Graded changes in central chemoceptor input by local temperature changes on the ventral surface of medulla

1. In cats under pentobarbitone anaesthesia the effects of focal temperature changes of the ;chemoceptive' areas on the ventral surface of medulla, described by Loeschcke and his associates, were studied with respect to tidal volume, V(T), tidal variation in efferent phrenic activity, Phr(T), and respiratory rate. The cats were either paralysed and ventilated at various constant P(A,CO2) and P(a,O2) levels, or breathing spontaneously.2. It was confirmed that focal bilateral cooling of the intermediate, ;I((S))', areas caused rapid depression of respiration even at constant artificial ventilation. In normocapnic and normoxic conditions apnoea usually ensued at brain surface temperatures of 20-22 degrees C.3. The effects were graded along continuous temperature-response curves with enhancements of ventilation above and depression below normal body temperature.4. The strongest effects on V(T) and Phr(T) were obtained from the I((S)) areas with no or only small effects on inspiratory or expiratory timing in the vagotomized animal. The Hering-Breuer inflation reflex and its effects on timing and amplitudes were not affected by cooling this area.5. Focal cooling of the caudal or the rostral ;chemoceptive' areas, ;C((L))' and ;R((M))' areas, caused smaller effects on V(T) and Phr(T) but produced significant effects on respiratory rate even after vagotomy.6. The effects of focal cooling of these areas could be mimicked by topical application of procaine solution which has been shown not to penetrate deeper than 100 mum from the surface.7. Moderate focal cooling of area I((S)) to temperatures above 28-30 degrees C caused a parallel shift in the CO(2)-response (V(T), Phr(T)) curves to the right with little change in slope. The P(CO2) thresholds for apnoea were correspondingly raised. These focal temperature effects could be compensated by changes in P(CO2) with, on the average, 2.7 torr/ degrees C. Focal temperatures below 28 degrees C usually caused some decrease in slope of the CO(2)-response curves in addition to further shifts.8. Added hypoxic stimulus or electrical stimulation of the carotid sinus nerves caused an almost parallel increase of Phr(T) at all P(CO2) levels and all focal temperatures suggesting an additive type of interaction between the input from the peripheral chemoreceptors and that from the central (CO(2), H(+)) sensing structures whether the latter was altered by changing P(CO2) or by focal temperature changes on the I((S)) areas.9. In contrast to these effects of hypoxia and stimulation of the carotid sinus nerves the reflex increase of inspiratory activity caused by lung deflation or by electrical stimulation of the glossopharyngeal nerve distal to the carotid sinus nerves was CO(2) dependent. These reflex effects decreased with focal cooling of the I((S)) areas as with hypocapnia, suggesting a mainly multiplicative or ;gain-changing' type of interaction with the central chemoceptive drive.10. The close similarities in effect of focal cooling and of hypocapnia on the different respiratory parameters even during constant artificial ventilation indicate that focal temperature changes of the I((S)) areas intervene effectively with the normal ventilatory response to CO(2) without changing the chemical or physical environment of those neural structures in the brain stem which set respiratory pattern.