Effects of adrenalectomy and corticosterone replacement on diurnal [3H]citalopram binding in rat midbrain

Nongenomic, glucocorticoid receptor-mediated regulation of serotonin transporter cell surface expression in embryonic stem cell derived serotonergic neurons

Depressive disorders have been linked to the combined dysregulation of the hypothalamus-pituitary-adrenal (HPA)-axis and the serotonergic system. The HPA-axis and serotonergic (5-HT) neurons exert reciprocal regulatory actions. It has been reported that glucocorticoid-glucocorticoid receptor (GR) signaling influences serotonin transporter (5-HTT) transcription but data also points to the fact that 5-HTT expression is regulated nongenomically via redistribution of 5-HTT from the cell surface into intracellular compartments. In order to analyze the acute effects of glucocorticoids on 5-HTT cell surface localization we differentiated serotonergic neurons from mouse embryonic stem (ES) cells derived from the C57BL/6N blastocysts. These postmitotic 5-HT neurons express all relevant serotonergic markers following the application of a growth factor-based differentiation protocol. Increasing concentrations of the GR agonist dexamethasone (Dex) resulted in enhanced, dose-dependent 5-HTT cell surface localization in the presence of the protein synthesis inhibitor cycloheximide already 1h after incubation. Inhibition of GR function by the specific GR-antagonist mifepristone abolished the increase in 5-HTT cell surface localization. Hence, our data account for a nongenomic upregulation of 5-HTT cell surface expression by glucocorticoid-GR interaction which likely constitutes a rapid physiological response to increased levels of glucocorticoids as seen during stress. Taken together, we provide a cellular model to analyze and dissect glucocorticoid-5HTT interactions on a molecular level that corresponds to in vivo animal models using C57BL/6N mice.

Arterial 5-hydroxytryptamine transporter function is impaired in deoxycorticosterone acetate and Nomega-nitro-L-arginine but not spontaneously hypertensive rats

We reported upregulation of the 5-hydroxytryptamine (HT) transporter (5-HTT) protein in peripheral arteries from deoxycorticosterone acetate (DOCA)-salt hypertensive rats. We hypothesized that upregulated 5-HTT may be generally elevated in hypertensive models and, as a consequence, a higher basal concentration of 5-HT, the 5-HT metabolite 5-hydroxyindoleacetic acid, and an increased 5-HT uptake would occur in peripheral arteries of hypertensive rats compared with normotensive rats. We examined 3 hypertension models: DOCA-salt rats, Nomega-nitro-L-arginine (LNNA) rats, and spontaneously hypertensive rats (SHRs) in our study (systolic blood pressure [mm Hg]: DOCA (D)=197+/-6, SHAM(D)=112+/-4, LNNA (L)=228+/-9, SHAM(L)=128+/-2, SHR=172+/-7, and Wistar-Kyoto [WKY]= 121+/-3). High-pressure liquid chromatography measurements showed lower basal 5-HT concentrations in aorta from DOCA-salt and LNNA rats compared with their SHAM rats but not in SHR compared with WKY. In all of the 5-HT-uptake studies, we used arteries isolated from rats treated with the monoamine oxidase-A inhibitor pargyline to minimize 5-HT metabolism. Exogenous 5-HT was taken up by aorta, and this was inhibited by the 5-HTT inhibitor fluoxetine (1 micromol/L) or fluvoxamine (1 micromol/L). Total 5-HT uptake and 5-HTT-dependent active 5-HT uptake were decreased in aorta from DOCA-salt and LNNA rats compared with SHAM rats, but this was not observed in SHRs compared with WKYs. Western analysis revealed similar expression of 5-HTT in aorta from WKYs and SHRs as opposed to an upregulated 5-HTT in aorta from DOCA-salt and LNNA-hypertensive rats. Our study suggested that an altered serotonergic system by impaired 5-HTT function might play a role in blood pressure regulation in DOCA-salt and LNNA-hypertensive rats.

Treatment enhances ultradian rhythms of CSF monoamine metabolites in patients with major depressive episodes

Unipolar and bipolar depressions show abnormal behavioral manifestations of ultradian (less than 24 h) rhythms, but abnormal rhythms of the central neurotransmitters thought to be important for depression pathophysiology (eg dopamine (DA) and serotonin (5-HT)) have not been shown in this time frame. Since antidepressant treatments normalize disrupted rhythms in depression (eg rapid-eye-movement sleep and hormonal rhythms), we hypothesized that depression-related changes in ultradian oscillations of DA and 5-HT might be revealed during antidepressant treatment. Cerebrospinal fluid (CSF) samples collected q10 min for 24 h in 13 patients experiencing major depressive episodes (MDE) before and after treatment for 5 weeks with sertraline or bupropion were assayed for levels of homovanillic acid (HVA) and 5-hydroxyindoleacetic acid (5-HIAA), and their ratio was calculated. Data were analyzed in the frequency domain using Fourier transforms and multivariate permutation testing. Antidepressant treatments were associated with decreased variance for 5-HIAA, increased variance for HVA, and markedly increased variance for the HVA : 5-HIAA ratio (p<0.05, p<0.02, and p<0.003, respectively). With treatment, the correlations between 5-HIAA and HVA weakened (p=0.06). Power spectral density (PSD-the Fourier magnitude squared) of the 5-HIAA signals at periods of 1.75 and 3.7 h (both p<0.05) decreased, while circadian cycling of HVA levels (p<0.05) and of the ratio (p<0.005) increased after treatment. The PSD of the full-length HVA : 5-HIAA ratio series after treatment increased in rapid variability (20-103 min periods, p<0.05). Spectrographic windowing demonstrated a focal span of enhanced HVA : 5-HIAA ratio variability following antidepressant treatment, in an approximately 84-min period through the evening (p<0.05). Periodic neurotransmitter relationships in depressed patients were altered by treatment in this analysis of a small data set. This may represent a baseline abnormality in the regulation of periodic functions involved in the depression pathophysiology, but it could also be due to an unrelated antidepressant effect. Further studies including comparisons with healthy subject data are in progress.

Chronopharmacological Study of Antidepressants in Forced Swimming Test of Mice

The influence of dosing time on the anti-immobility effect of antidepressants and mechanisms underlying this phenomenon were investigated in mice. In the forced swimming test (FST), the immobility time of mice treated with amitriptyline (15 mg/kg) and fluvoxamine (30 mg/kg) showed a significant 24-h rhythm. The anti-immobility effect of fluvoxamine in FST was potent at the early part of the dark phase without increasing locomotor activity. Concerning pharmacokinetics, although K(e) of fluvoxamine was approximately 1.3-fold higher in mice injected with fluvoxamine at 9:00 PM than at 9:00 AM, no dosing time dependence was demonstrated for either plasma or brain fluvoxamine concentration at 0.5 h after the drug injection. On the other hand, serotonin transporter (SERT) mRNA expression and 5-hydroxytryptamine (5-HT) uptake activity in the mouse midbrain showed significant time-dependent changes with higher levels during the dark phase and lower levels during the light phase. These results suggest that the reuptake of 5-HT might be more increased during the dark phase. Since the reuptake of 5-HT is inhibited almost completely by injection with 30 mg/kg fluvoxamine at any time, the extracellular 5-HT level may be more increased by the injection of fluvoxamine at the early part of the dark phase. The present results suggest that the anti-immobility effect of fluvoxamine in FST increases depending on dosing time. Furthermore, the time-dependent change of SERT mRNA expression and uptake activity in the midbrain is suggested to be the mechanism underlying the 24-h rhythm of anti-immobility effect of fluvoxamine.

Moderate differences in circulating corticosterone alter receptor-mediated regulation of 5-hydroxytryptamine neuronal activity

Circulating glucocorticoid levels vary with stress and psychiatric illness and play a potentially important role in regulating transmitter systems that regulate mood. To determine whether chronic variation in corticosterone levels within the normal diurnal range altered the control of 5-hydroxytryptamine (5-HT) neuronal activity, male rats were adrenalectomized and implanted with either a 2% or 70% corticosterone/cholesterol pellet (100 mg). Two weeks later, the regulation of 5-HT neuronal activity in the dorsal raphe nucleus was studied by in vitro electrophysiology. At this time, serum corticosterone levels approximated the low-point (2%) and mid-point (70%) of the diurnal range. The excitatory response of 5-HT neurones to the alpha1-adrenoceptor agonist phenylephrine (1-11 microM) was significantly greater in the 2% group compared to the 70% group. By contrast, the inhibitory response to 5-HT (10-50 microM) was significantly lower in the 2% group compared to the 70% group. Thus, chronic variation in circulating corticosterone over a narrow part of the normal diurnal range causes a shift in the balance of positive and negative regulation of 5-HT neurones, with increased alpha 1-adrenoceptor-mediated excitation and reduced 5-HT-mediated autoinhibition at lower corticosterone levels. This shift would have a major impact on control of 5-HT neuronal activity.

Altered glucocorticoid rhythm attenuates the ability of a chronic SSRI to elevate forebrain 5-HT: implications for the treatment of depression

Both glucocorticoids and selective serotonin reuptake inhibitors (SSRIs) alter aspects of 5-HT function including somatodendritic 5-HT1A autoreceptor sensitivity. Many depressed patients prescribed SSRIs have pre-existing flattened diurnal gluococorticoid rhythm. In these patients, interactions between flattened glucocorticoid rhythm and chronic SSRIs, which impact on the SSRI's ability to elevate forebrain 5-HT, may alter clinical efficacy. To address this issue rats underwent implantation of slow-release corticosterone (75 mg pellet s.c.) (to flatten the glucocorticoid rhythm) or sham surgery, and injection of fluoxetine (10 mg/kg/day i.p., 12 days) or vehicle. Using microdialysis in the frontal cortex we found that (21 h after the last injection) extracellular 5-HT was elevated in fluoxetine- or corticosterone-treated animals, but not in those treated with corticosterone plus fluoxetine. In fluoxetine-treated animals, blockade of terminal reuptake by local perfusion of fluoxetine increased 5-HT to the same level as it did in controls, suggesting normal terminal 5-HT release after chronic fluoxetine. However, 5-HT levels following local reuptake blockade in both the corticosterone and corticosterone plus fluoxetine groups were lower than controls, suggesting a corticosterone-induced decrease in terminal release. Finally in fluoxetine, corticosterone, and corticosterone plus fluoxetine groups, there was marked 5-HT1A receptor desensitization, evidenced by attenuation of the decrease in 5-HT release following systemic fluoxetine injection. The data indicate that, despite desensitization of 5-HT1A autoreceptors, concurrent flattened glucocorticoid rhythm compromises the ability of SSRIs to elevate forebrain 5-HT. These findings suggest a potential mechanism for the reduced antidepressant efficacy of SSRIs in those patients with pre-existing glucocorticoid abnormalities.

Influences of the corticotropic axis and sympathetic activity on neurochemical consequences of 3,4-methylenedioxymethamphetamine (MDMA) administration in Fischer 344 rats

The respective influences of the corticotropic axis and sympathetic activity on 3,4-methylenedioxymethamphetamine (MDMA, ecstasy) immediate effects on body temperature and long-term neurotoxicity, as assessed by decreases in hippocampal and striatal [(3)H]5-hydroxytryptamine ([(3)H]5-HT) reuptake, [(3)H]paroxetine binding at 5-HT transporters (5-HTT), and 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) levels, were examined in Fischer 344 rats. On each of the two injections of MDMA (5 or 10 mg/kg s.c. once a day for 2 consecutive days) body temperature rapidly increased in a dose-dependent manner. Six days after the last injection of 10 mg/kg MDMA, [(3)H]5-HT reuptake, [(3)H]paroxetine binding and 5-HT and 5-HIAA levels were decreased in the hippocampus and, to a lower extent, in striatum. Prior adrenalectomy (1 week beforehand), which weakened the immediate hyperthermic effect of MDMA, prevented the long-term MDMA-elicited reduction in hippocampal and striatal [(3)H]paroxetine binding. Supplementation of adrenalectomised Fischer 344 rats with corticosterone almost reinstated the immediate hyperthermic effect of MDMA and restored MDMA-elicited reduction in hippocampal and striatal [(3)H]paroxetine binding. In a final set of experiments, Fischer 344 rats were pretreated (30 min before each of the two injections of 10 mg/kg MDMA) with the ganglionic blocker chlorisondamine (2.5 mg/kg). This pretreatment markedly reduced the amplitudes of the immediate hyperthermia and long-term declines in hippocampal [(3)H]5-HT reuptake and [(3)H]paroxetine binding at 5-HTT, and in hippocampal and striatal 5-HT and 5-HIAA levels. These results suggest that sympathetic activity (possibly through its control of body temperature), but not corticotropic activity, plays a key role in MDMA-elicited neurotoxicity in Fischer 344 rats.

Biodistribution study of [(123)I] ADAM in mice: correlation with whole body autoradiography

Iodine-123 labeled 2-((2-((dimethylamino)methyl)phenyl)thio)-5-iodophenylamine ([(123)I] ADAM) has been suggested as a promising serotonin transporter (SERT) imaging agent. Much research has been accomplished, mainly focusing on the SERT binding sites in the central nervous system (CNS). However, the biodistribution of [(123)I] ADAM using whole body autoradiography (WBAR) has never been previously described, to the best of our knowledge. In this study, we assayed the biodistribution of [(123)I] ADAM in tissues/organs removed from mice, and measured their radioactivity with a scintillation counter (SC). The results showed that the liver has the highest uptake. On the other hand, the WBAR clearly demonstrated that [(123)I] ADAM was bound to SERT-rich sites including those in the brain stem, lung, adrenal glands and intestinal mucosa. This radiotracer also accumulated in the liver, kidney, and thyroid. The results from both methods were compared; each has its own complementary role in the biodistribution studies. The SC method revealed the total amount of radiotracer accumulation in each organ, and the WBAR demonstrated more anatomical details of the radiotracer's distribution. The whole body distribution results of the radioligand using both methods explore the usage of this novel radioligand for most possible SERT binding sites, not only in the CNS but also in the peripheral nervous system and neuroendocrine tissues. These findings suggest that [(123)I] ADAM is a potentially useful imaging agent for SERT.

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.

Effects of repeated fluoxetine on anxiety-related behaviours, central serotonergic systems, and the corticotropic axis axis in SHR and WKY rats

In keeping with the anxiolytic property of selective serotonin reuptake inhibitors (SSRIs) in humans, we have examined in the spontaneously hypertensive rat (SHR) and the Wistar-Kyoto (WKY) rat, which display low and high anxiety, respectively, some psychoneuroendocrine effects of a repeated treatment with the SSRI fluoxetine (5 or 10 mg/kg daily, for 3 weeks). Two days after the last injection, plasma levels of fluoxetine were not detectable whereas those of its metabolite, norfluoxetine, were present to similar extents in both strains. By means of the elevated plus-maze test (29-30 h after the 13th administration of fluoxetine) and an open field test (48 h after the last injection of fluoxetine), it was observed that fluoxetine pretreatment did not yield anxiolysis; hence, some, but not all, behaviours were indicative of anxiety and hypolocomotion (as assessed through principal component analyses and acute diazepam studies). In both strains, the 10 mg/kg dose of fluoxetine decreased hypothalamus 5-HT and 5-HIAA levels, and reduced midbrain and/or hippocampus [3H]citalopram binding at 5-HT transporters, but did not affect [3H]8-hydroxy-2-(di-N-propylamino)tetralin binding at hippocampal 5-HT1A receptors. However, the fluoxetine-elicited reduction in hippocampal 5-HT transporter binding was much more important in WKY than in SHR rats, this strain-dependent effect being associated in WKY rats with a reduction in cortical [3H]ketanserin binding at 5-HT2A receptors. Lastly, in WKY rats, repeated fluoxetine administration increased adrenal weights and the plasma corticosterone response to open field exposure, but did not affect the binding capacities of hippocampal mineralocorticoid and glucocorticoid receptors. These data show that key psychoneuroendocrine responses to repeated fluoxetine administration may be strain-dependent, and that repeated fluoxetine administration does not yield anxiolysis, as assessed by two standard tests of emotivity.