Peripheral fields of sympathetic vasoconstrictor neurons in guinea pigs

Abstract

We have combined retrograde axonal tracing using Fast Blue and DiI, with immunohistochemistry, to estimate the maximum size of peripheral fields of identified sympathetic vasoconstrictor neurons projecting to guinea-pig ear tips. Many neurons in the superior cervical ganglia were labelled with both Fast Blue and DiI after dye injections up to 7 mm apart. Few neurons were labelled when dye injections were 8–10 mm apart. Neurons labelled with both DiI and Fast Blue after dye injections 5–7 mm apart had, on average, larger somata (436±84 μm 2, mean±SEM, n=47) than neurons labelled with DiI only (388±11 μm 2, n=147). Typically, 50–100 neurons innervated a region of vasculature 1 mm in diameter. We conclude that sympathetic vasoconstrictor neurons branch widely before converging on to their target blood vessels. Progressive recruitment of vasoconstrictor neurons with increasing field size would provide an efficient mechanism for graded neural control of the circulation.