Presynaptic neurotransmitter and chemosensory responses to natural stimuli

We investigated the hypothesis that release of acetylcholine from presynaptic nerve terminals in the carotid body may be responsible for the excitation of carotid body chemoreceptors by hypoxia and hypercapnia and central ventilatory stimulation by hypercapnia. 4-Aminopyridine, an agent known to release presynaptic transmitters including acetylcholine, was administered intravenously (1 mg X kg-1) or by close intra-arterial injection to the carotid body (200 micrograms) in anesthetized cats. 4-Aminopyridine did not change the carotid chemosensory responses to any arterial PO2 or PCO2 levels studied, whereas it stimulated ventilation at all arterial PO2 and PCO2 levels. Atropine blocked the ventilatory effects of 4-aminopyridine but not the responses to hypoxia and hypercapnia. The results add to the evidence, which shows that the presynaptic cholinergic mechanism is not germane to carotid body chemoreception. Also, acetylcholine does not seem to mediate the central hypercapnic stimulation of ventilation.