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 studied 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 > 8 ms 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 < 8 ms, and this was unperturbed by cooling the cVLM in all cases. Further, there was no obvious difference in the baseline discharge rates between cells in the hemi-slice and those recorded in an intact slice. In a hemi-coronal slice cooling the cVLM also produced a 20% decrease in firing rate in identified SD neurons but no consistent change in AA cells. We conclude that (1) the ipsilateral cVLM contributes principally tonic excitatory drive to rhythmically active neurons in the dorsomedial NTS in vitro and this preferentially effects SD neurons; (2) other excitatory drives other than those from the ipsilateral cVLM impinge upon SD cells, the origin of which are relatively local and likely to be in the NTS; (3) in the slice the projection from the cVLM to the NTS appears to be present but the reciprocal connection is absent.