Partial regulation of serotonin transporter function by γ-synuclein

Human α-synuclein (α-Syn) is instrumental in maintaining homeostasis of monoamine neurotransmitters in brain, through its trafficking, and regulation of the cell surface expression and, thereby, activity of dopamine, serotonin and norepinephrine transporters. Here we have investigated whether other members of the synuclein family of proteins, γ-synuclein (γ-Syn) and β-synuclein (β-Syn) can similarly modulate the serotonin transporter (SERT). In Ltk⁻ cells co-transfected with SERT and γ-Syn, γ-Syn reduced [³H]5-HT uptake, in a manner dependent on its expression levels. The decrease in SERT activity was via decreased V_max of the transporter, without change in K_m, compared to cells expressing only SERT. By contrast, β-Syn co-expression failed to alter SERT uptake activity, and neither the V_max nor the K_m was changed in the presence of β-Syn. γ-Syn modulation of SERT was only partial, with a maximal ∼27% decrease in SERT activity seen even at high expression levels of γ-Syn. By contrast, α-Syn attenuated SERT activity by ∼65% at identical expression levels as γ-Syn. Co-immunoprecipitation studies showed the presence of heteromeric protein:protein complexes between γ-Syn or α-Syn and SERT, while β-Syn failed to physically interact with SERT. Both α-Syn and γ-Syn colocalized with SERT in rat primary raphae nuclei neurons. These studies document a novel physiological role for γ-Syn in regulating 5-HT synaptic availability and homeostasis, and may be of relevance in depression and mood disorders, where SERT function is dysregulated.     

Chronic antidepressant treatment selectively increases expression of plasticity-related proteins in the hippocampus and medial prefrontal cortex of the rat

Antidepressants protect against hippocampal volume loss in humans and reverse stress-induced atrophic changes in animals thus supporting the hypothesis that the pathophysiology of stress-related disorders such as depression involves reductions in neuronal connectivity and this effect is reversible by antidepressant treatment. However, it is unclear which brain areas demonstrate such alterations in plasticity in response to antidepressant treatment. The aim of the present study was to examine the effect of antidepressant treatment on the expression of three plasticity-associated marker proteins, the polysialylated form of nerve cell adhesion molecule (PSA-NCAM), phosphorylated cyclic-AMP response element binding protein (pCREB) and growth-associated protein 43 (GAP-43), in the rat brain. To this end, rats were treated either acutely (60 min) or chronically (21 days) with imipramine (30 and 15 mg/kg, respectively) and the expression of PSA-NCAM, pCREB, and GAP-43 was assessed using immunohistochemistry. Initial mapping revealed that chronic imipramine treatment increased expression of these plasticity-associated proteins in the hippocampus, medial prefrontal cortex and piriform cortex but not in the other brain regions examined. Since PSA-NCAM and pCREB are expressed in recently-generated neurons in the dentate gyrus, it is likely that chronic imipramine treatment increased their expression in the hippocampus at least partially by increasing neurogenesis. In contrast, since chronic imipramine treatment is not associated with neurogenesis in the medial prefrontal cortex, increased expression of PSA-NCAM and pCREB in the prelimbic cortex implicates changes in synaptic connectivity in this brain region. Acute treatment with imipramine increased the number of pCREB positive nuclei in the hippocampus and the prefrontal cortex but did not alter expression of GAP-43 or PSA-NCAM in any of the brain regions examined. Taken together, the results of the present study suggest that antidepressant treatment increases synaptic plasticity and connectivity in brain regions associated with mood disorders.     

Effects of single and multiple doses of dexnafenodone,imipramine and placebo on sleep of young healthy volunteers

The effects on polygraphically recorded sleep of single and repeated doses of dexnafenodone (20 mg daily) were determined in 12 young, healthy subjects, and compared to those of imipramine (75 mg daily: six subjects) and placebo (six subjects). After two adaptation nights, sleep was recorded at baseline (night 0), and after the first (night 1) and last (night 5) evening administration of the study drugs. REM sleep was substantially inhibited in both nights under the two active treatments, whereby the effect appeared immediately. With the exception of slow wave sleep (SWS), which was more reduced in night 1 under imipramine than under dexnafenodone, the other sleep stages were essentially unchanged. Time awake during bed rest increased under both active treatments, with a more rapid increase under dexnafenodone. Dexnafenodone, a potent inhibitor of noradrenaline, and to a lesser degree of serotonin reuptake, induced changes in the pattern of sleep which are comparable to those of non-sedating tricyclic antidepressants. The mode of action as well as the pharmacodynamic profile of dexnafenodone led to the expectation that this new substance will show antidepressive activity on a clinical level.     

Imipramine protects retinal ganglion cells from oxidative stress through the tyrosine kinase receptor B signaling pathway

Retinal ganglion cell (RGC) degeneration is irreversible in glaucoma and tyrosine kinase receptor B (TrkB)-associated signaling pathways have been implicated in the process. In this study, we attempted to examine whether imipramine, a tricyclic antidepressant, may protect hydrogen peroxide (H2O2)-induced RGC degeneration through the activation of the TrkB pathway in RGC-5 cell lines. RGC-5 cell lines were pre-treated with imipramine 30 minutes before exposure to H2O2. Western blot assay showed that in H2O2-damaged RGC-5 cells, imipramine activated TrkB pathways through extracellular signal-regulated protein kinase/TrkB phosphorylation. TUNEL staining assay also demonstrated that imipramine ameliorated H2O2-induced apoptosis in RGC-5 cells. Finally, TrkB-IgG intervention was able to reverse the protective effect of imipramine on H2O2-induced RGC-5 apoptosis. Imipramine therefore protects RGCs from oxidative stress-induced apoptosis through the TrkB signaling pathway.     

Changes induced by corticosterone and adrenalectomy in synaptosomal and platelet uptake and binding of 5-HT: The relationship to [3H]imipramine binding

A comparative study of the effect of adrenalectomy, treatment with corticosterone, and imipramine on platelet and synaptosomal uptake was undertaken. One day after adrenalectomy, uptake of 5-hydroxytryptamine (5-HT) in the platelet and the hypothalamus was significantly decreased with an increase in the apparent K_m, but the uptake of 5-HT of the cerebral synaptosomes was unchanged. In this group, the k_d) of low affinity binding of 5-HT was also increased in the hypothalamic synaptosomes and in the platelet preparations. Treatment with cortieosterone (5 $\text{mg}\text{kg}$, i.m.) restored the decrease in the uptake of 5-HT induced by adrenalectomy in the hypothalamic synaptosomes and in the platelets, and significantly increased the uptake of 5-HT of these fractions in sham-operated rats. The binding of 5-HT was unchanged by acute treatment with corticosterone. The effectiveness of imipramine varied with the preparation and treatment group. The IC₅₀ of the cerebral synaptosomal preparation was greater than that of the hypothalamic synaptosomal preparation. Using this latter preparation, the IC₅₀ for imipramine in adrenalectomised, sham-operated and corticosterone-treated rats were found to be 0.04, 0.09 and 0.25 μM, respectively. These changes in sensitivity to imipramine were not reflected in the binding of [³H]imipramine which was unchanged.     

A double-blind controlled study of viloxazine and imipramine in depression

Summary

Fifty-nine depressed patients entered a double-blind, between patient comparison of viloxazine hydrochloride (300?mg./day expressed as base) and imipramine hydro-chloride (150?mg./day expressed as salt). Twenty-nine patients took viloxazine and 30 imipramine. Depression was assessed at weekly intervals, using the Hamilton Scale over a period of 6 weeks, and both drugs produced a statistically significant antidepressant effect. No difference emerged between the response to the two agents. Imipramine was associated with a significantly higher incidence of side-effects than viloxazine and they were more persistent. Imipramine produced a mean increase in weight over 6 weeks of 5.9 kg. whereas viloxazine produced a rise of only 2.4 kg. Viloxazine produced few anticholinergic side-effects; imipramine, on the other hand, was associated with frequent and persistent side-effects such as dry mouth, blurred vision, disturbed micturition and constipation. Drowsiness was not seen with viloxazine but was seen in over half the patients on imipramine. It is concluded that, whereas viloxazine has antidepressant properties equal to those of imipramine, it has a much lower incidence of side-effects, and what side-effects it does have are transient.     

Effects of tranylcypromine enantiomers on monoamine uptake and release and imipramine binding

Summary Studies were carried outin vitro to determine effects of tranylcypromine enantiomers ([+]- and [–]-TCP) on uptake and release of 5-HT, DA and NA in rat synaptosomes and on imipramine binding to rabbit platelets. (+)-TCP was more potent than (–)-TCP as inhibitor of 5-HT uptake and imipramine binding, whereas (–)-TCP was more potent than (+)-TCP as inhibitor of DA and NA uptake. The enantiomers differed only slightly in their effects on monoamine release. The findings agree with previous reports on the stereoselectivity of monoaminergic mechanisms toward TCP enantiomers, and support the notion that the 5-HT uptake site may be associated with the imipramine binding site.     

Steady-state concentrations of imipramine and its metabolites in relation to the sparteine/debrisoquine polymorphism

Summary Thirty-five imipramine treated patients were phenotyped with regard to polymorphic drug oxidation using sparteine and/or debrisoquine.During treatment with 100 mg imipramine per day the mean steady-state concentrations and ratios in 28 extensive metabolizers were: imipramine 169 nmol/l; desipramine 212 nmol/l; 2-OH-imipramine/imipramine 0.25; 2-OH-desipramine/desipramine 0.57. The corresponding values in two poor metabolizers were: imipramine 455 and 302 nmol/l; desipramine 1148 and 1721 nmol/l; 2-OH-imipramine/imipramine 0.06 and 0.05; 2-OH-desipramine/desipramine: 0.09 and 0.04 respectively.The metabolic ratios (MR) sparteine/dehydrosparteine and debrisoquine/4-OH-debrisoquine (% of dose in 12-h urine samples) correlated poorly with the imipramine steady-state concentrations during administration of 100 mg per day, but quite well with the desipramine steady-state concentrations. Significant negative correlations were found between sparteine and debrisoquine MR and the 2-OH-imipramine/imipramine and 2-OH-desipramine/desipramine ratios.In most patients the initial dose was changed to obtain concentrations in the therapeutic range, and concentrations for imipramine + desipramine of (mean ± SD) 713±132 nmol/l were achieved in 33 patients. The therapeutic dose was 50 mg per day in one poor metabolizer and ranged from 50–400 mg per day in 32 extensive metabolizers. There was a weak negative correlation between sparteine MR and daily dose.Treatment with imipramine inhibited metabolism of both sparteine and debrisoquine (MR values about doubled), but did not affect the interpatient correlations.     

Further evidence supporting the role of the serotonin system in suicidal behavior: a preliminary study of suicide attempters

The 3H-imipramine binding parameters (Bmax and Kd) were studied in platelets from female suicide attempters and from healthy volunteers. The Bmax was significantly lower among suicide attempters, with no modification of the Kd. These results provide support of the hypothesis that decreased serotonin function may play a role in suicidal behavior.