Patterns of relationship between activity of sympathetic nerves in rabbits and rats

We used frequency domain analysis (power spectra, ordinary and partial coherence and phase spectra) of simultaneously recorded activity of postganglionic sympathetic nerves to investigate the construction of their central generators in rabbits and rats anesthetized with urethane. As found earlier in the cat, power spectra of sympathetic nerve discharge (SND) consisted of a wide-band component (1 to 10 Hz in rabbits and 1 to 15-20 Hz in rats) and superimposed cardiac and respiratory related peaks. The coherence between pairs of SNDs in the cardiac, vertebral, and renal nerves was significant over a wide range of frequencies, from 0 to 6-10 Hz in rabbits, and except for a sharp peak at the heart rate, was not explained by baroreceptor feedback. In rats, the coherence between distant nerves was relatively low (<0.2) except at the cardiac and respiratory frequencies. Analysis of partial coherences for the three nerves in rabbits revealed two main patterns; one characterized by dominance of the cardiac SND generator, and the other by strong coupling of the vertebral and cardiac SNDs, as compared with renal SND. Phase spectra of distant nerves contained a well-defined transportation lag corresponding to a delay of approximately 70 ms between upper and lower thoracic spinal cord segments. At frequencies close to heart rate however, the phase was constant in most experiments indicating that different mechanisms are involved in transmitting wide band and oscillatory components of resting SND. The similarities between sympathetic oscillators in cats, studied previously in great detail, and rabbits preferred in recent behavioral studies allow the translation of knowledge between these two species.