Voluntary exercise or amphetamine treatment, but not the combination, increases hippocampal brain-derived neurotrophic factor and synapsin I following cortical contusion injury in rats.

Traumatic brain injury (TBI) continues to be one of the leading causes of mortality and morbidity, with approximately 5.3 million Americans suffering from enduring disabilities resulting from TBI (Binder et al., 2005). Despite many experimental and clinical efforts using numerous therapeutic approaches, there are currently no proven treatments to alleviate the sequelae of TBI or to enhance recovery of function. Our laboratories have been investigating treatment strategies that may facilitate endogenous repair mechanisms and enhance functional recovery after experimental TBI in the rat. One approach has been to use spontaneous or voluntary exercise, obtained via running wheel (RW) exposure. Exercise is known to increase key molecules involved in neuroplasticity, such as brain-derived neurotrophic factor (BDNF) and synapsin I (Neeper et al., 1995; van Praag et al., 1999; Cotman and Berchtold, 2002), as well as reduce markers of oxidative stress (Navarro et al., 2004; Wu et al., 2004; Pan et al., 2007). While experimental TBI can acutely decrease expression of BDNF mRNA within the hippocampus (Hellmich et al., 2005), most studies have reported hippocampal increases in BDNF mRNA within the first 24 h following TBI (Oyesiku et al., 1999; Truettner et al., 1999; Morrison et al., 2000). At later post-injury times, neither hippocampal BDNF mRNA (Hicks et al., 2002) nor BDNF protein differs from that of controls (Griesbach et al., 2004b, 2007; Chen et al., 2005). However, spontaneous exercise in rats with TBI can increase the expression and/or levels of BDNF and synapsin I within the hippocampus (Hicks et al., 1998; Griesbach et al., 2004b, 2007), and this effect is associated with an improvement in cognitive performance (Griesbach et al., 2004b). The timing of this post-injury exercise has been shown to play an important role after experimental fluid percussion injury (FPI) to the brain. We previously reported that voluntary exercise during the first week after sham injury increased hippocampal BDNF and synapsin I protein, but exercise did not increase these molecules after a FPI (Griesbach et al., 2004a, 2007). The mechanisms underlying the inability of acute exercise to up-regulate BDNF after TBI are not known, but may be related to alterations in neurotransmitter systems during this acute period following injury. Transmitter depletion studies have shown that norepinephrine (NE) is necessary for voluntary wheel-running to up-regulate BDNF mRNA in the hippocampus (Garcia et al., 2003). Anti-depressant drugs, which increase synaptic levels of NE and 5-HT, have been reported to increase levels of hippocampal BDNF mRNA (Nibuya et al., 1995) and protein (Peng et al., 2008; Kozisek et al., 2008). In addition, anti-depressant drugs strengthen BDNF mRNA up-regulation when given in combination with exercise (Russo-Neustadt et al., 1999, 2000). Repeated treatments with d-amphetamine (AMPH) have also been reported to increase BDNF levels in multiple brain regions (Meredith et al., 2002). Brain injury induced by experimental stroke or cortical ablation reduces brain levels and/or turnover rates of NE, dopamine (DA) and/or 5-HT for days or weeks after injury (Feeney and Sutton, 1987). Experimental TBI in the rat produces early and widespread reductions in brain NE levels and turnover (Dunn-Meynell et al., 1994; Krobert et al., 1994; Levin et al., 1995). Hence, although exercise in non-injured rats is known to increase NE release (Dunn et al., 1996; Dishman et al., 2000; Garcia et al., 2003), a TBI-induced reduction of NE could conceivably prevent exercise-dependent increases in NE and BDNF during the acute period after injury. Another approach for the treatment of TBI has been to administer treatments that increase central levels of monoamines. Single or multiple treatments with AMPH have been frequently reported to accelerate functional recovery in various brain injury models (Feeney and Sutton, 1988; Sutton and Feeney, 1994), including cortical contusion (Sutton et al., 1987) or FPI (Prasad et al., 1994; Hovda, 1996). While AMPH increases levels of all the mono-amines (Fleckenstein et al., 2007), the beneficial effects of post-injury AMPH treatment appear to be mediated by NE release since intraventricular NE, but not DA (Boyeson and Feeney, 1990) or 5-HT (Boyeson et al., 1994), mimics the AMPH induced recovery of function in ablation models. In addition, drugs causing central NE release induce recovery similar to AMPH (Sutton and Feeney, 1992; Goldstein, 1993), and drugs that interfere with NE release or synaptic transmission can retard recovery or reinstate deficits in recovered animals (Feeney and Westerberg, 1990; Goldstein and Davis, 1990; Dunn-Meynell et al., 1997). In addition to AMPH, multiple treatments with methylphenidate (Kline et al., 2000), a D2 receptor agonist (Kline et al., 2002) or a 5- HT(1A) receptor agonist (Kline et al., 2004) are reported to improve outcome after experimental TBI. More recently, chronic intermittent vagus nerve stimulation, which increases NE release (Roosevelt et al., 2006) and BDNF mRNA levels (Follesa et al., 2007) in cortex and hippocampus, has been shown to improve outcome after experimental TBI (Smith et al., 2005, 2006; Clough et al., 2007). Given these preceding reports on the effects of TBI and the apparent links between AMPH, NE, exercise and BDNF, the current study was designed to determine if AMPH administration during the acute period after TBI would enable or facilitate spontaneous exercise-induced increases of hippocampal BDNF and synapsin I. Because oxidative stress plays an important role in neuronal injury after TBI (Shohami et al., 1997; Sullivan et al., 1998; Hall et al., 2004), we also determined if these treatments would alter oxidative stress after TBI. We employed the controlled cortical impact (CCI) model of TBI for this study, as this injury model is known to produce molecular changes and cellular damage in the hippocampus (Dash et al., 1995; Colicos et al., 1996; Oyesiku et al., 1999; McCullers et al., 2002; Saatman et al., 2006), induce hippocampal-dependent learning deficits (Hamm et al., 1992; Lindner et al., 1998; Dixon et al., 1999), and to reduce brain NE levels and turnover (Dunn-Meynell et al., 1994; Levin et al., 1995). While increased expression or production of hippocampal BDNF and synapsin I has been reported to occur after exercise in both intact rats and in rats with FPI (Timmusk et al., 1993; Neeper et al., 1996; Molteni et al., 2002; Griesbach et al., 2004b, 2007), exercise-induced effects of these markers of plasticity have not yet been evaluated in the CCI model. Based on prior studies we hypothesized that: 1) acute exercise after CCI would have detrimental or neutral effects on outcome measures, 2) low-dose AMPH treatment would improve these outcome measures, and 3) AMPH treatment combined with RW exposure would enable acute exercise to up-regulate BDNF and synapsin I and reduce oxidative stress.