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

Abstract

SUMMARY1. In cats under pentobarbitone anaesthesia the effects of focal temperature changes of the 'chemoceptive' areas on the ventral surface of medulla, described by Loescheke and his associates, were studied with respect to tidal volume, VT, tidal variation in efferent phrenic activity, PhrT, and respiratory rate. The cats were either paralysed and ventilated at various constant PA, co and Pa,O2 levels, or breathing spontaneously. It was confirmed that focal bilateral cooling of the intermediate, 'I(S)', areascaused rapid depression of respiration even at constant artificial ventilation. In normocapnic and normoxic conditions apnoea usually ensued at brain surface temperatures of 20-22 "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 VT and PhrT 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 VT and PhrT 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 pm from the surface. suggesting an additive type of interaction between the input from the peripheral chemoreceptors and that from the central (C02, H+) sensing structures whether the latter was altered by changing Peo2 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 C02 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 CO2 without changing the chemical or physical environment of those neural structures in the brain stem which set respiratory pattern.

Circadian pattern of ventilation during acute and chronic hypercapnia in conscious adult rats

“…A blunting effect of low T b on CO 2 chemosensitivity has been previously suggested, with a central, via the brain stem or hypothalamus (2,5,18), rather than a peripheral site of action (13). A reduction in T b has been associated with a decrease in not only the hyperpneic response (17) but also in the V E /V O 2 response (21,31).…”

“…In humans studied under 40 h of constant ambient conditions and wakefulness, V E per unit change in end-tidal CO 2 , assessed every few hours by rebreathing, followed a circadian pattern (33, 34). Different Table 2).…”

“…Variations in metabolism are also associated with changes in V E chemosensitivity (19). A higher V O 2 , as during muscular exercise (35) or cold exposure (12), is accompanied by a greater absolute increase in V E in response to acute hypoxia or hypercapnia, whereas reflex hyperpnea is lower whenVO 2 is reduced, such as with myxedema (36) or semi-starvation (6). Hence, the V E chemoreflex response seems to track V O 2 .…”

Central chemosensitivity and the reaction theory.

“…In the area between these two, the intermediate area, no positive ventilatory responses to acid were found, but rather a slight depression. This area has attracted much interest because either coagulation, application of procaine or cooling (to 12 'C) eliminated central chemosensitivity with the result that in a previously chemodenervated animal breathing stopped completely even during inhalation of CO2 Kao, 1979). When the central ends of the cut peripheral chemoreceptor nerves were stimulated electrically rhythmic breathing resumed (Loeschcke, Schlaefke, See & Herker-See, 1979) Dev & Loeschcke (1979a, b).…”

The respiratory role of the ventral surface of the medulla studied in the anaesthetized rat.

“…Mitchell, J. L. MALCOLM, I. H. SARELIUS AND J. D. SINCLAIR can affect circulatory control and thermoregulation as well as respiration. Cherniack, von Euler, Homma & Kao (1979) demonstrated a regional localization of thermal effects on respiration, present between 20 and 38 TC, and interacting with responses to inhaled CO2.…”