Sympathetic nerve fiber repulsion: testing norepinephrine, dopamine, and 17β-estradiol in a primary murine sympathetic neurite outgrowth assay

Loss of sympathetic nerve fibers (SNFs) occurs in inflamed tissue; and select semaphorins, upregulated during inflammation, stimulate repulsion/loss of SNFs. However, it is unknown whether other factors released locally in inflamed tissue, such as norepinephrine, dopamine, and 17β‐estradiol, are also repellent. In order to study the effects of hormones on SNF repulsion, an SNF outgrowth assay was used. The repellent activity of semaphorins 3C was weaker than of semaphorin 3F. Tumor necrosis factor α (TNF‐α) repelled nerve fibers with moderate to strong effects (from 0–100% repulsion). High concentrations of dopamine and norepinephrine (10−6 M) induced weak but significant nerve fiber repulsion (up to 20%). Norepinephrine at 10−8 M was comparable with 10−6 M at inducing nerve fiber outgrowth. Stimulation with low concentrations of 17β‐estradiol (10−10 M, but not 10−8 M) repelled SNFs. These results demonstrate that not only specific axon guidance molecules, such as semaphorins 3F and 3C, but also hormonal factors and TNF‐α influence SNF repulsion and outgrowth.