Distribution, origin and sensitivity to capsaicin of primary afferent substance P-immunoreactive nerves in the heart

This report is intended as an overview of the distribution, origin and sensitivity to capsaicin of substance P-immunoreactive (SP-I) primary afferent cardiac nerves. Immunohistochemical and physiological methods were employed to compare the presence and density of these nerve fibers in the guinea pig and rat hearts. SP-I fibers are numerous in the guinea pig heart including the parietal pericardium, atria, ventricles, valves, coronary arteries and around intrinsic cardiac ganglion cells. The rat heart contains few SP-I fibers. Vagotomy does not influence the number of intensity of immunoreactive fibers in the guinea pig heart. By stimulating the atrium or ventricle and recording from the second or third thoracic dorsal roots Ad1, Ad2 and C fibers were demonstrated in the atria, but only Ad fibers in the guinea pig ventricle; in addition, only Ad fibers were recorded from the vagus nerves. Only Ad1 fibers were demonstrated in the rat heart. Treatment with capsaicin depletes the SP-I and decreases the conduction velocity of C-fibers and some Ad2 fibers in the guinea pig heart. We suggest that SP-I primary afferent nerve fibers are unmyelinated (C-type) or small myelinated (Ad2-type) nerves in the guinea pig heart and that their cell bodies of origin are predominantly in dorsal root ganglia.