An epinephrine-containing pathway in avian spinal cord: development and localization

We have studied the uptake mechanism, biochemistry and autoradiographic localization of a descending epinephrine-containing pathway in the chick spinal cord. This epinephrine (E) projection has a developmental timetable (appears at 14 days in ovo) that is different from those of the serotonin (5-HT) and norepinephrine (NE) projections which appear at 8 and 12 days respectively. E possesses its own uptake mechanism with different pharmacological specificities from those of the NE and 5-HT uptake mechanisms. Phenylethanolamine-N-methyltransferase (PNMT), the enzyme that converts NE to E, is present in the cord at 14 days in ovo which is the same time that the uptake mechanism is detectable. Transection of the spinal cord at upper thoracic levels almost completely eliminates the uptake mechanism and PNMT activity below the transection, indicating a supraspinal origin of this pathway. E can first be detected fluorimetrically at 12 days in ovo but at this age E appears not to be of supraspinal origin since transmission at 5 days in ovo does not deplete the spinal cord of E. However, transection of the spinal cord at 3 days post-hatching does markedly reduce the E content by 12 days. Autoradiographic analysis after uptake with [3H]E shows a circumscribed localization of the uptake of E to the neuropil of the preganglionic sympathetic nucleus (nucleus of Terni). These observations demonstrate the presence of a separate descending epinephrine-containing projection in the avian spinal cord which terminates predominantly on preganglionic sympathetic neurons. This pathway may be the major central autonomic pathway in the avian spinal cord.