Activity of aortic chemoreceptors in the anaesthetized rat

1. It has been widely accepted that the rat aortic depressor nerve contains only baroreceptors. However, the experiments which have provided these neg ative data have employed whole aortic nerve recording. In the present study , the technical difficulties associated with recording single fibres in viv o, from the rat aortic nerve (diameter 25-50 mu m), have been surmounted us ing a small tip, glass suction electrode technique. 2. Upon switching from normocapnic hyperoxia to hypercapnic hypoxia, irregu larly firing units (n = 13) appeared and these were significantly excited b y intravenous injections of sodium cyanide (20 mu g) but not by rises in ar terial blood pressure induced by methoxamine (an alpha(1)-adrenoreceptor ag onist; 10 mu g). Inhalation of 100% oxygen rapidly and reversibly silenced, or profoundly reduced, ongoing activity. 3. Intravenous injection of phenylbiguanide (PBG; a 5-HT3 receptor agonist; 8 mu g) strongly stimulated the chemoreceptors and was followed by a perio d of chemodepression (3-21 s). In contrast none of the single fibre barorec eptors recorded (n = 15) were excited by PEG but all significantly increase d their discharge in response to the increases in arterial blood pressure a ssociated with methoxamine and cyanide. Both the excitatory and inhibitory effects of PEG on the chemoreceptor fibres were abolished by ondansetron (a 5-HT3 receptor antagonist: 1 mg kg(-1) I.V.; n=5 animals) whilst the chemo excitatory action of cyanide nas preserved. 4. It is concluded that there are chemoreceptor afferents contained in the aortic nerve of the Sprague-Dawley rat. The 5-HT3 receptor appears not to b e a pre-requisite for aortic body chemoexcitation.