Firing patterns of pre-Botzinger and Botzinger neurons during hypocapnia in the adult rat

Controversy exists about how a coordinated respiratory rhythm is generated in the brainstem. Some authors suggest that neurons in the pre-Botzinger co mplex are key to initiation of all types of breathing. While, on the other hand, it has been reported that some pre-Botzinger neurons fail to maintain a rhythmic discharge in phase with phrenic nerve discharge Juring mechanic al hyperventilation. Extracellular recordings were made from respiratory un its in the pre-Botzinger and Botzinger complexes of 13 anaesthetised. paral ysed and vagotomised rats. Central respiratory activity was monitored from the C5 phrenic nerve. During mechanical hyperventilation, several changes w ere observed in the phrenic neurogram. Firstly, the frequency and amplitude of integrated phrenic nerve discharge were reduced and reversibly stopped. Secondly, the patterned discharges changed from an augmenting to a variety of non-augmenting patterns in 53 of 60 cases. In some cases (n=9) we obser ved that the pattern appeared to have two components, an early short durati on discharge followed by a longer duration discharge. Respiratory units als o started to show different firing patterns during mechanical hyperventilat ion. In general, they were divided into those units that fired tonically (n = 28) and units that became silent (n = 32), before phrenic nerve discharg e ceased coincidently with complete apnoea. Of particular interest were tho se expiratory-inspiratory units in the pre-Botzinger complex (n=8) that nar rowed their firing period towards late expiration and early inspiration dur ing mechanical hyperventilation. Given their firing features, it is possibl e that these expiratory-inspiratory units may participate in generation of the early inspiratory component of phrenic nerve discharge. (C) 2001 Elsevi er Science B.V. All rights reserved.