ROLE OF CHLORIDE-MEDIATED INHIBITION IN RESPIRATORY RHYTHMOGENESIS INAN IN-VITRO BRAIN-STEM OF TADPOLE, RANA-CATESBEIANA

1. The isolated brainstem of larval Rana catesbeiana maintained in vit ro generates neural bursts that correspond to the lung and gill ventil atory activity generated in the intact specimen. To investigate the ro le of chloride channel-dependent inhibitory mechanisms mediated by GAB A(A) and/or glycine receptors on fictive lung and gill ventilation, we superfused the isolated brainstems with agonists, antagonists (bicucu lline and/or strychnine) or a chloride-free solution while recording m ulti-unit activity from the facial motor nucleus. 2. Superfusion with the agonists (GABA or glycine) produced differential effects on freque ncy, amplitude and duration of the neural bursts related to lung and g ill ventilation. At a GABA or glycine concentration of 1.0 mM, fictive gill bursts were abolished while fictive lung burets persisted, albei t with reduced amplitude and frequency. 3. At the lowest concentration s used (1.0-2.5 mu M), the GABA(A) receptor antagonist bicuculline pro duced an increase in the frequency of lung bursts. At higher concentra tions (5.0-2.0 mu M) bicuculline produced non-specific excitatory effe cts. The glycine antagonist strychnine, at concentrations lower than 5 .0 mu M, caused a progressive decrease in the frequency and amplitude of the gill bursts and eventually abolished the rhythmic activity. At higher concentrations (7.5 mu M), non-specific excitatory effects occu rred. Superfusion with bicuculline (10 mu M) and strychnine (5 mu M) c ombined abolished the neural output for gill ventilation but increased the frequency amplitude and duration of lung bursts. 4. Superfusion w ith Cl--free solution also abolished the rhythmic neural bursts associ ated with gill ventilation, while it significantly increased the ampli tude (228 +/- 51%; P < 0.05) (mean +/- S.E.M.) and duration of the lun g bursts (3.5 +/- 0.1 to 35.3 +/- 3.7 s; P < 0.05) and improved the re gularity of their occurrence. 5. We conclude that different neural sys tems generate rhythmic activity for lung and gill ventilation. Chlorid e-mediated inhibition may be essential for generation of neural bursts associated with gill ventilation. In contrast, the burst associated w ith lung ventilation can be generated in the absence of Cl--mediated i nhibition although the latter plays a role in shaping the normal lung burst.