THE SYNTHESIS OF L-DIHYDROXYPHENYLALANINE (L-DOPA) IN THE CEREBRAL GANGLIA OF THE COCKROACH, PERIPLANETA-AMERICANA L

We have measured L-DOPA levels and investigated the synthesis of L-DOPA in isolated cerebral ganglia of the American cockroach, Periplaneta americana L. Natural L-DOPA levels are between 300 and 400 pg/cerebral ganglion pair (including optic tracts). When the conversion of L-DOPA to dopamine is blocked by inhibition of DOPA decarboxylase activity the L-DOPA level in the ganglia rises rapidly, reaching 4-5 ng/cerebral ganglion pair after 2 h. Addition of L-tyrosine to the incubation medium results in even higher levels of L-DOPA (around 7 ng/cerebral ganglion pair). When alpha-methyl-p-tyrosine, an inhibitor of tyrosine hydroxylase, is added to the incubation medium with L-tyrosine then L-DOPA synthesis remains at control levels. We interpret these results as strong support for a synthesis pathway for insect neural dopamine in which L-tyrosine is hydroxylated to L-DOPA which is then decarboxylated to dopamine.