ELECTROPHYSIOLOGICAL STUDY OF DORSAL RESPIRATORY NEURONS IN THE MEDULLA-OBLONGATA OF THE RAT

There has been controversy whether the dorsal respiratory group (DRG), identified in the cat and several other species as a concentration of mainly inspiratory neurons located in the ventrolateral subnucleus of the solitary tract, also exists in the rat. The aim of this study was to re-examine this question by systematically exploring this region w ith extracellular microelectrodes, in anesthetized and artificially ve ntilated rats. One-hundred and forty-two units were recorded which fir ed in phase with central respiratory cycles (determined by recording f rom the phrenic nerve) and/or lung inflations. One-hundred and ninetee n recordings were thought to be from neuronal cell bodies (confirmed i n some cases by excitatory responses to microelectrophoretic administr ation of DL-homocysteic acid), while the remaining 23 were from lung v agal afferents. Most neurons in the former group (87/119) were inspira tory. Out of 96 neurons tested for spinal projections only 14 (12 insp iratory, 2 expiratory) responded antidromically following stimulation at C-3 segment. These results confirm the existence of the DRG in the rat and demonstrate that neurons located in this region have firing pa tterns generally similar to those previously described in the cat. The main difference is the relative paucity in the rat of neurons project ing spinally below the C-2 level, which indicates that most DRG neuron s in this species do not project directly to phrenic and intercostal m otoneurons, but to other, as yet unidentified, neuronal groups within the brainstem or upper cervical segments.