PROPERTIES OF PRESYMPATHETIC NEURONS IN THE ROSTRAL VENTROLATERAL MEDULLA IN THE RAT - AN INTRACELLULAR STUDY IN-VIVO

1. Intracellular recordings were made in pentobarbitone-anaesthetized rats from sixty-eight neurones located in the rostral ventrolateral me dulla (RVLM), which responded with inhibition (latency, 33.6 +/- 9.3 m s) after stimulation of the aortic depressor nerve with short bursts o f pulses. This inhibition was due to chloride- and voltage-dependent I PSPs. 2. Seventeen neurones could be excited antidromically after stim ulation in the T2 spinal segment (conduction velocity, 1.9-8.5 m s(-1) ) and were classified as RVLM presympathetic vasomotor neurones. 3. 'S pontaneously' active neurones (n = 29) displayed a largely irregular p attern of firing, with no clear relationship between the level of the membrane potential and cycles of phrenic nerve activity at end-tidal C O2 < 5.0%. Cardiac cycle-related shifts of the membrane potential were not considered indicative of baroreceptor input as they could be due to movement artifacts. 4. All neurones displayed synaptic activity (EP SPs and IPSPs, peak-to-peak large amplitude > 5.0 mV). The depolarizin g IPSPs observed during injection of chloride and/or negative current consisted of a phasic and a tonic component. 5. The on-going activity of these neurones resulted from synaptic inputs, with individual actio n potentials usually preceded by identifiable fast EPSPs. 6. No eviden ce was found for the presence of gradual depolarizations (autodepolari zations) between individual action potentials, and therefore under the se experimental conditions the activity of RVLM presympathetic neurone s did not depend on intrinsic pacemaker properties. 7. These results a re consistent with the 'network' hypothesis for the generation of symp athetic vasomotor tone.