Catecholaminergic system in the brain of the terrestrial slug Limax valentianus

Catecholamines, including dopamine, are thought to play important roles in the nervous system of mollusks. In the brains of the terrestrial gastropods, the procerebrum is the higher olfactory center involved in olfactory associative learning. Dopamine is known to affect the activities of interneurons in the procerebrum. However, little is known about from where and how dopaminergic neurons project to the procerebrum. In the present study, we generated a specific antibody against tyrosine hydroxylase (TH) and visualized catecholaminergic neurons, including dopaminergic neurons, in the central and peripheral nervous systems of the terrestrial slug Limax valentianus. We found that (1) the number of the cell bodies of catecholaminergic neurons was larger in the ventral aspect than in the dorsal aspect of the brain; (2) most of the TH-immunoreactive putative sensory afferents in the superior tentacle projected to the brain along the axis as bundles in a regularly spaced manner while bypassing the procerebrum following entry into the brain; (3) TH-immunoreactive nerves in the procerebrum were derived from at least three distinct origins outside and within the procerebrum; and (4) dopamine upregulated the oscillatory frequency of the local field potential oscillation in the procerebrum, which was occluded by pre-incubation with sulpiride, a mammalian D₂/D₃ receptor antagonist. This is the first study to investigate the catecholaminergic system, with a special focus on the procerebrum, using a specific antibody against TH in terrestrial gastropods. The present study uncovered catecholaminergic regulation of procerebrum activity through innervation from both the outside and inside of the procerebrum.