Study of verification and origin of a novel medullary respiratory neuron group (para-facial respiratory group)

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Aim of this study was to reveal cellular level entity (e.g. distribution, morphology, physiological and pharmacological characteristics) and the function of novel medullary respiratory neuron group (para-facial respiratory group, pFRG ; Onimaru ＆ Homma, 2003). For this analysis, we used the brainstem-spinal cord preparation isolated form newborn rats. The present study revealed the following : 1)The pFRG consisted predominantly of pre-inspiratory (Pre-I) neurons. 2)Cell soma of the pFRG/Pre-I neuron was located close to the ventral surface, immediately below the pia membrane, at the level of the facial nucleus. 3)Activity of the pFRG/Pre-I neurons was not inhibited by μ-opiate agonist, DAMGO. 4)Activity of the pFRG/Pre-I neurons was reflected in the facial nerve activity. 5)When the medulla was transversely cut between the pFRG and the pre-Botzinger complex (PBC), each block of the medulla could produced spontaneous rhythmic activity under the appropriate experimental conditions. 6)These rhythmic activity corresponded to the pFRG/Pre-I (in the rostral block) and PBC/inspiratory (in the caudal block) neuron activity. 7)Rhythm of the pFRG/Pre-I neurons was depressed by increasing extracellular potassium ion concentration, application of a persistent sodium channel blocker (riluzole) or lowering CO2 pressure, whereas the PBC/inspiratory activity was more resistant to these manipulations. 8)When the medulla was cut through the midline, rhythmic activity of the pFRG/Pre-I neurons was maintained independently in the right and left medulla, but PBC-inspiratory activity was stopped. These findings showed that mechanism of rhythm generation by the pFRG/Pre-I neurons are essentially different from that of PBC-inspiratory neurons and suggested that the pFRG/Pre-I neurons determine basic respiratory rhythm under normal experimental condition with normal (or lower) external potassium concentration.