Cardiac nociceptors innervated by vagal afferents in rats

It is reported that cardiac afferent information is transmitted through at least three different pathways to the CNS: sympathetic, parasympathetic, and somatic; however, there are few studies concerning the role of afferent fibers of vagus nerves for eliciting cardiac sensation including pain. Especially, receptive field properties innervated by single vagal nerve fiber and mechanical threshold of nociceptors on the cardiac surface are not yet quantitatively studied. Therefore, in this study, we systematically investigated characteristics of vagal units innervating cardiac nociceptors in rats. Using anesthetized and artificially ventilated rats, 37 single unit recordings were made from fine nerve filaments of the left vagal nerve. For quantitative mechanical stimulation, the cardiac surface was stimulated by a von Frey type device. In addition, bradykinin was used for checking the chemical sensitivity of the nociceptor. Electrical stimulation was used to estimate the conduction velocity of the recorded nerve fiber. All units recorded from the vagal nerve were either Adelta- or C-polymodal nociceptors. About 70% of the afferents had conduction velocities in the C-fiber range. In 60% of the units, the peripheral receptive field covered spot-like areas, but we also found larger and continuous receptive fields. Our results show that a majority of nociceptors innervated by vagal afferents are the C-polymodal type with spot-like receptive fields. We consider it to relate to the ambiguous and dull pain of angina pectoris.