Respiratory effects of high cervical cord cold blockade on efferent vagal and phrenic discharges in the rabbit.

A technique of reversible cold blockade was applied in decerebrate and vagotomized rabbits that were immobilized and artificially ventilated to study the modulation of spontaneous respiratory rhythms. Respiratory discharges were recorded from vagal and phrenic efferents before and during cold blockade at the second cervical segment (C2) with a coolant-circulated thermode (-15 degrees C). Measurement of the cooling profile demonstrated that there was significant hypothermia in the regions of the phrenic nucleus (+25 degrees C) and obex of the medulla (+26 degrees C). Arterial pressure was maintained by continual norepinephrine infusion, end-tidal carbon dioxide tension was held at hypercapnic levels, and rectal temperature was regulated near 38 degrees C. The cold blockade of descending respiratory drives to the cervical phrenic nucleus inhibited the spontaneous activity in the phrenic nerve for more than 90 min. Phrenic activity could be induced by the intravenous injection of strychnine, but not doxapram, although this was not of respiratory quality. These results show that in the absence of descending and pharmacologic drives, but in the presence of phrenic hypothermia, spinalized rabbits are incapable of generating rhythmic patterns of discharge. C2 cold blockade also significantly slowed the spontaneous central respiratory rhythm with no change in integrated vagal amplitude, presumably due to a direct cooling effect on brainstem oscillators for breathing.