Evidence for central entrainment of the medullary respiratory pattern by the locomotor pattern in the rabbit

1) Although periodic passive hindlimb movements can reproduce the enhancement of breathing frequency seen at the onset of muscular exercise, we have shown previously that they were unable to induce the 1:1 coupling which is observed between locomotion and respiration during galloping in quadrupeds. The purpose of this study was to investigate the existence of a central coupling in two experimental situations: first, decorticate - DOPA, and secondly, decerebrate rabbit preparations. 2) After DOPA administration in curarized, vagotomized, decorticate animals, an absolute coordination could be observed between the locomotor bursts (which developed in hindlimb muscle nerves) and phrenic activity. With the temporal evolution of the pharmacological activation, the coupling mode varied from 1:1 to 1:2 during the same experiment with a loss of coordination between these two forms. When the coordination between both motor activities was not produced in such conditions, it could be induced for some imposed frequencies of periodic passive motions applied to the contralateral hindlimb. 3) When the DOPA effects were completely over, a rostro-pontine decerebration allowed locomotor activity to be released and a tight 1:1 coupling could be obtained again between the two motor patterns in this new experimental situation. 4) An analysis of the data revealed that the various forms of coordination obtained in the different experimental situations are due to a central resetting of the respiratory and of the locomotor patterns. The capability of the hindlimb proprioceptive inputs to coordinate locomotor and respiratory patterns in the decorticate-DOPA preparation appeared simply linked to their ability to entrain the activity of the lumbar locomotion generator. It is suggested that these central reciprocal interactions, which have the properties of an entrainment process, are the result of interactions between the lumbar locomotion generator and the medullary respiratory one.