Central CO2 chemoreception: a mechanism involving P2 purinoceptors localized in the ventrolateral medulla of the anaesthetized rat

Abstract

1. The involvement of P2 purinoceptors in chemosensory function in the ventrolateral regions of the medulla oblongata was investigated in the anaesthetized rat. We have investigated the effect of antagonizing, or desensitizing, P2 receptors in the retrofacial area of the ventrolateral medulla on factors modifying respiratory activity. 2. Bilateral microinjection of suramin (50 nl, 0.02 M), a P2 purinoceptor antagonist, into the retrofacial area in the artificially ventilated rat reduced resting phrenic nerve discharge. It also markedly affected the response of the phrenic nerve to increases in arterial CO2. Under conditions of hyperoxic, hypocapnic apnoea, the mean threshold for inducing phrenic nerve activity was raised significantly (from an end-tidal CO2 of 2.5 % to 4.5 %, n = 9). 3. In addition, the slope of the respiratory response curve to increases in CO2 was reduced after suramin. A similar effect was observed after desensitization of certain P2X receptors with alphabeta-methyleneATP. As arterial levels of O2 were greater than 100 mmHg, and an equivalent pattern of response was observed in sino-aortically denervated and vagotomized animals, we believe any contribution of the peripheral chemoreceptors to be minimal. 4. Our data suggest that respiratory neurones within the retrofacial area (Botzinger complex) represent part of the central site of action of CO2 on respiration. Moreover, our observations lead us to suggest that CO2-evoked changes in respiration are mediated at least in part by P2X purinoceptors.

Figure 1. Raw data from an anaesthetized and ventilated rat showing effects of bilateral microinjection of suramin into the VLM on the phrenic nerve and cardiovascular responses to increases in inspired CO₂

A, response to increasing levels of CO₂ before suramin. Note that the threshold level of end-tidal CO₂ required to induce phrenic nerve activity from apnoea (shown by interrupted line) is 2.4 %. B, response to increasing CO₂ levels after suramin. Apnoeic threshold was 5.8 % end-tidal CO₂. Note a reduction in the amplitude of phrenic bursts in response to hypercapnia after suramin. IPNA, integrated phrenic nerve activity; HR, heart rate; ABP, arterial blood pressure.

Figure 3. Effect of bilateral microinjection of suramin into the VLM and inferior olives, and of αβ-meATP and saline into the VLM, on respiratory response curve to increasing levels of inspired CO₂

Examples of respiratory response curves to increasing end-tidal CO₂ are shown for five different animals. Respiratory activity is plotted against end-tidal CO₂. ○, control response; •, response after microinjection of suramin into the VLM (responses of two different animals shown) (A) and the inferior olives (B). C, ▴ indicate the response after microinjection of αβ-meATP into the VLM; D, ▪ indicate the response after microinjection of saline into the VLM. Numbers in parentheses give the slope in arbitrary units.

Figure 2. Raw data from an anaesthetized and ventilated rat showing effects of bilateral microinjection of suramin into the inferior olives on the phrenic nerve and cardiovascular responses to increases in inspired CO₂

A, response to increasing levels of CO₂ before suramin. B, response to increasing CO₂ levels after microinjections of suramin into the inferior olives. Suramin did not affect the apnoeic threshold or the amplitude of phrenic bursts in response to hypercapnia. Abbreviations are as in Fig. 1.

Figure 4. Distribution of histologically determined sites into which microinjections of suramin, αβ-meATP and saline were made into the VLM or inferior olives

•, sites of microinjection of suramin or αβ-meATP; ▴, sites of microinjection of saline. Distances from the interaural level are indicated. NTS, nucleus tractus solitarii; Amb, ambiguus nucleus; Bo, Bötzinger complex; PrBo, pre-Bötzinger complex; Io, inferior olives.