Abstract
Nitric oxide has been shown to react under physiologic conditions with norepinephrine (NE) to produce 6-nitro-norepinephrine (6-NO2-NE), a compound that enhances NE release in the brain. Previous studies suggest that 6-NO2-NE is formed in the spinal cord and stimulates spinal NE release to produce analgesia. The purpose of the current studies was to examine the mechanisms by which 6-NO2-NE stimulates NE release in the spinal cord. Crude synaptosomes were prepared from spinal cords of male Sprague-Dawley rats and loaded with [3H]NE. Incubation of synaptosomes with 6-NO2-NE resulted in a release of NE, with a threshold of 1 μM 6-NO2-NE and a maximum effect of 30% fractional release. NE transporter inhibitors desipramine and nomifensine blocked NE release from 6-NO2-NE, and desipramine exhibited an IC50 of 9.6 μM. NE release from 6-NO2-NE was dependent on external Na+, but not Ca2+ or the activity of guanylate cyclase. 6-NO2-NE also blocked uptake of [3H]NE into synaptosomes, with an IC50 of 8.3 μM. These data are consistent with a direct action of 6-NO2-NE on noradrenergic terminals in the spinal cord to release NE. This action is independent of guanylate cyclase activation, and most likely shares a common mechanism with classic monoamine releasers such as amphetamine that cause direct release of NE from vesicles into the nerve terminal cytoplasm, leading to extracellular release by reverse transport.
Footnotes
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Send reprint requests to: James C. Eisenach, M.D., Wake Forest University School of Medicine, Medical Center Blvd., Winston-Salem, NC 27157-1009. E-mail: eisenach{at}wfubmc.edu
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↵1 Supported in part by National Institutes of Health Grant GM35523 and a grant from the Max Kade Foundation.
- Abbreviations:
- NE
- norepinephrine
- NO
- nitric oxide
- 6-NO2-NE
- 6-nitro-norepinephrine
- Received August 13, 1999.
- Accepted November 10, 1999.
- The American Society for Pharmacology and Experimental Therapeutics
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