Activation of the 10-Hz sympathetic generator during the second phase of severe hypoxia-hypercapnia and Cushing reaction.

Under urethane anesthesia, as in freely moving cats, the sympathetic nerve discharge (SND) contains a 10-Hz component, either as a single peak in the autospectra or in addition to the cardiac-related or 2-6-Hz rhythm. In this study, we examined the changes in these rhythmic SND components during the reaction to asphyxia and to sudden elevation in the intracranial pressure (Cushing reaction). In all cats included in this study, resting SND was dominated by 2-6-Hz rhythm, but a peak at 10 Hz was also present in the coherence functions. During asphyxia or Cushing reaction, the 2-6-Hz component exhibited the usual two-phase pattern of activation followed by suppression. In phase 2, however, the SND did not desynchronize, as in chloralose-urethane anesthetized cats [Am. J. Physiol. 256 (1989) R120; J. Physiol. (London) 469 (1993) 37], and the massive SND activation and the resulting pressor reaction was due to strengthening of the 10-Hz rhythm. After the ischemic reaction, the 10-Hz component diminished and 2-6-Hz rhythm recovered. These findings suggest that two-phase response to hypoxia-hypercapnia is not due to hypoxic neuronal damage but represents a physiological sympathetic reaction involving different patterns of SND.