Proceedings of The Physiological Society

Trinity College Dublin (2003) J Physiol 551P, C54

Communications

Arterial chemoreceptors in the superior laryngeal nerve of the rat

Andrew Murphy, Deirdre M. O'Leary, Mark Pickering and James F.X. Jones

Department of Human Anatomy and Physiology, University College Dublin, Earlsfort Terrace, Dublin 2, Ireland


McDonald & Blewett (1981) have argued that in the rat, peripheral arterial chemoreceptor tissue is diffusely distributed along the main trunks and branches of the IXth and Xth cranial nerves. We have confirmed that the superior laryngeal nerve (SLN) of the rat contains abundant paraganglia. The question arises as to whether these SLN paraganglia are oxygen sensitive.

Five male Wistar rats (224-260 g in weight) were killed humanely by a sharp blow to the head followed by transection of the spinal cord (according to local animal ethics committee guidelines). The right SLN was quickly dissected and placed in a chamber containing Hepes-buffered Tyrode solution at pH 7.4 (Sigma T2145). The chamber had a capacity of 2 ml and was water-jacketed to maintain temperature at 35°C. The bath contained two glass microelectrodes; one for single axon recording and the other for pressure micro-ejection of sodium cyanide (NaCN; 250-500 ng in 5 µl) in the proximity of glomus tissue at the bifurcation of the SLN. A small WPI probe monitored PO2 continuously. Two reservoirs were used to superfuse the preparation (5 ml min-1) and were bubbled with oxygen or nitrogen in order to alter the bath PO2. It was found that this preparation was very robust, probably due to the small size of the glomi in the nerve. Single chemoreceptor units could be held for hours and as many as 15 NaCN excitatory responses made without deterioration of the preparation. It proved to be difficult, however, to lower the PO2 below 90 mmHg because of the gas-permeable tubing used in the experimental rig. Therefore, we tried to demonstrate oxygen sensitivity of NaCN excitation. The duration of the NaCN response, and the number of spikes in the 30 s after micro-ejection of cyanide, were compared during normoxia and hyperoxia using the Wilcoxon signed rank test in five animals. The criterion for statistical significance was P < 0.05. All values are means ± S.D.

Hyperoxia significantly shortened the response duration from 169 ± 52.9 s to 62.6 ± 46.2 s (P = 0.03) and reduced the number of spikes (30 s after application of NaCN) from 231 ± 173 to 123 ± 92. This latter reduction, however, was not statistically significant (P = 0.3). A linear regression was calculated for the data points of PO2 and response duration and the correlation was found to be strong, negative and significant (R = -0.82; P = 0.004).

In conclusion, we now have convincing morphological and electrophysiological data for the presence of arterial chemoreceptors in the SLN of the rat.

We wish to acknowledge support of University College Dublin.

Where applicable, experiments conform with Society ethical requirements