Perturbed pattern of catecholamine-containing neurons in mutant Drosophila deficient in the enzyme dopa decarboxylase

We have initiated a study of catecholamine-containing neurons in Drosophila melanogaster because of the potential, with this organism, to perturb catecholamine metabolism using genetic tools. The major objectives of this study were (1) to define the pattern of catecholamine-containing neurons and (2) to determine the effect of the absence of dopa decarboxylase (DDC) enzyme activity on the catecholamine-containing neurons. We chose to analyze the catecholamine-containing neurons in the ventral ganglion of the larval CNS. To define the catecholamine-containing neurons, CNSs were dissected and reacted with glyoxylic acid. The catecholamine histofluorescence (CF) neuronal pattern (normal-CF neurons) in the wild-type ventral ganglion is stereotypic. In the mutant ventral ganglia, in the absence of DDC enzyme activity, most normal-CF neurons still exhibit CF, probably indicating the presence of accumulated L-dopa. Interestingly, in the mutant CNSs, additional novel neuronal subsets also exhibit CF. Analysis of CNSs from early developmental stages revealed that the novel-CF neurons become fluorogenic earlier than the normal-CF neurons in the mutant CNS. To determine whether neuronal subsets, in addition to the normal-CF, neurons are able to sequester catecholamines, CNSs from wild-type larvae were incubated in exogenous catecholamine (L-dopa or dopamine). Incubations in L-dopa or dopamine revealed normally nonfluorogenic neurons that are able to take up the amine and become fluorogenic. Among the neurons able to sequester L-dopa or dopamine are subsets that are similar to the novel-CF neurons in the mutant CNS. This similarity is best characterized by a major novel-CF neuronal cluster in the subesophageal-thoracic region. These results suggest that in the absence of DDC activity, subsets of normally nonfluorogenic neurons capable of sequestering L-dopa or dopamine accumulate the fluorogenic catecholamine. Hypotheses that might explain the mode of accumulation of the catecholamine within the novel-CF neurons are considered.