Sympathoneural and skeletal muscle contributions to plasma dopa responses in pithed rats

Dihydroxyphenylalanine (DOPA) in plasma has been thought to originate from sympathetic nerve endings and to reflect catecholamine biosynthesis, because changes in DOPA levels follow pharmacologically- or environmentally-induced manipulations that alter turnover of the sympathetic neurotransmitter, norepinephrine (NE). Skeletal muscle may be an additional, non-neural source of circulating DOPA. In the present study we examined sympathoneural and skeletal muscle contributions to DOPA in arterial plasma in pithed rats. Electrical stimulation of the spinal cord causes discharges of sympathetic post-ganglionic neurons, with attendant release of NE into the bloodstream, and discharges of spinal motoneurons, which causes diffuse contraction of skeletal muscle. Stimulation of the spinal cord rapidly elevated arterial plasma concentrations of NE, dihydroxyphenylglycol (DHPG), and DOPA. Pre-treatment with curare, a skeletal muscle relaxant, did not affect the NE and DHPG responses but attenuated the DOPA responses by about 50%. Administration of chlorisondamine, a ganglionic blocker, abolished NE and DHPG responses to cord stimulation, and DOPA responses were decreased by about 90%. Adrenal-demedullation did not affect the stimulation-induced DOPA responses. The results demonstrate that in pithed rats undergoing spinal cord stimulation, DOPA is released into the bloodstream. Since this response is markedly inhibited after ganglionic blockade and also attenuated after skeletal muscle paralysis, the results provide indirect evidence that DOPA formed in sympathetic neurons can be stored in a non-neuronal pool and released during skeletal muscle contraction.