BDNF receptor blockade hinders the beneficial effects of exercise in a rat model of Parkinson’s disease

Abstract

Physical exercise is known to produce beneficial effects to the nervous system. In most cases, brain-derived neurotrophic factor (BDNF) is involved in such effects. However, little is known on the role of BDNF in exercise-related effects on Parkinson’s disease (PD). The aim of this study was to investigate the effects of intermittent treadmill exercise-induced behavioral and histological/neurochemical changes in a rat model of unilateral PD induced by striatal injection of 6-hydroxydopamine (6-OHDA), and the role of BDNF in the exercise effects. Adult male Wistar rats were divided into two main groups: (1) injection of K252a (a blocker of BDNF receptors), and (2) without BDNF receptor blockade. These groups were then subdivided into four groups: control (CLT), sedentary (SED, non-exercised with induction of PD), exercised 3×/week during four weeks before and four weeks after the induction of PD (EXB + EXA), and exercised 3×/week during four weeks after the induction of PD (EXA).

One month after 6-OHDA injections, the animals were subjected to rotational behavioral test induced by apomorphine and the brains were collected for immunohistochemistry and immunoblotting assays, in which we measured BDNF and tyrosine hydroxylase (TH) in the substantia nigra pars compacta (SNc) and the striatum (caudate-putamen, CPu).

Our results showed a significant reduction of rotational asymmetry induced by apomorphine in the exercised parkinsonian rats. BDNF decreased in the SNc of the SED group, and exercise was able to revert that effect. Exercised groups exhibited reduced damage to the dopaminergic system, detected as a decreased drop of TH levels in SNc and CPu. On the other hand, BDNF blockade was capable of substantially reducing TH expression postlesion, implying enhanced dopaminergic cell loss.

Our data revealed that physical exercise is capable of reducing the damage induced by 6-OHDA, and that BDNF receptors are involved in that effect.

Introduction

The etiology of Parkinson’s disease (PD) is still not well understood. Postmortem examinations reveal a loss of the dopaminergic neurons in the substantia nigra pars compacta (SNc), and the consequent loss of dopamine in the striatum in the brains of PD patients (Blum et al., 2001). Research on the pathogenesis of the PD has been mainly based on the development of animal models that reproduce the loss of dopaminergic neurons in the SNc. The most used animal model of PD is the injection of dopamine analog 6-hydroxydopamine (6-OHDA) into the caudate-putamen (striatum – CPu) or SNc, and which selectively destroys catecholaminergic neurons, resulting in a decrease of dopamine levels into CPu. That neurotoxin is found in the brains of PD patients (Jellinger et al., 1995) and the injection of 6-OHDA in the central–lateral portion of the CPu is the animal model that most closely resembles the human disease (Tillerson et al., 2002, Schober, 2004). In addition, the intrastriatal injection is less aggressive than other injection sites, and produces a slow evolution of symptoms, which seems to be more suitable for studies that aim therapeutic strategies (Blandini et al., 2008). The analysis of tyrosine hydroxylase (TH) levels is usually employed to follow the progression of the neurodegenerative process in dopaminergic fibers and cell bodies. TH activity is also known to be progressively decreased following the loss of dopamine neurons of the SNc in PD patients (Haavik and Toska, 1998).

With an increasing life expectancy worldwide and hence the number of people with neurodegenerative diseases, several efforts have been made in an attempt to understand the etiology of PD and to develop therapeutic approaches. Exercise is a simple and widely practiced behavior that activates molecular and cellular cascades that support and maintain brain plasticity (Cotman and Berchtold, 2002), and is inversely related to neurodegenerative diseases. However, the extent of the lesion and the type of the exercise (voluntary or forced) may affect the degree of neuroprotection and behavioral improvement induced by physical exercise (Hirsch and Farley, 2009, Alonso-Frech et al., 2011).

Animal studies with PD models have shown that several plasticity processes are triggered by exercise and are involved in neuroprotection mechanisms, such as enhanced angiogenesis (Al-Jarrah et al., 2010), increased anti-inflammatory (Cadet et al., 2003) and decreased inflammatory responses (Wu et al., 2011), improved mitochondrial functions (Lau et al., 2011), and increased neurogenesis in the striatum (Tajiri et al., 2010) and in the SNc (Steiner et al., 2006). Plasticity responses improve the neurochemical deficits, especially TH levels, and both cognitive and motor symptoms (Sutoo and Akiyama, 2003, Cohen et al., 2003, Petzinger et al., 2007, Yoon et al., 2007, O’Dell et al., 2007, Tajiri et al., 2010, Lau et al., 2011, Wu et al., 2011). In models, clinical studies with PD patients have shown that exercise decreases the risk of PD (Chen et al., 2005), improves life quality (Herman et al., 2007), motor coordination (Fisher et al., 2008) and balance (Toole et al., 2000, Goodwin et al., 2008).

It is possible that the neuroprotective effects of exercise are promoted by neurotrophins, such as the brain-derived neurotrophic factor (BDNF). This neurotrophin has a critical role in cell differentiation, neuronal survival, migration, dendritic arborization, synaptogenesis, and synaptic plasticity, acting through tyrosine kinase receptor B (TrkB) (Cotman and Berchtold, 2002).

BDNF expression has been shown to be decreased in animal models of PD (Tajiri et al., 2010, Lau et al., 2011, Wu et al., 2011) and postmortem studies in humans with PD (Howells et al., 2000). Exercise, on the other hand, is able to restore BDNF levels in animals (Tajiri et al., 2010, Lau et al., 2011, Wu et al., 2011) and parkinsonian patients (Ahlskog, 2011). In vitro, BDNF has neuroprotective effects against the neurotoxicity induced by 6-OHDA (Stahl et al., 2011). An in vivo study demonstrated similar results, as Wu and collaborators (2011) showed that BDNF injection in the CPu of mice prior to the injection of lipopolysaccharide (LPS) promoted an improvement of TH levels similar to what was observed in the exercised group. On the other hand, enhanced TH deficits were observed after intracerebroventricular administration of a BDNF receptor blocker in exercised mice prior to the administration of LPS (Wu et al., 2011) as well as in TrkB knockout mice (Baydyuk et al., 2011).

The above-mentioned studies reinforce the idea that BDNF is involved in PD neuroprotection induced by exercise, although they may not be the ideal models to study therapeutic strategies in the early stages of PD. For example, intraperitoneal injections of LPS promote a quick systemic and intense inflammation that activates microglial response in the SNc and other brain regions (Dutta et al., 2008, Wu et al., 2011), resulting in a severe loss of 76% of the TH-positive cells (Wu et al., 2011).

Furthermore, there are no studies in the literature showing the beneficial effects of intermittent exercise in PD animal models. An intermittent exercise routine, as proposed by this study, seems more feasible to PD patients and nonathletes that normally exercise 3 or 4 times a week (Herman et al., 2007). The present exercise protocol may therefore be closer to the reality of PD patients.

The aim of this study was to investigate the possible effects of intermittent treadmill exercise-induced behavioral and histological/neurochemical changes in a rat model of unilateral PD induced by striatal injection of 6-OHDA, and the role of BDNF in these effects.