RESPIRATORY ACTIVITY IN THE FACIAL NUCLEUS IN AN IN-VITRO BRAIN-STEM OF TADPOLE, RANA-CATESBEIANA

1. In studies of the central neural control of breathing, little advan tage has been taken of comparative approaches. We have developed an in vitro brainstem preparation using larval Rana catesbeiana which gener ates two rhythmic neural activities characteristic of lung and gill ve ntilation. Based on the pattern of the facial (VII) nerve activity bot h lung and gill rhythm-related respiratory cycles were divided into th ree distinct phases. The purpose of this study was to characterize and classify membrane potential trajectories of respiratory motoneurons i n the VII nucleus at intermediate stages (XII-XVII) of development. 2. Seventy five respiratory-modulated neurons were recorded intracellula rly within the facial motor nucleus region. Their resting membrane pot ential was between -40 and -80 mV. Sixty of them were identified as VI I motoneurons and fifteen were non-antidromically activated. Membrane potentials of fifty-six of the seventy-five neurons were modulated wit h both lung (5-27 mV) and gill rhythms (3-15 mV) and the remaining nin eteen neurons had only a modulation with lung rhythmicity (6-23 mV). N o cells with gill modulation alone were observed. 3. All of the cells modulated with lung rhythmicity had only phase-bound depolarizing or h yperpolarizing membrane potential swings which could be categorized in to four distinct patterns. In contrast, of the fifty-six cells modulat ed with gill rhythmicity, thirty-two were phasically depolarized durin g distinct phases of the gill cycle (four patterns were distinguished) , whereas the remaining: twenty-four were phase spanning with two dist inct patterns. The magnitudes of lung and gill modulations were propor tionally related to each other in the cells modulated with both rhythm s. 4. In all sixteen neurons studied, a reduction or a reversal of pha sic inhibitory inputs during a portion of the lung or gill respiratory cycle was observed following a negative cur rent or chloride ion (Cl- ) injection. The phasic membrane resistance modulation in relation to the gill rhythm was analysed in six neurons and a relative decrease in the somatic membrane resistance (0.7-8.1 M Omega) was detected during the periods of hyperpolarization. 5. We propose that, at these interm ediate stages of development: (a) both gill and lung respiratory oscil lations in motoneurons are generated by respiratory premotor neurons h aving only a few distinct activity patterns; (b) these patterns deline ate distinct portions of the centrally generated respiratory cycles; a nd (c) phasic synaptic inhibition, mediated by Cl-, contributes to sha ping the membrane potential trajectories of respiratory motoneurons.