Selective Depletion of Dopamine, Octopamine, and 5‐Hydroxytryptamine in the Nervous Tissue of the Cockroach ( Periplaneta americana )

High‐performance liquid chromatography with electrochemical detection was used to measure the concentrations of 3,4‐dihydroxyphenylethylamine (dopamine), 5‐hydroxytryptamine (5‐HT), p ‐hydroxyphenylethanolamine (octopamine), α‐methyl‐ p ‐tyrosine, and tryptophan in the cerebral ganglia of cockroaches ( Periplaneta americana ) after peripheral administration of α‐methyl‐ p ‐tyrosine and α‐methyltryptophan. In addition, the levels of dopamine, 5‐HT, octopamine, α‐methyl‐ p ‐tyrosine, and tryptophan were determined after injection of α‐methyl‐ p ‐tyrosine, 6‐hydroxydopamine, or 5,7‐dihydroxytryptamine directly into the cerebral ganglia by means of microinjection needles. Peripheral administration of α‐methyl‐ p ‐tyrosine (400–1,600 μg/insect) caused a reduction in dopamine and 5‐HT concentrations in cockroach cerebral ganglia, although the reduction in dopamine concentrations was more pronounced. Peripheral injections of α‐methyl‐ p ‐tyrosine also reduced octopamine levels in the cerebral ganglia. Peripheral injection of α‐methyltryptophan (400–1,600 μg/insect) caused a marked reduction in 5‐HT and tryptophan concentrations in cockroach cerebral ganglia without altering dopamine or octopamine concentrations. Central injections of α‐methyl‐ p ‐tyrosine (80 μg/insect) reduced dopamine concentrations in the cerebral ganglia. However, neither 6‐hydroxydopamine (20 μg/insect) nor 5,7‐dihydroxytryptamine (20 μg/insect) caused reductions in amine levels when applied near or directly into the cerebral ganglia. The results suggest that specific lesions of aminergic neurons in insects by either 6‐hydroxydopamine or 5,7‐dihydroxytryptamine are impractical. The specific, long‐lasting depletion of 5‐HT by α‐methyltryptophan suggests that this chemical may be useful in elucidating the functions of 5‐HT in insects.