Perturbations of Respiratory Rhythm and Pattern by Disrupting Synaptic Inhibition within Pre-Bötzinger and Bötzinger Complexes

Abstract

The pre-Bötzinger (pre-BötC) and Bötzinger (BötC) complexes are the brainstem compartments containing interneurons considered to be critically involved in generating respiratory rhythm and motor pattern in mammals. Current models postulate that both generation of the rhythm and coordination of the inspiratory-expiratory pattern involve inhibitory synaptic interactions within and between these regions. Both regions contain glycinergic and GABAergic neurons, and rhythmically active neurons in these regions receive appropriately coordinated phasic inhibition necessary for generation of the normal three-phase respiratory pattern. However, recent experiments attempting to disrupt glycinergic and GABAergic postsynaptic inhibition in the pre-BötC and BötC in adult rats in vivo have questioned the critical role of synaptic inhibition in these regions as well as the importance of the BötC, which contradicts previous physiological and pharmacological studies. To further evaluate the roles of synaptic inhibition and the BötC, we bilaterally microinjected the GABA_A receptor antagonist gabazine and glycinergic receptor antagonist strychnine into the pre-BötC or BötC in anesthetized adult rats in vivo and in perfused in situ brainstem-spinal cord preparations from juvenile rats. Muscimol was microinjected to suppress neuronal activity in the pre-BötC or BötC. In both preparations, disrupting inhibition within pre-BötC or BötC caused major site-specific perturbations of the rhythm and disrupted the three-phase motor pattern, in some experiments terminating rhythmic motor output. Suppressing BötC activity also potently disturbed the rhythm and motor pattern. We conclude that inhibitory circuit interactions within and between the pre-BötC and BötC critically regulate rhythmogenesis and are required for normal respiratory motor pattern generation.

Significance Statement: Defining functional roles of postsynaptic inhibition in respiratory and other mammalian central pattern generation circuits is a longstanding problem. Inhibitory circuit interactions within and between the brainstem respiratory pre-BötC and BötC have been proposed to be critically involved in normal rhythm and motor pattern generation. A fundamental role of postsynaptic inhibition in these regions has been questioned in recent experiments attempting to pharmacologically disrupt this inhibition. To resolve this contradiction, we applied similar approaches of microinjecting selective pharmacological antagonists of GABA_Aergic and glycinergic receptor-mediated inhibition in the pre-BötC and BötC in rats. Our results demonstrate large, site-specific perturbations of respiratory rhythm and motor pattern including disruption of rhythmic motor output and thus confirm the critical role of inhibitory circuit interactions.

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