Tyramine (Dopamine) β -Hydroxylase and Octopamine Immunoreactivities in the Central Nervous System of the Snail Lymnaea stagnalis

Research suggests that tyramine and octopamine have complementary distributions and roles in invertebrates somewhat analogous to those of norepinephrine and epinephrine in vertebrates. Interestingly, these groups of neurotransmitters/hormones share similar biosynthetic pathways that include the apparently homologous enzymes tyramine-β hydroxylase (TBH) and dopamine-β hydroxylase (DBH), which convert tyramine to octopamine and dopamine to norepinephrine, respectively. In this paper we report sequence alignments and Western blots that support the homology of TBH and DBH, as well as validate the use of an antibody raised against a DBH target sequence for immunohistochemical analysis of octopaminergic neural elements in molluscs. Accordingly, we found that DBH and octopamine immunoreactivities did indeed colocalize in several, but not all, neurons in the buccal, cerebral, and pedal ganglia of the pond snail Lymnaea stagnalis. Additional, apparently off-target labeling was also observed that warrants future examination. It was noteworthy, however, that no colocalization was found between DBH and tyrosine hydroxylase immunoreactivities, thus providing no evidence for norepinephrine synthesis within the central nervous system of this mollusc. These results expand on previous descriptions of neuroanatomical examinations of L. stagnalis and reinforce the argument that DBH and TBH are homologous proteins, thus supporting DBH/TBH as a target to investigate octopaminergic localization and function in molluscs and, more broadly, among invertebrates.