The iron‐deficient rat as a model of restless legs syndrome: Was anything lost in translation?

Abstract

We read with interest the article by Lai and colleagues 1 on an iron-deficient (ID) rat model for RLS, which might be promising for the discovery of novel RLS mechanisms.The diagnosis of RLS is entirely based on subjective symptoms, hindering the development of animal models.2 Appropriately therefore, Lai and colleagues 1 evaluated their model's face validity based on objective sleep derangements and periodic leg movements during sleep (PLMS), which are commonly associated to RLS. In this respect, we suggest three points that may be worth considering in future attempts to replicate and expand this seminal work.1. Patients with RLS experience leg discomfort and an urge to move that worsen in the evening-night. did not report significant nychtemeral distributions of sleep derangements and motor activity. The circadian component of RLS is crucial, and raw data, which are not available for the readers, are important to evaluate the possibility of a type II statistical error. 2. Lai and colleagues 1 defined PLMS as sequences of 4 hindlimb electromyographic (EMG) bursts with intermovement intervals (IMI) <90 seconds during non-rapid-eye-movement (NREM) sleep. However, the guidelines to define PLMS in humans recommended a lower IMI cutoff of 5 seconds, which was recently increased to 10 seconds, 5 and periodic limb movements falling within the 0-to 10-second IMI range have a clinical meaning different from that of the true PLMS occurring in the 10-to 90-second IMI range. We have shown that the tibialis anterior EMG bursts during NREM sleep in healthy rats, mice, and young humans have a single mode of the IMI distribution extending up to 10 seconds, and mainly consist of sequences of 2 to 3 events. 6 An analysis of the IMI distribution would clarify whether the ID rat model developed by Lai and colleagues 1 shows longer sequences of hindlimb EMG bursts with IMI <10 seconds and/or more frequent burst sequences with IMI 10 seconds during NREM sleep. 3. The PLMS shown by Lai and colleagues 1 in their 1A and supplementary figures were invariably bilateral and associated with decreased amplitude of the EEG and with neck EMG bursts, which might be interpreted as frank arousals fragmenting NREM sleep. On the contrary, we have shown that <50% of tibialis anterior EMG bursts during NREM sleep are bilateral in healthy rats.6 Lai and colleagues 1 also quantified periodic leg movements during quiet wakefulness. However, the quiet wakefulness periods shown in Figure 1A,B by Lai and colleagues 1 entailed sustained hindlimb EMG activity or phasic increases in EEG amplitude, which might be interpreted as active wakefulness or as light NREM sleep, respectively. A detailed and explicit definition of wake-sleep scoring criteria may help to understand whether hindlimb activity was clustered in the "twilight zone" of the wakesleep transition, or also occurred stably and with IMI >10 seconds in NREM sleep, which would strongly support the face validity of the model.

Reply: The iron‐deficient rat as a model of restless legs syndrome: Was anything lost in translation?

“…NREM sleep is scored if an epoch consisted of ≥50% of theta or lower frequency waves as well as behavioral observation on video recordings. We do not agree with Silvani et al [2] that this is in the “twilight zone” of the wake-sleep transition. An increased EEG K-alpha complex associated with PLMs is reported in RLS patients [4].…”

“…Silvani et al [2] suggested some critical issues for further study in the development of animal model of RLS.…”