AXONAL PROJECTIONS OF CAUDAL VENTROLATERAL MEDULLARY AND MEDULLARY RAPHE NEURONS WITH ACTIVITY CORRELATED TO THE 10-HZ RHYTHM IN SYMPATHETIC-NERVE DISCHARGE

1. This is the first study to map the axonal projections of medullary neurons that are elements of the network responsible for the 10-Hz rhy thm in sympathetic nerve discharge (SND) of urethan-anesthetized cats. Spike-triggered averaging and coherence analysis were used to identif y caudal ventrolateral medullary (CVLM) and medullary raphe neurons wi th activity correlated to this component of SND. Spike-triggered avera ging showed that CVLM neurons fired significantly earlier (17 ms on th e average) than raphe neurons during the 10-Hz slow wave in inferior c ardiac postganglionic SND. This observation raised the possibility tha t CVLM neurons are a source of the discharges of raphe neurons that ar e correlated to SND. 2. Nineteen of 47 CVLM neurons with activity corr elated to the 10-Hz rhythm in SND were antidromically activated by mic rostimulation of the raphe. The longest onset latency of antidromic ac tivation was 19.9 +/- 2.8 (SE) ms, a value comparable with the differe nce in firing times of CVLM and raphe neurons during the naturally occ urring 10-Hz slow wave in inferior cardiac SND. In most cases the resp onse likely reflected activation of an axonal branch of the CVLM neuro n, because the onset latency of antidromic activation could be changed dramatically by moving the stimulating microelectrode as little as 0. 2 mm within the raphe. Also, the onset latency of antidromic activatio n of nine CVLM neurons was significantly shortened (25.0 +/- 2.5 vs. 1 6.7 /- 2.7 ms) when the stimulus intensity was raised above threshold. 3. The hypothesis that the axons of CVLM neurons with activity correl ated to the 10-Hz rhythm in SND terminated on and excited raphe neuron s was supported by the following observations. First, CVLM neurons cou ld not be antidromically activated by stimuli applied to sites in trac ks located 1.5-2 mm lateral to the midline, contralateral to the neuro nal recording site; thus their axons did not cross the midline. Second , some CVLM neurons could be antidromically activated by stimuli appli ed to sites in only one of the tracks through the midline; thus it is unlikely that their axons were destined for more rostral or caudal por tions of the brain stem. Third, 37% of the raphe neurons with activity correlated to the 10-Hz rhythm were synaptically activated by microst imulation of the CVLM, with a minimum onset latency of 18.1 +/- 2.6 ms . This value was not significantly different than the longest onset la tency of antidromic activation of CVLM neurons by raphe stimulation. 4 . CVLM neurons with activity correlated to the 10-Hz rhythm in SND cou ld not be antidromically activated by microstimulation of the rostral ventrolateral medulla (RVLM) or thoracic spinal cord. Thus CVLM neuron s are not a direct source of the 10-Hz discharges of RVLM or pregangli onic sympathetic neurons. 5. Eight of 41 raphe neurons with activity c orrelated to the 10-Hz rhythm in SND were antidromically activated by microstimulation of the CVLM. The latency of the antidromic response o f six raphe neurons was shortened from 15.2 +/- 3.1 to 11.9 +/- 3.1 ms by raising stimulus current above threshold, implying the existence o f local axonal branching. The onset latency of antidromic activation o f five raphe neurons was changed by moving the stimulating microelectr ode within the CVLM. 6. The axons of at least some of these raphe neur ons likely terminated in the CVLM, because higher current was required to antidromically activate these neurons from sites in a track locate d 0.5 mm further laterally, and they were not antidromically activated by microstimulation of the RVLM. Also, 32% of the CVLM neurons were e ither excited or inhibited by microstimulation of the raphe. The minim um onset latency of synaptic activation (18.3 +/- 4.2 ms) or inhibitio n (10-20 ms) of CVLM neurons by raphe stimulation was similar to the l ongest onset latency of antidromic activation of raphe neurons by CVLM microstimulation. 7. These data are consistent with the view that CVL M and raphe neurons whose discharges are correlated to the 10-Hz rhyth m in SND are interconnected. Such a circuit would fit the model of a d istributed network of neurons responsible for this rhythm, as proposed in earlier studies from this laboratory.