In the present study we investigated a well

In the present study, we investigated a well-defined and selected group of NF1 patients fully active in daily life, with normal intelligence and without motor or cognitive impairments carrying the NF1 mutation and compared them to age-matched controls. Participants were investigated over an extended course while learning a novel and challenging motor skill. In addition, by applying a well-established double-pulse transcranial magnetic stimulation (dp-TMS) protocol, intracortical (GABAergic) inhibition in the contralateral M1 was non-invasively assessed (Ziemann et al., 1996; Mainberger et al., 2013), during resting and movement-related states, to determine underlying pathophysiological mechanisms (Heise et al., 2013; Hummel et al., 2009; Liuzzi et al., 2014). We hypothesized, that NF1 patients show an impairment in motor skill acquisition, and that these deficits will be paralleled by impaired modulation of inhibitory neurotransmission in the M1.
Methods
Results
Discussion Nf1 mutations lead to an increased risk for memory and attention problems. In addition, NF1-children display impairments in fine motor precision, upper limb coordination and fine motor integration (Johnson et al., 2010). Over 40% of these patients receive occupational therapy at some stage during their childhood to alleviate the developmental delays in motor skill (Krab et al., 2008). Recently, there have been groundbreaking advances in our understanding of the molecular, cellular, and neural systems underpinning NF1-associated cognitive deficits. Hyperactivation of the RAS signaling cascade resulting in increased GABA-mediated activity during periods of high-frequency neural stimulation in the hippocampus (Cui et al., 2008), medial prefrontal endothelin receptor and striatum (Shilyansky et al., 2010) have been proposed as the main mechanisms underlying Nf1+/− mice learning impairment. Interestingly, alteration in inhibition/excitation balance has been recently non-invasively demonstrated in the occipital and medial-frontal cortex of NF1 patients at rest, using MR-spectroscopy (Ribeiro et al., 2015; Violante et al., 2013). This suggests a region-specific abnormal GABAergic physiology in NF1 patients. As discussed in the paper of (Violante et al., 2013), MR-spectroscopy provides information about the overall concentration of GABA mainly including the cytosolic, extracellular and vesicular pools. GABA bound to macromolecules, such as to GABA receptors cannot be detected by this technique. dp-TMS allows to address non-invasively aspects of inhibitory (GABA-ergic) neurotransmission, not only in a static, resting state mode, but also during the performance of a task. This event-related approach provides information about fast dynamic changes in inhibitory neurotransmission, which can be associated with behavioral measures. Thus, although difficult to establish a direct parallelism with aforementioned studies and the present study due to technical aspects, the present results add on and extend the findings regarding the role of impaired inhibitory neurotransmission in NF1-patients. This small, but well defined study, adds to the understanding of the neuropathological mechanisms of NF1 by suggesting that not only the static GABA-ergic differences, but especially the impairments in dynamic GABA-ergic neurotransmission might underlie cognitive restrictions, such as demonstrated here within a motor skill acquisition task. Learning a procedural task is a cognitive process that leads to the acquisition of complex goal-oriented movements with practice. Based on behavioral studies, distinct stages for the process of acquiring a skill were proposed: an early stage, in which considerable fast improvement occurs within a single training session, and a late one, characterized by slow changes in performance that can be observed across several sessions including time- and sleep-dependent consolidation processes (Doyon and Benali, 2005). In this context, M1 has been demonstrated as one key brain structure engaged not only in the fast acquisition but also in consolidation and re-consolidation processes of a motor memory trace (Censor et al., 2010; Muellbacher et al., 2002). Increasing evidence supports the view that reduction in tonic GABA is essential for the induction of LTP-like plastic changes within M1 (Stagg et al., 2011). These processes are most likely based on ‘unmasking’ of existing horizontal connections within the cortex, an essential mechanism that underpins the rapid remodeling of motor representations seen in the early stages of plasticity and learning (Xu et al., 2009). To determine intracortical (GABAergic) inhibitory processes, we used a well-established dp-TMS paradigm (Kujirai et al., 1993a). A method with high spatial and temporal resolution to non-invasively explore changes in cortical excitability, inhibitory, and facilitatory neurotransmisson in the motor cortical system (Ziemann et al., 1996; Heise et al., 2013; Kujirai et al., 1993a). In addition, a recent study proposed dp-TMS as a rapid way of evaluating treatment response to Lovastatin in NF1 patients (Mainberger et al., 2013). For instance, while resting state evaluation is dominated by an inhibitory tone within M1, task-related evaluation shows a task specific modulation of inhibition (towards disinhibition) in healthy subjects associated with higher levels of skill acquisition. In contrast, NF1 patients did not show this pattern. Task-related evaluation of SICI was unaltered with a tendency towards less modulation, which was associated with reduced levels of skill acquisition (Fig. 3B).