Topographic organization of serotonergic and dopaminergic neurons in the cerebral ganglia and their peripheral projection patterns in the head areas of the snail Helix pomatia

The distribution of monoaminergic neurons within the cerebral ganglia was i nvestigated in the pulmonate snail Helix pomatia. Simultaneous serotonin an d tyrosine hydroxylase double immunostaining revealed that the immunoreacti ve cell groups are concentrated in a putative monoaminergic center on the v entral surface of the cerebral ganglia. Simultaneous cobalt (Co)- and nicke l (Ni)-lysine backfills of cerebral nerves were combined with 5,6-dihydroxy tryptamine pigment-labelling of serotonergic neurons, or with fluorescence immunocytochemistry of dopaminergic neurons. This showed that the serotoner gic and dopaminergic cell groups can be divided into smaller subgroups on t he basis of their axonal projections into different cerebral nerves. These subgroups show a topographic organization within the serotonergic and dopam inergic neuronal clusters. In the serotonergic system, the different region s of the head are represented in a rostrocaudal direction, whereas a caudor ostral organization is characteristic for the dopaminergic system. No serot onin- or dopamine-immunoreative cell bodies but numerous fibers were observ ed in the head areas, indicating that these are innervated by cerebral mono aminergic neurons and show different innervation patterns. Serotonin-immuno reactive fibers mostly innervate muscle fibers, whereas dopamine-immunoreac tive processes do not innervate effector cells, but terminate within the ne rve branches of the head areas. On the basis of their innervation pattern, we suggest that dopaminergic neurons may take part in en route modulation o f sensory afferent and efferent processes in an as yet unknown manner. The serotonergic neurons, on the other hand, may play a direct role in the modu lation of muscle function. (C) 1999 Wiley-Liss, Inc.