Identification of Substance P Immunoreactive Axons in the Aortic Depressor Nerve (ADN) of Normotensive Wistar Rats

The ADN conveys baroreceptor afferent input to the nucleus tractus solitarius (NTS) and contains unmyelinated and small myelinated fibers at a ratio of 4:1, respectively [1]. The consensus is that myelinated fibers provide rapid baroreflex control while unmyelinated fibers mainly inhibit vasomotor centers. Substance P (SP) is a primary neurotransmitter of the baroreflex. Nerve terminals with SP immunoreactivity occur in the aortic arch, nodose ganglia and NTS [2] but SP has not been demonstrated in the ADN. Twenty 20‐week old Wistar rats (10 male, 10 female) were anesthetized and had their left ADN isolated for recording spontaneous activity. ADN and phrenic nerves (positive controls) were removed, fixed in 2.5% glutaraldehyde to optimize preservation of axonal morphology and cryosectioned transversely (18 µm) or longitudinally (16 µm). Slide‐mounted sections were incubated in rabbit anti‐substance P (1:10.000), biotinylated donkey anti‐rabbit IgG (1:500) and finally Extravidin‐HRP (1:1.500), all for 24hr. A nickel‐intensified diaminobenzidine reaction [3] revealed immunoreactivity. Small SP‐positive axons, mainly unmyelinated, were present in all sections. These results show that SP can be detected in nerves that have been immersion‐fixed in a high concentration of glutaraldehyde. This method will allow quantification of ADN axons with a specific physiological role in the baroreflex. Support: FAPESP, CNPq, CAPES