Reply from M. Dutschmann and G. M. Stettner

Abstract

We appreciate the interest and comments regarding our recently published article in The Journal of Physiology (Stettner et al. 2007a). The manuscript described disturbances in vagal-pontine interactions in the control of the postinspiratory activity as a potential cause of breathing disturbances in MECP2-deficient mice, an animal model for Rett syndrome (Guy et al. 2001). The Letter by Poon & Song addresses an aspect of the publication that is illustrated in Fig. 7, which contrasts the response to repetitive electrical stimulation of the vagal nerve of wild-type (C57BL/6J) and MECP2-deficient mice. In both groups of mice, the intensity of electrical stimulation of the central end of the vagus nerve was determined by the same procedure and the stimuli elicited a postinspiratory reflex apnoea (Hering–Breuer reflex). In particular, the intensity of vagal stimulation in our study was 1–2 × above threshold (low intensity stimulation), which was similar to that used to study plasticity following repetitive vagal stimulation (Siniaia et al. 2000). The data showed that the apnoea after cessation of the electrical stimulus was significantly longer in MECP2-deficient mice than in C57BL/6J mice. Furthermore, the reflex apnoea progressively decreased with repeated vagal stimuli in the wild-type but not MECP2-deficient mice. We interpreted our data as Mecp2−/y knockout mice lacked ‘desensitization’ to vagal stimulation. This terminology was criticised in the letter to the Editor. We want to emphasize that disturbed processing of vagal afferent input remains a part of the phenotype of the MECP2-deficient mice, whether they lack desensitization or ‘classic habituation’. We agree that a full description of the underlying plasticity phenomenon would require analysis of the poststimulus rebound activity. At this point, we cannot confirm the correspondent's insights regarding Fig. 7. Differences in the stimulus intensities between our and other studies relate to different experimental tools, for instance the central vagal nerve was stimulated via suction electrodes in our experiment. This may clarify confusion regarding a potential secondary or non-specific sensitization of the vagally evoked reflex response due to activation of rapidly adapting pulmonary stretch receptor afferent fibres or cardiopulmonary C-fibres. Even though group data for the poststimulus rebound were not analysed, Fig. 7 shows that expiration lengthened after the stimulus. Poststimulus rebound activity as described for rats (Siniaia et al. 2000), however, did not occur in C57BL/6J and MECP2-deficient mice. The C57BL/6J mouse strain itself may have an imbalance in ponto-medullary circuits. We have observed spontaneous postinspiratory apnoeas in the perfused brainstem preparation in situ (Stettner et al. 2007b) and others report severe periodic breathing patterns during the recovery from a hypoxic challenge in vivo (Han et al. 2002; Gonsenhauser et al. 2004). We speculate that the pontine Kölliker–Fuse nucleus (KF) may be a part of this imbalance, not only because neurones in the KF express postinspiratory activity in many species (Dick et al. 1994; Ezure, 2006; Dutschmann & Herbert, 2006), but also because KF neuronal activity determines the timing of postinspiratory activities of laryngeal muscles in the absence of vagal afferent feedback (Dutschmann & Herbert, 2006). If pontine mechanisms in the control of breathing are similar in rats and mice, then spontaneous postinspiratory apnoeas in C57BL/6J mice indicate a genetic imbalance in pontine mechanisms controlling postinspiratory activity in C57BL/6J mice that may be accentuated under the condition of MECP2 deficiency. This may cause the unusual poststimulus rebound illustrated in Fig. 7. Interestingly, another Mecp2−/y knockout model (Chen et al. 2001) has a different genetic background. If we are correct about the intrinsic imbalance prevalent in C57BL/6J mice, then these other MECP2-deficient mice should show distinct differences in their breathing phenotype. In our opinion, further experiments are required in MECP2-deficient mice with different genetic backgrounds to fully understand disturbed synaptic plasticity associated with the processing of afferent vagal feedback.