Monosynaptic connection from caudal to rostral ventrolateral medulla in the baroreceptor reflex pathway

Experiments were done to test the hypothesis that caudal ventrolateral medulla (CVLM) neurons excited by activation of arterial baroreceptors and by stimulation of depressor sites in the nucleus tractus solitarii (NTS) project monosynaptically to the rostral ventrolateral medulla (RVLM). In urethan anaesthetized and artificially ventilated rats we recorded extracellular activity from 46 spontaneously firing units in the CVLM. Twenty of these units were excited by baroreceptor activation (1-3 micrograms phenylephrine i.v.) and of these 6 were excited (mean latency of 9.8 +/- 2.3 ms) by single pulses (0.1 ms, 30 +/- 8.3 microA) delivered once per second to a depressor site in the ipsilateral NTS. These 6 units were also antidromically activated with a latency of 4.1 +/- 0.12 ms by stimulation of a pressor region in the ipsilateral RVLM. These results provide evidence for the existence of an excitatory projection from the NTS to the CVLM which, in turn, projects monosynaptically to sympathoexcitatory neurons in the RVLM.