Effects of picrotoxin on apneustic respiration by tetrodotoxin in the rabbit.

The effects of carotid body chemoreceptor stimulation by NaCN and sustained lung inflation on TTX-induced apneustic respiration were studied before and after picrotoxin in the rabbit. Intravenous and intracarotid injections of TTX (30-100 micrograms) induced the apneusis. The characteristic phrenic responses induced by NaCN and lung inflation were not observed during the TTX-induced apneusis. The results indicate that apneustic respiration induced by TTX may cause a reduction of spontaneous cell activity in the medulla and pons respiratory neurons. In TXX-treated animals, picrotoxin partially restored the excitatory phrenic responses by NaCN, whereas the lung inflation effects were not restored. Therefore, it is most likely that the TTX-resistant brain stem respiratory neurons may be stimulated by picrotoxin and this effect also acts to elevate the sensitivity of inspiratory neurons closely related to the PC neurons.