CHEMICAL ACTIVATION OF CAUDAL MEDULLARY EXPIRATORY NEURONS ALTERS THEPATTERN OF BREATHING IN THE CAT

1. The purpose of this work was to ascertain whether the activation of caudal expiratory neurones located in the caudal part of the ventral respiratory group (VRG) may affect the pattern of breathing via medull ary axon collaterals. 2. We used microinjections of DL-homocysteic aci d (DLH) to activate this population of neurones in pentobarbitone-anae sthetized, vagotomized, paralysed and artificially ventilated cats. Bo th phrenic and abdominal nerve activities were monitored; extracellula r recordings from medullary and upper cervical cord respiratory neuron es were performed. 3. DLH (160 mM) microinjected (10-30 nl for a total of 1.6-4.8 nmol) into the caudal VRG, into sites where expiratory act ivity was encountered, provoked an intense and sustained activation of the expiratory motor output associated with a corresponding period of silence in phrenic nerve activity. During the progressive decline of the activation of abdominal motoneurones, rhythmic inspiratory activit y resumed, displaying a decrease in frequency and a marked reduction o r the complete suppression of postinspiratory activity as its most con sistent features. 4. Medullary and upper cervical cord inspiratory neu rones exhibited inhibitory responses consistent with those observed in phrenic nerve activity, while expiratory neurones in the caudal VRG o n the side contralateral to the injection showed excitation patterns s imilar to those of abdominal motoneurones. On the other hand, in corre spondence to expiratory motor output activation, expiratory neurones o f the Botzinger complex displayed tonic discharges whose intensity was markedly lower than the peak level of control breaths. 5. Bilateral l ignocaine blockades of neural transmission at C2-C3 affecting the expi ratory and, to a varying extent, the inspiratory bulbospinal pathways as well as spinal cord transections at C2-C3 or C1-C2, did not suppres s the inhibitory effect on inspiratory neurones of either the ipsi- or contralateral VRG in response to DLH microinjections into the caudal VRG. 6. The results show that neurones within the column of caudal VRG expiratory neurones promote inhibitory effects on phrenic nerve activ ity and resetting of the respiratory rhythm. We suggest that these eff ects are mediated by medullary bulbospinal expiratory neurones, which may, therefore, have a function in the control of breathing through me dullary axon collaterals.