The ventrolateral medulla as a source of synaptic drive to rhythmically firing neurons in the cardiovascular nucleus tractus solitarius of the rat

We sought to determine whether the caudal ventrolateral medulla (cVLM), at the level of area postrema, influences the rhythmically beating neurons found within the dorsomedial NTS in rat brainstem slices. Intra- or extracellular recordings of neurons firing rhythmically at around 5 Hz were characterized as either auto-active (i.e. pacemaker; AA) or synaptically driven (SD) by pharmacological interventions. The nature of inputs evoked from the ipsilateral cVLM were orthodromic and the majority were excitatory (latency 3–20 ms). Further, this excitatory influence was found to be tonically active in 25/47 cells studies since inactivating the ipsilateral cVLM by localized cooling reduced the firing rate by 0.5–3.0 Hz (23% on average). Neuronal characterization showed that the most consistent and pronounced effect occurred on SD rather than AA cells. Control experiments that cooled other areas of the slice closer to the recording site proved ineffective. Additional studies showed that most rhythmically firing cells in the NTS received an excitatory synaptic input from the solitary tract ( ts; latency 3–30 ms). This input was reduced or blocked by inactivating the cVLM in neurons in which the ts latency of activation was 8ms in half of the neurons tested. Subsequent pharmacological tests revealed that these neurons were predominantly SD. Identified AA neurons received an input from the ts at a shorter latency, typically