Anxiety is frequently reported in more

Anxiety is frequently reported in more than 50% of PD patients [6]. In general, all types of anxiety disorders found in PD are associated with generalized anxiety disorder, agoraphobia, specific phobia, and social anxiety disorder, panic disorder, and obsessive-compulsive disorder or related disorders (according to DSM-V) [7,8]. Anxiety disorder, in some patients, is a “reactive” response to the diagnosis of PD, as a result of stress and motor disability [9]. However, PD patients have greater risks developing anxiety before the diagnosis of PD [5,10]. In addition, loss of striatal serotonin [11], dopamine and noradrenaline [12] innervation are present in living patients with PD. Therefore, these evidence suggest that anxiety is an early non-motor symptom in Parkinson’s disease [13]. Moreover, another non-motor alteration that aggravates anxiety, in humans, and consequently PD, is rapid eye movement (REM) sleep loss [3]. Furthermore, REM sleep deprivation (REMSD) is known to increase anxiety behaviors in humans [14]. While preclinical studies, in rodents, are still inconsistent, suggesting that REMSD triggers anxiolytic-like [15–17] or anxiogenic-like effects [18–20]. These lack of consistency difficult translational applicability of preclinical data of anxiety and sleep deprivation [21]. The research of the motor and non-motor disturbances of PD frequently adopts a number of animal models based on neurotoxins injected within the brain. Rotenone is a pesticide which, freely crosses cellular membranes and accumulates in subcellular channel modulator such as mitochondria, where inhibits the mitochondrial complex Ι of electrical transport chain, in this way inducing apoptosis of the nigrostriatal pathway [22,23]. This dopaminergic neuronal loss mimics several motor [23] and non-motor features of PD, such as olfactory dysfunction [24] and depression [25,26]. However, studies reporting anxiety-like behaviors and their consequences after REMSD, in neurotoxin-based models, are sparse and somewhat inconsistent [6,27]. It is worth mentioning, that only one study, according to our knowledge, demonstrated the effects of rotenone in anxiety-like behaviors, after intraperitoneal injections of this neurotoxin [28]. There is a growing interest in exploring novel pharmacotherapeutic strategies for anxiety in the context of PD [13,26,29]. Among these emerging targets, melatoninergic drugs have gained considerable attention. Several reports have demonstrated anxiolytic-like effects of melatonin, observed in the elevated plus maze test (EPM) by an increase in the number of entries and time spent into the open arms [30–32]. The main roles of this neuropeptide are related to the control of the circadian sleep-wake states, regulation of sleep and seasonal biorhythm [33–35] by activating MT1 and MT2 receptors, two G-protein-coupled membrane receptors [36]. It has been hypothesized that dysregulations of these melatonin receptors may be involved in the installation of mood disorders [37]. As a matter of fact, it has been demonstrated that MT2 receptors play an important role in depression [38] as well as in anxiety [39]. Activation of MT2 receptors, by the partial agonist UCM765, elicits an anxiolytic-like effect in rats [39]. However, it is still unexplained if this effect is also observed in the intranigral rotenone model of PD. Besides, it remains to be clarified if striatal MT2 receptors could elicit anxiety-like effects, since these receptors are present within the striatum [40,41]. Hence, we hypothesize that striatal MT2 receptors associated to dopamine depletion might be implicated in the pathogenesis of anxiety in PD [42,43]. Therefore, in the present study we sought to investigate the existence of a purportedly anxiolytic-like effect generated by the striatal MT2 activation, achieved by the infusion of a selective agonist 8-methoxy-2-propionamidotetralin (8-M-PDOT), and counteracted by the selective antagonist 4-phenyl-2-propionamidotetralin (4-P-PDOT), after dopaminergic degeneration, induced by intranigral rotenone. This hypothesis was tested under the PD model and the REMSD protocol.