparations obtained 24 h after the last injection.

RESULTS

Repeated corticosterone administration resulted in decreased frequency, but not amplitude, of sIPSCs in DRN projection cells. There were no changes in the excitability of these cells; however, corticosterone treatment suppressed the 5-HT₇ receptor-mediated increase in sIPSC frequency. Administration of the 5-HT₇ receptor antagonist SB 269970 for 7 days beginning on the eighth day of corticosterone treatment reversed the detrimental effects of corticosterone on 5-HT₇ receptor reactivity and GABAergic transmission in the DRN.

CONCLUSIONS

Elevated corticosterone level reduces DRN 5HT₇ receptor reactivity and decreases GABAergic transmission within the DRN, which can be reversed by the 5-HT₇ receptor antagonist SB 269970.

Sensitization of restraint-induced corticosterone secretion after chronic restraint in rats: involvement of 5-HT₇ receptors

Serotonin (5-HT) modulates the hypothalamic-pituitary-adrenal (HPA) axis response to stress. We examined the effect of chronic restraint stress (CRS; 20 min/day) as compared to control (CTRL) conditions for 14 days, on: 1) restraint-induced ACTH and corticosterone (CORT) secretion in rats pretreated with vehicle or SB-656104 (a 5-HT₇ receptor antagonist); 2) 5-HT₇ receptor-like immunoreactivity (5-HT₇-LI) and protein in the hypothalamic paraventricular nucleus (PVN) and adrenal glands (AG); 3) baseline levels of 5-HT and 5-hydroxyindolacetic acid (5-HIAA), and 5-HIAA/5-HT ratio in PVN and AG; and 4) 5-HT-like immunoreactivity (5-HT-LI) in AG and tryptophan hydroxylase (TPH) protein in PVN and AG. On day 15, animals were subdivided into Treatment and No treatment groups. Treatment animals received an i.p. injection of vehicle or SB-656104; No Treatment animals received no injection. Sixty min later, Treatment animals were either decapitated with no further stress (0 min) or submitted to acute restraint (10, 30, 60 or 120 min); hormone serum levels were measured. No Treatment animals were employed for the rest of measurements. CRS decreased body weight gain and increased adrenal weight. In CTRL animals, acute restraint increased ACTH and CORT secretion in a time of restraint-dependent manner; both responses were inhibited by SB-656104. Exposure to CRS abolished ACTH but magnified CORT responses to restraint as compared to CTRL conditions; SB-656104 had no effect on ACTH levels but significantly inhibited sensitized CORT responses. In CTRL animals, 5-HT₇-LI was detected in magnocellular and parvocellular subdivisions of PVN and sparsely in adrenal cortex. Exposure to CRS decreased 5-HT₇-LI and protein in the PVN, but increased 5-HT₇-LI in the adrenal cortex and protein in whole AG. Higher 5-HT and 5-HIAA levels were detected in PVN and AG from CRS animals but 5-HIAA/5-HT ratio increased in AG only. Finally, whereas 5-HT-LI was sparsely observed in the adrenal cortex of CTRL animals, it strongly increased in the adrenal cortex of CRS animals. No TPH protein was detected in AG from both animal groups. Results suggest that CRS promotes endocrine disruption involving decreased ACTH and sensitized CORT responses to acute restraint. This phenomenon may be associated with increased function and expression of 5-HT₇ receptors as well as 5-HT turnover in AG.

Early parental deprivation in the marmoset monkey produces long-term changes in hippocampal expression of genes involved in synaptic plasticity and implicated in mood disorder

In mood disorder, early stressors including parental separation are vulnerability factors, and hippocampal involvement is prominent. In common marmoset monkeys, daily parental deprivation during infancy produces a prodepressive state of increased basal activity and reactivity in stress systems and mild anhedonia that persists at least to adolescence. Here we examined the expression of eight genes, each implicated in neural plasticity and in the pathophysiology of mood disorder, in the hippocampus of these same adolescent marmosets, relative to their normally reared sibling controls. We also measured hippocampal volume. Early deprivation led to decreases in hippocampal growth-associated protein-43 (GAP-43) mRNA, serotonin 1A receptor (5-HT(1A)R) mRNA and binding ([3H]WAY100635), and to increased vesicular GABA transporter mRNA. Brain-derived neurotrophic factor (BDNF), synaptophysin, vesicular glutamate transporter 1 (VGluT1), microtubule-associated protein-2, and spinophilin transcripts were unchanged. There were some correlations with in vivo biochemical and behavioral indices, including VGluT1 mRNA with reward-seeking behavior, and serotonin 1A receptor mRNA with CSF cortisol. Early deprivation did not affect hippocampal volume. We conclude that early deprivation in a nonhuman primate, in the absence of subsequent stressors, has a long-term effect on the hippocampal expression of genes implicated in synaptic function and plasticity. The reductions in GAP-43 and serotonin 1A receptor expressions are comparable with findings in mood disorder, supporting the possibility that the latter reflect an early developmental contribution to disease vulnerability. Equally, the negative results suggest that other features of mood disorder, such as decreased hippocampal volume and BDNF expression, are related to different aspects of the pathophysiological process.

Antidepressant-like effects of the mixture of honokiol and magnolol from the barks of Magnolia officinalis in stressed rodents

Honokiol and magnolol are the main constituents simultaneously identified in the barks of Magnolia officinalis, which have been used in traditional Chinese medicine to treat a variety of mental disorders including depression. In the present study, we reported on the antidepressant-like effects of oral administration of the mixture of honokiol and magnolol in well-validated models of depression in rodents: forced swimming test (FST), tail suspension test (TST) and chronic mild stress (CMS) model. The mixture of honokiol and magnolol significantly decreased immobility time in the mouse FST and TST, and reversed CMS-induced reduction in sucrose consumption to prevent anhedonia in rats. However, this mixture was unable to affect ambulatory or rearing behavior in the mouse open-field test. CMS induced alterations in 5-hydroxytryptamine (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) levels in various brain regions of rats. An increase in serum corticosterone concentrations and a reduction in platelet adenylyl cyclase (AC) activity were simultaneously found in the CMS rats. The mixture of honokiol and magnolol at 20 and 40 mg/kg significantly attenuated CMS-induced decreases of 5-HT levels in frontal cortex, hippocampus, striatum, hypothalamus and nucleus accumbens. And it markedly increased 5-HIAA levels in frontal cortex, striatum and nucleus accumbens at 40 mg/kg and in frontal cortex at 20 mg/kg in the CMS rats. A subsequent reduction in 5-HIAA/5-HT ratio was found in hippocampus and nucleus accumbens in the CMS rats receiving this mixture. Furthermore, the mixture of honokiol and magnolol reduced elevated corticosterone concentrations in serum to normalize the hypothalamic-pituitary-adrenal (HPA) hyperactivity in the CMS rats. It also reversed CMS-induced reduction in platelet AC activity, via upregulating the cyclic adenosine monophosphate (cAMP) pathway. These results suggested that the mixture of honokiol and magnolol possessed potent antidepressant-like properties in behaviors involved in normalization of biochemical abnormalities in brain 5-HT and 5-HIAA, serum corticosterone levels and platelet AC activity in the CMS rats. Our findings could provide a basis for examining directly the interaction of the serotonergic system, the HPA axis and AC-cAMP pathway underlying the link between depression and treatment with the mixture of honokiol and magnolol.

Hypothalamic?Pituitary?Adrenal Responses to 5-HT1Aand 5-HT2A/CAgonists Are Differentially Altered in Female and Male Rats Prenatally Exposed to Ethanol

BACKGROUND

Prenatal ethanol exposure alters the development of the hypothalamic-pituitary-adrenal (HPA) axis, resulting in HPA hyper-responsiveness to stressors in adulthood. Prenatal ethanol exposure also alters the development and activity of the serotoninergic (5-HT) system. We have previously shown that 5-HT(1A) and 5-HT(2A/C) receptor-mediated behavioral and physiological function are altered in fetal ethanol-exposed offspring. As there are extensive interactions between the HPA axis and the 5-HT system, the present study tested the hypothesis that prenatal ethanol exposure would alter 5-HT(1A) and 5-HT(2A/C) receptor-mediated HPA function.

METHODS

The 5-HT(1A) agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.2 mg/kg), and the 5-HT(2A/C) agonist, (+)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI; 0.3 mg/kg), or vehicle (1 mL/kg) were administered to adult female and male offspring from prenatal ethanol-exposed (E), pair-fed control (PF), and ad libitum-fed control (C) dams. The plasma concentration of adrenocorticotropin (ACTH) and corticosterone (CORT) were determined at 0, 15, 30, 60, and 120 minutes postinjection. In addition, corticotropin releasing hormone (CRH) mRNA expression in the paraventricular nucleus of the hypothalamus, and 5-HT(1A) and 5-HT(2A/C) receptor mRNA expression in the hippocampus and prefrontal cortex, respectively, were determined by in situ hybridization.

RESULTS

Ethanol-exposed females showed a blunted ACTH response to 8-OH-DPAT at 15 and 30 minutes, and conversely, an increased ACTH response to DOI at all time points postinjection, compared with PF and C females. Differences among E, PF, and C males failed to reach significance. Centrally, however, DOI resulted in a trend toward lower CRH mRNA levels in E and PF compared with C females, but higher CRH mRNA levels in E compared with control males. There were no differences among prenatal groups in 5-HT(2A) receptor expression in the prefrontal cortex following either 8-OH-DPAT or DOI treatment. However, following 8-OH-DPAT, hippocampal 5-HT(1A) receptor expression was higher in E than in PF females in CA1, with a trend toward higher expression in E than in C females in CA2, whereas following DOI, a prenatal group by subfield interaction suggests lower 5-HT(1A) mRNA levels in E and PF compared with C females in CA1 and the dentate gyrus.

CONCLUSIONS

These data are the first to demonstrate that prenatal ethanol exposure has differential long-term effects on 5-HT(1A)-mediated and 5-HT(2A)-mediated neuroendocrine function in females and males, and suggest a sex-specific ethanol-induced alteration in the interaction between the HPA axis and the serotonin system.

Structure of the human 5-HT7 receptor gene and characterization of its promoter region

The molecular mechanism regulating serotonin 5-HT(7) receptor expression is still unclear. In this study we provide evidence that transcription of the 5-HT(7) gene is at least partly regulated by Sp1 and Sp3. We isolated and sequenced >3000 bp of the upstream sequences and identified by RACE a number of transcriptional initiation sites over a region of 300 bp upstream of the coding region. This region has a high GC content, but contains no obvious TATA or a CAAT box. Besides a Sp1/Sp3 consensus motif, regulatory elements for AP2, Egr-1 and MAZ are also present. Transient transfection assays using deletion variants indicated that the GC-rich region is essential for full promoter activity. The role of Sp1 in this was confirmed by transient overexpression of both wild type Sp1 or dominant-negative forms. By gel shift and supershift analyses, targeting the Sp1 consensus sequence and the GC-rich region just upstream of the transcription initiation sites, binding of Sp1 and Sp3 was demonstrated. Both in vitro as well as in vivo experiments, using a cell line which endogenously expresses the 5-HT(7) receptor, indicated that mithramycin A, an inhibitor of Sp1/3 transcription factor binding, was able to inhibit 5-HT(7) promoter activity. Taken together these results support the essential role of Sp factors in regulating 5-HT(7) promoter activity.

Differential effects of chronic partial sleep deprivation and stress on serotonin-1A and muscarinic acetylcholine receptor sensitivity

Disrupted sleep and stress are often linked to each other, and considered as predisposing factors for psychopathologies such as depression. The depressed brain is associated with reduced serotonergic and enhanced cholinergic neurotransmission. In an earlier study, we showed that chronic sleep restriction by forced locomotion caused a gradual decrease in postsynaptic serotonin-1A receptor sensitivity, whilst chronic forced activity alone, with sufficient sleep time, did not affect receptor sensitivity. The first aim of the present study was to examine whether the sleep loss-induced change in receptor sensitivity is mediated by adrenal stress hormones. The results show that the serotonin-1A receptor desensitization is independent of adrenal hormones as it still occurs in adrenalectomized rats. The second aim of the study was to establish the effects of sleep restriction on cholinergic muscarinic receptor sensitivity. While sleep restriction affected muscarinic receptor sensitivity only slightly, forced activity significantly hypersensitized the muscarinic receptors. This hypersensitization is because of the stressful nature of the forced activity protocol as it did not occur in adrenalectomized rats. Taken together, these data confirm that sleep restriction may desensitize the serotonin-1A receptor system. This is not a generalized effect as sleep restriction did not affect the sensitivity of the muscarinic cholinergic receptor system, but the latter was hypersensitized by stress. Thus, chronic stress and sleep loss may, partly via different pathways, change the brain into a direction as it is seen in mood disorders.

Effects of the 5-HT7 receptor antagonist SB-269970 on rat hormonal and temperature responses to the 5-HT1A/7 receptor agonist 8-OH-DPAT

The physiological function of 5-HT(7) receptors is not yet fully determined. This study was designed to characterize the involvement of 5-HT(7) receptor in rat body temperature regulation and in adrenocorticotropic hormone (ACTH) and corticosterone secretion. In the first part of our study, acute administration of SB-269970 (0.1-1 mg/kg, i.p.), a potent and selective 5-HT(7) receptors antagonist, dose-dependently prevented 5-HT(1A/7) receptor agonist 8-OH-DPAT (0.1 mg/kg, s.c.)-induced hypothermia and when the 5-HT(1A) receptor antagonist WAY-100,635 was co-injected with SB-269970, a reduction of the latter hypothermia was obtained in an additive manner. In contrast, 1 mg/kg (i.p.) of SB-269970 failed to prevent 8-OH-DPAT (0.5 mg/kg, s.c.)-induced increase of ACTH and corticosterone plasma levels. In conclusion, the present results unveil an additive effect of both 5-HT(1A) and 5-HT(7) receptors in core body temperature regulation.

Glucocorticoid interaction with aggression in non-mammalian vertebrates: reciprocal action

Socially aggressive interaction is stressful, and as such, glucocorticoids are typically secreted during aggressive interaction in a variety of vertebrates, which may both potentiate and inhibit aggression. The behavioral relationship between corticosterone and/or cortisol in non-mammalian (as well as mammalian) vertebrates is dependent on timing, magnitude, context, and coordination of physiological and behavioral responses. Chronically elevated plasma glucocorticoids reliably inhibit aggressive behavior, consistent with an evolutionarily adaptive behavioral strategy among subordinate and submissive individuals. Acute elevation of plasma glucocorticoids may either promote an actively aggressive response via action in specialized local regions of the brain such as the anterior hypothalamus, or is permissive to escalated aggression and/or activity. Although the permissive effect of glucocorticoids on aggression does not suggest an active role for the hormone, the corticosteroids may be necessary for full expression of aggressive behavior, as in the lizard Anolis carolinensis. These effects suggest that short-term stress may generally be best counteracted by an actively aggressive response, at least for socially dominant proactive individuals. An acute and active response may be evolutionarily maladaptive under chronic, uncontrollable and unpredictable circumstances. It appears that subordinate reactive individuals often produce compulsorily chronic responses that inhibit aggression and promote submissive behavior.

5-HT7 receptor subtype as a mediator of the serotonergic regulation of luteinizing hormone release in the zona incerta

5-Hydroxytryptamine (5-HT) and the 5-HT(1A/7) receptor agonist (+)-8-hydroxy-2-(di-n-propylamino) tetralinHBr (8-OH-DPAT), injected into the zona incerta (an area in the dorsal hypothalamus) of the female rat, inhibit the release of luteinizing hormone (LH) and the effects of both are blocked by the 5-HT(2/7) receptor antagonist, ritanserin. As both 8-OH-DPAT and ritanserin have moderate activity at the 5-HT7 receptor subtype, the possibility that this subtype might mediate their effects in the zona incerta has been investigated. Ovariectomised rats were primed with 5 microg oestradiol benzoate followed at 48 h by 0.5 mg progesterone, which induces an LH surge. 5-Carboxamidotryptamine (5-CT), a potent but non-selective agonist at 5-HT7 receptors, like 5-HT and 8-OH-DPAT, inhibited the LH surge at 5 and 1.25 nmol injected bilaterally into the zona incerta. The non-selective 5-HT(2/7) receptor antagonist ritanserin and the selective 5-HT7 receptor antagonist, (R)-3-(2-(2-(4-methyl-piperidin-1-yl)-pyrrolidine-1-sulfonyl)-phenol (SB-269970-A) at 0.5 microg/side blocked all three receptor agonists when injected concurrently into the zona incerta. However, lower (0.2 microg) and higher doses (2 and 5 microg) of SB-269970-A were less effective, indicating a bell-shaped dose-response curve. SB-269970-A was also inhibitory when administered systemically (1 mg/kg intraperitoneally (i.p.)). When LH release was suppressed by 5 microg oestradiol benzoate, SB-269970-A (0.5 and 2 microg) did not elevate levels, indicating it is unlikely that 5-HT7 receptors mediate a tonic inhibition on release but rather are involved in terminating the pre-ovulatory LH surge. These data demonstrate that 5-HT7 receptors play a role in the regulation of LH by the zona incerta in rat brain.

Lack of effect of a single dose of hydrocortisone on serotonin(1A) receptors in recovered depressed patients measured by positron emission tomography with [11C]WAY-100635

BACKGROUND

Elevated cortisol levels might account for the reduction in central serotonin 1A (5-hydroxytryptamine [5-HT](1A)) receptor binding and function observed in patients with major depression. We tested this hypothesis by studying the effect of acute administration of hydrocortisone on 5-HT(1A) receptor binding potential (BP) in subjects recovered from depression.

METHODS

We studied 14 subjects (8 male, 6 female) who had recovered from at least two episodes of major depression and had been euthymic and drug free for at least 6 months. Serotonin 1A receptor BP was measured by [(11)C]WAY-100635 in conjunction with positron emission tomography. Subjects were tested on two occasions in a double-blind, random-order, crossover design after administration of either hydrocortisone (100 mg orally) or placebo 12 hours previously. Positron emission tomography scans were analyzed with a region of interest analysis.

RESULTS

Hydrocortisone treatment did not decrease 5-HT(1A) receptor BP either in the hippocampus, which was our a priori hypothesis, or in other cortical 5-HT(1A) regions; however, female subjects had a higher 5-HT(1A) receptor BP in certain brain areas compared with male subjects.

CONCLUSIONS

These data are consistent with an earlier study in healthy volunteers and do not support the proposal that decreased 5-HT(1A) receptor BP in patients with acute major depression is a consequence of cortisol hypersecretion.

Serotonin and the neuroendocrine regulation of the hypothalamic--pituitary-adrenal axis in health and disease

Serotonin (5-hydroxytryptamine, 5-HT)-containing neurons in the midbrain directly innervate corticotropin-releasing hormone (CRH)-containing cells located in paraventricular nucleus of the hypothalamus. Serotonergic inputs into the paraventricular nucleus mediate the release of CRH, leading to the release of adrenocorticotropin, which triggers glucocorticoid secretion from the adrenal cortex. 5-HT1A and 5-HT2A receptors are the main receptors mediating the serotonergic stimulation of the hypothalamic-pituitary-adrenal axis. In turn, both CRH and glucocorticoids have multiple and complex effects on the serotonergic neurons. Therefore, these two systems are interwoven and communicate closely. The intimate relationship between serotonin and the hypothalamic-pituitary-adrenal axis is of great importance in normal physiology such as circadian rhythm and stress, as well as pathophysiological disorders such as depression, anxiety, eating disorders, and chronic fatigue.

Implantation of a slow release corticosterone pellet induces long-term alterations in serotonergic neurochemistry in the rat brain

Many studies point to an involvement of deficits in the serotonergic nervous system and hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis function with depression. Indeed early life stress, involving HPA axis activation, may predispose susceptible individuals to develop depression in later life. This study investigates the effects of elevating the neuroendocrine stress hormone, corticosterone, for 1 week in adolescent rats on markers of serotonergic neurone function at adulthood. Slow release corticosterone pellets were implanted for 7 days and various serotonergic parameters, as well as plasma corticosterone levels, were measured on day 7 or on day 28 (21 days following removal of the pellet). The corticosterone implant attenuated weight gain and reduced adrenal weights compared to that in control rats implanted with a cholesterol pellet. After 7 days, with the implant still in place, the diurnal variation in plasma corticosterone was reduced so that the level was approximately at that of the evening peak throughout the day. Twenty-one days after removal of the implant, the diurnal variation in plasma corticosterone returned. Corticosterone treatment decreased [3H] 8-hydroxy-2-(di-n-propylamino)tetralin binding to the 5-hydroxytryptamine1A receptor in the cortex but not in the hippocampus. Corticosterone treatment also enhanced the circadian rhythm observed in 5-hydroxyindoleacetic acid level and the ratio of 5-hydroxyindoleacetic acid to the 5-hydroxytryptamine in the frontal cortex. Despite corticosterone pellet removal 21 days earlier, there was a persistent decrease in whole body and adrenal weight, cortical 5-hydroxytryptamine1A receptor binding and an alteration in the diurnal variation in the 5-hydroxytryptamine "turnover" in the frontal cortex.

5-Hydroxytryptamine7 (5-HT7) receptor immunoreactivity-positive 'stigmoid body'-like structure in developing rat brains

We examined the expression of 5-hydroxytryptamine(7) (5-HT(7)) receptor protein in developing and adult rats with immunohistochemical technique. In adult male rats, 5-HT(7) receptor immunoreactivity was detected in the septum, striatum, indusium griseum, tenia tecta, thalamus, hippocampus and hypothalamus in the forebrain as well as the pons and cerebellum. In brains of 1, 7, 15 and 21 days old male rats but not of adult ones, 5-HT(7) receptor immunoreactivity-positive dot-like structures were detected. The dot-like structures were visualized in hypothalamus, hippocampus, frontal cortex, brainstem and cerebellum at 1 day old male rats. In 7 days old male rats, the dot-like structures were found in the hypothalamus, medial preoptic area (MPA), bed nucleus of the stria terminalis (BST), amygdaloid nucleus and brainstem reticular formation. In 15 and 21 days old male and female rats, 5-HT(7) receptor immunoreactive dots were most clearly detected in MPA, hypothalamus, raphe pallidus, raphe magnus and brainstem reticular formation. The 5-HT(7) receptor immunoreactivity-positive dot-like structures were shown in the cytoplasm and they were less than 1 microm in diameter in 1 and 7 days old rats and became larger to 1-3 microm in 15 and 21 days old rats. From the distribution and morphologic features, the 5-HT(7) receptor immunoreactivity-positive dot-like structure found in developing rat brains is considered to be identical to a cytoplasmic inclusion named 'stigmoid body'.

Rapid glucocorticoid stimulation and GABAergic inhibition of hippocampal serotonergic response: in vivo dialysis in the lizard anolis carolinensis

Central serotonin (5-HT) is activated during stressful situations and aggressive interactions in a number of species. Glucocorticoids secreted peripherally during stressful events feed back on central systems and may affect 5-HT mediation of stress-induced behavioral events. To test the neuromodulatory effect of stress hormone secretion, serotonin overflow was measured from the hippocampus of the lizard Anolis carolinensis. Microdialysis was used to collect repeated samples from anesthetized lizards, with perfusate measured by HPLC with electrochemical analysis. Following initially high levels of 5-HT, concentrations stabilized to basal levels after approximately 2 h. Intracortical infusion of 200 ng/ml corticosterone evoked transient increases in 5-HT release of approximately 400%. The effect of corticosterone on 5-HT overflow appears to be dose dependent as 20 ng/ml stimulated an increase of 200%, whereas 2 ng/ml stimulated a 50% increase. Administration of 0.1 and 1 ng/ml GABA via the dialysis probe significantly inhibited 5-HT overflow by 20 and 40%, respectively. The duration of GABA inhibition is greater than the stimulatory response for glucocorticoids. Short-lived glucocorticoid stimulation of 5-HT release suggests a possible mechanism for endocrine mediation of continuously changing social behavioral events.

An RT-PCR study of 5-HT(6) and 5-HT(7) receptor mRNAs in the hippocampal formation and prefrontal cortex in schizophrenia

5-Hydroxytryptamine (5-HT; serotonin) 5-HT(6) receptors (5-HT(6)R) and 5-HT(7) receptors (5-HT(7)R) have been implicated in schizophrenia and as targets of atypical antipsychotic drugs. We have studied the expression of these receptors in the hippocampal formation and dorsolateral prefrontal cortex (DLPFC) of 17 subjects with schizophrenia and 17 controls using reverse transcription-polymerase chain reaction (RT-PCR) with cyclophilin co-amplification. In schizophrenia, 5-HT(6)R mRNA was decreased in the hippocampal formation, and 5-HT(7)R mRNA was decreased in the dorsolateral prefrontal cortex. The mRNAs were unchanged in rats treated for 2 weeks with haloperidol, chlorpromazine, risperidone, olanzapine or clozapine. Regional decreases in 5-HT(6)R and 5-HT(7)R expression in schizophrenia may contribute to the overall serotonergic alterations which occur in the disorder, in part through their interactions with other neurotransmitter systems including glutamate and acetylcholine.

Pharmacokinetic-pharmacodynamic modelling of the hypothermic and corticosterone effects of the 5-HT1A receptor agonist flesinoxan

The current investigation describes the pharmacokinetic-pharmacodynamic correlation of the hypothermic and the corticosterone effect of flesinoxan in the rat simultaneously. A specific objective was to determine the influence of handling the animal. The pharmacokinetic-pharmacodynamic correlation was determined following intravenous administration of 3 and 10 mg/kg flesinoxan in 5 or 15 min. Serial blood samples were obtained for determination of the time course of the flesinoxan and corticosterone concentrations by high performance liquid chromatography. Body temperature was monitored using a telemetric probe. The pharmacokinetics of flesinoxan were described using a three-compartment model. Both the hypothermic and the corticosterone response were successfully described using a physiological indirect response model. It is shown that customizing the animal prior to the experiment has no influence on the pharmacokinetic-pharmacodynamic parameter estimates. Furthermore, the similarity in potency between the hypothermic and corticosterone effects suggests that both are mediated via tissues with a similar receptor-effector coupling efficiency.

Role of 5-HT1A receptors in the control of food intake in obese Zucker rats of different ages

The present study describes the role of 5-HT1A receptors in the serotonergic control of food intake in obese Zucker rats of different ages. In addition, serotonin (5-HT) and cholecystokinin (CCK) content and 5-HT turnover were determined in various brain regions. The 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 100 microg/kg) stimulated food intake in 3-month-old lean control rats but inhibited feeding in obese Zucker rats (300 microg/kg). This pattern remained the same in 6-month-old rats. At 10 months of age, 8-OH-DPAT lost its inhibitory activity in the obese rats but still stimulated feeding in lean controls (300 microg/kg). 5-HT levels were higher in the hypothalamus and in the frontal and parietal cortices of 3-month-old obese Zucker rats and were associated with a lower cortical turnover. In the parietal cortex and the hypothalamus of 6-month-old rats, 5-HT levels were still higher, linked with a lower hypothalamic turnover. No differences were observed in 10-month-old rats. CCK content was not different between obese Zucker rats and lean rats. The persistently different feeding responses to 8-OH-DPAT in obese Zucker rats and lean controls may be related to changes in brain 5-HT metabolism in the obese Zucker rats.

Alteration in 5-hydroxytryptamine agonist-induced behaviour following a corticosterone implant in adult rats

Hypercortisolism and altered serotonergic function may account for the pathological symptoms seen in depression. This study examines the impact of 4 days continuous corticosterone treatment on 5-HT agonist-induced behaviour to delineate changes in 5-HT receptor function in the adult rat. The flat body posture, reciprocal forepaw treading, elevated corticosterone, hyperglycaemia, hypothermia and reduced hippocampal 5-HT induced by the 5-HT(1A) agonist 8-OHDPAT (0.3 mg/kg ip) were all significantly attenuated by the corticosterone implant. The elevation in plasma corticosterone and back muscle contractions evoked by the 5-HT(2A) agonist DOI (1 mg/kg ip) were attenuated, whilst wet-dog shakes were enhanced by corticosterone treatment. 5-HT(2B) agonist-induced behaviour and the hypolocomotion and hypophagia induced by the 5-HT(2C) agonist m-CPP (2.5 mg/kg ip) were unaltered but the mCPP-induced elevation in corticosterone was abolished by corticosterone treatment. Hypothalamic 5-HT receptors mediating corticosterone- and 5-HT(1A) receptors, whether on serotonergic nerve terminals or postsynaptic neurones, were downregulated by corticosterone treatment. In contrast, 5-HT(2A) receptors may be up- or downregulated dependent on whether they are on supraspinal or spinal neurones, respectively. A comparison of the brain region-dependent alteration in serotonergic function produced by hypercorticosterone in the rat with that seen in depression is discussed.

Effects of the 5-HT(7) receptor antagonist SB-258741 in animal models for schizophrenia

The 5-HT(7) receptor is targeted by several new antipsychotics such as clozapine and risperidone. We studied the effect of R-(+)-1-(toluene-3-sulfonyl)-2-[2-(4-methylpiperidin-1-yl)ethyl]-pyrrolidine (SB-258741), a specific 5-HT(7) receptor antagonist, in three models for positive symptoms, D-amphetamine-induced hyperactivity and D-amphetamine- and phencyclidine (PCP)-disrupted prepulse inhibition (PPI) in rats, with the aim of investigating the role of this receptor in the clinical effect of antipsychotics. We also tested this compound in a model for negative symptoms, PCP-disrupted social interaction (SIT) in rats. Induction of side effects by this compound was evaluated by testing its potency to reduce spontaneous motility and to induce catalepsy in rats. The effect of SB-258741 was compared to risperidone in all models. This study showed that SB-258741 had no beneficial effect on PCP-disrupted SIT. SB-258741 did not reverse D-amphetamine-disrupted PPI; however, it dose-dependently normalised PCP-disrupted PPI. SB-258741 antagonised D-amphetamine-induced hyperactivity but reduced motility of rats at similar doses. Thus, this specific 5-HT(7) receptor antagonist brought a clear positive outcome in only one model for positive symptoms of schizophrenia and had no beneficial effect in the model for negative symptoms. Consequently, it is clear that SB-258741 affects the PPI phenomenon but is not expected to have an antipsychotic effect on its own in clinic.

SB-258741: a 5-HT7 receptor antagonist of potential clinical interest

Recently, a series of 5-HT7 receptor antagonists have been developed (24,29,36,68). Among them SB-258741, R-(+)-1-(toluene-3-sulfonyl)-2-[2-(4-methylpiperidin-1-yl)ethyl]-pyrrolidine, (compound "13" in 36,37) was one of the most potent and specific compounds. Due to a lack of specific ligands the pharmacology of 5-HT7 receptor antagonists is still relatively unexplored. It has been suggested, however, that 5-HT7 receptor ligands could be useful in the therapy of various disorders such as sleep disorders, schizophrenia, depression, migraine, epilepsy, pain, or memory impairment. Many of these conceivable indications are not supported by pharmacological data. It is, therefore, of particular interest to review the data generated from studies of one of these most potent and specific 5-HT7 receptor antagonists, SB-258741, with a goal of testing the validity of the proposed clinical indications. In this review, the author describes pharmacology of this compound in order to define its potential clinical use. The available safety pharmacology data are discussed in an attempt to predict potential side effects of specific 5-HT7 receptor antagonists.

Hippocampal serotonin responses in short and long attack latency mice

Short and long attack latency mice, which are selected based on their offensive behaviour in a resident-intruder model, differ in their neuroendocrine regulation as well as in aspects of their brain serotonin system. Previous studies showed that the binding capacity and expression of serotonin-1A receptors in the hippocampal CA1 field of long attack latency mice are significantly lower than that found in short attack latency mice. We tested whether the functional responses of CA1 hippocampal cells to serotonin are also reduced in long attack latency mice. To this end, serotonin-induced changes in the membrane potential and input resistance were recorded in vitro with microelectrodes in CA1 pyramidal neurones of long and short attack latency mice. The data show that in long attack latency mice, along with a reduction of the serotonin-1A receptor mRNA expression, the serotonin-induced membrane hyperpolarization and decrease in resistance are attenuated. Basal membrane properties of CA1 neurones in the two.mice lines were comparable. Plasma corticosterone levels in response to a novelty stress were elevated in long compared to short attack latency mice and inversely related to the serotonin-induced responses. We tentatively conclude that long attack latency mice show attenuated functional responses to serotonin in the hippocampus, possibly linked to a chronic perturbation of hormonal levels.

Acute restraint stress increases 5-HT7 receptor mRNA expression in the rat hippocampus

The brain serotonin (5-HT) system interacts closely with the hypothalamic-pituitary-adrenal axis. We examined the effects of stress on hippocampal 5-HT7 receptor and corticosteroid receptor (mineralocorticoid receptor (MR) and glucocorticoid receptor (GR)) mRNA expression measured by in situ hybridisation histochemistry. Acute restraint stress increased 5-HT7 receptor mRNA in CA2 and CA3 hippocampal subregions (32% increase) and had a small effect on GR but not MR mRNA (19% fall in GR in CA1). In contrast, chronic stress (1 week of variable stressors) had little effect on hippocampal 5-HT7 receptor mRNA (9% rise in CA3) but decreased MR mRNA (e.g. 34% decrease in CA2) and GR mRNA expression selectively in the dentate gyrus (26% decrease). The rise in 5-HT7 receptor mRNA expression following restraint stress parallels our previously reported increase in expression after inhibiting the synthesis of adrenal steroids. These data suggest that acute but not chronic stress regulates 5-HT7 receptor mRNA expression in a manner that is likely to be independent of its glucocorticoid actions.

Role of 5-HT(1A) and 5-HT(7) receptors in the facilitatory response induced by 8-OH-DPAT on learning consolidation

The present study further explored the mechanisms involved in the facilitatory effect induced by (+/-)-8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) on learning consolidation. For this purpose, we analyzed in parallel the effects of LY215840 and ritanserin, two 5-HT(2) receptor antagonists with high affinity for the 5-HT(7) receptor, and WAY100635, a selective 5-HT(1A) receptor antagonist, on the facilitatory effect induced by 8-OH-DPAT on learning consolidation. We also determined whether LY215840 and/or ritanserin could be beneficial in restoring a deficient learning condition. Using the model of autoshaping task, post-training injection of LY215840 or WAY100635 had no effect on learning consolidation. However, both drugs abolished the enhancing effect of 8-OH-DPAT, with LY215840 being slightly more effective than WAY100635 in this respect. Ritanserin produced an increase in performance by itself and also abolished the effect of 8-OH-DPAT. Remarkably, selective blockade of 5-HT(2A) and 5-HT(2B/2C) receptors with MDL100907 and SB200646, respectively, failed to alter the 8-OH-DPAT effect. LY215840 and ritanserin, at the doses that inhibited the 8-OH-DPAT-induced response, reversed the learning deficits induced by scopolamine and dizocilpine. The present results suggest that the enhancing effect produced by 8-OH-DPAT on learning consolidation involves activation of 5-HT(1A) receptors and an additional mechanism, probably related to the 5-HT(7) receptor. Blockade of 5-HT(2) receptors, and perhaps of 5-HT(7) receptors as well, may provide some benefit in reversing learning deficits associated with decreased cholinergic and/or glutamatergic neurotransmission.

Corticosteroid effects on serotonin responses in granule cells of the rat dentate gyrus

Granule cells in the rat dentate gyrus contain mineralocorticoid and glucocorticoid receptors to which the adrenal hormone corticosterone binds with differential affinity. These cells also express various receptor-subtypes for serotonin (5-HT), including the 5-HT1A receptor which mediates a membrane hyperpolarization accompanied by a decrease in membrane resistance. Earlier studies have shown that removal of corticosterone by adrenalectomy, particularly in the dentate gyrus, results in enhanced expression of the 5-HT1A receptor mRNA and increased 5-HT1A receptor binding capacity. This was normalized by activation of mineralocorticoid receptors or concurrent activation of both receptor types. In the present, intracellular recording study in vitro, we examined if the altered levels of 5-HT1A receptor mRNA and protein are associated with changes in the response to 5-HT. We found that the hyperpolarization and resistance decrease induced in granule cells by a submaximal (10 microM) dose of 5-HT were unaltered 2-4 days after adrenalectomy, indicating a dissociation between corticosteroid actions on 5-HT1A receptor mRNA/protein levels and functional responses to 5-HT. Subsequent occupation of mineralocorticoid receptors in vitro significantly suppressed the 5-HT induced change in resistance, 1-4 h after steroid application. Compared to this, concurrent activation of glucocorticoid receptors led to large responses to 5-HT. This modulation by steroids was not observed with a higher dose of 5-HT (30 microM). The data suggest that with moderate amounts of 5-HT, corticosteroids affect the information flow through the dentate gyrus such that excitatory transmission is promoted with predominant mineralocorticoid receptor activation and attenuated with additional glucocorticoid receptor occupation.

Serotonergic gene expression and depression: implications for developing novel antidepressants

The development and configuration of several neural networks is dependent on the actions of serotonin (5-HT) acting through multiple hetero- and autoreceptor subtypes. During early brain development 5-HT modulates morphogenetic activities, such as neural differentiation, axon outgrowth, and synaptic modeling. In the adult brain, midbrain raphe serotonergic neurons project to a variety of brain regions and modulate a wide range of physiological functions. Several lines of evidence indicate that genetically determined variability in serotonergic gene expression, as it has been documented for the 5-HT transporter, influences temperamental traits and may lead to psychopathological conditions with increased anxiety, depression, and aggression. Investigation of the regulation of serotonergic gene transcription and its impact on neuronal development, synaptic plasticity, and neurogenesis spur interest to identify serotonergic gene-related molecular factors underlying disease states and to develop more effective antidepressant treatment strategies. Gene targeting strategies have increasingly been integrated into investigations of brain function and along with the fading dogma of a limited capacity of neurons for regeneration and reproducibility, it is realized that gene transfer techniques using efficient viral vectors in conjunction with neuron-selective transcriptional control systems may also be applicable to complex disorders of the brain. Given the fact that the 5-HT system continues to be an important target for drug development and production, novel strategies aiming toward the modification of 5-HT function at the level of gene expression are likely to be exploited by enterprises participating actively in the introduction of alternative therapeutic approaches.

Localization of 5-HT(7) receptors in rat brain by immunocytochemistry, in situ hybridization, and agonist stimulated cFos expression

5-HT(7) receptors are recently identified members of the serotonin receptor family that have moderate to high affinity for several important psychotropic drugs. However, the lack of selective ligands has impeded the study of the brain distribution of these receptors. In this report, we describe the localization of 5-HT(7) receptor in rat forebrain by immunocytochemistry, in situ hybridization of 5-HT(7) mRNA, and functional stimulation of cFOS expression by 5-HT(7) receptor activation. The anatomical localization of 5-HT(7) mRNA in situ hybridization signal. Prominent immunostaining was apparent in numerous sites within the cerebral cortex, hippocampal formation, tenia tecta, thalamus and hypothalamus. 5-HT(7) receptors were detected in suprachiasmatic nucleus by both immunocytochemistry and in situ hybridization. At a microscopic level, both cell bodies and proximal fibers were strongly stained in these regions, suggesting a somatodendritic subcellular distribution. 5-HT(7) receptor-like immunoreactivity was further compared with 5-HT(7) mediated biological function by administering 8-OH-DPAT intracerebroventricular injection (icv)with WAY 100135 (to block 5-HT(1A) receptors) followed by double immunostaining localization of cFos activation and 5-HT(7) receptors. In all regions examined, cFos stimulation and 5-HT(7)-like immunoreactivity colocalized to the same neurons. Furthermore, cFos activation by 8-OH-DPAT was blocked by pimozide--a 5-HT(7) antagonist. Therefore, by using multiple strategies, we were able to localize 5-HT(7) receptors in rat brain unequivocally. The distribution of these receptors is consistent with their involvement in the control of circadian activity and the action of anti-depressants and atypical neuroleptics.

Corticosteroids regulate 5-HT(1A) but not 5-HT(1B) receptor mRNA in rat hippocampus

The role of mineralocorticoid and glucocorticoid receptors (MR and GR, respectively) in the regulation of serotonin receptors has not been clearly delineated. There is no consensus regarding the regulation of 5-HT(1A) receptors, and corticosteroid regulation of 5-HT(1B) mRNA has not been previously studied. We compared the effects of long-term (two week) adrenalectomy (no MR or GR activation) and several hormone replacement protocols designed to stimulate MR selectively (ALDO), MR and GR (HCT), and continuous MR with cyclical GR activation (SHAM adrenalectomy). 5-HT(1A) and 5-HT(1B) mRNAs were measured by in situ hybridization in hippocampus and raphe nuclei. None of the experimental manipulations altered 5-HT(1B) mRNA levels in the hippocampus or dorsal raphe, and also had no effect on 5-HT(1A) mRNA in dorsal or median raphe. However, 5-HT(1A) mRNA levels were regulated in a complex manner in the different subfields of hippocampus. We conclude that both MR and GR play an integrated role in regulating 5-HT(1A) mRNA levels in hippocampus while having no effect on 5-HT(1B) mRNA levels under these conditions.

Changes in 5-HT(7) serotonin receptor mRNA expression with aging in rat brain

We examined 5-HT(7) receptor mRNA expression with in situ hybridization histochemistry in the brains of young (3 months), middle-aged (12 months) and old rats (24 months). In the ventral CA3 area of the hippocampus 5-HT(7) mRNA expression is reduced by approximately 30% between young and middle age without further decline between middle and old age. In other brain areas 5-HT(7) mRNA expression is unaffected by age.

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.

Effect of p-chloroamphetamine on 5-HT1A and 5-HT7 serotonin receptor expression in rat brain

The aim of this study was to investigate if p-chloroamphetamine (PCA), which is neurotoxic to serotonin (5-HT) nerve terminals, was able to induce, like 3,4-methylenedioxymethamphetamine, a region-specific regulation of 5-HT1A receptor mRNA expression. The effect of PCA on the expression of 5-HT7 receptors, which share some pharmacological properties with 5-HT1A receptors, was comparatively studied. PCA (2 x 5 mg/kg) produced a lasting depletion of 5-HT content in the rat frontal cortex and hippocampus. In the hippocampus, the maximal 5-HT depletion was found on day 21 (-70%), whereas in the cortex, the highest 5-HT depletion was found on day 14 (-73%), with a partial but significant recovery on day 21. At the latter time point, 5-HT1A receptor mRNA expression was increased by 80% in the cortex and decreased by 50% in the hippocampus. The 5-HT1A receptor mRNA expression was also enhanced after exposure to PCA of rat cortical but not of hippocampal primary cultures. In regard to 5-HT7 receptor mRNA expression, the most remarkable change after PCA was the great increase (+200%) in the brain-stem. Binding studies to 5-HT1A receptors matched the changes in receptor mRNA expression. Gel shift assays revealed enhanced nuclear protein binding to the KB sequence with use of cortical but not hippocampal extracts of PCA-treated rats. Overall, the data show region-specific changes in 5-HT receptor-type expression that may not be entirely dependent on the neurotoxic effect of PCA on 5-HT terminals.

5-HT7 receptors: current knowledge and future prospects

Identification of three splice variants of the 5-HT7 receptor suggests a possible diversity in 5-HT7 receptor action. Indeed, 5-HT7 receptors have been implicated in the pathophysiology of several disorders; they play a role in smooth muscle relaxation within the vasculature and in the gastrointestinal tract. However, most of these assignments are derived from receptor localization studies and investigations using nonselective ligands, and are therefore mainly suggestive. The development of selective 5-HT7 receptor antagonists will be of utmost importance in determining the actual physiological and pharmacological roles of this receptor. Major challenges of 5-HT7 receptor research are determination of the transcriptional regulation of the gene encoding the 5-HT7 receptor and elucidation of the differences in regulation and signalling of its four gene products.

Expression of 5-HT receptors and the 5-HT transporter in rat brain after electroconvulsive shock

Modulation of central 5-HT receptor sensitivity is implicated in the therapeutic response to electroconvulsive shock (ECS). Altered 5-HT receptor expression may play a role in this process. We have measured the mRNAs encoding 5-HT1A, 5-HT2A, 5-HT2C and 5-HT7 receptors, and the 5-HT transporter, in rat brain after single ECS, repeated ECS, and 3 weeks after repeated ECS. Hippocampal 5-HT1A receptor mRNA was decreased in CA4 and increased in dentate gyrus by single or repeated ECS, with parallel alterations in [3H]8-OH-DPAT binding site densities. Repeated ECS increased cortical [3H]ketanserin binding and 5-HT2A receptor mRNA. The other mRNAs were unchanged. The results show that ECS has subtype specific, anatomically discrete, and temporally selective effects on 5-HT receptor expression.

A review of central 5-HT receptors and their function

It is now nearly 5 years since the last of the currently recognised 5-HT receptors was identified in terms of its cDNA sequence. Over this period, much effort has been directed towards understanding the function attributable to individual 5-HT receptors in the brain. This has been helped, in part, by the synthesis of a number of compounds that selectively interact with individual 5-HT receptor subtypes--although some 5-HT receptors still lack any selective ligands (e.g. 5-ht1E, 5-ht5A and 5-ht5B receptors). The present review provides background information for each 5-HT receptor subtype and subsequently reviews in more detail the functional responses attributed to each receptor in the brain. Clearly this latter area has moved forward in recent years and this progression is likely to continue given the level of interest associated with the actions of 5-HT. This interest is stimulated by the belief that pharmacological manipulation of the central 5-HT system will have therapeutic potential. In support of which, a number of 5-HT receptor ligands are currently utilised, or are in clinical development, to reduce the symptoms of CNS dysfunction.

Repeated administration of dexamethasone increases phosphoinositide-specific phospholipase C activity and mRNA and protein expression of the phospholipase C beta 1 isozyme in rat brain

Altered hypothalamic-pituitary-adrenal (HPA) function has been shown to be associated with changes in mood and behavior. The enzyme phosphoinositide-specific phospholipase C (PI-PLC), an important component of the PI signal transduction system, plays a major role in mediating various physiological functions. In the present study, we investigated the effects of a single dose and of repeated administration (0.5 or 1.0 mg/kg for 10 days) of dexamethasone (DEX), a synthetic glucocorticoid, on PI-PLC activity and on expression of PLC isozymes (beta1, delta1, and gamma1) in rat brain. Repeated administration of DEX (1.0 mg/kg) caused a significant increase in PI-PLC activity and in protein expression of the PLC beta1 isozyme in both membrane and cytosol fractions of cortex and hippocampus; however, the repeated administration of a smaller dose of DEX (0.5 mg/kg) caused these changes only in hippocampus but not in cortex. The increase in PLC beta1 protein was associated with an increase in its mRNA level, as measured by competitive RT-PCR. A single administration of DEX (0.5 or 1.0 mg/kg) to rats had no significant effects on PI-PLC activity or on the protein expression of PLC isozymes. These results suggest that DEX up-regulates PI-PLC in rat brain, which presumably is due to a selective increase in expression of the PLC beta1 isozyme, and that these changes in PI-PLC may be related to HPA axis-mediated changes in mood and behavior.

Effects of chronic administration of interferon alpha A/D on serotonergic receptors in rat brain

The effects of chronic administration of interferon (IFN; recombinant human IFN-alphaA/D) on serotonergic binding sites in rat brain were investigated. IFN was injected daily for 2 weeks at a dose of 100000 I.U./kg, (i.p.) in male Wistar rats. IFN did not alter either [3H]ketanserin binding to 5-HT2A receptors or [3H]paroxetine binding to 5-HT transporters. Scatchard analysis of [3H]8-hydroxy-dipropylaminotetraline (8-OH-DPAT) binding to 5-HT1A receptors demonstrated the presence of high- and low-affinity binding sites in both treatment and control groups. IFN significantly increased both Kd and Bmax measures of [3H]8-OH-DPAT binding at low-affinity binding sites, but not at the high-affinity sites. These results suggest that IFN affects the low-affinity 5-HT1A receptors sites and may be involved in the development of IFN-induced psychiatric disturbances.

Administration of dexamethasone up-regulates protein kinase C activity and the expression of gamma and epsilon protein kinase C isozymes in the rat brain

Altered hypothalamic-pituitary-adrenal (HPA) function (increased plasma cortisol level) has been shown to be associated with mood and behavior. Protein kinase C (PKC), an important component of the phosphatidyl-inositol signal transduction system, plays a major role in mediating various physiological functions. The present study investigates the effects of acute (single) and repeated (10-day) administrations of 0.5 or 1.0 mg/kg doses of dexamethasone (DEX), a synthetic glucocorticoid, on Bmax and KD of [3H]phorbol 12,13-dibutyrate ([3H]PDBu) binding, PKC activity, and protein expression of PKC isozymes alpha, beta, gamma, delta, and epsilon in the membrane and the cytosolic fractions of rat cortex and hippocampus. It was observed that repeated administration of 1.0 mg/kg DEX for 10 days caused a significant increase in Bmax of [3H]PDBu binding to PKC, in PKC activity, and in expressed protein levels of the gamma and epsilon isozymes in both the cytosolic and the membrane fractions of the cortex and the hippocampus, whereas a lower dose of DEX (0.5 mg/kg for 10 days) caused these changes only in the hippocampus. On the other hand, a single administration of DEX (0.5 or 1.0 mg/kg) had no significant effect on PKC in the cortex or in the hippocampus. These results suggest that alterations in HPA function from repeated administration of glucocorticoids may modulate PKC-mediated functions.

Localization of 5-hydroxytryptamine1A and 5-hydroxytryptamine7 receptors in rabbit ocular and brain tissues

Serotonin is thought to play a physiological role in various tissues of the rabbit eye, yet little is known about the relative distribution of the different serotonin receptors. Demonstration of the receptor subtypes present in the various ocular tissues is essential in order to understand the function of serotonin in the eye. Using a combination of in situ hybridization histochemistry, in vitro receptor autoradiography and polymerase chain reaction studies, we have explored the distribution of the 5-hydroxytryptamine1A and 5-hydroxytryptamine7 receptors in the rabbit eye. As these receptors have not been sequenced in the rabbit, we initially established the suitability of the oligonucleotide probes by analysis of brain tissue. The distributions of 5-hydroxytryptamine1A and 5-hydroxytryptamine7 receptor messenger RNAs in rabbit brain correlated well with those in other species, confirming the specificity of the probes for detection of the messenger RNAs in rabbit tissues. In the eye, the expression of 5-hydroxytryptamine1A receptors appears to be restricted to the epithelial cell layer of the ciliary processes, although very low levels may appear in the retina. In contrast, the expression of 5-hydroxytryptamine7 receptor messenger RNA is more widespread with positive signals evident in the ciliary processes, retina and iris. The results confirm the existence of 5-hydroxytryptamine1A receptors in the ciliary body and their localization in the ciliary epithelium supports the hypothesis that they are involved in the secretion of aqueous humour. Unexpectedly, there was little evidence to support the idea that 5-hydroxytryptamine1A receptors are present in the retina and iris sphincter. However, the subsequent finding of 5-hydroxytryptamine7 receptor messenger RNA in the retina and iris may explain the apparent absence of 5-hydroxytryptamine1A receptors in these tissues. The presence of both 5-hydroxytryptamine1A and 5-hydroxytryptamine7 receptors in the ciliary processes may account for the complex intraocular pressure response of the rabbit to serotonin.

The molecular biology of serotonin receptors: therapeutic implications for the interface of mood and psychosis

This review summarizes the molecular biology of serotonin (5-HT; 5-hydroxytryptamine) receptors and indicates the potential relevance of this information for the treatment of mood and psychotic disorders. At least 15 separate subtypes of 5-HT receptors have been identified by molecular cloning techniques to be distinct genetic entities. Subtle differences in the primary amino acid sequences of these receptors can yield large differences in ligand selectivity. Additionally, it has recently been discovered that drugs such as atypical antipsychotic drugs and serotonin-selective reuptake inhibitors may interact with a large number of heretofore unknown 5-HT receptors. Thus clozapine, for instance, has high affinity for at least four separate 5-HT receptors, and it is unknown which of these receptors is essential for its unique therapeutic efficacy. One way to approach these questions is to test subtype-selective agents, although there are few of these currently available. Approaches to the design of subtype-selective ligands are described, including structure-based drug design and combinatorial approaches. Modes of regulation of 5-HT receptors are also summarized, and it is emphasized that antipsychotic drugs and antidepressants likely exert their effects via nontranscriptional and posttranslational means. Understanding the cellular mechanisms by which 5-HT receptors are regulated by psychopharmacologic agents is likely to yield novel insights into drug action.

Adrenalectomy-induced neuronal degeneration

The binding of glucocorticoids to CNS receptors results in the modulation of many processes, ranging from neurotransmission to cell birth and death. It is of no surprise, therefore, that the removal of these steroids following adrenalectomy disrupts a variety of physiological functions throughout the brain. It is the aim of this review to briefly describe the findings of research examining some of these glucocorticoid-mediated CNS effects; however, as many of these areas have been reviewed extensively by others, this review will focus on the recently described phenomenon, adrenalectomy-induced hippocampal cell death.