In recent years, enough evidence has accumulated to assert that cardiotonic steroids, Na⁺,K⁺-ATPase ligands, play an integral role in the physiological and pathophysiological processes in the body. However, little is known about the function of these compounds in the central nervous system. Endogenous cardiotonic steroids are involved in the pathogenesis of affective disorders, including depression and bipolar disorder, which are linked to dopaminergic system dysfunction. Animal models have shown that the cardiotonic steroid ouabain induces mania-like behavior through dopamine-dependent intracellular signaling pathways. In addition, mutations in the alpha subunit of Na⁺,K⁺-ATPase lead to the development of neurological pathologies. Evidence from animal models confirms the neurological consequences of mutations in the Na⁺,K⁺-ATPase alpha subunit. This review is dedicated to discussing the role of cardiotonic steroids and Na⁺,K⁺-ATPase in dopaminergic system pathologies—both the evidence supporting their involvement and potential pathways along which they may exert their effects are evaluated. Since there is an association between affective disorders accompanied by functional alterations in the dopaminergic system and neurological disorders such as Parkinson’s disease, we extend our discussion to the role of Na⁺,K⁺-ATPase and cardiotonic steroids in neurodegenerative diseases as well.

Cardiotonic steroids (CTSs), such as digoxin, are used for heart failure treatment. However, digoxin permeates the brain–blood barrier (BBB), affecting central nervous system (CNS) functions. Finding a CTS that does not pass through the BBB would increase CTSs’ applicability in the clinic and decrease the risk of side effects on the CNS. This study aimed to investigate the tissue distribution of the CTS ouabain following intraperitoneal injection and whether ouabain passes through the BBB. After intraperitoneal injection (1.25 mg/kg), ouabain concentrations were measured at 5 min, 15 min, 30 min, 1 h, 3 h, 6 h, and 24 h using HPLC–MS in brain, heart, liver, and kidney tissues and blood plasma in C57/black mice. Ouabain was undetectable in the brain tissue. Plasma: C_max = 882.88 ± 21.82 ng/g; T_max = 0.08 ± 0.01 h; T_1/2 = 0.15 ± 0.02 h; MRT = 0.26 ± 0.01. Cardiac tissue: C_max = 145.24 ± 44.03 ng/g (undetectable at 60 min); T_max = 0.08 ± 0.02 h; T_1/2 = 0.23 ± 0.09 h; MRT = 0.38 ± 0.14 h. Kidney tissue: C_max = 1072.3 ± 260.8 ng/g; T_max = 0.35 ± 0.19 h; T_1/2 = 1.32 ± 0.76 h; MRT = 1.41 ± 0.71 h. Liver tissue: C_max = 2558.0 ± 382.4 ng/g; T_max = 0.35 ± 0.13 h; T_1/2 = 1.24 ± 0.7 h; MRT = 0.98 ± 0.33 h. Unlike digoxin, ouabain does not cross the BBB and is eliminated quicker from all the analyzed tissues, giving it a potential advantage over digoxin in systemic administration. However, the inability of ouabain to pass though the BBB necessitates intracerebral administration when used to investigate its effects on the CNS.

Introduction. Cardiac glycosides are natural ligands of Na+/K+-ATPase, which regulate its activity and signaling. Intracerebroventricular administration of ouabain has been previously shown to induce hyperlocomotion in C57Bl/6 mice via a decrease in the rate of dopamine reuptake from the synaptic cleft. Materials and methods. This study involved forty C57BL/6 mice. 1.5 μL of 50 μM ouabain was administered daily into the left lateral cerebral ventricle over the course of 4 days. On day 5, open field, beam balance, and ladder rung walking tests were performed to assess the locomotor activity and motor impairments in the mice. We evaluated changes in the activation of signaling cascades, ratios of proapoptotic and antiapoptotic proteins, and the amount of α1 and α3 isoforms of the Na+/K+-ATPase α-subunit in brain tissue using Western blotting. Na+/K+-ATPase activity was evaluated in the crude synaptosomal fractions of the brain tissues. Results. We observed hyperlocomotion and stereotypic behavior during the open field test 24 hours after the last injection of ouabain. On day 5, the completion time and the number of errors made in the beam balance and ladder rung walking tests increased in the mice that received ouabain. Akt kinase activity decreased in the striatum, whereas the ratio of proapoptotic and antiapoptotic proteins and the number of Na+/K+-ATPase α-subunits did not change. Na+/K+-ATPase activity increased in the striatum and decreased in the brainstem. Conclusions. Long-term exposure to ouabain causes motor impairments mediated by changes in the activation of signaling cascades in dopaminergic neurons.

Cardiotonic steroids (CTS) are specific inhibitors and endogenous ligands of a key enzyme in the CNS—the Na⁺, K⁺-ATPase, which maintains and creates an ion gradient on the plasma membrane of neurons. CTS cause the activation of various signaling cascades and changes in gene expression in neurons and other cell types. It is known that intracerebroventricular injection of cardiotonic steroid ouabain causes mania-like behavior in rodents, in part due to activation of dopamine-related signaling cascades in the dopamine and cAMP-regulated phosphoprotein 32 (DARPP-32) expressing medium spiny neurons in the striatum. Dopaminergic projections in the striatum innervate these GABAergic medium spiny neurons. The objective of this study was to assess changes in the expression of all genes in human iPSC-derived expressing DARPP-32 and GABA receptors neurons under the influence of ouabain. We noted a large number of statistically significant upregulated and downregulated genes after a 16-h incubation with non-toxic concentration (30 nM) of ouabain. These changes in the transcriptional activity were accomplished with activation of MAP-kinase ERK1/2 and transcriptional factor cAMP response element-binding protein (CREB). Thus, it can be concluded that 30 nM ouabain incubated for 16 h with human iPSC-derived expressing DARPP-32 and GABA receptors neurons activates genes associated with neuronal maturation and synapse formation, by increasing the expression of genes associated with translation, vesicular transport, and increased electron transport chain function. At the same time, the expression of genes associated with proliferation, migration, and early development of neurons decreases. These data indicate that non-toxic concentrations of ouabain may induce neuronal maturation, neurite growth, and increased synaptogenesis in dopamine-receptive GABAergic neurons, suggesting formation of plasticity and the establishment of new neuronal junctions.

Objective: Ouabain, a cardiotonic steroid, induces hyperactivity mediated by alterations in dopamine turnover in the central nervous system. However, ouabain, a cardiotonic steroid, induces hyperactivity mediated by alterations in dopamine turnover in the central nervous system. However, the effects of prolonged ouabain administration remain poorly studied. Methods: Adult C57BL/6 mice received 20 μM ouabain via intracerebroventricular (ICV) infusion using 14-day osmotic pumps. Nest building was assessed on day 14 as a measure of general well-being. On day 15, locomotor activity was evaluated in square open field arenas; anxiety-like behavior was evaluated using the elevated plus maze. Brain regions including the cerebellum, frontal cortex, striatum, hippocampus, thalamus, and brain stem were collected for neurochemical analysis using high-performance liquid chromatography. Results: Chronic ICV ouabain led to elevated serotonin levels in the frontal cortex, thalamus, and hippocampus, and reduced noradrenaline in the thalamus. These changes were accompanied by decreased anxiety-like behavior and impaired nest building, without significant effects on locomotion. These effects differ from those observed following acute ICV ouabain injection, which affects dopamine turnover but not serotonin levels, causing an increase in locomotor activity and decrease in anxiety-like behavior. Conclusions: Unlike acute ICV ouabain administration, which alters dopamine turnover and induces persistent hyperactivity, sustained low-dose exposure elevates serotonin and reduces anxiety-like behavior unaccompanied by locomotor activation. These findings suggest that ouabain’s effects on anxiety and hyperactivity are mediated by different mechanisms, through alterations in serotonin and dopamine, respectively.

The relationship between stress and Parkinson’s disease is regarded as complex and multifaceted, although a direct causal link has not yet been conclusively proven. One prevailing hypothesis is based on the activation of the hypothalamic–pituitary–adrenal (HPA) axis and the consequent elevation of glucocorticoid levels. Prolonged exposure to these hormones may exacerbate oxidative stress, thereby rendering the dopaminergic neurons within the brain’s subcortical structures more susceptible to degeneration. Furthermore, stress may intensify neuroinflammation through the activation of microglia—a mechanism that could constitute a significant factor in the pathogenesis of Parkinson’s disease. Another important concept concerns the direct interaction of stressors with the dopaminergic system. Physiological and psychological stress can alter dopaminergic transmission by affecting both the synthesis and release of dopamine, as well as the sensitivity of dopamine receptors. Severe or chronic stress may contribute to the disruption of dopaminergic mechanisms and accelerate the onset of clinical symptoms in predisposed individuals. Furthermore, many researchers draw attention to the role of stress-induced aggregation of α-synuclein—a key protein implicated in the pathogenesis of Parkinson’s disease. Clinical data suggest a highly probable link between post-traumatic stress disorder and an increased risk of developing Parkinson’s disease, although these findings remain inconclusive. It is possible that stress acts not as a primary cause, but rather as a modifying factor that interacts with genetic predisposition, accelerating or triggering neurodegenerative processes. The aim of our narrative review was to examine these concepts and discuss possible directions for future research into the interaction between stress and Parkinson’s disease.