PRE-BOTZINGER COMPLEX IN THE CAT

1. Patterns of respiratory neuronal activity were examined in pentobarbitone anesthetized adult cats in a circumscribed area of the ventrolateral medulla, which has previously been defined as the pre-Botzinger complex (pre-BOTC) from electrophysiological and morphological criteria in the brain stem-spinal cord preparation of the neonatal rat. The pre-BOTC has been proposed to play a critical role in respiratory rhythm generation in mammals, but electrophysiological properties of the region have not been thoroughly characterized in the adult brain stem in vivo. 2. From intra- and extracellular recordings, we verified the existence of a well-defined zone with a distinct profile of neuronal activity between the rostral Botzinger complex containing expiratory neurons and the more caudal medullary pool of inspiratory neurons of the ventral respiratory group (VRG) in the para-ambigual region. This zone corresponds to the pre-BOTC. It was characterized by a concentration of the various types of respiratory neurons, particularly those proposed to be involved in respiratory phase transitions, including neurons discharging immediately before the onset of inspiratory phase activity (pre-inspiratory neurons), early-inspiratory, and postinspiratory neurons. The majority of these neurons were presumed interneurons because they were not antidromically activated by spinal cord or cranial nerve stimulation. 3. The locus of the pre-BOTC corresponded histologically to the rostral part of the nucleus ambiguus and ventrolateral reticular formation. It was located caudal to the retrofacial nucleus and rostral to the lateral reticular nucleus, extending 3.0-3.5 mm rostral to the obex, and 3.2-4.0 mm lateral from the midline. This location was homologous to that established in the neonatal rat. 4. Pre-inspiratory neurons (pre-I neurons) were specifically found in the pre-BOTC. Intracellular recordings from these neurons revealed two types of activity patterns. Type 1 of pre-I neurons exhibited a steady membrane depolarization during expiration and a steep membrane depolarization with a high-frequency burst of action-potential discharge during the phase transition from expiration to inspiration. This was followed by a decline of depolarization and spike discharge during the remainder of the inspiratory phase. A second type of pre-I neurons exhibited a secondary graded membrane depolarization and burst discharge during the late-inspiratory period. 5. Synaptic events were examined in other respiratory neurons during the 40-160 ms preceding the onset of phrenic nerve activity when pre-I neurons exhibited peak spike discharge. Early-inspiratory, throughout-inspiratory, and postinspiratory neurons were disinhibited during this period, whereas stage-2 expiratory neurons exhibited a decrease in spike activity and repolarization. 6. We conclude that the pre-BOTC is a specialized subregion of the VRG in the adult cat with electrophysiological and histological characteristics homologous to the pre-BOTC in the neonatal rat. All classes of interneurons postulated to be involved in respiratory rhythm generation are concentrated in this region. We hypothesize that pre-I neurons in the pre-BOTC may be critically involved in rhythmogenesis by promoting the expiratory-inspiratory phase transition, either by providing synaptic inhibition to stage-2 expiratory neurons causing expiratory phase termination, or as a source of excitatory drive to inspiratory neurons initiating inspiratory phase activity.