Inhibition of the serotonin transporter induces microglial activation and downregulation of dopaminergic neurons in the substantia nigra.

Drugs that selectively inhibit the serotonin transporter (SERT) are widely used in the treatment of depression and anxiety disorders. These agents are associated with a range of extrapyramidal syndromes such as akathisia, dystonia, dyskinesia and parkinsonism, suggesting an effect on dopaminergic transmission. We studied the time course of changes in dopaminergic neurons in the substantia nigra (SN) after initiation of two different SERT inhibitors, citalopram and fluoxetine. In the first experiment, groups of Sprague-Dawley rats received daily meals of rice pudding either alone (N = 9) or mixed with citalopram 5 mg/kg/day (N = 27). Rats were sacrificed after 24 h, 7 days or 28 days of treatment. Sections of SN were processed for tyrosine hydroxylase (TH) immunohistochemistry. Citalopram induced a significant decrease in TH-positive cell counts at 24 h (44%), 7 days (38%) and 28 days (33%). No significant differences among the citalopram treatment groups were observed in the SN. To determine whether these changes would occur with other SERT inhibitors, we conducted a second experiment, this time with a 28 day course of fluoxetine. As was observed with citalopram, fluoxetine induced a significant 21% reduction of TH cell counts in the SN. Immunoblot analysis showed that fluoxetine also induced a 45% reduction of striatal TH. To investigate a possible role for the innate immune system in mediating these changes, we also studied the microglial marker OX42 after administration of fluoxetine and noted a significant 63% increase in the SN of fluoxtine-treated animals. These results indicate that SERT inhibition can activate microglia and alter the regulation of TH, the rate limiting enzyme for dopamine biosynthesis. These changes may play a role in mediating the extrapyramidal side effects associated with SERT inhibitors.

Role of the dopamine transporter in mediating the neuroleptic-induced reduction of tyrosine hydroxylase-immunoreactive midbrain neurons.

We have previously reported a long-term downregulation of midbrain dopaminergic neurons following treatment with neuroleptic medications. The mechanism of this effect is not clear. The dopamine transporter (DAT) has been shown to play a role in the behavioural and biochemical action of neurotoxins such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). We postulated a role for the DAT in mediating the changes induced by neuroleptic (i.e., antipsychotic) drugs. In the first experiment, Sprague-Dawley rats simultaneously received twice daily sub-cutaneous injections of either saline or the DAT inhibitor GBR 12909 (GBR; 5 mg/kg/day) and haloperidol (HAL; 2 mg/kg/day) or vehicle. In the second experiment, the animals were treated with daily sub-cutaneous injections of saline or the DAT inhibitor GBR 129091 plus oral risperidone (RISP; 1.5 mg/kg/day) or vehicle. In a third experiment, animals were given normal drinking water or water with clozapine (CLZ, 20 mg/kg/day). Animals were sacrificed immediately after the last treatment. Sections of the substantia nigra (SN) and ventral tegmental area (VTA) were processed for tyrosine hydroxylase (TH) immunoreactivity. Cell counts were analyzed by one-way analysis of variance followed by post-hoc Tukey tests, with significance set at p<0.05. Treatment with HAL or RISP resulted in a significant reduction (HAL 27%; RISP 25%) in the number of TH-immunoreactive cells present in the medial SN pars compacta. This effect was in both cases completely blocked by administration of the DAT inhibitor. In the VTA, TH-positive cell counts were significantly decreased with RISP, but not with HAL. Once again, the RISP-induced changes were blocked by co-administration of the DAT inhibitor. CLZ treatment did not significantly affect TH-positive cell counts in the SN. These results indicate a role for the active dopamine transporter in mediating the suppression of TH expression in midbrain dopaminergic neurons by antipsychotic drugs. DAT inhibitors may prove useful in ameliorating the neurological side effects of antipsychotic medication.

Role of serotonin transporter inhibition in the regulation of tryptophan hydroxylase in brainstem raphe nuclei: time course and regional specificity.

Drugs that selectively inhibit the serotonin transporter (SERT) are widely prescribed for treatment of depression and a range of anxiety disorders. We studied the time course of changes in tryptophan hydroxylase (TPH) in four raphe nuclei after initiation of two different SERT inhibitors, citalopram and fluoxetine. In the first experiment, groups of Sprague-Dawley rats received daily meals of rice pudding either alone (n=9) or mixed with citalopram 5 mg/kg/day (n=27). Rats were sacrificed after 24 h, 7 days or 28 days of treatment. Sections of dorsal raphe nucleus (DRN), median raphe nucleus (MRN), raphe magnus nucleus (RMN) and caudal linear nucleus (CLN) were processed for TPH immunohistochemistry. Citalopram induced a significant reduction in DRN TPH-positive cell counts at 24 h (41%), 7 days (38%) and 28 days (52%). Similar reductions in TPH-positive cell counts were also observed at each timepoint in the MRN and in the RMN. In the MRN, citalopram resulted in significant reductions at 24 h (26%), 7 days (16%) and 28 days (23%). In the RMN, citalopram induced significant reductions of TPH-positive cell counts at 24 h (45%), 7 days (34%) and 28 days (43%). By contrast, no significant differences between control and treatment groups were observed in the CLN at any of the time points that we studied. To investigate whether these changes would occur with other SERT inhibitors, we conducted a second experiment, this time with a 28-day course of fluoxetine. As was observed with citalopram, fluoxetine induced significant reductions of TPH cell counts in the DRN (39%), MRN (38%) and RMN (41%), with no significant differences in the CLN. These results indicate that SERT inhibition can alter the regulation of TPH, the rate limiting enzyme for serotonin biosynthesis. This persistent and regionally specific downregulation of serotonin biosynthesis may account for some of the clinical withdrawal symptoms associated with drugs that inhibit SERT.

Establishing a liquid-phase IEF in combination with 2-DE for the analysis of Leishmania proteins.

The recent completion of genome sequencing projects for Leishmania major and near completion for two other species, L. infantum and L. braziliensis, has provided the needed genomic information for investigating the proteomes of Leishmania parasites. However, the design of effective 2-DE-based proteome mapping for complex protozoan parasites like Leishmania has proven to be severely compromised due to extensive overcrowding of spots especially in the acidic regions, coupled to a relatively low representation of basic proteins. In the present study, we optimized a liquid-phase IEF in combination with 2-DE for L. amazonensis promastigote as a way of reducing protein complexity and enhancing representation for low-abundance proteins on gels. Of 20 pH-based fractions eluted from Rotofor cells, 5 representative fractions selected from acidic, basic or neutral regions of the proteome and with adequate protein concentration were further analyzed by 2-DE using medium-range IPG strips. On this basis, we were able to generate high-resolution 2-DE maps encompassing both the acidic and basic ends of the proteome with enhanced spot representation.

Comparative two-dimensional gel electrophoresis maps for promastigotes of Leishmania amazonensis and Leishmania major.

The outcome of Leishmania infections is determined by both the parasite species and the host genetic makeup. While much has been learned regarding immune responses to this parasite, our knowledge on parasite-derived factors is limited. The recent completion of the L. major and L. infantum genome sequence projects and concurrent advancement in proteomics technology would greatly accelerate the search for novel Leishmania proteins. Using a proteomics-based approach to study species-specific Leishmania proteins, we developed high-resolution, broad pH (3-10) two-dimensional gel electrophoresis (2-DE) separations to determine protein-expression profiles between highly infectious forms of the parasitic species L. amazonensis (New World) and L. major (Old World). Approximately 1,650 and 1,530 distinct protein spots were detected in the L. amazonensis and L. major gels, respectively. While a vast majority of the spots had similar distribution and intensity, a few were computationally defined as preferentially expressed in L. amazonensis in comparison to L. major, or vice versa. These data attest to the feasibility of establishing a 2-DE-based protein array for inter-species profiling of Leishmania proteins and provide the framework for future design of proteome studies of Leishmania.

Leishmania species: evidence for transglutaminase activity and its role in parasite proliferation.

Albeit transglutaminase (TGase) activity has been reported to play crucial physiological roles in several organisms including parasites; however, there was no previous report(s) whether Leishmania parasites exhibit this activity. We demonstrate herein that TGase is functionally active in Leishmania parasites by using labeled polyamine that becomes conjugated into protein substrates. The parasite enzyme was about 2- to 4-fold more abundant in Old World species than in New World ones. In L. amazonensis, comparable TGase activity was found in both promastigotes and amastigotes. TGase activity in either parasite stage was optimal at the basic pH, but the enzyme in amastigote lysates was more stable at higher temperatures (37-55 degrees C) than that in promastigote lysates. Leishmania TGase differs from mouse macrophage (M Phi) TGase in two ways: (1) the parasite enzyme is Ca(2+)-independent, whereas the mammalian TGase depends on the cation for activity, and (2) major protein substrates for L. amazonensis TGase were found within the 50-75 kDa region, while those for the M Phi TGase were located within 37-50 kDa. The potential contribution of TGase-catalyzed reactions in promastigote proliferation was supported by findings that standard inhibitors of TGase [e.g., monodansylcadaverine (MDC), cystamine (CS), and iodoacetamide (IodoA)], but not didansylcadaverine (DDC), a close analogue of MDC, had a profound dose-dependent inhibition on parasite growth. Myo-inositol-1-phosphate synthase and leishmanolysin (gp63) were identified as possible endogenous substrates for L. amazonensis TGase, implying a role for TGase in parasite growth, development, and survival.

Comparative two-dimensional gel electrophoresis maps for promastigotes of Leishmania amazonensis and Leishmania major

The outcome of Leishmania infections is determined by both the parasite species and the host genetic makeup. While much has been learned regarding immune responses to this parasite, our knowledge on parasite-derived factors is limited. The recent completion of the L. major and L. infantum genome sequence projects and concurrent advancement in proteomics technology would greatly accelerate the search for novel Leishmania proteins. Using a proteomics-based approach to study species-specific Leishmania proteins, we developed high-resolution, broad pH (3-10) two-dimensional gel electrophoresis (2-DE) separations to determine protein-expression profiles between highly infectious forms of the parasitic species L. amazonensis (New World) and L. major (Old World). Approximately 1,650 and 1,530 distinct protein spots were detected in the L. amazonensis and L. major gels, respectively. While a vast majority of the spots had similar distribution and intensity, a few were computationally defined as preferentially expressed in L. amazonensis in comparison to L. major, or vice versa. These data attest to the feasibility of establishing a 2-DE-based protein array for inter-species profiling of Leishmania proteins and provide the framework for future design of proteome studies of Leishmania.