Role of nucleus isthmi in the ventilatory response to hypoxia of Bufo paracnemis.

Nucleus isthmi (NI) is a mesencephalic structure of the amphibian brain that has recently been reported to participate in CO2-ventilatory response. The present study was designed to test the hypothesis that NI is also involved in hypoxia-induced hyperventilation and in the breathing pattern of the toad Bufo paracnemis. Pulmonary ventilation was directly measured by pneumotachography method in control, sham-operated and NI-lesioned toads exposed to normoxia and hypoxia (7 and 5% inspired O2). Under normoxic conditions, NI lesion caused no significant change in the ventilatory pattern or in the pulmonary ventilation. Hypoxia caused a significant (P < 0.05) increase in ventilation in control and sham-operated animals mainly due to an elevated VT. The hypoxia-induced hyperventilation was greater (P < 0.05) in the NI-lesioned toads, due to increases in both fR and VT. Such increased fR under hypoxia was due to a higher number of breaths per burst. The data indicate that NI plays no role under normoxic conditions but is involved in the ventilatory response to hypoxia, exercising an inhibitory modulation on pulmonary ventilation.