First generation antipsychotics, such as haloperidol, act predominantly blocking D2 type dopaminergic receptors, thus reducing the positive symptoms of the disease (Stahl 2014). However, dopaminergic pathway antagonism, such as the nigrostriatal pathway, ends up generating adverse effects, which can be progressive and disabling, such as TD, thus compromising the quality of life of patients who need this medication (Moreno et al. 2004).
This study aimed to investigate the possible protective effect of isoflavones against involuntary movements induced by haloperidol in rats and the role of proinflammatory cytokines in this process. In clinical practice, treatment of extrapyramidal side effects (akathisia, tremor, parkinsonian stiffness, bradykinesia and TD) associated with the use of antipsychotics is generally performed with anticholinergic drugs such as biperiden (Reynoso et al. 2012; Valencia et al. 2012). However, these medications causing anticholinergic side effects (tachycardia, mydriasis, dryness of the mucous membranes and urinary retention), and there is some evidence that anticholinergic medications can increase the positive symptoms of schizophrenia (Ocaña-Zuritaet et al. 2016). According to Egan et al. (1996), anticholinergic drugs do not suppress late VCM.
In this context, natural compounds are used in the search for minimizing the adverse effects of drugs or as adjuvant treatments for diseases, and isoflavones, which are phenolic compounds found mainly in soybeans and are widely described in the literature for having several biological activities beneficial to the organism (Kawakami et al. 2004). Thus, we decided to evaluate its possible protective effect against the VCM induced by haloperidol in rats, a marker of OD in an animal model. After 28 days of treatment where the rats received isoflavones and/or haloperidol, the animals were subjected to the open field test and locomotion was assessed. Reduced locomotion in animals treated with haloperidol is already expected since it is known that blockade of dopamine D2 receptors and reduction of dopaminergic pathway function in nucleus accumbens led to suppression of locomotion and sedation (Kelley et al. 1989; Salamone et al. 2008). However, surprisingly co-treatment with isoflavones restored locomotor activity when compared to the control group.
Estrogens also have been shown to significantly affect mono-aminergic neurotransmitter pathways in the brain, and the agonism of estrogen receptors can up-regulated levels of neurotransmitters such as dopamine, serotonin and norepinephrine (Long et al. 2019). In addition, isoflavones have a similar molecular weight and structure to 17-β estradiol (E2), therefore the effects on rodent physiology and behavior may be a result of estrogen-dependent signaling pathways due to interaction with estrogen receptors (ERs) α and β (Morito et al. 2001; Cederroth and Nef, 2009). In view of this, we can hypothesize that co-treatment with isoflavones reversed the hypolocomotion caused by treatment with haloperidol by stimulating the release of dopamine through its agonism in estrogen receptors. Another hypothesis to explain the reversal of hypolocomotion observed by co-treatment with isoflavones, is that these phytoestrogens are capable of inhibiting the activity of the monoamine oxidase isoforms (Da Silva Schmitz et al. 2019), thus being able to modulate the amount of monoamines and restore locomotor activity.
Another important finding in this study was that the administration of 80 mg/kg of isoflavone powder per gavage for 28 days was able to reduce the number of VCM in animals treated with haloperidol. A hypothesis that may explain the onset of VCM is that the prolonged treatment with antipsychotics induces the production of proinflammatory mediators causing a neuroinflammation (Bishnoi et al. 2008; Peroza et al. 2016). In a previous study, we showed that rats treated with antipsychotics (haloperidol and risperidone) had high levels of pro-inflammatory cytokines [IL-1β, IL-6, TNF-α, interferon- γ (IFN-γ) and reduced levels of anti-inflammatory cytokine IL-10] (Peroza et al. 2016). In the study, when the animals were treated with isoflavones, it was observed a significant reduction in the levels of IL-1β and TNF-α, but not IL-6 levels induced in response to haloperidol. These results evidenced that treatment with isoflavones can have an immunomodulatory effect on the central nervous system These results evidenced an immunomodulatory effect in the nervous system induced by the treatment with isoflavones. In addition, the increase in pro-inflammatory cytokines induced by haloperidol was closely linked to the induction of VCM.
Other studies have also shown the anti-inflammatory effect of isoflavones in the nervous system. The effect of isoflavones in the improvement of inflammation induced by carrageenan by inhibiting the NF-κB pathway in mice and in the reduction of inflammatory cytokine levels is already known, as well as the expression of the p65 subunit in the brain of rats treated with β-amyloid peptide (Aβ) (Ding et al. 2011). Another study showed that rats that received soy isoflavones (80 mg / kg) for 14 days, reduced the high levels of IL-1β, TNFα, cyclooxygenase-2 (COX-2) and TNF -α in the brain treated with an intracerebroventricular injection of a single dose of colchicine (7.5 µg), a rat model of Alzheimer's disease (Essawy et al. 2019).
The cytotoxicity induced by the treatment with haloperidol has been associated with the appearance of extrapyramidal symptoms (Subramanyam et al. 2001). Moreover, an in vitro treatment with haloperidol (10 µM, 50 µM, 100 µM) in human dopaminergic neuroblastoma (SH-SY5Y) demonstrated cytotoxic effects in a dose dependent manner after 24 h of incubation (Yang and Lung 2011). In this context, a study published by Bains & Roberts (2016) reported that the pre-treatment with 17β-estradiol indirectly protects dopaminergic neurons by a mechanism that involves ER-alpha (ERα)-mediated PI3K signaling in astrocytes and subsequent neuroprotection of dopaminergic neurons against MPP + induced toxicity. Thus, knowing that isoflavones have a similar molecular weight and structure to 17β-estradiol and the ability to bind to estrogen receptors (Meza et al. 2015), they could also exert a neuroprotective mechanism against cytotoxicity caused by treatment with haloperidol, thus promoting the reduction of VCM induced by haloperidol found in this study.