Contribution of the parafacial Respiratory Group to the inspiratory, expiratory and cardiovascular responses of rats to muscle afferent fiber stimulation

Abstract

During basal conditions, breathing in mammals presents passive expiratory phase, with low activity of abdominal muscles. On the other hand, the exercise can recruit these expiratory muscles to produce active expiration. Active expiration in rats is dependent on the activity of neurons called late-Expiratory (late-E) located in the parafacial Respiratory Group (pFRG). During exercise, it is also possible to observe inspiratory and cardiovascular responses. These responses are dependent in part on the Exercise Muscle Reflex (EMR), mediated by two reflex mechanisms: the mechanoreflex and the metaborreflex, activated by muscle contraction. When activated, this reflex causes stimulation of the neurons located in the dorsal horn of the spinal cord, through afferent fibers, and consequently those located in the cardiovascular and respiratory centers in the brainstem. That is, the stimulation of afferent muscle fibers can evoke the EMR. However, the mechanisms responsible for the generation of active expiration, as well as the inspiratory and cardiovascular response during EMR activation are not yet fully established. In this sense, the hypotheses of the present research project are: (I) the stimulation of afferent muscle fibers, and consequently the EMR, in in situ preparations of rats evokes active expiration and inspiratory and cardiovascular responses; and (II) pFRG late-E neurons are involved in the central generation of active expiration and inspiratory and cardiovascular responses evoked by stimulation of afferent muscle fibers in rats. To test these hypotheses, we will analyze the expiratory, inspiratory and cardiovascular responses to stimulation of afferent muscle fibers, and consequently the EMR, before and after pFRG neurons silencing. These neurons will be selectively silenced acutely by Clozapine-N-Oxide (CNO) application following selective cellular transfection of these neurons in rats with an adeno-associated viral vector to express inhibitory G-protein-coupled receptors (hM4DiR) activated by CNO. We will also characterize the electrophysiological changes of pFRG late-E neurons in response to stimulation of afferent muscle fibers by whole cell patch clamp recordings in in situ preparations of rats. Besides, we will inject neuronal tracers into the rat´s medulla and spinal cord to investigate the afferent pathways of the EMR and its projection to pFRG. Thus, these results may contribute to a better understanding of afferent pathways involved in the central generation of inspiratory, expiratory and cardiovascular responses of rats to stimulation of afferent muscle fibers. (AU)