Origin of the central entrainment of respiration by locomotion facilitated by MK 801 in the decerebrate rabbit

In order to establish the origin of the central coupling between locomotion and respiration which operates in freely moving mammals during galloping, we sought experimental conditions that readily lead to such a coupling in decerebrate and curarised rabbit preparations. In such preparations, stimulation of the mesencephalic locomotor region (MLR) evokes locomotor activities, recorded from hindlimb muscle nerves, that are rarely totally coordinated with phrenic inspiratory activity. However, low doses (0.2 mg/kg i.v.) of MK 801, a non-competitive NMDA antagonist which has been shown to increase the activity of the spinal locomotion generators (Fenaux et al. 1991), dramatically enhanced this coupling during MLR stimulation in most experiments: 1/1 coupling was dominant but 2/1 and 3/1 couplings (i.e. two or three locomotor cycles per respiratory cycle) were also obtained. Compared with spontaneous respiratory activity, which was apneustic under these conditions, the respiratory period was drastically decreased during coupling. However, a further transection of the spinal cord at the C6 or C7 level, which isolated the spinal locomotion generators from the supraspinal levels, totally suppressed this reduction of the inspiratory period during MLR stimulation in the presence of MK 801. In experiments where locomotor activity was simultaneously recorded at forelimb and hindlimb levels, the 1/1 evoked locomotor-respiratory coupling remained after the lumbar cord had been isolated by L1 spinal transection. The present data do show that intact spinal mechanisms are required for entrainment to occur. They suggest either that a common supraspinal drive cannot entrain locomotion and respiration when being depressed, or that respiration is entrained at the locomotor rate by the spinal locomotion generators. If so, this capacity for entrainment does not appear to be limited to the lumbar locomotion generators, but rather, is likely to involve the cervico-thoracic locomotion generators too. MK 801 potentiates this coupling through an increase in rhythmogenesis of the spinal locomotion generators.