Changes from enkephalin-like to gastrin/cholecystokinin-like immunoreactivity in snail neurons

Coexistence of regulatory peptides is reported with increasing frequency in the mammalian neuroendocrine system. We have investigated the possible presence of such coexistence in the invertebrate Lymnaea stagnalis and have found evidence for concurrence of enkephalin- and gastrin/cholecystokinin (CCK)-like peptides in identified neurosecretory neurons. Our immunocytochemical studies reveal, however, that some of these neurons show variations in their content of immunoreactive peptides depending upon season or age. Thus, the light green cells of the Lymnaea cerebral ganglion, known to produce a body growth-stimulating hormone, contain enkephalin-like immunoreactivity during spring and gastrin/CCK-like immunoreactivity during fall. During winter, these cells contain both enkephalin-like and gastrin/CCK-like immunoreactivities. In another group of neurosecretory neurons, the caudodorsal cells, known to produce an ovulation hormone, age-dependent changes were noted. Thus, in young animals, these cells contain enkephalin-like immunoreactivity; in animals of intermediate age, they contain both enkephalin- and gastrin/CCK-like immunoreactivity and, in older animals, only gastrin/CCK-like immunoreactivity can be detected in these cells. Interestingly, the mediodorsal bodies, which are endocrine organs also implicated in regulating sexual function of Lymnaea, show age-dependent variations in immunocytochemically detectable enkephalin- and gastrin/CCK-like immunoreactivity which parallels that found in the caudodorsal cells. The changes noted in the light green cells and the caudodorsal cells are detected both in the cell bodies and in the terminals of these cells, suggesting that the immunoreactivity represents secretory material. The relation, if any, between the immunoreactivities detected and the growth and reproduction of Lymnaea remains to be established, as do the factors responsible for the observed age- and season-dependent variations.