Startle responses in Duchenne muscular dystrophy: a novel biomarker of brain dystrophin deficiency

Duchenne muscular dystrophy (DMD) is characterised by loss of dystrophin in muscle. Patients affected by DMD also have variable degree of intellectual disability and neurobehavioural co-morbidities. In contrast to muscle, in which a single full-length isoform (Dp427) is produced, multiple dystrophin isoforms are produced in the brain, and their deficiency accounts for the variability of CNS manifestations, with increased risk of comorbidities in patients carrying mutations affecting the 3’ end of gene, disrupting the shorter Dp140 and Dp71 isoforms. The mdx mouse model of DMD lacks Dp427 in muscle and CNS and exhibits exaggerated startle responses to threat, linked to the deficiency of dystrophin in limbic structures such as the amygdala, which normalise with postnatal brain dystrophin-restoration therapies. A pathological startle response is not a recognised feature of DMD, and its characterisation has implications for improved clinical management and translational research.To investigate startle responses in DMD, we used a novel fear-conditioning task in an observational study of 56 males aged 7-12 years (31 DMD, mean age 9.7±1.8 years; 25 Controls, mean age 9.6±1.4 years). Trials of two neutral visual stimuli were presented to participants: one ‘safe’ cue presented alone; one ‘threat’ cue paired with an aversive noise to enable conditioning of physiological startle responses (skin conductance response, SCR; heart rate, HR). Retention of conditioned physiological responses was subsequently tested with presentation of both cues without the aversive noise in an ‘extinction’ phase. Primary outcomes were the magnitude of the initial unconditioned SCR and HR change responses to the aversive ‘threat’ and acquisition and retention of conditioned responses after conditioning. Secondary outcomes were neuropsychological measures and genotype associations.The initial (unconditioned) mean SCR to threat was greater in DMD than Controls (Mean difference 3.0 µS (95% CI 1.0, 5.1), P=.004), associated with a significant threat-induced bradycardia only in the DMD group (mean difference -5.6 bpm (95% CI 0.51, 16.9); P=.04). DMD participants found the task more aversive than Controls, consequently early termination during the extinction phase occurred in 26% of the DMD group (vs. 0% Controls; P=.007).This study provides the first evidence that boys with DMD show increased unconditioned startle responses to threat, similar to the mdx mouse phenotype that also responds to brain dystrophin restoration. Our study provides new insights into the neurobiology underlying the complex neuropsychiatric co-morbidities in DMD and defines an objective measure of this CNS phenotype, which will be valuable for future CNS-targeted dystrophin-restoration studies.

Kinematic changes in goal-directed movements in a fear-conditioning paradigm

In individuals with a musculoskeletal disorder, goal-directed reaching movements of the hand are distorted. Here, we investigated a pain-related fear-conditioning effect on motor control. Twenty healthy participants (11 women and 9 men, 21.7 ± 2.7 years) performed a hand-reaching movement task. In the acquisition phase, a painful electrocutaneous stimulus was applied on the reaching hand simultaneous with the completion of reaching. In the subsequent extinction phase, the task context was the same but the painful stimulus was omitted. We divided the kinematic data of the hand-reaching movements into acceleration and deceleration periods based on the movement-velocity characteristics, and the duration of each period indicated the degree of impairment in the feedforward and feedback motor controls. We assessed the wavelet coherence between electromyograms of the triceps and biceps brachii muscles. In the acquisition phase, the durations of painful movements were significantly longer in both the acceleration and deceleration periods. In the extinction phase, painful movements were longer only in the acceleration period and higher pain expectation and fear were maintained. Similarly, the wavelet coherence of muscles in both periods were decreased in both the acquisition and extinction phases. These results indicate that negative emotional modulations might explain the altered motor functions observed in pain patients.

The Effect of High-Frequency Electrical Stimulation of Bilateral Nucleus Accumbens on the Behavior of Morphine-Induced Conditioned Place Preference Rats at Extinction and Reinstatement Phases

Objective. To explore the optimal time points for deep brain stimulation (DBS) on the treatment of morphine addiction and its possible mechanisms by investigating how high-frequency stimulation (HFS) in bilateral nucleus accumbens (NAc) at different time points influences the addictive behaviors of rats with drug addiction. Methods. The rats were randomly divided into extinction stimulation group (n = 20) and postextinction stimulation group (n = 20). Ten rats in the extinction stimulation group were treated using 120 Hz HFS during extinction stage while another 10 rats with pseudostimulation were served as control group. The CPP scores were evaluated at the second day after intervention, with total 9 sections accomplished. The CPP scores were evaluated at the second day of the intervention. In the postextinction stimulation group, 120 Hz HFS was intervened during the postextinction stage in 10 experimental rats and pseudostimulation was performed in 10 control rats. Stimulation was performed for 7 days continuously, and a small dose of morphine was administrated to induce relapse after the postextinction period. Results. During the extinction phase, CPP scores after HFS were significantly higher. During the postextinction phase, relapse CPP scores after HFS were dramatically lower. Conclusion. HFS of bilateral NAc inhibits the extinction of addictive behavior during the extinction phase, and HFS during the postextinction period suppresses relapse of drug seeking behavior.