Axonal mapping of the giant peptidergic neurons VD1 and RPD2 located in the CNS of the pond snail Lymnaea stagnalis, with particular reference to the innervation of the auricle of the heart.

VD1 and RPD2 are two giant neuropeptidergic neurons located respectively in the visceral and right parietal ganglion of the central nervous system (CNS) of the pond snail Lymnaea stagnalis. They are the most prominent representatives of a system of neurons expressing a gene that is similar to the gene expressed in R15 of Aplysia californica. Both neuronal systems are involved in the regulation of cardio-respiratory phenomena. In the present study the axonal branches of VD1 and RPD2 were mapped using immunocytochemical and tracer studies. To this end the alpha 1-antiserum (directed to one of the VD1/RPD2 neuropeptides) was used in combination with Lucifer yellow (LY) and Ni-lys tracers. In whole mount preparations of the CNS, immunostained axons of VD1 and RPD2 were observed to run to the pleural, cerebral and pedal ganglia and in several nerves. Upon LY injection of VD1 thin axon branches were observed in the internal right parietal nerve. These run to the skin in the mantle area near the pneumostome and osphradium. The skin of the lips appeared to receive a similar innervation via the lip nerves. Thick LY filled axons of VD1 and RPD2 were observed in the intestinal nerve. They could be traced to the heart region. The pericardial branch of the intestinal nerve innervates the pericardium and heart (Ni-lys tracing). Immunocytochemically, using the alpha 1-antiserum, it was demonstrated that this nerve branch carries the axons of VD1 and RPD2 to the venous side of the auricle, where they enter the pericardial cavity and ramify in the auricle (but not in the ventricle).(ABSTRACT TRUNCATED AT 250 WORDS)