Sympathoadrenal mechanisms underlying cardiovascular responses to intrathecal substance P in conscious rats

The sympathoadrenal mechanisms underlying cardiovascular responses to intrathecal (i.th.) injection of substance P (SP), at the T-9 spinal level, were investigated in conscious rats. Release of norepinephrine (NE), and epinephrine (EPI) could be induced by 6.5, 16.25, and 32.5 nmol SP correlated with a pressor and a positive chronotropic response. Release of neuropeptide Y (NPY) was evoked also by 32.5 nmol SP, and released NPY appears to derive from the adrenal medulla. The initial pressor response in intact rats is mediated by NE from sympathetic fibers, whereas the latent pressor response, observed in sympathectomized rats, is possibly due to NPY from the adrenal medulla. Although a functional balance between the sympathetic fibers and the adrenal glands is supported by the results, it appears that only the tachycardia can be efficiently produced by the adrenals in the absence of the sympathetic fibers. This study supports a functional role for SP in regulation of the sympathoadrenal system at the spinal level.