Tamoxifen Directly Interacts with the Dopamine Transporter

Tamoxifen in breast cancer and risk of Parkinson's disease: A meta-analysis

BACKGROUND

Tamoxifen is widely used for hormone-sensitive breast cancer, achieved by modulating the estrogen receptor activity in a tissue-specific manner. There is evidence to support the protective effects of estrogen against Parkinson's disease (PD), a common neurodegenerative condition. Some epidemiologic studies suggest the use of tamoxifen may modulate the PD risk. We conducted a meta-analysis to examine the association between tamoxifen and risk of PD.

METHODS

A search of PubMed using search terms synonymous with "tamoxifen" and "Parkinson's disease" was conducted. Outcomes of interest were the odds ratio (OR) of PD comparing tamoxifen-exposed to -unexposed women, as well as the incidence rate of PD in tamoxifen-exposed women.

RESULTS

A total of 37,932 subjects with breast cancer, comprising 17,233 tamoxifen-exposed subjects and 20,699 tamoxifen-unexposed subjects, satisfied the inclusion criteria. The exposure to tamoxifen ranged from 30-96 months. Using the common-effect model, the pooled OR of PD was 2.4, with (95% CI 1.91-3.01, P < 0.0001), with high heterogeneity (I2 = 81.5%, Cochran's Q test P = 0.001). Pooling 28,640 tamoxifen-exposed patients under the common-effect model found an incidence rate of 5.86 events per 10,000 person-years (95% CI 4.82-7.12) with minimal heterogeneity (I2 = 26%, Cochran's Q test P = 0.258).

CONCLUSIONS

Our meta-analysis suggests that tamoxifen use may be associated with an increased PD risk in women. However, due to heterogeneity and potential limitations of some of the studies, further clinical and functional validation will be needed. Longitudinal studies supported by imaging and biomarkers evaluation will be useful to identify the mechanisms linking tamoxifen and PD risk.

Unlocking the Potential of High-Quality Dopamine Transporter Pharmacological Data: Advancing Robust Machine Learning-Based QSAR Modeling

The dopamine transporter (DAT) plays a critical role in the central nervous system and has been implicated in numerous psychiatric disorders. The ligand-based approaches are instrumental to decipher the structure-activity relationship (SAR) of DAT ligands, especially the quantitative SAR (QSAR) modeling. By gathering and analyzing data from literature and databases, we systematically assemble a diverse range of ligands binding to DAT, aiming to discern the general features of DAT ligands and uncover the chemical space for potential novel DAT ligand scaffolds. The aggregation of DAT pharmacological activity data, particularly from databases like ChEMBL, provides a foundation for constructing robust QSAR models. The compilation and meticulous filtering of these data, establishing high-quality training datasets with specific divisions of pharmacological assays and data types, along with the application of QSAR modeling, prove to be a promising strategy for navigating the pertinent chemical space. Through a systematic comparison of DAT QSAR models using training datasets from various ChEMBL releases, we underscore the positive impact of enhanced data set quality and increased data set size on the predictive power of DAT QSAR models.

Early Effects of Alpha-Synuclein Depletion by Pan-Neuronal Inactivation of Encoding Gene on Electroencephalogram Coherence between Different Brain Regions in Mice

Inactivation of the Snca gene in young mice by chronic injections of tamoxifen (TAM), a selective estrogen receptor modifier, has been shown to decrease the level of alpha-synuclein, a key peptide in the pathogenesis of Parkinson's disease. In young mice, different time courses of the effect were observed in different brain areas, meaning associated disturbances in the intracerebral relations, namely in brain function after TAM-induced synucleinopathy.

METHODS

We analyzed electroencephalogram (EEG) coherence ("functional connectivity") between the cortex (MC), putamen (Pt), and dopamine-producing brain regions (ventral tegmental area, VTA, and substantia nigra, SN) in two groups of two-month-old male mice. We compared EEG coherences in the conditional knockout Sncaflox/flox mice with those in their genetic background (C57Bl6J) one, two, and three months after chronic (for five days) intraperitoneal injections of TAM or the vehicle (corn oil). The EEG coherences in the TAM-treated group were compared with those in the alpha-synuclein knockout mice.

RESULTS

A significant suppression of EEG coherence in the TAM-treated mice versus the vehicle group was observed in all inter-structural relations, with the exception of MC-VTA at one and three months and VTA-SN at two months after the injections. Suppressive changes in EEG coherence were observed in the alpha-synuclein knockout mice as well; the changes were similar to those in TAM-treated mice three months after treatment.

CONCLUSION

our data demonstrate a combined time-dependent suppressive effect induced by TAM on intracerebral EEG coherence.

Overview of the structure and function of the dopamine transporter and its protein interactions

The dopamine transporter (DAT) plays an integral role in dopamine neurotransmission through the clearance of dopamine from the extracellular space. Dysregulation of DAT is central to the pathophysiology of numerous neuropsychiatric disorders and as such is an attractive therapeutic target. DAT belongs to the solute carrier family 6 (SLC6) class of Na⁺/Cl^- dependent transporters that move various cargo into neurons against their concentration gradient. This review focuses on DAT (SCL6A3 protein) while extending the narrative to the closely related transporters for serotonin and norepinephrine where needed for comparison or functional relevance. Cloning and site-directed mutagenesis experiments provided early structural knowledge of DAT but our contemporary understanding was achieved through a combination of crystallization of the related bacterial transporter LeuT, homology modeling, and subsequently the crystallization of drosophila DAT. These seminal findings enabled a better understanding of the conformational states involved in the transport of substrate, subsequently aiding state-specific drug design. Post-translational modifications to DAT such as phosphorylation, palmitoylation, ubiquitination also influence the plasma membrane localization and kinetics. Substrates and drugs can interact with multiple sites within DAT including the primary S1 and S2 sites involved in dopamine binding and novel allosteric sites. Major research has centered around the question what determines the substrate and inhibitor selectivity of DAT in comparison to serotonin and norepinephrine transporters. DAT has been implicated in many neurological disorders and may play a role in the pathology of HIV and Parkinson's disease via direct physical interaction with HIV-1 Tat and α-synuclein proteins respectively.

Water Extract of Rhizoma Drynaria Selectively Exerts Estrogenic Activities in Ovariectomized Rats and Estrogen Receptor-Positive Cells

Our previous study demonstrated that the bone protective actions of herbal medicine Rhizoma Drynariae (Gusuibu, RD) were mainly mediated by flavonoid phytoestrogens via estrogen receptors, raising concerns about the safety of using RD as it may induce estrogen-like risk-benefit profile and interact with other ER ligands, such as selective estrogen receptor modulators (SERMs), when coadministered. The present study evaluated the estrogenic activities of RD and its potential interaction with tamoxifen, a SERM, in estrogen-sensitive tissues by using mature ovariectomized (OVX) rats and ER-positive cells. Similar to but weaker than tamoxifen, RD at its clinical dose dramatically ameliorated OVX-induced changes in bone and dopamine metabolism-related markers in OVX rats. However, tamoxifen, but not RD, induced uterotrophic effects. No significant alteration in mammary gland was observed in OVX rats treated with RD, which was different from the inhibitory actions of tamoxifen. The two-way ANOVA results indicated the interactions between RD and tamoxifen in the bone, brain, and uterus of OVX rats while RD did not alter their responses to tamoxifen. Our results demonstrate that RD selectively exerts estrogenic actions in a different manner from tamoxifen. Moreover, RD interacts with tamoxifen without altering its effects in OVX rats.

Prescribing Tamoxifen in Patients With Mood Disorders: A Systematic Review of Potential Antimanic Versus Depressive Effects

PURPOSE/BACKGROUND

Tamoxifen is a selective estrogen receptor modulator widely used for treatment and prevention of estrogenic receptor-positive breast cancer. Tamoxifen is an object of growing interest in psychopharmacology as an antimanic drug, because it inhibits the protein kinase C, a molecular target of bipolar disorder. Consistently, the potential depressive effect of tamoxifen has been repeatedly reported.

METHODS/PROCEDURES

This article systematically reviews studies examining tamoxifen impact on mood, exploring either its potential therapeutic use as antimanic agent or its potential depressive effect.

FINDINGS

Eight studies explored tamoxifen antimanic properties, all, but one, reported a rapid and efficacious antimanic action. As to the depressive effect, 9 cohort studies emerged among which 4 pointed out an increased risk of depression. Seven case reports described the onset or exacerbation of depressive episodes besides 1 case series study reported a high rate of depressive symptoms. In addition, 1 case report study described a tamoxifen-induced manic episode.

IMPLICATIONS/CONCLUSIONS

The present review highlights tamoxifen treatment as a possible trigger of mood symptoms onset or exacerbation in vulnerable patients. Accordingly, patients with a history of mood disorders may require a close clinical surveillance during tamoxifen use. At the same time, the use of tamoxifen as an antimanic agent in psychiatric settings requires caution, as available evidence came from small-sample studies with short observation time. More studies are needed to define how long-term tamoxifen use may affect the course of bipolar disorder.

Dopamine Neurons That Cotransmit Glutamate, From Synapses to Circuits to Behavior

Discovered just over 20 years ago, dopamine neurons have the ability to cotransmit both dopamine and glutamate. Yet, the functional roles of dopamine neuron glutamate cotransmission and their implications for therapeutic use are just emerging. This review article encompasses the current body of evidence investigating the functions of dopamine neurons of the ventral midbrain that cotransmit glutamate. Since its discovery in dopamine neuron cultures, further work in vivo confirmed dopamine neuron glutamate cotransmission across species. From there, growing interest has led to research related to neural functioning including roles in synaptic signaling, development, and behavior. Functional connectome mapping reveals robust connections in multiple forebrain regions to various cell types, most notably to cholinergic interneurons in both the medial shell of the nucleus accumbens and the lateral dorsal striatum. Glutamate markers in dopamine neurons reach peak levels during embryonic development and increase in response to various toxins, suggesting dopamine neuron glutamate cotransmission may serve neuroprotective roles. Findings from behavioral analyses reveal prominent roles for dopamine neuron glutamate cotransmission in responses to psychostimulants, in positive valence and cognitive systems and for subtle roles in negative valence systems. Insight into dopamine neuron glutamate cotransmission informs the pathophysiology of neuropsychiatric disorders such as addiction, schizophrenia and Parkinson Disease, with therapeutic implications.

Neuropsychiatric effects of tamoxifen: Challenges and opportunities

Epidemiological, clinical, and basic research over the past thirty years have described the benefits of estrogen on cognition, mood, and brain health. Less is known about tamoxifen, a selective estrogen receptor modifier (SERM) commonly used in breast cancer which is able to cross the blood-brain barrier. In this article, we review the basic pharmacology of tamoxifenas well as its effects on cognition and mood. The literature reveals an overall impairing effect of tamoxifen on cognition in breast cancer patients, hinting at central antiestrogen activity. On the other hand, tamoxifen demonstrates promising effects in psychiatric disorders, like bipolar disorder, where its therapeutic action may be independent of interaction with estrogen receptors. Understanding the neuropsychiatric properties of SERMs like tamoxifen can guide future research to ameliorate unwanted side-effects and provide novel options for difficult to treat disorders.

Mirtazapine reduces the expression of cocaine-induced locomotor sensitization in male and female Wistar rats

BACKGROUND

Epidemiological studies have described that women are more vulnerable to the reinforcing effects of cocaine. In animals, the findings are similar: female rats show higher levels of cocaine self-administration and increased cocaine-induced locomotor activity. In contrast, women with depression respond better to treatment with antidepressants, however their therapeutic response to tetracyclic antidepressants is lower. Several studies have shown that mirtazapine-a tetracyclic antidepressant-decreases the behavioral effects of cocaine in male rats. The objective of this study was to evaluate the efficacy of daily dosing of mirtazapine on cocaine-induced locomotor activity and sensitization in naive female rats compared to male rats.

METHODS

Male and female Wistar rats were daily dosed with 10 mg/kg of cocaine. During extinction, cocaine was withdrawn and the groups received daily mirtazapine (30 mg/kg, i.p.) or saline. Tamoxifen was administered during the antagonism phase. After each administration, locomotor activity for each animal was recorded for 30 min in transparent Plexiglass activity chambers.

RESULTS

In this study, a higher cocaine locomotor response was found in females than in males and the mirtazapine was equally effective in decreasing cocaine-induced locomotor activity and the expression of locomotor sensitization in male and female rats. In addition, co-administration of mirtazapine and tamoxifen enhanced the efficacy of mirtazapine in female rats.

CONCLUSION

The results suggest that mirtazapine may be considered an effective therapeutic option for the treatment of cocaine use disorder in men and women.

Deletion of the creatine transporter gene in neonatal, but not adult, mice leads to cognitive deficits

Creatine (Cr) is a guanidino compound that provides readily available phosphate pools for the regeneration of spent adenosine triphosphate (ATP). The lack of brain Cr causes moderate to severe intellectual disability, language impairment, and epilepsy. The most prevalent cause of Cr deficiency are mutations in the X-linked SLC6A8 (Creatine transporter; CrT) gene, known as CrT deficiency (CTD). One of the most critical areas that need to be addressed is whether Cr is necessary for brain development. To address this concern, the Slc6a8 gene was knocked out in either neonatal (postnatal day (P)5) or adult (P60) mice using a tamoxifen-inducible Cre recombinase driven by the human ubiquitin C (UBC) promoter. Mice were tested in the Morris water maze, novel, object recognition, and conditioned fear 60 days after Slc6a8 deletion. In addition, overnight locomotor activity was analyzed. Mice that had the gene deleted on P5 showed deficits in the Morris water maze and novel object recognition, while there were no deficits in P60 knockout mice. Interestingly, the P5 knockout mice showed hyperactivity during the dark phase; however, when examining control mice, the effect was due to the administration of tamoxifen from P5 to 10. Taken together, the results of this study show that Cr is necessary during periods of brain development involved in spatial and object learning. This study also highlights the continued importance of using proper control groups for behavioral testing.

Molecular Mechanisms of Amphetamines

There is a plethora of amphetamine derivatives exerting stimulant, euphoric, anti-fatigue, and hallucinogenic effects; all structural properties allowing these effects are contained within the amphetamine structure. In the first part of this review, the interaction of amphetamine with the dopamine transporter (DAT), crucially involved in its behavioral effects, is covered, as well as the role of dopamine synthesis, the vesicular monoamine transporter VMAT2, and organic cation 3 transporter (OCT3). The second part deals with requirements in amphetamine's effect on the kinases PKC, CaMKII, and ERK, whereas the third part focuses on where we are in developing anti-amphetamine therapeutics. Thus, treatments are discussed that target DAT, VMAT2, PKC, CaMKII, and OCT3. As is generally true for the development of therapeutics for substance use disorder, there are multiple preclinically promising specific compounds against (meth)amphetamine, for which further development and clinical trials are badly needed.