Age-dependent changes in fluorescent neurons in the brain of Notoplana acticola, a polyclad flatworm

The formaldehyde-glutaraldehyde-sucrose (FGS) method for in situ localization of catecholamines has been applied to the nervous system of the marine polyclad flatworm Notoplana acticola. This histochemical fluorescence technique revealed the presence of a small population of fluorescent cells within the brain. The number and positions of these neurons were constant in animals of the same size, but varied with the size of the worm. The brains of small animals (8 mm in length) were found to contain 20 fluorescent cells, whereas the largest animals studied (30 mm in length) were found to have 28 such cells. Various intermediate cell numbers were found in animals between these two sizes. The origin of the newly added fluorescent cells is uncertain. Peripheral fluorescence was found in association with the tentacular ocelli (eyespots) and interneurons within the ventral submuscular nerve plexus. The fluorescent spectrum from these cells measured in situ had a lambda max of 526 nm. Treatment with HCl shifts this peak to 530 nm. L-dopamine fluoresces with a similar peak emission before HCl treatment (525.5 nm) and shifts to the appropriate longer wavelength (530 nm) following acidification. This strongly suggests that the fluorescent substance in the neurons is dopaminergic in nature.