Effects of monoamine oxidase inhibition on catecholamine levels: evidence for synthesis but not storage of epinephrine in rat spinal cord

Epinephrine levels in the intermediolateral cell group in the rat spinal cord are very low, although there is a dense projection to this region from cells containing all the enzymes required for epinephrine biosynthesis. One explanation for this finding is that epinephrine in the nerve terminals is degraded as soon as it is synthesized, so that no epinephrine is actually stored in synaptic vesicles. To test this hypothesis, epinephrine levels were measured in spinal cord of rats pretreated with an inhibitor of monoamine oxidase, the major enzyme involved in epinephrine degradation. Selected other tissues (i.e. brainstem, hypothalamus, adrenal gland, superior cervical ganglion) were examined for comparison. Pargyline treatment (75 mg/kg i.p., 4 h prior to sacrifice) increased catecholamine levels in spinal cord, hypothalamus, and brainstem. However, the percent increase in epinephrine in the spinal cord and brainstem was much larger than that for dopamine and norepinephrine in the 3 central nervous system regions studied, as well as larger than that for epinephrine in the hypothalamus. These results suggest that phenylethanolamine N-methyltransferase (PNMT)-containing terminals in the rat spinal cord can synthesize epinephrine, but that little if any epinephrine is stored in synaptic vesicles due to the rapid metabolism of cytoplasmic catecholamines by monoamine oxidase. In contrast, pargyline pretreatment had no effect on catechol levels in the adrenal gland, suggesting that little metabolism of catecholamines takes place in those epinephrine-synthesizing cells. Furthermore, since pargyline pretreatment increased norepinephrine levels but decreased dopamine levels in the superior cervical ganglion, it is suggested that most of the dopamine in that sympathetic ganglion is present as a precursor to norepinephrine in noradrenergic postganglionic sympathetic neurons.(ABSTRACT TRUNCATED AT 250 WORDS)